Glycine and GABAA antagonists reduce the inhibition of primate spinothalamic tract neurons produced by stimulation in periaqueductal gray

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

Noradrenaline, Serotonin, GABA, and Glycine in Cerebrospinal Fluid during Labor Pain: A Cross-Sectional Prospective Study

Background and Aims

The inhibitory pathways that play a role in spinal modulation include local interneurons and descending control. Clinical data regarding the role of these pathways in acute pain is lacking. Accordingly, the aim of this study was to evaluate cerebrospinal fluid (CSF) levels of noradrenaline, serotonin, gamma-aminobutyric acid (GABA), and glycine in parturients with labor pain compared to those without labor pain.

Methods

One hundred term uncomplicated pregnant women receiving spinal anesthesia for cesarean section were enrolled in this prospective cross-sectional study. CSF noradrenaline, serotonin, GABA, and glycine levels were analyzed by enzyme-linked immunosorbent assay. Labor pain score was assessed by numerical rating scale.

Results

Median CSF serotonin concentration in parturients with labor pain was significantly lower than in those without pain (p < 0.001). Median CSF glycine level in the labor pain group was significantly higher than in the control group (p < 0.001). There were no significant differences in median CSF level of noradrenaline or GABA between parturients with and without labor pain. Subsequent analysis showed labor pain scores to be negatively correlated with CSF serotonin (r = −0.217, p = 0.04) but positively correlated with CSF glycine (r = 0.415, p < 0.001).

Conclusion

CSF serotonin and glycine were significantly correlated with labor pain scores. These findings suggest that the serotonergic and glycinergic systems may play a role in spinal modulation of visceral pain.

Defensive Vocalizations and Motor Asymmetry Triggered by Disinhibition of the Periaqueductal Gray in Non-human Primates

Rapid and reflexive responses to threats are present across phylogeny. The neural circuitry mediating reflexive defense reactions has been well-characterized in a variety of species, for example, in rodents and cats, the detection of and species-typical response to threats is mediated by a network of structures including the midbrain tectum (deep and intermediate layers of the superior colliculus [DLSC]), periaqueductal gray (PAG), and forebrain structures such as the amygdala and hypothalamus. However, relatively little is known about the functional architecture of defense circuitry in primates. We have previously reported that pharmacological activation of the DLSC evokes locomotor asymmetry, defense-associated vocalizations, cowering behavior, escape responses, and attack of inanimate objects (Holmes et al., 2012; DesJardin et al., 2013; Forcelli et al., 2016). Here, we sought to determine if pharmacological activation of the PAG would induce a similar profile of responses. We activated the PAG in three awake, behaving macaques by microinfusion of GABA-A receptor antagonist, bicuculline methiodide. Activation of PAG evoked defense-associated vocalizations and postural/locomotor asymmetry, but not motor defense responses (e.g., cowering, escape behavior). These data suggest a partial dissociation between the role of the PAG and the DLSC in the defense network of macaques, but a general conservation of the role of PAG in defense responses across species.

Functional magnetic resonance imaging of the cervical spinal cord during thermal stimulation across consecutive runs

The spinal cord is the first site of nociceptive processing in the central nervous system and has a role in the development and perpetuation of clinical pain states. Advancements in functional magnetic resonance imaging are providing a means to non-invasively measure spinal cord function, and functional magnetic resonance imaging may provide an objective method to study spinal cord nociceptive processing in humans. In this study, we tested the validity and reliability of functional magnetic resonance imaging using a selective field-of-view gradient-echo echo-planar-imaging sequence to detect activity induced blood oxygenation level-dependent signal changes in the cervical spinal cord of healthy volunteers during warm and painful thermal stimulation across consecutive runs. At the group and subject level, the activity was localized more to the dorsal hemicord, the spatial extent and magnitude of the activity was greater for the painful stimulus than the warm stimulus, and the spatial extent and magnitude of the activity exceeded that of a control analysis. Furthermore, the spatial extent of the activity for the painful stimuli increased across the runs likely reflecting sensitization. Overall, the spatial localization of the activity varied considerably across the runs, but despite this variability, a machine-learning algorithm was able to successfully decode the stimuli in the spinal cord based on the distributed pattern of the activity. In conclusion, we were able to successfully detect and characterize cervical spinal cord activity during thermal stimulation at the group and subject level.

Serotonergic modulation in neuropathy induced by oxaliplatin: effect on the 5HT2C receptor

Fluoxetine has been shown to be effective in clinical and experimental studies of neuropathic pain. Besides to increase serotonin levels in the synaptic cleft, fluoxetine is able to block the serotonergic 5-HT2C receptor subtype, which in turn has been involved in the modulation of neuropathic pain. This study investigated the effect of repeated treatments with fluoxetine on the neuropathic nociceptive response induced by oxaliplatin and the effects of both treatments on 5-HT2C receptor mRNA expression and protein levels in the rat spinal cord (SC), rostral ventral medulla (RVM), midbrain periaqueductal gray (PAG) and amygdala (Amy). Nociception was assessed by paw-pressure, cold plate and Von Frey tests. Fluoxetine prevented mechanical hypersensitivity and pain threshold alterations induced by oxaliplatin but did not prevent the impairment in weight gain induced by this anticancer drug. Ex vivo analysis revealed that oxaliplatin increased the 5-HT2C receptor mRNA expression and protein levels in the SC and PAG. Similar effects were observed in fluoxetine-treated animals but only within the PAG. While oxaliplatin decreased the 5-HT2C mRNA expression levels in the Amy, fluoxetine increased their protein levels in this area. Fluoxetine impaired the oxaliplatin effects on the 5-HT2C receptor mRNA expression in the SC and Amy and protein levels in the SC. All treatments increased of 5-HT2C receptor mRNA expression and protein levels in the PAG. These results suggest that the effects of fluoxetine on neuropathic pain induced by oxaliplatin are associated with quantitative changes in the 5-HT2C receptors located within important areas of the nociceptive system.

Neuroinflammation and comorbidity of pain and depression

Comorbid depression and chronic pain are highly prevalent in individuals suffering from physical illness. Here, we critically examine the possibility that inflammation is the common mediator of this comorbidity, and we explore the implications of this hypothesis. Inflammation signals the brain to induce sickness responses that include increased pain and negative affect. This is a typical and adaptive response to acute inflammation. However, chronic inflammation induces a transition from these typical sickness behaviors into depression and chronic pain. Several mechanisms can account for the high comorbidity of pain and depression that stem from the precipitating inflammation in physically ill patients. These mechanisms include direct effects of cytokines on the neuronal environment or indirect effects via downregulation of G protein-coupled receptor kinase 2, activation of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase that generates neurotropic kynurenine metabolites, increased brain extracellular glutamate, and the switch of GABAergic neurotransmission from inhibition to excitation. Despite the existence of many neuroimmune candidate mechanisms for the co-occurrence of depression and chronic pain, little work has been devoted so far to critically assess their mediating role in these comorbid symptoms. Understanding neuroimmune mechanisms that underlie depression and pain comorbidity may yield effective pharmaceutical targets that can treat both conditions simultaneously beyond traditional antidepressants and analgesics.

The role of dorsomedial and ventrolateral columns of the periaqueductal gray matter and in situ 5-HT₂A and 5-HT₂C serotonergic receptors in post-ictal antinociception

The periaqueductal gray matter (PAG) consists in a brainstem structure rich in 5-hydroxytryptamine (5-HT) inputs related to the modulation of pain. The involvement of each of the serotonergic receptor subtypes found in PAG columns, such as the dorsomedial (dmPAG) and the ventrolateral (vlPAG) columns, regarding post-ictal antinociception have not been elucidated. The present work investigated the participation of the dmPAG and vlPAG columns in seizure-induced antinociception. Specifically, we studied the involvement of serotonergic neurotransmission in these columns on antinociceptive responses that follow tonic-clonic epileptic reactions induced by pentylenetetrazole (PTZ), an ionophore GABA-mediated Cl(-) influx antagonist. Microinjections of cobalt chloride (1.0 mM CoCl2 /0.2 µL) into the dmPAG and vlPAG caused an intermittent local synaptic inhibition and decreased post-ictal antinociception that had been recorded at various time points after seizures. Pretreatments of the dmPAG or the vlPAG columns with the nonselective serotonergic receptors antagonist methysergide (5.0 µg/0.2 µL) or intramesencephalic microinjections of ketanserin (5.0 µg/0.2 µL), a serotonergic antagonist with more affinity to 5-HT2A/2C receptors, decreased tonic-clonic seizure-induced antinociception. Both dmPAG and vlPAG treatment with either the 5-HT2A receptor selective antagonist R-96544 (10 nM/0.2 µL), or the 5-HT2C receptors selective antagonist RS-102221 (0.15 µg/0.2 µL) also decrease post-ictal antinociception. These findings suggest that serotonergic neurotransmission, which recruits both 5-HT2A and 5-HT2C serotonergic receptors in dmPAG and vlPAG columns, plays a critical role in the elaboration of post-ictal antinociception.

