Phyllis E. Mann

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.

Prenatal stress has long-term effects on brain opiate receptors

Prenatal stress has been associated with a number of behavioral consequences including altered sensitivity to exogenous opiates. In the present study, mu opiate receptors were compared in the 42-day-old offspring from females stressed on days 15-22 of gestation and from females who were unstressed controls. Membrane homogenates from the prenatal stress group showed less binding of the mu opiate receptor ligand, [3H]DAGO in striatum but not in several other brain regions. Saturation studies suggest this difference is due to fewer striatal mu opiate receptors in offspring of prenatally stressed females. Using in vitro receptor autoradiography, the decreased binding in striatum was found mostly in the rostral striatum, extending into the nucleus accumbens with conservation of the normal anatomic distribution of receptor rich patches.

Prenatal stress alters morphine- and stress-induced analgesia in male and female rats.

Prenatal stress affects the expression of many opioid-regulated behaviors in adulthood, e.g., aggressive, maternal, regulatory, and sexual. In the present report we examined two forms of analgesia, morphine-induced (opioid receptor-mediated), and stress-induced [cold-water swim (CWS), nonopioid] analgesia in adult prenatally-stressed (P-S) male and female rats to determine whether and to what extent these analgesic responses might be altered. Timed-mated Sprague-Dawley females were exposed to heat and restraint stress (three daily 1/2 hour sessions, 0830, 1230, and 1630 hr) from days 15-22 of gestation. Control animals remained undisturbed throughout pregnancy. Between 120-150 days of age, baseline pain sensitivities were determined using a tail-flick monitor. P-S and Control animals were then exposed to 3.5 min cold-water swims (2 degrees C) and pain thresholds were again determined at 30 min intervals for 120 min. P-S females exhibited significantly lower pain thresholds than Control females at the 30 and 60 min marks, whereas P-S and Control males did not differ. Six to eight days later, analgesia was measured for 180 min following morphine (5.0 mg/kg) administration. P-S females exhibited significantly greater analgesia at each time-point after morphine treatment than Controls. Conversely, P-S males were significantly less analgesic than Control males from 60 to 180 min. These data suggest that prenatal stress alters the status of endogenous opiate systems. Such prenatal stress-induced alterations in opiate function may help account for some of the behavioral effects reported in P-S animals.

Neural and endocrine sensitivities to opioids decline as a function of multiparity in the rat

Hormonal changes during pregnancy regulate the onset of maternal behavior at parturition. In addition, the concentrations of beta-endorphin and mu opioid receptors are higher during pregnancy and lower during lactation. Previous studies have shown that sensitivity of female rats to the disruptive behavioral effects of morphine changes as a function of the number of pregnancies and/or lactations the females undergo. The objectives of the present study were to determine whether central infusions of the endogenous opioid, beta-endorphin, would disrupt maternal behavior. Next, we investigated the possibility that the neural sensitivity to beta-endorphin changes with repeated pregnancies. And finally, we examined whether opioid-mediated endocrine responses also change as a function of multiparity. In the first study, bilateral infusions of low doses (0.06-0.72 nmol) of beta-endorphin into the medial preoptic area (MPOA) of lactating, primiparous rats disrupted maternal behavior. When comparable doses of beta-endorphin were infused into the MPOA of age-matched, multiparous rats, the behavioral effects of beta-endorphin were significantly attenuated. In response to suckling stimulation, an opioid-mediated endocrine response, primiparous mothers secreted more prolactin than did multiparous rats. Moreover, multiparous, but not primiparous, mothers were insensitive to the ability of naloxone, an opiate antagonist, to block suckling-induced increases in prolactin. These findings indicate that reductions in neural sensitivity to opioids develop as females undergo repeated pregnancies and lactations, changes which affect both behavioral and endocrine functions.

