Pavlovian conditioning between co-administered drugs: elicitation of an apomorphine-induced antiparkinsonian response by scopolamine

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.

Stimulus gated cocaine sensitization: Interoceptive drug cue control of cocaine locomotor sensitization

Repeated cocaine treatments typically generate sensitization effects which are environment specific. In this study, we investigated whether drug treatments with highly selective receptor specificity can also function as contextual cues to control the expression of cocaine sensitization effects. Two experiments were conducted in which separate groups of rats (N=10) received ten paired or unpaired cocaine (10.0 mg/kg) treatments. In the experiments, autoreceptor preferring low doses of either the 5-HT1A agonist, 8-OHDPAT (8OH) (0.05 mg/kg) or the D1/D2 agonist apomorphine (APO) (0.05 mg/kg) were administered 20 min prior to cocaine administration and test environment placement (paired treatment). Under these conditions, the drug cues generated by the 8OH/APO treatments were associated with the cocaine stimulant effect in the test environment. The unpaired treatment groups received the same drug treatments but the cocaine was administered after testing, in the homecage. Consequently, for these groups, the 8OH/APO drug cues generated by the drug treatments would not become associated with the cocaine stimulant effect in the test environment. Critically, both 8OH and APO pretreatments elicited equivalent unconditioned response effects which were opposite to the cocaine unconditioned response effects; that is, behavioral inhibition vs. behavioral stimulation. Initially, the 8OH and APO pretreatments prevented the locomotor stimulant effects of cocaine; but, these inhibitory effects were reversed in the paired groups with repeated cocaine treatments, consistent with the emergence of cocaine sensitization effects. In the unpaired 8OH and APO pretreatment groups, behavioral suppression persisted throughout the treatment protocol. Subsequently, paired and unpaired groups were compared in four conditioning/sensitization tests. The conditioning tests included: a saline/saline test; and a 8OH/saline test (Experiment 1); and, a saline/saline test and a APO/saline test (Experiment 2). There were no paired/unpaired group differences in these conditioning tests. The sensitization tests included: a saline/cocaine test; and a 8OH/cocaine test (Experiment 1); and, a saline/cocaine test and a APO/cocaine test (Experiment 2). There were no paired/unpaired group differences in the saline/cocaine test for sensitization but paired/unpaired group differences were found in both the 8OH/cocaine and APO/cocaine sensitization tests. In these tests the paired but not the unpaired groups exhibited cocaine locomotor sensitization effects. Critically when, in an additional test, the pretreatments in the cocaine tests were reversed (i.e., 8OH paired group received APO and APO paired group received 8OH prior to cocaine), then there was no evidence for cocaine sensitization. Since the 8OH/APO pretreatments had equivalent inhibitory response effects, it was the stimulus properties of these drugs which controlled the expression of the cocaine locomotor sensitization effects. These findings support the critical role of associative processes in the stimulus-gating of psychostimulant drug sensitization. Importantly, this report incorporates a new methodology in which context can be specified in terms of highly specific brain receptor targets rather than in terms of global environmental situational cues.

Evidence that the 5-HT1A autoreceptor is an important pharmacological target for the modulation of cocaine behavioral stimulant effects

The psychostimulant effects of cocaine critically depend on the serotonergic (5-HT) system, of which the 5-HT1A receptor is an essential component. We recently showed divergent contributions of various pre- and postsynaptic 5-HT1A receptor populations to the behavioral effects of cocaine. Here, we further investigate the role of 5-HT1A autoreceptors in the acute and chronic stimulant effects of cocaine using 5-HT1A receptor ligands in autoreceptor preferring doses. In experiment 1, four groups of rats (N = 10) received either saline or the 5-HT1A agonist, 8-OHDPAT (0.05 mg/kg) 20 min prior to a saline or cocaine (10 mg/kg) injection on 9 consecutive days. In experiment 2, six groups (N = 10) were given either saline, the 5-HT1A antagonist, WAY 100635 (0.05 mg/kg) or 8-OHDPAT (0.05 mg/kg) plus WAY 100635 (0.05 mg/kg) 20 min before a saline or cocaine (10.0 mg/kg) treatment on 9 consecutive days. Initially, both the 8-OHDPAT and WAY 100635 pretreatments completely blocked the locomotor stimulant effects of cocaine whereas the combined 8-OHDPAT plus WAY 100635 pretreatment had no effect. In saline treated groups, neither the WAY 100635 nor the 8-OHDPAT plus WAY 100635 pretreatment influenced spontaneous activity levels, whereas the 8-OHDPAT alone severely reduced spontaneous activity. These effects persisted over the course of the 9 test sessions. A different pattern of results was obtained for the cocaine treatment groups. With repeated treatments, the WAY 100635 treatment always blocked the locomotor activation effect of cocaine, whereas the effects of 8-OHDPAT were transformed from an inhibition to an enhancement of cocaine locomotor stimulation. The combined 8-OHDPAT plus WAY 100635 pretreatment did not affect the stimulant effect of cocaine. These findings demonstrate that low dose autoreceptor preferring treatments with a 5-HT1A agonist and antagonist can strongly modify the behavioral stimulant effects of cocaine and suggest that the 5-HT1A autoreceptor may be an important pharmacological target for the development of treatments for cocaine addiction.

