Function of non-NMDA receptors and NMDA receptors in synaptic responses to natural somatosensory stimulation in the ventrobasal thalamus

Summary Sensory synaptic responses of rat ventrobasal thalamus neurones were challenged with iontophoretic applications of the excitatory amino acid antagonists CNQX and CPP. CNQX, applied with currents which were selective for non-NMDA receptors, antagonised responses of VB neurones to both 10 ms and 2000 ms air jet stimulation of the peripheral receptive field. In contrast, CPP only antagonised the latter type of response. These results suggest a differential involvement of excitatory amino acid receptors in sensory synaptic transmission to the ventrobasal thalamus, with an initial synaptic component being mediated by non-NMDA receptors (including kainate receptors), and a further NMDA receptor-mediated component being manifested upon maintained sensory stimulation. The expression of this latter component appears to be largely dependent upon the integrity of the non-NMDA receptor-mediated component.     

Excitatory amino acid receptors mediate synaptic responses to visual stimuli in superior colliculus neurones of the rat.

Excitatory amino acid receptors are involved in synaptic transmission throughout the central nervous system. As the specific synaptic pharmacology of visually responsive superior colliculus (SC) neurones has not been evaluated, we have attempted to antagonize visual responses of these neurones with selective excitatory amino acid antagonists. The N-methyl-D-aspartate (NMDA) receptor antagonist 3-((+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), and the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were applied iontophoretically in the vicinity of single visually responsive SC neurones. Visually evoked responses were antagonized by non-NMDA receptor selective currents of CNQX in 13 of 14 cells studied. Of 18 cells studied with NMDA receptor selective currents of CPP, visual responses were antagonized in only two cases. This study demonstrates that excitatory amino acid receptors are involved in synaptic transmission of visual information to the rat superior colliculus, but that NMDA receptors may play a relatively minor role.     

Electrophysiological evidence for N-methyl-D-aspartate excitatory amino acid receptors in the rat supraoptic nucleus in vitro.

The responses of neurons in slices of the rat supraoptic nucleus (SON) to afferent stimulation were recorded under current-clamp conditions. In magnesium (Mg2+)-free incubation medium, synaptic responses were prolonged and were partially antagonized by the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist (+)-5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). During blockade of non-NMDA excitatory amino acid (EAA) receptors, the synaptic responses in Mg(2+)-free medium were blocked by the competitive NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5). The results of these experiments provide electrophysiological evidence for the existence of NMDA receptors in the rat SON.     

NMDA and non-NMDA receptors contribute to synaptic transmission between the medial geniculate body and the lateral nucleus of the amygdala

We examined whether the NMDA class of excitatory amino acid receptors contribute to synaptic transmission in the pathway connecting the medial geniculate body (MGB) with the lateral nucleus of the amygdala (LA) using extracellular single unit recordings and microiontophoresis. Cells were identified in LA on the basis of responsivity to electrical stimulation of the MGB. For each cell, a level of current was found for the iontophoretic ejection of the NMDA antagonist AP5 that blocked responses elicited by iontophoresis of NMDA, but had no effect on responses elicited by AMPA. Iontophoresis of AP5 with this level of current blocked the excitatory response elicited by MGB stimulation in most cells tested. Microinfusion of AP5 (25, 50, or 100 M) also blocked the responses. Additional studies tested individual cells with both AP5 and the AMPA antagonist CNQX and showed that blockade of either NMDA or AMPA receptors interferes with synaptic transmission. Finally, iontophoretic ejection of either AP5 or CNQX blocked short-latency (<25 ms) responses elicited in LA by peripheral auditory stimulation. Together, these results suggest that the synaptic evocation of action potentials in the thalamo-amygdala pathway depends on both NMDA and non-NMDA receptors. We hypothesize that non-NMDA receptors are most likely required to depolarize the cell sufficiently to remove the blockade of NMDA channels by magnesium and NMDA receptors are required to further depolarize the membrane to the level required for action potential generation.     

