Involvement of alpha-2 adrenoceptors in the periaqueductal gray-induced inhibition of dorsal horn cell activity in rats.

The noradrenergic system is considered to be one of the major descending analgesia systems originating in the brainstem. The relationship between the periaqueductal gray (PAG) and the descending noradrenergic system is of great interest. A projection from the PAG to the locus ceruleus and parabrachial region has been reported. It is possible that electrical stimulation of the PAG activates spinally projecting collaterals of the noradrenergic cells in A5, the locus ceruleus, subceruleus and A7/Kölliker-Fuse nucleus. The current experiments were designed to study the role of alpha-2 adrenoceptors on spinal cord dorsal Horn cells in PAG-induced inhibition. A microdialysis fiber was introduced into the dorsal horn of the lumbar enlargement for drug administration. An electrode was placed in the vicinity of the microdialysis fiber for extracellular single-unit recording. Recordings were made from a total of 35 cells from 22 Sprague-Dawley rats (270-360 g). When the PAG was stimulated, responses to mechanical stimuli applied to the skin (brush, press and pinch) were inhibited. When an alpha-2 adrenoceptor agonist (clonidine) was administered, the background activity and the responses of the cell to brush, press and pinch stimuli were decreased. In contrast, administration of alpha-2 adrenoceptor antagonists (idazoxan or yohimbine) increased the responses of the cell to brush, press and pinch stimuli. The background activity was also increased by idazoxan. The PAG-induced inhibition of the response to mechanical stimuli was reduced by idazoxan and yohimbine. These results suggest an involvement of alpha-2 adrenoceptors in the mechanism of PAG-induced inhibition of dorsal horn cell activity.