TRPA1‐expressing primary afferents synapse with a morphologically identified subclass of substantia gelatinosa neurons in the adult rat spinal cord

The TRPA1 channel has been proposed to be a molecular transducer of cold and inflammatory nociceptive signals. It is expressed on a subset of small primary afferent neurons both in the peripheral terminals, where it serves as a sensor, and on the central nerve endings in the dorsal horn. The substantia gelatinosa (SG) of the spinal cord is a key site for integration of noxious inputs. The SG neurons are morphologically and functionally heterogeneous and the precise synaptic circuits of the SG are poorly understood. We examined how activation of TRPA1 channels affects synaptic transmission onto SG neurons using whole‐cell patch‐clamp recordings and morphological analyses in adult rat spinal cord slices. Cinnamaldehyde (TRPA1 agonist) elicited a barrage of excitatory postsynaptic currents (EPSCs) in a subset of the SG neurons that responded to allyl isothiocyanate (less specific TRPA1 agonist) and capsaicin (TRPV1 agonist). Cinnamaldehyde evoked EPSCs in vertical and radial but not islet or central SG cells. Notably, cinnamaldehyde produced no change in inhibitory postsynaptic currents and nor did it produce direct postsynaptic effects. In the presence of tetrodotoxin, cinnamaldehyde increased the frequency but not amplitude of miniature EPSCs. Intriguingly, cinnamaldehyde had a selective inhibitory action on monosynaptic C‐ (but not Aδ‐) fiber‐evoked EPSCs. These results indicate that activation of spinal TRPA1 presynaptically facilitates miniature excitatory synaptic transmission from primary afferents onto vertical and radial cells to initiate action potentials. The presence of TRPA1 channels on the central terminals raises the possibility of bidirectional modulatory action in morphologically identified subclasses of SG neurons.     

The cytokine TNFalpha increases the proportion of DRG neurones expressing the TRPV1 receptor via the TNFR1 receptor and ERK activation

TNFalpha is involved in the generation of hyperalgesia in pathological states such as neuropathy and inflammation. The pronociceptive action of TNFalpha may be mediated at least in part by activation of the TRPV1 receptor which transduces heat stimuli in primary nociceptive afferents and mediates thermal hyperalgesia. In the present study, we investigated in cultured dorsal root ganglion (DRG) neurones, the somata of primary afferent fibres, whether TNFalpha increases TRPV1 receptor expression. We found that long-term exposure of DRG neurones of both rat and mouse to TNFalpha significantly increased the proportion of DRG neurones expressing TRPV1 receptor-like immunoreactivity. This TNFalpha effect was abolished in mice DRG neurones when DRG cultures were obtained from tnfr1/2-/- and tnfr1-/-, but not from tnfr2-/- mice. Furthermore, we found that activation of ERK but not of p38 kinase or cyclooxygenases is critically involved in the TNFalpha-induced increase of TRPV1 receptor expression.     

Transient receptor potential A1 mediates acetaldehyde-evoked pain sensation

Six transient receptor potential (TRP) ion channels expressed in the sensory afferents play an important role as body thermosensors and also as peripheral pain detectors. It is known that a number of natural compounds specifically activate those sensory neuronal TRP channels, and a well-known example is cinnamaldehyde for TRPA1. Here we show that human and mouse TRPA1 are activated by acetaldehyde, an intermediate substance of ethanol metabolism, in the HEK293T cell heterologous expression system and in cultured mouse trigeminal neurons. Acetaldehyde failed to activate other temperature-sensitive TRP channels expressed in sensory neurons. TRPA1 antagonists camphor and gadolinium, and a general TRP blocker ruthenium red inhibited TRPA1 activation by acetaldehyde. Camphor, gadolinium and ruthenium red also suppressed the acute nociceptive behaviors induced by the intradermal administration of acetaldehyde into the mouse footpads. Intradermal co-application of prostaglandin E2 and acetaldehyde greatly potentiated the acetaldehyde-induced nociceptive responses, and this effect was reversed by treatment with the TRPA1 antagonist camphor. These results suggest that acetaldehyde causes nociception via TRPA1 activation. Our data may also help elucidate the mechanisms underlying acetaldehyde-related pathological symptoms such as hangover pain.     

