Biomedical subjects

D Crews

Publications and source records attributed to D Crews.

At least 127 records · Page 7Linked to original sources

PubMed1991

The effects of intracranial implantation of estrogen on receptivity in sexually and asexually reproducing female whiptail lizards, Cnemidophorus inornatus and Cnemidophorus uniparens.

The ventromedial hypothalamus (VMH) is an important site in the neuroendocrine control of sexual receptivity in mammals. This study was conducted to determine if the VMH was also involved in estrogen induction of receptivity in whiptail lizards. Estradiol benzoate (EB) was implanted into the VMH of ovariectomized Cnemidophorus inornatus, a sexually reproducing species, and C. uniparens, a parthenogenetic species which displays "pseudosexual" behaviors similar to the sexual behaviors typical of both male and female C. inornatus. In both species, EB was significantly more effective in eliciting receptivity when implanted in the VMH than in other locations in the brain. These results support the idea that, as in mammals, the VMH is an important location of estrogen action in the control of receptive behaviors in both sexually and asexually reproducing whiptail lizards.

Animals↗

PubMed1991

The effects of progesterone on sexual behavior in male green anole lizards (Anolis carolinensis).

It is well known that androgen-dependent sexual behaviors in male mammals and birds are inhibited by exogenous progesterone (P). However, recent research on male whiptail lizards (Cnemidophorus inornatus) indicates that P can stimulate sexual and copulatory behavior. We report here both antiandrogenic and synandrogenic actions of P on sexual behavior in males of another reptile, the green anole lizard (Anolis carolinensis). Earlier reports on birds and mammals are reviewed and discussed in relation to a possible physiological role of P in influencing sexual behavior in male vertebrates.

Animals↗

PubMed1991

Concentrations of sex steroid hormones in pregnant and fetal Mongolian gerbils.

Sex steroid concentrations in the plasma of 24-day pregnant Mongolian gerbils (Meriones unguiculatus) and their male and female fetuses were measured using radioimmunoassays. It was found that, on Day 24 of gestation: (a) androgen levels were higher in those male fetuses developing adjacent to no female fetuses than in those male fetuses developing between two female fetuses and (b) androgen levels were higher in those female fetuses developing between two male fetuses than in those female fetuses with no immediate, male neighbours. Further, plasma taken from 24-day pregnant dams that had exhibited vaginal opening at a relatively early age had significantly lower androgen levels and significantly higher estradiol levels than did plasma taken from 24-day pregnant dams that had exhibited relatively late vaginal opening. The data provide direct evidence of hormonal mediation of previously described differences both in the morphology and reproductive biology of male and female adult gerbils as a function both of their fetal intrauterine locations relative to members of the other sex and of the age at vaginal introitus of their respective dams.

Animals↗

PubMed1990

The effects of exogenous neuropeptide Y on feeding and sexual behavior in the red-sided garter snake (Thamnophis sirtalis parietalis).

Actively courting adult male red-sided garter snakes were injected with one of two dosages of neuropeptide Y (NPY) or saline into the third cerebral ventricle. Courtship and feeding responses were assessed prior to surgery, 4-5 h and 24 h post-surgery. Feeding behavior was tested by the presentation of a favored food, an earthworm. NPY significantly reduced courtship behavior scores at both low (0.16 nmol) and high (0.77 nmol) doses 4-5 h after surgery. Snakes also took much longer to reach the criterion courtship score at the high dose. Feeding occurred only in snakes treated with NPY. At the low dose 29% ate at 4-5 h and 24 h after surgery; 43% of the snakes treated with the high dose ate 4-5 h post-surgery. When tested the day following injection, animals were courting at levels comparable to presurgery levels and refused food.

Animals↗

PubMed1990

Sex-determining potencies vary among female incubation temperatures in a turtle.