Effects of excitatory amino acid antagonists on synaptic responses of supraoptic neurons in slices of rat hypothalamus

1. Intracellular recordings from magnocellular neurons in the supraoptic nucleus (SON) were obtained from rat hypothalamic slices to determine the effects of specific transmitter antagonists on evoked postsynaptic potentials (PSPs), action potential after-discharge, and spontaneously occurring PSPs. 2. Broad-spectrum excitatory amino acid (EAA) antagonists, kynurenic acid (KYN) and gamma-d-glutamylglycine (DGG), significantly diminished or eliminated electrically evoked depolarizing PSPs and spike discharges. These compounds also greatly reduced the amplitude and frequency of spontaneous PSPs. 3. The specific N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), did not significantly reduce these measures of synaptic activation under these experimental conditions. 4. The gamma-aminobutyric acid (GABA) antagonist, bicuculline methiodide (BIC), partially antagonized some PSPs when the cells were hyperpolarized (-75 to -80 mV) with steady injected currents; KYN antagonized BIC-resistant PSPs. 5. The involvement of a hypothetical cholinergic input to the SON in the responses to stimulation of the region dorsolateral to the SON was tested by bath application of nicotinic cholinergic antagonists, particularly d-tubocurarine (dTC). Nicotinic cholinergic antagonists, even after prolonged exposure to high concentrations, did not block the responses of SON cells to dorsolateral stimulation. 6. These findings strongly suggest that EAAs mediate fast excitatory synaptic responses of SON neurons to stimulation of cells and axons in the region dorsolateral to the SON. The blockade of almost all spontaneous EPSPs by broad-spectrum EAA antagonists likewise argues that EAAs are responsible for the majority of ongoing fast excitatory input. These responses appear to involve an interaction with kainate- and/or quisqualate-type EAA receptors.     

Excitatory and inhibitory responses of Purkinje cells, in the rat cerebellum in vivo, induced by excitatory amino acids

Responses of rat cerebellar Purkinje cells to iontophoretically administered excitatory amino acids have been studied in vivo. Responses to N-methyl-D-aspartate (NMDA) were either biphasic (excitation followed by inhibition) or purely inhibitory and were antagonized by the selective NMDA-receptor antagonist, 2-amino-5-phosphonovalerate. Quisqualate and kainate either excited or induced biphasic responses, in these neurones, which were only reduced by amino acid antagonists that acted at non-NMDA receptors. The excitatory amino acid-induced inhibitions were also antagonized by the selective gamma-aminobutyric acid (GABA) antagonist, picrotoxin, suggesting that they were indirectly mediated via GABAergic inhibitory interneurones, which could be excited via NMDA and non-NMDA receptors.     

Acousticolateral and visual processing and their interaction in the torus semicircularis of the trout, Salmo gairdneri

The hypothesis that synaptic transmission between the auditory nerve and the cochlear nucleus is mediated by an excitatory amino acid acting through N-methyl-D-aspartate (NMDA) receptors was examined in an in vitro preparation of the chicken brainstem. The ability of various bath-applied excitatory amino acid receptor antagonists to inhibit synaptically-evoked responses was assessed by recording field potentials from nucleus magnocellularis (NM) following electrical stimulation of the cochlear nerve. Antagonists that selectively block responses mediated by NMDA receptors, such as D-alpha-aminoadipate and 2-amino-5-phosphonovalerate, were without effect on evoked transmission in NM. In contrast, antagonists that additionally act on non-NMDA receptors, such as cis-2,3-piperidine dicarboxylate and gamma-D-glutamylglycine, reversibly suppressed transmission. The results indicate that (1) transmission in the chicken auditory system is mediated by non-NMDA receptors, and (2) a substance(s) chemically akin to aspartate and glutamate may be the transmitter used by the auditory nerve in NM.     

Differential activation of glutamate receptors by spontaneously released transmitter in slices of neocortex

Whole-cell recordings were made from neurons in neocortical brain slices in order to characterize excitatory synaptic currents mediated by glutamate receptors. Glutamate receptor antagonists, D-aminophosphonovalerate (D-APV) and CNQX, selectively attenuated distinct components in evoked synaptic currents, and were used to differentiate spontaneous synaptic currents mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Spontaneous excitatory synaptic currents were independent of action potentials, varied linearly with voltage, and were blocked by the non-NMDA receptor antagonist CNQX. An NMDA receptor-mediated component was not apparent in these spontaneous synaptic currents, however, when magnesium was omitted from the recording medium, fluctuations in current and sustained inward current became apparent, and these were blocked by the NMDA receptor antagonist D-APV. Based on these findings, we conclude that NMDA and non-NMDA receptors are activated differentially by transmitter released independently of action potentials.     

