Cortically evoked excitatory synaptic transmission in the cat red nucleus is antagonised by D-AP5 but not by D-AP7

Extracellular recordings were made from magnocellular neurones (mRN) in α-chloralose (50 mg/kg, iv) anaesthetised cats. Iontophoretically applied (NMDA) excited the neuronal firing which was antagonised by 4 selective NMDA receptor antagonists: 2-amino-5-phosphonophentanoate (AP5), 2-amino-7-phosphopnopheptanoate (AP7), RS-4(phosphonomethyl) piperazine-2-carboxylic acid (PMPC) and R-4-(3-phosphonoprophyl) piperazine-2-carboxylic acid (CPP), whereas AMPA responses were uneffected. Monosynaptic excitatory responses were produced by stimulation of the sensorimotor cortex. These responses were reduced and often abolished by AP5 and PMPC but not by AP7 or CPP. It is postulated that two NMDA receptor subtypes exist on mRN neurones.     

Reticulospinal neurones activate excitatory amino acid receptors

Paired intracellular recordings were used to study the monosynaptic excitatory postsynaptic potentials (EPSP) in lamprey motoneurones evoked by stimulation of single reticulospinal Müller and Mauthner cells. The chemical component of the synaptic potentials was depressed by both application of the non-selective excitatory amino acid antagonists kynurenic acid and cis-2,3-piperidine dicarboxylate. The N-methyl-D-aspartate (NMDA) antagonists Mg2+ and 2-amino-5-phosphonovalerate caused a selective depression of a late component of the EPSP. Thus, fast-conducting reticulospinal neurones appear to release an excitatory amino acid acting at both NMDA and non-NMDA receptors.     

Excitatory and inhibitory postsynaptic currents of the superior salivatory nucleus innervating the salivary glands and tongue in the rat

The excitatory and inhibitory synaptic inputs to parasympathetic preganglionic neurons in the superior salivatory (SS) nucleus were investigated in brain slices of neonatal (4-8 days old) rat using the whole-cell patch-clamp technique. The SS neurons innervating the submandibular and sublingual salivary glands and innervating the lingual artery in the anterior region of the tongue were identified by retrograde transport of a fluorescent tracer. Whole-cell currents were evoked by electrical stimulation of tissue surrounding the cell. These evoked postsynaptic currents were completely abolished by antagonists for N-methyl-D-aspartate (NMDA) glutamate, non-NMDA glutamate, gamma-aminobutyric acid type A (GABAA), and glycine receptors, suggesting that SS neurons receive glutamatergic excitatory, and GABAergic and glycinergic inhibitory synaptic inputs. In SS neurons for the salivary glands, the ratio of the NMDA component to the total excitatory postsynaptic current (EPSC) was larger than that of the non-NMDA component. This profile was reversed in the SS neurons for the tongue. In SS neurons for the salivary glands, the ratio of the GABAA component to the total IPSC was larger than the ratio of the glycine component to total inhibitory postsynaptic current (IPSC). The decay time constants of the GABAA component were slower than those for glycine. These characteristics of the excitatory and inhibitory inputs may be involved in determining the firing properties of the SS neurons innervating the salivary glands and the tongue.     

In vivo optical recordings of synaptic transmission and intracellular Ca2+ and Cl- in the superior colliculus of fetal rats

