The actions of nicotine and cocaine in a mouse model of anxiety

The failure of some rats to acquire intravenous cocaine self-administration is attributable to conditioned place aversion

Although cocaine administration in humans includes euphoric and anxiogenic effects, the latter are less well understood. Acute cocaine administration produces aversive effects including anxiogenic effects as well as appetitive effects in rats and mice. In the present study the self-administration and conditioned place preference paradigms were used to determine whether the failure of some rats to acquire intravenous cocaine self-administration is attributable to either an interference with learning or an aversion to cocaine. Rats were classified as self-administrators or non-self-administrators based on the mean number of cocaine self-infusions per session and whether or not rats exhibited either a stable high level of responding or a stable low level of responding. Intravenously administered cocaine produced place preference for the self-administrators, while intravenously administered cocaine produced place aversion for the non-self-administrators. The fact that the non-self-administrators showed place aversion is inconsistent with the interpretation that the failure of these rats to readily self-administer is attributable to cocaine-mediated interference of learning. This is the first study in which both the self-administration and the conditioned place preference paradigms have been used in the same animals to demonstrate that the effects of cocaine are appetitive for some rats and aversive for others, and are not an artifact of cocaine's interference with learning.

Chronic cocaine differentially affects diazepam's anxiolytic and anticonvulsant actions. Relationship to GABA(A) receptor subunit expression

Benzodiazepines are used to treat the anxiety associated with cocaine withdrawal, as well as cocaine-induced seizures. Since cocaine exposure was shown to affect BZ binding density, abuse liability, subjective hypnotic actions and seizure susceptibility, we assessed whether chronic cocaine alters diazepam's anxiolytic and anticonvulsant actions. Changes in GABA(A) receptor subunit protein expression were also assessed as they may relate to BZ activity at the receptor. Male Sprague-Dawley rats were injected with cocaine-HCl (15 mg/kg, i.p.) or saline once daily for 14 days. One day after the last injection, DZP (1 mg/kg i.p.) significantly increased time spent on and entries into open arms of an elevated plus maze in both saline- and cocaine-treated groups, yet the effect was greater in cocaine-treated rats. Eight days after cessation of treatment DZP did not have a significant anxiolytic effect in either group. To assess the effect of cocaine on DZP's anticonvulsant actions, PTZ was infused at a constant rate via the lateral tail vein and clonus onset was recorded in the presence and absence of DZP (5 mg/kg, i.p). DZP significantly elevated seizure threshold in both groups of rats. Chronic cocaine also had no effect on the beta-CCM seizure threshold. Quantitative immunohistochemistry of GABA(A) receptor subunit protein demonstrated significant regulation of alpha2 (-10%) and beta3 (+9%) subunits in the hippocampal dentate gyrus and CA1 regions, respectively. Small changes in GABAR subunit expression in specific brain areas may relate to DZP's enhanced anxiolytic effectiveness whereas it's anticonvulsant actions likely remain intact following cocaine administration.

Involvement of adrenergic and cholinergic systems in nicotine-induced anxiogenesis in mice

In the present study, the effect of alpha-adrenoceptor agents on response to nicotine in an anxiety model (elevated plus-maze) in mice was investigated. Administered nicotine reduced indices of anti-anxiety behaviour (percent open-arm time (%open-arm time) and percent open-arm entries (%open-arm entry)) and increased indices of anxiety behaviour (protected stretched attention posture and percent of protected head dipping (%protected dipping)), indicating that nicotine elicits an anxiogenic response. This response to the drug was obtained 7 min but not 30 min after drug injection and with doses of 0.25 and 0.5 mg/kg. Nicotinic receptor antagonists mecamylamine (0.5 and 1 mg/kg) and hexamethonium (5 and 10 mg/kg) reduced the response induced by nicotine (0.25 mg/kg). Mecamylamine (1 mg/kg; decreased %open-arm entry and increased protected stretched attention posture) and hexamethonium (10 mg/kg; decreased %open-arm time) showed an anxiogenic-like profile. A muscarinic receptor antagonist, atropine (2.5 and 5 mg/kg), did not alter the nicotine response but elicited an anxiogenic effect by itself. The alpha(1)-adrenoceptor antagonist prazosin (0.25 and 0.5 mg/kg), but not the alpha(1)-adrenoceptor agonist, phenylephrine (4 and 6 mg/kg), reversed the nicotine effect. Single administration of phenylephrine (6 mg/kg) increased %open-arm time, while prazosin did not alter the anxiety behaviour. The alpha(2)-adrenoceptor agonist clonidine (0.001 and 0.01 mg/kg), induced complete immobility when administered in combination with nicotine. However, an alpha(2)-adrenoceptor antagonist, yohimbine (0.5 and 1 mg/kg), appeared to reverse the nicotine response, but did not show interaction with nicotine's effect. Clonidine did not elicit any effect, but yohimbine (1 mg/kg) increased %open-arm entry and %open-arm time by itself. It can be concluded that certain doses of nicotine (0.25 and 0.5 mg/kg) 7 min after their injection induce an anxiogenic effect through nicotinic mechanism(s), and that involvement of alpha(1)- but not alpha(2)-adrenoceptors in the response to nicotine seems likely.

Phenotypic characterization of an alpha 4 neuronal nicotinic acetylcholine receptor subunit knock-out mouse

Neuronal nicotinic acetylcholine receptors (nAChR) are present in high abundance in the nervous system (Decker et al., 1995). There are a large number of subunits expressed in the brain that combine to form multimeric functional receptors. We have generated an alpha(4) nAChR subunit knock-out line and focus on defining the behavioral role of this receptor subunit. Homozygous mutant mice (Mt) are normal in size, fertility, and home-cage behavior. Spontaneous unconditioned motor behavior revealed an ethogram characterized by significant increases in several topographies of exploratory behavior in Mt relative to wild-type mice (Wt) over the course of habituation to a novel environment. Furthermore, the behavior of Mt in the elevated plus-maze assay was consistent with increased basal levels of anxiety. In response to nicotine, Wt exhibited early reductions in a number of behavioral topographies, under both unhabituated and habituated conditions; conversely, heightened levels of behavioral topographies in Mt were reduced by nicotine in the late phase of the unhabituated condition. Ligand autoradiography confirmed the lack of high-affinity binding to radiolabeled nicotine, cytisine, and epibatidine in the thalamus, cortex, and caudate putamen, although binding to a number of discrete nuclei remained. The study confirms the pivotal role played by the alpha(4) nAChR subunit in the modulation of a number of constituents of the normal mouse ethogram and in anxiety as assessed using the plus-maze. Furthermore, the response of Mt to nicotine administration suggests that persistent nicotine binding sites in the habenulo-interpeduncular system are sufficient to modulate motor activity in actively exploring mice.

The effects of nicotine on neural pathways implicated in depression: a factor in nicotine addiction?

