Selective blockade by yohimbine of locus coeruleus-induced inhibition of nociceptive reflex but not that of C responses of spinal dorsal horn neurons in rats.

The effect of alpha 2-adrenoceptor antagonist yohimbine (Yoh) on locus coeruleus (LC)-induced spinal antinociception was investigated in 18 anesthetized Wistar rats. Stimulation of LC markedly inhibited both nociceptive reflex of the posterior biceps semitendinosus (PBST) muscle and C responses of 16 wide-dynamic range (WDR) neurons of the dorsal horn. Application of Yoh (0.2%, 5-10 microliters) to the surface of spinal cord at L3-4 attenuated the LC-induced inhibition of nociceptive reflex without affecting that of C responses of 10 WDR neurons that were tested in 6 rats. The results suggested that LC may exert its inhibitory action on the nociceptive reflex via alpha 2 adrenoceptors somewhere other than the WDR neurons in the spinal dorsal horn.