Barbara E.H. Sumner

ESTROGEN CONTROL OF CENTRAL NEUROTRANSMISSION : EFFECT ON MOOD, MENTAL STATE, AND MEMORY

Summary1. Estrogen exerts profound effects on mood, mental state and memory by acting on both “classical” monoamine and neuropeptide transmitter mechanisms in brain. Here we review an example of each type of action.2. With respect to the effect of estrogen on central monoamine neurotransmission, low levels of estrogen in women are associated with the premenstrual syndrome, postnatal depression and post-menopausal depression. Sex differences in schizophrenia have also been attributed to estrogen. Previous studies have shown that estrogen stimulates a significant increase in dopamine2 (D2) receptors in the striatum. Here we show for the first time that estrogen also stimulates a significant increase in the density of 5-hydroxytryptamine2A (5-HT2A) binding sites in anterior frontal, cingulate and primary olfactory cortex and in the nucleus accumbens, areas of the brain concerned with the control of mood, mental state, cognition, emotion and behavior. These findings explain, for example, the efficacy of estrogen therapy or 5-HT uptake blockers such as fluoxetine in treating the depressive symptoms of the premenstrual syndrome, and suggest that the sex differences in schizophrenia may also be due to an action of estrogen mediated by way of 5-HT2A receptors.3. With respect to the effect of estrogen on central neuropeptide transmission, estrogen stimulates the expression of the arginine vasopressin (AVP) gene in the bed nucleus of the stria terminalis (BNST) in rodents. This results in a 100-fold increase in AVP mRNA in the BNST and a massive increase in AVP peptide in the BNST and its projections to the lateral septum and lateral habenula. The BNST-AVP system enhances and/or maintains “social” or “olfactory” memory, and thus provides a powerful model for correlating transcriptional control of neuropeptide gene expression with behavior. Whether similar mechanisms operate in the human remain to be determined.4. These two examples of the action of estrogen on central neurotransmission are discussed in terms of their immediate clinical importance for the treatment of depressive symptoms, their use as powerful models for investigations on the steroid control of central neurotransmitter mechanisms, and the role of estrogen as “Natures” psychoprotectant.

SEX STEROID CONTROL OF MOOD, MENTAL STATE AND MEMORY

1. Sex steroid hormones exert profound effects on mood and mental state. Thus, in women, oestrogen is thought to protect against depression and delay the onset of schizophrenia and Alzheimers disease.

Estrogen increases the density of 5-Hydroxytryptamine2A receptors in cerebral cortex and nucleus accumbens in the female rat

Estrogen exerts a profound effect on mood and mental function in man. Based on our finding that estradiol selectively stimulates the expression of 5-hydroxytryptamine(2A) (5-HT2A) receptor mRNA in the dorsal raphe nucleus of the female rat, we investigated the effects of estradiol on the density of 5-HT2A receptors in brain. The distribution and density of 5-HT2A receptors were determined by in vitro binding of [3H]ketanserin in the presence of prazosin to exclude binding to alpha 1-adrenoreceptors. Brains were collected, processed and analysed in pairs from six estradiol- and six vehicle-treated animals. Our results show that a single pulse of estradiol induces a significant increase in the density of 5-HT2A receptors in female rat forebrain, particularly the anterior frontal, anterior cingulate and primary olfactory cortex and the nucleus accumbens. Since these brain regions play a pivotal role in cognition and emotion, as well as neuroendocrine and motor control, our findings provide the first experimental evidence for the fact that estrogen could alter mood and mental state by increasing the density of 5-HT2A receptors in cerebral cortex and nucleus accumbens.

Androgen actions on central serotonin neurotransmission: relevance for mood, mental state and memory

Sex steroids exert potent effects on mood and mental state in the human. Our previous experimental findings in female rats suggest that these effects may be mediated, in part, by the action of estrogen on the 5-hydroxytryptamine2A receptor (5-HT(2A)R) and serotonin transporter (SERT) in brain. Here we review our recent findings on the effect of acute (approximately 32 h) testosterone manipulation on central 5-HT(2A)R and SERT in male rats. Castration decreased while testosterone or estrogen, but not 5alpha-dihydrotestosterone (5alpha-DHT), increased significantly the content of 5-HT(2A)R mRNA and SERT mRNA in the dorsal raphe nucleus (DR) and the density of 5-HT(2A)R and SERT binding sites in higher centers of the brain. The lack of effect of 5alpha-DHT, a potent androgen which cannot be converted to estrogen, suggests that the action of testosterone depends upon its conversion to estrogen by aromatase. This may also explain why estrogen, but not testosterone or 5alpha-DHT, increased the density of 5-HT(2A)R binding sites in the caudate-putamen, a brain region where aromatase is scarce. The estrogen induction of SERT mRNA is most prominent in the rostral DR and this together with the correlation between sensitivity of DR serotonin neurons to estrogen and neurotoxic amphetamine derivatives provides a potential topochemical handle with which to investigate testosterone/estrogen regulation of SERT gene expression. These findings are discussed in relation to the possible role of interactions between sex steroids and serotonin mechanisms in mood disorders, schizophrenia and Alzheimers disease.

