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.

The ultrastructure of each adenohypophysial secretory cell type was examined in pituitaries of adult female mollies (Poecilia latipinna) after various periods in vitro, and with varied medium osmotic pressure (OP), Na+, and Ca2+ concentrations. Prolactin (PRL) cells were markedly activated by 18 hr, and after 7 or 14 days were almost totally degranulated, with massive arrays of Golgi and RER. Reduction in OP, but not Na+ or Ca2+, caused an additional activation of PRL cells after periods of 18 hr or longer. Corticotroph (ACTH) cells became noticeably activated by 4 hr, and were possibly affected by OP, but not Na+ or Ca2+. Growth hormone (GH) cells were activated by 6 hr, and after 18 hr were quite degranulated with extensive arrays of RER. OP had no effect on GH cells before 3 days, when reduced OP appeared to cause an additional activation, with the appearance of large irregular secretory granule (SG)-like inclusions. Na+ and Ca2+ again had no effect. Gonadotrophic (GtH) cells appeared to be little affected by in vitro incubation; however, the very active cells from vitellogenic fish underwent a reduction in dilated RER after prolonged culture. Thyrotrophic (TSH) cells gradually became activated in vitro, but the response again varied with the sexual condition of the fish. Neither GtH nor TSH cells were affected by OP, Na+, or Ca2+. The findings are discussed in relation to hypothalamic control, via releasing/inhibiting factors, of adenohypophysial cell activity.

Male killifish adapted to calcium-free sea water developed hypocalcemia. In the pituitary the PAS-positive cells of the pars intermedia (PIPAS cells) displayed a strong reaction, increasing in numbers and becoming enlarged and more active. A very minor response was shown by a small minority of the prolactin cells which became marginally activated. No other adenohypophysial cell type responded to calcium deprivation, and no response was detected in the neurohypophysis. It is suggested that the PIPAS cell may secrete a hypercalcemic hormone which plays a major role in calcium regulation in sea water.

Fish were adapted to black (B) or white (W) backgrounds for 28 days, or moved between B and W for shorter periods. Compared to W, B induced a darker dorsal surface, the recruitment of new ventral melanophores, and the dispersion of melanin in denervated caudal fin melanophores. These responses were identical in fish adapted to seawater (SW), one-third seawater (13 SW), or fresh water (FW). The effects of B were abolished (melanophore recruitment, melanin dispersion) or attenuated (dorsal darkening) by hypophysectomy. B did not affect pituitary MSH content or plasma cortisol levels. Examination of the pituitary showed that while the prolactin cells were activated by reduced salinity, only the pars intermedia PAS-positive cell (PIPAS cell) was activated by B, the PI melanotrope (PIPbH cell) and all the pars distalis cells being similar in structure on B and W. Neither the PIPAS nor the PIPbH cell was affected by the different salinities. The neurohypophysis showed no response to background. It is concluded that the melanophore responses to B are mediated by an unknown hormone secreted by the PIPAS cell, and reasons are adduced for believing that prolactin, ACTH, and MSH are not involved in these responses.