Stimulation of the occipital or retrosplenial cortex reduces incision pain in rats

The electrical stimulation of the occipital (OC) or retrosplenial (RSC) cortex produces antinociception in the rat tail-flick and formalin tests. This study examined the antinociceptive effects of stimulating the OC or RSC in a rat model of post-incision pain. The involvement of the anterior pretectal nucleus (APtN) as intermediary for the effect of OC or RSC stimulation was also evaluated because the OC and RSC send inputs to the APtN, which is implicated in antinociception and nociception. It is shown that a 15-s period of electrical stimulation of the OC or RSC significantly reduced post-incision pain for less than 10 min and at least 15 min, respectively. The injection of 2% lidocaine (0.25 μl), naloxone (10 ng/0.25 μl), methysergide (40 pg/0.25 μl), or atropine (100 ng/0.25 μl) into the APtN produced a further increase in post-incision pain. The effect of RSC stimulation was shorter and less intense in rats pretreated with lidocaine, methysergide or naloxone. The effect of OC stimulation was shorter and less intense in lidocaine-treated rats, but remained unchanged in rats pretreated with methysergide or naloxone in the APtN. The effects of stimulating the OC or RSC were not changed in rats treated with atropine. We conclude that stimulation-induced antinociception from the RSC or OC in rat post-incision pain activates distinct descending pain inhibitory pathways. The pathway activated from the RSC utilizes serotonergic and opioid mediation in the APtN, whereas stimulation of the OC utilizes a non-serotonergic, non-cholinergic and non-opioid mediation in the same nucleus.