Electrophysiological characterisations of rat lamina I dorsal horn neurones and the involvement of excitatory amino acid receptors.

Lamina I of the spinal cord plays a key role in sensory transmission between afferent activity and the CNS. Studies have shown lamina I neurones to have distinct response properties compared to deep dorsal horn neurones, but little is known regarding excitatory amino acid mechanisms in their responses. Spinal electrophysiological recordings of lamina I neurones confirmed that the majority of these neurones (74%) are nociceptive specific (NS) in their responses, of which 18% can be termed polymodal nociceptive (HPC) (13% of the total population). The remainder (26%) were wide dynamic range. Lamina I neurones had smaller mechanical and heat-evoked responses compared to deeper dorsal horn neurones. The electrically evoked responses were also smaller, with a distinct lack of an NMDA-mediated 'wind-up' effect. NBQX (AMPA receptor antagonist, 0.5, 5, 50 microg/50 microl) produced dose-dependent inhibitions of the electrically evoked neuronal responses, but APV (NMDA receptor antagonist, 50, 100, 500 microg/50 microl) had minimal effects on their responses. These results implicate mainly AMPA receptors in the responses of lamina I neurones. Bicuculline (GABA(A) receptor antagonist, 0.5, 5, 50 microg/50 microl) demonstrated a role exerted by GABA(A) receptors in the control of A-delta fibre-mediated mechanical responses in lamina I. Overall, this study describes a high threshold, AMPA receptor possessing population of lamina I neurones, which seem to lack functional NMDA receptors, and are partially controlled by GABA(A) receptor activity.