Hormonal regulation of axonal sprouting in the hippocampus

Hormonal coordination of motor output and internal prediction of sensory consequences in an electric fish

Steroid hormones remodel neural networks to induce seasonal or developmental changes in behavior. Hormonal changes in behavior likely require coordinated changes in sensorimotor integration. Here, we investigate hormonal effects on a predictive motor signal, termed corollary discharge, that modulates sensory processing in weakly electric mormyrid fish. In the electrosensory pathway mediating communication behavior, inhibition activated by a corollary discharge blocks sensory responses to self-generated electric pulses, allowing the downstream circuit to selectively analyze communication signals from nearby fish. These pulses are elongated by increasing testosterone levels in males during the breeding season. We induced electric-pulse elongation using testosterone treatment and found that the timing of electroreceptor responses to self-generated pulses was delayed as electric-pulse duration increased. Simultaneous recordings from an electrosensory nucleus and electromotor neurons revealed that the timing of corollary discharge inhibition was delayed and elongated by testosterone. Furthermore, this shift in the timing of corollary discharge inhibition was precisely matched to the shift in timing of receptor responses to self-generated pulses. We then asked whether the shift in inhibition timing was caused by direct action of testosterone on the corollary discharge circuit or by plasticity acting on the circuit in response to altered sensory feedback. We surgically silenced the electric organ of fish and found similar hormonal modulation of corollary discharge timing between intact and silent fish, suggesting that sensory feedback was not required for this shift. Our findings demonstrate that testosterone directly regulates motor output and internal prediction of the resulting sensory consequences in a coordinated manner.

Effects of gonadotropin releasing hormone and estradiol on c-fos expression in the rat hippocampus

The effects of ovariectomy, estradiol, and gonadotropin releasing hormone (GnRH) on the protein synthetic activity of hippocampal neurons were studied with immunohistochemistry for the proto-oncogene c-fos. Ovariectomy caused a reduction in the number of c-fos-positive neurons to 16% in area CA(1) and to 25% in area CA(3) when compared to that in the intact control animal. The dentate gyrus was only slightly affected. The decline in the number of c-fos-immunoreactive neurons in the Ammon's horn was partially reversed by a single intravenous injection of estradiol which resulted in the expression of c-fos in 71% of the neurons in area CA(1) and 74% in CA(3) when compared to the numbers of positive cells in the control animals. Similarly, intracerebroventricular injections of 10 muM GnRH caused an increase in the number of c-fos-positive cells to 68% in area CA(1) and to 52% in area CA(3) compared to that in the control animals. The induction of c-fos synthesis after estradiol and GnRH was transient and reached a maximum after 1 to 2 h before it declined to pretreatment levels after 8 h. The results suggest that both estradiol and GnRH exert specific effects on protein synthesis in certain neurons of the hippocampus and that these effects of the hormones are, at least in part, mediated by c-fos.

RhoA, Rac1, and cdc42 intracellular distribution shift during hippocampal neuron development

Differences in cytoskeleton organization are key determinants of the architecture and dynamics of axons and dendrites. This is most clearly illustrated by the diverse pools of microtubule-associated proteins in axons and dendrites. Whether similar polarized organization occurs for actin regulatory proteins remains to be determined. To address this issue, we analyzed the intracellular distribution of the Rho GTPases, RhoA, Rac1, and cdc42 in hippocampal neurons in culture. We report that all three Rho members are evenly distributed during the time of axon and dendrite sprouting. This is not the case in mature neurons, as RhoA enriches in dendrites, Rac1 in axons, and Cdc42 is equally abundant in both domains. Polarized segregation of the actin regulatory machinery in mature neurons might play an important role in axonal and dendritic architectural plasticity.

Effects of sex and hormonal status on astrocytic basic fibroblast growth factor-2 and tyrosine hydroxylase immunoreactivity after medial forebrain bundle 6-hydroxydopamine lesions of the midbrain dopamine neurons

We examined astrocytic basic fibroblast growth factor immunoreactivity (FGF-2-IR) and tyrosine hydroxylase immunoreactivity (TH-IR) in the cell-body region of midbrain dopaminergic neurons after unilateral infusions of the neurotoxin 6-hydroxydopamine into the medial forebrain bundle in male and female rats. In addition, to determine whether neonatal exposure to gonadal hormones has consequences on the expression of astrocytic FGF-2 and cell loss in response to injury in adulthood, we studied the effects of these lesions in adult male and female rats that had been exposed or not to testosterone in the neonatal period. In both males and females there was a progressive loss of TH-expressing cells that peaked 5 weeks after the lesions. Females showed less loss of TH-expressing cells than males, but this effect was not estrogen dependent. Lesions led to an increase in expression of astrocytic FGF-2 that was greater in males than in females. Finally, it was found that, regardless of genetic sex, rats exposed to testosterone neonatally showed greater astrocytic FGF-2 expression after lesions than those not exposed, and that among those not exposed to testosterone, estrogen treatment had a modest protective effect. Analysis of behavior and striatal dopamine content showed that the percent of striatal dopamine depletion 14 days after the lesion correlated with the amount of behavioral asymmetry displayed by animals on all tests conducted after lesioning. In groups killed 2 and 5 weeks after the lesion, the amount of behavioral asymmetry correlated with the percent loss of TH-IR cells and with the percent increase in FGF-2-IR cells in the midbrain. These relationships were not evident in groups killed 3 and 7 days after the lesion, possibly because the changes in the number of FGF-2- and TH-IR cells were not fully manifested. The present findings show that hormonal events early in life can alter the response of midbrain dopamine neurons to insult and injury in adult life and suggest that the slow degeneration of these neurons may release signals triggering a sustained activation of adjacent astrocytes which, in turn, may lead to induction of astrocytic FGF-2.

