Involvement of adenohypophysial dopamine in the regulation of prolactin release during suckling.

Dopamine (DA) was measured in the anterior lobe of the pituitary and median eminence from lactating rats. The effect of pup separation and suckling was studied in order to correlate changes in DA levels with changes in serum PRL. In lactating rats separated from their pups, low levels of circulating PRL were found at 2,4, and 8 h. DA levels in the median eminence showed a decline at 2h; at 4 and 8 h of separation, a significant increase was observed. In the pars distalis, the concentration of DA increased with the length of the nonsuckling interval. Suckling induced a rapid rise in serum PRL levels in rats that were separated from their pups 4 h earlier. Under these conditions, a significant decrease in DA levels in the median eminence and pars distalis was observed as early as 5 min after the onset of suckling; at 30 min, the DA levels were still low. In the situations studied (suckling and pup separation), a negative correlation between serum PRL and DA levels in both the median eminence and pars distalis was always found.     

The hypothalamic paraventricular nucleus has a pivotal role in regulation of prolactin release in lactating rats

The affect of paraventricular nucleus (PVN) lesions on PRL secretory response to suckling was studied in adult female rats. Basal levels of PRL were similar in the control and lesioned groups. Substantial decreases in PRL levels occurred after separation of pups from their mothers in the control as well as lesioned animals. When mothers and pups were reunited, the circulating PRL concentrations of the control groups rose immediately from basal values of 50-100 micrograms/liter to reach peaks of 450-550 micrograms/liter. PVN lesions significantly decreased the suckling-induced rise of PRL levels. Furthermore, PVN lesions abolished the high amplitude, episodic pattern of PRL release in continuously lactating rats. These findings are consistent with the view that PVN neurons produce PRL releasing factor(s), which is (are) required for normal secretory patterns of PRL in lactating rats.     

Rat placental lactogens initiate and maintain lactation yet inhibit suckling-induced prolactin release

Mammalian reproduction is dependent on both a successful pregnancy and on the subsequent period of lactation. In the rat, ovulation occurs shortly after parturition making it possible for a dam to be simultaneously pregnant and lactating. The present studies investigate the effect of placental hormones on suckling-induced prolactin (PRL) release and the contribution of placental hormones to milk synthesis and secretion. A rat choriocarcinoma cell line, Rcho-1, which secretes placental lactogens (PLs) following transplantation in vivo, attenuated suckling-induced PRL release on both d 9 and d 14 of lactation by 43 and 58%, respectively. When PRL secretion was completely inhibited by bromocriptine, a dopamine agonist, Rcho-1-bearing dams still maintained a normal litters weight gain, demonstrating that placental lactogens can continue an established lactation. The Rcho-1 tumors also initiated milk synthesis and secretion in nulliparous rats continuously exposed to pups. Whereas none of the 11 control virgins began lactating and had an average pup weight loss of 2.07 g, the Rcho-1-bearing rats began lactating, as evidenced by a significant reduction in pup weight loss. Thirty percent of these rats became fully lactationally competent. Northern blot analysis showed that the Rcho-1 tumors expressed both PL-I and PL-II mRNA in all experimental groups. These tumors also secreted PL-I into the circulation, as shown by radioimmunoassay.     

