Showing papers on "Hypothalamus published in 1976"

An autoradiographic study of the efferent connections of the preoptic region in the rat.

Abstract: The normal morphology of the rat preoptic region has been briefly described on the basis of Nissl- and silver-stained preparations and its efferent connections have been studied autoradiographically in over 50 rat brains with single small injections of 3H-proline, or various mixtures of 3H-proline, 3H-leucine, and 3H-lysine. Injections in the anteroventral part of the lateral preoptic area labeled fibers projecting through, and perhaps to, the anterior and lateral hypothalamic areas and ending in the supramammillary region, and ventral fiber lamina of the mammillary complex; other labeled fibers ended in the periventricular hypothalamic gray and the internal lamina of the median eminence. The posteromedial lateral preoptic area projects to the same regions, as well as to the medial septal-diagonal band complex, and to the lateral habenula through the stria medullaris. Injections of the posterolateral lateral preoptic area labeled each of the above fiber systems as well as fibers to the main olfactory bulb, anterior olfactory nucleus and taenia tecta. Other fibers coursed over the genu of the corpus callosum, through the stria terminalis and ansa peduncularis to the medial, cortical and basal amygdaloid nuclei and the anterior amygdaloid area, and through the medial forebrain bundle to the substantia nigra. The transition region between the lateral preoptic and lateral hypothalamic areas at the level of the supraoptic nucleus has widespread connections as a whole (a) with the medial septal-diagonal band complex, lateral septum and bed nucleus of the stria terminalis, (b) through or to most of the hypothalamus, the substantia nigra, central tegmental field, central gray, superior central nucleus, and the locus coeruleus, (c) through the stria medullaris to the lateral habenula (bilaterally), parataenial, paraventricular, and mediodorsal nuclei of the thalamus, (d) through the stria terminalis and ansa peduncularis to the central, medial and cortical nuclei of the amygdala, and (e) to the main olfactory bulb, anterior olfactory nucleus, cingulate bundle, olfactory tubercle, medial septal-diagonal band complex and the lateral septum.

The efferent connections of the ventromedial nucleus of the hypothalamus of the rat.

Abstract: The efferent connections of the ventromedial nucleus of the hypothalamus (VMH) of the rat have been examined using the autoradiographic method. Following injections of small amounts (0.4-2.0 muCi) of tritium labeled amino acids, fibers from the VMH can be traced forward through the periventricular region, the medial hypothalamus and the medial forebrain bundle to the preoptic and thalamic periventricular nuclei, to the medial and lateral preoptic areas, to the bed nucleus of the stria terminalis and to the ventral part of the lateral septum. Some labeled axons continue through the bed nucleus of the stria terminalis into the stria itself, and hence to the amygdala, where they join other fibers which follow a ventral amygdalopetal route from the lateral hypothalamic area and ventral supraoptic commissure. These fibers terminate in the dorsal part of the medial amygdaloid nucleus and in the capsule of the central nucleus. A lesser number of rostrally directed fibers from the VMH crosses the midline in the ventral supraoptic commissure and contributes a sparse projection to the contralateral amygdala. Descending fibers from the VMH take three routes: (i) through the medial hypothalamus and medial forebrain bundle; (ii) through the periventricular region; and (iii) bilaterally through the ventral supraoptic commissure. These three pathways are interconnected by labeled fibers so that it is not possible to precisely identify their respective terminations. However, the periventricular fibers seem to project primarily to the posterior hypothalamic area and central gray, as far caudally as the anterior pole of the locus coeruleus, while the medial hypothalamic and medial forebrain bundle fibers apparently terminate mainly in the capsule of the mammillary complex, in the supramammillary nucleus and in the ventral tegmental area. The ventral supraoptic commissure fibers leave the hypothalamus closely applied to the medial edges of the two optic tracts. After giving off their contributions to the amygdala, they continue caudally until they cross the dorsal edge of the cerebral peduncle to enter the zona incerta. Some fibers probably terminate here, but others continue caudally to end in the dentral tegmental fields, and particularly in the peripeduncular nucleus. Within the hypothalamus, the VMH appears to project extensively to the surrounding nuclei. However, we have not been able to find evidence for a projection from the VMH to the median eminence. Isotope injections which differentially label the dorsomedial or the ventrolateral parts of the VMH have shown that most of the long connections (to the septum, amygdala, central tegmental fields and locus coeruleus) originate in the ventrolateral VMH, and there is also some evidence for a topographic organization within the projections of this subdivision of the nucleus.

