A pentylenetetrazol-like stimulus during cocaine withdrawal: Blockade by diazepam but not haloperidol

Diazepam Concurrently Increases the Frequency and Decreases the Amplitude of Transient Dopamine Release Events in the Nucleus Accumbens

Benzodiazepines are commonly prescribed anxiolytics that pose abuse liability in susceptible individuals. Although it is well established that all drugs of abuse increase brain dopamine levels, and benzodiazepines are allosteric modulators of the GABA_A receptor, it remains unclear how they alter dopamine release. Using in vivo fast-scan cyclic voltammetry, we measured diazepam-induced changes in the frequency and amplitude of transient dopamine release events. We found that diazepam concurrently increases the frequency and decreases the amplitude of transient dopamine release events in the awake and freely moving rat. The time course during which diazepam altered the frequency and amplitude of dopamine release events diverged, with the decreased amplitude effect being shorter lived than the increase in frequency, but both showing similar rates of onset. We conclude that diazepam increases the frequency of accumbal dopamine release events by disinhibiting dopamine neurons, but also decreases their amplitude. We speculate that the modest abuse liability of benzodiazepines is due to their ability to decrease the amplitude of dopamine release events in addition to increasing their frequency.

Current major depression is associated with greater sensitivity to the motivational effect of both negative mood induction and abstinence on tobacco-seeking behavior

BACKGROUND

Although depression and smoking commonly co-occur, the mechanisms underpinning this association are poorly understood. One hypothesis is that depression promotes tobacco dependence, persistence and relapse by increasing sensitivity to acute negative mood and abstinence induced tobacco-seeking behavior.

METHODS

Twenty nine daily smokers of >10 cigarettes per day, nine with major depression and 20 without, completed two laboratory sessions one week apart, smoking as normal prior to session 1 (sated session), and 6h abstinent prior to session 2 (abstinent session). In both sessions, tobacco-seeking was measured at baseline by preference to view smoking versus food images. Negative mood was then induced by negative ruminative statements and sad music, before tobacco-seeking was measured again at test.

RESULTS

In the sated session, negative mood induction produced a greater increase in tobacco choice from baseline to test in depressed (p<0.001, η_p²=0.782) compared to non-depressed smokers (p=0.045, η_p²=0.216, interaction: p=0.046, η_p²=0.150). Abstinence also produced a greater increase in baseline tobacco choice between the sated and abstinent sessions in depressed (p=0.002, η_p²=0.771) compared to non-depressed smokers (p=0.22, η_p²=0.089, interaction: p=0.023, η_p²=0.189). These mood and abstinence induced increases in tobacco choice were positively associated with depression symptoms across the sample as a whole (ps≤0.04, η_p²≥0.159), and correlated with each other (r=0.67, p<0.001).

CONCLUSIONS

Current major depression or depression symptoms may promote tobacco dependence, persistence and relapse by increasing sensitivity to both acute negative mood and abstinence induced tobacco-seeking behavior. Treatments should seek to break the association between adverse states and smoking to cope.

Withdrawal from chronic cocaine administration induces deficits in brain reward function in C57BL/6J mice

Anhedonia is a major symptom of cocaine withdrawal, whereas euphoria characterizes the effects of acute administration of this drug in humans. These mood states can be measured quantitatively in animals with brain reward thresholds obtained from the intracranial self-stimulation (ICSS) procedure. Studies have previously reported the reward-enhancing effects of acute cocaine administration using the ICSS procedure in mice, but the effects of chronic cocaine administration and withdrawal on brain reward thresholds have not been widely investigated in this species. Cocaine withdrawal was induced in C57BL/6J mice by removal of intraperitoneal osmotic minipumps that delivered cocaine (90 or 180 mg/kg/day, salt) for 72 h. Mice were tested in the ICSS procedure 3-100 h post-pump removal. Anxiety-like behavior was assessed in the light-dark box 24h post-pump removal. After an 18-day washout period, tolerance and sensitization to the reward-enhancing effects of cocaine were assessed by injecting bolus cocaine intraperitoneally (0, 2.5, 5, and 10 mg/kg). The results indicated that 72 h administration of 90 and 180 mg/kg/day cocaine significantly lowered brain reward thresholds. Withdrawal from 90 and 180 mg/kg/day of cocaine administration elevated ICSS thresholds to similar extents. No anxiety-like behavior was observed in the light-dark box during withdrawal from chronic cocaine administration, although the number of transitions between compartments and locomotion in the dark compartment markedly decreased. Chronic cocaine administration did not induce tolerance or sensitization to the reward-enhancing effects of acute cocaine. In conclusion, alterations in mood states induced by cocaine administration and withdrawal in mice can be measured using the ICSS procedure.

