Processing Of Nociceptive Information Research Articles

Transient receptor potential vanilloid type 1 receptor regulates glutamatergic synaptic inputs to the spinothalamic tract neurons of the spinal cord deep dorsal horn

The spinothalamic tract (STT) neurons in the spinal dorsal horn play an important role in transmission and processing of nociceptive sensory information. Although transient receptor potential vanilloid type 1 (TRPV1) receptors are present in the spinal cord dorsal horn, their physiological function is not fully elucidated. In this study, we examined the role of TRPV1 in modulating neuronal activity of the STT neurons through excitatory and inhibitory synaptic inputs. Whole-cell patch-clamp recordings were performed on STT neurons labeled by a retrograde fluorescent tracer injected into the ventral posterior lateral (VPL) nucleus of the thalamus. Capsaicin (1 μM) increased the frequency of miniature excitatory postsynaptic currents (mEPSC) without changing the amplitude or decay time constant of mEPSC. In contrast, capsaicin had no distinct effect on GABAergic miniature inhibitory postsynaptic currents (mIPSC). Capsazepine (10 μM), a TRPV1 receptor antagonist, abolished the effect of capsaicin on mEPSCs. Capsazepine itself did not affect the baseline amplitude and frequency of mEPSC. The effect of capsaicin on mEPSC was also abolished by removal of external Ca 2+, but not by treatment with Cd 2+. Furthermore, capsaicin increased the firing activity of the STT neurons and this increase in neuronal activity by capsaicin was abolished in the presence of non– N-methyl- d-aspartic acid (NMDA) and NMDA receptor antagonists, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) and (R)-amino-5-phosphonovaleric acid (APV). These data suggest that activation of TRPV1 potentiates the glutamate release from excitatory terminals of primary afferent fibers and subsequently increases the neural activity of STT neurons of the rat spinal cord deep dorsal horn.

Antinociceptive synergistic effect of spinal mGluR2/3 antagonist and glial cells inhibitor on peripheral inflammation-induced mechanical hypersensitivity

Metabotropic glutamate receptor (mGluR) 2/3 is distributed in neurons and glial cells in many regions of the nervous system, but its role in nociceptive processing is unclear. In this study, we examined the mRNA expressions of mGluR2 and mGluR3, by real-time RT-PCR, in the spinal cord. We further investigated the possible involvement of mGluR2/3 and mechanisms underlying peripheral inflammatory pain induced by subcutaneous complete Freund's adjuvant (CFA) injection. We demonstrate that compared to the controls, the mRNA expression levels of mGluR2 and mGluR3 were significantly higher 4 h after CFA injection. Functionally, blocking mGluR2/3 by their antagonist (2S)-2-amino-2-[(1S, 2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495) alleviated the CFA-induced mechanical allodynia and the inhibitory effects were reversed by mGluR2/3 agonist (2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate ((2R, 4R)-APDC). In addition, a glial metabolism inhibitor dl-fluorocitric acid barium salt (fluorocitric acid) also inhibited the CFA-induced mechanical allodynia in a dose-dependent manner. Remarkably, simultaneous inhibition of mGluR2/3 and glial metabolism had synergistic effects. The co-administration of LY341495 and fluorocitric acid with minimal dosages produced significant more inhibition than the additive effects by the individual inhibitor alone. In summary, our data suggest that spinal mGluR2/3 contributes to the generation of mechanical allodynia induced by peripheral inflammation. We also suggest that involvement of mGluR2/3 in the communication between glial cells and neurons takes part in the processing of nociceptive information.