The prevalence of tobacco smoking varies considerably between different groups within the community, tobacco smoking being particularly prevalent in patients with depressive disorder. This review will focus on results, derived from animal studies, which suggest that, in addition to its primary reinforcing properties, nicotine also exerts effects in stressful environments, which may account for its enhanced addictive potential in depressed patients. It focuses on the evidence that depression sensitises patients to the adverse effects of stressful stimuli, and that this can be relieved by drugs that stimulate dopamine release in the forebrain. This mechanism, it is proposed, contributes to the increased craving to smoke in abstinent smokers exposed to such stimuli, because they become conditioned to use this property of nicotine to produce rapid alleviation of the adverse effects of the stress. The review also explores the possibility that chronic exposure to nicotine elicits changes in 5-HT formation and release in the hippocampus which are depressogenic. It is postulated that smokers are protected from the consequences of these changes, while they continue to smoke, by the antidepressant properties of nicotine. However, they contribute to the symptoms of depression experienced by many smokers when they first quit the habit.

Selective c-fos induction and decreased dopamine release in the central nucleus of amygdala in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome

In the present study the neuronal expression of Fos, the protein product of c-fos, was used to study changes in neuronal activity in nerve terminal regions of the ascending dopaminergic system during nicotine withdrawal. Rats were infused for 14 days with nicotine (9 mg/kg/day nicotine hydrogen tartrate) via minipumps, whereas control animals carried empty pumps. Withdrawal was induced by the nicotinic receptor (nAChR) antagonist mecamylamine (1 mg/kg, s.c.). The behavior of each animal was observed after mecamylamine injection and subsequently its brain was processed for Fos-like immunoreactivity. Following mecamylamine, the score of abstinence signs increased in the nicotine-treated rats as compared to controls. The number of Fos-positive nuclei was substantially increased in the central nucleus of amygdala (CNA) in animals undergoing mecamylamine-precipitated withdrawal, whereas no significant changes in c-fos expression were observed in the basolateral amygdaloid nucleus, the core and the shell of the nucleus accumbens, the dorsolateral striatum, or the medial prefrontal cortex. Since there are indications of involvement of amygdaloid dopaminergic neurotransmission in anxiety-a core symptom of withdrawal from dependence-producing drugs-in a second experiment utilizing microdialysis we examined whether nicotine withdrawal affects dopaminergic neurotransmission in the CNA. Following mecamylamine injection, dopamine (DA) significantly decreased in nicotine-treated animals compared with controls. These results indicate that the mecamylamine-precipitated nicotine withdrawal reaction is accompanied by a selective induction of c-fos and a concurrent decrease in DA release in the CNA, which may have a bearing on symptoms such as anxiety and distress, which frequently are associated with the nicotine abstinence reaction in humans.

Evidence for opponent-process actions of intravenous cocaine

The present experiment was devised to test a prediction of the Opponent-Process Theory of drug action. This theory presumes that the initial affective experience of a subject treated with cocaine would be diametrically different immediately after administration compared to some point later in time when the positive impact of the drug had subsided. A conditioned place-preference procedure was employed in which a novel environment was paired with the effects of cocaine either immediately after, 5 min after, or 15 min after an intravenous injection of 0.75 mg/kg cocaine. It was hypothesized that animals would come to prefer environments associated with the immediate positive effects of cocaine and avoid environments associated with the drug's subsequent negative effects. The results confirmed this hypothesis. While the 0-min delay and 5-min delay groups exhibited conditioned preferences for the cocaine-paired environment, the 15-min delay group came to avoid the side of the preference apparatus paired with cocaine. These data, therefore, serve as additional support for an Opponent-Process account of cocaine's actions.

Cocaine potentiates defensive behaviors related to fear and anxiety

Cocaine use has been associated with a number of psychiatric disturbances, and an emerging literature attests to its ability to enhance anxiety-like behaviors in animal models. Ethoexperimental analyses of defensive behaviors, and tests designed specifically to provide individual measures of these behaviors, have been shown to respond very selectively and appropriately to anxiolytic and panicogenic or panicolytic drugs, suggesting that these tests, and this approach, might provide a more detailed and comprehensive description of the emotionality effects of cocaine than is currently available. In a Mouse Defense Test Battery (MDTB) using mouse subjects and an anesthetized rat as the threat stimulus, cocaine consistently enhanced flight and escape, with effects seen at 10-30 mg/kg (i.p.) dose levels. The effect was so potent that a lack of cocaine effect on other behaviors may have been due to response competition, or to early distancing of cocaine-dosed subjects from the threat stimulus. In a Rat Runway Test (RRT) similar to the MDTB but with rat subjects, 4 mg/kg cocaine, i.v. produced an explosive, but well directed, flight response. Flight was still elevated, although of lesser magnitude than originally, 30 min. after the i.v. cocaine, and defensive threat/attack to the oncoming threat stimulus were also reliably increased. Cocaine enhancement of defense was also seen in tests of sniffing "stereotypy" in rats. Sniffing after 30 mg/kg cocaine, i.p. was found to be appropriately oriented toward the direction of incoming air flow, suggesting that it may be part of a defensive risk assessment pattern. In undosed rats, risk assessment is suppressed by the presence of high-magnitude threat stimuli such as a cat, and the same, durable, phenomenon was obtained after 30 mg/kg (i.p.) cocaine. Toy cat exposure initially suppressed sniffing in cocaine-dosed rats, but this suppression was removed and sniffing increased, over repeated dose/toy cat exposures. Crouching in the same animals over these testing regimes supported a "sniffing-suppression" interpretation of these changes and also provided data suggesting that cocaine may enhance crouching. These data, indicating that cocaine enhances a number of defensive behaviors--some more strikingly than others--have implications for the involvement of cocaine in defense-linked psychopathologies; and for the involvement of defense in both conditioning and "sensitization" phenomena associated with cocaine. These effects raise the issue of the relationship between the defense-enhancing and the reinforcing consequences of cocaine.

Stimulation of nicotinic receptors in the lateral septal nucleus increases anxiety

The present study investigated the role of nicotinic receptors in the lateral septum in the modulation of anxiety. The effects of direct injections of nicotine into the lateral septum were first investigated in two tests of anxiety, social interaction and elevated plus-maze tests. Intra-septal injection of nicotine (1 and 4 microgram) induced consistent anxiogenic effects in both tests. The reversal of nicotinic effects with mecamylamine was then studied in the social interaction test. Intra-septal injection of mecamylamine at a low dose (15 ng) induced an anxiolytic effect, suggesting the presence of intrinsic cholinergic tone increasing anxiety. At higher doses (30-50 ng), mecamylamine was without effect in the social interaction test, but blocked the anxiogenic effects of nicotine (4 microgram). These findings provide further evidence for the role of the lateral septum in the modulation of anxiety and suggest that cholinergic projections to this brain area facilitate anxiety through nicotinic receptors.