Identification of 5-HT receptor subtypes enhancing inhibitory transmission in the rat spinal dorsal horn in vitro

Background

5-hydroxytryptamine (5-HT) is one of the major neurotransmitters widely distributed in the CNS. Several 5-HT receptor subtypes have been identified in the spinal dorsal horn which act on both pre- and postsynaptic sites of excitatory and inhibitory neurons. However, the receptor subtypes and sites of actions as well as underlying mechanism are not clarified rigorously. Several electrophysiological studies have been performed to investigate the effects of 5-HT on excitatory transmission in substantia gelatinosa (SG) of the spinal cord. In the present study, to understand the effects of 5-HT on the inhibitory synaptic transmission and to identify receptor subtypes, the blind whole cell recordings were performed from SG neurons of rat spinal cord slices.

Results

Bath applied 5-HT (50 μM) increased the frequency but not amplitudes of spontaneous inhibitory postsynaptic currents (sIPSCs) in 58% of neurons, and both amplitude and frequency in 23% of neurons. The frequencies of GABAergic and glycinergic mIPSCs were both enhanced. TTX (0.5 μM) had no effect on the increasing frequency, while the enhancement of amplitude of IPSCs was eliminated. Evoked-IPSCs (eIPSCs) induced by focal stimulation near the recording neurons in the presence of CNQX and APV were enhanced in amplitude by 5-HT. In the presence of Ba²⁺ (1 mM), a potassium channel blocker, 5-HT had no effect on both frequency and amplitude. A 5-HT_2A receptor agonist, TCB-2 mimicked the 5-HT effect, and ketanserin, an antagonist of 5-HT_2A receptor, inhibited the effect of 5-HT partially and TCB-2 almost completely. A 5-HT_2C receptor agonist WAY 161503 mimicked the 5-HT effect and this effect was blocked by a 5-HT_2C receptor antagonist, N-desmethylclozapine. The amplitudes of sIPSCs were unaffected by 5-HT_2A or 5-HT_2C agonists. A 5-HT₃ receptor agonist mCPBG enhanced both amplitude and frequency of sIPSCs. This effect was blocked by a 5-HT₃ receptor antagonist ICS-205,930. The perfusion of 5-HT_2B receptor agonist had no effect on sIPSCs.

Conclusions

Our results demonstrated that 5-HT modulated the inhibitory transmission in SG by the activation of 5-HT_2A and 5-HT_2C receptors subtypes located predominantly at inhibitory interneuron terminals, and 5-HT₃ receptors located at inhibitory interneuron terminals and soma-dendrites, consequently enhanced both frequency and amplitude of IPSCs.

Modulation of sensorimotor gating in prepulse inhibition by conditional brain glycine transporter 1 deletion in mice

Inhibition of glycine transporter 1 (GlyT1) augments N-methyl-D-aspartate receptor (NMDAR)-mediated transmission and represents a potential antipsychotic drug target according to the NMDAR hypofunction hypothesis of schizophrenia. Preclinical evaluation of GlyT1 inhibiting drugs using the prepulse inhibition (PPI) test, however, has yielded mixed outcomes. Here, we tested for the first time the impact of two conditional knockouts of GlyT1 on PPI expression. Complete deletion of GlyT1 in the cerebral cortices confers resistance to PPI disruption induced by the NMDAR blocker MK-801 (0.2mg/kg, i.p.) without affecting PPI expression in unchallenged conditions. In contrast, restricting GlyT1 deletion to neurons in forebrain including the striatum significantly attenuated PPI, and the animals remained sensitive to the PPI-disruptive effect of MK-801 at the same dose. These results demonstrate in mice that depending on the regional and/or cell-type specificity, deletion of the GlyT1 gene could yield divergent effects on PPI.

Nociceptin/Orphanin FQ in PAG modulates the release of amino acids, serotonin and norepinephrine in the rostral ventromedial medulla and spinal cord in rats

High density Nociceptin/Orphanin FQ (N/OFQ) and its receptor (NOPr) have been found in the ventrolateral periaqueductal gray (vlPAG), a main output pathway involved in the descending pain-control system. Our previous study demonstrated that the microinjection of N/OFQ into the vlPAG markedly facilitated nociceptive responses of spinal dorsal horn neurons. The aim of the present work was to further provide evidence for the supraspinal mechanisms of action for N/OFQ-mediated nociceptive facilitation by examining the effect of N/OFQ in the vlPAG on neurotransmitter release in the descending pain-control system, including the nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC) and dorsal horn of the spinal cord. The results showed that the microinjection of N/OFQ into the vlPAG produced robust decreases in 5-hydroxytryptamine (5-HT, serotonin), norepinephrine (NE), and gamma-aminobutyric acid (GABA), and increase in glutamate (Glu) release in the spinal dorsal horn. Spinal application of 5-HT, 2-Me-5-HT (5-HT(3) receptor agonist), muscimol (GABA(A) receptor agonist), and baclofen (GABA(B) receptor agonist) significantly blocked intra-vlPAG-induced facilitation on nociceptive responses. However, the extracellular concentrations of these neurotransmitters in the NRM and NGC exhibited diversity following intra-vlPAG of N/OFQ. In the NRM, intra-vlPAG injection of N/OFQ significantly decreased 5-HT, NE, and Glu, but increased GABA release. Differently, in the NGC, both NE and GABA releases were attenuated by intra-vlPAG of N/OFQ, whereas the concentration of 5-HT and Glu exhibited a trend to increase. These findings provide direct support for the hypothesis that intra-PAG of N/OFQ-induced facilitation of nociceptive responses is associated with the release of 5-HT, NE, and amino acids.

The antinociceptive effect of intrathecal administration of glycine transporter-2 inhibitor ALX1393 in a rat acute pain model

BACKGROUND

Glycinergic neurons in the spinal dorsal horn have been implicated in the inhibition of spinal pain processing in peripheral inflammation and chronic pain states. Neuronal isoform glycine transporter-2 (GlyT2) reuptakes presynaptically released glycine and regulates the glycinergic neurotransmission. In this study, we examined whether a selective GlyT2 inhibitor, ALX1393, elicits an antinociceptive effect in a rat acute pain model.

METHODS

Male Sprague-Dawley rats were implanted with a catheter intrathecally. The effects of intrathecal administration of ALX1393 (4, 20, or 40 microg) on thermal, mechanical, and chemical nociception were evaluated by tail flick, hot plate, paw pressure, and formalin tests. Furthermore, to explore whether ALX1393 affects motor function, a rotarod test was performed.

RESULTS

ALX1393 exhibited antinociceptive effects on the thermal and mechanical stimulations in a dose-dependent manner. The maximal effect of ALX1393 was observed at 15 min after administration, and a significant effect lasted for about 60 min. These antinociceptive effects were reversed completely by strychnine injected immediately after the administration of ALX1393. In the formalin test, ALX1393 inhibited pain behaviors in a dose-dependent manner, both in the early and late phases, although the influence was greater in the late phase. In contrast to antinociceptive action, ALX1393 did not affect motor function up to 40 microg.

CONCLUSIONS

This study demonstrates the antinociceptive action of ALX1393 on acute pain. These findings suggest that the inhibitory neurotransmitter transporters are promising targets for the treatment of acute pain and that the selective inhibitor of GlyT2 could be a novel therapeutic drug.

Increased phosphorylation of extracellular signal-regulated kinase in trigeminal nociceptive neurons following propofol administration in rats

Although propofol (PRO) is widely used in clinic as a hypnotic agent, the underlying mechanisms of its action on pain pathways is still unknown. Sprague-Dawley rats were assigned to receive PRO or pentobarbital (PEN) and were divided into 2 groups as LIGHT and DEEP hypnotic levels based on the EEG analysis. Rats in each hypnotic level received capsaicin injection into the face and phosphorylated extracellular signal-regulated kinase (pERK) immunohistochemistry was performed in subnucleus caudalis (Vc) and upper cervical spinal cord. In the rats with PEN or PRO administration, a large number of pERK-like immunoreactive (LI) cells was observed in the trigeminal spinal subnuclei interpolaris and caudalis transition zone (Vi/Vc), middle Vc, and transition zone between Vc and upper cervical spinal cord (Vc/C2) following capsaicin injection into the whisker-pad region. The number of pERK-LI cells in Vi/Vc, middle Vc, and Vc/C2 was significantly larger in rats with PRO infusion than those with PEN infusion. The number of pERK-LI cells was increased following an increase in the dose of PRO but not in PEN. The pERK-LI cells were mainly distributed in the Vi/Vc, middle Vc, and Vc/C2 after the bolus infusion of PRO. The expression of pERK-LI cells was depressed after the intravenous lidocaine application before bolus PRO infusion. The present findings suggest that PRO induced an enhancement of the activity of trigeminal nociceptive pathways through nociceptors innervating the venous structure, as indicated by a lidocaine-sensitive increase in pERK. This may explain deep pain around the injection regions during intravenous bolus infusion of PRO.

