Effects of 5-HT3 receptor antagonists on behavioural measures of naloxone-precipitated opioid withdrawal

The role of 5-HT modulation in opioid withdrawal and neonatal opioid withdrawal syndrome: mechanisms and potential serotonergic targets

INTRODUCTION

Opioid use disorder and neonatal opioid withdrawal syndrome are persisting consequences of the opioid epidemic in the United States. Current pharmacologic approaches primarily utilize opioid replacement therapy, but non-opioid therapeutics could have advantages. Preclinical and clinical data suggest modulation of the serotonergic system as a novel therapeutic approach in relieving opioid withdrawal syndromes.

AREAS COVERED

Serotonin receptors that have been reported to mediate opioid withdrawal signs based on preclinical findings are identified and described. Extant clinical studies assessing the modulation of these receptors on opioid withdrawal outcomes are then summarized.

EXPERT OPINION

While medications that serve as agonists or antagonists to serotonin receptor subtypes have the potential for treatment of opioid withdrawal and neonatal opioid withdrawal syndrome, much of the evidence is tenuous. The supportive data are mainly derived from preclinical studies, and the measured clinical efficacy has been variable, but reductions in symptom severity are consistently noted. Serotonergic modulation offers a non-opioid pathway to relieving opioid withdrawal manifestations, which is especially useful for neonates as changes in neuroplasticity have been noted with postnatal opioid use. Potential benefits warrant additional studies to clarify the mechanisms for their effect and for measuring how effective these agents are in human disease.

Non-Opioid Treatments for Opioid Use Disorder: Rationales and Data to Date

Opioid use disorder (OUD) represents a major public health problem that affects millions of people in the USA and worldwide. The relapsing and recurring aspect of OUD, driven by lasting neurobiological adaptations at different reward centres in the brain, represents a major obstacle towards successful long-term remission from opioid use. Currently, three drugs that modulate the function of the opioidergic receptors, methadone, buprenorphine and naltrexone have been approved by the US Food and Drug Administration (FDA) to treat OUD. In this review, we discuss the limitations and challenges associated with the current maintenance and medication-assisted withdrawal strategies commonly used to treat OUD. We further explore the involvement of glutamatergic, endocannabinoid and orexin signaling systems in the development, maintenance and expression of addiction-like behaviours in animal models of opioid addiction, and as potential and novel targets to expand therapeutic options to treat OUD. Despite a growing preclinical literature highlighting the role of these potential targets in animal models of opioid addiction, clinical and translational studies for novel treatments of OUD remain limited and inconclusive. Further preclinical and clinical investigations are needed to expand the arsenal of primary treatment options and adjuncts to maximise efficacy and prevent relapse.

Non-Opioid Neurotransmitter Systems that Contribute to the Opioid Withdrawal Syndrome: A Review of Preclinical and Human Evidence

Opioid misuse and abuse is a major international public health issue. Opioid use disorder (OUD) is largely maintained by a desire to suppress aversive opioid withdrawal symptoms. Opioid withdrawal in patients seeking abstinence from illicit or prescribed opioids is often managed by provision of a μ-opioid agonist/partial agonist in combination with concomitant medications. Concomitant medications are administered based on their ability to treat specific symptoms rather than a mechanistic understanding of the opioid withdrawal syndrome; however, their use has not been statistically associated with improved treatment outcomes. Understanding the central and/or peripheral mechanisms that underlie individual withdrawal symptom expression in humans will help promote medication development for opioid withdrawal management. To support focused examination of mechanistically supported concomitant medications, this review summarizes evidence from preclinical (N = 68) and human (N = 30) studies that administered drugs acting on the dopamine, serotonin, cannabinoid, orexin/hypocretin, and glutamate systems and reported outcomes related to opioid withdrawal. These studies provide evidence that each of these systems contribute to opioid withdrawal severity. The Food and Drug Administration has approved medications acting on these respective systems for other indications and research in this area could support the repurposing of these medications to enhance opioid withdrawal treatment. These data support a focused examination of mechanistically informed concomitant medications to help reduce opioid withdrawal severity and enhance the continuum of care available for persons with OUD.

Neuroprotective effects of 5-HT3 receptor antagonist ondansetron on morphine withdrawal induced brain edema formation, blood-brain barrier dysfunction, neuronal injuries, glial activation and heat shock protein upregulation in the brain

Morphine withdrawal response is associated with brain edema formation, blood-brain barrier (BBB) disruption, activation of glial cells and heat shock protein (HSP 72kDa) responses in the CNS. Thus, exploration of suitable therapeutic measures is the need of the hour to induce neuroprotection in morphine withdrawal cases. There are reports that 5-HT₃-receptor antagonists ondansetron attenuate some of the behavioral changes in morphine-withdrawal symptoms. However, brain protection in morphine withdrawal using pharmacological approaches is still not well known. In present investigation, effect of ondansetron the potent 5-HT₃ receptor antagonist on brain edema formation BBB disruption, glial activation and/or HSP response following morphine withdrawal was examined. Rats received ondansetron (1mg or 2mg/kg, s.c) or saline once daily from 2days before morphine administration (10mg/kg, s.c. once daily for 10days) that continued up to 2days after its withdrawal (day 13th). Cessation of morphine on day 11th results in withdrawal symptoms and BBB breakdown to proteins in the cerebral cortex, hippocampus, cerebellum, thalamus, hypothalamus, brain stem and spinal cord along with activation of glial fibrillary acidic protein (GFAP) and HSP immunoreactivity. In these animals brain edema and neurotoxicity are prominent on day 13th as compared to controls. Ondansetron treatment significantly reduced withdrawal symptoms on the day 13th in a dose dependent manner and attenuated BBB breakdown, edema formation, GFAP and HSP expression and neuronal injuries. These observations are the first to show that ondansetron is neuroprotective following morphine withdrawal indicating an important role of 5-HT₃ receptors in psychostimulants abuse.

Effects of repeated morphine on ultrasonic vocalizations in adult rats: increased 50-kHz call rate and altered subtype profile

RATIONALE

Adult rat 50-kHz vocalizations have been proposed to indicate a positive affective state, putatively revealed by a predominance of trill calls over flat calls. However, short-term exposure to non-sedative doses of the euphorigen morphine suppresses calling, with no discernible shift in trill or flat call prevalence.

