Biphasic alterations in Serotonin-1B (5-HT1B) receptor function during abstinence from extended cocaine self-administration

Profiling prefrontal cortex protein expression in rats exhibiting an incubation of cocaine craving following short-access self-administration procedures

INTRODUCTION

Incubation of drug-craving refers to a time-dependent increase in drug cue-elicited craving that occurs during protracted withdrawal. Historically, rat models of incubated cocaine craving employed extended-access (typically 6 h/day) intravenous drug self-administration (IV-SA) procedures, although incubated cocaine craving is reported to occur following shorter-access IV-SA paradigms. The notoriously low-throughput of extended-access IV-SA prompted us to determine whether two different short-access IV-SA procedures akin to those in the literature result in qualitatively similar changes in glutamate receptor expression and the activation of downstream signaling molecules within prefrontal cortex (PFC) subregions as those reported previously by our group under 6h-access conditions.

METHODS

For this, adult, male Sprague-Dawley rats were trained to intravenously self-administer cocaine for 2 h/day for 10 consecutive days (2-h model) or for 6 h on day 1 and 2 h/day for the remaining 9 days of training (Mixed model). A sham control group was also included that did not self-administer cocaine.

RESULTS

On withdrawal day 3 or 30, rats were subjected to a 2-h test of cue-reinforced responding in the absence of cocaine and a time-dependent increase in drug-seeking was observed under both IV-SA procedures. Immunoblotting of brain tissue collected immediately following the cue test session indicated elevated phospho-Akt1, phospho-CaMKII and Homer2a/b expression within the prelimbic subregion of the PFC of cocaine-incubated rats. However, we failed to detect incubation-related changes in Group 1 metabotropic glutamate receptor or ionotropic glutamate receptor subunit expression in either subregion.

DISCUSSION

These results highlight further a role for Akt1-related signaling within the prelimbic cortex in driving incubated cocaine craving, and provide novel evidence supporting a potential role also for CaMKII-dependent signaling through glutamate receptors in this behavioral phenomenon.

Dual Effect of 5-HT1B/1D Receptors on Dopamine Neurons in Ventral Tegmental Area: Implication for the Functional Switch After Chronic Cocaine Exposure

BACKGROUND

Serotonin (5-HT) 1B/1D receptor (5-HT_1B/1DR) agonists undergo an abstinence-induced switch in their effects on cocaine-related behaviors, which may involve changes in modulation of dopamine (DA) neurons in the ventral tegmental area (VTA). However, it is unclear how 5-HT_1B/1DRs affect VTA DA neuronal function and whether modulation of these neurons mediates the abstinence-induced switch after chronic cocaine exposure.

METHODS

We examined the ability of 5-HT_1B/1DRs to modulate D₂ autoreceptors (D₂ARs) and synaptic transmission in the VTA by slice recording and single unit recording in vivo in naïve mice and in mice with chronic cocaine treatment.

RESULTS

We report a bidirectional modulation of VTA DA neuronal firing through the interaction of VTA 5-HT_1B/1DRs and D₂ARs. In both VTA slices and the VTA of anesthetized mice, the 5-HT_1B/1DR agonist CP94253 decreased DA neuronal firing rate and evoked excitatory postsynaptic currents to DA neurons in slice. Paradoxically, CP94253 decreased quinpirole-induced inhibition of DA neurons by reducing D₂AR-mediated G protein-gated inwardly rectifying potassium current. This manifested decreased GABA_A (gamma-aminobutyric acid A) receptor-mediated evoked inhibitory postsynaptic currents in slice, resulting in disinhibition of DA neurons, in opposition to the 5-HT_1B/1DR-induced inhibition. This dual effect was verified in chronic cocaine-treated and mild stress-treated, male mice on days 1 and 20 posttreatment.

CONCLUSIONS

This study revealed dual effects of CP94253 on VTA DA neurons that are dependent on D₂AR sensitivity, with anti-inhibition under normal D₂AR sensitivity and inhibition under low D₂AR sensitivity. These dual effects may underlie the ability of CP94253 to both enhance and inhibit cocaine-induced behaviors.