PERSPECTIVE

The effect of propofol administration on ERK phosphorylation in the subregions of the spinal trigeminal complex and upper cervical spinal cord neurons were precisely analyzed in rats with PRO infusion. A large number of pERK-LI cells was observed following intravenous PRO administration, suggesting an enhancement of trigeminal nociceptive activity and that PRO may produce pain through nociceptors innervating the venous structures during infusion.

Facilitatory actions of serotonin type 3 receptors on GABAergic inhibitory synaptic transmission in the spinal superficial dorsal horn

Analgesic effects of serotonin (5-hydroxytryptamine [5-HT]) type 3 (5-HT3) receptors may involve the release of gamma-aminobutyric acid (GABA) in the spinal dorsal horn. However, the precise synaptic mechanisms for 5-HT3 receptor-mediated spinal analgesia are not clear. In this study, we investigated whether GABAergic neurons in the superficial dorsal horn (SDH) express functional 5-HT3 receptors and how these 5-HT3 receptors affect GABAergic inhibitory synaptic transmission in the SDH, by using slice preparations from adult glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mice. Tight-seal whole cell recordings from GFP-positive and -negative neurons showed that 5-HT3 receptor-specific agonist 2-methyl-serotonin (2-Me-5-HT) induced inward currents in a substantial population of both GFP-positive and -negative neurons. Additionally, we confirmed expression of 5-HT3 receptors in both types of neurons by single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. Further, GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs)-both those evoked by electrical stimulation and those occurring spontaneously in tetrodotoxin (i.e., miniature IPSCs [mIPSCs])-were recorded from GFP-negative neurons. 2-Me-5-HT increased the amplitude of the evoked IPSCs and the frequency of mIPSCs. The amplitude of mIPSCs was not affected by 2-Me-5-HT, suggesting that 5-HT augments GABAergic synaptic transmission via presynaptic mechanisms. The present observations indicate that 5-HT3 receptors are expressed on both somadendritic regions and presynaptic terminals of GABAergic neurons and regulate GABAA receptor-mediated inhibitory synaptic transmission in the SDH. Taken together, these results provide clues for the underlying mechanisms of the antinociceptive actions of 5-HT3 receptors in the spinal dorsal horn.

Increased glutamate and decreased glycine release in the rostral ventromedial medulla during induction of a pre-clinical model of chronic widespread muscle pain

Two injections of acidic saline into the gastrocnemius muscle produce long-lasting hyperalgesia that is initiated and maintained by changes in the rostroventromedial medulla (RVM). Potential underlying mechanisms could be increased release of excitatory neurotransmitters and/or reduced release of inhibitory neurotransmitters, in the RVM. We tested this hypothesis by measuring concentrations of aspartate, glutamate and glycine in response to the first and second injection of acidic saline and compared to intramuscular injections of normal saline using microdialysis with HPLC analysis. We show a significant increase in aspartate and glutamate during the second acidic saline injection compared to normal saline injections or the first injection of acidic saline. There were also long-lasting decreases in glycine concentrations in the RVM in response to both the first and second injection of acidic saline. It is possible that disinhibition after the first injection leads to long-lasting neuronal changes that allow a greater release of excitatory neurotransmitters after the second injection. We hypothesize that increased release of excitatory neurotransmitters in the RVM drives the release of excitatory neurotransmitters in the spinal cord, central sensitization and the consequent hyperalgesia.

Release of GABA and activation of GABA(A) in the spinal cord mediates the effects of TENS in rats

Transcutaneous electrical nerve stimulation (TENS) is a commonly utilized non-pharmacological, non-invasive treatment for pain. GABA is a neurotransmitter in the dorsal horn of the spinal cord that mediates analgesia locally, and also through activation of supraspinal sites. TENS reduces hyperalgesia through activation of receptor-mediated pathways at the level of the spinal cord, and supraspinally. The current study tested the hypothesis that either high or low frequency TENS applied to the inflamed knee joint increases GABA in the spinal cord dorsal horn and activates GABA receptors spinally. We utilized microdialysis to sample the extracellular fluid before, during and after TENS and analyzed GABA in dialysates with high performance liquid chromatography. We analyzed the extracellular GABA concentrations in animals with and without knee joint inflammation induced by intra-articular injection of kaolin and carrageenan. We further tested if spinal blockade of GABA receptors prevents the antihyperalgesia produced by TENS in rats with joint inflammation. We show that high frequency TENS increases extracellular GABA concentrations in the spinal cord in animals with and without joint inflammation. The increases in GABA do not occur in response to low frequency TENS, and there are no increases in glycine in response to low or high frequency TENS. However, the reduction in primary hyperalgesia by both high and low frequency TENS is prevented by spinal blockade of GABA(A) receptors with bicuculline. Thus, high frequency TENS increases release of GABA in the deep dorsal horn of the spinal cord, and both high and low frequency TENS reduce primary hyperalgesia by activation of GABA(A) receptors spinally.

Modulation of neuronal activity in CNS pain pathways following propofol administration in rats: Fos and EEG analysis

We studied Fos expression in the central nociceptive pathways at different sedative levels in order to clarify the central mechanism of propofol's nociceptive action. Sprague-Dawley rats received propofol (PRO) or pentobarbital (PEN) and were divided into two groups with different doses of drug administration (light and deep sedative levels) based on the electroencephalogram analysis. Rats at each sedative level received heat stimulation to their face and Fos immunohistochemistry was performed at various brain sites. We also infused lidocaine into the jugular vein to test whether PRO directly activated nociceptors distributed in the vein. Fos expression in two major ascending pain pathways (lateral and medial systems) and descending modulatory system were precisely analyzed following intravenous (i.v.) administration of PRO or PEN. Many Fos protein-like immunoreactive (Fos protein-LI) cells were expressed in the trigeminal spinal nucleus caudalis (Vc), parabrachial nucleus, parafascicular nucleus, a wide area of the primary somatosensory cortex, anterior cingulate cortex, amygdala, periaqueductal gray, solitary tract nucleus, and lateral hypothalamus following heating of the face during PRO or PEN infusion. The number of Fos protein-LI cells was significantly greater in many Central nervous system regions during PRO infusion compared with PEN. Fos expression was significantly greater in the Vc and Periaqueductal gray following greater amount of PRO infusions compared, whereas they were significantly smaller in the Vc in the rats with PEN infusion. The Fos expression was significantly depressed following i.v. infusion of lidocaine before PRO administration. The present findings suggest that PRO is involved in the enhancement of Vc activity through direct activation of the primary afferent fibers innervating veins, resulting in pain induction during infusion.

Mechanisms of orofacial pain control in the central nervous system

Recent advances in the study of pain have revealed somatotopic- and modality-dependent processing and the integration of nociceptive signals in the brain and spinal cord. This review summarizes the uniqueness of the trigeminal sensory nucleus (TSN) in structure and function as it relates to orofacial pain control. The oral nociceptive signal is primarily processed in the rostral TSN above the obex, the nucleus principalis (Vp), and the subnuclei oralis (SpVo) and interpolaris (SpVi), while secondarily processed in the subnucleus caudalis (SpVc). In contrast, the facial nociceptive signal is primarily processed in the SpVc. The neurons projecting to the thalamus are localized mostly in the Vp, moderately in the SpVi, and modestly in the ventrolateral SpVo and the SpVc. Orofacial sensory inputs are modulated in many different ways: by interneurons in the TSN proper, through reciprocal connection between the TSN and rostral ventromedial medulla, and by the cerebral cortex. A wide variety of neuroactive substances, including substance P, gamma-aminobutyric acid, serotonin and nitric oxide (NO) could be involved in the modulatory functions of these curcuits. The earliest expression of NO synthase (NOS) in the developing rat brain is observed in a discrete neuronal population in the SpVo at embryonic day 15. NOS expression in the SpVc is late at postnatal day 10. The neurons receiving intraoral signals are intimately related with the sensorimotor reflexive function through the SpVo. In summary, a better understanding of the trigeminal sensory system--which differs from the spinal system--will help to find potential therapeutic targets and lend to developing new analgesics for orofacial-specific pain with high efficacy and fewer side effects.

