Fitch & Denenberg (1995) have done an admirable job of marshalling evidence in support of the hypothesis that the full development of female-typical behavior requires exposure to ovarian steroids in the postnatal period. Although I am in agreement with this position, I feel that the authors have oversimplified the rich variation in the processes that might account for the production of sexually-dimorphic behaviors in the adult mammal.
2. Although I disagree with the authors on a number of points, I will choose one as an example to illustrate my main concern with their presentation of the material, namely that experimental results have often been oversimplified in support of the authors' thesis. Consider the discussion of amphetamine-induced rotation and stimulated dopamine (DA) release and the inferences made from those data about sex differences in the midbrain DA system (paragraph 14 of the target article). It is clearly the case that the DA system of adult female rats is more responsive to stimulation by DA-ergic drugs such as amphetamine (AMPH) than is that of male rats (e.g., Becker et al., 1982). The finding that castration does not alter AMPH-stimulated DA release concerns the effects of adult castration not castration at birth (Castner et al., 1993), a point which was not made clear by Fitch & Denenberg. These data, therefore, are evidence of a sexually dimorphic nervous system, not of the influence of ovarian steroids on the development of this sexual dimorphism. This is also true of the finding that estradiol does not have significant effects on DA release in castrated, adult male animals. Again, it shows that the two sexes are differentially responsive to steroids as adults, but it does not speak to the mechanism by which they become that way.
3. The suggestion that the sex difference in the ability of estradiol to enhance release of DA emerges at puberty, and the implication that the sex difference results from the influence of ovarian secretions at this time, is based on the data of Becker and Ramirez (1981) which was not cited in the target article. These authors found that castration of the male early in development did not influence the male pattern of AMPH-stimulated DA release in the adult rat. Those males, however, were castrated 2 days after birth, giving ample time for both pre and postnatal testicular secretions to have their effects. Tissue from females treated with testosterone propionate (TP) on day 2 and ovariectomized prepubertally (day 15), responded like that of male animals. Although prepubertal ovariectomy of the normal female rat (i.e., females that were not treated with TP at birth) clearly influenced the dynamics of AMPH-stimulated DA release from striatal tissue in adulthood (making it more male-like), and it may be true that the sex difference does not exist prior to puberty, it is not clear that perinatal testicular secretions are without effect in shaping the development of this response. Thus, it is misleading to imply that ovarian secretions are the main contributor to the sexually dimorphic development and functioning of this neural system.
4. Based on data that I have collected in collaboration with Jane Stewart of Concordia University, the development of sex differences in the behavioral response to AMPH (and by inference the functioning of the DA system) appears to be due to the interaction of a number of different factors (Forgie & Stewart, 1993; 1994a; 1994b). First, castration of male rats or administration of TP to females in the immediate postnatal period was sufficient to alter the responsiveness of adult animals to AMPH (Forgie & Stewart, 1993; 1994a). Thus, at least one component of the female's greater responsiveness to AMPH as an adult is due to the absence of testicular secretions in the postnatal period. Second, exposure of the female to ovarian secretions over the lifespan was also found to be important (Forgie & Stewart, 1993; 1994a). Circulating estradiol at the time of testing was found to be a major factor in the female's responsiveness to the drug, and like Becker and colleagues, we found evidence to suggest that males and females are differentially responsive to the effects of estradiol as adults. In our experiments, however, the ability of estradiol to have effects on AMPH-induced behavior was clearly dependent on whether the animal had received exposure to testosterone in the neonatal period. Thus, females ovariectomized in adulthood and males gonadectomized at birth showed greater responsiveness to AMPH in the presence of estradiol than males castrated in adulthood or females treated with TP at birth and ovariectomized in adulthood (Forgie & Stewart, 1993). Moreover, although prepubertal ovariectomy of the female was found to decrease responsiveness