E R Samuels

University of Nottingham, Nottingham, ENG, United Kingdom

Are you E R Samuels?

Claim your profile

Publications (9)35.25 Total impact

  • Source
    E R Samuels, E Szabadi
    [Show abstract] [Hide abstract]
    ABSTRACT: The locus coeruleus (LC), the major noradrenergic nucleus of the brain, gives rise to fibres innervating most structures of the neuraxis. Recent advances in neuroscience have helped to unravel the neuronal circuitry controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. Alterations in LC activity due to physiological or pharmacological manipulations or pathological processes can lead to distinct patterns of change in arousal and autonomic function. Physiological manipulations considered here include the presentation of noxious or anxiety-provoking stimuli and extremes in ambient temperature. The modification of LC-controlled functions by drug administration is discussed in detail, including drugs which directly modify the activity of LC neurones (e.g., via autoreceptors, storage, reuptake) or have an indirect effect through modulating excitatory or inhibitory inputs. The early vulnerability of the LC to the ageing process and to neurodegenerative disease (Parkinson's and Alzheimer's diseases) is of considerable clinical significance. In general, physiological manipulations and the administration of stimulant drugs, alpha(2)-adrenoceptor antagonists and noradrenaline uptake inhibitors increase LC activity and thus cause heightened arousal and activation of the sympathetic nervous system. In contrast, the administration of sedative drugs, including alpha(2)-adrenoceptor agonists, and pathological changes in LC function in neurodegenerative disorders and ageing reduce LC activity and result in sedation and activation of the parasympathetic nervous system.
    Current Neuropharmacology 10/2008; 6(3):254-85. · 2.03 Impact Factor
  • Source
    E R Samuels, E Szabadi
    [Show abstract] [Hide abstract]
    ABSTRACT: The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.
    Current Neuropharmacology 10/2008; 6(3):235-53. · 2.03 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A sudden loud sound evokes an electromyographic (EMG) response from the orbicularis oculi muscle in humans together with an auditory evoked potential (AEP) and an increase in skin conductance (SC). Startle responses are inhibited by weak prepulses (prepulse inhibition, (PPI)) and may also be modified by the level of alertness. We compared the sedative drug clonidine and the alerting drug modafinil on sound-evoked EMG, AEP, and SC responses, on the PPI of these responses and on level of arousal and autonomic functions. Sixteen healthy male volunteers participated in four weekly sessions (clonidine 0.2 mg, modafinil 400 mg, their combination, placebo) in a double-blind, cross-over, balanced design. Responses were evoked by sound pulses of 115 and 85 dB (PPI) for 40 ms and recorded conventionally. Level of alertness, autonomic functions (pupil diameter, blood pressure, heart rate, salivation, temperature) and the plasma levels of the hormones prolactin, thyroid-stimulating hormone and growth hormone were also measured. Data were analyzed with analysis of variance with multiple comparisons. Both prepulses and clonidine attenuated all three startle responses and modafinil antagonized clonidine's effects on the EMG and AEP responses. None of the drugs affected PPI. Clonidine showed sedative and sympatholytic effects, and modafinil showed alerting and sympathomimetic effects. In conclusion, startle responses were susceptible not only to PPI but also to the level of arousal.
    Neuropsychopharmacology 12/2007; 32(11):2405-21. · 8.68 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: There is a close relationship between arousal and pupil diameter, decrease in the level of arousal being accompanied by constriction of the pupil (miosis), probably reflecting the attenuation of sympathetic outflow as sedation sets in. Paradoxically, sedation induced by benzodiazepines is not accompanied by miosis. The objective of this study was to examine the hypothesis that diazepam may attenuate both the sympathetic and the opposing parasympathetic outflow to the iris, which may mask the miosis. Dapiprazole (sympatholytic) and tropicamide (parasympatholytic) were applied topically, together with the cold pressor test (CPT), to manipulate the sympathetic/parasympathetic balance. Sixteen healthy male volunteers participated in four weekly sessions according to a balanced double-blind protocol. Diazepam 10 mg (two sessions) and placebo (two sessions), associated with either 0.01% tropicamide or 0.5% dapiprazole eyedrops, were administered orally. Pupil diameter, light and darkness reflexes and pupillary sleepiness waves were recorded with infrared video pupillometry, alertness was measured by critical flicker fusion frequency (CFFF) and visual analogue scales (VAS), blood pressure and heart rate by conventional methods. CPT was applied after post-treatment testing. Data were analysed by analysis of variance, with multiple comparisons. Diazepam caused sedation (reduction in VAS alertness scores and CFFF, increase in sleepiness waves), dapiprazole had a sympatholytic and tropicamide a parasympatholytic effect on the pupil. Diazepam had no effect on pupil diameter and reflexes or their modifications by the antagonists. CPT increased pupil diameter, blood pressure and heart rate, and the increase only in systolic blood pressure was attenuated by diazepam. Diazepam-induced sedation is not accompanied by any change in either the sympathetic or parasympathetic influence on the iris.
