Sites of action of ondansetron to inhibit withdrawal from drugs of abuse

Diazepam withdrawal: effects of diazepam and gepirone on acoustic startle-induced 22 kHz ultrasonic vocalizations

It has proven difficult to demonstrate and study the "anxiogenic" quality of drug withdrawal states in animals. Ultrasonic vocalizations (USV) in response to acoustic startle stimuli have shown promise as a measure of affect and may represent "distress" responses during diazepam withdrawal. Three experiments evaluated the association between USV and "distress" by comparing the effects of diazepam as a prototypic benzodiazepine agonist and the putative anxiolytic gepirone with affinity for 5-hydroxytryptamine (5-HT1A) receptors in naive and diazepam-withdrawn subjects. Adult male Long-Evans rats were exposed to acoustic startle sessions consisting of nine 105 dB and nine 115 dB stimuli. USV at 20-30 kHz were readily emitted during startle and often commenced after the third or fourth stimulus presentation. Acutely, intraperitoneal (IP) administration of diazepam (0.1-3 mg/kg) and gepirone (0.1-1 mg/kg) decreased USV dose-dependently without affecting the startle reflex; gepirone also decreased tail flick latency. Startle-induced USV were also sensitive to the "anxiogenic" effects of withdrawal from diazepam exposure (0, 2.5, 5, 10 mg/kg b.i.d. IP x 5 days). Twenty-four hours after the last diazepam injection, rats were hyperreactive to startle stimuli and doubled their rate of USV over vehicle-treated controls. Gepirone (0.1-1 mg/kg IP), but not diazepam (3-20 mg/kg IP) antagonized the increased rate of USV in rats withdrawn from 10 mg/kg b.i.d. diazepam. Diazepam (2.5-10 mg/kg IP) antagonized the increased rate of USV in rats withdrawn from 2.5 mg/kg b.i.d. diazepam.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of physical dependence on diazepam by ondansetron in rats

The effects of ondansetron, a 5-HT3 antagonist, on the development of physical dependence on diazepam were examined in rats using a drug-admixed food method. Rats were treated with diazepam or diazepam in combination with ondansetron for 26 days. After an abrupt withdrawal from diazepam, the incidence of withdrawal signs, such as jerks, tremors and convulsions, and withdrawal scores, were potentiated by co-administration of ondansetron. On the other hand, rats which had been treated with ondansetron alone for 33 days did not show any withdrawal signs after abrupt withdrawal from ondansetron. These findings suggest that ondansetron does not possess physical dependence liability, but does potentiate the development of physical dependence on diazepam. Regulation of serotonergic neurons through 5-HT3 receptors may affect the development of physical dependence on diazepam.

Flumazenil induces localised increases in glucose utilization during diazepam withdrawal in rats

The quantitative [14C]2-deoxyglucose autoradiographic technique has been employed to identify the neural circuits involved in diazepam withdrawal. Local cerebral glucose utilization (LCGU) was assessed in parallel groups of rats chronically treated with diazepam (5 mg/kg i.p., daily for 28 days), in rats that were withdrawn from chronic diazepam 24 h previously and in those that received flumazenil (5 mg/kg i.v.) immediately or 24 h after the last dose of diazepam. Two further groups received chronic vehicle or acute flumazenil (5 mg/kg i.v.). Rats withdrawn from diazepam 24 h previously did not produce changes in LCGU in the 51 structures examined compared with both control and chronic diazepam treated groups, suggesting that spontaneous withdrawal from small doses of diazepam does not evoke marked alterations in functional activity. In contrast, flumazenil-precipitated diazepam withdrawal produced a marked increase in glucose use in structures of the Papez circuit of emotion (mammillary body, anterior thalamus, cingulate cortex), together with increases in the septal nucleus, basolateral amygdala and nucleus accumbens. Less widespread increases in glucose use occurred in primary auditory and visual areas and in extrapyramidal areas. This pattern resembles that produced after acute FG-7142 administration (Brain Res., 475 (1988) 218-231). In rats receiving flumazenil 24 h after the last dose of diazepam there was a similar, but more restricted, pattern of change in LCGU. Flumazenil had no effect on LCGU in drug naive rats. Thus, flumazenil could only exert an effect upon LCGU in rats chronically treated with diazepam. These data provide functional neuroanatomical evidence for a withdrawal shift in the inverse agonist direction after chronic diazepam and suggest that flumazenil-precipitated withdrawal changes may merely be a reflection of this phenomenon.

