ACTH-immunoreactive boutons form synaptic contacts in the hypothalamic arcuate nucleus of rat: evidence for local opiocortin connections.

ACTH-like immunoreactive structures were localized in rat hypothalamic arcuate nucleus using the unlabeled antibody, peroxidase-antiperoxidase method. At the ultrastructural level, immunoreactive presynaptic nerve terminals were observed forming symmetrical synaptic contacts with unlabeled dendrites and with ACTH-like immunoreactive perikarya. The results obtained are consistent with the hypothesis that ACTH in the brain acts as a synaptic regulator or transmitter, contributing not only to long projection pathways but also to a local circuit in the arcuate nucleus where ACTH cell bodies are localized.