Anxiogenic properties of cocaine withdrawal

What Is It Like to Be a Bass? Red Herrings, Fish Pain and the Study of Animal Sentience

Debates around fishes' ability to feel pain concern sentience: do reactions to tissue damage indicate evaluative consciousness (conscious affect), or mere nociception? Thanks to Braithwaite's discovery of trout nociceptors, and concerns that current practices could compromise welfare in countless fish, this issue's importance is beyond dispute. However, nociceptors are merely necessary, not sufficient, for true pain, and many measures held to indicate sentience have the same problem. The question of whether fish feel pain - or indeed anything at all - therefore stimulates sometimes polarized debate. Here, we try to bridge the divide. After reviewing key consciousness concepts, we identify "red herring" measures that should not be used to infer sentience because also present in non-sentient organisms, notably those lacking nervous systems, like plants and protozoa (P); spines disconnected from brains (S); decerebrate mammals and birds (D); and humans in unaware states (U). These "S.P.U.D. subjects" can show approach/withdrawal; react with apparent emotion; change their reactivity with food deprivation or analgesia; discriminate between stimuli; display Pavlovian learning, including some forms of trace conditioning; and even learn simple instrumental responses. Consequently, none of these responses are good indicators of sentience. Potentially more valid are aspects of working memory, operant conditioning, the self-report of state, and forms of higher order cognition. We suggest new experiments on humans to test these hypotheses, as well as modifications to tests for "mental time travel" and self-awareness (e.g., mirror self-recognition) that could allow these to now probe sentience (since currently they reflect perceptual rather than evaluative, affective aspects of consciousness). Because "bullet-proof" neurological and behavioral indicators of sentience are thus still lacking, agnosticism about fish sentience remains widespread. To end, we address how to balance such doubts with welfare protection, discussing concerns raised by key skeptics in this debate. Overall, we celebrate the rigorous evidential standards required by those unconvinced that fish are sentient; laud the compassion and ethical rigor shown by those advocating for welfare protections; and seek to show how precautionary principles still support protecting fish from physical harm.

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.

Ultrasonic Vocalizations as a Measure of Affect in Preclinical Models of Drug Abuse: A Review of Current Findings

The present review describes ways in which ultrasonic vocalizations (USVs) have been used in studies of substance abuse. Accordingly, studies are reviewed which demonstrate roles for affective processing in response to the presentation of drug-related cues, experimenter- and self-administered drug, drug withdrawal, and during tests of relapse/reinstatement. The review focuses on data collected from studies using cocaine and amphetamine, where a large body of evidence has been collected. Data suggest that USVs capture animals’ initial positive reactions to psychostimulant administration and are capable of identifying individual differences in affective responding. Moreover, USVs have been used to demonstrate that positive affect becomes sensitized to psychostimulants over acute exposure before eventually exhibiting signs of tolerance. In the drug-dependent animal, a mixture of USVs suggesting positive and negative affect is observed, illustrating mixed responses to psychostimulants. This mixture is predominantly characterized by an initial bout of positive affect followed by an opponent negative emotional state, mirroring affective responses observed in human addicts. During drug withdrawal, USVs demonstrate the presence of negative affective withdrawal symptoms. Finally, it has been shown that drug-paired cues produce a learned, positive anticipatory response during training, and that presentation of drug-paired cues following abstinence produces both positive affect and reinstatement behavior. Thus, USVs are a useful tool for obtaining an objective measurement of affective states in animal models of substance abuse and can increase the information extracted from drug administration studies. USVs enable detection of subtle differences in a behavioral response that might otherwise be missed using traditional measures.

Abuse potential and adverse cognitive effects of mitragynine (kratom)