Serotonin and dopamine sensitization in the nucleus accumbens, ventral tegmental area, and dorsal raphe nucleus following repeated cocaine administration

The present study was designed to examine the effects of chronic cocaine administration on the extracellular response of serotonin (5-HT) and dopamine (DA) to a peripheral cocaine injection using in vivo brain microdialysis in awake rats. Two different dual probe preparations were used: One group of animals had guide cannulae aimed at the ventral tegmental area (VTA) and nucleus accumbens (N ACC) and a second group of animals had guide cannulae aimed at the dorsal raphe nucleus (DRN) and N ACC. Rats from both groups were given daily injections of either cocaine (20 mg/kg i.p.) or saline (0.9%; 0.05 ml/kg i.p.) for 10 consecutive days. On day 11, baseline dialysate levels of DA, 5-HT, dihydroxyphenylacetic acid, and 5-hydroxyindoleacetic acid were obtained from either the N ACC and VTA or the N ACC and DRN, followed by a 10 mg/kg i.p. cocaine injection and an additional 150 min of dialysate sampling. The percent baseline increases of both 5-HT and DA were significantly higher in the N ACC, VTA, and DRN of animals that received daily injections of cocaine compared with saline controls (p < 0.05, in each region). Maximum dialysate 5-HT concentrations after cocaine challenge were significantly higher in the N ACC and VTA (p < 0.05) and DRN (p < 0.01) of chronically treated animals compared with saline controls. Maximum dialysate DA concentrations were significantly higher in the N ACC and DRN (p < 0.05) of chronically treated animals compared with saline controls. There was no significant difference between acute and chronic animals in the maximum dialysate DA concentration from the VTA after cocaine challenge.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and evaluation of [125I]-(S)-iodozacopride, a high affinity radioligand for 5HT3 receptors

We have developed a high specific activity radioiodinated ligand for the biochemical evaluation and autoradiographic localization of 5HT3 receptors in the brain. [125I]-(S)-iodozacopride was synthesized by radioiodination of deschloro-(S)-zacopride using chloramine-T, and the product was purified by HPLC. The equilibrium kinetics and pharmacology of the binding of this radioligand were studied in homogenates of rat cerebral cortex, while the distribution of binding was examined by quantitative autoradiography. [125I]-(S)-iodozacopride bound to a single, saturable, specific binding site (Kd = 192 +/- 9 pM, Bmax = 1.2 +/- 0.2 fmol/mg protein). The binding had the pharmacological properties of a 5HT3 receptor, being potently inhibited by a variety of 5HT3 agonists and antagonists including (S)-zacopride (Ki = 0.032 nM), Quipazine (Ki = 0.45 nM), LY278584 (Ki = 0.5 nM), (1-m-chlorophenyl)-biguanide (Ki = 0.6 nM) and ICS 205-930 (Ki = 1.0 nM). Autoradiographic studies were undertaken by incubating sections with 400 pM [125I]-(S)-iodozacopride and exposing them to film for 3-7 days to obtain suitable autoradiograms. Specific binding of [125I]-(S)-iodozacopride was found at various amounts in a variety of brain regions. The highest levels of binding were found in the brainstem, principally the nucleus of the solitary tract with somewhat lower levels in the area postrema, substantia gelatinosa of the trigeminal nucleus and dorsal motor nucleus of the vagus. In the rat forebrain, moderate levels of specific binding were found in the glomerular layer of the olfactory bulb, anterior olfactory nucleus and various subnuclei of the amygdala. Lower levels of binding were seen in the superficial laminae of the parietal cerebral cortex and diffusely distributed throughout the hippocampal formation. In conclusion, [125I]-(S)-iodozacopride binds to a receptor site with the pharmacological properties and distribution that is consistent with the 5HT3 receptor. [125I]-(S)-iodozacopride represents a significant improvement in autoradiographic studies of the 5HT3 receptor by reducing the required exposure time for producing autoradiograms from the 3-6 months required for [3H]-labeled ligands to 3-7 days.