Sex differences in response to kainic acid and estradiol in the hippocampus of newborn rats

Premature and full-term human infants are at considerable risk of excitotoxic-mediated brain damage due to hypoxia-ischemia, infection or other trauma. Glutamate receptor activation is a major source of excitoxicity in the adult and developing brain, and the hippocampus is particularly vulnerable to damage. The seven-day-old rat is a widely used model of pediatric brain damage, in large part due to the relative insensitivity of the brain to exogenous glutamate treatment prior to this age. We have reexamined the possible role of glutamate in pediatric brain damage in the newborn rat using kainic acid treatment and attending to the sex of the animal as well as the effects of pretreatment with the gonadal steroid estradiol. Consistent with previous studies, we found no evidence of damage 7 days posttreatment in the CA1 region of the hippocampus in males or females. There was also little to no damage in the CA2/3 or dentate gyrus of males. In females, however, kainic-acid treatment induced substantial damage in the dentate gyrus and moderate damage in CA2/3, as assessed by neuron number and regional volume. Pretreatment with estradiol was protective against kainic acid-induced damage in females but was permissive for damage in the dentate gyrus of males. Estradiol treatment in the absence of kainic acid treatment was also neuroprotective in females in that it increased neuron number and volume throughout the hippocampal formation, suggesting that the basis of the sex difference observed in hippocampal volume was hormonally mediated. There was no effect of exogenous estradiol given to males in the absence of kainic acid. We conclude that the newborn female rat brain, but not the male, is sensitive to glutamate-mediated toxicity and that gonadal steroids play a complex role in both naturally occurring sex differences in hippocampal volume and response to injury.

Sex steroids and the brain: lessons from animal studies

Gonadal steroid hormones have multiple effects throughout development on steroid responsive tissues in the brain. The belief that the cellular morphology of the adult brain cannot be modulated or that the synaptic connectivity is "hard-wired" is being rapidly refuted by abundant and growing evidence. Indeed, the brain is capable of undergoing many morphological changes throughout life and gonadal steroids play an important role in many of these processes. Gonadal steroids are implicated in the development of sexually dimorphic structures in the brain, in the control of physiological behaviors and functions and the brain's response to physiological or harmful substances. The effect of sex steroids on neuroprotection and neuroregeneration is an important and expanding area of investigation. Astroglia are targets for estrogen and testosterone and are apparently involved in the actions of sex steroids on the central nervous system. Sex hormones induce changes in the expression of glial fibrillary acidic protein, the growth of astrocytic processes and the extent to which neuronal membranes are covered by astroglial processes. These changes are linked to modifications in the number of synaptic inputs to neurons and suggest that astrocytes may participate in the genesis of gonadal steroid-induced sex differences in synaptic connectivity and synaptic plasticity in the adult brain. Astrocytes and tanycytes may also participate in the cellular effects of sex steroids by releasing neuroactive substances and by regulating the local accumulation of specific growth factors, such as insulin-like growth factor-I, that are involved in estrogen-induced synaptic plasticity and estrogen-mediated neuroendocrine control. Astroglia may also be involved in the regenerative and neuroprotective effects of sex steroids since astroglial activation after brain injury or after peripheral nerve axotomy is regulated by sex hormones.

Neonatal androgen manipulation differentially affects the development of monoamine systems in rat cerebral cortex, amygdala and hypothalamus

Neural tissue during perinatal life is sufficiently plastic to respond to the presence of testicular androgens. Here we studied the effect of neonatal androgen manipulation (castration of male and androgenization of female rats) on monoamine neurotransmitter systems in the cerebral cortex and sexually dimorphic regions of brain (hypothalamus and amygdala). Norepinephrine (NE) and dopamine (DA) concentrations in cortex, amygdala and hypothalamus of rats were assayed by HPLC at days 25, 60, 120, 180, 240 and 300. Results show that NE levels in all groups of rats at day 25 were higher in the hypothalamus (5-9 ng/mg protein) compared to the amygdala (0.5-3 ng/mg protein) and the cortex (0.5-1 ng/mg protein). Levels of DA at day 25 in the hypothalamus and the amygdala were comparable (up to 3.5 ng/mg) but higher than in the cortex (1.25-1.75 ng/mg protein). NE and DA concentrations in sham-castrated male and androgen-treated females were higher at day 25 compared to castrated male and control females in both amygdala and hypothalamus; however, levels of NE and DA remained unchanged in the cortex. Pattern of NE concentrations started reversing with increase in age, i.e., NE levels in control females and castrated males increased almost 4-fold in amygdala and 2-fold in hypothalamus by day 300, but there was no significant change in the cortex. Compared to that, NE levels decreased in sham-castrated male (2-fold) and androgen-treated females (3-fold) in amygdala as well as in the hypothalamus (2-fold) in both these groups. A similar pattern of reversal of DA levels was found in both amygdala and hypothalamus, however, at day 300 DA levels were comparable in all the four groups. These studies suggest that androgen manipulation (castration or androgen administration) induces age-dependent short- and long-term effects on the development of noradrenergic and dopaminergic systems in the sexually dimorphic regions of brain, amygdala and hypothalamus, without a significant change in the cerebral cortex.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Dendritic branching in cortical pyramidal cells in response to ovariectomy in adult female rats: suppression by neonatal exposure to testosterone

Female Long-Evans rats were treated with oil or testosterone propionate (TP) at birth (postnatal day zero, PN0) and PN1. As adults, animals from each group were ovariectomized or sham operated. Four months later the brains were prepared using a modified Golgi-Cox staining procedure. In neonatally oil-treated females, ovariectomy in adulthood increased the dendritic arbor of layer II/III pyramidal neurons of the parietal cortex; in addition, there were modest increases in apical dendritic spine density. The dendritic arbor of the pyramidal neurons of intact neonatally TP-treated females was greater than that of intact oil-treated females, but in these animals there was no increase in dendritic arbor in response to ovariectomy.

Gender-specific impairment on Morris water maze task after entorhinal cortex lesion

After unilateral entorhinal cortex lesion, deficits on a working spatial memory Morris water maze task were examined in male and female rats to determine if gender differences exist in response to hippocampal deafferentation. Brain-damaged males showed a persistent water maze deficit that persisted throughout the 10 days of testing. Brain-damaged females did not. The performance of the injured females was only slightly impaired relative to sham males and females, and was significantly better than males with EC damage. This lack of a water maze deficit in lesion females is hypothesized to be due either to gender differences in sprouting responses or to a more flexible use of multiple cues by females relative to males.

