Gastrocardiac afferent convergence in upper thoracic spinal neurons: a central mechanism of postprandial angina pectoris.

The aim of this study was to examine whether gastric afferent information converged onto upper thoracic spinal neurons that received noxious cardiac input. Extracellular potentials of single upper thoracic (T3) spinal neurons were recorded in pentobarbital-anesthetized, paralyzed, ventilated male rats. Gastric distension (GD) (20, 40, 60 mm Hg, 20 seconds) was produced by air inflation of a latex balloon surgically placed in the stomach. A catheter was placed in the pericardial sac to administer bradykinin solution (10 microg/mL, 0.2 mL, 1 minute) as a noxious cardiac stimulus. Noxious GD (> or =40 mm Hg) altered activity of 26 of 31 (84%) spinal neurons receiving cardiac input. Twenty-two (85%) gastrocardiac convergent neurons were excited, and 1 neuron was inhibited by both intrapericardial bradykinin and GD; the remainder exhibited biphasic response patterns. Twenty-three of 26 (88%) gastrocardiac neurons also received convergent somatic input from the chest, triceps, and upper back areas. Bilateral cervical vagotomy did not significantly affect excitatory responses to GD in 5 of 5 neurons tested. Spinal transection at the C1 segment after vagotomy did not affect excitatory responses to GD in 3 of 4 neurons but abolished the GD response in 1 neuron. These data showed that a gastric stimulus excited T3 spinal neurons with noxious cardiac input primarily by way of intraspinal ascending pathways. PERSPECTIVE: Convergence of gastric afferent input onto upper thoracic spinal neurons receiving noxious cardiac input that was observed in the present study may provide a spinal mechanism that explains stomach-heart cross-organ communication, such as postprandial triggering and worsening of angina pectoris in patients with coronary artery disease.     

Neuromodulation of thoracic intraspinal visceroreceptive transmission by electrical stimulation of spinal dorsal column and somatic afferents in rats.

Clinical studies have shown that neuromodulation therapies, such as spinal cord stimulation (SCS) and transcutaneous electrical nerve stimulation (TENS), reduce symptoms of chronic neuropathic and visceral pain. The neural mechanisms underlying SCS and TENS therapy are poorly understood. The present study was designed to compare the effects of SCS and TENS on spinal neuronal responses to noxious stimuli applied to the heart and esophagus. Direct stimulation of an intercostal nerve (ICNS) was used to simulate the effects of TENS. Extracellular potentials of left thoracic (T3) spinal neurons were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. SCS (50 Hz, 0.2 ms, 3-5 minutes) at a clinical relevant intensity (90% of motor threshold) was applied on the C1-C2 or C8-T1 ipsilateral spinal segments. Intercostal nerve stimulation (ICNS) at T3 spinal level was performed using the same parameters as SCS. Intrapericardial injection of bradykinin (IB, 10 microg/mL, 0.2 mL, 1 minute) was used as the noxious cardiac stimulus. Noxious thoracic esophageal distension (ED, 0.4 mL, 20 seconds) was produced by water inflation of a latex balloon. C1-C2 SCS suppressed excitatory responses of 16/22 T3 spinal neurons to IB and 25/30 neurons to ED. C8-T1 SCS suppressed excitatory responses of 10/15 spinal neurons to IB and 17/23 neurons to ED. ICNS suppressed excitatory responses of 9/12 spinal neurons to IB and 17/22 neurons to ED. These data showed that SCS and ICNS modulated excitatory responses of T3 spinal neurons to noxious stimulation of the heart and esophagus. PERSPECTIVE: Neuromodulation of noxious cardiac and esophageal inputs onto thoracic spinal neurons by spinal cord and intercostal nerves stimulation observed in the present study may help account for therapeutic effects on thoracic visceral pain by activating the spinal dorsal column or somatic afferents.     

Involvement of nicotinic and muscarinic receptors in synaptic transmission in cat superior cervical ganglions reinnervated by vagal primary afferent axons.

