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 tu berohypophysial dopaminergic (THDA) neurons are osmosensitive and regu late the secretion of POMC peptides (alpha-MSH and beta-endorphin) for m the intermediate lobe (IL) and dopamine is known to be the major phy siological regulator of PRL secretion we have investigated the effect of dehydration on plasma levels of alpha-MSH ACTH and beta-endorphin d etected after suckling stimulus in lactating rats. Changes in the hydr omineral balance induced by salt loading severly attenuated plasma lev er of alpha-MSH. This effect seems to be specific on peptides secreted by melanotropes because neither basal nor suckling-induced ACTH secre ted by corticotropes changed during dehydration. Reduction in plasma l evel of beta-endorphin due to 4 h separation was intensified and the s uckling-induced response was attenuated in dehydrated mothers. In summ ary, the simultaneous and resembling changes in the release of alpha-M SH, 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 h ypothalamic pathways.