The nuclear accumulation of [3H]testosterone and [3H]estradiol in the brain of the female primate: evidence for the aromatization hypothesis.

To identify the metabolites of estradiol (E2) and testosterone (T) in nuclei obtained from the female primate brain and, hence, to investigate the mechanism of their actions on behavior, 9 ovariectomized adult rhesus monkeys were studied. Two of these females were injected with 5.5 mCi [3H]T, and 30 min later, samples of 14 brain areas, pituitary gland, and peripheral tissues were removed and homogenized. Purified cell nuclei and a crude cytosol fraction were prepared, extracted with ether, and fractionated by HPLC to identify steroid metabolites. In nuclei from the hypothalamus, preoptic area, and amygdala, [3H]E2 formed locally was the major form of radioactivity. In nuclei from the clitoris, [3H]dihydrotestosterone was the major form of radioactivity, and in nuclei in all other brain samples and in the pituitary gland and uterus, [3H]T predominated. Two females (controls) were pretreated for 5 days with oil sc, injected with 1 mCi [3H]E2, and killed 60 min later. In these females, elevated nuclear concentrations of [3H]E2 were found in the hypothalamus, preoptic area, amygdala, pituitary gland, and uterus. Similar results were obtained in 2 females that were pretreated for 5 days with 2 mg/day dihydrotestosterone propionate, sc, and then injected with 1 mCi [3H]E2. In 3 females that were pretreated for 5 days with 2 mg/day T propionate, sc, and then injected with 1 mCi [3H]E2, levels of [3H]E2 were reduced by 100% (P less than 0.01) in nuclei from preoptic area and amygdala compared with control values and by 78% (P less than 0.05) in nuclei from the hypothalamus. There were no comparable reductions in steroid levels in cerebral cortex, pituitary gland, or uterus. This is the first direct evidence in the brain of a female primate that the actions of T and E2 involve the same receptor systems.

The uptake of [3H]testosterone and its metabolites by the brain and pituitary gland of the fetal macaque.

Testosterone is secreted by the fetal testis during gestation, and this is thought to influence certain aspects of the brain's subsequent development. To study this action at the neuronal level, nine macaque fetuses were injected with 250 microCi [3H]testosterone via the umbilical vein at about 120 days gestation. After 60 min, samples of brain and peripheral tissue were studied by autoradiography or HPLC. Purified nuclear pellets were prepared, and radioactivity in ether extracts was fractionated by HPLC and identified by coelution with internal standard steroids. Concentrations of radioactivity were significantly higher (P less than 0.05) in the hypothalamus-preoptic area than in amygdala, hippocampus, midbrain, and cerebral and cerebellar cortexes, and most of the radioactivity (75%) in the hypothalamus-preoptic area coeluted with 17 beta-estradiol. Radioactivity coeluting with 17 beta-estradiol was also detected in nuclear fractions from amygdala (44%). In contrast, 80% of the radioactivity extracted from pituitary gland nuclei coeluted with testosterone. Most of the neurons labeled in autoradiograms were located in the hypothalamus and preoptic area, fewer were found in the amygdala, and labeling in the frontal or motor cortex did not exceed chance levels. Results suggested that aromatization and, consequently, estrogen receptors play a role in the effects of testosterone on the hypothalamus and amygdala of the primate fetus at this stage of development.

Autoradiographic localization of radioactivity from [3-H]oxytocin in the rat mammary gland and oviduct.

[3-H]Oxytocin was incubated in vitro with pieces of oviduct and mammary gland and the tissue were subjected to autoradiography. Radioactivity was localized only in smooth muscle cells of the oviduct and in regions of mammary tissue where myoepithelial cells are found. In contrast, radioactivity was not concentrated in any region of skeletal muscle, a nontarget tissue for oxytocin. The localization of radioactivity in oviduct and mammary cells was absent when the tissues were incubated with [3-H]oxytocin and an excess of unlabeled oxytocin. These findings provide evidence for the presence of specific and high affinity receptors for oxytocin in its target tissues.

Brain cells of the male rhesus monkey accumulate 3H-testosterone or its metabolites.

