Lactating Females Research Articles

Do female degus communally nest and nurse their pups?

We examined the nesting and nursing behavior of females of the caviomorph rodent Octodon degus. We recorded the behavior of two, three-female groups of lactating degus kept in captivity and compared it with that of singly housed lactating females. Grouped females spontaneously nested communally. Five of six lactating females had non-offspring pups hanging from their teats, which suggests that some non-offspring nursing took place. Non-mothers increased their maternal behavior after the delivery of their own pups. Although time allocated to parental care by each communally nesting dam did not differ from that of solitary lactating females, young of communal litters spent less time alone and enjoyed more time cared for by a lactating female than young of single dams.

Begging and parent–offspring conflict in grey seals

Theoretical models, assuming an underlying conflict of interest between parents and offspring, suggest that begging can be an honest signal of offspring need provided that it is costly. Models on begging have been tested primarily on those birds in which parents provide care for several offspring. For these species, the underlying conflict selecting for extravagant (and apparently costly) begging displays might be sibling competition rather than parent–offspring conflict. We tested hypotheses on begging and parent–offspring conflict with the grey seal, Halichoerus grypus, as our model species, in which lactating females care for a single pup. As predicted from models of begging as an honest signal of need, pups begged at a significantly higher rate when hungry than when satiated. Furthermore, a mother was more likely to approach her pup shortly after the pup had uttered a begging call than within a randomly chosen period. We found no consistent support, however, for the assumption that there are strong conflicts of interest between mothers and pups. For example, the rate of suckling was not correlated with begging rate, suckling bouts terminated by pups were not longer than those terminated by mothers, and, although pups initiated the majority of suckling bouts, mothers did not terminate more suckling bouts than pups. These findings thus suggest that begging in species that care for a single offspring may evolve in the absence of strong parent–offspring conflict.

Maternal and mating-induced aggression is associated with elevated citrulline immunoreactivity in the paraventricular nucleus in prairie voles

Lactating female rodents are fiercely aggressive against intruders when they are rearing and protecting pups. In monogamous prairie voles, Microtus ochrogaster, males are parental and exhibit a dramatic increase in aggression, termed mating-induced aggression, in association with reproduction. In mice, the gas, nitric oxide (NO), inhibits male aggression, but may have an excitatory role in the production of maternal aggression. In this study, we combined aggressive behavioral testing of female and male prairie voles with immunohistochemistry for citrulline, a marker of NO synthesis, to examine NO synthesis indirectly during maternal and mating-induced aggression. A significant increase in the number of citrulline-positive cells was identified in the paraventricular nucleus (PVN) of the hypothalamus in aggressive lactating females compared with unstimulated lactating females. A significant increase in the number of citrulline-positive cells was also observed in the PVN of aggressive mated males compared with nonaggressive unmated males and unstimulated mated males. Both nonaggressive unmated males and unstimulated mated males show similar levels of citrulline immunoreactivity in the PVN. In other regions of the brain, no changes in the number of citrulline-positive cells were observed. These results suggest that NO is released specifically in the PVN during both maternal and mating-induced aggression in prairie voles.

Isotope recycling in lactating dogs (Canis familiaris).

Isotope-based techniques for the measurement of water turnover, energy expenditure, and milk intake often assume that there is no recycling of isotopes once they have left the labeled animal. In experiments involving lactating females or their suckling offspring, there are several possible routes of isotope recycling. These include the consumption of labeled milk by offspring, the ingestion of labeled excreta, and the rebreathing of exhaled labeled CO(2) or water vapor by both mother and offspring. Isotope recycling might be especially important during lactation because the offspring are in close contact with each other and their mother for prolonged periods. We show here in 24- to 30-day-old domestic dog Canis familiaris puppies that there was no detectable transfer of (18)O or (2)H from labeled to unlabeled pups in two litters (16 pups, 8 labeled, 8 unlabeled) that were weaned early and independent of their mother. However, there was a significant transfer of both isotopes from labeled to unlabeled pups and from labeled pups to their mothers in nine equivalent nursing litters of the same age (27 labeled, 26 unlabeled pups). The increases in enrichment of isotopes in unlabeled offspring were greater than the increases in enrichment of the mothers. This indicates that maternal ingestion of offspring excreta and subsequent transfer of isotope in milk is not the sole pathway of recycling. Additional routes must also be important, such as exchange of isotope between pups on saliva-coated nipples and perhaps direct ingestion of excreta by unweaned young. Recycling is unlikely to be an important factor when determining maternal metabolic rate during peak lactation in domestic dogs. However, experiments that are designed to assess the energy demands of pups and isotope-based estimates of water turnover in offspring may need to take into account any effects of isotope recycling. In a theoretical example, removing the effects of recycling increased the measured energy expenditure in pups by up to 7% and increased the calculated elimination rates of both isotopes by up to 11.1% in (18)oxygen and 10.9% in (2)hydrogen.

