Rostral ventral medulla 5-HT1A receptors selectively inhibit the somatosympathetic reflex

The role of the 5-hydroxytryptamine (5-HT1A) receptors in the rostral ventrolateral medulla (RVLM) on somatosympathetic, baroreceptor, and chemoreceptor reflexes was examined in anesthetized rats. Microinjection of the selective 5-HT1A agonist 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) decreased arterial blood pressure and splanchnic sympathetic nerve activity (SNA). Electrical stimulation of the hindlimb evoked early and late excitatory sympathetic responses. Bilateral microinjection in the RVLM of 8-OH-DPAT markedly attenuated both the early and late responses. This potent inhibition of the somatosympathetic reflex persisted even after SNA and arterial blood pressure returned to preinjection levels. Preinjection of the selective 5-HT1A antagonist NAN-190 in the RVLM blocked the sympathoinhibitory effect of 8-OH-DPAT and attenuated the inhibitory effect on the somatosympathetic reflex. 8-OH-DPAT injected in the RVLM did not affect baroreceptor or chemoreceptor reflexes. Our findings suggest that activation of 5-HT1A receptors in the RVLM exerts a potent, selective inhibition on the somatosympathetic reflex.     

Participation of GABA and glycine in the formation of somato-sympathetic reflexes]

Experiments were conducted on cats with intact spinal cord, and after spinalization a study was made of the action of strychnine and picrotoxin (synaptic GABA and glycine antagonists) on the components of somatosynaptic reflexes caused by stimulation of the skin and muscular intercostal afferents. Induced potentials were recorded in the white ramus of the third thoracic segment. The action of strychnine and picrotoxin on the values of different reflex components was not identical; this pointed to a different localization of the inhibitory action of GABA and glycine in the central link of the somatosympathetic reflex arch.     

Intrathecal melanin-concentrating hormone reduces sympathetic tone and blocks cardiovascular reflexes

Melanin-concentrating hormone (MCH) is a neuropeptide that acts to increase feeding behavior and decrease energy expenditure. The role of MCH in central cardiorespiratory regulation is still poorly understood. Experiments were conducted on urethane-anesthetized, vagotomized, and artificially ventilated male Sprague-Dawley rats (n = 22) to ascertain whether MCH modulates sympathetic vasomotor tone, as well as barosympathetic, chemosympathetic, and somatosympathetic reflexes at the level of the spinal cord. Intrathecal injection of 10 μl of MCH produced a dose-dependent hypotension, bradycardia, and sympathoinhibition. Peak response was observed following administration of 1 mM MCH, causing a decrease in mean arterial pressure of 39 ± 2 mmHg (P < 0.001), splanchnic sympathetic nerve activity of 78 ± 11% (P < 0.001), and heart rate of 87 ± 11 beats per minute (bpm) (P < 0.01). The two peaks of the somatosympathetic reflex were decreased by intrathecal MCH, 7 ± 3% (P < 0.01) and 31 ± 6% (P < 0.01), respectively, and the spinal component of the reflex was accentuated 96 ± 23% (P < 0.05), with respect to the baseline for MCH, compared with the two peaks and spinal component of the somatosympathetic reflex elicited following saline injection with respect to the baseline for saline. MCH decreased the sympathetic gain to 120 s of hyperoxic hypercapnea (10% CO(2) in 90% O(2)) and to 10-12 s poikilocapneic anoxia (100% N(2)) from 0.74 ± 0.14%/s to 0.23 ± 0.04%/s (P < 0.05) and 16.47 ± 3.2% to 4.35 ± 1.56% (P < 0.05), respectively. There was a 34% decrease in gain and a 62% decrease in range of the sympathetic baroreflex with intrathecal MCH. These data demonstrate that spinal MCH blunts the central regulation of sympathetic tone and adaptive sympathetic reflexes.     

Spinal mechanism of micturition reflex inhibition by naftopidil in rats

Aim

We investigated the spinal mechanism through which naftopidil inhibits the micturition reflex by comparing the effects of noradrenaline and naftopidil in rats.

