Ontogeny, sex dimorphism, and neonatal sex hormone determination of synapse-associated messenger RNAs in rat brain

Sex-specific effects of SNAP-25 genotype on verbal memory and Alzheimer's disease biomarkers in clinically normal older adults

INTRODUCTION

We tested sex-dependent associations of variation in the SNAP-25 gene, which encodes a presynaptic protein involved in hippocampal plasticity and memory, on cognitive and Alzheimer's disease (AD) neuroimaging outcomes in clinically normal adults.

METHODS

Participants were genotyped for SNAP-25 rs1051312 (T > C; SNAP-25 expression: C-allele > T/T). In a discovery cohort (N = 311), we tested the sex by SNAP-25 variant interaction on cognition, Aβ-PET positivity, and temporal lobe volumes. Cognitive models were replicated in an independent cohort (N = 82).

RESULTS

In the discovery cohort, C-allele carriers exhibited better verbal memory and language, lower Aβ-PET positivity rates, and larger temporal volumes than T/T homozygotes among females, but not males. Larger temporal volumes related to better verbal memory only in C-carrier females. The female-specific C-allele verbal memory advantage was evidenced in the replication cohort.

CONCLUSIONS

In females, genetic variation in SNAP-25 is associated with resistance to amyloid plaque formation and may support verbal memory through fortification of temporal lobe architecture.

HIGHLIGHTS

The SNAP-25 rs1051312 (T > C) C-allele results in higher basal SNAP-25 expression. C-allele carriers had better verbal memory in clinically normal women, but not men. Female C-carriers had higher temporal lobe volumes, which predicted verbal memory. Female C-carriers also exhibited the lowest rates of amyloid-beta PET positivity. The SNAP-25 gene may influence female-specific resistance to Alzheimer's disease (AD).

SNAP-25 isoforms differentially regulate synaptic transmission and long-term synaptic plasticity at central synapses

SNAP-25 exists as two developmentally regulated alternatively spliced isoforms, SNAP-25a and SNAP-25b. We explored the function of SNAP-25a and SNAP-25b at Schaffer collateral-CA1 synapses in hippocampus using 4-week-old wild-type (WT) and SNAP-25b-deficient (MT) mice. Characterizing the protein expression of individual SNAP-25 isoforms revealed that WT females had higher levels of SNAP-25a than WT males, suggesting a sex-dependent delay of the alternative splicing switch from SNAP-25a to SNAP-25b. MT mice expressed normal levels of total SNAP-25, Syntaxin 1A and SNAP-47 in the hippocampus, but females expressed lower levels of VAMP2. Electrophysiological recordings in in vitro hippocampal slices revealed significantly reduced magnitude of LTP in MT mice. We also found reduction in paired-pulse facilitation after induction of LTP in WT males, but not in WT females, possibly related to the difference in SNAP-25a/SNAP-25b ratios, suggesting that the splicing switch may play a sex-specific role in LTP-associated increases in presynaptic release probability. Basal synaptic transmission measured in input-output relations revealed that the ability to discriminate between the intensity of presynaptic stimuli was affected in SNAP-25b-deficient mice. Learning in a behavioural paradigm of active-avoidance was impaired in MT mice, strengthening the conclusion that SNAP-25b is important for cognitive performance by altering activity-dependent synaptic plasticity.

The Neural Mechanisms of Sexually Dimorphic Aggressive Behaviors

Aggression is a fundamental social behavior that is essential for competing for resources and protecting oneself and families in both males and females. As a result of natural selection, aggression is often displayed differentially between the sexes, typically at a higher level in males than females. Here, we highlight the behavioral differences between male and female aggression in rodents. We further outline the aggression circuits in males and females, and compare their differences at each circuit node. Lastly, we summarize our current understanding regarding the generation of sexually dimorphic aggression circuits during development and their maintenance during adulthood. In both cases, gonadal steroid hormones appear to play crucial roles in differentiating the circuits by impacting on the survival, morphology, and intrinsic properties of relevant cells. Many other factors, such as environment and experience, may also contribute to sex differences in aggression and remain to be investigated in future studies.

Variants in SNAP25 are targets of natural selection and influence verbal performances in women

Descriptions of genes that are adaptively evolving in humans and that carry polymorphisms with an effect on cognitive performances have been virtually absent. SNAP25 encodes a presynaptic protein with a role in regulation of neurotransmitter release. We analysed the intra-specific diversity along SNAP25 and identified a region in intron 1 that shows signatures of balancing selection in humans. The estimated TMRCA (time to the most recent common ancestor) of the SNAP25 haplotype phylogeny amounted to 2.08 million years. The balancing selection signature is not secondary to demographic events or to biased gene conversion, and encompasses rs363039. This SNP has previously been associated to cognitive performances with contrasting results in different populations. We analysed this variant in two Italian cohorts in different age ranges and observed a significant genotype effect for rs363039 on verbal performances in females alone. Post hoc analysis revealed that the effect is driven by differences between heterozygotes and both homozygous genotypes. Thus, heterozygote females for rs363039 display higher verbal performances compared to both homozygotes. This finding was replicated in a cohort of Italian subjects suffering from neuromuscular diseases that do not affect cognition. Heterozygote advantage is one of the possible reasons underlying the maintenance of genetic diversity in natural populations. The observation that heterozygotes for rs363039 display higher verbal abilities compared to homozygotes perfectly fits the underlying balancing selection model. Although caution should be used in inferring selective pressures from observed signatures, SNAP25 might represent the first description of an adaptively evolving gene with a role in cognition.

Neuropsycological gender differences in healthy individuals and in pediatric neurodevelopmental disorders. A role for SNAP-25

Synaptosomal-associated protein of 25 KD (SNAP-25) is a protein that participates in synaptic vesicle exocytosis through the formation of a SNARE complex; SNAP-25 also plays a pivotal role in modulating calcium homeostasis through negative regulation of voltage-gated calcium channels. SNAP-25 has been involved in different neuropsychiatric disorders, including attention deficit hyperactivity disorder. There are well known physiological gender differences in many neuropsychological skills, and there are even more striking gender differences in patients with attention deficit hyperactivity disorder and autism spectrum disorders. We hypothesize that these differences are the result of a mechanism involving SNAP-25 polymorphisms and its differential expression in specific brain areas.