Intravenous cocaine precipitates panic-like flight responses and lasting hyperdefensiveness in laboratory rats

There is an emerging body of clinical evidence that cocaine use in humans can result in serious fear or panic-related emotional disturbances. The present study evaluated the effects of intravenous cocaine administration upon defensive responses of rats to a threatening conspecific in a test situation, an oval runaway, permitting the display of the full range of the rat defensive repertoire. A battery of tests was employed to evaluate avoidance/escape, flight, freezing, defensive upright and defensive attack behaviors. In the first experiment male Long-Evans rats implanted with a chronic indwelling jugular catheter were placed in the runway and tested immediately after administration of either 0, 1, or 4 mg/kg of cocaine hydrochloride. The 4-mg/kg dose produced a dramatic flight response, the direction of which depended upon the direction of the approaching threat source. The same dose produced increased defensive upright postures during forced contact with the stimulus animal. Experiment 2 examined the time course for cocaine-induced hyperdefensiveness. Rats were administered either saline or 4 mg/kg cocaine intravenously and were tested following a delay of either 0, 5, 15, or 30 min following infusion. Cocaine-treated rats again displayed high levels of flight, which declined with increased time between infusion and testing. However, increased defensiveness persisted even at the 30 min delay for several defensive measures including avoidance, freezing, and defensive upright posture. Thus, following an initial period of rapid flight with intravenous cocaine administration, there was a lasting hyperdefensiveness in cocaine-treated rats. The present results suggest that cocaine may exert its panic-producing effects by acting upon neurobehavioral systems subserving defensive behavior, and that understanding of these systems is critical for understanding the neurobiology of panic disorder.

Preclinical research on cocaine self-administration: environmental determinants and their interaction with pharmacological treatment

It has been asserted that any comprehensive understanding of cocaine abuse and its treatment will require attention to both behavioral and pharmacological variables. Although the preclinical literature evaluating the effects of pharmacological variables on cocaine self-administration has been extensively reviewed, no comprehensive review of the effects of environmental variables on cocaine self-administration has been published. The present review summarizes and critiques the preclinical findings on environmental determinants of cocaine self-administration. The influence of environmental variables on the effects of pharmacological interventions on cocaine self-administration are also described. Several environmental variables have been shown to affect cocaine self-administration, including unit dose, schedule of cocaine delivery, schedules of nondrug stimuli, behavioral history, conditioned stimuli, food deprivation, exposure to stress, and rearing environment. Among these variables, unit dose, schedule of cocaine delivery, availability of alternative nondrug reinforcers, food deprivation, and rearing environment have also been shown to alter pharmacological treatment effects on cocaine self-administration. Thus, drug effects on cocaine self-administration are malleable and dependent upon the environmental context within which they occur. Suggestions for future research on the effects of these and other environmental variables on cocaine self-administration and its pharmacological treatment are presented.

Acute cocaine effects on stereotype and defense: an ethoexperimental approach

Cocaine administration to laboratory animals may produce locomotor hyperactivity and stereotypies that include sniffing and rearing, in addition to anxiety-like effects. A time-sampling study of the effects of 3, 10 or 30 mg/kg cocaine (i.p.) over time following injection indicated early enhancement of locomotion and crouching, with the latter most increased in low- and intermediate-dose cocaine groups, with increased rearing and standing during the second hour of the test period. Additional analyses at 30-60 min post-injection suggested qualitative changes in rearing, with high dose animals showing more, but shorter, rears, and a higher frequency of sniffing. The high dose cocaine enhancement of sniffing was strongly associated with rear and stand behaviors, but also occurred while the animal was crouching. This pattern of changes, with initial crouching/freezing and locomotion (flight?), followed by rearing, standing, and sniffing behaviors similar to those seen in risk assessment suggests that cocaine, particularly at high doses, may elicit defense. An additional study using only saline or the high (30 mg/kg) dose indicated that cocaine produced more sniffing regardless of the direction from which the air stream entered the test cage (i.e. top or bottom). However, cocaine animals oriented their sniffing behaviors toward the incoming air, with reliably more sniffs up in cages with the air stream entering from the top, and more sniffs down, when the air stream entered through a wire mesh cage bottom. Controls showed the same pattern, but their sniff orientation differences were not reliable. These results indicate that the sniffing that follows acute high dose cocaine administration is appropriately oriented toward relevant environmental stimuli, a factor disconsonant with the interpretation of sniffing as a stereotypical behavior, but one that is in agreement with the view that it may reflect a risk assessment component of the defense pattern.

Conditioned release of corticosterone by contextual stimuli associated with cocaine is mediated by corticotropin-releasing factor

Elevated blood concentrations of corticosterone (CORT), an adrenal steroid associated with stress responses, is one of the endocrine correlates of cocaine treatment. Experiment 1 confirmed and extended previous findings that chronic cocaine treatment does not alter corticosteroid responses to cocaine. In Experiment 2, conditioned endocrine effects of cocaine were examined in three groups of rats after 7 consecutive days of treatment. Cocaine-induced conditioning was achieved using a simple contextual design. In group 1 (paired), rats were injected with cocaine (30 mg/kg), then immediately placed into a locomotor activity chamber for 30 min. One hour after the rats were returned to their home cages, they received an injection of saline. In group 2 (unpaired), rats were injected with saline, then immediately placed into a locomotor activity chamber for 30 min. One hour after the rats were returned to their home cages, they received an injection of cocaine (30 mg/kg). Rats in group 3 (control) received only saline injections, but otherwise were treated as animals in the other treatment groups. On the test day (Day 8), all rats were placed immediately into the locomotor apparatus for 30 min prior to collection of a blood sample. Blood CORT concentrations and locomotor activity in the paired group were significantly higher than in the unpaired and control groups. However, pretreatment of the rats in Experiment 3 with the corticotropin-releasing factor (CRF) antagonist, alpha-helical CRF9-41 (1 microg, i.c.v.), on the test day, prior to exposure to cocaine-associated contextual cues, attenuated the subsequent conditioned increase in blood CORT concentrations. These data represent the first demonstration of classical conditioning of a steroid hormone response to stimuli associated with a psychoactive drug in rats and suggest that the effect is mediated by endogenous CRF. Because the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in modulating the actions of cocaine, it is plausible that such conditioned increases in CORT release by cocaine-associated cues may further predispose an organism to the reinforcing effects of the drug or enhance the susceptibility to drug-taking behavior. Alternatively, such conditioned effects may be related to the anxiogenic properties of cocaine. Further understanding of the conditioned effects of hormones in the development and expression of addictive behaviors may provide new insights into treatment of drug addiction.

Ethanol modulates cocaine-induced behavioral change in inbred mice

We recently conducted a study of the behavioral effects of combined cocaine and ethanol in genetically defined mice. Male and female C57BL/6 (B6) and DBA/2 (D2) were tested in an automated activity monitor on 2 consecutive days. On day 1, all animals received an IP injection of sterile saline and were placed into the activity monitor for 30 min. Behaviors measured were total distance traveled, stereotypy, nosepokes, and wall-seeking. On day 2, all animals were tested again for 15 min following injection of one of the following: saline, 10% v/v ethanol at 2.0 g kg(-1) or 2.0 g kg(-1) ethanol plus 5, 15, or 30 mg kg(-1) cocaine. Cocaine alone at the same doses was injected into separate groups of animals. For the B6 strain, the overall effect of ethanol was to reduce cocaine-induced locomotor stimulation; no consistent effect of ethanol on cocaine-induced locomotion was observed in D2 mice. Cocaine-induced inhibition of nosepokes in both strains and sexes was partially reversed by ethanol. Ethanol also partially reversed cocaine-elevated stereotypy in both strains and both sexes. In B6 mice, cocaine-increased wall seeking tended to be reversed by coadministration of ethanol, whereas no consistent pattern was observed in the D2s. Results from this study suggest that the several measures affected by cocaine (locomotor activity, stereotypy, exploration, thigmotaxis) were, in turn, differentially affected by concurrent treatment with ethanol. Furthermore, our results point to genetic-based differences in ethanol's effects on cocaine-related behaviors. We address the implications for combined ethanol and cocaine use in humans.