OBJECTIVES

This study aimed to determine whether morphine acutely increases 50-kHz call rates or alters the relative prevalence of trill or flat calls, after long-term morphine exposure or acute pharmacological pretreatment.

METHODS

Experiment 1 comprised 10 once-daily tests, alternating between saline and morphine, 1 mg/kg SC, followed by dose-response testing (0, 0.3, 1, and 3 mg/kg). Experiment 2 was similar but included additional testing with morphine in combination with the antinausea drug ondansetron or the peripheral opioid antagonist methylnaltrexone. In experiment 3, morphine was again combined with ondansetron or methylnaltrexone but in rats that were initially drug naïve.

RESULTS

In animals that were initially drug naïve, morphine tended to suppress calling and did not alter the 50-kHz call subtype profile. In morphine-experienced rats, morphine acutely increased the 50-kHz call rate and promoted trills over flat calls; short calls were also inhibited. Neither ondansetron nor methylnaltrexone detectably altered any effect of morphine on calling, nor did these two drugs affect 50-kHz calling when given alone.

CONCLUSIONS

With chronic exposure, morphine acutely enhances 50-kHz calling and differentially promotes trill calls, mainly at the expense of flat calls. These effects appear consistent with a positive affect interpretation of 50-kHz vocalizations.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

Inhibition of the acquisition of conditioned place aversion by dopaminergic lesions of the central nucleus of the amygdala in morphine-treated rats

The negative affective state of opiate abstinence plays an important role in craving and relapse to compulsive drug use. The dopamine system participates in the reward effects of opiate use and the aversive effect of opiate abstinence. The amygdala is an essential neural substrate for associative learning of emotion. To establish a model of conditioned place aversion (CPA) in morphine-treated rats, we used different visual and tactual cues as conditioned stimuli (CS) within a conditioning apparatus. An injection of naloxone served as the unconditioned stimulus (US). The 6-hydroxydopamine (6-OHDA) lesion technique was used to investigate the effects of the dopaminergic system of the central nucleus of the amygdala (CeA) on naloxone-induced CPA. Rats were rendered physically dependent via administration of increasing doses of morphine delivered via intraperitoneal injection. Doses increased by 20 % each day for 14 days, starting from an initial dose of 6 mg/kg. All rats also received a low dose of naloxone (0.1 mg/kg) by injection 4 hours after morphine treatment on days 11 and 13 to induce CPA in a biased two-compartment conditioned place apparatus. Morphine-dependent rats with sham lesions were found to develop significant CPA after naloxone treatment. Bilateral 6-OHDA lesions of the CeA impaired the acquisition of CPA but had no effect on locomotor activity. These results suggest that the dopaminergic system of the CeA plays an important role in the negative affective state of opiate abstinence.

Alleviating symptoms of withdrawal from an opioid

INTRODUCTION

Patients who are discharged following surgery on an oral opioid, and who have taken the drug for 2 or more weeks often experience withdrawal symptoms when they try to discontinue the drug.

CASE REPORT

Three weeks after discharge, a 44-year-old female patient decided to reduce her oxycodone (OxyContin(®)) dosage from 20 mg three times a day to 20 mg two times a day. She experienced severe withdrawal symptoms.

METHOD

To assist her in withdrawing from the remainder of the drug, a protocol using ondansetron was developed.

RESULTS

After 10 days, the patient was opioid and withdrawal-symptom free.

CONCLUSION

Use of ondansetron along with tapering of the opioid was safe and effective in preventing further withdrawal symptoms. This case should stimulate research with a larger, more diverse population including those with both short-term and chronic opioid dependence.

5-hydroxytryptamine type 3 receptor modulates opioid-induced hyperalgesia and tolerance in mice

BACKGROUND

Opioid-induced hyperalgesia (OIH) and tolerance are challenging maladaptations associated with opioids in managing pain. Recent genetic studies and the existing literature suggest the 5-hydroxytryptamine type 3 (5-HT3) receptor participates in these phenomena. The location of the relevant receptor populations and the interactions between the 5-HT3 system and other systems controlling OIH and tolerance have not been explored, however. We hypothesized that 5-HT3 receptors modulate OIH and tolerance, and that this modulation involves the control of expression of multiple neurotransmitter and receptor systems.

METHODS

C57BL/6 mice were exposed to a standardized 4-day morphine administration protocol. The 5-HT3 antagonist ondansetron was administered either during or after the conclusion of morphine administration. Mechanical testing was used to quantify OIH, and thermal tail-flick responses were used to measure morphine tolerance. In other experiments spinal cord and dorsal root ganglion tissues were harvested for analysis of messenger RNA concentrations by real-time polymerase chain reaction or immunochemistry analysis.

RESULTS

The results showed that (1) systemic or intrathecal injection of ondansetron significantly prevented and reversed OIH, but not local intraplantar injection; (2) systemic or intrathecal injection of ondansetron prevented and reversed tolerance; and (3) ondansetron blocked morphine-induced increases of multiple genes relevant to OIH and tolerance in dorsal root ganglion and spinal cord.

CONCLUSIONS

Morphine acts via a 5-HT3-dependent mechanism to support multiple maladaptations to the chronic administration of morphine. Furthermore, the use of 5-HT3 receptor antagonists may provide a new avenue to prevent or reverse OIH and tolerance associated with chronic opioid use.

α7 Nicotinic acetylcholine receptors in the central amygdaloid nucleus alter naloxone-induced withdrawal following a single exposure to morphine

RATIONALE

Negative motivational withdrawal from acute opiate dependence was induced by an opioid antagonist, and the withdrawal signs prevented by pretreatment with nicotine.

OBJECTIVES

The present study was undertaken to examine the mechanism of nicotine-induced attenuation of withdrawal precipitated by naloxone in rats administered a single dose of morphine.

METHODS

Conditioned place aversion (CPA) was precipitated by naloxone in rats exposed once to morphine. Nicotinic acetylcholine receptor (nAChR) agonists were microinjected into the central amygdaloid nucleus (CeA) before naloxone was administered. Additionally, c-Fos expression in the amygdala was measured in rats exposed to α7 nAChR ligands.