Hysterectomy-induced alterations in prolactin secretion of lactating rats

The contribution of the uterus to the regulation of PRL secretion in lactating dams and cycling female rats was investigated. Lactating animals were hysterectomized or sham operated 2 days after parturition, and the number of pups was adjusted to eight. Blood samples for PRL RIA were obtained through intra-atrial cannulae implanted 2 days before experimentation. In order to study the PRL secretory profile in undisturbed freely lactating rats, blood samples were taken every 2 h for 24 h starting at 1400 h. During early lactation (days 7-8), hysterectomy did not alter the PRL secretory profile compared to that of sham-operated controls. On days 14-15 post partum, PRL secretion followed a characteristic bimodal pattern showing two PRL surges at 1800 h and 0600 h. After hysterectomy, the early morning PRL surge disappeared and PRL secretion showed an unimodal daily rhythm reaching its peak at 1800 h. The possible effect of the absence of the uterus on suckling-induced PRL release at various stages of lactation was studied. On days 7-8, suckling stimuli after 4 h of pup deprivation induced robust PRL release. Hysterectomy did not significantly alter PRL release at this earlier stage of lactation. In control groups, the suckling-induced PRL secretory response markedly declined as the postpartum period advanced. On the other hand, the hysterectomized animals retained significantly greater responsiveness to suckling during the second half of lactation. These data indicate an inhibitory influence of the uterus on PRL secretion. The onset of this uterine effect is considerably delayed, and its influence became prominent only at a later phase of lactation. The effect of length of pup deprivation preceding the suckling stimulus, in combination with hysterectomy, was also investigated. Hysterectomy significantly increased suckling-induced PRL release after 4 and 24 h separation compared to the sham-hysterectomized animals. When the separation was longer than 48 h, the inducibility of PRL release by suckling declined and was not influenced by hysterectomy. In order to study the possible influence of the uterus on PRL secretion during the estrous cycle, regularly cycling female rats were hysterectomized at diestrus 1. Twelve days later the animals were cannulated, and serial blood samples were taken during the subsequent proestrus. Hysterectomy did not alter the PRL surge which occurred on the afternoon of proestrus indicating that the uterus does not have a major function in regulating PRL secretion on proestrus. In conclusion, hysterectomy significantly delayed the extinction of suckling-induced PRL release revealing the active role of the uterus in the regulation of this neuroendocrine reflex.     

Abolition of prolactin surge induced by ovarian steroid hormones in the lactating rat.

There are data indicating that stress-induced prolactin (PRL) release is blunted in the lactating rat like the release of other stress-associated hormones. In this experiment, the PRL release evoked by administration of estrogen, which is another principal stimulus for PRL release, was examined in ovariectomized lactating rats 8-15 days after delivery. Estradiol benzoate (EB, 20 micrograms) injections into ovariectomized nonlactating rats induced a PRL surge starting between 13:00 and 15:00 h with a peak at 17:00 h 2 days after the treatment, whereas the EB-induced PRL surge was absent in ovariectomized lactating rats separated from their pups at 09:00 h on the day or in mothers without separation from their pups. Injection of either thyrotropin-releasing hormone (TRH; 10 micrograms/kg) or pimozide (0.5 mg/kg) elevated serum PRL concentrations similarly in lactating and nonlactating rats when examined just before the beginning of the expected estrogen-induced PRL surge. Thus, the main cause for the reduced PRL response to estrogen in lactating rats seems not to be in the pituitary gland but in the brain. Progesterone, which is known to induce a PRL surge in ovariectomized estrogen-primed rats by acting on the mediobasal hypothalamus, also failed to evoke a PRL surge in lactating rats. Recovery from the inhibitory influence of suckling on PRL response to EB followed a time course similar to that observed in response to immobilization stress or to morphine injection; estrogen-induced PRL surge started to recover at 6 days and was almost fully recovered 8 days after weaning.(ABSTRACT TRUNCATED AT 250 WORDS)     

The endogenous stimulatory rhythm regulating prolactin secretion is present in the lactating rat

Prolactin (PRL) secretion in the female rat is regulated by an endogenous stimulatory rhythm (ESR), which is normally under the tonic inhibition of dopamine (DA). The ESR consists of a nocturnal (N) component which peaks at approximately 03.00 h, and a diurnal (D) component which peaks at approximately 17.00 h. This ESR has been shown to be present in ovariectomized and cervically stimulated rats. We have proposed that the ESR is continuously present in the female rat and that any suppression of the tonic inhibitory influence on PRL secretion can reveal its existence. In this study, the effects of the DA-lowering stimulus of suckling was investigated at different times of day in lactating rats. In addition, the pattern of PRL secretion in freely lactating rats throughout a 24-hour period was studied. Female rats were separated from their pups for 6 h prior to reunion at either 03.00 (coincident with the N component), 12.00 (control) or 17.00 h (coincident with the D component) at various stages of lactation. Blood samples were collected from intra-atrial cannulae immediately before separation of pups and dams, immediately before reunion of pups and dams (0 time), and 15, 30, 60 and 120 min following reunion of pups and dams. Four days following parturition, dams suckled at either 03.00 or 17.00 h secreted significantly greater PRL than rats suckled at 12.00 h. Peak levels of PRL were 60-, 90- and 25-fold greater than 0 time levels, at 03.00, 17.00 and 12.00 h, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of gonadectomy and chlorpromazine treatment on prolactin, LH, and FSH secretion in young and old rats of both sexes

