Antidromic activation of a peptidergic pathway in the limbic system of the male rat.

Stimulation of the medial amygdaloid nucleus (AME) produces a long-latency and long-lasting inhibition of pyramidal cells in both the dorsal and the ventral hippocampus. The inhibition is blocked by a specific antagonist to vasopressin, which is a candidate neurotransmitter in the system. Antidromic activation of the AME from the hippocampus occurs with a latency suggestive of the conduction velocity of small diameter unmyelinated fibers. Immunocytochemistry for vasopressin reveals small diameter, unmyelinated immunoreactive fibers in the vicinity of the stimulating electrode in the hippocampus, and immunoreactive cell bodies in the vicinity of the recording electrode in the AME.