Conditioning, habituation and behavioral reorganization factors in chronic cocaine effects

Behaviorally conditioned effects of psychoactive drugs in experimental animals: What we have learned from nearly a century of research and what remains to be learned

Continuous treatment with drugs is a crucial requirement for managing various clinical conditions, including chronic pain and neuropsychiatric disorders such as depression or schizophrenia. Associative learning processes, i.e. Pavlovian conditioning, can play an important role for the effects of drugs and could open new avenues for optimizing patient treatment. In this narrative literature review, we summarize available data in experimental animals regarding the behaviorally conditioned effects of psychostimulants such as d-amphetamine and cocaine, the dopamine receptor agonist apomorphine, the dopamine receptor antagonist haloperidol, morphine and antidepressant drugs. In each section, the drug under discussion is briefly introduced, followed by a detailed examination of conditioning features, including doses and dosing regimens, characteristics of the conditioning process such as test environments or specific conditioned stimuli, testing and conditioned response characteristics, possible extinction or reconditioning or reversal training, neural mechanisms, and finally, the potential clinical relevance of the research area related to the drug. We focus on key outcomes, delve into methodical issues, identify gaps in current knowledge, and suggest future research directions.

Repeated chemogenetic activation of dopaminergic neurons induces reversible changes in baseline and amphetamine-induced behaviors

RATIONALE

Repeated chemogenetic stimulation is often employed to study circuit function and behavior. Chronic or repeated agonist administration can result in homeostatic changes, but this has not been extensively studied with designer receptors exclusively activated by designer drugs (DREADDs).

OBJECTIVES

We sought to evaluate the impact of repeated DREADD activation of dopaminergic (DA) neurons on basal behavior, amphetamine response, and spike firing. We hypothesized that repeated DREADD activation would mimic compensatory effects that we observed with genetic manipulations of DA neurons.

METHODS

Excitatory hM3D(Gq) DREADDs were virally expressed in adult TH-Cre and WT mice. In a longitudinal design, clozapine N-oxide (CNO, 1.0 mg/kg) was administered repeatedly. We evaluated basal and CNO- or amphetamine (AMPH)-induced locomotion and stereotypy. DA neuronal activity was assessed using in vivo single-unit recordings.

RESULTS

Acute CNO administration increased locomotion, but basal locomotion decreased after repeated CNO exposure in TH-CrehM3Dq mice relative to littermate controls. Further, after repeated CNO administration, AMPH-induced hyperlocomotion and stereotypy were diminished in TH-CrehM3Dq mice relative to controls. Repeated CNO administration reduced DA neuronal firing in TH-CrehM3Dq mice relative to controls. A two-month CNO washout period rescued the decreases in basal locomotion and AMPH response.

CONCLUSIONS

We found that repeated DREADD activation of DA neurons evokes homeostatic changes that should be factored into the interpretation of chronic DREADD applications and their impact on circuit function and behavior. These effects are likely to also be seen in other neuronal systems and underscore the importance of studying neuroadaptive changes with chronic or repeated DREADD activation.

The Impact of Chronic Early Administration of Psychostimulants on Brain Expression of BDNF and Other Neuroplasticity-Relevant Proteins

ABSRACT

Frequently, healthy individuals, children, and students are using stimulants to treat attention deficit hyperactivity disorder (ADHD)-like symptoms or to enhance cognitive capacity, attention and concentration. Methylphenidate, the most common treatment for ADHD, similarly to cocaine, blocks the dopamine reuptake, leading to increase in dopamine level in the synaptic cleft. Brain-derived neurotrophic factor (BDNF) and other neuroplasticity-relevant proteins have a major role in cellular plasticity during development and maturation of the brain. Young Sprague Dawley rats (postnatal days (PND) 14) were treated chronically with either cocaine or methylphenidate. The rats were examined behaviorally and biochemically at several time points (PND 35, 56, 70, and 90). We found age-dependent, but stimulant-independent, alterations in the mRNA expression levels of microtubule-associated protein tau, doublecortin, and synaptophysin. The PND 90 rats, treated with methylphenidate at an early age, exhibited increased BDNF protein levels in the prefrontal cortex compared to the saline-treated group. Despite the treatment effects at the biochemical level, cocaine and methylphenidate treatments at an early age had only minor effects on the behavioral parameters measured at older ages. The biochemical alterations may reflect neuroprotective or neuroplastic effects of chronic methylphenidate treatment at an early age.

Apomorphine-induced context-specific behavioural sensitization is prevented by the D1 antagonist SCH-23390 but potentiated and uncoupled from contextual cues by the D2 antagonist sulpiride

RATIONALE

In the study of behavioural sensitization induced by dopamine agonists, D1 and D2 receptors have a critical, but a puzzling role.

OBJECTIVE

The objective of this study is to examine the effects of the D1 antagonist SCH-23390 and the D2 antagonist sulpiride given repeatedly alone or in combination with apomorphine upon apomorphine conditioning and sensitization.

METHODS

Apomorphine-induced (2.0 mg/kg) conditioning and sensitization were assessed following five paired/unpaired treatments. Sulpiride (10, 30 and 100 mg/kg) and SCH-23390 (0.01, 0.02 and 0.05 mg/kg) were administered alone or in combination with apomorphine. In experiment 1, the effect of 5 days of sulpiride and SCH-23390 treatments given alone were assessed on apomorphine reactivity. In experiment 2, sulpiride and SCH-23390 were co-administered with apomorphine for 5 days and subsequently, conditioning and sensitization tests were performed. In experiment 3, following five apomorphine treatment sessions, sulpiride and SCH-23390 were administered prior to the conditioning and sensitization tests.