Addiction motivation reformulated: an affective processing model of negative reinforcement

This article offers a reformulation of the negative reinforcement model of drug addiction and proposes that the escape and avoidance of negative affect is the prepotent motive for addictive drug use. The authors posit that negative affect is the motivational core of the withdrawal syndrome and argue that, through repeated cycles of drug use and withdrawal, addicted organisms learn to detect interoceptive cues of negative affect preconsciously. Thus, the motivational basis of much drug use is opaque and tends not to reflect cognitive control. When either stressors or abstinence causes negative affect to grow and enter consciousness, increasing negative affect biases information processing in ways that promote renewed drug administration. After explicating their model, the authors address previous critiques of negative reinforcement models in light of their reformulation and review predictions generated by their model.

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

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

Iboga compounds reverse the behavioural disinhibiting and corticosterone effects of acute methamphetamine: Implications for their antiaddictive properties

This study investigated the effects of pretreatment with the putative antiaddictive compound, ibogaine (IBO), and its synthetic derivative, 18-methoxycoronaridine (18-MC), on the changes in behaviour in an elevated plus maze and the changes in corticosterone (CORT) produced by a low dose of methamphetamine (METH). In the elevated plus maze, the acute administration of METH (0.1 mg/kg ip, -20 min) produced an increase in both the number and the duration of open arm entries relative to saline (SAL)-treated controls. No effect of METH administration was observed on the total number of arm entries. These data indicated that METH alone produced either anxiolysis or behavioural disinhibition in this paradigm. More consistent with the latter possibility, the open arm behaviour of METH controls was associated with an increase in plasma levels of CORT, supporting a facilitatory role for CORT in this METH-induced effect. Pretreatment with both IBO and 18-MC (40 mg/kg ip, 19 h earlier) antagonized the behavioural disinhibiting effects of acute METH without altering locomotor activity. In addition, both iboga agents antagonized the increase in CORT produced by METH. These data provide insight into yet another potential mechanism through which iboga compounds may exert their antiaddictive effects, a reversal of the behavioural disinhibiting properties of stimulant drugs. Furthermore, these data indicate that this reversal is related to effects of iboga compounds on the stimulation of neuroendocrine systems by stimulant drugs.

A discriminative stimulus produced by 1-(3-chlorophenyl)-piperazine (mCPP) as a putative animal model of anxiety

1. This study compares behavioral responses to serotonergic (5HT) agonists and pentylenetetrazol (PTZ) in two behavioral paradigms used as animal models of anxiety. PTZ and mCPP were compared for behavioral effects in elevated plus-maze and interoceptive discriminative stimuli they produce. 2. PTZ is a known anxiogenic drug. The discriminative stimuli of mCPP were selected for comparison because this drug produces "anxiety" in human subjects and "anxiety-like" behaviors in rats, and is a potent agonist at 5HT1B/2C receptors and a partial agonist at 5HT2A receptors. 3. In rats trained to discriminate mCPP (1.4 mg/kg, training dose) from saline, PTZ substituted for the mCPP suggesting the "anxiety-like" properties of the mCPP stimulus. The mCPP stimulus was blocked in a dose-related manner by methysergide, a 5HT2A/2C antagonist but not by the anxiolytic diazepam. TFMPP (a 5HT agonist) and DOI (a 5HT2A/2C agonist) substituted for mCPP, but 1-NP (a 5HT1 agonist and 5HT2C/2A antagonist) did not. 4. In animals trained to discriminate PTZ (16 mg/kg) from saline, mCPP and DOI substituted for PTZ, while TFMPP and 1-NP do not. 5. In the elevated plus maze, time spent on the open arms was reduced by mCPP, DOI and PTZ but there was no significant dose effect of TFMPP, or 1-NP. 6. Methysergide blocked the "anxiety-like" behavior in the EPM. 7. These data suggest that the discriminative stimuli produced by mCPP are based upon its selective actions on 5HT receptors and their use in behavioral pharmacology may offer another tool in studying pharmacology of 5HT based anxiogenic and anxiolytic drugs.