Preclinical support for the therapeutic potential of zolmitriptan as a treatment for cocaine use disorders

Serotonin 1B receptor (5-HT_1BR) agonists enhance cocaine intake in rats during daily self-administration but attenuate cocaine intake after prolonged abstinence. Here we investigated whether the less selective but clinically available 5-HT_1D/1BR agonist, zolmitriptan, produces similar effects. Male and free-cycling female Sprague-Dawley rats were trained to lever press for cocaine (0.75 mg/kg, i.v.) or sucrose (45 mg pellet) reinforcement until performance rates stabilized. Rats then received zolmitriptan (3.0, 5.6, and 10 mg/kg, s.c.) prior to testing for its effects on response and reinforcement rates. Under cocaine testing conditions, rats had access to the training dose for the first hour followed by a lower cocaine dose (0.075 mg/kg, i.v.) for the second hour. Zolmitriptan decreased cocaine intake at both cocaine doses and in both sexes even without a period of abstinence and without altering sucrose intake. A separate group of rats underwent identical training procedures and were tested for effects of the selective 5-HT_1B and 5-HT_1D receptor antagonists, SB224289 (3.2, 5.6, and 10 mg/kg, s.c.) and BRL15572 (0.3, 1.0, and 3.0 mg/kg, i.p.), respectively, alone or in combination with zolmitriptan (5.6 mg/kg, s.c.) under identical cocaine testing procedures as above. The zolmitriptan-induced decrease in cocaine intake was reversed by SB224289 and to a lesser extent by BRL15572, suggesting that the effects of zolmitriptan involve both 5-HT_1B and 5-HT_1D receptors. Neither zolmitriptan, SB224289, or BRL15572 altered locomotor activity at the doses effective for modulating cocaine intake. These findings suggest that zolmitriptan has potential for repurposing as a treatment for cocaine use disorders.

Amphetamine Self-Administration and Its Extinction Alter the 5-HT1B Receptor Protein Levels in Designated Structures of the Rat Brain

Manipulation of the serotonin (5-HT)_1B receptors can modify the behavioral effects of amphetamine including its reinforcing properties. Focus of this study was to examine changes in 5-HT_1B receptor protein expression in several brain structures linked to substance drug disorder in different stages of amphetamine addiction—single session of amphetamine self-administration, 20 consecutive days of amphetamine self-administration, and 3 and 14 days of extinction from chronic drug intake. “Yoked” procedure was employed to set apart pharmacological and motivational effects of amphetamine intoxication. Immunohistofluorescence was performed on brain slices containing the following regions: nucleus accumbens (NAc) shell and core, globus pallidum (GP) lateral and ventral, hippocampus (HIP), substantia nigra (SN), and ventral tegmental area (VTA). Single amphetamine session decreased the amount of 5-HT_1B receptors in SN, VTA, and HIP in active and yoked rats. On the contrary, 20 days of chronic amphetamine exposure triggered elevation of 5-HT_1B receptors exclusively in animals that voluntarily administered the drug in NAc core, GP ventral, and HIP. Furthermore, 14-day (but not 3-day) extinction from amphetamine increased the 5-HT_1B receptor expression in ventral and lateral GP, HIP, and SN. This study is the first to demonstrate that exposure to amphetamine and its extinction alter the expression of 5-HT_1B receptors in various rat brain regions, and those changes seem to be transient and region specific. Importantly, since increased expression of 5-HT_1B receptor after chronic amphetamine self-administration was limited only to active group of animals, we suggest that 5-HT_1B receptor is linked to motivational aspect of addiction.

MDMA self-administration fails to alter the behavioral response to 5-HT(1A) and 5-HT(1B) agonists

RATIONALE

Regular use of the street drug, ecstasy, produces a number of cognitive and behavioral deficits. One possible mechanism for these deficits is functional changes in serotonin (5-HT) receptors as a consequence of prolonged 3,4 methylenedioxymethamphetamine (MDMA)-produced 5-HT release. Of particular interest are the 5-HT(1A) and 5-HT(1B) receptor subtypes since they have been implicated in several of the behaviors that have been shown to be impacted in ecstasy users and in animals exposed to MDMA.

