Presynaptic dopaminergic neurotransmission mediates amphetamine-induced unconditioned but not amphetamine-conditioned locomotion and defecation in the rat

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.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Reduced presynaptic dopamine activity in adolescent dorsal striatum

Adolescence coincides with symptomatic onset of several psychiatric illnesses including schizophrenia and addiction. Excess limbic dopamine activity has been implicated in these vulnerabilities. We combined molecular and dynamic indices of dopamine neurotransmission to assess dopamine function in adolescent rats in two functionally distinct striatal subregions: nucleus accumbens (NAc) and dorsal striatum (DS). In adolescents, we find an overall reduction in dopamine availability selective to the DS. Dopamine release in the DS, but not in the NAc, was less responsive to amphetamine in adolescents compared to adults. The dopamine transporter (DAT) inhibitor, nomifensine, similarly inhibited basal and amphetamine-induced dopamine release in either regions of both the age groups, suggesting that the reduced effectiveness of amphetamine is not due to differences in DAT function. Furthermore, DAT and vesicular monoamine transporter-2 expressions were similar in the DS and NAc of adolescent rats. In contrast, expression of tyrosine hydroxylase (TH) was reduced in the DS, but not in the NAc, of adolescents compared to adults. Behaviorally, adolescents were less sensitive to amphetamine but more sensitive to a TH inhibitor. These data indicate that, in contrast to the general notion that dopamine is hyperactive in adolescents, there is diminished presynaptic dopamine activity in adolescents that is selective to the DS and may result from attenuated TH activity. Given recent reports of altered dopamine activity in associative/dorsal striatum of individuals at a clinically high risk of psychosis, our data further support the idea that dorsal, as opposed to ventral, regions of the striatum are a locus of vulnerability for psychosis.

Amphetamine-induced locomotion, behavioral sensitization to amphetamine, and striatal D2 receptor function in rats with high or low spontaneous exploratory activity: differences in the role of locus coeruleus

Individual differences in novelty-related behavior are associated with sensitivity to various neurochemical manipulations. In the present study the amphetamine-induced locomotor activity and behavioral sensitization to amphetamine (0.5 mg/kg) was investigated in rats with high or low spontaneous exploratory activity (HE- and LE-rats, respectively) after partial denervation of the locus coeruleus (LC) projections with a low dose of the selective neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine; 10 mg/kg). DSP-4 produced a partial depletion (about 30%) of noradrenaline in the frontal cortex of both HE- and LE-rats; additionally the levels of metabolites of dopamine and 5-HT were reduced in the frontal cortex and nucleus accumbens of the LE-rats. Amphetamine-stimulated locomotor activity was attenuated by the DSP-4 pretreatment only in the HE-rats and this effect persisted over repeated testing. Behavioral sensitization to repeated amphetamine was evident only in the LE-rats with intact LC projections. Repeated amphetamine treatment reduced D(2) receptor mediated stimulation of [(35)S]GTPgammaS-binding and dopamine-dependent change in GDP-binding affinity in the striatum, but only in HE-rats. The absence of amphetamine sensitization in HE-rats could thus be related to the downregulation by amphetamine of the G protein stimulation through D(2) receptors. Conclusively, acute and sensitized effects of amphetamine depend on the integrity of LC projections but are differently regulated in animals with high or low trait of exploratory activity. These findings have implications to the neurobiology of depression, drug addiction, and attention deficit hyperactivity disorder.

6-Hydroxydopamine lesions of nucleus accumbens core abolish amphetamine-induced conditioned activity

Environmental cues associated with drug experiences appear to play a critical role in drug dependence. We have previously reported that dopamine-depleting lesions of the nucleus accumbens medial shell inhibit amphetamine-conditioned place preference. Here, we examined the effects of analogous lesions on amphetamine-conditioned locomotor activity. Bilateral core, but not medial shell, lesions attenuated unconditioned locomotion and abolished the conditioned locomotor response. Taken with our previous results, these findings confirm a role for accumbens core in amphetamine-induced locomotor activity and suggest that the role of medial shell DA transmission in conditioned place preference is related to reward processing rather than conditioning in general.