Differences in the liability to self-administer intravenous cocaine between C57BL/6 x SJL and BALB/cByJ mice

Application of animal models of psychostimulant abuse for experimentation in mice is becoming increasingly important for studying the contribution of genetic differences, as well as the roles of selected (targeted) genes, in specific behaviors. The purpose of this study was to investigate strain differences in cocaine self-administration behavior between C57BL/6 x SJL hybrid mice and BALB/cByJ mice. These two strains were chosen because BALB/cByJ mice have a well-developed behavioral pharmacological profile, and hybrid strains on a C57BL/6 background are commonly used for generating transgenic expressing and knockout mutant mice. C57BL/6 x SJL mice dose-dependently acquired cocaine self-administration (1.0 mg/kg/injection but not 0.25 mg/kg/injection) by responding selectively in the active nose-poke hole and maintaining stable levels of daily drug intake; they also exhibited a characteristic inverted-U-shaped cocaine dose-effect function. BALB/cByJ mice failed to acquire cocaine self-administration at either dose under the same test conditions. The strain differences observed in self-administration did not seem to be attributed to other behavioral differences because the two strains exhibited similar amounts of spontaneous nose-poking in the absence of reinforcers, and BALB/cByJ mice responded more than C57BL/6 x SJL mice in a food-reinforced nose-poke operant task. Importantly, the dose-effect function for the motor stimulating effects of cocaine (3.8-30 mg/kg intraperitoneally) suggests enhanced sensitivity but reduced efficacy of cocaine in stimulating motor activity in BALB/cByJ mice relative to the C57BL/6 x SJL hybrid mice. These results indicate that the decreased liability of BALB/cByJ mice to acquire cocaine self-administration is not the result of differences in spontaneous activity or performance, but may reflect different sensitivities to the reinforcing, or rate-disrupting, properties of cocaine. The data support an influence of genetic background in the liability to self-administer cocaine. Thus, a hypothesis is proposed that the decreased liability of BALB/cByJ mice to acquire cocaine self-administration is related to differences in brain monoamine systems linked to the high "emotionality" profile of BALB/c mice in novel or fearful situations, including perhaps cocaine administration.

A neuroendocrine role in cocaine reinforcement

Cocaine stimulates the secretion of corticosterone and ACTH, probably through a CRF-related mechanism, indicating that the drug activates the HPA axis. Indeed, cocaine has been reported to produce anxiety and to precipitate episodes of panic attack during chronic use and withdrawal in humans and to induce anxiogenic behavior in animals. Cocaine also alters benzodiazepine receptor binding in discrete regions of the rat brain. Some of these changes in binding are obviously related to the convulsions and seizures which are often observed in an acute cocaine overdose. However, data from behavioral studies have suggested that some of these effects may be related directly to cocaine reinforcement since receptor changes also were observed when binding in the brains of rats that self-administered cocaine was compared with that from animals that had received identical yoked, but non-contingent infusions of the drug. In this regard, pretreatment with the benzodiazepine receptor agonists chlordiazepoxide and alprazolam decreased cocaine self-administration without decreasing food-reinforced responding, suggesting that these effects were specific for cocaine. Since this attenuation of self-administration was reversed by increasing the unit dose of cocaine, it is likely that these drugs were decreasing cocaine reinforcement. In contrast, exposure to stress increases vulnerability to self-administer psychostimulants. In these experiments, low-dose cocaine self-administration was related directly to stress-induced increases in plasma corticosterone, such that plasma corticosterone was always greater than 150 ng/ml for rats which subsequently self-administered cocaine at doses of 0.125 mg/kg/infusion or lower, suggesting a threshold for the hormone in cocaine reinforcement. In other experiments, bilateral adrenalectomy completely abolished the acquisition of intravenous cocaine self-administration in naive rats, while metyrapone decreased ongoing self-administration. In addition, ketoconazole pretreatment resulted in patterns of self-administration that were virtually indistinguishable from that observed during saline extinction, suggesting that plasma corticosterone is not only important, but may even be necessary for cocaine reinforcement. The mechanisms through which adrenocorticosteroids alter cocaine reinforcement remain to be determined, but there is increasing evidence that the mesocorticolimbic dopaminergic system is involved. In particular, the medial prefrontal cortex appears to be at least one brain region where dopamine and adrenocorticosteroids may interact to affect cocaine reinforcement.

Withdrawal from oral cocaine in rate: ultrasonic vocalizations and tactile startle

While anxiety appears to characterize humans who administer high doses of cocaine or experience withdrawal from cocaine, it is difficult to capture this aspect of cocaine effects in animals. The present study investigated if acute or protracted withdrawal from prolonged low-dose cocaine that is self-administered via the oral route could be detected in tactile startle and vocal "distress" responses of rats. Adult, male Long-Evans rats had access to cocaine solution (0.1 mg/ml) either for 24 or 4 h/day using the two-bottle choice technique. The amount of solution consumed from each bottle was measured daily for 30 or 60 days. On days 1, 3, 7, 14, 21, 28 of withdrawal, startle and ultrasonic vocal responses (USV, 15-35 kHz) were measured in response to 18 air-puff stimuli (20 psi). Rats drank an average of 5-20 mg/kg per day of the cocaine solution. On average, about half of the daily liquid was consumed from the cocaine solution-containing bottle. USVs were emitted at significantly increased rates on day 3 of withdrawal from 30 or 60 days of cocaine drinking. Startle reactions were slightly, but non-significantly increased on day 1 of withdrawal. Comparable to withdrawal from ethanol, morphine, and diazepam treatments, withdrawal from oral self-administration of low to moderate doses of cocaine increases the rate of ultrasonic vocalizations while increasing minimally the amplitude of startle responses to low-intensity tactile stimuli. Nevertheless, no correlation between the total amount of cocaine self-administered or the duration of treatment with the intensity of the withdrawal manifestations could be detected.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

Behavioral responses to low doses of cocaine are affected by genetics and experimental history

We recently conducted a set of two experiments to investigate the possible co-operation between genetics and exposure to novelty on the putative locomotor inhibiting effects of low doses of cocaine in male and female C57BL/6 and DBA/2 mice. Experiment one examined the effects of three low doses of cocaine (0.1, 0.5, and 1.0 mg/kg) on locomotion, exploration, stereotypy and wall-seeking in an automated activity monitor. Testing occurred on two consecutive days, with subjects receiving an IP injection of saline on day one, and one dose of cocaine on day 2 (S-C). Immediately following injection, subjects were placed into automated activity monitors, where four behaviors were recorded; total distance, nosepokes, stereotypy and margin time. Using this S-C injection regimen, we found significant decreases in measures of total distance and stereotypy when compared to saline in both male and female C57 mice. Experiment two was designed to determine if the observed decrease in locomotor activity was the result of low-dose cocaine or pre-exposure to the test procedure and apparatus. All conditions and procedures were identical to those in experiment one, with the exception of the injection regimen. In this experiment, we injected all subjects IP with 0.1 mg/kg cocaine on day one, followed by saline on day two (C-S). Additionally, a group of subjects receiving saline on both days (S-S) served as the control. In contrast to experiment one results, cocaine produced locomotor activation. Furthermore, significant sex and strain differences were found in both experiments. The results of our experiments suggest that the behavioral effects of low doses of cocaine are markedly influenced by both the genetic constitution of the experimental animal and by familiarity with the test apparatus.