Artificial synapses were established in the superior cervical ganglion reinnervated by vagal afferent fibers by heterologous cross-anastomosis between the cranial end of nodose ganglion and the caudal end of superior cervical ganglion in cats. Formation of functional synapses was evidenced by unilateral mydriasis and contraction of the nictitating membrane in response to inflation of the stomach with a balloon or to electrical stimulation of the afferent vagus. Electron microscopic findings indicated that the vagal afferent fibers terminated in the superior cervical ganglion after cross-anastomosis. In the superior cervical ganglion reinnervated by the afferent vagus, activities of choline acetyltransferase and cholinesterase were higher than those in the preganglionically denervated ganglion, but lower than those in the sympathetic preganglionically reinnervated ganglion. Contractions of the nictitating membrane and postganglionic action potentials evoked by electrical stimulation of the vagal artificial preganglionic trunk in the cross-anastomosed ganglion were blocked by treatment with tetraethylammonium and also with atropine. Atropine did not affect these responses in the normal and the preganglionically reinnervated ganglion, except at an early stage after operation. Comparisons of pharmacological properties in normal, anastomosed, preganglionically denervated and reinnervated ganglia indicated that activation of muscarinic receptors in the anastomosed ganglia is probably not secondary to an incomplete nerve supply, but may be dependent on the nature of the nonmyelinated vagal afferent fibers. The possibility that the transmitter involved may be acetylcholine is discussed.     

Neural Control of the Lower Esophageal Sphincter INFLUENCE OF THE VAGUS NERVES

We performed studies in the opossum to define the influence of the vagi in the control of lower esophageal sphincter (LES) function. Bilateral vagotomy caused transient sphincter hypertension which was prevented by phentolamine and by atropine. Stimulation of the peripheral end of vagus, after bilateral cervical vagotomy, caused relaxation of the LES over a wide range of frequency and intensity of electrical stimulation. The relaxation was less marked at the lower frequencies of stimulation, and atropine treatment did not enhance this relaxation. In other experiments, atropine treatment reversed the rise in gastric (fundic) pressure with the vagal stimulation, but atropine did not enhance the degree of LES relaxation. Stimulation of the central end of the vagus caused an increase in LES pressure due to a centrally mediated reflex; the efferents for this motor response were not present in the vagi, as the reflex contraction persisted after bilateral vagotomy. The LES contraction with the stimulation of the vagal afferents was antagonized by phentolamine as well as by atropine. These studies suggest that: (a) the vagi do not mediate any cholinergic excitatory influences to the LES and the vagal influence of the sphincter is entirely inhibitory; (b) the vagi carry afferent fibres for a centrally mediated neural reflex which contracts the LES, but the efferent path of this reflex arc does not lie in the vagi.     

The role of 5-hydroxytryptamine3 receptors in the vagal afferent activation-induced inhibition of the first cervical dorsal horn spinal neurons projected from tooth pulp in the rat

To test the hypothesis that vagal afferent (VA) stimulation modulates the first cervical dorsal horn (C(1)) neuron activity, which is projected by tooth pulp (TP) afferent inputs through the activation of a local GABAergic mechanism via 5-hydroxytryptamine(3) (5-HT(3)) receptors, we used the technique of microiontophoretic application of drugs. In pentobarbital-anesthetized rats, we recorded C(1) spinal neuron activity responding to TP stimulation. The TP stimulation-evoked C(1) spinal neuron excitation was inhibited by VA stimulation, and this inhibition was significantly attenuated by iontophoretic application of the 5-HT(3) receptor antagonist ICS 205-930 (3-tropanyl-indole-3-carboxylate hydrochloride [endo-8-methyl-8-azabicyclo [3.2.1] oct-3-ol indol-3-yl-carboxylate hydrochloride]) (40 nA) or the GABA(A) receptor antagonist bicuculline (40 nA). In another series of experiments, we determined that 60 nA iontophoretic application of glutamate produced a maximal increase in the C(1) spinal neuron activity at a minimal current. In 53 of 65 neurons (81.5%), VA conditioning stimulation (1.0 mA x 0.1 ms, 50 Hz for 30 s) caused a significant inhibition (35.1%) of the glutamate (60 nA) application-evoked C(1) spinal neuron excitation. Iontophoretic application of ICS 205-930 (40 nA) or bicuculline (40 nA) significantly attenuated the VA stimulation-induced inhibition of glutamate iontophoretic application (60 nA)-evoked C(1) spinal neuron excitation. These results suggest that VA stimulation-induced suppression of C(1) spinal neuron activity, responding to TP stimulation, involve 5-HT(3) receptor activation, possibly originating in the descending serotonergic inhibitory system, and postsynaptic modulation of inhibitory GABAergic neurons.     

