Evolutionary change in the endocrinology of behavioral receptivity: divergent roles for progesterone and prolactin within the genus Phodopus

Biology and Diseases of Hamsters

The hamster species used as research models include the Syrian (golden), Mesocricetus auratus; the Chinese (striped-back), Cricetulus griseus; the Armenian (gray), C. migratorius; the European, Cricetus cricetus; and the Djungarian, Phodopus campbelli (Russian dwarf) and P. sungorus (Siberian dwarf). Hamsters are classified as members of the order Rodentia, suborder Myomorpha, superfamily Muroidea and in family Cricetidae. Animals in this family are characterized by large cheek pouches, thick bodies, short tails, and an excess of loose skin. They have incisors that erupt continuously and cuspidate molars that do not continue to grow ((I 1/1, C 0/0, PM 0/0, M 3/3) × 2 = 16). In 2010, it was reported that approximately 146,000 hamsters were used in research in the United States (United States Department of Agriculture, 2010).

Individual variation in cortisol responses to acute "on-back" restraint in an outbred hamster

An outbred species of dwarf hamster (Phodopus campbelli) was used to assess between-individual variability in the response to, and recovery from, a one-time stressor of 6 min of physical restraint in a subordinate, on-back, position. Four repeated plasma samples were drawn under home-cage isoflurane anesthesia from 33 males and 38 females 50 min before, and then 10, 60, and 120 min after the stress onset. Plasma cortisol concentrations were higher in females than males, but there was no evidence for a sex difference in response to the stressor. The expected cross-sectional increase ( approximately 50 ng/ml) in response to the stressor, followed by recovery, was seen. However, there was extensive individual variation, ranging from no reaction to continuous decline from the initial to the final sample. Results were expressed in four ways (absolute concentration, relative concentration, and area under the curve relative to ground and relative to the stress-induced increase) and also standardized and subjected to hierarchical cluster analysis. Clusters failed to effectively partition the between-individual variation and did not cluster by sex, age, or housing conditions. The current study cautions against ignoring individual differences and suggests that outbred animal models might be particularly relevant to understanding stress-related pathological conditions.

Male and female prolactin receptor mRNA expression in the brain of a biparental and a uniparental hamster, phodopus, before and after the birth of a litter

Prolactin receptor (PRL-R) mRNA transcript level was quantified in the choroid plexus (ChP) of a naturally biparental hamster, Phodopus campbelli, and its otherwise similar, yet nonpaternal, sibling species, Phodopus sungorus. Pair-housed males and females on the day before the birth of their first litter (G17), the day after birth (L1), lactation day 5 (L5), and unpaired animals that were sexually naïve, were tested. PRL-R mRNA transcript level relative to total RNA, was evaluated by reverse transcriptase-polymerase chain reaction using primers common to the long- and short-form of the PRL-R in Phodopus. In the ChP, a region implicated in prolactin transport into the central nervous system, females had the expected increase in PRL-R mRNA transcript from dioestrus to L5, consistent with known actions of prolactin. As predicted, males and females of the biparental species were similar, although PRL-R mRNA in naive males was higher than in dioestrus females. Males of the two species also differed as predicted. PRL-R mRNA transcript levels were higher in the biparental males. In addition, P. campbelli males had low PRL-R mRNA at G17 compared to L5. By contrast, non-paternal P. sungorus males had elevated PRL-R mRNA transcript levels on G17 relative to unpaired males. We conclude that PRL-R mRNA in the ChP is differentially regulated before and after birth in a paternal and a nonpaternal male.

Ontogeny of the transition from killer to caregiver in dwarf hamsters (Phodopus campbelli) with biparental care

Biparental Phodopus campbelli and uniparental P. sungorus juvenile litters (2 males, 2 females) both consumed amniotic fluid and placenta during the birth of younger siblings. Three days later, P. campbelli juveniles were most responsive to a displaced younger sibling. Thus, P. campbelli are responsive to pups as juvenile alloparents and as new parents; however, at intervening ages, infanticidal attack (bite) was seen. At 5, 7, 9, 11, or 13 weeks of age, male and female P. campbelli were given a 5-min test with an unrelated, 3-day-old, anesthetized pup. Females attacked more often than males, yet pup-retrieval rates did not differ. Female aggression increased with age and was replaced by retrieval behavior 3 days after parturition. Male attack ceased after a birth, but parental behavior did not increase, remaining below the rate for new fathers tested with their own awake pup. Over repeated testing, behavior in one test did not predict behavior in another. Transitions from caregiving alloparent to infanticidal adult and back to parental care were clear in females, but less discrete with this stimulus paradigm in these highly paternal males.

