Glutamate Release In Rats Research Articles

Depolarization with high-K+ causes Ca2+-independent but partially Cl−-dependent glutamate release in rat hippocampal slice cultures

Depolarization with high-K+ causes Ca2+-independent but partially Cl−-dependent glutamate release in rat hippocampal slice cultures

Presynaptic modulation of cortical synaptic activity by calcineurin.

Synaptic plasticity is modulated by Ca(2+)-induced alterations in the balance between phosphorylation and dephosphorylation. Recent evidence suggests that calcineurin, the Ca(2+)-calmodulin-dependent phosphatase (2B), modulates the activity of postsynaptic glutamate receptors. However, in rat cortex, calcineurin is enriched mainly in presynaptic, not postsynaptic, fractions. To determine if calcineurin modulates glutamatergic neurotransmission through a presynaptic mechanism, we used whole-cell patch clamp experiments to test effects of two specific calcineurin inhibitors, cyclosporin A (CsA) and FK506, on synaptic activity in fetal rat cortical neurons. The rate of spontaneous action-potential firing was markedly increased by either CsA or FK506 but was unaffected by rapamycin, a structural analog of FK506 which has no effect on calcineurin. In voltage-clamp experiments, CsA increased the rate but not the amplitude of glutamate receptor-mediated, excitatory postsynaptic currents, suggesting an increased rate of glutamate release. CsA had no effect on the amplitude of currents evoked by brief bath application of selective glutamate receptor agonists, providing further evidence for a pre- rather than postsynaptic site of action. In conclusion, these data indicate that calcineurin modulates glutamatergic neurotransmission in rat cortical neurons through a presynaptic mechanism.

Glutamate: a major neuroendocrine excitatory signal mediating steroid effects on gonadotropin secretion.

The preovulatory gonadotropin surge is induced by progesterone in the cycling female rat or in the ovariectomized estrogen-treated female rat after adequate estrogen-priming activity is present. The source of progesterone under physiological conditions could be the ovary and/or the adrenal. Since the GnRH neuron does not possess estrogen and progesterone receptors, its function is modulated by other CNS neurotransmitters and neurosecretory products. Among these, excitatory amino acids (EAAs) have now been shown to play an important role in the regulation of pulsatile gonadotropin release, induction of puberty and preovulatory and steroid-induced gonadotropin surges. Glutamate, the major endogenous EAA exerts its action through ionotropic and metabotropic receptors. The ionotropic receptors consist of two major classes, the NMDA (N-methyl-D-aspartate) and non-NMDA: kainate and AMPA (DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors. EAA receptors are found in hypothalamic areas involved with reproduction. While both NMDA and non-NMDA receptors are involved in the regulation of LH secretion, the NMDA receptors appear to be involved with the regulation of puberty and FSH secretion as well. Steroids increase the release rates of glutamate and aspartate in the preoptic area during the gonadotropin surge. Steroids may also regulate the hypothalamic AMPA receptors.

Isoflurane Reduces Ischemia-induced Glutamate Release in Rats Subjected to Forebrain Ischemia

The release of excitatory neurotransmitters during ischemia is thought to contribute to ischemic neuronal injury. Volatile anesthetics have been shown to reduce excitatory neurotransmission in vitro, and it is conceivable that they reduce ischemia-induced neurotransmitter release. The current investigation was conducted to evaluate the effect of isoflurane and N2O-fentanyl anesthesia on ischemia-induced glutamate release in the rat and to compare it with that of mild hypothermia, an intervention known to reduce glutamate release significantly. Microdialysis probes were implanted into the parietal cortex and dorsal hippocampus of four groups of anesthetized rats (n = 5 per group). The hypothermic group was anesthetized with 1.2% halothane. The two isoflurane groups were anesthetized with 0.5 minimum alveolar concentration or electroencephalographic burst-suppression doses of isoflurane (approximately 2 minimum alveolar concentration). The control group was anesthetized with 70% N2O-30% O2 and fentanyl. The pericranial temperature was maintained at 34 degrees C in the hypothermic group and at 38 degrees C in the remaining groups. Ischemia was induced by bilateral carotid artery occlusion with simultaneous hypotension to 35 mmHg for 10 min, followed by a reperfusion period of 70 min. Dialysate was collected before, during, and after ischemia. The concentrations of glutamate and glycine in the dialysate were measured by high-performance liquid chromatography. Preischemic glutamate and glycine concentrations in the dialysate were similar among the groups. Ischemia resulted in a significant increase in glutamate and glycine concentrations in the N2O-fentanyl groups in the parietal cortex and in the hippocampus. This increase in neurotransmitter concentrations did not occur in the hypothermic group in either structure. Isoflurane reduced glutamate concentrations in both structures and glycine concentrations in the hippocampus. In the parietal cortex, glycine concentrations did not increase in either isoflurane group. Hypothermia inhibits ischemia-induced excitatory neurotransmitter release in the rat. Isoflurane, in comparison with a N2O-fentanyl-anesthetized state, significantly attenuates excitatory neurotransmitter release in the hippocampus. This effect of isoflurane is comparable to that of mild hypothermia.

