Involvement of noradrenergic and dopaminergic neurons in shock-induced jumping in rats

The role of central noradrenergic and dopaminergic neurons in shock-induced jumping behavior in the rat was examined using specific stimulants and blockers of the respective receptors. The administration of the alpha-noradrenergic receptor stimulant, clonidine, increased jumping frequency (10-100 micrograms/kg) and reduced latency (100-500 micrograms/kg). The effects of clonidine were counteracted by the postsynaptic alpha-adrenoceptive blocking agent, phenoxybenzamine (20 mg/kg). The dopaminergic receptor stimulant, apomorphine (0.1-5.0 mg/kg), did not increase jumping frequency, but prolonged jumping latency. On the other hand, combined administration of clonidine (30, 500 micrograms/kg) and apomorphine (2 mg/kg) increased jumping behavior more remarkably than did clonidine alone. The blockers of postsynaptic alpha-noradrenergic and dopaminergic receptors, phenoxybenzamine (20 mg/kg) and pimozide (2 mg/kg) respectively, had little effect on jumping when used alone, but each drug counteracted the enhanced jumping response caused by clonidine plus apomorphine. Combined administration of phenoxybenzamine and pimozide depressed jumping. These data suggest that excitation of dopaminergic neurons exerts a positive modulating effect on the noradrenaline-mediated jumping behavior induced by foot-shock.