Excitatory amino acid-evoked membrane currents and excitatory synaptic transmission in lamprey reticulospinal neurons

The characteristics of excitatory amino acid-evoked currents and of excitatory synaptic events have been examined in lamprey Müller neurons using voltage clamp and current clamp recording techniques. Application of glutamate evoked depolarizations associated with a decrease in input resistance. The reversal potential of the responses was -35 mV. Under voltage clamp conditions, a series of excitatory amino acid agonists evoked inward currents associated with little or no increase in baseline current noise. The order of potency of the excitatory amino acid agonists was quisqualate greater than kainate greater than glutamate greater than aspartate, while N-methyl-D-aspartic acid (NMDA) was inactive. Inward currents evoked by glutamate, as well as by kainate and quisqualate were attenuated reversibly by 1 mM kynurenic acid (KYN). In contrast, glutamate-evoked currents were not affected by 100 microM D(-)-2-amino-5-phosphonovaleric acid (APV), a selective NMDA antagonist. Spontaneously occurring and stimulus-evoked excitatory postsynaptic events were antagonized reversibly by 1 mM KYN. At this concentration, KYN had no effect on membrane potential, input resistance, or excitability of the cells. In contrast, excitatory postsynaptic currents were unaffected by APV. It is concluded that both glutamate responses and excitatory synaptic transmission in lamprey Müller neurons are mediated by non-NMDA-type receptors and that these receptors are associated with ionic channels with a low elementary conductance. The combined pharmacological and biophysical characteristics of these responses are therefore different from those previously reported in other preparations. Spontaneous (but not stimulus-evoked) inhibitory synaptic events in Müller neurons were blocked reversibly by 1 mM KYN but not by 100 microM APV, suggesting that excitation of interneurons inhibitory to Müller cells was also mediated by non-NMDA receptors.     

Pharmacological characterization of the glutamate receptor in cultured astrocytes

Cultured astrocytes from neonatal rat cerebral hemispheres are depolarized by the excitatory neurotransmitter glutamate. In this study we have used selective agonists of different neuronal glutamate receptor subtypes, namely, the N-methyl-D-aspartate (NMDA), kainate, and quisqualate type, to characterize pharmacologically the glutamate receptor in astrocytes. The agonists of the neuronal quisqualate receptor, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionic acid (AMPA) and quisqualate, depolarized the membrane. Kainate, an agonist of the neuronal kainate receptor, depolarized astrocytes more effectively than quisqualate. Combined application of kainate and quisqualate depolarized astrocytes to a level which was intermediate to that evoked by quisqualate and kainate individually. Agonists activating the neuronal NMDA receptor, namely NMDA and quinolinate, were ineffective. Application of NMDA did not alter the membrane potential even in combination with glycine or in Mg2+-free solution, conditions under which neuronal NMDA receptor activation is facilitated. The nonselective agonists L-cysteate, L-homocysteate, and beta-N-oxalylamino-L-alanine (BOAA) mimicked the effect of glutamate. Dihydrokainate, a blocker of glutamate uptake, did not, and several antagonists of neuronal glutamate receptors only slightly affect the glutamate response. These findings suggest that astrocytes express one type of glutamate receptor which is activated by both kainate and quisqualate, lending further support to the notion that cultured astrocytes express excitatory amino acid receptors which have some pharmacological similarities to their neuronal counterparts.     

Action of 3-((±)-2-car☐ypiperazin-4-yl)-propyl-1-phosphonic aci (CPP): a new and highly potent antagonist of N-methyl-d-aspartate receptors in the hippocampus

A new compound, 3-((±)-2-car☐ypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), has been evaluated as an excitatory amino acid receptor antagonist using electrophysiological assays and radioligand binding. In autoradiographic preparations, CPP reduces l-[³H]glutama binding in regions of the hippocampus rich in N-methyl-d-aspartate (NMDA) receptors, but not in regions richin kainate sites. In isolated membrane fraction preparations, CPP displaces l-[³H]glutamate binding to NMDA sites, but does not compete with the binding of selective kainate or quisqualate site ligands. CPP potently reduces depolarizations produced by application of NMDA but not depolarizations produced by quisqualate or kainate. Its order of potency against excitatory amino acid-induced responses in the hippocampus is NMDA > homocysteate > aspartate > glutamate > quisqualate. CPP has no efect on lateral perforant path responses or on inhibition of these responses by 2-amino-4-phosphonobutyrate. Finally, at doses that do not affect Schaffer collateral synpatic transmission, CPP reversibly blocks the induction of long-term potentiation of Schaffer synaptic responses. This new compounds is, therefore, a higly selective brain NMDA receptor blocker, and the most potent such by nearly an order of magnitude.     