Both cell-autonomous and cell non-autonomous functions of GAP-43 are required for normal patterning of the cerebellum in vivo

Growth-associated protein 43 (GAP-43) is required for development of a functional cerebral cortex in vertebrates; however, its role in cerebellar development is not well understood. Recently, we showed that absence of GAP-43 caused defects in proliferation, differentiation, and polarization of cerebellar granule cells. In this paper, we show that absence of GAP-43 causes defects in cerebellar patterning that reflect both cell-autonomous and non-autonomous functions. Cell-autonomous effects of GAP-43 impact precursor proliferation and axon targeting: In its absence, (1) proliferation of granule cell precursors in response to sonic hedgehog and fibroblast growth factor is inhibited, (2) proliferation of neuroepithelial precursors is inhibited, and (3) targeting of climbing fibers to the central lobe is disrupted. Cell non-autonomous effects of GAP-43 impact differentiated Purkinje cells in which GAP-43 has been downregulated: In its absence, both maturation and mediolateral patterning of Purkinje cells are inhibited. Both cell-autonomous and non-autonomous functions of GAP-43 involve its phosphorylation by protein kinase C. GAP-43 is phosphorylated in granule cell precursors in response to sonic hedgehog in vitro, and phosphorylated GAP-43 is also found in proliferating neuroepithelium and climbing fibers. Phosphorylated GAP-43 is specifically enriched in the presynaptic terminals of parallel and climbing fibers that innervate Purkinje cell bodies and dendrites. The cell-autonomous and non-autonomous effects of GAP-43 converge on the central lobe. The multiple effects of GAP-43 on cerebellar development suggest that it is a critical downstream transducer of signaling mechanisms that integrate generation of cerebellar structure with functional parcellation at the central lobe.

Early parental deprivation in the marmoset monkey produces long-term changes in hippocampal expression of genes involved in synaptic plasticity and implicated in mood disorder

In mood disorder, early stressors including parental separation are vulnerability factors, and hippocampal involvement is prominent. In common marmoset monkeys, daily parental deprivation during infancy produces a prodepressive state of increased basal activity and reactivity in stress systems and mild anhedonia that persists at least to adolescence. Here we examined the expression of eight genes, each implicated in neural plasticity and in the pathophysiology of mood disorder, in the hippocampus of these same adolescent marmosets, relative to their normally reared sibling controls. We also measured hippocampal volume. Early deprivation led to decreases in hippocampal growth-associated protein-43 (GAP-43) mRNA, serotonin 1A receptor (5-HT(1A)R) mRNA and binding ([3H]WAY100635), and to increased vesicular GABA transporter mRNA. Brain-derived neurotrophic factor (BDNF), synaptophysin, vesicular glutamate transporter 1 (VGluT1), microtubule-associated protein-2, and spinophilin transcripts were unchanged. There were some correlations with in vivo biochemical and behavioral indices, including VGluT1 mRNA with reward-seeking behavior, and serotonin 1A receptor mRNA with CSF cortisol. Early deprivation did not affect hippocampal volume. We conclude that early deprivation in a nonhuman primate, in the absence of subsequent stressors, has a long-term effect on the hippocampal expression of genes implicated in synaptic function and plasticity. The reductions in GAP-43 and serotonin 1A receptor expressions are comparable with findings in mood disorder, supporting the possibility that the latter reflect an early developmental contribution to disease vulnerability. Equally, the negative results suggest that other features of mood disorder, such as decreased hippocampal volume and BDNF expression, are related to different aspects of the pathophysiological process.

Sex differences in histone modifications in the neonatal mouse brain

Sex differences in neural development are established via a number of cellular processes (i.e., migration, death and survival). One critical factor identified is the neonatal rise in testosterone (T) which activates gene transcription via androgen (AR) and, after aromatization to estradiol, estrogen receptors (ERalpha and beta). Recent evidence shows that AR and ERs interact with histone modifying enzymes. Post-translational modifications of histones, including acetylation and methylation, are involved in transcriptional regulation during normal development. Therefore, we hypothesized that acetylation and/or methylation of histone H3 may underlie sexual differentiation, at least in some regions of the brain. We measured levels of acetylated (H3K9/14Ac) and trimethylated (H3K9Me3) H3 in whole neonatal mouse brains and in three regions: preoptic area + hypothalamus, amygdala and cortex + hippocampus (CTX/HIP). Sex differences in H3K9/14Ac and H3K9Me3 (males > females) were noted in the CTX/HIP on embryonic day 18, the day of birth, and six days later. To determine if T mediates these changes in H3 modifications, pregnant dams received vehicle or T for the final four days of gestation; pup brains were collected at birth. Methylation of H3 was sexually dimorphic despite hormone treatment. In contrast, H3 acetylation in the CTX/HIP of females from T-treated dams rose to levels equivalent to males. Thus, H3 modifications are sexually dimorphic in the developing mouse CTX/HIP and acetylation, but not methylation, is masculinized in females by T in utero. This is the first demonstration that histone modification is associated with neural sexual differentiation.

Regulation of complexin 1 and complexin 2 in the developing human prefrontal cortex

Complexin 1 (CX1) and complexin 2 (CX2) are presynaptic proteins that modulate neurotransmitter release and are used as markers of inhibitory and excitatory synapses, respectively. The aim of this study was to gain insight into the development of inhibitory and excitatory synapses in human prefrontal cortex (PFC) by examining the expression of CX1 and CX2 in postmortem tissues. Relative complexin protein levels were measured by Western blotting in postmortem dorsolateral prefrontal cortex (DLPFC) of 42 subjects without neurological or psychiatric disease ranging in age from 18 gestational weeks to 25 years. Samples were batched a priori into fetal, 0-12 month, 1-5 years, 6-10 years, 11-15 years, 16-20 years, and 21-25 years age groups. CX1 and CX2 expression and CX2/CX1 demonstrated a significant effect of age group by ANOVA. Group CX1 level increased progressively across development and was lowest in the fetal group and highest in the young adult group, whereas group CX2 level increased between the fetal and the 6-10 years groups and then plateaued. Consistent with these divergent patterns, there was a significant effect of age group on CX2/CX1, which was higher in fetal and infant groups than in the young adult group. Furthermore, regression analysis demonstrated linear relationships of CX1 and CX2/CX1 with age, whereas CX2 was better described as having a curvilinear relationship with age. These data indicate that complexin expression increases during synaptic maturation in human DLPFC and that an increase in the influence of inhibitory synapses relative to that of excitatory synapses occurs during development in this cortical region.

