Circadian patterns of locomotor activity and body temperature in blind mole-rats, Spalax ehrenbergi.

A wide variety of organisms exhibit circadian rhythms, regulated by internal clocks that are entrained primarily by the alternating cycle of light and darkness. There have been few studies of circadian rhythms in fossorial species that inhabit a microenvironment where day-night variations in most environmental parameters are minimized and where exposure to light occurs only infrequently. In this study, daily patterns of locomotor activity and body temperature (Tb) were examined in adult blind mole-rats (Spalax ehrenbergi). These fossorial rodents lack external eyes but possess rudimentary ocular structures that are embedded in the Harderian glands and covered by skin and fur. Most individual mole-rats exhibited circadian rhythms of locomotor activity, but some animals were arrhythmic. Individuals that did exhibit robust rhythms of locomotor activity also showed rhythms of Tb. In most cases, Tb was highest during the phase of intense locomotor activity. Locomotor activity rhythms could be entrained to light:dark cycles, and several mole-rats exhibited entrainment to non-24-h light cycles (T-cycles) with period lengths ranging from T = 23 h to T = 25 h. Some individuals also showed entrainment to daily cycles of ambient temperature. There was considerable interindividual variation in the daily patterns of locomotor activity among mole-rats in virtually all the conditions of environmental lighting and temperature employed in this study. Thus, whereas it appears likely that photic cues have a significant role in the entrainment of circadian rhythms in mole-rats, the amount of variability in rhythm patterns among individuals appears to be much greater than for most species that have been studied.

Endogenous inhibition of prolactin secretion in pregnant lactating rats.

The purpose of the present study was to test whether rat placental lactogen, proposed to be inhibitory for endogenous rhythmic PRL secretion during pregnancy, is also inhibitory for a neurally mediated PRL response to exogenous stimulation, i.e. suckling. Pregnant lactating rats were used as the model in this study. Blood samples for PRL were taken via a chronic atrial cannula before and after 30 min of suckling stimulation in the following groups of females: 1) bearing conceptuses of different degrees of development; 2) after removal of conceptuses; and 3) after injection of placental extract immediately before exposure to suckling stimulation. It was found that 1) the PRL response to suckling diminished in conjunction with placental development and the known concomitant rise in rat placental lactogen; 2) removal of the conceptuses permits elevation of PRL after suckling stimulation; and 3) injection of placental extract obtained on days 11-12 of pregnancy failed to block the elevation of PRL in response to suckling. It was concluded that 1) lactation can be maintained even in the presence of very low levels of PRL, provided developed placentae are present, and 2) the developed placentae are capable of inhibiting the acute PRL release in response to exogenous stimulation as well as endogenous rhythmic PRL secretion.

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.

Daily rhythm in secretion of prolactin in androgenized female rats.

Androgen-sterilized female rats were obtained by administering 10 micrograms testosterone propionate to pups on day 2 after birth. In contrast with ovariectomized adults, androgenized adult female rats are incapable of responding to cervical stimulation by secreting prolactin in the nocturnal surge pattern. In spite of the loss of this pattern the androgenized female rats still exhibited a daily circadian rhythm of prolactin secretion with afternoon levels three times higher than those after midnight.

Timing of termination of nocturnal prolactin surges in pregnant rats as determined by the number of fetuses.

In female rats bearing an average litter of 10+/-0.28 fetuses, pituitary prolactin is secreted in a pattern of two daily surges during the first 10 days of pregnancy only. When the number of fetuses was experimentally reduced, prolactin surges continued for a longer period and pregnancy was correspondingly lengthened. The relationship found between a reduced number of fetuses, and delay in termination of prolactin surges further supports the view that, during pregnancy, rat placental lactogen exerts a major influence in inhibiting secretion of prolactin by the maternal pituitary gland. The constant relationship found in all groups between termination of nocturnal surges of prolactin and parturition indicates that it is the interval between implantation and termination of prolactin surges which is prolonged in female rats with small litters.

