Demonstration of serotoninergic axons terminating on luteinizing hormone-releasing hormone neurons in the preoptic area of the rat using a combination of immunocytochemistry and high resolution autoradiography

Serotonin-immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus

Serotonin (5-HT) is an evolutionarily conserved monoaminergic neurotransmitter found in the central nervous system and peripheral nervous system across invertebrates and vertebrates. Although the distribution of 5-HT-immunoreactive (5-HT-ir) neurons is investigated in various fish species, the organization of these neurons in cichlid fishes is poorly understood. These fish are known for their adaptability to diverse environments, food habits, and complex mating and breeding behaviors, including parental care. In this paper, we describe the organization of 5-HT-ir neurons in the brain of the cichlid fish Oreochromis mossambicus. Aggregations of 5-HT-ir neurons were spotted in the granule cell layer of the olfactory bulb and near the ventricular border in the preoptic area and magnocellular subdivisions of the nucleus preopticus. Although the presence of 5-HT-ir cells and fibers in the hypothalamic and thalamic regions, cerebellum, and raphe nuclei was comparable to that of other teleosts, the current study reveals the occurrence of 5-HT-ir cells and fibers for the first time in some areas, such as the nucleus posterior tuberis, nucleus oculomotorius, and nucleus paracommissuralis in the tilapia. While the presence of 5-HT-ir cells and fibers in gustatory centers suggests a role for serotonin in the processing of gustatory signals, distinctive pattern of 5-HT immunoreactivity was seen in the telencephalon, pretectal areas, mesencephalic, and rhombencephalic regions, suggesting a cichlid fish specific organization of the serotonergic system. In conclusion, the 5-HT system in the tilapia brain may serve several neuroendocrine and neuromodulatory roles, including regulation of reproduction and sensorimotor processes.

Serotonin stimulates female preoptic area kisspeptin neurons via activation of type 2 serotonin receptors in mice

Background

The neuroendocrine control of ovulation is orchestrated by neuronal circuits that ultimately drive the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus to trigger the preovulatory surge in luteinizing hormone (LH) secretion. While estrogen feedback signals are determinant in triggering activation of GnRH neurons, through stimulation of afferent kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3VKISS1 neurons), many neuropeptidergic and classical neurotransmitter systems have been shown to regulate the LH surge. Among these, several lines of evidence indicate that the monoamine neurotransmitter serotonin (5-HT) has an excitatory, permissive, influence over the generation of the surge, via activation of type 2 5-HT (5-HT2) receptors. The mechanisms through which this occurs, however, are not well understood. We hypothesized that 5-HT exerts its influence on the surge by stimulating RP3VKISS1 neurons in a 5-HT2 receptor-dependent manner.

Methods

We tested this using kisspeptin neuron-specific calcium imaging and electrophysiology in brain slices obtained from male and female mice.

Results

We show that exogenous 5-HT reversibly increases the activity of the majority of RP3VKISS1 neurons. This effect is more prominent in females than in males, is likely mediated directly at RP3VKISS1 neurons and requires activation of 5-HT2 receptors. The functional impact of 5-HT on RP3VKISS1 neurons, however, does not significantly vary during the estrous cycle.

Conclusion

Taken together, these data suggest that 5-HT2 receptor-mediated stimulation of RP3VKISS1 neuron activity might be involved in mediating the influence of 5-HT on the preovulatory LH surge.

Neuro-pharmacological reinstatement of ovulation and associated neurobiology in a macaque model of functional hypothalamic amenorrhoea

STUDY QUESTION

What is the underlying neuropathology in a cynomolgus macaque model of functional hypothalamic amenorrhoea (FHA) and can it be normalized to restore ovulation?

SUMMARY ANSWER

Anovulatory monkeys exhibited increased hypothalamic norepinephrine (NE), kisspeptin and gonadotropin-releasing hormone (GnRH) in the early follicular phase, but administration of the NE reuptake inhibitor (NRI), reboxetine (REB), restored ovulation during stress and normalized NE, kisspeptin and GnRH.

WHAT IS KNOWN ALREADY

Female cynomolgus macaques, like women, show individual reproductive sensitivity to modest psychosocial and metabolic stress. During stress, resilient females ovulate through two menstrual cycles whereas stress-sensitive (SS) macaques immediately cease ovulation. On Day 5 of a non-stressed menstrual cycle, resilient macaques have less NE synthesizing enzyme [dopamine β-hydroxylase (DBH)], kisspeptin and GnRH innervation of the medial basal hypothalamus but more endogenous serotonin than SS macaques. Stress increased DBH/NE, kisspeptin and GnRH but did not alter serotonin.

STUDY DESIGN, SIZE, DURATION

In a longitudinal design, 27 adult (7-13 years) female cynomolgus macaques (Macaca fascicularis) with three different levels of sensitivity to stress were monitored with daily vaginal swabs and frequent serum progesterone (P) measurements. Three 90-day experimental periods called 'Cycle Sets' were monitored. A Cycle Set consisted of one ovulatory menstrual cycle without stress, and two cycles, or 60 days, with modest stress. Each Cycle Set was followed by a rest period. During a Cycle Set, individuals were either untreated (placebo) or administered escitalopram (CIT) or REB. Ultimately, half of each sensitivity group was euthanized during stress with CIT or REB treatment and the hypothalamus was obtained. Neurobiological endpoints were compared between CIT and REB treatment groups in stress resilient and SS monkeys.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The monkeys were housed at the University of Pittsburgh primate facility for the duration of the experiments. Upon euthanasia, their brains and serum samples were shipped to the Oregon National Primate Research Center. The hypothalamus was examined with immunohistochemistry for the expression of DBH (a marker for NE axons), kisspeptin and GnRH. P was measured in the serum samples by radioimmunoassay.

MAIN RESULTS AND THE ROLE OF CHANCE

Daily administration of REB restored ovulation in 9 of 10 SS animals during stress. Of note, REB significantly increased P secretion during stress in the most sensitive group (P = 0.032), which indicates ovulation. CIT lacked efficacy. REB significantly reduced DBH/NE, kisspeptin and GnRH axon density in the hypothalamus relative to CIT treatment (P = 0.003. 0.018 and 0.0001, respectively) on Day 5 of the menstrual cycle in resilient and sensitive groups.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The US FDA has not approved REB for human use, although it is used in Europe for the treatment of depression/anxiety as EdronaxTR. Whether REB could be useful for the treatment of FHA in women has not been determined.

WIDER IMPLICATIONS FOR THE FINDINGS

The use of an NRI to treat FHA is a novel approach and the potential reinstatement of ovulation could be straightforward compared to current treatment protocols. The underlying neurobiology provides a compelling case for treating the origin of the pathology, i.e. elevated NE, rather than circumventing the hypothalamus altogether with gonadotropins, which have associated risks such as hyperstimulation syndrome or multiple births.

STUDY FUNDING/COMPETING INTEREST(S)

Portions of this study were supported by NIH grant HD062864 to C.L.B., NIH grant HD62618 to J.L.C. and C.L.B. and 1P51 OD011092 for the operation of the Oregon National Primate Research Center. There were no competing interests.

Serum and follicular fluid levels of serotonin, kisspeptin, and brain-derived neurotrophic factor in patients undergoing in vitro fertilization: an observational study : Neurohormones in patients receiving IVF

OBJECTIVE

This study aimed to examine the effect of interactions between serotonin (5-HT), brain-derived neurotrophic factor (BDNF), and kisspeptin on the reproductive potential in women receiving in vitro fertilization (IVF).

METHODS

Paired serum and follicular fluid (FF) samples were obtained from 30 consecutive patients receiving IVF. Primary and secondary outcome measures were the rate of chemical/clinical pregnancy and the number of mature oocytes and embryos, respectively. Serum and FF 5-HT, BDNF, kisspeptin, and platelet-activating factor (PAF) levels were measured by enzyme-linked immunosorbent assay.

RESULTS

In response to ovarian hyperstimulation, serum 5-HT and kisspeptin levels significantly increased, whereas serum BDNF and PAF levels remained unchanged. These factors were detected in FF, but they were unrelated to serum levels. FF 5-HT and BDNF levels were positively correlated. Serum kisspeptin levels were negatively correlated with FF BDNF and serum and FF PAF levels. Women who were pregnant had significantly lower FF BDNF levels compared with women who were not pregnant (21.96±12.75 vs 47.63±52.90 µg/mL). Multivariate stepwise linear regression and logistic regression analyses showed that only 5-HT and kisspeptin improved IVF outcome.

