Neurokinin receptor 1-expressing spinal cord neurons in lamina I and III/IV of postnatal rats receive inputs from capsaicin sensitive fibers.

Dorsal horn neurons expressing receptor for substance P (SP), the neurokinin 1 (NK1) receptor, play an important role in transmission and processing of nociceptive stimuli. To identify and study these neurons in the rat spinal cord slice preparation, we used fluorescence-conjugated SP to label NK1 receptor-expressing neurons. Labeled neurons in lamina I and III/IV were patch clamped and the vanilloid receptor 1 (TRPV1) agonist, capsaicin, was applied to evoke glutamate release from central terminals of peripheral nociceptors. Capsaicin induced an increase in the frequency of miniature excitatory postsynaptic currents in 73% of lamina I and 43% of lamina III/IV neurons expressing NK1-receptor indicating that these neurons receive direct input from capsaicin and heat sensitive nociceptors.