Mitragynine is the major psychoactive alkaloid of the plant kratom/ketum. Kratom is widely used in Southeast Asia as a recreational drug, and increasingly appears as a pure compound or a component of 'herbal high' preparations in the Western world. While mitragynine/kratom may have analgesic, muscle relaxant and anti-inflammatory effects, its addictive properties and effects on cognitive performance are unknown. We isolated mitragynine from the plant and performed a thorough investigation of its behavioural effects in rats and mice. Here we describe an addictive profile and cognitive impairments of acute and chronic mitragynine administration, which closely resembles that of morphine. Acute mitragynine has complex effects on locomotor activity. Repeated administration induces locomotor sensitization, anxiolysis and conditioned place preference, enhances expression of dopamine transporter- and dopamine receptor-regulating factor mRNA in the mesencephalon. While there was no increase in spontaneous locomotor activity during withdrawal, animals showed hypersensitivity towards small challenging doses for up to 14 days. Severe somatic withdrawal signs developed after 12 hours, and increased level of anxiety became evident after 24 hours of withdrawal. Acute mitragynine independently impaired passive avoidance learning, memory consolidation and retrieval, possibly mediated by a disruption of cortical oscillatory activity, including the suppression of low-frequency rhythms (delta and theta) in the electrocorticogram. Chronic mitragynine administration led to impaired passive avoidance and object recognition learning. Altogether, these findings provide evidence for an addiction potential with cognitive impairments for mitragynine, which suggest its classification as a harmful drug.

Tales from the dark side: do neuromodulators of drug withdrawal require changes in endocannabinoid tone?

Environmental and interoceptive cues are theorized to serve as 'signals' that motivate drug seeking and effects that may be augmented in the withdrawn state. Phasic dopamine release events are observed in the nucleus accumbens in response to such motivational salient stimuli and are thought to be necessary for drug-associated cues to trigger craving. We recently demonstrated how dopamine neurons encode stimuli conditioned to a negative event, as might occur during conditioned withdrawal, and stimuli predicting the avoidance of negative events, as might occur as an addict seeks out drugs to prevent withdrawal. In this review we first discuss how the subsecond dopamine release events might process conditioned withdrawal and drug seeking driven by negative reinforcement processes within the context of our dopamine data obtained during conditioned avoidance procedures. We next describe how the endocannabinoid system modulates phasic dopamine release events and how it might be harnessed to treat negative affective states in addiction. Specifically, we have demonstrated that endocannabinoids in the ventral tegmentum sculpt cue-induced accumbal surges in dopamine release and, therefore, may also be mobilized during drug withdrawal.

The combination of metyrapone and oxazepam for the treatment of cocaine and other drug addictions

Although scientists have been investigating the neurobiology of psychomotor stimulant reward for many decades, there is still no FDA-approved treatment for cocaine or methamphetamine abuse. Research in our laboratory has focused on the relationship between stress, the subsequent activation of the hypothalamic-pituitary-adrenal (HPA) axis, and psychomotor stimulant reinforcement for almost 30 years. This research has led to the development of a combination of low doses of the cortisol synthesis inhibitor, metyrapone, and the benzodiazepine, oxazepam, as a potential pharmacological treatment for cocaine and other substance use disorders. In fact, we have conducted a pilot clinical trial that demonstrated that this combination can reduce cocaine craving and cocaine use. Our initial hypothesis underlying this effect was that the combination of metyrapone and oxazepam reduced cocaine seeking and taking by decreasing activity within the HPA axis. Even so, doses of the metyrapone and oxazepam combination that consistently reduced cocaine taking and seeking did not reliably alter plasma corticosterone (or cortisol in the pilot clinical trial). Furthermore, subsequent research has demonstrated that this drug combination is effective in adrenalectomized rats, suggesting that these effects must be mediated above the level of the adrenal gland. Our evolving hypothesis is that the combination of metyrapone and oxazepam produces its effects by increasing the levels of neuroactive steroids, most notably tetrahydrodeoxycorticosterone, in the medial prefrontal cortex and amygdala. Additional research will be necessary to confirm this hypothesis and may lead to the development of improved and specific pharmacotherapies for the treatment of psychomotor stimulant use.

Anxiety symptoms in crack cocaine and inhalant users admitted to a psychiatric hospital in southern Brazil

OBJECTIVE

The occurrence of psychiatric comorbidity among individuals with crack or inhalant dependence is frequently observed. The objective of this study was to investigate anxiety symptoms among crack cocaine and inhalant users in southern Brazil.

METHODS

The study investigated two groups of volunteers of equal size (n=50): one group consisted of crack cocaine users, and the other group consisted of inhalant users. Research volunteers completed the Portuguese versions of the State-Trait Anxiety Inventory (STAI), Hamilton Anxiety Rating Scale (HAM-A), and Self-Report Questionnaire (SRQ).

RESULTS

Both crack and inhalant users experience significant symptoms of anxiety. Inhalant users presented significantly more anxiety symptoms than crack users according to the HAM-A questionnaire only. In contrast to the results of the HAM-A, the STAI failed to demonstrate a significant difference between the two groups of substance users. SRQ scores revealed that crack and inhalants users had significant degrees of morbidity.