CONCLUSIONS

This study indicates a role of serotoninergic mechanisms in success of IVF, but the contribution of interacting neuropeptides requires additional investigation.

Altered Expression of Genes Encoding Neurotransmitter Receptors in GnRH Neurons of Proestrous Mice

Gonadotropin-releasing hormone (GnRH) neurons play a key role in the central regulation of reproduction. In proestrous female mice, estradiol triggers the pre-ovulatory GnRH surge, however, its impact on the expression of neurotransmitter receptor genes in GnRH neurons has not been explored yet. We hypothesized that proestrus is accompanied by substantial changes in the expression profile of genes coding for neurotransmitter receptors in GnRH neurons. We compared the transcriptome of GnRH neurons obtained from intact, proestrous, and metestrous female GnRH-GFP transgenic mice, respectively. About 1500 individual GnRH neurons were sampled from both groups and their transcriptome was analyzed using microarray hybridization and real-time PCR. In this study, changes in mRNA expression of genes involved in neurotransmitter signaling were investigated. Differential gene expression was most apparent in GABA-ergic (Gabbr1, Gabra3, Gabrb3, Gabrb2, Gabrg2), glutamatergic (Gria1, Gria2, Grin1, Grin3a, Grm1, Slc17a6), cholinergic (Chrnb2, Chrm4) and dopaminergic (Drd3, Drd4), adrenergic (Adra1b, Adra2a, Adra2c), adenosinergic (Adora2a, Adora2b), glycinergic (Glra), purinergic (P2rx7), and serotonergic (Htr1b) receptors. In concert with these events, expression of genes in the signaling pathways downstream to the receptors, i.e., G-proteins (Gnai1, Gnai2, Gnas), adenylate-cyclases (Adcy3, Adcy5), protein kinase A (Prkaca, Prkacb) protein kinase C (Prkca) and certain transporters (Slc1a4, Slc17a6, Slc6a17) were also changed. The marked differences found in the expression of genes involved in neurotransmitter signaling of GnRH neurons at pro- and metestrous stages of the ovarian cycle indicate the differential contribution of these neurotransmitter systems to the induction of the pre-ovulatory GnRH surge, the known prerequisite of the subsequent hormonal cascade inducing ovulation.

Maternal Dexamethasone Exposure Alters Synaptic Inputs to Gonadotropin-Releasing Hormone Neurons in the Early Postnatal Rat

Maternal dexamethasone [(DEX); a glucocorticoid receptor agonist] exposure delays pubertal onset and alters reproductive behavior in the adult offspring. However, little is known whether maternal DEX exposure affects the offspring's reproductive function by disrupting the gonadotropin-releasing hormone (GnRH) neuronal function in the brain. Therefore, this study determined the exposure of maternal DEX on the GnRH neuronal spine development and synaptic cluster inputs to GnRH neurons using transgenic rats expressing enhanced green fluorescent protein (EGFP) under the control of GnRH promoter. Pregnant females were administered with DEX (0.1 mg/kg) or vehicle (VEH, water) daily during gestation day 13-20. Confocal imaging was used to examine the spine density of EGFP-GnRH neurons by three-dimensional rendering and synaptic cluster inputs to EGFP-GnRH neurons by synapsin I immunohistochemistry on postnatal day 0 (P0) males. The spine morphology and number on GnRH neurons did not change between the P0 males following maternal DEX and VEH treatment. The number of synaptic clusters within the organum vasculosum of the lamina terminalis (OVLT) was decreased by maternal DEX exposure in P0 males. Furthermore, the number and levels of synaptic cluster inputs in close apposition with GnRH neurons was decreased following maternal DEX exposure in the OVLT region of P0 males. In addition, the postsynaptic marker molecule, postsynaptic density 95, was observed in GnRH neurons following both DEX and VEH treatment. These results suggest that maternal DEX exposure alters neural afferent inputs to GnRH neurons during early postnatal stage, which could lead to reproductive dysfunction during adulthood.

Dorsal and medial raphe nuclei participate differentially in reproductive functions of the male rat

BACKGROUND

Innervation of the hypothalamus and median eminence arise from the dorsal and medial raphe nuclei (DRN and MRN, respectively). The hypothalamus regulates the secretion of gonadotropins, which in turn regulate the reproductive function of males and females. However, it is not known the role of raphe nuclei in male reproductive function. Our goal was to investigate the role of the DRN and MRN in the regulation of the testicular function and secretion of gonadotropins in prepubertal rats.

METHODS

Dihydroxytryptamine (5,6-DHT) in ascorbic acid was used to chemically lesion the DRN or MRN. Rats were treated at 30 days-of-age and sacrificed at 45 or 65 days-of-age. Sham-treated controls were injected with ascorbic acid only. Negative controls were untreated rats. The damage induced by the 5,6-DHT was monitored in coronal serial sections of DRN and MRN; only the animals in which lesion of the DRN or MRN was detected were included in this study. As output parameters, we measured the concentrations of noradrenaline (NA), dopamine (DA) and serotonin (5-HT) in the anterior (AH) and medial (MH) hypothalamus by high performance liquid chromatography (HPLC); whereas, circulating concentrations of gonadotropins and sexual steroids were measured by radioimmunoassay. Seminiferous epithelium and sperm quality were also evaluated.

RESULTS

Lesion of DRN or MRN does not induced changes in concentrations of LH, progesterone, and testosterone. Compared with the control group, the sham or lesion of the DRN or MRN did not modify noradrenaline or dopamine concentrations in the AH and MH at 45 or 65 days of age. Meanwhile, serotonin concentrations decreased significantly in lesioned rats. Lesion of DRN induced significantly lower concentrations of FSH regardless of age; similar lesion in the MRN had no impact on FSH levels. Sperm concentration and motility were significantly decreased in the same animals. The lesion of the MRN does not induced changes in the seminiferous epithelium or gonadotropin levels. Our results suggest that raphe nuclei regulate differentially the male reproductive functions.

CONCLUSIONS

The DRN but not the MRN regulates the secretion of gonadotropins and testicular function.

Serotonin reuptake inhibitor citalopram inhibits GnRH synthesis and spermatogenesis in the male zebrafish

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressants for the treatment of depression. However, SSRIs cause sexual side effects such as anorgasmia, erectile dysfunction, and diminished libido that are thought to be mediated through the serotonin (5-hydroxytryptamine, 5-HT) system. In vertebrates, gonadotropin-releasing hormone (GnRH) neurons play an important role in the control of reproduction. To elucidate the neuroendocrine mechanisms of SSRI-induced reproductive failure, we examined the neuronal association between 5-HT and GnRH (GnRH2 and GnRH3) systems in the male zebrafish. Double-label immunofluorescence and confocal laser microscopy followed by three-dimensional construction analysis showed close associations between 5-HT fibers with GnRH3 fibers and preoptic-GnRH3 cell bodies, but there was no association with GnRH2 cell bodies and fibers. Quantitative real-time PCR showed that short-term treatment (2 wk) with low to medium doses (4 and 40 μg/L, respectively) of citalopram significantly decreased mRNA levels of gnrh3, gonadotropins (lhb and fshb) and 5-HT-related genes (tph2 and sert) in the male zebrafish. In addition, short-term citalopram treatment significantly decreased the fluorescence density of 5-HT and GnRH3 fibers compared with controls. Short-term treatment with low, medium, and high (100 μg/L) citalopram doses had no effects on the profiles of different stages of spermatogenesis, while long-term (1 mo) citalopram treatment with medium and high doses significantly inhibited the different stages of spermatogenesis. These results show morphological and functional associations between the 5-HT and the hypophysiotropic GnHR3 system, which involve SSRI-induced reproductive failures.

Possible role of serotonin and neuropeptide Y on the disruption of the reproductive axis activity by perfluorooctane sulfonate

Perfluorooctane sulfonate (PFOS) is an endocrine disruptor, whose exposure can induce several alterations on the reproductive axis activity in males during adulthood. This study was undertaken to evaluate the possible role of serotonin and neuropeptide Y (NPY) on the disruption of the hypothalamic-pituitary-testicular (HPT) axis induced by PFOS in adult male rats. For that, adult male rats were orally treated with 0.5; 1.0; 3.0 and 6.0mg of PFOS/kg/day for 28 days. After PFOS exposure, serotonin concentration increased in the anterior and mediobasal hypothalamus as well as in the median eminence. The metabolism of this amine (expressed as the ratio 5-hydroxyindolacetic acid (5-HIAA)/serotonin) was diminished except in the anterior hypothalamus, with the doses of 3.0 and 6.0mg/kg/day, being this dose 0.5mg/kg/day in the median eminence. In general terms, PFOS-treated rats presented a decrease of the hypothalamic concentration of the gonadotropin releasing hormone (GnRH) and NPY. A diminution of the serum levels of the luteinizing hormone (LH), testosterone and estradiol were also shown. These results suggest that both serotonin and NPY could be involved in the inhibition induced by PFOS on the reproductive axis activity in adult male rats.