Presession noise increases sensitivity to chlordiazepoxide's discriminative stimulus in pigeons

1. Pigeons were trained to discriminate chlordiazepoxide (CDP) from saline using two-key food reinforced drug discrimination procedures. Discriminative control by CDP was maintained despite extended training with vehicle-like doses of CDP, by using a modified "fading" procedure that provided for a mixture of drug discrimination training sessions preceded by an i.m. injection of either 8.0 mg/kg CDP, or a lower training dose of CDP (4.0, 2.8, 2.0, 1.4, 1.0, 0.7, or 0.5 mg/kg CDP), or saline. The lower training dose was decreased across blocks of sessions. 2. Four lower training doses (1.4, 1.0, 0.7, and 0.5 mg/kg CDP) were retrained, with 10 min of 98 dB of noise administered 75 min prior to each drug discrimination training session. Presession exposure to noise increased percent CDP-appropriate choices for each of the four lower training doses by 15-20% over those obtained previously. 3. It is concluded that brief presession exposure to loud noise increases sensitivity to the discriminative stimulus effects of low training doses of CDP.

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.

Aggression, anxiety and vocalizations in animals: GABAA and 5-HT anxiolytics

A continuing challenge for preclinical research on anxiolytic drugs is to capture the affective dimension that characterizes anxiety and aggression, either in their adaptive forms or when they become of clinical concern. Experimental protocols for the preclinical study of anxiolytic drugs typically involve the suppression of conditioned or unconditioned social and exploratory behavior (e.g., punished drinking or social interactions) and demonstrate the reversal of this behavioral suppression by drugs acting on the benzodiazepine-GABAA complex. Less frequently, aversive events engender increases in conditioned or unconditioned behavior that are reversed by anxiolytic drugs (e.g., fear-potentiated startle). More recently, putative anxiolytics which target 5-HT receptor subtypes produced effects in these traditional protocols that often are not systematic and robust. We propose ethological studies of vocal expressions in rodents and primates during social confrontations, separation from social companions, or exposure to aversive environmental events as promising sources of information on the affective features of behavior. This approach focuses on vocal and other display behavior with clear functional validity and homology. Drugs with anxiolytic effects that act on the benzodiazepine-GABAA receptor complex and on 5-HT1A receptors systematically and potently alter specific vocalizations in rodents and primates in a pharmacologically reversible manner; the specificity of these effects on vocalizations is evident due to the effectiveness of low doses that do not compromise other physiological and behavioral processes. Antagonists at the benzodiazepine receptor reverse the effects of full agonists on vocalizations, particularly when these occur in threatening, startling and distressing contexts. With the development of antagonists at 5-HT receptor subtypes, it can be anticipated that similar receptor-specificity can be established for the effects of 5-HT anxiolytics.

Defeat engenders pentylenetetrazole-appropriate responding in rats: antagonism by midazolam

