Adrenergic innervation of dopamine neurons in the hypothalamic arcuate nucleus of the rat

Abstract

Tuberoinfundibular dopaminergic (TIDA) neurons, which represent the final common pathway in the inhibitory neuronal control of prolactin (PRL) secretion, are regulated by synaptic input from various transmitter systems. Because adrenergic receptors at hypothalamic sites were implicated in the central regulation of lactotrophs, we hypothesized that a synaptic communication might exist between adrenergic pathways ascending from the brain stem and the TIDA system. Polyclonal antisera directed towards phenylethanolamine N-methyltransferase (PNMT) and tyrosine hydroxylase (TH), biosynthetic enzymes of catecholamines, were used for the simultaneous immunocytochemical detection of adrenergic fibers and TIDA neurons, respectively, in Vibratome sections of the rat hypothalamus. By the light microscopic evaluation of double-immunostained sections, PNMT-immunoreactive (IR) axon varicosities were localized in juxtaposition to TH-IR cell bodies and dendrites in the arcuate nucleus (AN) which contains perikarya and dendrites of TIDA neurons. The ultrastructural analysis of contacts provided firm evidence for the occurrence of synaptic interactions between the adrenergic and TIDA neuronal systems. These morphological findings show that adrenergic neurons are involved in the afferent regulation of the TIDA system and indicate a putative pathway of central adrenergic effects upon PRL secretion.