Female circulating sex hormones and hippocampal sympathetic ingrowth

Following cholinergic denervation of the hippocampal formation, via medial septal (MS) lesions, sympathetic fibers, originating from the superior cervical ganglia, growth into the hippocampus. Previous studies have demonstrated a sexually dimorphic effect of this neuronal rearrangement on recovery of a spatial-learning task, with this rearrangement being detrimental in male but protective in female rats. Circulating male sex hormones were found to interact with this effect in male animals. In this study we assessed the role of circulating female sex hormones on the behavioral and biochemical effects of hippocampal sympathetic ingrowth (HSI). For the behavioral studies female rats underwent either sham ovariectomy (sham OVARX) or OVARX and were taught a standard radial-8-arm maze task. Following attainment of criterion, animals underwent one of three surgical procedures: sham surgery; MS lesions+sham ganglionectomy (MS); HSI group; MS lesions+ganglionectomy (MSGx). As in our previous study, animals with HSI (i.e. MS group) were found to recover learning faster (in fact, these animals did not differ from controls) than animals with MS lesions without HSI. Gonadal status did not affect this behavioral recovery. For the biochemical studies hippocampal norepinephrine (NE) and choline acetyltransferase (ChAT) were measured in animals sham OVARX and OVARX, 8-12 weeks after the neurosurgical procedure. MS lesions (i.e. MSGx; MS) were found to reduce ChAT activity, regardless of circulating sex hormones. In controls NE levels were similar between OVARX and sham OVARX. NE levels were markedly elevated in the OVARX MS group compared to all other groups including sham OVARX. In the MSGx groups, NE levels were reduced compared to controls, while comparisons between these groups revealed a significant reduction in NE levels in the OVARX MSGx group compared to sham OVARX MSGx group. These studies suggest that female circulating sex hormones interact with brain injury in a very complex manner. However, this interaction does not appear to mediate the changes in behavior observed after HSI.

Neurotrophic effects of steroids on lesion-induced growth in the hippocampus. II. Hormone replacement

The mediation of lesion-induced sprouting in the nervous system is a complex interaction of local membrane factors and circulating hormones. This series of studies examines the reactivity of the sprouting response of both male and female subjects under different hormonal conditions. Young adult male and female Sprague-Dawley rats which were gonadectomized (GDX) and adrenalectomized (ADX) underwent a unilateral entorhinal cortex lesion, which partially denervates the molecular layer of the ipsilateral hippocampal denate gyrus. At the time of the lesion, each animal received hormonal therapy. Fifteen days post-ERC-ablation the brains were analyzed for changes in reactive fiber outgrowth in the hippocampal commissural/associational afferents. Fiber outgrowth in females in the "asteroidal" (GDX/ADX) condition was unaffected. Asteroidal males demonstrated a decreased response. Gonadal steroid replacement, estrogen or testosterone, enhanced outgrowth in both asteroidal males and females. Glucocorticoid replacement suppressed outgrowth in both asteroidal males and females. Gonadal steroids clearly have neurotrophic activity which is interactive with glucocorticoid activity. Glucocorticoids under the GDX/ADX conditions in vivo have a negative impact on fiber outgrowth in both sexes. The effect of glucocorticoids is most dramatic when compared to the outgrowth of asteroidal animals without additional hormonal supplementation.

Sex-related differences in dendritic branching of cells in the prefrontal cortex of rats

Abstract Male and female Sprague-Dawley rats were given either gonadectomies or sham surgery on the day of birth. In adulthood they were sacrificed and prepared for Golgi-Cox staining. Using a camera lucida technique, layer II/III pyramidal neurons were drawn in the anterior cingulate cortex areas 1 and 3 and the agranular insular cortex and the number of dendritic branches on the apical and basilar dendrites were then summarized. The results showed that the dendritic arbor of prefrontal cortical cells varied as a function of sex and neonatal gonadectomy. In cingulate 3, castrated males had a smaller arbor on the left relative to normal males, whereas, in cingulate 1 the reverse was found. In addition, females had less apical arbor than males in the cingulate areas, whereas, in the agranular insular cortex, females had greater apical arbor than the males. Furthermore, in this region, gonadectomized groups had less arbor than their respective control groups. Finally, in the agranular insular cortex, the dendritic arbor was greater in the left hemisphere.

Neonatal exposure to gonadal hormones affects the development of monoamine systems in rat cortex

Abstract The effects of neonatal gonadectomy of male, and testosterone propionate treatment of female rat pups on levels of monoamines and metabolites in the cerebral cortex were assessed using high-performance liquid chromatography with electrochemical detection. In Experiment 1, pups were killed at 0, 4, 10 and 21 days of age and the anterior and posterior portions of cortex in each hemisphere were removed. At 21 days of age the levels of dopamine in anterior cortex were higher in males and testosterone propionate-treated females than in females and gonadectomized males. However, dopaminergic activity developed earlier in females than in males and the gonadal hormone manipulations shifted the pattern of development to that of the other sex. In Experiment 2, the effects of these same gonadal hormone manipulations on the uptake, metabolism and storage capacity of catecholamine neurons in the cingulate, agranular insular, parietal and occipital cortex were estimated at 4 and 10 days of age by considering the difference between measured catecholamines in animals pretreated with vehicle or 2.5 mg/kg reserpine and then given 100 mg/kg L-DOPA. Again, the data indicated earlier development of catecholamine neurons in females, especially in the agranular insular cortex. Dopamine was found to account for group differences; for when dopamine levels alone were considered it was found that, at 4 days of age, females had the highest levels in every area with the exception of the occipital cortex where gonadectomized males had equally high levels. These data suggest a mechanism that might account for sex differences in the development of specific cortical regions.