Photoperiodic control of oestrous cycles in Syrian hamsters: mediation by the mediobasal hypothalamus

To assess whether the mediobasal hypothalamus (MBH) is necessary for photoperiodic control of oestrous cycles and prolactin secretion, we tested intact female Syrian hamsters (controls) and those that had sustained unilateral or bilateral lesions of the MBH. All hamsters displayed 4-day oestrous cycles postoperatively in the long-day photoperiod (14 h light/day); control females and those with unilateral MBH damage ceased to undergo oestrous cycles approximately 8 weeks after transfer to a short-day photocycle (10 h light/day), whereas 12 of 15 females with bilateral MBH lesions continued to generate 4-day oestrous cycles throughout 22 weeks in short days. Serum prolactin concentrations were either undetectable or low in all hamsters 8 or 14 weeks after the transfer to short-day lengths, but increased above long-day baseline values by week 22. We conclude that melatonin-binding sites in the MBH mediate suppression of oestrous cycles but not prolactin secretion by short-day lengths; recovery of prolactin secretion in females during prolonged exposure to short-day lengths reflects development of refractoriness to melatonin in a substrate distinct from the MBH. These findings suggest that separate neural pathways mediate photoperiodic control of gonadotropin and prolactin secretion in female hamsters.

Parental responsiveness is feminized after neonatal castration in virgin male prairie voles, but is not masculinized by perinatal testosterone in virgin females

We previously found a large sex difference in the parental responsiveness of adult virgin prairie voles (Microtus ochrogaster) such that most males are spontaneously parental, whereas most females are not. Because this sex difference is independent of the gonadal hormones normally circulating in adult virgin voles, the present study examined whether perinatal hormones influence the development of this sex difference. Males were treated prenatally (via their pregnant dam) with both the androgen receptor blocker flutamide (5 mg/day/dam) and the aromatase inhibitor ATD (1 mg/day/dam), or oil, for the last 2 weeks of gestation. Half of the subjects from each group were castrated on the day of birth and the other half received a sham surgery. As adults, intact males were castrated and all males received a silastic capsule filled with testosterone. Prenatal treatment with flutamide and ATD had no effect on males' behavior toward pups, but neonatal castration significantly reduced the percentage of males acting parentally. In a second experiment, females were exposed to testosterone propionate (TP; 50 microg/day/dam) or oil via their dam during the last 2 weeks of gestation. For the first neonatal week, half of the females from each group were injected with TP (1 mg/day) and the other half oil. As adults, females were ovariectomized and half from each group received a testosterone-filled capsule and the other half received an empty capsule. None of the perinatal TP treatments increased females' parental responsiveness, although females from all groups that received testosterone capsules as adults were highly parental. Therefore, although postnatal testicular hormones are necessary for high parental responsiveness in males, the behavior of females is not influenced by perinatal exposure to testosterone.

Changes in testosterone, cortisol, and estradiol levels in men becoming fathers

OBJECTIVE

To quantify longitudinally steroid hormone (testosterone, cortisol, and estradiol) concentrations in men becoming fathers for the first time ("dads").

SUBJECTS AND METHODS

Volunteer study subjects were recruited from first-trimester prenatal classes in Kingston, Ontario, in February 1999. Twenty-three dads provided saliva samples from recruitment through 3 months after the birth of their children. Fourteen men who were not fathers were recruited from the general population to serve as age-matched controls for season and time of day. Estradiol, testosterone, and cortisol levels were quantified.

RESULTS

After controlling for effects of time of day and season, dads had lower mean +/- SE testosterone (6.5+/-0.7 vs 10.0+/-0.9 ng/dL; P<.005) and cortisol (morning values, 0.30+/-0.05 vs 0.53+/-0.05 microg/dL; P<.005) concentrations, a higher proportion of samples with detectable estradiol concentrations (68% [308/454] vs 57% [87/154]; P=.01), and higher estradiol concentrations in those detectable samples (3.81+/-0.09 pg/mL [13 dads] vs 3.26+/-0.11 pg/mL [9 controls]; P<.002) than did control men. Within 10 individual dads with frequent samples before and after the birth, the percentage of samples with detectable estradiol was lower during the month before the birth than during the month after (51% vs 71%; P=.02), and cortisol concentration was increased in the week before the birth (to a mean of 0.16 microg/dL). In each of 13 dads providing frequent samples, testosterone concentration and variance were low immediately after the birth (no change from previous levels in 5, decrease after prebirth increase in 3, and decrease relative to all other times in 5).

CONCLUSIONS

In this population of Canadian volunteers attending prenatal classes, expectant fathers had lower testosterone and cortisol levels and a higher proportion of samples with detectable estradiol concentrations than control subjects. Individual patterns of testosterone variance relative to the birth and estradiol and cortisol concentrations immediately before the birth may be worthy of further investigation. The physiologic importance of these hormonal changes, if any, is not known. However, they are hormones known to influence maternal behavior.

