Central Modulation of Dental Pain in the Rat

J.E. BELFORTE, J.H. PAZO, AND A.C. BARCELO

San Diego, California, USA

Congreso; Anual; 2002

International Association for Dental Research

There is only limited knowledge of the central mechanisms that modulate orofacial noxious afferent inputs. Pain accompanying pathological states in the teeth is the most common cause of orofacial pain. Dental pulp is principally innervated by small-diameter, slow-conducting fibers related to nociceptive transmission (Ad and C-fiber). Thus, the stimulation of tooth pulp constitutes a high selective model for the study of oral pain. In previous work we found evidence that the striatum could inhibit the tooth pulp evoked jaw-opening reflex (JOR) probably, by modulating the activity of sensory neurons of the trigeminal spinal nucleus, subnucleus caudalis (Sp5c). This nucleus is considered to be an essential relay of craniofacial nociceptive information to higher levels of the brain. The aim of the present work was to explore the effect of striatal stimulation in the spontaneous and evoked activity of neurons of the trigeminal motor nucleus (Mo5) and the Sp5c recorded simultaneously. In order to extend our study, the evoked responses of Sp5c neurons were classified according with their latency and were correlated with the amplitude of the JOR during striatal stimulation. In rats anesthetized with urethane painful responses were evoked by electric stimulation of the tooth pulp of the lower incisors. Spontaneous single unit activity and the evoked by dental nociceptive stimulation was recorded in Sp5c and Mo5 before and after the chemical stimulation of the striatum by a microinjection of L-glutamate (163 nmol/0.5µl). Only neurons responding to dental pulp were analyzed The JOR was evaluated as the amplitude of the EMG activity recorded in the anterior belly of digastric muscles. The spontaneous activity of Mo5 neurons was not modified by striatal stimulation alone (6.06 ± 1.6 vs. 5.94 ± 1.8 Hz; t-test for paired samples t7=0.203, p=0.84). The Mo5 neurons respond to dental pulp stimulation with a peak of activity (latency 5.9 ± 0.4 msec). The striatal stimulation produces a significative inhibition of the evoked activity in Sp5c and Mo5 neurons simultaneously evaluated (37 ± 9 % and 56 ± 4 % respectively; ANOVA F1, 10=14.2, p<0.005). We found a correlation between the effect of striatal stimulation in JOR amplitude and the evoked responses in Sp5c neurons (Ad inputs R2=0.68, t40=9.01 p<0.0001; C inputs R2=0.67, t9=3.8 p<0.001). These results demonstrate that striatal stimulation can modulate an orofacial nociceptive reflex by acting over the sensory neurons rather than the motor ones. According to this, the striatum could be involved in the orofacial analgesia. Supported by grants from CONICET, National Agency for Scientific and Technological Promotion (FONCyT) and UBA. Orofacial pain, jaw-opening reflex, trigeminal system.