The partial D2-like dopamine receptor agonist terguride acts as a functional antagonist in states of high and low dopaminergic tone: evidence from preweanling rats

RATIONALE

In adult rats, the partial D(2)-like agonist terguride acts as an antagonist at normosensitive D(2)-like post-synaptic receptors, while it acts as an agonist at the same receptors during states of low dopaminergic tone.

OBJECTIVE

The purpose of the present study was to determine whether partial D(2)-like agonists exhibit both antagonistic and agonistic actions during the preweanling period.

METHODS

In experiments 1 and 2 (examining the agonistic actions of terguride), preweanling rats were either given an escalating regimen of amphetamine to induce a state of amphetamine withdrawal or pretreated with the tyrosine hydroxylase inhibitor AMPT. Distance traveled was measured after rats were injected with saline, terguride (0.4-1.6 mg/kg), or the full D(2)-like receptor agonist NPA (0.01 mg/kg). In experiment 3 (examining the antagonistic actions of terguride), preweanling rats were pretreated with terguride 30 min before they were tested with saline, NPA (0.05 mg/kg), or amphetamine (1.5 mg/kg).

RESULTS

NPA had an exaggerated locomotor activating effect when tested under conditions of amphetamine withdrawal, while the partial D(2)-like agonist did not enhance distance traveled under any circumstance. Similarly, NPA increased and terguride did not affect the distance-traveled scores of AMPT-pretreated rats. In experiment 3, terguride pretreatment significantly reduced the distance traveled of amphetamine-treated and NPA-treated rats.

CONCLUSIONS

The behavioral evidence indicates that, during the preweanling period, terguride antagonizes D(2)-like post-synaptic receptors in a state of high dopaminergic tone; however, there is no evidence that terguride is capable of stimulating D(2)-like post-synaptic receptors during states of low dopaminergic tone.

Effects of low dose N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine administration on exploratory and amphetamine-induced behavior and dopamine D2 receptor function in rats with high or low exploratory activity

Individual differences in behavioral traits are associated with sensitivity to various neurochemical and psychopharmacological manipulations. In this study exploratory and amphetamine-induced behavior in rats with persistently high or low exploratory activity (HE and LE, respectively) was examined before and after a partial denervation of the locus coeruleus (LC) projections with the selective neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine; 10 mg/kg). Partial LC denervation prevented the increase in exploratory activity over repeated test sessions in the LE animals, but had no effect in HE-rats. Amphetamine- (0.5 mg/kg) induced locomotor activity was attenuated by DSP-4 pretreatment only in HE-rats. These results suggest differential involvement of LC noradrenergic transmission in novelty- and amphetamine-induced behavior in animals with persistent differences in novelty-related behavior. In addition to partial noradrenaline depletion in the frontal cortex and hippocampus, which occurred in both HE- and LE-rats, DSP-4 treatment also decreased the content of dopamine and its metabolites in the nucleus accumbens, and the metabolite levels in striatum, but only in the LE-animals. 5-HIAA levels were also reduced in the nucleus accumbens and striatum in LE-rats by the neurotoxin. D(2) receptor function, as determined by dopamine-stimulated [(35)S]GTPgammaS binding, was increased by DSP-4 treatment in the striatum of LE-rats, but reduced in HE-rats. No effect of partial LC denervation was found on dopamine-stimulated [(35)S]GTPgammaS binding in the nucleus accumbens. Together these findings suggest that LC noradrenergic neurotransmission is differently involved in dopaminergic mechanisms which mediate novelty-related vs amphetamine-induced behavior.

Destruction of the noradrenergic system with DSP4 potentiates the behavioral effects of MK-801 in rats

In this study we investigated the effect of lesioning the noradrenergic systems on the behavioral effects of (5R, 10S)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo [a, d] cyclohepten-5,10-imine hydrogen maleate--MK-801, in rats. The noradrenergic system was lesioned with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride--DSP4 (60 mg/kg IP). MK-801 increased the locomotor activity and rearing. DSP4 significantly further increased the hyperlocomotor activity, circling (especially to the left side), sniffing, rolling, and falling that were induced by MK-801. These results showed that destruction of the noradrenergic system increased MK-801-hyperlocomotor activity, ataxia and stereotypy.