Maternal Aggression Is Reduced in Neuronal Nitric Oxide Synthase-Deficient Mice

Lactating females express rapid extremes in behavior, ranging from gentle nurturance toward offspring to fiercely protective aggression against intruders. Although males often behave aggressively against intruders, female rodents usually express aggression only when rearing and protecting pups. Nitric oxide (NO) inhibits male aggression; however, its role in maternal aggression is unknown. In the present study, female mice with targeted disruption of the neuronal nitric oxide synthase gene (nNOS-/-) displayed significant deficits in maternal aggression relative to wild-type (WT) mice in terms of percentage displaying aggression, the average number of attacks against a male intruder, and the total time spent attacking the male intruder. The nNOS-/- mice displayed normal pup retrieval behavior. Because the specific deficits in maternal aggression in the nNOS-/- mice suggested a possible role for NO in maternal aggression, we combined behavioral testing of WT mice with immunohistochemistry for citrulline, an indirect marker of NO synthesis, to examine indirectly NO synthesis during maternal aggression. A significant increase in the number of citrulline-positive cells was identified in the medial preoptic nucleus, the suprachiasmatic nucleus, and the subparaventricular zone regions of the hypothalamus in aggressive lactating females relative to control mice. In other regions of the brain, no changes in the number of citrulline-positive cells were observed across either groups or treatments. These results provide two indirect lines of evidence that NO release is associated with maternal aggression.

Reproductive Decisions in Female European Ground Squirrels: Factors Affecting Reproductive Output and Maternal Investment

Physiological and behavioural parameters associated with reproductive effort and success were investigated in female European ground squirrels Spermophilus citellus. The proportion of reproductive (lactating) females in the study population was over 90% and was not related to age. Timing of oestrus and ovulation was found to be affected by the female’s emergence date and condition. Females with low emergence mass showed delayed oestrus. Differences in ovulation dates were shown to affect reproductive output in terms of litter size and sex ratio. Early litters were larger and male biased. X‐ray techniques were used to determine intrauterine litter size in individual females. The results indicated that litter size and sex ratio were fixed prenatally. Lactation costs were reflected in the intensity of mass loss and duration of lactation. Mass loss varied with litter size, in that females with large litters showed a more rapid loss than others. The second parental investment parameter, lactation duration, varied among individual females and was dependent on the timing of reproduction and litter size (except yearlings). Early born litters, which were, in most cases, larger than later ones, were nursed longer. Prolonged lactation periods affected female condition in that they started prehibernation fattening later and entered hibernation with a lower mass than individuals that had shorter lactation periods. Yearling females probably could not afford the energetic costs of long lactation, independent of their offspring number. These results indicated that females with higher reproductive output and higher investment were unable to compensate these costs before hibernation. Consequences for these individuals could therefore be lower over‐winter survival or a delayed oestrus in the following season.

Lactation-induced reduction in rats' acoustic startle is associated with changes in noradrenergic neurotransmission.

The acoustic startle response (ASR) with or without fear conditioning was compared between cycling (CYC) and lactating (LACT) female rats. ASR sensitivity to changes in endogenous noradrenergic (NA) release was examined using the alpha-2 NA receptor drugs yohimbine and clonidine. Groups of CYC and LACT females were also tested in the open field. ASR was reduced in all LACT, compared with that in CYC females. Both groups exhibited a robust response to fear conditioning and unpotentiated ASR subsequent to conditioning was increased in LACT females. The lowest dose of yohimbine significantly increased ASR in LACT females, but not in CYC females. Clonidine reduced ASR in both groups of females, with a greater potency in CYC females. In the open field, LACT females displayed a shorter latency to emerge, less freezing behavior, and more entries into the field than did CYC females. The authors concluded that (a) LACT females are less anxious in a novel environment and that decreased anxiety can be efficiently counteracted by fear conditioning, and (b) changes in NA neurotransmission contribute to lactation-induced modifications in ASR.