Reptiles whose sex is determined by incubation temperature typically exhibit all-male or all-female sex ratios over a wide range of incubation temperatures. The question arises as to whether the various all-female temperatures (or the various all-male temperatures) are equivalent in their "potency", or capacity to effect female determination. In map turtles, warm incubation temperatures produce all females and cool ones produce all males. We compared sex determining potencies of two all-female temperatures, 31 degrees C and 32.5 degrees C, by incubating eggs first at a male-producing temperature (26 degrees C) and then shifting them to the warm temperatures. The resulting sex ratio was significantly more male biased in the 26 degrees C----31 degrees C shift than in the 26 degrees C----32.5 degrees C shift, indicating that 32.5 degrees C has the greater female potency. These results point to the possibility that sex determination depends on a quantitative rather than qualitative level of gene expression.

Animals↗

PubMed1990

Psychobiology of reptilian reproduction.

The key to understanding species variation in neuroendocrine controlling mechanisms lies in understanding the constraints that have given rise to diverse reproductive patterns. Recent research clearly shows that species evolving under different constraints exhibit different patterns of reproduction and have fundamentally different neuroendocrine mechanisms controlling reproduction. Diversity can be exploited profitably by those interested in the fundamental nature of biobehavioral mechanisms. The comparative approach also promises to increase our understanding of broader intellectual and theoretical issues in behavioral endocrinology.

Animals↗

PubMed1990

Mating-induced ovarian recrudescence in the red-sided garter snake.

In the female red-sided garter snake (Thamnophis sirtalis parietalis) mating initiates a neuroendocrine reflex that has both a short-term (within hours) effect on circulating estrogen concentrations and a long-term (6-7 weeks) effect on ovarian development. The perception of mating appears at least facultative, if not obligatory, for the initiation and maintenance of vitellogenesis and hence successful reproduction.

Animals↗

PubMed1990

Endocrine and behavioral responses to aggression and social dominance in the green anole lizard, Anolis carolinensis.

Adult males of the small arboreal iguanid lizard, Anolis carolinensis, will fight and form social dominance hierarchies when placed in habitats with limited resources. The relationships between time since initial aggressive interaction, relative social dominance, reproductive activity, and corticosterone and androgen levels were determined for 34 pairs of lizards. A discriminant analysis established a "dominance index" which indicated that over 90% of the difference between individuals who had won or lost aggressive interactions (putative social dominants and subordinates) was attributable to a single discriminant function reflecting altered body color, perch site selection, and circulating androgen. Animals that had darker body color also selected lower perch sites and had depressed rates of courtship relative to winners of fights and were thus designated as social subordinates. These animals also had levels of circulating androgen significantly lower than that of dominants, but circulating corticosterone was not significantly affected. Winners of fights showed a dramatic surge in circulating androgen at 1 hr but returned to near control values by 1 week; losers, however, showed depressed circulating androgen levels at 1 week.

Aggression↗

PubMed1990

Sexually dimorphic areas in the brain of whiptail lizards.

The whiptail lizard species Cnemidophorus inornatus exhibits sexually dimorphic mating behaviors. We report that complementary sexual dimorphisms exist in two hypothalamic regions in male and female C. inornatus. The anterior hypothalamus-preoptic area (AH-POA), which is involved in male-typical mounting and intromission behaviors, is larger in males. The ventromedial hypothalamus (VMH), which controls female-typical receptivity, is larger in females. The all-female whiptail lizard Cnemidophorus uniparens, a direct descendant of C. inornatus, regularly and reliably displays both male-like mounting and female-like receptive (pseudosexual) behaviors. Rather than having a male-like AH-POA, however, the AH-POA as well as the VMH of C. uniparens resemble that of female C. inornatus. The biological basis of pseudosexual behavior in the parthenogen appears to be due to an unusual sensitivity to ovarian progesterone rather than to a difference in brain morphology.

Animals↗

PubMed1989

Sex pheromones in snakes.

The majority of pheromones identified to date are insect pheromones, which are volatile in nature. Identification of nonvolatile pheromones have been relatively rare, especially in vertebrates. Male and female garter snakes use pheromones to mediate sexual behavior. The female sex attractiveness pheromone of the Canadian red-sided garter snake, Thamnophis sirtalis parietalis, consists of a novel series of nonvolatile saturated and monounsaturated long-chain methyl ketones, whereas the male sex recognition pheromone contains squalene. These compounds were isolated, identified, and partially synthesized, and field tests show them to be biologically active.

