cAMP Response Element Binding Protein
Phosphorylation in Nucleus Accumbens Underlies
Sustained Recovery of Sensorimotor Gating Following
Repeated D2-Like Receptor Agonist Treatment in Rats
Alison K. Berger, Thomas Green, Steven J. Siegel, Eric J. Nestler, and Ronald P. Hammer Jr.
acute treatment with dopamine D2-like receptor agonists and in patients with schizophrenia. Repeated treatment with a D2-like agonist,
examined the short- and long-term effects on PPI of treatment with quinpirole and ropinirole, dopamine D2/D3receptor agonists, and the
molecular mechanism by which they occur.
Methods: PPI was assessed in adult male Sprague–Dawley rats following acute and chronic treatment with quinpirole or ropinirole and 1,
binding protein (CREB) overexpression in the NAc on PPI following chronic quinpirole treatment was assessed.
dose. Repeated ropinirole treatment significantly increased PPI compared with acute treatment, and increased CREB phosphorylation in
NAc neurons. Subsequent ropinirole challenge had no effect as long as 28 days later, at which time NAc CREB phosphorylation had
normalized. Overexpression of dominant negative mutant CREB prevented PPI recovery induced by chronic quinpirole treatment.
NAc circuits to promote recovery of sensorimotor gating.
motor gating theoretically reflect sensory flooding and cognitive
fragmentation (1,2) and can be detected across species by quanti-
the reduction in startle response that occurs when the startling
a prepulse stimulus can inhibit the brainstem startle response to a
subsequent pulse stimulus.
phine and amphetamine disrupts PPI in rats (4), as do more selec-
ensorimotor gating, the neural mechanism underlying the
integration and processing of sensory information, is dis-
rupted in patients with schizophrenia (1). Defects in sensori-
tive D2-like receptor agonists, such as quinpirole (5) and ropinirole
(6). The importance of D2-like receptors in the NAc was confirmed
contrast to acute treatment, repeated treatment with indirect do-
pamine agonists such as cocaine or amphetamine attenuates PPI
pletely reverses acute disruption, which we term PPI recovery (9).
In addition to regulating PPI, acute D2-like receptor stimulation
reduces adenylate cyclase (AC) activity via Gi/Goprotein coupling,
which in turn decreases cyclic adenosine monophosphate (cAMP)
activity (10,11). Prolonged activation of these receptors in vitro
produces heterologous sensitization, characterized by enhanced
The molecular mechanisms underlying this effect are not well un-
derstood, although agonist-induced enhancement of Gspro-
quinpirole in vivo leads to recovery of PPI (9) accompanied by
tein (CREB) in the NAc (13). However, it is unknown whether CREB
phosphorylation and its subsequent regulation of target genes are
necessary to sustain PPI recovery even in the absence of further
Ropinirole is a selective D2/D3dopamine agonist used in the
treatment of Parkinson’s disease and restless leg syndrome (14).
ropinirole at the D3-receptor (15,16). Acute administration of ropi-
reduce PPI in rats (6) and in humans (17).
This study assessed the effects of pharmaceutical-grade ropini-
role compared with quinpirole on PPI and striatal CREB phosphor-
ylation after acute or repeated treatment, and examined the sus-
Medicine, Boston, Massachusetts; Department of Pharmacology and
Toxicology (TG), University of Texas Medical Branch, Galveston, Texas;
Department of Psychiatry and Pharmacology (SJS), University of Penn-
macology and Psychiatry (EJN), Mount Sinai School of Medicine, New
York, New York; Department of Basic Medical Sciences, Pharmacology
Arizona; and Department of Psychology and Neuroscience (RPH),
Arizona State University, Tempe, Arizona.
Address correspondence to Ronald P. Hammer Jr., Ph.D., Department of
nix, AZ 85004; E-mail: Ron.Hammer@arizona.edu.
Received Dec 30, 2009; revised Aug 29, 2010; accepted Aug 31, 2010.
BIOL PSYCHIATRY 2011;69:288–294
© 2011 Society of Biological Psychiatry
tained effect of repeated ropinirole treatment. The involvement of
CREB activation in the process of PPI recovery was examined di-
rectly using adeno-associated viral (AAV)-mediated gene transfer
to overexpress CREB or dominant-negative mutant CREB (mCREB)
in the NAc during chronic quinpirole treatment.
