Effects of isoflurane concentration on the activity of pontomedullary raphe and medial reticular neurons in the rat.

Neurons in the pontomedullary raphe magnus (RM) and adjacent nucleus reticularis paragigantocellularis pars alpha (NRPG alpha) are thought to participate in the modulation of spinal nociceptive transmission. In order to determine whether these cells also contribute to the suppression of nocifensive reflexes produced by general anesthetics, the spontaneous activity of RM/NRPG alpha cells was recorded in rats anesthetized with isoflurane (IF) at several steady state concentrations, corresponding to depths which are below, equal to, or above the threshold for blocking the motor response to noxious stimuli (minimum alveolar concentration, MAC). Neurons were classified by their spontaneous activity patterns and their responses to noxious stimulation as OFF, ON, REGULAR or NEUTRAL cells. After cell classification, unit activity, arterial blood pressure, heart rate, and EEG activity were simultaneously recorded, in the absence of somatic stimulation, for 1 h at each of two or three concentrations of IF. The concentrations tested were low (1.05-1.25%), medium (1.30-1.45%) and high (1.70-1.90%). ON, OFF and some NEUTRAL cells exhibited alternating periods of inactivity and activity when recorded during periods of low and medium anesthetic concentrations. At high steady state anesthetic concentrations, the mean discharge of most OFF, ON and NEUTRAL cells decreased by greater than 25% from their mean discharge rate at the low concentration. REGULAR cells maintained a uniform firing rate at all steady state anesthetic concentrations studied. Since high concentrations of IF do not activate OFF cells, the putative inhibitory output neuron of the RM/NRPG alpha, it is unlikely that the activity of RM/NRPG alpha neurons contributes to the suppression of nocifensive movement by the general anesthetic, IF.