RESULTS

SCH-23390 and sulpiride induced hyper-reactivity to apomorphine. SCH-23390 when given after the induction of apomorphine sensitization, blocked the expression of apomorphine sensitization. When given in combination with apomorphine, SCH-23390 blocked the apomorphine conditioning and sensitization, whereas low-dose sulpiride permitted conditioning and enhanced apomorphine sensitization and high-dose sulpiride blocked conditioning but permitted apomorphine sensitization. Both sulpiride doses transformed apomorphine sensitization from context-specific to context-independent sensitization.

CONCLUSION

The SCH-23390 findings are supportive of a critical role for D1 receptors in apomorphine effects whereas the sulpiride effects diminish the importance of conditioning and dopamine autoreceptor subsensitivity mechanisms in the mediation of apomorphine sensitization.

Response disinhibition evoked by the administration of nicotine and nicotine-associated contextual cues

Nicotine causes dose-dependent alterations in accuracy on the differential-reinforcement of low-rate responding (DRL) 29.5-s schedule in rats. The current investigation evaluated whether nicotine-associated contextual cues can produce nicotine-like perturbations in DRL-schedule performance in the absence of nicotine. Nicotine and saline administrations occurred just prior to DRL 29.5-s schedule responding for sucrose solution, and two different experimental contexts (differentiated by visual, olfactory, and tactile cues) were utilized. All subjects (N=16) experienced two consecutive sessions of DRL-schedule responding per day. The experimental group (n=8) was exposed to saline immediately prior to the first session and 0.3mg/kg nicotine before the second session, and the context was changed between sessions. This sequence of saline and then nicotine administration, paired with two reliable contexts, persisted for 12 consecutive days and successive nicotine administrations corresponded with increasingly poorer performance on the DRL 29.5-s schedule. No nicotine was administered for days 13-20 during context testing, and the nicotine-associated context produced response disinhibition on the DRL schedule. Two control groups were included in the design; subjects in one control group (n=4) received saline in each context to verify that the contexts themselves were not exerting control over operant responding. To assess how explicit and non-explicit pairings of nicotine and contextual cues influenced DRL behavior, subjects in a second control group (n=4) were given nicotine prior to the second session, but the contexts were not altered between sessions. The results from this experiment suggest that environmental stimuli associated with nicotine exposure can come to elicit nicotine-induced performance decrements on a DRL 29.5-s schedule.

The contribution of the central nucleus of the amygdala to individual differences in amphetamine-induced hyperactivity

Rats classified as high responders (HR) based on their response to an inescapable novel environment self-administer more amphetamine and have greater amphetamine-induced sensitization than rats classified as low responders (LR). Recent research suggests that the central nucleus of the amygdala (ACe) contributes to the elevated self-administration in HR rats. Therefore, the current study examined the role of the ACe in the expression of both amphetamine-induced sensitization and conditioned hyperactivity in HR and LR rats. Male Sprague-Dawley rats were screened for their response to inescapable novelty and classified as HR or LR rats. Rats were implanted with bilateral cannulae into the ACe and received amphetamine (1.0 mg/kg, s.c.) or saline injections immediately prior to 1-h locomotor sessions. Following five training sessions, all rats received an infusion of muscimol (0.5 microg/0.5 microl) or phosphate buffered saline (PBS) followed by a saline injection to measure conditioned hyperactivity. HR rats displayed conditioned hyperactivity, while LR rats did not, suggesting that HR and LR rats differ in the expression of conditioned hyperactivity. While ACe inactivation attenuated the expression of conditioned hyperactivity, it did not differentially affect HR and LR rats. Following additional training and a 10-day rest period, all rats were then tested for amphetamine-induced sensitization (1.0 mg/kg) following an infusion of muscimol or PBS. Inactivation of the ACe attenuated the expression of sensitization only in HR rats. These results suggest the ACe contributes to the greater amphetamine sensitization in HR rats.

Characterization of the rapid-onset type of behavioral sensitization to amphetamine in mice: role of drug-environment conditioning

A rapid-onset type of behavioral sensitization (ROBS) has been demonstrated in rats treated with a single 'priming' injection of amphetamine (AMP). In that species, however, this phenomenon was restricted to AMP-induced stereotyped behavior (SB), not occurring for the locomotor-stimulant effect (LSE) of AMP and not reflecting environment-specific sensitization. In the present study, the ROBS was characterized in the mouse. Mice received a single 'priming' intraperitoneal injection of 5.0 mg/kg AMP which was paired or not with environment. At different intervals (3, 4 or 5 h) subgroups were tested for AMP (1.5 or 5.0 mg/kg)-induced SB or AMP (1.5 mg/kg)-induced open-field LSE. Results showed that: (1) in the absence of drug-environment association, a priming injection of AMP increased the SB induced by a 1.5 mg/kg AMP challenge injection given 3 h (but not 4 or 5 h) later; (2) when the dose of AMP challenge injection was increased to 5.0 mg/kg, an enhancement of SB was verified at all the intervals tested (3, 4, and 5 h); (3) when animals were tested in an open field, the priming injection of AMP produced an increase in the LSE of a 1.5 mg/kg AMP challenge injection, given 4 h later; (4) drug-environment association increased both SB and locomotion after a saline challenge injection and potentiated the rapid-onset sensitization of both behaviors in AMP-challenged mice. Collectively, these results demonstrate that the ROBS phenomenon also occurs in mice, is extended to AMP-induced LSE, and is markedly potentiated by (but does not depend on) environmental conditioning.