Effects of electrical stimulation of vagal afferents on spinothalamic tract cells in the rat.

The effects of electrical stimulation of cervical vagal afferents (VAS) on the background activity and on the responses of 25 spinothalamic tract (STT) neurons to noxious stimuli were studied in anesthetized rats. Background (spontaneous) activity of 9 (36%) STT neurons was inhibited by all intensities of VAS. 6 (24%) units were facilitated at lesser and inhibited at greater intensities of VAS, 5 (20%) units were only facilitated by all intensities of VAS, and 5 (20%) units were not affected by VAS. Responses of 8 (36%) STT neurons to noxious stimuli were only inhibited by VAS, 9 (41%) were facilitated at lesser and inhibited at greater intensities of VAS, and 5 units (23%) were only facilitated by VAS. There were no significant differences in VAS-produced modulatory effects between STT neurons and 16 unidentified lumbar spinal dorsal horn neurons studied under the same conditions. These results reveal that descending facilitatory and inhibitory pathways engaged by activation of vagal afferents modulate rostrally projecting nociceptive transmission neurons in the spinal cord, constituting an important regulatory network for nociception.     

Convergence of cutaneous and pelvic visceral nociceptive inputs onto primate spinothalamic neurons

The responses of 66 primate spinothalamic neurons to natural stimulation of the urinary bladder and testicle were studied with extracellular recording techniques in order to elucidate the neural basis for referral of visceral pain. Thirty-eight out of 53 cells located at the thoraco-lumbar junction or in sacral segments responded to noxious cutaneous stimuli, and 84% of these also exhibited phasic and/or tonic excitatory responses to distension of the urinary bladder. Seventeen out of 20 of these units, all located at the thoraco-lumbar junction, were excited by compression of the ipsilateral testicle. The response was graded with the compressive force. Excitatory responses to noxious heat and an irritant chemical (KCl) applied to the exposed testicular surface were also observed. Twelve sacral units having inputs from deep receptors of the tail exhibited mixed excitatory and inhibitory responses to bladder distension. A further 2 cells located at the thoracolumbar junction responded only to cutaneous tactile stimuli, and 13 cells located at the lumbosacral enlargement were tonically inhibited by bladder distension.It is concluded that spinothalamic neurons that convey nociceptive input from the skin may also respond to noxious visceral stimuli. Such viscerosomatic convergence provides a neural substrate for the phenomenon of cutaneous referral of visceral pain.     

Relationship between mechano-receptive fields of dorsal horn convergent neurons and the response to noxious immersion of the ipsilateral hindpaw in rats

This study examines the relationship between mechano-receptive fields (inhibitory and excitatory, located on the ipsilateral hindpaw) of convergent dorsal horn neurons, and the responses of the neurons to noxious immersion of an entire paw in noxious hot water. In pentobarbital anesthetized rats with intact spinal cords and in unanesthetized decerebrate-spinalized rats, rat hindpaws were immersed in 50°C water for 10 s after the mechano-receptive fields had been delineated using 5-s noxious pinches. Convergent neurons were either excited or inhibited by noxious immersion of the hindpaw. In both groups, a significant association (χ², P<0.01) was found between the make-up of the mechano-receptive field and the response of the neuron to immersion. Immersion-inhibited neurons (intact=27, spinalized=13), always had both an excitatory and an inhibitory mechano-receptive field on the same hindpaw. Additionally, when the hindpaw was removed from the noxious water, these immersion-inhibited cells displayed a strong afterdischarge which was immediately inhibited once the paw was reimmersed. Pinch-induced and immersion-induced inhibition were found in both spinalized and intact rats suggesting spinal mechanisms were sufficient to mediate this effect. The majority of immersion-excited cells showed only an excitatory mechano-receptive field on the hindpaw (intact rats=18/23 or 78.3%, spinalized rats=24/36 or 66.7%). However, other immersion-excited cells had both an inhibitory and an excitatory mechano-receptive field on the hindpaw (intact rats=5/23 or 21.7%, spinalized rats=12/36 or 33.3%). The response of a convergent neuron, which has its excitatory receptive field located on a paw, to noxious immersion of the entire paw can be predicted by the make-up of the mechano-receptive fields. Additionally, since noxious paw immersion affects ipsilateral convergent neurons in two opposite manners, it suggests that other effects, such as heterotopic actions, might also not be uniform.     

