Synaptic Excitation Triggers Oscillations During NMDA Receptor Activation in Rat Abducens Motoneurons

Rat abducens motoneurons were intracellularly recorded in vivo during synaptic excitation and extracellular microionophoretic application of N -methyl- d -aspartate (NMDA). Trigeminal excitatory post-synaptic potentials (EPSPs) evoked in abducens motoneurons were studied during intracellular current injection. They were not sensitive to hyperpolarization or depolarization of the membrane potential in the range of –75 mV to –55 mV using current pulse intensities between –3 nA and + 1 nA. Microionophoretic applications of aminophosphonovalerate (APV), MK801 and i.v. injections of MK801 (1 – 3 mg/kg) or ketamine (10 mg/kg) did not modify trigeminal EPSPs, suggesting that NMDA receptors are not involved in this synaptic transmission. However, microionophoretic applications of NMDA on abducens motoneurons enhanced trigeminal EPSPs and gave rise to regenerative oscillations. The co-activation of NMDA receptors and trigeminal synapses induced these oscillations. The trigeminal EPSP may delay and reset the oscillations depending on where it was evoked in the oscillatory cycle. Depolarizing current pulses intracellularly applied to abducens motoneurons could trigger a post-hyperpolarization followed by rebound depolarization during NMDA application, confirming the activation of active membrane properties. However, depolarizing current pulses could not trigger oscillations similar to those entrained by the EPSPs. The importance of the location of trigeminal synapses in relation to those of NMDA receptors in the dendritic arborization of abducens motoneurons is discussed. Our results show that the same sensory stimulus may have different post-synaptic effects on abducens motoneurons during the co-activation of NMDA receptors. A complete modification of the motor output during NMDA receptor activation strongly supports an active role of abducens motoneurons provided that NMDA receptors are physiologically activated during motor pattern generation.     

Tonic Activation of NMDA Receptors Causes Spontaneous Burst Discharge of Rat Midbrain Dopamine Neurons In Vivo

Midbrain dopamine neurons in vivo discharge in a single-spike firing pattern or in a burst-firing pattern. Such activity in vivo strikingly contrasts with the pacemaker activity of the same dopamine neurons recorded in vitro. We have recently shown that burst activity in vivo of midbrain dopamine neurons is due to the local activation of excitatory amino acid receptors, as microapplication of the broad-spectrum antagonist of excitatory amino acids, kynurenic acid, strongly regularized the spontaneous firing pattern of these dopamine neurons. In the present study, we investigated which subtypes of excitatory amino acid receptors are involved in the burst-firing of midbrain dopamine neurons in chloral hydrate-anaesthetized rats, Iontophoretic or pressure microejections of 6-cyano,7-nitroquinoxaline-2,3-dione (CNQX), a non- N -methyl- d -aspartate (NMDA) receptor antagonist, did not alter the spontaneous burst firing of dopamine neurons ( n = 36). In contrast, similar ejections of (±)2-amino,5-phos-phonopentanoic acid (AP-5), a specific antagonist at NMDA receptors, markedly regularized the firing pattern by reducing the occurrence of bursts ( n = 52). In addition, iontophoretic ejections of NMDA, but not kainate or quisqualate, elicited a discharge of these dopamine neurons in bursts ( n = 20, 12 and 14, respectively). These data suggest that burst-firing of midbrain dopamine neurons in vivo results from the tonic activation of NMDA receptors by endogenous excitatory amino acids. In view of the critical dependency of catecholamine release on the discharge pattern of source neurons, excitatory amino acid inputs to midbrain dopamine neurons may constitute a major physiological substrate in the control of the dopamine level in target areas.     

Effect of upregulation of AMPA glutamate receptors on cerebral O2 consumption and blood flow in rat

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) receptors, in cerebral cortex, underwent upregulation (35% increase) following chronic blockade with a non-competitive AMPA receptor antagonist, GYKI 52466 (1-(aminophenyl)-4-methyl-7, 8-methylenedioxy-5H-2,3-benzodiazepine). Such upregulation did not alter basal cerebrocortical blood flow or O(2) consumption. There was a much higher increase in blood flow and O(2) consumption in the upregulated, agonist (AMPA) stimulated cortices of anesthetized rats.     

