Serotonergic-catecholaminergic interactions and foot shock-induced jumping behavior in rats

The role of serotonergic neurons in foot shock-induced jumping behavior was evaluated by studying the effects of serotonin agonists and depletors under conditions of increasing activity of catecholaminergic neurons by combining different shock intensities with methamphetamine or clonidine pretreatment. Low shock (40 V) jumping was suppressed by three different kinds of serotonergic agonists: 5-hydroxytryptophan (50 and 100 mg/kg), 5-methoxydimethyltryptamine (1 and 5 mg/kg) and clomipramine (5 mg/kg) but these drugs did not suppress high shock (60 V) jumping. A depletor of brain serotonin, p-chlorophenylalanine (200 mg/kg), failed to affect the jumping induced by both shock intensities. The inhibitory effect of serotonergic agonists on low shock jumping was abolished by pretreatment with methamphetamine (1 mg/kg) or clonidine (100 micrograms/kg). Furthermore, 5-methoxydimethyltryptamine (5 mg/kg), a serotonergic postsynaptic receptor stimulant, potentiated high shock jumping in rats pretreated with methamphetamine (3 and 5 mg/kg). The results indicated that activation of serotonergic neurons has a differential effect on jumping behavior ranging from inhibitory to facilitatory, depending upon the increasing activity of catecholaminergic neurons.