CONCLUSION

A significant difference regarding anxiety symptomatology, especially state anxiety, was observed among inhalant and crack users. Anxiety and overall mental psychopathology were significantly correlated in this sample. The results indicate that screening initiatives to detect anxiety and additional psychiatric comorbidities among crack and inhalant users are feasible and relevant.

Attenuation of methamphetamine seeking by the mGluR2/3 agonist LY379268 in rats with histories of restricted and escalated self-administration

Recent findings implicate group II metabotropic glutamate receptors (mGluR(2/3)) in the reinforcing effects of psychostimulants and have identified these receptors as potential treatment targets for drug addiction. Here, we investigated the effects of mGluR(2/3) stimulation on cue- and drug-primed reinstatement in rats with different histories of methamphetamine (METH) self-administration training, under two conditions: 16 daily sessions of short access (90 min/day, ShA), or 8 daily sessions of short access followed by 8 sessions of long access (6 h/day, LgA). Following self-administration and subsequent extinction training, rats were pretreated with the selective mGluR(2/3) agonist LY379268 (variable dose, 0-3 mg/kg), exposed to METH-paired cues or a priming injection of METH (1 mg/kg), and tested for reinstatement of METH-seeking behavior. LgA rats self-administered greater amounts of METH during the second half of training, but when pretreated with vehicle, ShA and LgA rats showed cue- and drug-primed reinstatement at equivalent response rates. However, LgA rats demonstrated greater sensitivity to mGluR(2/3) stimulation with attenuated responding during cue-induced reinstatement after 0.3 mg/kg and higher doses of LY379268, whereas ShA rats decreased cue-induced reinstatement behavior following 1.0 mg/kg and 3.0 mg/kg LY379268. Additionally, both LgA and ShA rats exhibited decreased METH-primed reinstatement behavior following 0.3 mg/kg and higher doses of LY379268. A separate group of control rats was trained to self-administer sucrose pellets, and demonstrated attenuated cue-induced sucrose-seeking behavior following 1.0 and 3.0 mg/kg LY379268. Together, the results indicate that LY379268 has differential attenuating effects on cue-induced reinstatement behavior in rats with different histories of METH intake. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

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.

Withdrawal from chronic administration of cocaine decreases delta opioid receptor signaling and increases anxiety- and depression-like behaviors in the rat

Chronic administration of cocaine has been shown to attenuate the functional capacity of delta opioid receptors to inhibit adenylyl cyclase activity. Abuse and withdrawal from cocaine in humans is associated with increases in anxiety and depression. Since recent research supports the role of delta opioid receptors in anxiety- and depression-like behaviors in rodents, we hypothesized that functional desensitization of delta opioid receptors contributes to anxiety- and depression-like behavioral phenotypes following short-term withdrawal from chronic administration of cocaine. To test this hypothesis, delta opioid receptor signaling and behaviors were evaluated 24h after 14days of binge-pattern cocaine administration (15mg/kg three times daily at 1h intervals) in male Sprague-Dawley rats. Results showed that the inhibition of adenylyl cyclase by delta opioid receptor agonists was attenuated in the frontal cortex, nucleus accumbens and caudate putamen 24h after cessation of cocaine administration. One day withdrawal from chronic administration of cocaine resulted in increased anxiety- and depression-like behaviors as measured by the elevated plus maze and the forced swim test respectively, and no change in locomotor activity. The anxiety- and depression-like behaviors were dose-dependently reduced by acute administration of the selective delta opioid receptor agonist, SNC80. These results demonstrate that early withdrawal from cocaine resulted in increased anxiety and depression, which accompanies the desensitization of delta opioid receptor function. Furthermore, cocaine-induced anxiety- and depression-like behaviors were reversible by the delta opioid receptor agonist SNC80.

