Working memory for emotional facial expressions: role of estrogen in humans and non-human primates

In recent years, numerous studies focusing on the role of sex steroid hormones on neuropsychological functions have been reported. The influence of estrogens on cognition can be explained by the widespread presence of estrogen receptors (ERs) in limbic and cortical areas, and the modulator role of estrogens on numerous neurotransmitter systems. There is a great deal of evidence suggesting that estrogen can enhance memory processes and improve performance in working memory (WM) tasks, including face-tasks, delayed matching-to-sample (DMTS) and delayed non-matching-to-sample (DNMTS). The modulator effects exerted by estrogens are complex and may vary according to task, gender, and phase of the menstrual cycle. For many years, physiological hormonal fluctuations during the menstrual cycle, postpartum, and menopause have been said to influence mood, cognition, and affective disorders. In addition, it has been hypothesised that estrogens have a protective role in schizophrenia and Alzheimer's disease (AD). Taking into account that women's performance in memory tasks can also fluctuate with circulating hormone levels across the menstrual cycle, data related to the cognitive performance in a WM task for emotional facial expressions were reported. The findings described in this review can contribute to clarifying the role of the steroid hormone estrogen in the modulation of cognitive functions, particularly memory, and indicate the possible usefulness of applying a new paradigm, employing a DNMTS task with emotional facial expressions, in studies focused on emotional WM in relevant models of nonhuman primates. Such data could also represent a useful tool in individualizing effective therapy for the treatment of disturbances linked to menstrual cycle phases and menopause in women.     

The neuroendocrinology of primate maternal behavior

In nonhuman primates and humans, similar to other mammals, hormones are not strictly necessary for the expression of maternal behavior, but nevertheless influence variation in maternal responsiveness and parental behavior both within and between individuals. A growing number of correlational and experimental studies have indicated that high circulating estrogen concentrations during pregnancy increase maternal motivation and responsiveness to infant stimuli, while effects of prepartum or postpartum estrogens and progestogens on maternal behavior are less clear. Prolactin is thought to play a role in promoting paternal and alloparental care in primates, but little is known about the relationship between this hormone and maternal behavior. High circulating cortisol levels appear to enhance arousal and responsiveness to infant stimuli in young, relatively inexperienced female primates, but interfere with the expression of maternal behavior in older and more experienced mothers. Among neuropeptides and neurotransmitters, preliminary evidence indicates that oxytocin and endogenous opioids affect maternal attachment to infants, including maintenance of contact, grooming, and responses to separation. Brain serotonin affects anxiety and impulsivity, which in turn may affect maternal behaviors such as infant retrieval or rejection of infants' attempts to make contact with the mother. Although our understanding of the neuroendocrine correlates of primate maternal behavior has grown substantially in the last two decades, very little is known about the mechanisms underlying these effects, e.g., the extent to which these mechanisms may involve changes in perception, emotion, or cognition.     

Effects of oral estrogen, raloxifene and arzoxifene on gene expression in serotonin neurons of macaques

The serotonin neural system contributes to cognition and affect, both of which exhibit pathologies with gender bias. We previously showed that estrogen (E) treatment of female macaques via Silastic implant alters gene expression for tryptophan hydroxylase (TPH), the serotonin reuptake transporter (SERT) and the 5HT1A autoreceptor. In addition, we have found that serotonin neurons of macaques express ER beta (ER beta). Together these studies suggest that the serotonin neural system could transduce the action of estrogen via ER beta on aspects of mood and cognition. However, estrogen replacement therapy can increase the risk for breast and uterine cancer. Therefore, we questioned whether the selective estrogen receptor modulators, raloxifene and arzoxifene, act in a manner similar to E on gene expression in serotonin neurons of a nonhuman primate model. Female rhesus macaques were ovariectomized and orally dosed with vehicle, estradiol 17beta, raloxifene or arzoxifene once per day by sipper bottles for 30 days. The animals were then euthanized and the midbrains were prepared for in situ hybridization for TPH, SERT and 5HT1A receptor mRNAs followed by densitometric analysis. There was a significant increase in TPH total signal (positive pixelsxOD) with E, raloxifene and arzoxifene, respectively. There was a significant decrease in SERT mRNA optical density with all treatments. 5HT1A autoreceptor mRNA did not change with any treatment. If these changes in gene expression are reflected by similar changes in the functional proteins, then raloxifene or arzoxifene could increase serotonin neurotransmission with little or no negative action in peripheral tissues. In conclusion, the selective estrogen receptor modulators, raloxifene and arzoxifene, act in a manner similar to natural E on TPH and SERT mRNA expression in serotonin neurons. This suggests that raloxifene and arzoxifene are agonists at ER beta in the context of the serotonin neuron. However, the responses to E were more variable and less robust with the oral dosing paradigm compared to a chronic implant paradigm.     