Serotonin acts through 5-HT1 and 5-HT2 receptors to exert biphasic actions on GnRH neuron excitability in the mouse

The effect of serotonin (5-HT) on the electrical excitability of GnRH neurons was examined using gramicidin perforated-patch electrophysiology in transgenic GnRH-green fluorescent protein mice. In diestrous female, the predominant effect of 5-HT was inhibition (70%) with 50% of these cells also exhibiting a late-onset excitation. Responses were dose dependent (EC(50) = 1.2μM) and persisted in the presence of amino acid receptor antagonists and tetrodotoxin, indicating a predominant postsynaptic action of 5-HT. Studies in neonatal, juvenile, peripubertal, and adult mice revealed that 5-HT exerted less potent responses from GnRH neurons with advancing postnatal age in both sexes. In adult male mice, 5-HT exerted less potent hyperpolarizing responses with more excitations compared with females. In addition, adult proestrous female GnRH neurons exhibited reduced inhibition and a complete absence of biphasic hyperpolarization-excitation responses. Studies using 5-HT receptor antagonists demonstrated that the activation of 5-HT(1A) receptors mediated the inhibitory responses, whereas the excitation was mediated by the activation of 5-HT(2A) receptors. The 5-HT-mediated hyperpolarization involved both potassium channels and adenylate cyclase activation, whereas the 5-HT excitation was dependent on protein kinase C. The effects of exogenous 5-HT were replicated using fluoxetine, which enhances endogenous 5-HT levels. These studies demonstrate that 5-HT exerts a biphasic action on most GnRH neurons whereby a fast 5HT(1A)-mediated inhibition occurs alongside a slow 5-HT(2A) excitation. The balance of 5-HT-evoked inhibition vs excitation is developmentally regulated, sexually differentiated, and variable across the estrous cycle and may play a role in regulation of hypothalamic-pituitary-gonadal axis throughout postnatal development.

Control of GnRH secretion: one step back

The reproductive system is controlled by gonadotropin releasing hormone (GnRH) secretion from the brain, which is finely modulated by a number of factors including gonadal sex steroids. GnRH cells do not express estrogen receptor α, but feedback is transmitted by neurons that are at least 'one step back' from the GnRH cells. Modulation by season, stress and nutrition are effected by neuronal pathways that converge on the GnRH cells. Kisspeptin and gonadotropin inhibitory hormone (GnIH) neurons are regulators of GnRH secretion, the former being a major conduit for transmission of sex steroid feedback. GnIH cells project to GnRH cells and may play a role in the seasonal changes in reproductive activity in sheep. GnIH also modulates the action of GnRH at the level of the pituitary gonadotrope. This review focuses on the role that kisspeptin and GnIH neurons play, as modulators that are 'one step back' from GnRH neurons.

Pharmaceuticals as neuroendocrine disruptors: lessons learned from fish on Prozac

Pharmaceuticals are increasingly detected in a variety of aquatic systems. One of the most prevalent environmental pharmaceuticals in North America and Europe is the antidepressant fluoxetine, a selective serotonin reuptake inhibitor (SSRI) and the active ingredient of Prozac. Usually detected in the range below 1 μg/L, fluoxetine and its active metabolite norfluoxetine are found to bioaccumulate in wild-caught fish, particularly in the brain. This has raised concerns over potential disruptive effects of neuroendocrine function in teleost fish, because of the known role of serotonin (5-HT) in the modulation of diverse physiological processes such as reproduction, food intake and growth, stress and multiple behaviors. This review describes the evolutionary conservation of the 5-HT transporter (the therapeutic target of SSRIs) and reviews the disruptive effects of fluoxetine on several physiological endpoints, including involvement of neuroendocrine mechanisms. Studies on the goldfish, Carassius auratus, whose neuroendocrine regulation of reproduction and food intake are well characterized, are described and represent a reliable model to study neuroendocrine disruption. In addition, fish studies investigating the effects of fluoxetine, not only on reproduction and food intake, but also on stress and behavior, are discussed to complement the emerging picture of neuroendocrine disruption of physiological systems in fish exposed to fluoxetine. Environmental relevance and key lessons learned from the effects of the antidepressant fluoxetine on fish are highlighted and may be helpful in designing targeted approaches for future risk assessments of pharmaceuticals disrupting the neuroendocrine system in general.

Pulsatile GnRH secretion: roles of G protein-coupled receptors, second messengers and ion channels

The pulsatile secretion of GnRH from normal and immortalized hypothalamic GnRH neurons is highly calcium-dependent and is stimulated by cAMP. It is also influenced by agonist activation of the endogenous GnRH receptor (GnRH-R), which couples to multiple G proteins. This autocrine mechanism could serve as a timer to determine the frequency of pulsatile GnRH release by regulating Ca(2+)- and cAMP-dependent signaling and GnRH neuronal firing. The firing of individual and/or bursts of action potentials (APs) in spontaneously active GnRH neurons is followed by afterhyperpolarization (AHP) that lasts from several milliseconds to several seconds. GnRH-induced activation of GnRH neurons causes a significant increase in medium AHP that is partially sensitive to apamin. GnRH-induced modulation of Ca(2+) influx and the consequent changes in AHP current suggest that the GnRH receptors expressed in hypothalamic GnRH neurons are important modulators of their neuronal excitability. The coexistence of multiple regulatory mechanisms could provide a high degree of redundancy in the maintenance of this crucial component of the reproductive process. It is also conceivable that this multifactorial system could reflect the gradation from simple to more complex neuroendocrine control systems for regulating hypothalamo-pituitary function and gonadal activity during the evolution of the GnRH pulse generator.

Morphology and distribution of neurons expressing serotonin 5-HT1A receptors in the rat hypothalamus and the surrounding diencephalic and telencephalic areas

Disorders of serotonergic neurotransmission are involved in disturbances of numerous hypothalamic functions including circadian rhythm, mood, neuroendocrine functions, sleep and feeding. Among the serotonin receptors currently recognized, 5-HT(1A) receptors have received considerable attention due to their importance in the etiology of mood disorders. While previous studies have shown the presence of 5-HT(1A) receptors in several regions of the rat brain, there is no detailed map of the cellular distribution of 5-HT(1A) receptors in the rat diencephalon. In order to characterize the distribution and morphology of the neurons containing 5-HT(1A) receptors in the diencephalon and the adjacent telencephalic areas, single label immunohistochemistry was utilized. Large, multipolar, 5-HT(1A)-immunoreactive (IR) neurons were mainly detected in the magnocellular preoptic nucleus and in the nucleus of diagonal band of Broca, while the supraoptic nucleus contained mainly fusiform neurons. Medium-sized 5-HT(1A)-IR neurons with triangular or round-shaped somata were widely distributed in the diencephalon, populating the zona incerta, lateral hypothalamic area, anterior hypothalamic nucleus, substantia innominata, dorsomedial and premamillary nuclei, paraventricular nucleus and bed nucleus of stria terminalis. The present study provides schematic mapping of 5-HT(1A)-IR neurons in the rat diencephalon. In addition, the morphology of the detected 5-HT(1A)-IR neural elements is also described. Since rat is a widely used laboratory animal in pharmacological models of altered serotoninergic neurotransmission, detailed mapping of 5-HT(1A)-IR structures is pivotal for the neurochemical characterization of the neurons containing 5-HT(1A) receptors.

Physiological endpoints for potential SSRI interactions in fish

Selective serotonin reuptake inhibitors (SSRIs) are among the pharmaceutical compounds frequently detected in sewage treatment plant effluents and surface waters, albeit at very low concentrations, and have therefore become a focus of interest as environmental pollutants. These neuroactive drugs are primarily used in the treatment of depression but have also found broader use as medication for other neurological dysfunctions, consequently resulting in a steady increase of prescriptions worldwide. SSRIs, via inhibition of the serotonin (5-hydroxytryptamine, 5-HT) reuptake mechanism, induce an increase in extracellular 5-HT concentration within the central nervous system of mammals. The phylogenetically ancient and highly conserved neurotransmitter and neurohormone 5-HT has been found in invertebrates and vertebrates, although its specific physiological role and mode of action is unknown for many species. Consequently, it is difficult to assess the impact of chronic SSRI exposure in the environment, especially in the aquatic ecosystem. In view of this, the current knowledge of the functions of 5-HT in fish physiology is reviewed and, via comparison to the physiological role and function of 5-HT in mammals, a characterization of the potential impact of chronic SSRI exposure on fish is provided. Moreover, the insight on the physiological function of 5-HT strongly suggests that the experimental approaches currently used are inadequate if not entirely improper for routine environmental risk assessment of pharmaceuticals (e.g., SSRIs), as relevant endpoints are not assessed or impossible to determine.