Fathers, fat, and maternal energetics in a biparental hamster: paternal presence determines the outcome of a current reproductive effort and adipose tissue limits subsequent reproductive effort

Djungarian hamster females, Phodopus campbelli, are severely constrained in their ability to reproduce successfully and lose 20% of their body weight by the time pups are weaned. In the wild and in the laboratory, biparental care improves maternal reproductive success. Two experiments quantified the effects of paternal presence and partial lipectomy [surgical depletion of parametrial white adipose tissue (PWAT) on day 8 of the 18-day gestation] on maternal energy balance, reproductive success, and investment in a subsequent reproductive attempt. Paired females reproduced successfully, maintained body weight, and invested in a second litter. Removal of the male decreased pup survival, growth, and readiness for dispersal by 18 days of age. Solitary females lost 10% of their body weight by the birth and a further 10% by day 18 after the birth. Thus, paternal presence balanced maternal energy budgets during reproduction and prevented a 20% loss in body weight. Equivalent weight loss occurs in response to other maternal stressors, therefore 20% may be the maximum tolerable weight loss in this species. Fresh weight of interscapular brown adipose tissue was predicted by the extent of maternal hyperthermia but not by maternal energy balance or lipectomy. Partial lipectomy did not adversely affect the female or the first litter but decreased the probability of investment in a second reproductive attempt and halved the size of the second litter. This effect may have been due to the 0.1% of body weight amount of lipid removed or may reflect a specialized role for PWAT in adjusting maternal investment.

Paternal hamsters mechanically assist the delivery, consume amniotic fluid and placenta, remove fetal membranes, and provide parental care during the birth process

We report the first description of a male mammal behaviorally participating in the birth of his young. Djungarian hamsters, Phodopus campbelli, are naturally biparental and males contribute to pup survival and growth through direct paternal care and indirect effects on maternal physiology. Males of the closely related Siberian hamster, P. sungorus, have only seasonal opportunities to interact with pups and neither contribute to pup survival and growth nor provide extensive paternal care under laboratory conditions. Male P. campbelli undergo a series of hormonal changes prior to the birth of their first litter which are not shared with P. sungorus males and may "prime" males to express paternal behavior. We hypothesized that P. campbelli males would behave paternally on first contact with pups during the birth process, whereas P. sungorus males would not. In addition to showing paternal behavior toward neonates, male P. campbelli licked amniotic fluid before the birth, mechanically assisted the delivery, opened an airway by clearing the nostrils (so that pups flushed red with oxygenation), licked and sniffed pups in the moments and minutes after birth to clean the pups of membranes, and ate the placenta. None of these behaviors were observed in P. sungorus. We anticipate that P. campbelli will not be the only species with highly developed paternal behavioral repertoires during the birth.

Divergent reproductive endocrinology of the estrous cycle and pregnancy in dwarf hamsters (phodopus)

Progesterone (P4) and prolactin (PRL) in peripheral circulation of Siberian hamsters (Phodopus sungorus) throughout an estrous cycle and pregnancy were determined by repeated, small volume sampling from individuals housed in modified home cages. As predicted, the endocrinology of P. sungorus reproduction is similar to the rat, mouse and golden hamster and shows none of the eight distinctive features known for Djungarian hamsters (Phodopus campbelli). Specifically, in P. sungorus there is no evidence for resumption of pituitary PRL surges in late pregnancy, P4 concentrations during the differentiation of the corpus luteum on day 2 of pregnancy are higher (as opposed to lower) than concentrations on the comparable day of an unmated estrous cycle (diestrus 1), P4 concentrations increase throughout pregnancy, PRL surges are common during the estrous cycle, including a reliable surge on proestrus, and P4 plays an important role in facilitating the expression of behavioral receptivity. We conclude that 'novel' P. campbelli reproductive endocrinology has evolved since a common ancestor was shared with P. sungorus. With a time frame (the available time since the divergence of the two species) and an ecological context (known niches and behavior in the wild) these species offer the opportunity to study endocrinological evolution in progress.

Hormonal changes in males of a naturally biparental and a uniparental mammal

Blood samples from male hamsters (Phodopus) during their mate's gestation and early lactation show that key hormones important in maternal behavior are also changing in males and differ for two closely related species with different levels of paternal care. Results of study 1 were consistent with a relationship between higher prolactin, lower testosterone and paternal behavior during early lactation in P. campbelli and provided no evidence for similar hormonal changes in P. sungorus. Study 2 sampled males before or after the birth. Prolactin did not increase until at least one day after the birth in P. campbelli but was high at the end of the pregnancy in P. sungorus. Increasing testosterone concentrations in P. campbelli as the birth approached were consistent with mate guarding, high testosterone concentrations on L5 were consistent with paternal aggression in defense of the litter, and the drop in testosterone after the birth was consistent with reduced aggression toward the new pups. Results confirmed that cortisol concentrations were reduced following the establishment of a pair-bond and found that P. campbelli males had elevated cortisol before the birth. Results support the hypothesis that mammalian paternal behavior has a hormonal basis which is analogous to maternal behavior.