Midbrain, diencephalic and cortical relationships of the basal nucleus of meynert and associated structures in primates

Abstract: The structure and connectivity of the basal nucleus of Meynert, the substantia innominata in which it lies, and certain related areas have been examined in New World and Old World Monkeys, using retrograde and anterograde axonal transport methods. Experiments using the retrograde, horseradish peroxidase method confirm the observations of Kievet and Kuypers ('75) that the basal nucleus and substantia innominata project directly, heavily and with a somewhat crude topography upon the neocortex. Experiments involving the anterograde, autoradiographic method show that the basal nucleus and substantia innominata form part of a complex pathway that links them together with the lateral hypothalamus, certain parts of the amygdala and the peripeduncular nucleus of the midbrain. The peripeduncular nucleus is often regarded as a part of the central auditory pathway; it gives rise to a fiber bundle of considerable size that ascends on the dorsal surface of the ipsilateral optic tract and terminates ultimately in the lateral hypothalamic area of both sides. As well as distributing fibers to the basal nucleus, substantia innominata and lateral hypothalamus, this pathway provides a heavy projection to a cytoarchitectonically distinct posterior part of the lateral nucleus of the amygdala, the medial and intercalated nuclei of the amygdala and a less dense projection to the bed nucleus of the stria terminalis. Certain parts of the hypothalamus and possibly the preoptic areas give rise to a complementary descending pathway that distributes fibers to the ipsilateral basal nucleus, substantia innominata and amygdala, and ends in the peripeduncular nuclei of both sides. Decussating fibers in both the ascending and descending pathways cross in the ventral supraoptic commissure. It is concluded that the basal nucleus should include most of the aggregated and unaggregated large cells that lie in the substantia innominata and which in places intrude upon the preoptic regions and the nucleus of the diagonal band of Broca. Together, these may form a complex that receives inputs from a variety of brainstem sources, and projects widely and diffusely upon all cortical structures of the telencephalon.

Ontogenesis of hypothalamic--pituitary--thyroid function and metabolism in man, sheep, and rat.

Abstract: Publisher Summary This chapter discusses the maturation of hypothalamus and thyroid function It also discusses the maturation of the neurovascular communication system between the hypothalamus and pituitary in a newborn rat It is a progressive process continuing throughout the period of hypothalamic maturation By 17 days of gestation, the pituitary gland is surrounded by a capillary network and separated from the brain by a thin layer of mesenchyme and capillaries—this is referred to as the supra-tuberal plexus, or the secondary plexus of the portal system The chapter illustrates Triiodothyronine (T3) production rates in rats at different times after birth In the illustration, T3 production rates, per 100 body weight are plotted as mean and SEM The general pattern of ontogenesis of the hypothalamic-pituitarythyroid system in the human, sheep, and rat species is similar The timing of the several stages relative to parturition differs in the rat Human and sheep fetuses are delivered toward the end of the stage of development of neuroendocrine control and early in the period of maturation of tissue metabolic systems In contrast, the rat is delivered soon after completion of embryogenesis of the thyroid and pituitary glands early in stage II maturation of the hypothalamus

Hormone concentrating cells in vocal control and other areas of the brain of the zebra finch ( Poephila guttata).

Abstract: Using the autoradiographic method in the zebra finch (Poephila guttata), areas of the brain were identified which contain cells which accumulate testosterone or its metabolites after intramuscular injection of tritiated testosterone. Among these areas are the caudal nucleus of the hyperstriatum ventrale, nucleus intercollicularis of the midbrain, and the tracheosyringeal portion of the hypoglossal nucleus of the medulla (nXIIts). These three are known to control or influence androgen dependent song and other vocalizations of passeriform birds, and nXIIts is composed of the motoneurons innervating the vocal (syringeal) muscles. Other areas containing hormone-concentrating cells are the medial preoptic area, nucleus periventricularis magnocellularis of the hypothalamus, dorsal infundibular layers, dorsomedial thalamus, lateral septum, magnocellular nucleus of the anterior neostriatum, periventricular medial neostriatum, nucleus taeniae of the archistriatum, and ventral paleostriatum augmentatum. Accumulation by cells in the preoptic area, hypothalamus, and limbic forebrain is consistent with a general vertebrate pattern of distribution of brain cells which accumulate sex steroids. Some of these same areas may be involved in the control of androgen dependent events such as courtship, copulation, aggression, and feedback regulation of the hypophysis.