RESULTS

The microinjection of nicotine (0.3 and 1.0 μg/μl) into the CeA dose-dependently inhibited naloxone-induced CPA. This inhibition of CPA was reversed by methyllycaconitine (MLA), an α7 nAChR antagonist. CPA was also significantly attenuated by the microinjection of tropisetron (3.0 μg/μl), an α7 nAChR agonist and 5-hydroxytriptamine 3 (5-HT(3)) receptor antagonist, but not by ondansetron (1.0 and 3.0 μg/μl), a 5-HT(3) receptor antagonist. The microinjection of PNU-282987 (3.0 μg/μl), a selective α7 nAChR agonist, into the CeA also inhibited CPA. Furthermore, nicotine increased c-Fos expression in the CeA, but not the medial or basolateral amygdaloid nucleus. The increase of c-Fos in the CeA was significantly inhibited by MLA.

CONCLUSION

Nicotine-induced attenuation of CPA precipitated by naloxone is mediated by the α7 nAChR subtype, and the CeA is one of the regions of the brain involved in the effect of nicotine on acutely opiate-dependent subjects.

Translational research in addiction: toward a framework for the development of novel therapeutics

The development of novel substance use disorder (SUD) therapeutics is insufficient to meet the medical needs of a growing SUD patient population. The identification of translatable SUD models and tests is a crucial step in establishing a framework for SUD therapeutic development programs. The present review begins by identifying the clinical features of SUDs and highlights the narrow regulatory end-point required for approval of a novel SUD therapeutic. A conceptual overview of dependence is provided, followed by identification of potential intervention targets in the addiction cycle. The main components of the addiction cycle provide the framework for a discussion of preclinical models and their clinical analogs, all of which are focused on isolated behavioral end-points thought to be relevant to the persistence of compulsive drug use. Thus, the greatest obstacle to successful development is the gap between the multiplicity of preclinical and early clinical end-points and the regulatory end-point of sustained abstinence. This review proposes two pathways to bridging this gap: further development and validation of the preclinical extended access self-administration model; inclusion of secondary end-points comprising all of the measures highlighted in the present discussion in Phase 3 trials. Further, completion of the postdictive validation of analogous preclinical and clinical assays is of high priority. Ultimately, demonstration of the relevance and validity of a variety of end-points to the ultimate goal of abstinence will allow researchers to identify truly relevant therapeutic mechanisms and intervention targets, and establish a framework for SUD therapeutic development that allows optimal decision-making and resource allocation.

Ondansetron blocks LiCl-induced conditioned place avoidance but not conditioned taste/flavor avoidance in rats

The ability of an experimental agent to support conditioned taste/flavor avoidance (CT/FA) in rats often is interpreted as sufficient evidence that the agent produced a state of malaise or nausea. Paradoxically, however, CT/FA also is induced by certain drugs that support conditioned preferences in rats, suggesting that CT/FA is insufficient to reveal a negative hedonic state. The present study tested the hypothesis that the anti-nausea drug ondansetron (OND) would block the ability of nauseogenic lithium chloride (LiCl) to support conditioned place avoidance (CPA), without attenuating LiCl-induced CT/FA. After pre-treatment with either OND or vehicle, rats were conditioned with i.p. injection of 0.15 M LiCl containing 2% saccharin (LiCl+sac) on conditioning day 1, and with 0.15 M NaCl alone on conditioning day 2. Rats were confined to a distinct chamber of a CPA apparatus after each conditioning injection. In other rats, OND or vehicle pre-treatment was followed by NaCl+sac on conditioning day 1, and LiCl alone on day 2. Subsequent testing revealed that OND blocked the ability of LiCl to support CPA. Conversely, in the same rats, OND did not alter the ability of LiCl to condition avoidance of 0.2% sac solution during a 60 min bottle test. In a separate experiment, a sensitive 2-bottle choice test was used to confirm that OND pre-treatment does not reduce the ability of LiCl to support CT/FA. These results support the view that CPA is an additional useful tool to reveal the experience of malaise and nausea in rats, whereas CT/FA demonstrated in bottle intake tests is insufficient for this purpose.

From mouse to man: the 5-HT3 receptor modulates physical dependence on opioid narcotics

OBJECTIVES

Addiction to opioid narcotics represents a major public health challenge. Animal models of one component of addiction, physical dependence, show this trait to be highly heritable. The analysis of opioid dependence using contemporary in-silico techniques offers an approach to discover novel treatments for dependence and addiction.

METHODS

In these experiments, opioid withdrawal behavior in 18 inbred strains of mice was assessed. Mice were treated for 4 days with escalating doses of morphine before the administration of naloxone allowing the quantification of opioid dependence. After haplotypic analysis, experiments were designed to evaluate the top gene candidate as a modulator of physical dependence. Behavioral studies as well as measurements of gene expression on the mRNA and protein levels were completed. Finally, a human model of opioid dependence was used to quantify the effects of the 5-HT3 antagonist ondansetron on signs and symptoms of withdrawal.

RESULTS

The Htr3a gene corresponding to the 5-HT3 receptor emerged as the leading candidate. Pharmacological studies using the selective 5-HT3 antagonist ondansetron supported the link in mice. Morphine strongly regulated the expression of the Htr3a gene in various central nervous system regions including the amygdala, dorsal raphe, and periaqueductal gray nuclei, which have been linked to opioid dependence in previous studies. Using an acute morphine administration model, the role of 5-HT3 in controlling the objective signs of withdrawal in humans was confirmed.

CONCLUSION

These studies show the power of in-silico genetic mapping, and reveal a novel target for treating an important component of opioid addiction.