Old male rats, and female rats in the states of senile persistent estrus (PE) and senile repetitive pseudopregnancy (RPD) were compared with young controls in the change of serum prolactin (PRL), LH, and FSH concentrations after gonadectomy and chlorpromazine (CPZ) treatment. Old male rats (18–21-month old) and PE (12–15-month old and 18–19-month old) and RPD (18–23-month old) female rats showed higher serum PRL levels. In both young and old male rats, orchidectomy did not cause any significant changes in serum PRL levels. The effect of ovariectomy on serum PRL levels was smaller in old female rats. CPZ treatment increased serum PRL levels in young controls of both sexes, 12-month old males and 12–15-month old PE rats. The sexual difference in PRL secretion in response to CPZ was observed at these ages. In 18–21-month old males and 18–23-month old females, CPZ did not significantly increase serum PRL levels. These results suggest that the higher serum PRL levels in intact old male and female rats may be partly due to the lowered activity of the hypothalamic dopaminergic inhibition. The changes in serum LH levels in response to ovariectomy were smaller in old female rats than in young controls, whereas male rats did not show any age-related changes in response to orchidectomy on serum LH levels. There was no appreciable age-difference in the increase in serum FSH levels after gonadectomy in either sex.     

A quantitative analysis of the roles of dosage, sequence, and duration of estradiol and progesterone exposure in the regulation of maternal behavior in the rat

The regulation of the onset of maternal behavior in the rat is under hormonal control. This study reports a new endocrine model for the study of the hormonal regulation of maternal responsiveness. The model employs the administration of physiological amounts of the steroids estradiol (E2) and progesterone (P) via Silastic implants to inexperienced nulliparous rats and measurement of the effects of these implants on maternal behavior. In the first two experiments, the levels of E2 and P in the sera of pregnant and hormone-treated rats were measured by RIA. Using known physiological treatments of E2 given in combination with P, the effects of E2 and P on maternal behavior were measured. Treatment with a combination of E2 at all dosages plus P for 2 weeks before P removal and behavioral testing stimulated a fast onset of maternal behavior in ovariectomized nulliparous rats. Exposure for 2 weeks to small E2 implants (1 or 2 mm; approximately 20-30 pg/ml serum) did not affect maternal responsiveness, whereas large E2 implants (10 mm; approximately 110 pg/ml serum) stimulated maternal behavior. P treatment alone had no behavioral effect. Simultaneous removal of E2 plus P before exposure to foster young also resulted in a stimulation of behavioral responsiveness, indicating that the presence of elevated titers of circulating E2 is not a requirement for stimulation to occur. In addition to facilitating a rapid onset of behavior, the quality of the response in steroid-primed rats was similar to that measured in lactating rats in a T-maze test. In another experiment, when female rats were treated with P before E2 administration, maternal behavior was rapidly induced. Thus, P itself can sensitize the female to the behavioral effects of E2. Finally, the duration of steroid-exposure before testing was found to influence maternal behavior. Increased durations of E2 plus P exposure before testing were accompanied by decreased latencies to respond maternally to foster young. These data indicate that during pregnancy, E2 and P prime the female to respond to her young at birth. The intensity of the steroidal priming increases as pregnancy progresses, and this primed potential is subsequently unmasked by the decline in P and the maintenance of E2 secretion around parturition. These findings demonstrate that behavioral processes can be modified in the adult animal as a result of long term changes in endocrine state, i.e. pregnancy.     

Evidence for a physiological role for oxytocin in the control of prolactin secretion

The presence of oxytocin (OT) in neuronal elements of the external layer of the median eminence and in hypophysial portal plasma suggests a role for the peptide in the control of anterior pituitary function. We have reported previously that OT stimulates PRL release in vitro; therefore, we attempted to establish evidence for a physiological PRL-releasing role for OT. Plasma OT levels rose significantly just before the PRL surges occurring during a suckling stimulus in lactating rats (10 min after pup reinstatement vs. 15 min for PRL) and 48 h after estrogen injection in ovariectomized (OVX) rats (at 1200 h vs. 1300 h). Dispersed anterior pituitary cells harvested from lactating female rats and OVX estrogen-primed rats released PRL in a specific, significant, and dose-related fashion when perifused in vitro with incubation medium containing 10(-7)-10(-9) M OT, doses similar to levels found previously in hypophysial portal plasma. Infusion of antiserum specific for OT into lactating females before pup reinstatement and into estrogen-primed OVX rats 2 h before the expected release of endogenous OT delayed and significantly reduced subsequent PRL surges compared to levels in saline-or normal rabbit serum-infused rats; however, PRL release was not completely abolished. These data indicate that OT plays a physiological role in the hypothalamic control of PRL secretion and further suggest the importance of multiple factors in coordinated regulation of PRL release.     

