ONTOGENY OF SOMATOSTATIN-IMMUNOREACTIVE SYSTEMS IN THE BRAIN OF THE BROWN TROUT (TELEOSTEI)

The development of somatostatin-immunoreactive neurons and fibres was studied, using immunocytochemistry, in the brain of the brown trout. S omatostatinergic perikarya were found in many regions including severa l telencephalic areas, the preoptic nucleus, anterior tuberal and late ral tuberal nuclei, the lateral recess nucleus, dorsal tuberal nucleus , the pre- and pseudoglomerular nuclei, central thalamic nucleus, opti c tectum, interpeduncular nucleus, several isthmal and reticular nucle i and the solitary fascicle nucleus. The ventrolateral area of the tel encephalon and the nucleus lateralis tuberis are the first immunoreact ive nuclei to appear in ontogeny, and cells of some telencephalic area s and of the lateral optic recess nucleus, the latest. Somatostatin-im munoreactive fibre tracts innervate the hypophysis and different regio ns of the brain. The most richly innervated areas in adults are the do rsolateral telencephalic area and the organon vasculosum laminae termi nalis. Two patterns of production of somatostatinergic cells were obse rved: that of populations in which cell numbers increase over the life time of the fish, and that of populations whose cell number is establi shed early in development or even diminishes in adulthood. These resul ts provide interesting contrasts to those previously reported in birds and mammals.