Chronic cocaine or ethanol exposure during adolescence alters novelty-related behaviors in adulthood

Adolescence is a time of high-risk behavior and increased exploration. This developmental period is marked by a greater probability to initiate drug use and is associated with an increased risk to develop addiction and adulthood dependency and drug use at this time is associated with an increased risk. Human adolescents are predisposed toward an increased likelihood of risk-taking behaviors [Zuckerman M. Sensation seeking and the endogenous deficit theory of drug abuse. NIDA Res Monogr 1986;74:59-70.], including drug use or initiation. In the present study, adolescent animals were exposed to twenty days of either saline (0.9% sodium chloride), cocaine (20 mg/kg) or ethanol (1 g/kg) i.p. followed by a fifteen-day washout period. All animals were tested as adults on several behavioral measures including locomotor activity induced by a novel environment, time spent in the center of an open field, novelty preference and novel object exploration. Animals exposed to cocaine during adolescence and tested as adults exhibited a greater locomotor response in a novel environment, spent less time in the center of the novel open field and spent less time with a novel object, results that are indicative of a stress or anxiogenic response to novelty or a novel situation. Adolescent animals chronically administered ethanol and tested as adults, unlike cocaine-exposed were not different from controls in a novel environment, indicated by locomotor activity or time spent with a novel object. However, ethanol-exposed animals approached the novel object more, suggesting that exposure to ethanol during development may result in less-inhibited behaviors during adulthood. The differences in adult behavioral responses after drug exposure during adolescence are likely due to differences in the mechanisms of action of the drugs and subsequent reward and/or stress responsivity. Future studies are needed to determine the neural substrates of these long lasting drug-induced changes.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Persistent anxiogenic effects of a single or repeated doses of cocaine and methamphetamine: interactions with endogenous cannabinoid receptor ligands

As persistent behavioural changes, such as increased anxiety-related behaviours, can be predicted based on the phenomenon of psychostimulant-induced neuronal plasticity, the time course (3-, 5- and 10-day time points) of the effects of both a single and repeated (daily for 7 days) i.p. administrations of cocaine (COC) and methamphetamine (MA) on anxiety-related behavioural symptoms in the elevated plus-maze test were examined in mice. Furthermore, based on the reported interactions between brain dopamine versus cannabinoid (CB) receptors and the contribution of CB receptors to the occurrence of persistent anxiety-related behavioural symptoms, the interactions of the agonist CP 55940 (CP) and the endogenous ligands anandamide (arachidonylethanolamide: AEA), 2-arachidonylglycerol (ARA), N-arachidonyldopamine (NADA), noladin ether (NL), and virodhamine (VA) with the COC- or MA-induced anxiety-related behaviours were also studied. In both an acute experiment using a single COC (30 mg/kg) or MA (4 mg/kg) dose and a chronic experiment using repeated COC (15 mg/kg) or MA (2 mg/kg) doses, anxiety-related behavioural symptoms were observed similarly at 3- and 5-day time points, but disappeared at the 10-day time point. Among the CB ligands, the agonists CP, AEA, ARA, NADA, and NL provided strong protective effects against each parameter at 3- and 5-day time points. Therefore, it was concluded that both COC and MA caused persistent anxiety-related behavioural symptoms following both a single and repeated treatments. Since these anxiogenic effects were attenuated by the endogenous CB agonists, the involvement of brain CB receptors was suspected.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

Behavioral depression during cocaine withdrawal is associated with decreased spontaneous activity of ventral tegmental area dopamine neurons

Withdrawal from an escalating-dose, bingelike regimen of cocaine administration in rats produced significantly depressed levels of locomotor activity during the nocturnal portion of the day-night cycle. This effect was observed during the first 48 hr of testing. Extracellular single-unit recordings of ventral tegmental area (VTA) dopamine (DA) neurons revealed no differences between saline- and cocaine-treated rats with respect to basal firing rates. However, significantly fewer spontaneously active VTA DA neurons were encountered in rats withdrawn from binge cocaine. As with the nocturnal hypoactivity, this effect was observed only during the first 48 hr of withdrawal. These findings suggest that short-term DA neuron dysfunction during cocaine withdrawal temporally corresponds to behavioral disruptions that are similar to those described in human addicts.

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.

Cocaine-induced anxiety: alleviation by diazepam, but not buspirone, dimenhydrinate or diphenhydramine

