5HT induces NMDA receptor-mediated intrinsic oscillations in embryonic amphibian spinal neurons

The existence and possible contribution of intrinsic membrane potential oscillations to the generation of locomotor rhythmicity was investigated in spinal cord neurons of newly hatched Rana temporaria tadpoles, by intracellular recording from immobilized animals. The bath application of 100 microM N-methyl-D-aspartate (NMDA) evoked continuous swimming-like activity in ventral motor roots and rhythmic synaptic drive to ventrally located spinal neurons, presumed to be motoneurons. In 0.5 microM tetrodotoxin-treated preparations, similar applications of NMDA depolarized neurons by ca. 20 mV, but did not lead to intrinsic oscillatory activity, although some evidence for voltage-dependent membrane bi-stability was obtained. However, bath application of the neuromodulatory amine, serotonin (5HT; 5 microM), in the presence of NMDA and TTX, reversibly induced sustained membrane potential oscillations (up to 40 mV in amplitude) that were similar in waveform to those already described in other adult vertebrate motor systems. The TTX-resistant oscillations were dependent upon the presence of magnesium ions in the bathing solution and were abolished by the NMDA receptor antagonist, D-2-amino-5-phosphonovaleric acid (APV). The results suggest that in this simple, developing vertebrate locomotor system, the activation of 5HT receptors on spinal cord neurons in turn modulates NMDA receptor activation to enable the expression of intrinsic oscillatory membrane properties which could contribute to the generation of locomotor behaviour.     

NMDA receptor-mediated oscillatory activity in the neonatal rat spinal cord is serotonin dependent

The effect of serotonin (5-HT) receptor blockade on rhythmic network activity and on N-methyl--aspartate (NMDA) receptor-induced membrane voltage oscillations was examined using an in vitro neonatal rat spinal cord preparation. Pharmacologically induced rhythmic hindlimb activity, monitored via flexor and extensor electroneurograms or ventral root recordings, was abolished by 5-HT receptor antagonists. Intrinsic motoneuronal voltage oscillations, induced by NMDA in the presence of tetrodotoxin (TTX), either were abolished completely or transformed to long-lasting voltage shifts by 5-HT receptor antagonists. Conversely, 5-HT application facilitated the expression of NMDA-receptor-mediated rhythmic voltage oscillations. The results suggest that an interplay between 5-HT and NMDA receptor actions may be critical for the production of rhythmic motor behavior in the mammalian spinal cord, both at the network and single cell level.     

Chronic interleukin-6 alters NMDA receptor-mediated membrane responses and enhances neurotoxicity in developing CNS neurons

Recent studies show that the cytokine interleukin-6 (IL-6) is expressed at elevated levels in the CNS in several disease states and contributes to the neuropathological process. The mechanisms through which IL-6 exerts its CNS effects are primarily unknown. We have investigated the pathophysiological effects of IL-6 on developing CNS neurons using a culture model system and a chronic treatment paradigm. Here, we show, using current- and voltage-clamp recordings, that chronic IL-6 treatment of developing cerebellar granule neurons increases the membrane and current response to NMDA and that these effects are the primary mechanism through which IL-6 produces an enhanced calcium signal to NMDA. We also show that calcium influx through voltage-sensitive calcium channels contributes to the enhanced calcium signal to NMDA in the IL-6-treated neurons in a developmentally regulated manner and that the membrane depolarization to NMDA is more sensitive to the NMDA receptor antagonist ifenprodil in the IL-6-treated neurons compared with control neurons at a late developmental stage, consistent with a larger proportion of NMDA receptors containing the NMDAR2B subunit in the IL-6-treated neurons. Additional studies show that IL-6 treatment reduces the number of granule neurons in culture and enhances neurotoxicity involving NMDA receptors. These results support a pathological role for IL-6 in the CNS and indicate that NMDA receptor-mediated functions are likely to play a critical role in neuropathological changes observed in CNS diseases associated with elevated CNS levels of IL-6.     