Definition of brainstem afferents to gonadotropin-releasing hormone neurons in the mouse using conditional viral tract tracing

Brainstem monoamines have long been considered to play a role in regulating the activity of GnRH neurons, although their neuroanatomical relationship with these cells has remained unclear. Using a Cre-dependent pseudorabies virus (Ba2001) technique that permits retrograde tracing selectively from GnRH neurons in the mouse, we have examined the organization of brainstem inputs to rostral preoptic area (rPOA) GnRH neurons. Two days after injection of Ba2001 into the rPOA of adult female GnRH-Cre transgenic mice, five to nine GnRH neurons located immediately adjacent to the injection site were found to express green fluorescent protein (GFP), the marker of virus infection, with no GFP expression anywhere else in the brain. In mice killed 24 h later (3 d after injection), GFP-expressing cells were identified (in order of density) in the raphe nuclei, periaqueductal grey, locus coeruleus, nucleus tractus solitarius, and area postrema. This time course is compatible with these neurons representing primary afferent inputs to the GnRH neurons. Four and 6 d after Ba2001 injection, GFP-expressing cells were found in additional brain regions. Dual-label immunofluorescence experiments in 3-d postinjection mice demonstrated that 100% of GFP-expressing neurons in the raphe were positive for tryptophan hydroxylase, whereas 100% and approximately 50% of GFP neurons in the locus coeruleus and nucleus tractus solitarius, respectively, expressed tyrosine hydroxylase. These observations demonstrate that rPOA GnRH neurons receive direct projections from brainstem A2 and A6 noradrenergic neurons and that, surprisingly, the largest afferent input from the brainstem originates from raphe serotonin neurons in the mouse.

Neural control of the synthesis and release of luteinizing hormone-releasing hormone

Preovulatory surges of luteinizing hormone (LH) depend upon neurotransmitter activation of neurons that secrete LH-releasing hormone (LHRH, gonadotropin-releasing hormone GnRH) and noradrenaline plays a pivotal role in this critical event. The interaction is amongst noradrenaline and other neurotransmitters such as GABA (gamma-aminobutyric acid), opiates, serotonin and excitatory amino acids (N-methyl-D-aspartate, NMDA) on LHRH neuronal activity are complex. GABA and opiates suppress the presynaptic release of noradrenaline but only GABA also directly affects the responsiveness of LHRH neurons to noradrenaline. Morphine induces the release of serotonin which either directly or indirectly via other neurotransmitters (e.g. dopamine) sensitizes LHRH neurons to the stimulatory effects of noradrenaline. NMDA rapidly induces LH release but whether this drug directly affects the activity of LHRH neurons is not known. The neuronal release of LHRH is modulated by the action of oestrogen on these various neurotransmitter systems. Antioestrogens, when placed into the medial preoptic area of otherwise completely oestrogenized rats, block LH surges; LHRH mRNA levels in such animals resemble those in 9-day castrated rats. Normally, LHRH message levels increase about the time of increased noradrenaline secretion just before the LH surge. NMDA rapidly releases LH and LHRH mRNA levels are significantly raised within 15 minutes and remain so over the next 45 minutes. Thus, it seems that stimuli which evoke LHRH release also increase LHRH mRNA transcription to replenish the hormone released during the LH surge.

Mechanisms mediating the effects of cytokines on neuroendocrine functions in the rat

Exposure to an antigen causes significant endocrine changes, some of which in turn affect immune functioning. Proteins produced by activated immune cells, cytokines, act as messengers between the immune and the endocrine systems, and convey to the brain the occurrence of immune activation. We have investigated the ability of interleukin 1 (IL-1) alpha and beta to alter endocrine functioning in the adult rat. Acute peripheral injection of IL-1 alpha or beta causes dose-dependent increases in plasma adrenocorticotropic hormone (ACTH) and corticosterone secretion. These changes are primarily dependent upon increased release of corticotropin-releasing factor (CRF) into the portal circulation, and recent studies have indicated that the paraventricular nucleus (PVN) of the hypothalamus is the main source of this CRF. This conclusion is based on our finding that intravenous injection of IL-1 increases CRF biosynthesis in the PVN, and that lesion of this hypothalamic area interferes with IL-1's stimulatory action on ACTH secretion. Indomethacin partially reverses the effect of IL-1, suggesting that increased prostaglandin synthesis plays some part in this activation. Administration of IL-1 beta into the brain, but not into the general circulation, interferes with secretion of luteinizing hormone (LH) and ovulation through mechanisms involving endogenous opiates. Because neither CRF antagonists, nor lesions of the PVN, alter the inhibitory effect of IL-1 on LH release, CRF perikarya in the PVN do not appear to be involved in this phenomenon. Central administration of IL-1 beta strongly increases c-Fos immunoreactivity in the PVN, mainly within CRF neurons. Infusion of IL-1 beta into the PVN does not induce measurable changes in release of gonadotropin-releasing hormone (GnRH), but infusion of IL-1 directly into the median preoptic area (MPOA), a region rich in GnRH perikarya, markedly decreases GnRH secretion in rats bearing a push-pull cannula in the median eminence. Furthermore, central administration of IL-1 beta during the critical phase of pro-oestrus (1600-1930) also inhibits the expression of c-fos in GnRH cell bodies in the MPOA. Thus, we suggest that IL-1 interferes with reproductive functioning through a direct action at the level of the MPOA. These results indicate that circulating cytokines can alter the activity of the hypothalamo-pituitary-adrenal axis by increasing CRF release, probably through both immediate stimulation of CRF terminals within the median eminence and stimulation of CRF synthesis in the PVN. In contrast, cytokine-induced changes in LH and GnRH secretion are mediated through pathways lying primarily beyond the blood-brain barrier.

Comparative study of the sources of neuronal projections to the site of gonadotrophin-releasing hormone perikarya and to the anteroventral periventricular nucleus in female rats

The rat ovulatory cycle is dependent on the preoptic region encompassing the gonadotrophin-releasing hormone (GnRH) perikarya and the anteroventral periventricular nucleus (AVPV). Retrograde tract tracing was used to identify and compare the sources of inputs to these sites in female rats. Within the telencephalon and diencephalon, the incidence of retrograde labelling from both sites was moderate to abundant in the ventral lateral septum, posteromedial bed nucleus of the stria terminalis, amygdalohippocampal area and the periventricular, medial preoptic, anterodorsal preoptic, dorsomedial suprachiasmatic, arcuate, and posterior ventrolateral ventromedial hypothalamic nuclei. In these regions, the incidence of retrograde labelling was either greater from the AVPV than from the GnRH perikarya site or similar from both sites. In the medial amygdaloid, parastrial, striohypothalamic, and ventral premammillary nuclei, the retrograde labelling from the AVPV greatly exceeded the sparse incidence from the GnRH perikarya site. In contrast, retrograde labelling from the GnRH perikarya site predominated in the median preoptic, lateroanterior and dorsomedial hypothalamic nuclei, subparaventricular zone, and retrochiasmatic area; it was abundant in the AVPV. Caudal to the diencephalon, retrograde labelling from either site was sparse, except in the lateral parabrachial nucleus, which displayed a particularly high incidence from the GnRH perikarya site. Other mesencephalic regions labelled from either site included the periaqueductal gray and dorsal and median raphe nuclei. The most caudal labelling was found in the ventrolateral medulla and region of the solitary tract nucleus; this was almost exclusively from the GnRH perikarya site. These findings further elucidate the neuroanatomical connections underlying the control of the ovulatory cycle.