Effects of TRPA1 activation and inhibition on TRPA1 and CGRP expression in dorsal root ganglion neurons

Transient receptor potential ankyrin 1(TRPA1) is a key player in pain and neurogenic inflammation, and is localized in nociceptive primary sensory dorsal root ganglion(DRG) neurons. TRPA1 plays a major role in the transmission of nociceptive sensory signals. The generation of neurogenic inflammation appears to involve TRPA1-evoked release of calcitonin gene-related peptide(CGRP). However, it remains unknown whether TRPA1 or CGRP expression is affected by TRPA1 activation. Thus, in this study, we examined TRPA1 and CGRP expression in DRG neurons in vitro after treatment with the TRPA1 activator formaldehyde or the TRPA1 blocker menthol. In addition, we examined the role of extracellular signal-regulated protein kinase 1/2(ERK1/2) in this process. DRG neurons in culture were exposed to formaldehyde, menthol, the ERK1/2 inhibitor PD98059 + formaldehyde, or PD98059 + menthol. After treatment, real-time polymerase chain reaction, western blot assay and double immunofluorescence labeling were performed to evaluate TRPA1 and CGRP expression in DRG neurons. Formaldehyde elevated mRNA and protein levels of TRPA1 and CGRP, as well as the proportion of TRPA1-and CGRP-positive neurons. In contrast, menthol reduced TRPA1 and CGRP expression. Furthermore, the effects of formaldehyde, but not menthol, on CGRP expression were blocked by pretreatment with PD98059. PD98059 pretreatment did not affect TRPA1 expression in the presence of formaldehyde or menthol.     

Spinal somatostatin SSTR2A receptors are preferentially up-regulated and involved in thermonociception but not mechanonociception

The present study was undertaken to investigate the role of spinal somatostatin SSTR2A receptors in nociceptive processing. SSTR2A receptor-like immunoreactivity was found in a dense network in the spinal cord of normal rats. With Western blot analysis a major band of approximately 80-85 kDa was detected. Both immunohistochemistry and immunoblot analysis indicated a significant increase in SSTR2A receptor content in the spinal cord 6 h after noxious thermal stimulation that lasted for at least 24 h. However, there were no notable changes in SSTR2A receptor content 3, 6, 12, or 24 h after noxious mechanical stimulation. Effects of intrathecally administered polyclonal antiserum to SSTR2A receptor (anti-SSTR2A) on thermal and mechanical pain thresholds were determined with behavioral tests. In normal rats, pretreatment with anti-SSTR2A (1 microl, intrathecal) did not affect paw withdrawal latency or pinch threshold. Hindpaw inflammation induced by complete Freund's adjuvant led to thermal and mechanical hyperalgesia as reflected by a robust decrease in paw withdrawal latency and pinch threshold. Significant attenuation of the thermal hyperalgesia was observed 3, 5, 7, 9, and 24 h after pretreatment with anti-SSTR2A. This effect disappeared in another 24 h. In contrast, pretreatment with anti-SSTR2A failed to exert any notable effect on adjuvant-induced mechanical hyperalgesia. The present findings provide the first evidence that SSTR2A receptors are responsible for thermal, but not mechanical, nociceptive transmission in the spinal cord. The results also suggest that somatostatin has an excitatory role in spinal nociceptive processing and that there are differential receptor responses to different types of noxious stimuli.     