Mediation of Thalamic Sensory Responses In Vivo by ACPD-activated Excitatory Amino Acid Receptors

The existence of the so-called metabotropic excitatory amino acid receptor has been known for some years. Various functions have been suggested for this receptor, but the lack of selective antagonists for (IS, 3R)-aminocyclopentane dicarboxylic acid (ACPD) has precluded the direct demonstration of a functional role for this receptor in synaptic processes. We describe here a specific antagonism of the excitatory responses of thalamic neurons to ACPD by two novel antagonists, and a parallel antagonism by these compounds of sensory synaptic responses to noxious stimuli of the same neurons. This provides the first direct pharmacological evidence for a functional role of ACPD-sensitive receptors in central neurotransmission, and indicates that these receptors may play an important part in central sensory processes.     

NMDA Receptor Activation Triggers Voltage Oscillations, Plateau Potentials and Burstinq in Neonatal Rat Lumbar Motoneurons ln Vitro

Whole-cell recordings of lumbar motoneurons in the intact neonatal rat spinal cord in vitro were undertaken to examine the effects of Kmethyl-D-aspartate (NMDA) receptor activation on membrane behaviour. Bath application of NMDA induced rhythmic voltage oscillations of 5.9 ± 2.1 mV (SD) at a frequency of 4.4 ± 1.5 Hz. Amplitude, but not frequency, of the voltage oscillations was membrane potential-dependent. Voltage oscillations could recruit action potentials and/or plateau potentials with or without superimposed bursting. Blockade of synaptic transmission with tetrodotoxin (TTX) sometimes resulted in a loss of oscillatory activity which could then be restored by increasing the NMDA concentration. After application of TTX, the trajectory of NMDA-induced oscillations was similar to the trajectory induced in the presence of intact synaptic networks, although the mean oscillation duration was longer and the oscillation frequency was slower (1.8 ± 1.1 Hz). Current ramps delivered after bath application of NMDA demonstrated bistable membrane properties which may underlie the plateau potentials. Injection of intracellular current pulses could initiate, entrain and terminate individual plateau potentials. The results suggest that membrane depolarization produced by oscillations may activate other intrinsic conductances which generate plateau potentials, thereby providing the neuron with enhanced voltage sensitivity, compared to that produced by NMDA receptor activation alone. These oscillatory events may have a role in the regulation of motor output in a variety of rhythmic behaviours including locomotion.     

Sensory Excitatory Postsynaptic Potentials Mediated by NMDA and non-NMDA Receptors in the Thalamus in vivo

Excitatory amino acid neurotransmitters, such as l-glutamate, act at several receptors in the brain, which are sometimes referred to as N-methyl-d-aspartate (NMDA) and non-NMDA receptors. Extensive in vitro work indicates that both NMDA receptors and non-NMDA receptors contribute to excitatory postsynaptic potentials (epsps). The contribution of NMDA receptors to epsps in vivo under physiological conditions is, however, almost unknown. The receptors that mediate the epsps evoked in thalamic relay cells by natural stimulation of sensory afferents have been investigated in anaesthetized rats, and we report the first pharmacological characterization of an excitatory amino acid receptor-mediated epsp in vivo involving both non-NMDA receptors and, in particular, NMDA receptors.     