Serotonin alters multi-segmental convergence patterns in spinal cord deep dorsal horn and intermediate laminae neurons in an in vitro young rat preparation.

Each spinal neuron has a receptive field that corresponds to stimulation of a specific area of skin or subcutaneous tissue. Receptive fields are plastic and can be altered during development and injury but the actions of neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) on receptive field properties are not well known. We used stimulation of multiple adjacent dorsal root spinal segments as a measure of "receptive field size" to determine the effects of 5-HT on multi-segmental convergent input onto neurons in laminae IV-VII. Whole-cell patch-clamp recordings were undertaken in the in vitro hemisected thoracolumbar spinal cord of rats aged 8-10 days old. Based on synaptic responses, neurons could be divided into two predominant groups and 5-HT exerted different effects on these groups. The first group received excitatory post-synaptic potentials (EPSPs) from the homonymous dorsal root but inhibitory post-synaptic potentials (IPSPs) with increasing amplitude from more distant dorsal roots. In this group, 5-HT preferentially depressed the IPSPs from adjacent nerve roots while leaving the EPSP intact. The second group received short-latency EPSPs from all segments stimulated and 5-HT potently depressed all synaptic input. In both populations the depressant actions of 5-HT increased with dose (0.1-10.0 microM). Bicuculline and strychnine did not affect the 5-HT induced short-latency synaptic depression. These results suggest that descending serotonergic systems depress spinal sensory convergence in a graded and differentiated manner. The findings are discussed in relation to the modulation of nociceptive signaling.     

Physiological characteristics of the projection pathway from the medial preoptic to the nucleus raphe magnus of the rat and its modulation by the periaqueductal gray.

Anatomical studies have shown a strong projection from the medial preoptic nucleus of the hypothalamus (MPO) to both the periaqueductal gray (PAG) and nucleus raphe magnus (NRM). In this study, we examined the physiological characteristics of MPO to NRM connections and examined how blockade of neuronal transmission and of the glutamatergic system within the PAG modifies this pathway. In deeply anesthetized rats, recordings were made from NRM neurons that were identified by their response to peripheral mechanical stimulation and designated as "E", "I", or "N" if they were excited, inhibited, or not activated by noxious stimulation. In addition, cells were identified as spinally projecting if they could be antidromically activated by stimulation of the dorsolateral funiculus at the thoracic level. The responses of 204 NRM neurons to electrical and 87 cells to both chemical and electrical stimulation of MPO were recorded. The response of NRM neurons to MPO stimulation was highly dependent on the sensory class of these cells. Chemical stimulation of MPO inhibited 50% (16/32) and excited 16% (5/32) of the I-cells. In contrast, 23% (9/39) of the E-cells were inhibited and 49% (19/39) were excited by chemical stimulation of MPO. Electrical stimulation at intensities below 80 microA at 100Hz had similar effects on the two classes of cells; 62% (24/39) of the E-cells and 31% (10/32) of the I cells were excited, and 31% (12/39) of the E-cells and 59% (19/32) of the I-cells were inhibited. The excitatory response to chemical stimulation lasted for an average of 136.8+/-73.2s and inhibitory response lasted for an average of 143.8+/-102.1s. Electrical stimulation of MPO at 1Hz excited 27%, inhibited 3%, and had no effect on 70% of NRM cells. The mean latency to peak excitation was 9.6+/-6.6ms. Antidromic activation of MPO neurons by NRM stimulation showed an average latency of 6.3+/-3.4ms. Blocking the glutamatergic transmission within the PAG (by injecting kynurenic acid (KYN) into the PAG) blocked the inhibitory response of 40% (6/15) of the I-cells and inhibitory response of 43% (3/7) of the E-cells. The excitatory response of 27% (3/11) of the I-cells and the excitatory response of 14% (1/7) of the E-cells were blocked by kynurenic injection into the PAG. It is concluded that: (1) in response to chemical stimulation of MPO, the number of I-cells that were inhibited was more than three times the number of I-cells that were excited; in contrast, the number of E-cells that were excited was more than twice the number of E-cells that were inhibited. (2) The interaction between MPO and NRM can be modulated by blockade of the neuronal transmission or blockade of the glutamatergic system in the PAG. (3) Simultaneous activity of many synapses is required for activation of the MPO-NRM pathway. (4) MPO to NRM interaction is mediated by fibers with a conduction velocity of less than 1m/s.     

