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Author manuscript:
Buchborn T, Grecksch G, Dieterich DC, Höllt V (2016). Tolerance to lysergic acid diethylamide (LSD):
Overview, correlates, and clinical implications. In: Preedy VR (ed.), Neuropathology of Drug Addictions and
Substance Misuse, Volume 2: Stimulants, Club and Dissociative Drugs, Hallucinogens, Steroids, Inhalants and
International Aspects, 846-858. Academic Press. doi:10.1016/B978-0-12-800212-4.00079-0.
Abstract
First reports on tolerance to the serotonergic hallucinogen lysergic acid diethylamide (LSD)
have been published half a century ago, yet hitherto, a systematic review on this topic is not
available. In this chapter, we discuss tolerance to LSD with regard to its psychedelic and
somatic effects in humans, as well as selected behaviours in animals. In humans, mental
tolerance to LSD substantially manifests 24 hours after its first application and reaches a
maximum by around the fourth day. Once established, tolerance cannot be overcome even if
the initial dose is quadrupled. Mental tolerance to LSD generalises to psilocybin and
mescaline but not to tetrahydrocannabinol or amphetamine. As to LSD’s somatic effects,
mental tolerance most reliably is accompanied by tolerance to mydriasis. Five days of
abstinence are sufficient for tolerance to be reversed; symptoms of withdrawal are not
encountered. In animals, LSD induced shaking behaviour, limb flicking, and hallucinogenic
pausing are undermined by tolerance, too; the first-mentioned behaviours, for instance, are
subject to tachyphylaxis. Mechanistically, pharmacodynamic adaptations of serotonin2A (5-
HT2A) and/or of (downstream) glutamate receptors are likely to account for tolerance; a
learning-related precipitation, however, has also been described. The rapid onset of mental
tolerance probably is a main reason why LSD by humans generally is not taken on an
everyday basis. Given its rapid reversal, on the other hand, a once-per-week abuse cannot be
excluded.
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Key Words: Serotonergic hallucinogen, LSD, (cross-)tolerance, tachyphylaxis, withdrawal,
5-HT2A receptor, (mGlu2/3) glutamate receptor, wet dog shakes, hallucinogenic pause, limb
flicks
List of abbreviations
• 5-HT2A: Serotonin2A receptor
• i.m.: Intramuscular
• i.p.: Intraperitoneal
• LSD: Lysergic acid diethylamide
• MDA: Methylenedioxyamphetamine
• mGlu2/3: Metabotropic glutamate2/3 receptor(s)
1. Introduction
Lysergic acid diethylamide (LSD) is a serotonergic hallucinogen and, as such, an agonist at
serotonin2A (5-HT2A) receptors that induces profound alterations of human consciousness and
stereotype-like (gross) motor outputs in animals. LSD, internationally, is very popular among
recreational drug users (Barratt, Ferris, & Winstock, 2014) and human research, after long
halt, recently has been resumed (Carhart-Harris et al., 2014; Schmid et al., 2015) with efforts
to re-implement the drug into psychotherapy (Gasser et al., 2014). Given that certain
psychopathologies, as implicated by animal research, might draw more benefit more from
repeated (rather than one-time) application of LSD (Buchborn, Schröder, Höllt, & Grecksch,
2014; Gorka, Wojtasik, Kwiatek, & Maj, 1979), it – in light of the current developments –
seems important to understand LSD’s basic neuropsychopharmacology not only as to its
acuteness, but also as to its chronicity. Repeated application of LSD, as with other substances
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of abuse, leads to a decline in the organism’s responsiveness to various of the drug’s effects.
This decline, commonly referred to as tolerance, might be acquired within the first hours after
a singular exposure (tachyphylaxis), or instead build up with multiple exposures over a few
days. Tolerance to LSD has first been described by the mid 1950's (e.g. Cholden, Kurland, &
Savage, 1955), yet – apart from a subsection in Hintzen and Passie’s comprehensive textbook
on LSD’s pharmacology (2010) – there are hitherto virtually no reviews specifically dedicated
to this topic. Bridging the gap, we here review the most important findings about tolerance to
LSD in humans and animals, discuss possible mechanisms, and outline clinical implications
of repeated LSD application.
2. Tolerance to LSD in humans
2.1. Tolerance to LSD’s psychedelic effect
Tolerance to LSD’s psychedelic effect, most comprehensively, has been investigated by Isbell
and colleagues. They applied LSD to patients formerly addicted to opioids (N=4-11) and
across multiple publications tested eleven different application regimens (Tab. 1: 1-11) (e.g.
Isbell, Belleville, Fraser, Wikler, & Logan, 1956). LSD’s psychedelic effect is characterised
by (visual) illusions and pseudo-hallucinations, formal thought disorders, ambivalence and
exaltation of affection, as well as distorted perceptions of time, space and body-self (e.g. Stoll,
1947). Isbell and colleagues quantified these by means of Abramson-et-al.’s 47-items
questionnaire, which asked the patients to self-rate their psychophysiological state (e.g. “Are
shapes and colours altered?”, “Do you feel as if in a dream?”, or “Do you tremble inside?”
(Abramson et al., 1955, p. 34)), as well as of a 4-graded rating system used by a physician to
externally estimate the severity of the patients’ perceptual distortions. Except from one
regimen, where LSD was given twice a day (Tab. 1: 2), Isbell and colleagues usually applied
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LSD once per day, per os or intramuscularly (i.m.). In most regimens, they started with a low
dose of around 0.3 μg/kg, gradually increased it over four to ten days to a final dose of around
1.4 μg/kg, which then was maintained (Tab. 1: 1-2, 5-11). Comparing the patients’ reactions
to the final dose before and at the end of a given regimen, Isbell and colleagues demonstrated
that significant tolerance to LSD was evident after four, seven to eight, eleven, 13-15, and 22
days of daily application (Tab. 1: 1-2, 5-11) (see Tab. for references, e.g. Isbell & Jasinski,
1969). On the average (referring to the results of both the questionnaire and the physician’s
rating), the patients’ mental responsiveness to LSD across the different regimens was reduced
by around 78%1 [1Note: Percent values in this review, if not directly extractable, were
calculated on basis of the mean values numerically or graphically reported in the original
papers]. Figure 1 depicts the rigorousness of this reduction. A 1.5-μg/kg dose of LSD, applied
in a pretest, induced a strong mental reaction; after two weeks of daily LSD treatment,
however, the same dose was virtually inactive (Tab. 1: 8) (Rosenberg, Wolbach, Miner, &
Isbell, 1963). In two further experiments, which Isbell et al. (1956) performed to outline the
course of tolerance development, LSD’s effects were quantified not only before and after, but
also at various time points during the regimens (Tab. 1: 3-4). In the first of these experiments,
they applied an average dose of 138 μg to their patients for two weeks. Quantifying LSD’s
psychoactivity on each day, they demonstrated that a 47% decrease in the questionnaire
responses occurred by day 2 already. Tolerance as to both parameters, the questionnaire
responses and the physician’s rating, became near-maximal by day 4 and thereafter remained
stable on any of the ten days that followed (Tab. 1: 3). In the second of their repeated-
quantification experiments, Isbell et al. (1956) applied LSD in daily doses of 1.28 to 1.55
μg/kg for 84 days, and challenged tolerance with increasing doses at weekly, later biweekly
intervals. The patients’ mental reactions to LSD were significantly reduced by days 7, 14, and
21. Although complete tolerance (especially in terms of the questionnaire responses) did not
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manifest in this experiment (Tab. 1: 4), LSD’s initial activity in the weeks to follow failed to
re-ignite even when the challenge doses were doubled, tripled, and quadrupled.