Defeat and the threat of defeat by an aggressive conspecific is stressful and may engender an anxiety- or fear-like state in animals; the present experiment investigated whether defeat generalized to the discriminative stimulus properties of PTZ and how benzodiazepine receptors were involved in this generalization. Separate groups of male Long-Evans rats (Rattus norvegicus) were trained to discriminate 20 mg/kg pentylenetetrazole (PTZ) or 0.4 mg/kg midazolam (MDZ) from saline in a two-choice drug-discrimination task. After establishing stimulus control, PTZ- and MDZ-trained rats were exposed to an aggressive conspecific which resulted in defeat, as defined by the display of defensive and submissive postures as well as audible and ultrasonic vocalizations. Administration of saline after defeat resulted in greater than 80% PTZ lever selection in 15 out of 25 PTZ-trained rats; this effect was attenuated through pretreatment with MDZ (1 mg/kg). Furthermore, short-term defeat substitution for the PTZ discriminative stimulus was not accompanied by long-term changes in the post-defeat generalization curves for PTZ and MDZ when compared to pre-defeat generalization curves. Nor did defeat alter the antagonism of PTZ by diazepam (2.5 mg/kg) or MDZ by flumazenil (10 mg/kg). In order further to characterize the necessary features for defeat substitution for the PTZ discriminative stimulus, exposure to a threatening conspecific was also attempted by PTZ-trained rats protected from physical contact with a wire mesh cage. In these tests, saline continued to engender greater than 50% PTZ lever responding in 15 of 25 rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacotherapy of cocaine abuse: preclinical development

Preclinical models of behavioral and toxic effects of cocaine are reviewed and their potential for predicting compounds with efficacy and safety in the medical management of cocaine abuse and toxicity is assessed. Many of the existing models appear to be good predictors of the effects of compounds against specific behavioral or toxicological actions of cocaine. However, the utility of the models for prediction of the efficacy of new therapeutic entities must await clinical validation as no accepted or standard pharmacotherapy currently exists. Preclinical data generated by these models with drugs currently under clinical investigation for cocaine abuse treatment as well as with other compounds are reviewed. These compounds include buprenorphine, bromocriptine, desmethylimipramine, carbamazepine, dopaminergic agonists, antagonists and partial agonists, dopamine reuptake inhibitors, sigma ligands, serotonin antagonists, and excitatory amino acid antagonists. Preclinical information on several drug classes appears sufficiently promising to warrant further evaluation. These include dopamine agonists and partial agonists, D1 receptor antagonists, selective sigma ligands, and modulators of the N-methyl-D-aspartate subtype glutamate receptor.

Withdrawal from chronic haloperidol substitutes for the pentylenetetrazol discriminative stimulus

The present study was designed to examine withdrawal from a therapeutic, non abused drug, haloperidol. Rats were trained to discriminate the anxiogenic compound pentylenetetrazol (PTZ) from water in a two lever, food reinforced, drug discrimination procedure. Dose effect curves were then determined for PTZ and the antipsychotic drug, haloperidol (0.1-1 mg/kg). Haloperidol did not substitute for PTZ, even at a dose that decreased rates of responding to approximately 15% of control values. Rats were then treated chronically with either 1 or 2 mg/kg/day haloperidol while training was suspended. After 5 days of chronic haloperidol 4/6 animals in the 1 mg/kg/day group and 5/7 in the 2 mg/kg/day group chose the PTZ lever when tested 24-48 hours after the last haloperidol injection. Haloperidol, 1 or 2 mg/kg, did not reverse PTZ-lever responding. After an additional 5 days of chronic haloperidol, 3/6 rats in the 1 mg/kg/day group and 5/7 rats in the 2 mg/kg/day group responded on the PTZ lever 24 hours after the last injection, and this was reversed with the anxiolytic, chlordiazepoxide (3.2-5.6 mg/kg). The current findings indicate that there is an anxiogenic component to withdrawal from haloperidol. In psychotic patients, abrupt discontinuation of haloperidol results in nausea, vomiting and sweating, as well as a "relapse into psychosis" characterized by anxiety, depression and internal chaos (1). Interestingly, the authors caution that the so-called relapse into psychosis may simply be a sign of withdrawal. The current findings support their view and suggest that abrupt discontinuation of psychoactive therapeutic agents may result in anxiety.

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.

