Prolactin in the postpartum rat: synthesis and release in the absence of suckling stimulation

Pituitary--adrenal responsiveness of rat mothers to noxious stimuli and stimuli produced by pups

Studies on the response of female rats to various stresses during the course of lactation showed a marked reduction in pituitary-adrenal activity. Maximum suppression coincided with the period of maximum lactation, about 14 days after parturition. Both pituitary corticotropin and the release of corticosterone were significantly reduced. Females showing maternal behaviour but not lactating did not exhibit this buffering of the stress response. Another aspect of pituitary-adrenal activity has been studied in lactating female rats. Females, although they have a buffered stress response, responded differently, in terms of pituitary-adrenal activity, to stimuli emitted by the pups: pups which are exposed to noxious stimuli elicit a much greater pituitary-adrenal response in lactating females than do pups which are merely handled. Separation from the pups does not elicit a pituitary-adrenal response. This difference in response is modified if the infants are malnourished and is not observed in virgin animals that are not lactating.

Regulation of anxiety during the postpartum period

Healthy mother-infant interactions are critical for the physical, cognitive, and psychological development of offspring. Such interactions rely on numerous factors, including a positive maternal emotional state. However, many postpartum women experience emotional dysregulation, often involving elevated anxiety. Neuroendocrine factors contributing to the onset of postpartum anxiety symptoms are mostly unknown, but irregularities in hypothalamic-pituitary-adrenal axis function, reduced prolactin and oxytocin signaling, or parturitional withdrawal of ovarian, placental and neural steroids could contribute to anxiety in susceptible women. Although the causes of initial onset are unclear, postpartum anxiety can be mitigated by recent contact with infants. Numerous neurochemical systems, including oxytocin, prolactin, GABA, and norepinephrine mediate this anxiolytic effect of infant contact. Insight into the etiology of postpartum anxiety disorders, and how contact with infants helps counter existing anxiety dysregulation, will surely facilitate the diagnosis and treatment of postpartum women at risk for, or experiencing, an anxiety disorder.

Reduced anxiety in postpartum rats requires recent physical interactions with pups, but is independent of suckling and peripheral sources of hormones

Changes in emotional behavior occur across the reproductive cycle in female rodents, with reduced anxiety found during the postpartum period, but relatively little is known about factors contributing to this decreased anxiety. Using increased duration of time spent in the open arms of an elevated plus-maze as an indicator of reduced anxiety, it was found in a series of experiments that (1) anxiety is significantly reduced in Long-Evans females during the first week of lactation, but not thereafter, (2) relatively recent contact with pups before testing (within 4 h) is necessary for their reduced anxiety, (3) dams that receive only distal sensory cues from pups for the 4 h prior to testing do not show reduced anxiety, (4) the absence of nipples, and therefore a lack of suckling by pups, has no effect on dams' anxiety, (5) cesarean delivery of pups 2 days prior to expected parturition did not alter later anxiety in dams, (6) hypophysectomy during mid-pregnancy or ovariectomy within 24 h after parturition also did not prevent reduced anxiety in dams, and (7) differences in anxiety between lactating and virgin females are greatest 4-8 min after being placed in the plus-maze. Therefore, exposure to their own peripheral hormones through mid-pregnancy is sufficient to prime female rats to show reduced anxiety, but only if they later have recent physical interaction with pups. Furthermore, because suckling and the peripheral hormones released during suckling appear to be unnecessary, decreased anxiety in maternal rats may instead be regulated by the transient intracerebral release of neuropeptides or neurotransmitters while dams receive other types of tactile inputs from their infants.