Apart from Isbell et al., there are four further groups that published research on mental
tolerance to LSD in humans (Tab. 1: 12-16, 18). Unfortunately, this research is restricted to
single-case or anecdotal-like reports and, accordingly, lacks proper statistical analysis. In
normal volunteers (N=2) and psychiatric/neurological patients (N=5), respectively, LSD’s
psychedelic effect during a four-to-seven-days treatment with increasing doses strongly was
undermined by tolerance and barely detectable (Tab. 1: 15-16) (Abramson, Jarvik, Gorin, &
Hirsch, 1956; Balestrieri & Fontanari, 1959). In schizophrenics (N=4), applying LSD in
increasing doses for five days, mental tolerance – as judged by the patients’ outward
behaviour – manifested by day 2, became maximal by 3 and 4, and slightly (if at all) reversed
with the highest dose on day 5 (Tab. 1: 12) (Cholden et al., 1955). Similarly in normal
volunteers (N=2), schizophrenics (N=4), and/or in a non-defined group of subjects,
respectively, tolerance to a 100-μg dose of LSD was (near-)complete by day 3, slightly (if at
all) varied on days 5 and 6, and thereafter (with continuous treatment) remained stable for up
to weeks or months (Tab. 1: 13-14, 18) (Abramson et al., 1956; Cholden et al., 1955; Hoffer
& Osmond, 1967).
2.1.1. Tachyphylaxis to LSD’s psychedelic effect
Tolerance to the mental effects of LSD, as described above, substantially manifests 24 hours
after a one-time application and can, therefore, be regarded as tachyphylaxis (see Mini-
Dictionary). Whether tachyphylaxis to LSD occurs at intervals shorter than 24 hours, on the
other hand, is largely unknown and can only be discussed with reference to anecdotal reports.
Balestrieri and Fontanari (1959) injected two 200-μg doses of LSD at an interval of six hours
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to two of their patients. The interval was chosen so that the second dose would challenge
tachyphylaxis right after the (main) effects of the first dose had worn off. As the responses
evoked by either dose were almost identical, though, tachyphylaxis at the given interval did
not seem to occur. In two other reports, the application of a second dose of LSD three hours
after the first dose (or at around the peak of the first dose’s effects) led to a prolongation
and/or intensification, and reignited (perceptual) changes characteristic for the earlier phases
of the first dose (Freedman, 1984; Hoffer, 1965). The acuteness of the second dose was
briefer, however, and the prolongation it brought fell short of additiveness (Freedman, 1984).
Thus, referring to the latter report, tachyphylaxis to LSD might not only occur at a 24-h but,
to a certain degree, also at a 3-h interval.
2.2. Tolerance to LSD’s somatic effects
Apart from alterations of human consciousness, LSD is known to stimulate and/or facilitate
certain autonomic and spinal functions. Typically, LSD’s somatic effects comprise a strong
dilatation of pupils (mydriasis), patellar hyperreflexia, and slight increases in pulse
(tachycardia), blood pressure (hypertension), and body core temperature (hyperthermia). As
with mental tolerance, somatic tolerance to LSD most comprehensively has been investigated
by Isbell and colleagues (Tab. 1: 3-11) (see Tab. for references, e.g. Isbell, Wolbach, Wikler,
& Miner, 1961). LSD induced mydriasis, in these investigations, showed a similar pattern of
tolerance development, as did the psychedelic effect. With a mean decrease of 57.97%,
tolerance gained significance after three, six, and ten days (Tab. 1: 3), could be demonstrated
when challenged on days 7-8, 11, 13-15, 21, and 22 (Tab. 1: 4-11), and failed to reverse even
when challenge doses were doubled, tripled, or quadrupled (Tab. 1: 4) (e.g. Isbell et al.,
1956). Tolerance to LSD induced patellar hyperreflexia and hypertension manifested by the
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days 3, 6, and 10 as well (Tab. 1: 3); as to the other regimens, however, results are more
inconsistent (Tab. 1: 4-10) (e.g. Rosenberg, Isbell, Miner, & Logan, 1964). Tolerance to LSD
induced tachycardia and hyperthermia has not been characterised as to its development over
three, six, and ten days; as to the other regimens, however, results again vary (Tab. 1: 5-11)
(e.g. Wolbach, Isbell, & Miner, 1962). Chessick, Haertzen, & Wikler (1964) – the only group
that apart from Isbell et al. employed a large enough sample to perform proper statistical
analysis (N=9) – demonstrated that schizophrenic patients, at the end of a one-week
treatment, largely were tolerant to LSD induced mydriasis and hyperreflexia. Partially, their
data conflict with those of Isbell and colleagues who, engaging an almost identical regimen,
only found mydriasis but not hyperreflexia to become tolerant (Tab. 1: 5 vs 17). Balestrieri &
Fontanari (1959), who for four to seven days applied LSD in increasing doses to a group of
psychiatric and neurologic patients (N=5) (Tab. 1: 16), found that – although a high degree of
mental tolerance was established – (not further defined) autonomic symptoms often remained.
Thus, given the latter report and the above-mentioned inconsistencies regarding hyperreflexia,
hypertension, tachycardia, and/or hyperthermia, tolerance to LSD’s somatic effects is less
clear-cut and (except from mydriasis) not necessarily coupled to mental tolerance.
2.3. Withdrawal and recovery from tolerance to LSD
Isbell and colleagues, following their 14- and 84-days regimens (Tab. 1: 3-4), withdrew LSD
without the patients’ knowledge and exchanged it to placebo. The exchange was not
recognised by the patients and symptoms of abstinence did not occur. Once withdrawn, three
days off of LSD were sufficient for the patients to fully recover from somatic and mental
tolerance (Isbell et al., 1956). Anecdotal reports from the other groups confirm that partial
(Abramson et al., 1956) to full recovery from tolerance to LSD manifests within three to six
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days after discontinuation (Cholden et al., 1955; Hoffer & Osmond, 1967), and that no
symptoms of withdrawal are encountered (Balestrieri & Fontanari, 1959).
2.4. Concluding remarks on tolerance to LSD in humans
Human research on tolerance to LSD has exclusively been performed in the 1950/60’s, and is
weakened by experimental drawbacks, including small samples sizes (of mostly psychiatric
patients), incomplete documentation, and/or the usage of unvalidated psychometrics.
Notwithstanding these, the given results overall clearly indicate that tolerance to LSD’s
psychedelic effect is rigorous. It requires three to four days of daily application to reach near-
maximum levels and five days of abstinence to completely reverse. As to long-term
application, although results and documentation are less dense, tolerance likewise appears to
persist and not to reverse even if challenged with doses as high as 500 μg. Mental tolerance to
LSD, most reliably, is accompanied by tolerance to mydriasis; across the different somatic
effects of LSD, however, tolerance only inconsistently manifests.
