Robert S. Bridges

Long-term effects of pregnancy and parturition upon maternal responsiveness in the rat ☆

Abstract Latency to retrieve and group rat pups was measured in nonlactating primiparous and nulliparous rats. Primiparous females that raised litters to weaning and were tested for responsiveness to donors pups 25 days postweaning exhibited an immediate onset of maternal responsiveness. The 21 day lactation period was found not be essential for the establishment of this level of responsiveness, since animals that experienced pregnancy and parturition, and interacted with their young for 1– 1 1 2 hr during parturition also retrieved and grouped pups immediately when tested 25 days postpartum. The establishment of this behavioral response in the newly parturient animal was found to be independent of lactogenesis and the presence of the ovaries during parturition. Virgin females with or without previous pup exposure, animals caesarean sectioned on Day 22 of pregnancy, and primiparous females whose pups were removed immediately at birth did not exhibit this level of maternal responsiveness immediately when tested later in adulthood. The data suggest that an organization of maternal behavioral responsiveness occurs during parturition in the rat.

Biochemical Basis of Parental Behavior in the Rat

Publisher Summary This chapter focuses on involvement of biochemical factors in the regulation of parental behavior in the Rattus norvegicus. Parturient rats display a set of behavioral responses over the course of lactation that helps to ensure the growth and survival of their offspring. These behaviors can be divided into those that are pup directed and others that are not. The chapter highlights role of hormones in parental behavior. Endogenous shifts in secretory patterns of hormones and neurochemicals are thought to underlie the shifts in incidences and intensities of parental behavior in female as well as male rats. A substantive body of research has explored the role of hormones and various neurotransmitters in the regulation of parental care. That female rats show pronounced shifts in latencies to display maternal behavior at the end of gestation initially led investigators to explore the relationships between the physiological changes of pregnancy and the expression of maternal behavior. The chapter also develops a working model that illustrates the biochemical factors which influence the expression of parental behavior in rats.

Effects of transecting lateral neural connections of the medial preoptic area on maternal behavior in the rat : nest building, pup retrieval and prolactin secretion

In lactating female rats bilateral parasagittal cuts transecting the dorsolateral neural connections of the medial preoptic area (MPOA) abolished nest building and retrieving components of maternal behavior, while crouching and nursing were unaffected. While few animals with these cuts were suckled when presented with pups the prolactin secretion response was undiminished when suckling did occur. Whereas previous studies have demonstrated that extensive surgical separation of the medial preoptic area-anterior hypothalamic continuum from the lateral preoptic area-lateral hypothalamus disrupts all aspects of maternal behavior, the present study has determined more specifically the zone of fibers essential for the active components of maternal behavior, i.e., nest building and retrieving. These fibers appear to enter/leave MPO dorsolaterally beneath the crossing of the anterior commissure in the region of the bed nucleus of the stria terminalis. These neural connections are not the same as those that regulate prolactin secretion in the lactating rat.

Neuroendocrine regulation of maternal behavior

The expression of maternal behavior in mammals is regulated by the developmental and experiential events over a females lifetime. In this review the relationships between the endocrine and neural systems that play key roles in these developmental and experiential processes that affect both the establishment and maintenance of maternal care are presented. The involvement of the hormones estrogen, progesterone, and lactogens are discussed in the context of ligand, receptor, and gene activity in rodents and to a lesser extent in higher mammals. The roles of neuroendocrine factors, including oxytocin, vasopressin, classical neurotransmitters, and other neural gene products that regulate aspects of maternal care are set forth, and the interactions of hormones with central nervous system mediators of maternal behavior are discussed. The impact of prior developmental factors, including epigenetic events, and maternal experience on subsequent maternal care are assessed over the course of the females lifespan. It is proposed that common neuroendocrine mechanisms underlie the regulation of maternal care in mammals.

Preoptic area opioids and opiate receptors increase during pregnancy and decrease during lactation

Opiate receptor and endogenous opioid content were determined in pregnant, lactating, ovariectomized, and ovariectomized and subsequently estradiol- and progesterone-treated adult female rats. Levels of estradiol and progesterone produced by Silastic capsules implanted in animals of the ovariectomized, hormone-treated group were similar to natural levels of those hormones induced during pregnancy. Quantitative receptor autoradiography and radioimmunoassay were used to determine [3H]naloxone binding density and immunoreactive beta-endorphin content, respectively, in the preoptic area of the hypothalamus. Both opiate receptor binding density and beta-endorphin content in the preoptic area varied in the same direction in all experimental groups. The highest levels of both were observed during pregnancy and the lowest levels during lactation. Ovariectomy without subsequent hormone treatment produced intermediate levels of both opiate receptor and beta-endorphin. Ovariectomy with experimentally-induced estradiol and progesterone levels similar to those of pregnancy produced opiate receptor density and beta-endorphin content similar to those observed in pregnant animals. These data suggest that gonadal steroids are capable of altering function of the endogenous opiate system in the preoptic area. Moreover, preoptic area levels of opioids and opiate receptors are normally elevated during pregnancy and reduced during lactation. Since opiates are known to disrupt ongoing maternal behavior, a reduction of preoptic opiate function during lactation may be required to promote normal maternal behavior. The specific preoptic region involved in opiate regulation of maternal behavior may be illustrated by the zone of opiate receptor alteration observed herein.