Excitatory amino acid-mediated components of synaptically evoked input from dorsal roots to deep dorsal horn neurons in the rat spinal cord slice

The participation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the responses of deep dorsal horn neurons to single shock stimulation of dorsal roots was investigated using current- and voltage-clamp techniques. In the presence of Mg2+, superfusion of rat spinal slices with 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX), a potent antagonist of non-NMDA receptors, reversibly blocks fast excitatory synaptic responses elicited by low-frequency stimulation of dorsal roots and to a greater extent the responses to quisqualate than to kainate or NMDA. The synaptic response elicited in a zero-Mg2+ medium is less sensitive to CNQX. The CNQX-resistant component is however abolished by D-APV, a selective antagonist of NMDA receptor. Under voltage-clamp, the excitatory postsynaptic currents also showed an initial fast (CNQX-sensitive) and a late slow (2-amino-5-phosphonovalerate (APV)-sensitive, Mg2+-sensitive) component, both of which had similar thresholds but differed in their latency, time-to-peak and duration. These results support the concept that both non-NMDA and NMDA receptor channels are present in a majority of deep dorsal horn neurons and could be simultaneously activated by transmitter released from stimulated primary afferents.     

Thalamic NMDA receptors and nociceptive sensory synaptic transmission

The responses of single thalamic neurones to noxious thermal stimulation were recorded in anaesthetized rats. The selective (NMDA) receptor antagonist, 3-((±)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), antagonised nociceptive responses when ejected iontophoretically with currents which produced selective antagonism at NMDA receptors. In contrast, the non-NMDA excitatory amino acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) had little or no effect on nociceptive responses, although it was able to reduce responses to non-nociceptive mechanoreceptor input. These results show that there is substantial NMDA receptor involvement in thalamic nociceptive responses, and that the contribution of CNQX-sensitive non-NMDA receptors to these responses is not extensive. Furthermore, it appears that nociceptive and non-nociceptive input to the thalamus have distinct synaptic pharmacologies.     

Characterization of L-homocysteate-induced currents in Purkinje cells from wild-type and NMDA receptor knockout mice

L-Homocysteic acid (HCA), an endogenous excitatory amino acid in the mammalian CNS, potently activates N-methyl-D-aspartate (NMDA) receptors in hippocampal neurons. However, the responses to HCA in Purkinje cells, which lack functional NMDA receptors, have been largely unexplored: HCA may activate conventional non-NMDA receptors by its mixed agonistic action on both NMDA and non-NMDA receptors, or it may activate a novel non-NMDA receptor that has high affinity for HCA. To test these possibilities, we compared the responses to HCA in cultured Purkinje cells with those in hippocampal neurons by using the whole cell patch-clamp technique. To clearly isolate HCA responses mediated by non-NMDA receptors, we complemented pharmacological methods by using neurons from mutant mice (NR(-/-)) that lack functional NMDA receptors. A moderate dose of HCA (100 microM) induced substantial responses in Purkinje cells. These responses were blocked by non-NMDA receptor antagonists but were insensitive to NMDA receptor antagonists. HCA also activated responses mediated by non-NMDA receptors in both wild-type and NR1(-/-) hippocampal neurons. HCA responses in Purkinje cells had a pharmacological profile (EC(50) and Hill coefficient) very similar to that of non-NMDA receptor components of HCA responses in hippocampal neurons. Moreover, the amplitude of the non-NMDA receptor component of HCA responses was directly correlated with that of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)- and kainate-induced responses in both types of neurons. Finally, in both types of neurons, HCA currents mediated by non-NMDA receptors were potently blocked by the AMPA receptor antagonist GYKI52466. These findings indicate that HCA-activated AMPA receptors in Purkinje cells are similar to those in hippocampal neurons and that there is no distinct HCA-preferring receptor in Purkinje cells. We also found that in hippocampal neurons, the EC(50)s of HCA for non-NMDA receptors and for NMDA receptors were more similar than originally reported; this finding indicates that HCA is similar to other mixed agonists, such as glutamate. HCA responses may appear to be selective at NMDA receptors in cells that express these receptors, such as hippocampal neurons; in cells that express few functional NMDA receptors, such as Purkinje cells, HCA may appear to be selective at non-NMDA receptors.     

Evidence for an involvement of NMDA and non-NMDA receptors in synaptic excitation of phrenic motoneurons in the rabbit

The action of endogenous excitatory amino acids on phrenic motoneurons was studied in anesthetized, vagotomized, paralyzed and artificially ventilated rabbits. The NMDA receptor antagonists APV and ketamine, as well as the non-NMDA receptor antagonists GAMS and DNQX were administered by microinjection into the ventral horn of the spinal segments C3-C5. Injection of each antagonist resulted in a reversible reduction of the phrenic nerve activity. Results suggest an important function of endogenous excitatory amino acids in the excitation of phrenic motneurons. NMDA as well as non-NMDA receptors are involved. The functional role of both receptor types in bulbospinal neurotransmission is discussed.     