Novel aspects of glutamatergic signalling in the neuroendocrine system

l -glutamate, the main excitatory neurotransmitter, influences virtually all neurones of the neuroendocrine hypothalamus via synaptic mechanisms. Vesicular glutamate transporters (VGLUT1–3), which selectively accumulate l -glutamate into synaptic vesicles, provide markers with which to visualise glutamatergic neurones in histological preparations; excitatory neurones in the endocrine hypothalamus synthesise the VGLUT2 isoform. Results of recent dual-label in situ hybridisation studies indicate that glutamatergic neurones in the preoptic area and the hypothalamic paraventricular, supraoptic and periventricular nuclei include parvocellular and magnocellular neurosecretory neurones which secrete peptide neurohormones into the bloodstream to regulate endocrine functions. Neurosecretory terminals of GnRH, TRH, CRF-, somatostatin-, oxytocin- and vasopressin-secreting neurones contain VGLUT2 immunoreactivity, suggesting the co-release of glutamate with hypophysiotrophic peptides. The presence of VGLUT2 also indicates glutamate secretion from non-neuronal endocrine cells, including gonadotrophs and thyrotrophs of the anterior pituitary. Results of in vitro studies show that ionotropic glutamate receptor analogues can elicit hormone secretion at neuroendocrine/endocrine release sites. Structural constituents of the median eminence, adenohypophysis and neurohypophysis contain elements of glutamatergic transmission, including glutamate receptors and enzymes of the glutamate/glutamine cycle. The synthesis of VGLUT2 exhibits robust up-regulation in response to certain endocrine challenges, indicating that altered glutamatergic signalling may represent an important adaptive mechanism. This review article discusses the newly emerged non-synaptic role of glutamate in neuroendocrine and endocrine communication.     

Ionotropic and Metabotropic Glutamate Receptor Agonist-Induced [Ca2+]i Increase in Isolated Rat Supraoptic Neurons

In the present study, the effects of glutamate and of agonists for ionotropic and metabotropic glutamate receptors on intracellular Ca²⁺ concentration ([Ca²⁺]_i) were investigated in neurons of the rat supraoptic nucleus (SON). We used the intracellular Ca²⁺ imaging technique with fura-2, in single magnocellular neurons dissociated from the SON of rats. Glutamate (10^?6?10^?4 M) evoked a dose-dependent increase in [Ca²⁺]_i. The glutamate agonists exerted similar effects, although with some differences in the characteristics of their responses. The [Ca²⁺]_i response to NMDA was smaller than those of glutamate or the non-NMDA receptor agonists, AMPA and kainate, but was significantly enhanced by the removal of extracellular Mg²⁺. Glutamate, as well as quisqualate, an agonist for both ionotropic and metabotropic glutamate receptors, evoked a [Ca²⁺]_i increase in a Ca²⁺-free condition, suggesting Ca²⁺ release from intracellular Ca²⁺ stores. This was further evidenced by [Ca²⁺]_i increases in response to a more selective metabotropic glutamate receptor agonist, t-ACPD, in the absence of extracellular Ca²⁺. Furthermore, the quisqualate-induced Ca²⁺ release was abolished by the selective metabotropic glutamate receptor antagonist, (S)-4-carboxyphenylglycine. The results suggest that metabotropic glutamate receptors as well as non-NMDA and NMDA receptors are present in the SON neurons, and that activation of the first leads to Ca²⁺ release from intracellular Ca²⁺ stores and the activation of the latter two types induces Ca²⁺ entry. These dual mechanisms of Ca²⁺ signalling may play a role in the regulation of SON neurosecretory cells by glutamate.     