State-dependent action of cocaine on brain temperature and movement activity: implications for movement sensitization

Because neural activity is highly energy consuming and heat producing, brain temperature offers a reliable, real-time measure of an animal's activity state and its changes induced by environmental and drug challenges. Therefore, it allows evaluation of the activity state of an animal preceding drug administration and its relation to subsequent drug-induced neural effects. This approach was used to explore the state dependency of cocaine's effects. Brain and body temperatures, as well as locomotion were measured simultaneously in rats during repeated, daily administration of cocaine (15 mg/kg i.p., daily for 5 days) under different experimental conditions. The drug was administered via (a). a chronically implanted catheter in quiet resting conditions, (b). an injection made under quiet rest or (c). an injection under activated conditions associated with placement in the cage. Although brain temperature and movement increased after cocaine administration in each condition, cocaine's action (evaluated as cocaine-saline difference for both parameters) was situational. Catheter-administered cocaine induced the strongest movement activation and robust, monophasic temperature increase, which remained relatively stable following each subsequent drug infusion. Cocaine injected during quiet and, especially, activated conditions, induced a weaker locomotor activation, while the temperature response (evaluated as drug-saline difference) had a biphasic pattern. Cocaine initially inhibited the temperature increases seen in saline-treated animals (0-20 min) and then induced a more prolonged hyperthermia, which was about twofold weaker than that seen after catheter-administered drug. Although movement activation gradually increased following repeated treatment in activated conditions, the magnitude of this sensitized motor response barely reached the levels induced by the initial cocaine administration via catheter. These data suggest that both the acute effects of cocaine in the brain and their change following repeated drug administration are dependent upon the ongoing neural activity state of the animal. Cocaine's interaction with this activity state is a crucial factor determining the behavioral effects of this drug, including state-dependent motor sensitization.

Intravenous self-administration of abused solvents and anesthetics in mice

Volatile organic solvents, fuels and anesthetics are subject to abuse. The aim of the present study was to evaluate i.v. self-administration of several of these chemicals in drug- and experiment-naive mice using a commercially available vehicle, intralipid. Two strains of mice (DBA/2 and Swiss) were allowed to self-administer toluene (0.0017-0.17 micromol/infusion), 1,1,1-trichloroethane (0.006-0.19 micromol/infusion), ethanol (0.32-1.6 micromol/infusion), cyclohexane (0.0017-0.052 micromol/infusion), propofol (0.01-0.53 micromol/infusion) and flurothyl (0.00042-0.072 micromol/infusion) or their vehicles during 30-min tests. During the test, each nose-poke of the master mouse resulted in a 1.88-microl i.v. infusion to the master mouse and a yoked control mouse. When the delivery line was loaded with a reinforcing drug solution, the number of nose-pokes of the master mice significantly exceeded that for yoked control mice. In the present experiments, significant differences in rates of nose-poking were observed between mice receiving response-contingent and response-noncontingent deliveries of ethanol and toluene in both strains of mice and of 1,1,1-trichloroethane in Swiss mice. These data suggest that the reinforcing effects of abused inhalants can be studied using i.v. self-administration procedures.

Quasi-asymptotic development of conditioned hyperactivity induced by intermittent injections of cocaine in C57BL/6J mice

The emergence of a conditioned cocaine-induced hyperlocomotion was examined in C57BL/6J mice using a procedure that has not been used previously. Two days after a session of preexposure to the test chambers under saline, a first group of mice (cocaine-cued) received five once-daily injections of 10-mg/kg s.c. cocaine every other day (on the odd days of the chronic treatment period) and a saline injection on the 5 days following each cocaine injection day (on the even days of the treatment period), in all cases before being placed in the test chamber. Another group of mice (saline-cued) received 10 injections of saline on both the even and the odd days in the same context, and a third group of mice (cocaine-uncued) received five injections of saline on the even days in the test context and five injections of cocaine on the odd days in an alternative context. On the odd days sessions, the cocaine-cued group showed significant repeated increases in locomotion without behavioural sensitisation being induced, whereas the saline-cued levels of locomotion remained on baseline levels. On the first even session, the three groups did not differ from each other and showed lower levels of locomotion than on the preexposure session. During the two following even sessions, the cocaine-cued group showed an increase in locomotion that levelled off on the two remaining sessions, whereas the saline-cued and the cocaine-uncued groups (which presented comparable values) exhibited significantly lower levels of locomotion. That pattern of successive placebo responses resembles the typical S-shaped development of a Pavlovian conditioned response, albeit the increase described here was quite rapid. The protocol used here may provide an additional method for the experimental analysis of stimulant-induced conditioned placebo activity.

Intravenous cocaine-induced activity in A/J and C57BL/6J mice: behavioral sensitization and conditioned activity

The stimulant properties of cocaine have been extensively investigated in the mouse using either intraperitoneal (i.p.) or subcutaneous (s.c.) administration of drug. However, cocaine use in humans often involves intravenous (i.v.) administration of drug. The purpose of this study was to develop a methodology for studying i.v. cocaine-induced activity in the mouse, which allows within-session determination of the dose-response function, and assessment of the development of behavioral sensitization and conditioned activity. The stimulant effects of i.v. cocaine (3-25 mg/kg) were investigated in C57BL/6J and A/J mice both acutely and following repeated treatments (four treatments at 48 hour intervals), in addition to the conditioned activating properties of the cocaine-paired context. Cocaine produced a dose-dependent increase in measures of motor activity in both strains of mice. Repeated cocaine treatments resulted in the development of behavioral sensitization to the stimulant properties of the drug at all doses tested, and exposure to the cocaine-paired context in the absence of drug revealed the development of conditioned activity. While both C57BL/6J and A/J strains displayed these phenomena, differences were observed between ambulation and total beam breaks, highlighting differences between multiple behavioral end-points. Both strains of mice displayed conditioned activity of a higher magnitude than their response to novelty, in addition to a positive relationship between the number of drug-environment pairings and the magnitude of the conditioned response. In summary, these data extend to the i.v. route of administration previous observations on cocaine-induced activity and conditioned activity.

