Ultrastructural relationships between serotonin and dopamine neurons in the rat arcuate nucleus and medial zona incerta: a combined radioautographic and immunocytochemical study.

Combined radioautographic and immunocytochemical detection of [3H]serotonin-labeled axon terminals and tyrosine hydroxylase-immunoreactive processes in the same thin sections allowed for electron microscopic demonstration of direct appositions between serotoninergic axonal varicosities and dopaminergic nerve cell bodies and/or dendrites in the anterior part of the arcuate nucleus and in the medial zona incerta. Although no junctional specializations were apparent at the sites of contacts, it is proposed that the observed appositions may represent a serotonin input onto tubero-infundibular and incerto-hypothalamic dopaminergic neurons. This innervation could account for some of the central neuroendocrine effects of serotonin, particularly its regulatory role on prolactin and gonadotropin secretion.