Prenatal testosterone improves the spatial learning and memory by protein synthesis in different lobes of the brain in the male and female rat

The high density of the steroid hormone receptors in the structures of temporal lobe involved in learning and memory, such as the hippocampus, perirhinal cortex, entorhinal cortex and amigdaloid complex, shows that there must be a direct relationship between gonadal hormones and organizational effects of steroid hormones in those structures during development of the nervous system. The present study was undertaken in order to investigate the effect of testosterone administration during the third week of gestation on the spatial memory formation of the offspring rats and the level of soluble proteins in the temporal lobe and frontal lobe of brain, as evidence of important organizational effects of androgens during prenatal development in brain sexual dimorphism. Animals have received testosterone undecanoate on days 14, 15, 16 and 19, 20, 21 of gestation. Learning and memory tests were started 100 days after the testosterone treatment. At the end of the experiments, the temporal and frontal lobes of brain were removed for assessing the level of soluble proteins. Testosterone treatment significantly improved spontaneous alternations percentage of male offspring in Y-maze task comparative with female offspring and reference memory in radial 8 arm-maze task (decreasing in number of reference memory errors in both male and female offspring groups), suggesting effects of both short and long-term memory. Also, testosterone significantly increased the brain soluble protein level of treated female rats in 14–16 prenatal days compared with the control group as well as the brain soluble protein level of treated male rats. These results suggest that steroid hormones play an important role in the spatial learning and memory formation by means of protein synthesis in different lobes of the brain.

Neonatal exposure to decabrominated diphenyl ether (PBDE 209) results in changes in BDNF, CaMKII and GAP-43, biochemical substrates of neuronal survival, growth, and synaptogenesis

Mammals have a marked period of rapid brain growth and development (BGS), which is postnatal in mice and rats, spanning the first 3-4 weeks of life and reaching its peak around postnatal day 10. CaMKII, GAP-43 and BDNF play important roles during the BGS in mammals. One class of flame retardants, polybrominated diphenyl ethers (PBDEs), are present and increasing in the environment and in human milk, which is also true for the only congener still in use, decabrominated diphenyl ether (PBDE 209). In the present study, the brains from 1, 3, 7, 10, 14 and 28 days old mice, were analysed for CaMKII and GAP-43. The level of CaMKII increases continuously during the neonatal period, while GAP-43 has a bell-shaped ontogeny curve, which peaks around postnatal day 10, in mouse brain. Furthermore, the effects of PBDE 209 on the developmental expression of CaMKII, GAP-43 and BDNF were examined in mice. Neonatal NMRI-male mice were orally exposed on days 3-20.1mgPBDE 209/kg body weight. The animals were euthanized 7 days after exposure to PBDE 209 and levels of CaMKII, GAP-43 and BDNF were analysed in different brain regions. The protein analysis showed that CaMKII increased significantly in hippocampus, but not in cortex, in animals 7 days after exposure to PBDE 209. GAP-43 showed a significant increase in hippocampus and a significant decrease in cortex of animals 7 days after exposure to PBDE 209. BDNF decreased significantly in hippocampus, but not in cortex, in mice 7 days after exposure to PBDE 209. This shows that PBDE 209 affects important proteins involved in normal maturation of the brain and further strengthen our findings concerning PBDE 209 as a developmental neurotoxicological agent.

Expression of estrogen receptor (ER) α and β in mouse cerebral cortex: Effect of age, sex and gonadal steroids

Estrogen receptor (ER), which mediates the multiple effects of estrogen in brain, is regulated by several factors including its own ligand. In the present study, we have examined the effect of age, sex and gonadal steroids (estrogen and testosterone) on the level of ERalpha and ERbeta in the cerebral cortex of AKR mice. Adult and old mice of both sexes were divided into four groups: intact, gonadectomized, 17beta-estradiol treated and testosterone treated. Western blot analysis showed higher level of ERalpha and ERbeta in the cerebral cortex of adult female than male mice. ERbeta level decreased significantly with advancing age in both sexes, whereas 17beta-estradiol supplementation decreased ERalpha level in old male and increased in old female, it also increased ERbeta level in old male and adult female. On the other hand, testosterone treatment decreased ERalpha level significantly in old female and ERbeta level in adult female but increased ERbeta level in male mice of both ages. Thus, these findings showed that the expression of ERalpha and ERbeta protein is differentially influenced by age, sex and gonadal steroids in the mouse cerebral cortex, suggesting differences in ER-mediated brain functions.

Differential expression pattern and steroid hormone sensitivity of SNAP-25 and synaptoporin mRNA in the telencephalic song control nucleus HVC of the zebra finch

Gonadal steroid hormones play an important role in the process of sexual differentiation of brain areas and behavior such as singing and song learning in songbirds. These hormones affect behavior controlling circuits on both the gross morphological and ultrastructural levels. Here we study whether the expression of genes coding for synaptic proteins is sensitive to gonadal steroid hormones and whether such altered expression coincides with changes in brain area size. We treated adult male zebra finches with the aromatase inhibitor fadrozole, to reduce estrogen synthesis and analyzed the mRNA expression of the synaptic proteins synaptoporin (SPO) and synaptosomal-associated protein 25 kDa (SNAP-25) in song control areas and surrounding tissues of adult male zebra finches. SPO and SNAP-25 are differently expressed throughout the song system. Generally, the telencephalic song nuclei expressed SNAP-25 at high intensity whereas SPO expression was area-specific. Elevated levels of SNAP-25 mRNA were present in the nucleus hyperstriatalis ventrale pars caudale (HVC) and in the robust nucleus of the archistriatum (RA). SPO mRNA was found in moderate levels in the HVC, in low levels in the lateral nucleus magnocellularis (lMAN) and Area X, and was absent in the RA. The treatment significantly increased the mRNA level of SPO in the HVC, whereas SNAP-25 expression level was not affected. These expression patterns are not explained by the decrease of HVC volume after treatment. The decreased HVC size is not area-specific but correlates with an overall reduction in size and an overall increase in cell density of the forebrain.

Regulation of Gene Expression in the Developing Midbrain by Estrogen: Implication of Classical and Nonclassical Steroid Signaling

Estrogen plays an important role during midbrain development. This is indicated by the presence of nuclear estrogen receptors and the transient expression of the estrogen-forming enzyme aromatase. A number of recent studies have shown that estrogen promotes the differentiation and survival, as well as physiological performance, of midbrain dopaminergic cells. In addition, we have reported that both ways of cellular estrogen signaling (classical and nonclassical) as well as interactions with nonneuronal target cells are involved in the transmission of intra- and intercellular estrogen effects in this brain region. This study provides additional evidence that (i) estrogen is capable of regulating gene expression in cultured embryonic neurons and astrocytes differently and (ii) both signaling mechanisms, i.e., classically through nuclear receptors and nonclassically through the stimulation of membrane-estrogen receptors, which are coupled to distinct intracellular signal transduction cascades, contribute diversely to gene regulation. These data reveal a high degree of complexity of estrogen action at the genomic level in the developing brain. Further studies are warranted to unravel the exact contribution of the differently regulated genes for developmental estrogen action.