Alterations in behaviour and glutamate transmission following presentation of stimuli previously associated with cocaine exposure

To study the role of glutamate in cocaine-conditioned responses, we developed a rat model in which conditioned locomotion is produced by repeated pairing of cocaine with discrete stimuli (flashing light and metronome). "Paired" subjects received cocaine (15 mg/kg) prior to six exposures to stimuli for 30 min in the test environment. "Unpaired" subjects received equivalent presentations of the stimuli yet received cocaine in home cages. Tests with the stimuli alone demonstrated that the conditioned locomotion displayed by Paired subjects was evident at 3 or 10 days post-training and resistant to two sessions of testing. The degree of conditioned locomotion was not correlated with the subjects' response to novelty or cocaine. Administration of the noncompetitive AMPA receptor antagonist GYKI 52466 (2.5 mg/kg, a dose without effect on spontaneous activity) attenuated the expression of conditioned activity. In vivo microdialysis revealed that Paired subjects had significantly lower basal glutamate levels in the nucleus accumbens (NAc) than did Unpaired subjects when no stimuli were presented. Presentation of the conditioned stimuli resulted in significant increases in glutamate levels in the NAc in the Paired group whilst glutamate levels in the Unpaired group remained unchanged. The associative control of glutamate levels in the NAc by stimuli formerly paired with a drug of abuse is an unprecedented finding. It is likely to reflect the convergence of excitatory inputs that the NAc receives from limbic structures.

Effects of repeated GBR 12909 administration on brain stimulation reward

Male rats were trained at three separate currents to bar press for intracranial self-stimulation. On days 1 and 15, all subjects were given 1-(2-bis(4-fluorophenyl)-methoxy)-ethyl-4-(3-phenylpropyl) piperazine, also known as GBR 12909 (10 mg/kg, i.p.), prior to test session. Between these days, the paired Chronic-before group was injected (every other day) with GBR 12909 prior to intracranial self-stimulation, while unpaired, Chronic-after group was given the drug just after the end of the session. A third group (Control) received saline injections (i.p.) 20 min following the session. Although GBR 12909 was found to be reward enhancing, neither sensitization nor tolerance developed to the rewarding and performance/motor effects regardless of the injection regimen. In addition, the rewarding effects of intracranial self-stimulation were found to be independent of both current and environment-specific pairing. The present data obtained for GBR 12909 agree with previous observations of the effects of repeated administration of drugs of abuse on intracranial self-stimulation.

Context determines the type of sensitized behaviour: a brief review and a hypothesis on the role of environment in behavioural sensitization

Behavioural sensitization to psychostimulants may develop context-dependency in certain circumstances. Animals given a stimulant repeatedly in a test cage but not in other environments may show enhanced drug-induced behaviour in the test cage. Conditioning mechanisms have been claimed to be responsible for these phenomena. However, several recent findings are not properly accounted for by conditioning. In addition, growing evidence supports the hypothesis that behavioural sensitization reflects neural changes induced by repeated exposure to psychostimulants (the pharmacological hypothesis). However, the pharmacological hypothesis itself fails to account for environmental influences. In this paper, we propose a hypothesis on the role of environment that is complementary to the pharmacological hypothesis. According to our hypothesis, environment does not have a causal role in the development of sensitization, but it modifies the mode of expression of the sensitized behaviour. Sensitization primarily reflects a neuroadaptive change induced by repeated exposure of the neural system to psychostimulants. However, psychostimulants are known to induce different behaviours in different environments. Therefore, repeated administration of a psychostimulant in different environments would result in augmentation of different behaviours. Our hypothesis potentially accommodates various previous observations. We briefly review the literature and present our hypothesis.

Repeated treatments with 7-OH-DPAT: context-independent behavioral sensitization and conditioned hyperactivity

The primary objective of this study was to determine whether the expression of behavioral sensitization to the putative dopamine D3 receptor agonist 7-OH-DPAT is context dependent. Three groups (n = 8 each) of male Wistar rats (250-350 g) were given nine injections (at 48-h intervals) of 7-OH-DPAT (1.0 mg/kg, SC) or vehicle 15 min before and after activity testing. The paired group received 7-OH-DPAT before activity testing and vehicle after testing. The unpaired group received vehicle before and 7-OH-DPAT after testing, and the vehicle control group received two vehicle injections. Locomotor activity was measured in photocell arenas for 2 h. After the first seven sessions, all rats were tested for activity following a vehicle injection to test for possible conditioning effects. Prior to the 11th session, all rats were given a challenge injection of 7-OH-DPAT (1.0 mg/kg, SC) to test for sensitization. Major findings were as follows: (a) the 7-OH-DPAT/paired group displayed a progressively greater increase in locomotor activity with repeated treatments; (b) the 7-OH-DPAT/paired group was significantly more active than either the vehicle control group or the 7-OH-DPAT/unpaired group during the vehicle test session; and (c) after the 7-OH-DPAT challenge injection, the paired and unpaired 7-OH-DPAT groups were significantly, and equally, more active than the vehicle control group. In contrast to previous findings with the D2-type dopamine agonists bromocriptine and quinpirole, these results suggest that the expression of behavioral sensitization to 7-OH-DPAT is not context dependent. Moreover, these results suggest that the apparent conditioned hyperactivity and context dependency often observed after repeated dopamine agonist treatments may not be related to the same associative and/or nonassociative mechanisms.