Testosterone as well as estrogen increases serotonin2A receptor mRNA and binding site densities in the male rat brain

Our previous findings in female rats suggest that the potent effects of sex steroids on mood and mental state may be mediated, in part, by the effect of estrogen on the 5-hydroxytryptamine2A receptor (5-HT2AR) in brain. The aim of the present study was to determine the effect of acute (approximately 32h) sex steroid manipulation on central 5-HT2AR in the adult male Wistar rat. Castration (under halothane anesthesia) decreased while testosterone or estrogen, but not 5alpha-dihydrotestosterone (5alpha-DHT), increased significantly the 5-HT2AR mRNA content in dorsal raphe nucleus and the density of 5-HT2AR binding sites in frontal, cingulate and primary olfactory cortex and nucleus accumbens. The lack of effect of 5alpha-DHT, a potent androgen which cannot be converted to estrogen, suggests that the action of testosterone depends upon its conversion to estrogen by aromatase. This may also explain why estrogen, but not testosterone or 5alpha-DHT, increased the density of 5-HT2AR binding sites in the caudate-putamen, a brain region where aromatase is scarce. These findings are discussed in relation to the possible role of the 5-HT2AR in depression, schizophrenia and Alzheimers Disease.

Effects of tamoxifen on serotonin transporter and 5-hydroxytryptamine2A receptor binding sites and mRNA levels in the brain of ovariectomized rats with or without acute estradiol replacement

Estradiol-17beta (E(2)), in its positive feedback mode for gonadotropin release in the female rat, induces expression of the genes for the 5-hydroxytryptamine(2A) receptor (5-HT(2A)R) and the serotonin transporter (SERT) in the dorsal raphe nucleus (DRN) with a concomitant increase in the densities of 5-HT(2A)R and the SERT in rat forebrain. The forebrain regions affected are those which, in humans, are concerned with the control of mood, mental state, cognition and emotion. Here we have used the mixed estradiol agonist/antagonist, tamoxifen, to determine whether this action of estradiol is mediated by cytoplasmic estradiol receptors. Acute treatment ( approximately 32 h) of ovariectomized rats with estradiol benzoate (EB) increased significantly the amount of 5-HT(2A)R mRNA and SERT mRNA in the DRN and the densities of 5-HT(2A)R and SERT binding sites in the forebrain. These effects of EB were completely blocked by tamoxifen. Treatment with tamoxifen alone had no effect on either gene expression or the density of binding sites. Together, these data show that tamoxifen acts as a pure estradiol antagonist with respect to serotonergic mechanisms in brain. Detailed analysis of the effects of estradiol and tamoxifen on the DRN showed that SERT gene expression is constitutive only in the posterior DRN; in the anterior DRN, SERT gene expression appears to depend upon estrogen induction which is blocked by tamoxifen. Our findings strongly suggest that estradiol receptors are involved in mediating estradiol action on central serotonergic mechanisms and are relevant for our understanding of the effects of antiestrogens as well as estradiol on mood, mental state and cognition.

Serotonin transporter (SERT) mRNA and binding site densities in male rat brain affected by sex steroids

Estrogen increases serotonin transporter (SERT) mRNA and binding sites in female rat brain. In order to determine whether changes in SERT are gender- and steroid-specific we have now carried out studies on adult male Wistar rats which were either intact or castrated (under halothane anesthesia) and injected with arachis oil, estradiol benzoate (EB), testosterone propionate (TP) or the non-aromatizable androgen, 5alpha-dihydrotestosterone (5alpha-DHT). The number of SERT mRNA-expressing cells in the dorsal raphe (DR) nucleus was decreased by castration and increased by treatment (for approximately 32 h) with EB or TP, but not 5alpha-DHT. Sex steroids had no effect on the number of SERT mRNA-expressing cells in the median raphe nucleus. The density of SERT sites, assessed by autoradiography of [3H]paroxetine binding, was significantly reduced in arcuate nucleus and median raphe after castration, and increased in arcuate, basolateral amygdala and ventromedial hypothalamic nucleus by treatment with EB or TP, but not 5alpha-DHT. Estradiol, but not testosterone or 5alpha-DHT reduced the density of SERT sites in midbrain central grey. These data show that testosterone as well as estrogen affects SERT expression in male brain, and that the action of testosterone probably depends upon its enzymatic conversion, by aromatase, to estradiol. Our findings may have implications for sex steroid control of mood and behavior, and the action of neurotoxic derivatives of amphetamine, such as 3, 4-methylenedioxymethamphetamine, in the human.

