Antinociception induced by stimulating the anterior pretectal nucleus in two models of pain in rats.

1. This study examined whether different parts of the rat anterior pretectal nucleus (APtN) may be involved in the spinal control of brief (tail flick test) or persistent (surgical incision of the plantar aspect of a hind paw) noxious inputs via activation of descending pathways. 2. We have confirmed that stimulation of the dorsal APtN produces a strong antinociceptive effect in the tail flick test, as opposed to a very weak effect obtained from the ventral APtN. Stimulation at the ventral APtN was the most effective part of the nucleus against a persistent incisional pain. 3. The incisional pain was significantly increased following injection of 1 or 2% lignocaine (0.25 microL) into the nucleus, but the effect was more intense after neural block of the ventral rather than the dorsal APtN. Injection of 2% lignocaine (0.10 microL) into the ventral, but not dorsal, APtN significantly increased the perception of the incisional pain. 4. We conclude that the effect of stimulating the APtN depends on the site of stimulation and model of pain used. Sustained noxious stimuli activate pathways from the ventral APtN to reduce further noxious spinal inputs. The noxious stimulation produced during the tail flick test may be not enough to activate the same circuitry, but electrical stimulation at the dorsal APtN is very effective in inhibiting brief thermal noxious inputs at the spinal level.