AMPA and NMDA receptors expressed by differentiating Xenopus spinal neurons

N-methyl--aspartate (NMDA) receptors are often the first ionotropic glutamate receptors expressed at early stages of development and appear to influence neuronal differentiation by mediating Ca2+ influx. Although less well studied, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors also can generate Ca2+ elevations and may have developmental roles. We document the presence of AMPA and NMDA class receptors and the absence of kainate class receptors with whole cell voltage-clamp recordings from Xenopus embryonic spinal neurons differentiated in vitro. Reversal potential measurements indicate that AMPA receptors are permeable to Ca2+ both in differentiated neurons and at the time they first are expressed. The PCa/Pmonocation of 1.9 is close to that of cloned Ca2+-permeable AMPA receptors expressed in heterologous systems. Ca2+ imaging reveals that Ca2+ elevations are elicited by AMPA or NMDA in the absence of Mg2+. The amplitudes and durations of these agonist-induced Ca2+ elevations are similar to those of spontaneous Ca2+ transients known to act as differentiation signals in these cells. Two sources of Ca2+ amplify AMPA- and NMDA-induced Ca2+ elevations. Activation of voltage-gated Ca2+ channels by AMPA- or NMDA-mediated depolarization contributes approximately 15 or 30% of cytosolic Ca2+ elevations, respectively. Activation of either class of receptor produces elevations of Ca2+ that elicit further release of Ca2+ from thapsigargin-sensitive but ryanodine-insensitive stores, contributing an additional approximately 30% of Ca2+ elevations. Voltage-clamp recordings and Ca2+ imaging both show that these spinal neurons express functional AMPA receptors soon after neurite initiation and before expression of NMDA receptors. The Ca2+ permeability of AMPA receptors, their ability to generate significant elevations of [Ca2+]i, and their appearance before synapse formation position them to play roles in neural development. Spontaneous release of agonists from growth cones is detected with glutamate receptors in outside-out patches, suggesting that spinal neurons are early, nonsynaptic sources of glutamate that can influence neuronal differentiation in vivo.     

NMDA receptor-mediated inspiratory off-switching in pneumotaxic-disconnected cats

BACKGROUND: National morbidity surveys provide valuable data for monitoring the health needs of populations, health policy planning and design of medical curricula. In order to meet a long-standing need for such information in Sri Lanka, a nationwide general practice morbidity survey was conducted for the first time in 1996. OBJECTIVES: We aimed to identify people's needs by determining the reasons for encounter or the demand for care with GPs/family physicians, to illustrate the pattern of morbidity in general practice and to determine the average daily workload of GPs in Sri Lanka. METHOD: A random sample of 75 GPs were requested to complete a Practitioner Profile Questionnaire (PPQ) and to record on an encounter form (EF) the reason/s for encounter (RFE) and problems managed during all consultations on 4 July. Central coding of the RFEs and problems defined was done using the International Classification of Primary Care (ICPC). RESULTS: Forty GPs (53.3%) completed the PPQ, while the EF received a response from 34 (43.3%). The GP profile showed a male to female ratio of 7:1, none below 35 years and none qualified after 1984. The average daily workload was 74. It was estimated that GPs handle at least 26.5% of the primary care morbidity. Children accounted for 32% of consultations. There was a significantly higher proportion of children (P < 0.0001) and of the elderly (P < 0.05) in the consulting population compared with the general population. In 2068 encounters, 3448 RFEs and 2087 problems had been recorded. Respiratory and General chapters included 55% of the RFEs. By ICPC rubrics, 27 of the top thirty RFEs were for common symptoms. In addition to acute illnesses, asthma, hypertension and diabetes as well as preventive care activities were within the top 12 problems managed. CONCLUSIONS: The fact that many common illnesses, chronic diseases and preventive treatments are dealt with in general practice shows the necessity to include family medicine in the undergraduate curriculum of all medical schools. Undergraduate and postgraduate training in family medicine should concentrate more on child care and care of the elderly. Suitable incentives may be necessary to motivate younger doctors to become GPs to meet the medical care needs of the community.     