Methods

The following were investigated: the influence of oral naftopidil on plasma monoamine and amino acid levels, the distribution of oral 14C-naftopidil, the effects of intravenous (IV) or intrathecal (IT) injection of noradrenaline or naftopidil on isovolumetric bladder contractions, amino acid levels in the lumbosacral spinal cord after IT noradrenaline or naftopidil, and the effects of IT naftopidil and strychnine and/or bicuculline on isovolumetric bladder contractions.

Key findings

Oral naftopidil decreased the plasma adrenaline level, while it increased the serotonin and glycine levels. After oral administration, 14C-naftopidil was detected in the spinal cord and cerebrum, as well as in plasma and the prostate gland. When the bladder volume was below the threshold for isovolumetric reflex contractions, IV (0.1 mg) or IT (0.1 μg) noradrenaline evoked bladder contractions, but IV (1 mg) or IT (0.01–1 μg) naftopidil did not. When the bladder volume was above the threshold for isovolumetric reflex contractions, IV or IT noradrenaline transiently abolished bladder contractions. IT noradrenaline decreased the levels of glycine and gamma-aminobutyric acid (GABA) in the lumbosacral cord, while IT naftopidil increased the GABA level. IT strychnine and/or bicuculline blocked the inhibitory effect of IT naftopidil on bladder contractions.

Significance

Naftopidil inhibits the micturition reflex by blocking α1 receptors, as well as by the activation of serotonergic, glycinergic, and GABAergic neurons in the central nervous system.     

Neuromedin U causes biphasic cardiovascular effects and impairs baroreflex function in rostral ventrolateral medulla of spontaneously hypertensive rat

Neuromedin U (NMU) causes biphasic cardiovascular and sympathetic responses and attenuates adaptive reflexes in the rostral ventrolateral medulla (RVLM) and spinal cord in normotensive animal. However, the role of NMU in the pathogenesis of hypertension is unknown. The effect of NMU on baseline cardiorespiratory variables in the RVLM and spinal cord were investigated in urethane-anaesthetized, vagotomized and artificially ventilated male spontaneously hypertensive rats (SHR) and Wistar–Kyoto rats (WKY). Experiments were also conducted to determine the effects of NMU on somatosympathetic and baroreceptor reflexes in the RVLM of SHR and WKY. NMU injected into the RVLM and spinal cord elicited biphasic response, a brief pressor and sympathoexcitatory response followed by a prolonged depressor and sympathoinhibitory response in both hypertensive and normotensive rat models. The pressor, sympathoexcitatory and sympathoinhibitory responses evoked by NMU were exaggerated in SHR. Phrenic nerve amplitude was also increased following intrathecal or microinjection of NMU into the RVLM of both strains. NMU injection into the RVLM attenuated the somatosympathetic reflex in both SHR and WKY. Baroreflex sensitivity was impaired in SHR at baseline and further impaired following NMU injection into the RVLM. NMU did not affect baroreflex activity in WKY. The present study provides functional evidence that NMU can have an important effect on the cardiovascular and reflex responses that are integrated in the RVLM and spinal cord. A role for NMU in the development and maintenance of essential hypertension remains to be determined.     

GABAB receptors in the NTS mediate the inhibitory effect of trigeminal nociceptive inputs on parasympathetic reflex vasodilation in the rat masseter muscle