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.

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.

Nimodipine and haloperidol attenuate behavioural sensitization to cocaine but only nimodipine blocks the establishment of conditioned locomotion induced by cocaine

The classical conditioning of the behavioural effects of cocaine has been shown to contribute to behavioural sensitization. In the present experiments, it was demonstrated that the effects of cocaine in rats can be conditioned to contextual stimuli. Furthermore, sensitization to cocaine's locomotor effects were demonstrated, and shown to be context specific. Nimodipine (10 mg/kg, SC), an L-type dihydropyridine Ca2+ channel antagonist, appeared to completely block the establishment of conditioning of cocaine's effects, but only partially blocked sensitization to cocaine. Haloperidol (0.05 mg/kg, IP), a relatively specific D2 dopamine receptor antagonist, attenuated behavioral sensitization but had no influence on the establishment of the conditioned component of cocaine. These results indicate that the sensitization to, and the development of classical conditioning of, cocaine's behavioural effects can be pharmacologically dissociated, but that a non-associative process involved in sensitization is normally overridden by conditioning factors.

Calcium channel blockade interacts with a neuroleptic to attenuate the conditioning of amphetamine's behavioral effects in the rat

Conditioned responses to drug-related cues appear to be related to the maintenance of stimulant addiction. These conditioned responses are not blocked by treatments that block the direct effects of stimulants and may contribute to the high rate of relapse of addicts. Rats administered (+)-amphetamine in a specific environment exhibit conditioned locomotion when subsequently placed in that environment without drugs. The neuroleptic haloperidol significantly attenuated amphetamine-induced locomotor activity but failed to reduce conditioned locomotion. Nimodipine, an L-type calcium channel antagonist, had no effect on amphetamine-induced unconditioned or conditioned locomotion. However, combined nimodipine and haloperidol treatment blocked the unconditioned and attenuated the conditioned locomotor response to amphetamine. Conjunctive therapy with nimodipine and haloperidol may provide an efficacious treatment for stimulant addiction. In addition, nimodipine may provide an important adjunctive therapy for schizophrenia, allowing the use of lower doses of neuroleptic to avoid extrapyramidal side effects.

Evidence for presynaptic dopamine mechanisms underlying amphetamine-conditioned locomotion

Rats with a history of receiving (+)-amphetamine in a specific environment exhibit a conditioned psychomotor response when subsequently placed in that environment without drug treatment. Previous work has shown that while the unconditioned effects of amphetamine can be blocked by dopamine D1 or D2 receptor antagonists or with alpha-methyl-p-tyrosine, conditioned locomotion is not influenced by these treatments. In the present experiment, alpha-methyl-p-tyrosine (50 mg/kg, s.c.) was given in conjunction with amphetamine (1.5 mg/kg, s.c.) for 8 days before testing for conditioned locomotion. alpha-Methyl-p-tyrosine completely blocked amphetamine-induced locomotion but only attenuated amphetamine-conditioned locomotion. Reserpine (reduced over the 8 days from 2.5 to 1.25 mg/kg, i.p.) did not block amphetamine-induced locomotion; indeed, potentiation of amphetamine-induced locomotor activity was observed on the last 3 days of treatment. Reserpine treatment in conjunction with alpha-methyl-p-tyrosine treatment blocked amphetamine-induced locomotion for the first 4 days only, with full recovery of amphetamine-induced unconditioned locomotion by the last treatment day. Reserpine alone had no effect on amphetamine-conditioned locomotion, but completely blocked amphetamine-conditioned locomotion when given with alpha-methyl-p-tyrosine. It is concluded that the alpha-methyl-p-tyrosine-sensitive pool of dopamine mediates the immediate psychomotor effects of amphetamine, but that both the alpha-methyl-p-tyrosine- and reserpine-sensitive pools of dopamine are involved in the establishment of amphetamine-conditioned locomotion. In addition, the occurrence of amphetamine-conditioned locomotion is independent of the direct effects of amphetamine on locomotion.