Postischemic enhancements of N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor-mediated responses in hippocampal CA1 pyramidal neurons

Glutamate receptor-mediated responses were investigated by using a whole-cell recording and an intracellular calcium ion ([Ca2+]i) imaging in gerbil postischemic hippocampal slices prepared at 1, 3, 6, 9, 12, and 24 hours after 5-minute ischemia. Bath application of N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), and kainate showed that NMDA-, AMPA- and kainate-induced currents were enhanced in postischemic CA1 pyramidal neurons at 1 to 12 hours after 5-minute ischemia. NMDA and non-NMDA receptor-mediated excitatory postsynaptic currents (EPSC) were examined in postischemic CA1 pyramidal neurons at 3 hours after 5-minute ischemia to confirm whether synaptic responses are enhanced in the postischemic CA1 pyramidal neurons. The amplitudes of NMDA- and non-NMDA-receptor-mediated EPSC were enhanced in the postischemic CA1 pyramidal neurons. NMDA-, AMPA-, and kainate-induced [Ca2+]i elevations were also examined to determine whether the enhancement of currents is accompanied by the enhancement of [Ca2+]i elevation. The enhancements of NMDA-, AMPA-, and kainate-induced [Ca2+]i elevations were shown in the postischemic CA1. These results indicate that NMDA and non-NMDA receptor-mediated responses are persistently enhanced in the CA1 pyramidal neurons 1 to 12 hours after transient ischemia, and suggest that the enhancement of glutamate receptor-mediated responses may act as one of crucial factors in the pathologic mechanism responsible for leading postischemic CA1 pyramidal neurons to irreversible neuronal injury.     

Chronic spinal nerve ligation induces changes in response characteristics of nociceptive spinal dorsal horn neurons and in their descending regulation originating in the periaqueductal gray in the rat

We studied whether a chronic neuropathy induced by unilateral spinal nerve ligation changes the response characteristics of spinal dorsal horn wide-dynamic range (WDR) neurons or their periaqueductal gray (PAG)-induced descending modulation. Experiments were performed in rats with behaviorally demonstrated allodynia induced by spinal nerve ligation and in a group of nonneuropathic control rats. The stimulus-response functions of WDR neurons for mechanical and thermal stimuli and the modulation of their peripherally evoked responses by electrical stimulation of the PAG were determined under pentobarbital anesthesia. The results showed that neuropathy caused a significant leftward shift in stimulus-response functions for mechanical stimuli. In contrast, stimulus-response functions for noxious heat stimuli in the neuropathic limb were, if anything, shifted rightward, although this shift was short of statistical significance. In neuropathic rats, PAG stimulation produced a significantly stronger attenuation of spinal neuronal responses induced by noxious heat in the unoperated than in the operated side. At the intensity that produced attenuation of noxious heat stimuli, PAG stimulation did not produce any significant change in spinal neuronal responses evoked by mechanical stimuli either from the operated or the nonoperated hindlimb of the neuropathic rats. Spontaneous activity of WDR neurons was higher in the operated side of neuropathic rats than in control rats. Afterdischarges evoked by peripheral stimuli were observed in 1/16 of the WDR neurons ipsilateral to spinal nerve ligation and not at all in other experimental groups. The WDR neurons studied were not activated by innocuous or noxious cold stimuli. The results indicate that spinal nerve ligation induces increased spontaneous activity and enhanced responses to mechanical stimuli in the spinal dorsal horn WDR neurons, whereas noxious heat-evoked responses are not significantly changed or if anything, attenuated. Moreover, the inhibition of noxious heat stimuli by PAG stimulation is attenuated in the neuropathic side. It is proposed that the observed changes in the response characteristics of the spinal dorsal horn WDR neurons and in their descending modulation may contribute to the neuropathic symptoms in these animals.     

