Colocalization of galanin and luteinizing hormone-releasing hormone in a subset of preoptic hypothalamic neurons: anatomical and functional correlates.
Colocalization of neurotransmitters, including neuropeptides and amines, in the same neuron of certain areas or well-defined nuclei of the central and peripheral nervous systems appears to be the rule rather than the exception. The coexistent neurotransmitters can be coreleased and interact at pre- and postsynaptic levels in a synergistic or antagonistic manner. Galanin is a recently isolated and characterized "gut-brain" peptide. It is colocalized with many neurotransmitters in both the central and the peripheral nervous systems. Among other regions in the central nervous system, galanin is present in neuronal perikarya of the septum and the hypothalamus. The dense accumulation of nerve terminals in the external zone of the median eminence suggests that galanin is an important peptide regulating neuroendocrine functions. Although most galanin and luteinizing hormone-releasing hormone (LHRH) neurons have a distinctly different morphology, a subset of galanin-immunoreactive perikarya in the diagonal band of Broca and the medial preoptic area, near the organum vasculosum of the lamina terminalis, have morphological features similar to those of LHRH neurons. By using double-labeling immunocytochemistry, we have found that in the preoptic region of the male rat brain approximately 15-20% of these "LHRH-like" galanin-immunoreactive neurons are also immunopositive for LHRH. Moreover, in the medial preoptic area and the diagonal band of Broca, some of the single-labeled LHRH cells are surrounded with galanin-immunoreactive nerve terminals, suggesting that LHRH perikarya have synaptic contacts with galanin-immunoreactive terminals. Additional studies indicated that galanin can readily enhance in vitro release of LHRH from nerve terminals in the median eminence. The observations that (i) galanin is coexpressed with LHRH, (ii) galanin seems to innervate LHRH-producing neurons, and (iii) galanin acts as a putative neurotransmitter to enhance the release of LHRH suggest that galanin should be considered an important regulator of LHRH-containing neurons and, therefore, of reproductive functions.