The present study was designed to examine whether trigeminal nociceptive inputs are involved in the modulation of parasympathetic reflex vasodilation in the jaw muscles. This was accomplished by investigating the effects of noxious stimulation to the orofacial area with capsaicin, and by microinjecting GABA(A) and GABA(B) receptor agonists or antagonists into the nucleus of the solitary tract (NTS), on masseter hemodynamics in urethane-anesthetized rats. Electrical stimulation of the central cut end of the cervical vagus nerve (cVN) in sympathectomized animals bilaterally increased blood flow in the masseter muscle (MBF). Increases in MBF evoked by cVN stimulation were markedly reduced following injection of capsaicin into the anterior tongue in the distribution of the lingual nerve or lower lip, but not when injected into the skin of the dorsum of the foot. Intravenous administration of either phentolamine or propranolol had no effect on the inhibitory effects of capsaicin injection on the increases of MBF evoked by cVN stimulation, which were largely abolished by microinjecting the GABA(B) receptor agonist baclofen into the NTS. Microinjection of the GABA(B) receptor antagonist CGP-35348 into the NTS markedly attenuated the capsaicin-induced inhibition of MBF increase evoked by cVN stimulation, while microinjection of the GABA(A) receptor antagonist bicuculline did not. Our results indicate that trigeminal nociceptive inputs inhibit vagal-parasympathetic reflex vasodilation in the masseter muscle and suggest that the activation of GABA(B) rather than GABA(A) receptors underlies the observed inhibition in the NTS.     

Study of tonic and evoked electrical activity in efferent fibers of the sympathetic nerve in white rats

In anaesthetised Wistar rats, electrical sympathetic activity and a somatosympathetic reflex in the cervical sympathetic trunk elicited by a single electrical shock to forelimb or hindlimb afferent nerves, were recorded. The spontaneous activity was shown to conform with the pulse and respiratory waves of arterial pressure. Somatosympathetic reflex consists of early and late discharges evoked by somatic myelinated afferent fibres stimulation, and C-response elicited by stimulation of unmyelinated afferent fibres in spinal nerves.     

Differences in activation of the motoneurone inhibitory receptors in a frog Rana ridibunda by GABA and glycine and their interaction

The membrane potential responses evoked by GABA and glycine bath applications were studied intracellularly in the motoneurons of the isolated frog spinal cord. The amplitude of glycine-evoked responses was 1.5-2.0 larger than that of GABA-evoked response at the same concentration. EC50 were 0.75 mM and 1.57 mM for glycine and GABA, respectively. The amplitude of responses induced by the simultaneous applications of both agonists were 79.1 +/- 2.4% (n = 19) of the sum of individual responses and 130.1 +/- 1.5% (n = 19) of individual glycine-induced responses (partial occlusion). GABA-evoked responses were decreased by 85.3 +/- 0.2% (n = 10) as a result of glycine preliminary application while preapplication of GABA reduced glycine-evoked responses only by 52.9 +/- 0.3% (n = 11). Glycine- and GABA-evoked responses were selectively suppressed by strychnine and bicuculline, respectively. These results suggest that amphibian spinal motoneurones express (less specifically than those of mammals) both glycine (predominantly) and GABAA receptors, with asymmetric cross-inhibition possibly taking place in them.     

Activation of NTS A1 adenosine receptors inhibits regional sympathetic responses evoked by activation of cardiopulmonary chemoreflex

Previously we have shown that adenosine operating via the A(1) receptor subtype may inhibit glutamatergic transmission in the baroreflex arc within the nucleus of the solitary tract (NTS) and differentially increase renal (RSNA), preganglionic adrenal (pre-ASNA), and lumbar (LSNA) sympathetic nerve activity (ASNA>RSNA≥LSNA). Since the cardiopulmonary chemoreflex and the arterial baroreflex are mediated via similar medullary pathways, and glutamate is a primary transmitter in both pathways, it is likely that adenosine operating via A(1) receptors in the NTS may differentially inhibit regional sympathetic responses evoked by activation of cardiopulmonary chemoreceptors. Therefore, in urethane-chloralose-anesthetized rats (n = 37) we compared regional sympathoinhibition evoked by the cardiopulmonary chemoreflex (activated with right atrial injections of serotonin 5HT(3) receptor agonist phenylbiguanide, PBG, 1-8 μg/kg) before and after selective stimulation of NTS A(1) adenosine receptors [microinjections of N(6)-cyclopentyl adenosine (CPA), 0.033-330 pmol/50 nl]. Activation of cardiopulmonary chemoreceptors evoked differential, dose-dependent sympathoinhibition (RSNA>ASNA>LSNA), and decreases in arterial pressure and heart rate. These differential sympathetic responses were uniformly attenuated in dose-dependent manner by microinjections of CPA into the NTS. Volume control (n = 11) and blockade of adenosine receptor subtypes in the NTS via 8-(p-sulfophenyl)theophylline (8-SPT, 1 nmol in 100 nl) (n = 9) did not affect the reflex responses. We conclude that activation of NTS A(1) adenosine receptors uniformly inhibits neural and cardiovascular cardiopulmonary chemoreflex responses. A(1) adenosine receptors have no tonic modulatory effect on this reflex under normal conditions. However, when adenosine is released into the NTS (i.e., during stress or severe hypotension/ischemia), it may serve as negative feedback regulator for depressor and sympathoinhibitory reflexes integrated in the NTS.     

