Cholecystokinin synapses in the sexually dimorphic central part of the medial preoptic nucleus.

The distribution of immunoreactive cholecystokinin synapses in the medial preoptic nucleus was studied. At the light microscopic level, the relative density of cholecystokinin in the nucleus corresponds to its three subdivisions. The sexually dimorphic central part of the nucleus was filled with a very dense plexus of cholecystokinin fibers/terminals. At the ultrastructural level, cholecystokinin-positive boutons contained labeled, dense core vesicles and clear vesicles as well as a population of unlabeled vesicles. The cholecystokinin boutons formed axodendritic, axosomatic, and axoaxonic interactions. Very few of the boutons were found in apposition with unlabeled elements but without any evidence of contact, i.e., synaptoid-type. Interestingly, cholecystokinin-positive, dendrite-like structures occasionally extended thin processes that were observed to end in a bouton containing both light and dense core vesicles. These varicosities were observed to have synaptoid endings. The dense innervation and synaptoid interaction of cholecystokinin elements in the central part of the medial preoptic nucleus may help to clarify the peptide's function in the medial preoptic area.