Hormonal contributions to sexually dimorphic behavioral development in humans

Nineteen studies on the behavioral effects of prenatal exposure to hormones administered for the treatment of at-risk human pregnancy are reviewed. Because the role of prenatal exposure to hormones in the development of human behavioral sex differences is potentially confounded by society's differential treatment of the sexes, comparisons between exposed and unexposed subjects were evaluated and summarized separately for male and female subjects. Therefore, this review focuses on data for individuals whose prenatal hormone environments were atypical relative to what is normal for their own sex. Overall, it appears that prenatal exposure to androgen-based synthetic progestin exerted a masculinizing and/or defeminizing influence on human behavioral development, whereas prenatal exposure to natural progesterone and progesterone-based synthetic progestin had a feminizing and/or demasculinizing influence, particularly among female subjects. The data on prenatal exposure to synthetic estrogen derive primarily from subjects exposed to diethylstibestrol (DES). DES-exposed male subjects appeared to be feminized and/or demasculinized, and there is some evidence that exposed female subjects were masculinized. These findings are discussed in the context of prenatal hormonal contributions to sexually dimorphic behavioral development both within and between the sexes. Recommendations for the conduct of future research in developmental behavioral endocrinology are presented.

Transient elevation of estrogen receptors in the neonatal rat hippocampus

The presence of sex differences in hippocampal morphology and function suggests that this brain region may be sensitive to the organizational actions of gonadal steroids. We therefore examined the postnatal development of estrogen receptor (ER) in the rat hippocampal formation. ER was measured by the in vitro binding of [3H]estradiol to a cytosolic preparation. Radioinert R2858 (moxestrol) was used to determine nonspecific binding. Hippocampal ER concentrations increased from birth through postnatal day (PND) 4 when levels peaked (10.05 +/- 1.2 fmol/mg protein); these were maintained through PND-7 (9.45 +/- 1.4) and declined thereafter to low levels characteristic of the adult (2.05 +/- 0.35). This ontogenic profile is similar to that found in several neocortical regions, as well as in the cingulate cortex, but is distinct from that observed in the hypothalamus, where ER levels remain high in the adult. Saturation analysis of PND-7 hippocampal cytosols demonstrated a single, high affinity binding site (Kd: 5.51 +/- 1.7 X 10(-10) M). [3H]Estradiol binding was specific in that it was displaced by radioinert R2858, diethylstilbestrol (DES), and 17 beta-estradiol but not by nonestrogenic steroids. Significantly greater ER levels were found in hippocampal nuclear extracts from DES-treated PND-7 animals compared to controls (9.74 +/- 2.27 vs. 0.49 +/- 0.24 fmol/mg DNA, P less than 0.01). The presence of functional ER was also shown by the ability of receptors to be retained on DNA cellulose. DNA cellulose column chromatography elution profiles for PND-7 hippocampal and medial basal hypothalamic (MBH) cytosols following incubation with [3H]estradiol were similar. The presence of elevated hippocampal ER levels during the perinatal critical period and evidence of functional transformation to the DNA binding state following DES treatment in vivo or estrogen incubation in vitro suggests that the hippocampus is a potential substrate for estrogen-mediated organizational events.

The effect of gonadal steroids on the behavioral and biochemical effects of hippocampal sympathetic ingrowth

Following cholinergic denervation of the hippocampal formation, via medial septal lesions, sympathetic fibers, originating from the superior cervical ganglia, grow into the hippocampus. Previous studies have demonstrated a detrimental effect of these fibers on recovery of a spatial-learning task in male but not female animals. In this study we assessed the role of circulating male sex hormones on the behavioral and biochemical effects of hippocampal sympathetic ingrowth (HSI). For the behavioral studies male Sprague-Dawley rats underwent either sham gonadectomy or gonadectomy and were taught a standard radial 8-arm maze task. Following attainment of criterion animals underwent one of three surgical procedures: sham surgery, MS lesions, MS lesions + ganglionectomy. MS lesions, regardless of the presence of HSI, were found to severely impair reacquisition of the task in both sham-gonadectomized and gonadectomized animals. As expected, in the sham-gonadectomized group, MSGx animals reacquired the task faster than the MS group. However, gonadectomy was found to block the detrimental effect of HSI on behavior. For the biochemical studies hippocampal norepinephrine (NE) and choline acetyltransferase (ChAT) were measured eight weeks after surgery in sham-gonadectomized and gonadectomized animals. MS lesions were found to significantly reduce the ChAT activity, regardless of circulating sex hormones. Gonadectomy was found to significantly reduce the level of NE associated with HSI, while having no effect on central NE in CON or MSGx animals. These studies suggest that circulating male sex hormones can influence both the behavioral and biochemical processes associated with HSI.

Organizational role of testosterone on the biochemical and morphological development of the hypogastric ganglion

Previous reports have demonstrated that after postnatal day 10 testosterone influences hypogastric ganglion (HG) development by 'activating' morphological and biochemical indices. We now report an 'organizational' influence on the developing HG during the first 10 postnatal days. To investigate the organizational effects of testosterone, male rats were castrated within 12 h of birth. Testosterone replacement therapy initiated following castration maintained the normal number of neurons in the HG. Conversely, delayed replacement therapy starting at day 10 or vehicle treatment only, resulted in a significant decrease in neuron number. Castration also produced a significant decrease in somal and nuclear cross-sectional areas. Testosterone replacement, whether initiated immediately or if delayed until day 10, restored somal and nuclear cross-sectional areas to normal. Tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT) activities were sensitive to both testosterone dosage and the time of administration. Testosterone decanoate administered subsequent to castration was not able to completely reverse the enzyme activity deficits, while delayed replacement therapy was even less effective in restoring enzyme activities. In contrast, higher doses of testosterone completely reversed enzyme activity deficits, and in fact produced a significant increase in TH activity. Again, delayed testosterone replacement did not fully restore deficits in enzyme activity. In summary, the hormonal environment during the first 10 days of life is critical for the organization of HG cell number; in contrast, nuclear and cell size appear to be dependent on testosterone for activation. TH and ChAT activities also appear to be organized during this dose- and time-dependent developmental period.

