Behavioral sensitization to cocaine is not associated with changes in serotonin (5-HT) fiber immunoreactivity in rat forebrain

PPARγ agonism attenuates cocaine cue reactivity

Cocaine use disorder is a chronic relapsing condition characterized by compulsive drug seeking and taking even after prolonged abstinence periods. Subsequent exposure to drug-associated cues can promote intense craving and lead to relapse in abstinent humans and rodent models. The responsiveness to these cocaine-related cues, or 'cue reactivity', can trigger relapse and cocaine-seeking behaviors; cue reactivity is measurable in cocaine-dependent humans as well as rodent models. Cue reactivity is thought to be predictive of cocaine craving and relapse. Here we report that PPARγ agonism during abstinence from cocaine self-administration reduced previously active lever pressing in Sprague Dawley rats during cue-reactivity tests, while administration of the PPARγ antagonist, GW9662, reversed this effect. PPARγ agonism also normalized nuclear ERK activity in the medial prefrontal cortex and hippocampus which was reversed with GW9662. Our results support the utility of PPARγ agonism as a relapse prevention strategy to maintain abstinence in the presence of cocaine-associated cues.

Effects of cocaine and reserpine administration on RNA editing of rat 5-HT2C receptor estimated by primer extension combined with denaturing high-performance liquid chromatography

Serotonin 2C receptor (5-HT(2C)R) transcripts undergo RNA editing, generating pharmacologically different isoforms. To test whether the RNA editing of 5-HT(2C)R is regulated by serotonergic activity, effects of cocaine or reserpine administration in the rat cerebral cortex were examined. Although these drugs have been known to alter serotonin metabolism, no alterations in the RNA editing were found by the sequencing analysis. Towards high throughput analysis, we developed a non-RI method that allows accurate and rapid estimation of RNA editing by combining the primer extension with denaturing high-performance liquid chromatography (DHPLC). By using this, RNA editing efficiencies of 5-HT(2C)R in the midbrain and hippocampus as well as the cerebral cortex were examined, and no alterations were found among these regions. Our method using DHPLC is applicable to examine association of RNA editing with various diseases.

Alterations in serotonergic responsiveness during cocaine withdrawal in rats: similarities to major depression in humans

BACKGROUND

Withdrawal from long-term cocaine use is accompanied by symptoms resembling major depression. Because acute cocaine affects serotonin (5-HT) neurons, and 5-HT dysfunction is implicated in the pathophysiology of depression, we evaluated the effects to 5-HT agonists in rats withdrawn from repeated injections of cocaine (15 mg/kg i.p., b.i.d., 7 days) or saline.

METHODS

In the first study, prolactin (PRL) responses elicited by the 5-HT-releasing agent fenfluramine, the 5-HT1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) were examined as indices of postsynaptic 5-HT receptor function. In a second study, specific responses induced by 8-OH-DPAT, namely inhibition of brain 5-HT synthesis and stimulation of feeding, were examined as correlates of 5-HT1A autoreceptor function.

RESULTS

Prior treatment with cocaine did not modify fenfluramine-evoked PRL release; however, the PRL secretory response to 8-OH-DPAT was blunted and the PRL response to DOI was potentiated after chronic cocaine treatment. Cocaine exposure did not alter the inhibitory effect of 8-OH-DPAT on 5-HT synthesis. 8-OH-DPAT-induced feeding was influenced by prior cocaine, but this effect was secondary to pronounced baseline hyperphagia in the cocaine-treated group.

CONCLUSIONS

These data indicate that withdrawal from chronic cocaine renders specific subpopulations of postsynaptic 5-HT1A receptors subsensitive and 5-HT2A/2C receptors supersensitive. No evidence for cocaine-induced changes in 5-HT1A autoreceptor responsiveness was found. A survey of the literature reveals similarities in the profile of 5-HT dysfunction between rats withdrawn from cocaine and humans diagnosed with depression. We propose that withdrawal from chronic cocaine in rats may serve as a useful animal model of depressive disorders.

Chronic cocaine exposure potentiates prolactin and head shake responses to 5-HT2 receptor stimulation in rats

The effect of repeated cocaine administration on serotonin2 (5-HT2) receptor function was examined in male rats. Rats were fitted with indwelling jugular catheters and subsequently received cocaine (15 mg/kg, i.p., b.i.d.) or saline for 7 days. Rats were challenged with the 5-HT2 agonist DOI (25, 100, 400 micrograms/kg, i.v.) or saline 42 hr and 8 days after cessation of chronic treatment. Serial blood samples were collected at various times after DOI challenge and analyzed for prolactin levels. DOI-induced head shakes and skin jerks were examined concurrently in the same subjects. After 42 hr of withdrawal, the stimulatory effects of DOI on prolactin release and shaking behavior were significantly enhanced in cocaine-treated rats. Conversely, the skin jerk response to DOI was not altered by prior cocaine exposure. After 8 days of withdrawal, the prolactin and head shake responses to DOI were still potentiated in cocaine-treated rats, but this effect was no longer statistically significant. The data indicate that chronic cocaine enhances the sensitivity of 5-HT2 receptor mechanisms. Our findings further suggest the possibility that altered 5-HT2 receptor function may be involved in the mood disturbances experienced by abstinent cocaine addicts.

