Responses Of Nociceptive Neurons Research Articles

The rostral ventromedial medulla mediates the facilitatory effect of microinjected orphanin FQ in the periaqueductal gray on spinal nociceptive transmission in rats

Single unit extracellular recordings from spinal dorsal horn neurons were obtained with glass micropipettes in pentobarbital-anesthetized rats. A total of 50 wide dynamic range (WDR) neurons were studied in 25 rats. Microinjection of orphanin FQ (OFQ, 0.1 μg/0.1 μl) (a potent endogenous ligand of the opioid receptor-like receptor (ORL-1)) into the ipsilateral ventrolateral parts of periaqueductal gray (vlPAG) significantly increased C-response and post-discharge activity in most of the WDR neurons. Pre-microinjection of lidocaine (4%) into the nucleus raphe magnus (NRM) (0.5 μl), ipsilateral nucleus reticularis gigantocellularis (NGC) (0.6 μl), or nucleus gigantocellularis pars alpha (NGCα) and nucleus reticularis paragigantocellularis lateralis (NPGL) (0.5 μl) markedly reduced intra-vlPAG microinjection of OFQ-induced facilitatory effects on nociceptive responses of WDR neurons. Furthermore, if the NRM and ipsilateral NGC were simultaneously pre-microinjected with lidocaine, the intra-vlPAG microinjection of OFQ-induced facilitation on nociceptive responses of WDR neurons was eliminated. Also, a similar effect was observed when all the NRM, ipsilateral NGC, NGCα and NPGL were blocked with lidocaine. No significant effect on nociceptive responses of WDR neurons per se was found after blocking the NRM, ipsilateral NGC, NGCα/NPGL, or all the NRM, ipsilateral NGC, and NGCα/NPGL with lidocaine. These results indicate that (1) the facilitatory effect evoked by microinjection of OFQ into the vlPAG on nociceptive responses of WDR neurons in the spinal dorsal horn is primarily mediated by the NRM and ipsilateral NGC; (2) the NRM, ipsilateral NGC, and NGCα/NPGL do not mediate tonic descending inhibition of the spinal dorsal horn neurons.

The medullary dorsal reticular nucleus enhances the responsiveness of spinal nociceptive neurons to peripheral stimulation in the rat.

Single-unit spinal recordings combined with application of glutamate into the medullary dorsal reticular nucleus were used to assess the action of this nucleus upon deep dorsal horn neurons in rats. Injection of high glutamate concentrations (10 and 100 mm) induced a dramatic and long-lasting increase of the responses of wide-dynamic range neurons to electrical stimulation of the sciatic nerve in the noxious range, without affecting ongoing discharges. Post-stimulus time histograms revealed that this increase concerned the post-discharge, but not A- or C-fibre-mediated responses, which remained unchanged independently of the stimulation frequency applied. The onset of the glutamate-induced response enhancement occurred with a concentration-dependent time delay and developed slowly until its maximum. These data indicate that the medullary dorsal reticular nucleus exerts a facilitating action upon deep dorsal horn wide-dynamic range neurons by enhancing their capacity to respond to peripheral stimulation through prolongation of their discharge. This action is accompanied by the strengthening of wind-up of deep dorsal horn wide-dynamic range neurons, hence providing a plausible substrate for chronic pain states. These results are in agreement with previous behavioural studies suggesting a pronociceptive role for the dorsal reticular nucleus [Almeida et al. (1996) Brain Res. Bull., 39, 7-15; Almeida et al. (1999) Eur. J. Neurosci., 11, 110-122], and support the involvement of a reverberating circuit, previously described in morphological studies [Almeida et al. (1993) Neuroscience, 55, 1093-1106; Almeida et al. (2000) Eur. J. Pain, 4, 373-387], which probably operates only at a certain threshold of activation.

