SUPPRESSION OF PROLACTIN-RELEASE IN-VITRO FROM THE RAINBOW-TROUT PITUITARY, WITH SPECIAL REFERENCE TO THE STRUCTURAL ARRANGEMENT OF THE PITUITARY-CELLS

In teleosts, prolactin (PRL) cells are localized in rostral region of the pars distalis (PD) forming clusters or follicles. The present stud y was undertaken to examine whether interactions resulting from the ar rangement of PRL cells might be involved in the regulation of PRL rele ase. To this end, the release of PRL from the rainbow trout (Oncorhync hus mykiss) and tilapia (Oreochromis mossambicus) was compared under t hree different conditions of incubation: 1) the organ culture of PRL c ells in intact PD; 2) the incubation of individual PD cells that were dispersed and subsequently attached to the culture plate; 3) the incub ation of PD cells that had been allowed to aggregate after dispersion. For the trout, PRL release from the dispersed cells was greater than that from either the organ-cultured PD or the cell aggregates. For the tilapia, by contrast, the release of PRL from dispersed cells was sim ilar to that observed during the incubation of either the organ-cultur ed PD or the cell aggregates. In both the trout and tilapia, growth ho rmone (GH) cells form clusters in the proximal PD. For both species, t he release of GH from the dispersed cells was similar to that from eit her the organ-cultured PD or the cell aggregates. For the trout, but n ot the tilapia, it would appear that the close association of PRL cell s in pituitary follicles has considerable importance in establishing b asal hormone release. The release of newly synthesized (pulse-labeled) PRL from dispersed trout PRL cells was reduced when the incubation me dium was conditioned by previous incubation of the trout PD. Neverthel ess, the release of newly synthesized PRL was not diminished by the ad dition of salmon PRL to the incubation medium. Taken together, our fin dings suggest that, for the trout, the suppression of PRL release from the PD and cell aggregates was mediated through (an) inhibitory facto r(s) within the PD other than PRL itself.