Trans-synaptic Modulation of Striatal ACh Release In Vivo by the Parafascicular Thalamic Nucleus

Electrical stimulation of the parafascicular but not the ventrolateral or dorsomedial thalamic nucleus (ten 0.5 ms, 10 V pulses, 140 pA) of freely moving rats induced a frequency-dependent (2.5, 5,10 and 20 Hz) increase in the extracellular acetylcholine (ACh) content of the dorsal striatum, assessed by trans-striatal microdialysis. The time-dependent effect of 10 Hz stimulation was studied. The peak increase, 39% above baseline, was attained during 4 min of stimulation. This was blocked by coperfusion with 5 pM tetrodotoxin, indicating that the release we measured represents a physiological process. The facilitatory effect of parafascicular nucleus stimulation does not appear to be associated with indirect action through the cerebral frontal cortex because acute lesion of the excitatory corticostriatal afferents, which by itself reduced basal ACh release by 40%, did not modify the effect of 10 Hz stimulation. The possible involvement of the fasciculus retroflexus in the facilitation of ACh release was also ruled out. The non-competitive NMDA-type receptor antagonist MK-801, applied by reversed dialysis (30 pM) or systemically injected (0.2 mg/kg), significantly reduced the basal ACh output and prevented the tetanus-evoked increase in ACh release. The results provide in vivo evidence that the activity of the cholinergic neurons in the dorsal striatum is trans-synaptically modulated by parafascicular nucleus excitatory afferents through activation of the NMDA subtype of glutamate receptors that is probably located in the striatum.     

Activation of N-methyl-d-aspartate Receptors Induces Endogenous Rhythmic Bursting Activities in Nucleus Tractus Solitarii Neurons: An Intracellular Study on Adult Rat Brainstem Slices

A brainstem slice preparation and intracellular recording techniques were used to examine the effects of N-methyl-d-aspartate (NMDA) application on neurons within the swallowing area of the nucleus tractus solitarii (NTS). According to their cellular properties, NTS neurons were classified into type I and type II neurons. The most striking difference was the occurrence of delayed excitation in type I but not in type II neurons, when they were depolarized from membrane potentials more negative than -60 mV. Bath application of NMDA (30 - 60 microM) elicited depolarization and triggered stable repetitive firing in all the NTS neurons but one. During the NMDA-induced depolarization, hyperpolarization below -60 mV elicited, in some type I neurons, a rhythmic bursting pattern. The duration of the bursts (300 - 1000 ms) and their frequency (0.5 - 2 Hz) depended on the membrane potential. With hyperpolarizations below -75 mV, rhythmic bursting was converted into rhythmic single discharges, a pattern elicited directly in the other type I neurons. In all cases, rhythmic patterns were superimposed on cyclic depolarizations of the membrane potential characterized by an initial ramp-shaped phase. In type II neurons, rhythmic bursting discharges, superimposed on rhythmic oscillations of the membrane potential, were also obtained upon hyperpolarization during the NMDA-induced depolarization. In all type I neurons tested, NMDA-induced cyclic ramp-shaped depolarizations continued after addition of tetrodotoxin to the medium. Rhythmic bursting was not elicited by bath application of kainate (10 - 20 microM). Application of d-2-amino-5-phosphonovalerate (50 microM) blocked NMDA-induced depolarizations without modifying those elicited by kainate, which were selectively depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Moreover, removal of Mg2+ from the medium suppressed NMDA-induced cyclic depolarizations. Results demonstrate that both NMDA and non-NMDA receptors are present in NTS neurons and that selective activation of NMDA receptors induced rhythmic bursting and/or rhythmic single discharges. Rhythmic patterns were not driven by synaptic mechanisms but originated from endogenous properties of NTS neurons activated by NMDA. Thus, NTS neurons can be considered as conditional pacemakers. According to the location of the neurons, the conditional properties shown in these in vitro experiments might be involved in vivo in the generation of rhythmic motor activities set up at the NTS level, such as swallowing.     

