Optical responses evoked by single-pulse stimulation to the dorsal root in the rat spinal dorsal horn in slice

Neuronal excitation evoked after dorsal-root (DR) stimulation in the spinal dorsal horn (DH) of rats was visualized with a high-resolution optical-imaging method, and the propagation mechanism was studied. Transverse slices of the spinal cord were obtained from 2–4 week-old rats and stained with the voltage-sensitive dye RH-482. Single-pulse stimulation to the primary-afferent A fibers in the DR attached to the slice evoked a weak, brief (<10 ms) excitatory optical response in the laminae I and III–V. When the stimulus intensity and duration were increased to activate both A and C fibers, an additional, much greater, and longer-lasting (>100 ms) excitatory response was generated in the laminae I–III, most intensely in the lamina II. A treatment with excitatory amino acid (EAA) antagonists, -2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, significantly reduced the amplitude and duration of the response in the lamina II. The optical response in the antagonists-containing solution was quite similar to that recorded in a Ca²⁺-free solution that blocked afferent synaptic transmission. The late component (>10 ms) was, however, slightly greater than that in the Ca²⁺-free solution. Treatment with the ATP-receptor antagonist, suramin, had a minimal effect on the response in the presence of EAA antagonists. These results suggested that the propagation of the DR-stimulus-elicited excitation was contributed largely by EAA receptors, but also by other receptors to a much lesser extent.