Localization of Ca2+ channel subtypes on rat spinal motor neurons, interneurons, and nerve terminals

Ca2+ channels in distinct subcellular compartments of neurons mediate voltage-dependent Ca2+ influx, which integrates synaptic responses, regulates gene expression, and initiates synaptic transmission. Antibodies that specifically recognize the alpha1 subunits of class A, B, C, D, and E Ca2+ channels have been used to investigate the localization of these voltage-gated ion channels on spinal motor neurons, interneurons, and nerve terminals of the adult rat. Class A P/Q-type Ca2+ channels were present mainly in a punctate pattern in nerve terminals located along the cell bodies and dendrites of motor neurons. Both smooth and punctate staining patterns were observed over the surface of the cell bodies and dendrites with antibodies to class B N-type Ca2+ channels, indicating the presence of these channels in the cell surface membrane and in nerve terminals. Class C and D L-type and class E R-type Ca2+ channels were distributed mainly over the cell soma and proximal dendrites. Class A P/Q-type Ca2+ channels were present predominantly in the presynaptic terminals of motor neurons at the neuromuscular junction. Occasional nerve terminals innervating skeletal muscles from the hindlimb were labeled with antibodies against class B N-type Ca2+ channels. Staining of the dorsal laminae of the rat spinal cord revealed a complementary distribution of class A and class B Ca2+ channels in nerve terminals in the deeper versus the superficial laminae. Many of the nerve terminals immunoreactive for class B N-type Ca2+ channels also contained substance P, an important neuropeptide in pain pathways, suggesting that N-type Ca2+ channels are predominant at synapses that carry nociceptive information into the spinal cord.     

Peripheral target specification of synaptic connectivity of muscle spindle sensory neurons with spinal motoneurons

The source of environmental cues determining the central connections of muscle sensory neurons was investigated by manipulating chick embryos so that sensory neurons supplied a duplicate set of dorsal thigh muscles. These neurons projected out ventral nerve pathways and along motor axons that normally project to ventral muscles but their ultimate target tissue was the duplicate set of dorsal muscles. The central connections of these sensory neurons to motoneurons supplying normal dorsal muscles were then determined with intracellular recordings in isolated spinal cord preparations. Sensory neurons supplying individual duplicate dorsal muscles made the same connections as those supplying the corresponding normal dorsal muscles; the pattern of these connections was different than that made by afferents supplying ventral muscles. Sensory neurons thus made synaptic connections appropriate for their target muscle rather than for their more proximal ventral environment. These findings suggest that the target muscle is the source of cues that determine the central connections of the sensory neurons projecting to it. Motoneurons forced to innervate novel muscle received many of the same sensory inputs they would normally receive, suggesting that motoneurons are less influenced by their target tissue than sensory neurons.     

NMDA receptor-mediated pilocarpine-induced seizures: characterization in freely moving rats by microdialysis