OBJECTIVES

This study aimed to determine the effect of extensive MDMA self-administration on behavioral responses to the 5-HT(1A) agonist, 8-hydroxy-2-(n-dipropylamino)tetralin (8-OH-DPAT), and the 5-HT(1B/1A) agonist, RU 24969.

METHODS

Male Sprague-Dawley rats self-administered a total of 350 mg/kg MDMA, or vehicle, over 20-58 daily self-administration sessions. Two days after the last self-administration session, the hyperactive response to 8-OH-DPAT (0.03-1.0 mg/kg) or the adipsic response to RU 24969 (0.3-3.0 mg/kg) were assessed.

RESULTS

8-OH-DPAT dose dependently increased horizontal activity, but this response was not altered by MDMA self-administration. The dose-response curve for RU 24969-produced adipsia was also not altered by MDMA self-administration.

CONCLUSIONS

Cognitive and behavioral deficits produced by repeated exposure to MDMA self-administration are not likely due to alterations in 5-HT(1A) or 5-HT(1B) receptor mechanisms.

Attenuated methamphetamine-induced locomotor sensitization in serotonin transporter knockout mice is restored by serotonin 1B receptor antagonist treatment

Repeated administration of methamphetamine (METH) enhances acute locomotor responses to METH administered in the same context, a phenomenon termed as 'locomotor sensitization'. Although many of the acute effects of METH are mediated by its influences on the compartmentalization of dopamine, serotonin systems have also been suggested to influence the behavioral effects of METH in ways that are not fully understood. The present experiments examined serotonergic roles in METH-induced locomotor sensitization by assessing: (a) the effect of serotonin transporter (SERT; Slc6A4) knockout (KO) on METH-induced locomotor sensitization; (b) extracellular monoamine levels in METH-treated animals as determined by in-vivo microdialysis; and (c) effects of serotonin (5-HT) receptor antagonists on METH-induced behavioral sensitization, with focus on effects of the 5-HT1B receptor antagonist SB 216641 and a comparison with the 5-HT2 receptor antagonist ketanserin. Repeated METH administration failed to induce behavioral sensitization in homozygous SERT KO (SERT-/-) mice under conditions that produced substantial sensitization in wild-type or heterozygous SERT KO (SERT+/-) mice. The selective 5-HT1B antagonist receptor SB 216641 restored METH-induced locomotor sensitization in SERT-/- mice, whereas ketanserin was ineffective. METH-induced increases in extracellular 5-HT (5-HTex) levels were substantially reduced in SERT-/- mice, although SERT genotype had no effect on METH-induced increases in extracellular dopamine. These experiments demonstrate that 5-HT actions, including those at 5-HT1B receptors, contribute to METH-induced locomotor sensitization. Modulation of 5-HT1B receptors might aid therapeutic approaches to the sequelae of chronic METH use.

Serotonin/dopamine interactions in a hyperactive mouse: reduced serotonin receptor 1B activity reverses effects of dopamine transporter knockout

Knockout (KO) mice that lack the dopamine transporter (SL6A3; DAT) display increased locomotion that can be attenuated, under some circumstances, by administration of drugs that normally produce psychostimulant-like effects, such as amphetamine and methylphenidate. These results have led to suggestions that DAT KO mice may model features of attention deficit hyperactivity disorder (ADHD) and that these drugs may act upon serotonin (5-HT) systems to produce these unusual locomotor decreasing effects. Evidence from patterns of brain expression and initial pharmacologic studies led us to use genetic and pharmacologic approaches to examine the influence of altered 5-HT_1B receptor activity on hyperactivity in DAT KO mice. Heterozygous 5-HT_1B KO and pharmacologic 5-HT_1B antagonism both attenuated locomotor hyperactivity in DAT KO mice. Furthermore, DAT KO mice with reduced, but not eliminated, 5-HT_1B receptor expression regained cocaine-stimulated locomotion, which was absent in DAT KO mice with normal levels of 5-HT_1B receptor expression. Further experiments demonstrated that the degree of habituation to the testing apparatus determined whether cocaine had no effect on locomotion in DAT KO or reduced locomotion, helping to resolve differences among prior reports. These findings of complementation of the locomotor effects of DAT KO by reducing 5-HT_1B receptor activity underscore roles for interactions between specific 5-HT receptors and dopamine (DA) systems in basal and cocaine-stimulated locomotion and support evaluation of 5-HT_1B antagonists as potential, non-stimulant ADHD therapeutics.