3. Tolerance to LSD in animals
Tolerance to LSD in animals critically varies with the strain and species used, the regimen of
application, as well as the behaviour in question. Given the general overview provided by
Hintzen and Passie (2010), we here restrict our discussion to three selected behaviours (i.e.
shaking behaviour, limb flicking, and hallucinogenic pausing) which most elaborately have
been investigated in terms of tolerance and, like the human psychedelic effect, are thought to
primarily arise from LSD’s interaction with 5-HT receptors (see section 4.2.).
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3.1. Tolerance to LSD induced shaking behaviour in mammals
Tolerance to LSD induced shaking behaviour, a repetitive movement of the mammal’s head
(and trunk) around the long-axis of its body (Buchborn, Schröder, Dieterich, Grecksch, &
Höllt, 2015), has been investigated in rats, cats, and monkeys. In a recent investigation in
Sprague-Dawley rats, we applied a single 25-μg/kg dose of LSD (intraperitoneally [i.p.]) and
challenged acute tolerance, i.e. tachyphylaxis, by re-applying the same dose after four, eight,
or 24 hours (Tab. 2: 12). As depicted in figure 2a, tachyphylaxis to LSD – with a 46.6%
decrease in shaking behaviour – manifested at the early interval, decreased to 32.3% at the
intermediate interval, and totally was absent when both doses were separated by 24 hours.
Tachyphylaxis did not seem to represent an artefact of LSD-accumulation because a 50-μg/kg
dose of the drug (as investigated in non-tachyphylaxed rats) evoked even more shaking
behaviour than a 25-μg/kg dose (Fig. 2b) (Buchborn et al., unpublished). Having learnt about
the rapid onset and the rapid reversal of tachyphylaxis, we in a next step focused on tolerance
as it would develop over the course of four days. Applying LSD (25 μg/kg, i.p.) once or twice
per day, we found tolerance to shaking behaviour (with a maximal reduction by 40.5% on day
3) only to arise from the twice-per-day regimen (Tab. 2: 13-14). The reduction was dose-
dependent; when the second of each day’s two doses was increased by an order of magnitude,
the degree of tolerance likewise increased, and by day 3 acquired a maximum of 66.8% (Tab.
2: 15) (Buchborn et al., 2015). In macaques, tolerance to LSD induced shaking behaviour
manifested on day 3 as well, yet in contrast to Sprague-Dawley rats, a once-per-day regimen
was sufficient to that end; results on tachyphylaxis are inconsistent (Tab. 2: 4-6) (Schlemmer
& Davis, 1986; Schlemmer, Nawara, Heinze, Davis, & Advokat, 1986). In cats, tachyphylaxis
to LSD, as compared to Sprague-Dawley rats, developed with a delay. Although a decrease in
shaking behaviour was present two and six hours after the first dose’s application already, it
became significant only at the 24-hours interval and reversed when both doses were separated
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by three days (Tab. 2: 1-2) (Trulson & Jacobs, 1977). Rabbits, after an eight-days treatment
with LSD, showed significant (cross-)tolerance to head bobs (a variant of shaking behaviour)
challenged by DOI (dimethoxyiodoamphetamine) or endogenous serotonin (Tab. 2: 7, 9-10)
(Aloyo & Dave, 2007; Romano et al., 2010).
3.2. Tolerance to LSD induced limb flicking in cats
Tolerance to LSD induced limb flicking, a paw-movement cats repeatedly exhibit as if to
remove a foreign substance, has been investigated by Trulson and colleagues. They injected a
single 10-μg/kg dose of LSD (i.p.) and challenged tolerance, or rather tachyphylaxis2, by
injecting a second, slightly higher dose of the hallucinogen (50 μg/kg, i.p.) at varying
intervals [2Note: Trulson and Crisp (1983) rejected the term tachyphylaxis and instead spoke
of rapidly-developing tolerance. Regarding both termini as synonymous, however (see Mini-
Dictionary), we discuss their results in terms of tachyphylaxis]. Tachyphylaxis to LSD (with a
23.7% decrease in limb flicking) could be detected as early as half-an-hour after the first
dose’s application; it increased with longer intervals (-48.1% at one hour, -51.3% at two
hours, and -74.2% at six hours) and became near-complete (-91.5%) when the second dose
was applied 24 hours after the first dose. Tachyphylaxis, similarly as noted for shaking
behaviour in rats (see section 3.1.), did not seem to relate to an accumulation of LSD. If there
had been such a relation, a 60-μg/kg dose of LSD (in non-tachyphylaxed cats) would have
had to induce significantly less limb flicking than a 50-μg/kg dose; this, however, was not the
case (Trulson & Crisp, 1983). Mirroring results for LSD’s human psychedelic effect and
shaking behaviour in cats (see sections 2.3. and 3.1.), three to five days of abstinence were
needed for limb flicking to recover from tachyphylaxis (Trulson & Jacobs, 1977).
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3.3. Tolerance to LSD induced hallucinogenic pausing in rats
In rats, operantly conditioned to press a lever for food or water reinforcement, LSD interrupts
the constancy of lever pressing and induces periods of non-responding, i.e. hallucinogenic
pausing (Rech, Tilson, & Marquis, 1975). Tolerance to LSD induced hallucinogenic pausing
has been investigated in various rat strains. Sprague-Dawley rats, daily treated with a 100- or
130-μg/kg dose of LSD (i.p.), within three to six days became near-maximally tolerant to the
pause-inducing effect of the drug (Commissaris, Lyness, Cordon, Moore, & Rech, 1980;
Freedman, Appel, Hartman, & Molliver, 1964; Rech et al., 1975); tachyphylaxis, applying
LSD three times at one-hour intervals, only as a trend occurred (Freedman et al., 1964). When
higher doses of LSD were used, tolerance was protracted. Thus, with 150 μg/kg as a daily
dose, tolerance did not manifest before six to ten days; with a 195-μg/kg dose, a ten-days
regimen failed and up to two weeks were needed (Freedman et al., 1964; Rech et al., 1975).
The protraction of tolerance, as demonstrated for the 195-μg/kg dose, depended on the
schedule of reinforcement; when a variable-interval (instead of a fixed-ratio) schedule was
employed, the onset of tolerance was accelerated to one week (Freedman et al., 1964). As to
other rat strains, although literature is less dense, tolerance had similar time- and dose-
requirements as in Sprague-Dawley rats. In CFN, hooded, and Wistar rats – with daily
injection of 96, 100, and 130 μg/kg (i.p.), respectively – (near-complete) tolerance to LSD
manifested within three days (Murray, Craigmill, & Fischer, 1977; Silva, Carlini, Claussen, &
Korte, 1968; Winter, 1971). Again, however, no such manifestation could be observed when a
high-dose regimen (250 μg/kg/d, i.p. [hooded rats]) was employed at a fixed-ratio schedule
(Murray et al., 1977). Holtzman rats, the only strain that markedly fell out of the alignment,
even after three weeks of daily LSD application (100 μg/kg, i.p.), exhibited undiminished
hallucinogenic pausing. Only with a three-times-a-day regimen thereafter, tolerance at the end
of the fourth week finally manifested (Kovacic & Domino, 1976).