Animals↗

PubMed1989

Absence of temperature-dependent sex determination in congeneric sexual and parthenogenetic Cnemidophorus lizards.

Eggs from congeneric gonochoristic and parthenogenetic whiptail lizards were incubated at various temperatures. There was no significant deviation from a sex ratio of one-half in the sexual species at any temperature, whereas no males were produced in the parthenogenetic species. Temperature-dependent sex determination appears to be completely absent in these lizards.

Animals↗

PubMed1989

Action of sex steroid hormones on temperature-induced sex determination in the snapping turtle (Chelydra serpentina).

Administration of exogenous estradiol benzoate (EB) or an estrogen agonist, R2858, into eggs of snapping turtles caused all embryos incubated at male-producing temperatures to develop as females, whereas testosterone propionate (TP) caused 42% of the embryos to develop as females. Some of the embryos treated with EB, R2858, or TP also had hypertrophied oviducts. Neither dihydrotestosterone (DHT) nor cholesterol had any apparent effect on the sex determination of embryos incubated at male-producing temperatures. Injections of TP, DHT, the androgen agonist R1881, or cholesterol had no apparent effect on sex determination of embryos incubated at female-producing temperatures. Administration of estradiol antiserum or testosterone antiserum resulted in some individuals having undifferentiated or ambiguous gonads. Although both exogenous estrogens and androgens can induce embryonic gonads to develop as ovaries, the findings of this study indicate that estrogen is the female-inducing hormone and that androgens may feminize gonads via aromatization to estrogen. Furthermore, the results of the antisera injection suggest that endogenous steroid hormones may have a natural role in gonadal differentiation of reptiles with environmental sex determination.

Animals↗

PubMed1989

Ovarian and adrenal function in the parthenogenetic whiptail lizard Cnemidophorus uniparens in the field and laboratory.

The purpose of this study was to document circulating concentrations of corticosterone in the plasma in relation to the ovarian follicular cycle in animals in the field and to examine the effects of captivity and laboratory manipulations on plasma corticosterone in the parthenogenetic whiptail lizard Cnemidophorus uniparens. In nature, circulating concentrations of corticosterone are highest in previtellogenic individuals when concentrations of progesterone are basal and estradiol-17 beta is undetectable. In vitellogenic lizards, circulating concentrations of corticosterone decline from previtellogenic levels while concentrations of progesterone and estradiol-17 beta increase. In preovulatory individuals, corticosterone concentrations are basal, while progesterone and estradiol-17 beta levels are maximal. Following ovulation, corticosterone and progesterone remain at preovulatory concentrations and estradiol-17 beta is undetectable. Ovariectomy elevates circulating concentrations of corticosterone, as does cold and handling. Individuals housed either in isolation or in groups of three per cage exhibit circulating concentrations of corticosterone similar to those measured in the plasma of animals immediately after being captured in the field. The effects of ovariectomy and cold/handling manipulations were additive. No nonadditive effect (statistical interaction) was revealed. These results document complementary and reciprocal effects of ovarian and adrenal gland function with respect to hormonal changes both in the field and as a result of experimental manipulations in the laboratory. However, relationships between ovarian and adrenal function appear to be complex. Several overlapping hypotheses are offered as possible explanations of the results and as potential avenues for further investigations.

Adrenal Glands↗

PubMed1989

Effects of intracranial implantation of dihydrotestosterone on sexual behavior in male Cnemidophorus inornatus, a direct sexual ancestor of a parthenogenetic lizard.

Dihydrotestosterone was implanted directly into the brain of castrated male Cnemidophorus inornatus, a direct sexual ancestor of the parthenogenetic species C. uniparens. Only implants located in the anterior hypothalamus--preoptic area (AH-POA) induced male-typical sexual behavior. Implants in other brain regions, including the ventromedial hypothalamus, failed to elicit courtship or copulatory behavior. Radioimmunoassay revealed no significant difference in the concentrations of circulating androgens between the responding and nonresponding animals. Previous data from this laboratory demonstrated that the AH-POA controls male-like pseudosexual behavior in C. uniparens. The current results support the hypotheses that (i) the AH-POA is the major area of hormone action in the brain controlling male-typical sexual behavior in C. inornatus as in other vertebrates and (ii) the neural circuits controlling male-typical behavior have been conserved in the evolution of the parthenogen C. uniparens.

