Primary afferent depolarization in the rat spinal cord is mediated by pathways utilising NMDA and non-NMDA receptors

Effects of niflumic acid on γ-aminobutyric acid-induced currents in isolated dorsal root ganglion neurons of neuropathic pain rats

Niflumic acid (NFA) is a type of non-steroidal anti-inflammatory drug. Neuropathic pain is caused by a decrease in presynaptic inhibition mediated by γ-aminobutyric acid (GABA). In the present study, a whole-cell patch-clamp technique and intracellular recording were used to assess the effect of NFA on GABA-induced inward current in dorsal root ganglion (DRG) neurons of a chronic constriction injury (CCI) model. It was observed that 1–1,000 µmol/l GABA induced a concentration-dependent inward current in DRG neurons. Compared with pseudo-operated rats, the thermal withdrawal latency (TWL) of CCI rats significantly decreased (P<0.01); however, the TWLs of each NFA group (50 and 300 µmol/l) were significantly longer than that of the CCI group (P<0.01). In the CCI group, the response evoked by GABA (10⁻⁶-10⁻³ mol/l) was reduced in a concentration dependent manner compared with a normal control group (P<0.01), and the current amplitudes of CCI rats activated by the same concentrations of GABA (10⁻⁶-10⁻³ mol/l) were significantly decreased compared with the control group (P<0.05). The inward currents activated by 100 µmol/l GABA were suppressed by treatment with 1, 10 and 100 µmol/l NFA (5.32±3.51, 33.8±5.20, and 52.2±6.32%, respectively; P<0.05). The inverse potentials of GABA-induced currents were 9.87±1.32 and 9.64±1.24 mV with and without NFA, respectively (P<0.05). Pre-treatment with NFA exerted a strong inhibitory effect on the peak value of GABA-induced current, and the GABA-induced response was inhibited by the same concentrations of NFA (1, 10 and 100 µmol/l) in the control and CCI groups (P<0.05). The results suggest that NFA reduced the primary afferent depolarization (PAD) associated with neuropathic pain and mediated by the GABA_A receptor. NFA may regulate neuropathic pain by inhibiting dorsal root reflexes, which are triggered PAD.

Presynaptic inhibition of primary afferents by depolarization: observations supporting nontraditional mechanisms

Primary afferent neurotransmission is the fundamental first step in the central processing of sensory stimuli and is controlled by pre- and postsynaptic inhibitory mechanisms. Presynaptic inhibition (PSI) is probably the more powerful form of inhibitory control in all primary afferent fibers. A major mechanism producing afferent PSI is via a channel-mediated depolarization of their intraspinal terminals, which can be recorded extracellularly as a dorsal root potential (DRP). Based on measures of DRP latency it has been inferred that this primary afferent depolarization (PAD) of low-threshold afferents is mediated by minimally trisynaptic pathways with pharmacologically identified GABAergic interneurons forming last-order axo-axonic synapses onto afferent terminals. There is still no "squeaky clean" evidence of this organization. This paper describes recent and historical work that supports the existence of PAD occurring by more direct pathways and with a complex pharmacology that questions the proprietary role of GABA and GABA(A) receptors in this process. Cholinergic transmission in particular may contribute significantly to PAD, including via direct release from primary afferents.

Early alterations in the electrophysiological properties of rat spinal motoneurones following neonatal axotomy

Early in development, motoneurones are critically dependent on their target muscles for survival and differentiation. Previous studies have shown that neonatal axotomy causes massive motoneurone death and abnormal function in the surviving motoneurones. We have investigated the electrophysiological and morphological properties of motoneurones innervating the flexor tibialis anterior (TA) muscle during the first week after a neonatal axotomy, at a time when the motoneurones would be either in the process of degeneration or attempting to reinnervate their target muscles. We found that a large number ( approximately 75%) of TA motoneurones died within 3 weeks after neonatal axotomy. Intracellular recordings revealed a marked increase in motoneurone excitability, as indicated by changes in passive and active membrane electrical properties. These changes were associated with a shift in the motoneurone firing pattern from a predominantly phasic pattern to a tonic pattern. Morphologically, the dendritic tree of the physiologically characterized axotomized cells was significantly reduced compared with age-matched normal motoneurones. These data demonstrate that motoneurone electrical properties are profoundly altered shortly after neonatal axotomy. In a subpopulation of the axotomized cells, abnormally high motoneurone excitability (input resistance significantly higher compared with control cells) was associated with a severe truncation of the dendritic arbor, suggesting that this excitability may represent an early electrophysiological correlate of motoneurone degeneration.