Absence of nocturnal autonomic prolactin secretion during lactation in the rat.

During pseudopregnancy in the rat prolactin is secreted autonomously in a characteristic pattern of two daily surges, one nocturnal and one diurnal. The present study investigated whether, during lactation, prolactin secretion occurred in an autonomous nightly surge pattern, in addition to secretion resulting from suckling stimulation. Nocturnal levels of prolactin were measured daily for 2 weeks in lactating rats which were separated from their young from 21.00 to 06.00 h. It was found that no nocturnal prolactin surges were present throughout the 2 weeks of lactation. In addition, it was observed that after pups were removed for 3 h prolactin levels were lower during the first week of lactation than during the second week. It was concluded that lactational dioestrus is distinguished from pseudopregnancy by the absence of autonomous secretion of prolactin in the pattern of a nocturnal surge.

Failure of rat placenta to inhibit prolactin secretion by ectopic pituitary gland.

At mid-pregnancy in the rat, episodic secretion of pituitary prolactin ceases when the placenta is sufficiently developed. At this time, sufficient placental lactogen is secreted by the placenta to inhibit prolactin secretion. The present study tested whether the fully developed placenta at mid-pregnancy can inhibit prolactin secreted by a donor pituitary gland implanted under the kidney capsule. Three pituitary glands were implanted in rats on day 7 of pregnancy; muscle fragments were implanted in controls. Blood was collected during the first and second halves of pregnancy. It was found that prolactin concentrations in the animals with the pituitary implants were high on days 9 and 10 of pregnancy and remained raised during the second half of pregnancy while in control animals nocturnal surges were absent in the second half of pregnancy, the last one occurring on day 10. This observation indicates that the placental hormone cannot act directly on the pituitary gland to inhibit prolactin secretion, but presumably exerts its suppressive effect on prolactin secretion through the hypothalamus.

Neuroendocrine processes in the establishment of pregnancy and pseudopregnancy in rats.

Rats, like mice, hamsters and other small rodents, have a short, incomplete estrous cycle. They lack a spontaneous luteal phase, and the corpora lutea (CL) of the estrous cycle are short-lived and nonfunctional. The activation of the luteal phase in these incompletely cycling females depends on the neuroendocrine reflex provided by the male at the time of mating. In this article we review our research on the cervix-brain-pituitary-ovary interaction at the endocrine, neuroendocrine, neurotransmitter and behavioural levels, dealing with the chain of events initiated by the male and necessary for the establishment of the progestational state.

Effect of the neuroactive steroid alpha-THDOC on staircase test behavior in mice.

This study examined the effect of the neuroactive steroid 3 alpha, 5 alpha-tetrahydrodeoxycorticosterone (alpha-THDOC) as compared to the benzodiazepines diazepam and midazolam and the barbiturate phenobarbital on the number of rearing events and the number of steps ascended in the mouse staircase test. The benzodiazepines, phenobarbital and alpha-THDOC all reduced rearing activity at doses that did not affect climbing. The rearing-suppression effect of the benzodiazepines and alpha-THDOC, but not of phenobarbital, was blocked by the benzodiazepine antagonist flumazenil. It appears that, although such neuroactive steroids, like barbiturates, bind to distinct sites within the chloride ion channel of the gamma-aminobutyric acid type A (GABAA) receptor complex, alpha-THDOC behavioral activity is modulated by the benzodiazepine recognition site.

Neuroendocrinology of coitally and noncoitally induced pseudopregnancy.