Ethanol- and diazepam-withdrawal hyperlocomotion is not due to 5-HT3 receptor stimulation

The effect of 5-HT3 receptor antagonists such as ondansetron, ICS 205-930, MDL 72222, metoclopramide, and zacopride was investigated on the ethanol as well as diazepam withdrawal phenomena in the present study. There was a significant increase in locomotor activity in the ethanol- as well as diazepam-withdrawn rats. The treatment of rats with 5-HT3 receptor antagonists during withdrawal phase did not modify the effect. The ethanol-withdrawn rats were more sensitive to pentylenetetrazole (PTZ)-induced convulsions as compared to control animals. 5-HT3 receptor antagonists did not attenuate the increased sensitivity of ethanol-withdrawn rats to PTZ. These observations indicated that 5-HT3 receptor antagonists are ineffective in attenuating hyperlocomotor activity following abrupt termination of chronic administration of ethanol or diazepam, and increased sensitivity to PTZ in the ethanol-withdrawn rats.

Nervous system distribution of the serotonin 5-HT3 receptor mRNA

The serotonin 5-HT3 receptor subtype has been implicated in many brain functions. Antagonists of this receptor have anxiolytic and antiemetic effects in humans and in animal models. To determine with cellular resolution the distribution of 5-HT3 receptor mRNA, in situ hybridization was performed in sections of mouse brain and dorsal root ganglia. Scattered labeled cells were observed throughout cortical regions, with highest densities in the piriform, cingulate, and entorhinal areas. Strong hybridization signals were seen in the hippocampal formation, where expression appeared primarily in interneurons. Labeled cells were most abundant in the posteroventral hippocampal region, particularly in the lacunosum moleculare layer of CA1. This distribution suggests that 5-HT3 receptors may mediate the known serotonergic inhibition of pyramidal cell populations via excitation of inhibitory interneurons. Labeled cells were also observed in the major subdivisions of the amygdaloid complex, the olfactory bulb, the trochlear nerve nucleus, the dorsal tegmental region, the facial nerve nucleus, the nucleus of the spinal tract of the trigeminal nerve, and the spinal cord dorsal horn. In the periphery, intense hybridization signals were seen in a subpopulation of cells in dorsal root ganglia. The data correlate generally with physiological, behavioral, and receptor autoradiographic studies, provide cellular resolution, and reveal regions of receptor expression not previously observed. The distribution of 5-HT3 receptor mRNA is consistent with roles for the receptor in cognition and affect and in the modulation of sensory input.

Increased 5-HT release mediates the anxiogenic response during benzodiazepine withdrawal: a review of supporting neurochemical and behavioural evidence

This paper reviews the biochemical and behavioural evidence that the increased anxiety that occurs during benzodiazepine withdrawal is caused by increased 5-HT activity. In hippocampal slices taken from rats withdrawn for 24 h from chronic diazepam treatment (2 mg/kg/day for 21 days) there was a significant increase in K(+)-evoked release of [3H]5-HT and in 45Ca2+ uptake and both of these changes were reversed by the GABAB agonist, baclofen. Baclofen also reversed the anxiogenic response that is detected on withdrawal from chronic diazepam treatment. Other drugs that reduce 5-HT function (tianeptine which increases 5-HT uptake; buspirone, a 5-HT1A receptor agonist/partial agonist; zacopride, a 5-HT3 receptor antagonist) also reversed this anxiogenic response. Finally, we present data from a group of rats that did not develop tolerance to the anxiolytic effects of diazepam (2 mg/kg), even after 5 weeks treatment. This group failed to show an anxiogenic response on withdrawal from diazepam, nor was there an increase in hippocampal 5-HT release. We discuss the extent to which increased hippocampal 5-HT release can be causally linked to the increased anxiety during benzodiazepine withdrawal.

Discharge dependencies of amygdala central nucleus neurons to the cardiac and respiratory cycle following local cocaine administration

We examined dependencies of amygdala central nucleus neuronal discharge to the cardiac and respiratory cycles in freely behaving cats following local microinjection of cocaine (100 micrograms/0.2 microliter). Cross-correlation histograms showed cycle-by-cycle dependencies between neuronal discharge and the cardiac and respiratory cycles in 10 of 30 cells and 7 of 30 cells, respectively, during baseline periods. After cocaine delivery, the discharge rate of half of the central nucleus of the amygdala cells (16/30, 53%) were partly or completely inhibited in a reversible manner. Excluding cardiac- and respiratory-dependent neurons which ceased firing after cocaine, more than half (5/8) of the remaining cardiac and two-thirds (4/6) of respiratory-dependent neurons altered discharge dependencies following cocaine administration. Of the cells that did not exhibit cardiac and respiratory dependencies pre-cocaine, 2 of 20 developed cardiac correlations and 3 of 23 developed respiratory correlations following cocaine administration. We speculate that a portion of the cardiac and respiratory responses induced by cocaine may be mediated through the central nucleus of the amygdala.