Clinical reports and animal experiments indicate that both cocaine administration and cocaine withdrawal increase anxiety. We investigated the ability of a number of putative anxiolytic agents to alleviate these anxiety states using the elevated plus-maze. Rats in the cocaine condition received either saline or cocaine (20 mg/kg) 40 min prior to testing; those in the withdrawal condition were tested 48 h following a chronic treatment regime (saline or cocaine 20 mg/kg per day for 14 days). Prior to testing, animals received a benzodiazepine (1.0 or 2.0 mg/kg diazepam), a serotonergic agonist (0.5 or 1.0 mg/kg buspirone), an antihistamine (50 mg/kg dimenhydrinate or 27 mg/kg diphenhydramine) or a saline injection. All drugs were administered intraperitoneally. Cocaine administration and cocaine withdrawal reduced the percentage time spent on and the number of entries into the open arms. Diazepam dose-dependently alleviated cocaine withdrawal-induced anxiety and non-significantly attenuated cocaine-induced anxiety. Buspirone, dimenhydrinate and diphenhydramine did not consistently alleviate the anxiety caused by either cocaine pre-treatment regime; in the saline conditions, however, each of these treatments was anxiogenic. In summary, benzodiazepines alleviated cocaine-induced anxiety, while future research on the ability of serotonergic and antihistaminergic drugs to alleviate these anxiety states is warranted.

The discriminative stimulus effects of pentylenetetrazol as a model of anxiety: recent developments

Pentylenetetrazol (PTZ), a GABA(A) receptor antagonist and prototypical anxiogenic drug, has been extensively utilized in animal models of anxiety. PTZ produces a reliable discriminative stimulus which is largely mediated by the GABA(A) receptor. Several classes of compounds can modulate the PTZ discriminative stimulus including 5-HT(1A), 5-HT(3), NMDA, glycine, and L-type calcium channel ligands. Spontaneous PTZ-lever responding is seen in trained rats during withdrawal from GABA(A) receptor compounds such as chlordiazepoxide and diazepam, and also ethanol, morphine, nicotine, cocaine, haloperidol, and phencyclidine. This effect is largely mediated by the GABA(A) receptor, which suggests that anxiety may be part of a generalized withdrawal syndrome mediated by the GABA(A) receptor. There are also important hormonal influences on PTZ. Corticosterone plays some role in mediation of its anxiogenic effects. There is a marked sex difference in response to the discriminative stimulus effects of PTZ, and estrogens appear to protect against its anxiogenic effects. Further work with the PTZ drug discrimination is warranted for characterization of anxiety during withdrawal, and the hormonal mechanisms of anxiety.

The HPA axis and cocaine reinforcement

Scientists have been aware of the existence of a complex relationship between stress and the subsequent activation of the hypothalamic-pituitary-adrenal (HPA) axis and the endocrine and neurobehavioral effects of cocaine for many years now. Our research program has focused on the involvement of HPA axis activation in cocaine reinforcement using the intravenous self-administration model. Behaviorally, there are at least three general phases in the etiology of drug self-administration to consider: acquisition, maintenance and reinstatement. We have investigated the role for the HPA axis during each of these three phases. Corticosterone is necessary during acquisition; self-administration does not occur unless this stress-related hormone is increased above a threshold critical for reward. Sensitivity to low doses of cocaine falling on the ascending limb of the acquisition dose-response curve can be augmented by increasing circulating levels of corticosterone, but similar treatments do not affect responding maintained by higher doses. In a similar vein, ongoing, low-dose cocaine self-administration is decreased by drugs affecting the synthesis and/or secretion of corticosterone. When higher doses falling on the descending limb of the cocaine dose-response curve are self-administered, plasma corticosterone can still reach this hypothetical reward threshold even when synthesis is inhibited, and drug intake is not affected. On the other hand, the self-administration of doses falling on both the ascending and descending limbs of the cocaine dose-response curve can each be attenuated by drugs that block central corticotropin-releasing hormone (CRH) receptors. Finally, corticosterone and CRH are also critical for the stress- and cue-induced reinstatement of extinguished cocaine-seeking behavior, demonstrating an involvement of the HPA axis in the relapse to cocaine use as well. Continued investigations into how stress and the subsequent activation of the HPA axis affect cocaine self-administration will likely result in the identification of more effective and efficient treatment for cocaine addiction.

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.

