Spinal sensory transmission is under descending biphasic modulation, and descending facilitation is believed to contribute to chronic pain. Descending modulation from the brainstem rostral ventromedial medulla (RVM) has been the most studied, whereas little is known about direct corticospinal modulation. Here, we found that stimulation in the anterior cingulate cortex (ACC) potentiated spinal excitatory synaptic transmission and this modulation is independent of the RVM. Peripheral nerve injury enhanced the spinal synaptic transmission and occluded the ACC-spinal cord facilitation. Inhibition of ACC reduced the enhanced spinal synaptic transmission caused by nerve injury. Finally, using optogenetics, we showed that selective activation of ACC-spinal cord projecting neurons caused behavioral pain sensitization, while inhibiting the projection induced analgesic effects. Our results provide strong evidence that ACC stimulation facilitates spinal sensory excitatory transmission by a RVM-independent manner, and that such top-down facilitation may contribute to the process of chronic neuropathic pain.

Presynaptic ATP P2X receptors have been proposed to play a role in modulating glutamate release from the first sensory synapse in the spinal cord. Using spinal cord slice preparations and patch-clamp recordings from dorsal horn neurons in lamina V of the rat spinal cord, we showed that the activation of P2X receptors by ␣,␤-methylene-ATP (␣␤m-ATP) resulted in a large increase in the frequency of spontaneous EPSCs (sEPSCs) and miniature EPSCs (mEPSCs). The increases in mEPSC frequency by ␣␤m-ATP were not blocked by the Ca 2ϩ channel blocker, 30 M La 3ϩ , but were abolished in a bath solution when Ca 2ϩ was omitted. The increases in mEPSC frequency by ␣␤m-ATP were blocked completely by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (PPADS) at 10 M. Furthermore, the EPSCs evoked by dorsal root stimulation were potentiated by ␣␤m-ATP as well as by the ecto-ATPase inhibitor ARL67156 and were depressed in the presence of P2 receptor antagonists PPADS (10 M) and suramin (5 M). The effects of these compounds on the evoked EPSCs were associated with the changes in glutamate release probability of primary afferent central terminals. Our results indicate that ␣␤m-ATP-sensitive P2X receptors play a significant role in modulating excitatory sensory synaptic transmission in the spinal cord, and the potential role of endogenous ATP is suggested.

No abstract

TRPA1 is expressed in primary sensory neurons and hair cells, and it is proposed to be activated by cold stimuli, mechanical stimuli, or pungent ingredients. However, its role in regulating synaptic transmission has never been documented yet. In the present study, we examined whether activation of the TRPA1 channels affects synaptic transmission in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. A chief ingredient of mustard oil, allyl isothiocyanate (AITC), superfused for 2 min markedly increased the frequency and amplitude of spontaneous EPSCs (sEPSCs), which was accompanied by an inward current. Similar actions were produced by cinnamaldehyde and allicin. The AITC-induced increases in sEPSC frequency and amplitude were resistant to tetrodotoxin (TTX) and La 3ϩ , whereas being significantly reduced in extent in a Ca 2ϩ -free bath solution. In the presence of glutamate receptor antagonists CNQX and AP5, AITC did not generate any synaptic activities. The AITC-induced increases in sEPSC frequency and amplitude were reduced by ruthenium red, whereas being unaffected by capsazepine. AITC also increased the frequency and amplitude of spontaneous inhibitory postsynaptic currents; this AITC action was abolished in the presence of TTX or glutamate receptor antagonists. These results indicate that TRPA1 appears to be localized not only at presynaptic terminals on SG neurons to enhance glutamate release, but also in terminals of primary afferents innervating onto spinal inhibitory interneurons, which make synapses with SG neurons. This central modulation of sensory signals may be associated with physiological and pathological pain sensations.

Background and purpose: Although tramadol is known to exhibit a local anaesthetic effect, how tramadol exerts this effect is not understood fully. Experimental approach: The effects of tramadol and its metabolite mono-O-demethyl-tramadol (M1) on compound action potentials (CAPs) were examined by applying the air-gap method to frog sciatic nerves, and the results were compared with those of other local anaesthetics, lidocaine and ropivacaine. Key results: Tramadol reduced the peak amplitude of the CAP in a dose-dependent manner (IC 50 ¼ 2.3 mM). On the other hand, M1 (1-2 mM), which exhibits a higher affinity for m-opioid receptors than tramadol, did not affect CAPs. These effects of tramadol were resistant to the non-selective opioid receptor antagonist naloxone and the m-opioid receptor agonist, DAMGO, did not affect CAPs. This tramadol action was not affected by a combination of the noradrenaline uptake inhibitor, desipramine, and the 5-hydroxytryptamine uptake inhibitor, fluoxetine. Lidocaine and ropivacaine also concentrationdependently reduced CAP peak amplitudes with IC 50 values of 0.74 and 0.34 mM, respectively. Conclusions and implications: These results indicate that tramadol reduces the peak amplitude of CAP in peripheral nerve fibres with a potency which is less than those of lidocaine and ropivacaine, whereas M1 has much less effect on CAPs. This action of tramadol was not produced by activation of m-opioid receptors nor by inhibition of noradrenaline and 5-hydroxytryptamine uptake. It is suggested that the methyl group present in tramadol but not in M1 may play an important role in producing nerve conduction block.

