In asserting that a masculinizing role of estrogen (derived via intracellular aromatization) is paradoxically incompatible with a feminizing role of estrogen, Rucklidge (1995) has overlooked the critical temporal distinction between the sensitive windows for these effects. The fact that the neural substrate (including but not limited to estrogen receptor populations) is profoundly different in
P10 female rats, combined with the fact that physiological levels of ovarian estrogen are much lower than those used to exogenously induce masculinization, it should not be surprising that estrogen could exert different effects (masculinizing versus feminizing) on males versus females in two different developmental time-frames.
(a) Androgen-based masculinization begins prenatally in male rats, as supported, for example, by data showing the critical role of the gestational day 18 (G18) fetal androgen surge in masculinizing reproductive behavior (Ward, 1983) and cortical thickness asymmetry (Fleming et al., 1986).
(b) Masculinizing effects seen with administration of aromatizable testosterone propionate (TP) to female rodents prior to postnatal days 8-10 (P8 - P10) are NOT seen when TP is administered outside this early sensitive window (e.g., Fitch & Denenberg, 1995; Rhees et al., 1990a & 1990b; Wagner & Clemens, 1989).
(c) Ovarian secretion of estrogen does not begin until approximately P12 in female rats (Carson & Smith, 1986; Dohler & Wuttke, 1975). Thus the window of masculinization in the rodent appears to have "closed" before the ovaries even become active.
(d) Levels of alpha-fetoprotein start to decline around P10 in the rat, and are at trace levels by weaning, consistent with a potentially active role of ovarian estrogen in this later time frame (Raynaud et al., 1971; Raynaud, 1973; MacLusky & Naftolin, 1981).
2. In sum, males appear to be masculinized by testicular androgens converted intracellularly to estrogen in the prenatal/early postnatal period (in rodents). Both males and females are apparently protected against maternal estrogen by alpha-fetoprotein (AFP, aka feto-neonatal estrogen binding protein, or FEBP) during this time frame. At some point following the "close" of this sensitive window to masculinizing estrogen effects, levels of AFP begin to decline and the neonatal ovary begins to secrete estrogen. The convergence of these factors can explain how feminization via ovarian estrogen can be temporally separated from estrogenic masculinization, an assertion supported by numerous findings of later than P10 feminizing effects of estrogen in rodents (Bloch & Gorski, 1988; Fitch & Denenberg, 1995; Gerall et al., 1973; Munoz-Cueto et al., 1990; Pappas et al., 1979; Stewart & Cygan, 1980).
3. The next question is how the same hormone could have different effects at different times in males and females (i.e., why doesn't ovarian estrogen exert masculinizing effects in females?). There are three likely mediating factors for these temporally dichotomous effects.
(a) Neural estrogen receptor populations vary in topographic distribution and density as a function of gender (Brown et al., 1990; DonCarlos & Handa, 1994; Kuhnemann et al., 1994; Sandhu et al., 1986) and postnatal age (MacLusky et al., 1979a & b; Miranda & Toran-Allerand, 1992; O'Keefe & Handa, 1990; Shugrue et al., 1990).
(b) Physiological levels of ovarian estrogen are significantly lower than exogenous estrogen levels typically used to induce experimental masculinizing effects.
(c) The impact of the same variable on a substrate in developmental "flux" is likely to differ dramatically depending on the time-point. For example, ischemic neural injury induced in rats on P1, during critical periods of neuromigration, exerts profoundly different effects from these same insults administered just 4 days later (Dvorak & Feit, 1977; Suzuki & Choi, 1991).
4. In sum, it appears that early "high-dose" levels of intra-cellular estrogen masculinize, while later and lower levels of estrogen feminize, at least in the rodent model, and at least for the variety of structural and behavior phenomena that have been studied. Thus different sensitive windows can apparently produce different developmental consequences from the same hormone (estrogen). A perfect example of this temporal distinction is provided by Stewart and Cygan (1980; see also Stewart et al., 1979), who showed significant masculinizing effects on open field activity in female rats with early high dose estrogen treatment (25 ug estradiol benzoate on P2 and 3), as compared to feminizing effects on open field behavior with later low dose estrogen replacement in ovariectomized rats (Silastic implants of estradiol 17B on P30-40, delivering physiological levels of about 108 pg/ml serum).
5. This distinction is recognized, to some degree, by Rucklidge herself in asserting that "estrogen exerts different effects depending on ... the behavior being investigated, and the critical period of that behavior." (1995, par. 8) We would assert that this principle can be more generally applied, in as much as the sensitive window for masculinizing and feminizing effects of estrogen on a wide variety of functional and structural phenomena appear to be temporally separate (G18 to P8 - 10 in males, and > P10 in females) in the rodent models which have been well studied.
Bloch, G.J. and Gorski, R.A. (1988) Estrogen/progesterone treatment in adulthood affects the size of several components of the medial preoptic area in the male rat. Journal of Comparative Neurology, 275, 613-622.
Brown, T.J., MacLusky, N.J., Shanabrough, M. and Naftolin, F. (1990) Comparison of age- and sex-related changes in cell nuclear estrogen-binding capacity and progestin receptor induction in the rat brain. Endocrinology, 126, 2965-2972.
Carson, R. and Smith, J. (1986). Development and steroidogenic activity of preantral follicles in the neonatal rat ovary. J Endocrinol, 1190, 87-92.
