In vivo patch clamp analysis of pain transmission in the spinal dorsal horn

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In vivo patch-clamp recording technique has been developed to analyze synaptic responses evoked by noxious or non-noxious stimuli applied to the skin. Under artificial ventilation of rats anesthetized with urethane and monitoring the blood pressure and temperature, L4 or L5 lumbar spinal cord was exposed by laminectomy and then the rat was placed on a stereotoxic apparatus. Using a micromanipulator, recording electrodes were inserted into the dorsal horn and giga-ohm sealing was established. Patch-clamp recordings were obtained from neurons located in substantia gelatinosa (SG, lamina II), which had a resting membrane potential of -50〜-75 mV and an input membrane resistance of 250〜600 MΩ. Under a voltage-clamp condition, they exhibited spontaneous fast excitatory postsynaptic currents (EPSCs) at holding potential of -70 mV and inhibitory postsynaptic currents (IPSCs) at 0 mV. Pinch and air stimuli applied to the ipsilateral hind limb increased the fast EPSCs in frequency and amplitude … More in all SG neurons tested, some of which initiated an actin potential under a current clamp condition. In contrast, all SG neurons examined, did not respond to noxious heat stimuli. To know where the noxious heat information is transmitted, patch-clamp recordings were made from deeper neurons. The deep dorsal horn neurons responded to noxious heat stimuli with increasing fast EPSCs in frequency and amplitude. The responses were, however, mediated solely by glutamatergic fast EPSCs, and no slow responses, supposed to be mediated by peptides, were detected in all neurons tested. These observations suggest that SG neurons receive nociceptive and non-nociceptive mechanical, but not noxious thermal inputs. The mechanical information may be transmitted by Aδ and C primary afferent fibers through an activation of non-NMDA receptor. Whereas, the noxious heat information is transmitted to the deeper laminae through activating glutamatergic receptors. Mechanisms of peptidergic transmission are required to be clarified. Less