Hypogastric ganglion perinatal development: evidence for androgen specificity via androgen receptors

The hypogastric ganglion (HG) has previously been shown to be sensitive to both the organizational and activational influences of testosterone. The current investigations examined whether testosterone exerts similar effects prenatally, whether these events are specifically controlled by androgen, and whether androgens might directly masculinize the HG. Prenatal treatment with an anti-androgen, flutamide, resulted in significant decreases in the adult levels of tyrosine hydroxylase (TH) activity, an index of postsynaptic noradrenergic ontogeny, and choline acetyltransferase (ChAT) activity, a marker for presynaptic terminal formation. In addition, testosterone propionate and dihydrotestosterone benzoate reversed the effects of neonatal castration on the development of TH and ChAT activities. In contrast estradiol benzoate was unable to restore enzyme activities. To determine whether the above observations might be produced by direct effects on the HG, androgen cytosol receptor characteristics were studied. Competition and saturation analyses demonstrate that the affinity and specificity of the androgen cytosol receptor in the HG are similar to that displayed in the pituitary, which has previously been shown to contain androgen receptors. These results suggest that the adult levels of TH and ChAT activities are organized during prenatal and early postnatal development. In addition, the organization of the HG appears to be androgen specific. The presence of cytosol androgen receptors suggests that the organizational effects of androgens are possibly induced by a direct mechanism.

Prenatal ethanol exposure impairs lesion-induced plasticity in a dopaminergic synapse after maturity

This study examined the consequences of alcohol (ethanol) exposure during fetal life on lesion-induced dopaminergic synapse responsiveness (plasticity) in the olfactory tubercle of the adult rat. Normally, in the olfactory tubercle, olfactory bulbectomy elicits alterations in pre- and postsynaptic dopaminergic markers, including, respectively, (1) increased tyrosine hydroxylase activity and immunoreactivity, which is associated with dopaminergic axon sprouting, and (2) increased dopaminergic receptor density and potentiated dopamine activation of adenylate cyclase. We have utilized biochemical and quantitative immunocytochemical methodology to examine these synaptic markers in olfactory bulbectomized or sham-operated adult rats. These animals were offspring of dams which were administered one of the following diets during pregnancy: (1) liquid diet containing 35% ethanol-derived calories ad libitum; (2) liquid diet containing an isocaloric amount of maltose-dextrin instead of ethanol, pair-fed; or (3) unaltered liquid diet ad libitum. The results show that prenatal alcohol exposure leads to suppression of the lesion-elicited dopaminergic synapse responsiveness in the olfactory tubercle. There were no significant differences between offspring born to control and pair-fed animals, indicating that the observed abnormalities were not due to alterations in their nutritional status. In conclusion, the present data are a biochemical and quantitative immunocytochemical demonstration of impaired lesion-induced synaptic responsiveness. This renders a new dimension in support of previous evidence indicating that prenatal alcohol exposure leads to altered neuroanatomical, neuroendocrinological and behavioral responsiveness to various challenges. Such impaired synaptic responsiveness may underlie brain functional abnormalities characteristic of fetal alcohol syndrome.

Neurotrophic effects of steroids on lesion-induced growth in the hippocampus. I. The asteroidal condition

Young adult male and female Sprague-Dawley rats were randomly assigned to be gonadectomized (GDX), adrenalectomized (ADX), gonadectomized and adrenalectomized (GDX/ADX) or left intact. One week following initial surgery all animals were subjected to a lesion of the entorhinal cortex (ERC) which partially denervates the ipsilateral hippocampal dentate gyrus. Fifteen days after the ERC ablation, the brains were analyzed for changes in reactive outgrowth of the hippocampal commissural-associational (C-A) afferents. Under intact conditions there were no differences between male and female subjects. Female subjects demonstrated a suppression of sprouting following GDX alone and an enhancement of growth following ADX alone. Reactive growth in male subjects was unaffected by GDX or ADX alone. When placed in the 'asteroidal' (GDX/ADX) condition, reactive outgrowth of male subjects is significantly impaired as compared to female subjects. The results suggest a complex inter-relationship between the gonadal and adrenal hormones and a possible neurotrophic effect for the gonadal steroids.

Autoradiographic localization of estradiol-binding neurons in the rat hippocampal formation and entorhinal cortex

This study has examined the distribution of [3H]estradiol and [1 alpha,2 alpha-3H]testosterone uptake in the hippocampal formation and entorhinal cortex of male and female rats. In both males and females, [3H]estradiol-binding neurons in Ammon's horn are located deep in stratum pyramidale and may correspond either to polymorphic interneurons or to early maturing pyramidal cells. Interneurons of strata oriens, lucidum and radiatum of Ammon's horn and of stratum moleculare of the subiculum also bind [3H]estradiol, as do basket cell interneurons in the polymorphic, infragranular layer of the dentate gyrus. While no granule cells appear to accumulate [3H]estradiol, these cells may be affected transsynaptically by gonadal steroids via their afferent contacts with the entorhinal cortex, which, of the areas examined, contains the greatest number of [3H]estradiol-binding neurons. While relatively few neurons concentrate [3H]estradiol in the hippocampal formation, these are localized to specific subpopulations, which may enhance their functional significance. Because there is no significant nuclear accumulation of [3H]-alpha-testosterone in either the entorhinal cortex or hippocampal formation, it appears that aromatase enzyme activity is not a major contributor to estrogen receptor occupancy in adult rats.

Role of gender in the behavioral effects of peripheral sympathetic ingrowth

Following cholinergic denervation of the hippocampal formation, peripheral sympathetic nerves originating from the superior cervical ganglia grow into the hippocampus. As gender is known to alter the anatomy of hippocampal sympathetic ingrowth, we assessed the effect of this variable on the behavioral recovery following ingrowth. Adult male or female rats were trained on a standard version of a radial-8-arm maze task until they reached a specific learning criterion. Animals from each sex then underwent one of three surgical procedures: sham surgery, medial septal lesions plus superior cervical ganglionectomy, or medial septal lesion plus sham ganglionectomy. Reacquisition of the maze was then assessed. Prior to surgery, male animals acquired the task significantly faster than female animals. Following surgery male and female rats recovered overall performance at similar rates. However, marked group differences were observed. In males, the control group recovered faster than the group with medial septal lesion plus ganglionectomy, which recovered faster than the medial septal lesion group. In females, the control group recovered faster than the medial septal lesion group, which in turn recovered faster than the medial septal ganglionectomy group. The results of this study clearly demonstrate that gender can influence the behavioral effects of hippocampal sympathetic ingrowth. We believe that this is the first report in which gender has been shown to alter the behavioral effect of a neuronal reorganization.