Nathusius' pipistrelle bats (Pipistrellus nathusii, Keyserling & Blasius 1839) breeding in Ireland

AbstractWe describe the most westerly known maternity colony of Nathusius' pipistrelle bats (Pipistrellus nathusii). The bats were identified by using morphometric measurements and analysis of time‐expanded echolocation and social calls. The roost, containing approximately 150 individuals, was located in a mid 19th century farm stable block and store house situated in parkland in County Antrim, Northern Ireland. The roost was visited on 30 April, 1 May and 22 June 1997. Over this period, 11 bats were caught: one adult male, five pregnant females, four lactating females and a juvenile male. Direct observation of behaviour patterns suggests that mating groups of P. nathusii may occur in Ireland as late as May. The migratory nature of this species is discussed.

Reduced noradrenergic tone to the hypothalamic paraventricular nucleus contributes to the stress hyporesponsiveness of lactation.

Lactation in mammals is accompanied by a marked decrease in stress responsiveness that we previously attributed, in part, to a reduction in noradrenergic (NA) innervation of hypothalamic paraventricular nucleus (PVN) neurons controlling neuroendocrine stress responses. In the present study, we compared in-vivo PVN catecholamine secretion by microdialysis between nonlactating and lactating females and tested the effects of NA alpha-1 and alpha-2 receptor antagonists (corynanthine and idazoxan, respectively) on the acute stress response of lactating and virgin female rats. To determine if PVN alpha-adrenoreceptor density, affinity, or synthesis, changes as a function of lactation, we performed receptor autoradiography, Scatchard analysis and in situ hybridization of alpha-adrenoreceptors. Densitometric analysis of the alpha-adrenoreceptors in the supraoptic nucleus (SON) was used to evaluate changes in magnocellular neurons. Endogenous PVN norepinephrine release under basal conditions was lower in lactating females than in females who had their pups removed for 2 days, and microdialysate concentrations of adrenaline and MHPG were attenuated in lactating females. Alpha-2 adrenoreceptor density in the PVN showed a significant decrease from lactation day 3 to lactation days 10-12 and a reduction to 40% of virgin controls on days 10-20 of lactation. A similar pattern was observed for the SON. The affinity of hypothalamic alpha-2 adrenoreceptors was reduced as a function of lactation. Alpha-1 adrenoreceptor density in the PVN and in the hypothalamus rose as a function of lactation, although the affinity of these receptors was not altered. In contrast, alpha-1D adrenoreceptor subtype mRNA expression in the PVN decreased in middle lactating females (day 10) compared to virgins. Intracerebroventricular (i.c.v.) application of idazoxan, significantly increased the ACTH response to swim stress in virgin females, but had the opposite effect in lactating females. In contrast, i.c.v. corynanthine treatment significantly decreased the ACTH response in virgins, but not in lactating females. Overall, these data suggest that the secretion of NA in the PVN is reduced during lactation, and that the ability of PVN parvocellular neurons to respond to changes in synaptic NA levels (i.e. after stress) is also altered.

Consequences of lead exposure of rats during pregnancy, lactation, and postweaning. A combined behavioural and neurotoxicological study

Behavioural and electrophysiological changes caused by inorganic lead treatment at different phases of the ontogenesis were investigated in Wistar rats. They were treated by gavage with 80.0, 160.0, and 320.0 mg/ kg lead during the 5th-15th days of pregnancy, or the 5th-15th days of pregnancy + 4 weeks of lactation (females of P generation), or the 5th-15th days of pregnancy + 4 weeks of lactation (females of P generation) + 8 weeks after weaning (males of F1 generation). The behavioural (open field) and electrophysiological (electrocorticogram, cortical evoked potentials, etc.) parameters of F1 male rats in the above groups were investigated at the age of 12 weeks. Only rats exposed to lead during pregnancy exhibited a significant hyperactivity in the OF test expressed as a higher ambulation rate in the first minute in the OF and as longer running distances. Grooming was also increased. The spontaneous and evoked electrophysiological functions showed dose- and treatment-dependent changes (e.g. decreased mean amplitude and increased frequency of the electrocorticogram, lengthened latency and duration of the evoked potentials, etc.). The results indicate that the low-level pre- and postnatal inorganic lead exposure considerably affects both the behaviour and the function of the nervous system.