Transient Sensitivity of Rat Cerebellar Purkinje Cells to N-methyl-D-aspartate during Development. A Voltage Clamp Study in in vitro Slices

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PC) to L-glutamate (Glu) and N-methyl-D-aspartate (NMDA). In 8-day-old animals, all PCs recorded in magnesium-free medium responded to iontophoretic applications of both agonists by transient and dose dependent inward currents which, in both cases, were heavily contaminated by a Glu and NMDA-induced synaptic noise. When 5 x 10-6 M tetrodotoxin (TTX) was added to the perfusing medium, this evoked synaptic noise was completely abolished in most cells whereas clear-cut inward currents induced in PCs by Glu and NMDA applications on their dendrites were still visible. These responses were selectively antagonized by the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by the NMDA receptor antagonist D-2-aminophosphono-5-valeric acid (2APV) respectively. Excitatory responses induced by aspartate in 8 - 10-day-old PCs were also markedly antagonized by CNQX. At this stage, the sensitivity of PCs to NMDA was about one order of magnitude less than that to Glu. In 15 - 20-day-old animals, all PCs were still responsive to Glu whereas only 70% of them were still excited by NMDA in the presence of TTX in the bath. Furthermore, the sensitivity of PCs to Glu was higher than at 8 days of age, whereas that to NMDA was significantly lower, even when considering only those cells which still responded to this agonist. This trend was still accentuated later on since at 2 months of age, only 25% of PCs were excited by NMDA whereas their sensitivity to Glu was similar to that observed in 15 - 20-day-old animals. Therefore, the present results are fully consistent with the view that PCs have a transient expression of NMDA receptors during development.     

Dorsal root and dorsal column mediated synaptic inputs to reticulospinal neurons in lampreys: Involvement of glutamatergic,glycinergic, and GABaergic transmission

This study was aimed at characterizing the inputs from dorsal roots and dorsal columns to reticulospinal neurons within the posterior rhombencephalic reticular nucleus in the lamprey. The in vitro isolated brainstem and spinal cord preparation was used. Microstimulation of dorsal roots and columns on both sides induced, in identified reticulospinal neurons, synaptic responses which consisted of large IPSPs mixed with excitation, particularly from stimulation on the ipsilateral side. When the spinal cord was selectively exposed to kynurenic acid or to Ca²⁺, synaptic responses to stimulation of dorsal roots and columns were not modified, whereas the same responses were abolished when the brainstem was exposed selectively to kynurenic acid, thus suggesting that the responses were carried by long fibres ascending directly to the brainstem. The excitatory and inhibitory synaptic responses are relayed by interneurons located in the brainstem. The ascending excitatory inputs to inhibitory interneurons and, most likely, also to excitatory interneurons, use excitatory amino acid transmission. Inhibitory responses were abolished by adding the glycinergic antagonist strychnine (5 μM) to the physiological solution, thus suggesting that inhibitory interneurons use glycine transmission. The synaptic transmission was depressed by (?)-baclofen, a GABA_u agonist, probably acting at a presynaptic site. Taken together, the present results suggest that dorsal root and dorsal column stimulations give rise to disynaptic inhibition and excitation of reticulospinal neurons mediated by excitatory and inhibitory amino acid transmission via brainstem interneurons. © 1993 Wiley-Liss, Inc.     

Low Doses of NMDA Receptor Antagonists Synergistically Increase the Anticonvulsant Effect of the AMPA Receptor Antagonist NBQX in the Kindling Model of Epilepsy

Excitatory amino acid transmitters are involved in the initiation of seizures and their propagation. Most attention has been directed to synapses using N -methyl-d-aspartate (NMDA) receptors, although more recent evidence indicates potential roles for the a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors as well. In the present experiments in amygdala-kindled rats, i.e. a model of partial epilepsy, competitive and uncompetitive NMDA antagonists exerted only weak anticonvulsant effects, whereas the AMPA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX) potently increased focal seizure threshold and inhibited seizure spread from the focus. These effects of NBQX were dramatically increased by pretreatment with low doses of NMDA antagonists, whereas adverse effects of NBQX were not potentiated. These data suggest that both non-NMDA and NMDA receptors are critically involved in the kindled state, and that combinations of AMPA and NMDA receptor antagonists provide a new strategy for treatment of epileptic seizures.     

