Larry J. Young

Gonadal Steroids have Paradoxical Effects on Brain Oxytocin Receptors

Specific brain receptors for oxytocin have been described in several mammalian species. The distribution of these receptors differs greatly across species and in the rat, receptor binding in specific brain regions appears to depend upon gonadal steroids. This study used in vitro receptor autoradiography to examine the effects of testosterone on oxytocin receptor binding in the mouse forebrain. Three groups of male mice were compared: castrates treated with blank capsules, castrates treated with testosterone filled capsules, and intact males. Irrespective of steroid treatment, the distribution of oxytocin receptors in mouse forebrain differed markedly from patterns previously described in the rat. In addition to these species differences in receptor distribution, testosterone had effects in the mouse which differed from the induction of receptors previously reported in the rat. In the mouse ventromedial nucleus of the hypothalamus, binding in the untreated castrate males was approximately double that observed in either the intact or the testosterone‐treated castrates. In other regions of the mouse brain, such as the intermediate zone of the lateral septum, binding to oxytocin receptors was increased with testosterone treatment. These results suggest that the brain oxytocin receptor varies across species not only in its distribution but also in its regional regulation by gonadal steroids. These apparently paradoxical changes in oxytocin receptor binding may result from either direct or indirect effects of gonadal steroids in mouse brain.

Progesterone modulation of androgen-dependent sexual behavior in male rats

The present study examines the effects of physiological levels of progesterone (P) on copulatory behavior in sexually naive male rats. Two weeks after gonadectomy males were implanted with either empty Silastic capsules (BL) or Silastic capsules containing testosterone (T), P, or both (P+T). When tested with an estrous female, all of the gonadally intact males (intact) and none of the BL controls exhibited mounting/intromission behaviors. Mounting was observed in 75% of the T-alone males. More than half (64%) of the P-alone males and 100% P+T males exhibited mounting. In most cases, mounting was followed by intromission responses. Subsequently, intact and gonadectomized males received daily injections of the P antagonist RU486 along with hormone treatment. After receiving RU486, only 63% of the intact males and 71% of the T-alone males mounted successfully. The facilitatory effects of P on copulatory behavior were completely abolished by RU486 treatment. The present studies provide the first evidence in mammals suggesting that P-dependent mechanisms influence neurochemical pathways involved in copulation.

Progesterone and sexual behavior in males.

Previous investigations into the effects of progestins on copulatory behavior have suggested that progesterone inhibits the expression of androgen-dependent sexual behaviors in males. However, virtually all of those studies utilized pharmacological dosages of progesterone. Such experiments, although essential for understanding the behavioral effects of progesterone, yield little insight into the function of endogenous progesterone in masculine sexual responses. In this brief review, attention is focused on the role of physiological levels progesterone in copulatory behavior in male reptiles and mammals. Efforts are made to promote a reevaluation of the behavioral effects of progestins in males, similar to ongoing studies which are reexamining neural mechanisms involved in progestin-mediated reproductive behavior in the female.

Comparative neuroendocrinology of steroid receptor gene expression and regulation: Relationship to physiology and behavior

Great diversity exists among vertebrates in reproductive behaviors and the neuroendocrine mechanisms underlying these behaviors. Comparisons of species with different hormone-brain-behavior relationships reveal three factors which may explain species differences in endocrine physiology and behavior: (a) sensitivity to sex steroid hormones, (b) hormone-dependent regulation of sex steroid hormone receptor gene expression, and (c) neuroanatomical distribution of steroid receptor gene expression, especially in nonlimbic structures.

Reptilian Sex Steroid Receptors: Amplification, Sequence and Expression Analysis

Sex steroid hormones secreted by the gonads play a central role in the reproduction of all vertebrates. In addition to direct effects on gametogenesis, sex steroid hormones are important in sexual development, brain organization, and sexual behavior. The actions of sex steroid hormones are mediated primarily by ligand-dependent transcription factors, or receptors which bind to specific sequences of the DNA and alter the transcription rates of nearby genes. We have used the polymerase chain reaction to amplify cDNA fragments of the estrogen receptor, progesterone receptor and androgen receptor from the unisexual whiptail lizard, Cnemidophorus uniparens. The lizard steroid hormone receptors share a high degree of sequence homology to the steroid hormone receptors of other vertebrates. Ribonuclease protection assays demonstrate that both estrogen receptor mRNA and progesterone receptor mRNA are increased in the oviduct during vitellogenesis and after estrogen treatment. This report demonstrates the utility of the polymerase chain reaction to generate species specific probes for comparative molecular studies and provides the first report of cDNA sequences for reptilian steroid hormone receptors.

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.