A morphogenetic role for acetylcholine in mouse cerebral neocortex

Recently, cholinergic afferents to cerebral cortex have met renewed attention regarding the regulation of plasticity as well as cognitive processing. My laboratory has developed a mouse neonatal basal forebrain lesion paradigm that has contributed considerably to the understanding of cholinergic mechanisms in cortical development. We have shown that transient cholinergic deafferentation, beginning at birth, precipitates alterations in neuronal differentiation and synaptic connectivity that persist into maturity, and contribute to altered cognitive behavior. These data are in general agreement with studies in rats in which the cholinergic basal forebrain is lesioned very early in development but contrast with effects of later developmental lesions. Moreover, in mouse, both morphological and behavioral consequences of the lesion are sex dependent. Studies of receptors and secondary messengers that are instrumental in morphogenesis and plasticity suggest that sex dependent molecular alterations occur within days if not hours following cortical cholinergic deafferentation.

AMPA receptor potentiation by acetylcholinesterase is age-dependently upregulated at synaptogenesis sites of the rat brain

We have used radioligand binding to synaptic membranes from distinct rat brain regions and quantitative autoradiography to investigate the postnatal evolution of acetylcholinesterase (AChE)-evoked up-regulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in CNS areas undergoing synaptogenesis. Incubation of synaptosomal membranes or brain sections with purified AChE caused a developmentally modulated enhancement in the binding of [3H]-(S)-AMPA and the specific AMPA receptor ligand [3H]-(S)-5-fluorowillardiine, but did not modify binding to kainate neither N-methyl-D-aspartate receptors. In all postnatal ages investigated (4, 7, 14, 20, 27, 40 days-old and adult rats), AChE effect on binding was concentration-dependent and blocked by propidium, BW 284c51, diisopropylfluorophosphonate and eserine, therefore requiring indemnity of both peripheral and active sites of the enzyme. AChE-mediated enhancement of [3H]-fluorowillardiine binding was measurable in all major CNS areas, but displayed remarkable anatomical selectivity and developmental regulation. Autoradiograph densitometry exhibited distinct temporal profiles and peaks of treated/control binding ratios for different cortices, cortical layers, and nuclei. Within the parietal, occipital and temporal neocortices, hippocampal CA1 field and cerebellum, AChE-potentiated binding ratios peaked in chronological correspondence with synaptogenesis periods of the respective AMPA-receptor containing targets. This modulation of AMPA receptors by AChE is a molecular mechanism able to transduce localized neural activity into durable modifications of synaptic molecular structure and function. It might also contribute to AChE-mediated neurotoxicity, as postulated in Alzheimer's disease and other CNS disorders.

Sexual differentiation of the brain: genes, estrogen, and neurotrophic factors

Based on evidence obtained during the past 50 years, the current hypothesis to explain the sexual dimorphism of structure and function in the brain of vertebrates maintains that these differences are produced by the epigenetic action of gonadal hormones. However, evidence has progressively accumulated suggesting that genetic mechanisms controlling sexual-specific neuronal characteristics precede, or occur in parallel with, hormonal effects. 1. In cultures of hypothalamic neurons taken from gestation day 16 (GD16) embryos, treatment of sexually segregated cultures with estradiol (E2) induces axon growth in neurons from male neurons, but not from female neurons. In these cultures treatment with E2 increased the levels of tyrosine kinase type B (TrkB) and insulin-like growth factor I (IGF-I) receptors in male but not in female neurons. This and other sex differences cannot be explained by differences in hormonal environment, because the donor embryos were obtained when gonadal secretion of steroids is just beginning, before the perinatal surge of testosterone that determines development of the male brain beginning at GD17/18. 2. The response to estrogen is contingent upon coculture with heterotopic glia (mostly astrocytes) from a target region (amygdala) harvested from same-sex fetuses at GD16, whereas in the presence of homotopic glia or in cultures without glia, E2 had no effect. It was concluded that the axogenic effect of E2 depends on interaction between neurons and glia from a target region and that neurons from fetal male donors appear to mature earlier than neurons from females, a differentiated response that takes place prior to divergent exposure to gonadal secretions. 3. The effects of target and nontarget glia-conditioned media (CM) on the E2-induced growth of neuronal processes of hypothalamic neurons obtained from sexually segregated fetal donors were also studied. Estrogen added to media conditioned by target glia modified the number of primary neurites and the growth of axons of hypothalamic neurons of males but not of females. 4. Neither the Type III steroidal receptor blocker tamoxifen nor Type I antiestrogen ICI 182,780 prevented the axogenic effects of the hormone. Estradiol made membrane-impermeable by conjugation to a protein of high molecular weight (E2-BSA) preserved its axogenic capacity, suggesting the possibility of a membrane effect responsible for the action of E2. 5. Western blot analysis of the tyrosine kinase type A (TrkA), type B (TrkB), type C (TrkC), and insulin-like growth factor (IGF-I R) receptors in extracts from homogenates of cultured hypothalamic neurons showed that in cultures of male-derived neurons grown with E2 and CM from target glia, the amounts of TrkB and IGF-I R increased notably. Densitometric quantification showed that these cultures had more TrkB than cultures with CM alone or E2 alone. On the contrary, in cultures of female-derived neurons, the presence of CM alone induced maximal levels of TrkB, which were not further increased by E2; female-derived neurons in all conditions did not contain IGF-I R. Levels of TrkC were not modified by any experimental condition in male- or female-derived cultures and Trk A was not found in the homogenates. These results are compared with similar data from other laboratories and integrated in a model for the confluent interaction of estrogen and neurotrophic factors released by glia that may contribute to the sexual differentiation of the brain.