Glutamate: a major excitatory transmitter in neuroendocrine regulation.

Excitatory amino acids (EAAs), such as glutamate and aspartate, are found in large concentrations in presynaptic boutons of a variety of important hypothalamic nuclei, including the arcuate nucleus, supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, organum vasculosa of the lamina terminalis (OVLT) and preoptic area (POA). Likewise, the different ionotropic/metabotropic EAA receptor subtypes are found in the same regions of the hypothalamus although there are differences in their individual patterns of localization. Furthermore, there is evidence supporting the presence of ionotropic N-methyl-D-aspartate (NMDA) receptors and non-NMDA (kainate and AMPA) receptors in the anterior lobe, intermediate lobe and posterior lobe of the pituitary. The majority of work to date has focused on the role of EAAs in the control of LH secretion. Administration of glutamate, NMDA, kainate or AMPA leads to rapid LH release mediated through the stimulation of hypothalamic GnRH release. The major site of NMDA action appears to be the OVLT/preoptic area--where GnRH cell bodies reside, whereas AMPA and kainate have been suggested to act primarily at the arcuate nucleus/median eminence--the site of GnRH nerve terminals. There is evidence that some of the effects of glutamate on GnRH release may involve activation of the novel neurotransmitter nitric oxide and possibly catecholamines. The physiological importance of EAAs in the control of LH surge expression is evidenced by the findings that the steroid-induced LH surge in ovariectomized animals and the preovulatory LH surge in cycling animals and in PMSG-primed animals is blocked by treatment with specific NMDA receptor antagonists, or non-NMDA receptor antagonists. EAAs also appear to be important in regulating the normal pulsatile pattern of LH release as evidenced by the finding that both the NMDA antagonist, AP5, and the AMPA/kainate antagonist, DNQX, lower mean LH levels, LH pulse amplitude and LH pulse frequency in the adult ovariectomized rat. A role for NMDA receptors in the achievement of puberty has been suggested since activation of NMDA receptors has been shown to advance the time of vaginal opening in the immature female rat, while kainate and DNQX were without effect. Steroids have been reported not to affect NMDA receptor binding in the hypothalamus; however, steroids appear to up-regulate AMPA receptor GluR1 subunit levels and non-NMDA receptor binding in the hypothalamus. Steroids also increase the release rates of glutamate and aspartate in the POA during the steroid-induced LH surge in the ovariectomized adult rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of neuroactive compounds, noxious and cardiovascular stimuli on the release of amino acids in the rat locus coeruleus

The release of excitatory amino acids (glutamate, aspartate), inhibitory amino acids (GABA, taurine) and arginine was determined in the locus coeruleus (LC) of anaesthetized rats. The neuronal origin of stimulated amino acid release was verified by superfusion with neuroactive compounds. Electrical stimulation of the sciatic nerve, as well as mechanical footshock, enhanced LC release rates of glutamate and aspartate without influencing those of taurine and arginine. GABA release rate was increased slightly after some delay. Excitatory amino acid release was not influenced by changes in blood pressure. The results provide direct neurochemical evidence that noxious stimuli activate LC neurons via the glutamate and aspartate input into this nucleus.