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3.4. Concluding remarks on tolerance to LSD in animals
A generalised conclusion about tolerance to LSD in animals, given the variability of regimens
engaged across the different studies, is difficult. All three behavioural effects of LSD
discussed are subject to tolerance, yet the temporal patterns of development vary. In cats and
Sprague-Dawley rats, respectively, tolerance to limb flicking and/or shaking behaviour
manifests within a few hours after a singular application of LSD. Cats require three days of
abstinence to recover, for rats, one day is enough. Tolerance to hallucinogenic pausing in rats
is not as rapid; it varies with the dose, strain, and schedule of reinforcement, and may need up
to weeks to manifest.
4. Possible mechanisms of tolerance to LSD
General pharmacology differentiates three basic mechanisms of tolerance – pharmacokinetic,
pharmacodynamic, or learning-related in nature – that engage adaptations of either the
metabolism of the drug, the (molecular) targets of the drug, or the organism’s capacity to
expect and compensate for the effects of the drug.
4.1. Pharmacokinetic adaptations
Trulson and Jacobs (1977) addressed the possibility that tolerance to LSD arises from
adaptations of the drug’s metabolism. Applying two doses of LSD at a two- or a 24-hours
interval to cats, they demonstrated that tachyphylaxis to limb flicking and shaking behaviour
manifested at the early and/or the late interval, respectively (see sections 3.1.-3.2.). Since the
plasma and brain concentrations of LSD in the tachyphylaxed cats, at either interval, were
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virtually the same as they were in non-tachyphylaxed animals, however, the decrease in
behaviour did not seem to be accounted for by an upregulation of its degradation. Similarly,
when CFN rats were treated for three days with a 96-μg/kg dose of LSD, the hallucinogenic
pausing it induced was undermined by tolerance (see section 3.3.), yet the degradation of LSD
– as indicated by its liver and plasma levels – remained unaffected (Winter, 1971).
4.2. Pharmacodynamic adaptations
LSD interacts with a variety of different monoamine receptors (Ray, 2010) and tolerance, if
pharmacodynamic, theoretically could arise from adaptations of either of these receptors.
Despite LSD’s promiscuity in receptor binding, however, shaking behaviour in rodents
(Buchborn et al., 2015), limb flicking in cats (Heym, Rasmussen, & Jacobs, 1984), as well as
LSD’s psychedelic effect in humans (Nichols, 2004) are thought to be primarily mediated by
activation of (cortical) 5-HT2(A) receptors. For hallucinogenic pausing and shaking behaviour,
respectively, LSD’s activity at 5-HT1(A) receptors (Rech, Commissaris, & Mokler, 1988) and
heterocomplexation between 5-HT2A and metabotropic glutamate2 (mGlu2) receptors might
additionally play a role (Moreno, Holloway, Albizu, Sealfon, & Gonzalez-Maeso, 2011).
Repeated application of LSD (130 or 260 μg/kg, i.p.) for five or ten days has been shown to
downregulate 5-HT2(A) binding sites in various brain areas of Sprague-Dawley rats, including
brain stem, mesencephalon, hippocampus, and cortex. Other receptors, such as cortical 5-
HT1A, 5-HT1B, adrenergic alpha1, alpha2 and beta receptors, cortical serotonin transporters, or
striatal dopamine2 receptors were not affected (Tab. 3: 4, 13) (Buckholtz, Freedman, &
Middaugh, 1985; Buckholtz, Zhou, Freedman, & Potter, 1990). In rabbits, repeated LSD
application downregulated 5-HT2A (but not 5-HT2C) receptors in the frontal cortex (Aloyo,
Dave, Rahman, & Harvey, 2001), which co-occurred with tolerance to shaking behaviour
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induced by endogenous serotonin (Tab. 3: 14, 16) (Aloyo & Dave, 2007). In Wistar and
Sprague-Dawley rats, respectively, frontocortical 5-HT2(A) downregulation and/or
desensitisation similarly were paralleled by tolerance to the discriminative cue of LSD (Tab.
3: 15) (Gresch, Smith, Barrett, & Sanders-Bush, 2005) and shaking behaviour induced by the
serotonergic hallucinogens DOM and DOB (dimethoxymethyl- and
dimethoxybromoamphetamine) (Buchborn et al., 2015; Leysen, Janssen, & Niemegeers,
1989). Given these results in rodents and the finding that hallucinogen/entactogen-
experienced humans – in a first positron-emission-tomography study – exhibited (a trend for)
reduced cortical 5-HT2A binding sites, too (Erritzoe et al., 2011), it is overall very likely that
(fronto-)cortical 5-HT2(A) downregulation indeed is one of the pharmacodynamic key
adaptations tolerance to serotonergic hallucinogens, so LSD, arises from. In Sprague-Dawley
rats, after a three-days application, on the other hand, LSD (130 μg/kg, i.p.) failed to reduce
cortical 5-HT2(A) binding sites (Tab. 3: 11) (Buckholtz et al., 1990). Thus, tolerance to LSD
(especially with application regimens shorter than five days) might not solely be accounted
for by mere 5-HT2A downregulation. In a recent study, we applied LSD (25+250 μg/kg/d, i.p.)
for four days to Sprague-Dawley rats and demonstrated that – although LSD induced shaking
behaviour increasingly was undermined by tolerance (see section 3.1.) – a reduction in
frontocortical 5-HT2(A) signalling and binding sites, respectively, did not or only as trend
occur. In contrast to the lack of significant 5-HT2(A) regulation, however, LSD significantly
reduced frontocortical glutamate binding sites and mGlu2/3 signalling (Tab. 3: 18) (Buchborn
et al., 2015). LSD has no affinity (Ray, 2010) but is thought to indirectly affect glutamate
receptors via 5-HT2A-mGlu2 crosstalk and 5-HT2A related glutamate release (see Fig. 3)
(Moreno et al., 2011; Muschamp, Regina, Hull, Winter, & Rabin, 2004). As the reduction in
glutamate binding sites, in our study, highly correlated with tolerance to LSD induced shaking
behaviour (r=.86), whereas unregulated 5-HT2(A) receptors did not (Buchborn et al., 2015), it
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appears that glutamatergic transmission stimulated downstream of LSD-5-HT2A interaction
(Fig. 3) can adapt as a substitute of 5-HT2A receptors and, thus, complement 5-HT2A
downregulation in certain phases of tolerance development.
The pharmacodynamics of tachyphylaxis to LSD are less well characterised. A one-time
application of LSD, in Sprague-Dawley rats, only in an unphysiological dose (650 μg/kg, i.p.)
induced cortical 5-HT2(A) downregulation (Tab. 3: 8-10) (Buckholtz, Zhou, & Freedman,
1988). Furthermore, the mechanisms of tachyphylaxis to LSD induced limb flicking, as
suggested by an ontogeny study in cats, are different from the mechanisms of limb flicking
itself (Trulson & Howell, 1983). LSD, by activation of somatodendritic 5-HT1A receptors
inhibits the activity of serotonergic neurons of the dorsal raphe nucleus (Fig. 3), and
tachyphylaxis to limb flicking and shaking behaviour co-occurred with a sensitisation of this
inhibition (Tab. 3: 3) (Trulson, Heym, & Jacobs, 1981). Whether the given 5-HT1A
sensitisation, a depletion of glutamatergic vesicles, and/or any other adaptation beyond 5-
HT2A-glutamate interaction underlies tachyphylaxis to LSD, however, remains to be
elucidated.