17beta-estradiol enhances cortical cholinergic innervation and preserves synaptic density following excitotoxic lesions to the rat nucleus basalis magnocellularis

Estradiol exerts beneficial effects on neurodegenerative disorders associated with the decline of cognitive performance. The present study was designed to further investigate the effect of 17beta-estradiol on learning and memory, and to evaluate its neuroprotective action on cholinergic cells of the nucleus basalis magnocellularis, a neural substrate of cognitive performance. Female rats were ovariectomized at an age of 6 months. Three weeks later they received injections of either a mid-physiological dose of 17beta-estradiol or vehicle (oil), every other day for 2 weeks. The effect of estradiol on cognitive performance was tested in two associative learning paradigms. In the two-way active shock avoidance task estradiol-replaced animals learned significantly faster, while in the passive shock avoidance test no differences were observed between the experimental groups. Subsequent unilateral infusion of N-methyl-D-aspartate in the nucleus basalis magnocellularis resulted in a significant loss of cholinergic neurons concomitant with the loss of their fibers invading the somatosensory cortex. Estradiol treatment did not affect the total number of choline-acetyltransferase-immunoreactive neurons and their coexpression of the p75 low-affinity neurotrophin receptor either contralateral or ipsilateral to the lesion. In contrast, cholinergic fiber densities in estradiol-treated animals were greater both in the contralateral and ipsilateral somatosensory cortices as was detected by quantitative choline-acetyltransferase and vesicular acetylcholine transporter immunocytochemistry. However, estradiol treatment did not affect the lesion-induced relative percentage loss of cholinergic fibers. A significant decline of synaptophysin immunoreactivity paralleled the cholinergic damage in the somatosensory cortex of oil-treated animals, whereas an almost complete preservation of synaptic density was determined in estradiol-treated rats. Our results indicate that estradiol treatment enhances the cortical cholinergic innervation but has no rescuing effect on cholinergic nerve cells in the basal forebrain against excitotoxic damage. Nevertheless, estradiol may restore or maintain synaptic density in the cerebral cortex following cholinergic fiber loss. This estradiol effect may outweigh the lack of cellular protection on cholinergic cells at the functional level.

Regional, laminar, and cellular distribution of immunoreactivity for ER alpha and ER beta in the cerebral cortex of hormonally intact, adult male and female rats

Behavioral, biochemical and anatomical studies suggest that estrogen stimulates structure and/or function in the adult cerebral cortex. The studies presented here used immunocytochemistry to map the alpha and beta isoforms of intracellular estrogen receptors (ER alpha, ER beta) in major subdivisions of adult rat cortex to identify potential sites for relevant receptor-mediated hormone actions. These studies revealed that immunoreactivity for ER alpha (ER alpha-IR) and ER beta (ER beta-IR) was present in most cortical areas, was associated exclusively with neurons, and was similar in males and females. Each receptor isoform also had its own unique distribution with respect to cortical regions, layers, and cells. In sensorimotor areas, for example, ER beta-IR was more prominent than ER alpha-IR, and was concentrated in layer V neurons that were immunoreactive for parvalbumin. In contrast, ER alpha-IR was scattered among parvalbumin-immunonegative cells in layers II/III and V/VI. Likewise, in entorhinal cortex, ER beta-IR was present in calbindin-containing cells in layers III-VI, while ER alpha-IR was restricted to small numbers of calbindin-negative neurons in infragranular layers. In sum, ER beta-IR and ER alpha-IR were differentially distributed both with respect to cortical compartments and with respect to each other. Accordingly, estrogen activation at these two sites may be anticipated to impact disparate sets of cortical circuits, cells, and functions.

Developmental sex differences in glutamic acid decarboxylase (GAD(65)) and the housekeeping gene, GAPDH

Previous work has demonstrated that the GABAergic system is involved in sexual differentiation of the rodent hypothalamus. The present study was designed to further examine this involvement by investigating developmental sex differences in GAD(65) protein levels in hypothalamic and extrahypothalamic brain regions known to be sexually dimorphic in adulthood. Brain nuclei were micro-dissected and GAD(65) protein levels were quantified using western immunoblotting. Sex differences in levels of GAD(65) were found in the dorsomedial nucleus and preoptic area of the hypothalamus and also the medial amygdaloid nucleus and CA1 subfield of the hippocampus. Unexpectedly, there were sex differences in protein levels of the housekeeping gene, GAPDH, cautioning against the use of GAPDH for standardizing protein samples during western immunoblotting.

Invited review: Estrogens effects on the brain: multiple sites and molecular mechanisms

Besides their well-established actions on reproductive functions, estrogens exert a variety of actions on many regions of the nervous system that influence higher cognitive function, pain mechanisms, fine motor skills, mood, and susceptibility to seizures; they also appear to have neuroprotective actions in relation to stroke damage and Alzheimer's disease. Estrogen actions are now recognized to occur via two different intracellular estrogen receptors, ER-alpha and ER-beta, that reside in the cell nuclei of some nerve cells, as well as by some less well-characterized mechanisms. In the hippocampus, such nerve cells are sparse in number and yet appear to exert a powerful influence on synapse formation by neurons that do not have high levels of nuclear estrogen receptors. However, we also find nonnuclear estrogen receptors outside of the cell nuclei in dendrites, presynaptic terminals, and glial cells, where estrogen receptors may couple to second messenger systems to regulate a variety of cellular events and signal to the nuclear via transcriptional regulators such as CREB. Sex differences exist in many of the actions of estrogens in the brain, and the process of sexual differentiation appears to affect many brain regions outside of the traditional brain areas involved in reproductive functions. Finally, the aging brain is responsive to actions of estrogens, which have neuroprotective effects both in vivo and in vitro. However, in an animal model, the actions of estrogens on the hippocampus appear to be somewhat attenuated with age. In the future, estrogen actions over puberty and in pregnancy and lactation should be further explored and should be studied in both the hypothalamus and the extrahypothalamic regions.

Tracking the estrogen receptor in neurons: implications for estrogen-induced synapse formation

Estrogens (E) and progestins regulate synaptogenesis in the CA1 region of the dorsal hippocampus during the estrous cycle of the female rat, and the functional consequences include changes in neurotransmission and memory. Synapse formation has been demonstrated by using the Golgi technique, dye filling of cells, electron microscopy, and radioimmunocytochemistry. N-methyl-d-aspartate (NMDA) receptor activation is required, and inhibitory interneurons play a pivotal role as they express nuclear estrogen receptor alpha (ERalpha) and show E-induced decreases of GABAergic activity. Although global decreases in inhibitory tone may be important, a more local role for E in CA1 neurons seems likely. The rat hippocampus expresses both ERalpha and ERbeta mRNA. At the light microscopic level, autoradiography shows cell nuclear [3H]estrogen and [125I]estrogen uptake according to a distribution that primarily reflects the localization of ERalpha-immunoreactive interneurons in the hippocampus. However, recent ultrastructural studies have revealed extranuclear ERalpha immunoreactivity (IR) within select dendritic spines on hippocampal principal cells, axon terminals, and glial processes, localizations that would not be detectable by using standard light microscopic methods. Based on recent studies showing that both types of ER are expressed in a form that activates second messenger systems, these findings support a testable model in which local, non-genomic regulation by estrogen participates along with genomic actions of estrogens in the regulation of synapse formation.