Biochemical and morphological effects of castration on the postorganizational development of the hypogastric ganglion

The biochemical and morphological development of the sympathetic hypogastric ganglion (HG) was examined subsequent to postnatal castration at 10-11 days of age. Previous studies suggested that tyrosine hydroxylase (T-OH) activity, an index of noradrenergic maturation, and choline acetyltransferase (ChAT) activity, a marker for preganglionic terminal formation, were dependent on gonadal hormones during normal ontogeny. In the present studies, morphometric analyses of the HG revealed that the cross-sectional area of the cell soma and nucleus were significantly reduced following postnatal castration at day 10. Conversely, castration produced no change in the number of HG neurons. In addition, postnatal castration prevents the development of postsynaptic T-OH activity to a greater extent than ganglionic protein resulting in a significant loss of T-OH specific activity. In contrast, presynaptic ChAT activity was reduced in parallel with ganglionic protein, thus ChAT specific activity was unchanged. Testosterone replacement therapy, even in groups where treatment was delayed for up to 2 weeks after castration, completely reversed deficits in both T-OH and ChAT activities. These studies suggest that altered development of ganglion protein subsequent to postnatal castration is related to decreases in the size of neurons and not to the loss of neurons. The lack of cell loss also suggest that decreased levels of postsynaptic T-OH activity results from a loss of enzyme activity per cell and the decreased levels of ChAT activity probably represent fewer presynaptic terminals per neuron. In addition, delayed testosterone replacement subsequent to castration was effective in restoring enzyme activities suggesting an 'activational' not 'organizational' role for testosterone after postnatal day 10.

Sexually dimorphic development of cholinergic enzymes in the rat septohippocampal system

Cholinergic enzyme activity is sexually dimorphic in the rat hippocampal formation. Acetylcholinesterase (AChE) activity is greater in females than males in Ammon's horn/subiculum on the day of birth, but is equivalent in males and females at all older ages, suggesting an earlier maturation. Choline acetyltransferase (ChAT) activity also reaches adult levels earlier in female septum, and at day 18 and in adults is greater in the female dentate gyrus compared to males. Hippocampal weight relative to body weight is consistently greater in females at all ages, particularly in Ammon's horn/subiculum. Such regional sex differences during development and in adulthood suggests that cholinergic enzyme activity is regulated locally in the hippocampal target tissue.

Sympathetic sprouting in the central nervous system: a model for studies of axonal growth in the mature mammalian brain

Sympathetic fibers innervate many peripheral tissues but are normally confined to extracerebral structures within the cranial cavity, e.g. blood vessels. The invasion of the central nervous system by vascular sympathetic axons is a unique example of neuronal plasticity which provides new information concerning the regulation and mechanisms of neuronal sprouting in both the peripheral and central nervous systems. In this paper, the principal findings concerning the conditions under which such sprouting occurs, the mechanisms which may be involved, and the question of its possible function are reviewed. Of special interest is the fact that a nerve growth factor-like brain factor may be involved in this growth response. The principles gleaned from studies of this sprouting phenomenon may be applicable to other models of neuronal plasticity and may have clinical relevance.

Altered sympathetic-salivary gland development: delayed response to postnatal castration

These studies defined the normal and hormonally altered development of activity for tyrosine hydroxylase (T-OH), the rate-limiting enzyme in catecholamine biosynthesis (Levitt et al., 1965), and choline acetyltransferase activity (CAT) in the male rat superior cervical ganglion (SCG). Additionally, salivary gland weight was monitored. Two distinct developmental plateaus for postsynaptic T-OH activity exist. The first plateau represents the prepubertal level, which is significantly lower than the second postpubertal plateau. In contrast, presynaptic CAT activity displayed only a single plateau, commencing at approximately 45 days of age. The effects of postnatal castration (at 10 or 11 days of age) on the submandibular gland and T-OH activity were delayed until after puberty. No change in T-OH activity was seen at two and four post-operative weeks between control and castrated animals; however, T-OH activity was significantly less in castrated animals at 12 and 16 post-operative weeks. Testosterone replacement reversed the effect of castration on T-OH activity. Conversely, CAT activity in the SCG was unchanged by postnatal castration for at least 16 post-operative weeks, the longest time point studied. The failure of castrated animals to display the normal developmental increase in T-OH activity following puberty was comparable with the effect of castration on the development of submandibular salivary gland weight. These results suggest that in postpubertal male rats, development of T-OH activity in the superior cervical ganglion is influenced by testosterone. The parallel effects of castration on submandibular gland weight imply that testosterone regulates T-OH activity via an indirect mechanism. In contrast to noradrenergic enzyme activity and target tissue size, the ontogeny of presynaptic CAT activity appears to be insensitive to testosterone levels.

Sex steroids promote neurite growth in mesencephalic tyrosine hydroxylase immunoreactive neurons in vitro

The influence of steroid hormones on the differentiation of catecholaminergic and serotonergic (5-HT) neurons was studied in dissociated cell cultures from embryonic day 14 (E14) rat diencephalon, mesencephalon and metencephalon treated for 6 days with 17 beta-estradiol (E), testosterone (T), 5 alpha-dihydrostestosterone (DHT), progesterone (P), dexamethasone (DEX), or E + T. The effects of these hormones on morphologic differentiation were determined by morphometric measurements of total length of neurites of immunocytochemically identified neurons in culture, which were stained with antisera against tyrosine hydroxylase (TH) or 5-HT. A significant increase in neurite length was observed in cultures of TH-immunoreactive (TH-IR) neurons from the mesencephalon treated with E, T, E + T, but not with P, DHT or DEX. Based on labeling with [3H]dopamine (DA) uptake and competition with specific inhibitors, these mesencephalic TH-IR cells appear to represent DA neurons of the A8-A10 groups (which includes the substantia nigra). No statistically significant effects of these steroids were observed on TH-IR neurons from the diencephalon (assumed to be precursors of the tuberoinfundibular and incertohypothalamic dopaminergic groups). The 5-HT neurons of the raphe nuclei (metencephalon) showed no statistically significant response to steroids. We conclude that during the early fetal period, sex steroids can affect the morphologic differentiation of mesencephalic DA neurons in vitro, indicating that these hormones are capable of selectively influencing the development of a specific population of monoamine neurons during this critical period.