Effects of amino acid antagonists on spontaneous dorsal root activity and evoked dorsal horn field potentials in an isolated preparation of rat spinal cord

Fast and slow dorsal horn field potentials and spontaneous dorsal root activity were recorded from 19-23-day-old rat isolated spinal cord preparations. The effects of GABA, glycine, and glutamate antagonists were tested on these recordings. CNQX, an AMPA/kainate antagonist, reduced all 3 components of the dorsal horn field potential whereas MK801, an NMDA ion channel antagonist, reduced the fast S2 component and the slow wave. Both reduced spontaneous dorsal root activity. NMDA antagonists, D-AP5, 7-chlorokynurenic acid and arcaine, and the metabotropic glutamate antagonists L-AP3 and ethylglutamic acid, while having little effect on the fast components of the field potential, all reduced the slow component. The GABA antagonist, bicuculline, and the glycine antagonist, strychnine, while having no effect on the fast S1 and slow components of the field potential, reduced both the fast S2 component of the field potential and spontaneous dorsal root activity. These results suggest that non-NMDA glutamate receptors are involved in low and high threshold transmission to dorsal horn neurones while NMDA and metabotropic glutamate receptors are primarily involved in high threshold transmission and both GABA and glycine have roles in the transmission or modulation of sensory information within the dorsal horn of the cord.

Two prodrugs of potent and selective GluR5 kainate receptor antagonists actives in three animal models of pain

Amino acids 5 and 7, two potent and selective competitive GluR5 KA receptor antagonists, exhibited high GluR5 receptor affinity over other glutamate receptors. Their ester prodrugs 6 and 8 were orally active in three models of pain: reversal of formalin-induced paw licking, carrageenan-induced thermal hyperalgesia, and capsaicin-induced mechanical hyperalgesia.

The effect of a kainate GluR5 receptor antagonist on responses of spinothalamic tract neurons in a model of peripheral neuropathy in primates

The responses of antidromically identified spinothalamic tract (STT) neurons to mechanical and thermal stimuli were compared in anesthetized normal and neuropathic monkeys before and after administration of a GluR5 kainate receptor antagonist (LY382884) into the spinal cord dorsal horn through a microdialysis fiber. Peripheral neuropathy was induced by tight ligation of the L7 spinal nerve 13-15 days prior to the experiment. STT neurons recorded in the animals with neuropathy showed increased responsiveness to weak mechanical stimuli and to heating and cooling of the skin compared to STT cells in normal animals. In both normal and the neuropathic monkeys the responses of the STT neurons to mechanical and thermal stimuli were attenuated by LY382884 application in a concentration-dependent manner. Intraspinal application of LY382884 in the neuropathic animals led to a potent reduction of those responses of the STT neurons that were aggravated by the peripheral neuropathy (weak mechanical, heat and innocuous cooling stimuli). These results suggest that kainate receptors are involved in synaptic activation of STT cells in the normal state and may also play an important role in pathological pain states such as peripheral neuropathy in primates. Kainate receptor antagonists could thus be useful for the treatment of certain forms of allodynia and hyperalgesia.

Synaptic relationships between hair follicle afferents and neurones expressing GABA and glycine-like immunoreactivity in the spinal cord of the rat

gamma-Aminobutyric acid (GABA) and glycine have been implicated in the inhibition of sensory pathways in the dorsal horn of the spinal cord. The object of this study is to investigate the interactions between neurones immunoreactive for GABA and/or glycine and hair follicle afferent terminals labelled by intracellular injection with neurobiotin. GABA and glycine-like immunoreactivity in axons and dendrites in synaptic contact with the afferent terminals was demonstrated by using a postembedding immunogold method, and serial section reconstruction was used to show the distribution and nature of these interactions in lamina III of the dorsal horn. Most afferent boutons (94%) were postsynaptic at axo-axonic synapses: 67% of presynaptic boutons presynaptic to the afferent terminals were immunoreactive for GABA and glycine, 24% for GABA alone, and 7% for glycine alone. Only a small percentage of dendrites postsynaptic to afferent boutons appeared to belong to inhibitory interneurones: 3% were immunoreactive for GABA and glycine, 10% for glycine alone, but 87% were immunoreactive for neither antibody. Many afferent boutons were the central terminals of what appeared to be type IIb glomeruli and were involved triadic synaptic arrangements at which boutons presynaptic to an afferent terminal also made axodendritic contacts with dendrites postsynaptic to the afferent. Many of the presynaptic boutons involved in the triads were immunoreactive for GABA and glycine. Because afferent terminals do not themselves express glycine receptors (Mitchell et al. [1993] J. Neurosci. 13:2371-2381), glycine may therefore act on dendrites postsynaptic to hair follicle afferent terminals at these triads.

Ethyl (3S,4aR,6S,8aR)-6-(4-ethoxycar- bonylimidazol-1-ylmethyl)decahydroiso-quinoline-3-carboxylic ester: a prodrug of a GluR5 kainate receptor antagonist active in two animal models of acute migraine

Amino diacid 3, a highly selective competitive GluR5 kainate receptor antagonist, exhibited high GluR5 receptor affinity and selectivity over other glutamate receptors. Its diethyl ester prodrug 4 was orally active in two models of migraine: the neurogenic dural plasma protein extravasation model and the nucleus caudalis c-fos expression model. These data suggest that a GluR5 kainate receptor antagonist might be an efficacious antimigraine therapy with a novel mechanism of action.

