Organization of galanin‐immunoreactive inputs to the paraventricular nucleus with special reference to their relationship to catecholaminergic afferents

Abstract

Immunohistochemical and axonal transport techniques were used to characterize the origin and distribution of galanin‐immunoreactive inputs to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus in the rat. In the parvicellular division of the PVH, the most prominent inputs were confined to the anterior and periventricular parts of the nucleus rostrally and the dorsal and ventral medial subdivisions caudally; the galaninergic imputs to the magnocellular division of PVH and SO were very sparse and were preferentially distributed to regions containing predominantly oxytocinergic neurons. A combined retrograde transport‐immunohistochemical method was employed to identify sources of these projections. Galanin immunoreactivity was found to coexist with dopamine‐β‐hydroxylase (DBH) immunoreactivity in subsets of retrogradely labeled neurons of the A1 and A6 (locus coeruleus) catecholamine cell groups; no evidence was adduced for the presence of galanin in adrenergic (i.e., phenylethanolamine‐N‐methyltransferase‐positive) neurons that project to the PVH. Apart from minor contributions from the mesencephalic raphe nuclei, no other brain‐stem cell groups contributed to the galaninergic innervation of the PVH. In the forebrain, the most prominent grouping of doubly labeled cells was centered in the rostral part of the dorsomedial nucleus of the hypothalmus (DMH), though significant numbers were also found in the lateral hypothalamic area, the arcuate nucleus, and the medial preoptic area. In experiments designed to define the subnuclear specificity of some galanin‐containing inputs to the PVH, iontophoretic deposits of the anterogradely transported plant lectin, Phaseolus vulgaris‐leucoagglutmin (PHA‐L), were placed in the A1 and A6 cell groups and in the DMH. Sections through the PVH were prepared so as to allow colocalization of anterogradely transported PHA‐L and galanin immunoreactivity in individual fibers and vari‐cosities. Consistent with the retrograde transport data, the greatest degree of galanin‐PHA‐L correspondence was seen after lectin deposits in the DMH, and over 80% of the doubly labeled varicosities were confined to the anterior, periventricular, and medial parvicellular subdivisions of the nucleus. The galanin‐containing projection from the locus coeruleus was most circumscribed, with the vast majority of doubly labeled varicosities confined to the periventricular and adjoining aspects of the anterior and medial parvicellular subdivisions. Finally, the galaninergic component of the A1 projection to the PVH was most broadly distributed, with some 25% of the doubly stained elements found in magnocellular regions, and the remainder distributed rather uniformly throughout the parvicellular division. These results implicate galanin as an important neuroactive agent in a highly ordered set of afferent pathways that are in a position to influence selectively distinct neuroendocrine and autonomic effector neuron populations in the PVH.