A pharmacological study of the modulation of neuronal and behavioural nociceptive responses in the rat trigeminal region

Electrical stimulation of the brain, particularly in the periventricular grey areas, caused long-lasting increases in behavioural escape thresholds to heating and mechanical stimuli applied to the facial region of the rat. The brain stimulation selectively suppressed responses to noxious stimuli. Responses to non-noxious stimuli, evoked by low threshold brush, were unaffected. The same animals that were studied in the behavioural tests were then anaesthetized with urethane and the inhibitory effect of the same brain stimulation was studied in single neurones recorded in the caudal trigeminal nucleus. A clear correlation (r_s = 0.63) emerged between degree of behavioural antinociception and the amount of inhibition seen in nociceptive neurones. In addition the mean duration of the inhibition (6 min) was similar to the mean duration of the antinociceptive effect (7.3 min). Other classes of non-nociceptive neurones were unaffected by the stimulation. The neurones were also studied using iontophoretically applied monoamine candidates for the inhibitory neurotransmitter, noradrenaline (NA) and 5-hydroxytryptamine (5-HT). The profile of the effects of NA most closely fitted that of the inhibitory neurotransmitter. This profile was expressed in terms of depression and excitation of different classes of neurones, and by the duration of effects. The depressant effects could be antagonized by iontophoretic idazoxan. In addition clonidine induced long-lasting depression of firing. 5-HT was more likely than NA to excite nociceptive neurones and to depress non-nociceptive neurones. Only NA consistently elevated thermal response thresholds in a similar manner to that produced by brain stimulation. These results provide some support for the hypothesis that selective descending inhibition of nociceptive responses in neurones of the rat caudal trigeminal nucleus is mediated by NA, possibly by an action at α₂-adrenoceptors.     

MOR-1-immunoreactive neurons in the dorsal horn of the rat spinal cord: evidence for nonsynaptic innervation by substance P-containing primary afferents and for selective activation by noxious thermal stimuli

A direct action of mu-opioid agonists on neurons in the spinal dorsal horn is thought to contribute to opiate-induced analgesia. In this study we have investigated neurons that express the mu-opioid receptor MOR-1 in rat spinal cord to provide further evidence about their role in nociceptive processing. MOR-1-immunoreactive cells were largely restricted to lamina II, where they comprised approximately 10% of the neuronal population. The cells received few contacts from nonpeptidergic unmyelinated afferents, but many from substance P-containing afferents. However, electron microscopy revealed that most of these contacts were not associated with synapses. None of the MOR-1 cells in lamina II expressed the neurokinin 1 receptor; however, the mu-selective opioid peptide endomorphin-2 was present in the majority (62-82%) of substance P axons that contacted them. Noxious thermal stimulation of the foot induced c-Fos expression in approximately 15% of MOR-1 cells in the medial third of the ipsilateral dorsal horn at mid-lumbar level. However, following pinching of the foot or intraplantar injection of formalin very few MOR-1 cells expressed c-Fos, and for intraplantar formalin injection this result was not altered significantly by pretreatment with systemic naloxone. Although these findings indicate that at least some of the neurons in lamina II with MOR-1 are activated by noxious thermal stimulation, the results do not support the hypothesis that the cells have a role in transmitting nociceptive information following acute mechanical or chemical noxious stimuli.     

Neonatal capsaicin and thermal nociception: a paradox

Thermal and mechanical nociceptive thresholds and somato-visceral reflexes have been studied in normal rats and in rats treated neonatally with capsaicin. Mechanical nociceptive thresholds were increased in capsaicin treated rats whereas thermal nociceptive thresholds remained unchanged. In contrast, somato-visceral reflexes evoked by thermal noxious stimulation of the skin could not be elicited in capsaicin treated rats whereas mechanical noxious stimulation was effective in evoking visceral reflexes.     