Limits to the Energy Budget in a Rodent, Peromyscus maniculatus: The Central Limitation Hypothesis

The maximum rate of energy assimilation in lactating Peromyscus maniculatus (100 kJ/d, body mass = 25 g) was lower than in cold-exposed females (115 kJ/d, body mass = 20 g). This pattern differs from that found in a few species of voles and mice, in which lactating females achieve higher rates of assimilation than cold-exposed animals. The results do not support the hypothesis that energy budgets of animals are limited centrally by the rate of energy assimilation from food, irrespective of which metabolic function constitutes the main energy expenditure. The discussion about physiological limits to energy budgets must involve information about the life strategies of the animals.

The Effect of Dimethoate on Bone Marrow Cell Chromosomes of Rats in Subchronic Four-Generation Experiments

The effect of the insecticide dimethoate (O,O-dimethylS-/N-methylcarbamoylmethyl phosphorodithioate) on bone marrow cells of four generations of rats was investigated. Male and female Wistar rats were treated for 6 weeks with 5 treatments per week via gavage at doses of 7.0, 9.33, and 14.0 mg/kg (1/100, 1/75, and 1/50 of the LD50). Chromosomes of 10 of the treated males were prepared. Other males (6–8 males per dose) were paired with females (11–16 females per dose). This represented the first (P) generation. During pregnancy and lactation females were treated with adequate doses of dimethoate. In F1 (70 males and 56 females), F2 (71 males and 62 females), and F3 generations (62 males and 44 females) treatment with dimethoate and the preparation of chromosomes (40 males per generation) were the same as those in the P generation. Following treatment with dimethoate in P and F1 generations, the number of numerical chromosome aberrations increased significantly. No significant changes in chromosome aberrations were found in F2 and F3 generations compared to the control groups. The frequency of numerical chromosome aberrations decreased considerably compared to the P generation at the dose of 14.0 mg/kg (1/50 of LD50).

Heart and lung adaptations to pregnancy and lactation in a crocidurine shrew

Heart and lung mass, rate of oxygen consumption (V̇ O 2 ), respiration rate (f r), tidal volume ( Vt), and heart rate (f h), were measured at rest and thermoneutrality in the shrew Crocidura russula monacha [(This shrew is claimed to be Crocidura suaveolens (Catzeflis, F., T. Maddalena, S. Hellwing and P. Vogel (1985). Unexpected findings on the taxonomic status of East Mediterranean Crocidura russula auct. (Mammalia, Insectivora). Zeitschrift fur Säugetierkunde 50: 185–201)] in nullipar (N), pregnant (P) and lactating (L) females. The heart mass of N females is large (0.86% of body mass) but f h is slow (70% of that expected by mammalian allometry), while lung mass is small (87% of expected) and f r is high (47% above expected). The 31% higher than expected expired ventilation (V̇ e) matches the 25% higher than expected V̇ O 2 . In P females the f h of 560 min −1 did not change but it increased in L females to 620 min −1. V̇ e did not change in P females but decreased in L females from 11.3 ml · min −1 to 10.1 ml min −1. The normal Vt decreased in P and L females from 47.3 μl to 38.4 μl and 37.8 μl respectively. The lower than expected resting f h of N females may provide sufficient scope for increased heart work and oxygen supply during exercise. The large heart may be more efficient. Hyperventilation in N females is indicated by the calculated relatively high fractional concentration of oxygen in expired air ( Fe O 2 ), and low fractional concentration of CO 2 in expired air ( Fe CO 2 ) (17.7% and 3.2% respectively), the high oxygen partial pressure in alveolar gas ( Pa O 2 ) and low alveolar CO 2 ( Pa CO 2 ), 119 and 34 Torr respectively, which facilitate O 2 transport through the lung air-blood barrier. The elevated V̇O 2 in P and L females is achieved by increased respiratory efficiency from the normal 15%, to 24% and 29% respectively.

