Immunocytochemical investigation of forebrain control by somatostatin of the pituitary in the teleost Poecilia latipinna

Summary An extensive system of somatostatin-immunoreactive neurons has been localized in the forebrain and pituitary of the molly (Poecilia latipinna), using the unlabelled antibody immunocytochemical method.In the hypothalamus, reactive perikarya were scattered throughout the parvocellular divisions of the preoptic nucleus. These cells were smaller in size and more ventral in position than those which stained with antisera to the neurohypophysial hormones, vasotocin and isotocin. A few very small somatostatin-immunoreactive cells were observed in the tuberal region and in the nuclei of the lateral and posterior recesses — areas which were rich in somatostatin-immunoreactive fibres.Somatostatin cells were also found in a small area of the ventral thalamus, mainly in the dorsolateral nucleus. Some of these neurons were large and multipolar, and appeared to form tracts of fibres into the posterior hypothalamus. In the telencephalon there were a few stained cells in the ventral area, with a complex pattern of fibres occurring in parts of the dorsal area.Somatostatin-immunoreactivity was intense in the central and posterior neurohypophysis, and particularly in its finger-like projections into the proximal pars distalis, around groups of growth hormone cells. Examination of material from fishes under various experimental conditions provided evidence for the somatostatin fibres originating from the preoptic neurons being involved in the control of growth hormone secretion.