Effects of calcium channel blockers on pentylenetetrazol drug discrimination in rats

The effects of the dihydropyridine L-type calcium channel blockers nitrendipine and nimodipine on the pentylenetetrazol (PTZ) drug discrimination, an operant model of anxiety, were investigated. Male Long-Evans rats were trained to discriminate PTZ (16 mg/kg, i.p.) from saline. Both nitrendipine (5.0-25 mg/kg, i.p.) and nimodipine (5.0-25 mg/kg, i.p.) partially substituted for the PTZ discriminative stimulus. However, pretreatment with nitrendipine (25 mg/kg, i.p.) or nimodipine (25 mg/kg, i.p.) produced no change in the PTZ dose-effect function. Rats were given a nutritionally balanced liquid diet containing 6.5% ethanol for 10 days. Rats selected the PTZ drug lever during withdrawal. Subchronic coadministration of nitrendipine (1.25-5.0 mg/kg, i.p., b.i.d.) with ethanol failed to dose-dependently reduce PTZ-lever responding, but it did reverse withdrawal signs. Acute administration of nitrendipine (5, 10, and 20 mg/kg, i.p.) produced marked suppression of lever responding, but it failed to significantly reduce levels of PTZ-lever responding. Although calcium channel blockers reduce signs of ethanol withdrawal, they also markedly reduce rates of behavior and produce no clear effects on anxiety-like behaviors induced by ethanol withdrawal.

Prepulse inhibition of the acoustic startle response in cocaine-withdrawn rats

Prepulse inhibition (PPI) of startle is a sensorimotor gating task in which a low-intensity acoustic stimulus presented prior to a high-intensity, startle-eliciting stimulus can attenuate the acoustic startle response (ASR). Previous studies on startle reactivity in cocaine-withdrawn rats have found minimal changes; the present study extends this work to the gating of ASR. In Experiment 1, rats were injected daily with either saline or cocaine (30 mg/kg i.p.) for 2 weeks. ASR and PPI were measured prior to, and at 3- and 14-day withdrawal from, the chronic treatment. No effect of cocaine treatment was found on either measure. In Experiment 2, treatment was extended to 8 weeks, and an earlier withdrawal time point (1 day) was added. Rats treated with cocaine for 8 weeks exhibited lower startle reactivity during withdrawal compared with saline-treated controls. PPI did not differ between treatment groups. Thus, extended chronic treatment with cocaine rendered significant effects on startle responsivity. Further, this finding mirrors the blunted ASR exhibited in chronic cocaine users [Neuropsychopharmacology 22 (2000) 89.].

Effects of ritanserin on ethanol withdrawal-induced anxiety in rats

This study investigated the ability of ritanserin, a 5-HT2 antagonist, to modify ethanol withdrawal (EW) symptoms in two animal models of anxiety: the elevated plus-maze (EPM) and the pentylenetetrazol (PTZ) discrimination assay. Long-Evans hooded rats were given a nutritionally balanced liquid diet containing 4.5% ethanol for 10 days. Twelve hours after removal of the ethanol diet, rats were tested in the EPM. A significant reduction in the open-arm activity and the number of total arm entries was observed, which is indicative of EW. Acute ritanserin (0.16-0.64 mg/kg, i.p., 60 min) had no effect on EW-induced anxiety-like behavior on the EPM. Ritanserin (0.08-0.64 mg/kg, i.p., b.i.d. 12 h) administered concurrently with the last 5 days of ethanol diet produced an increase in the time spent on the open arms of the EPM and reversed the EW-induced reduction in total arm entries. Rats trained to discriminate between saline and PTZ (an anxiogenic drug), selected the PTZ lever during EW. Chronic ritanserin (0.32 mg/kg, i.p., b.i.d. ) did not block PTZ lever responding during EW. On the rotorod, ritanserin (0.32 mg/kg, i.p.) increased the motor incoordination induced by ethanol. In conclusion, coadministration of ritanserin with ethanol prevented the development of EW-induced anxiety as measured by the EPM, but not in the PTZ drug discrimination.

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.

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.

Opponent process model and psychostimulant addiction

There are many sources of reinforcement in the spectrum of cocaine dependence that contribute to the compulsive cocaine self-administration or loss of control of cocaine intake that constitutes the core of modern definitions of dependence. The development of withdrawal has long been considered an integral part of drug addiction but has lost its impact in the theorization of drug dependence because of new emphasis on the neurobiological substrates for the positive-reinforcing properties of drugs. The present treatise reviews the neurobiological substrates for the acute positive reinforcing effects of cocaine and what is beginning to be known about the neurobiological substrates of cocaine withdrawal. The concept of motivational or affective withdrawal is reintroduced, which reemphasizes opponent process theory as a model for the motivational effects of cocaine dependence. The same neural substrates hypothesized to be involved in the acute reinforcing properties of drugs (basal forebrain regions of nucleus accumbens and amygdala) are hypothesized to be altered during chronic drug treatment to produce the negative motivational states characterizing drug withdrawal. Within these brain regions, both the neurochemical system(s) on which the drug has its primary actions and other neurochemical systems may undergo adaptations to chronic presence of the drug. An understanding of the adaptations of the motivational systems of the brain accompanying cocaine dependence leads to important predictions not only about the etiology, treatment, and prevention of cocaine addiction but also about the vulnerability of these motivational systems in non-drug-induced psychopathology.