Regulation of Estrogen Receptor and Progesterone Receptor Messenger Ribonucleic Acid by Estrogen in the Brain of the Whiptail Lizard (Cnemidophorus uniparens)

Receptive behavior in females vertebrates is controlled by hormones, principally estrogen, secreted by the ovary. Estrogen influences behavior by interacting with a specific estrogen binding protein, or receptor, located in target cells in certain hypothalamic nuclei. To better understand the molecular mechanisms involved in the control of receptive behavior in whiptail lizards, we investigated the effects of exogenous estrogen on t h e regulation of estrogen receptor and progesterone receptor expression in several regions of the brains of Cnernidophorus uniparens. First we determined a dosage of 17β‐estradiol 3‐benzoate (0.5 μg) which reliably induced receptive behavior in ovariectomized C. uniparens. Then using in situ hybridization, we examined the effects of that dosage on the expression of estrogen receptor and progesterone receptor mRNA in the brain 24 h after injection. Estrogen treatment resulted in a significant up‐regulation of estrogen receptor mRNA expression in the ventromedial nucleus of the hypothalamus and torus semicircularis, down‐regulation of estrogen receptor mRNA expression in the lateral septum, and no change in the periventricular nuclei of the hypothalamus, the periventricular nucleus of the preoptic area, and the dorsal hypothalamus. The same dosage resulted in increased progesterone receptor mRNA expression in the ventromedial nucleus of the hypothalamus and the periventricular nucleus of the preoptic area; no significant changes in progesterone receptor mRNA expression were observed in the periventricular nuclei of the hypothalamus or the torus semicircularis, although the differences in progesterone receptor expression in the torus semicircularis approached statistical significance. The up‐regulation of estrogen receptor gene expression by estrogen in the ventromedial nucleus of the hypothalamus of lizards is opposite to that reported in female rats in which estrogen down‐regulates estrogen receptor expression in the ventromedial nucleus of the hypothalamus. We propose that this may be related to differences in reproductive physiology between vertebrates which have extended follicular phases and prolonged periods of estrus and rats, which have an abbreviated follicular phase and brief periods of estrus.

Species Differences in Behavioral and Neural Sensitivity to Estrogen in Whiptail Lizards: Correlation with Hormone Receptor Messenger Ribonucleic Acid Expression

Cnemidophorus uniparens is a unisexual species of whiptail lizard of hybrid origin whereas C. inornatus is a sexual species and the maternal ancestor of C. uniparens. Together they represent an excellent model system for investigating the evolution of hormone-brain-behavior relationships. Normal circulating estradiol (E) concentrations in C. uniparens are approximately 5-fold lower than those of female C. inornatus in a similar reproductive state. Experiments were performed to determine whether (i) C. uniparens is more sensitive to E, and (ii) whether the difference in sensitivity is correlated with differences in estrogen receptor (ER)-mRNA expression in the brain. Dose-response curves reveal that ovariectomized C. uniparens are more responsive than ovariectomized C. inornatus to exogenous estradiol 17 beta-benzoate (EB). EB is more effective in C. uniparens at inducing receptive behavior and progesterone receptor (PR) gene expression in the ventromedial nucleus of the hypothalamus (VMH). In situ hybridization analysis of ER-mRNA expression revealed no species differences in ER-mRNA content in the VMH of ovariectomized animals. Treatment of ovariectomized animals with EB resulted in a greater induction of ER-mRNA expression in the VMH of C. uniparens compared to C. inornatus. These results indicate that the differences in behavioral sensitivity to E lie in the estrogen target neurons in the brain region controlling receptive behavior, the VMH, and that the difference in sensitivity cannot be explained by species differences in the basal rate of ER gene expression.

Species Differences in Estrogen Receptor and Progesterone Receptor-mRNA Expression in the Brain of Sexual and Unisexual Whiptail Lizards

Circulating concentrations of gonadal steroid hormones and reproductive behavior in female vertebrates vary as a function of ovarian state. Steroids secreted by the ovary, specifically estrogen and progesterone, influence the expression of behaviors associated with reproduction by interacting with intracellular sex steroid receptors located in specific regions of the brain. Using in situ hybridization, we analyzed estrogen receptor and progesterone receptor messenger RNA expression in several brain regions of ovariectomized, vitellogenlc, and postovulatory individuals from two species of whiptail lizards (Cnemidophorus uniparens and C. inornatus). Although these species are genetically very similar, they differ in two aspects of their reproductive biology: (i) the unisexual C. uniparens alternate between expressing female‐typical and male‐like pseudosexual behaviors while female C. inornatus normally express only female receptive behavior, and (ii) circulating estradiol concentrations in reproductively active female C. uniparens are approximately five‐fold lower than in reproductively active female C. inornatus. We found that the regulation of sex steroid receptor gene expression was region specific, with receptor‐mRNA expression being increased, unchanged, or decreased during vitellogenesis depending on the area. Furthermore, Several species differences in the amount of sex steroid receptor‐mRNA were found that may be relevant to the species differences in circulating estrogen concentrations and sexual behavior.

Transgenic Models for Oxytocin and Vasopressin

Oxytocin (OT) and arginine vasopressin (AVP) were among the first peptide hormones to be isolated and sequenced (1). Both OT and AVP consist of nine amino acids that form a ring structure from disulfide bonds, bridging two cysteine residues. Although the two hormones are similar in structure, differing at only two positions, their functions appear to be quite distinct. Oxytocin has been implicated in the induction of labor during parturition, milk ejection during lactation (2), and the control of reproductive and maternal behaviors (3). Arginine vasopressin, also known as antidiuretic hormone, plays a crucial role in maintaining osmotic homeostasis and vascular tone, and has more recently been implicated in cognition and social behaviors (4–6).