The Psychopharmacology of 5-HT3Receptors

5-HT3 receptors have an exclusive neuronal location and evidence is presented of their involvement in behaviour. 5-HT3 receptor antagonists such as ondansetron, tropisetron and zacopride have provided the critical pharmacological tools to reveal a potent and efficacious ability to regulate disturbed behaviour. Thus the 5-HT3 receptor antagonists will restore to normal rodent and primate behaviour disturbed by increasing limbic dopamine function, aversive situations, cognitive impairments and drug abuse. The remarkable feature of their action is a failure to modify normal behaviour. This unique pharmacological signature has ensured a wide interest in the potential role of the 5-HT3 receptor antagonists in the treatment of schizophrenia, anxiety, age related memory impairment and the problems of withdrawal from drugs of abuse. The preclinical data and preliminary clinical observations are presented.

Effects of cocaine alone and in combination with prazosin or ondansetron on multiple fixed-interval fixed-ratio performance in pigeons

Three pigeons were trained to respond on a two-component multiple schedule in which the components alternated regularly. In one component of the schedule, food was presented when the pigeon successfully completed a fixed-interval 120-s schedule within 150 s. In the other component of the schedule, food presentation occurred when the pigeon managed to complete a fixed-ratio 30 schedule within 30 s. Once responding had stabilized under both components of the schedule, pigeons were challenged with different doses of cocaine alone or cocaine in combination with 1.0 mg/kg prazosin (a selective alpha 1-adrenergic antagonist) or 0.10 or 0.50 mg/kg ondansetron (a selective 5-hydroxytryptamine3 antagonist). All drugs were injected intramuscularly 5 min before the start of selected experimental sessions. For two subjects, low doses of cocaine increased the low response rates maintained by the fixed-interval schedule while decreasing the high rates maintained by the fixed-ratio schedule. At intermediate doses, both high and low rates decreased but higher rates were more susceptible to disruption than low rates. The highest doses of cocaine completely eliminated responding in both schedule components. The high-rate behavior of the third subject was not affected by low or intermediate doses of cocaine, while low rates decreased at doses up to 5.6 mg/kg. The higher doses of cocaine eliminated responding in this subject as well. Prazosin and both doses of ondansetron antagonized the behavioral effects of cocaine at doses that ranged from 1.0-3.0 mg/kg. Redetermination of the dose-effect curve for cocaine at the conclusion of the experiment revealed that the curve had significantly shifted to the right.

Failure of ondansetron to block the discriminative or reinforcing stimulus effects of cocaine in the rat

Ondansetron (GR38032F), a serotonin 5HT3 antagonist, is active in numerous behavioral paradigms and neurochemical systems. Since 5HT3 antagonists have been suggested as therapeutic agents for the treatment of drug abuse, the action of ondansetron on cocaine drug discrimination and self-administration paradigms in rats was investigated. Doses of ondansetron (0.001 - 1.0 mg/kg) had no effect on the discriminative stimulus properties of 10 mg/kg cocaine. In contrast SCH23390, a dopamine D1 antagonist known to block cocaine discrimination, acted as previously reported. Ondansetron did not augment the effects of SCH23390, but at higher doses, combinations of ondansetron and SCH23390 produced disruption of lever pressing in the presence of cocaine. Ondansetron (0.001-1.0 mg/kg) had no effect on the self-administration of various doses of cocaine, nor did it have any effect on reacquisition of cocaine self-administration in animals with a history of active administration followed by a period of abstinence. As before, SCH23390, known to block cocaine self-administration, acted as previously reported. Although other 5HT antagonists may prove to be efficacious in cocaine abuse, ondansetron appears unlikely to alter the subjective or rewarding stimulus properties of cocaine.

