DOES ALPHA-MELANOCYTE-STIMULATING HORMONE FROM THE PARS-INTERMEDIA REGULATE SUCKLING-INDUCED PROLACTIN-RELEASE - SUPPORTIVE EVIDENCE FROM MORPHOLOGICAL AND FUNCTIONAL-STUDIES

Recent evidence suggests that substances derived from the hypophyseal intermediate lobe (IL) play a crucial role in the regulation of suckli ng-induced PRL secretion. The purpose of the present study was to expl ore this possibility further by determining whether the suckling stimu lus acutely increases the secretory activity of the IL and whether alp haMSH, a major secretory product of the IL, plays a specific role in s uckling-induced PRL release. Light microscopic morphometric analysis o f serial pituitary sections obtained from lactating rats revealed that as little as 1 min of suckling caused a significant increase in the p roportion of the IL that was in secretory configuration (11.8 +/- 0.7% vs. 6.7 +/- 0.5%; 1-min suckled vs. nonsuckled control; mean +/- SE). Moreover, the fraction of the IL in secretory configuration continued to increase after 5 and 10 min of nursing (to 16.0 +/- 0.8% at 5 min and 18.2 +/- 0.7% at 10 min). In contrast, serum PRL was not significa ntly elevated above the control level after 1 min of suckling (18.1 +/ - 13.5 vs. 9.9 +/- 6.5 ng/ml, 1-min suckled vs. control). In fact, a s ignificant rise in PRL levels (to 314.4 +/- 19.4 ng/ml) could be detec ted only after 10 min of nursing. Thus, secretion by the IL in respons e to suckling preceded the release of adenohypophyseal PRL, suggesting that a secretory product(s) from the pars intermedia is involved in t he modulation of nursing-induced PRL release. Having established a seq uential temporal relationship between these two phenomena, we next inv estigated whether alphaMSH was the IL factor involved in the regulatio n of suckling-induced PRL secretion. To this end, lactating rats were injected either with antiserum to alphaMSH or preimmune serum and then allowed to nurse their pups. Serial blood samples were taken from the mothers 15, 30, 60, and 90 min after the litters were returned, and s erum PRL was measured by RIA. We found that the suckling-induced rise in serum PRL was severely attenuated in animals that received anti-alp haMSH serum. This suppression was most evident at 15 min (70.1 +/- 13. 4 vs. 323.5 +/- 127.0 ng/ml, antibody treated vs. preimmune serum cont rol) and persisted throughout the entire 90-min test period. When take n together, our results suggest that suckling-induced PRL secretion is mediated at least in part by alphaMSH released from the hypophyseal I L.