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

The 5-HT1A receptor and behavioral stimulation in the rat: effects of 8-OHDPAT on spontaneous and cocaine-induced behavior

RATIONALE

The contribution of the 5-HT(1A) somato-dendritic autoreceptor populations to spontaneous and cocaine-induced locomotion is unclear.

OBJECTIVES

To use a low dose range of +/-8-hydroxy-2-(di- n-propylamino)tetralin (8-OHDPAT) to preferentially stimulate 5-HT(1A) autoreceptors and a medium 8-OHDPAT dose range to stimulate both 5-HT(1A) autoreceptors and postsynaptic receptors as pretreatments prior to either saline or cocaine.

METHODS

In experiment 1, either a medium dose of 8-OHDPAT (0.4 mg/kg) or a low dose (0.05 mg/kg) was given as pretreatments 20 min before five separate 20-min open-field tests. In experiment 2, the pretreatments were changed to a low dose range of 8-OHDPAT (0.01-0.05 mg/kg), with or without WAY 100635 (0.01-0.05 mg/kg). In experiment 3, the 8-OHDPAT pretreatments (0.01, 0.025 or 0.05 mg/kg) were administered 20 min prior to saline or cocaine (10 mg/kg) tests. In experiment 4, a medium dose range (0.2-0.3 mg/kg) was given 20 min prior to saline or cocaine (10 mg/kg) tests.

RESULTS

Experiment 1 showed that 8-OHDPAT (0.4 mg/kg) tended to increase locomotor activity but that pretreatment with 0.05 mg/kg severely suppressed locomotor activity. In experiment 2, 8-OHDPAT in the low dose range inhibited locomotor activity and this effect was reversed by co-administration of WAY 100635. Experiment 3 showed that the low-dose 8-OHDPAT pretreatment reduced locomotor activity in saline but not cocaine tests. In experiment 4, 8-OHDPAT in the medium dose range enhanced locomotor activity in cocaine tests.

CONCLUSIONS

It is suggested that the facilitatory effect of 8-OHDPAT on cocaine-induced locomotor stimulation is mediated by inhibition of 5-HT(1A) somato-dendritic autoreceptors.

Cocaine-conditioned behavioral effects: a role for habituation processes

Cocaine has potent locomotor stimulant effects in rodents, which seemingly can become conditioned to test environment cues. In two experimental protocols, we measured the effects of cocaine on locomotor activity and grooming behavior, and subsequently tested whether these cocaine effects became conditioned to contextual cues. In the first experiment, three groups of rats received 14 injections of either saline or cocaine (10 mg/kg) paired or unpaired to the test environment. Cocaine increased locomotion and decreased grooming during treatment and on the conditioning test. Over the course of the treatment phase, however, the saline- and cocaine-unpaired groups but not the cocaine paired group developed progressively lower locomotion and higher grooming scores indicative of substantial habituation effects. To examine whether the cocaine may have impaired the acquisition of habituation effects rather than induce a Pavlovian cocaine conditioned response, an additional experiment was conducted in which two additional non-habituation saline and cocaine control groups were added to the experimental design. On a conditioning test, the two non-habituation control groups were equivalent in activity and grooming behavior to the cocaine-paired group. The findings were consistent with a failure by cocaine-paired animals to acquire habituation effects, which could transfer to the non-cocaine state. The connection between cocaine and novelty/habituation may have substantial importance for understanding cocaine effects.

Effects of low dosages of apomorphine on maternal responsiveness in lactating rats

Lactating rats (day 7 +/- 1 postpartum) were observed during a 1-h reunion with their pups 4 h after separation from them and 10 min after subcutaneous injection of saline (SAL; 0.1 ml) or low dosages of the dopamine agonist, apomorphine (APO; 0.1 or 0.25 mg/kg). Although APO did not affect latency to sniff pups or retrieve the first pup, there were dosage-dependent delays in onset of licking and nursing pups, and decreases in retrieval and grouping of pups, nursing duration, and litter weight gain. The alterations in maternal responsiveness among APO-treated dams were related to increased carrying and mouthing of pups and markedly increased sniffing of pups, bedding, and cage. Duration of time spent licking pups, exploring, and self-grooming did not differ between groups. Thus, certain APO-induced stereotypic behaviors interfered with the normal sequence of maternal behavior by exaggerating some components and delaying others. These results are relevant to disturbances in maternal behavior caused by hyperreactivity or by other drugs that increase dopaminergic activity, such as cocaine.

The 5-HT(1A) antagonist WAY 100635 can block the low-dose locomotor stimulant effects of cocaine

Cocaine treatments of 2.5, 5.0, 10.0 and 15.0 mg/kg induced dose-dependent increases in locomotor behavior. This cocaine-induced increase in locomotion was blocked if the animals were administered the selective 5-HT(1A) antagonist, WAY 100635 (0.4 mg/kg) prior to the cocaine treatment. The 0.4-mg/kg dose of WAY 100635 did not affect locomotor behavior or alter cocaine availability in brain.

Issues in the pharmacological modification of cocaine conditioning: evidence that the stimulus properties of drugs can interact with contextual cues to activate or inactivate cocaine conditioned stimuli

Cocaine conditioned stimuli are capable of eliciting cocaine craving in individuals with a history of cocaine use. As a consequence, there have been a number of attempts using animal models to identify pharmacological treatments which can attenuate cocaine conditioned effects. The emphasis in these studies has been to employ drug doses which do not have response effects that could directly alter the conditioned drug response. A drug treatment may not have a response effect but still have drug stimulus effects which could interact with and modify the cocaine conditioned stimulus. In order to experimentally investigate this important issue, two experiments are reported. In one experiment, rats were co-administered 0.1 mg/kg MK-801 either with cocaine (10 mg/kg) or with saline; in the other experiment 3.0 mg/kg buspirone was co-administered with either cocaine (10 mg/kg) or with saline. The MK-801 and buspirone treatments did not affect spontaneous activity levels or alter the unconditioned cocaine stimulant effect. In tests for conditioning, however, the effects of buspirone and MK-801 depended upon their association with cocaine. If MK-801 and buspirone had no association with cocaine then these drugs inactivated the cocaine conditioned stimulant response. If MK-801 and buspirone had been co-administered with cocaine, then, in saline conditioning tests, no cocaine conditioning was observed. If the conditioning tests were conducted following MK-801 or buspirone treatment, however, cocaine conditioning was elicited. Altogether, these studies demonstrate that the stimulus properties of drugs can interact with contextual stimuli to inactivate or activate cocaine conditioned stimuli. In the search for drugs which may prevent cocaine craving, therefore, the stimulus properties of drugs provide an important mechanism for the modification of cocaine conditioned stimuli.