Tropisetron attenuates naloxone-induced place aversion in single-dose morphine-treated rats: role of alpha7 nicotinic receptors

We have previously reported that acute dependence can occur when naloxone is administered 24 h after even a single dose of morphine, and that nicotine attenuates this naloxone-precipitated withdrawal syndrome. In the present study, we studied the effect of tropisetron, an alpha7 nicotinic receptor agonist and 5-hydroxytryptamine 3 (5-HT(3)) receptor antagonist, on place aversion induced by naloxone in morphine-treated rats. Place aversion was significantly attenuated by pre-administered tropisetron (1.0 and 2.0 mg/kg, i. p.) in a dose-dependent manner, however tropisetron alone had no effect in a place-conditioning paradigm. This attenuation was completely antagonized by mecamylamine (1.0 mg/kg, s.c.), which is a central nicotinic receptor antagonist, but not by ondansetron (0.3 and 1.0 mg/kg, s.c.), a 5-HT(3) receptor antagonist. Furthermore, methyllycaconitine (1.0 and 2.0 mg/kg, s.c.), an alpha7 nicotinic acetylcholine receptor antagonist, but not dihydroxy-beta-erithroidine (1.0 and 2.0 mg/kg, s.c.), an alpha4beta2 nicotinic acetylcholine receptor antagonist, also antagonized the inhibitory effect of tropisetron. These findings suggest that tropisetron attenuates place aversion induced by naloxone in single-dose morphine-treated rats via alpha7 nicotinic receptors.

Vocalizations during withdrawal from opiates and cocaine: possible expressions of affective distress

Intense anxiety has been postulated to trigger relapse to abuse of opiates and psychomotor stimulants. Preclinical research methodologies need to be developed to adequately characterize the affective or emotional component of withdrawal. Classically, withdrawal from psychomotor stimulants and opiates focuses on somatic and autonomic indices, foremost based on observational assessments and, additionally, on measures of disrupted conditioned behavior. These measures depict the intensity and time course of withdrawal from specific doses of opiates and psychomotor stimulants, but require large numbers of subjects due to single use of each individual. Behavioral disruptions have been attributed to anhedonia, a core symptom of drug withdrawal, as well as major depressive and psychotic disorders. In spite of some pharmacological validation, inferences about anxiety-like disturbances, based on observed somatic and autonomic signs or on changes in conditioned responses, have to remain tentative. High-pitched vocalizations may communicate affective expressions and, in rodents, different kinds of ultrasonic vocalizations communicate maternal separation distress in infants, accompany the intensely arousing phases of agonistic confrontations, signal submission and distress in defensive responses to threats and painful events, and are part of the excitatory and inhibitory phases of sexual behavior. While acute treatment with opiates, psychomotor stimulants, alcohol and benzodiazepines suppresses ultrasonic vocalizations in the 22-25-kHz range, rats emit high rates of ultrasonic vocalizations upon withdrawal from prolonged exposure to these drugs, particularly if they have been startled. Peak rates of ultrasonic distress calls occur ca. 1-3 days after cessation of cocaine or opiate treatment and decline within 5-7 days. Ultrasonic vocalizations during withdrawal from cocaine, alcohol or benzodiazepines can be attenuated by renewed access to the drug. It will be informative to learn how the neural circuit mediating vocalizations interacts with the ones subserving self-administration of alcohol, opiates and psychomotor stimulants.

Inhibition of 5-HT neurotransmission increases clonidine protective effects on naloxone-induced conditioned place aversion in morphine-dependent rats

Previous pharmacological studies have implicated serotonergic brain systems in opiate-withdrawal-precipitated conditioned place aversion. To assess this hypothesis, we tested the effects of either (i). a near-total 5,7-dihydroxytryptamine-induced lesion (90% depletion) or (ii). an acute serotonin (5-HT) inhibition induced by the specific stimulation of 5-HT1A autoreceptors (8-OHDPAT 5-100 microg/kg), on naloxone-induced conditioned place aversion in morphine-dependent rats. Morphine dependence was induced by the implantation of morphine slow-release pellets. The protective properties of clonidine (an alpha-2 adrenergic agonist classically given for opiate detoxification) were also tested after inhibition of 5-HT transmission. Serotonergic lesions in morphine-dependent rats failed to alter naloxone-induced conditioned place aversion but increased the sensitivity to the protective effects of clonidine. Acute neuropharmacological blockade of serotonin transmission also potentiated the clonidine effects on naloxone-induced conditioned place aversion. When combined with the 5-HT1A agonist 8-OHDPAT, clonidine was also found to be more potent. Further understanding of this serotonin/noradrenaline interaction might help devise new therapeutic treatments for the acute opiate withdrawal syndrome.

Changes in dopaminergic neurotransmission do not alter somatic or motivational opiate withdrawal-induced symptoms in rats

Opiate withdrawal has been correlated with decreased extracellular dopamine (DA) levels in the nucleus accumbens (NAC) of morphine-dependent rats. The authors tested the hypothesis that DA transmission plays a critical role in the induction of motivational and somatic withdrawal symptoms. First, the authors used a 6-hydroxydopamine-induced lesion of the NAC to chronically disrupt mesolimbic DA transmission. Second, global DA neurotransmission was acutely stimulated by the nonselective DA agonist (apomorphine) or inhibited by nonselective DA antagonists (droperidol or flupentixol). Morphine-dependent rats bearing 6-hydroxydopamine-induced lesions displayed naloxone-precipitated motivational and somatic withdrawal symptoms similar to those of sham-lesioned rats. Administration of apomorphine did not reduce naloxone-induced opiate withdrawal. Moreover, in total absence of naloxone, DA antagonists did not precipitate either conditioned place aversion or somatic abstinence signs in dependent rats. Taken together, these findings suggested that DA transmission is not critical for the induction of opiate withdrawal syndrome.

Dorsal and median raphe serotonergic system lesion does not alter the opiate withdrawal syndrome

Previous pharmacological studies have implicated serotonergic brain systems in opiate withdrawal. To test the hypothesis that serotonin (5-HT) has a critical role in the development of opiate withdrawal, we have employed a near-total brain 5-HT system lesion technique (90% depletion) using 5,7-dihydroxytryptamine combined with induction of opiate dependence by implantation of morphine pellets or by repeated injections of increasing doses of morphine. The effects of serotonergic neuron lesion were examined on spontaneous opiate withdrawal (changes in circadian locomotor activity) and naloxone-precipitated opiate withdrawal syndrome (the somatic aspect). The antiwithdrawal properties of clonidine, an alpha(2)-adrenoceptor agonist currently used for clinical treatment for the somatic signs of opiate withdrawal, were tested also in the lesioned rats. Our findings show that serotonergic lesions in morphine-dependent rats did not alter either the spontaneous or the naloxone-induced withdrawal syndrome (with exception of jumping behavior). Moreover, clonidine alleviated the naloxone-induced withdrawal syndrome in lesioned as well as in sham-operated morphine-dependent rats. These results demonstrate that 5-HT systems are not directly responsible for the development of the somatic opiate withdrawal syndrome in morphine-dependent rats.

