Separation of the associative and non-associative effects of brain serotonin released by p-chloroamphetamine: dissociable serotoninergic involvement in avoidance learning, pain and motor function.

Abstract

p-Chloramphetamine (PCA, 0.63-5 mg/kg IP) injected 30-60 min before testing produced a dose-related impairment of avoidance acquisition, prolonged reaction time in the hot-plate test and increased locomotor activity. Pretreatment with the selective serotonin (5-HT) uptake inhibitor zimeldine (10 mg/kg IP) blocked these behavioral effects. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 X 10 mg/kg IP) or inhibition of tryptophan hydroxylase by p-chlorophenylalanine (300 mg/kg IP) also blocked the behavioral effects of PCA. There was a complete blockade of the PCA-induced avoidance deficit following pretreatment with metergoline, a central 5-HT receptor blocking agent. On the other hand, metergoline failed to block the hot-plate analgesia and the increased locomotion caused by PCA. Depletion of brain NA and DA by the tyrosine hydroxylase inhibitor H44/68 did not counteract the PCA effect on avoidance or hot-plate performance, but reduced the locomotor stimulating effect. The selective NA neurotoxin DSP4 (50 mg/kg IP) or the opiate antagonist naloxone (1 mg/kg) failed to affect the PCA-induced modulations of the behaviours studied. In addition, PCA administration in doses that caused avoidance deficits, did not result in motor impairment as assessed by the tread mill test. The above results support the hypothesis that the PCA-induced impairment of active avoidance acquisition does not involve changes in nociception or altered locomotor activity. It is concluded that behavioural processes related to serotonergic neurotransmission can be independently modified, suggesting differences in the underlying 5-HT mechanisms.