Involvement of serotonin 5HT1 and 5HT2 receptors and nitric oxide synthase in the medial preoptic area on gonadotropin secretion

Due to the stimulatory action of serotonin (5HT) and nitric oxide (NO) on the secretion of gonadotropins and PRL, this work aimed at investigating the participation of serotoninergic receptors 5HT(1) and 5HT(2) of the medial preoptic area (MPOA) in the control of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) secretion and the possible modulation by ovarian steroids as well as the possible participation of NO as a mediator of the stimulatory effects of serotonin in the MPOA on LH secretion. Microinjections of three different doses (0.02, 0.2, and 2 ug) of methiothepin, a serotoninergic 5HT(1) antagonist or ketanserin, a seretoninergic 5HT(2) antagonist, were carried out into the MPOA in ovariectomized rats treated or not with estrogen or estrogen plus progesterone. Other groups of ovariectomized rats treated with estrogen, estrogen plus progesterone or vehicle were prepared to evaluate NOS activity in the MPOA. Plasma LH, FSH, and PRL in ovariectomized rats were not altered by the microinjection of methiothepin or ketanserin in the MPOA. Methiothepin microinjection in the MPOA reduced LH but did not change plasma FSH and PRL in ovariectomized rats treated with estrogen or estrogen plus progesterone. On the other hand, ketanserin microinjection in the MPOA reduced plasma LH and FSH but did not change plasma PRL in the animals submitted to the same steroidal treatment. NOS activity in the MPOA was significantly reduced by methiothepin or ketanserin in ovariectomized rats treated with estrogen or estrogen plus progesterone. In conclusion, this work showed that in the studied conditions, serotonin in the MPOA: (1) does not work in the control of PRL secretion through 5HT(1) and 5HT(2) receptors; (2) integrates the control of FSH secretion by 5HT(2) receptors, but not 5HT(1); (3) in the presence of estrogen, stimulates LH secretion by 5HT(1) and 5HT(2) receptors, which can be differentially modulated by progesterone; (4) at least partly, stimulates LH secretion by nitric oxide activity.

Influence of monoamines on differentiating gonadotropin-releasing hormone neurones in foetal mice

This study evaluated the influence of monoamines, serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline, on differentiating gonadotropin-releasing hormone (GnRH)-producing neurones in foetal mice. The differentiation and migration of GnRH neurones were compared in Tg8 mice (the knocked-out gene encoding monoamine oxidase A) with increased levels of 5-HT and noradrenaline and in C3H mice with normal metabolism of monoamines in C3H mice. To achieve this, immunocytochemistry for GnRH combined with quantitative and semiquantitative image analysis were employed. GnRH neurones in foetuses at the 18th embryonic day were detected in the forebrain along the trajectory of their migration from the olfactory bulbs to the hypothalamic retrochiasmatic region. The total number of GnRH neurones in the forebrain in knockout mice was significantly lower compared to C3H mice, suggesting an inhibiting influence of monoamines on the proliferation of precursor cells. The fraction of GnRH neurones in the caudal part of the trajectory of their migration in Tg8 mice exceeded significantly those in C3H foetuses, whereas there was a reverse in the rostral part of the trajectory. These data suggest that an excess of 5-HT and noradrenaline served to accelerate the GnRH neurone migration in Tg8 mice. Moreover, an excess of 5-HT and noradrenaline provided a minor effect on the area and optical density of GnRH neurones (i.e. on GnRH neurone differentiation). Thus, an excess of 5-HT and noradrenaline appears to inhibit the proliferation of the precursor cells of GnRH neurones and stimulates the GnRH neurone migration to the place of their final location in the septo-preoptic region.

Black cohosh acts as a mixed competitive ligand and partial agonist of the serotonin receptor

Extracts of the rhizome of black cohosh [Actaea racemosa L., formerly called Cimicifuga racemosa (L.) Nutt.] were evaluated for potential mechanisms of action in the alleviation of menopausal hot flashes. Ovariectomized Sprague-Dawley rats were administered a 40% 2-propanol extract of black cohosh [4, 40, and 400 mg/(kg.day)] by gavage for 2 weeks with or without estradiol [50 microg/(kg.day)] to determine if black cohosh could act as an estrogen or antiestrogen on the basis of an increase in uterine weight or vaginal cellular cornification. No effects were observed on uterine weight or on vaginal cellular cornification in rats treated with black cohosh alone or in combination with 17beta-estradiol, indicating this black cohosh extract had no estrogenic or antiestrogenic properties in the ovariectomized rat model. To evaluate other potential pathways by which black cohosh might reduce menopausal hot flashes, serotonin activity was first assessed by the inhibition of radioligand binding to cell membrane preparations containing recombinant human serotonin receptor (5-HT) subtypes. A 40% 2-propanol extract of black cohosh was tested against 10 subtypes of the serotonin receptor, revealing the presence of compounds with strong binding to the 5-HT(1A), 5-HT(1D), and 5-HT(7) subtypes. Subsequent binding studies were carried out using 5-HT(1A) and 5-HT(7) receptors because of their association with the hypothalamus, which has been implicated in the generation of hot flashes. The black cohosh 40% 2-propanol extract inhibited [(3)H]lysergic acid diethylamide (LSD) binding to the human 5-HT(7) receptor (IC(50) = 2.4 +/- 0.4 microg/mL) with greater potency than binding of [(3)H]-8-hydroxy-2-(di-N-propylamino)tetralin to the rat 5-HT(1A) receptor (IC(50) = 13.9 +/- 0.6 microg/mL). Analysis of ligand binding data indicated that components of a black cohosh methanol extract functioned as a mixed competitive ligand of the 5-HT(7) receptor. In addition, a black cohosh methanol extract elevated cAMP levels in 293T-5-HT(7)-transfected HEK cells, suggesting the extract acted as a partial agonist at the receptor. The elevation in cAMP mediated by the black cohosh extract could be reversed in the presence of the antagonist methiothepin, indicating a receptor-mediated process. These data suggest that reductions in hot flashes in some women taking black cohosh may not be due to estrogenic properties. This study identifies other possible biological targets of black cohosh that could account for reported biological effects.

Influence of serotonin on the development and migration of gonadotropin-releasing hormone neurones in rat foetuses

This study used a pharmacological approach to evaluate the consequences of the metabolic perturbations of neurotransmitters on brain development. Pregnant rats received p-chlorophenylalanine (pCPA), an inhibitor of serotonin (5-hydroxytryptamine, 5-HT) synthesis, or saline (control) from the 11th day of gestation once or daily up to the 15th, 17th and 20th day, followed by processing of the forebrain and/or nasal cranium of foetal males and females for high-performance liquid chromatography of monoamines, radioimmunoassay of gonadotropin-releasing hormone (GnRH) and quantitative and semiquantitative immunocytochemistry for GnRH. The pCPA treatment resulted in a 50-70% depletion of 5-HT in the nasal crania and forebrains at any studied age. Radioimmunoassay showed no change in GnRH content in 5-HT deficient foetuses at E16 compared to controls, being higher in both cases in the rostral forebrain than in the hypothalamus. In controls at E21, the GnRH content in the hypothalamus exceeded that in the rostral forebrain, whereas in the 5-HT deficient group the opposite was found. These data suggest that 5-HT provided a stimulating effect on GnRH neurone migration, and this was confirmed by quantification of GnRH-immunoreactive neurones in the forebrain along the trajectory of their migration. At E18 and E21, the fractions of GnRH neurones in the rostral part of the trajectory in pCPA-treated foetuses were greater than those in control foetuses but the opposite was true for the caudal part of the trajectory. Moreover, 5-HT appeared to control the proliferation of the precursor cells of GnRH neurones and their differentiation, as derived from the observations of the increased number of GnRH neurones in the forebrain of foetuses of both sexes, as well as the region-specific decreased neuronal size and content of GnRH in 5-HT-deficient females. Thus, 5-HT appears to contribute to the regulation of the origin, differentiation and migration of GnRH neurones.

Serotonergic activation rescues reproductive function in fasted mice: does serotonin mediate the metabolic effects of leptin on reproduction?