Functional expression of AMPA receptors on central terminals of rat dorsal root ganglion neurons and presynaptic inhibition of glutamate release

No direct evidence has been found for expression of functional AMPA receptors by dorsal root ganglion neurons despite immunocytochemical evidence suggesting they are present. Here we report evidence for expression of functional AMPA receptors by a subpopulation of dorsal root ganglion neurons. The AMPA receptors are most prominently located near central terminals of primary afferent fibers. AMPA and kainate receptors were detected by recording receptor-mediated depolarization of the central terminals under selective pharmacological conditions. We demonstrate that activation of presynaptic AMPA receptors by exogenous agonists causes inhibition of glutamate release from the terminals, possibly via primary afferent depolarization (PAD). These results challenge the traditional view that GABA and GABA(A) receptors exclusively mediate PAD, and indicate that PAD is also mediated by glutamate acting on presynaptically localized AMPA and kainate receptors.

NMDA or non-NMDA receptor antagonists attenuate increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of capsaicin in rats

GABAergic neurons play an important role in the generation of primary afferent depolarization, which results in presynaptic inhibition and, if large enough, triggers dorsal root reflexes. Recent electrophysiological studies by our group have suggested that increased excitation of spinal GABAergic neurons by activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors following intradermal injection of capsaicin results in the generation of DRRs that contribute to neurogenic inflammation. The present study was to determine if changes in the expression of Fos protein occur in GABAergic neurons in the lumbosacral spinal cord following injection of capsaicin into the glabrous skin of one hind paw of anesthetized rats and if pretreatment with an NMDA receptor antagonist, D-(-)-2-amino-7-phosphonoheptanoic acid (AP7) or a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocks Fos expression in these neurons. The experiments used western blots and immunofluorescence double labeling staining following capsaicin or vehicle injection. Western blots showed that Fos protein was increased on the ipsilateral side in spinal cord tissue 0.5 h after capsaicin injection. Pretreatment with AP7 or CNQX caused a decrease in capsaicin-induced Fos expression. Immunofluorescence double labeling showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following capsaicin injection (48.8+/-4.8%) compared to the vehicle injection (23.8+/-5.1%) in superficial laminae on the ipsilateral side in lumbosacral spinal cord (P<0.05). However, when the spinal cord was pretreated with AP7 (5 microg) or CNQX (0.2 microg), only 9.1+/-0.6% or 7.1+/-0.8% of GABA-immunoreactive neuronal profiles were stained for Fos following capsaicin injection. The blockade of the capsaicin-evoked Fos staining was dose-dependent. These findings suggest that GABAergic neurons take part in dorsal horn circuits that modulate nociceptive information and that the function of GABAergic neurons following capsaicin injection is partially mediated by NMDA and non-NMDA receptors.

Dorsal root reflexes and cutaneous neurogenic inflammation after intradermal injection of capsaicin in rats

The role of dorsal root reflexes (DRRs) in acute cutaneous neurogenic inflammation induced by intradermal injection of capsaicin (CAP) was examined in anesthetized rats. Changes in cutaneous blood flow (flare) on the plantar surface of the foot were measured using a laser Doppler flowmeter, and neurogenic edema was examined by measurements of paw thickness. To implicate DRRs in neurogenic inflammation after CAP injection, the ipsilateral sciatic and femoral nerves were sectioned, dorsal rhizotomies were performed at L(3-)-S(1), and antagonists of GABA or excitatory amino acid receptors were administered intrathecally. Intradermal injection of CAP evoked a flare response that was largest at 15-20 mm from the injection site and that spread >30 mm. Acute transection of the sciatic and femoral nerves or dorsal rhizotomies nearly completely abolished the blood flow changes 15-20 mm from the CAP injection site, although there was only a minimal effect on blood flow near the injection site. These procedures also significantly reduced neurogenic edema. Intrathecal bicuculline, 6-cyano-7-nitroquinoxaline-2,3-dione, (CNQX) or D(-)-2-amino-7-phosphonoheptanoic acid (AP7), but not phaclofen, also reduced dramatically the increases in blood flow 15-20 mm from the CAP injection site, but had only a minimal effect on blood flow near the injection site. Neurogenic edema was reduced by the same agents that reduced blood flow. Multiunit DRRs recorded from the central stumps of cut dorsal rootlets in the lumbar spinal cord were enhanced after CAP injection. This enhanced DRR activity could be reduced significantly by posttreatment of the spinal cord with bicuculline, CNQX or AP7, but not phaclofen. It is concluded that peripheral cutaneous inflammation induced by intradermal injection of CAP involves central nervous mechanisms. DRRs play a major role in the development of neurogenic cutaneous inflammation, although a direct action of CAP on peripheral nerve terminals or the generation of axon reflexes also may contribute to changes in the skin near the injection site.