Somatosympathetic reflex in rats with various models of arterial hypertension

Spontaneous and reflex activities of sympathetic nerve were compared in animals with arterial hypertension of different aetiology. Reflex discharges elicited by single-shock stimulation of afferent fibres were recorded. In acute experiences on anaesthetized rats with renovascular and spontaneous (SHR) model of arterial hypertension, electric basal and evoked activity (somatosympathetic reflex) in cervical sympathetic trunk were recorded. It is shown, that the spontaneous electric activity in sympathetic nerve of hypertensive rats is larger than spontaneous activity of normotensive control animals. The somatosympathetic reflex in hypertensive rats differs from that of control animals. In rats with renovascular model of hypertension, the reflex magnitude is reduced, and in the SHR the reflex is increased. Time characteristics of the reflex in hypertensive rats differed among them. It is suggested that functional activities of the brain stem in rats with different arterial hypertension model are unequal.     

Sympathoinhibition from ventrolateral periaqueductal gray mediated by the caudal midline medulla

Activation of neurons in the ventrolateral region of the periaqueductal gray (vlPAG) can elicit a decrease in renal sympathetic nerve activity and blood pressure. The present study investigated whether the vlPAG-evoked sympathoinhibitory response depends on neurons in the caudal midline medulla (CMM). In pentobarbital-anesthetized rats, activation of neurons in the vlPAG evoked a decrease in renal sympathetic nerve activity to 29.4 +/- 4.8% below baseline levels and arterial blood pressure fell 8.9 +/- 1.6 mmHg (n = 20). Microinjection of the GABA agonist muscimol into sympathoinhibitory regions of the CMM significantly attenuated the vlPAG-evoked sympathoinhibition to 17.9 +/- 4.1% below baseline and the depressor response to 4.3 +/- 1.2 mmHg. At 65% (13/20) of the sites examined, the vlPAG-evoked sympathoinhibition was responsive to CMM muscimol microinjection and attenuated from 34.2% to 11.5%, with the depressor response reduced from 14.8 to 3 mmHg. Microinjection of muscimol at the remaining 35% of the CMM sympathoinhibitory sites was ineffective on the vlPAG-evoked sympathoinhibition and depressor response. These data indicate that sympathoinhibitory and hypotensive responses elicited by activation of neurons in the vlPAG can be mediated by neurons in the sympathoinhibitory region of the CMM. The finding that the vlPAG-evoked response is not affected by muscimol at all CMM sympathoinhibitory sites also suggests that sympathoinhibitory sites in the CMM are not homogeneous and can mediate functionally different responses.     