Excitatory Amino Acids: Function and Significance in Reproduction and Neuroendocrine Regulation

Excitatory amino acid neurotransmission is an essential component of the neuroendocrine transmission line that regulates anterior pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion. Excitatory amino acids (EAAs), such as glutamate and aspartate, are found in largo concentrations in presynaptic boutons of a variety of important hypothalamic nuclei, including the arcuate nucleus, the suprachiasmatic nucleus, the supraoptic nucleus, the paraventricular nucleus, and the preoptic area. EAA receptors can be divided into two broad groups, namely, ionotropic and metabotropic receptors. Ionotropic receptors are subdivided into NMDA ( N-methyl-D-aspartate), kainate, and AMPA (DL-α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors. Their main mode of action is by the modulation of Na +, K +, and Ca 2+ ion channels. Metabotropic receptors, on the other hand, act by a G-protein-stimulated release of intracellular Ca 2+ or modulation of adenylate cyclase activity. The different EAA receptor subtypes are found in a variety of areas of the hypothalamus and the brain. In a variety of species, the administration of glutamate, NMDA, or kainate leads to LH release mediated through the stimulation of hypothalamic gonadotropin hormone-releasing hormone (GnRH) release. The major site of NMDA action appears to be the preoptic area—where GnRH cell bodies reside. AMPA and kainate appear to act primarily at the arcuate nucleus/median eminence, the site of GnRH nerve terminals. NMDA may also act upon noradrenergic neurons in the locus coeruleus to influence hypothalamic GnRH release. The steroid-induced LH surge in ovariectomized animals and the preovulatory surge of LH in cycling animals and in pregnant mare's serum gonadotropic-primed animals are blocked by the NMDA antagonist MK801 and the AMPA/kainate antagonist DNQX. MK801 also suppressed FSH surges in most instances, whereas DNQX had no effect on FSH surges. In the ovariectomized female rat, both the NMDA antagonist AP5 and the AMPA/kainate antagonist DNQX, lowered mean LH levels, LH pulse amplitude, and LH pulse frequency. Activation of NMDA receptors advanced the time of vaginal opening in the immature female rat, while kainate and DNQX were without effect. Gonadal steroid removal (castration) did not alter NMDA receptor levels or affinity in the hypothalamus of female or male rats. Likewise, steroid replacement to castrate rats did not affect hypothalamic NMDA receptor levels or NMDA R 1 mRNA levels. Similarly, NMDA and kainate receptor levels in the hypothalamus did not change during the time of puberty in the female rat. In contrast, AMPA receptor (GluR 1) immunoreactive levels in the magnocellular preoptic area (mPOA), the arcuate nucleus (ARC), and the suprachiasmatic nucleus (SCN) were found to be markedly elevated during the time of the LH surge in estradiol-progesterone-treated castrate rats compared to those of the vehicle-only-treated castrate rat. The release rates of glutamate and aspartate in the POA were found to be significantly elevated during the steroid-induced LH surge in the ovariectomized adult rat. Hence, the induction of the LH surge by steroids may involve enhanced EAA neurotransmission mediated by steroid-induced elevations in the release of glutamate and aspartate in the POA and AMPA receptors in the POA, the ARC, and the SCN. Evidence also exists supporting a role of EAAs in the release of other pituitary hormones, such as prolactin, ACTH, growth hormone, oxytocin, and vasopressin, as well as circadian rhythm induction and regulation. The above evidence points to a pivotal role of EAAs acting at the level of the hypothalamus in the neuroendocrine regulation of a variety of hormonal systems with the largest amount of data available on the control of gonadotropin secretion.

Release of glutamate and aspartate from the preoptic area during the progesterone-induced LH surge: in vivo microdialysis studies.