Kainate GluR5 receptor subtype mediates the nociceptive response to formalin in the rat

In order to study the roles of the AMPA and kainate subtypes of non-NMDA glutamate receptors in the processing of persistent nociceptive information, compounds with varying activities at these receptors were examined for effects on the formalin-induced paw-licking behavior in rats. The selective AMPA antagonist, LY300164 and the mixed AMPA/kainate antagonist, NBQX, were compared for their effects on formalin-induced pain behavior. NBQX (3, 10, 20 mg/kg, i.p.), caused antinociception as well as ataxia whereas the selective AMPA antagonist, LY300164 (3,5,10 mg/kg, i.p.), did not cause antinociception at doses that did not produce ataxia. In view of the well documented distribution of kainate receptors on C fibres and of the kainate-preferring iGluR5 subtype on dorsal root ganglia (DRG), we tested a series of three decahydroisoquinolines with different profiles of activity between iGluR5 and AMPA receptors and all without activity on iGluR6, iGluR7 or KA2 subtypes. LY293558 (0.1, 1, 3, 5 mg/kg, i.p.), which had low micromolar affinity for both iGluR5 and 2 caused, like NBQX, both antinociceptive and ataxic effects. However, the selective iGluR5 antagonist LY382884 (5, 10, 30, 100 mg/kg, i.p.), exhibited antinociceptive actions without ataxia while the iGluR2 preferring antagonist LY302679 (5 mg/kg, i.p), caused ataxia but did not produce antinociceptive effects at that dose. These actions were stereoselective since the enantiomeric compounds, LY293559 and LY302680, were ineffective in these tests. The data strongly suggest an involvement of iGluR5 in the processing of nociceptive information.

Involvement of AMPA receptors in posterior locomotor activity in the rabbit: an in vivo study

Although AMPA receptors are known to be widely involved in excitatory synaptic neurotransmission at the spinal level, very little is known about their role in modulating motor activity in mammals. In curarized decerebrate or spinalized rabbit preparations, fictive locomotion was monitored on hindlimb nerves after either activation or blockade of AMPA receptors. In decerebrate preparations, the administration of the antagonist, NBQX (3.5 mg/kg i.p.) or the agonist, AMPA (0.5 mg/kg i.v.) produced, in both cases, a depression of locomotor activities induced by stimulation of cutaneous afferents (evoked locomotor activity). This potent effect was transient with AMPA (recovery after 20 min) and followed by the occurrence of spontaneous locomotor sequences, while no recovery was observed with NBQX treatment. In spinal preparations where a continuous 'spontaneous' locomotor activity resulted from the pharmacological activation of noradrenergic descending pathways (nialamide-DOPA pretreatment), the same drugs injected at higher doses (5 mg/kg NBQX i.p. and 1 mg/kg AMPA i.v.) only weakly affected the frequency of 'spontaneous' and evoked locomotor bursts while they exerted inhibitory and facilitatory effects on the burst amplitude respectively. The results suggest that AMPA receptors are involved at spinal level: 1) in direct mediation of cutaneous afferent excitatory effects on the posterior locomotor generators (pLG); 2) in indirect mediation of a supraspinal descending inhibition controlling, likely presynaptically, the cutaneous afferent activation; and 3) in transmission to motoneurons of the output signals from the pLG. Finally, tight spinal interactions between potent descending noradrenergic pathways and spinal AMPA neurotransmission were disclosed.

Mechanisms of touch-evoked pain (allodynia): a new model

In this paper we review the current neurophysiological models of touch-evoked pain and present a new proposal that addresses the mechanisms of allodynia. The new model is based on the notion that A-beta mechanoreceptors can gain access to nociceptive neurones by means of a presynaptic link, at central level, between low threshold mechanoreceptors and nociceptors. We propose that the excitation of nociceptors provoked by a peripheral injury activates the spinal interneurones that mediate primary afferent depolarization (PAD) between low threshold mechanoreceptors and nociceptors. As a consequence of the increased and persistent barrage driving these neurones their excitability is increased such that, when activated by low threshold mechanoreceptors from areas surrounding the injury site, they produce a very intense PAD in the nociceptive afferents which is capable of generating spike activity. This activation would be conducted antidromically in the form of dorsal root reflexes (DRRs) but would also be conducted forward activating the second order neurones normally driven by nociceptors. The sensory consequence of this mechanism is pain evoked by the activation of low threshold mechanoreceptors from an area surrounding an injury site (allodynia).

Pre- and post-synaptic actions of 5-hydroxytryptamine in the rat lumbar dorsal horn in vitro: implications for somatosensory transmission

Since the relative contribution of pre- versus post-synaptic actions of 5-hydroxytryptamine (5-HT) to modulation of somatosensory processing in the dorsal hom is not known, recordings from primary afferents and dorsal hom neurons from in vitro rat spinal cord were used to address this issue. 5-HT produced a depression of spontaneous dorsal root potentials and a slow primary afferent depolarization (PAD): the PAD versus 5-HT concentration-response curve was bell shaped (maximum at 5 microM; 250 +/- 41.5 microV). In 28/40 dorsal horn neurons, 5-HT elicited a slow depolarization not clearly associated with a specific input resistance change. Excitatory synaptic transmission from primary afferents to dorsal horn neurons was depressed by 5-HT in 40/45 neurons. 5-HT > or = 5 microM significantly (P < or = 0.05) decreased the amplitude, shortened the total duration and half-decay time of the excitatory post-synaptic potential (EPSP). A dominant effect of 5-HT on longer latency EPSP components was evident. There was not direct relationship between the magnitude of PAD and the reduction of the EPSP by 5-HT. 5-Carboxamidotryptamine, an agonist for 5-HT1 receptors, mimicked the depression of neurotransmission in the dorsal horn without producing PAD. A sample of dorsal horn neurons (n = 8) was injected with biocytin and their morphology described. All had somata within laminae III-VI. In five of these neurons 5-HT depressed the EPSP but in one interneuron-like and one unclassed neuron the EPSP was potentiated. These data suggest that whilst depression of synaptic transmission is the predominant effect of 5-HT in the deep dorsal horn, this is not easily related to PAD or cellular actions of 5-HT on dorsal horn neurons.