The ontogenesis of pituitary hormones and hypothalamic factors in the human fetus: maturation of central nervous system regulation of anterior pituitary function.

Abstract: Publisher Summary Early in fetal life there is relatively autonomous secretion of hypothalamic hypophysiotropic releasing factors and later in development the maturation of inhibitory or restraining influences, mediated via the central nervous system (CNS), modulates pituitary secretion. Even at birth, CNS regulation of pituitary function is incomplete. Therefore, the maturation of CNS control of anterior hypophysial function can be looked upon as a continuum, extending from fetal life into a variable period in infancy and in relation to gonadotropin secretion, into adolescence. This chapter emphasizes on the limitations of measuring pituitary content and serum concentration of hormones in human fetuses obtained largely after therapeutic abortion. It also provides an overview of some of the factors that can influence the plasma concentration of pituitary hormones. The chapter discusses the morphological changes in the hypothalamic-pituitary complex in the fetus, human fetal hypothalamic-hypophysiotropic factors, hypothalamic content and concentration of releasing factors in human fetuses, human fetal pituitary hormones, placental polypeptide hormones, and pituitary hormones in anencephaly. The developmental pattern of secretion of fetal pituitary hormones is not only diverse, but the hormones apparently develop independently of each other.

Effect on various putative neurotransmitters on the secretion of corticotrophin-releasing hormone from the rat hypothalamus in vitro-a model of the neurotransmitters involved.

Abstract: The effect of incubating the hypothalamus of adult male rats with various neurotransmitters upon the release of corticotrophin-releasing hormone (CRH) was studied. The CRH activity in the incubation medium was assayed in 48 h median eminence-lesioned rats and the corticosteroidogenesis of excised adrenals in vitro was used as the end-point. 5-Hydroxytryptamine (100 pg/ml-10ng/ml) caused a dose-dependent release of CRH which was antagonized by methysergide (30-100 ng/ml). The response to 5-hydroxytryptamine was also inhibited by hexamethonium and atropine which indicated that it was acting through a cholinergic interneurone. Melatonin (10 ng) did not alter the basal release of CRH but inhibited the action of both 5-hydroxytryptamine (10 ng) and acetylcholine (3 pg). Thus it appears that both 5-hydroxytryptamine and melatonin play a role in the control of CRH release. Noradrenaline blocked the release of CRH induced by both acetylcholine and 5-hydroxytryptamine and presumably this inhibition was caused by direct action on the CRH neurone. gamma-Aminobutyric acid (GABA) also inhibited the release of CRH and may also be involved in the regulation of CRH secretion. The inhibitory neurotransmitters, noradrenaline, GABA and melatonin, act via independent receptor mechanisms. A model based on the above data is presented.

The effect of dopamine on neurohypophysial hormone release in vivo and from the rat neural lobe and hypothalamus in vitro.

Abstract: 1. The rat hypothalamus (containing the supra-optic nuclei, paraventricular nuclei, median eminence and proximal pituitary stalk) has been incubated in vitro and shown to be capable of releasing the neurohypophysial hormones, oxytocin and arginine vasopressin, at a steady basal rate about one twentieth that of the rat neural lobe superfused in vitro. 2. The hypothalamus and neural lobe in vitro released both hormones in a similar arginine vasopressin/oxytocin ratio of about 1-2:1. However, when release was expressed relative to tissue hormone content, the hypothalamus was shown to release about three times as much arginine vasopressin and six times as much oxytocin as the neural lobe. 3. Dopamine in a concentration range of 10(-3)-10(-9)M caused graded increases in hormone release from the hypothalamus in vitro to a maximum fivefold increase over preceding basal levels. The demonstration that apomorphine also stimulated hormone release whereas noradrenaline was relatively ineffective suggested that a specific dopamine receptor was involved. A separate cholinergic component in the release process was indicated by the finding that acetylcholine stimulated release to a maximum fivefold increase in concentrations of 10(-3)-10(-9)M. 4. The fact that the isolated hypothalamus can be stimulated by dopamine and acetylcholine to release increased amount of oxytocin and arginine vasopressin raises the question of the origin and fate of the hormones released in this way. The possibility that they could be released into the hypophysial portal circulation from median eminence to affect the anterior lobe of the pituitary is discussed. 5. In similar doses, both dopamine and noradrenaline injected into the lateral cerebral ventricles of the brain of the anaesthetized, hydrated, lactating rat caused the release of arginine vasopressin and oxytocin. Apomorphine release both hormones but at a higher dose level and to less effect than the catecholamines. 6. The hormone release induced in vivo by dopamine could be prevented by the prior administration of haloperidol or phentolamine and these antagonists were equally effective in blocking the hormone release due to noradrenaline. The involvement of a specific dopamine receptor was more clearly implicated by the use of pimozide which completely inhibited the hormone release due to dopamine and apomorphine but not that due to noradrenaline. 7. It is suggested that the release of neurohypophysial hormones can be stimulated via a dopaminergic nervous pathway in addition to a cholinergic one. The possibility that the osmoreceptor mechanism for the release of antidiuretic hormone from the neural lobe of the pituitary may involve such a dopaminergic pathway is discussed.