Suckling-induced activation of neural c-fos expression at lower thoracic rat spinal cord segments

Suckling stimulation is essential for neuroendocrine and sympathetic reflex activation during lactation. In the present study, the induction of c-fos gene expression was used to identify neuronal populations in the spinal cord activated by acute 5 min suckling or by electrical stimulation of the central stump of the first abdominal mammary nerve in lactating rats previously separated from their litters for 6 or 18 h. In addition, to investigate whether spinal sympathetic preganglionic neurons are activated by suckling, dual immunostaining (Fos and choline acetyltransferase) was performed. Fos was expressed at low levels in continuously suckled and 6 h nonsuckled mothers, but no expression was found after 18 h of nonsuckling. On the other hand, in 6 h nonsuckled rats, significant increments in Fos expression occurred in several regions after acute suckling and after electrical stimulation. Also, the pattern of Fos expression in each spinal laminae was different for the two stimuli, i.e. more intense effects of suckling in deep laminae V-X and more intense effects in laminae I-IV with electrical stimulation. Double-labeling after suckling was found only in sympathetic preganglionic neurons from the intermedio-medial cell column, whereas after electrical stimulation, double label was observed only in neurons from the intermedio-lateral cell column. On the other hand, no effect upon Fos protein expression was observed after suckling and only a minor effect after electrical stimulation of mammary nerve in 18 h nonsuckled rats. These results are consistent with previous findings on the sympathetic reflex regulation of the mammary gland, as well as on the importance of the nonsuckling interval for optimal functioning of lactation.

Dorsolateral columns of the spinal cord are necessary for both suckling-induced neuroendocrine reflexes and the kyphotic nursing posture in lactating rats

Maternal behavior in rats consists of active behaviors, such as retrieval and licking of pups, and quiescent nursing, including the suckling-induced kyphotic (upright, dorsally-arched) posture. Because lesions of the dorsolateral, but not of the dorsal, columns are known to prevent the suckling-induced milk-ejection reflex, we asked whether the same is true for kyphosis as well. Bilateral lesions of the dorsolateral funiculus (DLF) or dorsal columns (DC) at spinal segments C(4-6) were made on day 5-8 postpartum; controls (CON) were subjected to a sham procedure. All aspects of maternal behavior and lactation were present in CON and DC dams soon after treatment. Among DLF dams, two had poor postural, ambulatory, and ingestive recovery that was associated with large lesions extending to the ventrolateral columns, while one with very small lesions continued to lactate. Of the remaining eight DLF dams, milk ejection was lost while recovery of retrieval and licking of pups occurred in all (between 1 and 4 days after surgery). All eight were quiescent for long periods in response to suckling but they did not display sustained kyphosis; rather, they nursed while prone or hunched over the pups, with little or no leg support, or while supine. Ventral trunk cutaneous sensitivity was present in all subjects. These data suggest that the dorsolateral funiculus relays both suckling-induced neuroendocrine and postural nursing reflexes that are mediated by separate supraspinal regions, hypothalamus and the ventrolateral sectors of the caudal periaqueductal gray, respectively.

Pharmacological evidence that prolactin acts from late gestation to promote maternal behaviour in rabbits

We investigated the role of prolactin and suckling stimulation in the expression of maternal behaviour of primiparous rabbits. Bromocriptine (1 mg/kg/day), given to intact mothers across postpartum days 1-5, decreased serum concentrations of prolactin to undetectable levels, reduced crouching, and increased time inside the nest. Failure of maternal nest-building, provoked by bromocriptine injections from pregnancy day 26 to parturition or to postpartum day 5, correlated with a stronger reduction in crouching and an increased time inside the nest, measures of disturbed maternal behaviour, on postpartum days 3 and 5. Preventing suckling by thelectomy did not prevent prolactin release but reduced crouching incidence and increased the time spent inside the nest on postpartum days 3 and 5. Bromocriptine, injected in thelectomized mothers across postpartum days 1-5, further reduced the incidence of crouching and increased the time spent inside the nest on postpartum days 3 and 5. We conclude that prolactin acting prepartum facilitates maternal behaviour initiation in rabbits and, together with pup stimulation, maintains this behaviour across lactation.