8-OHDPAT effects upon cocaine unconditioned and conditioned behaviors: a role for drug stimulus effects

The effects of the 5-HT1A agonist, (+/-)-8-hydroxy-dipropylaminotetralin (8-OHDPAT) upon the unconditioned and conditioned behavior induced by cocaine were assessed in rats. Separate groups (n=7) received saline, cocaine (10 mg/kg), 8-OHDPAT (0.2 mg/kg), or 8-OHDPAT (0.2 mg/kg) plus cocaine (10 mg/kg) for eight treatment sessions (two per week) in which the rats were tested for 20 min in an open-field. On the eighth treatment session, cocaine enhanced locomotion and rearing but decreased grooming. 8-OHDPAT also decreased grooming and, when given in combination with cocaine, enhanced locomotion but attenuated cocaine-induced rearing. The two 8-OHDPAT groups differed substantially from each other and from the cocaine group in terms of locomotion during the drug treatment phase. Subsequently, all groups received a series of conditioning tests in which they received saline, 0.1, 0.2, or 0.4 mg/kg 8-OHDPAT prior to testing. Groups which had received either 8-OHDPAT or cocaine prior to the conditioning tests exhibited equivalent conditioned effects on the saline conditioning test. When conditioning tests were conducted with 8-OHDPAT, however, only the group which had previously received the combined 0.2 mg/kg 8-OHDPAT plus cocaine treatment exhibited a conditioned response and this effect only occurred at the 0.2 8-OHDPAT dose level. These observations indicate the important influence of the stimulus properties of drugs for the study of drug conditioning and for understanding drug interactions with cocaine.

Scopolamine prevents tolerance to the effects of caffeine on rotational behavior in 6-hydroxydopamine-denervated rats

Continuous administration of caffeine has been shown to induce tolerance to its psychostimulant effects. In this study, using unilateral 6-hydroxydopamine nigrostriatal denervated rats, we tested the hypothesis that the muscarinic receptor antagonist, scopolamine, would prevent the tolerance to caffeine-induced contralateral rotational behavior. For that purpose we administered either caffeine (40 mg/kg) plus saline or scopolamine (5, 10 and 20 mg/kg) plus saline, as well as caffeine in combination with the various doses of scopolamine for 7 consecutive days, and measured ipsilateral and contralateral rotational behavior. The results showed that acute injections of scopolamine plus saline produced similar levels of both ipsilateral and contralateral turning, while caffeine produced more contralateral than ipsilateral turning. Tolerance to caffeine-induced contralateral turning was observed as of the second administration, while scopolamine plus saline injections did not produce significant changes in rotational behavior with repeated treatment. Scopolamine co-administered with caffeine significantly attenuated the increased contralateral turning produced by acute injections of caffeine plus saline, but significantly prevented the tolerance effects with repeated administration. These findings strongly suggest that muscarinic cholinergic processes may be involved in tolerance to caffeine-induced contralateral turning. The results are interpreted in terms of the possible interactions between dopamine, adenosine and acetylcholine neurotransmitter systems within the basal ganglia circuitry involved in motor behavior.

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.