Dopamine/Neuroleptic Receptors in Basal Hypothalamus and Pituitary

Abstract: In order to determine whether the basal hypothalamus or the pituitary (or both) is the likely locus of action of the tuberoinfundibular (TI) dopamine neurons, these regions were examined for dopamine and neuroleptic receptors. High affinity receptors for haloperidol and dopamine were found in the rat pituitary while none were detected in rat basal hypothalamus. The relative ability of two neuroleptics, chlorpromazine and haloperidol, to displace (3H)haloperidol from the receptor in monkey pituitary is similar to that for rat striatum. The lack of receptors capable of binding (3H)haloperidol or (3H)dopamine in the basal hypothalamus strongly suggests that the TI neurons do not produce postsynaptic effects in this region. The pituitary receptors for (3H)haloperidol and (3H)dopamine have the characteristics of a functional system. The presence of neuroleptic/dopamine receptors in the pituitary and lack of such receptors in the basal hypothalamus supports the hypothesis that dopamine may act directly as a pro...

Somatostatinergic neurons in anterior hypothalamus: immunohistochemical localization.

Abstract: The distribution of somatostatin was studied in the rat with an immunoperoxidase technique and rabbit anti-somatostatin. Somatostatinergic neurons were identified in the preoptic and anterior periventricular hypothalamus between the anterior commissure, optic chiasm, and the anterior portion of the ventromedial nucleus.

Autoradiographic localization of hormone-concentrating cells in the brain of the female rhesus monkey.

Abstract: With autoradiographic procedures, cells which bind 3H-estradiol were found in preoptic, hypothalamic and limbic structures in the brains of ovariectomized, adult female rhesus monkeys. Estrogen-binding cells were seen in the medial preoptic area, medial anterior hypothalamus, ventromedial nucleus, and especially heavy labelling was seen throughout the extent of the arcuate (infundibular) nucleus of the hypothalamus. In limbic structures, cells in the bed nucleus of the stria terminalis and in the medial nucleus of the amygdala were well labelled. Systematic charting also revealed smaller numbers of estrogen-concentrating cells in other specific hypothalamic and limbic locations. In the anterior pituitary, significant numbers of basophils and acidophils were found to bind estrogen. Pars intermedia and the posterior lobe were virtually unlabelled. In the uterus, heavily labelled cells were seen in the endometrial stroma and in the myometrium. These autoradiographic findings agree with results of parallel biochemical experiments. In monkeys injected with 3H-corticosterone, the most extensive high-intensity binding found with autoradiography was in the hippocampus. Both pyramidal neurons and dentate gyrus granule cells were labelled. Biochemical experiments, also, showed highest cell nuclear accumulation of corticosterone in the hippocampus. Findings with estradiol in the rhesus monkey extend to primates conclusions based on autoradiographic experiments with steroid sex hormones in a wide variety of vertebrates, including fish, amphibians, birds, and various mammalian species (Morrell et al., '75a). All of these vertebrate forms have sex hormone-concentrating neurons, which are found in specific preoptic, hypothalamic and limbic structures. In the species studied, such hormone-concentrating neurons appear to be involved in the hormonal control of behavioral and pituitary function.

A radioimmunoassay for growth hormone release-inhibiting hormone: method and quantitative tissue distribution.

Abstract: Summary A specific antiserum has been produced to haemocyanin conjugated synthetic growth hormone release inhibiting hormone (GHRIH). This has allowed the development of a radioimmunoassay for GHRIH sensitive to 5 pg/tube. GHRIH content in 2 m acetic acid extracts of rat tissues have been measured and show the majority to be in CNS—especially hypothalamus and septum and preoptic areas with substantial amounts in spinal cord and thalamus. Extra neurological localization in pancreas, gastric antrum, colon and thyroid have also been demonstrated.

