FIGURE 6.

The role of spinal α_2A-adrenoceptors in clonidine-induced electrophysiological antinociception. In (A), a pair of raw tracings of a single second-order WDR neuron cluster response induced by one electrical stimulus (upper tracing), the raster plot (middle), and peri-stimulus time histogram (PSTH) depicting the effect of BRL 44408 on the clonidine-induced antinociception. Notice that under α_2A-adrenoceptors blockade with BRL 44408 (1 nmol), the antinociceptive action of clonidine is not elicited. (B–E) show the time-course changes in the percentage average of the different fibers activating WDR cell responses and post-discharge elicited by receptive field (RF) electrical stimulation and the effects of spinal clonidine (1 and 10 nmol/10 μl; n = 5 each dose). Notice that 1 nmol clonidine only inhibits the neuronal firing of Aδ-fibers (C), while 10 nmol clonidine inhibits the activity of Aδ-fibers (C), C-fibers (D), and post-discharge (E). (F–I) show the global neuronal activity (obtained from the respective time course figures) of Aβ-, Aδ-, C-fibers, and post-discharge in response to spinal clonidine. Clonidine preferentially blocks the neuronal activity associated with Aδ-, C-fibers, and post-discharge but not Aβ-fibers. On the other hand, blockade of the α_2A-adrenoceptors with BRL 44408 (1 nmol) inhibits the clonidine-induced electrophysiological antinociception.