Luteinizing hormone releasing hormone containing synapses in the diagonal band and preoptic area of the guinea pig.

Light microscopic immunocytochemical processing of the region of the ventral diagonal band of Broca (vertical limb) and the medial preoptic area of the guinea pig for LHRH revealed complex axonal bundles. The axons were of several different shapes and widths and showed varicosities of varying form. When such tissue was embedded and processed for electron microscopic localization of immunoreaction product, numerous LHRH-positive synapses were seen. The synapses were made by large varicosities. They were all axodendritic and showed a well-defined synaptic cleft, as well as a clustering of immunoreactive large granules and immunonegative small, clear vesicles. Postsynaptic densities were either very small or absent. These data show that LHRH is present in direct synapses within the CNS and is, therefore, in a position to be released and act as a neurotransmitter/neuromodulator as well as a neurohormone.