Evidence for alterations in presynaptic serotonergic function during withdrawal from chronic cocaine in rats

The effects of repeated cocaine administration on serotonin (5-hydroxytryptamine, 5-HT) function were investigated by comparing the corticosterone response to 5-HT receptor agonists in cocaine-treated and vehicle-treated rats. Male rats were fitted with indwelling jugular catheters and received cocaine (15 mg/kg i.p., b.i.d.) or saline for 7 days. Rats were challenged with either saline, the 5-HT releaser fenfluramine (1.2 mg/kg i.v.), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 50 micrograms/kg i.v.), or the 5-HT2A/2C receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 100 micrograms/kg i.v.) 42 h and 8 days after the final chronic treatment. Repeated blood samples were withdrawn immediately before and at 15, 30 and 60 min after acute challenge injections. All 5-HT receptor agonists increased plasma corticosterone, but the fenfluramine-induced rise in corticosterone was significantly attenuated in cocaine-treated rats withdrawn for 42 h. This blunted response to fenfluramine exhibited only partial recovery when examined at 8 days postchronic treatment. Corticosterone responses to 8-OH-DPAT and DOI were not affected by cocaine exposure. Our data suggest that chronic cocaine produces deficits in presynaptic 5-HT function, and alterations in 5-HT neurotransmission may underlie the dysphoria experienced by abstinent cocaine users. Neuroendocrine challenge tests should be performed in human addicts to evaluate potential 5-HT dysfunction associated with cocaine abuse.

Repeated cocaine administration reduces 5-HT1A-mediated prolactin secretion in rats

The prolactin and behavioral responses elicited by the 5-HT1A agonist 8-OH-DPAT (8-hydroxy-2-[di-n-propylamino]tetralin) were examined in male rats previously exposed to chronic cocaine (15 mg/kg, i.p., b.i.d., 7 days) or saline. After 42 h of withdrawal, cocaine-treated rats exhibited a reduced prolactin response to 8-OH-DPAT challenge (50 micrograms/kg, i.v.). A 5-fold higher dose of 8-OH-DPAT stimulated maximal prolactin secretion that was similar in cocaine- and saline-treated rats. Prior cocaine treatment had no effect on the 5-HT syndrome induced by 8-OH-DPAT. Our data agree with the findings of others and suggest that 5-HT1A receptors mediating neuroendocrine secretion become subsensitive after repeated cocaine administration.

Differing neurotoxic potencies of methamphetamine, mazindol, and cocaine in mesencephalic cultures

The potent reinforcing effects of methamphetamine and cocaine are thought to be mediated by their interactions with CNS dopamine neurons. Both stimulants share the ability to block dopamine uptake potently, and methamphetamine can release cytoplasmic dopamine as well. There is also abundant evidence demonstrating the neurotoxic effects of methamphetamine. There are, however, limited studies that attempt to discern the neurotoxic mechanisms of these agents. The purpose of the present study was to characterize and compare the chronic in vitro effects of methamphetamine, cocaine, and the dopamine uptake blocker, mazindol, on cultured fetal mesencephalic dopamine neurons. Our studies examined biochemical mechanisms to evaluate the contribution of reuptake blockade versus release of dopamine. Using a dispersed cell preparation of fetal mesencephalon, cultures were treated for 5 days with the three uptake blockers. Dopamine function was assessed by measuring high-affinity [3H]dopamine uptake and by examining cultures for the presence of tyrosine hydroxylase-immunopositive neurons. Nonspecific neurotoxicity was assessed by staining for neuron-specific enolase and measuring lactate dehydrogenase activity. The results indicate that repeated administration of high concentrations of methamphetamine (10(-4) and 10(-3) M) caused a generalized neurotoxicity whereas the effects of 10(-5) M methamphetamine appeared to be specific to dopamine cells. Likewise, treatment of the cultures with mazindol (10(-6) M) resulted in reduced dopamine uptake while not significantly affecting neuron-specific enolase or tyrosine hydroxylase immunostaining. On the other hand, repeated exposure to cocaine (10(-5) and 10(-4) M) did not alter dopaminergic function in these cultures. The different mechanisms of action of these stimulants may explain the differences in neurotoxic potency of these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic cocaine enhances serotonin autoregulation and serotonin uptake binding

Repeated cocaine intoxication can result in the development of behavioral sensitization in animals and psychosis in humans, phenomena that have been associated with alterations in dopamine (DA) function. Using electrophysiologic and autoradiographic techniques, modifications of central serotonin (5-hydroxytryptamine; 5-HT) systems were investigated in rats treated with a regimen of cocaine administration that produced behavioral sensitization. The inhibitory response of single 5-HT neurons in the dorsal raphe (DR) to (-)-cocaine, the 5-HT uptake inhibitor fluoxetine or the 5-HT1A agonist 8-hydroxy-2-[di-N-propylamino]tetralin (8-OHDPAT) was significantly enhanced in cocaine-treated rats. Furthermore, several brain areas that contain either cell bodies (DR) or terminals for 5-HT (medial and sulcal prefrontal cortex, frontal cortex) showed cocaine-induced elevations in [3H]imipramine-labeled 5-HT uptake sites, while [3H]-8-OHDPAT-labeled 5-HT1A receptors were decreased only in the central medial amygdala. These results suggest that modifications of autoregulatory mechanisms secondary to alterations of 5-HT uptake processes may contribute to the development of cocaine sensitization.