N-Methyl-beta-carboline-3-carboxamide (FG 7142): An anxiogenic agent in cigarette smoke condensate and its mechanism of formation

beta-Carboline-3-carboxylic acid methylamide (FG 7142), an anxiogenic agent has been found in cigarette smoke condensate, but not in the cigarette itself. When a cigarette, except its filter portion, was immersed in 20 ml of potassium phosphate buffer, pH 7.4, then heated at 60 degrees C for 2 days with or without presence of methylamine, FG 7142 was detected only in the mixture containing methylamine. Furthermore, when the mixtures of beta-carboline derivatives and various amounts of methylamine hydrochloride were heated at 60 degrees C for 5 days, FG 7142 was formed only in the mixtures containing methylamine and 1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-caroxylic acid (MTCA) or 1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid (TCCA). FG 7142 was also produced in the mixture of glucose, l-tryptophan and methylamine when heated at 200 degrees C in a dry condition. These observations suggest that FG 7142 is formed through the smoking process and that methylamine in cigarette smoke may play an important role in the formation of FG 7142.

Diversity of neuronal nicotinic acetylcholine receptors: lessons from behavior and implications for CNS therapeutics

Although the molecular biology of neuronal nicotinic acetylcholine receptors (nAChRs) provides evidence for multiple receptor subtypes, few selective pharmacological tools exist to identify these subtypes in vivo. However, the diversity of behavioral effects of available nAChR agonists and antagonists reviewed in this paper suggests that neuronal nAChR subtypes may play distinct roles in a variety of behavioral outcomes. Further characterization of the behavioral effects of the activation of discrete nAChR subtypes may eventually provide information useful in designing selective nAChR ligands targeting a variety of CNS disorders.

Benzodiazepine receptor mediation of the anxiolytic-like effect of (-)-nicotine in mice

The anxiolytic-like effect of (-)-nicotine (1.9 mumol/kg, IP) on the elevated plus-maze in CD1 mice was blocked by the benzodiazepine receptor antagonist flumazenil (1 and 10 mumol/kg, IP). On the other hand, the cholinergic nicotinic channel blocker mecamylamine (1 to 15 mumol/kg, IP), did not affect the anxiolytic-like properties of diazepam in the same test. These data suggest that the reduction in anxiety induced by (-)-nicotine occurs indirectly via the release of endogenous substances that can activate the benzodiazepine receptor.

Neural mechanisms underlying nicotine dependence

There is little doubt that many habitual smokers find it difficult to quit the habit because they have become addicted to the nicotine present in the smoke. This paper addresses some of the pharmacological mechanisms underlying this addiction and discusses how an understanding of these mechanisms may contribute to the more effective use of nicotine replacement therapy during smoking cessation. It considers critically the evidence that the "rewarding" properties of nicotine, which serve to reinforce drug-seeking behaviour, are related to stimulation of the mesolimbic dopamine system of the brain. The critique focuses specifically on the evidence that many central nicotinic receptors, including those which mediate the effects of the drug on dopamine secretion, are readily desensitized by chronic exposure to agonist and that hypotheses which assume that nicotine inhaled from tobacco smoke invariably results in stimulation of the receptors must be treated with caution. Nicotinic receptors in the brain are, however, heterogeneous in nature with different molecular structures and pharmacologies. It is concluded that the reinforcing properties of nicotine sought by smokers may reflect both stimulation and desensitization of the different nicotinic receptor populations, and that smokers may adjust their smoking habits to achieve the balance of receptor stimulation and desensitization which they find most reinforcing. It seems likely that the efficacy of the different nicotine formulations during the treatment of smoking cessation may also reflect their ability to stimulate or desensitize brain nicotinic receptors.

Actions of 5-hydroxytryptophan to inhibit and disinhibit mouse behaviour in the light/dark test

The involvement of 5-HT receptors in behavioural responding to an aversive situation was investigated in the mouse light/dark test. The administration of 5-hydroxytryptophan (5-HTP) (12.5-50 mg/kg i.p.) increased brain 5-HT turnover and inhibited mouse behaviour in the light/dark test box. The 5-HT2C/5-HT2A receptor antagonists methysergide (1.0 and 5.0 mg/kg i.p.) and ritanserin (0.1-1.0 mg/kg i.p.) antagonised (methysergide) or reversed (ritanserin) the effects of 5-HTP to an increased exploration of the light compartment; a low dose of the 5-HT3 receptor antagonist ondansetron (0.01 mg/kg i.p.) had a similar effect. The disinhibitory effect of the 5-HTP/ritanserin interaction was antagonised by the 5-HT3/5-HT4 receptor antagonists SDZ205-557 (0.001-0.1 mg/kg) and a high dose of tropisetron (1.0 mg/kg i.p.) but not by ondansetron (1.0 mg/kg i.p.). At these doses tropisetron and ondansetron had no effect in their own right. Thus the dominant effect of 5-HTP in the mouse is to inhibit behaviour, a response mediated via 5-HT2C/5-HT2A and 5-HT3 receptors. A 5-HT4 receptor may effect an opposing disinhibitory potential as revealed by ritanserin.

Behavioural consequences of cocaine withdrawal in rats

This study was designed to examine the impact of cocaine withdrawal on several behavioural parameters in rats. After 1 and 3 day withdrawal from continuous cocaine administration (50 mg kg-1 for 28 days, subcutaneous infusion via osmotic minipumps), rats showed significant changes in spontaneous locomotor activity, conditioned avoidance response and increased levels of anxiety. However, cocaine withdrawal did not alter the motor co-ordination, body weight, food and water consumption of these animals. The neurochemical effects of cocaine on central dopaminergic neuronal systems may account for locomotor deficit observed in these cocaine-withdrawal animals.

Chronic nicotine reverses age-associated increases in tail-flick latency and anxiety in rats

The chronic consumption of low doses of nicotine in drinking water for two years consistently increased the sensitivity of rats to a nociceptive thermal stimulus (tail-flick test), but reduced aversiveness in the elevated plus-maze test, relative to the responses of age-matched controls in these tests. The responses of aged nicotine-consuming rats were indistinguishable from those of young adult rats that did not receive nicotine. To determine whether these effects were due to a nicotine-induced retardation of age-related changes, young adult rats were similarly treated with nicotine for three months and similar changes in the tail-flick latency and performance in the plus-maze test were observed during nicotine consumption. These changes were reversed following withdrawal from nicotine. It is concluded that the maintenance of circulating low levels of nicotine (and/or its metabolites) increased the nociceptive sensitivity of the rats and reduced their aversions in the plus-maze test regardless of their age.