The effect of exteroceptive pup stimuli on the responsiveness of prolactin release mechanisms to suckling stimuli in the lactating rat

In the lactating rat, the neural stimulus of suckling not only acutely releases PRL but also maintains the responsiveness of PRL regulatory mechanisms to subsequent nursing stimuli. Beginning near midlactation exteroceptive pup stimuli (ECS) can acutely release PRL. We have examined the capacity of this signal also to maintain the responsiveness of PRL release mechanisms to subsequent suckling stimuli. On day 14 postpartum lactating rats were either isolated from their young or exposed to ECS (without suckling) for approximately 24 h. When both groups were later nursed, plasma PRL of mothers earlier exposed to ECS rose significantly higher than that of subjects previously isolated from their young. Suckling produced a significant depletion in pituitary PRL and GH concentrations of ECS-exposed mothers; it did not produce a similar depletion in the pituitaries of the previously isolated group. When the pups were returned for suckling, ECS-exposed mothers began to nurse their pups substantially faster than did females of the isolated group. During the 6 h after nursing, the mammary glands of ECS-exposed mothers secreted milk at twice the rate of mammary glands of the isolated females. We conclude that ECS can maintain the capacity of neuroendocrine mechanisms to respond to galactopoetic hormone-releasing stimuli (consequently enhancing milk secretion) and support the maternal behavior pattern necessary for suckling to occur. As a result, ECS may become an important factor during later stages of lactation, compensating for the decline in suckling stimuli known to occur at that time.     

The role of the suckling stimulus in regulating pituitary prolactin mRNA in the rat

Prolactin (PRL) gene expression and the synthesis and secretion of PRL were examined in ovarian-intact lactating rats suckling eight pups on 10 days postpartum. Plasma samples were assayed for PRL concentrations, and pituitary glands were analyzed for total PRL content and PRL mRNA levels. We found that suckling-induced hyperprolactinemia was associated with very high levels of plasma PRL and a doubling in pituitary PRL mRNA levels, whereas pituitary PRL content was not changed. Removal of the suckling pups decreased plasma PRL concentrations 15-fold within 24 h. This decrease in PRL secretion was not accompanied by any significant change in pituitary PRL content. Evidently, both synthesis and secretion of PRL were decreased in the pituitary gland within 24 h following cessation of suckling, as pituitary PRL mRNA content had returned to diestrous levels at this time. To determine whether or not ovarian steroids might have contributed to the changes in PRL synthesis and secretion during lactation and after withdrawal of the suckling stimulus, the experiments were repeated in lactating rats ovariectomized (OVX) on day 2 postpartum. The results in these OVX rats were qualitatively similar to those described in ovarian-intact rats. We concluded from these findings that the stimulus of suckling induces increases in PRL mRNA levels in the pituitary which provides for the increased PRL synthesis accompanying increased PRL secretion. The cessation of suckling led to prompt decreases in PRL synthesis and secretion within 24 h.     

Inhibition of in vivo pituitary release of luteinizing hormone in lactating rats by exogenous prolactin.

The mechanism whereby the reciprocal relationship between the plasma levels of prolactin and HL is maintained in lactating rats under different degrees of suckling stimulus has been investigated in the present study. Plasma levels of luteinizing hormone (LH) in lactating rats suckling two pups could be reduced significantly by injecting prolactin (PRL). This reduction was also evident in ovariectomized and ovariectomized-adrenalectomized lactating rats, thus excluding mediation of the inhibitory effect by steroids from end-organs. The in vivo response of the pituitary to exogenous LHRH was lower in rats suckling eight pups than those suckling two pups. Prolactin administered prior to LHRH caused an inhibition of the response of the pituitary to exogenous LHRH in rats suckling two pups. These data provide evidence for the hypothesis that in the lactating rat, in the presence of minimal suckling stimulus, the causative factor for reducing serum LH levels is prolactin, which acts by altering the pituitary responsiveness to LHRH.     