Trigeminal reflex regulation of the glottis depends on central glycinergic inhibition in the rat

In an unanesthetized decerebrate in situ arterially perfused brain stem preparation of mature rat, strychnine (0.05-0.2 microM) blockade of glycine receptors caused postinspiratory glottal constriction to occur earlier, shifting from early expiration to inspiration. This resulted in a paradoxical inspiratory-related narrowing of the upper airway. Stimulation of the trigeminal ethmoidal nerve (EN5; 20 Hz, 100 micros, 0.5-2 V) evoked a diving response, which included a reflex apnea, glottal constriction, and bradycardia. After strychnine administration, this pattern was converted to a maintained phrenic nerve discharge and a reduced glottal constriction that was interrupted intermittently by transient abductions. The onset of firing of postinspiratory neurons shifted from early expiration into neural inspiration in the presence of strychnine, but neurons maintained their tonic activation during EN5 stimulation, as observed during control. Inspiratory neurons that were hyperpolarized by EN5 stimulation in control conditions were powerfully excited after loss of glycinergic inhibition. Thus the integrity of glycinergic inhibition within the pontomedullary respiratory network is critical for the coordination of cranial and spinal motor outflows during eupnea but also for protective reflex regulation of the upper airway.     

Diet-induced obesity severely impairs myelinated aortic baroreceptor reflex responses

Diet-induced obesity (DIO) attenuates the arterial cardiac baroreceptor reflex, but the mechanisms and sites of action are unknown. This study tested the hypothesis that DIO impairs central aortic baroreceptor pathways. Normal chow control (CON) and high-fat-chow obesity-resistant (OR) and obesity-prone (OP) rats were anesthetized (inactin, 120 mg/kg) and underwent sinoaortic denervation. The central end of the aortic depressor nerve (ADN) was electrically stimulated to generate frequency-dependent baroreflex curves (5-100 Hz) during selective activation of myelinated (A-fiber) or combined (A- and C-fiber) ADN baroreceptors. A mild stimulus (1 V) that activates only A-fiber ADN baroreceptors induced robust, frequency-dependent depressor and bradycardic responses in CON and OR rats, but these responses were completely abolished in OP rats. Maximal activation of A fibers (3 V) elicited frequency-dependent reflexes in all groups, but a dramatic deficit was still present in OP rats. Activation of all ADN baroreceptors (20 V) evoked even larger reflex responses. Depressor responses were nearly identical among groups, but OP rats still exhibited attenuated bradycardia. In separate groups of rats, the reduced heart rate (HR) response to maximal activation of ADN A fibers (3 V) persisted in OP rats following pharmacological blockade of β(1)-adrenergic or muscarinic receptors, suggesting deficits in both parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) reflex pathways. However, the bradycardic responses to direct efferent vagal stimulation were similar among groups. Taken together, our data suggest that DIO severely impairs the central processing of myelinated aortic baroreceptor control of HR, including both PNS and SNS components.     

Analgesic action of nicotine on tibial nerve transection (TNT)-induced mechanical allodynia through enhancement of the glycinergic inhibitory system in spinal cord

The activation of cholinergic pathways by nicotine elicits various physiological and pharmacological effects in mammals. For example, the stimulation of nicotinic acetylcholine receptors (nAChRs) leads to an antinociceptive effect. However, it remains to be elucidated which subtypes of nAChR are involved in the antinociceptive effect of nicotine on nerve injury-induced allodynia and the underlying cascades of the nAChR-mediated antiallodynic effect. In this study, we attempted to characterize the actions of nicotine at the spinal level against mechanical allodynia in an animal model of neuropathic pain, tibial nerve transection (TNT) in rats. It was found that the intrathecal injection of nicotine, RJR-2403, a selective alpha4beta2 nAChR agonist, and choline, a selective alpha7 nAChR agonist, produced an antinociceptive effect on the TNT-induced allodynia. The actions of nicotine were almost completely suppressed by pretreatment with mecamylamine, a non-selective nicotinic antagonist, or dihydro-beta-erythroidine, a selective alpha4beta2 nAChR antagonist, and partially reversed by pretreatment with methyllycaconitine, a selective alpha7 nAChR antagonist. Furthermore, pretreatment with strychnine, a glycine receptor antagonist, blocked the antinociception induced by nicotine, RJR-2403, and choline. On the other hand, the GABAA antagonist bicuculline did not reverse the antiallodynic effect of nicotine. Together, these results indicate that the alpha4beta2 and alpha7 nAChR system, by enhancing the activities of glycinergic neurons at the spinal level, exerts a suppressive effect on the nociceptive transduction in neuropathic pain.     

