In vivo patch-clamp analysis of developmental changes in the spinal nociceptive circuit.

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Developmental changes in the spinal nociceptive synaptic transmission were examined using slice and in vivo patch-clamp recordings. In transverse spinal cord slices of young and adult rats at lumbar spinal levels with an attached dorsal root, electrical dorsal root stimulation was applied to record the evoked-synaptic responses in substantia gelatinosa (SG, lamina II of the spinal cord)and deep lamina (lamina III-IV) neurons. In the in vivo patch-clamp analysis, patch electrodes were inserted into the SG at the spinal level of L3-L5 after a thoracolumbar laminectomy was performed under urethane anesthesia. Whole-cell recordings were obtained from SG neurons, and then modality-dependent synaptic responses evoked by cutaneous natural mechanical stimuli applied to the ipsilateral hind limb were recorded. In adult rats, monosynaptic Aβ and C fiber-evoked EPSCs were elicited in SG neurons. Additionally, monosynapitc Aβ fiber-evoked EPSCs were evoked in the deeper lamina neurons but not in SG neurons. In young rats, on the other hand, monosynaptic Aβ fiber-evoked EPSCs were observed in SG neurons in addition to Aβ and C fiber-evoked responses. In the in vivo patch-clamp analysis, pinch stimulation elicited a barrage of EPSCs in SG neurons in young and adult rats. Even in SG neurons that received synaptic inputs from only Aβ fibers, pinch stimulation evoked a barrage of EPSCs. Most deep neurons in adult rats responded to touch stimulation. The present results suggest that Aβ fibers, in addition to Aδ and C fibers, convey noxious mechanical information to SG neurons in young rats. In adult rats with further development of neuronal circuits in the spinal dorsal horn, innocuous mechanical synaptic responses are mediated only through Aβ fibers in deep lamina neurons.