Pharmacological evidence for an abstinence-induced switch in 5-HT1B receptor modulation of cocaine self-administration and cocaine-seeking behavior

Studies examining serotonin-1B (5-HT_1B) receptor manipulations on cocaine self-administration and cocaine-seeking behavior initially seemed discrepant. However, we recently suggested based on viral-mediated 5-HT_1B-receptor gene transfer that the discrepancies are likely due to differences in the length of abstinence from cocaine prior to testing. To further validate our findings pharmacologically, we examined the effects of the selective 5-HT_1B receptor agonist CP 94,253 (5.6 mg/kg, s.c.) on cocaine self-administration during maintenance and after a period of protracted abstinence with or without daily extinction training. We also examined agonist effects on cocaine-seeking behavior at different time points during abstinence. During maintenance, CP 94,253 shifted the cocaine self-administration dose–effect function on an FR5 schedule of reinforcement to the left, whereas following 21 days of abstinence CP 94,253 downshifted the function and also decreased responding on a progressive ratio schedule of reinforcement regardless of extinction history. CP 94,253 also attenuated cue-elicited and cocaine-primed drug-seeking behavior following 5 days, but not 1 day, of forced abstinence. The attenuating effects of CP 94,253 on the descending limb of the cocaine dose–effect function were blocked by the selective 5-HT_1B receptor antagonist SB 224289 (5 mg/kg, i.p.) at both time points, indicating 5-HT_1B receptor mediation. The results support a switch in 5-HT_1B receptor modulation of cocaine reinforcement from facilitatory during self-administration maintenance to inhibitory during protracted abstinence. These findings suggest that the 5-HT_1B receptor may be a novel target for developing medication for treating cocaine dependence.

Serotonin at the nexus of impulsivity and cue reactivity in cocaine addiction

Cocaine abuse and addiction remain great challenges on the public health agendas in the U.S. and the world. Increasingly sophisticated perspectives on addiction to cocaine and other drugs of abuse have evolved with concerted research efforts over the last 30 years. Relapse remains a particularly powerful clinical problem as, even upon termination of drug use and initiation of abstinence, the recidivism rates can be very high. The cycling course of cocaine intake, abstinence and relapse is tied to a multitude of behavioral and cognitive processes including impulsivity (a predisposition toward rapid unplanned reactions to stimuli without regard to the negative consequences), and cocaine cue reactivity (responsivity to cocaine-associated stimuli) cited as two key phenotypes that contribute to relapse vulnerability even years into recovery. Preclinical studies suggest that serotonin (5-hydroxytryptamine; 5-HT) neurotransmission in key neural circuits may contribute to these interlocked phenotypes well as the altered neurobiological states evoked by cocaine that precipitate relapse events. As such, 5-HT is an important target in the quest to understand the neurobiology of relapse-predictive phenotypes, to successfully treat this complex disorder and improve diagnostic and prognostic capabilities. This review emphasizes the role of 5-HT and its receptor proteins in key addiction phenotypes and the implications of current findings to the future of therapeutics in addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Behavioral evidence for the significance of serotoninergic (5-HT) receptors in cocaine addiction

Cocaine addiction has somatic, psychological, psychiatric, socio-economic and legal implications in the developed world. Presently, there is no medication approved for the treatment of cocaine addiction. In recent years, data from the literature (pre-clinical studies and clinical trials) have provided several lines of evidence that serotonin (5-HT) and 5-HT receptors play a modulatory role in the mechanisms of action of cocaine. Here we review the contribution of 5-HT receptor subtypes to cocaine sensitization, discrimination, conditioned place preference, self-administration, reinstatement of seeking behavior and withdrawal symptoms in laboratory animals. Additionally, the consequences of chronic cocaine exposure on particular 5-HT receptor-assigned functions in pre-clinical studies are presented.