Autoradiography was used to localize target cells for 3H-testosterone or its labeled metabolites in the brain of the rhesus monkey. Two castrated males were injected intravenously with 3H-testosterone (2 mCi/monkey) and were killed 1 hour later. In both animals, neurons that concentrated radioactivity in their nuclei were located in a well-circumscribed system that included the bed nucleus (n.) of the stria terminalis, medial preoptic n., suprachiasmatic preoptic n., anterior hypothalamic area, ventromedial hypothalamic n., cortical, medial, and basal accessory amygdaloid n., mesencephalic reticular n., and periaqueductal gray matter. In contrast to reports in some nonprimate mammalian species, little or no cellular labeling was observed in the lateral septum, arcuate n.-median eminence, motor nuclei of cranial nerves, or spinal cord.

Localization of the synthetic progestin 3H-ORG 2058 in neurons of the primate brain: evidence for the site of action of progestins on behavior.

The location of neurons that concentrate progestin in the brains of female cynomolgus monkeys was mapped by autoradiography using the specific synthetic progestin receptor ligand 3H-ORG 2058. Three females were ovariectomized and treated with estrogen (20 micrograms estradiol benzoate daily for 7 days), and one of them was also pretreated with progesterone. Each received an i.v. injection of 1 mCi 3H-ORG 2058 and was killed 1 hour later. Thaw-mount autoradiograms revealed intense accumulation of radioactivity in the nuclei of many neurons in the mediobasal hypothalamus, particularly in the ventromedial nucleus (n.), arcuate n., and premammillary n. Neuronal labeling was also observed frequently in the medial preoptic n., and occasionally in the anterior hypothalamic area, paraventricular n., and organum vasculosum of the lamina terminalis. In the pituitary gland, about 5% of cells in the pars distalis were intensely labeled. In the female pretreated with progesterone, however, labeling was almost completely blocked. Analysis of samples by high-performance liquid chromatography demonstrated that the radioactivity extracted from brain and pituitary gland cell nuclei was almost entirely unmetabolized 3H-ORG 2058. The nuclear concentration of progestin was much greater in the pituitary gland than in the brain, and was greater in the hypothalamus than in any other brain area. These results revealed well-localized groups of progestin-concentrating neurons in the primate brain which presumably mediate the effects of progesterone on both gonadotropin secretion and female sexual behavior.

The estrogen-sensitive neural system in the brain of female cats.

Estrogens participate in the regulation of gonadotropin secretion and in the control of sexual and aggressive behavior. To elucidate the neuroanatomical substrate for these actions in the female cat, the topographical distribution of estrogen-concentrating cells in the brain was studied by autoradiography. One hour after the intravenous injection of 1 mCi (3 microgram) of 3H-estradiol in three adult ovariectomized cats, radioactivity was concentrated in the nuclei of neurons in several brain areas. Within the telencephalon, high concentrations of moderately to heavily labeled cells were found in the lateral and posterior septal nuclei (n.), bed n. of the stria terminalis (medial division), medial and central amygdaloid n., and amygdalo-hippocampal area. There were also many less strongly labeled cells in the ventral dentate gyrus. Within the diencephalon, many heavily labeled cells were distributed in the medial and periventricular preoptic n., anterior hypothalamic area, and infundibular, ventromedial, and periventricular hypothalamic n. In the brain stem, some labeled cells were located throughout the periaqueductal gray, and in the laminar spinal trigeminal n., n. of the solitary tract, n. incertus, marginal n. of the superior cerebellar peduncle, rostral linear n., and interfascicular n. Although minor differences were noted, the topographical distribution of estradiol-concentrating cells in the female cat brain agreed closely with that reported in other vertebrate species.

LIM kinase 1 accumulates in presynaptic terminals during synapse maturation.