Reticular thalamic responses to nociceptive inputs in anesthetized rats

The present study compares nociceptive responses of neurons in the reticular thalamic nucleus (RT) to those of the ventroposterior lateral nucleus (VPL). Extracellular single-unit activities of cells in the RT and VPL were recorded in anesthetized rats. Only units with identified tactile receptive fields in the forepaw or hindpaw were studied. In the first series of experiments, RT and VPL responses to pinching with a small artery clamp were tested with the rats under pentobarbital, urethane, ketamine, or halothane anesthesia. Under all types of anesthesia, many RT units were inhibited. Second, the specificity of the nociceptive response was tested by pinching and noxious heating of the unit’s tactile receptive field. Of the 39 VPL units tested, 20 were excited by both types of noxious stimuli. In sharp contrast, of the 30 RT units tested, none were excited and 17 were inhibited. In a third series of experiments, low-intensity and beam-diffused CO 2 laser irradiation was used to activate peripheral nociceptive afferents. Wide-dynamic-range VPL units responded with short- and long-latency excitations. In contrast, RT units had short-latency excitation followed by long-latency inhibition. Nociceptive input inhibited RT units in less than 500 ms. We conclude that a significant portion of RT neurons were polysynaptically inhibited by nociceptive inputs. Since all the cells tested were excited by light tactile inputs, the somatosensory RT may serve in the role of a modality gate, which modifies (i.e. inhibits) tactile inputs while letting noxious inputs pass.

Responses of diencephalic nociceptive neurones to orofacial stimuli and effects of internal capsule stimulation in the rat

The effect of conditioning stimulation of the internal capsule on nociceptive neurones in the rat diencephalon was investigated. The animals were anaesthetised with N 2O/O 2 (2:1) and 0.5% halothane, and immobilised with pancuronium bromide. Nociceptive neurones responding to noxious stimulation of the face and oral structures were recorded in the ventral posteromedial nucleus, posterior group and zona incerta. These neurones were classified into wide dynamic range and nociceptive-specific types. Functional segregation of these nociceptive neurones was not apparent within the nucleus or between nuclei. A test stimulus with a single rectangular pulse (5–70 V) was applied to the centre of the receptive field; the nociceptive neurones exhibited short- and/or long-latency responses. Both responses in about 45% of the nociceptive neurones were inhibited by conditioning stimuli to the contralateral internal capsule with trains of 33 pulses (300 μA) at 330 Hz. The percent inhibitory effects on the nociceptive neurones of each area were 68.0±14.8% ( n=6) in the ventral posteromedial nucleus, 72.8±12.4% ( n=4) in the posterior group and 61.5±7.5% ( n=4) in the zona incerta. Effective sites for conditioning stimulation were concentrated in the lateral side of the internal capsule, through which the corticofugal fibres from the somatosensory cortex pass. These findings suggest that the transmission of nociceptive information to the diencephalon is modulated by stimulation of the internal capsule at the level of the trigeminal sensory complex in the brainstem. They might provide a novel way to elucidate the neurophysiological basis for antinociception by stimulation of the internal capsule observed in clinical studies.

Central sensitization of nociceptive neurons in trigeminal subnucleus oralis depends on integrity of subnucleus caudalis.

Our recent studies have shown that application to the tooth pulp of the inflammatory irritant mustard oil (MO) produces a prolonged (>40 min) "central sensitization" reflected in neuroplastic changes in the mechanoreceptive field (RF) and response properties of nociceptive brain stem neurons in subnuclei oralis (Vo) and caudalis (Vc) of the trigeminal spinal tract nucleus. In view of the previously demonstrated ascending modulatory influence of Vc on Vo, our aim was to determine whether the Vo neuroplastic changes induced by MO application to the tooth pulp depend on an ascending influence from Vc. In chloralose/urethan-anesthetized rats, MO application to the pulp produced significant increases in Vo nociceptive neuronal orofacial RF size and responses to mechanical noxious stimuli that lasted as long as 40-60 min. These changes were not affected by vehicle (saline) microinjected into Vc at 20 min after MO application, but 0.3 microl of a 5 mM CoCl(2) solution microinjected into the ipsilateral Vc produced a reversible blockade of the MO-induced Vo neuroplastic changes. A similar volume and concentration of CoCl(2) solution injected into subnucleus interpolaris of the trigeminal spinal tract nucleus did not affect the MO-induced neuroplastic changes in Vo. These findings indicate that inflammatory pulp-induced central sensitization in Vo is dependent on the functional integrity of Vc.

