Alterations of medial preoptic area neurons following pregnancy and pregnancy-like steroidal treatment in the rat.

There is a marked increase in the maternal behavior displayed by a female rat following pregnancy-due primarily to exposure to the gonadal hormones progesterone and estradiol (P and E(2), respectively). We examined Golgi-Cox silver-stained, Vibratome-sectioned neurons visualized and traced using computerized microscopy and image analysis. In Part One, we examined the hormonal-neural concomitants in the medial preoptic area (mPOA), an area of the brain that regulates maternal behavior, by comparing cell body size (area in microm(2); also referred to as soma and perikaryon) in the mPOA and cortex of five groups (n = 4-6/group) of ovariectomized (OVX-minus), diestrous, sequential P and E(2)-treated (P+E(2)), late-pregnant, and lactating rats; for Part Two, we examined a subset of mPOA neurons, which were traced in their entirety, from these same subjects. In Part One, whereas there was no difference between OVX-minus and diestrous females, both had smaller somal areas compared to OVX+P+E(2)-treated and late-pregnant females. The area of the soma returned to diestrous/OVX-minus levels in the lactating females. We found no change among the five groups in area of cell body in cortical neurons, which generally lack steroid receptors. In Part Two, which included a more detailed morphometric analysis of mPOA neurons, we examined several additional measures of dendritic structure, including number of proximal dendritic branches (the largest proximal dendrite was defined as the one with the largest diameter leaving the soma); cumulative length of the largest proximal dendrite; area of the cell body; number of basal dendrites; cumulative basal dendritic length; number of basal dendritic branches; and branch-point (distance from cell body to first branch of largest proximal dendrite). Again, we found similar effects on cell body size as in Part One, together with effects on number of basal dendritic branches and cumulative basal dendritic length in pregnant and P+E(2)-treated groups compared to OVX, diestrous, and lactating. An increase in somal area denotes increased cellular activity, and stimulatory effects on additional neuronal variables represents modifications in information processing capacity. Pregnancy and its attendant hormonal exposure, therefore, may stimulate neurons in the mPOA, which then contribute (in an as yet undetermined manner) to the display of maternal behavior. During the postpartum lactational period, when cues from pups primarily maintain maternal attention, the neuronal soma appears to return to a pre-pregnancy, non-hormonally dependent state, whereas other aspects of the dendrite remain altered. Collectively, these data demonstrate a striking plasticity in the brains of females that may be reflected in modifications in behavior.

Opiate disruption of maternal behavior: morphine reduces, and naloxone restores, c-fos activity in the medial preoptic area of lactating rats.

Morphine significantly impairs maternal behavior; naloxone, an opiate antagonist, restores it. Maternal behavior is associated with c-fos expression, an immediate early gene product, in the medial preoptic area (mPOA) of females. In two experiments, the effects of morphine-alone and morphine plus naloxone on the expression of c-fos were examined. On postpartum day 5, females were injected with morphine or saline (experiment 1), and morphine + naloxone or morphine + saline (experiment 2) and placed back in the home-cage, separated from their pups by a wire-mesh partition. A separate group in experiment 1 was injected but not exposed to pups. Processing for c-fos immunohistochemistry commenced, and c-fos positive cells in a proscribed portion of mPOA were counted. Morphine-treated females had fewer c-fos cells in mPOA compared to saline-treated females, and the presence of pups accounted for a significant increase in c-fos-expressing neurons, whereas in females not exposed to pups, morphine treatment did not significantly reduce baseline c-fos expression (experiment 1). Furthermore, naloxone mitigated the effect as morphine + naloxone-treated females expressed more c-fos cells compared to morphine + saline females (experiment 2). Morphine-treated females, therefore, may exhibit reductions in maternal behavior because of relative opiate-induced inactivation of areas of the brain devoted to the regulation of maternal behavior.