LIM kinase 1 (LIMK1) is a cytoplasmic protein kinase that is highly expressed in neurons. In transfected cells, LIMK1 binds to the cytoplasmic tail of neuregulins and regulates the breakdown of actin filaments. To identify potential functions of LIMK1 in vivo, we have determined the subcellular distribution of LIMK1 protein within neurons of the rat by using immunomicroscopy. At neuromuscular synapses in the adult hindlimb, LIMK1 was concentrated in the presynaptic terminal. However, little LIMK1 immunoreactivity was detected at neuromuscular synapses before the 2nd week after birth, and most motoneuron terminals were not strongly LIMK1 immunoreactive until the 3rd week after birth. Thus, LIMK1 accumulation at neuromuscular synapses coincided with their maturation. In contrast, SV2, like many other presynaptic terminal proteins, can be readily detected at neuromuscular synapses in the embryo. Similar to its late accumulation at developing synapses, LIMK1 accumulation at regenerating neuromuscular synapses occurred long after these synapses first formed. In the adult ventral spinal cord, LIMK1 was concentrated in a subset of presynaptic terminals. LIMK1 gradually accumulated at spinal cord synapses postnatally, reaching adult levels only after P14. This study is the first to implicate LIMK1 in the function of presynaptic terminals. The concentration of LIMK1 in adult, but not nascent, presynaptic terminals suggests a role for this kinase in regulating the structural or functional characteristics of mature synapses.

Muscarinic receptor subtypes in the lateral geniculate nucleus: a light and electron microscopic analysis.

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, ml-m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3-positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2-positive synaptic contacts were classified as F2-type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype-specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic-mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons.

Immunohistochemical localization of subtype 4a metabotropic glutamate receptors in the rat and mouse basal ganglia.

Recent studies suggest that metabotropic glutamate receptors (mGluRs) may play a significant role in regulating basal ganglia functions. In this study, we investigated the localization of mGluR4a protein in the mouse and rat basal ganglia. Polyclonal antibodies that specifically react with the metabotropic glutamate receptor subtype mGluR4a were produced and characterized by Western blot analysis. These antibodies recognized a native protein in wild-type mouse brain with a molecular weight similar to the molecular weight of the band from a mGluR4a-transfected cell line. The immunoreactivity was absent in brains of knockout mice deficient in mGluR4. mGluR4a immunoreactivity was most intense in the molecular layer of the cerebellum. We also found a striking mGluR4a immunoreactivity in globus pallidus, and moderate staining in substantia nigra pars reticulata and entopeduncular nucleus. Moderate to low mGluR4a immunoreactivity was present in striatum and other brain regions, including hippocampus, neocortex, and thalamus. Double labeling with mGluR4a antibodies and antibodies to either a dendritic marker or a marker of presynaptic terminals suggest a localization of mGluR4a on presynaptic terminals. Immunocytochemistry at electron microscopy level confirmed these results, revealing that in the globus pallidus, mGluR4a is mainly localized in presynaptic sites in axonal elements. Finally, quinolinic acid lesion of striatal projection neurons decreased mGluR4a immunoreactivity in globus pallidus, suggesting a localization of mGluR4a on striatopallidal terminals. These data support the hypothesis that mGluR4a serves as a presynaptic heteroreceptor in the globus pallidus, where it may play an important role in regulating g-amino-n-butyric acid (GABA) release from striatopallidal terminals.

Estrogen target neurons in the forebrain of the cat during postnatal development.

The postnatal ontogeny of estrogen binding neurons in the forebrain of the kitten was studied quantitatively by counting 14,000 neurons in thaw-mount autoradiograms. One hour after the i.v. injection of [3H]estradiol into male and female kittens at 3, 10 and 22 days of age, concentrations of radioactivity were observed in the nuclei of neurons in the septal-bed nucleus-preoptic area, hypothalamus, and amygdala; these are the same regions that have been shown to contain estrogen target neurons in adult female cats. In addition, however, estrogen target neurons were present in the lateral third of the caudate nucleus (n.) and in the putamen in 3-day-old kittens; regions that are devoid of any estrogen target cells in the adult. Between 3 and 10 days of age, the numbers of labeled neostriatal neurons decreased markedly, and they were down to about 15% of 3-day values in kittens 22 days of age. The transient presence of estrogen target neurons during the phase of rapid dendritic development and synaptogenesis in the caudate n. immediately after birth raises the question of whether gonadal hormones may influence sexual differentiation in the neostriatum. In the medial preoptic n., a greater percentage of neurons were labeled in males than in females. There were no consistent sex differences in labeling in six other regions examined: ventromedial n., arcuate n., medial amygdaloid n., septofimbrial n., caudate n. and putamen.