Immunocytochemical distribution of ionotropic glutamate receptor subunits in the spinal cord of the rabbit

Several histochemical and physiological studies in the literature suggest that ionotropic glutamate receptors are involved in various sensory and motor control mechanisms at the spinal level. The present immunocytochemical study used three specific antibodies to GluR2,4, GluR5,6,7 and to NMDAR1 to differentiate between the regional distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors throughout the rabbit spinal cord. All of these immunoreactivities were prominent in the superficial dorsal horn and motor column. Each antibody gave rise to regionally specific immunostaining patterns but which were similar at all spinal levels. Numerous small neurons in superficial laminae were immunostained with GluR2,4 antibody while only neuropilar elements were immunostained with the two other antibodies. Cell bodies of the intermediate zone and fibres in the motor column were particularly densely immunostained with GluR5-7. Such an immunostaining pattern, which was particularly abundant with the GluR5-7 antibody, suggests the presence, at the spinal level, of an extensive population of neurons exhibiting a high density of kainate receptors. Immunostaining with NMDAR1 antibody was less dense in comparison with the two others and especially in the motoneuron area. The present results provide the first immunohistochemical comparison between the respective regional distributions of the three types of ionotropic glutamate receptors in the spinal cord. Their parallel distributions throughout the spinal cord support the concept of a tight functional cooperation between NMDA and non-NMDA receptors which has been extensively described for spinal events.

Role of A delta afferent fibers in modulation of primary afferent input to the adult rat spinal cord

To address the question of whether fine myelinated and unmyelinated primary afferent fibers contribute to the mechanism of presynaptic inhibition in the spinal cord, we studied dorsal root-evoked dorsal root potentials (DR-DRPs) using a newly developed longitudinal spinal cord slice preparations in the adult rat. Single stimuli applied to the L6 dorsal root elicited a DR-DRP in the L5 dorsal root which had an amplitude of 50-150 microV and had a half decay time of 20-66 ms. The DR-DRP was depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-20 microM), while DL-2-amino-5-phosphonovaleric acid (APV, 50-100 microM) had no significant effect. DR-DRP was markedly depressed by bicuculline or picrotoxin. The evoked DR-DRP was unchanged in rats treated with capsaicin which eliminated the majority of unmyelinated C afferent fibers. Taken together with the higher voltages (> or = 1.9 V) required to elicit DR-DRP, this observation strongly suggests that the A delta afferent fibers are primarily responsible for producing and receiving the DR-DRP. The present study shows that the DR-DRP mediated by the A delta fibers in the slice preparation is analogous to those described for larger myelinated fibers in vivo. This pathway may contribute importantly to synaptic modulation of somatosensory information, including nociception at the superficial dorsal horn through an interneuronal connection which are mediated by the non-NMDA and GABAA receptors.

The role of glutamate and GABA receptors in the generation of dorsal root reflexes by acute arthritis in the anaesthetized rat

1. In rats anaesthetized with pentobarbitone sodium, a unilateral acute arthritis was produced by the injection of kaolin and carrageenan into one knee-joint cavity. Four hours after injection, the medial articular nerve (MAN) was sectioned distally and recordings obtained from the proximal stump of the nerve. 2. Centrifugally conducted action potentials were recorded from the cut MAN following the development of arthritis. Acute dorsal rhizotomy, but not sympathectomy, prevented the action potentials, and so it is concluded that the action potentials represent dorsal root reflexes. 3. Central administration of either the GABAA receptor antagonist, bicuculline, or the non-NMDA receptor antagonist, CNQX, also prevented dorsal root reflexes in the MAN. 4. Neither the GABAB receptor antagonist, CGP35348, nor the NMDA receptor antagonist, AP7, altered the dorsal root reflexes in the MAN. 5. It is concluded that arthritis causes excess primary afferent depolarization in the dorsal horn of the spinal cord leading to dorsal root reflexes. It is proposed that these dorsal root reflexes contribute to the inflammation.

Correlates of anesthetic properties in isolated spinal cord: cyclobutanes

Two halogenated cyclobutanes, one anesthetic and one not, were compared on receptor-specific pathways in isolated neonatal rat spinal cord. The anesthetic 1-chloro-1,2,2-trifluorocyclobutane depressed the monosynaptic reflex (glutamate non-NMDA receptors) and abolished a slow ventral root potential (glutamate NMDA, non-NMDA and tachykinin receptors). This compound slightly enhanced the muscimol-evoked dorsal root potential (GABAA) but reversibly depressed the dorsal root potential elicited by dorsal root stimulation. The non-anesthetic 1,2-dichlorohexafluorocyclobutane increased monosynaptic reflex, depressed slow ventral root potential approximately 50%, had little effect on muscimol-evoked dorsal root potential, and irreversibly depressed dorsal root-evoked dorsal root potential. Hypoxia accounts for slow ventral root potential depression, but not monosynaptic reflex enhancement. In this preparation and for this pair of compounds, anesthetic properties are related to blockade of transmission at glutamate synapses, with a small component of GABAA enhancement. Monosynaptic reflex increase may be related to the non-anesthetic cyclobutane's convulsant and anti-anesthetic properties.

