Characterization of inhibition of a spinal nociceptive reflex by stimulation medially and laterally in the midbrain and medulla in the pentobarbital-anesthetized rat.

Inhibition of the spinal nociceptive tail flick (TF) reflex by electrical stimulation throughout the midbrain and medulla was examined and characterized in pentobarbital-anesthetized rats. The TF reflex in the lightly anesthetized state is of significantly shorter latency (1.63 s vs 2.36 s) and of greater amplitude than in the unanesthetized state. Systematic mapping studies revealed that inhibition of the TF reflex could be produced from widespread areas in the midbrain and medulla. Midbrain areas having the lowest thresholds for inhibition of the TF reflex were found lateral and ventrolateral to the periaqueductal gray matter, including nucleus cuneiformis, the lateral reticular formation, and extending into the central tegmental area. In the rostral medulla, the lowest thresholds for inhibition of the TF were distributed mediolaterally across the dorsal one-third of the nucleus raphe magnus and into the adjacent nucleus reticularis gigantocellularis. Thresholds for inhibition of the TF reflex were slightly higher in the ventral nucleus raphe magnus and adjacent nucleus reticularis paragigantocellularis. Chronaxies of stimulation in the midbrain and medulla were virtually the same, indicating that the same neural elements were affected by stimulation in both brainstem areas. The thresholds of stimulation for inhibition of the TF reflex in the lightly anesthetized state were not significantly different from the thresholds of stimulation at the same midbrain sites in the awake state in the same animals. These findings contribute to a growing body of literature establishing (1) the utility of the lightly pentobarbital-anesthetized rat model for investigations of antinociceptive mechanisms and (2) the presence of multiple loci and pathways in the brainstem capable of modulating spinal nociceptive processes.