Neuropathic pain and the endocannabinoid system in the dorsal raphe: pharmacological treatment and interactions with the serotonergic system

We used a model of neuropathic pain consisting of rats with chronic constriction injury (CCI) of the sciatic nerve, in order to investigate whether endocannabinoid levels are altered in the dorsal raphe (DR) and to assess the effect of repeated treatment with (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate, a synthetic cannabinoid agonist, or N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM404), an inhibitor of endocannabinoid reuptake, on DR serotonergic neuronal activity and on behavioural hyperalgesia. CCI resulted in significantly elevated anandamide but not 2-arachidonoylglycerol levels in the DR. Furthermore, as well as thermal and mechanical hyperalgesia, CCI caused serotonergic hyperactivity (as shown by the increase of basal activity of serotonergic neurones, extracellular serotonin levels and expression of 5-HT1A receptor gene). Repeated treatment with either (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate or AM404 reverted the hyperalgesia and enhanced serotonergic activity induced by CCI in a way attenuated by N-piperidino-5-(4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-3-pyrazolecarboxamide, a selective cannabinoid subtype 1 (CB1) receptor antagonist. Despite the elevated levels of anandamide following CCI, N-piperidino-5-(4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-3-pyrazolecarboxamide did not produce hyperalgesia or any other effect on serotonergic neuronal activity when administered alone. Furthermore, the effects of AM404 were not accompanied by an increase in endocannabinoid levels in the DR. In conclusion, following CCI of the sciatic nerve, the endocannabinoid and serotonergic systems are activated in the DR, where repeated stimulation of CB1 receptors with exogenous compounds restores DR serotonergic activity, as well as thermal and mechanical nociceptive thresholds, to pre-surgery levels. However, an elevated level of endogenous anandamide in the DR does not necessarily contribute to the CB1-mediated tonic control of analgesia and serotonergic neuronal activity.     

Corneal innervation and sensitivity to noxious stimuli in trkA knockout mice

Most primary sensory neurones depend on neurotrophins for survival. Mutant mice in which TrkA, the high-affinity receptor for nerve growth factor (NGF), has been inactivated lack nociceptive neurones in sensory ganglia and do not respond to noxious stimuli. The cornea of the eye is innervated by trigeminal neurones that are activated by noxious mechanical, thermal and chemical stimuli. In the human cornea, these stimuli evoke only sensations of pain. We have analysed the innervation pattern and the response to noxious stimulation of the cornea of trkA (–/–) mutant mice. Corneal nerves were stained with the gold chloride impregnation method. Corneal sensitivity to noxious stimuli was assessed by counting blinking movements evoked by von Frey hairs, topical application of saline at different temperatures and application of acetic acid and capsaicin at different concentrations. In the cornea of trkA (–/–) mutant animals, we observed a drastic reduction in the number of nerve trunks and branches in the corneal stroma. Furthermore, quantitative analysis of the number of thin nerve terminals revealed a marked decrease in the corneal epithelium of trkA (–/–) mice when compared to those present in wild type and trkA (+/–) animals. The blinking response of trkA (–/–) mice to mechanical, thermal and chemical noxious stimuli was also significantly reduced. These results indicate that the population of corneal sensory neurones is markedly depleted in trkA (–/–) mutant mice. However, a small portion of corneal sensory neurones survive in these mice suggesting that they may be NGF independent. On the basis of our results, we propose that these surviving cells are polymodal nociceptive neurones, sensitive to mechanical stimulation, noxious heat and acid.     

The primary somatosensory cortex and the insula contribute differently to the processing of transient and sustained nociceptive and non‐nociceptive somatosensory inputs

