Bradley M. Cooke

Finger-length ratios and sexual orientation

Animal models have indicated that androgenic steroids acting before birth might influence the sexual orientation of adult humans. Here we examine the androgen-sensitive pattern of finger lengths, and find evidence that homosexual women are exposed to more prenatal androgen than heterosexual women are; also, men with more than one older brother, who are more likely than first-born males to be homosexual in adulthood, are exposed to more prenatal androgen than eldest sons. Prenatal androgens may therefore influence adult human sexual orientation in both sexes, and a mothers body appears to ‘remember’ previously carried sons, altering the fetal development of subsequent sons and increasing the likelihood of homosexuality in adulthood.

Sexual Differentiation of the Vertebrate Brain: Principles and Mechanisms☆☆☆

A wide variety of sexual dimorphisms, structural differences between the sexes, have been described in the brains of many vertebrate species, including humans. In animal models of neural sexual dimorphism, gonadal steroid hormones, specifically androgens, play a crucial role in engendering these differences by masculinizing the nervous system of males. Usually, the androgen must act early in life, often during the fetal period to masculinize the nervous system and behavior. However, there are a few examples of androgen, in adulthood, masculinizing both the structure of the nervous system and behavior. In the modal pattern, androgens are required both during development and adulthood to fully masculinize brain structure and behavior. In rodent models of neural sexual dimorphism, it is often the aromatized metabolites of androgen, i.e., estrogens, which interact with estrogen receptors to masculinize the brain, but there is little evidence that aromatized metabolites of androgen play this role in primates, including humans. There are other animal models where androgens themselves masculinize the nervous system through interaction with androgen receptors. In the course of masculinizing the nervous system, steroids can affect a wide variety of cellular mechanisms, including neurogenesis, cell death, cell migration, synapse formation, synapse elimination, and cell differentiation. In animal models, there are no known examples where only a single neural center displays sexual dimorphism. Rather, each case of sexual dimorphism seems to be part of a distributed network of sexually dimorphic neuronal populations which normally interact with each other. Finally, there is ample evidence of sexual dimorphism in the human brain, as sex differences in behavior would require, but there has not yet been any definitive proof that steroids acting early in development directly masculinize the human brain.

Sexually Dimorphic Synaptic Organization of the Medial Amygdala

The medial amygdala is important in social behaviors, many of which differ between males and females. The posterodorsal subnucleus of the medial amygdala (MeApd) is particularly sensitive to gonadal steroid hormones and is a likely site for gonadal hormone regulation of sexually dimorphic social behavior. Here we show that the synaptic organization of the MeApd in the rat is sexually dimorphic and lateralized before puberty. With the use of whole-cell voltage-clamp recording and quantitative electron microscopy, we found that, specifically in the left hemisphere, prepubertal males have ∼80% more excitatory synapses per MeApd neuron than females. In the left but not the right MeApd, miniature EPSC (mEPSC) frequency was significantly greater in males than in females; mEPSC amplitude was not sexually dimorphic. Paired-pulse facilitation of EPSCs, an index of release probability, also was not sexually dimorphic, suggesting that greater mEPSC frequency is caused by a difference in excitatory synapse number. Electron microscopy confirmed that the asymmetric synapse-to-neuron ratio and the total asymmetric synapse number were significantly greater in the left MeApd of males than of females. In contrast to results for excitatory synapses, we found no evidence of sexual dimorphism or laterality in inhibitory synapses. Neither the frequency nor the amplitude of mIPSCs was sexually dimorphic or lateralized. Likewise, the number of symmetric synapses measured with electron microscopy was not sexually dimorphic. These findings show that the excitatory synaptic organization of the left MeApd is sexually differentiated before puberty, which could provide a sexually dimorphic neural substrate for the effects of hormones on adult social behavior.

Differences in Finger Length Ratios Between Self-Identified "Butch" and "Femme" Lesbians

There is indirect evidence that heightened exposure to early androgen may increase the probability that a girl will develop a homosexual orientation in adulthood. One such putative marker of early androgen exposure is the ratio of the length of the index finger (2D) to the ring finger (4D), which is smaller in male humans than in females, and is smaller in lesbians than in heterosexual women. Yet there is also evidence that women may have different sexual orientations at different times in their lives, which suggests that other influences on female sexual orientation, presumably social, are at work as well. We surveyed individuals from a gay pride street fair and found that lesbians who identified themselves as “butch” had a significantly smaller 2D:4D than did those who identified themselves as “femme.” We conclude that increased early androgen exposure plays a role in only some cases of female homosexuality, and that the sexual orientation of “femme” lesbians is unlikely to have been influenced by early androgens.

