LOCUS-COERULEUS STIMULATION MODULATES THE NOCICEPTIVE RESPONSE IN PARAFASCICULAR NEURONS - AN ANALYSIS OF DESCENDING AND ASCENDING PATHWAYS

The nociceptive responses in parafascicular neurons (PF) were recorded and studied following electrical stimulation of locus coeruleus (LC) combined with intrathecal (IT) or intracerebroventricular (ICV) admini stration of phentolamine (Ph), an cr-adrenoceptor antagonist. The resu lts revealed the following. (1) Three different PF neuronal population s were observed according to their response pattern following noxious stimulation: nociceptive-on, nociceptive-off, and nonresponsive units. Only the nociceptive-on units were studied further. (2) The nocicepti ve discharges in majority of PF neurons (66/87) were inhibited by elec trical stimulation of the LC. (3) The inhibitory effect of LC stimulat ion was prevented and even reversed by pretreatment of IT Ph (40 nmol) in 22 units, or by dorsolateral funiculi transection in 24 units test ed. (4) The inhibitory effect of LC stimulation was strengthened by pr eadministration of ICV Ph (40 nmol) in 17 units tested. (5) ICV admini stration of norepinephrine (NE 30 nmol) resulted in PF neurons a bipha sic response to nociceptive stimulation: an early brief inhibition and a late long-lasting facilitation. (6) Pretreatment of ICV Ph (40 nmol ) prior to NE injection prevented the NE-induced biphasic response. Th e results suggest that stimulation of LC modulates the nociceptive res ponse of PF neurons through both ascending and descending routes. Thes e two diverse routes exert two different effects: a predominantly inhi bitory role on the nociceptive transmission at the spinal cord level b y descending NE-ergic fibers, and a facilitatory role on the responsiv eness of PF to noxious inputs by ascending fibers. Copyright (C) 1997 Elsevier Science Inc.