Dohler, K.D. and Wuttke, W. (1975) Changes with age in levels of serum gonadotropins, prolactin, and gonadal steroids in prepubertal male and female rats. Endocrinology 97: 898-908.
DonCarlos, L.L. and Handa, R.J. (1994) Developmental profile of estrogen receptor mRNA in the preoptic area of male and female neonatal rats. Brain Res Dev brain Res, 79, 283-289.
Dvorak, K. and Feit, J., (1977) Migration of neuroblasts through partial necrosis of the cerebral cortex in newborn rats - contribution to the problems of morphological development and developmental period of cerebral microgyria. Acta Neuropathol. (Berl.), 38, 203-212.
Fleming, D., Anderson, R.H., Rhees, R., Kinghorn, E. and Bakaitis, J. (1986) Effects of prenatal stress on sexually dimorphic asymmetries in the cerebral cortex of the male rat. Brain Research Bulletin, 16, 395-398.
Fitch, R.H. and Denenberg, V. (1995) A role for ovarian hormones in sexual differentiation of the brain. PSYCOLOQUY 6(5) sex-brain.1.fitch.
Gerall, A., Dunlap, J. and Hendricks, S. (1973) Effects of ovarian secretions on female behavioral potentiality in the rat. Journal of Comparative and Physiological Psychology, 82,449-465.
Kuhnemann, S., Brown, T.J., Hochberg, R.B. and MacLusky, N.J. (1994) Sex differences in the development of estrogen receptors in the brain. Horm Behav, 28, 483-491.
MacLusky, N.J., Chaptal, C. and McEwen, B.S. (1979a) The development of estrogen receptor estrogen receptor systems in the rat brain and pituitary: postnatal development. Brain Research 178: 143-160.
MacLusky, N.J., Lieberburg, I. and McEwen, B.S. (1979b) The development of estrogen receptors in the rat brain and pituitary: perinatal development. Brain Research, 178, 129-142.
MacLusky, N.J. and Naftolin, F. (1981) Sexual differentiation of the nervous system. Science, 211, 1294-1303.
Miranda, R.C. and Toran-Allerand, C.D. (1992) Developmental expression of estrogen receptor mRNA in the rat cerebral cortex: a nonisotopic in situ hybridization histochemistry study. Cereb Cortex, 2, 1-15.
Munoz-Cueto, J.A., Garcia-Segura, L.M. and Ruiz-Marcos, A. (1990) Developmental sex differences and effect of ovariectomy on the number of cortical pyramidal cell dendrite spines. Brain Research, 515, 64-68.
O'Keefe, J.A. and Handa, R.J. (1990) Transient elevation of estrogen receptors in the neonatal rat hippocampus. Brain Res Dev Brain Res, 57, 119-127.
Pappas, C.T.E., Diamond, M.C. and Johnson, R.E. (1979) Morphological changes in the cerebral cortex of rats with altered levels of ovarianhormones. Behavioral and Neural Biology, 26, 298-310.
Raynaud, J.P. (1973) Influence of rat estradiol binding protein on uterotrophic activity. Steroids, 21, 249-258.
Raynaud, J.P., Mercier-Bodard, C. and Baulieu, E.E. (1971) Rat estradiol binding plasma protein. Steroids, 18, 767-788.
Rhees, R.W., Shryne, J.E., and Gorski, R.A. (1990a) Onset of the hormone-sensitive perinatal period for sexual differentiation of the sexually dimorphic nucleus of the preoptic area. Journal of Neurobiology, 21, 781-786.
Rhees, R.W., Shryne, J.E. and Gorski, R.A. (1990b) Termination of the hormone-sensitive period for differentiation of the sexually dimorphic nucleus of the preoptic area in male and female rats. Developmental Brain Research, 52, 17-23.
Rucklidge, J. (1995) Incorporating estrogen's masculinizing role: Commentary on Fitch and Denenberg on Sex-Brain. PSYCOLOQUY 6(5) sex-brain.2.rucklidge.
Sandhu, S., Cook, P. and Diamond, M. (1986) Rat cerebral coretical estrogen receptors: male-female, right-left. Experimental Neurology, 92, 186-196.
Shugrue, P.J., Stumpf, W.E., MacLusky, N.J., Zielinski, J.E. and Hochberg, R.B. (1990) Developmental changes in estrogen receptors in mouse cerebral cortex between birth and postweaning: studied by autoradiography with 11b-Methoxy-16a-[125I] Iodoestradiol. Endocrinology, 126, 1112 - 1124.
Stewart, J. and Cygan, D. (1980) Ovarian hormones act early in development to feminize open field behavior in the rat. Hormones and Behavior, 14, 20-32.
Stewart, J., Vallentyne, S. and Meaney, M.J. (1979) Differential effects of testosterone metabolities in the neonatal period on open-field behavior and lordosis in the rat. Hormones and behavior, 13, 282-292.
Suzuki, M. and Choi, B.H. (1991) Repair and reconstruction of the cortical plate following closed cryogenic injury to the neonatal rat cerebrum. Acta Neuropathol. (Berl), 82, 93-101.
Wagner, C.K. and Clemens, L.G. (1989) Perinatal modification of a sexually dimorphic motor nucleus in the spinal cord of the B6D2F1 house mouse. Physiology and Behavior, 45, 831-835.
Ward, I. (1983) Effects of maternal stress on the sexual behavior of male offspring. Monographs in Neural Sciences, 9, 169-175.