Although the N-methyl-D-aspartate (NMDA) receptor is known to play a crucial role in activity-dependent remodeling of synaptic connections in the fetal superior colliculus (SC), its contribution to the electrical activity of fetal SC neurons has not been determined. Furthermore, whether gamma-aminobutyric acid (GABA)-mediated inhibition occurs either as early as prenatal periods or only after eye opening has been controversial. We therefore performed optical recordings using voltage-, Ca2+- and Cl--sensitive fluorescent dyes to analyse synaptic transmission and changes in intracellular Ca2+ and Cl- in the SC of fetal rats that were still connected with the dams by the umbilical cord. Excitatory and inhibitory responses were evoked by focal SC stimulation. The excitatory synaptic responses are composed of early and late components. The early component was mediated by both non-NMDA and NMDA receptors, whereas the late component occurred mainly via NMDA receptors. Train pulse stimulation at higher currents was required for induction of the inhibition, which was antagonized by bicuculline, and blocking of the GABA-mediated inhibition by bicuculline uncovered masked excitatory synaptic responses. Focal SC stimulation induced increases in [Cl-]i and [Ca2+]i that were mediated by GABA-A receptors and mainly by NMDA receptors, respectively. GABA antagonists augmented SC-induced increases in [Ca2+]i. These results indicate that, in the fetal SC, excitatory and inhibitory synaptic transmissions occur before birth, that the NMDA receptor is a major contributor to excitatory synaptic transmission and increased [Ca2+]i, and that the GABA-A receptor is already functioning to inhibit excitatory neurotransmission.     

Activation of locus coeruleus neurons by nucleus paragigantocellularis or noxious sensory stimulation is mediated by intracoerulear excitatory amino acid neurotransmission

The nucleus paragigantocellularis (PGi), located in the rostral ventrolateral medulla, is one of two major afferents to the nucleus locus coeruleus (LC). Electrical stimulation of PGi exerts a robust, predominantly excitatory influence on LC neurons that is blocked by intracerebroventricular (i.c.v.) administration of the broad spectrum excitatory amino acid (EAA) antagonists kynurenic acid (KYN) or gamma-D-glutamylglycine (DGG), but not by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-7-phosphonoheptanoate (AP7). I.c.v. injection of KYN or DGG also blocked activation of LC neurons evoked by noxious somatosensory stimuli. These results indicate that activation of LC neurons by PGi and noxious stimuli may be mediated by an EAA acting at a non-NMDA receptor in LC. In the present study, microiontophoretic techniques were used to determine the sensitivity of LC neurons in vivo to the selective EAA receptor agonists kainate (KA), NMDA and quisqualate (QUIS). Microinfusion and microiontophoresis were also used to determine whether direct application of KYN, the preferential non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3 dione (CNQX) or the selective NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) onto LC neurons blocked excitation elicited by stimulation of PGi or the sciatic nerve. The results demonstrated that individual LC neurons were robustly activated by direct application of KA, NMDA and QUIS. Iontophoretically applied KYN reduced or completely antagonized responses evoked by all 3 agonists. In contrast, iontophoretically applied AP5 strongly attenuated NMDA-evoked excitation, while KA-and QUIS-evoked responses were not affected by this agent. Furthermore, direct application of KYN or the specific non-NMDA receptor antagonist, CNQX, onto LC neurons substantially attenuated or completely blocked synaptic activation produced by PGi or sciatic nerve stimulation in nearly every LC neuron tested. Microinfusion of the selective NMDA receptor antagonist AP5 had no effect on sciatic nerve-evoked responses. These results confirm our hypothesis that activation of LC neurons from PGi is mediated by an EAA operating primarily at a non-NMDA receptor subtype on LC neurons. Furthermore, these findings provide additional support for the hypothesis that this pathway mediates at least some sensory-evoked responses of LC neurons.     

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.     

Excitatory synaptic inputs to pyramidal neurons of the lateral amygdala

Whole-cell patch clamp recordings were made from pyramidal neurons in the rat lateral amygdala (LA). Synaptic currents were evoked by stimulating in either the external capsule (ec), internal capsule (ic) or basolateral nucleus (BLA). Stimulation of either the ic, ec or BLA evoked a glutamatergic excitatory synaptic current (EPSC) which was mediated by both non-NMDA and NMDA (N-methyl-d-aspartic acid) receptors. The ratio of the amplitude of the NMDA receptor-mediated component measured at +40 mV to the amplitude of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) component measured at -60 mV was similar regardless of whether EPSCs were evoked in the ec, ic or BLA. At resting membrane potentials, excitatory synaptic potentials evoked from either the ec or putative thalamic inputs were unaffected by application of the NMDA receptor antagonist APV. Spontaneous glutamatergic currents had two components to their decay phase. The slow component was selectively blocked by the NMDA receptor antagonist D-APV, indicating that AMPA and NMDA receptors are colocalized in spiny neurons. We conclude that pyramidal cells of the LA receive convergent inputs from the cortex, thalamus and basal nuclei. At all inputs, both AMPA/kainate and NMDA-type receptors are active and colocalized in the postsynaptic density.     

