Withdrawal from chronic cocaine administration causes prolonged deficits in L-tryptophan-induced head-twitch response in mice

Early postnatal cocaine exposure causes sequential, dose-dependent, enduring but reversible supersensitivity in 5-HT2A receptor-mediated function during development in male mice

This report investigated whether postnatal exposure to cocaine affects the index of 5-HT2A receptor function during development by utilizing the ability of the 5-HT2A/C agonist DOI to induce the head-twitch response (HTR) in mice. Thus, several groups of mice litters were treated with varying doses of cocaine (0, 0.5, 1, 5, and 10 mg/kg, IP) twice daily from postnatal days 5 to 14. Then, different groups of cocaine-exposed male mice pups along with their corresponding age-matched vehicle-exposed control groups were HTR tested once during development on the following postnatal test days: 15, 16, 18, 20, 30, 45, and 60. The HTR testing involved administration of DOI (0.5 mg/kg, IP) and counting the frequency of the behavior for the next 20 min. Cocaine exposure caused bell-shaped, dose-dependent, enduring but reversible increase in DOI-induced HTR frequency (mean +/- SEM) during development. The developing pups were most sensitive to low and intermediate doses of cocaine (0.5-5 mg/kg). The greatest degree of increase in HTR frequency in response to DOI challenge occurred in the 1 mg/kg cocaine-exposure group on most test days. The onset of HTR supersensitivity varied from 48 h (5 mg/kg) to 144 h (0.5 mg/kg) following the termination of chronic cocaine exposure. Moreover, maximal supersensitivity for the latter doses of cocaine occurred 96 and 384 h postcocaine treatment, respectively. Other cocaine exposure groups attained their maxima sometime between the latter time periods. The duration of persistence of 5-HT2A receptor supersensitivity varied with different doses of cocaine: the 10-mg/kg group was supersensitive up to 384 h postcocaine treatment, the 1- and 5-mg/kg groups up to 744 h; and the 0.5-mg/kg group up to 1104 h. Although developmentally cocaine-exposed pups exhibit some similarities (i.e., exquisite sensitivity and bell-shaped dose-response) in 5-HT2A receptor adaptation to mature adult mice exposed to cocaine, they also differ from mature adult cocaine-exposed mice in the onset of appearance as well as the enduring persistence of the induced supersensitivity.

Prolonged deficits in presynaptic serotonin function following withdrawal from chronic cocaine exposure as revealed by 5-HTP-induced head-twitch response in mice

Recent in vivo microdialysis studies have indicated that presynaptic deficits occur in brain 5-HT neurochemistry during cocaine withdrawal. The purpose of the present study was to utilize the head-twitch response (HTR) produced by 5-hydroxytryptophan (5-HTP) to investigate the dose- and time-response effects of this deficit. The HTR is considered to be a sensitive model for activation of central postsynaptic 5-HT2A receptors in rodents. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg/kg, i.p.) for 7 or 13 days. During HTR testing, at 24 h following last injection, the treated mice received either 1) no cocaine; 2) their corresponding daily dose as challenge injection; or 3) a 10 mg/kg challenge dose. In a second series of experiments, extended abstinence studies were performed under the conditions of experimental protocols 1 and 2 for both 7- and 13-day cocaine (0, 0.5 and 5 mg/kg, twice daily) exposure regimens at 24, 48, 72 and 96 h following last cocaine injection. In protocol 3, the effects of a 10 mg/kg challenge dose of cocaine were studied following prolonged withdrawal from chronic cocaine exposure (0, 0.5, 5 and 10 mg/kg, twice daily for 7 and 13 days) at 24, 96 and 240 h abstinence. In experimental protocol 1 at 24 h abstinence in the 7 day exposure group, only lower doses of cocaine (0.5-2.5 mg/kg) significantly attenuated the 5-HTP-induced HTR. The deficit in 0.5 mg/kg group persisted up to 72 h abstinence. Although in the 13 day cocaine exposure groups (experimental paradigm 1) mean HTRs were generally reduced, they however failed to attain statistical significance throughout the 96 h abstinence. In protocol 2 very low challenge doses of cocaine (0.1-0.5 mg/kg) in their corresponding pretreatment groups significantly reduced the behavior at diverse abstinence intervals in both 7- and 13-day exposure regimens relative to their chronically vehicle-treated controls which had received a vehicle challenge injection during HTR testing. Unlike small doses of cocaine, larger challenge doses (5-10 mg/kg) of the stimulant potentiated the HTR score at various abstinence periods. However, the degree of the potentiations are considerably less than the ability of acute cocaine administration in enhancing the 5-HTP-induced HTR. The 10 mg/kg challenge injection in experimental protocol 3 at 24 h abstinence in the 7-day exposed mice attenuated the 5-HTP-induced HTR in 0.5, 5 and 10 mg/kg cocaine-treated groups relative to their chronic vehicle-treated controls receiving a 10 mg/kg challenge cocaine injection. The deficit in chronic 10 mg/kg cocaine-exposed mice persisted up to 240 h postcocaine abstinence. On the other hand, in the 13-day regimen, the challenge 10 mg/kg dose exhibited significant potentiations at 24 h and at 96 h for 5 and 0.5 mg/kg chronic cocaine doses respectively, but it also produced significant deficits in 0.5 and 10 mg/kg chronic doses of cocaine at 240 h abstinence. Overall, the present results suggest that enduring deficits occur in presynaptic serotonin neurochemistry and serotonergic adaptive mechanisms are exquisitely sensitive to chronic administration of low- and high-doses of cocaine.

Deficits in D-fenfluramine-sensitive pool of brain 5-HT following withdrawal from chronic cocaine exposure

Recent head-twitch response (HTR) studies in mice have indicated that withdrawal from chronic cocaine exposure produces deficits in CNS conversion of L-tryptophan to 5-HT. In the present study, the ability of 5-HT releaser, d-fenfluramine, was utilized to induce the HTR in mice following abstinence from chronic cocaine exposure. d-Fenfluramine-induced HTR, is a 5-HT2A receptor-mediated phenomenon and its induction frequency can be regarded as an indirect but in vivo measure of basal brain 5-HT concentration. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg/kg, i.p.) for either 7 or 13 days. At 24 h after last cocaine injection, the treated mice received d-fenfluramine (5 mg/kg, i.p.) and the induced HTR (mean+/-SEM) was recorded for the next 30 min. Cocaine attenuated the d-fenfluramine-induced HTR frequency by 30-37% in the 13-day regimen and significant effects were observed from 0.5 mg/kg dose. At 24 h withdrawal in the 7-day cocaine exposure group, the mean HTR frequencies were attenuated, however, they did not achieve statistical significance. Extended abstinence studies (i.e. 24, 48, 72 and 96 h postwithdrawal) from chronic cocaine exposure (0, 0.5 and 5 mg/kg/day for either 7 or 13 days) indicated that in the 7-day exposure group, significant reductions (26, 39 and 22%) in HTR frequency occurred at 48, 72 and 96 h following withdrawal from 0.5 mg/kg cocaine, whereas its 5 mg/kg dose failed to induce a significant effect. In the 13-day exposure group significant reductions in HTR frequency were observed at 24 h abstinence (27%) for the 0.5 mg/kg cocaine dose and at 24 and 48 h for the 5 mg/kg. Overall, these results indicate that abstinence from chronic exposure to cocaine produces enduring deficits in basal 5-HT concentration. Lastly, serotonergic function appears to be uniquely sensitive to chronic administration of low doses of cocaine.