Neural basis for a heritable phenotype: differences in the effects of apomorphine on startle gating and ventral pallidal GABA efflux in male Sprague-Dawley and Long-Evans rats

RATIONALE

Prepulse inhibition (PPI) of startle is a measure of sensorimotor gating that is heritable and deficient in certain psychiatric disorders, including schizophrenia. Sprague-Dawley (SD) rats are more sensitive to PPI disruptive effects of dopamine (DA) agonists at long interstimulus intervals (60-120 ms) and less sensitive to their PPI-enhancing effects at short (10-30 ms), compared with Long-Evans (LE) rats. These heritable strain differences in sensitivity to the PPI disruptive effects of DA agonists must ultimately reflect neural changes "downstream" from forebrain DA receptors.

OBJECTIVE

The current study evaluated the effects of the DA agonist, apomorphine (APO), on ventral pallidal (VP) gamma-aminobutyric acid (GABA) and glutamate efflux and PPI in SD and LE rats.

METHODS

PPI was tested in SD and LE rats after vehicle or APO (0.5 mg/kg, subcutaneously (s.c.)) in a within-subject design. In different SD and LE rats, VP dialysate was collected every 10 min for 120 min after vehicle or APO (0.5 mg/kg, s.c.) and analyzed for GABA and glutamate content by capillary electrophoresis (CE) coupled with laser-induced fluorescence (LIF).

RESULTS

As predicted, SD rats exhibited greater APO-induced PPI deficits at long intervals and less APO-induced PPI enhancement at short intervals compared to LE rats. APO significantly reduced VP GABA efflux in SD but not in LE rats; glutamate efflux was unaffected in both strains.

CONCLUSION

Heritable strain differences in PPI APO sensitivity in SD vs LE rats parallel, and may be mediated by, strain differences in the VP GABA efflux.

Stimulation of 5-HT(1B) receptors enhances cocaine reinforcement yet reduces cocaine-seeking behavior

Paradoxically, stimulation of 5-HT(1B) receptors (5-HT(1B)Rs) enhances sensitivity to the reinforcing effects of cocaine but attenuates incentive motivation for cocaine as measured using the extinction/reinstatement model. We revisited this issue by examining the effects of a 5-HT(1B)R agonist, CP94253, on cocaine reinforcement and cocaine-primed reinstatement, predicting that CP94253 would enhance cocaine-seeking behavior reinstated by a low priming dose, similar to its effect on cocaine reinforcement. Rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. For reinstatement experiments, they then underwent daily extinction training to reduce cocaine-seeking behavior (operant responses without cocaine reinforcement). Next, they were pre-treated with CP94253 (3-10 mg/kg, s.c.) and either tested for cocaine-primed (10 or 2.5 mg/kg, i.p.) or cue-elicited reinstatement of extinguished cocaine-seeking behavior. For reinforcement, effects of CP94253 (5.6 mg/kg) across a range of self-administered cocaine doses (0-1.5 mg/kg, i.v.) were examined. Cocaine dose-dependently reinstated cocaine-seeking behavior, but contrary to our prediction, CP94253 reduced reinstatement with both priming doses. Similarly, CP94253 reduced cue-elicited reinstatement. In contrast, CP94253 shifted the self-administration dose-effect curve leftward, consistent with enhanced cocaine reinforcement. When saline was substituted for cocaine, CP94253 reduced response rates (i.e. cocaine-seeking behavior). In subsequent control experiments, CP94253 decreased open-arm exploration in an elevated plus-maze suggesting an anxiogenic effect, but had no effect on locomotion or sucrose reinforcement. These results provide strong evidence that stimulation of 5-HT(1B)Rs produces opposite effects on cocaine reinforcement and cocaine-seeking behavior, and further suggest that 5-HT(1B)Rs may be a novel target for developing medications for cocaine dependence.