Endocrine Communication between Conceptus and Mother: Placental Lactogen Stimulation of Maternal Behavior

The possible role of the conceptus in stimulating the onset of maternal behavior through its secretion of placental lactogens and their passage into the brain was investigated in female rats. In the first study, significant mitogenic activity in the Nb2 lymphoma cell bioassay was detected in cerebrospinal fluid (CSF) samples collected by push-pull perfusion from rats on days 12-21 of pregnancy, coincident with the establishment of placental function. In contrast, mitogenic activity was absent from CSF in lactating and gonadectomized, virgin females. In a second study the mitogenic activity in day 12 pregnant samples was neutralized 71% with antibodies to rat placental lactogen-I (rPL-I) and > 90% with a combination of antibodies to rPL-I plus rPL-II. In contrast, activity on day 21 of pregnancy, 1 day prepartum, was reduced by antibodies to rPL-II (> 85%), but not by antibodies to rPL-I, indicating that the predominant lactogen in the CSF prepartum is rPL-II. The behavioral actions of placental secretions were assessed in the third experiment by infusing recombinant rPL-I and purified rPL-II directly into the medial preoptic area of the brain of steroid-primed, nulliparous rats. Latencies to respond maternally to foster young were significantly reduced in rPL-I- and rPL-II-treated rats (2- to 3-day latencies) when compared with latencies in control females (5- to 6-day latencies). Thus, the conceptus through its secretion of rPLs which apparently gain access to the CSF helps to prime the pregnant females brain to respond maternally at the end of gestation. This endocrine communication between the developing conceptus and pregnant female appears to be an important part of the biological system which helps to establish successful maternal care.

Opioid Receptor Subtype Involvement in Maternal Behavior in Lactating Rats

Central or systemic administration of morphine disrupts maternal behavior in steroid-primed, pup-induced virgin and lactating rats. Morphine, the prototypical mu agonist, also interacts with different opioid receptor subtypes. The present study examined the effectiveness of five receptor-selective agonists, in addition to morphine, to disrupt maternal behavior in primiparous lactating rats following intracerebroventricular (i.c.v.) infusions in order to characterize opioid receptor subtype involvement in maternal behavior in the female rat. Virgin, Sprague-Dawley rats were mated and implanted with lateral ventricle cannulae on days 13-15 of gestation. On postpartum day 5, mothers were tested for maternal behavior 30 min after i.c.v. vehicle infusion (5 microliters). On day 6, rats received one of the following opioid receptor agonists 30 min before testing: beta-endorphin (mu/epsilon receptor subtype; 0.29, 0.72, 1.45, 2.9 nmol), DAGO (mu; 0.29, 0.72, 1.45, 2.9 nmol), morphine (mu; 0.29, 0.72, 1.45, 2.9, 14.5 nmol), DPDPE (delta; 2.9, 29 nmol), U50488H (kappa l; 2.9, 29, 145 nmol) and SKF10047 (sigma; 2.9, 29, 145 nmol). Only activation of mu opioid receptors dose-dependently disrupted maternal behavior in primiparous lactating rats. DPDPE, U50488 and SKF10047 had no discernible effect on maternal behavior. DAGO, a highly selective mu agonist, was even more potent than beta-endorphin and morphine in disrupting maternal behavior suggesting that maternal behavior is regulated by opioids interacting with the mu opioid receptor.

Prolactin-brain interactions in the induction of maternal behavior in rats

Most adult female mammals display an immediate onset of maternal care toward their offspring at parturition, whereas the responses of inexperienced, nulliparous females are often less intense or absent. The shift from being a slow or nonresponder in nulliparous females to a rapid responder at parturition in primiparous animals is induced in part by the endocrine changes of pregnancy. This report reviews recent evidence demonstrating a role for prolactin in the stimulation of maternal behavior in the rat. Moreover, new findings are presented that indicate that endogenous rat prolactin acts centrally to stimulate maternal behavior in steroid-primed, nulliparous rats and that the ventromedial hypothalamus in addition to the medial preoptic area are important neural substrates regulating the rapid induction of maternal behavior at parturition.

