Resistance of gonadotropin-releasing hormone neurons to glutamatergic neurotoxicity

Getting his act together: roles of glutamate, nitric oxide, and dopamine in the medial preoptic area

Gonadal hormones have primarily slow, genomically mediated effects, but copulation requires rapid interactions with a partner. A major way in which hormones facilitate male sexual behavior is by increasing production of neurotransmitter receptors or of enzymes that regulate neurotransmitter synthesis or release. Dopamine is an important facilitative neurotransmitter, and the medial preoptic area (MPOA) is a critical integrative site for male sexual behavior. MPOA dopamine is released before and during mating and facilitates copulation, genital reflexes, and sexual motivation. Gonadal hormones regulate dopamine release in the MPOA of male rats in part by increasing nitric oxide synthase (NOS) in the MPOA; the resultant increase in production of nitric oxide (NO) increases both basal and female-stimulated dopamine release. Glutamate also increases dopamine release via increased production of NO. At least some of the glutamatergic inputs to the MPOA are from the medial amygdala (MeA) and bed nucleus of the stria terminalis (BNST), which mediate the female-stimulated increase in dopamine, which in turn enhances copulatory ability. Extracellular glutamate in the MPOA increases during copulation, especially during ejaculation, and increased glutamate facilitates copulation and genital reflexes. Previous sexual experience also facilitates copulation and confers resistance to impairment by various lesions, drugs, and stress. Experience enhances processing of sexual stimuli, and its effects require activation of glutamate NMDA receptors and NOS in the MPOA. Neuronal NOS is increased in the MPOA of experienced males. Therefore, glutamate, NO, and dopamine interact in the MPOA to facilitate mating and to enhance future sexual responsiveness.

Nitric oxide mediates glutamate-evoked dopamine release in the medial preoptic area

Dopamine (DA) release in the medial preoptic area (MPOA) of the hypothalamus is an important facilitator of male sexual behavior. The presence of a receptive female increases extracellular DA in the MPOA, which increases further during copulation. However, the neurochemical events that mediate the increase of DA in the MPOA are not fully understood. Here we report that glutamate, reverse-dialyzed into the MPOA, increased extracellular DA, which returned to baseline after the glutamate was removed. This increase was prevented by co-administration of the nitric oxide synthase inhibitor NG-nitro-l-arginine methyl ester (L-NAME), but not by the inactive isomer, Nw-nitro-d-arginine methyl ester (D-NAME). In contrast, extracellular concentrations of the major metabolites of DA were decreased by glutamate, suggesting that the DA transporter was inhibited. These decreases were also inhibited by L-NAME, but not D-NAME. These results indicate that glutamate enhances extracellular DA in the MPOA, at least in part, via nitric oxide activity. Therefore, glutamatergic stimulation of nitric oxide synthase may generate the female-induced increase in extracellular DA in the MPOA, which is important for the expression of male sexual behavior.

Seasonal plasticity within the gonadotropin-releasing hormone (GnRH) system of the ewe: changes in identified GnRH inputs and glial association

The annual reproductive cycle in sheep may reflect a functional remodeling within the GnRH system. Specifically, changes in total synaptic input and association with the polysialylated form of neural cell adhesion molecule have been observed. Whether seasonal changes in a specific subset(s) of GnRH inputs occur or whether glial cells specifically play a role in this remodeling is not clear. We therefore examined GnRH neurons of breeding season (BS) and nonbreeding season (anestrus) ewes and tested the hypotheses that specific (i.e. gamma-aminobutyric acid, catecholamine, neuropeptide Y, or beta-endorphin) inputs to GnRH neurons change seasonally, and concomitant with any changes in neural inputs is a change in glial apposition. Using triple-label immunofluorescent visualization of GnRH, glial acidic fibrillary protein and neuromodulator/neural terminal markers combined with confocal microscopy and optical sectioning techniques, we confirmed that total numbers of neural inputs to GnRH neurons vary with season and demonstrated that specific inputs contribute to these overall changes. Specifically, neuropeptide Y and gamma-aminobutyric acid inputs to GnRH neurons increased during BS and beta-endorphin inputs were greater during either anestrus (GnRH somas) or BS (GnRH dendrites). Associated with the changes in GnRH inputs were seasonal changes in glial apposition, glial acidic fibrillary protein density, and the thickness of glial fibrils. These findings are interpreted to suggest an increase in net stimulatory inputs to GnRH neurons during the BS contributes to the seasonal changes in GnRH neurosecretion and that this increased innervation is perhaps stabilized by glial processes.