Direct and indirect 5-HT receptor agonists produce gender-specific effects on locomotor and vertical activities in C57 BL/6J mice

It is well established that the dopamine (DA) and serotonin (5-HT) systems have extensive and complex interactions. However, the effects of specific 5-HT receptor agonists on traditionally DA-related behaviors remain unclear. Our goal in these studies was to characterize the effects of 5-HT receptor agonists on measures of locomotor activity and vertical rearing. The SSRIs fluoxetine and citalopram produced significant decreases in locomotor activity and vertical rearing at the highest doses used with females significantly more sensitive to citalopram. The 5-HT(1A) agonist 8-OH-DPAT and the 5-HT(2C) agonist MK 212 significantly decreased activity in both male and female mice, with females more sensitive to 8-OH-DPAT. In contrast, the 5-HT(1B) agonist RU 24969 and the 5-HT(2A) agonist DOI both increased activity, with DOI exhibiting differential effects with regard to sex. Finally, the 5-HT(3) agonist SR 57227 produced significant locomotor increases only in female mice at the lowest dose. The results of these experiments define locomotor profiles of several 5-HT agonists in male and female C57BL/6J mice, providing a foundation for further explorations of 5-HT receptor effects on activity.

Extended daily access to cocaine results in distinct alterations in Homer 1b/c and NMDA receptor subunit expression within the medial prefrontal cortex

Human cocaine addicts show altered function within the basal ganglia and the medial prefrontal cortex (mPFC) and altered glutamate function within these areas is postulated to be critical for cocaine addiction. The present project utilized a highly valid animal model of cocaine addiction, to test whether excessive use of cocaine alters glutamate function within these brain areas. Rats were trained to lever-press for i.v. saline vehicle or cocaine (0.25 mg/infusion) over seven 1-h daily sessions, after which, saline controls and half of cocaine self-administering animals (brief access condition) received 10 more 1-h daily sessions, whereas the remaining cocaine animals received 10 additional 6-h daily sessions (extended access condition). One, 14, or 60 days after the last self-administration session, animals were sacrificed. Tissue samples from the ventral tegmental area (VTA), nucleus accumbens (N.Acc) core and shell, and mPFC were analyzed by immunoblotting for expression of Homer1b/c, Homer2a/b, mGluR1, mGluR5, NR2a, and NR2b subunits of the NMDA receptor. Brief and extended access to cocaine failed to alter protein levels within the VTA, and produced transient and similar changes in N.Acc protein expression, which were more pronounced in the core subregion. In contrast, extended access to cocaine resulted in distinct and long lasting alterations of protein expression within the mPFC that included: increased levels of Homer1b/c at 1 day, NR2b at 14 days, and NR2a at 60 days, of withdrawal. These data support the notion that altered NMDA function within the mPFC may contribute, in part, to the transition to excessive uncontrolled consumption of cocaine.

Acquisition of and withdrawal from cocaine self-administration regulates 5-HT mRNA expression in rat striatum

This study investigated how different stages of cocaine self-administration in rats affect the expression of two serotonin receptors in dorsal and ventral striatum, the 5-HT(1B) and 5-HT(6) subtypes, which have both been implicated in mediating some aspects of cocaine-related behaviors. In the first experiment, rats were trained to work for saccharin (oral) or cocaine (i.v.) reinforcers. We found that continuous access to cocaine for 23 days did not change the level of 5-HT(1B) mRNA expression compared to control animals receiving saccharin. However, a single cocaine session, given either by self-administration or non-contingently, increased 5-HT(1B) mRNA in dorsal striatum, whereas forced abstinence for two weeks after cocaine reduced 5-HT(1B) mRNA expression in the same subregion. 5-HT(6) mRNA was not changed by any of these treatments. A follow-up experiment investigated the effects of limited versus extended access to cocaine as well as forced abstinence, and we found that 14 days of forced abstinence significantly reduced 5-HT(1B) mRNA throughout the dorsal and ventral striatum compared to no withdrawal. These results suggest that the influence of 5-HT(1B) receptors in striatal projection neurons may be increased during cocaine acquisition and reduced after forced abstinence and may therefore be targets for pharmacological intervention in addiction.