Effects of Acute Estradiol on 5-Hydroxytryptamine and Dopamine Receptor Subtype mRNA Expression in Female Rat Brain

The aim of the present study was to determine the effect of estradiol-17beta, in its positive feedback mode for the release of prolactin and luteinizing hormone (LH), on gene expression of 5-hydroxytryptamine (5-HT) and dopamine receptors. Gene expression was determined by measurement of the levels of receptor mRNA by in situ hybridization in brain sections from adult female rats. The animals were ovariectomized under halothane anesthesia on the morning of diestrus, given a sc injection of either estradiol benzoate (EB) or oil (vehicle), and killed between 1600-1700 h of the next day, presumptive proestrus. The injection of EB in this preparation is known to stimulate a massive surge of LH and prolactin in the late afternoon of presumptive proestrus. The injection of EB produced a significant increase in silver grain densities representing 5-HT(2) receptor mRNA in the dorsal raphe nucleus (DRN) and a significant reduction in these grain densities in the medial septum and diagonal band of Broca. The number of cells expressing 5-HT(2) receptor mRNA was increased by 290% in the DRN and decreased by 25% in the medial preoptic area. Estradiol had no effect on silver grain densities, or numbers or percentages of cells expressing 5-HT(2) receptor mRNA in any of the other nine brain regions or choroid plexus studied, and had no effect on the levels of 5-HT(1A), 5-HT(1C), D(1), or D(2) receptor mRNA in any region studied. These data suggest that the 5-HT(2) receptor is a key receptor involved in mediating the positive feedback stimulation by estradiol-17beta of LH and prolactin release. Since estradiol increases 5-HT levels in the DRN, the data raise the intriguing possibility that estrogen positive feedback on LH and prolactin release is mediated by a feed-forward 5-HT drive which results in increased 5-HT release at DRN-derived nerve terminals on hypothalamic neurons which control LH and prolactin release.

The density of 5-hydoxytryptamine2A receptors in forebrain is increased at pro-oestrus in intact female rats

We have shown previously that in ovariectomised rats, oestradiol-17beta, in its positive-feedback mode for luteinizing hormone (LH) release, induces a significant increase in the density of 5-hydroxytryptamine2A (5-HT2A) receptors in the forebrain. Here we investigated whether there are any changes in 5-HT2A receptor density in relation to the spontaneous surge of oestradiol-17beta in female COB Wistar rats between dioestrus and pro-oestrus. Using [3H]RP62203-binding and autoradiography, we found that 5-HT2A binding sites were significantly increased at 1630-1800 h on pro-oestrus compared with 0900-1130 h on dioestrus in frontal and cingulate cortex, olfactory tubercle and nucleus accumbens. The densities of 5-HT2A binding sites in male rats were similar to dioestrous female values in cortex, and to pro-oestrous female values in nucleus accumbens. The changes in the density of 5-HT2A binding sites in the forebrain of female rats may be relevant to oestrogen effects on mood and mental state.

Relative density of 5-hydroxytryptamine receptor subtype mRNAs in female rat neuroendocrine brain determined by in situ hybridization histochemistry

The relative distribution and density of three 5-hydroxytryptamine (5-HT) receptor subtype mRNAs in female rat brain were investigated by in situ hybridization histochemistry using (36)S-labeled riboprobes. Special attention was focused on the hypothalamus and other parts of the brain involved in neuroendocrine control. Perikarya of the diagonal band of Broca (DBB), medial septum (MS), medial preoptic area (MPOA), and ventromedial hypothalamic nucleus (VMH) contained high concentrations of 5-HT(2) receptor mRNA, only moderate amounts of 5-HT(1A) receptor mRNA, and variable amounts of 5-HTP(1c) receptor mRNA. 5-HT(1c) receptor mRNA content was high in certain neurons of the lateral septum. The suprachiasmatic (SCN) and arcuate nuclei contained moderate concentrations of 5-HT(1A) receptor mRNA, but little or none of the other two subtype mRNAs. The supraoptic (SON) and paraventricular nuclei (PVN) also contained moderate amounts of 5-HT(1A) receptor mRNA, with small to moderate amounts of 5HT(1c) and 5-HT(2) receptor mRNAs. In other brain regions the highest contents of 5-HT(1A), 5-HT(1c), and 5-HT(2) receptor mRNAs were found in hippocampus, choroid plexus, and cingulate and frontal cortices, respectively. In the dorsal and median raphe nuclei 5-HT(1A) receptor mRNA content was moderately high, 5-HT(2) receptor mRNA moderate, and 5-HT(1c), receptor mRNA low. This distribution of 5-HT receptor mRNA is consistent with a role of 5-HT(2) receptors in the control of ovulation (high density in DBB, MS, and MPOA) and mating (high density in VMH) and 5-HT(1A) receptors in the 5-HT control of circadian rhythms (SCN) and in some functions of the SON and PVN.