Functional type I vanilloid receptor expression by substantia gelatinosa neurons of trigeminal subnucleus caudalis in mice

The aim of this study was to investigate the existence of functional TRPV1 receptor by substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis (Vc), which is implicated in the processing of nociceptive information from orofacial regions. The direct membrane effects of a TRPV1 receptor agonist, capsaicin, were examined by gramicidin-perforated patch clamp recording using a trigeminal brainstem slice preparation containing Vc from immature mice. Capsaicin (1–2 μM) induced a membrane depolarization in 58 out of 71 SG neurons tested (82%). Capsaicin-induced depolarization was maintained in 20 out of 32 (63%) SG neurons in the presence of amino acid and voltage-dependent sodium channel blockers (10 μM CNQX, 20 μM AP-5, 0.5 μM TTX, 50 μM picrotoxin and 2 μM strychnine). In addition, capsaicin-induced depolarization was maintained in the presence of L-732,138 (1 μM), an NK1 receptor antagonist, in 14 out of 17 neurons (82%) tested. The capsaicin-induced depolarizing effects were blocked by a TRPV1 receptor antagonist, capsazepine (10 μM). These results indicate that a sub-population of SG neurons in the Vc express functional TRPV1 receptors, and that capsaicin can directly activate the TRPV1 receptor on the postsynaptic membrane of SG neurons.

Evidence for a Critical Period in the Development of Excitability and Potassium Currents in Mouse Lumbar Superficial Dorsal Horn Neurons

The output of superficial dorsal horn (SDH; laminae I-II) neurons is critical for processing nociceptive, thermal, and tactile information. Like other neurons, the combined effects of synaptic inputs and intrinsic membrane properties determine their output. It is well established that peripheral synaptic inputs to SDH neurons undergo extensive reorganization during pre- and postnatal development. It is unclear, however, how membrane properties or the subthreshold whole cell currents that shape SDH neuron output change during this period. Here we assess the intrinsic membrane properties and whole cell currents in mouse SDH neurons during late embryonic and early postnatal development (E15-P25). Transverse slices were prepared from lumbar spinal cord and whole cell recordings were obtained at 32 degrees C. During this developmental period resting membrane potential (RMP) became more hyperpolarized (by approximately 10 mV, E15-E17 vs. P21-P25) and input resistance decreased (1,074 +/- 78 vs. 420 +/- 27 MOmega). In addition, action potential (AP) amplitude and AP afterhyperpolarization increased, whereas AP half-width decreased. Before and after birth (E15-P10), AP discharge evoked by intracellular current injection was limited to a single AP at depolarization onset in many neurons (>41%). In older animals (P11-P25) this changed, with AP discharge consisting of brief bursts at current onset ( approximately 46% of neurons). Investigation of major subthreshold whole cell currents showed the rapid A-type potassium current (I(Ar)) dominated at all ages examined (90% of neurons at E15-E17, decreasing to >50% after P10). I(Ar) expression levels, based on peak current amplitude, increased during development. Steady-state inactivation and activation for I(Ar) were slightly less potent in E15-E17 versus P21-P25 neurons at potentials near RMP (-55 mV). Together, our data indicate that intrinsic properties and I(Ar) expression change dramatically in SDH neurons during development, with the greatest alterations occurring on either side of a critical period, P6-P10.

Short-Term Synaptic Plasticity in the Nociceptive Thalamic-Anterior Cingulate Pathway