1. Pilocarpine administration has been used as an animal model for temporal lobe epilepsy since it produces several morphological and synaptic features in common with human complex partial seizures. Little is known about changes in extracellular neurotransmitter concentrations during the seizures provoked by pilocarpine, a non-selective muscarinic agonist. 2. Focally evoked pilocarpine-induced seizures in freely moving rats were provoked by intrahippocampal pilocarpine (10 mM for 40 min at a flow rate of 2 microl min(-1)) administration via a microdialysis probe. Concomitant changes in extracellular hippocampal glutamate, gamma-aminobutyric acid (GABA) and dopamine levels were monitored and simultaneous electrocorticography was performed. The animal model was characterized by intrahippocampal perfusion with the muscarinic receptor antagonist atropine (20 mM), the sodium channel blocker tetrodotoxin (1 microM) and the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine maleate, 100 microM). The effectiveness of locally (600 microM) or systemically (10 mg kg(-1) day(-1)) applied lamotrigine against the pilocarpine-induced convulsions was evaluated. 3. Pilocarpine initially decreased extracellular hippocampal glutamate and GABA levels. During the subsequent pilocarpine-induced limbic convulsions extracellular glutamate, GABA and dopamine concentrations in hippocampus were significantly increased. Atropine blocked all changes in extracellular transmitter levels during and after co-administration of pilocarpine. All pilocarpine-induced increases were completely prevented by simultaneous tetrodotoxin perfusion. Intrahippocampal administration of MK-801 and lamotrigine resulted in an elevation of hippocampal dopamine levels and protected the rats from the pilocarpine-induced seizures. Pilocarpine-induced convulsions developed in the rats which received lamotrigine perorally. 4. Pilocarpine-induced seizures are initiated via muscarinic receptors and further mediated via NMDA receptors. Sustained increases in extracellular glutamate levels after pilocarpine perfusion are related to the limbic seizures. These are arguments in favour of earlier described NMDA receptor-mediated excitotoxicity. Hippocampal dopamine release may be functionally important in epileptogenesis and may participate in the anticonvulsant effects of MK-801 and lamotrigine. The pilocarpine-stimulated hippocampal GABA, glutamate and dopamine levels reflect neuronal vesicular release.     

Involvement of spinal NMDA receptors in capsaicin-induced nociception

On poststress images with 99mTc-sestamibi (MIBI), increased lung uptake of the radiotracer may reflect severe or multivessel coronary artery disease. METHODS: We measured pulmonary/myocardial ratios of MIBI at standardized times on immediate poststress acquisitions and on delayed tomographic acquisitions. In 1500 sequential patients referred for rest and stress myocardial tomography, ancillary planar images were obtained 4 min postinjection at peak stress with exercise, either alone (exercise, n = 674), or after intravenous dipyridamole (dipyridamole, n = 826). RESULTS: Based on 95% confidence limits in the angiographic normals, high values for immediate acquisitions were found in 17% of dipyridamole studies and 15% of exercise studies. High values for delayed acquisitions were found in 10% of dipyridamole studies and 9% of exercise studies. For both stress modes, increased values were related (p < 0.001) to ischemic perfusion defects for immediate images, to fixed defects for delayed images, and to ventricular dilation in both cases. By logistic regression analysis, body weight and history of infarction were also minor independent determinants (p < 0.01) of delayed acquisitions. In a subset of 250 cases with angiographic correlation (163 with dipyridamole; 87 with exercise), immediate lung uptake was highly correlated with ventricular dysfunction and with coronary stenoses (p < 0.0001). Relationships were similar to those in a historic control series imaged with 201TI. Values for delayed poststress images, and for corresponding rest images, showed strong relationships to ventricular dysfunction but not to stenosis severity. CONCLUSION: The relationships of immediate lung uptake to scintigraphic and angiographic disease patterns suggest its possible diagnostic use as an indicator of stress-induced ventricular decompensation.     

Peripheral nerve entrapment in Paget''s disease

Thirteen anthropometric measurements of Kaffa district schoolchildren in Ethiopia were factor analysed. Two factors emerged which may be interpreted as (a) body size and (b) fat mass.     

Whole body hyperthermia selectively induces heat shock protein 72 in neurons of the rat spinal cord

Rabbits with lesions of either medial prefrontal cortex (mPFC) or amygdala central nucleus (ACN) were compared with sham-lesioned animals during differential and reversal classical conditioning of the eyeblink (EB) and heart rate (HR) response. Lesions of the mPFC, but not ACN, produced a severe impairment in EB reversal conditioning, but neither lesion affected original discrimination. However, both mPFC and ACN lesions produced a severe attenuation of accompanying HR decelerations during both initial differentiation and reversal. These results suggest that mPFC processing of Pavlovian conditioning contingencies affects not only the autonomic component of learning but preservative somatomotor conditioning as well, whereas ACN processing affects only the autonomic component.     

NMDA receptor involvement in spatial delayed alternation in developing rats.

