The suckling stimulus increases the responsiveness of mammotropes located exclusively within the central region of the adenohypophysis.

Studies from several groups, including our own, have shown that the suckling stimulus increases the responsiveness of pituitary cells to PRL-releasing stimuli. These findings, when viewed in light of differences in PRL cell responsiveness from one pituitary region to another, raised the possibility that suckling may influence responsiveness of cells in only a specific portion of the gland rather than in the entire pituitary. To address this issue, we evaluated cell responsiveness by performing plaque assays [with and without TRH, Angiotensin II (AII), and dopamine] on cells from two different regions of pituitaries from suckled and nonsuckled rats. These pituitary regions consisted of the inner zone, which is a central area proximate to the neurointermediate lobe, and an outer zone, which encompasses the remaining peripheral area of the anterior lobe. We found that inner zone cells from nonsuckled animals were highly responsive to dopamine and relatively unresponsive to TRH and AII. However, after suckling, a complete shift occurred with inner zone cells becoming sensitive to TRH and AII and resistant to dopamine. In contrast to these inner zone alterations, outer zone cells did not change after suckling, but remained responsive to TRH and AII and unresponsive to dopamine. Our results demonstrate clearly that suckling-induced alterations in PRL cell responsiveness to certain modulatory agents can be attributed to a discrete subpopulation of cells located in a specific region of the pituitary.