BackgroundAlthough the mechanisms of short- and long-term potentiation of nociceptive-evoked responses are well known in the spinal cord, including central sensitization, there has been a growing body of information on such events in the cerebral cortex. In view of the importance of anterior cingulate cortex (ACC) in chronic pain conditions, this review considers neuronal plasticities in the thalamocingulate pathway that may be the earliest changes associated with such syndromes.ResultsA single nociceptive electrical stimulus to the sciatic nerve induced a prominent sink current in the layer II/III of the ACC in vivo, while high frequency stimulation potentiated the response of this current. Paired-pulse facilitation by electrical stimulation of midline, mediodorsal and intralaminar thalamic nuclei (MITN) suggesting that the MITN projection to ACC mediates the nociceptive short-term plasticity. The short-term synaptic plasticities were evaluated for different inputs in vitro where the medial thalamic and contralateral corpus callosum afferents were compared. Stimulation of the mediodorsal afferent evoked a stronger short-term synaptic plasticity and effectively transferred the bursting thalamic activity to cingulate cortex that was not true for contralateral stimulation. This short-term enhancement of synaptic transmission was mediated by polysynaptic pathways and NMDA receptors. Layer II/III neurons of the ACC express a short-term plasticity that involves glutamate and presynaptic calcium influx and is an important mechanism of the short-term plasticity.ConclusionThe potentiation of ACC neuronal activity induced by thalamic bursting suggest that short-term synaptic plasticities enable the processing of nociceptive information from the medial thalamus and this temporal response variability is particularly important in pain because temporal maintenance of the response supports cortical integration and memory formation related to noxious events. Moreover, these modifications of cingulate synapses appear to regulate afferent signals that may be important to the transition from acute to chronic pain conditions associated with persistent peripheral noxious stimulation. Enhanced and maintained nociceptive activities in cingulate cortex, therefore, can become adverse and it will be important to learn how to regulate such changes in thalamic firing patterns that transmit nociceptive information to ACC in early stages of chronic pain.

Expression of a Prrxl1 alternative splice variant during the development of the mouse nociceptive system

Gene expression can be differentially regulated by alternatively spliced transcription factors, providing a mechanism for precise control of diverse morphogenetic events. The paired-type homedomain transcription factor Prrxl1 (formerly known as Drg11) was described as a key regulator of the differentiation of the spinal cord neuronal circuit dedicated to the processing of nociceptive information. Here, we report the characterization of a Prrxl1 alternative splice variant that we termed Prrxl1-b. Mouse Prrxl1 isoform mRNA sequences were obtained by Rapid Amplification cDNA Ends (RACE) analysis. The distribution and the amount of Prrxl1-b at different developmental ages were analyzed by in situ hybridization and quantitative real-time PCR, and compared with those of Prrxl1. The amount of Prrxl1 was higher than that of the Prrxl1-b isoform both in the DRG and the spinal cord. Prrxl1-b contains the N-terminal homeodomain but differs from the previously identified Prrxl1 in the C-terminal part due to alternative mRNA processing. This results in the lack of the OAR domain in the Prrxl1-b primary structure. Prrxl1-b is exclusively localized in neurons primarily involved in the processing of the pain somatosensory modality. Prrxl1-b presents the same regional distribution pattern as Prrxl1, but differs as to the qualitative and quantitative expression profile at distinct developmental ages in the dorsal root ganglion and spinal cord. We suggest that the tissue-specific role of the Prrxl1 gene may be sustained by an accurate balance in the ratio between the amount of Prrxl1 and its OAR-lacking variant, Prrxl1-b, which may be critical during nociceptive circuit development.

Preventing Extracellular Diffusion of Trigeminal Nitric Oxide Enhances Formalin-induced Orofacial Pain

Nitric oxide (NO), a diffusible gas, is produced in the central nervous system, including the spinal cord dorsal horn and the trigeminal nucleus, the first central areas processing nociceptive information from periphery. In the spinal cord, it has been demonstrated that NO acts as pronociceptive or antinociceptive mediators, apparently in a concentration-dependent manner. However, the central role of NO in the trigeminal nucleus remains uncertain in support of processing the orofacial nociception. Thus, we here investigated the central role of NO in formalin (3%)-induced orofacial pain in rats by administering membrane-permeable or -impermeable inhibitors, relating to the NO signaling pathways, into intracisternal space. The intracisternal pretreatments with the NO synthase inhibitor L-NAME, the NO-sensitive guanylate cyclase inhibitor ODQ, and the protein kinase C inhibitor GF109203X, all of which are permeable to the cell membrane, significantly reduced the formalin-induced pain, whereas the membrane-impermeable NO scavenger PTIO significantly enhanced it, compared to vehicle controls. These data suggest that an overall effect of NO production in the trigeminal nucleus is pronociceptive, but NO extracellularly diffused out of its producing neurons would have an antinociceptive action.