Calcium-calmodulin-dependent protein kinase II contributes to spinal cord central sensitization.

Calcium/calmodulin-dependent protein kinase II (CaMK II) is found throughout the CNS. It regulates calcium signaling in synaptic transmission by phosphorylating various proteins, including neuronal membrane receptors and intracellular transcription factors. Inflammation or injuries to peripheral tissues cause long-lasting increases in the responses of central nociceptive neurons to innocuous and noxious stimuli. This change can occur independently of alterations in the responsiveness of primary afferent neurons and has been termed central sensitization. Central sensitization is a form of activity-dependent plasticity and results from interactions in a set of intracellular signaling pathways, which modulate nociceptive transmission. Here we demonstrate an increased expression and phosphorylation of CaMK II in rat spinal dorsal horn neurons after noxious stimulation by intradermal injection of capsaicin. Local administration of a CaMK II inhibitor in the spinal cord significantly inhibits the enhancement of responses of spinal nociceptive neurons and changes in exploratory behavior evoked by capsaicin injection. In addition, spinal CaMK II activity enhances phosphorylation of AMPA receptor GluR1 subunits during central sensitization produced by capsaicin injection. This study reveals that CaMK II contributes to central sensitization in a manner similar to its role in the processes underlying long-term potentiation.

Calcium–Calmodulin-Dependent Protein Kinase II Contributes to Spinal Cord Central Sensitization

Calcium/calmodulin-dependent protein kinase II (CaMK II) is found throughout the CNS. It regulates calcium signaling in synaptic transmission by phosphorylating various proteins, including neuronal membrane receptors and intracellular transcription factors. Inflammation or injuries to peripheral tissues cause long-lasting increases in the responses of central nociceptive neurons to innocuous and noxious stimuli. This change can occur independently of alterations in the responsiveness of primary afferent neurons and has been termed central sensitization. Central sensitization is a form of activity-dependent plasticity and results from interactions in a set of intracellular signaling pathways, which modulate nociceptive transmission. Here we demonstrate an increased expression and phosphorylation of CaMK II in rat spinal dorsal horn neurons after noxious stimulation by intradermal injection of capsaicin. Local administration of a CaMK II inhibitor in the spinal cord significantly inhibits the enhancement of responses of spinal nociceptive neurons and changes in exploratory behavior evoked by capsaicin injection. In addition, spinal CaMK II activity enhances phosphorylation of AMPA receptor GluR1 subunits during central sensitization produced by capsaicin injection. This study reveals that CaMK II contributes to central sensitization in a manner similar to its role in the processes underlying long-term potentiation.

Plastic changes in nociceptive transmission of the rat spinal cord with advancing age.

To understand characteristics of the pain system in the elderly, we investigated the electrophysiological properties of nociceptive neurons in the lumbar spinal dorsal horn of aged (29-34-mo old) and adult (7-13-mo old) rats. The responses of nociceptive neurons to noxious thermal stimulation, as well as the spontaneous firing rate, were significantly higher in the aged as compared with adult rats. Furthermore, the size of the high-threshold receptive field area of wide dynamic range neurons was larger (P < 0.01) and that of the low-threshold area was smaller (P < 0.05) in aged rats than in adult rats. The increased nociceptive neuronal activity in the aged rats correlated with the finding that the paw withdrawal latency was significantly shorter in the aged rats than those of the adult rats following heat stimulation of the hind paw (P < 0.05). Reversible local anesthetic block of descending pathways resulted in a dramatic increase in neuronal activity in adult rats but had little effect in aged rats. There was also a significant loss of serotoninergic and noradrenergic fibers in the spinal dorsal horn of the aged rats. These results demonstrate an age-related plasticity in spinal nociceptive processing that is related to impairment of descending modulatory pathways.