Negative energy balance inhibits reproduction by restraining GnRH secretion. Leptin is a permissive metabolic signal for reproduction, but GnRH neurons do not appear to express leptin receptors, suggesting that interneurons transmit leptin signals to these cells. Serotonin (5HT) has satiety effects similar to those of leptin and alters LH release, and serotonergic neurons, which have been shown to express leptin receptors, terminate on GnRH neurons. We hypothesized that serotonergic neurons convey leptin signals to the reproductive neuroendocrine axis. To test this, mice were fasted for 48 h beginning on Diestrous Day 1. While fasting, mice received saline or leptin every 12 h or the 5HT-selective reuptake-inhibitor fluoxetine once at the start of the fast. Estrous cycles of fasted mice were longer (mean +/- SEM, 10.2 +/- 0.5 days; P < 0.0001) than those of fed mice (4.5 +/- 0.2 days). As previously reported, leptin prevented fasting-induced cycle lengthening (4.6 +/- 0.7 days). Fluoxetine also rescued estrous cycles in fasted mice (4.7 +/- 0.6 days), suggesting that 5HT and leptin have similar positive effects on reproduction. Coadministration of the 5HT 1/2/7 receptor-antagonist metergoline blocked rescue of cycle length by fluoxetine and by leptin. Treating leptin-deficient ob/ob and leptin receptor-deficient db/db mice with fluoxetine did not normalize body weight or rescue fertility, perhaps due to altered serotonergic tone in these animals. Together, these data demonstrate a permissive role for serotonergic systems in the metabolic control of reproduction and are consistent with the hypothesis that serotonergic neurons convey leptin signals to GnRH neurons.

Serotonin inhibits luteinizing hormone release via 5-HT1A receptors in the zona incerta of ovariectomised, anaesthetised rats primed with steroids

The zona incerta (ZI), an area in the dorsal hypothalamus, contains neuronal systems that appear to control gonadotropin release. Previous findings show that there is an inverse relationship between serotonin (5-HT) activity in the ZI and plasma luteinizing hormone (LH) levels, indicating that the 5-HT system in this area has an inhibitory effect on LH release. Employing anaesthetised, ovariectomised rats primed with 5 microg oestradiol benzoate followed at 48 h by 0.5 mg progesterone, we have shown that 2 microg/side 5-HT in the ZI inhibits the LH surge that normally occurs 4 h after the progesterone treatment. This effect was mimicked by 2 microg/side 8-OH-DPAT, a 5-HT1A agonist, but not by DOI, a 5-HT2 agonist, BMY7378, a presynaptic 5-HT1A agonist or MCPP, a 2B & 2C agonist. The inhibitory effect of 5-HT and 8-OH-DPAT was prevented by pretreatment, 1 h before, with either 2 mg/kg i.p. WAY100135, a 5-HT1A antagonist or 0.25 mg/kg i.p. ritanserin, a 5-HT2 antagonist. These results indicate that 5-HT in the ZI exerts its inhibitory effect on LH release via 5-HT1A receptors but that another 5-HT subtype may also be involved.

Distribution of serotonin in the brain of the Senegalese sole, Solea senegalensis: an immunohistochemical study

We report the distribution of serotonin immunoreactive (5-HT-ir) structures in the brain of the adult Senegalese sole, Solea senegalensis, using the streptavidin-biotin-peroxidase complex immunohistochemical method. We have found a wide distribution of immunoreactive fibers throughout the entire brain. 5-HT-ir cell bodies appeared restricted to some periventricular nuclei associated with the diencephalic recesses, and in the rhombencephalic reticular formation and inferior olivary region. Specifically, cerebrospinal fluid-contacting serotoninergic cells were found within the pars dorsalis and pars ventralis of the nucleus recessus lateralis, in the paraventricular organ and in the nucleus recessus posterioris. In the brainstem, 5-HT-ir perikarya appear within the superior and inferior raphe, the nucleus reticularis superioris, the nucleus interpeduncularis and the inferior olive. Although positive fibers were not found in the neurohypophysis, a few 5-HT-ir cells were identified in the adenohypophysis. This distribution is compared with those found in other fishes and discussed in the context of putative roles of 5-HT as a neuroendocrine factor and neurotransmitter in the Senegalese sole.

Prevention of inhibitory effect of dorsal raphe nucleus lesions on ovulation and LH surge by 5-HT 2A/2C receptor agonists in female rats

In order to investigate the role of the dorsal raphe nucleus and the serotonergic system in the regulation of ovulation, the number of ova and plasma luteinizing hormone (LH) concentrations were measured in female rats after making lesions in this nucleus (DRL) and/or treatment with 5-hydroxytryptamine (5-HT) receptor agonists or antagonists. DRL or sham lesion was made on the afternoon of proestrous (12:00-14:00 h) under ether anesthesia and the number of ova in the oviduct was counted on the next estrous and diestrous morning. In some animals, blood samples were taken via the atrial cannula during the proestrous evening for the radioimmunoassay of LH. All intact control and sham-operated females ovulated and plasma LH increased between 19:00 and 21:00 h. In contrast, ovulation was seen in only 36% of DRL rats. LH surge did not occur in this group. However, 80% of DRL rats ovulated after treatment with (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride [(+/-)-DOI; a 5-HT 2A/2C receptor agonist] at 15:00 h on proestrous day. LH surge was also observed in the DRL rats with (+/-)-DOI. On the other hand, only 8% of DRL rats ovulated after treatment with buspirone (5-HT 1A receptor agonist). Furthermore, when mianserin (5-HT 2A/2C receptor antagonist) was administered at 16:00 h on proestrous day, ovulation was not seen in all rats without DRL. These results suggest that the dorsal raphe nucleus plays an important role in induction of LH surge and ovulation and the 5-HT 2A/2C receptor system is involved in this mechanism.

Serotonergic 5-HT2A receptors important for the oestradiol-induced surge of luteinising hormone-releasing hormone in the rat

Serotonin (5-HT) plays a role in mediating the oestradiol-induced surge of luteinising hormone (LH), but so far the 5-HT receptor subtype involved has not been identified. Our previous in-situ hybridization and pharmacological studies suggest that the action of 5-HT involves the 5-HT2A receptor. The aim of the present study was to investigate this possibility by the direct approach of determining whether 5-HT2A receptor antagonists block the oestradiol-induced surge of luteinising hormone releasing hormone (LHRH). Adult female Wistar rats, which had shown at least two consecutive 4-day oestrous cycles, were ovariectomised under halothane anaesthesia in the morning of dioestrus and injected with vehicle (arachis oil) alone or oestradiol benzoate (OB). At 12.00 h of the next day, presumptive pro-oestrus, the animals were injected intraperitoneally with one of three 5-HT2A antagonists, a selective 5-HT reuptake inhibitor (fluoxetine), or the appropriate vehicles; hypophysial portal blood was then collected under alphaxalone anaesthesia between 15.00 and 19.00 h. The amount of LHRH released into hypophysial portal blood during consecutive 30-min periods was determined by radioimmunoassay. As expected, oestradiol, but not oil, triggered a surge of LHRH in hypophysial portal blood with a peak at about 16.00 h of presumptive pro-oestrus. This oestradiol-induced surge of LHRH was blocked by ketanserin, ritanserin and the highly selective 5-HT2A receptor antagonist, RP62203, but not by fluoxetine. These results provide the first direct evidence that the 5-HT2A receptor plays an important role in the oestradiol-induced surge of LHRH.

The effects of oestrogen and progesterone on serotonin and its metabolite in the lateral septum, medial preoptic area and ventromedial hypothalamic nucleus of female rats

The effects of sex steroid hormones on serotonin and its metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA) in the lateral septal nucleus (LS), the medial preoptic area (MPA) and the ventromedial nucleus of the hypothalamus (VMH) of female rats were investigated, using immunohistochemistry and high-performance liquid chromatography (HPLC). Female rats were divided into three groups: ovariectomized rats (OVX group); OVX-rats treated with estradiol benzoate alone (E2 group); and OVX-rats treated with E2 plus progesterone (E2 + P group). We analysed the density of serotonin-immunoreactive fibres with a computer-assisted image analysis system, and measured the tissue concentrations of serotonin and 5-HIAA. Many serotonin-immunoreactive fibres were observed in the LS, MPA and VMH in all three groups. The density of serotonin-immunoreactive fibres in the MPA and VMH was significantly lower in the E2 and E2+P groups compared to the OVX group, whereas the LS showed no detectable differences among the three groups. In the HPLC study, the concentrations of serotonin in the MPA and VMH of the E2 and E2+P groups were significantly lower than that in the OVX group. There was no significant difference in the concentration of serotonin in the LS. The concentration of 5-HIAA and the ratio of 5-HIAA/serotonin in the LS, MPA and VMH showed no significant differences among the OVX, E2 and E2+P groups. The present results suggest that E2 priming for sexual behaviour can affect the serotonergic system by decreasing serotonin content, but not the turnover rate, in the MPA and VMH of female rats.