Selective Actions of Prolactin on Catecholamine Turnover in the Hypothalamus and on Serum LH and FSH

Abstract: The effects of prolactin (PRL) administration on catecholamine turnover in various brain regions of ovariectomized rats were determined by observing the decline of dopamine and norepinephrine concentrations after α-methyltyrosine (αMT) administration PRL had no effect on the steady state concentration of dopamine in the median eminence, anterior hypothalamus and corpus striatum or on the norepinephrine concentration in the anterior hypothalamus However, PRL selectively enhanced dopamine turnover in the median eminence and anterior hypothalamus after a latent period of 10–26 h In addition, PRL administration significantly decreased serum concentrations of LH and FSH These results suggest that the PRL-induced increase in activity of dopaminergic neurons in the median eminence or anterior hypothalamus may be responsible for the reduction of the post-castration rise in serum concentrations of LH and FSH

Androgen Receptors in the Anterior Pituitary and Central Nervous System of the Androgen “Insensitive” (Tfm) Rat: Correlation Between Receptor Binding and Effects of Androgens on Gonadotropin Secretion

Abstract: The cytosol fractions of the anterior pituitary, hypothalamus, preoptic area and brain cortex of androgen “insensitive” (Tfm) rats possess androgen receptors. However, in the Tfm rats the androgen binding per mg protein was only 10–15% of that in the corresponding normal littermates (Nl). The physicochemical properties of the androgen receptors in the anterior pituitary of the Tfm rat were indistinguishable from those of the normal rat. Thus, no distinctive differences were observed with regard to electrophoretic mobility in 3.25% polyacrylamide gels, isoelectric point (pI = 5.8), binding affinity (KD = 1.5 × 10−9M), temperature stability, sulfhydryl dependence and steroid specificity. It is, therefore, likely that the very low androgen binding capacity by the anterior pituitary and the central nervous system is due to an extreme reduction in the receptor number rather than to the presence of abnormal receptors. Since in the Tfm animals the androgen receptor number is reduced by 85–90%, il: is to be expec...

Catecholamine Involvement in Episodic Luteinizing Hormone Release in Adult Ovariectomized Rats

Abstract: This study was intended to examine the role of hypothalamic norepinephrine (HNE) and dopamine (HDA) in episodic luteinizing hormone (LH) release in adult ovariectomized rats. Unrestrained, unanesthetized rats with indwelling right atrial cannulae were bled continuously (30, 50, or 100 mul of whole blood/4-6 min for 3-4 hours), and the blood samples were analyzed for LH by radio-immunoassay. In other individual rats, changes in the hypothalamic levels of norepinephrine and dopamine after drug administration were determined by a radioisotopic-enzymatic catechol-O-methyl transferase assay. alpha-Methyl-p-tyrosine significantly decreased HNE and HDA concentrations but failed to alter episodic LH release. Two dopamine-beta-hydroxylase inhibitors (U-14,624 and FLA-63) caused marked reductions in HNE, small but not statistically significant increases in HDA, and an inhibition of episodic LH secretion. Apomorphine, a dopamine receptor stimulator, caused a transient (50-60 min) but marked inhibition of episodic LH release. Saline injection had no effect. Pimozide, a blocker of dopamine receptors, prevented the inhibitory effects seen following apomorphine. Although not studied in detail, pimozide alone did not appear to alter episodic LH secretion. These data suggest that in adult ovariectomized rats norepinephrine may be an excitatory neurotransmitter in the modulation of episodic LH release. The activation of dopamine receptors may be capable of inhibiting this release process. However, the apparent inability of pimozide alone to alter episodic LH discharge suggests that under physiological conditions dopamine may not play a role in the modulation of episodic LH secretion.

Neurosecretion by Exocytosis

Abstract: Publisher Summary This chapter discusses the neurosecretion by exocytosis. The neurosecretion theory was based on the selective staining of secretory substances within some neurons of the central nervous system. Typical neurosecretory cells appear to offer special advantages for the study of controlled membrane fusion. Their secretory and electrophysiological activities lend themselves to experimental manipulation, and the elements are of a convenient size for electron microscope analysis. Neurophysins are synthesized and stored within neurosecretory granules, together with their hormone. Although hormone and neurophysin may interact noncovalently, evidence obtained by Sachs and co-workers strongly suggests that hormone and binding protein share a common precursor. Neurophysin, together with the hormone, travels down the axon from the cell body in the hypothalamus to the posterior pituitary at a rate of at least 2-3 mm per hour in the rat. Observations on the role and fate of neurophysin are also, however, relevant when exocytosis is under consideration.