Superficial and deep convergent nociceptive neurons are differentially affected by N-methyl- D-aspartate applied on the brainstem surface of the rat medullary dorsal horn

The activation of N-methyl- D-aspartate receptors is implicated in the spinal and trigeminal processing of nociceptive information conveyed by convergent (wide dynamic range) neurons and particularly in C-fiber-evoked responses elicited by repetitive and high-intensity electrical stimulation of the neuronal receptive field. In this study, the effects of intrathecal NMDA application on the electrically evoked nociceptive responses of trigeminal subnucleus caudalis convergent neurons have been investigated. The total C-fiber-evoked activity triggered by 30 successive stimuli was divided into two components: the C-fiber input response and the ‘wind-up’ response. Application of 0.1 μg (in 50 μl) of NMDA evoked a bi-directional effect on the total C-fiber-evoked activity of 19 neurons tested. A significant increase in the total C-fiber-evoked activity was observed 15–25 min after the NMDA application for nine neurons located in superficial laminae II and III. In contrast, a significant decrease in the total C-fiber-evoked activity was observed 5–25 min after the NMDA application for 10 neurons located more deeply, in lamina V. The NMDA-induced modifications seen in the total C-fiber-evoked activity were likely a reflection of a significant increase or decrease in neuronal activity evoked by the C-fiber input rather than wind-up of the responses since the latter was not significantly modified by the NMDA application. These results provide evidence for a possible inhibitory role for NMDA-dependent interneurons of the superficial laminae of the medullary dorsal horn on the nociceptive activity of deep convergent neurons.

Selective depression of nociceptive responses of dorsal horn neurones by SNC 80 in a perfused hindquarter preparation of adult mouse

Detailed electrophysiological characterisation of spinal opioid receptors in the mouse has been limited due to various technical difficulties. In this study, extracellular single unit recordings were made from dorsal horn neurones in a perfused spinal cord with attached trunk-hindquarter to investigate the role of δ-opioid receptor in mediating nociceptive and non-nociceptive transmission in mouse. Noxious electrical shock, pinch and heat stimuli evoked a mean response of 20.8±2.5 ( n=10, P<0.005), 30.1±5.4 ( n=58, P<0.005) and 40.9±6.3 ( n=29, P<0.005) spikes per stimulus respectively. In 5 of 22 cells, repetitive noxious electrical stimuli applied to the hindpaw for 20 s produced a progressive increase in spike number, the phenomenon known as ‘wind-up’ and/or hyperactivity. When the selective δ-opioid receptor agonist (+)-4-[(α R)-α-((2 S,5 R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]- N, N-diethylbenzamide (SNC 80) was perfused for 8–10 min, these evoked nociceptive responses were reversibly depressed. SNC 80 (2 μM) depressed the nociceptive responses evoked by electrical shock, pinch and heat by 74.0±13.7% ( n=8, P<0.01), 66.5±16.6% ( n=10, P<0.01) and 74.1±17.0% ( n=10, P<0.01) respectively. The maximum depression by 5 μM SNC 80 was 92.6±6.8% ( n=3). SNC 80 at 5 μM also completely abolished the wind-up and/or hypersensitivity ( n=5). The depressant effects of SNC 80 on the nociceptive responses were completely blocked by 10 μM naloxone ( n=5) and 3 μM 17-(cyclopropylmethyl)-6,7-dehydro-4,5α-epoxy-14β-ethoxy-5β-methylindolo [2′,3′:6′,7′] morphinan-3-ol hydrochloride (HS 378, n=8), a novel highly selective δ-opioid receptor antagonist. Interestingly, HS 378 (3 μM) itself potentiated the background activity and evoked responses to pinch and heat by 151.8±38.4% ( P<0.05, n=8), 34.2±6.1% ( P<0.01, n=7) and 45.5±11.8% ( P<0.05, n=5) respectively. In contrast, the responses of non-nociceptive dorsal horn neurones were not inhibited by SNC 80 at a dose of up to 10 μM ( n=5). These data demonstrate that δ-opioid receptor modulate nociceptive, but not non-nociceptive, transmission in spinal dorsal horn neurones of the adult mouse. The potentiation of neuronal activity by HS 378 may reflect an autoregulatory role of the endogenous δ-opioid in nociceptive transmission in mouse.