Aspartate- and Glutamate-like Immunoreactivities in Rat Hippocampal Slices: Depolarization-induced Redistribution and Effects of Precursors

The light microscopic localization of aspartate-like immunoreactivity (Asp-LI) was compared to that of glutamate-like immunoreactivity (Glu-LI) in hippocampal slices by means of specific polyclonal antibodies recognizing the amino acids fixed by glutaraldehyde. After incubation in Krebs' solution with normal (5 mM) or depolarizing concentrations of K+, and various additives, the slices were fixed with glutaraldehyde, resectioned and processed according to the peroxidase - antiperoxidase procedure. At 5 mM K+, Glu-LI was localized in nerve-terminal like dots with a conspicuous laminar distribution, the highest Glu-LI concentrations coinciding with the terminal fields of major excitatory pathways thought to use glutamate or aspartate as transmitters. The localization of Asp-LI showed some similarity to that of Glu-LI, but the laminar distribution was less differentiated and the immunoreactivity was much weaker. At 40 and 55 mM K+ the nerve terminal localizations of Glu-LI and Asp-LI were strongly reduced. Concomitantly, both immunoreactivities appeared in astroglial cells. These changes were Ca2+-dependent. The nerve ending staining patterns of Asp-LI and Glu-LI could be sustained during depolarization if the medium was supplemented with glutamine (0.5 mM). Under these conditions Asp-LI became more intense and its distribution approached that of Glu-LI. This suggests that, when stimulated, some nerve endings can increase their reservoir of releasable aspartate. The presence of glutamine during depolarization strongly reduced glial Asp-LI and Glu-LI, possibly due to its providing nitrogen for conversion of glutamate to glutamine. alpha-Ketoglutarate, another glia-derived precursor of neuronal glutamate, was virtually ineffective in supporting Glu-LI and Asp-LI in nerve endings, and did not suppress Glu-LI or Asp-LI in glia. Our findings provide morphological support for the view that excitatory nerve endings under certain conditions can contain high levels of both aspartate and glutamate (possibly in the same terminals), and that aspartate as well as glutamate can be released synaptically. Further, they underline the importance of the glial supply of the nerve endings with precursor glutamine, which allows them to build up and sustain high concentrations of transmitter amino acids during release.     

Synaptic coexistence of AMPA and NMDA receptors in the rat hippocampus: A postembedding immunogold study

Synaptic distributions of N-methyl-d -aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) receptor subunits, NMDAR1 and GluR2, respectively, were examined by electron microscopy with the high spatial resolution of postembedding immunogold localization. We provide direct evidence for colocalization at individual axodendritic asymmetric synapses within the CA1 subfield of rat hippocampus. AMPA/ NMDA receptor colocalization was found both in γ-aminobutyric acid (GABA)ergic dendrites and non-GABAergic dendritic shafts, as well as dendritic spines. Some asymmetric synapses were found to contain only NMDAR1 or GluR2; however, most immunopositive synapses contained both subunits. Many NMDAR1 and/ or GluR2 immunopositive profiles received GABAergic innervation at an adjacent synapse, providing a substrate for GABAergic modulation of both GluR classes. These data suggest that excitatory neuronal transmission in CA1 neurons may generally involve activation of both NMDA and AMPA receptor subunits at a single synapse, however, they also offer ultrastructural evidence for NMDAR1-only synapses that might represent silent synapses. J. Neurosci. Res. 54:444–449, 1998. © 1998 Wiley-Liss, Inc.     

