Journal of Neuroendocrinology, 1997, Vol. 9, 859–865 Changes in Oxytocin Receptor mRNA in Rat Brain During Pregnancy and the Effects of Estrogen and Interleukin-6 Larry J. Young, Scott Muns, Zuoxin Wang and Thomas R. Insel* Department of Psychiatry and Behavioral Sciences and *Yerkes Regional Primate Center, Emory University School of Medicine, Atlanta, GA, 30322, USA. Key words: maternal behavior, cytokines, hypothalamus, preoptic area, amygdala. Abstract Changes in brain oxytocin receptor (OTR) binding sites during the course of pregnancy may influence the sudden onset of maternal behavior in female rats at parturition. In situ hybridization was used to identify changes in OTR messenger ribonucleic acid (mRNA) expression during pregnancy and parturition. Two patterns of mRNA regulation were observed. Relative to diestrus virgin control females, OTR mRNA was elevated in the lateral septum and medial preoptic area at days 13–15 of pregnancy but not on the morning of parturition. In the central nucleus of the amygdala and ventromedial nucleus of the hypothalamus (VMH), OTR mRNA was most abundant on the morning of parturition. Strong signals for OTR mRNA were detected in the bed nucleus of the stria terminalis, hypothalamic paraventricular nucleus, supraoptic nucleus and suprachiasmatic nucleus. However no group differences were detected in these areas. As estrogen and interleukin-6 have been suggested to modulate OTR gene expression and both are elevated at the time of parturition, their effects on OTR mRNA in the brain were examined. Estrogen and interleukin-6, given simultaneously, significantly elevated the concentration of OTR mRNA in the VMH, but not in the amygdala. The increase in the VMH was significantly greater than that produced by estrogen alone, and interleukin-6 alone had no effects. These results demonstrate that transcriptional regulation of OTR gene expression mediates changes in receptor density in the brain in a region specific manner during pregnancy and suggests a potential mechanism for some of these changes. Oxytocin (OT ) plays a crucial role in several functions necessary and IL-6 increase at the onset of spontaneous labor in humans (19), these substances could mediate the changes in OTR gene for reproduction in the female rat, including sexual behavior (1, 2), uterine contractions during parturition (3 ), lactation (4) and expression in the uterus and brain associated with parturition. Therefore, we examined the influences of estrogen and IL-6 on maternal behavior ( 5, 6). Both peripheral and central actions of OT are mediated by a member of a G-protein coupled, seven the expression of brain OTR in ovariectomized rats. transmembrane domain family of receptors (7, 8). Oxytocin receptor (OTR) mRNA is increased several-fold at parturition in Results rat and human uterus (9, 10 ). Oxytocin receptor binding and mRNA expression in the uterus and in select brain regions are In situ hybridization with the antisense probe resulted in specific regulated by estrogen (9, 11–13). Quantitative analysis of OTR labeling in each of the areas which exhibit strong binding with by receptor autoradiography has demonstrated increases in recep- receptor autoradiography, including the lateral septum (LS ), bed tor binding in the hypothalamus, amygdala and bed nucleus of nucleus of the stria terminalis (BnST ), central amygdala (AmC ), the stria terminalis at parturition (14, 15). In the rat, the onset and the ventromedial nucleus of the hypothalamus (VMH) of maternal behavior coincides with these increases in OTR (Fig. 1). In addition, a strong signal was detected in several other binding. Central infusions of OT antiserum ( 16) or a selective regions previously reported to express OTR mRNA, including the paraventricular nucleus of the hypothalamus ( PVN), medial antagonist (5) delays the onset of maternal behavior. The present study was designed to characterize the changes in preoptic area (MPOA ), suprachiasmatic nucleus (SCN ) and the supraoptic nucleus (SON ), although strong binding was not OTR mRNA expression during pregnancy and at parturition as well as to investigate a possible mechanism of these changes. In detected with receptor autoradiography in these areas. Sense strand control hybridization gave a uniform background over all situ hybridization was used to quantify changes in mRNA expres- sion during pregnancy in specific brain regions. The 5∞ flanking of these regions. The results of the quantitative analysis of OTR mRNA expres- region of the OTR gene contains several sequence elements known to mediate genomic responses to estrogen as well as interleukin sion in the brains of diestrus virgins, pregnant (days 13–15), and parturient females are presented in Fig. 2. Significant group 6 (IL-6) (17, 18). As circulating concentrations of both estrogen Correspondence to: Larry J. Young, Department of Psychiatry and Behavioral Sciences, AF, Emory University School of Medicine, Atlanta, GA 30322, USA. © 1997 Blackwell Science Ltd