Entorhinal cortex long-term potentiation evoked by theta-patterned stimulation of associative fibers in the isolated in vitro guinea pig brain

Long-term potentiation (LTP) induced in the lateral entorhinal cortex by theta-patterned tetanic stimulation of the piriform cortex was analyzed in the isolated guinea pig brain maintained in vitro. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulation of the piriform cortex are composed of an early and late component selectively blocked bynon-N-methyl-d-aspartate (non-NMDA) and NMDA receptor antagonists, respectively. LTP induction was dependent on NMDA receptor activation, being blocked by perfusing the preparation with 2-amino-5-phosphonovalerate (AP-5). LTP was expressed through synaptic enhancement of both early non-NMDA and late, possibly NMDA receptor-mediated responses.     

Pharmacological characterization of calcium currents and synaptic transmission between thalamic neurons in vitro.

We recorded from pairs of cultured, synaptically connected thalamic neurons. Evoked excitatory postsynaptic currents (EPSCs) reversed at +17 mV and were blocked reversibly by 1 mM kynurenic acid, a glutamate receptor antagonist. NMDA and non-NMDA receptors mediated excitatory post-synaptic responses, as shown by selective block of EPSC components with 50 microM (+/-)-2-amino-5-phosphonopentanoic acid and 10 microM 6,7-dinitroquinoxaline-2,3-dione, respectively. Inhibitory postsynaptic responses were evoked less frequently and were blocked by the GABAA receptor antagonist (-)-bicuculline methochloride. The pharmacological profiles of whole-cell calcium currents and evoked EPSCs were compared. With 50 microM cadmium chloride (Cd), whole-cell low voltage-activated (LVA) calcium currents were reduced in amplitude and high voltage-activated (HVA) calcium currents and excitatory synaptic transmission were completely blocked. This suggests that the residual calcium influx through LVA channels into the presynaptic terminal does not suffice to trigger transmitter release. A saturating concentration of omega-conotoxin GVIA (omega-CgTx) (2.5 microM) blocked one-third of whole-cell HVA calcium currents and evoked EPSCs. The dihydropyridine nifedipine (50 microM) reversibly reduced whole-cell HVA calcium currents in a voltage-dependent manner but not excitatory synaptic transmission. Cd and omega-CgTx did not alter amplitude distributions of miniature EPSCs, demonstrating that the inhibition of synaptic transmission was due to block of presynaptic calcium channels. We conclude that excitatory glutamatergic transmission in thalamic neurons in vitro was mediated mainly by HVA calcium currents, which were insensitive to omega-CgTx and nifedipine.     

Preferential potentiation by nitric oxide of spontaneous inhibitory postsynaptic currents in rat supraoptic neurones

Magnocellular neurones in the supraoptic nucleus and paraventricular nucleus express mRNA for nitric oxide synthase (NOS) and the expression becomes more prominent when the release of vasopressin or oxytocin is stimulated. It has also been reported that NO donors inhibit the electrical activity of supraoptic nucleus neurones, but the mechanism involved in the inhibition remains unclear. In the present study, to know whether modulation of synaptic inputs into supraoptic neurones is involved in the inhibitory effect of NO, we measured spontaneous excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) from rat supraoptic nucleus neurones in slice preparations identified under a microscope using the whole-cell mode of the slice-patch-clamp technique. The NO donor, S-nitroso-N-acetylpenicillamine (SNAP), reversibly increased the frequency of spontaneous IPSCs mediated by GABAA receptors, without affecting the amplitude, indicating that NO potentiated IPSCs via a presynaptic mechanism. The NO scavenger, haemoglobin, suppressed the potentiation of IPSCs by SNAP. On the other hand, SNAP did not cause significant effects on EPSCs mediated by non-NMDA glutamate receptors. The membrane permeable analogue of cGMP, 8-bromo cGMP, caused a significant reduction in the frequency and amplitude of both IPSCs and EPSCs. The results suggest that NO preferentially potentiates the inhibitory synaptic inputs into supraoptic nucleus neurones by acting on GABA terminals in the supraoptic nucleus, possibly via a cGMP-independent mechanism. The potentiation may, at least in part, account for the inhibitory action of NO on the neural activity of supraoptic neurones.     