Morphological sex differences and laterality in the prepubertal medial amygdala

The medial amygdala (MeA) is crucial in the expression of sex‐specific social behaviors. In adult rats the regional volume of the MeA posterodorsal subnucleus (MeApd) is ≈50% larger in males than in females. The MeApd is also sexually dimorphic in prepubertal rats. We have recently shown that the left MeApd is significantly larger in prepubertal males than females. In contrast with volumetric sex differences elsewhere in the brain, however, we found no sex difference in the number of left MeApd neurons. In the present study we investigated the cellular bases of the sex difference in MeApd regional volume by quantifying the volume occupied by dendrites, axons, synapses, or glia, and by measuring MeApd dendritic morphology in 26–29‐day‐old male and female rats. We find that the volume occupied by dendritic shafts and glia completely accounts for the sex difference in left MeApd regional volume. Dendritic length measurements in the left hemisphere confirm that males have greater overall dendritic length, which is due to greater branching rather than to longer dendrite segments. In the right hemisphere the pattern of sex differences was different: Males have more MeApd neurons than females, whereas the dendritic morphology of individual neurons is not sexually dimorphic. These results highlight the importance of evaluating laterality in the MeA and suggest that the left and right MeA could play different roles in neuroendocrine regulation and sexually dimorphic social behaviors. J. Comp. Neurol. 501:904–915, 2007.

Sex difference and laterality in the volume of mouse dentate gyrus granule cell layer

Sex differences in spatial learning have been reported in both humans and rodents. Correspondingly, there have been reports of sexual dimorphism in the morphology of the hippocampal formation (HF), a brain structure implicated in spatial cognition. In Experiment 1, we confirmed earlier reports that the overall volume of the granule cell layer (GCL) of the dentate gyrus (DG) of A/J mice is larger in males than in females. We also found that male A/J mice have a larger GCL volume in the right hemisphere than the left. Female A/J mice displayed no such laterality. A similar pattern of laterality, favoring the right HF, had been reported previously in male, but not female, rats. In Experiment 2, we examined mice with a defective structural gene for androgen receptors (testicular feminization mutant, or tfm mice) on a C57/BL6J background. The C57/J strain had not previously been examined for hippocampal sexual dimorphism. We found no sexual dimorphism in the left, right, or total volume of the GCL in C57/BL6J mice whether they were wildtype or tfm. However, the right GCL volume was greater than the left in wildtype C57/BL6J mice of either sex. No lateralization of GCL volume was found in the androgen-insensitive tfm-affected males or the partially androgen-insensitive tfm-carrier females. These findings confirm earlier reports that sexual dimorphism in mouse HF is found in some inbred strains but not others, and indicate for the first time that mouse HF structures are lateralized. The absence of lateralization in partially or wholly androgen-insensitive mice suggests that androgen receptors may play a role in development of laterality in the GCL independently of any sexual dimorphism in this structure.

Post-weaning social isolation of male rats reduces the volume of the medial amygdala and leads to deficits in adult sexual behavior

At 21 days of age, gonadally intact male Long Evans rats were weaned and placed into standard laboratory conditions (three per cage) or housed singly. They were tested for noncontact erections and sexual performance at 90 and 220 days of age. Rats raised in isolation displayed significantly fewer noncontact erections in response to sensory cues from an estrous female and fewer intromissions when allowed to mate with a female than did males raised in groups. The volume of the posterodorsal component of the medial amygdala (MePD) and the size of neurons within the MePD were significantly smaller in the isolated males than in socially housed males. Similarly, neurons in the sexually dimorphic nucleus of the preoptic area (SDN-POA) were smaller in isolate animals than in controls. As both MePD volume and SDN-POA soma size are responsive to sex steroids, these differences could result if the isolates experienced lower testosterone levels. Finally, the volume of the overall medial amygdala (MeA) correlated significantly with the number of noncontact erections, a relationship that was not explained by housing condition. These findings highlight the role of social experience as a factor in the sexual differentiation of the brain and suggest a positive relationship between the volume of a brain structure and the display of sexual behaviors.