    Psychopharmacology 12/2007; 195(1):41-59. · 4.06 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: To investigate the effects of the D2-receptor agonist pramipexole with and without the co-administration of the peripherally acting D2-receptor antagonist domperidone on measures of alertness, autonomic and endocrine function. Sixteen male volunteers participated in four weekly sessions of pramipexole 0.5 mg, domperidone 40 mg, their combination, and placebo administered according to a balanced, double-blind design. Alertness (visual analogue scales (VAS), critical flicker fusion frequency, pupillographic sleepiness test), autonomic (pupil diameter, light and darkness reflexes, blood pressure, heart rate, salivation, temperature) and endocrine (prolactin, thyroid-stimulating hormone (TSH), growth hormone (GH)) functions were assessed. Data were analyzed with anova with multiple comparisons. The pre-post treatment changes in VAS alertness were reduced by pramipexole with and without domperidone (mean difference from placebo (95% confidence interval), mm): pramipexole -15.75 (-23.38, -8.13), combination -11.84 (-20.77, -2.91). Treatment condition significantly affected pupil diameter measured in different ways (resting pupil diameter (F(3,45) = 8.39, P < 0.001), initial diameter of the light reflex response (F(3,42) = 3.78, P < 0.05), and light (F(3,45) = 5.21, P < 0.005) and dark (F(3,45) = 3.36, P < 0.05) diameters of the darkness reflex response). Pramipexole without domperidone consistently increased pupil diameter on all measures (P < 0.05), whereas with domperidone only the increase in resting and dark diameters reached significance. Pramipexole reduced light reflex amplitude and increased latency, whereas the combination affected latency only. Concentrations of prolactin and TSH were increased by domperidone. Pramipexole reduced prolactin and increased GH concentrations. The attenuation of the central pupillary effects of pramipexole by domperidone indicates that domperidone had access to some central D2-receptors.
    British Journal of Clinical Pharmacology 12/2007; 64(5):591-602. · 3.58 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Classical fear conditioning involves pairing a neutral conditional stimulus (CS) with an aversive unconditional stimulus (US). Subsequent presentation of the CS alone induces fear responses. Acquisition of conditioned fear is thought to involve learning of the CS/US association, followed by memory consolidation. Recently we reported that the N1/P2 auditory evoked potential was enhanced by fear conditioning in humans. Diazepam 10 mg, given before CS/US pairing, prevented subsequent expression of fear potentiation when the response was elicited, 1 week later, in the presence of the CS. In this experiment, we examined whether this effect of diazepam was caused by disruption of the formation of CS/US associations or by disruption of consolidation. The benzodiazepine antagonist flumazenil was used to block the effect of diazepam either during the association period or during subsequent consolidation. Forty-two male volunteers (18-35 years) participated in two sessions separated by 7 days. In Session One, they ingested diazepam 10 mg or placebo: 60 minutes later they received flumazenil 1 mg or saline intravenously (i.v.). Then they received 20 presentations of a light (CS), 50% of which terminated with electric shock (US). This was followed by a second infusion of flumazenil or saline. Subjects received placebo/saline/saline (Group 1), diazepam/saline/saline (Group 2), diazepam/flumazenil/saline (Group 3) and diazepam/saline/flumazenil (Group 4). In Session Two, the CS was presented without the US; 50% of CS presentations terminated with a sound pulse; an equal number of sound pulses were presented without the CS. Auditory evoked potentials were recorded at Cz. In Session Two, CS presentation enhanced the auditory N1/P2 potential in placebo-treated subjects (Group 1). This enhancement was prevented by diazepam (Group 2). Flumazenil reversed diazepam's effect on fear potentiation if it was administered before conditioning (Group 3), but not if it was administered afterwards (Group 4). The results confirm that diazepam prevents the acquisition of fear conditioning in humans, and suggest that it disrupts the formation of CS/US associations, rather than the consolidation of fear memory.