Changes in leptin levels during lactation: implications for lactational hyperphagia and anovulation

In these studies we investigated the time course of changes in circulating leptin levels in lactating rats and the dependence of these changes on the energetic cost of lactation and evaluated the contribution of changes in leptin levels to lactational hyperphagia and infertility. In the first experiment, plasma leptin levels were measured on Days 5, 10, 15, 20, and 25 postpartum in freefeeding lactating rats and age-matched virgin females. Retroperitoneal and parametrial fat pads weights were obtained from the same females. In the second experiment the same measures, together with plasma insulin and prolactin levels, were taken on Days 15 and 20 postpartum from galactophore-cut and sham-operated females. In Experiments 3 and 4, the effects of exogenous leptin administration, either subcutaneously (sc) or intracerebroventricularly (icv), on lactational anovulation, maternal food intake, and dam and litter weights were examined. Circulating leptin levels decreased in lactating rats. Leptin levels were highly positively correlated with fat pad weight. Eliminating the energetic costs of lactation by preventing milk delivery induced dramatic increases in plasma leptin and insulin levels and also increased adiposity. Exogenous leptin administration did not affect length of lactational anovulation but reduced food intake, maternal body weight, and litter weight gain when given centrally and maternal body weight when given systemically. Together, these data show that the energetic costs of lactation are associated with a fall in circulating leptin levels but that these do not make a major contribution to the suppression of reproduction in lactating rats; however, they may be permissive to the hyperphagia of lactation.

The contribution of changes in milk delivery to the prolongation of lactational infertility induced by food restriction or increased litter size

In rats, the length of lactational anovulation is prolonged when litter size is increased or when the dam is food restricted. In both of these situations the energetic demand on the dam is increased, milk delivery to each pup is reduced, and consequently, patterns of pup suckling may be changed. We investigated the contribution of these factors to the maintenance of lactational anovulation by examining the effect of eliminating milk delivery on the length of lactational diestrus in food restricted and ad lib-fed females nursing litters of 8 pups and in females nursing large (14 pups) and small (6 pups) litters. The results of these studies show that preventing milk delivery neither extends the period of lactational infertility in ad lib-fed females nursing eight pups nor eliminates the effects of increasing litter size on this period.

Suckling-induced changes in responsivity to the hypoalgesic effect of morphine

The hypoalgesic effect of morphine in lactating rats was assessed using the hot-plate test. At midlactation (days 12 and 18 postpartum) females nursing litters of 8 pups were less responsive to the hypoalgesic effect of morphine than ovariectomised or cycling females and females on days 6 or 24 of lactation. Subsequent studies showed that the hypoalgesic response to morphine was inhibited in lactating rats at a number of time points after drug administration and across a variety of doses. This effect was not dependent on milk delivery but was dependent on the hormonal state of the female since separation of dams and their litters for 96 h was sufficient to reinstate the response to morphine if it resulted in a reappearance of vaginal estrus.

Maternal aggression of rats is impaired by cutaneous anesthesia of the ventral trunk, but not by nipple removal

Aggression toward conspecific and allospecific individuals by female mammals is much more likely during lactation than during other reproductive states, a behavior that serves to protect the young. Previous research revealed that removal of nipples (thelectomy) prepartum greatly reduces the likelihood of postpartum aggression in house mice, but not in Sprague-Dawley Norway rats. The present work shows that prepartum thelectomy has no effect on the likelihood or intensity of postpartum aggression toward a strange male intruder in Long-Evans rats. In contrast, anesthesia of each nipple and surrounding skin prevents or severely impairs the elicitation of biting and attacking by the intruder, but does not impair normal retrieval of pups. Following removal of the litter, maternal aggression occurs readily at 1 h and somewhat less so at 5 h, but is absent at 24 h. These data suggest that while maternal aggression in postpartum rats does not require suckling, it is dependent on somatosensory stimulation of the ventral trunk by pups; this stimulation apparently produces a motivational change that lasts several hours.