Qualitative changes in luteinizing hormone and prolactin responses to N-methyl-aspartic acid during lactation in the rat

The suppression during lactation of pulsatile LH release and pituitary GnRH receptors has been attributed to a primary deficit in hypothalamic GnRH release. In the present investigation we have attempted to characterize the responsiveness of the lactational hypothalamus using the excitatory amino acid receptor agonist N-methyl-aspartic acid (NMA) to stimulate LH and PRL secretion. Lactating rats were ovariectomized on day 2 postpartum, and their litters were adjusted to eight pups. Dual venous catheters were implanted 6-7 days later, and rats were fitted with protective tethers and jackets for chronic pulsatile infusions of GnRH and NMA. GnRH pulses (5 or 10 ng/pulse once every 50 min) were administered for 20 h to up-regulate GnRH receptors and restore pituitary responsiveness to GnRH. Rats were then infused with NMA (40 mg/kg BW.pulse) once every 50 min for four pulses or once every 2 h over a 24-h period. Blood samples were collected at 10-min intervals at times surrounding the final two GnRH pulses, the first several NMA pulses, and the final three NMA pulses 24 h later. Samples were analyzed for LH and PRL by RIA. Procedural control experiments were performed in normal adult rats with NMA administered at 20 mg/kg BW.pulse in males and at 20 and 40 mg/kg BW.pulse in females. Whereas normal rats responded to NMA pulses with unambiguous LH and PRL peaks, lactating rats failed to show LH responses either acutely or after 24 h of treatment. PRL responses to the drug depended upon the circulating levels of the hormone immediately preceding each NMA pulse. When levels were elevated (presumably due to intermittent suckling by the pups), NMA infusion resulted in an acute suppression of PRL. When PRL levels were low, NMA appeared to neither stimulate nor inhibit this hormone. These data suggest that GnRH release from the hypothalamus of the lactating rat is refractory to NMA stimulation, perhaps due to suckling-induced activation of endogenous opioid peptide or gamma-aminobutyric acid systems that could suppress GnRH neurons. Conversion of the PRL response from stimulation by NMA in normal animals to inhibition during lactation might be attributed to simultaneous activation of both dopamine neurons and the PRL-releasing factor system. According to this hypothesis, the response to NMA would be dominated by PRL-releasing factor in normal rats and by dopamine in lactating animals, which have a lower dopamine turnover rate and thus a greater potential for becoming activated by NMA.     

Antagonist of pituitary adenylate cyclase activating polypeptide suppresses prolactin secretion without changing the activity of dopamine neurons in lactating rats

Pituitary adenylate cyclase activating polypeptide (PACAP) is a relatively new neuropeptide, and it has a potent stimulatory effect on adenylate cyclase activity in rat pituitary cells. However, the role of PACAP in the physiological control of prolactin (PRL) secretion is still unclear. In the present study, we investigated the physiological significance of endogenous PACAP on PRL secretion in lactating rats. On lactation days 7-8, pups were separated from their mother rats for 5 h before the onset of suckling and PACAP6-38 (16 microg), a receptor antagonist, was injected through the lateral ventricle cannula just after the removal of pups. The effects of PACAP6-38 on PRL and oxytocin secretion, and on the activity of tyrosine hydroxylase (TH), were examined after the onset of suckling. Administration of PACAP6-38 inhibited PRL levels in response to suckling, but it did not affect the activity of TH, as measured by DOPA accumulation at 15 min after administration of NSD 1015 (25.0 mg/kg), an L-aromatic amino acid decarboxylase inhibitor, or the plasma concentrations of oxytocin in lactating rats. Injection of alpha-methyl-p-tyrosine (alpha-MT; 50 mg/kg), an inhibitor of dopamine synthesis, increased PRL levels, and suckling caused a further increase in the plasma concentrations of PRL. An injection of PACAP6-38 (i.c.v.) also inhibited the PRL response to suckling under dopamine depletion. These results suggest that endogenous PACAP acts as a neurotransmitter or neuromodulator within the hypothalamus and plays an important role for PRL secretion in lactating rats. Endogenous PACAP may regulate PRL secretion, possibly mediated by PRL-releasing factors such as vasoactive intestinal polypeptide or vasopressin.     