A genetic comparison of behavioral actions of ethanol and nicotine in the mirrored chamber

Human alcoholics are almost invariably heavy users of tobacco, perhaps because both ethanol and nicotine may have anxiolytic activity. However, studies in humans have not uniformly detected anxiolytic effects because significant individual differences in anxiolytic actions of these agents seem to exist. One factor that seems to contribute to these individual differences is tolerance to ethanol. Individuals who are more sensitive to depressant actions of alcohol seem to show anxiolytic actions more readily. Consequently, we examined the relative sensitivities of the ethanol-sensitive (to the anesthetic actions of ethanol) long-sleep (LS) and ethanol-resistant short-sleep (SS) mouse lines to diazepam, ethanol, nicotine, and ethanol-nicotine combinations in the mirrored chamber test. This test measures approach-conflict behavior. Ethanol and nicotine evoked changes in mirrored chamber activities that resembled those elicited by diazepam. These effects were seen at doses that did not markedly affect locomotor activity, thereby suggesting that these changes in behavior represent anxiolytic actions. The LS-SS mice did not differ in sensitivity to diazepam, but the SS were more uniformly responsive to the other drugs. Only the SS showed clear evidence for interactions between ethanol and nicotine. If the changes in mirrored chamber behavior elicited by ethanol, nicotine, and combinations of the two drugs occur because of anxiety reduction, it seems that the SS mouse line is more responsive to anxiolytic actions of these drugs.

Nicotinic receptor agonists exhibit anxiolytic-like effects on the elevated plus-maze test

The effects of nicotinic receptor agonists on the elevated plus-maze test of anxiety were investigated in CD1 mice after intraperitoneal injections. Nicotine and lobeline, but not cytisine, exhibited a significant increase in the time spent by the mice in the open arms, a measure of anxiolytic activity. Nicotine also increased the total number of arm entries, a measure of general activity, but this effect was secondary to its anxiolytic-like properties. Nicotinic receptor antagonists on their own did not modify the behavior of mice in the maze. The effect of nicotine was mediated by central nicotinic receptors as it was blocked by the centrally-acting nicotinic antagonists mecamylamine and chlorisondamine, but not by hexamethonium (a peripherally acting blocker). Cotinine, the major metabolite of nicotine, was evaluated at different times after systemic injections and had no effect in the plus-maze. The anxiolytic-like profile induced by nicotinic receptor stimulation was not associated with potentiation of alcohol effects, a liability associated with the benzodiazepine therapy. This study demonstrates the anxiolytic-like properties of nicotine and lobeline in mice, and suggests that central nicotinic receptors are involved in the expression of emotional behavior.

The effect of the 5-HT3 receptor antagonist, RS-42358-197, in animal models of anxiety

The S-isomer of the novel 5-HT3 receptor antagonist RS-42358 ((S)-N-(1-azabicyclo[2.2.2]oct-3-yl)-2,4,5,6-tetrahydro-1-H- benzo[de]isoquinolin-1-one, RS-42358-197) disinhibited behaviour in the mouse suppressed by the aversive situation of the light/dark test box. RS-42358-197 was effective at sub-ng/kg dose levels and the efficacy was maintained over a 100 million-fold dose range. In contrast, the R-isomer was ineffective at all doses studied. The S-isomer also disinhibited a suppressed behaviour in social interaction and elevated X-maze tests in the rat and reduced anxiety-related behaviours in a marmoset human threat test. RS-42358-197 prevented the exacerbation of the suppression of behaviour in the mouse light/dark test following withdrawal from treatment with alcohol, nicotine, cocaine and diazepam. Thus, the S-isomer of RS-42358 has a consistent non-sedating anxiolytic profile in rodent and primate models. It is exceptionally potent and a maintained efficacy at high doses distinguishes its actions from many other 5-HT3 receptor antagonists.

Effects of alprazolam on intravenous cocaine self-administration in rats

The specificity of benzodiazepine pretreatment on the reinforcing efficacy of cocaine was investigated using a multiple schedule of cocaine and food presentation. Cocaine was available under a fixed-ratio 4 schedule of reinforcement during 1 h of the session, while food was delivered under a discrete-trial, fixed-ratio 10 schedule during the other. Following initial exposure to alprazolam, responding maintained by both cocaine and food was significantly reduced. However, tolerance quickly developed to the sedative effects of alprazolam on food-maintained responding, while no reduction in the effects of the drug on cocaine self-administration was observed. Alprazolam (0.5 to 4.0 mg/kg, IP) significantly reduced cocaine intake without affecting food-maintained responding following subsequent testing with the drug. These data suggest a potential specific effect (e.g., anxiolytic) of alprazolam in cocaine reinforcement.

Nicotine withdrawal: a behavioral assessment using schedule controlled responding, locomotor activity, and sensorimotor reactivity

Three different behavioral measures were used to assess the effects of abrupt cessation of chronic nicotine treatment. Nicotine (0, 3, or 6 mg/kg per day) was continuously administered for 12 days in rats by surgically implanting Alzet osmotic mini-pumps subcutaneously. Experiment 1 employed a light/dark discrimination task. There were no significant effects on number of responses or percent correct responding either during nicotine administration, or following cessation of nicotine. Experiment 2 examined ambulatory (locomotor) and nonambulatory activity. Chronic nicotine administration produced significant dose-dependent increases in both ambulatory and nonambulatory activity during the first 3 days of exposure. However, no significant alterations were seen in activity levels following nicotine cessation. Experiment 3 examined sensorimotor reactivity using the auditory startle response. During nicotine withdrawal, significant increases were seen in startle amplitude in both nicotine groups for 4 days. Nicotine (0.4 mg/kg, IP) administered before startle testing during the withdrawal phase attenuated the increased reactivity seen during nicotine cessation. These studies indicate that 1) rats display increased sensorimotor reactivity after cessation of chronic nicotine exposure, and 2) the expression of nicotine dependence and withdrawal is dependent on the behavioral task employed.

Anxiogenic effects of acute and chronic cocaine administration: neurochemical and behavioral studies

The effects of cocaine on defensive withdrawal behavior in rats and elevated plus-maze behavior in mice were investigated. Cocaine (20 mg/kg IP) injected daily for 7 or 14 days induced defensive withdrawal; that is, the latency to emerge from a small chamber in an open field and the mean time in the chamber were both significantly increased. Acute cocaine administration also induced defensive withdrawal, and this effect was prevented by prior treatment with chlordiazepoxide (5 mg/kg IP). Both acute and chronic cocaine treatments significantly increased plasma concentrations of corticosterone and reduced the ratios of 3,4-dihydroxyphenylacetic acid to dopamine and 5-hydroxyindoleacetic acid to serotonin in several brain regions. Further evidence for an acute anxiogenic effect of cocaine was obtained from mice studied in the elevated plus-maze. Acute cocaine administration decreased both the number of entries into and the time spent in the open arms of the maze. These results taken together strongly support an anxiogenic action of acute and chronic cocaine administration.

