The involvement of nigral serotonin innervation in the control of punishment-induced behavioral inhibition in rats.

In rats, serotonergic innervation of the substantia nigra plays a role in the control of experimentally-elicited anxiety: punishment-induced inhibition is lessened following bilateral intra-nigral infusion of 5,7-dihydroxytryptamine (2 micrograms; 0.5 microliter). A significant correlation (0.62) is found between the loss of nigral, but not hippocampal, tryptophan hydroxylase activity and the release of behavior in two situations of shock-induced suppression of responding. Likewise, infusion of this neurotoxin (1 microgram; 0.4 microliter) into the nucleus raphé dorsalis causes an attenuation of punishment-induced suppression. These findings suggest an involvement of serotonergic raphé-nigral neurons in experimentally-elicited anxiety.