Long-term effects of nerve growth factor and neural transplants on behavior of rats with medial septal lesions

The present experiment investigated the interaction between exogenous nerve growth factor (NGF) and intrahippocampal septal grafts on the behavior of rats after a medial septum lesion. Young female rats received a bilateral injection of a fetal septal cell suspension into the dorsal hippocampus either immediately (immediate grafts) or 8 days after the lesion (delayed grafts). For delayed grafts, a higher concentration of endogenous neurotrophic factors can be assumed to be present in the deafferentated host tissue at the time of transplantation. One group of rats with lesions received NGF with the immediate grafts, another group received NGF alone. A sham-operated group and 3 groups with lesions (and given either immediate or delayed intrahippocampal saline injections, or no other treatment) constituted controls. The animals were tested for spontaneous alternation and for performance in a radial 8-arm maze, 1, 5 and 9 months postoperatively. Medial septal lesions reduced spontaneous alternation but, 9 months after surgery, recovery was observed in both lesion-control rats and in rats with delayed grafts (but not with immediate grafts). In the radial maze task, lesions produced a persistent impairment, although both immediate and delayed grafts reduced this deficit several months after surgery (more markedly and rapidly in the case of delayed grafts). NGF, however, increased the maze learning deficit especially 5 months postoperatively. These latter results are in contrast to findings of earlier studies showing transient beneficial effects of NGF administration. It is suggested that the effects of NGF in the present study might be due to an enhanced sprouting of sympathetic fibers into the hippocampal formation.

Hippocampal sympathetic ingrowth in rats and guinea pigs: quantitative morphometry and topographical differences

Sympathetic ingrowth is an unusual neural rearrangement in response to damage of the septohippocampal pathway in which peripheral noradrenergic nerves grow into the hippocampal formation. Hippocampal ingrowth has been extensively studied in rats and has been suggested to be regulated by the mossy fibers of the dentate granule cells, hippocampal interneurons, or glial cells. Sympathetic ingrowth was found to occur in both rats and guinea pigs; however, a discrepancy between the species was observed in the topographical distribution of sympathetic ingrowth. Ingrowth fibers were found in the dentate hilus and area CA3 of guinea pigs and rats. However, in the guinea pig fibers extended into area CA1. Quantitative estimates of fiber number confirmed these observations and identified significant differences between the species in the intrahippocampal lamellar distribution of ingrowth fibers. The topographical differences in sympathetic ingrowth could not be explained by differences in the distribution of the mossy fibers (Timms stain), cholinergic septal afferents (anterograde HRP), or in hippocampal interneurons (GAD-immunoreactive neurons). These species differences are challenging to current theories concerning the regulation of sympathetic ingrowth and may provide a useful model for testing further hypotheses about axonal guidance and target selection.

Incoming synapses and size of small granule-containing cells in a rat sympathetic ganglion after post-ganglionic axotomy

A quantitative ultrastructural study has been made of the reaction of the incoming synapses of small granule-containing cells after axotomy of the major post-ganglionic branches of the superior cervical ganglion of the young adult rat. These cells are intrinsic and interneurone-like in this ganglion, receiving a preganglionic input and giving outgoing synapses to principal post-ganglionic neurones. Unlike their outgoing synapses, which are lost after post-ganglionic axotomy (Case & Matthews, 1986), the incoming synapses of the small granule-containing cells in axotomized ganglia increased in incidence post-operatively. The increase first became clearly evident 5-7 days post-operatively and was greater, being both more sustained and progressive, after bilateral than after unilateral axotomy. After bilateral axotomy the incidence of incoming synapses rose to more than four times that of normal ganglia and was still elevated at 128 days post-operatively, but was within normal limits at 390 days. After a unilateral lesion, increases of similar extent and time course to those in the axotomized ganglia were seen in the incoming synapses of small granule-containing cells in the uninjured contralateral ganglia. The incoming synapses of the small granule-containing cells are multifocal, i.e. show several points or active foci of synaptic specialization. The increase in synapses expressed itself both through an increased incidence of these synaptic active foci per nerve terminal and through an increase in the number of presynaptic nerve terminal profiles associated with the cells. Control observations indicated that the increase in synapses was not due to surgical stress, nor was it attributable solely to post-operative ageing. The nerve terminals which were presynaptic to the small granule-containing cells post-operatively were all of preganglionic origin: no incoming synapses or presynaptic nerve terminals remained at 2 days after a preganglionic denervation of axotomized or contralateral ganglia, at whatever stage this was performed throughout the range of survival intervals. There was some evidence that the synapses had increased by sprouting, including terminal sprouting, of the preganglionic nerve fibres. In the shorter term there was an increase in the proportion of small nerve terminal profiles. In the longer term the mean size of the terminal profiles increased, and very large terminals of unusual form were seen. After post-ganglionic axotomy, and in particular after a bilateral lesion, the small granule-containing cells became hypertrophied.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic remodelling in arcuate nucleus after injection of estradiol valerate in adult female rats

Adult female rats were injected with a single dose (20 mg/kg) of estradiol valerate (EV). The number of synapses was evaluated in thin sections of arcuate nucleus fixed 3, 8, 16 and 32 weeks after EV treatment and compared with the values obtained in the arcuate nucleus of uninjected proestrus control rats. By 8 weeks after EV treatment a significant (P less than 0.001) decrease was found in the number of axo-somatic and axo-dendritic synapses on dendritic shafts, but not in the number of axo-dendritic synapses on dendritic spines. However, by 32 weeks postinjection, the number of axo-somatic and axo-dendritic synapses had returned to control values. This transient decrease in the number of synapses was preceded by a massive appearance of neuronal degenerative images by 3 weeks after EV injection. These results are interpreted as reflecting a process of circuitry remodelling in the arcuate nucleus after a neuronal lesion induced by estrogen.