Elimination of neurokinin-1 receptor neurons in caudal nucleus reverses the effects of systemic bicuculline on c-Fos expression in rat trigeminal sensory nucleus: I. High intensity electrical stimulation of the trigeminal ganglion

Although neurokinin-1 receptor (NK-1)-bearing neurons are distributed in lamina I of the trigeminal caudal nucleus (Vc) and constitute major projection neurons, little is known about their fundamental role(s) in nociceptive processing. This study examines the effect of intra cisterna magna injection of substance P (SP) conjugated to saporin (SP-Sap; 5 microM, 5 microl) [with/without systemic administration of bicuculline] on c-Fos expression in the trigeminal sensory nucleus (TSN) induced 2 h after 10 min repetitive electrical stimulation of the trigeminal ganglion (TG) at high intensity (1.0 mA, 5 Hz, 5 ms) in the urethane-anesthetized rat. In the SP-Sap-treated rats, the numbers of NK-1-immunopositive neurons in laminae I and III of the Vc decreased compared with rats similarly pretreated with saline (Sal; 5 microl) or blank-saporin (Bl-Sap; 5 microM, 5 microl). In Sal- or Bl-Sap-treated controls, high intensity stimulation induced c-Fos expression in neurons throughout the full extent of ipsilateral superficial layers of the Vc (VcI/II), magnocellular zone of the Vc (VcIII/IV) and the dorsal or dorsomedial subdivisions of the rostral TSN above the obex (trigeminal principal, oral (Vo) and interpolar nuclei). Preadministration of bicuculline (2 mg/kg, i.p.) decreased the numbers of c-Fos-immunopositive neurons in the VcI/II, VcIII/IV and Vo in Sal- or Bl-Sap-treated controls. In contrast, high intensity stimulation induced less c-Fos-immunopositive neurons in the VcI/II and Vo of rats treated with SP-Sap compared with those in Sal- or Bl-Sap-treated controls. In SP-Sap-treated rats preadministered with bicuculline, the numbers of c-Fos-immunopositive neurons in the VcI/II and Vo were increased compared with the SP-Sap-treated rats preadministered with Sal. These results suggest that NK-1-immunopositive neurons in laminae I and III of Vc play a pivotal role in the nociceptive specific processing in the TSN through GABA(A) receptors.

The functional anatomy of neuropathic pain

The generation of neuropathic pain is a complex phenomenon involving a process of peripheral and central sensitization producing enhanced transmission of nociceptive inputs to the brain associated with the loss of discriminatory processing of noxious and innocuous stimuli. This increased flow of abnormally processed nociceptive inputs to the brain may overcome the ability of descending modulatory pathways to produce analgesia, causing further worsening of the pain. Several crucial locations involved in the physiologic generation of pain inputs (eg, peripheral nociceptors, dorsal horns, thalamus, cortex) show evidence of functional reorganization and altered nociceptive processing in association with chronic pain. These locations present the best targets for therapeutic intervention, including systemic administration of drugs able to counteract the chemical storm induced by neural injuries in the nociceptive afferents and dorsal horns, or for more focused intervention, such as neuroablative procedures; intrathecal drug delivery; and spinal cord, deep brain, or motor cortex stimulation.

Adenosine suppresses GABAA receptor-mediated responses in rat sacral dorsal commissural neurons

The modulatory effect of adenosine on gamma-aminobutyric acid (GABA)-activated whole-cell currents were investigated in the neurons acutely dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin perforated patch recording configuration under the voltage-clamp conditions. The results showed that: (1) GABA acted on GABAA receptor and elicited inward Cl- currents (IGABA) at a holding potential (VH) of -40 mV; (2) adenosine suppressed GABA-induced Cl- current without affecting the reversal potential of IGABA and the apparent affinity of GABA to its receptor; (3) N6-cyclohexyladenosine mimicked the suppression effect of adenosine on IGABA, whereas 8-cyclopentyl-1,3-dipropylxanthine blocked the suppression effect of adenosine; (4) adenosine fails to suppress IGABA on the neurons that were pretreated with bisindolylmaleimide I (BIM), while after pretreatment with H-89, the inhibitory effect of adenosine on IGABA were not affected; (5) the suppression effect of adenosine on IGABA remained in the presence of BAPTA-AM. The present results indicate that the suppression of adenosine on IGABA is mediated by adenosine A1 receptor and through a Ca2+-independent protein kinase C transduction pathway, and that the interactions between adenosine and GABA might participate in the modulation of nociceptive information transmission at the SDCN.

Bicuculline and strychnine suppress the mesencephalic locomotor region-induced inhibition of group III muscle afferent input to the dorsal horn

We examined the effect of iontophoretic application of bicuculline methiodide and strychnine hydrochloride on the mesencephalic locomotor region (MLR)-induced inhibition of dorsal horn cells in paralyzed cats. The activity of 60 dorsal horn cells was recorded extracellularly in laminae I, II, V-VII of spinal segments L7-S1. Each of the cells was shown to receive group III muscle afferent input as demonstrated by their responses to electrical stimulation of the tibial nerve (mean latency and threshold of activation: 20.1+/-6.4 ms and 15.2+/-1.4 times motor threshold, respectively). Electrical stimulation of the MLR suppressed transmission in group III muscle afferent pathways to dorsal horn cells. Specifically the average number of impulses generated by the dorsal horn neurons in response to a single pulse applied to the tibial nerve was decreased by 78+/-2.8% (n=60) during the MLR stimulation. Iontophoretic application (10-50 nA) of bicuculline and strychnine (5-10 mM) suppressed the MLR-induced inhibition of transmission of group III afferent input to laminae I and II cells by 69+/-5% (n=10) and 29+/-7% (n=7), respectively. Likewise, bicuculline and strychnine suppressed the MLR-induced inhibition of transmission of group III afferent input to lamina V cells by 59+/-13% (n=14) and 39+/-11% (n=10), respectively. Our findings raise the possibility that GABA and glycine release onto dorsal horn neurons in the spinal cord may play an important role in the suppression by central motor command of thin fiber muscle afferent-reflex pathways.

Peripheral inflammation increases the functional coherency of spinal responses to tactile but not nociceptive stimulation

Reorganization of central networks and plasticity of neuronal representations have been implicated in recent years in the dynamic expression of somatosensory responses. The functional properties of spinal cells were shown to change in the scale of minutes after peripheral high-intensity stimulations and to undergo profound alterations in their responses in experimental models of chronic pain. These observations, however, are restricted to recordings from individual cells, and no information exists on how these changes may be reflected on the activity of somatosensory neuronal networks involved in pain processing. To understand how spinal cord networks may be altered after the onset of hyperalgesia, we extracellularly recorded from groups of five to nine neighboring neurons in the hindlimb representation area of the dorsal horn. The multineuronal activity evoked by cutaneous innocuous and noxious stimulation was compared before and for 3 h after the subcutaneous injection of diluted formalin. Formalin caused immediate changes in response properties and mechanical threshold of activation for the majority of the neurons and induced the incorporation of previously unresponsive neighboring neurons to the functional network. Analysis of the temporal correlation within the neuronal population revealed that formalin-induced inflammation increased the functional coherence of the network to the nonnociceptive stimulation but not to the painful stimuli. This increase in the tactile acuity of populations of nociceptive neurons may be a basis for the emergence of touch-evoked pain.

Serotonin potentiation of glycine-activated whole-cell currents in the superficial laminae neurons of the rat spinal dorsal horn is mediated by protein kinase C

The modulatory effects of serotonin (5-HT) on glycine (Gly)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (I and II) of the rat spinal dorsal horn using the nystatin-perforated patch recording configuration under voltage-clamp conditions. Our results demonstrate that (1). Gly acted on strychnine (STR)-sensitive Gly receptors and elicited inward Cl(-) currents (I(Gly)) at a holding potential of -40 mV; (2). 5-HT potentiated I(Gly) without affecting the reversal potential of I(Gly); (3). the agonist (alpha-methyl-5-HT) and antagonist (ketanserine) of 5-HT(2) receptor mimicked and blocked the potentiating effect of 5-HT on I(Gly), respectively; (4). bisindolylmaleimide I (BIM), a selective inhibitor of protein kinase C (PKC), reduced the potentiating effect of 5-HT on I(Gly); and (5). 5-HT-induced enhancement of I(Gly) was not affected by pretreatment with 1,2-bis-(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis (acetoxy-methyl) ester (BAPTA AM), a Ca(2+) chelator. These results indicate that (1). the potentiation of 5-HT on I(Gly) is mediated by 5-HT(2) receptor and through Ca(2+)-independent PKC intracellular signal transduction pathway; and (2). the interactions between 5-HT and Gly might modulate the transmission of nociceptive information through the spinal cord.