Delayed postnatal development of NMDA receptor function in medium-sized neurons of the rat striatum

During early postnatal development, the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor plays a dominant role in excitatory amino acid-mediated synaptic transmission in essentially every brain region that has been examined. In contrast, we have found that in the rat striatum, NMDA receptor-mediated current develops later in the medium-sized neurons (MSNs) than currents mediated by activation of non-NMDA receptors. MSNs were identified using infrared video microscopy, and voltage-clamped in a slice preparation using the whole-cell patch-clamp technique. Intrastriatal stimulation was used to evoke excitatory synaptic currents from slices in animals ranging in age from postnatal day (PND) 5 to 60. Though most cells from animals younger than PND 10 failed to respond to synaptic stimulation, postsynaptic responses were occasionally evoked in cells as young as PND 5. Synaptic currents from cells between PNDs 5 and 7 had a significant contribution due to activation of non- NMDA receptors, as evidenced by sensitivity to the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and rapidly rising and falling response components. The relative contribution of NMDA receptors increased approximately twofold from the first to the third postnatal week; no further change was observed through PND 60. At the same ages that the NMDA receptors contributed maximally to the synaptic current, the decay time constant of the NMDA receptor-mediated current decreased significantly. The increasing weight of NMDA receptor-mediated current may reflect a change in the number of functional receptors at the synapse since there was no apparent change in the voltage dependence of the current. To more completely examine receptor function early in postnatal development, NMDA and kainate were applied either iontophoretically or in the bath. Iontophoretic application of NMDA onto cells obtained from rats between PNDs 3 and 5 only occasionally evoked current, provided that the membrane was held at depolarized potentials to remove the Mg(2+) block. In contrast, application of kainate consistently evoked a response from cells of the same age group. Bath application of the same agonists provided similar results. Taken together, the present experiments demonstrate that striatal non-NMDA receptor-mediated currents are more mature than NMDA receptor-mediated currents early in development.     

NMDA and non-NMDA receptors on rat supraoptic nucleus neurons activated monosynaptically by olfactory afferents

The recently discovered efferent projections from the main and accessory olfactory bulbs to the supraoptic nucleus (SON) were further investigated. Intracellular electrophysiological methods were used to determine (a) if these projections are monosynaptic, (b) which excitatory amino acid (EAA) receptor subtypes mediate responses to activation of these pathways and (c) whether the same receptor subtypes mediate responses of phasically firing (vasopressin) and continuously firing (putative oxytocin) neurons. Recordings were made from SON neurons in large explants and 500 μm thick horizontal slices, containing 2–6 mm of the piriform cortex and lateral olfactory tract (LOT). This allowed recording of synaptic responses to selective stimulation of the LOT. EPSPs in SON neurons faithfully followed stimulus frequencies of 50–100 Hz, indicating that these inputs were monosynaptic. Stimulus-evoked EPSPs were blocked by the non-specific EAA antagonist, kynurenate. Perifusion of the slice with Mg²⁺-free medium revealed the presence of NMDA receptors in addition to the non-NMDA receptors on both phasically and continuously firing cells, indeed, on all cells tested. Partial blockade of these EPSPs in Mg²⁺-free medium could be achieved with either the NMDA antagonist, AP5, or the non-NMDA antagonists, CNQX or NBQX. Full blockade of the stimulus-evoked EPSPs was effected by adding both types of antagonists to the medium, although spontaneous EPSPs were still observed in several cells. These results are consistent with prior studies showing both receptor subtypes in the SON. This is the first demonstration that afferent stimulation activates both subtypes in the same SON neuron regardless of its peptide content.     