Imaging CNS Modulation of Pain in Humans

Pain is a highly complex and subjective experience that is not linearly related to the nociceptive input. What is clear from anecdotal reports over the centuries and more recently from animal and human experimentation is that nociceptive information processing and consequent pain perception is subject to significant pro- and anti-nociceptive modulations. These modulations can be initiated reflexively or by contextual manipulations of the pain experience including cognitive and emotional factors. This provides a necessary survival function since it allows the pain experience to be altered according to the situation rather than having pain always dominate. The so-called descending pain modulatory network involving predominantly medial and frontal cortical areas, in combination with specific subcortical and brain stem nuclei appears to be one key system for the endogenous modulation of pain. Furthermore, recent findings from functional and anatomical neuroimaging support the notion that an altered interaction of pro- and anti-nociceptive mechanisms may contribute to the development or maintenance of chronic pain states. Research on the involved circuitry and implemented mechanisms is a major focus of contemporary neuroscientific research in the field of pain and should provide new insights to prevent and treat chronic pain states.

A Role For Wind-Up in Trigeminal Sensory Processing: Intensity Coding of Nociceptive Stimuli in the Rat

Wind-up is a progressive, frequency-dependent increase in the excitability of trigeminal and spinal dorsal horn wide dynamic range (WDR) nociceptive neurons evoked by repetitive stimulation of primary afferent nociceptive C-fibres. The correlate of wind-up in humans is temporal summation, which is an increase in pain perception to repetitive constant nociceptive stimulation. Although wind-up is widely used as a tool for studying the processing of nociceptive information, including central sensitization, its actual role is still unknown. Here, we recorded from trigeminal WDR neurons using in vivo electrophysiological techniques in rats and assessed the wind-up phenomenon in response to stimuli of different intensities and frequencies. First, we found that the amplitude of C-evoked responses of WDR neurons to repetitive stimulation increased progressively to reach a peak, then consistently showed a stable or slightly decreasing plateau phase. Only the first phase of this time course fitted in with the wind-up description. Therefore, to assess wind-up, we measured a limited number of initial responses. Second, we showed that wind-up, i.e. the slope of the frequency-dependent increase in the response to C-fibre stimulation, was linearly correlated to the stimulus intensity. Intensities of brief C-fibre inputs were thus coded into frequencies of action potentials by second-order neurons through frequency-dependent potentiation of the evoked responses. Third, wind-up also occurred at stimulation intensities below the threshold for C-evoked responses in WDR neurons, suggesting that wind-up can amplify subthreshold C-fibre inputs to WDR neurons. This might account for the observation that sparse, subliminal, neuronal activity in nociceptors can become painful via central integration of neural responses. Altogether, the present results show that wind-up can provide trigeminal WDR neurons with the capability to encode the intensity of short-duration orofacial nociceptive stimuli and to detect subthreshold nociceptive input. Thus, not only may wind-up play a physiological role in trigeminal sensory processing, but its enhancement may also underlie the pathophysiology of chronic orofacial pain conditions.

Long-lasting enhancement in the intrinsic excitability of deep dorsal horn neurons

Intrinsic excitability (IE) can be defined as an output of action potentials from a given input signal. Changes to the IE of a neuron are an important aspect of the cellular plasticity that underlies learning and memory process. In this study, long-term plastic change in IE of deep dorsal horn neurons (DHNs) was investigated. Associative spike pairing stimulation (PS) induced a long-lasting increase in IE. Buffering intracellular calcium with BAPTA (10 mM) prevented the induction of a long-lasting increase in IE. PS failed to induce a long-lasting increase in IE in the presence of either D-APV (50 μM) or cadmium chloride (100 μM). Apamin (100 nM) partially blocked the induction of a long-lasting increase in IE. This intrinsic plasticity requires a rise in postsynaptic Ca 2+ and NMDA receptor activation during the induction period, and this process might be mediated by the down-regulation of small-conductance calcium-dependent potassium (SK) channels. In deep DHNs, PS induced excitatory postsynaptic potential (EPSP)-spike (E-S) potentiation, which increases the firing probability and the number of spikes, by consistent dorsal rootlet stimulation. Under bath application of bicuculline (10 μM) and strychnine (1 μM), PS induced E-S potentiation and long-lasting increases in IE. These results suggest that an increase in IE might underlie E-S potentiation, while a reduction in inhibitory transmission does not contribute to E-S potentiation and long-lasting increases in IE. We conclude that PS enhances the IE of deep DHNs, which may play an important role in spinal processing of nociceptive information.