The role of the pituitary-adrenal system in the footshock-induced increase of [3H]lysine incorporation into mouse brain and liver proteins.

The brains of mice subjected to electric footshocks took up more free [3H]lysine and incorporated more [3H]lysine into protein than did the brains of undisturbed mice, suggesting that footshocks caused increased cerebral protein synthesis. The possible mediation of these effects by hormones of the pituitary-adrenocortical system was investigated. Despite adrenalectomy, footshock treatment still elevated the poolcorrected [3H]lysine incorporation into cerebral protein. Pretreatment with dexamethasone, which greatly suppressed the pituitary-adrenal response to footshocks, prevented the increase in [3H]lysine incorporation. It is suggested that ACTH itself may mediate this stress-induced increase in net free [3H]lysine uptake. Although the lysine uptake response thus appeared to depend upon the presence of the adrenal glands, the response was not mimicked by injection of corticosterone. The response of liver [3H]lysine metabolism was studied in the same experiments. Footshock treatment decreased the net uptake and increased the incorporation into protein, in normal, adrenalectomized and dexamethasone-pretreated mice. Corticosterone did not mimic these effects. Thus, the metabolic response to footshock of liver, unlike that of brain, appeared to be independent of the pituitary-adrenal system.

Estrogen binding and the actions of testosterone in the brain of the male rhesus monkey.

Autoradiography and high performance liquid chromatography (HPLC) were used to determine where metabolites of testosterone interact with estrogen binding sites in the brain of the male primate. Three days after castration, animals received a subcutaneous injection of either estradiol benzoate (EB, 200 micrograms/kg, n = 4) or oil vehicle (controls, n = 4). Three hours later, 5 mCi [3H]testosterone was administered as an intravenous bolus. At 60 min, brains were rapidly removed, left halves were used for autoradiography and right halves were dissected into 14 samples for HPLC of nuclear and supernatant fractions. In control males, labeled neurons were observed in preoptic area, hypothalamus and amygdala. In EB-pretreated males, the number of labeled neurons was reduced by 35% in the anterior hypothalamus and ventromedial nucleus, and by 65% in the cortical and accessory basal amygdaloid nuclei, but was not significantly reduced in other brain regions. In hypothalamus, preoptic area and amygdala, EB-pretreatment reduced nuclear concentrations of [3H]estradiol to 37-55% of control levels, but reduced neither the nuclear concentrations of [3H]testosterone nor the supernatant concentrations of [3H]estradiol and [3H]testosterone. The data suggest that the actions of testosterone in regions such as the arcuate nucleus and lateral septal nucleus primarily involve unchanged testosterone or dihydrotestosterone, while in regions such as the amygdala, aromatization and interaction with estrogen receptors is involved also.

Sites in the male primate brain at which testosterone acts as an androgen.

Quantitative autoradiographic analysis was used to identify regions in the brain of the male primate where androgen binding sites may be involved in the actions of testosterone. Three days after castration, adult male rhesus monkeys received a subcutaneous injection of either dihydrotestosterone propionate (DHTP, 20 mg, n = 6), testosterone propionate (TP, 100 mg, n = 2), or oil vehicle (control males, n = 4). Three hours later, 5 mCi [3H]testosterone was administered as an i.v. bolus. At 60 min, brains were rapidly removed and the left halves were used for autoradiography. In control males, highest percentages of labeled neurons (20-84% using a rigorous Poisson criterion) were observed in the ventromedial, arcuate and premammillary nuclei (n.) of the hypothalamus, medial preoptic n., bed n. of stria terminalis, intercalated mammillary n., lateral septal n. and the medial, cortical and accessory basal n. of the amygdala. Pretreatment with DHTP eliminated labeling in androgen target tissues of the genital tract, and reduced the percentages of labeled neurons to 4-22% of control values in the arcuate, lateral septal, premammillary and intercalated mammillary n., indicating that in these regions testosterone acted predominantly at androgen binding sites. However, in the medial preoptic n., the ventromedial hypothalamic n. and the accessory basal amygdaloid n., DHTP pretreatment resulted in much less blocking which, together with other data, suggested that in these sites, testosterone's actions involved aromatization and interaction with estrogen-binding sites.

