Suppression of nociceptive responses in parafascicular neurons by stimulation of substantia nigra: an analysis of related inhibitory pathways

Abstract

A total of 166 neurons in parafascicular nucleus (PF) were studied, 85 from intact animals, 72 following dorsal spinal cord transection (D.Sp.C.X.) and 9 following complete transection of the spinal cord. Two patterns of nociceptive response were identified following noxious stimulation and these responses were classified as ‘nociceptive-off’ and ‘nociceptive-off’ neurons, respectively. The effect of stimulating the substantia nigra (SNS) on the spontaneous and on the nociceptive evoked discharges were observed and compared in intact, D.Sp.C.X. and completely transected spinal cord rats. The results show that SNS significantly suppresses both the spontaneous and the nociceptive evoked discharge elicited by peroneal nerve stimulation. With an intact spinal cord, SNS suppressed both the spontaneous [−37±3.2.% ( P < 0.05)] and the nociceptive evoked discharges [−52.8±2.8% ( P < 0.01)] of the the same stimulation elicited an even more prominent suppression upon both discharges (−4.7.±5.4.%, P < 0.01 and −64.9±5.0%, P < 0.01), respectively. After D.Sp.C.X., the suppressive effects on the ‘nociceptive-on’ cells following SNS were diminished (−28.1±3.5% and −36.9±2.6%, respectively) but not abolished, while in the ‘nociceptive-off’ cells, the inhibitory effects on SNS were unchanged. In addition, the suppressive effects of SNS on the spontaneous activity of PF neurons in cases with completely cut spinal cords remains unchanged. These results suggest that SNS modulates the spontaneous and the noxious evoked responses of the PF neurons by way of supraspinal connections besides the previously described descending projecting pathways.