Innervation of somatostatin synthesizing neurons by adrenergic, phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive axons in the anterior periventricular nucleus of the rat hypothalamus

The adrenergic innervation of somatostatin synthesizing neurons located in the anterior region of the rat hypothalamic periventricular nucleus was studied by means of a light and electron microscopic immunocytochemical double labelling technique. This region which is the source of hypophysiotrophic somatostatin immunoreactive (IR) neurons also receives a dense plexus of adrenergic axons as determined by immunocytochemistry of phenylethanolamine-N-methyltransferase (PNMT), the marker enzyme for the central adrenergic system. The simultaneous detection of PNMT and somatostatin antigens in hypothalamic sections of colchicine pretreated animals revealed a congruency in the distribution of the labelled elements and also close juxtaposition of PNMT-IR axons to somatostatin producing neurons. At the ultrastructural level, axo-somatic and axo-dendritic synaptic connections were found between PNMT-containing axons and somatostatin expressing neurons. These morphological findings support the view that the central adrenergic system might influence the production and secretion of growth hormone in the pituitary gland by a direct monosynaptic interaction with somatostatin synthesizing neurons.