Modulation of GABA-mediated Synaptic Potentials by Glutamatergic Agonists in Neonatal CA3 Rat Hippocampal Neurons

Intracellular recordings were made from slices of adult and neonatal hippocampal neurons. During the first 2 weeks of life the majority of pyramidal cells exhibited spontaneous gamma-aminobutyric acid (GABA)-mediated synaptic potentials, which were depolarizing at birth and became hyperpolarizing by the end of the first postnatal week. These synaptic potentials were reduced in frequency or blocked by the N-methyl-d-aspartate (NMDA) receptor antagonist d(-)2-amino-5-phosphonovalerate (AP-5, 50 microM) (13/15 cells). The non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5 - 10 microM) abolished the GABA-mediated synaptic potentials in all the cells tested (n=12), Superfusion of l-glutamate (up to 100 microM) increased the frequency of both depolarizing and hyperpolarizing GABA-mediated synaptic potentials. This effect was reduced by AP-5 or dl-2-amino-7-phosphonoheptanoate (AP-7, 50 microM) and fully blocked by concomitant application of AP-5 (50 microM) and CNQX (5 - 10 microM). NMDA (0.5 - 2 microM) increased the frequency of the GABA-mediated synaptic potentials. These effects were blocked by AP-5 (50 microM) and by bicuculline (10 microM). Quisqualate (100 - 300 nM), (RS)-alpha-amino-3-hydroxy-5-methyl-4-izopropionate (AMPA, 100 - 300 nM) and kainate (100 nM) also increased the frequency of the GABA-mediated synaptic potentials. These effects were blocked by CNQX (5 - 10 microM) and by bicuculline (10 microM) but not by AP-5 (50 microM). In the presence of tetrodotoxin (TTX, 1 microM), quisqualate (up to 300 nM), AMPA (up to 500 nM) and kainate (100 nM) had no effect on membrane potential or input resistance. In conclusion, our experiments suggest that, in early postnatal life, NMDA and non-NMDA receptors located on GABAergic interneurons modulate GABAergic synaptic potentials.     

Microinfusion of the Metabotropic Glutamate Receptor Agonist 1S, 3R–1-Aminocyclopentane-1, 3-dicarboxylic Acid Into the Nucleus Accumbens Induces Dopamine-dependent Locomotor Activation in the Rat

Although the striatum has one of the highest densities of metabotropic glutamate receptor (mGluR) binding sites in the brain, little is known about their physiological role. In this study we characterized the contribution of mGluRs located in the ventral part of the striatum (the nucleus accumbens) to the control of extrapyramidal motor function. Activation of mGluRs by local infusion of the selective agonist 1S, 3R–1-aminocyclopentane-1, 3-dicarboxylic acid (1 S, 3R-ACPD; 25, 50 and 100 nmol/0.5 μl) into the nucleus accumbens induced a dose-dependent increase in locomotor activity in rats. Intra-accumbens infusion of a selective antagonist of mGluRs, a-methyl-4-carboxyphenylglycine (MCPG) did not modify spontaneous locomotion but decreased the locomotor response to 1S, 3R-ACPD. This effect appeared to be mediated by dopamine, since blockade of dopamine receptors with haloperidol (0.05 and 0.1 mg/kg i.p.) dose-dependently reduced 1 S3R-ACPD-induced locomotor activation. Furthermore, D-amphetamine (0.5 mg/kg, i.p.) combined with intra-accumbens infusion of 1S, 3R-ACPD (100 nmol) potentiated the locomotor hyperactivity response to a higher level than that seen with a single treatment with either drug. In contrast, D-amphetamine-induced hypermotility was abolished by infusion of MCPG (100 nmol) into the nucleus accumbens. These results demonstrate that glutamate may control extrapyramidal motor function through metabotropic receptors. Furthermore, activation of metabotropic glutamate receptors appears to act in synergy with the dopamine system at the level of the nucleus accumbens to produce a motor stimulant response.     

Flip and Flop Variants of AMPA Receptors in the Rat Lumbar Spinal Cord

The expression of eight messenger RNA splice forms encoding the Flip and Flop variants of AMPA receptor subunits GluR-A to -D in the rat lumbar spinal cord was examined by in situ hybridization using specific oligonucleotides. In the dorsal horn (laminae I, II and III) the predominant mRNA was GluR-B Flip. Much lower levels of GluR-A Flip were found in lamina I and in superficial parts of lamina II outer. In the ventral horn, motor neurons expressed mainly GluR-B Flip, GluR-C Flip and Flop, and GluR-D Flip. Serial sectioning through large motor neurons indicated that a given cell contained, for example, both GluR-C Flip and Flop splice types.     

