Responsiveness of rat substantia gelatinosa neurones to mechanical but not thermal stimuli revealed by in vivo patch‐clamp recording

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Abstract
1 Synaptic responses of 46 substantia gelatinosa (SG) neurones in the spinal dorsal horn to cutaneous mechanical and/or thermal stimuli were investigated in an in vivo rat preparation with whole‐cell patch‐clamp recordings. The clamped neurones were identified as being in the SG based on either their morphological features by intrasomatic injection of biocytin or the depth of the neurones from the surface of the spinal cord. 2 In all SG neurones examined where spontaneous EPSCs occurred, pinch (noxious) and air (innocuous) stimuli applied to the ipsilateral hindlimb elicited a barrage of EPSCs (some of which initiated an action potential under current‐clamp conditions), which subsided just after cessation of the stimuli without any residual slow current (or after‐discharge). The spontaneous and evoked EPSCs were reversibly abolished by a non‐N‐methyl‐D‐aspartate (non‐NMDA) receptor antagonist, CNQX (20 μm). 3 Noxious (≥ 45 °C) or innocuous (≤ 40 °C) thermal stimuli did not elicit any synaptic responses in all 18 SG neurones tested which were sensitive to mechanical stimuli. Noxious cold stimulation (≤ 10 °C) also failed to produce any responses (n= 6). 4 It is concluded that both noxious and innocuous mechanical information to SG neurones are transmitted primarily by activation of non‐NMDA receptors, probably without any involvement of slow synaptic transmission, and that thermal information is conveyed to areas of the dorsal horn other than SG.