Postnatal changes in the role of NMDA in the isolated spinal cord of the hamster, Mesocricetus auratus

The frequency of spontaneous activity recorded from lumbar dorsal roots and the lumbar dorsal horn of isolated spinal cord preparations taken from hamsters aged between 2 days to 8 weeks showed an age dependent sensitivity to 1 mM Mg2+ and 5 microM AP5. Spontaneous dorsal root and dorsal horn activity in cords from animals less than 3 weeks of age was depressed by 1 mM Mg2+ and 5 microM AP5. Cords taken from animals older than 3 weeks showed significantly less depression of spontaneous activity. The application of 10 microM NMDA to the cord produced a small (33%) depression in spontaneous dorsal root and dorsal horn activity in cords from 4 to 6 week old animals. Cords from younger animals exhibited a complex response to NMDA, with an initial increase in spontaneous activity followed by a profound (77%) depression of the firing rate. These results indicate that there are substantial changes taking place in the pharmacology of the dorsal horn during the early weeks of life, and care must be exercised when extrapolating results obtained from neonatal preparations to adult animals.

The effects of central myorelaxants on synaptically-evoked primary afferent depolarization in the immature rat spinal cord in vitro

1. In the immature rat in vitro hemisected spinal cord preparation the dorsal root-evoked depolarizing potential recorded from an adjacent dorsal root DR-DRP had a mean peak amplitude (+/- s.e.mean, n = 27) of 2.9 +/- 0.2 mV and a mean latency to peak amplitude of 106 +/- 3 ms. The DR-DRP amplitude was maximal with a stimulus intensity of four times the threshold intensity required to activate the lowest threshold fibres. The peak amplitude and/or integral over a time-source of 0.5 s were used to assess the effects of applied drugs. 2. The DR-DRP was abolished by baclofen (mean IC50 190 +/- 46 nM, n = 7). The depressant effect of baclofen was reversed by CGP35348 (1 mM). The mean apparent Kd value calculated from dose-ratios was 16.7 +/- 6.4 microM (n = 3). 3. At a maximally effective concentration, tizanidine (1 microM) produced at the most only a 14% depression of the DR-DRP (n = 4). Clonidine (0.3 microM) had an effect similar to that of tizanidine. These depressant effects were reversed by idazoxan (1 microM). 4. The DR-DRP was potentiated by diazepam in a flumazenil (1 microM)-reversible manner. A maximal potentiation of 23.2 +/- 2.7% (n = 5) was produced by 1 microM diazepam. 5. Diazepam (1 microM) induced a mean bicuculline- (10 microM, n = 2) and flumazenil- (1 microM, n = 8) sensitive depolarization in the dorsal root of 0.25 +/- 0.03 mV (n = 8). However, diazepam failed to depolarize dorsal roots (n = 3) which had been excised from the spinal cord. 6. Comparison of the above effects with previously reported depressant effects of these drugs on the synaptic output from ventral roots suggests that actions on presynaptic inhibition, as reflected in the DR-DRP, are of subsidiary importance in explaining the muscle relaxant actions of tizanidine or diazepam.

Spinal cord amino acid release and content in an arthritis model: the effects of pretreatment with non-NMDA, NMDA, and NK1 receptor antagonists

An experimental arthritis, induced by injection of the knee joint with kaolin and carrageenan, results in guarding of and decreased weight bearing on the limb. At the time of injection, a transient increased release of all amino acids examined is measurable in samples collected by microdialysis. A second and prolonged increase of aspartate (ASP), glutamate (GLU), and glutamine (GLN) concentrations follows after 3 h. The increased release at time of injection is blocked by microdialysis application of a non-N-methyl-D-aspartate (non-NMDA) or an NMDA receptor antagonist, and the release of ASP, GLU, and GLN in the late phase is blocked by pretreatment with a non-NMDA (CNQX), an NMDA (AP7) or a neurokinin 1 (NK1; CP-96,345) antagonist. Dorsal horn immunoreactive staining of GLU, substance P (SP), and calcitonin gene-related peptide (CGRP) is reflective of the events occurring in the late phase of amino acid release since GLU release is positively correlated with GLU staining density. Increased immunoreactivity for GLU, SP, and CGRP at 8 hr in the arthritic animals is differentially altered by pretreatment of the spinal cord dorsal horn with non-NMDA, NMDA, or NK1 receptor antagonists. The differential staining pattern for GLU, SP, and CGRP, the differential release of ASP and GLU, and the differential activation of the EAA and NK1 receptors implies that ASP, GLU, SP, and CGRP are each involved in the processing of sensory information and that their roles in the central sensitization occurring with the inflammatory process, are unique.