Effects of ovarian hormones on levels of luteinizing hormone in plasma and on serotonin concentrations in discrete brain nuclei

Levels of serotonin were measured in microdissected, individual brain nuclei in ovariectomized rats after treatment with ovarian hormones. Regions sampled included nuclei in the forebrain, rostral and medial hypothalamus, and midbrain tegmentum. Estradiol benzoate decreased levels of luteinizing hormone in plasma but did not affect serotonin levels in any region. Progesterone alone elevated serotonin content in the nucleus tractus diagonal's and ventral tegmental area. The combined estrogen plus progesterone regime produced a surge in plasma luteinizing hormone and also markedly elevated serotonin in the median eminence. These results may be of significance for ovarian hormonal regulation of gonadotropin secretion and reproductive behavior.     

Ovarian hormones,sleep and cognition across the adult female lifespan: An integrated perspective

Loss of ovarian function in women is associated with sleep disturbances and cognitive decline, which suggest a key role for estrogens and/or progestins in modulating these symptoms. The effects of ovarian hormones on sleep and cognitive processes have been studied in separate research fields that seldom intersect. However, sleep has a considerable impact on cognitive function. Given the tight connections between sleep and cognition, ovarian hormones may influence selective aspects of cognition indirectly, via the modulation of sleep. In support of this hypothesis, a growing body of evidence indicates that the development of sleep disorders following menopause contributes to accelerated cognitive decline and dementia in older women. This paper draws from both the animal and human literature to present an integrated view of the effects of ovarian hormones on sleep and cognition across the adult female lifespan.     

Neuroactive steroid effects on cognitive functions with a focus on the serotonin and GABA systems

This article will review neuroactive steroid effects on serotonin and GABA systems, along with the subsequent effects on cognitive functions. Neurosteroids (such as estrogen, progesterone, and allopregnanolone) are synthesized in the central and peripheral nervous system, in addition to other tissues. They are involved in the regulation of mood and memory, in premenstrual syndrome, and mood changes related to hormone replacement therapy, as well as postnatal and major depression, anxiety disorders, and Alzheimer's disease. Estrogen and progesterone have their respective hormone receptors, whereas allopregnanolone acts via the GABA(A) receptor. The action of estrogen and progesterone can be direct genomic, indirect genomic, or non-genomic, also influencing several neurotransmitter systems, such as the serotonin and GABA systems. Estrogen alone, or in combination with antidepressant drugs affecting the serotonin system, has been related to improved mood and well being. In contrast, progesterone can have negative effects on mood and memory. Estrogen alone, or in combination with progesterone, affects the brain serotonin system differently in different parts of the brain, which can at least partly explain the opposite effects on mood of those hormones. Many of the progesterone effects in the brain are mediated by its metabolite allopregnanolone. Allopregnanolone, by changing GABA(A) receptor expression or sensitivity, is involved in premenstrual mood changes; and it also induces cognitive deficits, such as spatial-learning impairment. We have shown that the 3beta-hydroxypregnane steroid UC1011 can inhibit allopregnanolone-induced learning impairment and chloride uptake potentiation in vitro and in vivo. It would be important to find a substance that antagonizes allopregnanolone-induced adverse effects.     

Ovarian hormones differentially influence immunoreactivity for dopamine beta- hydroxylase, choline acetyltransferase, and serotonin in the dorsolateral prefrontal cortex of adult rhesus monkeys.