The neuropharmacology of centrally-acting analgesic medications in fibromyalgia

As demonstrated above, the anatomy and neuropharmacology of the pain pathways within the CNS, even to the level of the midbrain, are extraordinarily complex. Indeed, discussions of the effects of these agents on the neuropharmacology of the thalamus, hypothalamus, and cortex were excluded from this review owing to their adding further to this complexity. Also, the dearth of data regarding FMS pain pathophysiology necessitated a relatively generic analysis of the pain pathways. As mentioned in the introduction, the current thought is that central sensitization plays an important role in FMS. However, we see in this chapter that the behavioral state of central sensitization may be a result of alterations in either the ascending systems or in one or more descending systems. Studies to assess the presence or relative importance of such changes in FMS are difficult to perform in humans, and to date there are no animal models of FMS. Accepting these limitations, it is apparent that many drugs considered to date for the treatment of FMS do target a number of appropriate sites within both the ascending and descending pain pathways. The data regarding clinical efficacy on some good candidate agents, however, is extremely preliminary. For example, it is evident from the present analysis that SNRIs, alpha 2 agonists, and NK1 antagonists may be particularly well suited to FMS, although current data supporting their use is either anecdotal or from open-label trials [114,149]. Other sites within the pain pathways have not yet been targeted. Examples of these include the use of CCKB antagonists to block on-cell activation or of nitric oxide synthetase antagonists to block the downstream mediators of NMDA activation. Efficacy of such agents may give considerable insight into the pathophysiology of FMS. Finally, as indicated previously, FMS consists of more than just chronic pain, and the question of how sleep abnormalities, depression, fatigues, and so forth tie into disordered pain processing is being researched actively. Future research focusing on how the various manifestations of FMS relate to one another undoubtedly will lead to a more rational targeting of drugs in this complex disorder.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Reduction in opioid- and cannabinoid-induced antinociception in rhesus monkeys after bilateral lesions of the amygdaloid complex

The amygdaloid complex is a prominent temporal lobe region that is associated with "emotional" information processing. Studies in the rodent have also recently implicated the amygdala in the processing and modulation of pain sensation, the experience of which involves a considerable emotional component in humans. In the present study, we sought to establish the relevance of the amygdala to pain modulation in humans by investigating the contribution of this region to antinociceptive processes in nonhuman primates. Using magnetic resonance imaging guidance, the amygdaloid complex was lesioned bilaterally in six rhesus monkeys (Macaca mulatta) through microinjection of the neurotoxin ibotenic acid. This procedure resulted in substantial neuronal cell loss in all nuclear subdivisions of this structure. In awake unoperated control monkeys, systemic administration of the prototypical opioid morphine or the cannabinoid receptor agonist WIN55,212-2 produced dose-dependent antinociception on a warm-water tail-withdrawal assay. The antinociceptive effects of each drug were reversible with an appropriate antagonist. In monkeys with bilateral amygdala lesions, however, the antinociceptive effects of each drug were significantly reduced. These results constitute the first causal data demonstrating the necessity of neurons in a specific brain region for the full expression of opioid- and cannabinoid-induced antinociception in the primate. Because our amygdala-lesioned monkeys exhibited both a reduction in antinociception and a reduction in behavioral indices of fear (Emery et al., 2001), the possibility should be considered that, in the primate, "antinociceptive circuitry" and "fear circuitry" overlap at the level of the amygdala.

The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaqueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is postulated that the PAG exerts this inhibitory effect on spinal nociceptive functions through the activation of descending serotonergic and noradrenergic pathways that arise from the rostral ventromedial medulla (RVM) and pontine noradrenergic nuclei. To investigate the neuroanatomical substrate of this functional link between the PAG and RVM, as well as the pontine noradrenergic nuclei in the rat, we labelled axons that project from the ventrolateral PAG to various regions of the pons and medulla oblongata using the anterograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstrated that some of PAG efferents really do terminate in the RVM and pontine noradrenergic nuclei, but a substantial proportion of them project to the intermediate subdivision of the pontobulbar reticular formation. Combining the axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemistry, we also found that, in contrast to previous results, PAG efferents make relatively few appositions with serotonin- and tyrosine-hydroxylase-immunoreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the ventrolateral PAG may activate a complex pontobulbar neuronal assembly including neurons in the intermediate subdivision of the pontobulbar reticular formation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunoreactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulbar neural circuitry, then, may mediate the PAG-evoked activities towards the spinal dorsal horn resulting in the inhibition of spinal nociceptive functions.

Topical bicuculline to the rat spinal cord induces highly localized allodynia that is mediated by spinal prostaglandins

The purpose of this study was to investigate the allodynic effect of bicuculline (BIC) given topically to the dorsal surface of the rat spinal cord, and to determine if spinal prostaglandins (PGs) mediate the allodynic state arising from spinal GABA(A)-receptor blockade. Male Sprague-Dawley rats (325-400 g) were anaesthetized with halothane and maintained with urethane for the continuous monitoring of blood pressure (MAP), heart rate (HR) and cortical electroencephalogram (EEG). A laminectomy was performed to expose the dorsal surface of the spinal cord. Unilateral application of BIC (0.1 microg in 0.1 microl) to the L5 or L6 spinal segment induced a highly localized allodynia (e.g. one or two digits) on the ipsilateral hind paw. Thus, hair deflection (brushing the hair with a cotton-tipped applicator) in the presence, but not absence of BIC, evoked an increase in MAP and HR, abrupt motor responses (MR; e.g. withdrawal of the hind leg, kicking, and/or scratching) on the affected side, and desynchrony of the EEG. BIC-allodynia was dose-dependent, yielding ED(50)'s (95% CI's) of 45 ng (31-65) for MAP; 68 ng (46-101) for HR and 76 ng (60-97) for MR. Allodynia was sustained for up to 2 h with repeated BIC application without any detectable change in the location or area of peripheral sensitization. Pretreatment with either the EP(1)- receptor antagonist, SC-51322, the cyclooxygenase (COX)-2 selective inhibitor, NS-398, or the NMDA-receptor antagonist, AP-7, inhibited BIC-allodynia in a dose-dependent manner. The results demonstrate: (a) BIC, applied to the dorsal surface of the spinal cord, induces highly localized allodynia; (b) this effect can be sustained with repeated BIC application; (c) it is evoked by NMDA-dependent afferent input; (d) spinal PGs are synthesized by constitutive COX-2 during BIC-allodynia; and (e) spinal PGs contribute to the abnormal processing of tactile input via spinal EP1-receptors.

GABA(A) and 5-HT(3) receptors are involved in dorsal root reflexes: possible role in periaqueductal gray descending inhibition

The dorsal root reflex (DRR) is a measure of the central excitability of presynaptic inhibitory circuits in the spinal cord. Activation of the periaqueductal gray (PAG), a center for descending inhibition of spinal cord nociceptive transmission, induces release of variety of neurotransmitters in the spinal cord, including GABA and serotonin (5-HT). GABA has been shown to be involved in generation of DRRs. In this study, pharmacological agents that influence DRRs and their possible mechanisms were investigated. DRRs were recorded in anesthetized rats from filaments teased from the cut central stump of the left L(4) or L(5) dorsal root, using a monopolar recording electrode. Stimulating electrodes were placed either on the left sciatic nerve or transcutaneously in the left foot. Animals were paralyzed and maintained by artificial ventilation. Drugs were applied topically to the spinal cord. A total of 64 units were recorded in 34 Sprague-Dawley rats. Peripheral receptive fields were found for nine of these units. In these units, DRRs were evoked by brush, pressure, and pinch stimuli. Nine units were tested for an effect of electrical stimulation in the periaqueductal gray on the DRRs. In eight cases, DRR responses were enhanced following PAG stimulation. The background activity was 4.2 +/- 1.9 spikes/s (mean +/- SE; range: 0-97.7; n = 57). The responses to agents applied to the spinal cord were (in spikes/s): artificial cerebrospinal fluid, 7.1 +/- 3.6 (range: 0-86.9; n = 25); 0.1 mM GABA, 16.8 +/- 8.7 (range: 0-191.0; n = 22); 1.0 mM GABA, 116.0 +/- 26.5 (range: 0.05-1001.2; n = 50); and 1.0 mM phenylbiguanide (PBG), 68.1 +/- 25.3 (range: 0-1,073.0; n = 49). Bicuculline (0.5 mM, n = 27) and ondansetron (1.0 mM, n = 10) blocked the GABA and PBG effects, respectively (P < 0.05). Significant cross blockade was also observed. It is concluded that GABA(A) receptors are likely to play a key role in the generation of DRRs, but that 5-HT(3) receptors may also contribute. DRRs can be modulated by supraspinal mechanisms through descending systems.