Methods and Materials
Animals and Drug Treatment
For all the following experiments, animals were provided with
food and water ad libitum while housed in a climate-controlled
facility with reverse light–dark cycles (lights off at 9 AM, on at 7 PM).
Rats were allowed to acclimate to the laboratory for 7 days before
handling and habituation to the behavioral testing apparatus. All
experiments were approved by the Tufts–New England Medical
Center and the University of Arizona Institutional Animal Care and
Male Sprague–Dawley rats (Charles River Laboratories, King-
ston, Rhode Island) weighing 250–300 g were habituated to han-
dling and subcutaneous (SC) saline injection and placed into a
California) with 70-dB ambient noise for 5 min daily on each of 2
days before baseline PPI testing. Baseline PPI was assessed as de-
(1.0 mL/kg SC) on 2 consecutive days to ensure a reliable mean
value. Treatment groups were normalized according to the mean
acoustic startle response observed during baseline testing.
Prepulse Inhibition Testing
All PPI testing was conducted during the dark phase (10 AM–2
PM). Each animal was exposed to 70-dB ambient noise for 5 min,
followed by a test session, which consisted of four pulse trials (120
dB, 40-msec pulses), followed by a randomized presentation of 10
pulse and 30 prepulse trials (10 each of 73, 76, and 82 dB, 20-msec
pulse trials. The intertrial interval was an average of 15 sec (range:
8–22 sec). Percent PPI was calculated using the equation 100-
compared using repeated-measures analysis of variance in each
treatment group, followed by a Fisher’s least squares difference
post hoc test.
Several days after normalization, rats received either quinpirole
HCl (.0, .05, .1, or .3 mg/kg SC; Sigma-Aldrich, St. Louis, Missouri) or
GlaxoSmithKline, Research Triangle Park, North Carolina) in .9%
sterile saline followed by PPI testing 10 min later. For 10 days, the
vice versa, followed by PPI testing 10 min later.
Following habituation and normalization of groups, rats re-
ceived repeated treatment once daily for 28 consecutive days with
the same dose of ropinirole HCl (.0, .05, .1, or .5 mg/kg SC) in .9%
sterile saline vehicle. PPI was assessed on the first and last day of
drug administration as described earlier.
On the 29th day, rats were chal-
lenged with the same dose of ropinirole they received previously.
Twenty minutes after the challenge injection, sodium pentobarbi-
tal (100 mg/kg intraperitoneal) was administered, rats were per-
fused, and brains were removed and processed for immunohisto-
chemistry to label CREB phosphorylated at serine 133 (Ser133
using National Institutes of Health ImageJ following background
correction and application of a set labeling and size threshold
across all sections (see Supplement 1, Methods and Materials, for
Following habituation and normalization of groups, rats re-
with ropinirole HCl (.0 or .1 mg/kg SC) in .9% sterile saline vehicle.
Startle amplitude was assessed on the first and last day of drug
administration as described earlier. Thereafter, PPI testing was ac-
Another group of rats received repeated drug treatment once
in .9% sterile saline vehicle. Startle amplitude was assessed on the
handled daily, rats that previously received .1 mg/kg ropinirole
vehicle treatment received either .0 or .1 mg/kg of ropinirole, fol-
lowed by PPI testing. Brains were removed on the 29th day after
termination of drug treatment and processed for CREB and phos-
phoCREB immunohistochemistry as described earlier.
Following habituation and normalization of groups, rats under-
phosphorylation, coexpressed with enhanced green fluorescent
protein (eGFP), or eGFP alone bilaterally in the NAc. The next day,
PPI was assessed 10 min after an injection of .9% sterile saline (1.0
ml/kg SC) to examine the effect of surgery as described above.
Three weeks after stereotaxic surgery, rats received quinpirole
HCl (.0 or .1 mg/kg SC; Sigma-RBI, St. Louis, Missouri) in .9% sterile
rats underwent subcutaneous implantation of custom-made con-
trol or quinpirole pellets, which released quinpirole HCl (.0 or ap-
testing following saline injection (1.0 ml/kg SC) 1 and 28 days after
pellet implantation. PPI was reassessed the next day 10 min after
challenge with quinpirole HCl (.0 or .1 mg/kg SC).
casting and compression molding of 75:25 poly-lactide-co-glycol-
whereas control pellets contained the polymer alone.
AAV Vector Construction and Stereotaxic Injections. AAV
vectors were produced using Stratagene’s helper-free system to
add an eGFP tag to the N-terminus of CREB and mCREB, as de-
als, for details.