Enhanced responses of spinal dorsal horn neurons to heat and cold stimuli following mild freeze injury to the skin.

The effects of a mild freeze injury to the skin on responses of nociceptive dorsal horn neurons to cold and heat stimuli were examined in anesthetized rats. Electrophysiological recordings were obtained from 72 nociceptive spinal neurons located in the superficial and deep dorsal horn. All neurons had receptive fields (RFs) on the glabrous skin of the hindpaw, and neurons were functionally divided into wide dynamic range (WDR) and high-threshold (HT) neurons. Forty-four neurons (61%) were classified as WDR and responded to both innocuous and noxious mechanical stimuli (mean mechanical threshold of 12.8 +/- 1.6 mN). Twenty-eight neurons (39%) were classified as HT and were excited only by noxious mechanical stimuli (mean mechanical threshold of 154.2 +/- 18.3 mN). Neurons were characterized for their sensitivity heat (35 to 51 degrees C) and cold (28 to -12 degrees C) stimuli applied to their RF. Among WDR neurons, 86% were excited by both noxious heat and cold stimuli, while 14% responded only to heat. For HT neurons, 61% responded to heat and cold stimuli, 32% responded only to noxious heat, and 7% responded only to noxious cold. Effects of a mild freeze injury (-15 degrees C applied to the RF for 20 s) on responses to heat and cold stimuli were examined in 30 WDR and 22 HT neurons. Skin freezing was verified as an abrupt increase in skin temperature at the site of injury due to the exothermic reaction associated with crystallization. Freezing produced a decrease in response thresholds to heat and cold stimuli in most WDR and HT neurons. WDR and HT neurons exhibited a mean decrease in response threshold for cold of 9.0 +/- 1.3 degrees C and 10.0 +/- 1.6 degrees C, respectively. Mean response thresholds for heat decreased 4.0 +/- 0.4 degrees C and 4.3 +/- 1.3 degrees C in WDR and HT neurons, respectively. In addition, responses to suprathreshold cold and heat stimuli increased. WDR and HT neurons exhibited an 89% and a 192% increase in response across all cold stimuli, and a 93 and 92% increase in responses evoked across all heat stimuli, respectively. Our results demonstrate that many spinal neurons encode intensity of noxious cold as well as noxious heat over a broad range of stimulus temperatures. Enhanced responses of WDR and HT neurons to cold and heat stimuli after a mild freeze injury is likely to contribute to thermal hyperalgesia following a similar freeze injury in humans.

Orphanin FQ produces gender-specific modulation of trigeminal nociception: behavioral and electrophysiological observations

The present study aimed to determine if orphanin FQ, an endogenous ligand for the opioid receptor like-1 receptor, produces gender-specific effects in the modulation of N-methyl- D-aspartate (NMDA)-evoked responses of trigeminal nociceptive neurons, and in the NMDA-induced nociceptive behavior. Single-unit extracellular recordings were made from nociceptive-specific and wide dynamic range neurons in the superficial and deeper dorsal horn of the medulla (trigeminal nucleus caudalis) in anesthetized (1.5 g/kg urethane) rats. In the proestrous female, orphanin FQ applied microiontophoretically produced facilitation of the NMDA-evoked responses in 50% (16/32) of nociceptive neurons, inhibition in 31% (10/32), and biphasic effects in 19% (6/32). In contrast, in the male, it inhibited the responses in 86% (18/21), and facilitated the responses in 14% (4/21). In ovariectomized animals, orphanin FQ inhibited the responses in 75% (9/12) of nociceptive neurons, facilitated the responses in 17% (2/12) and produced biphasic effects in 8% (1/12). In contrast, in estradiol-treated ovariectomized rats, it facilitated the responses in 46% (5/11), inhibited the responses in 36% (4/11) and produced biphasic effects in 18% (2/11). For behavioral studies, NMDA-induced scratching behavior was used to assess the effects of orphanin FQ. Twenty-eight male, ovariectomized and estradiol-treated ovariectomized rats were microinjected with NMDA (2 nmol in 10 μl) alone through a cannula implanted in the medullary region, while another 27 rats were microinjected with orphanin FQ (10 nmol in 10 μl) 10 min prior to giving NMDA. Orphanin FQ reduced the NMDA-induced nociceptive scratching behavior by 92% in the male, and by 96% in ovariectomized rats. In contrast, in estradiol-treated ovariectomized animals, orphanin FQ facilitated the NMDA-induced scratching behavior by 210%. We conclude from these studies that orphanin FQ is primarily pronociceptive in the female and primarily antinociceptive in the male. Furthermore, we suggest that estrogen is involved in generating the gender-specific effects of orphanin FQ.