Dissociation between behavioral and hormonal responses to the forced swim stress in lactating rats.

Retention of immobility in the Porsolt forced swim test is believed to be dependent upon glucocorticoid secretion in male rats. Because lactating females exhibit increased basal glucocorticoid secretion and blunted stress responses, we tested the hypothesis that lactation-induced changes in adrenal glucocorticoid and in circulating estrogen and progesterone levels would improve retention and/or acquisition of immobility. Immobility was recorded during 3 intervals of 5 min on day 1 (acquisition) and one 5 min interval 24 h later (retention). Blood samples were collected before the swim test and at various times after the onset of stress for plasma ACTH and corticosterone (B) determinations. Male rats (young=200 g, old=325 g) were compared to virgin females (V) and to lactating females in early (day 8-10, EL) and late (day 17-19, LL) lactation. Adrenalectomy (ADX) and ovariectomy (OVX) were performed 5 and 10 days prior to testing, respectively. All animals acquired immobility at the end of the 15 min swim on day 1, but only the young male group exhibited a significant retention of immobility on day 2. Total immobility was higher in males than females (V) although basal and stress-induced ACTH and B secretion were comparable on both testing days. Lactational status did not affect immobility in either the acquisition or retention phases. However, stress-induced ACTH secretion was greatly diminished in intact and ADX lactating females (EL and LL) compared to virgins (LL < EL < virgin), demonstrating a clear dissociation between behavioral and neuroendocrine responses. Following ADX, immobility in the retention phase was either decreased in males or increased in lactating females. Finally, OVX decreased immobility in both lactating (EL) and virgin females without significantly altering the magnitude of the ACTH and B responses to stress. In summary, our results demonstrated both sex-related and lactation-related differences in the behavioral and endocrine responses to he forced swim test of Porsolt. Although retention of the immobile response is thought to involve glucocorticoids and/or opioids secreted during the first testing session, we did not find evidence for a direct relationship between basal or stress-induced total corticosterone secretion, the magnitude of ACTH response to stress and behavioral scores in the retention period. However, experimental variables such as body weight, sex and water depth could significantly modify the outcome of behavioral testing and question the validity of glucocorticoid-mediated retention processes. Since the effect of ADX was reversed in lactating females compared to male rats, we hypothesize that glucocorticoid sensitivity of cognitive processes controlling behavioral reactivity is different from that controlling hypothalamic-adrenocortical function. Our results also demonstrated a clear dissociation between behavioral and neuroendocrine responses to the swim test, in particular during lactation. In early and late lactation, blunted responsiveness to stress was not caused by enhanced glucocorticoid feedback but might result from modifications in the inhibitory and/or stimulatory inputs to hypothalamic neurons controlling adrenocortical activity.

Dietary energetics of the insectivorous Mexican free-tailed bat (Tadarida brasiliensis) during pregnancy and lactation.

Stomach content analysis of 20 pregnant (x body mass=13.4 g) and 18 lactating (x body mass=11.5 g) female Tadarida brasiliensis revealed that the diet, expressed as percent volume, consists largely of lepidopterans, coleopterans, hymenopterans, and dipterans, in decreasing order of importance. We found no significant difference in the diet of pregnant and lactating females when expressed as percent volume. However, when expressed as percent frequency, proportionately more pregnant females fed on lepidopterans, coleopterans, and dipterans than did lactating bats, and proportionately more lactating females fed on hymenopterans. We found no significant differences in the percentages of water, lean dry mass, fat, and energy density in the stomach contents of pregnant and lactating females. Water in stomach contents averaged 62.7% and fat and lean dry mass averaged 22.2% and 15.2%, respectively (expressed as percentage of wet mass); energy density averaged 31.2 kJ g-1 dry mass. This relatively high energy density of stomach contents, as compared to whole insects, can be attributed to the consumption of insects high in fat (especially flying ants) and the abdomens only of moths and beetles (other body parts being discarded). Estimates of nightly food intake increased markedly from mid- to late pregnancy, stabilized or decreased during late pregnancy, and increased again during early to mid-lactation. Average nightly feeding rate doubled from pregnancy to lactation and increased threefold during the first half of lactation. By adjusting our gravimetric estimates of nightly feeding rate upwards by 40% (based on estimates of field metabolic rate), we estimate that the average nightly energy intake of female T. brasiliensis ranges from 57 kJ day-1 in early lactation to 104 kJ day-1 in mid-lactation. These estimates represent nightly feeding rates averaging 39% and 73% of a female's body mass in the period from early to mid-lactation, respectively.