Rats were anesthetized and placed into a stereotaxic frame, a
sterile incision was made through the dermis, a bore hole was
placed into the skull, and a microsyringe (Hamilton; Reno, Nevada)
was lowered into the NAc at the following coordinates: ? 1.2 mm
anterior to bregma, ? 1.3 mm lateral, and 7.1 mm from the pial
A.K. Berger et al.
BIOL PSYCHIATRY 2011;69:288–294 289
surface. AAV was infused at a rate of .01 ?L/min for 10 min on each
side, after which the needle remained in place for 5 min.
Immunolabeling. Thirty days after pellet implantation (the
itated, brains were rapidly frozen in 2-methyl butane and stored at
(21) was collected in a –20°C cryostat, and tissues were processed
mined and normalized relative to the housekeeping gene, glycer-
Methods and Materials, for details.
cryostat, and sections were mounted on slides and stored at –35°C
before immunohistochemical processing. Sections were postfixed
with 4% paraformaldehyde in phosphate-buffered saline and then
processed for immunohistochemical quantification of phospho-
CREB as described earlier.
PPI Disruption Following Acute Quinpirole or Ropinirole
Experiment 1 demonstrates that acute treatment with the D2-
like receptor agonist, quinpirole, significantly (p ? .001) disrupted
doses of .05, .1, and .3 mg/kg reduced PPI by 17%, 30%, and 40%,
respectively, compared with vehicle treatment. Higher doses of
lowest dose. Acute ropinirole also produced significant (p ? .001)
PPI disruption (Figure 1). However, the magnitude of ropinirole-
induced PPI disruption was similar (25%, 33%, and 34% reduction
(46% reduction after 1.0 mg/kg). Neither mean acoustic startle re-
sponse to pulse only trials (Supplement 1, Table S1) nor responses
in the presence of ambient noise alone (data not shown) were
altered by acute quinpirole or ropinirole treatment.
PPI Attenuation and CREB Phosphorylation Following
Repeated Ropinirole Administration
In Experiment 2, the initial ropinirole treatment reduced PPI at
effect. In contrast, repeated ropinirole treatment significantly in-
creased PPI compared with Day 1 treatment levels (Figure 2), indi-
cating that complete recovery of PPI occurred with repeated ropi-
nirole treatment at all doses. Neither mean acoustic startle
response to pulse-only trials (Supplement 1, Table S2) nor re-
sponses in the presence of ambient noise alone (data not shown)
time regardless of treatment; repeated saline treatment increased
pulse response by 125%, whereas average pulse response to re-
peated ropinirole treatment increased by 116%.
Immunohistochemical data revealed that the phosphorylation
state of CREB was altered by repeated ropinirole treatment in the
NAc but not in the caudatoputamen. CREB phosphorylation was
and increased after treatment with higher ropinirole doses in the
NAc (Figure 3). For example, repeated ropinirole treatment at a
dose of .1 mg/kg increased phosphoCREB labeling by 90% in the
NAc core (p ? .05) and by 73% in the NAc shell (p ? .05) compared
ment had no significant effect on the number of phosphoCREB
immunoreactive profiles in either the dorsolateral (DL) or medial
caudatoputamen. Repeated ropinirole treatment also had no sig-
nificant effect on the number of CREB immunoreactive profiles in
the caudatoputamen (Supplement 1, Table S3).
Sustained Effect of Repeated Ropinirole Administration
a challenge with ropinirole on the 28th day of treatment was with-
out effect (Figure 4). Moreover, ropinirole challenge had no signifi-
cant effect 1, 2, 3, or 4 weeks after repeated treatment compared
with baseline or saline challenge (Supplement 1, Figure S1). Fur-
thermore, when ropinirole challenge was given 28 days after the
ropinirole. Animals were challenged with quinpirole (.0, n ? 36; .05, n ? 12;
a significant effect of quinpirole dose [F(11,204) ? 4.399, p ? .001] and
ropinirole dose [F(14,297) ? 2.383, p ? .005] using one-way analysis of
Figure 2. Recovery of prepulse inhibition (PPI) after 28 days of repeated
ropinirole treatment. Animals were treated daily for 28 consecutive days
with ropinirole (.0, n ? 10; .05, n ? 11; .1, n ? 11; or .5 mg/kg, n ? 10).