    Journal of Psychopharmacology 02/2007; 21(1):93-101. · 3.37 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The noradrenergic locus coeruleus is a major wakefulness-promoting nucleus of the brain, which is also involved in the regulation of autonomic and endocrine functions. The activity of the locus coeruleus is believed to be tonically enhanced by a mesocoerulear dopaminergic pathway arising from the ventral tegmental area of the midbrain. Both modafinil, a wakefulness-promoting drug, and pramipexole, a D(2)/D(3)receptor agonist with sedative properties, may act on this pathway, with modafinil increasing and pramipexole decreasing locus coeruleus activity. The aim of this study was to compare the two drugs on alertness, autonomic and endocrine functions in healthy volunteers. Pramipexole (0.5mg), modafinil (200mg), and their combination were administered to 16 healthy males in a double-blind, placebo-controlled design. Methods included tests of alertness (pupillographic sleepiness test, critical flicker fusion frequency, visual analogue scales), autonomic functions (resting pupil diameter, light and darkness reflex responses, heart rate, blood pressure, salivation, core temperature), and endocrine functions (blood concentrations of prolactin, growth hormone, and thyroid stimulating hormone). Data were analysed by ANOVA. Pramipexole reduced alertness, caused pupil dilatation, increased heart rate, reduced prolactin and thyroid stimulating hormone, and increased growth hormone level. Modafinil caused small increases in blood pressure and core temperature, and reduced prolactin levels. The sedative effect of pramipexole and the autonomic effects of modafinil are consistent with altered activity in the mesocoerulear pathway; the pupil dilatation following pramipexole suggests reduced dopaminergic excitation of the Edinger-Westphal nucleus.
    Journal of Psychopharmacology 12/2006; 20(6):756-70. · 3.37 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: In a previous study in healthy volunteers, the anti-Parkinsonian drug pramipexole caused sedation and pupil dilatation, consistent with the stimulation of inhibitory D(2)/D(3) autoreceptors on the ventral tegmental area dopaminergic neurones. The sedation may be related to the removal of the dopaminergic excitation of the locus coeruleus (via the meso-coerulear pathway), whereas the pupil dilatation may be due to the removal of the dopaminergic excitation of the Edinger-Westphal nucleus (via a putative meso-pupillomotor pathway). We investigated the hypothesis that amisulpride, a D(2)/D(3) receptor antagonist, would have effects opposite to those of pramipexole on alertness, pupillary and endocrine functions. Pramipexole (0.5 mg), amisulpride (50 mg), and their combination were administered to 16 healthy males in a balanced, cross-over, double-blind design. Tests included measures of alertness (Pupillographic Sleepiness Test, critical flicker fusion frequency, visual analogue scales), pupillary functions (resting pupil diameter, light and darkness reflex responses), non-pupillary autonomic functions (heart rate, blood pressure, salivation, core temperature), and endocrine functions [blood concentrations of prolactin, growth hormone (GH) and thyroid stimulating hormone (TSH)]. Data were analysed by ANOVA. Pramipexole reduced alertness and pupillary light reflex response amplitude, tended to reduce core temperature, reduced prolactin levels and increased GH levels. Amisulpride reduced pupil diameter, increased the amplitude of the light reflex response and prolactin and TSH levels. The opposite effects of pramipexole and amisulpride on alertness, pupillary function and pituitary hormone levels are consistent with their interactions with inhibitory D(2)/D(3) receptors on VTA neurones and in the tuberoinfundibular system.
    Psychopharmacology 10/2006; 187(4):498-510. · 4.06 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Patients suffering from Alzheimer's disease (AD) may show increased sensitivity to tropicamide, a muscarinic cholinoceptor antagonist. AD is associated with a severe loss of noradrenergic neurones in the locus coeruleus (LC), which can be "switched off" experimentally by the alpha(2)-adrenoceptor agonist clonidine. The possibility arises that increased pupillary sensitivity to tropicamide in AD may be due to diminished LC activity. To examine the hypothesis that clonidine may potentiate tropicamide-evoked mydriasis. Sixteen healthy male volunteers participated in two experimental sessions (0.2 mg clonidine or placebo) conducted 1 week apart. In each session tropicamide (0.01% 10 microl x 2) was applied to the left eye and artificial tear (10 microl x 2) was applied to the right eye. Pupillary functions (resting pupil diameter and light and darkness reflexes), alertness and non-pupillary autonomic functions (blood pressure, heart rate, core temperature and salivary output) were measured. Data were analysed by ANOVA, with multiple comparisons. Tropicamide increased resting pupil diameter, velocity and amplitude of the darkness reflex response, and decreased recovery time of the light reflex response. Clonidine affected all these pupillary measures in the opposite direction with the exception of the recovery time. The mydriatic response to tropicamide was potentiated by pre-treatment with clonidine. Clonidine reduced critical flicker fusion frequency, subjective alertness, blood pressure, salivation and temperature. The potentiation of tropicamide-evoked pupil dilatation by clonidine may be due to the abolition of the increase in parasympathetically mediated pupil constriction due to reduced LC activity.
    Psychopharmacology 02/2006; 184(1):95-106. · 4.06 Impact Factor