Excitatory amino acid receptor mediation of sensory inputs to functionally identified dorsal horn neurons in cat spinal cord

As excitatory amino acid receptors have been implicated in nociceptive sensory transmission, the principal objective of the present study was to investigate the effects of various excitatory amino acid antagonists on naturally evoked responses in spinal dorsal horn neurons. Extracellular single unit activity was recorded from functionally identified, spinal dorsal horn neurons in unanesthetized, decerebrated cats and in alpha-chloralose-anesthetized cats. The tests included iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV), the non-N-methyl-D-aspartate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and kynurenate, and also the intravenous administration of the N-methyl-D-aspartate receptor antagonist, ketamine. In addition, attempts were made to determine the effects on these neurons of iontophoretic application of the excitatory amino acid agonists, L-glutamate, N-methyl-D-aspartate, quisqualate, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate. Marked differences were noted in the actions of agonists and antagonists between the responses observed in the unanesthetized, decerebrated and the anesthetized animals. In decerebrated cats, responses to hair afferent stimulation were blocked by kynurenate, 6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were attenuated by 2-amino-5-phosphonovaleric acid and in one unit also by ketamine. Neither 6-cyano-7-nitroquinoxaline-2,3-dione nor kynurenate affected the responses to noxious thermal stimulation. The proportion of cells responding to the agonists were: N-methyl-D-aspartate 24/27 (89%), quisqualate 12/13 (92%) and domoate 6/7 (86%). In chloralose-anesthetized cats, responses to hair afferent stimulation were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and kynurenate but not by 2-amino-5-phosphonovaleric acid. Responses to noxious thermal stimulation were not affected by any of these antagonists, while the response to non-noxious thermal stimulation was blocked by 2-amino-5-phosphonovaleric acid, ketamine and kynurenate in the one neuron studied. The proportion of cells excited by the agonists differed from those observed in decerebrated cats: N-methyl-D-aspartate 9/32 (28%), quisqualate 50/54 (93%), (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate 19/23 (83%) and domoate 17/38 (45%). Application of the putative endogenous excitatory amino acid precursor N-acetyl-aspartyl-glutamate (NAAG) did not elicit a response in any of the neurons studied.(ABSTRACT TRUNCATED AT 400 WORDS)

The non-NMDA antagonist CNQX prevents release of amino acids into the rat spinal cord dorsal horn evoked by sciatic nerve stimulation

Basal extracellular concentrations of 9 amino acids (AAs: aspartate, Asp; glutamate, Glu; asparagine, Asn; serine, Ser; glycine, Gly; threonine, Thr; alanine, Ala; taurine, Tau; and glutamine, Gln) were determined in the spinal cord dorsal horn of anesthetized rats using microdialysis and HPLC techniques. The concentrations of all measured AAs but Gln increased significantly (P < 0.05) during sciatic nerve stimulation at C-fiber strength. The concentration of Tau remained elevated following stimulation, while the other AAs returned to prestimulation values. Addition of the specific non-NMDA antagonist, CNQX, to the perfusing solution prevented the nerve stimulation-evoked AA release. Since the measured increases in extracellular AA concentrations are probably mainly due to activation of interneurons, these results suggest that blockade of non-NMDA receptors prevented activation of interneurons in the dorsal horn and support a major role of non-NMDA receptors at the first synapse of primary afferent fibers in the dorsal horn. Complete block of AA release and decreased basal levels of Glu after infusion of TTX into the dorsal horn also implies increased neuronal activity as the main source of higher AA levels during nerve stimulation.

Excitatory amino acid binding sites in the trigeminal principal sensory and spinal trigeminal nuclei of the rat

Quantitative autoradiography was used to examine the density and distribution of excitatory amino acid (EAA) binding site subtypes in the principal sensory and spinal trigeminal nuclei of the rat trigeminal complex. The highest densities of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), kainate and metabotropic receptors were found in the superficial laminae (I and II) of subnucleus caudalis, a region known to be densely innervated by primary afferent nociceptive terminals. Lower densities of EAA binding sites were observed in spinal subnuclei interpolaris and oralis and within the principal sensory nucleus. These results are consistent with the hypothesis that EAAs are involved in primary afferent nociceptive neurotransmission.

NMDA as well as non-NMDA receptor antagonists can prevent the phase-shifting effects of light on the circadian system of the golden hamster

The present experiments were designed to evaluate whether the intraventricular administration of excitatory amino acid (EAA) receptor antagonists would prevent light-induced phase shifts of the circadian rhythm of wheel-running activity in the hamster. Administration of the non-N-methyl-D-aspartate (non-NMDA) antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) blocked light-induced phase advances and delays. Similarly, administration of the competitive NMDA receptor antagonist, 3(2-carboxypiperazin-4-yl)-propyl-l-phosphonic acid (CPP), prevented light-induced phase advances and delays. Neither drug by itself caused any consistent effect on the phase of the rhythm. These data provide further evidence that EAA receptors mediate the effects of light on the circadian system, and suggest that both NMDA and non-NMDA receptor types may be involved.