4.3. Learning-related adaptations
The role of behavioural accommodation for tolerance development has been investigated in
terms of LSD induced hallucinogenic pausing and shaking behaviour. As discussed in section
3.3., Sprague-Dawley and hooded rats within three to six days became tolerant to LSD
induced hallucinogenic pausing. Tolerance occurred solely, though, when LSD on each of the
application days was given right before the lever-press session. When LSD on the first days
was given after, and only on the last day before the session, tolerance on the last day
completely was absent. Thus, although both the pre- and the post-session rats received the
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same total amount of LSD, only the pre-session animals – which were able to anticipate and
behaviourally compensate for the drug effect – became tolerant (Commissaris et al., 1980;
Murray et al., 1977). Addressing a simpler form of learning, we recently challenged tolerance
to LSD induced shaking behaviour with regard to habituation (Buchborn et al., 2015). Rats,
over the course of a multiple-days experiment, usually habituate and become less excited
about the experimental procedure. To make sure that this decrease in excitation would not
confound with tolerance, we for four days (before the actual LSD experiment) habituated a
group of animals to daily saline injections and the experimental non-home cages. As the
overall-decline in shaking behaviour, however, did not significantly differ between the so-
habituated and non-habituated animals (Tab. 2: 15-16), tolerance did not seem to represent an
artefact of contextual habituation.
4.4. Concluding remarks on mechanisms of tolerance to LSD
Tolerance to LSD, as indicated by the given literature, is of the pharmacodynamic rather than
the pharmacokinetic type and, in cases the drug interferes with the organism’s strive for
reinforcement, can also be precipitated by behavioural accommodation. Regarding effects of
LSD that originate in 5-HT2A related glutamate release in the cortex, cortical 5-HT2A and/or
(downstream) glutamate receptor downregulation is likely to constitute the key adaptation
tolerance arises from. As LSD’s regulatory activity across the monoaminergic systems
appears to be very complex, though (Tab. 3: 13 vs 17) (Buchborn et al., 2014; Buckholtz et
al., 1990), further correlative-design studies are needed to more clearly establish the
interrelation between tolerance and receptor regulation. Likewise, given the lack of human
research into this field, imaging studies correlating mental tolerance to adaptations of receptor
binding and/or brain metabolism appear desirable.
17
5. Pathological and therapeutic implications of repeated LSD application
It is generally assumed that LSD, compared to other substances of abuse, rather has a low
addiction liability (e.g. Gable, 1993; Nutt, King, Saulsbury, & Blakemore, 2007) and
consequently rarely is taken on a regular basis. That near-complete tolerance to the
psychedelic effect of LSD manifests within three to four days (see section 2.1.), might be one
of the main reasons for recreational users to avoid its everyday-application. As tolerance to
LSD reverses within three to six days of abstinence, however (see section 2.3.), a weekend-
based pattern of usage cannot be excluded (Barron, Lowinger, & Ebner, 1970; Ludwig &
Levine, 1965). Likewise, as doses above 500 μg have not been challenged, yet, an
overcoming of tolerance with even higher doses of LSD (and a subsequent abuse) cannot be
ruled out.
Given its apparently low incidence, any putative sequelae and/or benefits of frequent LSD use
are largely unknown, and only a few publications can be referred to. In two retrospective
studies, where subjects had a median of 75 lifetime exposures (at a mean age of 40 [N=16]) or
an average of 29.3 exposures within 12.2 months (N=20), no (consistent) evidence of
cognitive impairments and/or brain damages could be detected on basis of a
neuropsychological test battery (McGlothlin, Arnold, & Freedman, 1969; Wright & Hogan,
1972). In a third retrospective study, where subjects (at a mean age of 20 [N=21]) had a mean
of 65 LSD lifetime exposures, electroencephalogram measurements revealed an increased
sensitivity to visual stimuli, the auditory two-tone evoked potential (which is sensitive to
cognitive disorganisation), on the other hand, was normal (Blacker, Jones, Stone, &
Pfefferbaum, 1968). Bender (1970), who for weeks, months, sometimes even years applied
LSD in daily doses up to 150 μg to autistic and schizophrenic children, qualitatively resumed
18
that the drug improved the well-being and the psychosocial adjustment of her patients.
Although tolerance to LSD’s perceptual effects in most children occurred by day 2, the
clinical benefit they drew from the drug appeared to persist for months. Taking account of the
fact that there is cross-tolerance between LSD and certain drugs of the antidepressant-class
(which is indicative of a mechanistic overlap), we – engaging the olfactory-bulbectomy rodent
model of depression – recently evaluated the antidepressant-like property of repeated LSD
treatment. Bulbectomised rats, reminiscent on negatively biased cognitions of depressed
patients, exhibit a deficiency to learn negative-stimulus avoidance. LSD (130 μg/kg,
subcutaneous), given on eleven days in a row, ameliorated this avoidance learning deficiency,
such as it in former publications only had been found for antidepressant drugs, and
additionally normalised the bulbectomy-related disruption of hippocampal 5-HT2 signalling
(Buchborn et al., 2014). In contrast to these salutogenic-like adaptations after short-term LSD
treatment in bulbectomised Wistar rats, unimpaired Sprague-Dawley rats – after a three-
months every-other-day treatment with a similar dose of the drug (160 μg/kg, i.p.) – exhibited
pathological adaptations which in behaviour and neurogenetics had a schizophrenia-like
appeal (Martin, Marona-Lewicka, Nichols, & Nichols, 2014).
5.1. Concluding remarks on clinical implications of repeated LSD application
The given literature indicates that repeated LSD application at high frequency is uncommon
among recreational drug users and, if performed at (weekly to) monthly intervals, does not
seem to precipitate gross neuropsychological dysfunctions. Beyond once-in-a-while use, daily
short-term application of LSD, as implicated by experimental data in rats, might – if
alternated with stimulus-contexts that favour cognitive plasticity – entail therapeutic benefit
for defined pathological conditions, such as depression; long-term use at an every-other-day
19
rate (which probably impedes tolerance development), as opposed, might harbour pathology
itself.
Applications to other addictions and substance misuse
• Tolerance to a drug can generalise to another drug, a phenomenon called cross-
tolerance, and therefore influence the clinical course of polydrug abuse.
• Mental tolerance to LSD in humans generalises to psilocybin and mescaline (and vice
versa) (Isbell et al., 1961; Wolbach et al., 1962), moderately to DMT
(dimethyltryptamine) (Rosenberg et al., 1964), slightly (i.e. in terms of peak intensity)
to scopolamine (Isbell, Rosenberg, Miner, & Logan, 1964), but not to amphetamine or
THC (tetrahydrocannabinol) (Isbell & Jasinski, 1969; Rosenberg et al., 1963). LSD-
tolerant dogs, moreover, exhibit cross-tolerance to several effects of MDA
(methylenedioxyamphetamine) (Nozaki, Vaupel, & Martin, 1977).