L-5-hydroxytryptophan does not stimulate LH secretion directly from the pituitary in patients with gonadotrophin releasing hormone deficiency

OBJECTIVE

There is abundant histological and physiological evidence that serotonin plays a role in the regulation of LH secretion in rats. Studies in human subjects have been few, but their results include the finding that pulsatile administration of L-5-hydroxytryptophan (5-HTP, the immediate precursor of serotonin) amplifies LH secretion in women in the medium-late follicular phase, and that this effect is not due to 5-HTP directly inducing LH secretion by the pituitary. We have investigated whether 5-HTP amplifies LH secretion by enhancing the response of the pituitary to GnRH.

PATIENTS

Seven patients aged 20-40 years with hypogonadotrophic hypogonadism (HH) of hypothalamic origin (3 men with Kallmann's syndrome, 2 women without anosmia and with GH deficiency, and 2 women with anorexia nervosa).

DESIGN

To prime the pituitary, subcutaneous pulsatile GnRH was administered for 7 days at the rate of one 5-20 micrograms pulse every 90 min. The day before the investigation, this regimen was replaced by 1.5-3 micrograms intravenous pulses at the same frequency. On the day of the investigation, 3 ml blood samples were taken every 10 min from 0850 to 19:00 hours. After the first two samples, the intravenous GnRH pulse frequency was increased to one per hour and was maintained at this level throughout the rest of the study. The first 4 h of the study acted as a control phase allowing determination of the pituitary response to GnRH. At 1300 h, 75 mg of the aromatic-L-amino-acid decarboxylase inhibitor carbidopa was administered orally; carbidopa does not cross the blood-brain barrier, and prevents peripheral conversion of 5-HTP to serotonin. At 1600 h, another 75 mg dose of carbidopa was administered, and administration of 8-20 mg pulses of 5-HTP at a rate of one pulse per hour was begun.

MEASUREMENTS

LH was determined in triplicate by an immunoradiometric assay (IRMA), and LH pulses identified by means of a program developed in our laboratory.

RESULTS

When pulsatile administration of GnRH was accompanied by administration of carbidopa and 5-HTP, LH pulse amplitude (2.32 +/- 0.71 IU/I) did not differ significantly from its value in either the GnRH+ carbidopa phase (2.58 +/- 1.12 IU/I) or the unaccompanied GnRH phase (2.77 +/- 1.76 IU/I).

CONCLUSIONS

L-5-hydroxytryptophan-induced amplification of LH secretion in humans is not due to enhancement of the pituitary response to GnRH. The effect of L-5-hydroxytryptophan must therefore be due to its action on the hypothalamus, where it may be hypothesized that it increases GnRH release.

Photoperiodically driven changes in Fos expression within the basal tuberal hypothalamus and median eminence of Japanese quail

The rapid photoperiodic response in Japanese quail is so precise that it allows neural analyses of how photoperiodic information is transduced into an endocrine response. After transfer from short [SD; 6L:18D (6:18 hr light/dark cycle)] to long (LD; 20L:4D) days, luteinizing hormone (LH) first rises 20 hr after dawn. Using Fos immunocytochemistry, we examined the basal tuberal hypothalamus (BtH) to determine the relationship between brain cell activation and the first endocrine changes. Two separate cell populations within the BtH expressed Fos-like immunoreactivity (FLI) by hour 18 of the first LD. Importantly, this activation occurred before the LH rise. Median eminence activation appeared within glial cells, whereas activated infundibular nucleus cells were neuronal, providing support to the view that gonadotropin-releasing hormone (GnRH) release can be controlled at the terminals by glia. The FLI induction parallels LH changes, suggesting that gene expression may be involved in events preceding photostimulation and is the earliest photoperiodically stimulated physiological change yet reported. Additional experiments provided further support for this hypothesis. First, photoperiodically induced activation is not a result peculiar to castrates because intact birds displayed similar results. Second, the critical length of 14 hr of light had to be exceeded to cause both BtH activation and a LH rise 30 hr from dawn. Finally, valuable evidence of the response specificity was provided by using a unique property of the quail photoperiodic clock in which exposure to 10L:26D, but not 10L:14D, causes photoinduction. The 36 hr paradigm increased both plasma LH and BtH activation.

Serotonin directly stimulates luteinizing hormone-releasing hormone release from GT1 cells via 5-HT7 receptors

Luteinizing hormone-releasing hormone (LHRH release, which serves as the primary drive to the hypothalamic-pituitary gonadal axis, is controlled by many neuromediators. Serotonin has been implicated in this regulation. However, it is unclear whether the central effect of serotonin on LHRH secretion is exerted directly on LHRH neurosecretory neurons or indirectly via multisynaptic pathways. The present studies were undertaken in order to examine whether LHRH secretion from immortalized LHRH cell lines is directly regulated by serotonin and, if so, to identify the receptor subtype involved. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A/7 receptor agonist, stimulated LHRH release from GT1-1 cells. This effect was blocked by ritanserin, a 5-HT2/7 receptor antagonist, but not by SDZ-216-525, a 5-HT1A antagonist. Basal LHRH release was not affected by the 5-HT2 agonist DOI. Reverse transcription and polymerase chain reaction technique (RT-PCR) was used in order to identify 5-HT1A and 5-HT7 receptor mRNA in immortalized LHRH cell lines. GT1-1 cells express mRNA for the 5-HT7, but not the 5-HT1A receptor subtypes. These results demonstrate a direct stimulatory effect of serotonin on LHRH release via 5-HT7 receptor.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

Role of serotoninergic receptors in gonadotropin secretion in male rats

Experimental data concerning the role of serotonin (5-HT) in the control of gonadotropins secretion remain controversial: different receptors subtypes should mediate the serotoninergic action. In the present work we analyzed in prepubertal and adult male rats the FSH and LH responses after administration of the following drugs: 5-hydroxy-L-tryptophan (5-HTP), precursor of 5-HT synthesis, the 5-HT1 receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), the 5-HT2 receptor agonists alpha-methylserotonin maleate (alpha-CH3-HT) and (+/-)-DOI hydrochloride (DOI), the 5-HT3 agonists, quipazine-N-methyldimaleate (QUIP), 2 methyl-serotonin maleate (2-CH3-HT), trimethylserotonin iodide (5-HTQ) and 1-phenylbiguanide (PHE). We found that: a) FSH secretion in adult males was stimulated by 5-HTP, inhibited by 5-HT2 agonists and unaffected by 5-HT3 agonists; b) LH secretion in adult males was stimulated by 5-HTP and alpha-CH3-HT, while DOI and 5-HT3 agonists were ineffective; c) in prepubertal males, 5-HT2 and 5-HT3 agonists inhibited LH release. All these results taken together suggest that: (1) 5-HT2 and 5-HT3 receptors are involved in the control of gonadotropin secretion; (2) FSH and LH secretion are differently modulated by agonists of 5-HT2 and 5-HT3 receptors; (3) the 5-HT2 agonists alpha-CH3-HT and DOI elicit different responses; and (4) the responses obtained vary with animal age, since the inhibitory effect of 5-HT2 and 5-HT3 agonists on LH secretion was observed only in prepubertal males.

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

8-OH-DPAT regulates the amplitude and the phase of LH surge in ovariectomized steroid-primed rats

Precise interactions between ovarian steroids and neurotransmitters are required for the secretion of phasic LH surge. Previous data suggested the existence of an interactive stimulatory effect of progesterone (P) and serotonin (5-HT) on LH release. In the present work the effects of 8-OH-DPAT, a selective 5-HT(1A) agonist, on phasic LH secretion were tested in ovariectomized rats implanted for 6 days with a pellet of 17 β estradiol (OVX-E(2)) and in OVX-E(2) treated with progesterone (OVX-E(2)-P). Intraperitoneal injection of 8-OH-DPAT at 11.00 h in the morning of the expected LH surge had no effect on circadian plasma levels of LH in OVX-E(2) rats, whereas it induced a phase advance and an increase in LH surge in OVX-E(2)-P rats. Administration of the antiprogestin RU 38486 in OVX-E(2)-P rat, totally abolished the combined effects of P and 8-OH-DPAT on phasic LH release. SDZ 216-525, a specific 5-HT(1A) antagonist administered 60 min before 8-OH-DPAT, inhibited the stimulatory effect of the 5-HT(1A) agonist on the amplitude of LH surge. The present data suggest that progesterone is required for the regulation of phasic LH release by 5-HT(1A) agonists and that under this hormonal condition the activation of 5-HT(1A) receptors induces a phase advance and an increase in LH surge.