In this paper, I have outlined our research perspective on coitally and noncoitally induced PSP. Coitally induced PSP requires a minimal threshold of vaginal stimulation to establish the repetitive nocturnal PRL surges which are essential for converting the corpora lutea from a nonfunctional to a functional state. The induction of PSP requires the activation of the adrenergic system and simultaneous inhibition of the cholinergic system. Reduction of neural activity in the medial preoptic area occurs at the time of coital stimulation, and is believed to be associated with the initiation of the nocturnal PRL surges. Apparently, cervical stimulation acts by inhibiting the nocturnal PRL surge inhibitory neurons at this brain site, thereby instituting PSP. It is proposed that the specific mating pattern of cervical stimulation activates a "mnemonic system" which retains information from vaginocervical stimulation permitting the expression of the repeated nightly PRL surges. In our view, each pregnancy originates as a direct continuation of the initial "hypothalamic PSP" triggered by the neural signal of mating. The mechanisms involved in the activation of the "mnemonic system" and the hypothalamically pseudopregnant state remain to be elucidated. Pseudopregnancy can be initiated by a number of socioenvironmental factors as well, such as cohabitation in all-female groups in mice or concaveation with foster pups in virgin rats. However, unlike coitally induced PSP which activates the "mnemonic system," noncoitally induced pseudopregnancies lack a "mnemonic system," and therefore require the continued presence of the initiating stimulus until progesterone levels become sufficiently elevated to provide positive feedback on PRL secretion.

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

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

Effects of small medial preoptic area lesions on maternal behavior: retrieving and nest building in the rat.

Previous studies in rats have demonstrated that large lesions in the medial preoptic area (MPOA) disrupt all aspects of maternal behavior. In the present study, small bilateral electrolyte lesions in the MPOA of lactating females abolished nest building and retrieving components of maternal behavior while crouching and nursing were unaffected. Animals which failed to show retrieval and nest building behaviors tended to have a greater area of lesion within the more dorsal part of the MPOA. Although the dorsal MPOA may play a role in the maintenance of the active components of maternal behavior, i.e. nest building and retrieving, another critical factor in determining which components of maternal behavior are disrupted may be the size of the lesion. As one increases the area of damaged tissue there is also an accompanying increase in the components of maternal behavior which are disrupted. No correlation was found between damage to the Sexually Dimorphic Nucleus of the preoptic area and retrieving and nest building.

Retinal projections in the blind mole rat: a WGA-HRP tracing study of a natural degeneration.

The mole rat Spalax ehrenbergi is a fossorial rodent. Although its peripheral visual system--eye and optic nerve--is highly degenerated, it shows some sensitivity to light. However, in the usual sense, it is essentially blind. An auditory take-over of the visual lateral geniculate nucleus and at least part of the visual cortices was recently demonstrated. In order to visualize the retinal projections during ontogeny, we used an anterograde tracing technique, with monocular injection of wheat germ agglutinin-labeled horseradish peroxidase (WGA-HRP). In the newborn mole rat the retina projects to most of its normal targets as compared with seeing rodents, with bilateral projections to the suprachiasmatic nuclei, the dorsal and ventral lateral geniculate nuclei, the lateroposterior nuclei, the optic tract nuclei and the superior colliculi. During the course of ontogeny, the retinohypothalamic connection is stabilized but the main optic tract undergoes progressive degeneration. In adults, only a few retinal fibers enter the contralateral ventral lateral geniculate nucleus, the lateroposterior nucleus, the optic tract nucleus and the superior colliculus. No retinal fibers could be detected in the dorsal lateral geniculate nucleus. Thus, the retinofugal projections in the adult mole rat could explain its reduced sensitivity to light, whereas the complete degeneration of the retino-dorsal lateral geniculate nucleus projection could underlie the invasion of auditory input into this normally visual center.

Effect of photoperiod variation on testes and accessory sex organs in the male blind mole rat Spalax ehrenbergi.