Nociceptive behavioral responses to chemical, thermal and mechanical stimulation after unilateral, intrastriatal administration of 6-hydroxydopamine

The basal ganglia are involved not only with motor processes such as posture, pre-movement planning and movement initiation, but also with the processing and modulation of nociceptive somatosensory information. In the current studies, unilateral, intrastriatal 6-hydroxydopamine (6-OHDA) was used to investigate how dopamine depletion alters nociceptive behavioral responses to chemical, thermal and mechanical stimulation in rats. Compared to control rats injected with intrastriatal saline, rats depleted of dopamine displayed increased nociceptive responses to chemical stimulation of the face and hyperalgesic responses to thermal stimulation of the hind paw without alterations in rearing behavior or body weight gain. Minor changes were observed in the response to mechanical stimulation of the hind paws and face. These data provide further evidence that the dopaminergic nigrostriatal pathway plays a role in the modulation of nociceptive information.

Genetic Mutations that Prevent Pain: Implications for Future Pain Medication

Part of the interindividual variability in pain therapy has been associated with genetic polymorphisms. Several genetic variants prevent or at least decrease pain in their carriers as compared with carriers of the respective wild-type or common alleles by impeding the generation, transmission and processing of nociceptive information or by increasing the local availability of active analgesics or their pharmacodynamic effects. Complete prevention of pain has so far been seen in six distinct rare hereditary syndromes, namely the 'channelopathy-associated insensitivity to pain', caused by 13 currently identified variants in the SCN9A gene coding for the alpha-subunit of the voltage-gated sodium channel, and five maladies belonging to the hereditary sensory and autonomic neuropathy (HSAN) I-V syndromes, caused by various mutations in several genes. Reduced pain in the average population has been associated with frequent variants in the micro-opioid receptor gene (OPRM1), catechol-O-methyltransferase gene (COMT), guanosine triphosphate cyclohydrolase 1/dopa-responsive dystonia gene (GCH1), transient receptor potential cation channel, subfamily V, member 1 gene (TRPV1) or the melanocortin-1 receptor gene (MC1R). Duplications/amplifications of the cytochrome P450 2D6 (CYP2D6) gene leading to increased enzyme function may cause intense opioid effects of codeine up to toxicity. The COMT V158M variant has been associated with decreased morphine requirements for analgesia. Inactivating MC1R variants have been associated with increased opioid analgesia of the micro-opioid receptor agonist morphine-6-glucuronide and, in women only, of kappa-opioid agonists. Finally, variants in the P-glycoprotein gene (ABCB1) conferring decreased transporter function have been associated with increased respiratory depressive effects of fentanyl. In summary, a finite number of genetic variants that prevent pain by decreasing nociception or increasing analgesia have been identified. Given the complex biological and psychological nature of pain, we will see in the near future how much of the interindividual variance in pain and analgesia is due to identifiable genetic causes, and to what extent genetics enters clinical pain therapy.