Transient nociceptive stimuli elicit consistent brain responses in the primary and secondary somatosensory cortices (S1, S2), the insula and the anterior and mid‐cingulate cortex (ACC/MCC). However, the functional significance of these responses, especially their relationship with sustained pain perception, remains largely unknown. Here, using functional magnetic resonance imaging, we characterize the differential involvement of these brain regions in the processing of sustained nociceptive and non‐nociceptive somatosensory input. By comparing the spatial patterns of activity elicited by transient (0.5 ms) and long‐lasting (15 and 30 s) stimuli selectively activating nociceptive or non‐nociceptive afferents, we found that the contralateral S1 responded more strongly to the onset of non‐nociceptive stimulation as compared to the onset of nociceptive stimulation and the sustained phases of nociceptive and non‐nociceptive stimulation. Similarly, the anterior insula responded more strongly to the onset of nociceptive stimulation as compared to the onset of non‐nociceptive stimulation and the sustained phases of nociceptive and non‐nociceptive stimulation. This suggests that S1 is specifically sensitive to changes in incoming non‐nociceptive input, whereas the anterior insula is specifically sensitive to changes in incoming nociceptive input. Second, we found that the MCC responded more strongly to the onsets as compared to the sustained phases of both nociceptive and non‐nociceptive stimulation, suggesting that it could be involved in the detection of change regardless of sensory modality. Finally, the posterior insula and S2 responded maximally during the sustained phase of non‐nociceptive stimulation but not nociceptive stimulation, suggesting that these regions are preferentially involved in processing non‐nociceptive somatosensory input. Hum Brain Mapp 36:4346–4360, 2015. © 2015 Wiley Periodicals, Inc.     

Intrathecal neuropeptide Y exacerbates nerve injury-induced mechanical hyperalgesia

In normal animals, spinal administration of neuropeptide Y induces analgesia to thermal stimuli, but has no effect on mechanical thresholds. Recent anatomical studies, however, have shown that following nerve injury there is an altered expression of neuropeptide Y and its receptors. The aim of this behavioural study, therefore, is to examine the effect of intrathecal administration of neuropeptide Y, its agonists and an antagonist on mechanical nociceptive thresholds in rats with partial injury to the sciatic nerve. Test agents were administered for 14 days via osmotic pumps (0.5 μl/day) attached to intrathecal catheters and the nociceptive flexion reflex was quantified using an Ugo Basile Analgesymeter. Partial injury to the sciatic nerve, in animals treated intrathecally with saline, induces a significant decrease in mechanical threshold as compared to the sham operated, contralateral paw. The nerve injury-induced hyperalgesia is exacerbated by 2 μM neuropeptide Y and by 2 μM [Leu³¹,Pro³⁴]-neuropeptide Y, a Y₁ receptor agonist. The Y₂ receptor agonist, N-acetyl-[Leu²⁸,Leu³¹]-neuropeptide Y_24–36 (2 μM), had no effect on the nerve injury-induced hyperalgesia. The putative neuropeptide Y antagonist, -trinositol (10 μM), significantly attenuated the nerve injury-induced hyperalgesia. This study suggests that neuropeptide Y may contribute to nerve injury-induced mechanical hyperalgesia via the Y₁ receptor and provides further insight into the possible mechanisms underlying nerve injury-induced hyperalgesia to mechanical stimuli.     

Properties of functionally identified nociceptive and nonnociceptive facial primary afferents and presynaptic excitability changes induced in their brain stem endings by raphe and orofacial stimuli in cats

The activity of 221 single primary afferent units was recorded extracellularly in the trigeminal (V) ganglion of chloralose-anaesthetized cats to examine the receptive field properties of nonnociceptive and nociceptive cutaneous afferents and the effect of conditioning stimulation of the raphe system and orofacial afferents on the antidromic excitability of their brain stem endings in V subnucleus caudalis. In addition to slowly adapting and rapidly adapting low-threshold mechanosensitive afferents, we functionally identified three classes of cutaneous nociceptive afferents: these included A-delta high-threshold mechanoreceptive afferents (A-delta HTMs), C-fiber high-threshold mechanoreceptive afferents (C-HTMs), and C-polymodal nociceptive afferents (CPNs). Most of the CPNs could be activated by light tactile stimuli as well as by heavy pressure and pinch and noxious radiant heat applied to their mechanoreceptive field which usually involved a localized spot (approximately 1 mm in diameter) of skin. In contrast, the C-HTMs and A-delta HTMs could not be activated by radiant heat stimuli although some did show sensitization which was also a feature of the CPNs; they did respond to noxious mechanical stimulation of a localized area of skin. We noted that orofacial conditioning stimulation could produce an increase in antidromic excitability which was considered a reflection of primary afferent depolarization (PAD) in both nociceptive and nonnociceptive afferents innervating the cat's facial skin; nonnoxious mechanical stimuli and electrical stimuli were particularly effective in the low-threshold mechanosensitive afferents and noxious mechanical and high-intensity electrical stimuli were especially effective in the cutaneous nociceptive afferents. Raphe conditioning stimulation also was very effective in inducing PAD in these nociceptive afferents; however, the raphe conditioning effects were not limited to these nociceptive afferents since PAD was also frequently demonstrated in the low-threshold mechanosensitive afferents.     

