VENTROLATERAL ORBITAL CORTEX AND PERIAQUEDUCTAL GRAY STIMULATION-INDUCED EFFECTS ON ON-CELLS AND OFF-CELLS IN THE ROSTRAL VENTROMEDIAL MEDULLA IN THE RAT

On- and off-cells of the rostral ventromedial medulla are thought to b e involved in bulbospinal inhibition of ascending nociceptive informat ion. Experiments were carried out in lightly anaesthetized rats to ass ess the effects of prefrontal cortex stimulation on the responses of n eurons in the rostral ventromedial medulla. For comparison purposes, e ffects of periaqueductal gray stimulation were also investigated. Sing le unit activity was recorded in the rostral ventromedial medulla and on-, off- and neutral-cells were identified based on the tail nocifens or reflex to noxious heat. Short (0.1-1 s) and long (10-15 s) trains o f bipolar electrical stimulation (100-300 Hz) were delivered to the ve ntrolateral orbital,cortex of the rat forebrain and the periaqueductal gray. Short-train stimulation of the periaqueductal gray (including d orsolateral, ventrolateral and the dorsal raphe regions) excited 58% ( 25 of 43) of on-cells and 44% (seven of 16) of off-cells in the rostra l ventromedial medulla. Long trains blocked the noxious stimulus-evoke d pause of all seven off-cells tested and blocked the excitatory respo nse of two, and enhanced one of three on-cells. Such stimulation also inhibited or abolished the tail-flick reflex at currents below 100 mu A. Glutamate microinjections into the periaqueductal gray inhibited th e noxious-evoked response of two off- and two on-cells and increased t he tail-Rick latency. Short-train stimulation of the ventrolateral orb ital cortex (100-400 mu A) excited eight of 25 on-cells and inhibited the ongoing activity of 10 of 14 off-cells, Long-train ventrolateral o rbital cortex stimulation (5-15 s, 100-200 mu A, 200-300 Hz) enhanced the noxious evoked responses of 10 of 11 on-cells, prolonged the noxio us heat-evoked pause of all of four off-cells and decreased the tail-f lick latency (pronociception). The results of this study support the p roposed role of on- and off-cells in descending inhibition of nocicept ion from the periaqueductal gray and implicate the ventrolateral orbit al cortex in the control of this pathway.