Synaptic activation of dorsal horn neurons by selective C-fibre excitation with capsaicin in the mouse spinal cord in vitro.

Low doses (0.2-0.8 microM) of capsaicin were used to achieve selective excitation of C-fibres and the consequent synaptic activation of dorsal horn neurons (laminae I-VI) in the spinal cord of the 12-20-day-old mouse, maintained in vitro. Most dorsal horn cells were activated by application of capsaicin to dorsal root ganglia. The response consisted of a long-lasting membrane depolarization with increased regenerative (synaptic) activity in 79% of the cells, and in a further 7% only an increased synaptic activity was evoked. These effects of capsaicin were completely blocked by removing extracellular calcium ions from the superfusate to the spinal cord, or by the addition of 1 microM tetrodotoxin, suggesting a presynaptic origin of the capsaicin action. Only 67% of cells excited by capsaicin were sensitive to exogenous substance P. The excitatory amino acid antagonists, kynurenic acid (50-100 microM) or (-)-2-amino-5-phosphonovaleric acid (10-20 microM) completely blocked the capsaicin-evoked response in deep dorsal horn cells, indicating the involvement of excitatory amino acid receptors in the synaptic pathway. However, in superficial dorsal horn neurons these antagonists attenuated, but never completely abolished, the capsaicin-evoked depolarization. The kynurenic acid-resistant component of the capsaicin-evoked excitation in superficial dorsal horn cells suggests the involvement of non-amino acid excitatory transmitters--possibly neuropeptides--in the synaptic transmission. Activation of primary afferents by high-intensity electrical stimulation of the dorsal roots induced a prolonged (0.5-3 s) postsynaptic excitation in the majority of deep dorsal horn cells. The duration of the synaptic response was significantly reduced by (-)-2-amino-5-phosphonovaleric acid. Following repeated application of capsaicin, desensitization of the capsaicin-evoked synaptic activation of dorsal horn cells occurred. This effect was paralleled with the loss of the prolonged (-)-2-amino-5-phosphonovaleric acid-sensitive phase of the excitatory postsynaptic potential evoked by the high-intensity electrical stimulation of dorsal roots. This observation suggested that activation of the N-methyl-D-aspartate receptors in the dorsal horn can be activated by small-calibre capsaicin-sensitive fibres. In summary, our data suggest that the selective activation of C-fibre afferents with capsaicin produces synaptic activity in the dorsal horn which has a strong excitatory amino acid component as well as a non-excitatory amino acid, possibly peptidergic, component.