Catecholaminergic system innervates galanin-immunoreactive neurons in the human diencephalon

Galanin released into the hypophysial portal circulation in the hypothalamus may function as a hypophysiotropic factor regulating the anterior pituitary function or it may function as a neurotransmitter/neuromodulator acting at synaptic sites regulating neuronal activity of many neurons in the brain. Catecholamines (adrenaline, noradrenaline, and dopamine) primarily regulate anterior pituitary functions indirectly via innervating hypophysiotropic neurons. The aim of the present studies was to explore with double-label immunocytochemistry if, as in rodents, catecholamines interact with galanin in the human diencephalon. Due to the long post-mortem period and subsequent lack of optimal preservation of the cell membranes in the brain, electron microscopy could not be employed to show the presence of catecholaminergic-immunoreactive synapses on galanin-immunoreactive neurons. Therefore, we used light microscopic immunocytochemistry and high-magnification microscopy with oil immersion to identify putative juxtapositions between catecholamines and galanin-utilizing antisera against key enzymes of catecholamine synthesis (tyrosine hydroxylase (TH), representing all three catecholamines; dopamine-beta-hydroxylase (DBH), representing noradrenaline; and phenylethanolamine-N-methyltransferase (PNMT), representing adrenaline) and galanin. Our studies show that among the three catecholamines, dopamine is the most abundant and the vast majority of catecholaminergic contacts on galanin-immunoreactive neurons is dopaminergic. The number of DBH-immunoreactive contacts is less and the number of PNMT-immunopositive contacts is negligible. Among the hypothalamic regions, the periventricular region above the infundibulum (infundibular or arcuate nucleus) contained the largest number of contacts. These en passant-type intimate associations between catecholamine- and galanin-immunoreactive neuronal elements may be functional synapses and may provide the morphological basis for the catecholamine-mediated galanin release.