Activation of transient receptor potential vanilloid 2‐expressing primary afferents stimulates synaptic transmission in the deep dorsal horn of the rat spinal cord and elicits mechanical hyperalgesia

Probenecid, an agonist of transient receptor vanilloid (TRPV) type 2, was used to evaluate the effects of TRPV2 activation on excitatory and inhibitory synaptic transmission in the dorsal horn (DH) of the rat spinal cord and on nociceptive reflexes induced by thermal heat and mechanical stimuli. The effects of probenecid were compared with those of capsaicin, a TRPV1 agonist. Calcium imaging experiments on rat dorsal root ganglion (DRG) and DH cultures indicated that functional TRPV2 and TRPV1 were expressed by essentially non‐overlapping subpopulations of DRG neurons, but were absent from DH neurons and DH and DRG glial cells. Pretreatment of DRG cultures with small interfering RNAs against TRPV2 suppressed the responses to probenecid. Patch‐clamp recordings from spinal cord slices showed that probenecid and capsaicin increased the frequencies of spontaneous excitatory postsynaptic currents (sEPSCs) and spontaneous inhibitory postsynaptic currents in a subset of laminae III–V neurons. In contrast to capsaicin, probenecid failed to stimulate synaptic transmission in lamina II. Intrathecal or intraplantar injections of probenecid induced mechanical hyperalgesia/allodynia without affecting nociceptive heat responses. Capsaicin induced both mechanical hyperalgesia/allodynia and heat hyperalgesia. Activation of TRPV1 or TRPV2 in distinct sets of primary afferents increased the sEPSC frequencies in a largely common population of DH neurons in laminae III–V, and might underlie the development of mechanical hypersensitivity following probenecid or capsaicin treatment. However, only TRPV1‐expressing afferents facilitated excitatory and/or inhibitory transmission in a subpopulation of lamina II neurons, and this phenomenon might be correlated with the induction of thermal heat hyperalgesia.     

Response properties of nociceptive and low-threshold mechanoreceptive neurons in the hamster superior colliculus

There are many somatosensory neurons in the hamster superior colliculus (SC); some respond to innocuous tactile stimuli, while others respond either preferentially, or solely, to noxious stimuli. Yet, there are little quantitative data describing the responses of these neurons. We sought to provide such information by relating stimulus intensity to the magnitude of the neural response using controlled innocuous and noxious mechanical and thermal stimuli. Of 122 somatosensory SC neurons studied in urethane-anesthetized hamsters, the majority (52%) had low-threshold mechanoreceptive properties (LT). LT neurons had force thresholds less than 1 gm, adapted rapidly to maintained stimuli, and did not respond with higher numbers of impulses to noxious mechanical or thermal stimuli. A smaller, though substantial, proportion of neurons (45%) responded either preferentially, or solely, to noxious stimuli. A few neurons (3%) were inhibited by either light tactile or noxious mechanical stimuli. Two populations of nociceptive neurons were found and classified either as wide dynamic range (WDR) neurons (n = 25), those that responded to gentle mechanical, noxious mechanical, and/or thermal stimuli; or nociceptive-specific (NS) neurons (n = 30), those that responded solely to high-intensity mechanical or noxious thermal stimuli. WDR neurons responded monotonically to increases in the intensity of innocuous mechanical stimuli, and displacement-response relationship for this population was a slightly negatively accelerating power function with an exponent of 0.785. However, the thermal stimulus-response relationships (to graded skin temperatures) of both WDR and NS neurons were positively accelerating power functions with exponents of 2.3 and 2.5 (r2 = 0.988), respectively. These values are consistent with both electrophysiological data from dorsal horn nociceptive neurons and from human psychophysical results using the same range of thermal stimuli. These experiments demonstrate that SC neurons are capable of signaling not only the presence and location of a noxious stimulus but its intensity as well. Presumably, these neurons play a significant role in the animal's reactions to potentially harmful stimuli. The partial laminar segregation of WDR and NS neurons may reflect different involvements of particular nociceptive subtypes in the various overt responses mediated by the SC.     