Presynaptic noradrenergic regulation of glutamate inputs to hypothalamic magnocellular neurones

Glutamate and norepinephrine transmitter systems play critical roles in the synaptic control of hypothalamic magnocellular neurones. We recently reported on a norepinephrine-sensitive glutamate circuit within the paraventricular nucleus (PVN) that projects to magnocellular neurones. Here, we present evidence for norepinephrine regulation of glutamate release in the PVN and supraoptic nucleus (SON) via actions on presynaptic terminals. Whole-cell synaptic currents were recorded in magnocellular neurones of the SON and PVN in an acute slice preparation. Bath application of norepinephrine (100 microm) caused a robust, reversible increase in the frequency of spontaneous glutamatergic excitatory postsynaptic currents in 100% of SON neurones (246%) and in 88% of PVN magnocellular neurones (259%). The norepinephrine-induced increase in glutamate release was mediated by activation of both presynaptic alpha1 receptors and alpha2 receptors, but the alpha1-receptor component was the predominant component of the response. The presynaptic actions of norepinephrine were predominantly, although not completely, resistant to blockade of Na-dependent spikes, implicating a presynaptic terminal locus of action. Interestingly, the spike-dependent component of the response was greater in PVN than in SON magnocellular neurones. This robust presynaptic facilitation of glutamate release by norepinephrine, combined with the known excitatory postsynaptic actions of norepinephrine, activational effects on local glutamate circuits, and inhibitory effects on gamma-aminobutyric acid release, indicate a strong excitatory role of norepinephrine in the regulation of oxytocin and vasopressin release during physiological stimulation.     

Interaction between glutamate and GABA on H-noradrenaline release from rat hypothalamus

Glutamate has been shown to stimulate noradrenaline (NA) release from hypothalamic nerve terminals. In the present study, we evaluated the possible interaction between the excitatory amino acid glutamate and gamma-aminobutyric acid (GABA), an inhibitory transmitter, on noradrenaline (NA) release from mediobasal hypothalamus (MBH) of adult male rats. Hypothalamic slices loaded in vitro with ³H-NA were superfused and exposed to glutamate, N-methyl- -aspartic acid (NMDA), or kainate (KA). We found that ³H-NA release evoked by the excitatory amino acids glutamate and NMDA was dramatically decreased by GABA. The facilitatory effects of NMDA and KA were prevented concentration-dependently by the GABA_B receptor antagonist 2-hydroxy saclofen which restored the NMDA effect. In addition, beclofen blocked K⁺-induced ³H-NA release. Activation of GABA_A receptors by muscimol and THIP was ineffective. In conclusion, glutamate and GABA, through GABA_B receptors, may interact to modulate NA release from the rat mediobasal hypothalamus.     

Phasic bursts in rat magnocellular neurosecretory cells are not intrinsically regenerative in vivo

Vasopressinergic hypothalamic magnocellular neurosecretory cells fire in phasic bursts. Burst initiation involves summation of postsynaptic potentials to generate action potentials. Action potentials are each followed by a nonsynaptic depolarizing after-potential that summates temporally to generate a plateau potential and so sustain activity throughout the burst. It is unknown whether this plateau potential exceeds spike threshold in vivo to cause intrinsic regenerative firing or simply approaches threshold to increase the probability that excitatory postsynaptic potentials will trigger further action potentials. Here we show that pharmacological blockade of ionotropic glutamatergic transmission by microdialysis application of kynurenic acid into the supraoptic nucleus of anaesthetized rats prevents spontaneous bursts and bursts (after-discharge) evoked by short trains of antidromically stimulated action potentials in magnocellular neurosecretory cells. Even during prolonged depolarization induced by 1 m NaCl infusion, kynurenic acid microdialysis application still blocked after-discharge. The ability of kynurenic acid to block after-discharge during osmotic stimulation was not caused by an unmasking of inhibitory postsynaptic potentials as kynurenic acid was equally effective in the presence of the ionotropic gamma-aminobutyric acid receptor antagonist, bicuculline, nor did it result from inhibition of plateau potential amplitude as this was unaffected by kynurenic acid and bicuculline in vitro, as was after-discharge evoked in vitro. We conclude that phasic bursts are nonregenerative in vivo but rather require continued excitatory synaptic input activity superimposed upon a subthreshold plateau potential to sustain burst activity.     