Recent studies have shown that ovariectomy reduces, and subsequent hormone replacement restores the density of axons immunoreactive for tyrosine hydroxylase in the dorsolateral prefrontal cortex of adult female rhesus monkeys. The present study indicates that three additional extrathalamic frontal lobe afferents are also sensitive to changes in the ovarian hormone environment. Specifically, the combination of hormone manipulation with qualitative and quantitative analysis of immunocytochemistry for dopamine beta-hydroxylase, choline acetyltransferase, and serotonin in the primate prefrontal cortex revealed quantitative responses in both cholinergic and monoaminergic axons to changing ovarian hormone levels. However, whereas ovariectomy produced a modest net decrease in the density of fibers immunoreactive for choline acetyltransferase, this same treatment markedly increased the density of axons immunoreactive for dopamine beta-hydroxylase and for serotonin. Further, the effects of ovariectomy on these afferent systems were differentially attenuated by estrogen verses estrogen plus progesterone hormone replacement. Estrogen was as effective as estrogen plus progesterone in stimulating normal prefrontal immunoreactivity for choline acetyltransferase and dopamine beta-hydroxylase. The dual replacement of estrogen plus progesterone, however, was a much more potent influence than estrogen alone for serotonin immunoreactivity. Thus, ovarian hormones appear to provide stimulation that differentially affects each of four chemically identified extrathalamic prefrontal afferent systems examined to date, and may have roles in maintaining the normal balance and functional interactions between these neurotransmitter systems.     

Neuroprotective actions of estradiol and novel estrogen analogs in ischemia: translational implications

This review highlights our investigations into the neuroprotective efficacy of estradiol and other estrogenic agents in a clinically relevant animal model of transient global ischemia, which causes selective, delayed death of hippocampal CA1 neurons and associated cognitive deficits. We find that estradiol rescues a significant number of CA1 pyramidal neurons that would otherwise die in response to global ischemia, and this is true when hormone is provided as a long-term pretreatment at physiological doses or as an acute treatment at the time of reperfusion. In addition to enhancing neuronal survival, both forms of estradiol treatment induce measurable cognitive benefit in young animals. Moreover, estradiol and estrogen analogs that do not bind classical nuclear estrogen receptors retain their neuroprotective efficacy in middle-aged females deprived of ovarian hormones for a prolonged duration (8 weeks). Thus, non-feminizing estrogens may represent a new therapeutic approach for treating the neuronal damage associated with global ischemia.     

Neuroimaging the menstrual cycle: A multimodal systematic review

Increasing evidence indicates that ovarian hormones affect brain structure, chemistry and function of women in their reproductive age, potentially shaping their behavior and mental health. Throughout the reproductive years, estrogens and progesterone levels fluctuate across the menstrual cycle and can modulate neural circuits involved in affective and cognitive processes. Here, we review seventy-seven neuroimaging studies and provide a comprehensive and data-driven evaluation of the accumulating evidence on brain plasticity associated with endogenous ovarian hormone fluctuations in naturally cycling women (n = 1304). The results particularly suggest modulatory effects of ovarian hormones fluctuations on the reactivity and structure of cortico-limbic brain regions. These findings highlight the importance of performing multimodal neuroimaging studies on neural correlates of systematic ovarian hormone fluctuations in naturally cycling women based on careful menstrual cycle staging.     

Sexually Dimorphic Neurosteroid Synthesis Regulates Neuronal Activity in the Murine Brain

Sex steroid hormones act on hypothalamic kisspeptin neurons to regulate reproductive neural circuits in the brain. Kisspeptin neurons start to express estrogen receptors in utero, suggesting steroid hormone action on these cells early during development. Whether neurosteroids are locally produced in the embryonic brain and impinge onto kisspeptin/reproductive neural circuitry is not known. To address this question, we analyzed aromatase expression, a key enzyme in estrogen synthesis, in male and female mouse embryos. We identified an aromatase neuronal network comprising ∼6000 neurons in the hypothalamus and amygdala. By birth, this network has become sexually dimorphic in a cluster of aromatase neurons in the arcuate nucleus adjacent to kisspeptin neurons. We demonstrate that male arcuate aromatase neurons convert testosterone to estrogen to regulate kisspeptin neuron activity. We provide spatiotemporal information on aromatase neuronal network development and highlight a novel mechanism whereby aromatase neurons regulate the activity of distinct neuronal populations expressing estrogen receptors.SIGNIFICANCE STATEMENT Sex steroid hormones, such as estradiol, are important regulators of neural circuits controlling reproductive physiology in the brain. Embryonic kisspeptin neurons in the hypothalamus express steroid hormone receptors, suggesting hormone action on these cells in utero. Whether neurosteroids are locally produced in the brain and impinge onto reproductive neural circuitry is insufficiently understood. To address this question, we analyzed aromatase expression, a key enzyme in estradiol synthesis, in mouse embryos and identified a network comprising ∼6000 neurons in the brain. By birth, this network has become sexually dimorphic in a cluster of aromatase neurons in the arcuate nucleus adjacent to kisspeptin neurons. We demonstrate that male aromatase neurons convert testosterone to estradiol to regulate kisspeptin neuron activity.     