To examine the role of excitatory amino acid neurotransmitters in the regulation of LH and FSH secretion in ovariectomized (ovx) adult rats treated with estradiol and progesterone, we measured the release rates of the excitatory amino acid neurotransmitters, glutamate and aspartate, as well as the neurally inactive amino acid serine in microdialysis perfusate samples of the preoptic area (POA) of the hypothalamus collected at 30-min intervals during the LH surge induced by progesterone. The concentrations of the amino acids were measured by assay using orthophthalaldehyde prederivatization followed by HPLC coupled with fluorometric detection. The release rates of glutamate in the POA increased at 15:30 and 16:00 h in the progesterone-treated estrogen-primed ovx rat as compared to estrogen controls. Release rates of aspartate in the POA were increased at 12:00, 12:30, 15:00 and 16:00 h in the progesterone-treated estrogen-primed ovx rat as compared to estrogen controls. The increase in POA release rates of glutamate and aspartate in the progesterone-treated rats occurred immediately prior to peak serum LH levels induced by progesterone. The specificity of the increase in POA release rates of glutamate and aspartate was evidenced by the finding that the preoptic release of neurally inactive neurotransmitter serine was unchanged by steroid treatment. Estradiol alone had no significant effect on the POA release rates of glutamate, aspartate or serine as compared to ovariectomized controls. In conclusion, the present study demonstrates that the POA release rates of glutamate and aspartate are increased immediately preceding the peak of the progesterone-induced LH surge in the estrogen-primed adult ovx rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Preoptic release of amino acid neurotransmitters evaluated in peripubertal and young adult female rats by push-pull perfusion.

In order to evaluate the participation of amino acid neurotransmitters (AANT) in pubertal development, we measured the preoptic release rates of glutamate (Glu), aspartate (Asp), homocysteic acid (HCA), gamma-aminobutyric acid (GABA), glycine (Gly), and taurine (Tau) in female rats aged 26, 35 and 42 days. Push-pull cannulae were implanted in the preoptic area (POA) after reinforcing the os calvarium by cementing a slightly curved 2-mm thick polyethylene plaque. Four days later, 120-min perfusion experiments (0-90 min: artificial CSF; 90-120 min: CSF containing 50 mM KCl) were performed. Amino acid concentrations were measured in 15-min fractions after derivatization with phenylisothiocyanate, using reverse-phase high-performance liquid chromatography and UV detection. Rats aged 42 days showed significantly (p < 0.01) higher preoptic Asp release rates, as compared to 26-day-old rats; Glu output increased significantly (p < 0.05) in animals aged 35 and 42 days, as compared to 26-day-old rats. No significant changes in HCA release were observed. Preoptic release rates of GABA, Gly and Tau were significantly higher in 26-day-old animals as compared to 35-day-old (GABA, p < 0.05) and 35- and 42-day-old rats (Gly, Tau, p < 0.01). Basal release of Asp, Glu, HCA, Tau and GABA, but not of Gly, was significantly stimulated (p < 0.05) by KCl in all age groups. It is suggested that in peripubertal female rats, increased preoptic Asp and Glu release rates and decreased GABA and Tau output might enhance luteinizing hormone-releasing hormone secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

O-117 - ATP-stimulated glutamate release in rat cultured hippocampal neurons

O-117 - ATP-stimulated glutamate release in rat cultured hippocampal neurons

Intracellular CA 2+ and excitatory amino acids in hippocampal neurons : Robert K.S. Wong, Department of Neurology, Columbia University, NY

