Modulatory effect of p-chlorophenylalanine microinjected into the dorsal and median raphe nuclei on cocaine-induced behaviour in the rat

Abstract

The present study examined whether a potentiation of cocaine-induced behaviour in rats following peripheral pretreatment with the 5-hydroxytryptamine (5-HT) biosynthesis inhibitor p-chlorophenylalanine may be due to depletion of 5-HT in the dorsal raphe nucleus and/or median raphe nucleus. Following peripheral pretreatment with p-chlorophenylalanine (100 mg/kg, i.p.) for 3 consecutive days, a potentiation of cocaine-induced locomotor activity and rears was observed. To investigate a possible involvement of serotonergic neurones arising in the midbrain raphe nuclei in the observed potentiation, p-chlorophenylalanine (0.5 μg) was microinjected in either the dorsal raphe nucleus or median raphe nucleus followed by behavioural testing 48 h later. Application of p-chlorophenylalanine in the dorsal raphe nucleus resulted in an enhancement of cocaine-induced locomotor activity and head bobs. In contrast, the stimulant effect of cocaine on behaviour was not altered by microinjection of p-chlorophenylalanine in the median raphe nucleus. Peripheral and central administration of p-chlorophenylalanine did not consistently alter the baseline behaviour of saline-treated animals. Biochemical results indicated only a moderate depletion of 5-HT in the midbrain raphe nuclei following peripheral p-chlorophenylalanine administration. Surprisingly, the central application of p-chlorophenylalanine in the dorsal raphe nucleus and median raphe nucleus did not alter the 5-HT levels in the midbrain raphe nucleus investigated. In addition, peripheral and central administration of p-chlorophenylalanine did not alter the 5-HT levels in the nucleus accumbens. In conclusion, the behavioural results suggest that the potentiation of cocaine-induced behaviour following peripheral p-chlorophenylalanine administration may be attributed to the dorsal raphe nucleus but not the median raphe nucleus suggesting that, serotonergic dorsal raphe nucleus neurones may normally mediate a tonic inhibitory effect on cocaine-induced behaviour. Furthermore, the biochemical data may indicate the existence of neurochemical resistance of the midbrain raphe nuclei to the 5-HT depleting effects of p-chlorophenylalanine.