Although hyperalgesia elicited by inflammation has been shown to be partly due to central sensitization, the cellular mechanisms are not clear at the moment. The present study was designed to address this issue using the blind whole-cell patch-clamp technique; glutamatergic primary-afferent inputs to substantia gelatinosa (SG) neurons were compared between spinal cord slices of naive rats and rats inflamed by an intraplantar injection of complete Freund's adjuvant. In naive rats, a large number of SG neurons examined received monosynaptic A delta- (69% of 41 neurons innervated by A fibers) and/or polysynaptic C- (94% of 36 neurons innervated by C fibers) afferent inputs, and only a few neurons received monosynaptic A beta inputs (7%). In addition, when examined in neurons which have both of the A- and C-afferent inputs, A afferent-evoked excitatory postsynaptic currents (EPSCs) were larger in amplitude than C afferent-induced ones; a ratio (A/C ratio) of the former to latter amplitude was 1.8 +/- 0.1 (n = 36). In inflamed rats, a change in the synaptic responses was observed: (1) SG neurons receiving monosynaptic A delta-afferent inputs decreased in number (to 20% of 30 neurons tested, innervated by A fibers), whereas those having monosynaptic A beta-afferent inputs increased to 33%, and (2) the A/C ratio decreased to 0.7 +/- 0.1 (n = 33). These results suggest that after inflammation, a substantial number of A beta-afferents sprout into the SG from their original location (laminae III-V) and that sensory information that used to be conveyed directly to the SG through A delta afferents is transmitted there indirectly through interneurons. These reorganizations of sensory pathway may contribute, at least in part, to underlying mechanisms for the development of hyperalgesia due to inflammation.

Using a spinal cord slice preparation and patch-clamp recordings from spinal cord dorsal horn neurons, we examined excitatory and inhibitory circuits connecting to lamina V neurons after the activation of afferent central terminal vanilloid receptor-1 (VR1) receptors and P2X receptors. We found that single neurons in lamina V often received excitatory inputs from two chemically defined afferent pathways. One of these pathways was polysynaptic from capsaicin-sensitive afferent terminals. In this pathway the capsaicin-sensitive afferent input first activated interneurons in superficial laminas, and then the excitatory activity was transmitted onto lamina V neurons. The second excitatory input was monosynaptic from alpha(beta)m-ATP-sensitive/capsaicin-insensitive afferent terminals. Both capsaicin-sensitive and alpha(beta)m-ATP-sensitive/capsaicin-insensitive pathways also recruited polysynaptic inhibitory inputs to lamina V neurons. Furthermore, we demonstrated that simultaneous activation of both capsaicin-sensitive afferent pathways and alpha(beta)m-ATP-sensitive/capsaicin-insensitive pathways could generate a temporal summation of excitatory inputs onto single lamina V neurons. These convergent pathways may provide a mechanism of sensory integration for two chemically defined sensory inputs and may have implications in different sensory states.

To elucidate the mechanisms of antinociception mediated by the dopaminergic descending pathway in the spinal cord, we investigated the actions of dopamine (DA) on substantia gelatinosa (SG) neurons by in vivo whole-cell patch-clamp methods. In the voltage-clamp mode (V(H)=-70mV), the application of DA induced outward currents in about 70% of SG neurons tested. DA-induced outward current was observed in the presence of either Na(+) channel blocker, tetrodotoxin (TTX) or a non-NMDA receptor antagonist, CNQX, and was inhibited by either GDP-β-S in the pipette solution or by perfusion of a non-selective K(+) channel blocker, Ba(2+). The DA-induced outward currents were mimicked by a selective D2-like receptor agonist, quinpirole and attenuated by a selective D2-like receptor antagonist, sulpiride, indicating that the DA-induced outward current is mediated by G-protein-activated K(+) channels through D2-like receptors. DA significantly suppressed the frequency and amplitude of glutamatergic spontaneous excitatory postsynaptic currents (EPSCs). DA also significantly decreased the frequency of miniature EPSCs in the presence of TTX. These results suggest that DA has both presynaptic and postsynaptic inhibitory actions on synaptic transmission in SG neurons. We showed that DA produced direct inhibitory effects in SG neurons to both noxious and innocuous stimuli to the skin. Furthermore, electrical stimulation of dopaminergic diencephalic spinal neurons (A11), which project to the spinal cord, induced outward current and suppressed the frequency and amplitude of EPSCs. We conclude that the dopaminergic descending pathway has an antinociceptive effect via D2-like receptors on SG neurons in the spinal cord.