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 descendi ng analgesia systems originating in the brainstem. The relationship be tween the periaqueductal gray (PAG) and the descending noradrenergic s ystem is of great interest. A projection from the PAG to the locus cer uleus and parabrachial region has been reported. It is possible that e lectrical stimulation of the PAG activates spinally projecting collate rals of the noradrenergic cells in A5, the locus ceruleus, subceruleus and A7/Kolliker-Fuse nucleus. The current experiments were designed t o study the role of alpha-2 adrenoceptors on spinal cord dorsal horn c ells in PAG-induced inhibition. A microdialysis fiber was introduced i nto the dorsal horn of the lumbar enlargement for drug administration. An electrode was placed in the vicinity of the microdialysis fiber fo r extracellular single-unit recording. Recordings were made from a tot al of 35 cells from 22 Sprague-Dawley rats (270-360 g). When the PAG w as stimulated, responses to mechanical stimuli applied to the skin (br ush, press and pinch) were inhibited. When an alpha-2 adrenoceptor ago nist (clonidine) was administered, the background activity and the res ponses of the cell to brush, press and pinch stimuli were decreased. I n contrast, administration of alpha-2 adrenoceptor antagonists (idazox an or yohimbine) increased the responses of the cell to brush, press a nd pinch stimuli. The background activity was also increased by idazox an. The PAG-induced inhibition of the response to mechanical stimuli w as reduced by idazoxan and yohimbine. These results suggest an involve ment of alpha-2 adrenoceptors in the mechanism of FAG-induced inhibiti on of dorsal horn cell activity.