Ibogaine enhances the expression of locomotor sensitization in rats chronically treated with cocaine

Pretreatment (19 h) with the putative antiaddictive agent, ibogaine, has been shown previously to potentiate cocaine-induced locomotion in rats. The present study demonstrates that the magnitude of this effect of ibogaine is dependent on the previous cocaine history of the animal, on the time following ibogaine treatment, and on the number of ibogaine treatments. Compared to rats with no previous cocaine experience, ibogaine pretreatment (40 mg/kg, IP, 19 h earlier) markedly enhanced the expression of locomotor sensitization in response to a cocaine challenge injection (7.5 mg/kg) in rats that were chronically treated with cocaine (15 mg/ kg, IP, daily for 5 days). Tolerance to cocaine-induced locomotor sensitization appeared to occur in vehicle-pretreated chronic cocaine controls. Following a second series of identical treatments (beginning 3-4 days after the initial treatment series), locomotor responding to the cocaine challenge was further enhanced by a second ibogaine injection in chronically cocaine-treated animals. Twenty-four hours later, when animals were challenged again with cocaine in the absence of any further ibogaine pretreatment, the effect of ibogaine had dissipated. Consistent with previous studies from this laboratory, these data demonstrate that ibogaine can enhance sensitivity to the psychomotor stimulant effect of cocaine. The results of the present study further indicate that the extent of this effect depends on the animal's history of exposure to both ibogaine and cocaine.

Drug conditioning induced by intrastriatal apomorphine administration

The present study examined (1) whether the neostriatum is involved in a drug-induced conditioned locomotor response and; (2) whether this structure participates in the development of behavioral sensitization. Moreover, the present study addressed the question whether the development of behavioral sensitization is necessary for the induction of conditioning. Rats received injections of either apomorphine (2 microg) or vehicle (solution of 0.1% ascorbate/saline) into the dorsal neostriatum daily for 7 days. These treatments were performed immediately prior to (apomorphine-paired group and vehicle group) or 30 min following (apomorphine-unpaired group) 10-min placement in an open field which served as the test environment. After a 3-day drug withdrawal period, the animals were given a 10-min non-drug vehicle test trial in the test environment. Three days later, a drug test with apomorphine was administered to the animals of the paired and unpaired treatment groups; the vehicle group again received an injection of vehicle. The analysis of locomotor activity in the open field (measured as the distance traversed) revealed that locomotor activity in the apomorphine-paired group was higher than in the other groups. There were no indications for behavioral sensitization to intrastriatal apomorphine, since the locomotor response in the apomorphine-paired group did not increase, but rather decreased with daily repeated injections of apomorphine. Furthermore, only the apomorphine-paired animals showed a higher locomotor response when tested after an intrastriatal injection of vehicle in the previously apomorphine-paired environment, which is indicative of a conditioned drug effect. These results suggest that the neostriatum is directly involved in the development of drug-induced conditioning of locomotor behavior but not in the establishment of behavioral sensitization.

Cocaine conditioning and cocaine sensitization: what is the relationship?

With repeated cocaine use, cocaine conditioned behavior develops to associated stimuli, and in addition, sensitization can occur to the unconditioned stimulant effects of cocaine. To investigate the relationship between the conditioned and unconditioned behavioral effects of repeated cocaine use, two groups of rats (n = 7) were given five daily paired cocaine treatments (10 mg/kg i.p.) immediately before a 20-min placement in an open-field environment. Other groups received either saline before testing or cocaine unpaired which was administered 30 min after testing in the homecage. When tested in the open-field with saline for conditioned effects, the two cocaine paired groups selectively exhibited substantial and equivalent cocaine conditioned responses. One of these groups was subjected to an extinction procedure which was effective in eliminating the cocaine conditioned responses. Subsequently, all the rats which had received cocaine in the first phase of the experiment, paired and unpaired, along with a subset of saline animals were given a cocaine (10 mg/kg i.p.) challenge test. The paired cocaine animals exhibited an earlier onset of the cocaine induced behavioral response (sensitization) as compared with the saline and the unpaired cocaine animals. Critically, the sensitization effects were unaffected by extinction, and in addition, the conditioned response did not contribute to the sensitization effect. It is suggested that the cocaine drug response occludes the cocaine conditioned response. Subsequent to this sensitization test, the animals were retested for conditioning. In this test, the paired cocaine animals which had not been subjected to the extinction procedure still exhibited a conditioned cocaine response, whereas, the paired cocaine group subjected to extinction was indistinguishable from saline controls. Although the present results show that Pavlovian conditioned responses to exteroceptive contextual cues do not directly contribute to cocaine induced behavioral sensitization effects, the sensitization effects were context-specific, and therefore, were tied to associative processes. It is suggested that context specificity is mediated by a compound stimulus complex comprised of exteroceptive stimuli and interoceptive cocaine drug cues. Furthermore, these exteroceptive and interoceptive cues associated with cocaine effectively expedite the onset of cocaine effects, and thereby, facilitate the addictive potency of cocaine.