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 role of dopamine in drug abuse viewed from the perspective of its role in motivation

Drugs of abuse share with conventional reinforcers the activation of specific neural pathways in the CNS that are the substrate of their motivational properties. Dopamine is recognized as the transmitter of one such neural pathway, being involved in at least three major aspects of motivation: modulation of motivational state, acquisition (incentive learning) and expression of incentive properties by motivational stimuli. Drugs of abuse of different pharmacological classes stimulate in the low dose range dopamine transmission particularly in the ventral striatum. Apart from psychostimulants, the evidence that stimulation of dopamine transmission by drugs of abuse provides the primary motivational stimulus for drug self-administration is either unconvincing or negative. However, stimulation of dopamine transmission is essential for the activational properties of drugs of abuse and might be instrumental for the acquisition of responding to drug-related incentive stimuli (incentive learning). Dopamine is involved in the induction and in the expression of behavioural sensitization by repeated exposure to various drugs of abuse. Sensitization to the dopamine-stimulant properties of specific drug classes leading to facilitation of incentive learning of drug-related stimuli might account for the strong control over behaviour exerted by these stimuli in the addiction state. Withdrawal from drugs of abuse results in a reduction in basal dopamine transmission in vivo and in reduced responding for conventional reinforcers. Although these changes are likely to be the expression of a state of dependence of the dopamine system their contribution to the motivational state of drug addiction is unclear.

Brain corticotropin-releasing factor mediates 'anxiety-like' behavior induced by cocaine withdrawal in rats

Anxiety is a key symptom of the cocaine withdrawal syndrome in human addicts, and it is considered to be one of the major factors in precipitating relapse to chronic cocaine abuse. Corticotropin-releasing factor (CRF) plays an important role in the pathophysiology of anxiety and depression, and it may also be involved in the acute behavioral and neuroendocrine actions of cocaine. The role of endogenous CRF in cocaine withdrawal-induced anxiety was investigated in the present study. Animals were subjected to chronic cocaine (20 mg/kg, intraperitoneally, once a day for 14 days) administration. Rats tested 30 min after the last cocaine injection did not show withdrawal anxiety on the elevated plus maze or any alterations in brain CRF levels. Withdrawal (48 h) from chronic cocaine administration produced an intense anxiety-like behavior characterized by decreased open arm exploration. Immunoreactive CRF (CRF-LI) levels were selectively altered in the hypothalamus, in the amygdala and in the basal forebrain structures at the time of the behavioral anxiety, reflecting an increased activity of brain CRF systems. Daily intracerebroventricular (i.c.v.) pretreatment with an immunoserum raised against CRF completely prevented the development of anxiety induced by cocaine withdrawal. These data suggest that extrahypothalamic-limbic CRF hypersecretion may be involved in the development of anxiety related to cocaine withdrawal and that the CRF system may be a useful target for new pharmacotherapies for cocaine withdrawal and relapse.

Neurobiology of cocaine abuse

The recent escalation of cocaine abuse has increased awareness of the need to understand the behavioral effects of cocaine and the determinants of those effects. Cocaine alters both conditioned and unconditioned behavior, and has prominent reinforcing and subjective effects that are particularly relevant to its abuse. An increase in CNS dopamine neurotransmission, resulting from a competitive blockade of high-affinity dopamine uptake mediated by both D1 and D2 dopamine receptors, is a primary determinant of the behavioral effects of cocaine. Either tolerance or sensitization may develop with repeated administration of cocaine. Dependence also develops, although the behavioral changes associated with cocaine withdrawal are subtle. Although numerous CNS changes have been associated with repeated administration of cocaine, the neuropharmacological mechanisms that underlie the behavioral changes that occur with repeated administration remain to be firmly established. Bill Woolverton and Ken Johnson stress that continued collaboration between behavioral pharmacologists and neuroscientists is critical for a complete understanding of the effects of cocaine.

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.