Withdrawal from chronic phencyclidine produces a pentylenetetrazol-like discriminative stimulus

Rats trained to discriminate the anxiogenic compound pentylenetetrazol (PTZ) from water were implanted with osmotic mini-pumps containing 7 mg/kg/day phencyclidine (PCP) or water. Rats were tested for generalization to PTZ 24 hours prior to pump removal, and 4 to 96 hours after pump removal. While the pumps were in place, rats did not generalize to PTZ. When the pumps were removed on day 10, rats in the water group did not generalize to PTZ, but 69% of the rats in the chronic PCP group responded on the PTZ lever at 4 and/or 24 hours after pump removal, suggesting that the PCP withdrawal state mimics the interoceptive cue produced by PTZ. This withdrawal phenomenon was repeatable, in that rats that generalized once to PTZ during PCP withdrawal, generalized a second time when the procedure was repeated. In addition, the phenomenon was dose-dependent, as rats that did not generalize to PTZ after 7 mg/kg/day PCP did generalize when the chronic dose of PCP was increased to 10 mg/kg/day. These findings suggest that there is an anxiogenic component of PCP withdrawal and that tolerance does not develop to this effect.

Pentylenetetrazole can induce a conditioned place preference

Thirty-two male Sprague-Dawley rats were randomly selected into 4 groups (n = 8/group) and conditioned in a standard place preference task. The groups differed in the dose of pentylenetetrazole (PTZ) administered prior to conditioning trials. With respect to the three treatment groups, placement into the, initially, nonpreferred side of the CPP apparatus was preceded by injections of 5.6, 10, or 17.8 mg/kg PTZ. The control group was injected with hypertonic saline (1.8% w/v) on the rats', initially, nonpreferred sides and isotonic saline (0.9% w/v) on their preferred sides, to control for any irritative effects of PTZ injections in the treatment groups. Six pairs of drug-saline conditioning trials were conducted with each subject. PTZ produced a dose-dependent increase in the amount of time spent in the drug-associated environment. Saline control subjects' preference scores did not change over the course of the study. These data suggest that PTZ is not aversive in the place learning task; more importantly, the data suggest that a dose-dependent shift in the hedonic valences associated with environmental stimuli can occur when these stimuli are repeatedly paired with PTZ administration. The data are discussed in terms of the stimulus properties of PTZ and the hypothetical "anxiety" state the drug may produce.

Chronic cocaine administration alters corticotropin-releasing factor receptors in the rat brain

Two groups of 12 rats received daily injections of cocaine (20 mg/kg, i.p.) or saline for 15 days following bilateral infusions of 6-hydroxydopamine or vehicle into the lateral ventricles. Cocaine administration resulted in significant decreases in CRF receptor labeling primarily in brain areas associated with the mesolimbic/mesocortical dopaminergic system. All of the cocaine-induced changes in CRF binding were attenuated in the lesioned animals, suggesting that these effects may be mediated, in part, through the actions of the drug on dopaminergic neuronal activity. Dopamine may also be involved in the release of CRF in the rat brain since the purported loss of dopaminergic innervations resulted in increased CRF binding in the saline-treated animals.

Animal models of drug withdrawal symptoms

There have been few attempts to model subjective symptoms of drug withdrawal using animals as subjects. Two approaches for developing such models are reviewed. First, using drug discrimination methodology, it may be possible to train animals to detect the effects of withdrawal. This method has two difficulties: 1) the only discriminations trained to date involve precipitated withdrawal, and 2) the stimulus controlling behavior is difficult to specify. Second, withdrawal from many drugs of abuse produces the symptom of anxiety, and it seems likely that animal models of anxiety could be useful for studying drug withdrawal. This hypothesis has been explored most fully using subjects trained to detect the discriminative stimulus properties of the putative anxiogenic drug pentylenetetrazole (PTZ). Withdrawal from benzodiazepines or ethanol substitutes fully for PTZ, and withdrawal from cocaine, morphine, and nicotine substitutes partially for PTZ. Emerging data suggest that other animal models of anxiety may also be useful for detecting drug withdrawal. The final portion of this review examines a behavioral test that is very sensitive for detecting physical signs of withdrawal in animals. In subjects maintained on an operant baseline using food as a reinforcer, withdrawal from a drug of dependence frequently is associated with disruption of that operant behavior. For example, tetrahydrocannabinol and cocaine, drugs that are not traditionally seen as having significant withdrawal signs, produce disruption of operant responding when high-dose administration is terminated, and their readministration reverses this behavioral disruption. Based on the observation that withdrawal is associated with anxiogenic stimuli, we suggest a method to determine if disruption of operant behavior may be related to these stimuli.