Anat Embryol (1995) 191:119-137 9 Springer-Verlag 1995 Manuela Becerra 9 Maria Jesfis Manso Isabel Rodriguez-Moldes - Ramdn Anad6n Ontogeny of somatostatin-immunoreactive systems in the brain of the brown trout (Teleostei} Accepted: 18 July 1994 Abstract The development ofsomatostatin-immunore- active neurons and fibres was studied, using immunocy- tochemistry, in the brain of the brown trout. Somato- statinergic perikarya were found in many regions in- cluding several telencephalic areas, the preoptic nucleus, anterior tuberal and lateral tuberal nuclei, the lateral recess nucleus, dorsal tuberal nucleus, the pre- and pseudoglomerular nuclei, central thalamic nucleus, op- tic tectum, interpeduncular nucleus, several isthmal and reticular nuclei and the solitary fascicle nucleus. The ventrolateral area of the telencephalon and the nucleus lateralis tuberis are the first immunoreactive nuclei to appear in ontogeny, and cells of some telencephalic ar- eas and of the lateral optic recess nucleus, the latest. Somatostatin-immunoreactive fibre tracts innervate the hypophysis and different regions of the brain. The most richly innervated areas in adults are the dorsolateral telencephalic area and the organon vasculosum laminae terminalis. Two patterns of production of somatostatin- ergic cells were observed: that of populations in which cell numbers increase over the lifetime of the fish, and that of populations whose cell number is established early in development or even diminishes in adulthood. These results provide interesting contrasts to those pre- viously reported in birds and mammals. Key words Somatostatin - Immunohistochemistry Ontogeny 9 Brain - Teleost Introduction Somatostatin-14 is a tetradecapeptide that appeared early in vertebrate phylogeny (Vale et al. 1976; King and Millar 1979; Dubois 1981). Although salmonids have M. Becerra 9 I. Rodriguez-Moldes 9 R. Anaddn ([~l) Departamento de Biologia Fundamental, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, Spain M.J. Manso Departamento de Biologia Celular y Molecular, Universidad de La Corufia, E-15007 La Corufia, Spain two different somatostatins, somatostatin-14 and so- matostatin-25, only the former is expressed in the brain (Nozaki et al. 1988; see Andrew et al. 1988). The pres- ence of somatostatin in the hypothalamo-hypophysial system of most vertebrates suggests a role for this pep- tide in hypophysiotropic regulation. Somatostatin-14 immunoreactive (SOM) neurons and fibres are also widely distributed in extrahypothalamic areas, suggest- ing a neuromodulatory and/or neurotransmitter role for this peptide in the CNS. Biochemical studies have shown that somatostatin inhibits brain adenylate cy- clase, which may represent one of the postreceptor mechanisms mediating somatostatin modulation of brain functions (Schettini et al. 1989). Immunocytochemical studies of development of so- matostatinergic systems have shown different patterns of differentiation in mammals (Shiosaka et al. 1981a,b, 1982; Yamashita et al. 1989; Forloni et al. 1990) and birds (Ambrosi et al. 1992; Nicolardi et al. 1992; Fontanesi et al. 1993). Previous developmental studies in teleosts have reported the ontogeny of some aminer- gic and peptidergic systems (serotonin: Ekstr6m et al. 1985; catecholamines: Ekstr6m et al. 1992; Manso et al. 1993; substance P: Vecino and Sharma 1992) but, as far as we are aware, no similar studies exist on somato- statinergic systems. The aim of this study was to determine the ontogeny of the somatostatin-immunoreactive system of the brown trout, and to compare it with that of other verte- brate groups. The evolution of this system in vertebrates is also discussed. Material and methods Embryos (8 mm, 9 ram, 10 mm, 14 mm; stages 24, 25, 26 and 29), alevins (16 ram, 18 mm and 22 mm; stages 31, 32 and 35), fry (28 mm and 32 mm) and adult (125 mm to 360 mm) brown trout (Salmo truttafario) were obtained from a local fish farm. Embryos, alevins and fry were deeply anaesthetized with tricaine methane sulphonate (MS-222, Sigma) and fixed by immersion in Bouin's fluid without acetic acid, embedded in paraffin, serially sectioned (8 12 gm thick in either transverse or sagittal planes) and mount-