Effect of sensory stimulation on the uptake and incorporation of radioactive lysine into protein of mouse brain and liver.

Alterations in incorporation of tritiated lysine into protein of mouse brain and liver were observed following brief exposure to a variety of sensory stimuli. During a 15-min session, subjects were trained to perform a one-way active avoidance response or else were exposed to one of the stimulus components of the situation, including shocks, buzzers, lights, handling, and the apparatus alone. Twenty min after these behavioral treatments, tritiated lysine was injected subcutaneously, and its incorporation into total protein during a 10-min pulse was measured. Quiet mice, undisturbed until injection of the precursor, constituted the baseline group for biochemical comparisons. Most behavioral treatments increased the total amount of radioactivity in brain and liver. The treatments increased the incorporation of radioactivity into protein of both organs even more, thereby producing elevations of relative radioactivity (RR) of protein, a measure of the amount of radioactivity incorporated into protein relative to that in the acid-soluble pools. The RR increases following most of the behavioral experiences were approximately equal; however, exposure to lights or to the apparatus were less effective than the other treatments in eliciting these metabolic changes. The responses were greatly diminished in mice previously exposed to the treatments. Thus, the effectiveness of a stimulus in producing these metabolic alterations may depend upon its apparent magnitude and its novelty. The total radioactivity increases were larger in brain than in liver, while the RR increases were smaller in brain than in liver. Brain RR increases were of equal magnitude when the precursor was injected 5, 20, or 35 min after behavioral treatment, whereas the liver RR responses declined markedly over this period. Despite these differences, strong positive correlations between brain and liver across the various behavioral treatments existed. The RR changes occurred about equally in the cerebellum-brain stem, basal ganglia, hippocampus-septum, and ventral cortex, while the thalamus-hypothalamus and dorsal cortex showed smaller differences.

Stimulation increases incorporation of [3H]lysine into rat brain and liver proteins.

Exposure of rats to electric footshocks or merely to the footshock apparatus increased the incorporation of [3H]lysine into brain and liver protein. The effect was present in both fore-and hindbrain. The footshock treatment was the more effective stimulus, producing larger and more significant changes. These results resemble those previously observed in C57Bl/6J mice, and thus suggest that altered protein or amino acid metabolism is a general response to stress in rodents.

Autoradiographic studies with a behaviorally potent 3H-ACTH4--9 analog in the brain after intraventricular injection in rats.

Autoradiographic studies aimed at identifying target cells in the brain for ACTH-like peptides were performed using (3H-7-Phe)-4-Met(O2),8-D-Lys, 9-Phe-ACTH4--9, a behaviorally potent analog of ACTH4--9. The 3H-peptide was injected into the lateral ventricle of hypophysectomized rats that were sacrificed 5, 30, 60, 180, and 240 min later. Dry-mount autoradiograms of brain showed the highest density of silver grains in the ventricular lumen and choroid plexus. In addition, radioactivity penetrated brain tissue as far as 100 microns from the ventricles, and was distributed predominantly over neuropil. Within 5 min after the injection, an intracellular concentration of radioactivity above background levels was observed in a small proportion of cells near the ventricles in the septum, caudate-putamen, preoptic area, hypothalamus, thalamus, amygdala, and hippocampus. The cellular labeling decreased in intensity at greater distances from the injection site and at longer survival intervals, and was no longer evident 4 hr after the injection. The labeled cells were usually small, dark, and often elongated, suggesting that ACTH peptides may act preferentially upon a morphologically distinct class of cells in the brain.

Neurons in the brain of fetal rhesus monkeys accumulate 3H-testosterone or its metabolites.

Autoradiography was used to map sites in the primate brain at which testosterone may have sexual differentiating actions on brain function and behavior during fetal development. Two female rhesus monkey fetuses were injected in utero on days 112 and 114 of gestation respectively with 3H-testosterone, and were killed 30 and 60 minutes later. Thaw-mount autoradiography of the brains revealed the accumulation of radioactivity, representing 3H-testosterone or its metabolites, in neurons of the medial preoptic-anterior hypothalamic area, bed nucleus (n.) of the stria terminalis, ventromedial hypothalamic n., and corticomedial amygdala. Thus, it appears that steroid receptors are present in a circumscribed system in the brain of the primate fetus at this stage of development.