Role of Non-NMDA Receptors in Osmotic and Glutamate Stimulation of Vasopressin Release: Effect of Rapid Receptor Desensitization

Previous studies demonstrated that the increase in vasopressin (VP) release and induction of VPmRNA content by osmotic stimulation was blocked by kynurenic acid, a non-specific antagonist of excitatory amino acid (EAA) receptors. In order to identify the type of EAA receptor involved, perifused explants of the hypothalamo-neurohypophyseal system (HNS) were exposed to a ramp increase in osmolality (40 mOsm over 6  h achieved by increasing NaCl) in the presence and absence of 10  μ m 6,7-dinitroquinoxaline-2,3-dione (DNQX), an antagonist of non- n -methyl-d-aspartate (NMDA) excitatory amino acid receptors. Vasopressin release and VP mRNA content were significantly increased by exposure to the osmotic stimulus. 6,7-dinitroquinoxaline-2,3-dione inhibited osmotically stimulated VP release ( F =16.65, P=0.0008) without significantly reducing basal release. It also prevented the osmotically stimulated increase in VP mRNA content (P<0.05). Although these results implicated glutamate, the primary endogenous ligand for EAA receptors, in the regulation of VP, exogenous glutamate was ineffective in stimulating VP release from HNS explants in either low-Mg²⁺ or Mg²⁺-replete medium. However, blockade of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor desensitization with cyclothiazide (100  μ m ) caused a marked increase in VP release in response to 100  μ m glutamate, and blockade of kainate receptor desensitization with concanavalin A resulted in a small, but significant increase in VP release in response to 1  m m glutamate. These results support a role for non-NMDA receptor activation in osmotic regulation of VP release.     

Evidence for the Existence of a Functional Polysynaptic Pathway From Trigeminal Afferents to Lumbar Motoneurons in the Neonatal Rat

Stimulation of trigeminal afferents has been reported to have powerful effects on the spinal cord in adult animals of several species. In the present study, the pathway transmitting these influences was investigated in the neonatal rat. Experiments were performed on in vitro brainstem/spinal cord preparations. Stimulation of the trigeminal nerve evoked bilateral polysynaptic discharges in lumbar ventral roots. Intracellular recordings from lumbar motoneurons showed mainly excitatory responses, although a few inhibitory responses were also observed. Experiments with perfusion of different parts of the preparation with general or selective synaptic blockers revealed a synaptic relay under GABAergic control in the brainstem, and at least one synapse in the cervical and in the thoracic spinal cord. The involvement of lumbar interneurons was established by perfusing the lumbar enlargement with saline containing either a high concentration of divalent ions or mephenesin in order to reduce transmission along polysynaptic pathways. The contribution of excitatory amino acid transmission was evaluated and was found to evoke mixed receptor responses. The course of the pathway was traced by using different lesions to the brainstem and spinal cord. The pathway was found to be ipsilateral in the brainstem and to become bilateral in the spinal cord. The results of the present study demonstrate that polysynaptic sensorimotor pathways are present at birth. The results are discussed in relation to the pontomedullary locomotor strip, which has been thought to share many features with the trigeminal system.     

Amino acids in the dorsal lateral geniculate nucleus of the cat—collection in vivo

A dialysis sampling probe was used to collect amino acids from the dorsal lateral geniculate nucleus (LGN) in vivo. The sampling probe was equipped with an electrode to allow local stimulation and recording of nerve activity. The amino acids in the dialysates were determined fluorimetrically by precolumn derivation and hple-separation. Local electrical stimulation of the LGN caused a multifold increase in glutamate, aspartate and GABA levels. Smaller changes were observed for taurine, alanine and glycine. The results indicate that the dialysis sampling probe is rather atraumatic and can be used to detect stimulation-induced changes in extracellular amino acid concentrations.