Properties of inhibitory and excitatory synapses between hippocampal neurons in very low density cultures

The whole cell patch clamp technique was used to examine the electrophysiological properties of embryonic hippocampal neurons maintained in a very low density (VLD) culture prejparation. The goal of these experiments was to establish the viability of the VLD culture as a model system in which to study regulation of neurotransmission at single monosynaptic connections, in the absence of polysynaptic innervation. Depolarization of neurons in the VLD culture revealed voltage-dependent sodium, calcium, and potassium currents which were blocked with, respectively, tetrodotoxin (TTX), cobalt, and tetraethylammonium and 4-aminopyridine. When pairs of neurons were simultaneously recorded, action potentials evoked in presynaptic neurons elicited either excitatory or inhibitory postsynaptic currents (EPSCs or IPSCs, respectively). The dual component “EPSCs” were due to the activation of both types-of postsynaptic, ionotropic glutamate receptors: N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Evoked IPSCs were due to the activation of postsynaptic γ-aminobutyric acid (GABA) receptors. Both excitatory and inhibitory synapses exhibited short term depression in response to high frequency stimulation, although IPSCs were routinely decreased to a much greater degree than EPSCs. Spontaneous miniature EPSCs and IPSCs were found to persist in TTX, were blocked by the same pharmacological antagonists which blocked evoked responses, increased in frequency in response to hyperosmotic solution, and were unaffected by changes in extracellular calcium concentration. mIPSCS were found to occur at a significantly lower frequency than mEPSCs. These experiments indicated that neurotransmission in the VLD cultures occurs in a manner consistent with the quantal hypothesis and, therefore, the VLD culture is a good model for studying excitatory and inhibitory neurotransmission between isolated pairs of neurons. In addition, these experiments, performed under comparable physiological conditions, demonstrated that there are fundamental differences underlying neurotransmitter release between excitatory and inhibitory neurons. © 1994 Wiley-Liss, Inc.     

Crocin antagonizes ethanol inhibition of NMDA receptor-mediated responses in rat hippocampal neurons

We have previously found that crocin (crocetin di-gentiobiose ester) antagonizes the inhibitory effect of ethanol on long-term potentiation in the rat hippocampus in vivo and in vitro. To explore mechanisms underlying the antagonism of crocin against ethanol, we investigated the effects of ethanol and crocin on synaptic potentials mediated by N-methyl-d-aspartate (NMDA) receptors in the dentate gyrus of rat hippocampal slices. Synaptic potential mediated by non-NMDA receptors was recorded in normal medium (1.3 mM Mg²⁺), while NMDA receptor-mediated synaptic potential was isolated in low (0.13 mM) Mg²⁺ medium containing the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 μM). Crocin (10 μM) alone did not affect synaptic potentials mediated by non-NMDA nor NMDA receptors. Non-NMDA response was slightly inhibited by 100 mM ethanol, while NMDA response was selectively inhibited by lower concentrations (10–50 mM) of ethanol. Crocin (10 μM) did not affect the inhibition of non-NMDA response by 100 mM ethanol, but significantly blocked the inhibition of NMDA response by 10–50 mM ethanol. In addition, we performed whole-cell patch recording with primary cultured rat hippocampal neurons, and confirmed that crocin blocked ethanol inhibition of inward currents evoked by application of NMDA. These results suggest that crocin specifically antagonizes the inhibitory effect of ethanol on NMDA receptor-mediated responses in hippocampal neurons.     