Autoradiographic Localization of Estradiol-Concentrating Cells in the Female Hamster Brain

Abstract: Autoradiographic methods were used to study the location of estrogen-concentrating cells in the brain of the female hamster. In the hypothalamus, well-labelled cells were reliably found in the posterior medial preoptic area (MPOA), the anterior hypothalamus (AHA), and the ventromedial (VM), arcuate (ARC) and ventral premammillary nuclei (VPM). In the limbic system, well-labelled cells were found in the ventro-lateral septum, bed nucleus of the stria terminalis, and the medial and cortical nuclei of the amygdala. Labelled cells, in small numbers, were also detected in the mesencephalic central gray (CG), lateral hypothalamus, subiculum and entorhinal cortex. The neuro-anatomical pattern of estrogen-concentrating cells in the hamster supports the concept of a generalized vertebrate pattern. Furthermore, a comparison of hamster and rat patterns of cellular 3H-estradiol (3H-E2) concentration appears to suggest that species differences in their responsiveness to estrogen may be paralleled by differences in estrogen binding.

Edward T. Tyler Prize Oration: LH-releasing hormone and its analogues: recent basic and clinical investigations.

Abstract: Literature on luteinizing hormone-releasing hormone (LH-RH) and its analogues is reviewed LH-RH which stimulates the release of both luteinizing hormone (LH) and follicle stimulating hormone (FSH) is active in numerous species It has been postulated that the release and synthesis of FSH and LH is mediated by an interaction between sex steroids and hypothalamic LH-RH Administration of antiserum to LH-RH has been shown to block the preovulatory LH surge and ovulation in laboratory animals which suggests an immunological approach to fertility control It appears that LH-RH is rapidly broken down by dispersed liver anterior pituitary and kidney cells Cyclic AMP has been shown to act as a mediator of LH-RH and FSH release Although LH-RH has potential diagnostic value its effectiveness in differentiating various types of hypogonadism is not clearly established Generally sex steroids diminsh the responses to LH-RH though estrogens have both a stimulatory and inhibitory effect Glucocorticoids have been shown to suppress the LH response to LH-RH LH-RH has been effective in inducing ovulation in anovulatory women and in treating men with hypogonadotropic hypogonadism Stimulatory and inhibitory analogues of LH-RH and their clinical application are reviewed

Effects of morphine and methadone on serum testosterone and luteinizing hormone levels and on the secondary sex organs of the male rat.

Abstract: The effects of morphine and methadone on the endocrine control of the male rat's sexual function were examined. The results indicate that these narcotics markedly reduce the structural and functional integrity of the secondary sex organs by producing a pronounced reduction in serum testosterone levels. Serum levels of luteinizing hormone (LH) were not detectable in narcotic-treated animals, whereas serum levels of the follicle stimulating hormone (FSH) were unaltered. On the basis of these observations, it seems reasonable to conclude that the narcotics inhibit the secretion of LH, by an action either in the hypothalamus (e.g., suppression of LH-releasing hormone) or directly in the pituitary gland, which leads to a reduction in serum testosterone levels and a subsequent reduction in the wet-tissue weight and secretory activity of the secondary sex organs.

Interactions between hormones and nerve tissue.

Abstract: The interactions between steroid hormones particularly sex hormones and nerve tissue are discussed. Steroid hormones act on the genetic material of the cell nucleus. Estradiol is responsible for the reversible activation of the lordosis response and ovulation in adult female rats. When certain areas of the developing brain are exposed to hormones such as estradiol permanent structural changes in the brain lead to a male pattern of behavior in the adult. if the developing rat male brain is deprived of testosterone female behavior results. These differences are probably due to the state of differentiation of the neurons at the time of hormonal interaction. The nerve cell circuits which may be established by steroid hormone stimulation during sexual differentiation of the brain become permanent and are not susceptible to further hormonal influence. Rather the hormones affect the functional efficacy of the circuits. Further research is needed to understand the chemical properties of the target neurons necessary for sexual behavior response.