The blind mole rat is a seasonally breeding fossorial rodent that is perceptionally blind. This study examines the effect of photoperiod on the morphology and histology of the male mole rat reproductive system, three groups of male mole rats were maintained in the laboratory under short day (SD) conditions (9L: 15D); long day (LD) conditions (15L:9D); and constant darkness (CD), and compared to animals trapped in the field (FL). It was found that the field animals revealed higher testes and prostate gland weights, higher prostate tubuli volume (v*) and lower testes tubuli volume (v*) compared to the other three groups. Distribution of the tubuli in the testes (Vv) was low in the FL group compared to the SD and LD groups but still higher than in the CD group. Distribution of lumen in the testes (Vv) was higher in the CD group in comparison to the other three groups. Distribution of interstitial tissue in the testes (Vv) was higher in the FL group than in the other three groups. Electrolytes and elements secreted from the prostate gland did not differ among the four groups. In the FL group distribution of the tubuli (Vv) in the prostate gland was low and lumen ratio (Vv) was high compared to the other three groups. Distribution of connective tissue in the prostate gland did not differ among all four groups. Testosterone levels and total sperm count was highest in the FL group. Sperm production was noted in all groups; however spermatid and spermatozoa cell production was higher in the FL group. This study shows that photoperiod could be important in initiating timing in the breeding season but that certain other conditions which are absent in the laboratory appear to be responsible for successful breeding in the field.

Grooming secretions and seasonal adaptations in the blind mole rat (Spalax ehrenbergi).

The postulated thermoregulatory function of grooming in the "blind" mole rat (Spalax ehrenbergi), a solitary, highly aggressive subterranean rodent, was tested by subjecting individuals to extreme hot and cold environments and measuring their grooming frequency and thermoregulatory capacity. It was found that exposure to heat stress during both the winter and the summer failed to elicit grooming in isolated mole rats, even though their body temperature was significantly elevated. Thus, unlike Mongolian gerbils (Meriones unguiculatus), mole rats do not seem to have a mechanism for lowering body temperature by grooming. However, at low temperatures, grooming behavior significantly increased during both seasons, although a decline in body temperature was recorded only during the summer. Because grooming in mole rats expresses Harderian lipids onto the fur, it is suggested that in the winter, under cold stress, this behavior might prevent a drop in body temperature through spreading insulating Harderian lipids.

Transition from pup killing to parental behavior in male and virgin female albino rats.

In the first part of this study, the effect of habituation to pups was examined in virgin female and male Sprague-Dawley rats that committed infanticide in a screening test. With repeated exposure to test pups (5-10 days old), the rats ceased to commit infanticide and came to behave parentally. Preexposure to inaccessible pups (confined inside wire-mesh baskets) did not accelerate the rate of disappearance of infanticide during subsequent contact with young, which suggests that pup killing is not a neophobic response to the novelty of young. In the second part of the study, three groups of infanticidal male Wistar rats were mated and tested for their responses towards unrelated pups after different intervals of cohabitation with their mates. The males continued to commit infanticide at the time that their mates were at midpregnancy or 24 hr before parturition, but males that cohabited with their mates till day 9 postpartum no longer attacked the young. Thus, the mother rat, presumably by means of postpartum aggression, renders her mate noninfanticidal, thereby reducing the likelihood of her offspring being harmed when she is away from the nest.

Social stress in neighboring and encountering blind mole-rats (Spalax ehrenbergi).