Differential effects of a 5-HT2 receptor blocker on alcohol intake in rats selectively bred for high and low catecholamine responses to stress

The effect of the selective 5-HT2 receptor blocker ritanserin on alcohol consumption was investigated in two strains of rats selectively bred for high and low catecholamine responses to stress. Rats were forced to drink a 5% alcohol solution for 10 days. For the subsequent six days, animals were injected subcutaneously with 2.5 mg/kg/2 ml ritanserin or vehicle only, and both a 5% solution of alcohol and water were presented to the animals. Ritanserin affected neither water nor total fluid intake. Furthermore, no effect of ritanserin on alcohol consumption could be demonstrated in high-responding rats, whereas in low-responding rats a very pronounced ritanserin-induced reduction in alcohol intake was observed. Results are discussed in terms of mediating effects of serotonergic neurons on mesolimbic dopaminergic reward systems related to drug addiction.

Cocaine addiction: psychology and neurophysiology

Cocaine was considered incapable of producing dependence in 1980 but was recently proclaimed the drug of greatest national health concern. Recent clinical and preclinical investigations demonstrate that cocaine produces unique abuse and withdrawal patterns that differ from those of other major abused drugs and suggest that long-term cocaine abuse produces neurophysiological alterations in specific systems in the central nervous system that regulate the capacity to experience pleasure. It will be necessary to develop clinically pertinent research models before these findings can be considered definitive, but these evolving ideas have already led to applications of promising experimental treatments for cocaine abuse.

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.

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

The acute administration of nicotine (0.01-1.0 mg/kg IP) to the mouse increased the time spent and rearings and line crossings in the aversive brightly illuminated white area of a two compartment white/black test box, with a corresponding decrease in the black. This profile of change was maintained during twice daily administration (0.1 mg/kg IP) for 14 days. Eight to 96 hr following withdrawal of nicotine (14-day treatment), the behavioural profile was reversed to a preference for the black area: by 240 hr values had returned to control levels. In contrast to the effects of nicotine, an acute injection of cocaine (0.1-10 mg/kg IP) exacerbated the aversive response to the white area. However, similarly to nicotine, the administration of cocaine (1.0 mg/kg IP) twice daily for 14 days reduced the aversion to the white area and exacerbated the response following cocaine withdrawal. The effects of nicotine and cocaine to reduce and enhance responsiveness to the aversive properties of the white area are discussed in terms of an anxiolytic and anxiogenic response and the possibility of a serotonergic involvement.

Effects of cocaine on conflict behavior in the rat

The present studies examined the effects of acute cocaine administration, chronic cocaine administration and cocaine withdrawal on behavior in the Conditioned Suppression of Drinking (CSD) conflict paradigm, an animal model for the study of anxiety. In daily 10-minute sessions, water deprived rats were trained to drink from a tube that was occasionally electrified (0.25 mA), electrification being signalled by a tone. Within 3-4 weeks, control (i.e., non-drug) CSD behavior stabilized (30-50 shocks and 10-12 ml/session) and drug studies were initiated. Acute administration of cocaine (30-min pretreatment) produced a selective pro-conflict effect only at a dose of 10 mg/kg cocaine, with lower doses (2.5, 5 mg/kg) exerting no effect on CSD behavior and a higher dose (20 mg/kg) depressing both punished and unpunished responding. In a second experiment, cocaine (10 mg/kg, IP, 2/day) or saline was administered to separate groups of subjects for 7 weeks. In this chronic treatment study, CSD testing was conducted 12 hours after each evening cocaine administration. Although it had no effect on CSD behavior during the first week of treatment, this chronic cocaine administration produced a significant and selective pro-conflict effect which was stable during the period from Weeks 2-7. In a final experiment, a high dose of cocaine (20 mg/kg, 3/day) or saline was given to separate groups of subjects for 2 weeks and the behavioral effects of these treatments and their subsequent termination were examined. In this study, CSD testing was conducted 8 hours after each evening cocaine treatment. During the first week of high dose cocaine treatment, a decrease in punished responding was observed; this parameter returned to baseline levels by Week 2. Discontinuation of this high dose chronic cocaine treatment resulted in a selective decrease in punished responding. This pro-conflict effect was greatest at 3 days, and lasted for 6 days after the last cocaine dose. These data are consistent with clinical findings demonstrating the anxiogenic effects of both acute and chronic cocaine treatment as well as cocaine withdrawal and suggest that conflict paradigms such as the CSD may be useful for the study of cocaine-induced anxiety states.