Sex differences in the use of daily torpor and foraging time by big brown bats (Eptesicus fuscus) during the reproductive season

Temperate zone bats can use daily torpor as a means of saving energy. Some argue, however, that torpor is costly for both males and females and that individuals should only use it during times of poor foraging conditions. Others hypothesize that the costs are greater for females and that males should enter torpor more regularly. We tested these alternative hypotheses by using temperature‐sensitive radiotransmitters to monitor use of torpor and foraging by free‐ranging big brown bats (Eptesicus fuscus). During the pregnancy period, males used torpor at night more and foraged less often than did females. Males also went into deep torpor more often and remained in torpor longer than did females. When they foraged, males and females were away from the roosts for equal periods of time. During the lactation period, males and females rarely failed to forage and foraging times were again no different between the sexes, although males may roost at night away from the maternity colonies. Males again used torpor and deep torpor more often and for longer than females did. These results support the hypothesis that the fitness costs of using torpor are lower for males than for reproductive females and that males regularly use torpor as an energy‐saving mechanism. Females enter torpor only when foraging conditions are poor, presumably because torpor prolongs gestation and slows neonatal growth thereby leaving less time for females and their young to prepare for hibernation.

Effects of protein and energy intake, body condition, and season on nutrient partitioning and milk production in caribou and reindeer

We used captive caribou (Rangifer tarandus granti) and reindeer (Rangifer tarandus tarandus) to study the effects of energy intake, protein intake, dietary protein:energy ratio, date, and body condition on (i) body fat versus body protein deposition and (ii) maternal tissue deposition versus milk production. Energy intake was the only variable significantly affecting body mass (BM) changes in either breeding or nonbreeding adult females. Lactating and nonlactating females had comparable efficiency coefficients for net energy retention (60 and 65%, respectively), but the daily maintenance requirement for lactating females (457 kJ/BM0.75) was twice that for nonlactating individuals (232 kJ/BM0.75). In both lactating and nonlactating females, the proportion of tissue deposited as fat rather than protein increased between spring and fall but decreased with increasing fatness. Energy intake increased protein deposition in lactating females but increased fat deposition in nonlactating females. Milk water volume increased with maternal energy intake and decreased with calf age. However, production of milk dry matter, milk fat, and milk energy were not affected by maternal energy or protein intake, maternal body condition, or calf age. Production of milk lactose correlated with maternal energy intake, while production of milk protein correlated with the maternal dietary protein:energy ratio.

Retinoic acid and vitamin A: Effect of low levels on outcome of pregnancy in Guinea Pigs

To study the effect of marginal vitamin A status during pregnancy on the health of guinea pig neonates the level of vitamin A required to produce viable offspring was determined. In previous work adult animals fed a vitamin A-deficient diet for one month before mating and throughout pregnancy delivered dead pups. In this experiment depletion was followed by supplementation with three levels of vitamin A after day 30 (d30) of pregnancy (gestation=68±2d). Young adult guinea pigs were fed a vitamin A-deficient diet for 1 mo prior to mating and throughout pregnancy. Because approximately 6 wk was required for all females to mate under our conditions, they were dosed with retinoic acid (RA) from the end of the one month depletion until day 30 of pregnancy. RA was given to spare the females' remaining stores of vitamin A to reduce differences that might arise between females that mated first and those that mated last. All-trans RA was given orally three times a week; the weekly dose approximated 1.5 mg/kg body weight. After d30 RA was discontinued, and females were dosed weekly with 0, 175, 350 or 1166 retinol equivalents (RE) as retinol palmitate. At delivery malformations were observed in 28 to 66 percent of the litters irrespective of vitamin A supplementation. No malformations occurred in litters of 11 females of the same age fed a commercial guinea pig diet. Because the occurrence of malformations complicated determination of the amount of vitamin A required to produce viable neonates, females continued to receive the same treatment they received from d30 of the first pregnancy through remating, a second pregnancy, and a 2 wk lactation. Malformations were observed in litters of only 2/28 pregnancies. Under these conditions guinea pigs required a minimum of 175 RE/wk (equivalent to 25 RE/d) to produce viable neonates. To maintain normal weight gains of pups through a 2 wk lactation females required at least 45 RE/kg body wt/d. Since the dose of RA used was much less than the teratogenic dose in other animals, the guinea pig may be unusually sensitive to RA, or prior depletion of vitamin A stores may have increased the teratogenicity of RA.