Parallel Processing of Nociceptive A-δ Inputs in SII and Midcingulate Cortex in Humans

The cingulate cortex (CC) as a part of the "medial" pain subsystem is generally assumed to be involved in the affective and/or cognitive dimensions of pain processing, which are viewed as relatively slow processes compared with the sensory-discriminative pain coding by the lateral second somatosensory area (SII)-insular cortex. The present study aimed at characterizing the location and timing of the CC evoked responses during the 1 s period after a painful laser stimulus, by exploring the whole rostrocaudal extent of this cortical area using intracortical recordings in humans. Only a restricted area in the median CC region responded to painful stimulation, namely the posterior midcingulate cortex (pMCC), the location of which is consistent with the so-called "motor CC" in monkeys. Cingulate pain responses showed two components, of which the earliest peaked at latencies similar to those obtained in SII. These data provide direct evidence that activations underlying the processing of nociceptive information can occur simultaneously in the "medial" and "lateral" subsystems. The existence of short-latency pMCC responses to pain further indicates that the "medial pain system" is not devoted exclusively to the processing of emotional information, but is also involved in fast attentional orienting and motor withdrawal responses to pain inputs. These functions are, not surprisingly, conducted in parallel with pain intensity coding and stimulus localization specifically subserved by the sensory-discriminative "lateral" pain system.

Role of Spinal Cord Alpha-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic Acid Receptors in Complete Freund's Adjuvant-Induced Inflammatory Pain

Spinal cord α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) mediate acute spinal processing of nociceptive and non-nociceptive information, but whether and how their activation contributes to the central sensitization that underlies persistent inflammatory pain are still unclear. Here, we examined the role of spinal AMPARs in the development and maintenance of complete Freund's adjuvant (CFA)-induced persistent inflammatory pain. Intrathecal application of two selective non-competitive AMPAR antagonists, CFM-2 (25 and 50 μg) and GYKI 52466 (50 μg), significantly attenuated mechanical and thermal hypersensitivities on the ipsilateral hind paw at 2 and 24 h post-CFA injection. Neither CFM-2 nor GYKI 52466 affected the contralateral basal responses to thermal and mechanical stimuli. Locomotor activity was not altered in any of the drug-treated animals. CFA-induced inflammation did not change total expression or distribution of AMPAR subunits GluR1 and GluR2 in dorsal horn but did alter their subcellular distribution. The amount of GluR2 was markedly increased in the crude cytosolic fraction and decreased in the crude membrane fraction from the ipsilateral L4–5 dorsal horn at 24 h (but not at 2 h) post-CFA injection. Conversely, the level of GluR1 was significantly decreased in the crude cytosolic fraction and increased in the crude membrane fraction from the ipsilateral L4–5 dorsal horn at 24 h (but not at 2 h) post-CFA injection. These findings suggest that spinal AMPARs might participate in the central spinal mechanism of persistent inflammatory pain.

Effect of acetylcholine on pain-related electric activities in hippocampal CA1 area of normal and morphinistic rats

To examine the effect of acetylcholine (ACh) on the electric activities of pain-excitation neurons (PEN) and pain-inhibitation neurons (PIN) in the hippocampal CA1 area of normal rats or morphinistic rats, and to explore the role of ACh in regulation of pain perception in CA1 area under normal condition and morphine addiction. The trains of electric impulses applied to sciatic nerve were set as noxious stimulation. The discharges of PEN and PIN in the CA1 area were recorded extracellularly by glass microelectrode. We observed the influence of intracerebroventricular (i.c.v.) injection of ACh and atropine on the noxious stimulation-evoked activities of PEN and PIN in the CA1 area. Noxious stimulation enhanced the electric activity of PEN and depressed that of PIN in the CA1 area of both normal and addiction rats. In normal rats, ACh decrease the pain-evoked discharge frequency of PEN, while increased the frequency of PIN. These effects reached the peak value at 4 min after injection of ACh. In morphinistic rats, ACh also inhibited the PEN electric activity and potentialized the PIN electric activity, but the maximum effect appeared at 6 min after administration. The ACh-induced responses were significantly blocked by muscarinic receptor antagonist atropine. Cholinergic neurons and muscarinic receptors in the hippocampal CA1 area are involved in the processing of nociceptive information and they may play an analgesia role in pain modulation. Morphine addiction attenuated the sensitivity of pain-related neurons to the noxious information.