Sensory processing in the deep spinal dorsal horn of neurokinin-1 receptor knockout mice.

The neurokinin-1 receptor and its primary ligand, substance P, are widely recognized as contributing to the spinal processing of nociceptive stimuli, yet the specific function of the neurokinin-1 receptor remains unclear. To better clarify these functions, the authors examined the neurophysiologic responses of L4-L5 neurons in the deep dorsal horn to acute mechanical, thermal, and electrical stimuli in knockout and wild-type mice. In addition, the capacity of knockout and wild-type mice to show wind-up to repeated C-fiber stimuli and to show sensitization after cutaneous mustard oil was assessed. A total of 68 nociceptive neurons (35 in knockout, 33 in wild type) in laminae III-V were studied. No differences in the acute responses of neurons in knockout and wild-type mice to graded mechanical, thermal, or electrical stimuli or in the acute responses to mustard oil were observed. However, wind-up to repeated electrical stimulation at C-fiber intensity was significantly attenuated in the knockout mice compared with wild type controls. In addition, mustard oil-induced mechanical hypersensitivity was significantly reduced in the knockout mice. These results indicate that neurokinin-1 receptors do not play a significant role in the responses of nociceptive neurons in the deep spinal dorsal horn to acute noxious mechanical, thermal, electrical, or chemical stimuli. On the other hand, neurokinin-1 receptors are critical for the central hyperexcitability that is observed in these neurons with repeated C-fiber inputs and to the central sensitization induced by topical mustard oil application.

Modulation of pain by [1DMe]NPYF, a stable analogue of neuropeptide FF, in neuropathic rats

The pain modulatory effects of ( d-Tyr) l(Me-Phe)QPQRF-amide ([1DMe]NPYF), a stable analogue of neuropeptide FF were studied in rats with a chronic neuropathy induced by unilateral ligation of two spinal nerves. According to behavioral assessments, intrathecal (i.t.) administration of [1DMe]NPYF induced mechanical antiallodynic and thermal antinociceptive effects in a parallel and dose-dependent fashion, whereas following administration in the periaqueductal gray (PAG) it produced only mechanical antiallodynia. I.t. or PAG administration of FLFQPQRF, a non-amidated form of NPFF, or intraplantar injection of [1DMe]NPYF into the neuropathic paw had no effects. Electrophysiological results indicated that administration of [1DMe]NPYF suppressed responses of nociceptive spinal dorsal horn neurons in a submodality selective way and without an effect on their spontaneous activity; PAG administration predominantly suppressed brush-evoked responses and i.t. administration heat-evoked responses. The descending inhibitory effect by conditioning electrical stimulation of the PAG was enhanced by i.t. administration of [1DMe]NPYF. The reversibility of [1DMe]NPYF-induced effects by naloxone (1 mg/kg subcutaneously) depended on the submodality of test stimulation and the route of drug administration. The amplitude of the innocuous H-reflex was not changed by [1DMe]NPYF administered i.t. in control rats. The present results indicate that [1DMe]NPYF produces a selective attenuation of pain in neuropathic animals due to naloxone-sensitive or -insensitive central mechanisms depending on the submodality of pain and route of drug administration. The amide-group is essential for the [1DMe]NPYF-induced attenuation of pain.

