Changes in the local metabolism of dopamine in the anterior and neural lobes but not in the intermediate lobe of the pituitary gland during nursing

Abstract

Dopamine (DA), produced by tubero-infundibular dopaminergic (TIDA) neurons of the arcuate nucleus (ARN) is the established inhibitor of the secretion of prolactin (PRL). Changes in dopaminergic (DAergic) neuronal activity in the median eminence-long portal vessels (ME-LPV) and/or the concentration of DA in the anterior lobe (AL) are inversely related to the secretion of PRL. However, conflicting reports concerning DAergic neuronal activity during the suckling-induced release of PRL persist. In addition to TIDA neurons, PeVN-hypophysial DAergic (PHDA) and tubero-hypophysial DAergic (THDA) neurons which, respectively, innervate the intermediate lobe (IL) and the IL/neural lobe (NL) also have a significant role. We measured the concentrations of DA and its main metabolite, 3, 4-dihydroxyphenylacetic acid (DOPAC), in the median eminence and the three pituitary lobes of lactating mothers. Concentrations of DA and DOPAC from tissues and the concentration of PRL in plasma were measured by HPLC-EC and RIA, respectively. There were no changes in the concentration of DA and DOPAC of the IL due to the suckling stimulus. In the NL, a decrease in the concentration of DOPAC was detected due to the suckling stimulus. In addition, there were no changes of DA or DOPAC in the outer zone of the AL (AL-OZ) due to suckling. However, a decrease in the concentrations of DA and DOPAC was detected in the inner zone of the AL (AL-IZ). These data suggest lactotrophs from the AL-IZ are responsible for the changes in the concentration of plasma PRL in response to the suckling stimulus. In addition TIDA and THDA neurons, but not PHDA neurons, regulate the control of the secretion of PRL in response to suckling.