Acute phenobarbital administration induces hyperalgesia: pharmacological evidence for the involvement of supraspinal GABA-A receptors

The aim of the present study was to determine if phenobarbital affects the nociception threshold. Systemic (1-20 mg/kg) phenobarbital administration dose dependently induced hyperalgesia in the tail-flick, hot-plate and formalin tests in rats and in the abdominal constriction test in mice. Formalin and abdominal constriction tests were the most sensitive procedures for the detection of hyperalgesia in response to phenobarbital compared with the tail-flick and hot-plate tests. The hyperalgesia induced by systemic phenobarbital was blocked by previous administration of 1 mg/kg ip picrotoxin or either 1-2 mg/kg sc or 10 ng icv bicuculline. Intracerebroventricular phenobarbital administration (5 microg) induced hyperalgesia in the tail-flick test. In contrast, intrathecal phenobarbital administration (5 microg) induced antinociception and blocked systemic-induced hyperalgesia in this test. We suggest that phenobarbital may mediate hyperalgesia through GABA-A receptors at supraspinal levels and antinociception through the same kind of receptors at spinal levels.

Antiallodynic effects produced by stimulation of the periaqueductal gray matter in a rat model of neuropathic pain

It has been well documented that there is opioid resistance in neuropathic pain. This indicates that the endogenous opioid system may not be involved effectively in modulating neuropathic pain. The present study sought to determine if activation of the descending pain inhibition system might produce analgesia in the animal neuropathic model we developed. Under ketamine anesthesia, male Sprague-Dawley rats were chronically implanted with stimulating electrodes in the ventral periaqueductal gray matter (PAG) and both the tibial and sural nerves of the sciatic nerve branches were severed. Pain sensitivity was measured with a von Frey filament and acetone applied to the sensitive area for 1 week postoperatively. Rats with neuropathic pain syndrome after transection of the tibial and sural nerves were tested as to the analgesic effects of ventral PAG stimulation for an additional two weeks. Electrical stimulation of the ventral PAG turned out to be highly effective in alleviating neuropathic pain. Mechanical allodynia and cold allodynia were reduced by PAG stimulation. Naloxone reversed the antiallodynic effects of ventral PAG stimulation. These results suggest that activation of the descending pain inhibition system including the ventral PAG reduces neuropathic pain syndrome and that opiates are involved in this system.

GABA(A) receptor-mediated effects on expression of c-Fos in rat trigeminal nucleus following high- and low-intensity afferent stimulation

We examined the effects of systemic administration of a GABA(A) receptor agonist, muscimol, or antagonist, bicuculline, on the expression of c-Fos protein induced 3h after electrical stimulation of the trigeminal ganglion at low (0.1 mA) or high intensities (1. 0 mA) in the urethane-anesthetized rat. In saline-treated rats, 10 min stimulation of the trigeminal ganglion induced c-Fos-immunopositive neurons throughout the full extent of the ipsilateral superficial layers of the trigeminal nucleus caudalis, and dorsal or dorsomedial part of the nuclei rostral to obex (trigeminal nucleus principalis, dorsomedial nucleus of trigeminal nucleus oralis, dorsomedial nucleus of trigeminal nucleus interpolaris). Animals stimulated at 1. 0 mA induced a significantly higher number of labeled neurons in all trigeminal sensory nucleus than animals stimulated at 0.1 mA. In rats treated with 1mg/kg i.p. muscimol and stimulated at 0.1 mA, the numbers of Fos-positive neurons in trigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus interpolaris, and dorsomedial nucleus of trigeminal nucleus oralis were significantly decreased. However, after stimulation at 1.0 mA, the numbers of Fos-positive neurons in the superficial layers of trigeminal nucleus caudalis was increased and no changes occurred in the numbers of Fos-positive neurons in the magnocellular zone of trigeminal nucleus caudalis, the dorsomedial nucleus of trigeminal nucleus interpolaris, or dorsomedial nucleus of trigeminal nucleus oralis compared to saline-treated controls. In rats treated with 2mg/kg i.p. bicuculline and stimulated at 0.1 mA, the number of Fos-positive neurons increased in the superficial layers of trigeminal nucleus caudalis and trigeminal nucleus principalis. However, after stimulation at 1.0 mA, the number of Fos-positive neurons was unchanged in superficial layers of trigeminal nucleus caudalis, but decreased in the magnocellular zone of trigeminal nucleus caudalis, dorsomedial nucleus of trigeminal nucleus interpolaris and dorsomedial nucleus of trigeminal nucleus oralis. There was a specific loss of Fos-positive neurons in the maxillary and ophthalmic divisions (ventrolateral half) of trigeminal nucleus caudalis. These results indicate that the expression of c-Fos in the trigeminal nucleus is differentially regulated through GABA(A) receptors in a manner that is dependent on the nucleus and the type of primary afferents that are activated by different stimulus intensities.

Serotonin potentiates the response of neurons of the superficial laminae of the rat spinal dorsal horn to gamma-aminobutyric acid

Employing the Nystatin-perforated whole-cell patch-clamp recording technique, the modulatory effects of serotonin (5-HT) on gamma-aminobutyric acid (GABA)-activated whole-cell currents were investigated in neurons acutely dissociated from the superficial laminae (laminae I and II) of the rat spinal dorsal horn. The results showed: (1) GABA acted on GABA(A) receptors and elicited inward Cl(-) currents (I(GABA)) at a holding potential (V(H)) of -40 mV; (2) 5-HT potentiated GABA-induced Cl(-) current without affecting the reversal potential of I(GABA) and the apparent affinity of GABA to its receptor; (3) alpha-methyl-5-HT, a selective agonist of 5-HT(2) receptor, mimicked the potentiation effect of 5-HT on I(GABA), whereas ketanserine, an antagonist of 5-HT(2) receptor, blocked the potentiation effect of 5-HT; (4) Chelerythrine, an inhibitor of protein kinase C, reduced the potentiation effect of 5-HT on I(GABA). The present results indicate: (1) The potentiation of 5-HT on I(GABA) is mediated by 5-HT(2) receptor and through a protein kinase-dependent transduction pathway; (2) The interactions between 5-HT and GABA might play an important role in the modulation of nociceptive information transmission at spinal cord level.

Nitric oxide-mediated spinal disinhibition contributes to the sensitization of primate spinothalamic tract neurons

This study concentrated on whether an increase in spinal nitric oxide (NO) diminishes inhibition of spinothalamic tract (STT) cells induced by activating the periaqueductal gray (PAG) or spinal glycinergic and GABAergic receptors, thus contributing to the sensitization of STT neurons. A reduction in inhibition of the responses to cutaneous mechanical stimuli induced by PAG stimulation was seen in wide dynamic range (WDR) STT cells located in the deep layers of the dorsal horn when these neurons were sensitized during administration of a NO donor, 3-morpholinosydnonimine (SIN-1), into the dorsal horn by microdialysis. In contrast, PAG-induced inhibition of the responses of high-threshold (HT) and superficial WDR STT cells was not significantly changed by spinal infusion of SIN-1. A reduction in PAG inhibition when STT cells were sensitized after intradermal injection of capsaicin could be nearly completely blocked by pretreatment of the dorsal horn with a NO synthase inhibitor, 7-nitroindazole. Moreover, spinal inhibition of nociceptive activity of deep WDR STT neurons elicited by iontophoretic release of glycine and GABA agonists was attenuated by administration of SIN-1. This change paralleled the change in PAG-induced inhibition. However, the inhibition of HT and superficial WDR cells induced by glycine and GABA release did not show a significant change when SIN-1 was administered spinally. Combined with our recent results, these data show that the effectiveness of spinal inhibition can be reduced by the NO/cGMP pathway. Thus disinhibition may constitute one mechanism underlying central sensitization.