Days 1 and 28. Percent PPI (mean ? SEM) are mean values collapsed across
vehicle [F(8,81) ? 1.056, p ? .05], however, there was a significant effect of
? 6.787, p ? .005; .5 mg/kg: F(8,90) ? 5.220, p ? .05] using repeated-
measures analysis of variance. *p ? .05 compared with baseline PPI within
290 BIOL PSYCHIATRY 2011;69:288–294
A.K. Berger et al.
weekly challenges, there was no significant effect compared with
ambient noise alone (data not shown) were altered by termination
of ropinirole treatment.
Immunohistochemical data revealed that phosphorylation of
CREB decreased slightly with acute ropinirole challenge compared
with saline; however, this effect was not statistically significant.
Twenty-nine days after termination of repeated ropinirole treat-
ment, CREB phosphorylation did not differ significantly compared
with saline in the NAc or caudatoputamen (Figures 5 and 6). Fur-
thermore, there was no significant change in the number of CREB
repeated ropinirole treatment (Supplement 1, Table S5).
mCREB Prevents PPI Recovery After Chronic Quinpirole
Intra-NAc AAV infusions had no effect on PPI 24 hours after
surgery in any group compared with baseline (Figure 7). Subse-
quent quinpirole challenge significantly reduced PPI by 30%, 40%,
and 46% compared with baseline (p ? .05) in AAV-eGFP-, AAV-
mCREB-eGFP-, and AAV-CREB-eGFP-treated groups, respectively;
however, there was no effect of saline challenge. After 28 days of
significantly increased PPI upon quinpirole challenge compared
with preimplantation levels (Figure 7). In contrast, PPI remained
challenge in rats overexpressing mCREB.
pared with baseline 24 hours after implantation of quinpirole-con-
taining pellets by 29%, 31%, and 24% in AAV-eGFP-, AAV-mCREB-
eGFP and AAV-CREB-eGFP-treated groups, respectively (Table 1).
in the presence of ambient noise was altered by viral or quinpirole
treatment (data not shown).
CREB Phosphorylation and Total CREB Protein in the NAc
After Chronic Quinpirole Treatment
expression consistently enhanced CREB immunolabeling in NAc
tissue compared with eGFP groups, although the extent of this
effect did not reach significance (Supplement 1, Table S6). AAV-
mCREB infusion resulted in 53% to 57% induction, whereas AAV-
CREB infusion produced 22% to 33% enhancement of regional
CREB labeling. Chronic quinpirole treatment consistently, but non-
compared with blank pellet implantation by 46%, 51%, and57%in
AAV-eGFP-, AAV-mCREB-eGFP-, and AAV-CREB-eGFP-treated groups,
tein (CREB) phosphorylation increased after repeated ropinirole treatment
in the nucleus accumbens (NAc) but not in the caudatoputamen (CP). Im-
munohistochemistry was performed on tissue obtained from rats treated
10). Data are expressed as number of nuclear profiles (mean ? SEM) per
and shell [F(3,38) ? 3.653, p ? .05] but not in the caudatoputamen [dorso-
.05] using repeated-measures analysis of variance. *p ? .05 compared with
.0 mg/kg dose within brain region. pCREB, phosphoCREB.
Figure 4. Ropinirole challenge 28 days after repeated ropinirole treatment
does not disrupt prepulse inhibition (PPI). Rats were treated daily for 28
testing was accomplished 28 days after termination of daily treatment fol-
lowing challenge with the same dose received previously. Percent PPI
(mean ? SEM) are mean values collapsed across three prepulse intensities.
There was a significant effect on Day 1 [F(1,40) ? 4.442, p ? .05] but not on
any other day [baseline: F(1,40) ? .002, p ? .05; day 28: F(1,40) ? .002, p ?
analysis of variance. There was no significant effect over time of saline
*p ? .05 compared with .0 mg/kg dose.
nation of repeated ropinirole treatment. Immunohistochemistry was per-
after termination of daily treatment. Rats that previously received vehicle
treatment received either .0 or .1 mg/kg of ropinirole, whereas rats that
previously received .1 mg/kg ropinirole received the same challenge dose
(n ? 9 in each group). Data are expressed as number of nuclear profiles
(mean ? SEM) per mm2. There was no significant effect in any brain region
of variance. pCREB, phosphoCREB.
A.K. Berger et al.
BIOL PSYCHIATRY 2011;69:288–294 291
Immunohistochemical data indicate that chronic quinpirole
treatment altered CREB phosphorylation in the NAc but not in the
caudatoputamen (Figure 8). PhosphoCREB labeling increased sig-
nificantly (p ? .05) after treatment with chronic quinpirole in AAV-
eGFP-, AAV-mCREB-eGFP-, and AAV-CREB-eGFP-treated groups by
and 47%, respectively, in the NAc shell.