Autoradiographic localization of 3H-dihydrotestosterone in the preoptic area, hypothalamus, and amygdala of a male rhesus monkey.

In a preliminary study, autoradiography was used to localize target cells for 3H-dihydrotestosterone (DHT), a non-aromatizable androgen, in the brain of the rhesus monkey. One castrated male was injected intravenously with 2 mCi of 3H-DHT (0.42 microgram/kg), and was killed one hour later. Neurons that concentrated radioactivity in their nuclei were located in widespread areas of the brain, which included the medial and suprachiasmatic preoptic nuclei, bed nucleus of the stria terminalis, lateral septal nucleus, anterior hypothalamic area, ventromedial, arcuate, dorsomedial, and paraventricular hypothalamic nuclei, ventral premammillary nucleus, and medial, cortical, basal accessory, and lateral amygdaloid nuclei. These results indicate that the topographic distribution of androgen target neurons is considerably wider than that observed in a study using 3H-testosterone (T) in the male rhesus monkey (1). However, further work is needed to elucidate these differences before attempting correlations between behavioral activity and androgen receptors in the brain.

Pre-optic and hypothalamic neurons accumulate [3H]medroxyprogesterone acetate in male cynomolgus monkeys.

Medroxyprogesterone acetate (MPA) is a synthetic progestin that is reported to be effective in the treatment of paraphilic behavior, including paraphilic aggression, in men. The mechanisms and sites of action for its behavioral effects are not known. Thaw-mount autoradiography was used to help identify sites in the brain at which MPA may act in a male primate. Two adult, castrated male cynomolgus monkeys were administered [3H]MPA and killed one hour later. Radioactivity was concentrated in the nuclei of many neurons in the medial preoptic nucleus (n.), anterior hypothalamic area, ventromedial hypothalamic n., and arcuate n. Virtually no labeled cells were observed in the bed n. of the stria terminalis, lateral septal n., or amygdala. Analysis by high performance liquid chromatography of brain samples from the same animals demonstrated that 84% of the extractable radioactivity in cell nuclei from the hypothalamus and preoptic area was in the form of unmetabolized [3H]MPA. The localization of MPA-concentrating neurons in regions of the brain known to be implicated in regulating both sexual behavior and pituitary function suggests that, among other sites of action, MPA may act directly upon the brain.

Characterization by high performance liquid chromatography of nuclear metabolites of testosterone in the brains of male rhesus monkeys.

Testosterone (T) restores the potency of castrated male rhesus monkeys, and our autoradiographic data have demonstrated that 3H-T or its metabolites concentrate in cell nuclei in the corticomedial amygdala, bed nucleus of stria terminalis, preoptic area, and hypothalamus. In rat, 3H-estradiol (3H-E2) is a major nuclear metabolite of 3H-T in areas of the limbic system, but comparable data are lacking for the primate. We have therefore developed an improved technique using high performance liquid chromatography for investigating metabolites of 3H-T that accumulate in cell nuclei in small amounts of tissue obtained from the brain of the rhesus monkey. Two castrated male rhesus monkeys were injected with 5 mCi of 3H-T and were killed 30 min later. In amygdala, preoptic area-bed nucleus of stria terminalis, and hypothalamus, 48-70% of the nuclear radioactivity was in the form of 3H-E2 (Type I tissues). In six other brain areas and in pituitary, 35-85% of the nuclear radioactivity was in the form of 3H-T (Type II tissues), whereas in genital tract tissues, 86-99% of the nuclear radioactivity was in the form of 3'-dihydrotestosterone (3H-DHT) (Type III tissues). In plasma and in supernatants from both Type I and Type II tissues, the proportions of 3H-T were high, and 3H-E2 did not exceed 10% of the total extractable radioactivity. These data suggest that, as in rodents, some of the central actions of T in primates may be mediated by estrogen target neurons.