Some effects of excitatory amino acid receptor antagonists on synaptic transmission in the rat olfactory cortex slice

A study has been made of the effects of a series of excitatory amino acid receptor antagonists on the field potentials evoked on electrical stimulation of the lateral olfactory tracts of olfactory cortex slices perfused in vitro. The antagonists studied included (+/-)-2-amino-5-phosphonovaleric acid, a potent, specific antagonist of N-methyl-D-aspartate (NMDA) receptors, gamma-D-glutamylglycine, an antagonist of NMDA and kainate receptors and (+/-)-cis-2,3-piperidine dicarboxylic acid and 2-amino-4-phosphonobutyric acid, drugs which in addition to antagonizing NMDA and kainate receptors also block responses to quisqualic acid. From the patterns of effects of the drugs it is proposed that quisqualate and NMDA but not kainate receptors are involved in mediating excitatory transmission in the olfactory cortex; quisqualate receptors are located at the lateral olfactory tract - superficial pyramidal cell synapse whereas NMDA receptors are present at the synapses of the superficial pyramidal cell collaterals with the deep pyramidal cell dendrites and/or at the synapses of the pyramidal cell collaterals and inhibitory interneurones. The results are discussed in terms of possible presynaptic and/or postsynaptic sites of antagonist action.     

Modulation of synaptic inputs in magnocellular neurones in a rat model of cancer cachexia

In cancer cachexia, abnormal metabolism and neuroendocrine dysfunction cause anorexia, tissue damage and atrophy, which can in turn alter body fluid balance. Arginine vasopressin, which regulates fluid homeostasis, is secreted by magnocellular neurosecretory cells (MNCs) of the hypothalamic supraoptic nucleus. Arginine vasopressin secretion by MNCs is regulated by both excitatory and inhibitory synaptic activity, alterations in plasma osmolarity and various peptides, including angiotensin II. In the present study, we used whole‐cell patch‐clamp recordings of brain slices to determine whether hyperosmotic stimulation and/or angiotensin II potentiate excitatory synaptic input in a rat model of cancer cachexia, similar to their effects in normal (control) rats. Hyperosmotic (15 and 60 mmol L^‐1  mannitol) stimulation and angiotensin II (0.1 μmol L^‐1) increased the frequency, but not the amplitude, of miniature excitatory postsynaptic currents in normal rats; in model rats, both effects were significantly attenuated. These results suggest that cancer cachexia alters supraoptic MNC sensitivity to osmotic and angiotensin II stimulation.     

Kainate and NMDA toxicity for cultured developing and adult rat spiral ganglion neurons: further evidence for a glutamatergic excitatory neurotransmission at the inner hair cell synapse.

In the inner ear, the excitatory amino acid glutamate is a proposed neurotransmitter acting at the synapse between hair cells and afferent auditory neurons. Using cultures of 5-day-old rat auditory neurons, we show that the afferent auditory neuronal population can be divided, on the basis of its sensitivity to the neuronotoxic effect of glutamate and its analogs, in at least 3 subpopulations, one responding to N-methyl-D-aspartate (NMDA), one responding to kainate and a third minor one unresponsive to NMDA, kainic acid and glutamate. No toxic effect of quisqualate is observed. The use of specific antagonists (kynurenate and 2-amino-5-phosphonovalerate (DAP-5) demonstrates the specificity of the receptors to the excitatory amino acids on the afferent auditory neurons. Afferent auditory neurons from adult rats can also be cultured and in these preparations only the large neurons are sensitive to glutamate, kainate and NMDA while the small neurons are not responsive, suggesting that a glutamatergic neurotransmission occurs only at this synapse between the inner hair cells and the large radial afferent auditory neurons. We also show that, in vitro, the organ of Corti releases, in response to an increased potassium concentration and in the presence of calcium, a toxic activity for the afferent auditory neurons that is antagonized by kynurenate and DAP-5. Pathophysiological implications are discussed.     