The neurobiology of reproductive development

This brief review has highlighted some of the major advances in the last decade or so in understanding the central control of puberty. These include the discovery that GnRH-I neurons develop in the olfactory placode and migrate into the forebrain, the recognition that puberty is a reactivation of GnRH secretion, the identification of leptin as a metabolic signal which may permit puberty to occur, unraveling the molecular basis of the circadian clock which underlies photoperiodic control of puberty in seasonal species, the identification of the structure of pheromones in urine, and the discovery of other populations of GnRH neurons in mammals expressing the GnRH-II gene. Such advances generate further questions: what regulates the migratory pathways of GnRH neurons, and what controls axon outgrowth and targeting to the median eminence? What is the mechanism which causes GnRH secretion to decline between the neonatal and pubertal phase of development? How do leptin and other sensory inputs finally communicate to the GnRH neuron? How do GnRH neurons communicate with each other such that co-ordinated pulsatile release of GnRH occurs? What is the function of GnRH-II? Some of these issues may be better addressed using the transgenic technologies which allow the identification and thus the recording, sampling and observation of GnRH neurons in living tissue, but in order to understand how internal and external cues influence puberty it will also be important to study a variety of other mammalian models in which the relative importance of such inputs differs.

Baroreceptor sensitivity of rat supraoptic vasopressin neurons involves noncholinergic neurons in the DBB

Previous studies suggest that cholinergic neurons in the diagonal band of Broca (DBB) participate in the baroreceptor-mediated inhibition of phasic vasopressin neurons in the supraoptic nucleus (SON). To test this hypothesis, extracellular recordings were obtained from putative vasopressin SON neurons of anesthetized rats injected with the cholinergic immunotoxin 192 IgG-saporin (0.8 microg/microl) in the DBB. Baroreceptor sensitivity of neurons was tested with brief phenylephrine-induced (10 microg/10 microl iv) increases in blood pressure of at least 40 mmHg. In rats injected with vehicle or unconjugated saporin, 19 of 21 and 18 of 20 phasic neurons, respectively, were inhibited by increased blood pressure. In rats injected with 192 IgG-saporin, which significantly reduced the number of choline acetyltransferase (ChAT)-positive DBB neurons, 33 of 36 phasic neurons were inhibited. Normal rats and rats with DBB saporin injections received rhodamine bead injections into the perinuclear zone (PNZ) to retrogradely label DBB neurons, and their brains were stained for ChAT. ChAT-positive DBB neurons were not retrogradely labeled from the PNZ. Together, these results indicate that the pathway relaying baroreceptor information to the SON involves noncholinergic DBB neurons.

Glutamatergic regulation of gonadotropin releasing hormone mRNA levels during development in the mouse

The aims of these studies were to investigate the time course of the increase in gonadotropin releasing hormone (GnRH) mRNA levels during sexual development in the mouse, and to test the hypothesis that the neurotransmitter glutamate regulates the GnRH secretory system via actions at the level of GnRH gene expression. GnRH mRNA abundance was estimated by measuring silver grains generated by in situ hybridization of an 35S-labelled oligonucleotide probe. There was a significant increase in GnRH mRNA abundance between the day of birth (P0) and postnatal day 2 (P2) in male mice, but no further increases at later ages when overt pubertal changes are manifest. GnRH mRNA levels also increased significantly between P0 and P2 in female mice. Treatment with the glutamate agonist NMDA caused a significant increase in GnRH mRNA levels in neonatal (P0) mice and adult male mice within 30 min of treatment, which is consistent with previous studies in the rat implicating glutamate in the regulation of GnRH mRNA stability. Treatment with the glutamate antagonist CGP40116 caused an equally rapid decrease in GnRH mRNA levels in adult mice and in mice on P5 after the neonatal increase in GnRH gene expression, but was without effect in mice on P0, prior to the developmental increase. These observations that the effect on GnRH mRNA levels of blocking endogenous glutamatergic signalling depends upon the developmental stage suggest that endogenous glutamate maintains GnRH mRNA levels in adult mouse, and is a potential regulator of the developmental increase seen in the neonatal period.

Effects of N-methyl-D-aspartate on luteinizing hormone release and Fos-like immunoreactivity in the male White-crowned sparrow (Zonotrichia leucophrys gambelii)

Seasonal breeding is terminated in the White-crowned sparrow by the onset of absolute photorefractoriness, a condition in which the reproductive system is switched off indefinitely until it is dissipated by short day lengths. Absolute photorefractoriness is controlled by the central nervous system; however, the mechanisms underlying GnRH quiescence in photorefractory birds have yet to be elucidated. Using the excitatory amino acid glutamate agonist N-methyl-D-aspartate (NMDA), plasma LH levels in White-crowned sparrows were significantly elevated regardless of the reproductive or photoperiodic condition. NMDA also significantly induced Fos-like immunoreactivity (FLI) within the infundibular nucleus and median eminence, regions previously shown to express FLI after a photoperiodically driven LH rise. NMDA did not induce FLI within GnRH I neurons; instead, it significantly activated cells within the organum vasculosum of the lamina terminalis in close proximity to GnRH I perikarya. These findings provide the first evidence that photorefractoriness is not due to depletion of GnRH stores, as LH and presumably GnRH were secreted in response to excitatory amino acid stimulation. NMDA activation of FLI in the region of the organum vasculosum of the lamina terminalis and the basal tuberal hypothalamus suggests that seasonal reproductive neuroendocrine control may be mediated via cells in the region of the GnRH I perikarya and terminals.