Gastric excitation by stimulation of the vagal trunk after chronic supranodose vagotomy in cats

Experiments were performed on cats anesthetized with thiopental sodium and gallamine triethiodide and ventilated artificially. Gastric motility was recorded by a balloon method. Electrical stimulation of the vagal trunk in cats with chronic supranodose vagotomy for 11 to 32 days caused an excitatory response of the stomach. The pulse duration of electrical stimulation to obtain a maximal excitatory response of the stomach was 3 msec. Administration of hexamethonium (10 mg/kg i.v.) did not inhibit but enhanced the excitatory response of stomach. Atropine (3, 10 and 30 micrograms/kg i.v.), hemicholinium (10 mg/kg i.v.) and morphine (5 mg/kg i.v.) inhibited this hexamethonium-resistant excitatory response of the stomach, whereas treatment with physostigmine (300 mu/kg i.v.) augmented it. A substance P antagonist, (D-Pro2, D-Trp7.9)-substance P (250 and 500 micrograms/kg i.v.), did not affect the hexamethonium-resistant excitatory response. Acetylcholine content of the nodose ganglion 6 to 8 days after supranodose vagotomy was assayed using the radioenzymatic method, and the level was about 48% that of the intact ganglion. These results suggest that the gastric excitatory response to stimulation of the supranodose denervated vagal trunk is produced by activation of vagal afferent fibers probably originating from the nodose ganglion; the fibers involved are cholinergic.     

Evidence that raphe-spinal neurons mediate opiate and midbrain stimulation-produced analgesias.

The present experiments were undertaken to assess the role of neurons in the nucleus raphe magnus (NRM) in mediating opiate and stimulation-produced analgesias. A cannode for both electric stimulation and local opiate microinjection was placed in the midbrain preiaqueductal gray region of decerebrate or chloralose-urethane anesthetized cats. Microelectrodes recorded the responses of medullary NRM neurons. Raphe-spinal neurons were identified by antidromic activation from the cerevical spinal cord. Fifteen of 20 raphe-spinal cells tested were excited by electrical stimulation of the midbrain. Of 49 NRM neurons tested, 26 were excited by either systemic or midbrain injection of opiate agonist. Twelve of 33 NRM cells tested by midbrain microinjection were excited. In 10 the effect was reversed by naloxone. Seventeen raphe-spinal neurons were tested; 5 showed naloxone-reversible excitation to either midbrain or intravenous injection of opiates. NRM neurons respond to auditory and somatic stimuli; at least half respond maximally to somatic stimuli of noxious intensity. These findings are consistent with the hypothesis that the raphe-spinal projection mediates opiate and electrical stimulation-produced effects from midbrain sites. The properties of raphe-spinal neurons suggest that they are part of a negative feedback system which monitors and limits the output of spinal dorsal horn pain-transmission neurons.     