Nigrostriatal collaterals to thalamus degenerate in parkinsonian animal models

Movement, cognition, emotion, and positive reinforcement are influenced by mesostriatal, mesocortical, and mesolimbic dopamine systems. We describe a fourth major pathway originating from mesencephalic dopamine neurons: a mesothalamic system. The dopamine transporter, specific to dopamine containing axons, was histochemically visualized in thalamic motor and limbic-related nuclei and regions that modulate behavioral state as opposed to sensory nuclei in rats, nonhuman primates, and humans. Anatomical tracing established this innervation's origin via axon collaterals from the mesostriatal pathway. These findings implicate the thalamus as a novel site for disease specific alterations in dopamine neurotransmission, such as exist with nigral degeneration attending Parkinson's disease. This was confirmed in hemiparkinsonian animals where reduction of thalamic dopamine innervation occurred coincident with signs of active axonal degeneration. Individual mesencephalic dopamine neurons therefore have the potential to modulate normal and pathologic behavior not only through traditional nigrostriatal pathways but also by way of axon collaterals that innervate the thalamus.     

Vesicular acetylcholine transporter knock-down mice show sexual dimorphism on memory

The key neural substrates involved in memory and cognitive tasks have been reported to receive important modulation from ovarian hormones. In fact, neurochemical systems associated with cognitive functions, such as the cholinergic system, are, at least in part, under modulation of estrogens. Here we show that vesicular acetylcholine transporter (VAChT) mutant mice, which express lower levels of the VAChT (VAChT KD) and reduced acetylcholine release, present sexual dimorphism on memory. We evaluate short- and long-term object recognition memories (STM and LTM) in both sexes. We have showed previously, and confirm here, that VAChT KDHET male mice present deficits in both STM and LTM object recognition memories in comparison with WT. In contrast, VAChT KDHET female mice present deficit in LTM, but not in STM. To test if the female hormones levels could be a determinant factor on sexual dimorphism observed, we submitted female mice to ovariectomy (OVX) or sham-surgery. After 1 week (1 w), we evaluate STM. Female hormone deprivation promotes STM impairment in VAChT KDHET, but not in WT female mice. Our results strongly suggest that the sexual dimorphism observed in VAChT KDHET mice on STM is due to modulation of cholinergic system by ovarian hormones.     

The psychological, neurochemical and functional neuroanatomical mediators of the effects of positive and negative mood on executive functions

In this review we evaluate the cognitive and neural effects of positive and negative mood on executive function. Mild manipulations of negative mood appear to have little effect on cognitive control processes, whereas positive mood impairs aspects of updating, planning and switching. These cognitive effects may be linked to neurochemistry: with positive mood effects mediated by dopamine while negative mood effects may be mediated by serotonin levels. Current evidence on the effects of mood on regional brain activity during executive functions, indicates that the prefrontal cortex is a recurrent site of integration between mood and cognition. We conclude that there is a disparity between the importance of this topic and awareness of how mood affects, executive functions in the brain. Most behavioural and neuroimaging studies of executive function in normal samples do not explore the potential role of variations in mood, yet the evidence we outline indicates that even mild fluctuations in mood can have a significant influence on neural activation and cognition.     

17β-estradiol replacement in young, adult and middle-aged female ovariectomized rats promotes improvement of spatial reference memory and an antidepressant effect and alters monoamines and BDNF levels in memory- and depression-related brain areas