Differential contribution of GABAergic and glycinergic components to inhibitory synaptic transmission in lamina II and laminae III–IV of the young rat spinal cord

Using whole-cell patch-clamp recordings from spinal cord slices of young (10-15 days old) rats, we have characterized and compared the properties of inhibitory synaptic transmission in lamina II and laminae III-IV of the dorsal horn, which are involved in the processing of nociceptive and non-nociceptive sensory information, respectively. All (100%) of laminae III-IV neurons, but only 55% of lamina II neurons, received both gamma-aminobutyric acid (GABA)ergic and glycinergic inputs. The remaining 45% of lamina II neurons received only GABAergic synapses. Neurons receiving only glycinergic synapses were never observed. Among the 55% of lamina II neurons receiving both GABAergic and glycinergic inputs, all displayed a small proportion (approximately 10%) of mixed miniature inhibitory postsynaptic currents (mIPSCs), indicating the presence of a functional GABA/glycine co-transmission at a subset of synapses. Such a co-transmission was never observed in laminae III-IV neurons. The presence of mixed mIPSCs and the differences in decay kinetics of GABAA-type receptor mIPSCs between lamina II and laminae III-IV were due to the endogenous tonic production of 3alpha5alpha-reduced steroids (3alpha5alpha-RS) in lamina II. Stimulation of the local production of 3alpha5alpha-RS was possible in laminae III-IV after incubation of slices with progesterone, subcutaneous injection of progesterone or induction of a peripheral inflammation. This led to the prolongation of GABAergic mIPSCs, but failed to induce the appearance of mixed mIPSCs in laminae III-IV. Our results indicate that, compared with lamina II, inhibitory synaptic transmission in laminae III-IV is characterized by a dominant role of glycinergic inhibition and the absence of a functional GABA/glycine co-transmission.

Validation of an automated punctate mechanical stimuli delivery system designed for fMRI studies in rodents

Functional magnetic resonance imaging (fMRI) is increasingly being used for animal studies studying the transmission of nociceptive information. Application of noxious mechanical stimuli is widely used for animal and human assessment of pain processing. Any accessory hardware used in animal imaging studies must, however, be sufficiently small to fit in the magnet bore diameter and be non-magnetic. We have developed a system that can apply mechanical stimuli simultaneously with fMRI. This system consists of a standardized instrument to deliver mechanical stimuli (VonFrey monofilament) and a gas-pressured mechanical transducer. These components were integrated with a computer console that controlled the period of stimuli to match acquisition scans. Preliminary experiments demonstrated that the force-stimulus transducer did not influence MRI signal to noise ratio. Mechanical stimulation of the hindpaw significantly increased blood oxygen level dependent (BOLD) signal intensity in several midbrain regions involved in the processing of nociceptive information in the rat ( p < 0.001, uncorrected for multiple comparisons). This system can be applied to both animal and human imaging studies and has a wide range of applications for studies of nociceptive processing.

Capsaicin-induced thermal hyperalgesia and sensitization in the human trigeminal nociceptive pathway: An fMRI study

The aim of this study was to differentiate the processing of nociceptive information, matched for pain intensity, from capsaicin-induced hyperalgesic vs. control skin at multiple levels in the trigeminal nociceptive pathway. Using an event-related fMRI approach, 12 male subjects underwent three functional scans beginning 1 h after topical application of capsaicin to a defined location on the maxillary skin, when pain from capsaicin application had completely subsided. Brush and two levels of painful heat (low—Thermal-1 and high—Thermal-2) were applied to the site of capsaicin application and to the mirror image region on the opposite side. Temperatures for each side were set to evoke perceptually matched pain (mean temperatures [capsaicin/control]: Thermal-1 = 38.4/42.8 °C; Thermal-2 = 44.9/47.8 °C). We found differences in activation patterns following stimuli to treated and untreated sides in sensory circuits across all stimulus conditions. Across the trigeminal nociceptive pathway, Thermal-2 stimulation of hyperalgesic skin evoked greater activation in trigeminal ganglion and nucleus, thalamus, and somatosensory cortex than the control side. Thus, trigeminal nociceptive regions showed increased activation in the context of perceptually equal pain levels. Beyond these regions, contrast analyses of capsaicin vs. control skin stimulation indicated significant changes in bilateral dorsolateral prefrontal cortex and amygdala. The involvement of these emotion-related regions suggests that they may be highly sensitive to context, such as prior experience (application of capsaicin) and the specific pain mechanism (hyperalgesic vs. normal skin).