Amphetamine-induced conditioned activity does not result from a failure of rats to habituate to novelty

Psychostimulant-induced conditioned activity is characterized by the presence of a hyperactivity in drug-free rats exposed to an environment previously paired with the effects of a psychostimulant. According to the habituation hypothesis, conditioned activity arises not through a Pavlovian conditioning process but rather because rats under the effects of the psychostimulant would be unable to habituate normally to the environment paired with these effects. This hypothesis predicts that conditioned activity should not develop in a previously habituated environment. This prediction was tested using a within-subject design. In this design, conditioned activity is evidenced when a group of rats, following a vehicle injection, was more active in a previously amphetamine-paired environment than in a previously vehicle-paired environment. The drug-environment pairing involved administering rats with d-amphetamine (1.25 mg/kg; SC) immediately prior to their placement in one of two distinctive environments. On alternate days, the rats received the vehicle and were placed in the other environment. With this design, it was found that: a) conditioned activity developed in a previously habituated environment; b) its magnitude was independent of the number of amphetamine-environment pairings (two, four or eight pairings); c) this development of conditioned activity did not result from a forgetting of the habituated environment due to a state-dependent retention of the habituation. Taken together, these results do not support the habituation hypothesis of psychostimulant-induced conditioned activity.

A new methodological approach to the study of habituation: the use of positive and negative behavioral indices of habituation

This report details a new method to measure habituation in an open-field. In addition to the measurement of spontaneous locomotor activity, time spent per visit to the center zone (CZ) is also measured. Critically, a small object is placed in the CZ to modulate an animal's investigatory response. When an animal is first exposed to the open-field, the presence of the object does not affect the duration of its visits to the CZ but, if the animal is given one additional 10-min exposure to the open-field, then the presence of the object substantially increases the duration of its visits to the CZ. The presence of the object, however, has no effect on the rat's locomotor activity. Thus, habituation could be observed by two different measures: a decrease in locomotor activity and an increase in an animal's investigatory response to a stimulus object. A basic problem with a reliance solely upon a decrease in locomotor activity to measure habituation is that it represents a negative change in behavior. The present method circumvents this shortcoming by incorporating a positive behavioral measure of habituation in conjunction with the measurement of locomotor activity. This modification of the open-field test offers substantial utility for studies of neurotoxicology and memory because one can assess concurrently treatment effects on motor activity, attention to an object and memory.

Behavioral sensitization and tolerance to cocaine and the occupation of dopamine receptors by dopamine

Data from the authors' laboratory on the neural substrates of Pavlovian conditioning and behavioral sensitization to psychomotor stimulants are reviewed. The findings of a recent experiment on the role of occupation of dopamine receptors by dopamine and its association to behavioral sensitization are reported. Daily intermittent injections of cocaine produced behavioral sensitization to the locomotor response in rats, whereas continuous cocaine infusions produced behavioral tolerance. Behavioral sensitization to cocaine was blocked by coadministration of nimodipine, an L-type calcium channel blocker. The increase in locomotion produced by cocaine was associated with an increase in the occupation of striatal dopamine D1 and D2 receptors, measured as the density of receptors protected from denaturation by N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ). This association was not observed when rats were given a challenge injection of cocaine 10 d after withdrawal from similar treatment regimens. Rats given a cocaine challenge after withdrawal from either intermittent or continuous cocaine treatments regimens exhibited increased occupation of striatal D1 and D2 receptors. This increase was similar in magnitude to that observed in rats without a history of cocaine treatments after a challenge injection of cocaine. This suggests that the differences in occupancy of striatal dopamine receptors by dopamine observed in the prewithdrawal condition are likely the results of differences in brain levels of cocaine achieved by the two treatment regimens. Occupancy of striatal dopamine D1 and D2 receptors does not appear to be related to the development of sensitization to the motor-stimulating effects of cocaine.

Evidence for N-methyl-D-aspartate receptor mediation of cocaine induced corticosterone release and cocaine conditioned stimulant effects

The role of the N-methyl-D-aspartate (NMDA) receptors in cocaine conditioning and sensitization of locomotor activity was studied in four groups of Sprague-Dawley rats. A sub-motoric dose of the NMDA antagonist MK-801 (0.1 mg/kg, i.p.) was employed using a novel dual-compartment Pavlovian drug conditioning paradigm. The animals were placed sequentially in two different test environments in which locomotor activity was monitored. In the first compartment, the animals always received a non-drug test for 20 min. Upon completion of this test, the animals received either saline, cocaine (10 mg/kg i.p.), MK-801 or MK-801 plus cocaine depending on group assignment and were then placed immediately into the second compartment and again tested for 20 min. A total of six non-drug and six drug tests were conducted every other day over a 12-day period. Across all drug/saline treatment and post-treatment tests for conditioning, there were no statistical differences in locomotor activity among the saline and drug treatment groups in the non-drug test environment. In the drug/saline associated environment, however, cocaine had a reliable stimulant effect on locomotion when administered alone or in combination with MK-801. Following a 1-day and again after 21-days of withdrawal, all animals were administered a non-drug test for conditioning in which no injections were administered. On both tests, all groups had equivalent activity levels in the non-drug environment. In the drug/saline environment, only the cocaine group of the three drug treatment groups exhibited conditioned hyperlocomotion. Importantly, MK-801 blocked conditioned hyperlocomotion in the combined cocaine+MK-801 group. MK-801 did not alter serum or brain cocaine concentration or the cocaine effects on dopamine metabolism in limbic brain tissue. The co-administration of MK-801 with cocaine, however, blocked the corticosterone release effect of cocaine. Thus, the NMDA receptor site appears critical for cocaine induced conditioning and for corticosterone release.