to AMPH in adulthood, it did not alter the effect of estradiol on AMPH-induced behavior. These effects of prepubertal ovariectomy were also dependent on the prior exposure of the female animal to testosterone postnatally (Forgie & Stewart, 1994a). Therefore, it is misleading of Fitch & Denenberg to imply that the female responsiveness to AMPH, and by inference the sexual differentiation of the midbrain DA neurons, is solely dependent on the exposure of the female to ovarian steroids peripubertally. In fact, the sexual differentiation of this neural system appears to have its foundation in the differential exposure of the two sexes to testicular secretions in the neonatal period. This produces a divergence in the course of development of the two sexes and ultimately alters the way in which the DA system responds to steroids later in development. Ovarian secretions in the female at later postnatal times influence the level of responsiveness to AMPH in adulthood and are important in producing full female-typical patterns of response, but they do so against the backdrop of a nervous system that has already taken a "female" course. Finally, from the very provocative data of Ingrid Reisert and her colleagues (e.g., Reisert & Pilgrim, 1991) we have evidence that the course of development in this neural system in the rat may already be sexually dimorphic prior to embryonic day 17, possibly indicating sexually dimorphic genetic effects on DA development (see also Pilgrim & Hutchinson, 1994). Thus, although I agree with the authors' call to "broaden the concept of sexual differentiation by recognizing that both testicular and ovarian hormones are active participants" (paragraph 28) and with the spirit of their conclusion that "In order for the brain to become sexually differentiated, males need exposure to testicular androgens during the perinatal period ... and females need exposure to ovarian secretions... during a late period that may extend to or even beyond puberty" (paragraph 54) I believe that they have greatly oversimplified the issues involved. This is unfortunate given the richness of the variations in hormonal influences apparent in their own data.
5. I would argue then, that for sexual differentiation of the brain and behavior the male animal "needs" an XY genotype, and testicular androgens in the pre and postnatal period (acting as androgens or aromatized estrogens or both), the peripubertal period and in adulthood. The female animal, on the other hand, "needs" an XX genotype, the relative absence of large amounts of testicular androgens (or estrogens) in the perinatal period, the presence of ovarian secretions in the postnatal period and the presence of circulating ovarian secretions in adulthood. Furthermore, different neural systems (or different components of the same neural system) will be more or less sensitive to these different events, and the influence of the different events will depend in large part on the state of the neural system at the time that each event occurs. This "state" of the nervous system by definition will be dependent on all the prior events that have occurred. In conclusion, although it is clear that ovarian secretions play a role in the sexual differentiation of the female animal, it is also clear that these effects do not occur in isolation from the other genetic and hormonal events that characterize the development of the mammalian nervous system. Only when we begin to acknowledge and understand the importance of the interaction of all these factors over the lifespan of the animal will we begin to understand the true underpinnings of sexually dimorphic differentiation of brain and behavior.
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Becker, J.B., Robinson, T.E. & Lorenz, K.A. (1982) Sex differences and estrous cycle variations in amphetamine-elicited rotational behavior. European Journal of Pharmacology 80: 65-72.
Castner, S.A., Xiao, L. & Becker, J.B. (1993) Sex differences in striatal dopamine: in vivo microdialysis and behavioral studies. Brain Research 610: 127-134.
Fitch, R. H. & Denenberg, V.H. (1995) A Role for Ovarian Hormones in Sexual Differentiation of the Brain. PSYCOLOQUY 6(5) sex-brain.1.fitch.
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Forgie, M.L. & Stewart, J. (1993) Sex differences in amphetamine-induced locomotor activity in adult rats: Role of testosterone exposure in the neonatal period. Pharmacology Biochemistry & Behavior 46: 637-645.
Reisert, I. & Pilgrim, C. (1991) Sexual differentiation of monoaminergic neurons-genetic or epigenetic? Trends in Neurosciences 14: 468-473.
Pilgrim, C. & Hutchinson, J.B. (1994) Developmental regulation of sex differences in the brain: Can the role of gonadal steroids be redefined? Neuroscience 60: 843-855.