Recovery of Sensorimotor Gating Deficits by Chronic D2-Like
Receptor Agonist Treatment
PPI, an indication of sensorimotor gating deficits (6,9,22). Here we
role and ropinirole, significantly reduce PPI in drug-naive rats. Both
magnitude of ropinirole-induced PPI disruption did not increase fur-
ther except after a very high dose (1.0 mg/kg). Ropinirole obtained
dose-dependent (6). Thus, ropinirole may have a moderate effect on
In contrast to acute administration, repeated ropinirole treatment
reverses sensorimotor gating deficits, as observed previously follow-
nirole-mediated PPI disruption significantly recovered at all doses
(.05–.5 mg/kg). The recovery of sensorimotor gating in response to
repeated exposure to a D2-like receptor agonist suggests that toler-
agonist apomorphine or the indirect dopamine agonists amphet-
tive effects on repeated treatment. Our results using selective direct
These studies also reveal that both intermittent daily treatment
D2-like agonist produced PPI recovery. Therefore, this is likely a
compensatory response to steady-state drug exposure achieved
with both treatment paradigms.
by repeated ropinirole treatment; however, there was a significant
Although increased startle response to pulse-only trials may alter the
calculated percent PPI, the magnitude of pulse response increased
even more after repeated saline than ropinirole treatment. Thus, the
Figure 6. Cyclic adenosine monophosphate response el-
in the nucleus accumbens (NAc) immediately following
repeated ropinirole but not 4 weeks after termination of
treatment. Image of phosphoCREB labeling in the NAc
core from (A) saline-, (B) acute ropinirole, or (C) repeated
ropinirole-treated rats, or (D) repeated ropinirole-treated
missure is shown at lower left in each illustration. Scale
bar: 100 ?m.
chronic quinpirole treatment. PPI was determined before and 24 hour after
intracranial infusion of adeno-associated virus for gene transfer of en-
hanced green fluorescent protein (eGFP) alone, mCREB-eGFP (mCREB), or
CREB-eGFP (CREB). Rats were challenged with quinpirole (.0 or .1 mg/kg) 3
weeks later. Pellets that released quinpirole were then implanted (.0 or .1
implantation and then following quinpirole challenge on Day 29 (.0 or .1
mg/kg). Percent PPI (mean ? SEM) are mean values collapsed across three
prepulse intensities. There was no significant effect in the control group
[F(8,207) ? 1.669, p ? .05]; however, there was a significant effect of quin-
pirole treatment [GFP/quinpirole: F(8,207) ? 3.211, p ? .005; mCREB/quin-
pirole: F(8,207) ? 5.337, p ? .005; CREB/quinpirole: F(8,207) ? 4.894, p ?
postvirus surgery within treatment group.†p ? .05 compared with chal-
lenge one within treatment group.
292 BIOL PSYCHIATRY 2011;69:288–294
A.K. Berger et al.
cAMP Response Element Activation in the NAc Is Necessary
for PPI Recovery
plasticity after long-term drug administration (24). For example,
chronic cocaine upregulates NAc dynorphin expression via indirect
and CREB phosphorylation (25). Increased CREB phosphorylation ob-
served in the NAc following repeated ropinirole administration may
regulate gene expression, which could be responsible for the induc-
To determine whether sensorimotor gating adaptation de-
pends on increased CREB phosphorylation in the NAc, we used
AAV-mediated gene transfer to induce prolonged overexpression
of mCREB in the NAc. We show here that mCREB overexpression
pirole challenge, even though chronic treatment increased neuro-
nal labeling of phosphoCREB in the NAc but not in the caudatopu-
tamen. By contrast, chronic quinpirole treatment induced
complete PPI recovery after eGFP or CREB overexpression, with
similar selective regional increases of phosphoCREB in the NAc.
mCREB dimerizes with endogenous phosphoCREB, and although
resulting dimers can bind to cAMP response elements, they are
functionally inactive. mCREB thereby acts as a dominant negative
appears to be necessary for PPI recovery.