A new method to assess Pavlovian conditioning of psychostimulant drug effects

Experimental studies of psychoactive drugs by pavlovian drug-conditioning methods, which originally began with investigations of drug-induced responses mediated by the autonomic nervous system, have now been expanded to include drug-induced response effects expressed as modulations of spontaneous motoric behaviors. In the latter application, however, equivalent behavioral response outcomes in post-treatment tests for conditioning can occur following a psychostimulant drug treatment either through drug interference effects on habituation processes, drug-induced stress effects and/or by pavlovian conditioning of the drug-induced motoric activation effect. Current methodologies for the study of pavlovian conditioned drug effects and/or drug sensitization cannot distinguish among these possibilities. This methodological inadequacy was addressed by a modification of the conventional paired-unpaired treatment protocol. In the new protocol, the animal is sequentially placed into two test compartments with the drug treatment administered in conjunction with placement into the second test compartment. This design permits a differentiation of a pavlovian conditioned drug responses from non-conditioned drug effects through continuous measurement of the non-drug behavioral baseline in both the drug and non-drug control treatment groups combined with multiple response measurements and post-treatment tests for conditioning at variable post-conditioning intervals. The present study details the use of the new modified pavlovian protocol with repeated cocaine (10 mg/kg) treatment. A cocaine conditioned response at 1, 7, and 21 days post-conditioning was identified and distinguished from habituation and stress effects.

Conditioned cocaine induced hyperactivity: an association with increased medial prefrontal cortex serotonin

Two sets of contextual stimuli, only one of which was associated with drug treatment, were employed in a new paradigm to assess the conditioned effects of ten cocaine (20 mg/kg) treatments on motoric behavior. Two matched groups of Sprague-Dawley rats were sequentially placed into two similar but distinct test compartments. The drug treatment was administered immediately prior to placement into the second test compartment in one group (paired group) but 2 h later in the other group (unpaired group). Changes in the non drug behavioral baseline were assessed in the non drug compartment immediately prior to each drug treatment and provided the referent baseline for determining stimulus specificity of the observed behavioral effects in the drug compartment. Both groups had equivalent activity levels in the non-drug compartment. In the drug compartment, however, the paired group showed an overall cocaine stimulant effect expressed as a marked increase in distance traversed in the drug box. Additionally, the repeated cocaine treatment induced a conditioned drug effect as revealed by the higher level of motor activity in the paired compared to the unpaired treatment group exclusively in the drug compartment in two post-treatment non-drug tests for conditioning after 1-day and 21-day withdrawal and non-testing intervals. The biochemical assay results showed no significant group differences in DA and dopamine metabolites, DOPAC, HVA and 3-MT, in striatal, limbic or cortical tissue. No differences were observed either in NE or plasma corticosterone levels. Importantly, the analysis for serotonin revealed higher levels of 5-HT and 5-HIAA selectively in the prefrontal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential blockade of chronic versus acute effects of intravenous cocaine by dopamine receptor antagonists

The objectives of this study were to investigate behavioral sensitization to repeated once daily IV injections of cocaine, and to determine whether dopamine receptor antagonists differentially block chronic versus acute cocaine effects. Acute cocaine (0.3-3.0 mg/kg) produced a dose-dependent increase in both horizontal and stereotypic movements in male Sprague-Dawley rats. Repeated once daily injections of 1 or 3 mg/kg of cocaine augmented these effects. Pretreatment with either the D2 dopamine receptor antagonist haloperidol (0.03-0.3 mg/kg) or the D1 dopamine receptor antagonist R(+)-SCH-23390 (0.003-0.1 mg/kg) dose dependently attenuated cocaine's behavioral effects in both sensitized and cocaine-naive animals. There was a rightward shift in the dose-effect relationship of these antagonists in blocking the expression of behavioral sensitization as compared to their ability to block the acute behavioral effects of cocaine. These results indicate that repeated once daily IV injections of cocaine produced behavioral sensitization and both D1 and D2 dopamine receptor antagonists attenuated the expression of this sensitization. The data also suggest that dopamine receptor antagonists were more potent in blocking cocaine's effects in cocaine-naive animals than in cocaine-sensitized animals.

Effects of nimodipine and/or haloperidol on the expression of conditioned locomotion and sensitization to cocaine in rats

The development of classical conditioning of cocaine's locomotor effects can be dissociated from the development of sensitization to cocaine by co-administration of haloperidol, a dopamine D2-like receptor antagonist, and nimodipine, an L-type calcium channel antagonist. The effects of these agents on the expression of conditioning and sensitization are described in the present report. Rats were given injections of vehicle or cocaine (10 mg/kg, IP) for 10 days before placement in a specific context in which locomotor activity was recorded. Neither haloperidol (0.05 mg/kg, IP) nor nimodipine (10 mg/kg, SC) influenced the expression of classical conditioning of cocaine's locomotor effects to the situational context on a subsequent cocaine-free test. Combined treatment of rats with both drugs did block classical conditioning with cocaine. Nimodipine, but not haloperidol, blocked the expression of behavioural sensitization to cocaine after a cocaine challenge. It is concluded that the expression of cocaine-induced classical conditioning can be pharmacologically dissociated from the expression of behavioural sensitization to cocaine. Furthermore, the effects of nimodipine and haloperidol on the expression of conditioning and sensitization are different from their effects on the development of these phenomena.