Medullary pathways mediating the parasubthalamic nucleus depressor response

The parasubthalamic nucleus (PSTN) projects extensively to the nucleus of the solitary tract (NTS); however, the function of PSTN in cardiovascular regulation is unknown. Experiments were done in alpha-chloralose anesthetized, paralyzed, and artificially ventilated rats to investigate the effect of glutamate (10 nl, 0.25 M) activation of PSTN neurons on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA). Glutamate stimulation of PSTN elicited depressor (-20.4 +/- 0.7 mmHg) and bradycardia (-26.0 +/- 1.0 beats/min) responses and decreases in RSNA (67 +/- 17%). Administration (intravenous) of atropine methyl bromide attenuated the bradycardia response (46%), but had no effect on the MAP response. Subsequent intravenous administration of hexamethonium bromide blocked both the remaining bradycardia and depressor responses. Bilateral microinjection of the synaptic blocker CoCl(2) into the caudal NTS region attenuated the PSTN depressor and bradycardia responses by 92% and 94%, respectively. Additionally, prior glutamate activation of neurons in the ipsilateral NTS did not alter the magnitude of the MAP response to stimulation of PSTN, but potentiated HR response by 35%. Finally, PSTN stimulation increased the magnitude of the reflex bradycardia to activation of arterial baroreceptors. These data indicate that activation of neurons in the PSTN elicits a decrease in MAP due to sympathoinhibition and a cardiac slowing that involves both vagal excitation and sympathoinhibition. In addition, these data suggest that the PSTN depressor effects on circulation are mediated in part through activation of NTS neurons involved in baroreflex function.     

Primitive nervous systems: action of aminergic drugs and blocking agents on activity in the ventral nerve cord of the flatworm Notoplana acticola.

Electrically evoked activity in the submuscular ventral longitudinal nerve cords of Notoplana acticola is depressed by GABA and glycine in the presence of high magnesium concentrations. This inhibition occurs with 0.001--0.01 millimolar concentrations of these putative aminergic neurotransmitters and is reversible when washed out. The action of GABA and glycine was reversed nonspecifically by picrotoxin, bicuculline, and strychnine. PTZ (Pentylenetetrazole) was shown to mimic the effects that these blocking agents had on evoked activity when they were tested alone. The release of inhibition by these blocking agents is similar to that of decerebration. Three possible mechanisms responsible for synaptic activity in high Mg2+ concentrations are discussed and the possibility that the effector site of interaction may be the chloride ionophore is explored.     

Electrophysiological and pharmacological properties of single spinal neurons isolated from adult bullfrogs

1. We developed an isolated spinal cell preparation from adult bullfrogs. 2. The average resting membrane potential was -60 mV, and an action potential was activated by positive current injection. 3. The cells retained their tetrodotoxin-sensitive Na+ channels and at least two kinetically different types of K+ channel. 4. Under K(+)-free conditions, responses to GABA were blocked by bicuculline while responses to glycine, taurine or beta-alanine were blocked by strychnine. 5. The potency of excitatory amino acids decreased in the order: kainic acid greater than glutamate greater than NMDA. 6. These studies demonstrated that the isolated cells are applicable for electrophysiological and pharmacological investigations.     

Central inhibition of the aortic baroreceptors-heart rate reflex at the onset of spontaneous muscle contraction.