Sexual differentiation of oxytocin stress responsiveness: effect of neonatal androgenization, castration and a luteinizing hormone-releasing hormone antagonist

The plasma OT increment following stress in rats is sexually dimorphic, females exhibiting greater responses than males. We have investigated the role of neonatal androgen secretion in determining the sex-typical level of response. Castration of male pups either surgically or functionally (GnRH antagonist treatment) within either 2 h or 5 days of birth did not elevate the OT responses of adult males. In contrast, androgenization of female pups (testosterone, 1.25 mg/pup) within 5 days of birth markedly reduced the OT stress responses of adults to a level insignificantly different to males. The results show that neonatal androgens can exert organizational effects on OT regulatory mechanisms. Since neonatal castration was ineffective it would appear that a prenatal defeminization or masculinization event determines OT stress responsiveness in males.     

Effects of morphine and naloxone on serum levels of luteinizing hormone and prolactin in prepubertal male and female rats

The effects of morphine (M) and naloxone (N) on serum levels of luteinizing hormone (LH) and prolactin (PRL) in prepubertal male and female rats were investigated. N raised serum LH concentrations in female rats at 10, 15, 20, 25 and 30 days of age, but increased serum LH levels in male prepubertal rats only at 30 days of age. M significantly depressed serum LH values in both sexes only at 15 days of age. M increased serum PRL levels in immature rats of both sexes in all age groups, except in 25-day-old males, whereas N decreased serum PRL only in 25-day-old male rats. These data show that there are differences in the pituitary LH and PRL responses to M or N of immature as compared to the responses previously reported in mature rats, and suggest that the endogenous opioid peptides may have a role in regulating LH and PRL secretion in immature rats.     

Central prolactin infusions stimulate maternal behavior in steroid-treated, nulliparous female rats.

A series of experiments were conducted to determine whether and under what conditions central prolactin (PRL) administration would stimulate the onset of maternal behavior in female rats and to identify possible neural sites of PRL action. In each experiment ovariectomized, nulliparous rats whose endogenous PRL levels were suppressed with bromocriptine were tested for maternal behavior toward foster young. In experiments 1, 2, and 4, females were also exposed to pregnancy-like levels of progesterone (days 1-11) followed by estradiol (days 11-17). In experiment 1 infusions (days 11-13) of four doses of ovine PRL (400 ng, 2 micrograms, 10 micrograms, or 50 micrograms, but not 80 ng) into the lateral ventricle resulted in a rapid onset of maternal behavior (behavioral testing, days 12-17). The stimulatory action of these doses of PRL appears to be central, since subcutaneous injections of 50 micrograms of ovine PRL failed to affect maternal responsiveness (experiment 2). Experiment 3 indicated that the stimulatory effect of intracerebroventricularly administered PRL is steroid dependent. Infusions of either 10 micrograms of ovine PRL or 10 micrograms of rat PRL failed to induce maternal behavior in nonsteroid-treated animals. In the final experiment (no. 4) bilateral infusions of 40 ng of ovine PRL into the medial preoptic area of steroid-treated rats resulted in a pronounced stimulation of maternal behavior. These findings demonstrate a central site of PRL action in the stimulation of maternal responsiveness and point to the medial preoptic area as a key neural site for PRL regulation of maternal behavior.     

Pup removal suppresses estrogen-induced surges of LH secretion and activation of GnRH neurons in lactating rats

During lactation, the suckling stimulus exerts profound influences on neuroendocrine regulation in nursing rats. We examined the acute effect of pup removal on the estrogen-induced surge of LH secretion in ovariectomized lactating rats. Lactating and nonlactating cyclic female rats were given an estradiol-containing capsule after ovariectomy, and blood samples were collected through an indwelling catheter for serum LH determinations. In lactating, freely suckled ovariectomized rats, estrogen treatment induced an afternoon LH surge with a magnitude and timing comparable to those seen in nonlactating rats. Removal of pups from the lactating rats at 0900, 1100, or 1300 h, but not at 1500 h, suppressed the estrogen-induced surge that normally occurs in the afternoon of the same day. The suppressive effect of pup removal at 0900 h was completely abolished when the pups were returned by 1400 h. In contrast, pup removal was ineffective in abolishing the stimulatory effect of progesterone on LH surges. Double immunohistochemical staining for gonadotropin-releasing hormone (GnRH) and c-Fos, a marker for neuronal activation, revealed a decrease, concomitantly with the suppression of LH surges, in the number of c-Fos-immunoreactive GnRH neurons in the preoptic regions of nonsuckled rats. An LH surge was restored in nonsuckled rats when 0.1 microg oxytocin was injected into the third ventricle three times at 1-h intervals during pup removal. These results suggest that the GnRH surge generator of lactating rats requires the suckling stimulus that is not involved in nonlactating cyclic female rats.     