Actions of excitatory amino acid antagonists on synaptic inputs to the rat medial vestibular nucleus: an electrophysiological study in vitro

Summary The actions of excitatory amino acid (EAA) antagonists on synaptic inputs to neurons in the rat medial vestibular nucleus (MVN) from ipsilateral vestibular afferents and vestibular commissures were studied in brain stem slice preparations. Antagonists used were 2-amino-5-phosphonovalerate (APV), a selective antagonist for the N-methyl-D-aspartate (NMDA) type of EAA receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective antagonist for the quisqualate/kainate (non-NMDA) type of EAA receptors and kynurenate (KYNA), a broad spectrum antagonist for the three types of EAA receptors. MVN neurons were classified as having mono- or polysynaptic inputs from vestibular afferents and commissural fibers by calculating synaptic delay. An application of APV through the perfusion medium suppressed 82% of cells activated monosynaptically from commissures, while it suppressed only 9% of cells activated monosynaptically from vestibular afferents. The application of KYNA proved much less selective, suppressing 83% of the former group of cells and 93% of the latter. CNQX suppressed almost all the cells of both groups. The sensitivity of monosynaptic inputs to KYNA, CNQX or APV was not significantly different from that of polysynaptic inputs irrespective of sources of inputs. These results suggest that excitatory synaptic inputs to MVN neurons are mediated mainly through non-NMDA type of EAA receptors from vestibular afferents and through NMDA as well as non-NMDA types of EAA receptors from commissures.     

Role of excitatory amino acids in mediating burst discharge of red nucleus neurons in the in vitro turtle brain stem-cerebellum.

1. Bursts of discharge have been recorded in the red nucleus in several species and are thought to represent the expression of motor commands. A cerebellorubral circuit comprised of recurrent connections among the cerebellum, red nucleus, and reticular formation was postulated to function as a positive feedback loop that generates these motor commands and transmits them to the spinal cord via the rubrospinal pathway. We have used an in vitro preparation from the turtle that leaves the circuitry connecting the cerebellum, brain stem, and spinal cord intact to study the role of excitatory amino acid neurotransmitters and recurrent excitation in mediating the generation of burst discharges in the red nucleus. 2. Burst discharges were recorded extracellularly from single cells in the red nucleus in response to single pulse or brief train stimulation of the contralateral spinal cord or brief train stimuli applied to the ipsilateral cerebellar cortex. The firing characteristics and pharmacologic sensitivities of the bursts were independent of the type of stimulus used. The bursts had long durations ranging from 2 to 17 s and showed spike frequency adaptation. 3. Transection of the cerebellar peduncle, which eliminates inhibition impinging onto the cerebellorubral circuit, greatly enhanced the spontaneous activity and burst discharges recorded in the contralateral red nucleus. Furthermore, bath application of a solution containing elevated levels of calcium and magnesium blocked the expression of burst discharges even though synaptic activation of the neurons was not blocked. 4. The possibility that excitatory amino acid receptors mediate burst responses in the red nucleus was investigated in light of the antagonistic effects of elevated magnesium ions on bursting. Bath application of 100 microns DL-2-amino-5-phosphonovaleric acid (APV), a specific N-methyl-D-aspartate (NMDA) receptor antagonist; [10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)], a specific non-NMDA receptor antagonist; or 100 microM, DL-2-amino-4-phosphonobutyric acid (AP4), an agonist of a fourth class of excitatory amino acid receptor, blocked burst activity in the red nucleus. With a multibarreled pipette for simultaneous ejection of drug and recording, iontophoresis of APV or CNQX into the red nucleus blocked bursting whereas AP4 failed to show a significant effect. These data suggest that red nucleus neurons have both NMDA and non-NMDA receptors. The site of action of the AP4-sensitive receptor appears to be elsewhere in the cerebellorubral circuit. 5. Iontophoretic application of excitatory amino acid receptor agonists NMDA and quisqualate (Q) induced excitation of red nucleus neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

A microiontophoretic study on the nature of the putative synaptic neurotransmitter involved in the pedunculopontine-substantia nigra pars compacta excitatory pathway of the rat