Effects of repeated treatment with methamphetamine plus scopolamine and methamphetamine on behavioral sensitization and conditioning

This study compared repeated treatment with methamphetamine (4.0 mg/kg, i.p.) plus scopolamine (0.5 mg/kg, i.p.) and methamphetamine alone in behavioral sensitization and drug conditioning with respect to a reciprocal balance between the dopaminergic and cholinergic systems. Repeated methamphetamine plus scopolamine treatment induced a more progressive and enduring enhancement of stereotyped behavior than repeated methamphetamine treatment. Methamphetamine plus scopolamine-induced stereotyped behavior was reproduced by challenge injections of not only methamphetamine plus scopolamine and methamphetamine, but also, to a lesser extent, by scopolamine challenges. The methamphetamine plus scopolamine-sensitized rats were conditioned to a low-frequency tone (300 Hz, 100 dB) as conditioned stimulus associated with the drug state. They responded to pairings of the tone and placebo injections, but not to the tone alone or the placebo alone. The methamphetamine-sensitized rats failed to exhibit conditioning. These results suggest that methamphetamine plus scopolamine-induced pronounced behavioral sensitization may produce an enhanced conditioning. Exteroceptive conditioned stimulus-interoceptive unconditioned stimulus associations may provide an important source for drug conditioning. We concluded that behavioral sensitization may be mediated via a reciprocal balance between the dopaminergic and cholinergic systems, in favor of a dopaminergic dominance. Conditioning to the drug-associated tone may operate via a reciprocal balance between the dopaminergic and cholinergic systems.

The relative salience of morphine and contextual cues as conditioned stimuli

Two experiments were conducted to delineate further the properties of conditioning when morphine is used as a conditioned stimulus (CS) in the conditioned suppression of drinking paradigm. Experiment 1 used a test for overshadowing designed to compare the relative salience of contextual cues (metal box) and morphine induced cues (6 mg/kg, IP) as CSs when each was paired with a foot shock unconditioned stimulus (US) in water deprived rats. Six groups (six rats each) were exposed to conditioning procedures during which the conditioning context was present 19 h (groups 1 and 2), 90 min (groups 3 and 4), or 5 min (groups 5 and 6) before shock onset, and morphine (in groups 1, 3, and 5) or saline (in groups 2, 4, and 6) was injected 10 min before shock. Subsequently, the magnitude of suppression of drinking in response to morphine, to the metal box, and to morphine plus the metal box was measured. Only group 1 (19 h group) suppressed drinking in response to morphine, while groups 3-6 suppressed drinking whenever tested in the metal box. The results indicate that morphine cues acted as a CS that elicited suppression of drinking in group 1, and that contextual cues present up to 90 min before morphine cues overshadowed morphine. Experiment 2 showed that expression of the conditioned response to morphine was blocked by naloxone.

Comparison of behavioural effects of repeated treatment with methamphetamine plus scopolamine and methamphetamine alone on behavioural sensitization and conditioned response

We investigated how repeated treatments with methamphetamine (4.0 mg kg-1, i.p.) plus scopolamine (0.5 mg kg-1, i.p.) and methamphetamine alone effected behavioural sensitization and conditioned response in rats. Repeated methamphetamine plus scopolamine treatment induced a more progressive and enduring enhancement of focused stereotyped behaviour than repeated methamphetamine treatment. Stereotyped behaviour induced by methamphetamine plus scopolamine was reproduced by challenge injections of methamphetamine plus scopolamine, methamphetamine, and to a lesser extent by scopolamine challenges. The methamphetamine plus scopolamine-sensitized rats were conditioned to a low frequency tone (300 Hz, 100 dB) associated with the drug state. They exhibited a conditioned response to pairings of the tone (conditioned stimulus) and placebo injections. However, they did not respond to the tone alone or the placebo injections alone. The methamphetamine-sensitized rats failed to demonstrate any conditioning; only the repeated methamphetamine plus scopolamine treatment induced sensitization to the drug-associated tone. Pairings of exteroceptive conditioned stimulus-interoceptive unconditioned stimulus associations may provide an important source for conditioning to the tone associated with the drug state. We conclude that behavioural sensitization may operate via a reciprocal balance between the dopaminergic and cholinergic inhibitory systems, in favour of a dopaminergic dominance. Conditioning to the drug-associated tone may be mediated via a reciprocal balance between the two transmitter systems.