Two experiments examined the effect of the noncompetitive NMDA receptor antagonist, dizocilpine maleate (MK-801), on spatial working memory during development. Rats were trained on spatial delayed alternation (SDA) in a T-maze after ip administration of 0.06 mg/kg MK-801, 0.1 mg/kg MK-801, or saline on postnatal days (P) P23 and P33 (Experiment 1), or following bilateral intrahippocampal administration of 2.5 or 5.0 υg per side MK-801 or saline on P26 (Experiment 2). In Experiment 1, MK-801 dose-dependently impaired SDA learning at both ages. Because the same doses of systemic MK-801 have no effect on T-maze position discrimination learning, impairment of SDA by MK-801 likely reflects disruption of spatial working memory. Both doses of MK-801 abolished acquisition of SDA performance in Experiment 2. Disruption of hippocampal plasticity may account for the effects produced by systemic MK-801 administration. These results confirm and extend earlier lesion studies by implicating plasticity of hippocampal neurons in the ontogeny of spatial delayed alternation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Voltage oscillations in Xenopus spinal cord neurons: developmental onset and dependence on co-activation of NMDA and 5HT receptors

The development of intrinsic, N-methyl-D-aspartate (NMDA) receptor-mediated voltage oscillations and their dependence on co-activation of 5-hydroxytryptamine (5HT) receptors was explored in motor neurons of late embryonic and early larval Xenopus laevis. Under tetrodotoxin, 100 microM NMDA elicited a membrane depolarization of around 20 mV, but did not lead to voltage oscillations. However, following the addition of 2-5 microM 5HT, oscillations were observed in 12% of embryonic and 70% of larval motor neurons. The voltage oscillations depended upon co-activation of NMDA and 5HT receptors since they were curtailed by selectively blocking NMDA receptors with D-2-amino-5-phosphonovaleric acid (APV) or by excluding Mg2+ from the experimental saline. 5HT applied in the absence of NMDA also failed to elicit oscillations. Oscillations could be induced by the non-selective 5HT1alpha receptor agonist, 5-carboxamidotryptamine (5CT) and both 5HT- and 5CT-induced oscillations were abolished by pindobind-5HT1, a selective 5HT1alpha receptor antagonist. To test whether 5HT enables voltage oscillations by modulating the voltage-dependent block of NMDA channels by Mg2+, membrane conductance was monitored under tetrodotoxin. Although 5HT caused membrane hyperpolarization of 4-8 mV, there was little detectable change in conductance. NMDA application caused an approximate 20 mV depolarization and an 'apparent' decrease in conductance, presumably due to the conductance pulse bringing the membrane into a voltage region where Mg2+ blocks the NMDA ionophore. 5HT further decreased conductance, which we propose is due to its enhancement of the voltage-dependent Mg2+ block. When the membrane potential was depolarized by approximately 20 mV via depolarizing current injection (to mimic the NMDA-induced depolarization), 5HT increased rather than decreased membrane conductance. Furthermore, 5HT did not affect the increase in membrane conductance following NMDA applications in zero Mg2+ saline. The results suggest that intrinsic, NMDA receptor-mediated voltage oscillations develop in a brief period after hatching, and that they depend upon the co-activation of 5HT and NMDA receptors. The enabling function of 5HT may involve the facilitation of the voltage-dependent block of the NMDA ionophore by Mg2+ through activation of receptors with 5HT1alpha-like pharmacology.     

Extracellular K+ induces locomotor-like patterns in the rat spinal cord in vitro: comparison with NMDA or 5-HT induced activity