Actions of the systemically active metabotropic glutamate antagonist MPEP on sensory responses of thalamic neurones

It is known that metabotropic glutamate receptors of the subtypes mGlu1 and mGlu5 participate in nociceptive processing in the thalamus, an area of prime importance in supra-spinal sensory processing. Antagonists of these receptors thus have potential as centrally-acting analgesics. We have investigated whether intravenous administration of the novel mGlu5-receptor antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP) is able to reduce nociceptive responses of thalamic neurones. Extracellular recordings were made from single thalamic neurones of adult male Wistar rats anaesthetised with urethane. MPEP (1 mg/kg) reduced neuronal responses to noxious thermal stimuli to a mean of 24±4% of control within 10 min, whereas saline injections had no significant effect. Partial recovery was seen within 30–45 min after injection. Responses of neurones to non-noxious stimuli were not significantly affected by MPEP administration. In addition, MPEP caused an increase in the power of the slow-wave component (<1 Hz) of the electroencephalogram (EEG), but had no significant effect on peak frequency of the EEG or on heart rate. These results confirm that nociceptive responses of thalamic neurones are mediated in part by mGlu5 receptors. Furthermore, the effectiveness of intravenous MPEP suggests that such antagonists may be useful as centrally-acting analgesics.

A fMRI study of brain activations during non-noxious and noxious electrical stimulation of the sciatic nerve of rats

An acute pain animal model for fMRI study would provide useful spatial and temporal information for studying the supraspinal nociceptive neuronal responses. The aim of the present study was to investigate whether the nociceptive responses in different brain areas can be differentiated by using functional magnetic resonance imaging (fMRI) in anesthetized rats. Functional changes in brain regions activated by noxious or non-noxious stimuli of the sciatic nerve were investigated using fMRI in a 4.7 T MR system in α-chloralose anaesthetized rats. To determine the electrical intensity for noxious and non-noxious stimuli, compound action potential recording was employed to reveal the type of fibers activated by graded electrical stimulation of sciatic nerve. It showed that innocuous A-β fibers were excited by two times the muscle twitch threshold and nociceptive A-δ and C fibers were recruited and excited by 10 and 20 times threshold, respectively. A series of four-slice gradient echo images were acquired during innocuous (two times threshold) and noxious (10 and 20 times threshold) stimuli in a 4.7 T MR system. Contralateral somatosensory cortex was the most prominent brain area activated by innocuous stimuli. Both signal intensity and activated areas were significantly increased in the somatosensory cortex, cingulate cortex, medial thalamus and hypothalamus during noxious stimuli. These four brain areas activated by noxious stimuli were significantly suppressed by prior intravenous injection of morphine (5 mg/kg). The present findings demonstrated that the difference of the innocuous and nociceptive responses in the brain could be detected and localized by an in vivo spatial map using fMRI. Results suggest that fMRI may be an invaluable tool for studying pain in anesthetized animals.

Modulation of visceral nociceptive responses of rat spinal dorsal horn neurons by sympathectomy.

We determined whether sympathectomy modulates visceral nociception under physiological or inflammatory conditions. Recordings of sacral spinal dorsal horn neurons with sustained responses were performed in pentobarbitone-anesthetized rats. Graded colorectal distension (CRD, 20-100 mmHg) was used as a visceral nociceptive stimulus. Inflammation was induced by intracolonic instillation of turpentine (25%). Sympathectomy was produced by administering 6-hydroxydopamine. Inflammation produced an increase in the CRD-evoked responses. The CRD-evoked responses were attenuated following sympathectomy both under control and inflammatory conditions. These changes in the CRD-evoked responses were associated with corresponding changes in spontaneous discharge rate. The convergent input evoked by noxious pinch of the skin was not changed by any of the experimental conditions. The results indicate that sympathectomy attenuates visceral nociceptive responses and spontaneous activity of sacral spinal cord neurons, without effect on convergent cutaneous inputs, both under physiological and inflammatory conditions.

Variable effects of nitrous oxide at multiple levels of the central nervous system in goats.