Maternal protein reserves and their influence on lactational performance in rats

To determine the contribution of tissue protein reserves to lactational performance, multiparous female Sprague-Dawley rats were mated, caged individually and offered a diet high in protein (215 g crude protein (N x 6.25; CP)/kg dry matter (DM); H) ad lib. until day 12 of gestation. Subsequently half the rats continued to receive diet H while the remainder were offered a diet low in protein (65 g CP/kg DM; L) until parturition. This treatment aimed to produce a difference in carcass protein at parturition. On day 1 of lactation females were allocated to either diet H or a low-protein diet (90 g CP/kg DM; L2) offered until day 13 of lactation, giving four lactation treatment groups HH, HL2, LH and LL2. Groups of females were slaughtered on days 2 and 12 of gestation and days 1 and 13 of lactation and carcass and major organs were analysed. Weight gain of standardized litters was used as an indicator of lactational performance. Maternal carcass protein contents at parturition were 43.5 (SE 1.2) and 38.7 (SE 0.8) g (P < 0.01) for diets H and L respectively. During lactation there was little change in carcass protein content of HH rats while LH rats appeared to replenish their depleted reserves. Food intake or lactational performance did not differ between these two groups. HL2 and LL2 rats lost carcass protein with HL2 rats losing more than LL2 rats (P < 0.05). Intake and lactational performance were reduced compared with that on diet H (P < 0.05) but for the first 6 d of lactation were both greater (P < 0.05) for diet HL2 than for diet LL2. All four groups showed a considerable loss of body fat during lactation which was not affected by diet. The ability of HL2 rats to catabolize more protein and consume more food allowed them to sustain a greater lactational performance. Previous maternal protein depletion had no influence on lactational performance as long as an adequate supply of dietary protein was provided.

The Role of the Mu1 Opioid Receptor Subtype in the Regulation of Prolactin and Growth Hormone Secretion by Beta‐Endorphin in Female Rats: Studies with Naloxonazine

The μ opioid receptor subtype has been reported to mediate the prolactin secretory response to opioids. This receptor subtype has been implicated in the morphine-induced prolactin increase, as well as the prolactin response to μ-specific opioid peptides. Subtypes of the μ receptor have been proposed and the μ(1) , site has been postulated as the receptor subtype involved in the morphine-induced prolactin secretory response. However, the role of this receptor subtype in mediating the endocrine effects of the endogenous opioid peptides has not been characterized. In order to determine the physiological significance of this receptor subtype, animals were pretreated with saline, WIN 44,441-3 (a μ, δ and κ antagonist) or naloxonazine (a μ(1) antagonist) followed by a stimulatory dose of morphine or β-endorphin. A dose response study for β-endorphin was conducted to determine the minimal stimulatory dose of β-endorphin on the prolactin and growth hormone (GH) secretory response. The dose response study indicated that β-endorphin is a more potent stimulus for prolactin release than for GH. A dose as low as 25 ng increased prolactin levels as much as 100-fold in both lactating and diestrous female rats. In contrast, 2.5 μg β-endorphin was required in order to consistently and significantly increase circulating levels of GH by 2- to 3-fold. WIN 44,441-3 antagonized the stimulatory effects of β-endorphin on both prolactin and GH secretion. Naloxonazine pretreatment abolished the morphine-induced prolactin secretory response, without affecting the GH increase in diestrous females. Naloxonazine also antagonized the prolactin response to β-endorphin in both lactating and diestrous females. In addition, it attenuated the GH secretory response but did not totally abolish it. These data indicate that β-endorphin elicits an increase in prolactin release through an opioid specific receptor which appears to be the μ(1) opioid receptor subtype. They further suggest that β-endorphin may increase GH levels, at least partially, via its action at this μ(1) site.