Quantitative autoradiographic mapping of 5-HT3 receptors in the rat CNS using [125I]iodo-zacopride and [3H]zacopride as radioligands

Substitution of the chlorine atom by a radio-iodine in position 5 in the zacopride molecule yielded [125I]iodo-zacopride that bound with high affinity (Kd = 4.3 nM) to 5-HT3 receptors in the rat central nervous system. Assays with membranes from the posterior (mainly entorhinal) cortex confirmed that the pharmacological properties and regional distribution of [125I]iodo-zacopride-specific binding sites were identical with those of 5-HT3 sites labelled by the reference radioligand [3H]zacopride. Autoradiographic investigations for the visualization and quantification of 5-HT3 receptors yielded similar results with both radioligands, but autoradiograms could be obtained after only 1-3 days of exposure of sections labelled with [125I]iodo-zacopride, instead of 4-6 months using [3H]zacopride. The highest density of 5-HT3 sites was found in the nucleus tractus solitarius followed by, in decreasing order, the dorsal motor nucleus of the vagus nerve, the superficial layers of the dorsal horn in the spinal cord, the nucleus of the spinal tract of the trigeminal nerve, and the area postrema. Significant labelling of 5-HT3 receptors was also observed in limbic areas (amygdala, hippocampus, frontal and entorhinal cortex), and to a much lower extent in the dorsal raphe nucleus, striatum, and substantia nigra. These multiple locations further support the idea that 5-HT3 receptors are probably involved in several 5-HT-mediated functions in the central nervous system.

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.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

The role of the amygdala in fear-potentiated startle: implications for animal models of anxiety

Over the past several years, major advances have been made in understanding the pharmacology of anxiety, involving three broad classes of experimental approach. One approach studies the mechanism of action of drugs that are known to treat anxiety clinically, such as the benzodiazepines. A second approach uses various animal models of fear or anxiety that are sensitive to known anxiolytic drugs, to see if they will detect new compounds. A third approach involves describing the neural pathways and neurotransmitters that are active in a state of fear or anxiety; importantly, this approach is not derived from the mechanisms of known anxiolytics. In this review, Michael Davis describes such a 'neural systems' approach to the study of fear or anxiety that uses the paradigm of fear-potentiated startle.

Characteristics of 5-HT3 binding sites in NG108-15, NCB-20 neuroblastoma cells and rat cerebral cortex using [3H]-quipazine and [3H]-GR65630 binding

1. The biochemical and pharmacological properties of 5-HT3 receptors in homogenates of NG108-15 and NCB-20 neuroblastoma cells and rat cerebral cortex have been ascertained by the use of [3H]-quipazine and [3H]-GR65630 binding. 2. In NG108-15 and NCB-20 cell homogenates, [3H]-quipazine bound to a single class of high affinity (NG108-15: Kd = 6.2 +/- 1.1 nM, n = 4; NCB-20: Kd = 3.0 +/- 0.9 nM, n = 4; means +/- s.e.means) saturable (NG108-15: Bmax = 1340 +/- 220 fmol mg-1 protein; NCB-20: Bmax = 2300 +/- 200 fmol mg-1 protein) binding sites. In rat cortical homogenates, [3H]-quipazine bound to two populations of binding sites in the absence of the 5-hydroxytryptamine (5-HT) uptake inhibitor, paroxetine (Kd1 = 1.6 +/- 0.5 nM, Bmax1 = 75 +/- 14 fmol mg-1 protein; Kd2 = 500 +/- 300 nM, Bmax2 = 1840 +/- 1040 fmol mg-1 protein, n = 3), and to a single class of high affinity binding sites (Kd = 2.0 +/- 0.5 nM, n = 3; Bmax = 73 +/- 6 fmol mg-1 protein) in the presence of paroxetine. The high affinity (nanomolar) component probably represented 5-HT3 binding sites and the low affinity component represented 5-HT uptake sites. 3. [3H]-paroxetine bound with high affinity (Kd = 0.02 +/- 0.003 nM, n = 3) to a site in rat cortical homogenates in a saturable (Bmax = 323 +/- 45 fmol mg-1 protein, n = 3) and reversible manner. Binding to this site was potently inhibited by 5-HT uptake blockers such as paroxetine and fluoxetine (pKi s = 8.6-9.9), while 5-HT3 receptor ligands exhibited only low affinity (pK; < 7). No detectable specific [3H]-paroxetine binding was observed in NG108-15 or NCB-20 cell homogenates. 4. [3H]-quipazine binding to homogenates of NG108-15, NCB-20 cells and rat cortex (in the presence of 0.1 microM paroxetine) exhibited similar pharmacological characteristics. 5-HT3 receptor antagonists competed for [3H]-quipazine binding with high nanomolar affinities in the three preparations and the rank order of affinity was: (S)-zacopride > quarternized ICS 205-930 2 granisetron > ondansetron > ICS 205-209 (R)-zacopride > quipazine > renzapride > MDL-72222 > butanopride > metoclopramide. 5. [3H]-GR65630 labelled a site in NCB-20 cell homogenates with an affinity (Kd = 0.7 + 0.1 nms n = 4) and density (B__ = 1800 + 1000 fmol mg- protein) comparable to that observed with [3H]-quipazine. Competition studies also indicated a good correlation between the pharmacology of 5-HT3 binding sites when [3H]-GR65630 and [3H]-quipazine were used in these cells. 6. In conclusion, [3H]-quipazine labelled 5-HT3 receptor sites in homogenates of NG108-15 cells, NCB-20 cells and rat cerebral cortex. In rat cortical homogenates, [3H]-quipazine also bound to 5-HT uptake sites, which could be blocked by 0.1 microM paroxetine. The pharmacological specificity of the 5-HT3 receptor labelled by [3H]-quipazine was similar in the neuroblastoma cells and rat cortex and was substantiated in NCB-20 cells by the binding profile of the selective 5-HT3 receptor antagonist, [3H]-GR65630.