Endogenous ligands for the quisqualate receptor: presence in rat brain cortex synaptic vesicles

The presence in highly purified rat brain cortex synaptic vesicles of endogenous ligands for rat brain quisqualate receptors was investigated. The vesicles were extracted, and their contents fractionated by high voltage electrophoresis. Endogenous ligands were detected by a radioreceptor assay in which such ligands competed with 50 nM -[³H]glutamate for binding to quisqualate receptors present in rat brain postsynaptic densities (PSDs). Binding of -[³H]glutamate to (NMDA) receptors, also present in PSDs, was blocked by 100 μM NMDA. We found that the endogenous ligands present in brain cortex synaptic vesicles for quisqualate receptors, were glutamate and aspartate, in a molar ratio of about two to one. The quisqualate receptor had an affinity 130-fold higher for glutamate (K_d 0.3 μM) than for aspartate, and the latter amino acid also showed a marked negative cooperativity for binding (Hill number 0.29, against 0.67 for glutamate). These findings suggest that glutamate is the natural transmitter that activates quisqualate receptors at some central excitatory synapses, and also that aspartate may be a classical transmitter, the receptor for which still remains to be shown.     

Glutamate receptors in striatum and substantia nigra: Effects of medial forebrain bundle lesions

We examined NMDA-sensitive [³H]glutamate, [³H]AMPA, [³H]kainate and metabotropic-sensitive [³H]glutamate binding sites in neostriatum and substantia nigra pars reticulata (SNr) in rats after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle. One week after the lesion, NMDA, AMPA, kainate and metabotropic receptors were decreased in the ipsilateral neostriatum, whereas at three months NMDA receptors were increased while AMPA, kainate and metabotropic receptors were not changed. In the SNr at one week, only AMPA and metabotropic receptors were significantly decreased whereas three months after the lesion NMDA, AMPA and kainate binding sites were decreased. The early decrease of excitatory amino acid receptors in the striatum is likely to reflect degeneration of dopaminergic fibers, suggesting that specific subpopulations of excitatory amino acid binding sites are located on dopaminergic terminals.     

Glutamergic Action on Alpha-Melanocyte-Stimulating Hormone Release from the Rat Hypothalamus

Release of α-melanocyte-stimulating hormone (α-MSH) synthesized in the hypothalamus is regulated by monoaminergic neuronal systems. An endogenous dopaminergic system inhibits α-MSH release (1, 2) whilst serotoninergic systems exert a biphasic effect on peptide release (3). The toxic effects of neonatal peripheral administration of monosodium glutamate on hypothalamic neurons containing proopiomelanocortin- (POMC-) derived peptides (4, 5) suggest additionally the presence of glutamate receptors on or indirectly influencing the POMC neuron. By comparison of the effect of the excitatory amino-acid agonists N-methyl-D-aspartate (NMDA), quisqualate and kainate on the release of α-MSH from superfused slices of rat hypothalamus, we have demonstrated a stimulatory glutamergic action on α-MSH release mediated through NMDA-type receptors.     

Structure-activity relationships for amino acid transmitter candidates acting at N-methyl-D-aspartate and quisqualate receptors

Dose-response curves for activation of excitatory amino acid receptors on mouse embryonic hippocampal neurons in culture were recorded for 15 excitatory amino acids, including the L-isomers of glutamate, aspartate, and a family of endogenous sulfur amino acids. In the presence of 3 microM glycine, with no extracellular Mg, micromolar concentrations of 11 of these amino acids produced selective activation of N-methyl-D-aspartate (NMDA) receptors. L-Glutamate was the most potent NMDA agonist (EC50 2.3 microM) and quinolinic acid the least potent (EC50 2.3 mM). Dose-response curves were well fit by the logistic equation, or by a model with 2 independent agonist binding sites. The mean limiting slope of log-log plots of NMDA receptor current versus agonist concentration (1.93) suggests that a 2-site model is appropriate. There was excellent correlation between agonist EC50S determined in voltage clamp experiments and KdS determined for NMDA receptor binding (Olverman et al., 1988). With no added glycine, and 1 mM extracellular Mg, responses to NMDA were completely blocked; responses to kainate and quisqualate were unchanged. Under these conditions, glutamate and the sulfur amino acids activated a rapidly desensitizing response, similar to that evoked by micromolar concentrations of quisqualate and AMPA, but mM concentrations of L-aspartate, homoquinolinic acid, and quinolinic acid failed to elicit a non-NMDA receptor-mediated response. Except for L-glutamate (EC50 480 microM), the low potency of the sulfur amino acids prevented the study of complete dose-response curves for the rapidly desensitizing response at quisqualate receptors. Small-amplitude nondesensitizing quisqualate receptor responses were activated by much lower concentrations of all quisqualate receptor agonists. Full dose-response curves for the nondesensitizing response were obtained for 9 amino acids; L-glutamate was the most potent endogenous agonist (EC50 19 microM). Domoate (EC50 13 microM) and kainate (EC50 143 microM) activated large-amplitude, nondesensitizing responses.     