The effect of the serotonergic system on opioid withdrawal-like syndrome in a mouse model of cholestasis

There is a marked elevation of endogenous opioid levels in plasma of human subjects with biliary cirrhosis as well as animal model of cholestasis. In addition, development of morphine tolerance and dependence has been shown to be inhibited by drugs which reduce brain serotonin levels. However, intracerebroventricular injection of serotonin increases the morphine analgesia. In the present study we have investigated the role of the serotonergic pathway in determining the withdrawal syndrome in a mouse model of cholestasis. There were three experimental groups: unoperated mice, sham operated mice and mice in which the main bile duct was ligated. Physical dependency was assessed by precipitating a withdrawal syndrome (writing, climbing, rearing, grooming and jumping) by naloxone (2 mg/kg) 5 days after induction of cholestasis. In separate experimental same groups, the antinociception was evaluated by the tail flick latency (TFL) test. Administration of serotonin receptors antagonists, cyproheptadine (10 mg/kg), methysergide (6 mg/kg) and ondansetron (10 mg/kg) attenuated withdrawal signs and decreased the antinociception. However, treatment by fluoxetine (15 mg/kg), an inhibitor of serotonin reuptake, increased the withdrawal signs and antinociception. These experiments lead us to conclude that the naloxone-precipitated withdrawal signs which occur in the mouse model of cholestasis are potentially dependent on the serotonergic pathway. Copyright 2000 John Wiley & Sons, Ltd.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

Continued trends in the conditioned place preference literature from 1992 to 1996, inclusive, with a cross-indexed bibliography

In light of the overwhelming response to the previous publication in Neuroscience and Biobehavioral Reviews (1993, 17, 21-41) regarding trends in place conditioning (either preference or aversion), the present work constitutes a five-year follow-up to review the empirical research in this behavioral paradigm from 1992 to 1996, inclusively. The behavioral technique has grown as indicated by the number of publications over the last five years which equals those authored over the 35 years covered by our last survey. The previous work used descriptive statistics to explore topical issues, whereas the present work discusses trends since that time and hopes to provide an exhaustive bibliography of the CPP literature, including articles, published abstracts, book chapters and reviews, as well as providing a cross-index of identified key words/drugs tested.

Effects of ondansetron administration on opioid withdrawal syndrome observed in rats

This study tested whether a 5-HT3 receptor antagonist could reverse the signs of precipitated opioid withdrawal. Rats were treated with either saline or morphine for 4 days. After the four days, half of the rats in each group received naloxone and half received saline. Each animal also received one of four doses of ondansetron (0, 1, 2 and 4 mg/kg i.p.). Administration of ondansetron to rats receiving naloxone after chronic morphine decreased the intensity of withdrawal signs such as increased defecation, jumping and wet-dog shakes, elevated the nociceptive threshold values which were decreased by precipitated withdrawal, but produced no change in urination, rectal temperature or salivation. The effects exhibited by ondansetron administration may be explained through interference of its 5-HT3 receptor antagonist activity with serotoninergic mechanisms involved in the regulation of these withdrawal symptoms. The use of this drug is thus suggested as a possible treatment of opioid withdrawal signs in heroin addicts.

Attenuation of mecamylamine-precipitated nicotine-withdrawal aversion by the 5-HT3 receptor antagonist ondansetron

The effect of ondansetron (0.01-0.1 mg/kg, s.c.), a selective 5-HT3 receptor antagonist, on mecamylamine-precipitated nicotine-withdrawal aversion was examined in the conditioned place preference paradigm. Male Sprague-Dawley rats were chronically treated subcutaneously with 9 mg/kg/day (-)-nicotine tartrate using an osmotic minipump. After nicotine treatment for 7 days, mecamylamine (1 mg/kg, s.c.), a nicotinic receptor antagonist, produced place aversion in nicotine-dependent rats. This aversive effect was dose-dependently antagonized by pretreatment with ondansetron 30 min prior to the conditioning. These results suggest that ondansetron may attenuate the place aversion associated with nicotine withdrawal, and may be useful for the treatment of nicotine dependence.

Role of delta-opioid receptors in mediating the aversive stimulus effects of morphine withdrawal in the rat

An unbiased place preference conditioning procedure was used to examine the role of delta-opioid receptors in mediating the aversive effects of opioid withdrawal. Rats were implanted s.c. with two pellets each containing placebo or 75 mg morphine. Single-trial conditioning sessions with saline and the opioid receptor antagonists naloxone (0.001-1.0 mg/kg, s.c.), naltrindole (0.01-3.0 mg/kg, s.c.) or naltriben (0.01-3.0 mg/kg, s.c.) commenced 4 days later. During these conditioning sessions, physical signs of withdrawal were also quantified. Tests of conditioning were conducted on day 5. Naloxone in doses of 0.01-1.0 mg/kg produced significant conditioned place aversions in morphine-implanted animals. A dose of 0.01 mg/kg produced few physical withdrawal signs whereas higher doses resulted in marked wet dog shakes, body weight loss ptosis and diarrhea. No such effects were observed in control (placebo-implanted) animals. Administration of the selective delta-opioid receptor antagonists naltrindole and naltriben produced dose-related place aversions in morphine-implanted animals. The magnitude of these effects did not differ from that observed with naloxone. The minimum effective doses of naltrindole and naltriben were 0.1 mg/kg. Doses of 0.1-1.0 mg/kg produced few, if any, somatic signs of withdrawal whereas higher doses of these antagonists only produced diarrhea and wet-dog shakes. Other withdrawal signs were absent. In contrast to the opioid receptor antagonists tested, the dopamine D1 receptor antagonist SCH23390 failed to produced conditioned place aversions or physical signs of withdrawal in morphine-pelleted animals. These data demonstrate that the selective blockade of either delta- or mu-opioid receptors is sufficient to induce conditioned aversive effects in morphine-dependent animals. They also indicate that physical symptoms associated with precipitated morphine withdrawal differ depending upon the opioid receptor antagonist employed.