Selective antagonism of dopamine D1 and D2 receptors does not block the development of behavioral sensitization to cocaine

The objective of this study was to determine whether the development of behavioral sensitization to cocaine could be prevented by either D1 or D2 selective dopamine receptor antagonists. Male Wistar rats were treated daily for 7 days with either cocaine (15 mg/kg, IP) or vehicle in combination with the D1 dopamine antagonist SCH 23390 (0.3 mg/kg, SC), the D2 dopamine antagonist sulpiride (100 mg/kg, IP), or vehicle. After the daily injections, the rats were tested for locomotor activity in photocell arenas. Twenty-four hours after the last pre-exposure test session, all rats were given a challenge injection of cocaine (15 mg/kg, IP) and tested for activity. Cocaine treatments produced a greater relative increase in locomotor activity with repeated exposure (i.e. sensitization). Moreover, this increase in cocaine-induced locomotor activity was attenuated by both SCH 23390 and sulpiride. In contrast, neither sulpiride nor SCH 23390 blocked the development of behavioral sensitization to cocaine. That is, rats pretreated with sulpiride or SCH 23390 and cocaine did not differ from rats pre-exposed only to cocaine when given a cocaine challenge injection. These results suggest that behavioral sensitization to cocaine may develop through either D1 or D2 dopamine receptor stimulation or possibly through stimulation of some non-dopaminergic receptor.

Cocaine fatalities increased by restraint stress

Laboratory rats injected daily with a moderate dose of cocaine hydrochloride (30 mg/kg, i.p.) showed increased fatalities when cocaine injections were followed by 30 min of restraint stress. The 5-day mortality rate was 58% for the cocaine-plus-stress group, while 17% of the animals receiving cocaine without restraint stress died. This finding suggests that stress can augment the toxic effect of cocaine and that minimizing stress may be an important consideration in the clinical management of cocaine overdose.

Apomorphine sensitization effects: evidence for environmentally contingent behavioral reorganization processes

Apomorphine-induced behavioral sensitization was investigated with a Pavlovian conditioning paradigm. Rats were administered apomorphine (2.0 mg/kg SC) daily for 7 days either paired or unpaired with a 10-min test environment placement. Initially, apomorphine induced hypolocomotion, but by treatment day 5, hyperlocomotion developed. Utilizing a videoimage analysis program which quantitated angular movement, it was determined that the increase in locomotion induced by repeated apomorphine treatment was due to an increase in rotational locomotion. Critically, rotation per se did not increase, but rather wide angle rotation toward the periphery of the test environment increased. Furthermore, a directional bias of rotation developed and stabilized which was unrelated to the animal's initial asymmetry bias. This emergence of a new locomotion pattern in conjunction with hyperlocomotion pointed to the need to reconceptualize behavioral sensitization phenomena into a new framework consistent with a progressive change in behavioral structure. Behavioral reorganization is presented as an alternative formulation to that of behavioral sensitization, as a drug-environment interactive process which is more compatible with the behavioral dynamics that emerge with repeated intermittent dopaminergic psychostimulant drug treatment.

Activation of an apomorphine behavior pattern by cocaine: a new behavioral test for drug substitution

In current tests for drug substitution using the 2-choice operant bar press paradigm, an animal can be trained to respond differentially between an interoceptive drug state stimulus vs. saline. Despite the long history and extensive number of investigations, the interpretation of such test results, nevertheless, is limited by number of potential sources of stimulus drug effects which can occur with repeated drug administration but which cannot be distinguished by this paradigm: (1) local effects at the injection site; (2) peripheral effects; (3) central activity effects; and/or (4) sensory-motor dysfunctional effects. By using a video-image analysis of behavior in an intact animal model and a pavlovian drug treatment protocol, new, spontaneous but environmentally contingent and stable behavior patterns emerge selectively in the paired treatment group with repeated drug administration which can be used as a behavioral marker to assess dose-response effects and drug substitution test results with a novel drug. A study with repeated administration of the DA agonist apomorphine and with a drug substitution assessment by a cocaine drug test is presented as a reference example to illustrate the new response-based methodological approach to the study of central mechanisms mediating the stimulus and response effects of drugs. Additionally, the observed efficacy of cocaine to activate a complex acquired behavioral pattern unique to repeated apomorphine treatment supports a critical role for dopaminergic mechanisms in the mediation of cocaine induced stereotypy.

Opponent-process theory and drug conditioning: an assessment for conditioned stimulant-induced movement

Opponent-process theory occupies an important place in drug conditioning because it accounts for conditioned drug effects which are opposite to those induced by the drug itself. It has not been established, however, whether there is an opponent-process component to stimulant drug induced conditioned effects. In the present study the unilateral 6-hydroxydopamine (6-OHDA) rat model was used to examine this issue. Two groups of Sprague-Dawley rats with equivalent 6-OHDA lesions were administered five apomorphine treatments (0.05 mg/kg s.c.) either paired or unpaired to a 10-min test chamber placement. Apomorphine induced vigorous contralateral rotation and suppressed all ipsilateral rotation. While the apomorphine-induced contralateral rotation response can be conditioned to the test environment cues, the critical test of opponent-process theory in the present study was whether the opposite response of ipsilateral rotation would also become conditioned as a latent opponent-process response to the exteroceptive test environment cues associated with the apomorphine drug state. The postacquisition saline test for conditioning showed that the paired group exhibited higher rates of contralateral and ipsilateral rotation compared to the unpaired group. In addition, when the animals were subsequently tested with the dopaminergic receptor antagonist, haloperidol (0.5 mg/kg), unexpectedly, contralateral rotation was enhanced in the paired group, whereas, ipsilateral rotation was suppressed in both groups. While these findings are, in part, compatible with an opponent-process mechanism, the data supported a simpler explanation; namely, the mechanism of differential habituation in the two groups due to a blocking effect of apomorphine on habituation selectively in the paired group.(ABSTRACT TRUNCATED AT 250 WORDS)