Activation of excitatory amino acid receptors reduces thymidine incorporation and cell proliferation rate in primary cultures of astrocytes

Addition of quisqualate (a heterocyclic analogue of glutamate) reduced [methyl-3H]thymidine incorporation and cell proliferation in primary cultures of rat cortical astrocytes. The inhibitory action of quisqualate was mimicked by glutamate and ibotenate, whereas kainate, N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) were inactive. These results suggest that activation of a specific class of excitatory amino acid receptors contributes to the regulation of growth and proliferation of glial cells in primary culture.     

Localization of putative glutamatergic/aspartatergic neurons projecting to the supraoptic nucleus area of the rat hypothalamus

Oxytocin and vasopressin neurosecretory neurons of the supraoptic nucleus receive a rich glutamatergic innervation. The nerve cells of this prominent structure express various ionotropic and metabotropic glutamate receptor subtypes and there is converging evidence that glutamate acts as an excitatory transmitter in the control of release of oxytocin and vasopressin synthesized in this cell group. The location of the glutamatergic neurons projecting to this hypothalamic region is unknown. The aim of the present investigation was to study this question. [(3)H]D-aspartate, which is selectively taken up by high-affinity uptake sites at presynaptic endings that use glutamate as a transmitter, and is transported back to the cell body, was injected into the supraoptic nucleus area. The neurons retrogradely labelled with [(3)H]D-aspartate were detected autoradiographically. Labelled nerve cells were found in several diencephalic and telencephalic structures, but not in the brainstem. Diencephalic cell groups included the supraoptic nucleus itself, its perinuclear area, hypothalamic paraventricular, suprachiasmatic, ventromedial, dorsomedial, ventral premammillary, supramammillary and thalamic paraventricular nuclei. Within the telencephalon, labelled neurons were detected in the septum, amygdala, bed nucleus of the stria terminalis and preoptic area. The findings provide neuromorphological data on the location of putative glutamatergic neurons projecting to the supraoptic nucleus and its perinuclear area. Furthermore, they indicate that local putative glutamatergic neurons as well as several diencephalic and telencephalic structures contribute to the glutamatergic innervation of the cell group and thus are involved in the control of oxytocin and vasopressin release by neurosecretory neurons of the nucleus.     

Excitation of supraoptic vasopressin cells by stimulation of the A1 noradrenaline cell group: failure to demonstrate role for established adrenergic or amino acid receptors

The effects of adrenergic and excitatory amino acid antagonists on supraoptic nucleus (SON) neurosecretory cell responses to stimulation of the A1 noradrenaline (NA) cell group were examined in anaesthetized male rats. As in previous studies, delivery of cathodal pulses (100 microA, 1 ms pulses, 1 Hz) to the A1 region of the caudal ventrolateral medulla excited spontaneously active, antidromically identified neurosecretory cells, the majority of which were identified as arginine vasopressin (AVP) secreting on the basis of basal discharge patterns and responses to abrupt increases in arterial blood pressure. Administration of alpha- and beta-adrenoreceptor antagonists, by systemic or intracerebroventricular delivery of a bolus, or by direct pressure injection into the SON, did not alter neurosecretory cell responses to A1 stimulation, even when doses applied exceeded that required for blockade of excitations elicited by local application of NA. Application of the broad spectrum excitatory amino acid antagonist kynurenic acid (5-40 mM) blocked the excitatory effects of locally applied glutamate (100 microM) and transiently inhibited spontaneous activity, but failed to alter the excitatory effects of A1 region stimulation on SON cells. Identical effects were obtained with a selective kainate/quisqualate receptor antagonist. These data indicate that neurosecretory cell responses to activation of the A1 cell group are unaltered by antagonists of alpha- and beta-adrenoreceptors, or excitatory amino acid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differences in the properties of ionotropic glutamate synaptic currents in oxytocin and vasopressin neuroendocrine neurons.