Viral-mediated mCREB overexpression in the NAc is known to en-
hance the rewarding effects of cocaine and intracranial self-stimula-
tion and to produce antidepressant-like effects (26-28). These behav-
ioral effects generally coincide with altered dynorphin expression in
the NAc, implicating NAc neurons expressing predominantly D1-like
receptors. In contrast, PPI is regulated by D2-like receptors in the NAc
It should be noted that although mCREB overexpression blocked
lets was sufficient to cause initial PPI disruption, which was reduced
over time. Nevertheless, the larger challenge dose still produced dis-
Long-Lasting Effects of Repeated D2-Like Agonist Treatment
Our results reveal that recovery of sensorimotor gating is
present for at least 28 days after the termination of repeated ropi-
nirole treatment in the presence or absence of weekly challenges
that produced no PPI deficit. This suggests that PPI recovery is
long-lasting and independent of weekly “priming” injections. Fur-
thermore, NAc phosphoCREB labeling is elevated immediately
after termination of repeated treatment, but not 28 days later,
even though PPI recovery remains. Thus, NAc phosphoCREB is
necessary to induce PPI recovery, but it need not be present to
maintain PPI recovery. This suggests that activation of CREB might
induce transcriptional regulation of NAc protein expression, the
D2-like receptor-G protein function is unaffected by repeated ago-
nist treatment, but cAMP-dependent protein kinase activity is in-
creased (9,13). Other proteins might be altered in such a way as to
offset D2-like receptor action. Alternatively, transcriptional regula-
tion might induce plasticity of NAc neuronal connections or func-
tion with long-term effects. Although it might seem paradoxical
the nucleus accumbens (NAc) but not in the caudatoputamen (CP). Immu-
days with blank or quinpirole pellets (.0 or .1 mg/kg/day, n ? 9 per virus
or CREB-eGFP (CREB). Data are expressed as number of nuclear profiles
(mean ? SEM) per mm2. There was a significant effect of quinpirole treat-
.01], but not in the CP [dorsolateral (DL) CP: F(5,22) ? .9656, p ? .05; medial
(Med) CP: F(5,22) ? .39, p ? .05] using repeated-measures analysis of vari-
Table 1. Effect of Viral Overexpression and Quinpirole Treatment on PPI
Baseline Pellet Day 1Pellet Day 28
42.9 ? 3.8
41.5 ? 3.5
45.7 ? 5.3
46.9 ? 3.6
55.7 ? 3.9
53.6 ? 4.1
47.3 ? 4.6
47.3 ? 4
47.7 ? 4.1
31.8 ? 4.5a
50.5 ? 4.2
50.9 ? 5.2
44.4 ? 3.7 26.3 ? 5.4a
52.6 ? 3.4
40.0 ? 4.1 21.6 ? 5.6a
50.4 ? 5.5
effect over time of blank pellets; however, quinpirole-containing pellets
reduced PPI in all groups on implantation Day 1, which normalized by Day
28 [eGFP/blank pellet: F(2,70) ? 2.314, p ? .05; mCREB/blank pellet: F(2,72)
? 2.522, p ? .05; CREB/blank pellet: F(2,71) ? .1678, p ? .05; eGFP/quinpi-
role pellet: F(2,78) ? 4.856, p ? .01; mCREB/quinpirole pellet: F(2,75) ?
ed-measures analysis of variance.
tein; eGFP, enhanced green fluorescent protein; mCREB, mutant CREB; PPI,
ap ? .01 compared with baseline PPI.
A.K. Berger et al.
BIOL PSYCHIATRY 2011;69:288–294 293
which also reverse PPI disruption (31), may underlie this persistent Download full-text
neuroadaptive behavioral response.
In humans, some direct dopamine agonists, such as apomor-
ter tolerated and has clinical efficacy for other dopaminergic disor-
to PPI recovery, which is long-lasting even without further treat-
ment. Both drugs appear to use the same mechanism of selective
ery after chronic steady-state pharmacotherapy. A prolonged-re-
lease formulation of ropinirole that provides steady-state pharma-
cokinetics in a single daily dose (33) is now available (34,35). Our
results suggest that this prolonged-release formulation of ropini-
role might be efficacious for treating sensorimotor gating deficits.
This work was supported by U.S. Public Health Service Awards
MH060251 and MH073930 and a research grant from GlaxoSmith-
EJN. AKB is currently affiliated with the Department of Neurology,
the content of this manuscript from AstraZeneca. Dr. Hammer has
received lecture fees from Merck and research funding from Glaxo-
SmithKline and reports no other biomedical financial interests or po-
tential conflicts of interest. All other authors report no biomedical fi-
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