• Given that psilocybin and MDMA (methylenedioxymethamphetamine), which in the
body is converted to MDA, like LSD downregulate cortical 5-HT2(A) receptors
(Buckholtz et al., 1990; Reneman et al., 2002), an overlap in pharmacodynamic
regulation is likely to account for cross-tolerance. Given that mescaline, on the other
hand, fails to induce 5-HT2(A) downregulation (Buckholtz et al., 1990), yet unknown
(pharmacodynamic and/or pharmacokinetic) principles might play a complementary
role.
Mini-Dictionary of Terms
20
• Serotonergic hallucinogen: Psychedelic. A drug that by activation of serotonin2(A)
receptors provokes shaking behaviour in (certain) mammals and alterations of
consciousness in humans.
• Tolerance: In consequence of repeated or long-term exposure to a drug, the organism
over days, weeks, and months becomes less responsive to the effect(s) of the drug.
• Tachyphylaxis: Acute tolerance. In consequence of a single exposure or multiple
short-interval exposures to a drug, the organism within minutes and hours becomes
less responsive to the effect(s) of the drug.
• Cross-tolerance: Tolerance to a drug can diminish the effect(s) of another drug if the
enzymes that degrade both drugs and/or the (receptor) targets that mediate the drugs’
effects overlap.
• 5-HT2A receptor: A transmembrane protein highly enriched in the mammalian cortex
cerebri that (upon occupancy by serotonin or serotonergic hallucinogens) regulates the
cell’s physiology through interaction with G-protein(s).
• Shaking behaviour: Head twitches, wet dog shakes. A behaviour of (certain) mammals
that is stereotyped by serotonergic hallucinogens. The mammal shows brisk rotational
movements of head (and trunk) around the long-axis of its body.
• Limb flicking: A behaviour of cats that is stereotyped by serotonergic hallucinogens.
The cat lifts its paw, rapidly shakes it, or flicks it away from the body as if to remove a
foreign substance.
• Hallucinogenic pausing: Hallucinatory pausing. Intermittent interruption of operant
lever-press responding of rats induced by serotonergic hallucinogens.
21
Key facts of LSD
• Semi-synthetic derivative of the ergot alkaloid ergotamine; first synthesised in the year
1938 by the Swiss Sandoz-chemist Albert Hofmann; first human research, published
in 1947, by Werner Stoll in Switzerland.
• Federal prohibition in the USA effective by 1966, other countries followed; after long
halt, recent resumption of human research by Robin Carhart-Harris and David Nutt in
England, and Peter Gasser and Matthias Liechti in Switzerland.
• Serotonergic hallucinogen; as such to be differentiated from anticholinergic delirants,
antiglutamtergic dissociatives, serotonin-releasing entactogens, and atypical
hallucinogens (including the GABAergic muscimol, the cannabinoid
tetrahydrocannabinol, and the kappa-opioid salvinorin A).
Summary points
• In humans, mental tolerance to LSD manifests 24 hours after its first application,
reaches a maximum by around the fourth day, remains relatively constant thereafter,
and does not reverse unless a few days of abstinence are interspersed. Symptoms of
abstinence, even after a 12-weeks treatment, do not occur.
• Recreational drug users, given the rapid onset of mental tolerance, generally do not
apply LSD on an everyday basis; given the rapid reversal of tolerance, on the other
hand, a once-per-week abuse may be encountered.
22
• In cats and rats, tolerance to LSD induced limb flicking and/or shaking behaviour
manifests in a tachyphylaxis-like manner; hallucinatory pausing in rats, as opposed, is
more resistant to tolerance.
• The mechanisms of tolerance to LSD are of the pharmacodynamic rather than the
pharmacokinetic type and (in terms of 5-HT2A related behaviours) appear to involve
adaptations of (cortical) 5-HT2A and/or (downstream) glutamate receptors.
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26
Figures and tables
Title of Figure 1: Tolerance to the psychedelic effect of LSD in humans
Legend to Figure 1. Mean time course of the psychedelic effect of a 1.5-μg/kg intramuscular
dose of LSD (as determined by a self-rating questionnaire) before (control) and after (test)
two weeks of daily LSD treatment (N=10). Recreated from: Psychopharmacologia, 5, 1963, p.
11, fig. 2a, Observations on direct and cross tolerance with LSD and d-amphetamine in man,
by Rosenberg et al..
27
Title of Figure 2: Tachyphylaxis to LSD induced shaking behaviour in rats
Legend to Figure 2. (a) Shaking behaviour in Sprague-Dawley rats, as induced by two
separate 25-μg/kg intraperitoneal doses of LSD at a 4-h, 8-h, or 24-h interval. (b) Shaking
behaviour induced a by 25- and 50-μg/kg dose of LSD (i.p.). Each mean+SEM (n=5-7 [per
group], two littermates per cage). Wilcoxon comparison to 0-h effect, #p<.05, ##p<.01 (a).
Mann-Whitney comparison to control, **p<.01, and to 50-μg/kg effect, #p<.05, respectively
(b). For interpretation, see section 3.1. (unpublished; for general methods Buchborn et al.,
2015).
28
Title of Figure 3: Pharmacodynamics affected by repeated application of LSD
Legend to Figure 3. (1) LSD inhibits the dorsal raphe nucleus via activation of 5-HT1A
autoreceptors and (consequently) interferes with the release of serotonin. LSD via 5-HT2A
activation promotes glutamatergic transmission, which might involve a (2) facilitation of
mGlu2/3 sensitive glutamate release, as well as (3) postsynaptic amplification. Tolerance to
LSD co-occurs with adaptations of 5-HT1A autoreceptors, frontocortical 5-HT2A, mGlu2/3
and/or overall-glutamate receptors. Adaptations of 5-HT2A and overall-glutamate receptors
might be complementary to one another (for discussion, see section 4.2.).
29
LSD regimen
Tolerance
Reference
(+ sample
[size])
Challenge
Noted for
1
7 ds: 20 μg daily
increasing to
75 μg p.o. (by 7th d)
8th d: 75 μg p.o.
Mentally (somatic effects n.d.)
(Isbell et al.,
1956)
n=8
2
1st d: 2x 10 μg
4th d: 75 μg p.o.
Mentally (somatic effects n.d.)
n=11
2nd d: 2x 20 μg
3rd d: 2x 30 μg p.o.
3
7-8 ds: 90-130 μg →
3 ds: 150 μg →
3 ds: 180 μg p.o.
Daily for mental
effects; ds 3, 6, and 10
for somatic effects
Mentally (Ø -81.41% for R, Ø
-78.51% for Q), mydriasis (Ø -
57.97%), HTN (Ø -63.89%),
and PTR (Ø -131.11%)*
n=4-5
4
7 ds: Ø 1.28 μg/kg →
77 ds: Ø 1.55 μg/kg
p.o.
7th d: Ø 1.28 μg/kg
Mentally (Ø -73,42% for R, Ø
-45.83% for Q) and mydriasis
(Ø -55.9%); inconsistent for
HTN (Ø -29.9%) and PTR (Ø -
7.3%)*
n=7 FOA
14th d: Ø 1.55 μg/kg
21st d: Ø 1.55 μg/kg
35th d: 3 μg/kg
49th d: 4.5 μg/kg
63rd d: 6 μg/kg p.o.