Inhibition of primate spinothalamic tract neurons by spinal glycine and GABA is modulated by guanosine 3',5'-cyclic monophosphate

Our recent work has suggested that the nitric oxide/guanosine 3', 5'-cyclic monophosphate (NO/cGMP) signal transduction system contributes to central sensitization of spinothalamic tract (STT) neurons in part by influencing the descending inhibition of nociception resulting from stimulation in the periaqueductal gray. This study was designed to examine further whether activation of the NO/cGMP cascade reduces the inhibition of the activity of STT neurons mediated by spinal inhibitory amino acid (IAA) receptors. Responses of STT cells to noxious cutaneous stimuli were inhibited by iontophoresis of glycine and GABA agonists in anesthetized monkeys. Administration of 8-bromoguanosine-3',5'-cyclophosphate sodium (8-bromo-cGMP), a membrane permeable analogue of cGMP, either by microdialysis or by iontophoresis reduced significantly the IAA-induced inhibition of wide dynamic range (WDR) STT cells in the deep layers of the dorsal horn. The reduction in inhibition lasted for up to 1-1.5 h after the cessation of drug infusion. In contrast, IAA-induced inhibition of WDR STT cells in the superficial dorsal horn and high-threshold (HT) cells in superficial or deep layers was not significantly changed during 8-bromo-cGMP infusion. Iontophoresis of 8-bromo-cGMP onto STT cells produced the same actions as produced by microdialysis of this agent, but the effect was not as long-lasting nor as potent. Finally, an attenuation of the IAA receptor-mediated inhibition of STT cells produced by iontophoretic release of a NO donor, 3-morpholinosydnonimine, could be blocked by pretreatment of the spinal cord with a guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. These results suggest that an increased spinal cGMP level contributes to the sensitization of WDR STT neurons in the deep dorsal horn in part by down-regulating spinal IAA receptors. However, no evidence is provided in this study that the NO/cGMP cascade regulates IAA receptors on HT and superficial WDR neurons. Combined with the preceding studies, our data support the view that NO and cGMP function in the same signal transduction cascade and play an important role in central sensitization.

Quantitative immunoelectron microscopic colocalization of GABA and enkephalin in the ventrocaudal periaqueductal gray of the rat

In the present ultrastructural study in the ventrocaudal periaqueductal gray (PAG) of the rat, the relationship and the association between GABAergic and enkephalinergic neuronal elements were investigated using postembedding colocalization immunogold electron microscopic technique in order to establish the precise relationship between these two important neurotransmitters in this part of the brain stem. The GABA-like neuronal elements were immunoreacted with 20 nm gold particles and the enkephalin (ENK)-like immunoreactive neurons were labeled with 10 nm gold particles. Double labeling of sections with ENK and GABA produced colocalization in 23.3% and 1.2% of axon terminals and dendrites, respectively. Most of the double-labeled terminals contained more GABA-like than ENK-like immunolabeling. Approximately 19.4% of the labeled axon terminals and 8.5% of the labeled dendrites contained only GABA-like immunoreactivity, while 24% of the immunolabeled dendrites were immunoreactive with only ENK-like immunoreactivity. The synapses between the two kinds of immunolabeled neuronal profiles appear to be both asymmetrical and symmetrical. GABA-like immunolabeled terminals contained small, clear, pleomorphic or round vesicles and were found to make synapses with ENK-like immunolabeled and nonimmunolabeled dendrites, whereas most of the ENK-like immunolabeled axon terminals contained dense-cored vesicles. Approximately half of the axon terminals (51%) and dendrites (56%) in the ventrolateral PAG were not labeled for either GABA or for ENK immunoreactivity. The results are discussed in terms of GABAergic inhibition of antinociceptive mechanisms in the ventrolateral PAG and of the activation of these mechanisms by ENK neurotransmitter.

Gamma-aminobutyric acid-induced responses in acutely dissociated neurons from the rat sacral dorsal commissural nucleus

The electrophysiological and pharmacological properties of GABA-activated Cl- currents (IGABA) were investigated in enzymatically dissociated rat sacral dorsal commissural nucleus (SDCN) neurons using the nystatin perforated patch recording configuration under voltage-clamp conditions. Exogenous application of GABA to SDCN neurons induced Cl- currents which increased in a concentration-dependent manner. Bicuculline (BIC) and strychnine (STR) antagonized the IGABA in a concentration-dependent manner. Zn2+ suppressed the IGABA with an IC50 of 2.8 X 10(-5) M. Muscimol mimicked the IGABA, while baclofen evoked no response. Pentobarbital (PB) and 5beta-pregnan-3alpha-ol-20-one (pregnanolone, PGN) also induced GABAA-mimic Cl- currents. Diazepam (DZP), PB and PGN all enhanced the IGABA by increasing the apparent affinity of the GABAA receptors to GABA. Moreover, spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) were observed in mechanically dissociated SDCN neurons attached with synaptic boutons, so called 'synaptic bouton preparation'. These results indicate that SDCN neurons express GABAA receptors with relatively low sensitivity to Zn2+ inhibition, and that GABA may have a functional role as an inhibitory transmitter in the SDCN regulating nociceptive, analgesic, and autonomic functions.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Serotonergic modulation of spinal ascending activity and sacral reflex activity evoked by pelvic nerve stimulation in cats

Serotonin (5-HT) may be inhibitory to micturition at a spinal level. A potential mechanism of action for serotonergic inhibition of bladder function is a depression of the ascending limb of the supraspinal reflex mediating micturition. Ascending activity evoked by pelvic nerve stimulation was recorded in the thoracic spinal cord of anesthetized cats. For comparison, spinal reflex activity evoked by pelvic nerve stimulation was recorded on the pudendal nerve. The effects of intrathecal administration of serotonergic agents were examined to determine whether spinal and supraspinal responses to bladder afferent activation were modulated by 5-HT. Methysergide (60 nmol), a non-selective serotonergic antagonist, increased ascending activity by 61+/-7% and depressed spinal reflex activity by 38+/-6%. Zatosetron (10 nmol), a 5-HT3 antagonist had a similar effect on both activities (increased by 93+/-24% and decreased by 77+/-7%, respectively). The effect on ascending activity of blocking 5-HT3 receptors was also confirmed with ICS 205930 and MDL 72222. 2-Methyl-5-HT (800 nmol), a 5-HT3 agonist, depressed ascending activity to 46+/-9% of control, but enhanced spinal reflex activity by 73+/-92%. These results demonstrate that stimulation of 5-HT3 and methysergide-sensitive 5-HT receptors can inhibit ascending activity and facilitate spinal reflex activity elicited by activation of bladder afferents. It is suggested that descending serotonergic pathways may participate in the spinal coordination of urinary continence.

5-HT potentiation of the GABA(A) response in the rat sacral dorsal commissural neurones

1. The modulatory effect of 5-hydroxytryptamine (5-HT) on the gamma-aminobutyric acid(A) (GABA(A)) response was investigated in the neurones freshly dissociated from the rat sacral dorsal commissural nucleus (SDCN) using the nystatin perforated patch recording configuration under the voltage-clamp conditions. 2. 5-HT potentiated GABA-induced Cl- current (IGABA) without affecting the reversal potential of IGABA and the apparent affinity of GABA to its receptor. 3. Alpha-Methyl-5-HT mimicked the potentiation effect of 5-HT on IGABA while ketanserine blocked it. 1-Oleoyl-2-acetyl-glycerol (OAG) potentiated IGABA, and the effect of 5-HT on IGABA was occluded by OAG pretreatment. In the presence of chelerythrine, 5-HT failed to potentiate IGABA, suggesting that protein kinase C (PKC) is involved in the pathway through which the activation of the 5-HT2 receptor potentiates the IGABA. 4. The facilitatory effect of 5-HT on IGABA remained in the presence of BAPTA-AM. LiCl also had no effect on 5-HT-induced potentiation of IGABA. 5. H-89, genistein, okadaic acid and pervanadate all had no effects on 5-HT potentiation of IGABA. Pertussis toxin treatment for 6-8 h did not block the facilitatory effect of 5-HT on IGABA. 6. The present results show that GABA(A) receptor in the rat SDCN could be modulated in situ by 5-HT, one of the major transmitters involved in the supraspinal control of nociception, and that the phosphorylation of GABA(A) receptor by PKC may be sufficient to support such modulation. The results also strongly support the hypothesis that the cotransmission by 5-HT and GABA has an important role in the spinal cord.