Involvement of serotonin in the regulation of GnRH gene expression in the male rat brain

The role of serotonin (5-HT) in the regulation of the hypothalamo-pituitary-gonadal axis is still controversial. In order to evaluate the influence of 5-HT on gonadotropin-releasing hormone (GnRH) neurons, we have investigated the effects of repeated administration (during 2 days) of 5-HT, the 5-HT1+2 receptor antagonist methysergide, the 5-HT2 receptor antagonist ketanserin, and the 5-HT3 receptor antagonist ondansetron on GnRH mRNA levels in the male rat medial preoptic area (MPOA), as measured by quantitative in situ hybridization. The treatment with 5-HT decreased by 32% the number of silver grains overlying labelled neurons. The administration of methysergide and ketanserin increased the hybridization signal by 32% and 29%, respectively. On the other hand, the 5-HT3 receptor antagonist did not modify GnRH mRNA levels. The present results clearly indicate that the serotoninergic system exerts a negative tonic influence on the biosynthesis of GnRH as evaluated by mRNA level measurements. They also strongly suggest that the influence of 5-HT in the regulation of GnRH neuronal activity is mediated via activation of 5-HT2 receptor, although an involvement of 5-HT1 receptors cannot be totally excluded.

Comparison of ultrastructural characteristics of gonadotropin-releasing hormone neurons in prepubertal and adult male rats

Gonadotropin-releasing hormone neurons from prepubertal (29-day-old) and adult (three-month-old) male rats were demonstrated immunocytochemically using the LR1 antibody, and prepared for electron microscopic examination. Gonadotropin-releasing hormone neurons were equally immunoreactive in the two age groups, but there were heavy deposits of reaction product in the outer nuclear envelope of these neurons in prepubertal animals. Point count stereology on electron micrographic montages of gonadotropin-releasing hormone neurons at x25,000 was used to compare the relative proportion of cytoplasm containing various subcellular organelles. More of the cytoplasm was occupied by Golgi apparatus and secretory vesicles in the prepubertal animals. The representation of mitochondria was equal in the two age groups, while there were more lysosomes in the gonadotropin-releasing hormone neurons from adult animals. The density of synaptic input to the neurons was estimated using quantitative morphometrics on electron micrographs of three levels of section through the neuron, magnified x25,000. The percentage of the perikaryal membrane with synaptic contacts was greater in the gonadotropin-releasing hormone neurons from adults. Most strikingly, there were gonadotropin-releasing hormone terminals on gonadotropin-releasing hormone soma of these neurons in prepubertal animals, but not in the adults. The highly immunoreactive outer nuclear envelope and relative larger representation of Golgi and secretory vesicles in gonadotropin-releasing hormone neurons in prepubertal animals suggest that these cells are actively synthesizing peptides, including gonadotropin-releasing hormone. The large representation of Golgi apparatus may also reflect the active biosynthesis of membrane in association with the elaboration of neuronal processes.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural evidence for changes in synaptic input to the hypothalamic luteinizing hormone-releasing hormone neurons in photosensitive and photorefractory starlings

Neural input to the hypothalamic luteinizing hormone-releasing hormone (LHRH) neurons was investigated in male starlings (Sturnus vulgaris) using electron microscopy combined with immunocytochemistry. Birds (4 to 6 in each group) were sampled at four stages of a photoperiodically induced reproductive cycle: (a) sexually immature but photosensitive, under short days; (b) during sexual maturation after 7 to 25 long days; (c) during gonadal regression after 35 to 50 long days; and (d) when fully photorefractory after 11 months exposure to long days. The length of the perikaryal membrane, the number and length of axo-somatic terminals in contact with it and the number and length of synaptic modifications within the terminals were measured for a minimum of six LHRH neurons in each brain. The number of axo-somatic terminals per neuron and the number per unit length of perikaryal membrane did not differ in birds of groups (a), (b) and (c), but was significantly greater (P < 0.05) in the fully refractory birds (group d). Similarly, the number of synaptic modifications was significantly greater (P < 0.05) in group (d) than in the other groups. These results are consistent with increased neural input to the LHRH perikarya in photorefractory birds after prolonged exposure to long days, although there was no indication of a change in input at the time of gonadal regression.

Neuronal projections to the medial preoptic area of the sheep, with special reference to monoaminergic afferents: immunohistochemical and retrograde tract tracing studies

The preoptic area contains most of the luteinizing hormone releasing hormone immunoreactive neurons and numerous monoaminergic afferents whose cell origins are unknown in sheep. Using tract tracing methods with a specific retrograde fluorescent tracer, fluorogold, we examined the cells of origin of afferents to the medial preoptic area in sheep. Among the retrogradely labeled neurons, immunohistochemistry for tyrosine hydroxylase, dopamine-beta-hydroxylase, phenylethanolamine N-methyltransferase, and serotonin was used to characterize catecholamine and serotonin fluorogold labeled neurons. Most of the afferents came from the ipsilateral side to the injection site. It was observed that the medial preoptic area received major inputs from the diagonal band of Broca, the lateral septum, the thalamic paraventricular nucleus, the lateral hypothalamus, the area dorsolateral to the third ventricle, the perimamillary area, the amygdala, and the ventral part of the hippocampus. Other numerous, scattered, retrogradely labeled neurons were observed in the ventral part of the preoptic area, the vascular organ of the lamina terminalis, the ventromedial part of the hypothalamus, the periventricular area, the area lateral to the interpeduncular nucleus, and the dorsal vagal complex. Noradrenergic afferents came from the complex of the locus coeruleus (A6/A7 groups) and from the ventro-lateral medulla (group A1). However, dopaminergic and adrenergic neuronal groups retrogradely labeled with fluorogold were not observed. Serotoninergic fluorogold labeled neurons belonged to the medial raphe nucleus (B8, B5) and to the serotoninergic group situated lateral to the interpeduncular nucleus (S4). In the light of these anatomical data we hypothesize that these afferents have a role in the regulation of several functions of the preoptic area, particularly those related to reproduction. Accordingly these afferents could be involved in the control of luteinizing hormone releasing hormone (LHRH) pulsatility or of preovulatory LHRH surge.

Increase in number of LHRH neurones in septal-preoptic area of rats following chronic amitriptyline treatment: implication in antidepressant effect

Recent studies have implicated the peptide LHRH in a variety of actions including a role in modulation of affective behavior. The present study has been undertaken to determine its involvement in the action of antidepressants, if any, using amitriptyline (AMT) as the model antidepressant drug. The repeated administration of AMT (10 mg/kg/day) in rats increased the number of LHRH neurones in the septal-preoptic area. While 1 week of AMT treatment slightly augmented the number of LHRH neurones, the rise was not statistically significant, however, following 2 weeks of AMT treatment, a significant (P < 0.05) increase (41.05%) was observed. Three and four weeks of AMT treatment further increased the number of neurones by 60.84% and 72.96% respectively; a remarkable rise in the LHRH immunoreactivity around organum vasculosum of lamina terminalis (OVLT) was also noticed. Acute AMT treatment had no effect on the number of neurons; however, the intensity of immunoreaction in the OVLT was slightly decreased. In the behavior despair test, a single dose of AMT displayed an immobility reducing effect which was also shown by a single dose of LHRH (1 mg/kg). The combination of LHRH (1 mg/kg) and AMT also reduced the immobility; the effect was the same as one produced by each drug given separately. The results suggest that chronic AMT treatment may induce transcription and translation in LHRH cells and that the peptide LHRH may be involved in the mediation of the antidepressant effect, characteristic of AMT.

Increased synaptic input to gonadotropin-releasing hormone neurons in aged, virgin, male Sprague-Dawley rats

Using double-label ultrastructural immunocytochemistry, we found the synaptic input to gonadotropin-releasing hormone (GnRH) neurons in the preoptic area of aged (20 months old), virgin, male Sprague-Dawley rats to be denser than that in young adults (3 months old). These results confirmed earlier observations on F-344 virgin male rats. The aging F-344 rat, however, is prone to testicular tumor and so it was essential to see if the phenomenon was reproducible in another rat strain. In the first study, a portion of the increase in synaptic density was due to an increase in the proportion of synapses containing pleiomorphic vesicles, frequently associated with the neurotransmitter GABA. We tested the possibility directly using a double-label protocol for GnRH and glutamic acid decarboxylase (GAD). However, in the present study the density of input by GABA did not change with age. This inhibitory amino acid represented about 10% of the total innervation in young animals; but, in aged animals, because the total synaptic input was greater, GABA represented only about 4% of the innervation. Synaptic vesicles within GAD-immunoreactive terminals were uniformly clear and spherical, suggesting that pleiomorphic vesicle shape is not an appropriate criterion for GABAergic innervation.