NEUROANATOMICAL ANALYSIS OF THE VISUAL AND HYPOPHYSIOTROPHIC SYSTEMS IN ATLANTIC SALMON (SALMO-SALAR) WITH EMPHASIS ON POSSIBLE MEDIATORS OF PHOTOPERIODIC CUES DURING PARR SMOLT TRANSFORMATION

To identify the neural components in the brain of the Atlantic salmon (Salmo salar) that receive and relay photoperiodic cues, the neuroanat omical and neurochemical properties of the visual and hypophysiotrophi c system have been investigated by means of neural tract tracing in co mbination with immunocytochemical localization of tyrosine hydroxylase , arginine vasotocin and isotocin. The systems were characterized in p arr and were then studied during parr-smolt transformation. The data s how that: (1) different putative dopaminergic diencephalic optic nucle i receive both retinal and pineal innervation and possess projections to the optic tectum, (2) a dopaminergic neuronal population in the lat eral habenular nucleus seems to receive retinal, tectal and pineal inp uts, (3) an extensive retino-hypothalamic innervation coincides with p utative dopaminergic and peptidergic neurons, (4) several of the pepti dergic neurosecretory neurons innervate the pituitary but possess also projections to central optic nuclei and the optic tectum, (5) a disti nct group of putative dopaminergic neurons in the ventral pre-optic ar ea receives extensive retinal innervation and possesses major projecti ons to the pituitary, (6) during parr-smolt transformation, additional pinealofugal termination fields appear that coincide with retinal ter mination fields in the dopaminergic and peptidergic neuronal portion o f the hypothalamic optic nucleus. The data suggest that dopaminergic a nd peptidergic neurons may serve as interfaces between the visual syst em, the pituitary and extra-hypothalamic brain areas. The discovery of additional pineal projections to the hypophysiotrophic portion of the retinorecipient hypothalamic optic nucleus in smolts further emphasiz es that the hypothalamic optic nucleus constitutes a photo-neuroendocr ine control center that may play an important role during parr-smolt t ransformation.