Activation of 5-HT(1B/1D) receptor in the periaqueductal gray inhibits nociception

It is considered that the site of action of the abortive antimigraine compounds acting at serotonin, 5-HT(1B/1D,) receptors (triptans) is the trigeminovascular system. We tested whether there is a non-trigeminal site of action. The 5-HT(1B/1D) agonist, naratriptan, was microinjected into the ventrolateral periaqueductal gray (vlPAG), and activity in the trigeminal nucleus caudalis (TNC) was monitored. Recordings were made from 20 nociceptive neurons in the dorsal horn of the TNC that received convergent input from the dura mater and face. Responses of neurons to dural, facial cutaneous and corneal stimulation were studied before and after injection of naratriptan. Naratriptan decreased the excitability to electrical stimulation of the dura mater as the A-fiber response decreased by 24 +/- 4.1% (p < 0.001) and the C-fiber response decreased by 42 +/- 8.2% (p < 0.001). Spontaneous activity was decreased by 38 +/- 7.5% (p < 0.001). After injection, the mechanical thresholds of the dura mater increased from (n = 14, p < 0.01). Responses to stimulation of the face and cornea were not altered by injection of naratriptan. These results suggest that 5-HT(1B/1D) receptor activation in the vlPAG activates descending pain-modulating pathways that inhibit dural, but not facial and corneal nociceptive input. These findings have implications for the understanding of the action of triptans in migraine and cluster headache, suggesting that brain loci other than the trigeminal nucleus may play a role in the clinical action of triptans.     

Dorsal column postsynaptic neurons in the cat are excited by myelinated nociceptors

Some (25-50%) dorsal column postsynaptic (DCPS) neurons respond only to innocuous mechanical stimuli; the remainder (50-75%) responds to both innocuous and noxious mechanical stimuli. Those that respond to noxious mechanical stimuli (pinch) are assumed to be excited by input from nociceptive primary afferents, but it is conceivable that their pinch-evoked responses are produced by the inadvertent activation of those low-threshold mechanoreceptive primary afferents that respond to stretching the skin. Because nociceptive primary afferents respond reliably to noxious heat and low-threshold mechanoreceptors do not, we tested DCPS neurons in the cat lumbar spinal cord with a series of noxious heat stimuli (48 degrees C or 50 degrees C-56 degrees C; 30 s duration). Seven of eight pinch-responsive neurons responded to noxious heat, but only after their receptive fields had been sensitized by prolonged or repeated heating. The results show that (1) many DCPS neurons in the cat are excited by nociceptive primary afferents and (2) these nociceptive afferents are probably myelinated high-threshold mechanoreceptors.     

