Received March 14, 2002; Revised April 24, 2002; Accepted April 24, 2002. Author to whom all correspondence and reprint requests should be addressed: Dr. Flavio Mena, Institute of Neurobiology, National University of Mexico, Campus UNAM, Juriquilla, Queretaro, 76001, Mexico. E-mail: fmena@ servidor.unam.mx Regional Mechanisms Within Anterior Pituitary of Lactating Rats May Regulate Prolactin Secretion Nestor Diaz, Icnelia Huerta, Nephtali Marina, Nilda Navarro, and Flavio Mena Department of Cellular and Molecular Neurobiology, Institute of Neurobiology, National University of Mexico, Campus UNAM, Juriquilla, Queretaro, Mexico Endocrine, vol. 18, no. 1, 41–46, June 2002 0969–711X/02/18:41–46/$11.50 © 2002 by Humana Press Inc. All rights of any nature whatsoever reserved. 41 Prolactin (PRL) release was compared after incubating the central and peripheral regions of the anterior pitui- tary of lactating rats, either nonsuckled or suckled in conditioned medium obtained by incubating medium with the same anterior pituitary regions from nonsuckled or suckled rats. To collect conditioned media, anterior pituitary regions were incubated in Earle’s medium for 4 h, and conditioned medium was filtered and employed double concentrated. Each anterior pituitary was incu- bated in conditioned medium for 30 min. PRL released in vitro was determined by polyacrylamide gel electro- phoresis. As a control, anterior pituitary regions from lactators were incubated in medium conditioned by male rat anterior pituitary regions, and they showed no changes of PRL release compared with those cultured in Earle’s medium. In general, conditioned media from both anterior pituitary regions of nonsuckled and suck- led rats inhibited PRL release in peripheral anterior pitu- itary regions, whereas PRL release was stimulated in central regions of both nonsuckled and suckled rats. A higher number of stimulatory effects was provoked by conditioned media from suckled than from nonsuckled rats, and most of these effects were from conditioned media of the peripheral region of suckled rats. Together, these results suggest the existence within anterior pitu- itary regions of factors that regulate PRL secretion and that their action depends on the physiologic condition of the animal. Key Words: Anterior pituitary; prolactin; lactation; regionalization. Introduction The secretion of prolactin (PRL) by the anterior pituitary is a complex neuroendocrine phenomenon whose regula- tion is systemic, hypothalamic, and locally produced; that is, autocrine and paracrine factors participate (see refs. 1 and 2 for reviews). Regional and lactotroph heterogeneity has been documented (3–5) regarding cell size, secretory capacity, responsiveness to secretagogues, as well as the influ- ences of other PRL secretagogues from the posterior and/ or neurointermediate lobes of the pituitary (PNIL), which, reaching the anterior pituitary via the short portal vessels, may physiologically stimulate PRL release and may influ- ence PRL regionalization ([6–8]; see ref. 5 for review). In a previous study, we analyzed the effect of remov- ing the PNIL of the pituitary gland of lactating rats on the suckling-induced transformation and release of PRL and on regionalization of its secretion (9). The results of this and other studies (10–13) suggested that the PNIL may play a complementary role on in vivo and in vitro PRL release, and it may influence PRL regionalization, even though fac- tors from the PNIL showed no effect on suckling-induced PRL transformation (9). In the present study, we used a different approach to determine whether within the central (i.e., the region of the anterior pituitary surrounding the neurointermediate and posterior lobes of the pituitary) and the rest of the gland of lactating rats there exist mechanisms capable of influenc- ing each others’ secretion of PRL and whether the type of effect exerted varied according to the physiologic condi- tion of the animal (i.e., suckled or nonsuckled). Conditioned media obtained by incubation of each region of anterior pituitaries from rats previously nonsuckled 6 h or from rats nonsuckled 6 h and then suckled for 15 min were employed to incubate anterior pituitary regions from animals in the same (i.e., nonsuckled and suckled) physiologic condition, and their effects were determined on PRL release. Results Effect of Conditioned Media from Incubation of Anterior Pituitary Regions of Male Rats on PRL Release from Anterior Pituitary Regions of Lactating (nonsuckled and suckled) Rats As shown in Table 1, conditioned media from anterior pitu- itary regions of male rats had no effect on PRL release from anterior pituitary regions of either nonsuckled or suckled rats.