Differential proopiomelanocortin gene expression in the medial basal hypothalamus of rats during pregnancy and lactation

Hypothalamic proopiomelanocortin (POMC) gene expression was determined using in situ hybridization histochemistry (ISHH) during pregnancy and lactation in rats with and without prior reproduction experience. POMC mRNA levels in the arcuate nucleus were compared between primigravid (first pregnancy) and multigravid (second pregnancy) and primiparous and multiparous lactating rats, and between these groups and age-matched, regularly cycling, nulliparous females in diestrus. Hybridizations were performed using a digoxigenin-labeled riboprobe complementary to 837 bp of the POMC gene. The number of cells expressing POMC mRNA in the arcuate nucleus decreased in primiparous rats on day 12 of lactation when compared with the number of POMC cells in the arcuate nucleus of nulliparous rats in diestrus. In addition, the number of cells expressing POMC mRNA in multigravid animals was significantly less than in the primigravid group on days 7 and 21 of pregnancy, and on day 12 of lactation in primiparous animals. Repeated reproductive experience affected the number of POMC mRNA positive cells; there were fewer cells expressing POMC mRNA in the multigravid females on day 7 of pregnancy and an increase in the number of POMC cells in the multiparous group on day 12 of lactation compared to the primiparous animals. Optical density measurements revealed a significant increase in reaction product in the labeled cells on all days of pregnancy compared with virgin females in diestrus and a significant decrease in reaction product on day 12 of lactation in the multiparous group. The results of the present study indicate that POMC gene expression changes across pregnancy and lactation and that repeated reproductive experience has long-term, possibly permanent, effects on the endogenous opioid system.

Intracerebroventricular cholecystokinin infusions block beta-endorphin-induced disruption of maternal behavior

Recent work has shown that infusions of beta-endorphin, an endogenous opioid, into the ventricular system of lactating rats blocks normal maternal behavior. Other behavioral and biochemical studies have demonstrated that sulfated cholecystokinin-octapeptide (CCK-8) can have effects opposite those of opioids. The present study evaluated whether intracerebroventricular (ICV) administration of CCK-8 is able to antagonize the inhibitory effect of beta-endorphin on maternal behavior. The results of this study demonstrated that CCK-8 (14.5 nmol) prevented the beta-endorphin (1.45 nmol)-induced increase in latencies to retrieve the first pup, retrieve all pups, and to group and crouch over rat pups. In addition, reductions in the percentage of rats retrieving all pups and displaying full maternal behavior were prevented by CCK-8. These data suggest that CCK-8 can act as an opioid antagonist in neural systems that control maternal behavior.

Long-term effects of parity on opioid and nonopioid behavioral and endocrine responses

Parity (number of parturitions) affects the endogenous opioid system. Multiparous lactating rats are less sensitive to the effects of morphine (MOR) on maternal behavior (MB) and analgesia than primiparous lactating rats. In order to determine whether these changes in opiate sensitivity persist beyond the lactational state, the present study compared the sensitivity of ovariectomized nulliparous and nonlactating primiparous rats to MORs effects on MB (Experiment 1), analgesia (Experiment 2) and prolactin release (Experiment 3) in addition to stress-induced analgesia (Experiment 2). In Experiments 1 and 2 primiparous rats were allowed to give birth and remain with their litter (culled to 6 pups) until weaning. At that time the pups were removed and the dams and age-matched nulliparous rats were ovariectomized. Four weeks later animals were exposed to foster pups daily in order to induce MB (Experiment 1). On day 5 or 6 of full MB the primiparous and nulliparous rats received either saline or one of four doses of MOR (0.625, 1.25, 2.5, or 5.0 mg/kg, SC) and 60 min later MB was assessed. MOR, at the 2.5 mg/kg dose, disrupted MB in a significantly greater percentage of nulliparous as compared to primiparous animals (100% vs. 55%, respectively). In Experiment 2, nulliparous and nonlactating primiparous animals received 2.5 mg/kg of MOR four weeks after ovariectomy. Analgesia was assessed on a tail-flick apparatus 30, 60, 90, 120 and 150 min postinjection. One week later the same animals were exposed to cold-water swims (CWS, 2 degrees C, 3.5 min) and tail-flick latencies were again recorded.(ABSTRACT TRUNCATED AT 250 WORDS)