Reactions of the neurons of various hypothalamic structures to tooth pulp and sciatic nerve A beta-fiber stimulation in the cat

Responses of single units from lateral and medial areas of the posterior, tuberal and anterior hypothalamus to electrical stimulation of dental pulp and sciatic nerve A beta afferents were recorded in anesthetized curarized cats. 80.7%, 81.5% and 71.4% of units, respectively, responded to stimulation in posterior tuberal and anterior hypothalamus. The shortest latency of responses was recorded in the postero-lateral hypothalamus. Nociceptive responses in the lateral hypothalamus were of shorter latency than responses of medial hypothalamic units. Marked predominance of excitatory responses in the posterior hypothalamus and almost an equal proportion of excitatory and inhibitory responses in the tuberal and anterior hypothalamus were found. High degree of convergence (85.8%) of noxious and nonnoxious (sciatic nerve A beta afferents) impulses was revealed. Unidirectional, similar pattern of responses of convergent type neurons to stimulation of the tooth pulp and sciatic nerve A beta afferents indicate a nonspecific character of responses of the majority of polysensory neurons. 14.2% of hypothalamic units were monomodal, specific nociceptive neurons. Hypothalamic nociceptive units were characterized by a long recovery cycle (200-500 ms) and low reproducibility of responses to repetitive stimulation of the tooth pulp afferents (1.5-2/s). The neuronal organization of the nociceptive afferent system of the hypothalamus and the functional role of convergent and specific nociceptive neurons of the hypothalamus are discussed.     

Characterization of cat nasal afferents and brain stem neurones receiving ethmoidal input

Stimulation of the nasal mucous membrane can initiate protective reflexes, particularly sneezing and apnea. Very little is known about the receptors in the nasal cavity responsible for initiating these reflexes, which are thought to be mediated by trigeminal rather than olfactory pathways. In the cat, the ethmoidal branch of the ophthalmic division of V carries this afferent information. The objectives of the present study were to determine a set of adequate nasal receptor stimuli capable of initiating these protective reflexes; to characterize the types of information carried in the ethmoidal nerve afferents, by recording from single fibers dissected from the nerve; to determine the conduction velocities of these afferents, by recording from the cell bodies of ethmoidal afferents in the trigeminal ganglion; and to study the second-order neurones in the brain stem on which these afferents make contact. Results of single fiber recording indicated that, in addition to being tactile, approximately half of the neurones studied also responded to noxious chemical stimuli. The conduction velocity of the majority of these afferents were found to be in the A-delta range. Second-order neurones in spinal trigeminal nucleus which had an input from the ethmoidal nerve could be divided into two classifications: (i) low threshold mechanoreceptive (LTM) neurones which received light tactile input and did not respond to noxious chemical or mechanical stimuli applied to the nasal cavity, and (ii) wide dynamic range (WDR) neurones which responded to both noxious and nonnoxious chemical or mechanical stimuli applied to the nasal cavity.     

5,6-EET is released upon neuronal activity and induces mechanical pain hypersensitivity via TRPA1 on central afferent terminals

Epoxyeicosatrienoic acids (EETs) are cytochrome P450-epoxygenase-derived metabolites of arachidonic acid that act as endogenous signaling molecules in multiple biological systems. Here we have investigated the specific contribution of 5,6-EET to transient receptor potential (TRP) channel activation in nociceptor neurons and its consequence for nociceptive processing. We found that, during capsaicin-induced nociception, 5,6-EET levels increased in dorsal root ganglia (DRGs) and the dorsal spinal cord, and 5,6-EET is released from activated sensory neurons in vitro. 5,6-EET potently induced a calcium flux (100 nm) in cultured DRG neurons that was completely abolished when TRPA1 was deleted or inhibited. In spinal cord slices, 5,6-EET dose dependently enhanced the frequency, but not the amplitude, of spontaneous EPSCs (sEPSCs) in lamina II neurons that also responded to mustard oil (allyl isothiocyanate), indicating a presynaptic action. Furthermore, 5,6-EET-induced enhancement of sEPSC frequency was abolished in TRPA1-null mice, suggesting that 5,6-EET presynaptically facilitated spinal cord synaptic transmission by TRPA1. Finally, in vivo intrathecal injection of 5,6-EET caused mechanical allodynia in wild-type but not TRPA1-null mice. We conclude that 5,6-EET is synthesized on the acute activation of nociceptors and can produce mechanical hypersensitivity via TRPA1 at central afferent terminals in the spinal cord.