Study of the mechanisms involved in behavioral changes induced by flunitrazepam in morphine withdrawal

1. The attenuation of morphine withdrawal syndrome by acute benzodiazepine administration has been well documented. However, the pharmacological mechanisms implicated in this effect remain unclear. 2. In this study, the possible participation of noradrenergic, serotonergic and benzodiazepine receptors on flunitrazepam-modified morphine withdrawal syndrome was investigated in mice. Flunitrazepam was associated to the noradrenergic antagonists prazosin (1 mg/kg) and propranolol (0.5 mg/kg), the serotonergic agents ritanserine (1 mg/kg) and p-chloro phenylalanine (600 mg/kg), the benzodiazepine antagonist flumazenil (10 mg/kg), and the benzodiazepine partial inverse agonist Ro 15-4513 (5 mg/kg). 3. The decrease in jumping behavior-induced by flunitrazepam was potentiated by prazosin, while ritanserine, flumazenil and Ro 15-4513 blocked this effect. 4. Flunitrazepam-induced increase on wet dog shake frequency was partially reduced by flumazenil, and strongly antagonized by ritanserine and Ro 15-4513. 5. Noradrenergic and serotonergic systems seem to be primarily implicated in the changes induced on jumping and wet dog shakes respectively. These modifications are induced through the activation of the benzodiazepine receptors.

Antidepressant-like effects of SR 57227A, a 5-HT3 receptor agonist, in rodents

We have investigated the effect of SR 57227A, a selective 5-HT3 receptor agonist which crosses the blood brain barrier, on three rodent models in which antidepressants are active. In the forced swimming test, SR 57227A dose-dependently reduced the duration of immobility in mice and rats after i.p. administration. (ED50 = 14.2 mg/kg i.p. in mice, and 7.6 mg/kg i.p. in rats.) The compound was also active in both species after oral administration. In a time-course study in mice, SR 57227A (20 mg/kg p.o.) produced a significant effect lasting 6 h. SR 57227A (1 and 3 mg/kg i.p.) reduced the elevation of the escape failures in the learned helplessness model in rats by 50-60% on the last two days of the avoidance task, and reduced isolation-induced aggressivity in mice by 50 to 85%, an effect which was antagonised by zacopride (1 mg/kg i.p.). These results suggest that the stimulation of 5-HT3 receptors can produce antidepressant-like effects in behavioral tests in rodents.

The role of 5-HT3 receptors in drug dependence

Since the discovery of serotonin receptor subtypes in 1957, the classification of serotonin receptors now includes 5-HT1 through 5-HT7 receptors, with further subtypes of receptors in each family. Unique among this expanding group of 5-HT receptor subtypes is the 5-HT3 receptor, which is the only known 5-HT receptor that directly gates an ion channel. The channel conducts primarily Na+ and K+, resulting in rapid depolarization followed by a rapid desensitization. The immediate consequence of neuronal depolarization resulting from 5-HT3 receptor activation is the release of stored neurotransmitter. The subsequent release of stored neurotransmitter, particularly dopamine in the mesolimbic pathways, suggest a potentially important role for this receptor system in neuronal circuitry involved in drug abuse. The following review broadly covers the structure, function and distribution of the 5-HT3 receptor system in the CNS and data addressing the potential role of this receptor system in modulating the effects of a wide variety of abused drugs. Most of the evidence indicates an association between the ability of 5-HT3 antagonists to decrease mesolimbic dopamine levels and to attenuate the psychomotor stimulant effects of drugs. However 5-HT3 receptor antagonists are less robust at attenuating other drug effects that are believed to be related to their abuse liability, such as discriminative stimulus and reinforcing effects. The one exception may be ethanol, which directly potentiates the effects of 5-HT at the 5-HT3 receptor channel complex. In addition to the implications of an interaction with the mesolimbic dopaminergic system, the ability of 5-HT3 receptor antagonists to function as anxiolytics suggest they could be useful pharmacotherapies during drug withdrawal. However, further studies are needed since currently available 5-HT3 receptor antagonists do not have uniform behavioral effects, may interact with other receptor systems, and have atypical dose-response effects.

Contrasting actions of acute or chronic paroxetine and fluvoxamine on morphine withdrawal-induced place conditioning

The acute and chronic effects of paroxetine and fluvoxamine on naloxone withdrawal-induced place aversion in morphine dependent rats were investigated. Acutely administered fluvoxamine (25 mg/kg s.c. given 30 min prior to naloxone withdrawal pairing) and chronic daily paroxetine (10 mg/kg s.c.) coadministration with a morphine induction protocol, both attenuated morphine withdrawal place aversion. Conversely, acutely administered paroxetine (up to 25 mg/kg s.c.) or chronic daily fluvoxamine (10 mg/kg s.c.) coadministration with morphine did not modify subsequent withdrawal place aversion. Previous radioligand binding studies indicate that fluvoxamine has opioid-displacing properties. It is suggested therefore that acute fluvoxamine may have decreased withdrawal aversion, probably through serotonin and also, in part, via an opioid-like mechanism whereas chronic paroxetine decreased withdrawal aversion by a serotonergic mechanism, but it is not clear whether opioid systems play any role in the action of paroxetine.

Clinical efficacy of the 5-HT3 antagonist ondansetron in alcohol abuse and dependence

Medications that act on the serotonergic system have been found to be of benefit in the treatment of alcohol-dependent individuals. In a randomized, placebo-controlled study, the efficacy of 6 weeks of ondansetron, a 5-HT3 antagonist (0.25 mg bid or 2.0 mg bid), in the treatment of 71 nonseverely alcohol-dependent males was tested. The results showed reduction of drinking differences were steadily increasing toward the end of the treatment period approached significance at week 7 in the 0.25 mg group (p = 0.06). Twice as many patients in this group showed > or = 2 standard deviations decrease in drinking compared with the other groups. When patients drinking > 10 drinks/drinking day at baseline (n = 11) were excluded from the analysis, significant group differences were found at both treatment and follow-up, with the lower ondansetron dose producing the greatest reduction from baseline (i.e., 2.8 standard drinks; -35% compared with baseline and -21% compared with placebo; p < 0.02-0.001). Within this group, there was an almost 4-fold greater number of patients showing a clinically meaningful decrease in drinking. Lower baseline drinking and higher level of education were significant and strong predictors of drinking reduction during treatment. Ondansetron was very well tolerated; hence, further long-term studies with 5-HT3 antagonists alone or in combination with other treatment components may offer promise for treatment of alcoholism.

