Effects of somatostatin and morphine on the responses of dorsal horn neurons to noxious peripheral nerve stimulation in cats

ABSTRACT

Painful nociceptive informations are well known to be transferred from nociceptors through spinal dorsal horn not only in different pathways but also in diverse nature depending on the type of noxious stimuli. There have been some controversies about the role of neuropeptide somatostatin in the transmission of the nociceptive information to the dorsal horn cells of the spinal cord. We performed the study in order to elucidate the effects of somatostatin on transmission of noxious stimuli in the spinal dorsal horn, comparing with those of morphine. Using carbon-filamented microelectrode, the single cell activities of wide dynamic range(WDR) neuron were recorded extracellularly at the lumbosacral enlargement of the spinal cord in cats after noxious mechanical(squeeze), thermal(heat lamp), and cold(dry ice) stimulation to the receptive field. The sciatic nerve was stimulated electrically to evoke, A4-fiber and C-fiber each other. Data were compiled into single pass time histograms or postsimulus time histograms. Twenty micro-gram of somatostatin was injected intravenously to study the changes of single cell activities in 20 minutes, which were compared with the effects of morphine(2m/kg). Then naloxone was administrated(0.1mg/kg) to know whether it antagonized the effects of somatostatin and morphine And those finding were also observed in inverted WDR cells. In WDR cell, somatostain decreased the cellular responses to noxious heat stimuli in 6cell(n=9), but increased those to cold stimuli in 4 cells(n=6). And the responses to noxious mechanical stimuli were so diverse that they were slightly increased in 7 cells(164%), decreased in 5 cells, and were not changed in 6 cells(n=18). A-response, the response to peripheral Ad-afferent activation, showed a tendency to be facilitated(n=6/9), while C-response had a slightly depressed tendency(n=4/9). Morphine strongly suppressed the responses of dorsal horn neurons to noxious heat(n=9/13), cold(n=2/2), mechanical stimuli(n=16/19) and electrical A-response(n=7/10), C-response(n=6/7). Following subsequent injection of naloxone, the effects of morphine on noxious stimuli evoked response were fully reversed but those of somatostatin were not antagonized. There was significant difference between the reversal effects of naloxone on morphine and somatostatin(p<0.05). From the above results it is concluded that somatostatin suppresses the transmission of nociceptive heat stimuli, especially via C-fiber, while facilitates that of nociceptive mechanical and cold stimuli via Adelta-fiber in spinal dorsal horn cells. Also the somatostatin appears to have different nociceptive mechanism from morphine.