Animals decerebrated at the precollicular-premammillary body level exhibit spontaneous locomotion without any artificial stimulation. Our laboratory reported that the cardiovascular and autonomic responses at the onset of spontaneous locomotor events are evoked by central command, generated from the caudal diencephalon and the brain stem (Matsukawa K, Murata J, and Wada T. Am J Physiol Heart Circ Physiol 275: H1115-H1121, 1998). In this study, we examined whether central command and/or a reflex resulting from muscle afferents modulates arterial baroreflex function using a decerebrate cat model. The baroreflex was evoked by stimulating the aortic depressor nerve (ADN) at the onset of spontaneous muscle contraction (to test the possible influence of central command) and during electrically evoked contraction or passive stretch (to test the possible influence of the muscle reflex). When the ADN was stimulated at rest, heart rate and arterial blood pressure decreased by 40 +/- 2 beats/min and 11 +/- 1 mmHg, respectively. The baroreflex bradycardia was attenuated to 55 +/- 4% at the onset of spontaneous contraction. The attenuating effect on the baroreflex bradycardia was not observed at the onset and middle of electrically evoked contraction or passive stretch. The depressor response to ADN stimulation was identical among resting and any muscle interventions. The inhibition of the baroreflex bradycardia during spontaneous contraction was seen after beta-adrenergic blockade but abolished by muscarinic blockade, suggesting that the bradycardia is mainly evoked through cardiac vagal outflow. We conclude that central command, produced within the caudal diencephalon and the brain stem, selectively inhibits the cardiac component, but not the vasomotor component, of the aortic baroreflex at the onset of spontaneous exercise.     

Reduction in the mechanonociceptive response by intrathecal administration of glycine and related compounds

We have previously reported that enhanced glycine release is produced by epidural spinal cord stimulation, a clinical method for treating neuropathic pain. Our current hypothesis is that glycine administered intrathecally reduces neuropathic pain as measured by the Randall-Selitto method. Neuropathic rats created by unilateral partial ligation of the sciatic nerve were treated with intrathecal infusion of glycine, strychnine, MK-801, or 5,7-DKA at 0.1 μmol, or artificial CSF for 2 hours at a rate of 10 μl/min. Force required to produce the pain response was significantly increased after glycine administration and reduced using strychnine, a specific glycine receptor (Gly 1) antagonist. Strychnine blocked the response to glycine when infused together. Administration of the non-specific NMDA receptor MK-801 antagonist and 5,7-DKA, a specific glycine-NMDA receptor (Gly 2) antagonist, however, failed to block the response to glycine. Our results provide evidence for the use of glycine and related compounds to treat neuropathic pain.     

Evidence for GABA-mediated control of putative nociceptive modulating neurons in the rostral ventromedial medulla: iontophoresis of bicuculline eliminates the off-cell pause.

This study was undertaken to test the hypothesis that gamma-aminobutyric acid (GABA) is an endogeneous neurotransmitter regulating the activity of a class of putative nociceptive modulatory neurons (termed "off-cells") in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. Off-cells, which are believed to correspond to the RVM output neuron that inhibits nociceptive processing at the level of the spinal cord, exhibit an abrupt pause in firing that begins immediately prior to the occurrence of the tail flick response (TF), a nocifensive reflex evoked by application of noxious heat to the tail. Single-unit recording and iontophoretic techniques were used to examine the ability of the GABAA receptor antagonist bicuculline methiodide (BIC) to antagonize selectively the characteristic off-cell pause. Iontophoretic application of BIC (5-30 nA) blocked the TF-related pause in each of the off-cells tested. This effect of BIC was generally slow in onset, and outlasted the period of application by several minutes. BIC iontophoresis also eliminated the cyclic alternation between active and silent periods that is often displayed by off-cells in lightly anesthetized rats. BIC application did not have a consistent effect on the firing of two other classes of RVM neurons ("on-cells" and "neutral cells"). Iontophoretically applied BIC antagonized the inhibitory effect of iontophoretically applied GABA, but not that produced by glycine. The glycine receptor antagonist strychnine did not mimic the action of BIC on off-cell activity. These data demonstrate antagonism of a synaptically evoked response using iontophoretic application of BIC, and provide strong evidence that the inhibitory neurotransmitter GABA mediates the TF-related off-cell pause. Taken together with behavioral experiments demonstrating that a GABA-mediated inhibitory process within RVM is crucial in permitting execution of the TF response, the present observations point to the significant functional relevance of GABA transmission within RVM in modulation of nociception.