Clinical and experimental evidence suggest that estrogens have a major impact on cognition, presenting neurotrophic and neuroprotective actions in regions involved in such function. In opposite, some studies indicate that certain hormone therapy regimens may provoke detrimental effects over female cognitive and neurological function. Therefore, we decided to investigate how estrogen treatment would influence cognition and depression in different ages. For that matter, this study assessed the effects of chronic 17β-estradiol treatment over cognition and depressive-like behaviors of young (3 months old), adult (7 months old) and middle-aged (12 months old) reproductive female Wistar rats. These functions were also correlated with alterations in the serotonergic system, as well as hippocampal BDNF. 17β-Estradiol treatment did not influence animals’ locomotor activity and exploratory behavior, but it was able to improve the performance of adult and middle-aged rats in the Morris water maze, the latter being more responsive to the treatment. Young and adult rats displayed decreased immobility time in the forced swimming test, suggesting an effect of 17β-estradiol also over such depressive-like behavior. This same test revealed increased swimming behavior, triggered by serotonergic pathway, in adult rats. Neurochemical evaluations indicated that 17β-estradiol treatment was able to increase serotonin turnover rate in the hippocampus of adult rats. Interestingly, estrogen treatment increased BDNF levels from animals of all ages. These findings support the notion that the beneficial effects of 17β-estradiol over spatial reference memory and depressive-like behavior are evident only when hormone therapy occurs at early ages and early stages of hormonal decline.     

Progestins inhibit the neuroprotective effects of estrogen in rat hippocampus

Although estrogen has beneficial actions in brain, recent clinical trials demonstrated adverse neural effects of hormone therapy in postmenopausal women. The cause(s) of this disconnect between experimental and clinical findings may include unanticipated effects of progestins. We report that both natural progesterone and the clinical progestin medroxyprogesterone acetate block estrogen neuroprotection. These findings underscore the need to evaluate neural actions of progestins in the rational design of hormone therapy.     

Estrogen selectively increases tryptophan hydroxylase-2 mRNA expression in distinct subregions of rat midbrain raphe nucleus: association between gene expression and anxiety behavior in the open field.

BACKGROUND: Ovarian steroids modulate anxiety behavior, perhaps by regulating the serotonergic neurons in the midbrain raphe nucleus. The regulation of the brain-specific isoform of rat tryptophan hydroxylase (TPH2) by ovarian hormones has not yet been investigated. Therefore, we examined the effects of estrogen and progesterone on TPH2 mRNA in the rat dorsal and median raphe nuclei (DRN and MRN, respectively) and whether TPH2 mRNA levels correlated with anxiety behavior. METHODS: Ovariectomized rats were treated for two weeks with placebo, estrogen or estrogen plus progesterone, exposed to the open field test, and TPH2 mRNA was quantified by in situ hybridization histochemistry. RESULTS: Estrogen increased TPH2 mRNA in the mid-ventromedial and caudal subregions of the DRN and the caudal MRN. Combined estrogen and progesterone treatment did not change TPH2 mRNA relative to ovariectomized controls. TPH2 mRNA in caudal DRN was associated with lower anxiety-like behavior, whereas TPH2 mRNA in rostral dorsomedial DRN was associated with increased anxiety-like behavior. CONCLUSIONS: These results suggest that estrogen may increase the capacity for serotonin synthesis in discrete subgroups of raphe neurons, and reinforce previous observations that different subregions of DRN contribute to distinct components of anxiety behavior.     

The GnRH System of Seasonal Breeders: Anatomy and Plasticity

Seasonal breeders, such as sheep and hamsters, by virtue of their annual cycles of reproduction, represent valuable models for the study of plasticity in the adult mammalian neuroendocrine brain. A major factor responsible for the occurrence of seasonal reproductive transitions is a striking change in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the inhibitory effects of gonadal steroids. However, the neural circuitry mediating these seasonal changes is still relatively unexplored. In this article, we review recent findings that have begun to define that circuitry and its plasticity in a well-studied seasonal breeder, the ewe. Tract tracing studies and immunocytochemical analyses using Fos and FRAs as markers of activation point to a subset of neuroendocrine GnRH neurons in the MBH as potential mediators of pulsatile GnRH secretion. Because the vast majority of GnRH neurons lack estrogen receptors, seasonal changes in responsiveness to estradiol are most probably conveyed by afferents. Two possible mediators of this influence are dopaminergic cells in the A14/A15 cell groups of the hypothalamus, and estrogen receptor-containing cells in the arcuate nucleus that project to the median eminence. The importance of GnRH afferents in the regulation of season breeding is underscored by observations of seasonal changes in the density of synaptic inputs onto GnRH neurons. Thyroid hormones may participate in this remodeling, because they are important in seasonal reproduction, influence the morphology of other brain systems, and thyroid hormone receptors are expressed within GnRH neurons. Finally, in the hamster, neonatal hypothyroidism affects the number of caudally placed GnRH neurons in the adult brain, suggesting that thyroid hormones may influence development of the GnRH system as well as its reproductive functions in the adult brain.     