5
6-7 ds: 0.25 μg/kg
daily increasing to 1.5
μg/kg p.o. (by 6th d)
7-8th d: 1.5 μg/kg p.o.
Mentally, mydriasis,
hyperthermia, HTN, and
TACH; not for PTR
(Isbell et al.,
1961)
n=10
6
12 ds: 0.15 μg/kg
daily increasing to 1.5
μg/kg p.o. (by 10th d)
13th d: 1.5 μg/kg p.o.
Mentally, mydriasis,
hyperthermia, HTN, and
TACH; not for PTR
n=9 FOA
7
14 ds: 0.3 μg/kg daily
increasing to 1.5 μg/kg
i.m. (by 5th d)
15th d: 1.5 μg/kg i.m.
Mentally, mydriasis, HTN, and
PTR; not for hyperthermia or
TACH
(Wolbach et al.,
1962)
N=10 FOA
8
13 ds: 0.3 μg/kg daily
increasing to 1.5 μg/kg
i.m. (by 5th d)
14th d: 1.5 μg/kg i.m.
Mentally, mydriasis;
trend for TACH and PTR;
not for HTN or hyperthermia
(Rosenberg et
al., 1963)
N=10 FOA
9
21 ds: Increasing to
1.5 μg/kg i.m. once
daily**
22th d: 1.5 μg/kg i.m.
Mentally, mydriasis, HTN, and
TACH (hyperthermia and PTR
n.d.)
(Isbell et al.,
1964)
N=6 FOA
10
13 ds: Daily
increasing to 1.5 μg/kg
i.m. (by 6th d)
14th d: 1.5 μg/kg i.m.
Mentally and mydriasis; not
for TACH, HTN, or PTR
(hyperthermia n.d.)
(Rosenberg et
al., 1964)
N=6 FOA
11
10 ds: 0.5 μg/kg daily
increasing to 1.5 μg/kg
i.m. (by 5th d)
11th d: 1.5 μg/kg i.m.
Mentally, mydriasis, and
TACH (HTN, PTR, and
hyperthermia n.d.)
(Isbell &
Jasinski, 1969)
N=10 FOA
12
5 ds: 100 μg daily
increasing to 500 μg
i.m. (by 5th d)
Daily
Mentally (estimated by
outward gross behavioural
change)
(Cholden et al.,
1955)
n=4
13
2 weeks: 100 μg i.m.
Daily
n=4
schizophrenics
…continued
30
14
3-6 ds: 100 μg p.o.
Daily
Mentally (somatic effects n.d.)
(Abramson et
al., 1956) n=2
15
5 ds: 10 μg (1st d)
daily increasing to 75
or 100 μg (by 5th d)
p.o.
Daily
Mentally (somatic effects n.d.)
n=2 college
graduates
16
4-7 ds: 25-50 μg (1st d)
daily increasing to 200
μg p.o.***
Daily
Mentally; partially for
(undefined) autonomic effects
(Balestrieri &
Fontanari, 1959)
N=5 PNP
17
6 ds: 0.25 μg/kg (1st d)
daily increasing to
1.25 μg/kg (by 6th d)
p.o.
7th d: 1.5 μg/kg p.o.
Mydriasis, PTR (mental
effects n.d.)
(Chessick et al.,
1964) N=9
schizophrenics
18
1st d: 300 μg →
6 ds: 100 μg →
months: 100 μg#
Daily(?)#
Mentally
(Hoffer &
Osmond, 1967)#
*Percent values (averaged across the different challenge days) were calculated on basis of the mean values graphically presented
in the original paper. **Exact regimen details not stated. ***Regimens varied between subjects, exact details not stated. #Exact
details, application route, and sample (size) not stated. 2x: Twice; → Followed by; Ø Mean; HTN: Hypertension; FOA: Former
opioid addicts; i.m.: Intramuscular; n.d.: Not determined; PNP: Psychiatric and neurological patients; PTR: Patellar
hyperreflexia; p.o.: Per os; Q: 47-items self-rating questionnaire; R: Rating by physician; TACH: Tachycardia.
Title of Table 1: Human studies on tolerance to LSD
Legend to Table 1. Each row (1-18) contains the LSD regimen employed, the day(s) when
tolerance was challenged, the results of challenge, samples and the corresponding reference.
31
LSD regimen
Shaking Behaviour
(+ species/strain)
Tolerance
Reference
1
1x 10 μg/kg i.p. →
50 μg/kg challenge after
2, 6, or 24 h
Cat: Head and body
shakes
Tachyphylaxis at 24 h (-
85.6%); n.s. at 2 h (-
57.2%) and 6 h (-
67.6%)*
(Trulson &
Jacobs, 1977)
2
1x 10 or 50 μg/kg i.p.
→ 50 μg/kg challenge
after 1, 3, 5, or 7 ds
Tachyphylaxis only at
24 h
3
1st d: 50 μg/kg
Cat: Head and body
shakes
Tachyphylaxis (-90.5%)
(Trulson et al.,
1981)
2nd d: 50 μg/kg i.p.
4
5 ds: 10 μg/kg i.m.
Macaque: Body
shakes
Tolerance on 3rd d (-
41.4%)* (other ds n.d.)
(Schlemmer et
al., 1986)
5
5 ds: 10 μg/kg i.m.
Macaque: Body
shakes
Tachyphylaxis on 2nd d
(-51.3%)* (other ds
n.d.)
(Schlemmer &
Davis, 1986)
6
2 ds: 10 μg/kg i.m.
No tachyphylaxis (?) (-
11.9%)* (sign. n.d.)
7
8 ds: 14.2 µg/kg i.v.
Rabbit: Open-field
(endogenous 5-HT)
related head bobs
Cross-tolerance on 10th
d (-40%) (other ds n.d.)
(Aloyo & Dave,
2007)
8
8 ds: 0.43 μg into each
site of dHC
Rabbit: DOI induced
head bobs
No cross-tolerance on
9th d (other ds n.d.)
(Romano et al.,
2010)
9
8 ds: 1.3 μg into each
site of dHC
Cross-tolerance on 9th d
(-43.5%) (other ds n.d.)
10
8 ds: 4.3 μg into each
site of dHC
Cross-tolerance on 9th d
(-44.4%) (other ds n.d.)
11
8 ds: 13 μg into each
site of dHC
No cross-tolerance on
9th d (other ds n.d.)
12
1x 25 μg/kg i.p. →
25 μg/kg challenge after
4, 8, or 24 h
Sprague-Dawley rat:
Head twitches and wet
dog shakes
Tachyphylaxis at 4 h (-
46.6%) and 8 h (-32.3%)
(Buchborn et al.,
unpublished
[Fig. 2a])
13
4 ds: 25 μg/kg i.p.
