DEHYDRATION DECREASES PLASMA-LEVEL OF ALPHA-MELANOCYTE-STIMULATING HORMONE (ALPHA-MSH) AND ATTENUATES SUCKLING-INDUCED BETA-ENDORPHIN BUT NOT ACTH RESPONSE IN LACTATING RATS

When lactating rats are exposed to a short term dehydration, anterior lobe (AL) prolactin (PRL) secretion is immediately depressed. Since tuberohypophysial dopaminergic (THDA) neurons are osmosensitive and regulate the secretion of POMC peptides (alpha-MSH and beta-endorphin) form the intermediate lobe (IL) and dopamine is known to be the major physiological regulator of PRL secretion we have investigated the effect of dehydration on plasma levels of alpha-MSH ACTH and beta-endorphin detected after suckling stimulus in lactating rats. Changes in the hydromineral balance induced by salt loading severly attenuated plasma lever of alpha-MSH. This effect seems to be specific on peptides secreted by melanotropes because neither basal nor suckling-induced ACTH secreted by corticotropes changed during dehydration. Reduction in plasma level of beta-endorphin due to 4 h separation was intensified and the suckling-induced response was attenuated in dehydrated mothers. In summary, the simultaneous and resembling changes in the release of alpha-MSH, beta-endorphin and PRL support our assumption that the regulation of AL and IL is closely coupled and THDA system is one of the common hypothalamic pathways.