Effects of ritanserin and chlordiazepoxide on sleep-wakefulness alterations in rats following chronic cocaine treatment

The effects of ritanserin, a 5-hydroxytryptamine-2 (5-HT2) receptor antagonist, and chlordiazepoxide, a benzodiazepine agonist, on sleep-wakefulness disturbances in rats after acute administration of cocaine and after discontinuation of chronic cocaine treatment were examined. Intraperitoneal (IP) injection of chlordiazepoxide (10 mg/kg) but not ritanserin (0.63 mg/kg) prevented the increase of wakefulness (W) and the reduction of light slow wave sleep (SWS1) and deep slow wave sleep (SWS2) induced by an acute injection of cocaine (20 mg/kg IP). Daily injection of cocaine (20 mg/kg for 5 days, then 30 mg/kg for 5 days IP) at the onset of the light phase elicited an increase of W and a concomitant decrease of SWS1, SWS2 and paradoxical sleep (PS) in the light phase, followed by a rebound in SWS2 and PS in the subsequent dark phase. Following cocaine discontinuation, the circadian distribution of sleep-wakefulness states remained disturbed in saline-treated rats for at least 5 days. Both ritanserin (0.63 mg/kg IP/day) and chlordiazepoxide (10 mg/kg IP/day) reduced the alteration in the distribution of W and SWS2 throughout the light-dark cycle from the first day of administration on, but failed to prevent PS alterations. The mechanisms by which both compounds exert their effect are probably quite different. For chlordiazepoxide sedative and sleep-inducing properties probably play a major role. In contrast, for ritanserin SWS2-increasing properties and its ability to reverse preference for drugs of abuse without inducing aversion might be key factors.

Cocaine actions, brain levels and receptors in selected lines of mice

The effects of cocaine (15 mg/kg IP) versus IP saline on open-field behaviors were evaluated using a crossover design in long-sleep (LS) and short-sleep (SS) mice. Under treatment order 1, mice received saline injection on day 1 followed 24 h later by cocaine (saline-cocaine, S-C). Under treatment order 2, animals received cocaine on day 1 and saline on day 2 (cocaine-saline, C-S). Immediately following injection, animals were placed into an automated open-field apparatus with behavioral samples taken at 5-min intervals for 30 min. The behaviors measured were distance traveled, stereotypy and time spent in proximity to the margins of the test apparatus (thigmotaxis). Cocaine increased locomotor activity in both lines of mice, with S-C producing more pronounced initial activation than C-S in LS mice. Compared to S-C, C-S also increased thigmotaxis, an effect more pronounced in SS mice. In a separate experiment, brain cocaine levels were measured in brains of adapted and nonadapted LS and SS mice 5 min following injection of 15 mg/kg cocaine. Regardless of order, SS mice had significantly higher brain cocaine levels than did LS mice. Mazindol and cocaine binding studies in the forebrain indicated higher Bmax values for both ligands in LS compared to SS mice. The results of this study indicate that genetically based differences in cocaine receptors as well as treatment order contribute to behavioral actions of cocaine.

The influence of stress on psychopharmacological responses to nicotine

This essay considers the mechanisms which may mediate the apparent anxiolytic properties of nicotine and which are thought to be responsible for the 'calming' effect of tobacco smoke experienced by many smokers. It summarizes the evidence that, in many tests for anxiolytic activity, the effects of nicotine do not resemble those of established anxiolytic drugs such as diazepam and concludes that it is likely that neural systems other than those which mediate the responses to the benzodiazepines are responsible for the putative anxiolytic properties of nicotine. Circumstantial evidence which suggests that the increase in mesolimbic dopamine secretion evoked by nicotine may not only be rewarding per se but may also contribute to the ability of the drug to alleviate the effects of stress is presented. The essay also summarizes results which suggest that chronic nicotine evokes changes in the mesolimbic dopamine system which resemble those seen in animals treated chronically with antidepressant drugs and proposes that the mechanisms which mediate the ability of antidepressant drugs to alleviate the effects of stress may also mediate the apparent anxiolytic properties of nicotine. The possible consequences of this hypothesis for future research are considered.

Animal model for investigating the anxiogenic effects of self-administered cocaine

Male albino rats were trained to traverse a straight alley for a reward of five intravenous injections of cocaine (0.75 mg/kg/injection in a volume of 0.1 ml/injection delivered over 4 s). Animals were tested one trial per day with the following dependent measures assessed on each trial: start latency, running time, the number of retreats, and the location within the alley where each retreat occurred. While start latencies remained short and stable, running times tended to increase over days. This effect was apparently related to a concomitant increase in the number of retreats occurring in the alley (r = 0.896). Retreats tended to occur in very close proximity to the goal box, suggesting that animals working for IV cocaine come to exhibit a form of conflict behavior (i.e., retreats) putatively stemming from the drug's well documented rewarding and anxiogenic properties. Consistent with this hypothesis was the demonstration that diazepam (0.5, 1.0, 2.0 mg/kg IP) pretreatment dose-dependently reduced the incidence of retreat behaviors in the alley. In addition, the rewarding efficacy of the cocaine dosing parameters was subsequently confirmed in the runway subjects by conditioned place preference. The present paradigm, therefore, provides a useful method for investigating the anxiogenic effects of self-administered cocaine in laboratory animals.

Ketotifen and its analogues reduce aversive responding in the rodent

The abilities of ketotifen and other 4-piperidylidene derivatives (HF200-184, HE36-953, SDZ209-321 and SDZ206-703) to inhibit aversive responding were compared in the mouse light/dark test box and in the rat social interaction test. Ketotifen and HF200-184 reduced aversive responding of the mouse to the brightly illuminated area of the test box and facilitated rat social interaction; HF200-184 was approximately 100 times more potent than ketotifen. The chronic administration and withdrawal from treatment with diazepam, ethanol, nicotine and cocaine in the mouse was associated with increased behavioural suppression which was prevented by the administration of ketotifen and HF200-184 during the period of withdrawal. HE36-953 also prevented the behavioural consequences of withdrawal from diazepam and cocaine. The relative potencies of ketotifen and its analogues to inhibit aversive responding did not correlate with their affinities for the 5-HT3 recognition site. It is concluded that compounds within the 4-piperidylidene series can reduce behavioural suppression in rodent models of anxiety and attenuate the behavioural consequences of withdrawing from treatment with drugs of abuse.

Ondansetron inhibits a behavioural consequence of withdrawing from drugs of abuse

The ability of the selective 5-HT3 receptor antagonist ondansetron to influence the behavioural consequences of withdrawal from chronic treatment with ethanol, nicotine or cocaine was investigated in the light/dark exploration test in the mouse and social interaction test in the rat. In both tests acute and chronic (7 days) treatments with ondansetron (0.01-1.0 microgram.kg-1 IP) disinhibited suppressed behaviour; withdrawal from chronic treatment (0.1 mg/kg IP b.i.d.) did not exacerbate the behavioural suppression. Chronic treatment for 14 days with ethanol (8% w/v in the drinking water), nicotine (0.1 mg/kg b.i.d.) or cocaine (1.0 mg/kg b.i.d.) released suppressed behaviour in the mouse and rat tests. Behavioural suppression was increased following withdrawal from ethanol, nicotine and cocaine. The administration of ondansetron (0.01 mg/kg IP b.i.d.) during the period of ethanol, nicotine and cocaine withdrawal prevented the exacerbation in suppressed behaviour. It is concluded that ondansetron potently reduces behavioural suppression during acute and chronic treatments in the rodent models, does not cause a rebound exacerbation of behavioural suppression following withdrawal, and is a highly effective inhibitor of the increased behavioural suppression following withdrawal from the drugs of abuse: ethanol, nicotine and cocaine.