The effects of pentobarbital and benzodiazepines on GABA-responses in the periphery and spinal cord in vitro

Pentobarbital and benzodiazepines were compared in their interaction with the gamma-aminobutyric acid (GABA) antagonists picrotoxin and bicuculline on GABAA receptor-mediated events. On excised vagal nerves and dorsal roots pentobarbital, in contrast to the benzodiazepines diazepam, lorazepam and flurazepam, was able to enhance GABA-induced depolarizations recorded in the presence of picrotoxin or bicuculline. On hemicord preparations picrotoxin simultaneously depressed the electrically evoked dorsal root-dorsal root potential and enhanced the dorsal root-ventral root potential. Pentobarbital overcame the effects of picrotoxin, whereas diazepam and midazolam were without effect. These results may be explained by the suggestion that the GABA receptors in these test systems are not tightly associated with the benzodiazepine receptor activated by diazepam, lorazepam, midazolam and flurazepam, and correspond to the recently described GABAA2 subdivision of GABA receptors.

Quantitative autoradiographic analysis of excitatory amino acid receptors in the cat spinal cord

Using quantitative autoradiography, we have studied the density and distribution of N-methyl-D-aspartate (NMDA), kainate and AMPA receptors and the binding site for the sodium-dependent EAA transporter in sections from the cat spinal cord. NMDA, kainate and AMPA receptors were found in highest concentrations in laminae I and II of the dorsal horn. Lower levels of all receptors were seen in other regions of the spinal cord grey matter. The distribution of the sodium-dependent transporter was unlike that of any of the receptor populations with highest levels found in the ventral horn with slightly lower levels in other regions of grey matter. The pattern of binding sites was consistent throughout all levels of the spinal cord.

Potentiation of synaptic reflexes by D-serine in the rat spinal cord in vitro

6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) depressed dorsal root-evoked ventral and dorsal root potentials of the in vitro immature rat spinal cord to 26.3 +/- 5.2 S.E.M. and 40.8 +/- 2.7% of control values respectively. These depressant effects of CNQX were partially reversed by D-serine (EC50 values 39.7 microM +/- 8.7 S.E.M. N = 6 and 34.9 +/- 12.5 microM, N = 5 for ventral root potential and dorsal root potential respectively). Under our experimental conditions, which included the presence of Mg2+ (0.75 mM) in the bathing medium, no measurable potentiation of these synaptic reflexes by D-serine was recorded in the absence of CNQX. These data indicate that CNQX, in addition to its depressant effect at non-NMDA receptors, depresses an NMDA receptor-mediated component of segmental transmission through its action at the glycine site of the NMDA receptor complex.

Dorsal root potentials in the isolated frog spinal cord: amino acid neurotransmitters and magnesium ions

Sucrose gap techniques recorded dorsal root potentials evoked by supramaximal dorsal root stimulation in in vitro, hemisected frog spinal cords. In 0 mM Mg2+ large (mean 13.0 mV), long lasting (mean 8.1 s) dorsal root potentials were recorded which consisted of two components: (1) an early component sensitive to picrotoxin, bicuculline, and low [Cl-]o and presumably produced by activation of GABAA receptors; and (2) a long-duration second component enhanced and lengthened by picrotoxin, bicuculline and low [Cl-]o and thought to result from increased interneuron discharges resulting from depression of GABA-mediated pre- and postsynaptic inhibition. Both the early and late components were reduced by over 90% in amplitude and duration by 20 mM Mg2+ or by kynurenate and bicuculline. The early component of the dorsal root potential may depend mainly upon activation of non-N-methyl-D-aspartate receptors. Thus, the N-methyl-D-aspartate antagonist D-(-)-2-amino-5- phosphonovalerate caused only a modest reduction in the amplitude of the early dorsal root potential component while the non N-methyl-D-aspartate antagonist 6-cyano-7-nitroquinoxaline-2,3-dione caused a much more substantial reduction. Exposure of the spinal cord to a "physiological" concentration of Mg2+ (1.0 mM) greatly reduced the duration and somewhat reduced the amplitude of the dorsal root potential. The reduction of dorsal root potentials by 1.0 mM Mg2+ appears to be caused by both pre- and postsynaptic factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal kynurenate reduces arterial pressure, heart rate and baroreceptor-heart rate reflex in conscious rats

In the present study, an excitatory amino acid (EAA) pathway in the spinal cord which maintains sympathetic vasomotor tone in conscious rats has been investigated. To this end, the cardiovascular effects of an intrathecally administered EAA antagonist, kynurenate (KYN), were studied in conscious rats. KYN (0.5 mumol in 10 microliters) caused a dramatic reduction in mean arterial pressure (MAP) and heart rate (HR) that persisted for 2-3 h, and also resulted in extensor paralysis of the hindlimbs. The time courses of fall in MAP and HR and hindlimb paralysis were similar. Baroreceptor-HR reflex activity was also markedly impaired after KYN, suggesting functional diminution of sympathetic outflow at the level of the spinal cord after blockade of EAA receptors by KYN. Xanthurenate, a metabolite of KYN without EAA antagonistic properties, produced negligible effects at the same dose of KYN. While these findings do not identify the putative EAA pathway, they do provide the first demonstration that this system is tonically active in conscious rats.