Oxytocin (OT) and vasopressin (VP) hormone release from neurohypophysial terminals is controlled by the firing pattern of neurosecretory cells located in the hypothalamic supraoptic (SON) and paraventricular nuclei. Although glutamate is a key modulator of the electrical activity of both OT and VP neurons, a differential contribution of AMPA receptors (AMPARs) and NMDA receptors (NMDARs) has been proposed to mediate glutamatergic influences on these neurons. In the present study we examined the distribution and functional properties of synaptic currents mediated by AMPARs and NMDARs in immunoidentified SON neurons. Our results suggest that the properties of AMPA-mediated currents in SON neurons are controlled in a cell type-specific manner. OT neurons displayed AMPA-mediated miniature EPSCs (mEPSCs) with larger amplitude and faster decay kinetics than VP neurons. Furthermore, a peak-scaled nonstationary noise analysis of mEPSCs revealed a larger estimated single-channel conductance of AMPARs expressed in OT neurons. High-frequency summation of AMPA-mediated excitatory postsynaptic potentials was smaller in OT neurons. In both cell types, AMPA-mediated synaptic currents showed inward rectification, which was more pronounced in OT neurons, and displayed Ca2+ permeability. On the other hand, NMDA-mediated mEPSCs of both cell types had similar amplitude and kinetic properties. The cell type-specific expression of functionally different AMPARs can contribute to the adoption of different firing patterns by these neuroendocrine neurons in response to physiological stimuli.     

Presynaptic and postsynaptic modulation of glutamatergic synaptic transmission by activation of α1‐ and β‐adrenoceptors in layer V pyramidal neurons of rat cerebral cortex

Adrenergic agonists have different modulatory effects on excitatory synaptic transmission depending on the receptor subtypes involved. The present study examined the loci of α₁‐ and β‐adrenoceptor agonists, which have opposite effects on excitatory neural transmission, involved in modulation of glutamatergic transmission in layer V pyramidal cells of rat cerebral cortex. Phenylephrine, an α₁‐adrenoceptor agonist, suppressed the amplitude of AMPA receptor‐mediated excitatory postsynaptic currents evoked by repetitive electrical stimulation (eEPSCs, 10 pulses at 33 Hz). The coefficient of variation (CV) of the 1st eEPSC amplitude and paired‐pulse ratio (PPR), which were sensitive to extracellular Ca²⁺ concentration, were not affected by phenylephrine. Phenylephrine suppressed miniature EPSC (mEPSC) amplitude without changing its frequency. In contrast, isoproterenol, a β‐adrenoceptor agonist, strongly increased the amplitude of the 1st eEPSC compared with that of the 2nd to 10th eEPSCs, which resulted in a decrease in PPR. Isoproterenol‐induced enhancement of eEPSC amplitude was accompanied by a decrease in CV. Isoproterenol increased the frequency of mEPSCs without significant effect on amplitude. Phenylephrine suppressed inward currents evoked by puff application of glutamate, AMPA, or NMDA, whereas isoproterenol application was not accompanied by significant changes in these inward currents. These findings suggest that phenylephrine decreases eEPSCs through postsynaptic AMPA or NMDA receptors, while the effects of isoproterenol are mediated by facilitation of glutamate release from presynaptic terminals without effect on postsynaptic glutamate receptors. These two different mechanisms of modulation of excitatory synaptic transmission may improve the “signal‐to‐noise ratio” in cerebral cortex. Synapse 63:269–281, 2009. © 2008 Wiley‐Liss, Inc.