Differentiation of coital behavior in mammals: a comparative analysis

Genetic and hormonal factors in sex differentiation of the brain

The mode of chromosomal sex determination is opposite in mammals and in birds, the heterogametic sex being male in the former and female in the latter. This difference is parallel by a basic program of development of sex characters in the absence of gonads; this program is feminine in mammals and male in birds. 'Defeminization' is produced by the testes in male mammals, and 'demasculinization' is produced by the ovary in female birds. The same difference prevails in the permanent organizational effects of sex hormones on the neural structures mediating sex behavior, during a critical period. In mammals, especially in the rat, testosterone released by the testis controls the various sex characters. It can work as such, or after being converted locally into dihydrotestosterone or estradiol, in the cells of the end organ. Estradiol seems to be the actual intracellular 'defeminizing' agent in the central nervous system. In birds, especially in quail, accumulating data suggest that estradiol is the intracellular 'demasculinizing' agent.

Cytoarchitectonic sexual dimorphisms of the medial preoptic and anterior hypothalamic areas in guinea pig, rat, hamster, and mouse

A cytoarchitectonic analysis is presented of a configuration of cells which extends through the medial preoptic and rostral part of the anterior hypothalamus of the guinea pig, rat, hamster, and mouse. Within that configuration, called here the sexually dimorphic nuclear complex of the medial preoptic-anterior hypothalamic area (SDNC-MPAH), there is nearly continuous variation in cellular distribution or packing density as one proceeds rostrocaudally, as well as variations between females and males at corresponding levels. The major cell groups comprising SDNC-MPAH are the medial preoptic nucleus (MP) and the preventricular portion of the periventricular nucleus (Pep) rostrally, and the anterior hypothalamic nucleus (AH), anterior portion of periventricular nucleus, (Pea), and bed nucleus of stria terminalis (ST) caudally. The most striking cytoarchitectonic dimorphisms usually involve MP and AH. In all four species examined, MP is larger and, except for the mouse, appears to be more densely cellular in the female than in the male. Most prominently in the rat and guinea pig, AH is densely cellular throughout in the female, including the subependymal zone, whereas in the male AH lies at a distance from the third ventricle, separated from the ependyma by a cell-poor zone. The significance of these findings may lie in the fact that the region exhibiting the sexual dimorphisms has been shown to be involved in the regulation of reproductive functions which are sexually differentiated and developmentally influenced by the fetal or perinatal hormonal milieu in some species.

Testosterone induced mounting behavior in adult female rats born in litters of different female to male ratios

Female rats can show mounting behavior towards isosexual or heterosexual conspecifics. The present experiments were designed to study whether the prenatal presence of male fetuses would affect display of this mounting behavior in adulthood. Therefore mounting behavior, after gonadectomy and during continuous treatment with testosterone propionate (TP) was studied in female rats which were born in litters without male siblings ("all female" litters) and in litters with a variable number of male siblings. Litter composition at birth did not affect display of adult mounting behavior during protracted tests (a total of 6 tests during 8 weeks of treatment with TP). The data indicate that TP induced mounting behavior in adulthood occurs independent of the prenatal presence of male fetuses. If mounting behavior in adulthood has to be "organized" by prenatally present androgen (the current way of thinking), then the present data indicate that female rat fetuses provide themselves with these hormones. It would then seem inappropriate to judge adult mounting behavior as a sign of "masculinization" of the rat brain.

Sex differences in drug-induced rotation in two strains of rats

Female Fischer and Sprague-Dawley rats rotate significantly more in response to D-amphetamine sulfate (1 mg/kg, i.p.) than their male littermates. Although Sprague-Dawley males rotate more than females when injected with apomorphine hydrochloride (10 mg/kg, i.p.), Fischer males and females respond similarly to this drug. The sexually dimorphic behavioral response to amphetamine appears to be due to differences in brain sensitivity to this drug, and may relate to reports of sex-related differences in nigrostriatal dopaminergic activity.

Neural gonadal steroid actions

Neurons sensitive to gonadal steroids are located strategically within neural circuits that mediate behaviors broadly related to the reproductive process. Some neuronal events and properties are regulated by these hormones. Variability in the occurrence and distribution of particular neural hormonal sensitivities across species may be related to variations in the hormonal requirements for sexual differentiation and for activation of reproductive behaviors.

Prenatal exposure to synthetic progestins increases potential for aggression in humans

Seventeen females and eight males exposed during gestation to synthetic progestins showed a significantly higher potential for physical aggression than their sex-matched unexposed siblings. Exposure to androgen-based compounds appeared to be most closely associated with aggressive responses. There were no differences in verbal aggression or IQ between exposed and unexposed siblings.