Effects of opioid agonists on sympathetic and parasympathetic transmission to the dog heart

Effects of leu- and met-enkephalin, pentazocine and morphine on negative or positive chronotropic response to vagal nerve stimulation or cardiac sympathetic nerve stimulation were examined in anesthetized dogs in order to determine whether opioid receptors modulate vagal and sympathetic transmission. Leu- and met-enkephalin (10-100 micrograms/kg i.v.) and pentazocine (100-1000 micrograms/kg i.v.) inhibited bradycardic response to vagal nerve stimulation (1-4 Hz) in a dose-dependent manner. Morphine (300 and 1000 micrograms/kg i.v.) did not affect vagal bradycardia. The inhibitory effect of leu-enkephalin (30 micrograms/kg) and pentazocine (300 micrograms/kg) was effectively antagonized by naloxone (1000 micrograms/kg i.v.). Bradycardic response to intracoronary injection of methacholine (0.1, 0.3 and 1 microgram) into the right coronary artery was unaffected by leu-enkephalin (30 micrograms/kg). On the other hand, leu-enkephalin and pentazocine did not modify tachycardic response to sympathetic nerve stimulation (1-8 Hz). Morphine attenuated sympathetic tachycardia only slightly. These results suggest that presynaptic opioid receptors, probably delta type, are present in the vagus nerves, and that the activation of opioid receptors inhibit vagal transmission to the dog heart. In contrast, the presence of opioid receptors in the cardiac sympathetic nerves is not evident.     

Effects of opiate agonists and antagonists on central neurons of the cat.

Morphine, naloxone, nalorphine, levorphanol, dextrorphan and levallorphan were ejected electrophoretically from micropipettes near cholinoceptive and noncholinoceptive cells of the spinal cord, ventrobasal thalamus and cerebral cortex of decerebrate and barbiturate-anesthetized cats. Morphine excited those cells having nicotinic receptors for acetylcholine. Naloxone and nalorphine reduced the action of morphine and acetylcholine on these cells but not the effects of excitant amino acids. Levorphanol excited spinal neurons also excited by acetylcholine, an effect antagonized by naloxone, but also showed atropine-like activity when ejected for prolonged periods. Dextrorphan depressed the firing of both cholinoceptive and noncholinoceptive spinal neurons. Levallorphan reduced the effects of both acetylcholine and excitant amino acids on spinal neurons. The depressant effects of morphine and levorphanol on noncholinoceptive spinal neurons were not antagonized by naloxone.     

Sympathetic Control of Lower Esophageal Sphincter Function in the Cat: ACTION OF DIRECT CERVICAL AND SPLANCHNIC NERVE STIMULATION

The purpose of this study was to determine the effect of direct stimulation of the sympathetic nerves on the lower esophageal sphincter (LES) in the anesthetized cat. Neither unilateral nor bilateral cervical sympathectomy, or splanchnicectomy significantly modified basal LES pressure in animals with intact vagi, or animals having undergone bilateral cervical vagotomy. Electrical stimulation of the cut, peripheral, cervical sympathetic trunk increased mean arterial blood pressure, but had no effect on LES pressure or LES relaxation as induced by vagal stimulation. Stimulation of the central end of the cervical sympathetic trunk had no effect on LES pressure. Stimulation of the central end of the cut splanchnic nerve produced a decrease in LES pressure with a maximal response of 69.1+/-16.0% (mean+/-SEM). This inhibitory response was not modified by either propranolol or bilateral cervical vagotomy. Stimulation of the peripheral end of the cut, greater splanchnic nerve gave an increase in LES pressure with a maximal response of 38.2+/-7.19 mm Hg. Guanethidine, in the presence or absence of the adrenal glands, significantly augmented this excitatory response. This response was also slightly increased by phentolamine alone at 10 V, 1 Hz, but was not altered by propranolol. The excitatory response was completely antagonized by atropine or by trimethaphan camsylate. Stimulation of the peripheral end of the splanchnic nerve inhibited LES relaxation as induced by vagal stimulation. The results of this study suggest that: (a) the LES in the cat is not affected by either central or peripheral stimulation of the cervical sympathetic trunk; (b) the central portion of the splanchnic nerve carries an afferent inhibitory response to the LES through yet unknown pathways; (c) the peripheral splanchnic nerve carries an atropine-sensitive excitatory response to the LES; and (d) the splanchnic nerves may modulate LES relaxation as induced by vagal stimulation.     