The effect of ketotifen in rodent models of anxiety and on the behavioural consequences of withdrawing from treatment with drugs of abuse

Ketotifen was compared to diazepam to inhibit aversive responding of the mouse in a black and white test box and in the rat social interaction test. Both drugs reduced aversive responding in the mouse to the brightly illuminated area of the test box and facilitated social interaction in the rat; ketotifen was approximately 100 times more potent than diazepam. The chronic administration of diazepam, ethanol, nicotine and cocaine in the mouse also reduced aversive responding but their withdrawal was associated with an increased behavioural suppression. The administration of ketotifen during the period of withdrawal from diazepam, ethanol, nicotine and cocaine prevented the exacerbation in aversive responding. It is concluded that ketotifen, like diazepam and 5-HT3 receptor antagonists, can reduce behavioural suppression in rodent models of anxiety and attenuate the behavioural consequences of withdrawal from treatment with drugs of abuse.

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

The cerebral site of action of the selective 5-HT3 receptor antagonist ondansetron to influence the behavioural consequences of withdrawal from subchronic treatment with diazepam, ethanol, nicotine or cocaine was studied in the light/dark exploration test in the mouse. The aversive response to the light compartment of the test box was reduced during a subchronic treatment with peripherally administered diazepam, ethanol, nicotine and cocaine, but was exacerbated following withdrawal from the 4 treatments. The behavioural consequences of withdrawal from diazepam (10 mg/kg IP b.i.d. 14 days), ethanol (8%/w/v drinking water for 14 days), nicotine (0.1 mg/kg IP b.i.d. 14 days) or cocaine (1.0 mg/kg IP b.i.d. 14 days) were antagonised by ondansetron injected into the amygdala and dorsal raphe nucleus (1-10 ng); injections of ondansetron (10 ng) into the median raphe nucleus, the nucleus accumbens and striatum were ineffective. It is concluded that the amygdala and dorsal raphe nucleus may be sites of action for ondansetron to antagonise the aversive behaviour caused by withdrawal from 4 common drugs of abuse in a mouse model, and that 5-HT projections from the dorsal raphe nucleus may be involved in aversive behaviour.

Effects of captopril and SQ29,852 on anxiety-related behaviours in rodent and marmoset

The abilities of the ACE inhibitors captopril and SQ29,852 to modify aversive behaviour was compared to the effects of diazepam in the light/dark exploration test in the mouse, the elevated plus maze and social interaction test in the rat, and in anxiety-related behaviours induced by human threat in the marmoset. In the four tests the acute administration of captopril, SQ29,852 and diazepam had the same profiles of action to reduce aversive responding. This was also observed during chronic administration with the three agents in the mouse. However, withdrawal from a chronic treatment with diazepam precipitated a syndrome of increased aversion, whereas withdrawal from treatment with captopril and SQ29,852 was uneventful, values waning to control levels. Withdrawal from treatment with ethanol, nicotine and cocaine also enhanced aversive responding. Treatment with captopril and SQ29,852 antagonised the behavioural consequences of withdrawal from treatment with diazepam and nicotine and SQ29,852 also blocked the consequences of withdrawal from ethanol and cocaine. It is concluded that captopril and SQ29,852 have an anxiolytic profile of action in 3 species, that cessation of treatment is not associated with a withdrawal syndrome, that the ACE inhibitors cross tolerate with diazepam and can antagonise the behavioural consequences of withdrawal from treatment with drugs of abuse.

Smoking-associated changes in the serotonergic systems of discrete regions of human brain

This paper describes the results of a postmortem study of the effects of tobacco smoking on the concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) as well as the binding of [3H]-8-hydroxy-(di-n-propylamino)-tetralin ([3H]-8-OH-DPAT) and [3H]-ketanserin in six discrete regions of human brain. Smoking was associated with significant decreases in the concentrations of 5-HIAA in the hippocampal neocortex (P less than 0.001), hippocampal formation (P less than 0.05) and the median raphe nuclei (P less than 0.05). The 5-HT level of the hippocampal formation was also significantly reduced in smokers (P less than 0.05). These changes were accompanied by significant increases in the binding of [3H]-8-OH-DPAT in the hippocampal neocortex (P less than 0.01) and hippocampal formation (P less than 0.05). [3H]-Ketanserin binding in the brain regions studied was unaffected by smoking. It is concluded that smoking is associated with a regionally selective decrease in the activity of the serotonergic system of the human hippocampus.

The psychopharmacology of 5-HT3 receptors

The review presents evidence that 5-HT3 receptors within the brain may contribute to the control of behavior. 5-HT3 receptor antagonists GR38032F, zacopride, ICS 205-930 and other agents are very potent in reducing mesolimbic dopamine hyperactivity caused by the injection of amphetamine or infusion of dopamine into the rat nucleus accumbens and amygdala, and the ventral striatum of the marmoset. Such actions are distinguished from those of neuroleptic agents by a failure to reduce normal levels of activity or to induce a rebound hyperactivity after discontinuation of treatment. Indeed, the 5-HT3 receptor antagonists can prevent the neuroleptic-induced rebound hyperactivity. Further evidence that 5-HT3 receptors moderate limbic dopamine function is shown by their ability to reduce both the behavioral hyperactivity and changes in limbic dopamine metabolism caused by DiMe-C7 injection into the ventral tegmental area. The 5-HT3 receptor antagonists also have an anxiolytic profile in the social interaction test in the rat, the light/dark exploration test in the mouse, the marmoset human threat test and behavioral observations in the cynomolgus monkey. They differ from the benzodiazepines by an absence of effect in the rat water lick conflict test and a withdrawal syndrome. Importantly, the 5-HT3 receptor antagonists are highly effective to prevent the behavioral syndrome following withdrawal from treatment with diazepam, nicotine, cocaine and alcohol. Intracerebral injection techniques in the mouse indicate that the dorsal raphe nucleus and amygdala may be important sites of 5-HT3 receptor antagonist action to inhibit aversive behavior. Studies with GR38032F indicate an additional effect in reducing alcohol consumption in the marmoset. The identification and distribution of 5-HT3 receptors in the brain using a number of 5-HT3 receptor ligands, [3H]65630, [3H]zacopride and [3H]ICS 205-930 correlates between studies, and the 5-HT3 recognition sites in cortical, limbic and other areas meet the criteria for 5-HT3 receptors to mediate the above behavioral effects. Thus the use of 5-HT3 receptor antagonists reveals an important role for 5-hydroxytryptamine in the control of disturbed behavior in the absence of effect on normal behavior. The profile of action of the 5-HT3 receptor antagonists has generated a major clinical interest in their potential use for schizophrenia, anxiety and in the control of drug abuse.