Hormonal regulation of adult sympathetic neurons: the effects of castration on tyrosine hydroxylase activity

The effects of the hormone testosterone on neurotransmitter synthesis in peripheral sympathetic ganglia were examined in adult male Sprague-Dawley rats. Tyrosine hydroxylase (T-OH), the rate limiting enzyme in catecholamine biosynthesis was examined in the hypogastric (HG), coeliac (CG), and superior cervical ganglion (SCG) subsequent to castration. Initial studies indicated that 2 weeks after surgery, HG T-OH activity fell to approximately 30% of control. In order to more clearly define the pattern of testosterone effects, HG was examined 1, 2 and 4 weeks after surgery. T-OH activity was 67%, 50% and 11% of control at these 3 respective time points, and the observed alteration in T-OH activity appeared to parallel changes in the size of pelvic target organs. Similar hormonal effects did not occur in other peripheral sympathetic ganglia; T-OH activity was unchanged in SCG and CG when examined 1 month after castration. Enzyme activity was restored following replacement therapy with testosterone, whereas the neural metabolite 17-beta estradiol was without effect. The recovery in T-OH activity was associated with partial recovery of target organ size. These studies suggest that hormonal factors regulate neurotransmitter synthesizing enzymes in adult sympathetic neurons and may do so via consequences of alterations in target organs. These observations parallel similar events in the developing nervous system.

Behavioral sensitization: characterization of enduring changes in rotational behavior produced by intermittent injections of amphetamine in male and female rats

Factors influencing the behavioral sensitization ("reverse tolerance") produced by intermittent amphetamine (AMPH) injections were studied by quantifying rotational behavior in rats that had a unilateral 6-hydroxy-dopamine lesion of the substantia nigra. The results indicate that: a single injection of a low dose of AMPH enhances rotational behavior induced by a second injection of AMPH for up to 12 weeks; multiple, weekly injections of AMPH produce a progressive enhancement in rotational behavior, over-and-above that produced by a single injection; female rats show more robust sensitization than males following single or multiple injections of AMPH; this sex difference may be due to the suppression of sensitization by an androgen, because removal of testicular hormones potentiates sensitization; the long-lasting sensitization of rotational behavior produced by infrequent injections of AMPH is not due to drug-environment conditioning effects, but perhaps to a persistent AMPH-induced change(s) in brain catecholamine systems; and a simple change in DA receptors is probably not involved, because the sensitization produced by infrequent injections of AMPH does not influence the rotation produced by a subsequent injection of apomorphine. The results illustrate an intriguing example of neuroplasticity that may have clinical relevance.

Hormonal regulation of sympathetic neuron development. The effects of neonatal castration

The effects of neonatal castration on neuronal ontogeny were examined in peripheral sympathetic ganglia in male Sprague-Dawley rats. Tyrosine hydroxylase (T-OH) activity, the rate-limiting enzyme in catecholamine biosynthesis and a marker of noradrenergic maturation, was examined in the hypogastric (HG) and superior cervical ganglion (SCG). Initial studies characterized the normal development of T-OH activity in HG ganglia. Neonatal castration at 10-11 days of age prevented the normal ontogeny of HG T-OH activity: T-OH activity failed to develop normally and was 17% of sham-operated littermate controls when examined at 8 weeks of age, and less than 5% when studied 10 weeks after surgery. In contrast to the effects in HG, there was no change in enzyme activity in the SCG. Replacement therapy with testosterone decanoate completely reversed the developmental alteration in enzyme activity. These observations suggest that hormonal factors modulate noradrenergic ontogeny in peripheral sympathetic ganglia but these effects appear restricted to ganglia whose targets include hormonally dependent sex organs.

Steroid suppression of axon sprouting in the hippocampal dentate gyrus of the adult rat: dose-response relationship

Various doses of adrenal steroids were given to adult male rats for 8 days following unilateral ablation of the entorhinal cortex in order to test their effects on axon sprouting. Seven days after termination of the steroid treatment the brains were analyzed for changes in the outgrowth of the commissural-associational fiber plexus in the hippocampal dentate gyrus. Analysis of the data revealed a dose-dependent suppression of the reactive fiber growth in this region of the hippocampus. Deoxycorticosterone had no suppressive effect, indicating that suppression was not related to mineralocorticoid properties. On the other hand dexamethasone, a synthetic glucocorticoid devoid of mineralocorticoid properties, had a maximal effect. These results suggest that glucocorticoids and their anti-inflammatory effects may have a significant influence on the response of the nervous system to damage.

Neonatal testosterone treatment increases neuron and synapse numbers in male rat superior cervical ganglion

Neonatal treatment with gonadal steroids has been reported to alter morphological as well as functional development in various regions of the brain and spinal cord. Among the observed alterations are changes in numbers of neurons and in the organization and numbers of synapses. These regions have been found to be sexually dimorphic, and the dimorphism dependent upon gender differences in circulating levels of gonadal steroids. Neonatal treatment with testosterone has been shown to produce an increase in the number of neurons in the superior cervical sympathetic ganglion in female rats. The present studies were designed to investigate the possibility of a normally occurring sexual dimorphism in the SCG of the rat, and to characterize the effect of neonatal treatment with testosterone on neurons and synapses in the male rat. We report a sexual dimorphism in the number of neurons but not in the number of preganglionic axons or ganglionic synapses. In addition, neonatal administration of testosterone propionate results in a 40% increase in the number of superior cervical ganglion neurons in treated male rats over the control male number at 15 and 30 days of age. The testosterone propionate treatment results in a 66% increase in the number of synapses in male superior cervical ganglia, without a concomitant increase in the number of preganglionic axons.