Summary The nature of the synaptic transmitter involved in the excitatory fibers linking the nucleus tegmenti pedunculopontinus (PPN) to the pars compacta of the substantia nigra (SNPC) was investigated using microiontophoretic techniques in rats anesthetized with ketamine. Among the SNPC cells activated orthodromically by PPN electrical stimulation, only a few cells were weakly excited by iontophoretically administered acetylcholine (Ach) while most were not affected. Conversely all cells were promptly and powerfully excited by short pulses of glutamate (GLU). The administration of the GLU antagonists glutamic acid diethylester (GDEE) and D--aminoadipic acid (DAA) reversibly and simultaneously suppressed both the PPN-evoked orthodromic response and the GLU-induced excitation of SNPC cells without affecting their response to iontophoretic Ach. GDEE was more effective than DAA in counteracting the synaptically evoked excitation. On the other hand, atropine, while antagonizing the Ach response in those cells which were cholinoceptive, did not affect either the PPN-evoked or the GLU-induced excitation. Hence, despite the presence of cholinergic cells in the PPN region, Ach does not appear to be involved in the excitatory PPNSNPC pathway. The present findings suggest that the excitatory PPN fibers innervating the SNPC may utilize GLU or a closely related amino acid as a neurotransmitter.Supported by grants from the Ministero della Pubblica Istruzione     

Subthreshold receptive fields and baseline excitability of ”silent” S1 callosal neurons in awake rabbits: contributions of AMPA/kainate and NMDA receptors

 The contribution of NMDA and non-NMDA receptors to excitatory subthreshold receptive fields was examined in callosal efferent neurons (CC neurons) in primary somatosensory cortex of the fully awake rabbit. Only neurons showing no traditional (suprathreshold) receptive fields were examined. Subthreshold responses were examined by monitoring the thresholds of efferent neurons to juxtasomal current pulses (JSCPs) delivered through the recording microelectrode. Changes in threshold following a peripheral conditioning stimulus signify a subthreshold response. Using this method, excitatory postsynaptic potentials and inhibitory postsynaptic potentials are manifested as decreases and increases in JSCP threshold, respectively. NMDA and non-NMDA agonists and antagonists were administered iontophoretically via a multibarrel micropipette assembly attached to the recording/stimulating microelectrode. Receptor-selective doses of both AMPA/kainate and NMDA antagonists decreased the excitability of CC neurons in the absence of any peripheral stimulation. Threshold to JSCPs rose by a mean of 20% for both classes of antagonist. Despite the similar effects of NMDA and non-NMDA antagonists on baseline excitability, these antagonists had dramatically different effects on the subthreshold excitatory response to activation of the receptive field. Whereas receptor-selective doses of AMPA/kainate antagonists either eliminated or severely attenuated the subthreshold excitatory responses to peripheral stimulation, NMDA antagonists had little or no effect on the subthreshold evoked response. Received: 12 September 1996 / Accepted: 6 December 1996     

Quisqualate-induced changes in extracellular sodium and calcium concentrations persist in the combined presence of NMDA and non-NMDA receptor antagonists in rat hippocampal slices

In the CA1 stratum pyramidale of rat hippocampal slices we have used Ca2(+)- and Na(+)-sensitive microelectrodes to measure the changes in [Ca2+]o, [Na+]o and associated slow field potentials elicited by the iontophoretic application of the excitatory amino acids N-methyl-D-aspartic acid (NMDA), quisqualic acid (Quis), alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) and glutamic acid (Glu) in the presence of the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (AP5) and ketamine and the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Although a combination of these antagonists blocked stimulation-induced synaptic field potentials as well as AMPA- and NMDA-induced ionic changes and associated field potentials, Quis-induced ionic changes and associated field potentials could still be observed after 60 min. These residual Quis signals were also resistant to 2-amino-3-phosphonopropionic acid (AP3) and 2-amino-4-phosphonobutyric acid (AP4) suggesting that metabotropic receptors do not play a role in the generation of these signals. We suggest that the receptor class activated under these conditions may play a role in CNQX/AP5-resistant epileptogenesis as well as other pathophysiological conditions.     

Differential roles of ionotropic glutamate receptors in canine medullary inspiratory neurons of the ventral respiratory group.