Response of neurons in the thalamic nucleus submedius (Sm) to noxious stimulation and electrophysiological identification of on- and off-cells in rats

Previous studies have indicated that thalamic nucleus submedius (Sm) is involved in nociceptive modulation and plays an important role in an endogenous analgesic system (a feedback loop) consisting of spinal cord (Sc)-Sm-ventrolateral orbital cortex-periaqueductal gray-Sc. However, the function of different types of Sm neurons in nociceptive modulation is unclear. For this reason, on the basis of further studies of properties of the Sm neurons responding to noxious stimuli, the different effects of systemic morphine on the Sm neurons were examined and two classes of nociceptive modulatory neurons, named as off- and on-cells, in this region were identified in lightly anesthetized rats. The results showed that (1) most (84%, 132/157) of the Sm neurons responded to peripheral noxious stimuli. Of these neurons, 66% (n = 87) were inhibited, 34% (n = 45) excited. All neurons had very large and bilateral, even all body receptive fields. No neuron was found to be responsive to innocuous stimulation; (2) systemic morphine increased the firing rate of neurons inhibited by noxious stimulation, but decreased that of neurons excited by the same stimulation. Furthermore, the effects of morphine could be reversed by systemic naloxone; (3) 45 of Sm neurons examined could be divided into three different classes: off-cells that decreased the firing rate from tail heating just prior to occurrence of the tail-flick (TF) reflex (3140 +/- 167 ms, n = 27), on-cells that increased the firing rate just before the TF reflex (1720 +/- 240 ms, n = 8), and neutral-cells that did not respond to any stimuli and neuronal activities were not related to the TF reflex (n = 10). Findings of this study provided electrophysiological evidence for involvement of Sm neurons, as those in the rostral ventromedial medulla, in the opioid-receptor-mediated descending nociceptive modulation.     

Picrotoxin- and bicuculline-sensitive inhibition of cardiac vagal reflexes.

Transmission in the cardiac vagal reflex pathway can be inhibited by stimulation of the hypothalamic defense region or somatic afferent nerves. A pharmacological analysis of inhibitory modulation of reflex vagal bradycardia was undertaken in the present study. Picrotoxin (0.5--1.5 mg/kg i.v.) or bicuculline (0.5--1.5 mg/kg i.v.) produced a dose-related blockade of inhibition of reflex vagal bradycardia elicited by stimulation of the lateral hypothalamus or branches of the brachial plexus in spinal (C1 or C8 transected) cats. In contrast, strychnine and pentylenetetrazol failed to change the heart rate responses produced by stimulation of the hypothalamus or brachial plexus afferents. Picrotoxin and bicuculline also blocked inhibition of reflex vagal bradycardia produced by stimulation of the inferior olive in decerebrate spinal cats. This observation supports the contention that these agents act in the brain stem to block inhibitory modulation of reflex vagal bradycardia. In addition, picrotoxin and bicuculline lowered basal heart rate in spinal cats but not in decerebrate spinal cats. This observation suggests that tonic suprabulbar inhibition of reflex vagal bradycardia also is sensitive to blockade by picrotoxin and bicuculline.     

Hypocretin-1 (orexin-A) facilitates inhibitory and diminishes excitatory synaptic pathways to cardiac vagal neurons in the nucleus ambiguus

Hypocretin-1 is a neuropeptide recently shown to be involved in autonomic regulation. Hypocretin-1 is expressed by hypothalamic neurons, which project to many regions of the central nervous system, including the nucleus ambiguus. One possible site of action of hypocretin-1 could be cardioinhibitory parasympathetic vagal neurons within the nucleus ambiguus. This study examines whether hypocretin-1 modulates inhibitory and excitatory postsynaptic currents in cardiac vagal neurons in the rat nucleus ambiguus. GABAergic, glycinergic, and glutamatergic activity to cardiac vagal neurons was examined using whole-cell patch-clamp recordings in an in vitro brain slice preparation. Hypocretin-1 (1 microM) produced a significant increase in the frequency and amplitude of both GABAergic and glycinergic inhibitory postsynaptic currents and a significant decrease in the frequency of glutamatergic excitatory postsynaptic currents. Application of tetrodotoxin (0.5 microM) blocked all of the responses to hypocretin-1, indicating the changes in neurotransmission with hypocretin-1 do not occur at presynaptic terminals but rather occur at the preceding GABAergic, glycinergic, and glutamatergic neurons that project to cardiac vagal neurons. The increase in GABAergic and glycinergic inhibitory postsynaptic currents, and the decrease in glutamatergic excitatory postsynaptic currents, could be mechanisms by which hypocretin-1 affects heart rate and cardiac function.     

