Neural interaction between galanin-like peptide (GALP)- and luteinizing hormone-releasing hormone (LHRH)-containing neurons.

Galanin-like peptide (GALP), commonly known as an appetite-regulating peptide, has been shown to increase plasma luteinizing hormone (LH) through luteinizing hormone-releasing hormone (LHRH). This led us to investigate, using both light and electron microscopy, whether GALP-containing neurons in the rat brain make direct inputs to LHRH-containing neurons. As LHRH-containing neurons are very difficult to demonstrate immunohistochemically with LHRH antiserum without colchicine treatment, we used a transgenic rat in which LHRH tagged with enhanced green fluorescence protein facilitated the precise detection of LHRH-producing neuronal cell bodies and processes. This is the first study to report on synaptic inputs to LHRH-containing neurons at the ultrastructural level using this transgenic model. We also used immunohistochemistry to investigate the neuronal interaction between GALP- and LHRH-containing neurons. The experiments revealed that GALP-containing nerve terminals lie in close apposition with LHRH-containing cell bodies and processes in the medial preoptic area and the bed nucleus of the stria terminalis. At the ultrastructural level, the GALP-positive nerve terminals were found to make axo-somatic and axo-dendritic synaptic contacts with the EGFP-positive neurons in these areas. These results strongly suggest that GALP-containing neurons provide direct input to LHRH-containing neurons and that GALP plays a crucial role in the regulation of LH secretion via LHRH.