Sex, steroids, and stimulant sensitivity

The current study investigated ovarian modulation of the locomotor response to cocaine in rats. Ovariectomy in females lowered the response to cocaine (10 mg/kg i.p.), whereas castration did not change that of males. The locomotor responses of prepubertal males and females to cocaine were similar. However, the postpubertal sex difference resulted from a fall in cocaine-stimulated locomotion in males rather than a rise in females. Neonatal testosterone treatment of female rat pups decreased the response to cocaine in adulthood. These findings suggest that both the activational and organizational effects of gonadal steroids contribute to the greater response of females to cocaine.

Estrogen-induced remodeling of hypothalamic neural circuitry

For decades, sexual behavior has been a valuable model system for behavioral neuroscientists studying the neural basis of motivated behaviors. One striking example of a change in motivation is the binary switch in sexual receptivity that occurs during the estrous cycle in female rats. Investigations of the neural basis of this change in behavior have fundamentally advanced our understanding of both behaviorally relevant neural pathways and basic mechanisms of steroid action in the brain. These advances have made this behavioral model system a staple of neuroendocrinology. A challenge that remains before us, given our current understanding of the circuitry and chemistry, is to develop a coherent model of how neural plasticity in the hypothalamus contributes to the dependence of this behavior on motivational state. This review will focus on the ventromedial nucleus of the hypothalamus, especially its ventrolateral subdivision. First, the anatomical, neurochemical, and functional aspects of the macro- and microcircuitry of this brain region will be discussed, followed by a discussion of the likely mechanisms of estrogen action within the ventrolateral VMH. Then, the evidence for estrogen-induced neural plasticity will be considered, including a comparison with the effects of estrogen on synaptic organization in other brain regions. Finally, a working model of neural plasticity within the ventrolateral VMH microcircuitry will be presented as a starting point for future experiments to verify or, more likely, revise and expand.

Special relationship of gamma-aminobutyric acid to the ventromedial nucleus of the hypothalamus during embryonic development

The ventromedial nucleus of the hypothalamus (VMH) is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. We conducted immunocytochemical analyses by using antisera directed against gamma-aminobutyric acid (GABA), its synthetic enzyme glutamic acid decarboxylase (GAD67), GABA-A receptor subunits (alpha2, beta3, epsilon), estrogen receptor-alpha, and Neuropeptide Y (NPY) in the region of the VMH in embryonic mice to identify potential patterning elements for VMH formation. Cells and fibers containing GABA and GAD67 encircled the primordial VMH as early as embryonic day 13 (E13) when the cytoarchitecture of the VMH was not recognizable by Nissl stain. At E16-17 the cytoarchitecture of the VMH became recognizable by Nissl stain as GABAergic fibers invaded the nucleus, continued postnatally, and by adulthood the density of GABAergic fibers was greater inside than outside the VMH. GABA-A receptor subunit expression (beta3 by E13 and alpha2 by E15) within the primordial VMH suggested potential sensitivity to the surrounding GABA signal. Brain slices were used to test whether fibers from distal or proximal sites influenced VMH development. Coronal Vibratome slices were prepared and maintained in vitro for 0-3 days. Nissl stain analyses showed a uniform distribution of cells in the region of the VMH on the day of plating (E15). After 3 days in vitro, cellular aggregation suggesting VMH formation was seen. Nuclear formation in vitro suggests that key factors resided locally within the coronal plane of the slices. It is suggested that either GABA intrinsic to the region nearby the VMH directly influences the development and organization of the VMH, or along with other markers provides an early indicator of pattern determination that precedes the cellular organization of the VMH.

Increased synaptic sprouting in response to estrogen via an apolipoprotein E-dependent mechanism: implications for Alzheimer's disease

Estrogen replacement therapy appears to delay the onset of Alzheimer's disease (AD), but the mechanisms for this action are incompletely known. We show how the enhancement of synaptic sprouting by estradiol (E2) in response to an entorhinal cortex (EC) lesion model of AD may operate via an apolipoprotein E (apoE)-dependent mechanism. In wild-type (WT) mice, ovariectomy decreased commissural/associational sprouting to the inner molecular layer of the dentate gyrus, with synaptophysin (SYN) as a marker. E2 replacement returned SYN in the inner layer to levels of EC-lesioned, ovary-bearing controls and increased the area of compensatory synaptogenesis in the outer molecular layer. In EC-lesioned apoE-knock-out (KO) mice, however, E2 did not enhance sprouting. We also examined apoJ (clusterin) mRNA, which is implicated in AD by its presence in senile plaques, its transport of Abeta across the blood-brain barrier, and its induction by neurodegenerative lesioning. ApoJ mRNA levels were increased by E2 replacement in EC-lesioned WT mice but not in apoE-KO mice. These data suggest a mechanism for the protective effects of estrogens on AD and provide a link between two important risk factors in the etiology of AD, the apoE epsilon4 genotype and an estrogen-deficient state. This is also the first evidence that SYN, a presynaptic protein involved in neurotransmitter release, is regulated by E2 in the adult brain, and that apoE is necessary for the induction of apoJ mRNA by E2 in brain injury.

Estrogen down-regulates mRNA encoding the exocytotic protein SNAP-25 in the rat pituitary gland

Exocytosis is dependent on specific proteins that are located at the secretory granule membrane, in the cytoplasm or at the plasma membrane. The mRNA expression of synaptosomal-associated protein of 25 kDa (SNAP-25) isoforms SNAP-25a and SNAP-25b, vesicle associated membrane protein (VAMP) 2, mammalian homologue of unc-18 (munc-18) and Hrs-2 was studied in the pituitary of ovariectomized rats after subcutaneous insertion of capsules containing estrogen or placebo using in situ hybridization. Estrogen treatment (0.25 mg estradiol) significantly decreased SNAP-25a (32%; 10%) and SNAP-25b (25%; 22%) mRNA levels in the anterior and intermediate lobes, respectively, whereas VAMP-2, munc-18 and Hrs-2 mRNA levels remained unchanged. The results suggest that estrogen selectively regulates SNAP-25 transcription in the pituitary gland, but leaves VAMP-2, munc-18 and Hrs-2 mRNA levels unaffected.