Sprague-Dawley rat:
Head twitches and wet
dog shakes
No tolerance
(Buchborn et al.,
unpublished)
14
4 ds: 25 μg/kg (a.m.) +
25 μg/kg i.p. (p.m.)**
Tolerance on 2nd d (-
27.8%), 3rd d (-40.5%),
and 4th d (-28.6%)
15
4 ds: 25 μg/kg (a.m.) +
250 μg/kg i.p. (p.m.)**
Tolerance on 2nd d (-
46.5%), 3rd d (-66.8%),
and 4th d (-66.8%)
(Buchborn et al.,
2015)
16
4 ds: 10 ml saline/kg →
Tolerance on 2nd d (-
55.4%), 3rd d (-50.4%),
and 4th d (-59.5%)
4 ds: 25 μg/kg (a.m.) +
250 μg/kg i.p. (p.m.)**
*Percent values were calculated on basis of the mean values presented in the original paper. **P.m. dose only on ds 1, 2,
and 3 (shaking behaviour was quantified each day after the a.m. dose). → Followed by; 1x: Once; 5-HT: Serotonin; dHC:
Dorsal hippocampus; DOI: Dimethoxyiodoamphetamine; i.m.: Intramuscular; i.p.: Intraperitoneal; i.v.: Intravenous; n.d.:
Not determined; n.s.: Not significant; sign.: Significance.
32
Title of Table 2: Studies on tolerance to LSD induced shaking behaviour in animals
Legend to Table 2. Each row (1-16) contains the LSD regimen employed, species and
shaking-behaviour component(s) investigated for tolerance, results of tolerance investigation,
and the corresponding reference.
33
LSD regimen
Behavioural
tolerance
Pharmacodynamic correlate
Reference
1
1x 100 μg/kg i.p.
Sprague-Dawley
rat: N.d.
FB, BS + SC: [3H]5-HT binding
UC
(Trulson &
Jacobs, 1979)
2
4 ds: 100 μg/kg/6 h
i.p.
FB, BS + SC: [3H]5-HT and
[3H]LSD binding↓
STR, LFB: D2 binding UC
3
1st d: 50 μg/kg →
Cat: Shaking
behaviour, limb
flicking
LSD induced inhibition of dorsal
raphe unit activity↑
(Trulson et al.,
1981)
2nd d: 50 μg/kg i.p.
4
10 ds: 260 µg/kg i.p.
Sprague-Dawley
rat: N.d.
Cortex, HC, STR, DE/ME, P/MO:
[3H]LSD binding to 5-HT2↓ and to
5-HT(1) UC
(Buckholtz et al.,
1985)
5
1st d: 50 μg/kg i.p.
Cat: Limb
flicking
FB, BS + SC: [3H]5-HT binding
UC
(Trulson, 1985)
(→ 2nd d: 50 μg/kg for
challenge of tolerance)
6
5 ds: 50 μg/kg i.p.
(→ 6th d: 50 μg/kg for
challenge of tolerance)
7
5 ds: 2x 50 μg/kg i.p.
FB, BS + SC: [3H]5-HT binding↓
(→ 6th d: 50 μg/kg for
challenge of tolerance)
8
1x 130 μg/kg i.p.
Sprague-Dawley
rat: N.d.
Cortex: 5-HT2(A) binding UC
(Buckholtz et al.,
1988)
9
1x 650 μg/kg i.p.
Cortex: 5-HT2(A) binding↓
10
1x 130 μg/kg i.p.
Sprague-Dawley
rat: N.d.
Cortex: 5-HT2(A) binding UC
(Buckholtz et al.,
1990)
11
3 ds: 130 μg/kg i.p.
Cortex, STR, HT, BS: 5-HT2(A) and
5-HT1A binding UC
ME, HC: 5-HT2(A) binding↓, 5-HT1A
binding UC
12
5 ds: 16.25, 32.5, or
65 μg/kg i.p.
Cortex: 5-HT2(A) binding UC
13
5 ds: 130 μg/kg i.p.
Cortex, ME, HC, BS: 5-HT2(A)
binding↓ 5-HT1A binding UC
STR, HT: 5-HT2(A) and 5-HT1A
binding UC
Cortex: 5-HT1B, α1/2, β, and SERT
binding UC
STR: D2 UC
14
4 ds: 0.03 µM/kg →
Rabbit: N.d.
FC: 5-HT2A binding↓ 5-HT2C
binding UC
(Aloyo et al.,
2001)
2 ds: No injection →
4 ds: 0.03 µM/kg i.v.
15
5 ds: 130 μg/kg s.c.
(→ 6th d: 60 μg/kg for
challenge of tolerance)
Sprague-Dawley
rat: LSD-vs-
saline
discrimination
mPFC, ACC: LSD and DOI
induced [35S]GTPγS binding↓
(Gresch et al.,
2005)
(m)PFC, ACC, FPC, CL, and EN:
[125l]LSD binding to 5-HT2A↓
…continued
34
16
8 ds: 14.2 µg/kg i.v.
Rabbit: Open-field
(endogenous 5-HT)
related head bobs
FC: 5-HT2A binding↓
(Aloyo & Dave,
2007)
17
11 ds: 130 μg/kg s.c.
Wistar rat (sham-
operated): No
tolerance to LSD
induced open-field
hypolocomotion
(unpublished)
FC: 5-HT2A binding↑ α-MS, 8-
OH-DPAT, 5-HT, DA, NA, and
isoprenaline induced [35S]GTPγS
binding↑
(Buchborn et al.,
2014)
HC: 5-HT2A binding UC; α-MS,
DA, and isoprenaline induced
[35S]GTPγS binding↓
18
4 ds: 25 μg/kg (a.m.) +
250 μg/kg i.p. (p.m.)*
Sprague-Dawley
rat: Shaking
behaviour
FC: 5-HT2A binding(↓),
[3H]glutamate binding↓, DOB
induced [35S]GTPγS binding UC,
and LY35 induced [35S]GTPγS
binding↓
(Buchborn et al.,
2015)
*P.m. dose only on ds 1, 2, and 3. 1x: Once; 5-HT: Serotonin; 5-HT1A/1B/2A/2C: Serotonin1A/1B/2A/2C receptor(s); 8-OH-DPAT: 8-
hydroxy-2-[di-n-propylamino]tetralin (5-HT1A); ↑ Increase; ↓ Decrease; (↓) Trend for decrease; ACC: Anterior cingulate cortex;
α1/2: Alpha adrenergic1/2 receptor; α-MS: Alpha-methylserotonin (5-HT2); β: Beta adrenergic receptor; BS: Brain stem; CL:
Claustrum; D2: Dopamine2 receptor; DA: Dopamine; DE: Diencephalon; DOB: Dimethoxybromoamphetamine (5-HT2); DOI:
Dimethoxyiodoamphetamine (5-HT2); EN: Endopiriform nucleus; FC: Frontal cortex; FB: Forebrain; FPC: Frontal parietal cortex;
GTPγS: Guanosine 5'-(γ-thio)triphosphate; HC: Hippocampus; HT: Hypothalamus; i.p.: Intraperitoneal; Isoprenaline: Beta
adrenergic; i.v.: Intravenous; LFB: Limbic forebrain; LY35: LY354740 (mGlu2/3); ME: Mesencephalon; mPFC: Medial prefrontal
cortex; NA: Noradrenaline; n.d.: Not determined; P/MO: Pons/medulla oblongata; s.c.: Subcutaneous; SC: Spinal cord; SERT:
Serotonin transporter; STR: Striatum; UC: Unchanged.
Title of Table 3: Pharmacodynamics of repeated LSD application in animals
Legend to Table 3. Each row (1-18) contains the LSD regimen employed, species and
behaviour investigated for tolerance, pharmacodynamics investigated for adaptation, and the
corresponding reference.
