HYPOPHYSIOTROPHIC SYSTEMS IN THE BRAIN OF THE ATLANTIC SALMON - NEURONAL INNERVATION OF THE PITUITARY AND THE ORIGIN OF PITUITARY DOPAMINE AND NONAPEPTIDES IDENTIFIED BY MEANS OF COMBINED CARBOCYANINE TRACT TRACING AND IMMUNOCYTOCHEMISTRY

The neuroanatomical organization of neurons projecting to the pituitar y and the origin of pituitary dopamine and nonapeptides were investiga ted in the brain of the Atlantic salmon (Salmo salar). Carbocyanine tr act tracing in combination with tyrosine hydroxylase, arginine vasotoc in and isotocin immunocytochemistry for double labelling revealed a pr eviously unknown organization of hypophysiotrophic cell groups and the ir extrahypothalamic projections, and provide the first direct identif ication in a teleost fish of the origin of the dopaminergic and nonape ptidergic innervation of the pituitary. The present data include ident ification of (1) hypophysiotrophic neurons in the ventral telencephalo n and in the periventricular preoptic nucleus, (2) large (magnocellula r) vasotocinergic hypophysiotrophic neurons in the most rostral extens ion of the preoptic area, (3) a distinct neuronal group located in a s upraoptic/suprachiasmatic position in the anterior periventricular nuc leus, that seems to be the major source of dopaminergic innervation of the pituitary, (4) the nonapeptidergic hypophysiotrophic neurons in t he preoptic nucleus, (5) hypophysiotrophic neurons in the ventral and posterior hypothalamus of which some are of liquor-contacting type, (6 ) projections from hypophysiotrophic and non-hypophysiotrophic neurons in the preoptic nucleus to extrahypothalamic areas such as thalamic a nd periventricular pretectal nuclei, and (7) subdivisions within the p reoptic nucleus that exhibit different combinations of hypophysiotroph ic and extrahypothalamic efferent connections. Together with previous studies of retinohypothalamic projections and neurochemical organizati on of hypothalamic/preoptic areas, the present data suggest that the p reoptic nucleus and the anterior periventricular nucleus in teleosts p ossess functional subdivisions with features that resemble those of th e paraventricular, supraoptic and suprachiasmatic nuclei of other vert ebrates. In the Atlantic salmon, specific dopaminergic and nonapeptide rgic neuronal subdivisions are proposed to play a role for photoperiod control of endocrine activity.