Glutamatergic synaptic inputs activate neurons in the subfornical organ through non-NMDA receptors

The subfornical organ (SFO) plays an important role in central regulation of the autonomic nervous system. The synaptic transmission properties of neurons in the SFO were studied with intracellular and whole-cell patch clamp recordings in the rat slice preparations. Both the spontaneous and evoked excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) were almost completely suppressed by the glutamate receptor antagonist kynurenic acid and the non-NMDA (N-methyl-D-aspartic acid) antagonist CNQX. The non-NMDA agonist kainic acid depolarized the membrane most potently, compared with NMDA and quisqualic acid. These suggest that glutamate is a main excitatory neurotransmitter in the SFO and that its action is at least partly mediated through non-NMDA receptors.     

Mechanisms of excitation and inhibition in the nigrostriatal system

The extracellular responses of neurones in the neostriatum following single pulse stimulation of the substantia nigra were investigated in urethane anaesthetized rats. Low intensity stimulation (< 10 V) evoked single large amplitude spikes while higher intensities (10–20 V) elicit a high frequency burst of small amplitude spikes or waves. When spontaneous or glutamate-induced large spikes are recorded, nigral stimulation causes their inhibition coincidentally with the development of a burst. If the burst is prevented, the inhibitory response disappears. Both the nigral evoked inhibition and burst response are unaffected by iontophoretically or systemically administered antagonists of dopamine or by chemical lesions of the dopamine-containing nigral neurones. The monosynaptic activation of large amplitude striatal neurones, which could also be identified antidromically by stimulation of the globus pallidus, was reversibly blocked by dopamine antagonists.It is concluded (a) that the burst responses are induced through the antidromic excitation of striatonigral axons within the striatum; (b) that the striatal neurones thus activated are inhibitory interneurones and (c) that the dopamine-containing neurones of the nigra make excitatory synaptic contact with a population of striatal output cells, some of which at least project to the globus pallidus.     

Participation of low-threshold calcium spikes in excitatory synaptic transmission in guinea pig medial frontal cortex

We studied the activation of low-threshold calcium spikes (LTS) by excitatory postsynaptic potentials in pyramidal neurons from guinea pig medial frontal cortex with intracellular recording. We used extracellular bicuculline and phaclofen and intracellular QX-314 to block inhibitory synaptic potentials and sodium currents. Postsynaptic potentials were evoked by stimulation of layer I. We found that large (> 10-15 mV) excitatory synaptic potentials evoked from membrane potentials more negative than -75 mV were able to trigger LTS. The activation of LTS resulted in an increase of the rising slope or amplitude of the synaptic potentials depending on the size of the excitatory postsynaptic potential (EPSP). We used 100 microM NiCl2 to confirm the presence of LTS as part of the EPSPs. The N-methyl-D-aspartate (NMDA) and non-NMDA components of the excitatory synaptic potentials were isolated using (+/-)2-amino-5-phosphonovaleric acid (APV; 50 microM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 20 microM); both components could, independently, trigger an LTS. With recordings made with K+ acetate-filled electrodes, we show that the activation of LTS was critical to allow excitatory synaptic potentials to reach the threshold of action potential firing; also, this amplification of synaptic responses produced the firing of more than a single action potential by the postsynaptic cell. These results demonstrate that in cortical pyramidal neurons the activation of low-threshold calcium spikes results in the amplification of synaptic responses.     

Pharmacological characterization of excitatory synaptic potentials in rat basolateral amygdaloid neurons.

The pharmacological properties of synaptic responses in rat basolateral amygdaloid (BLA) neurons were studied using intracellular recording techniques. Three distinct types of synaptic potential were evoked by stimulation of the adjacent ventral endopyriform nucleus: 1) a fast excitatory postsynaptic potential (f-EPSP); 2) a late EPSP (1-EPSP) following the f-EPSP; and 3) a multiphasic hyperpolarization following the initial depolarizing potential. Superfusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-N-methyl-D-aspartate (non-NMDA) receptor antagonist, blocked the f-EPSP in a concentration-dependent manner. The ED50 for this effect was around 4 microM. In the presence of CNQX, however, a small depolarizing potential remained. This residual depolarizing component was markedly enhanced on removing Mg++ from the perfusing medium and could subsequently be abolished by DL-2-amino-5-phosphonovaleate (DL-APV, 50 microM) indicating its mediation via NMDA receptor-coupled ionophore. The l-EPSP was reversibly blocked by DL-APV. These results suggest that the pyriform cortex-amygdala pathway is mediated through excitatory amino acids acting on non-NMDA as well as NMDA receptors located on the BLA neurons.     