Excitatory amino acid neurotransmission is an essential component of the neuroendocrine transmission line that regulates anterior pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion. Excitatory amino acids (EAAs), such as glutamate and aspartate, are found in largo concentrations in presynaptic boutons of a variety of important hypothalamic nuclei, including the arcuate nucleus, the suprachiasmatic nucleus, the supraoptic nucleus, the paraventricular nucleus, and the preoptic area. EAA receptors can be divided into two broad groups, namely, ionotropic and metabotropic receptors. Ionotropic receptors are subdivided into NMDA (N-methyl-D-aspartate), kainate, and AMPA (DL-α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors. Their main mode of action is by the modulation of Na+, K+, and Ca2+ ion channels. Metabotropic receptors, on the other hand, act by a G-protein-stimulated release of intracellular Ca2+ or modulation of adenylate cyclase activity. The different EAA receptor subtypes are found in a variety of areas of the hypothalamus and the brain. In a variety of species, the administration of glutamate, NMDA, or kainate leads to LH release mediated through the stimulation of hypothalamic gonadotropin hormone-releasing hormone (GnRH) release. The major site of NMDA action appears to be the preoptic area—where GnRH cell bodies reside. AMPA and kainate appear to act primarily at the arcuate nucleus/median eminence, the site of GnRH nerve terminals. NMDA may also act upon noradrenergic neurons in the locus coeruleus to influence hypothalamic GnRH release. The steroid-induced LH surge in ovariectomized animals and the preovulatory surge of LH in cycling animals and in pregnant mare's serum gonadotropic-primed animals are blocked by the NMDA antagonist MK801 and the AMPA/kainate antagonist DNQX. MK801 also suppressed FSH surges in most instances, whereas DNQX had no effect on FSH surges. In the ovariectomized female rat, both the NMDA antagonist AP5 and the AMPA/kainate antagonist DNQX, lowered mean LH levels, LH pulse amplitude, and LH pulse frequency. Activation of NMDA receptors advanced the time of vaginal opening in the immature female rat, while kainate and DNQX were without effect. Gonadal steroid removal (castration) did not alter NMDA receptor levels or affinity in the hypothalamus of female or male rats. Likewise, steroid replacement to castrate rats did not affect hypothalamic NMDA receptor levels or NMDA R1 mRNA levels. Similarly, NMDA and kainate receptor levels in the hypothalamus did not change during the time of puberty in the female rat. In contrast, AMPA receptor (GluR1) immunoreactive levels in the magnocellular preoptic area (mPOA), the arcuate nucleus (ARC), and the suprachiasmatic nucleus (SCN) were found to be markedly elevated during the time of the LH surge in estradiol-progesterone-treated castrate rats compared to those of the vehicle-only-treated castrate rat. The release rates of glutamate and aspartate in the POA were found to be significantly elevated during the steroid-induced LH surge in the ovariectomized adult rat. Hence, the induction of the LH surge by steroids may involve enhanced EAA neurotransmission mediated by steroid-induced elevations in the release of glutamate and aspartate in the POA and AMPA receptors in the POA, the ARC, and the SCN. Evidence also exists supporting a role of EAAs in the release of other pituitary hormones, such as prolactin, ACTH, growth hormone, oxytocin, and vasopressin, as well as circadian rhythm induction and regulation. The above evidence points to a pivotal role of EAAs acting at the level of the hypothalamus in the neuroendocrine regulation of a variety of hormonal systems with the largest amount of data available on the control of gonadotropin secretion.

Preoptic catecholamine, GABA, and glutamate release in ovariectomized and ovariectomized estrogen-primed rats utilizing a push-pull cannula technique.

The push-pull cannula technique was used to evaluate the role of the medial preoptic/anterior hypothalamic area (MPO) in regulating pituitary luteinizing hormone (LH) and prolactin release. The concentrations of the three catecholamines--dopamine, norepinephrine (NE), epinephrine (E)--and gamma-aminobutyric acid (GABA) and glutamate could be measured in 15-min fractions at which interval blood samples for LH and prolactin determination were also collected. Comparison of neurotransmitter release rates into the MPO were made between ovariectomized and ovariectomized estradiol benzoate treated rats. Release of the neurotransmitters occurred in a pulsatile manner, the release episodes for each transmitter appeared to be independent of the others. No direct correlation between neurotransmitter release episodes and blood LH or prolactin levels could be established. The release of GABA was significantly lower and that of NE and E higher in ovariectomized animals in comparison to estrogen-primed ovariectomized animals under negative feedback conditions. In the afternoon, however, when the estrogen stimulated LH and prolactin release, preoptic GABA release was low, whereas preoptic NE and particularly E release rates were high. Conspicuously high dopamine and NE release episodes were observed in estrogen-primed animals at noon, i.e., prior to the expression of the positive feedback signal. This may reflect a biochemical correlate to the so-called critical period. No consistent differences between ovariectomized and ovariectomized estradiol-17 beta benzoate treated animals were observed for preoptic glutamate release rates. The data show that preoptic GABA release rates show generally an inverse pattern to NE and E release and therefore also to blood LH and prolactin levels. No direct mathematical correlation between any of the neurotransmitter release rates and blood hormone levels could be established.