Neuroethology of olfactory preference development

Young mammals come to approach the odor of their mother, a response that facilitates their survival during early life. Young rats induce a cascade of events in their mother to induce the emission of her odor. The pups increase circulating prolactin levels, which increases food intake and the emission of large quantities of cecotrophe containing the maternal odor. This odor is synthesized by the action of cecal microorganisms and changes with maternal diet. The diet-dependence of the odor requires the pups to acquire their attraction to the odor postnatally. The acquisition of this preference occurs when an odor is paired with the tactile stimulation that pups receive during maternal care. The action of the tactile stimulation appears to be mediated by noradrenaline. The development of this type of olfactory attraction is accompanied by changes in the regions of the olfactory bulb that are responsive to the attractive odor. Metabolic, anatomical, and neurophysiological changes in response to the attractive odor emerge in such regions of the bulb after early olfactory preference training.

Ventral somatosensory determinants of nursing behavior in Norway rats. I. Effects of variations in the quality and quantity of pup stimuli

By varying the quality and quantity of tactile input that rat dams, between Days 2 and 14 postpartum, received from their pups, we found the following: a) After a 4-hr separation from their litter, mothers continuously display an array of activities, until the onset of nursing, in response to displaced pups and to pups gathered in the nest. b) While the dam hovers over the gathered pups, engaged in an activity such as pup-licking, the young gain access to the dam's ventrum and root for a nipple. c) Pups capable of effective rooting, nipple attachment, and suckling thereby stimulate the dam's immobility (i.e., inhibition of head and limb movements), assumption of the upright crouching posture, and milk ejections. d) Rat dams do not become immobile or crouch in response to pups that are inactive or that are active but incapable of rooting effectively or suckling. e) The likelihood and speed of assuming the quiescent nursing posture, as well as of having milk ejections subsequently, are directly related to the number of effective pups in the nest. We propose that the initiation, maintenance and termination of nursing behavior are related to the spatial and temporal summation of effective ventral somatosensory afferents.

Pregnancy-induced autogrooming in mice: the effects of nipple removal

As pregnancy advances, Rockland-Swiss albino mice spend increasingly more time engaged in autogrooming. By gestation day 18 (the day prior to parturition), the total duration of autogrooming (a composite measure) as well as the duration of nipple line grooming is significantly longer than that displayed by virgin and early pregnant females. Nipple excision (i.e., thelectomy) performed prior to pregnancy reduces the time GD 18 dams spend licking nipple line regions without producing a concomitant decrease in the duration of grooming all regions. It is unlikely, therefore, that peripheral stimulation of the nipple line regions via self-licking contributes to the heightened display of grooming by pregnant mice.

The lactation-blocking drug bromocriptine and its application to studies of weaning and behavioral development

An experimental method for blocking maternal lactation is reviewed and the possible application of this technique for experimentally manipulating weaning is considered. Maternal milk production can be inhibited using the prolactin-suppressing drug bromocriptine. The suitability of bromocriptine for use in behavioural experiments is considered. The pharmacology of bromocriptine (CB 154) is briefly outlined and a compilation of the reported lactation-inhibiting doses for various species is presented. The possible endocrine and behavioral side-effects and the toxicity of the drug are discussed. It is concluded that, in most species studied so far, the drug is relatively free from significant side-effects at the low doses needed to suppress lactation. Guidelines for the practical use of bromocriptine are suggested. Finally, some ideas about the possible application of the drug to the study of behavioral development and parent-offspring relationships are discussed.

Postnatal undernutrition: an alternative method

Rat pups were undernourished from birth by placing them for 12 hr/day with a normal lactating mother and 12 hr/day with a nipple-ligated mother each day for 25 days. The method resulted in a marked delay in the body growth of the undernourished pups, especially during the first 2 weeks of life. Observations of the behavior of the mothers towards the underfed pups were made at different times of the day and compared to the behavior of the mothers suckling well-fed pups. The results show that (1) nipple-ligated mothers are able to provide adequate maternal care for undernourished pups, and (2) both ligated and nonligated mothers caring for underfed pups spend more time with those pups than mothers caring for well-fed pups.