Methamphetamine plus scopolamine potentiates behavioral sensitization and conditioning

The effects of repeated methamphetamine (4.0 mg/kg) plus scopolamine (0.5 mg/kg) treatment on behavioral sensitization and drug conditioning in rats were compared with the effects of repeated methamphetamine treatment. Behavioral sensitization induced by repeated methamphetamine plus scopolamine treatment was more vigorous than that induced by repeated methamphetamine treatment. Repeated methamphetamine plus scopolamine treatment produced sensitized responses, not only to methamphetamine plus scopolamine and methamphetamine but also, to a lesser extent, to scopolamine. Methamphetamine plus scopolamine-sensitized rats but not methamphetamine-sensitized rats exhibited conditioned responses to a low-frequency tone (300 Hz, 100 dB) associated with the drug state, suggesting that robust methamphetamine plus scopolamine-induced behavioral sensitization may lead to enhanced conditioning. It is plausible that robust behavioral sensitization might operate via a reciprocal balance between the dopaminergic and cholinergic systems in favor of dopaminergic dominance. Conditioning to the drug-associated tone may be mediated via a reciprocal balance between the two transmitter systems.

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.

Conditioned effects of apomorphine are manifest in regional EEG of rats both in hippocampus and in striatum

Previously, in own studies, it was shown that stereotyped behaviour produced by apomorphine can be conditioned if the drug is repeatedly paired with defined environmental stimuli (conditioned stimuli, CS). Eventually, the presentation of CS alone produces stereotyped behaviour as conditioned response (CR). Furthermore, in electrocorticographic recordings it could be demonstrated that the characteristic pattern following acute apomorphine treatment, namely a selective increase in the power of the alpha-1 band, could be conditioned as well. In the present study, regional EEG was recorded in the striatum and in the hippocampus of freely moving rats. For conditioning, apomorphine (0.5 mg/kg s.c.) was paired with auditory and olfactory stimuli as CS for seven times, and on the eighth day the drug was substituted by the solvent in the presence of the CS. The effects were compared with those obtained in "pseudoconditioned" controls. Acute apomorphine administration led to an increase in power in the alpha-1 band (7.00-9.50 Hz), which effect was obvious in the hippocampus, above the cortex and in the striatum. After performing the conditioning procedure, these effects in regional EEG were found to be conditioned as well: as CR, activation in power in the alpha-1 band in hippocampus and striatum were manifest in the presence of the CS, but in absence of the drug. These effects occurred sporadically, but with a significantly higher frequency than in the pseudoconditioned controls. The results suggest that both the hippocampus and the striatum play important roles in classical conditioning of apomorphine effects which are primarily mediated by the striatum.

Morphine as a conditioned stimulus in a conditioned emotional response paradigm

A Pavlovian conditioning experiment was conducted to determine whether morphine (6 mg/kg, IP) could act as a conditioned stimulus (CS) when paired with an electric shock unconditioned stimulus (US), and later produce a conditioned suppression of drinking (CR) in water deprived rats. Seven groups were tested for conditioning after exposure to one of the following conditioning procedures: (1) morphine paired with shock; (2) morphine alone with no shock; (3) shock but no morphine; (4) no shock and no morphine; (5) morphine paired with vocalizations of shocked rats; (6) saline paired with shock; (7) saline alone with no shock. Groups 1 and 2 tested whether morphine could act as a CS. Groups 3 and 4 tested for sensitization. Group 5 tested whether exposure to the vocalizations of other rats could act as a US when paired with a morphine CS. Groups 6 and 7 tested whether cues associated with the injection procedure could act as a CS. Only subjects in group 1 showed conditioned suppression of drinking, when compared to control groups. Overall, the results indicate that morphine could act as a conditioned stimulus and that several of the more obvious possible sources of artifact did not significantly contribute to the CR that is produced.

Rotational behavior as a classically conditioned response to pentobarbital administration

Pentobarbital stimulus control of rotational behavior was investigated in rats with unilateral 6-hydroxydopamine lesions of substantia nigra. In conditioning trials, lesioned rats were injected simultaneously with 10 mg/kg pentobarbital and 0.05 mg/kg apomorphine and their rotational (circling) behavior observed and counted. Subsequent to three consecutive daily conditioning sessions, animals were re-introduced to the rotation environment and tested with saline or pentobarbital. Pentobarbital, but not saline, administration was followed by a brief epoch of rapid contralateral rotation. After additional conditioning trials in which pentobarbital and apomorphine administration were paired, test sessions with 1 g/kg ethanol and with 10 mg/kg chlordiazepoxide were conducted. Most animals did not rotate in response to ethanol administration and most did rotate in response to chlordiazepoxide. Finally, in order to determine the persistence of the conditioned effect, animals were tested with pentobarbital 15 weeks after their last conditioning session and were found to rotate actively in response.