Blind mole-rats (Spalax ehrenbergi) are solitary aggressive subterranean rodents. They inhabit individual territories, comprised of branched tunnels. Each such tunnel system is completely separate from that of any neighboring mole-rat. Although intraspecific encounters between neighbors are infrequent, when they do occur, they may result in the injury or death of one or both animals. Avoidance of encounters may be due to the awareness of a neighbor's whereabouts through scent-marking and/or seismic (vibratory) communication. The present study was intended to examine whether encounters between individual mole-rats result in physiological stress. Two experimental conditions were designed to simulate natural situations: a brief encounter between two neighboring mole-rats, taking place either once or several times and long-term residency of neighbors whose only contact was either vibratory or vibratory plus odor communication. Blood samples were taken before, during, and after encounters in the first experiment and at set intervals in the second. The blood variables measured were blood glucose levels (BGL) and neutrophil/lymphocyte ratio (N/L). Blood glucose levels and neutrophil/lymphocyte ratio ratios increased in both members of encountering pairs. Long-term residency with a neighbor resulted in the establishment of a dominant-subordinate relationship through vibratory communication only, with increased neutrophil/lymphocyte ratio ratio in the subordinate males. However, long-term residency of males exposed to both vibrations and odors of neighboring males resulted in the death of both individuals. It seems that brief direct encounters and long-term neighboring conditions without physical contact are sufficient to cause severe stress to mole-rats. It is possible that in the wild, in some situations in which neighboring mole-rats cannot avoid constant exposure to each other's vibratory and odor signals, the consequent extensive stress may result in death.

Urinary testosterone levels in the male blind mole rat (Spalax ehrenbergi) affect female preference.

This study investigated the sexual attraction of female blind mole rats to four groups of male mole rats: (a) intact males raised in captivity; (b) intact males trapped in the field; (c) captive males injected with testosterone; (d) captive castrated males. In the first part we measured blood testosterone, androstenedione, and dihydrotestosterone (DHT) levels, by radioimmunoassay; and urine testosterone levels, measured by GC-MS. The second part examined the relationship between urine testosterone levels in males and their attractiveness to females. Higher blood and urine testosterone levels were found in the field animals and in those injected with testosterone compared to captive intact or castrated animals: urine testosterone levels in the two other groups were not detectable. Blood androstenedione levels were also higher in the field animals and in those injected with testosterone compared to captive intact or castrated mole rats. Blood dihydrotestosterone levels were not detectable in all four experimental groups. Female mole rats chose to spend a longer period of time next to males with high blood and urine testosterone levels and high blood androstenedione levels than next to those with lower levels of these hormones. Because courtship and sexual behavior are influenced both by high levels of blood and urine testosterone and high levels of blood androstenedione, we suggest that the low levels of courtship and other sexual behavior in captive mole rats may be related to the lack of female attraction to these males, which display low levels of all three parameters.

Incidence of pup killing and parental behavior in virgin female and male rats (Rattus norvegicus): differences between Wistar and Sprague-Dawley stocks.

The responses toward young shown by males and nulliparous females differed substantially between two outbred stocks of laboratory rats. Sprague-Dawley females showed maternal behavior either spontaneously (35% of the naive rats) or through concaveation (92% of the initially neutral virgins). Of the Wistar females, however, only 10% showed maternal behavior spontaneously, and only 29% of the neutral virgins came to behave maternally during 15 days of concaveation. Prepubertal cohabitation with lactating rats did not facilitate maternal responsiveness in adulthood in the Wistar virgin females. Of the Sprague-Dawley males, 50% showed paternal behavior spontaneously, and only 4% killed the young. Among the Wistar males, however, only 4% showed paternal behavior spontaneously, and 76% killed pups. Such profound differences between outbred stocks of rats may be a source of discrepancies between the results of studies dealing with the induction of parental behavior in nonlactating rats.

Effect of postweaning social experience on response of Siberian hamsters (Phodopus sungorus sungorus) toward young.

Between 18 and 50 days of age, Siberian hamsters were housed either with same-sex littermates, opposite-sex littermates, opposite-sex littermates and parents, or opposite-sex littermates, parents, and a younger litter. When adults, these animals were presented with two 3-8-day-old pups for 8 hr, and their responses were recorded. Rearing conditions did not affect behavior of females. Males housed with female littermates in the absence of a younger litter showed fewer pup attacks and more nesting with pups than did males housed only with other males. Thus, housing with females in the early postweaning period may influence males' later (adult) responses toward pups. Only among hamsters housed with same-sex littermates did males and females differ in their response to pups, a result emphasizing that sex differences in behavior may depend on early social rearing.