Central infusions of the recombinant human prolactin receptor antagonist, S179D-PRL, delay the onset of maternal behavior in steroid-primed, nulliparous female rats

The expression of maternal behavior in the newly parturient rat is under endocrine regulation. Blocking endogenous PRL secretion with bromocriptine delays the normal rapid expression of maternal care shown toward foster young in steroid-primed virgin female rats. The recent development of the PRL receptor antagonist S179D-PRL, a mutant of human PRL in which the serine residue at the 179 position is replaced with aspartate, provides a potentially useful tool to examine the role of PRL in neural processing. In the present report, three experiments were conducted that examined the effects of this PRL antagonist on the induction of maternal behavior. In each experiment, ovariectomized, nulliparous rats were treated sequentially with SILASTIC capsules implanted sc with progesterone (days 1-11) and estradiol (days 11-17), a treatment that stimulates a rapid onset of maternal behavior in virgin rats. On day 11, females were implanted with Alzet miniosmotic pumps connected to cannulae directed unilaterally at the lateral ventricle (Exp 1) or bilaterally at the medial preoptic area (MPOA; Exp 2 and 3). Pumps contained either doses of S179D-PRL (0.115 or 1.15 mg/ml; Exp 1 and 2), wild-type human PRL (1.15 mg/ml; Exp 3), or the saline vehicle (Exp 1-3). Testing for maternal behavior began on day 12, a day after pump insertion, and animals were tested daily for 6 days. Latencies to contact, retrieve, and group foster test young were recorded. Administration of both the high and low doses of S179D-PRL infused into the lateral ventricle (Exp 1) or MPOA (Exp 2) significantly delayed the onset of maternal behavior. In contrast, MPOA infusions of the control hormone, wild-type human PRL, in Exp 3 did not delay the onset of maternal behavior. These findings support the concept that the effects of S179D-PRL are caused by its actions as a PRL receptor antagonist rather than by a nonspecific effect of the protein. Overall, these results demonstrate the effectiveness of S179D-PRL acting at the level of the central nervous system (and, more specifically, within the MPOA) to regulate maternal behavior, a PRL-mediated response.     

Prolactin release and milk ejection in rats suckling underfed pups

Mother rats chronically exposed to underfed pups (obtained daily from nonlactating mothers) show increased milk production which is associated with increased consumption of food and water. The purpose of the present study was 2-fold: to examine whether the magnitude of maternal PRL discharge is related to the nutritional state of the pups and to differentiate between the role of the mother and that of the pups themselves in the regulation of milk intake. PRL release in response to reunion with pups (preceded by 4-h separation from pups) was measured between days 12 and 14 postpartum; each mother was tested twice, once with hungry pups (deprived of food for 24 h) and once with satiated (normally fed) pups. The magnitude of PRL discharge elicited by hungry pups in lactating as well as in thelectomized rats was twice as large as the discharge elicited by satiated pups. For each type of stimulus pups, the magnitude of PRL release was smallest in thelectomized rats, largest in rats that had nursed normally fed pups before testing, and intermediate in rats that had nursed underfed pups. The frequency of milk ejection was considerably reduced in mothers that were chronically exposed to underfed pups compared with mothers that had nursed normally fed pups. Regardless of the mother's previous history, and independent of the milk ejection frequency or maternal food and water intake, hungry pups ingested more milk than did satiated pups over the same suckling period even when both types of pups were suckled simultaneously by the same female. Milk ingestion in satiated pups was not enhanced by the vigorous sucking exerted by hungry littermates. It is suggested that hungry pups are able to meet their increased nutritional demands both directly, by withdrawing a larger portion of the milk available in the mammary glands, and indirectly, by enhancing maternal PRL secretion which in turn may promote maternal consummatory behavior and milk production.