Estradiol Facilitates the Release of Neuropeptide Y to Suppress Hippocampus-Dependent Seizures

About one-third of women with epilepsy have a catamenial seizure pattern, in which seizures fluctuate with the menstrual cycle. Catamenial seizures occur more frequently when the ratio of circulating estradiol to progesterone is high, suggesting that estradiol is proconvulsant. We used adult female rats to test how estradiol-induced suppression of GABAergic inhibition in the hippocampus affects behavioral seizures induced by kainic acid. As expected, estradiol decreased the latency to initiate seizures, indicating increased seizure susceptibility. At the same time, however, estradiol also shortened the duration of late-stage seizures, indicating decreased seizure severity. Additional analyses showed that the decrease in seizure severity was attributable to greater release of the anticonvulsant neuropeptide, neuropeptide Y (NPY). First, blocking hippocampal NPY during seizures eliminated the estradiol-induced decrease in seizure duration. Second, light and electron microscopic studies indicated that estradiol increases the potentially releasable pool of NPY in inhibitory presynaptic boutons and facilitates the release of NPY from inhibitory boutons during seizures. Finally, the presence of estrogen receptor-α on large dense-core vesicles (LDCVs) in the hippocampus suggests that estradiol could facilitate neuropeptide release by acting directly on LDCVs themselves. Understanding how estradiol regulates NPY-containing LDCVs could point to molecular targets for novel anticonvulsant therapies.

Effects of prepubertal gonadectomy on a male-typical behavior and excitatory synaptic transmission in the amygdala

Mammalian puberty entails the emergence of behaviors such as courtship, coitus, and territorial aggressiveness. In adult rodents, the medial amygdala (MeA) is an important site for gonadal steroid hormone regulation of social behaviors and is sensitive to changes in the level of gonadal steroids. Here we show that prepubertal gonadectomy of male rats reduces the expression of a sexually dimorphic behavior, juvenile rough‐and‐tumble play, as well as the level of excitatory synaptic transmission assayed in adulthood. Behavioral observations in juveniles showed that gonadectomy reduced the initiation of playful attacks, particularly between postnatal days 31–35. Whole‐cell voltage clamp recordings made in slices from adults showed that gonadectomy also reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) in MeA neurons without affecting paired pulse facilitation, an index of vesicle release probability. As mEPSC frequency can reflect the number of excitatory synapses per neuron, we also compared the dendritic morphology of Lucifer Yellow filled neurons from intact and gonadectomized adults. This showed that gonadectomy significantly reduced the density of dendritic spines without affecting overall dendritic length or branching of MeA neurons, which is consistent with a gonadectomy‐induced reduction in the number of excitatory synapses. These findings suggest that peripubertal androgens activate rough‐and‐tumble play and promote the maintenance and/or development of new excitatory synapses in the MeA.

Juvenile social subjugation induces a sex-specific pattern of anxiety and depression-like behaviors in adult rats

Child abuse is the most significant environmental risk factor for the development of mood disorders, which occur twice as frequently in women as in men. To determine whether juvenile social subjugation (JSS) of rats induces mood disorder-like symptoms, we exposed 28 day-old male and female rats to daily aggressive acts from aggressive male residents. Each rat received pins, kicks, and dominance postures from the resident for 10 min per day for 10 days. When the rats were adults, we tested their anxiety- and depression-like behaviors. In addition, we measured circulating basal and stress-evoked corticosterone (CORT) levels, and weighed the adrenal glands. Although the amount of JSS was indistinguishable between males and females, females were nonetheless more severely affected by the experience. Subjugated females became immobile more quickly during forced swim tests, and made fewer investigatory approaches during the social interaction test than control females. Juvenile social subjugation increased closed arm time in the elevated plus maze of males and females, but the effect of social subjugation was greater in females. Finally, stress-evoked CORT levels were significantly higher, and adrenal gland weights were significantly heavier, in subjugated females relative to their controls and to subjugated males. Our results demonstrate that JSS increases depression- and anxiety-like behaviors and sensitizes the stress response system in a sex-specific manner.