Evidence for excitatory amino acid transmission between mesencephalic nucleus of V afferents and jaw-closer motoneurons in the guinea pig

Previous studies have suggested that monosynaptic transmission between spinal primary afferent fibers and motoneurons is mediated by an excitatory amino acid, most likely glutamate or aspartate. No such comparable studies have been carried out in the trigeminal system. In an attempt to elucidate the neurotransmitter(s) mediating monosynaptic transmission between mesencephalic of V nucleus afferents (Mes V) and trigeminal jaw-closer motoneurons, the effect of iontophoretic application of excitatory amino acid antagonists on the Mes V-induced field potential, recorded in the trigeminal motor nucleus (Mot V), was examined. Application of DL-2-amino-4-phosphonobutyrate (APB) and the broad spectrum amino acid antagonists, kynurenic acid (KYN) and gamma-D-glutamylglycine (DGG), for 3-4 min reversibly reduced the amplitude of the Mes V induced field potential. The effect of APB was much greater than any of the other compounds tested. On the other hand, the specific N-methyl-D-aspartate (NMDA) receptor blocker DL-2-amino-5-phosphonovaleric acid (APV), was without effect on the field potential. Based on current-response curves for each antagonist tested, the order of potency was determined to be APB greater than KYN greater than DGG greater than APV. These antagonists were also compared with respect to their efficacy in blocking individual jaw-closer motoneuron activity induced by iontophoretic application of amino acid receptor excitants glutamate (Glut), aspartate (Asp), kainate (K), and quisqualate (Q). NMDA application was without effects on these motoneurons. The profile of activity of these antagonists on these amino acid excitants was similar to that found in other areas of the CNS by other investigators. KYN and DGG both significantly reduced responses induced by all excitants tested, whereas APB had more modest effects on K and Q excitation and was without effect on Glut and Asp excitations in most cells tested. The data suggest that an excitatory amino acid, activating non-NMDA receptors, mediates some component of synaptic transmission between Mes V afferents and jaw-closer motoneurons. The data is also consistent with the proposal made in other systems that APB blocks synaptic transmission by a mechanism other than postsynaptic receptor blockade.     

Electrophysiological and neuropharmacological study of tectoreticular pathways in lampreys

Tectoreticular (TR) cells along the diencephalic–mesencephalic border are the origin of prominent crossed and uncrossed pathways that project to the middle (MRRN) and posterior (PRRN) rhombencephalic reticular nuclei in juvenile and adult lampreys [I.C. Zompa, R. Dubuc, Diencephalic and mesencephalic projections to rhombencephalic reticular nuclei in lampreys, Brain Res. (1998) in press.]. This study investigated the synaptic contacts between TR axons and the reticular cells. Intracellular recordings were carried out in reticular neurones (n=124) while microstimulating the TR regions. Tectoreticular inputs were recorded in all reticular cells studied (248 PSPs); although stronger responses were evoked in the MRRN neurones. The majority of responses were excitatory, but increasingly mixed and inhibitory when recorded in the middle and caudal part of the reticular nuclei. The excitation had the shortest onset latencies and sharpest slopes measured in both reticular nuclei, while the inhibition was longer and smoother. The characteristics of TR inputs to different reticular cell types is also presented. The transmission of evoked responses was isolated to the crossed and uncrossed TR pathways by studying the effects of 1% Xylocaine ejections and surgical lesions. The TR inputs were transmitted to reticular cells through monosynaptic and polysynaptic contacts. The synaptic transmission involved excitatory amino acids, acting through AMPA and NMDA receptors, while the inhibition was glycinergic. Comparisons with other sensory systems in lampreys are discussed.