B-50, the growth associated protein-43: modulation of cell morphology and communication in the nervous system

The growth-associated protein B-50 (GAP-43) is a presynaptic protein. Its expression is largely restricted to the nervous system. B-50 is frequently used as a marker for sprouting, because it is located in growth cones, maximally expressed during nervous system development and re-induced in injured and regenerating neural tissues. The B-50 gene is highly conserved during evolution. The B-50 gene contains two promoters and three exons which specify functional domains of the protein. The first exon encoding the 1-10 sequence, harbors the palmitoylation site for attachment to the axolemma and the minimal domain for interaction with G0 protein. The second exon contains the "GAP module", including the calmodulin binding and the protein kinase C phosphorylation domain which is shared by the family of IQ proteins. Downstream sequences of the second and non-coding sequences in the third exon encode species variability. The third exon also contains a conserved domain for phosphorylation by casein kinase II. Functional interference experiments using antisense oligonucleotides or antibodies, have shown inhibition of neurite outgrowth and neurotransmitter release. Overexpression of B-50 in cells or transgenic mice results in excessive sprouting. The various interactions, specified by the structural domains, are thought to underlie the role of B-50 in synaptic plasticity, participating in membrane extension during neuritogenesis, in neurotransmitter release and long-term potentiation. Apparently, B-50 null-mutant mice do not display gross phenotypic changes of the nervous system, although the B-50 deletion affects neuronal pathfinding and reduces postnatal survival. The experimental evidence suggests that neuronal morphology and communication are critically modulated by, but not absolutely dependent on, (enhanced) B-50 presence.

Gene expression and neuronal activity in schizophrenia: a study of polyadenylated mRNA in the hippocampal formation and cerebral cortex

The abundance of polyadenylated messenger RNA (poly(A)+mRNA) reflects overall gene expression and provides an index on neuronal activity. Poly(A)+mRNA was measured in the hippocampal formation and in occipital, temporal, cingulate and frontal cortices (Brodmann areas 17, 22, 24 and 46, respectively) of 11 schizophrenic and 17 control subjects post mortem by in situ hybridization histochemistry with a 35S-oligodeoxythymidine probe. There were no differences in poly(A)+mRNA between cases and controls, except for a modest decrease in the parahippocampal gyrus of the schizophrenics which may be attributable to cytoarchitectural differences in this area in the disease. The unchanged level of poly(A)+mRNA in all other regions argues against the existence of a widespread or sustained alteration in the metabolic activity of cortical neurons in schizophrenia. It also provides a further indication that the differential expression of individual transcripts reported in the disease is not merely reflecting changes in overall gene expression.

Enterocytic gene expression is altered in experimental gastroschisis

Gastroschisis is a congenital anomaly in which exposure of the intestines to amniotic fluid throughout fetal life results in nutrient malabsorption. To begin to understand the molecular basis underlying epithelial changes in this condition, we investigated enterocytic gene expression during fetal development. Gastroschisis was surgically created at 24 days gestation (term = 31 days) in fetal rabbits; sham-operated and unoperated fetuses served as controls. Bowel was harvested at 28 and 31 days gestation. Cellular lactase expression was detected using immunohistochemistry, and apolipoprotein A-I and cellular retinol binding protein II (CRBPII) mRNA levels were quantitated using Northern blot analysis. Despite absence of gross histological changes in the mucosa, lactase protein expression and apolipoprotein A-I and CRBPII mRNA expression were decreased in intestine from gastroschisis compared to sham-operated animals. Expression of GAPDH (a housekeeping gene) increased over the same period, suggesting that the changes in enterocytic absorptive gene expression associated with gastroschisis were relatively specific. In conclusion, a decrease in expression of a variety of genes involved in nutrient absorption and trafficking within the enterocyte may contribute to the absorptive defects seen in this gastroschisis.

In vitro models for the effects of sex hormones on neurons

The results of these two in vitro models share some striking similarities. In both, estrogen was able to induce or promote the formation of either dendrites themselves in hippocampal neurons or dendritic specializations in PC12 neurites, and these specializations were then able to induce interneural interactions. In both models, androgen was able to promote the development of axons that branched frequently, while not directly fostering interneuronal contact. These findings recapitulate in part some of the effects of estrogen and androgen on neurons in vivo and suggest the inherent ability of cells of neural crest origin to respond to these hormones with specific neural morphogenetic programs designed to alter interneuronal communication. In these ways, it seems likely that both sex hormones are acting as neural growth factors in cells that express the appropriate receptor, leading to stereotyped changes in neural growth and pattern formation. Through the examination of such subcellular mechanisms, we hope to further understand the effects of sex hormones on brain development and the ontogeny of behavioral, cognitive, and reproductive differences between the sexes.

Differential subcellular localization of SNAP-25a and SNAP-25b RNA transcripts in spinal motoneurons and plasticity in expression after nerve injury

Synaptosomal-associated protein of 25 kDa (SNAP-25) is involved in the molecular regulation of neurotransmitter release. SNAP-25 exists in two isoforms, which arise from alternative splicing of exon 5. In situ hybridization was used to examine whether SNAP-25 isoform mRNA expression may be altered by experimental manipulations. The effect of unilateral nerve injury on SNAP-25 mRNA levels was studied in motoneurons of the rat lumbar spinal cord. In all animals, SNAP-25a RNA transcripts were demonstrated in the nucleus of motoneurons, whereas SNAP-25b mRNA was present mainly in the cytoplasm. Cloning of the rat Snap gene intron spacing the alternative exon 5a and 5b sequences and generation of an intron-specific oligonucleotide probe used for in situ hybridization did not point to the presence of unspliced variants of SNAP-25b mRNA. After unilateral sciatic nerve transection (axotomy), SNAP-25a and SNAP-25b expression decreased in axotomized motoneurons compared with corresponding motoneurons on the unlesioned side. A significant decrease was demonstrated 2 days after axotomy, which reached a maximum after 7 days (62% for SNAP-25a and 67% for SNAP-25b), while levels had slightly recovered by 14 and 28 days. Ventral root avulsion also induced a decrease in levels of SNAP-25 RNA transcripts, suggesting that the axonal injury in itself was responsible for the down-regulation of Snap gene expression. This study shows that, in spinal motoneurons, SNAP-25a and SNAP-25b RNA transcripts have different subcellular localization and that levels of SNAP-25 RNA transcripts are down-regulated after axonal injury.