Blockade of the serotonin 5-HT2A receptor suppresses cue-evoked reinstatement of cocaine-seeking behavior in a rat self-administration model

The serotonin 5-HT2A receptor (5-HT-sub(2A)R) may play a role in reinstatement of drug-seeking. This study investigated the ability of a selective 5-HT-sub(2A)R antagonist to suppress reinstatement evoked by exposure to cues conditioned to cocaine self-administration. Cocaine self-administration (0.75 mg/kg/0.1 mL/6 s infusion; FR 4) was trained in naïve, free-fed rats to allow interpretation of results independent from changes related to food deprivation stress. Pretreatment with the selective 5-HT-sub(2A)R antagonist M100907 (volinanserin) failed to reduce rates of operant responding for cocaine infusions. On the other hand, M100907 (0.001-0.8 mg/kg ip) significantly suppressed the cue-induced reinstatement of cocaine-seeking behavior following extinction; effective M100907 doses did not alter operant responding for cues previously associated with sucrose self-administration. Importantly, a greater magnitude of active lever presses on the initial extinction session (high extinction responders) predicted the maximal susceptibility to M100907-induced suppression of cue-evoked reinstatement. The findings indicate that blockade of the 5-HT-sub(2A)R attenuates the incentive-motivational effects of cocaine-paired cues, particularly in high extinction responders, and suggests that M100907 may afford a therapeutic advance in suppression of cue-evoked craving and/or relapse.

Serotonergic mechanisms in addiction-related memories

Drug-associated memories are a hallmark of addiction and a contributing factor in the continued use and relapse to drugs of abuse. Repeated association of drugs of abuse with conditioned stimuli leads to long-lasting behavioral responses that reflect reward-controlled learning and participate in the establishment of addiction. A greater understanding of the mechanisms underlying the formation and retrieval of drug-associated memories may shed light on potential therapeutic approaches to effectively intervene with drug use-associated memory. There is evidence to support the involvement of serotonin (5-HT) neurotransmission in learning and memory formation through the families of the 5-HT(1) receptor (5-HT(1)R) and 5-HT(2)R which have also been shown to play a modulatory role in the behavioral effects induced by many psychostimulants. While there is a paucity of studies examining the effects of selective 5-HT(1A)R ligands, the available dataset suggests that 5-HT(1B)R agonists may inhibit retrieval of cocaine-associated memories. The 5-HT(2A)R and 5-HT(2C)R appear to be integral in the strong conditioned associations made between cocaine and environmental cues with 5-HT(2A)R antagonists and 5-HT(2C)R agonists possessing potency in blocking retrieval of cocaine-associated memories following cocaine self-administration procedures. The complex anatomical connectivity between 5-HT neurons and other neuronal phenotypes in limbic-corticostriatal brain structures, the heterogeneity of 5-HT receptors (5-HT(X)R) and the conflicting results of behavioral experiments which employ non-specific 5-HT(X)R ligands contribute to the complexity of interpreting the involvement of 5-HT systems in addictive-related memory processes. This review briefly traces the history of 5-HT involvement in retrieval of drug-cue associations and future targets of serotonergic manipulation that may reduce the impact that drug cues have on addictive behavior and relapse.

Quinelorane, a dopamine D3/D2 receptor agonist, reduces prepulse inhibition of startle and ventral pallidal GABA efflux: time course studies

Startle is inhibited when the startling stimulus is preceded 30-300 ms by a weak prepulse. Prepulse inhibition (PPI), an operational measure of sensorimotor gating, is deficient in schizophrenia patients, and reduced in rats and humans by dopamine agonists. The neural basis for the PPI-disruptive effects of dopamine agonists in rats is studied to understand neural circuitry regulating PPI and its deficits in schizophrenia. Existing data suggest that ventral pallidal (VP) GABAergic transmission regulates PPI and its disruption by dopamine agonists. We measured changes in VP GABA efflux and PPI in rats in response to the D2/D3 agonist, quinelorane. Wistar rats were administered quinelorane (vehicle, 0.003 or 0.01 mg/kg). In some rats, VP dialysate was analyzed for GABA content. In others, PPI was assessed using 120 dB(A) startle pulses and prepulses 10 dB over a 70 dB(A) background. Quinelorane reduced GABA efflux, with significant effects for 0.01 but not 0.003 mg/kg, persisting for at least 100 min. Quinelorane reduced PPI for 50 min, an effect significant for both the 0.003 (p < 0.05) and 0.01 mg/kg doses (p < 0.015). Differences in time course and dose sensitivity of quinelorane effects on VP GABA efflux and PPI are discussed.