Endogenous kappa-opioid systems in opiate withdrawal: role in aversion and accompanying changes in mesolimbic dopamine release

The kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI) was recently shown to potentiate certain overt withdrawal signs in morphine-dependent rats. The present study sought to further assess this phenomenon by examining the influence of nor-BNI treatment upon the conditioned place aversion associated with the naloxone-precipitated withdrawal syndrome. In addition, in vivo microdialysis studies were conducted in morphine-dependent rats to determine whether nor-BNI treatment can modify withdrawal-induced changes in basal dopamine (DA) release within the mesolimbic system. Rats were pretreated with either saline or a single dose of nor-BNI and then received ascending doses of morphine for 10 days. A withdrawal syndrome was then precipitated by the administration of naloxone (1 mg/kg SC). In rats which received chronic morphine injections, administration of naloxone produced a characteristic withdrawal syndrome and a marked aversion for an environment previously associated with naloxone-precipitated withdrawal. Nor-BNI treatment potentiated most overt signs of physical dependence. This treatment also resulted in a greater withdrawal-induced place aversion. Morphine-dependent rats exhibited a marked reduction in basal mesolimbic DA release. An even greater decrease in basal DA release was observed in nor-BNI treated rats. These results suggest that endogenous kappa-systems are important in the modulation of mesolimbic DA release and the accompanying place aversion which occurs during opiate withdrawal.

Antagonist-precipitated opioid withdrawal in rats: evidence for dissociations between physical and motivational signs

In rats made opioid dependent by the implantation of a single morphine 75 mg base pellet, an attempt was made to determine whether any correlational existed between physical and motivational withdrawal signs by adjusting the dose of naloxone used to precipitate withdrawal. The models used to study motivational signs were taste (one- and two-bottle) conditioning and operant responding for food under an FR15 schedule of reinforcement. Naloxone at doses of 0.01 mg/kg and above produced both a conditioned taste aversion (two-bottle test only) and reduced food responding in morphine pellet, but not placebo pellet, implanted animals. No physical withdrawal signs, e.g., wet dog shakes, diarrhoea, were noted until naloxone doses of 0.05 mg/kg and above were used. It is concluded that the difference in naloxone doses required to elicit physical and motivational withdrawal components provides further support for their dissociation.

The 5-HT3 antagonist MDL-72222 exacerbates ethanol withdrawal seizures in mice

Ethanol-dependent mice were treated with the 5-HT3 antagonist MDL 72222 after withdrawal from ethanol. Treatment with unit doses (0, 5.6, 10, and 17.0 mg/kg) of MDL 72222 at 0, 4, and 7 hr after withdrawal dose-dependently exacerbated the severity of ethanol withdrawal seizures. Treatment with a single dose (17 mg/kg) of MDL 72222 at 5 hr after withdrawal also exacerbated the severity of ethanol withdrawal seizures. Ethanol naive mice treated with MDL 72222 (56 mg/kg) did not display any seizures. Treatment with another 5-HT3 antagonist, ICS 205-930 (23 and 46 mg/kg), or the 5-HT2 receptor antagonist ketanserin, did not affect ethanol withdrawal seizures. The findings suggest MDL 72222 selectively enhances sensitivity to withdrawal seizures following chronic ethanol exposure.

Clonidine attenuates conditioned aversion produced by naloxone-precipitated opiate withdrawal

Clonidine, an alpha 2-adrenoreceptor agonist, is used to alleviate withdrawal severity during detoxification from chronic opiate use. The effectiveness of clonidine has been challenged because it attenuates some, but not all, withdrawal signs in humans and animals. Because somatic assessments may not reflect opiate withdrawal aversion, this study used a one-trial place conditioning procedure. Naloxone was paired with the most preferred side and the amount of time spent on this side and on the vehicle-paired side was measured after training. This procedure led to a dose-related decrease in the time spent on the naloxone-paired side in morphine-exposed rats, but not in non-morphine-exposed rats. This conditioned place aversion shown by morphine-exposed rats was significantly attenuated by clonidine. These results suggest that in addition to clonidine's ability to lessen some somatic signs of opiate withdrawal, it also attenuates the aversive aspects of this state.

Voluntary oral morphine self-administration in rats: effect of haloperidol or ondansetron

Rats were exposed to increasing concentrations of morphine hydrochloride (up to 0.4 mg/ml) in 5% w/v sucrose solution as their sole source of drinking water. Physical dependence was established as determined by the precipitation of withdrawal behaviour following administration of 1 mg/kg IP naloxone hydrochloride on day 23. The choice between either a 5% w/v sucrose solution or a 5% w/v sucrose solution containing 0.4 mg/ml morphine hydrochloride 4 days following withdrawal resulted in rats being categorized into two groups based on their respective consumption of the morphine-containing solution. The amount of morphine solution voluntarily consumed by approximately half the rats were sufficiently high as to lead to a relapse into physical dependence to morphine. The high preference for morphine shown by these rats could not be attributed to the taste of the morphine solution. Naive rats or rats exposed to a 5% w/v sucrose solution for 23 days failed to consume significant quantities of the morphine-containing solution when provided with a choice. The administration of either an IM slow-release formulation of 70.5 mg/kg haloperidol decanoate (= 50 mg/kg haloperidol) or 10 micrograms/kg IP ondansetron hydrochloride daily did not alter morphine ingestion in the high morphine-preferring rats.