The relative roles of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in supplying excitatory drive to inspiratory (I) augmenting pattern neurons of the ventral respiratory group were studied in anesthetized, ventilated, paralyzed, and vagotomized dogs. Multibarrel micropipettes were used to record simultaneously single-unit neuronal activity and pressure microeject the NMDA antagonist, 2-amino-5-phosphonovalerate (AP5; 2 mM), the non-NMDA antagonist 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX; 0.25 mM), and an artificial cerebrospinal fluid vehicle. Ejected volume-rates were measured directly via meniscus level changes. The moving time average of phrenic nerve activity was used to determine respiratory phase durations and to synchronize cycle-triggered histograms of the discharge patterns. Both AP5 and NBQX produced dose-dependent reductions in peak spontaneous I neuronal discharge frequency (Fn). The average (+/- SE) maximum reduction in peak Fn produced by AP5 was 69.1 +/- 4.2% and by NBQX was 47.1 +/- 3.3%. Blockade of both glutamate receptor subtypes nearly silenced these neurons, suggesting that their activity is highly dependent on excitatory synaptic drive mediated by ionotropic glutamate receptors. Differential effects were found for the two glutamatergic antagonists. AP5 produced downward, parallel shifts in the augmenting pattern of discharge, whereas NBQX reduced the slope of the augmenting discharge pattern. These results suggest that time-varying excitatory input patterns to the canine I bulbospinal neurons are mediated by non-NMDA glutamate receptors and that constant or tonic input patterns to these neurons are mediated by NMDA receptors.     

NMDA, non-NMDA and glycine receptors mediate binaural interaction in the lateral superior olive: physiological evidence from mouse brain slice.

Binaural interaction was investigated in a 400 microns brain slice taken through the mouse lateral superior olive (LSO). Ipsilateral excitatory and contralateral inhibitory inputs to LSO neurons were examined by recording physiological responses to electrical stimulation of the trapezoid body. Bath application of non-N-methyl-D-aspartate (non-NMDA) antagonists blocked ipsilateral excitation and strychnine blocked contralateral inhibition. N-methyl-D-aspartate (NMDA) had little effect on ipsilateral responses but completely blocked contralateral inhibition. These results suggest that ipsilateral excitation is mediated by non-NMDA receptors and contralateral inhibition by strychnine dependent glycine receptors. NMDA receptors may play a role by modulating contralateral inhibition in LSO.     

Excitatory and inhibitory synaptic inputs shape the discharge pattern of pump neurons of the nucleus tractus solitarii in the rat

The second-order relay neurons of the slowly-adapting pulmonary stretch receptors (SARs) are called pump neurons (P cells) and are located in the nucleus tractus solitarii (NTS). We have shown recently that P cells do not act merely as simple relay neurons of SAR afferents but also receive rhythmic inputs from the central respiratory system. This study aimed to analyze two aspects of the respiratory inputs to P cells: (1) suppression of P cell firing at early inspiration (eI suppression) and (2) facilitation of P cell firing at around the period from late inspiration to early expiration (IE facilitation). This study employed extracellular recordings combined with iontophoretic applications of neuroactive drugs to single P cells, in Nembutal-anesthetized, paralyzed, and artificially ventilated rats. The results showed that several excitatory and inhibitory neurotransmitters were involved in these synaptic events. First, the glycine antagonist strychnine and the GABA_A antagonist bicuculline were applied to identify the neurotransmitters acting in eI suppression. Strychnine greatly diminished eI suppression, but bicuculline had little effect. This suggested that eI suppression was elicited by inspiratory neurons that were glycinergic and had a decrementing firing pattern. Second, on the other hand bicuculline markedly enhanced IE facilitation as well as the baseline frequency of P cell firing. The enhancement of IE facilitation was distinctive even when the effects of increased baseline firing on this enhancement were taken into account. Third, IE facilitation was diminished by applications of the NMDA glutamate receptor antagonists D-2-amino-5-phosphonovaleric acid (APV) and dizocilpine (MK-801). These results suggested that glutamatergic synapses on P cells from some unidentified respiratory neurons form excitatory inputs for IE facilitation and GABA_A receptor-mediated processes control the strength of IE facilitation, possibly at the presynaptic level. Finally, iontophoretic application of the non-NMDA glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2, 3-dione disodium (CNQX), almost completely abolished P cell firing in response to both lung inflation and electrical stimulation of the vagus nerve. This confirmed the previous report that glutamate is the primary neurotransmitter at the synapses between SAR afferents and P cells. We concluded that complicated synaptic inputs involving glycinergic and GABAergic inhibitions, and non-NMDA and NMDA glutamate receptor-mediated excitations form the basic pattern of P cell firing. Received: 31 March 1999 / Accepted: 8 June 1999