Bath-application of increasing concentrations of extracellular K+ elicited alternating motor patterns recorded from pairs of various lumbar ventral roots of the neonatal rat (0-2 days old) spinal cord in vitro. The threshold concentration of K+ for this effect was 7.9 +/- 0.8 mM (mean +/- SD). The suprathreshold concentration range useful to evoke persistent motor patterns (lasting >/=10 min) was very narrow ( approximately 1 mM) as further increments elicited only rhythmic activity lasting from 20 s to a few minutes. On average, the fastest period of rhythmic patterns was 1.1 +/- 0.3 s. Intracellular recording from lumbar motoneurons showed that raised extracellular K+ elicited membrane potential oscillations with superimposed repetitive firing. In the presence of N-methyl--aspartate (NMDA) or non-NMDA receptor blockers [R(-)-2-amino-phosphonovaleric acid or 6-cyano-7-nitroquinoxaline-2,3-dione, respectively] extracellular K+ increases could still induce motor patterns although the threshold concentration was raised. Serotonin (5-HT) also induced alternating motor patterns (threshold 15 +/- 7 microM) that were consistently slower than those induced by high K+ or NMDA. Ritanserin (1 microM) prevented the locomotor-like activity of 5-HT but not that of high K+ provided the concentration of the latter was further increased. Subthreshold concentrations of K+ became effective in the presence of subthreshold doses of 5-HT or NMDA, indicating mutual facilitation between these substances. The fastest pattern frequency was observed by raising K+ or by adding NMDA. In the presence of 5-HT, the pattern frequency was never as fast even if NMDA (or high K+) was coapplied. Furthermore, application of 5-HT significantly slowed down the K+- or NMDA-induced rhythm, an effect strongly potentiated in the presence of ritanserin. It is suggested that the operation of the spinal locomotor network was activated by rises in extracellular K+, which presumably led to a broad increase in neuronal excitability. Whenever the efficiency of excitatory synaptic transmission was diminished (for example by glutamate receptor antagonism), a larger concentration of K+ was required to evoke locomotor-like patterns. The complex effect (comprising stimulation and inhibition) of 5-HT on alternating pattern generation appeared to result from a dual action of this substance on the spinal locomotor network.     

No change in NMDA receptor-mediated response rise-time during development: evidence against transmitter spillover

Glutamatergic transmission was examined in tadpole optic tectum to test the possibility that transmitter concentration reaching N-methyl-D-aspartate (NMDA) receptors increases over development. Pharmacologically isolated NMDA receptor-mediated transmission was monitored with whole-cell recordings. Synaptic responses were recorded from cells at different locations in the optic tectum, corresponding to different stages of development. Rise-times and decay-times of NMDA currents were analyzed. We found no significant correlation between rise-time and developmental stage. As NMDA rise-times can correlate with concentration for glutamate concentrations below 200 microM, these results argue that, if there is developmental variation in transmitter concentration, this occurs for values greater than 200 microM. Furthermore, we found a correlation between rise-times and decay-times, consistent with a model in which transmitter concentration is high and rise-time is controlled by channel closings. These results argue against synaptic models in which low concentrations of transmitter (as by spillover from nearby release sites) selectively activates NMDA receptors.     

Sustained exposure to a glycine receptor partial agonist differentially alters NMDA receptor agonist and antagonist potencies in cultured spinal cord neurons

The present study examined the effects of K+ channel inhibitors on the basal tone and on KCl- or methacholine-induced contraction of the mouse-isolated trachea. Glibenclamide and iberiotoxin, procaine, quinine and tetraethylammonium did not induce any contraction of the indomethacin-treated mouse trachea. 4-Aminopyridine induced concentration-dependent contraction. This action of 4-aminopyridine was abolished by atropine and reduced by tetrodotoxin and nifedipine. Glibenclamide failed to modify KCl- or methacholine-induced contraction. Iberiotoxin and 4-aminopyridine potentiated KCl- and methacholine-induced contractions. Nifedipine, procaine, quinine and tetraethylammonium inhibited KCl- and methacholine-induced contractions. These data suggest that the closure of large Ca2+-dependent K+ channels can potentiate KCI- and methacholine-induced contraction. The effects of 4-aminopyridine on the mouse trachea reflect chiefly activation of muscarinic receptors. Procaine, quinine and tetraethylammonium inhibit depolarization-induced and receptor-mediated contractions of the mouse-isolated trachea.