Interactions between 5-HT3 receptors and cerebral dopamine function: implications for the treatment of schizophrenia and psychoactive substance abuse

This article reviews current knowledge on the interaction between 5-hydroxytryptamine (5-HT), acting at 5-HT3 receptors in the CNS, and cerebral dopamine systems. Since 1987, a growing body of behavioural, neurochemical and electrophysiological evidence from animal studies has demonstrated a clear role for 5-HT3 receptors in the modulation of activity of mesolimbic and mesocortical dopamine neurones. This evidence has led to the suggestion that 5-HT3 receptor antagonists have potential as novel antipsychotic agents and may also find use in the treatment of psychoactive substance abuse. Data emerging from clinical studies generally support this hypothesis and suggest that 5-HT3 antagonists may prove to be among the first agents available to treat schizophrenia which are not dopamine D2 antagonists and hence lack their side-effect problems.

Effect of 5-HT3 receptor antagonists on the discriminative stimulus properties of morphine in rats

5-HT3 receptor antagonists, e.g. MDL72222, ondansetron and ICS205-930, have been previously reported to block a morphine (1.5 mg/kg)-induced conditioned place preference in rats. This finding suggests that these drugs may modify the morphine discriminative stimulus which underlies place conditioning. To study this further we have examined the effects of MDL72222, ondansetron and ICS205-930 against a morphine discriminative stimulus using a two-choice, food reinforced, operant paradigm. In an attempt to provide consistency with previous place conditioning studies, a morphine training dose of 1.5 mg/kg was used in addition to a higher 3 mg/kg dose which was studied in separate animals. Stimulus control of behaviour was attained at both morphine training doses, the characteristics of each being consistent with an effect at the mu opioid receptor. Ondansetron (0.001-1 mg/kg), MDL72222 (0.1-3 mg/kg), and ICS205-930 (0.001-1 mg/kg) all failed to consistently antagonise the morphine cue at both training doses, although a mild attenuation was seen in the 1.5 mg/kg group following pretreatment with an intermediate dose of ondansetron and ICS205-930 (both 0.01 mg/kg). The present results therefore suggest hat 5-HT3 antagonists do not block a morphine discriminative state, at least in rats.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Behavioural pharmacology of 5-HT3 receptor ligands

Extensive studies have ascribed a role for the central 5-HT3 receptor in the modulation of behaviour. Much of the work stems from the actions of potent and selective 5-HT3 receptor antagonists; these agents reduce mesolimbic dopamine initiated hyperactivity, release suppressed behaviour, reduce the reinforcing properties and withdrawal symptoms of drugs of abuse, enhance cognitive performance and modulate appetite. This article reviews the preclinical and clinical evidence implicating the 5-HT3 receptor in these indications and discusses the potential neurochemical mechanisms underlying the behavioural changes.

Effect of the 5-HT3 receptor antagonists, MDL72222 and ondansetron on morphine place conditioning

The purpose of the present study was to reassess the original findings of Carboni et al. (1988) who suggested that 5-HT3 receptor antagonists may block morphine-induced place conditioning in rats. These workers used a biased protocol with treatments allocated to compartments based on initial preference. In the present study we have adopted an unbiased approach with treatments randomly assigned to conditioning compartment in a counter-balanced fashion. Thus treatments were equally paired between distinct environmental cues. Using this protocol, morphine produced a dose-related place preference (0.3-3 mg/kg SC). Thirty-minute pretreatment with the selective 5-HT3 antagonists, MDL72222 (1 mg/kg SC) and ondansetron (0.01 mg/kg SC) before morphine (1.5 mg/kg SC), significantly antagonized the place conditioning to this treatment. However, with higher doses of ondansetron (0.1-1 mg/kg SC), the antagonism of morphine-induced place preference became variable and dependent on the conditioning compartment. This was probably a reflection of the fact that ondansetron when administered alone also appeared to produce an environmentally dependent place conditioning at these doses. Therefore it is concluded that at certain doses, 5-HT3 receptor antagonists may antagonize morphine place conditioning in a manner consistent with a blockade of the appetitive effects of this drug. However, at higher doses, at least with ondansetron, this antagonism became non-specific and dependent on the training environment. It is suggested that other animal models of opioid reinforcement (e.g., self-administration) are now needed to validate the hypothesis that 5-HT3 receptor antagonists may modify opioid reward.

The NMDA antagonist dizocilpine (MK801) attenuates motivational as well as somatic aspects of naloxone precipitated opioid withdrawal

The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine has recently been reported to antagonize certain overt withdrawal signs in morphine dependent rats. The purpose of the present study was to reassess this response and examine the effect of this drug in a model presumably reflective of the motivational impact of withdrawal using the place conditioning technique. Rats were made opiate dependent by the subcutaneous implantation of a 75 mg morphine pellet. Three-4 days later withdrawal was precipitated by naloxone 0.5 mg/kg. Dizocilpine (0.1-0.5 mg/kg) attenuated many of the subsequent behaviours elicited by naloxone, notably diarrhoea, mouth movements, paw shakes and ptosis. In a separate group of morphine dependent rats, naloxone (0.05 mg/kg) precipitated withdrawal produced a clear place aversion. This place aversion was blocked by dizocilpine (0.02-0.1 mg/kg) pre-treatment prior to conditioning. Therefore dizocilpine may modify both motivational and somatic aspects of opioid withdrawal.

Effects of the 5-HT3 antagonist ondansetron on benzodiazepine-induced operant behavioural dependence in rats

This study was designed to assess whether rats made tolerant to the suppressant action on Fixed Ratio operant responding of the benzodiazepine (BZ) chlordiazepoxide (CDP) would show behavioural disruption on drug withdrawal--so-called operant behavioural dependence. In addition, the study examined the effects of the 5-HT3 antagonist ondansetron on such operant behavioural dependence. During 42 consecutive days of CDP treatment, at doses escalated from 10 to 30 mg/kg/day, marked tolerance developed to the rate-suppressant action of CDP. On subsequent days, during spontaneous withdrawal, response rates declined significantly by around 30% in animals treated with saline, although some recovery of responding was seen over successive days of withdrawal. Similar reductions in responding followed by recovery were seen in rats treated with the 5-HT3 antagonist ondansetron (0.01-0.1 mg/kg, b.i.d.). These findings demonstrate for the first time that it is possible to use operant procedures to detect spontaneous BZ withdrawal. They also suggest, in agreement with recent studies from this laboratory (Leathley and Goudie 1992), that 5-HT3 antagonists may have relatively limited utility in treating some signs of BZ dependence.