Disruption of ongoing maternal responsiveness in rats by central administration of morphine sulfate

The ability of central morphine administration to disrupt established maternal responsiveness in rats was examined. Studies focused on the direct administration of morphine sulfate (M) to the preoptic area (POA), a region known to be involved in the expression of maternal behavior in this species. In the first experiment, crystalline M was administered via bilateral 28 g cannulae to the POA or ventromedial hypothalamus (VMH) of 31 ovariectomized, estrogen-primed, nulliparous, pup-induced, maternal females. All subjects were tested twice for maternal responsiveness; once after M-filled cannulae and once after blank inserts were lowered into place. Behavioral tests lasted for 1 h after foster young were introduced into the test cage, and retrieval, grouping and crouching responses were recorded. Thirteen of 14 females with cannulae placements in the POA showed disruption of maternal responsiveness following morphine treatment. In contrast, only 4 of 17 females with VMH implants showed some deficit in maternal behavior following opiate administration. Results of a second experiment established that M-filled implants placed in the POA of lactating females were capable of disrupting the responsiveness of nursing females toward their own young. Finally, the specificity of morphines central effects were examined in a third experiment in which lactating females received bilateral POA infusions of morphine (0.5 microgram, n = 7), dextrorphan (an inactive stereoisomer of an active opioid compound; 1.0 microgram, n = 7) or saline alone (n = 6). Whereas maternal behavior was disrupted in all morphine-treated subjects, infusions of dextrorphan or saline had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Prior parity reduces post-coital diurnal and nocturnal prolactin surges in rats.

Mating stimuli received by female rats activate a neuroendocrine mnemonic system which produces daily diurnal and nocturnal prolactin (PRL) surges for the first half of gestation, surges which help maintain corpora lutea function and a viable pregnancy. Since these PRL surges may be regulated in part by endogenous opioids and opioid sensitivity declines as a function of multiple births, we decided to investigate the possibility that prior parity might affect the post-coital diurnal and nocturnal PRL surges, reducing their magnitude and/or occurrence. Age-matched, nulliparous and primiparous rats were mated to males from our colony. On days 5 or 10 of pregnancy females received jugular catheters. Blood samples were collected at regular intervals from 1000 h on day 7 to 1000 h on day 8, and from 1000 h on day 12 to 1000 h on day 13 of gestation in separate sets of multigravid and primigravid rats. Measurement of plasma PRL by radioimmunoassay revealed that prior reproductive experience altered the patterns and levels of plasma PRL. Plasma PRL levels were significantly reduced during both the diurnal and nocturnal surges on days 7-8 in multigravid rats when compared with levels in primigravid rats. No differences in PRL levels were found between primigravid and multigravid groups on days 12 to 13 of gestation. The changes in diurnal and nocturnal PRL surges during early pregnancy indicate that prior parity reduces the subsequent secretion of PRL, possibly by altering the neuroendocrine regulation of this hormone.

Immunoreactive Beta-Endorphin Concentrations in Brain and Plasma during Pregnancy in Rats: Possible Modulation by Progesterone and Estradiol

Changes in opioid concentrations in brain and plasma as well as opioid activity have been reported to occur as a function of pregnancy and lactation in rats. The present study examines the status and steroidal regulation of the endogenous opioid, beta-endorphin, in the behaviorally and neuroendocrinologically important preoptic area (POA) and hypothalamus, and in the plasma of pregnant and nonpregnant rats. In the first study, concentrations of beta-endorphin-like immunoreactivity (beta-EP-LI-Ir) in POA and hypothalamic tissues as well as in plasma were measured throughout gestation in rats. beta-EP-LI-Ir concentrations in the POA were significantly higher in rats from day 6 to 18 of gestation than in nonpregnant, diestrous females. beta-EP-LI-Ir concentrations in the POA declined significantly between day 18 and 22 of gestation. Changes in hypothalamic beta-EP-LI-Ir concentrations were not detected either as a function of pregnancy or during pregnancy, while plasma beta-EP-LI-Ir concentrations declined gradually from day 6 to 18 of pregnancy and then increased significantly prepartum (day 22 of gestation). In the second study, the effects of 2 weeks of exposure to pregnancy levels of progesterone and estradiol on brain and plasma beta-EP-LI-Ir were measured. Exposure to the combination of progesterone and estradiol (administered subcutaneously via Silastic capsule implants) resulted in a significant increase in beta-EP-LI-Ir concentrations in the POA, but did not affect beta-EP-LI-Ir concentrations in either the hypothalamus or plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Parturition: Its role in the long term retention of maternal behavior in the rat

Abstract The role of parturition in the establishment of the long-term retention of maternal behavior in the rat was investigated. Primigravid rats caesarean sectioned on Day 22 of gestation that responded maternally immediately (

A developmental study of maternal responsiveness in the rat

Abstract Male and female rats will retrieve and group rat pups as early as 24 days of age. Injections of testosterone propionate failed to inhibit the onset of this behavior in the male.