Nitrous oxide and the inhibitory synaptic transmission in rat dorsal horn neurons

The analgesic effect of nitrous oxide (N₂O) is thought to depend on noradrenaline release in the spinal cord following activation of descending inhibitory neurons. In addition to this indirect facilitation of inhibition in the spinal cord, we previously showed direct inhibition of glutamate receptors in dorsal horn neurons by N₂O. Since general anesthetics could possibly affect excitatory and/or inhibitory components of synaptic transmission, we sought to evaluate the direct effect of N₂O on inhibitory transmission in spinal cord neurons. Using whole‐cell patch‐clamp recording from rat transversal spinal cord slices, we investigated the actions of 50% N₂O and 0.5% isoflurane (both 0.3 rat MAC; minimum alveolar concentration) on exogenously applied γ‐aminobutyric acid (GABA)‐ and glycine‐induced currents in rat dorsal horn lamina II neurons. The amplitudes and integrated areas of GABA‐ and glycine‐induced currents were not significantly affected by N₂O, but were increased in the presence of isoflurane. N₂O did not affect the amplitude, frequency or decay time probability distribution of either GABA or glycine receptor‐mediated miniature postsynaptic currents. We further sought to determine the effect of N₂O on focal stimulation‐evoked synaptic currents mediated by GABA and glycine receptors, and found no effect in the majority of neurons. These and other findings suggest that N₂O has a discrete action in the spinal cord, distinct from the effects of the volatile anesthetics, consisting of inhibition of excitation in SG neurons through an action on ionotropic glutamatergic receptors and potentiation of inhibition through the descending noradrenergic system.     

MrgC agonism at central terminals of primary sensory neurons inhibits neuropathic pain

Chronic neuropathic pain is often refractory to current pharmacotherapies. The rodent Mas-related G-protein-coupled receptor subtype C (MrgC) shares substantial homogeneity with its human homologue, MrgX1, and is located specifically in small-diameter dorsal root ganglion neurons. However, evidence regarding the role of MrgC in chronic pain conditions has been disparate and inconsistent. Accordingly, the therapeutic value of MrgX1 as a target for pain treatment in humans remains uncertain. Here, we found that intrathecal injection of BAM8-22 (a 15-amino acid peptide MrgC agonist) and JHU58 (a novel dipeptide MrgC agonist) inhibited both mechanical and heat hypersensitivity in rats after an L5 spinal nerve ligation (SNL). Intrathecal JHU58-induced pain inhibition was dose dependent in SNL rats. Importantly, drug efficacy was lost in Mrg-cluster gene knockout (Mrg KO) mice and was blocked by gene silencing with intrathecal MrgC siRNA and by a selective MrgC receptor antagonist in SNL rats, suggesting that the drug action is MrgC dependent. Further, in a mouse model of trigeminal neuropathic pain, microinjection of JHU58 into ipsilateral subnucleus caudalis inhibited mechanical hypersensitivity in wild-type but not Mrg KO mice. Finally, JHU58 attenuated the miniature excitatory postsynaptic currents frequency both in medullary dorsal horn neurons of mice after trigeminal nerve injury and in lumbar spinal dorsal horn neurons of mice after SNL. We provide multiple lines of evidence that MrgC agonism at spinal but not peripheral sites may constitute a novel pain inhibitory mechanism that involves inhibition of peripheral excitatory inputs onto postsynaptic dorsal horn neurons in different rodent models of neuropathic pain.