Antinociception produced by microinjection of morphine in the rat periaqueductal gray is enhanced in the foot, but not the tail, by intrathecal injection of alpha1-adrenoceptor antagonists

Antinociception produced by microinjection of morphine in the ventrolateral periaqueductal gray is mediated in part by alpha2-adrenoceptors in the spinal cord dorsal horn. However, several recent reports demonstrate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the feet. More specifically, alpha2-adrenoceptors appear to mediate the antinociception produced by morphine using the tail-flick test, but not that using the foot-withdrawal or hot-plate tests. The present study extended these findings and determined the role of alpha1-adrenoceptors in mediating the antinociceptive effects of morphine microinjected into the ventrolateral periaqueductal gray using both the foot-withdrawal and the tail-flick responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of selective antagonists was used to determine whether the antinociceptive effects of morphine were modulated by alpha1-adrenoceptors. Injection of the selective alpha1-adrenoceptor antagonists prazosin or WB4101 potentiated the increase in the foot-withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray. In contrast, either prazosin or WB4101 partially reversed the increase in the tail-flick response latency produced by morphine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray modulates nociceptive responses to noxious heating of the feet by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the tail. More specifically, alpha1-adrenoceptors mediate a pro-nociceptive action of morphine using the foot-withdrawal response, but in contrast, alpha1-adrenoceptors appear to mediate part of the antinociceptive effect of morphine determined using the tail-flick test.

The effects of strychnine, bicuculline, and ketamine on 'immersion-inhibited' dorsal horn convergent neurons in intact and spinalized rats

In both intact and spinalized rats, this study examined the effects of strychnine (a glycine antagonist), bicuculline (a GABAA antagonist), and ketamine (a non-competitive NMDA receptor antagonist) on one particular class of lumbar dorsal horn convergent neurons. This group of convergent neurons are inhibited when a rat's entire ipsilateral hindpaw is immersed in 50 degrees C water and has a strong afterdischarge as soon as the paw is removed from the water. Strychnine (2 mg/kg, iv) increased ongoing activity and blocked the 'inhibition phase' in both intact and spinalized rats demonstrating that a spinal-related glycine mechanism was involved in the inhibition. However, only in intact rats did the firing rate of the 'afterdischarge phase' increase significantly from pre-drug levels, suggesting that supraspinal sites may be involved in modulating this phase. Ketamine (15 mg/kg, iv) depressed ongoing activity and the firing rate in the afterdischarge phase of these neurons. Additionally, ketamine reversed the strychnine-induced increase in ongoing activity. Bicuculline (2 mg/kg, iv) had no effect on the activity of this cell class. As shown previously, and replicated here, these 'immersion-inhibited' neurons invariably have both inhibitory and excitatory mechano-receptive fields on the ipsilateral hindpaw. Thus, the response of this class of convergent neurons to noxious stimulation may be a function of relative inputs of glycine and EAA's, each possibly triggered by the stimulation of different receptive fields/regions on the same paw. Furthermore, when both fields are co-stimulated during noxious immersion of the entire paw, glycine has a stronger influence on activity than does the EAA's.

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

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

Spinal modulation of the induction of central sensitization

Peripheral tissue injury results in a change in the excitability of spinal dorsal horn neurons, central sensitization, and the behavioral correlate, hyperalgesia. It is proposed here that a dynamic balance exists between excitatory and inhibitory synaptic input to the spinal dorsal horn that functions to prevent central sensitization following brief, mild, noxious stimulation. Following more severe stimulation and injury, there is a loss of these inhibitory mechanisms that allow central sensitization to proceed. Single-unit recordings were made from L4-L5 deep dorsal horn neurons (wide dynamic range and nociceptive specific) from barbiturate-anesthetized rats that were non-inflamed or had a carrageenan-inflamed hindpaw. Baseline test responses to mechanical stimuli were obtained and normalized to 100%. An electrical conditioning stimulus (1 Hz, 20 s, C-fiber strength) was applied to the tibial nerve or the neuronal receptive field. Five seconds later the test stimulus was repeated and the magnitude of response compared to baseline. During the conditioning stimulus, 46% of the neurons from non-inflamed and inflamed rats showed wind-up although the magnitude of wind-up was significantly greater for inflamed rats. The remaining neurons showed no change (36-46%) or wind-down (8-18%). Five seconds following the end of the conditioning stimulus 67% of the neurons from non-inflamed rats had attenuated responses to mechanical stimuli (36% of baseline). The remaining neurons were either unaffected (30%) or facilitated (3%). Following inflammation significantly fewer neurons (28%) had attenuated responses and the magnitude of attenuation was significantly less than in non-inflamed rats (54% of baseline). The responses of the remaining neurons were unaffected (54%) or facilitated (18%). During subsequent test stimuli, the responses of 30% of the neurons from non-inflamed rats were facilitated to 140% of baseline. The responses of 46% of neurons from inflamed rats were facilitated to 160% of baseline. In these neurons there was significantly less initial attenuation following inflammation compared to non-inflamed rats. The response of the neuron during the electrical conditioning had no effect upon the response following conditioning. The conditioning stimulus given transcutaneously within the receptive field produced qualitatively similar results to tibial nerve stimulation. In non-inflamed rats, when the conditioning/test-stimulus interval was increased from 5 s to 10-30 s, the responses of 20% of the neurons were attenuated (compared to 67%) and the mean magnitude of attenuation was 52% of baseline (compared to 36% of baseline). However, the responses of only 33% of the neurons were ultimately facilitated (compared to 30%). The present study documents a short period following a low-frequency C-fiber input in which the response to natural stimuli is suppressed. It is suggested that this attenuation, whether or not expressed, prevents a significant portion of deep dorsal horn neurons from becoming sensitized to C-fiber input. This functions to prevent central sensitization when the noxious stimulus does not produce inflammation and it is not beneficial to the animal to become hyperalgesic (i.e., to alter its behavior in order to protect an injured limb and reduce painful sensations). Following injury-producing tissue damage and inflammation the mechanisms that produce the attenuation are reduced, with a concomitant increase in excitation to electrical and natural stimuli, suggesting that the attenuation is inhibitory modulation of nociceptive input and injury results in a disinhibition producing an increase in excitability and central sensitization.

Altered receptive fields and sensory modalities of rat VPL thalamic neurons during spinal strychnine-induced allodynia

Altered receptive fields and sensory modalities of rat VPL thalamic neurons during spinal strychnine-induced allodynia. J. Neurophysiol. 78: 2296-2308, 1997. Allodynia is an unpleasant sequela of neural injury or neuropathy that is characterized by the inappropriate perception of light tactile stimuli as pain. This condition may be modeled experimentally in animals by the intrathecal (i.t.) administration of strychnine, a glycine receptor antagonist. Thus after i.t. strychnine, otherwise innocuous tactile stimuli evoke behavioral and autonomic responses that normally are elicited only by noxious stimuli. The current study was undertaken to determine how i.t. strychnine alters the spinal processing of somatosensory input by examining the responses of neurons in the ventroposterolateral thalamic nucleus. Extracellular, single-unit recordings were conducted in the lateral thalamus of 19 urethan-anaesthetized, male, Wistar rats (342 +/- 44 g; mean +/- SD). Receptive fields and responses to noxious and innocuous cutaneous stimuli were determined for 19 units (1 per animal) before and immediately after i.t. strychnine (40 microgram). Eighteen of the animals developed allodynia as evidenced by the ability of otherwise innocuous brush or air jet stimuli to evoke cardiovascular and/or motor reflexes. All (3) of the nociceptive-specific units became responsive to brush stimulation after i.t. strychnine, and one became sensitive to brushing over an expanded receptive field. Expansion of the receptive field, as determined by brush stimulation, also was exhibited by all of the low-threshold mechanoreceptive units (14) and wide dynamic range units (2) after i.t. strychnine. The use of air jet stimuli at fixed cutaneous sites also provided evidence of receptive field expansion, because significant unit responses to air jet developed at 13 cutaneous sites (on 7 animals) where an identical stimulus was ineffective in evoking a unit response before i.t. strychnine. However, the magnitude of the unit response to cutaneous air jet stimulation was not changed at sites that already had been sensitive to this stimulus before i.t. strychnine. The onset of allodynia corresponded with the onset of the altered unit responses (i.e., lowered threshold/receptive field expansion) for the majority of animals (9), but the altered unit response either terminated concurrently with symptoms of allodynia (6) or, more frequently, outlasted the symptoms of allodynia (10) as the effects of strychnine declined. The present results demonstrate that the direct, receptor-mediated actions of strychnine on the spinal processing of sensory information are reflected by changes in the receptive fields and response properties of nociceptive and nonnociceptive thalamic neurons. These changes are consistent with the involvement of thalamocortical mechanisms in the expression of strychnine-induced allodynia and, moreover, suggest that i.t. strychnine also produces changes in innocuous tactile sensation.