Effects of Ovariectomy on GnRH Neuronal Morphology inRhesus Monkey (Macaca mulatta)

Gonadotropin releasing hormone (GnRH) neurons are typically simple, fusiform cells; however, over the course of prepubertal development increasing numbers take on a 'spiny' appearance. Following gonadectomy there is a decrease in the frequency of these spiny GnRH neurons. These observations which were made in the rat suggest that GnRH neurons are directly affected by the gonadal steroid milieu, though they do not themselves contain receptors for these steroidal hormones. In that there are important species differences in the hypothalamic-pituitary-gonadal axis between rats and primates, the present study was undertaken to determine whether a reduction in ovarian hormones would produce similar changes in the morphology of GnRH neurons in the monkey. A further aim was to determine whether such changes were localized to a specific brain region.     

Ovarian hormones influence the morphology,distribution, and density of tyrosine hydroxylase immunoreactive axons in the dorsolateral prefrontal cortex of adult Rhesus monkeys

The maturation, adult functioning and dysfunction of the prefrontal cortex in disorders such as schizophrenia show gender biases in human and non-human primates. Although the basis for the hormone influence suggested in these observations is unknown, one possibility is that circulating hormones stimulate catecholamine innervation in the frontal lobe. This innervation is essential for prefrontal cortical function, and gonadal, especially ovarian hormones, profoundly influence catecholamine function and physiology in subcortical structures. This study was undertaken to determine whether influence is also exerted upon the catecholamine innervation of the association cortex by combining ovarian hormone manipulation with immunocytochemistry for tyrosine hydroxylase in the dorsolateral prefrontal cortex of adult female macaque monkeys. Qualitative and quantitative analyses of immunoreactive fibers were carried out and compared in cortices of ovariectomized animals, ovariectomized animals treated with estrogen, ovariectomized animals treated with estrogen followed by progesterone, and in intact, age- and sex-matched controls. These analyses revealed striking, layer-specific anomalies in fiber morphology and profound reductions in fiber density in ovariectomized animals. While hormone replacement with estrogen alone had limited influence, estrogen followed by progesterone was particularly effective in restoring tyrosine hydroxylase innervation in ovariectomized animals. Thus, ovarian hormones appear to be potent regulators of the catecholamine innervation of the primate prefrontal cortex. Such regulation is anticipated in the gender differences observed in prefrontal cortical development and function, and may also be relevant for the prefrontal dysfunction in disorders such as schizophrenia. J. Comp. Neurol. 395:1–17, 1998. © 1998 Wiley-Liss, Inc.     

Serotonin, via HTR2 receptors, excites neurons in a cortical-like premotor nucleus necessary for song learning and production

Serotonin (5-HT) is a neuromodulator that is important for neural development, learning and memory, mood, and perception. Dysfunction of the serotonin system is central to depression and other clinically important mood disorders and has been linked with learning deficits. In mammals, 5-HT release from the raphe nuclei in the brainstem can modulate the functional properties of cortical neurons, influencing sensory and motor processing. Birds also have serotonergic neurons in the dorsal raphe, suggesting that 5-HT plays similar roles in sensory and motor processing, perhaps modulating brain circuitry underlying birdsong. To investigate this possibility, we measured the effects of 5-HT on spontaneous firing of projection neurons in the premotor robust nucleus of the arcopallium in brain slices from male zebra finches. These neurons are thought be akin to cortical layer V pyramidal neurons. 5-HT dramatically and reversibly enhanced the endogenous firing of RA neurons. Using pharmacological agonists and antagonists in vitro, we determined this action is mediated via HTR2 receptors, which we verified are expressed by in situ hybridization. Finally, focal administration of the serotonin selective reuptake inhibitor fluvoxamine revealed that endogenous 5-HT is sufficient to mediate this effect in vivo. These findings reveal a modulatory action of serotonin on the physiology of the song system circuitry and suggest a novel role of serotonin in regulating song production and/or learning; further understanding of the role of 5-HT in this system may help illuminate the complex role of this neuromodulator in social interactions and motor plasticity in humans.