Lactation decreases angiotensinogen mRNA expression in the midcaudal arcuate nucleus of the rat brain

In lactating rats, ANG II receptor binding in the arcuate nucleus (ARH) and median eminence is decreased. To further evaluate brain angiotensinergic a ctivity during lactation, we assessed angiotensinogen (AON) mRNA by in situ hybridization in forebrains of day 10 or 11 postpartum lactating and diest rous rats. AON mRNA was abundantly expressed in the ARH, preoptic, suprachi asmatic, supraoptic, paraventricular, and dorsomedial hypothalamic nuclei, and other regions, similar to that reported in male rat brains. AON mRNA le vels were decreased 27% in the midcaudal ARH of lactating rats but did not differ between lactating or diestrous rats in any of the other brain areas examined. Immunofluorescence for AON and glial fibrillary acidic protein or tyrosine hydroxylase confirmed that the AON immunoreactivity in the ARH wa s limited to astrocytes. Confocal microscopy revealed close appositions of AON-positive astrocytes to dopaminergic neurons in the ARH. The decrease in AON mRNA in the midcaudal ARH during lactation coupled with decreased ARH ANG II receptor binding suggests that lactating rats are less subject to AN G II-mediated inhibition of prolactin secretion.