Galanin immunoreactivity in hypothalamic histamine neurons: Further evidence for multiple chemical messengers in the tuberomammillary nucleus

Abstract

By using a specific antibody against the 29 amino‐acid peptide galanin (Gal) with light and electron microscopic immunocytochemistry, we have studied the distribution of Gal immunoreactivity in the posterior hypothalamic magnocellular neurons of the rat. In colchicine‐treated rats, a large number of Gal‐immunoreactive cells were identified within all subdivisions of the tuberomammillary nucleus. The majority of these cells are large multipolar or fusiform neurons, with long, sparsely branching dendrites. A small number project to the ventral hippocampus, as shown by experiments with the retrograde tracing of Fast Blue. Ultrastructural examination of the Gal‐immunoreactive cells confirms their indentity as magnocellular neurons, with dense deposits of immunoreaction product, particularly in small ribosomal arrays and in large, dense‐cored vesicles. Axosomatic synapses occur on these neurons. The axonal boutons synapse with asymmetric and symmetric junctions and contain small synaptic vesicles as well as numerous large, dense‐cored vesicles, which display Gal immunoreactivity. Sequential staining of thin, alternate sections with antibodies against Gal and Lhistidine decarboxylase (HDCase; EC 4.1.1.22) showed colocalization of Galand HDCase‐immunoreactivities in a majority of tuberomammillary neurons. The finding of Gal immunoreactivity within histamine‐producing neurons of the tuberomammillary nucleus adds to the multiplicity of potential neuronal messengers utilized by these cells.