Group I metabotropic glutamate receptor-induced Ca 2+-gradients in rat superficial spinal dorsal horn neurons

Here, we investigated changes in the free cytosolic Ca 2+ concentration ([Ca 2+] i), induced by the pharmacological activation of metabotropic glutamate receptors (mGluRs), in nociceptive neurons of the superficial spinal dorsal horn. Microfluorometric Ca 2+ measurements with fura-2 in a lumbar spinal cord slice preparation from young rats were used. Bath application of the specific group I mGluR agonist ( S)-3,5-dihydroxyphenylglycine (( S)-3,5-DHPG) resulted in a distinct increase of [Ca 2+] i in most of the neurons in superficial dorsal horn. In contrast, activation of groups II or III mGluRs by DCG-IV or l-AP4, respectively, failed to evoke any significant change in [Ca 2+] i. The effect of ( S)-3,5-DHPG was mediated by both group I subtypes mGluR1 and mGluR5, since combined pre-treatment with the subtype antagonists ( S)-4-CPG and MPEP was necessary to abolish the [Ca 2+] i increase. Depleting intracellular Ca 2+ stores with CPA or inhibiting IP 3-receptors with 2-APB, respectively, reduced the ( S)-3,5-DHPG-evoked [Ca 2+] i increase significantly. Inhibition of voltage-dependent L-type Ca 2+ channels (VDCCs) by verapamil or nicardipine reduced the ( S)-3,5-DHPG-induced [Ca 2+] i rise likewise. Thus, in rat spinal cord, ( S)-3,5-DHPG enhances Ca 2+ signalling in superficial dorsal horn neurons, mediated by the release of Ca 2+ from IP 3-sensitive intracellular stores and by an influx through L-type VDCCs. This may be relevant to the processing of nociceptive information in the spinal cord.

Reversible attenuation of neuropathic-like manifestations in rats by lesions or local blocks of the intralaminar or the medial thalamic nuclei

Background and aim: Thalamic somatosensory nuclei have been classified into medial and lateral systems based on their role in nociception. An imbalance between these two systems may result in abnormal somatic sensations and spontaneous pain. This study aims to investigate the effects of transient or permanent block of the medial and intralaminar nuclear groups on the neuropathic-like behavior in a rat model for mononeuropathy. Methods: Neuropathy was induced on one hind paw in different groups of rats following the spared nerve injury model. When the resulting hyperalgesia and allodynia (tactile and cold) reached a maximum plateau, the rats received either chemical or electrolytic lesion or lidocaine (2%) microperfusion, placed in the various thalamic nuclear groups. Results: All procedures produced transient but significant decrease of neuropathic manifestations. The magnitude and duration of decrease depended on the type and the site of the block. These effects can be ranked in increasing order as follows, electrolytic < chemical < lidocaine micro-perfusion according to the procedure, and as rostro-medial < ventro-median < parafascicular nuclei, according to the site of the block. Thermal hyperalgesia was the most affected while cold allodynia showed the least attenuation. Neuropathic manifestations returned to their pre-lesion levels after 2–3 weeks, along with frequently observed delayed hyper-responsiveness to the hotplate test. Conclusion: The observed results demonstrate the involvement of the medial and intralaminar thalamic nuclei in the processing of neuropathic-like manifestations, and the reversibility of the effects suggests the flexibility of the neural network involved in supraspinal processing of nociceptive information.