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

The effect of the selective 5-HT3 receptor antagonists, ondansetron and MDL 72,222, against various behaviours elicited by naloxone-precipitated morphine withdrawal were examined. Rats made dependent upon morphine by the subcutaneous implantation of a 75 mg pellet, when challenged with naloxone (0.5 mg/kg SC), 3 or 4 days later exhibited a wide range of behaviours including wet dog shakes, paw shakes, salivation and a marked weight loss. Pre-treatment with ondansetron (0.01-1 mg/kg SC) or MDL 72,222 (1-3 mg/kg SC) failed to affect the incidence of these responses except weight loss, which was attenuated by both treatments. At doses similar to and below those required to elicit the withdrawal syndrome, naloxone produced a single-trial place aversion in morphine dependent rats. The place aversion produced by naloxone (0.05 mg/kg SC) was antagonized by pre-treatment of ondansetron (0.1-1 mg/kg SC) and MDL 72,222 (1 mg/kg SC) prior to conditioning. Chlordiazepoxide (10 mg/kg IP) but not gepirone (3-10 mg/kg SC) was similarly effective. It is concluded that 5-HT3 antagonists may attenuate some but not all behavioural signs associated with morphine withdrawal. Reasons for this apparent selectivity are discussed.

Characterisation and autoradiographic localisation of 5-HT3 receptor recognition sites identified with [3H]-(S)-zacopride in the forebrain of the rat

The pharmacological characterisation and topographical distribution of [3H]-(S)-zacopride recognition sites in the forebrain of the rat was studied using homogenate and autoradiographic radioligand binding techniques. [3H]-(S)-Zacopride labelled a single, saturable, specific binding site (defined by 10.0 microM granisetron) in homogenates prepared from the entorhinal cortex of the rat (pKD = 9.51 +/- 0.08; Bmax = 104 +/- 7 fmol mg-1 protein; mean +/- SEM, n = 8). Pharmacological characterisation of the recognition site, within the entorhinal cortex, suggested that [3H]-(S)-zacopride selectively labelled the recognition site of the 5-HT3 receptor. Specific binding of [3H]-(S)-zacopride (defined by 1.0 microM granisetron) was differentially distributed throughout the forebrain of the rat; highest densities were located within sub-nuclei of the amygdala (cortical amygdaloid nucleus, amygdalohippocampal area, posterior medial cortical amygdaloid nucleus, posterior lateral amygdaloid nucleus), cortical areas (primary olfactory cortex, entorhinal cortex) and hippocampus. Non-specific binding was distributed homogeneously, although lower in myelinated structures. It is concluded that [3H]-(S)-zacopride selectively labels 5-HT3 receptor recognition sites within the forebrain of the rat; the topographical distribution of these sites, within the limbic nuclei, is consistent with the behavioural actions in animal models of the selective 5-HT3 receptor antagonists.

5-HT3 receptor antagonists attenuate cocaine-induced locomotion in mice

The 5-HT3 receptor antagonists ICS 205-930 and zacopride attenuated cocaine-induced locomotor activity in C57BL/6ByJ mice. In contrast, the aselective 5-HT1 and 5-HT2 receptor antagonist, methysergide did not affect the response to cocaine. The effect of the 5-HT3 receptor antagonists was not due to general sedation, because zacopride did not alter the locomotor response to caffeine.