The direct and indirect effects of nitrous oxide (N2O) on the nociceptive responses of lumbar dorsal horn neurons, and the indirect effects on midbrain reticular formation (MRF) neurons and thalamic neurons were determined in goats anaesthetized with isoflurane. The technique used enabled the differential delivery of N2O to either the torso or the cerebral circulation, thus allowing assessment of the direct spinal and indirect brain effects of N2O. Systemic delivery of N2O appeared to have divergent effects, facilitating (4/11) or depressing (7/11) the responses of dorsal horn neurons. Such divergent effects were also observed when N2O was differentially delivered to the circulation in the torso (i.e. the spinal cord). Likewise, MRF and thalamic responses to noxious stimulation were variably affected by administration of N2O to the torso, with some cells facilitated (7/13 MRF neurons, 3/8 thalamic neurons) and others depressed (6/13 MRF neurons, 5/8 thalamic neurons). It appears that N2O has variable effects on the caprine CNS. The facilitatory action of N2O might partially explain why it is a relatively weak anaesthetic.

Left vagus nerve stimulation suppresses experimentally induced pain.

To test whether electric stimulation of the vagus nerve has an antinociceptive effect in humans. In a variety of animal studies, vagus nerve stimulation was shown to inhibit nociceptive behavior as well as electric responses of spinal nociceptive neurons. In humans, chronic left vagus nerve stimulation is used to treat pharmacologically refractory epilepsy. The authors investigated experimental pain in 10 patients with seizures before and twice after implantation of a vagus nerve stimulator by using different controlled stimuli, including noxious heat, tonic pressure, and short impact. Pain was quantified on a visual analogue scale. Twelve nonepileptic age- and gender-matched individuals served as control subjects. Vagus nerve stimulation reduced increasing pain associated with trains of five consecutive stimuli at 1.5-second intervals ("wind-up"; p < 0.001). In a similar manner, pain on tonic pressure was reduced by vagus nerve stimulation (p < 0.03). Pain associated with single-impact stimuli as well as heat pain thresholds were unaltered under vagus nerve stimulation. Thus, vagus nerve stimulation led to pain relief predominantly in experimental procedures in which pain magnitude was amplified by central processing. The antinociceptive effect was independent of the acute on-off cycles of vagus nerve stimulation. Vagus nerve stimulation is effective in reducing pain in humans. In humans, the antinociceptive effect might rely on central inhibition rather than alterations of peripheral nociceptive mechanisms. These results indicate a promising, potential future role of vagus nerve stimulation in pain treatment.

Contributions of mGlu1 and mGlu5 receptors to interactions with N-methyl-d-aspartate receptor-mediated responses and nociceptive sensory responses of rat thalamic neurons

The nociceptive responses of rat ventrobasal thalamus neurons can be reduced by N-methyl-d-aspartate antagonists and by selective metabotropic glutamate receptor mGlu1 antagonists. The recent development of the mGlu5-selective antagonist 6-methyl-2-(phenylethynyl)-pyridine now allows the direct probing of the possible involvement of mGlu5 receptors in thalamic nociceptive responses. Extracellular recordings were made from single neurons in the ventrobasal thalamus and immediately overlying dorsal thalamic nuclei of adult urethane-anaesthetized rats using multi-barrel electrodes. Responses of neurons to iontophoretic applications of the mGlu5-selective agonist (R,S)-2-chloro-5-hydroxyphenylglycine were selectively reduced during continuous iontophoretic applications of 6-methyl-2-(phenylethynyl)-pyridine. Similar applications of 6-methyl-2-(phenylethynyl)-pyridine reduced neuronal responses to noxious thermal stimuli to 53±9.5% of control responses. Co- application by iontophoresis of N-methyl-d-aspartate and metabotropic glutamate receptor agonists resulted in a mutual potentiation of excitatory responses. This effect could be reduced by either 6-methyl-2-(phenylethynyl)-pyridine or the mGlu1 antagonist LY367385.These results, taken together with previous data, suggest that acute thalamic nociceptive responses are mediated by a combination of mGlu1, mGlu5 and N-methyl-d-aspartate receptor activation, and that co-activation of these receptors produces a synergistic excitatory effect. Thus blockade of any of these receptor types would have a profound effect on the overall nociceptive response.