Animals↗

PubMed1989

Mating increases plasma levels of prostaglandin F2 alpha in female garter snakes.

Plasma levels of prostaglandin F2 alpha (PGF2 alpha) in female red-sided garter snakes (Thamnophis sirtalis parietalis) were measured at intervals after mating or exposure to males. PGF2 alpha levels increased significantly within 15 minutes of mating and peaked 6-24 hr after mating. Females that did not mate, but received similar amounts of male courtship, had levels of PGF2 alpha significantly lower than those of females that mated. These results extend previous findings that unmated female garter snakes injected with PGF2 alpha exhibit sexual behavior characteristic of females that have mated. Together these data indicate that female garter snakes elaborate PGF2 alpha in response to stimuli associated with mating and that PGF2 alpha has a functional role in inducing post-mating declines in sexual behavior of this species.

Animals↗

PubMed1989

Effect of fall mating on ovarian development in the red-sided garter snake.

Although spring mating is an important factor in initiating vitellogenesis in female red-sided garter snakes (Thamnophis sirtalis parietalis), some females can become vitellogenic without having mated in the spring. Two hypotheses have been proposed to explain this phenomenon: 1) long-term storage of copulatory stimuli from mating the previous fall, or 2) additional cue(s) overcoming the lack of mating to stimulate vitellogenesis. Through oviductal biopsy, the presence of sperm in females returning to the hibernaculum in the fall was assessed. Laparotomies performed just before and 6 wk after hibernation indicated that although fall mating may stimulate slight ovarian development in the fall, there appears to be no effect of fall mating on spring ovarian recrudescence. Spring mating seems to be the most important factor in determining ovarian maturation. Therefore, it appears that there is no long-term storage of copulatory cues and that other as yet unknown cues are responsible for initiating vitellogenesis in spring unmated females.

Animals↗

PubMed1989

Effect of castration on epididymal sperm storage in male musk shrews (Suncus murinus) and mice (Mus musculus).

Reproductively mature male musk shrews and mice were bilaterally castrated. Epididymal sperm numbers and motility were assessed 0, 2, 4 and 6 weeks after surgery. Seminal vesicle weights and plasma concentrations of total androgens were also measured. In male musk shrews, 30% of the original epididymal sperm numbers were still present 2 weeks after castration and motile spermatozoa were present in 2 of 7 individuals. By 4 and 6 weeks after castration the numbers of spermatozoa remaining declined to about 10% and no sperm motility was noted. Seminal vesicle weights were maintained at about 30% of their original size even up to 6 weeks after castration. In male mice, epididymal sperm numbers, seminal vesicle weights, and androgen levels declined more dramatically after castration. Although androgen concentrations in gonadally intact male musk shrews were approximately 50% of the values in male mice, after castration the concentrations in musk shrews were approximately 2-fold higher than in mice at all times. The results suggest that post-castration retention of epididymal sperm and seminal vesicle weights in the male musk shrew as compared with male mice, is facilitated either by a relatively greater adrenal contribution to circulating androgen levels and/or greater target tissue sensitivity.

Androgens↗

PubMed1989

2-Deoxyglucose uptake following visual stimulation in squamate reptiles.

Unilateral visual stimulation following 2-deoxyglucose injection was used to delineate functional visual pathways in red-sided garter snakes (Thamnophis sirtalis parietalis) and whiptail lizards (Cnemidophorus inornatus and C. uniparens). Stimulation enhanced both overall uptake and particularly uptake in visual pathways contralateral to the side of stimulation. Bilaterally symmetrical uptake was observed in retinae, septum, nucleus sphericus, and brainstem nuclei. Overall uptake and lateralization of uptake were consistently greater in garter snakes than in whiptail lizards.

Animals↗