Roles of steroid hormones and their receptors in structural organization in the nervous system

Due to their chemical properties, steroid hormones cross the blood-brain barrier where they have profound effects on neuronal development and reorganization both in invertebrates and vertebrates, including humans mediated through their receptors. Steroids play a crucial role in the organizational actions of cellular differentiation representing sexual dimorphism and apoptosis, and in the activational effects of phenotypic changes in association with structural plasticity. Their sites of action are primarily the genes themselves but some are coupled with membrane-bound receptor/ion channels. The effects of steroid hormones on gene transcription are not direct, and other cellular components interfere with their receptors through cross-talk and convergence of the signaling pathways in neurons. These genomic and non-genomic actions account for the divergent effects of steroid hormones on brain function as well as on their structure. This review looks again at and updates the tremendous advances made in recent decades on the study of the role of steroid (gonadal and adrenal) hormones and their receptors on developmental processes and plastic changes in the nervous system.

Immediate upstream promoter regions required for neurospecific expression of SNAP-25

The promoter structure and regulation of Snap, a gene encoding the presynaptic t-SNARE SNAP-25 implicated in synaptic vesicle docking and fusion, was studied. Transcription start-site analysis revealed two major start sites located 42 nucleotides apart. Nucleotide sequence of a promoter region 2073 nucleotides upstream of the first transcription site contains three AP-1, one CRE sequence, and three Sp1-like sites close to the TATA box. Further upstream of these sites two TG repeats were found. The ability of regions within the 5' upstream sequence to promote basal neural-specific expression in tissue culture cells was evaluated using a series of constructs containing both Snap gene start sites with progressively restricted 5' sequence linked to the chloramphenicol acetyl transferase (CAT) reporter gene. CAT expression was maximal in neuron-like undifferentiated ND7 and PC12 cells transfected with constructs containing Snap sequences up to 127 bp from the start site. In contrast, nonneuronal fibroblast cell lines did not express significant amounts of CAT, suggesting that this short 127-bp sequence is sufficient to drive neural specific expression of SNAP-25. Band shift analysis of oligonucleotides spanning from -127 to -41 bp of the Snap promoter revealed three distinct DNA-protein complexes generated by brain nuclear extracts and one by liver nuclear extracts, indicating that transcription factors that bind to this 86-bp sequence located just upstream of the TATA box are involved in regulation of basal neurospecific expression of this gene.

Synaptophysin gene expression in human brain: a quantitative in situ hybridization and immunocytochemical study

Synaptophysin is a presynaptic vesicle protein. Its quantitative detection has become established as a molecular marker of synaptic density. We have studied synaptophysin messenger RNA in the neocortex, hippocampus and cerebellum using in situ hybridization histochemistry to see if the encoding transcript can be detected in post mortem human brain and to investigate factors which might influence its abundance. Synaptophysin was also measured immunocytochemically in the hippocampus. The level of synaptophysin messenger RNA expression was uniform in all neocortical areas examined. Strong correlations were found for the amount of synaptophysin messenger RNA between individual regions and between homologous areas in the two hemispheres. Synaptophysin messenger RNA declined with increasing age and prolonged post mortem interval. Synaptophysin immunoreactivity also reduced with age, as well as with duration of formalin fixation but not post mortem interval. Synaptophysin immunoreactivity correlated with the abundance of the messenger RNA in neurons within, and projecting to, each hippocampal subfield. Significantly greater synaptophysin immunoreactivity was seen in the left than the right CA4 and CA1 regions. These data show that quantitative detection of synaptophysin messenger RNA as well as synaptophysin itself can reliably be carried out in post mortem human brain sections. They are in keeping with other findings that synaptic density is relatively uniform through the neocortex and decreases with age. They also suggest a possible asymmetry of hippocampal synaptophysin expression. The level of synaptophysin messenger RNA paralleled that of synaptophysin immunoreactivity, indicating that changes in gene expression contribute to variations in the latter observed in aging and other situations. Detection of synaptophysin messenger RNA broadens the range of methods by which synaptic protein gene products can be studied and used as markers of synaptic density and synaptic involvement during physiological and pathological processes in human brain.

The ontogeny of GAP-43 (neuromodulin) mRNA in postnatal rat brain: evidence for a sex dimorphism

GAP-43 is a membrane-bound protein selectively concentrated in axonal growth cones during brain development and implicated in axonal outgrowth and elongation. A sex dimorphism in the number of synapses in certain regions of the adult rat brain has been attributed to differences in gonadal steroid hormone action during early postnatal life. The results of recent studies have demonstrated that gonadal steroids modulate GAP-43 mRNA in regions of the postnatal and adult brain where steroid hormone receptors are concentrated. Since gonadal steroids influence the development of the sexually undifferentiated brain during the first few weeks of postnatal life, the present study investigated the ontogeny of GAP-43 mRNA in the male and female rat brain between postnatal days 1 and 25. On postnatal days 1, 3, 6, 12, 18, and 25, brains were collected from male and female postnates and frozen, and 16 microns cryostat sections were processed and hybridized with a 35S-labeled antisense riboprobe complementary to GAP-43 mRNA. Evaluation of film autoradiograms demonstrated a widespread distribution of GAP-43 mRNA in postnatal brain regions, including the cerebral cortex; bed nucleus of the stria terminalis; and medial preoptic area, ventromedial nucleus, and arcuate nucleus of the hypothalamus. Densitometric measurements revealed that GAP-43 mRNA was transiently elevated during early postnatal life, with a subsequent decrease during brain maturation, although the pattern of change varied among the brain regions investigated. In addition, the level of GAP-43 hybridization signal was significantly higher in the male cortex, bed nucleus, and medial preoptic nucleus, but not the ventromedial and arcuate nuclei, than in postnatal females. Analysis of slide autoradiograms demonstrated that the change in GAP-43 mRNA during postnatal development was due to changes at the cellular level. The present results indicate that expression of GAP-43 mRNA is transiently elevated and sexually dimorphic in certain regions of the early postnatal rat brain. The results further suggest that the differential expression of GAP-43 in the male and female postnatal brain may be related to sex differences in neuronal outgrowth and connectivity resulting in a dimorphism in the pattern of adult neuronal circuitry.