Central antiaversive and antinociceptive effects of anterior pretectal nucleus stimulation: attenuation of autonomic and aversive effects of medial hypothalamic stimulation.

Previous studies have shown that stimulation of the rat anterior pretectal nucleus (APtN) strongly depresses a spinal reflex to noxious heat without causing significant aversion or depression of other motor responses. It is not known if APtN stimulation can similarly reduce the aversiveness of electrical stimulation of the brain, nor is it known if APtN stimulation is itself rewarding or aversive. This study used a simple switch-off paradigm to examine the rewarding properties of APtN stimulation at different sites throughout the nucleus and also used the tail-flick test to determine if the stimulation produced antinociception. The effects of APtN stimulation on the behavioural and autonomic responses to electrical stimulation of the medial hypothalamus (MH) and the nucleus raphe magnus (NRM) were also examined. The results show that electrical stimulation of dorsal APtN was rewarding and also caused antinociception which lasted for 50 min. However, sites which gave the strongest reward were not necessarily those which gave the greatest antinociception, as these effects were not correlated. Electrical stimulation of ventral APtN induced only aversive effects. The aversive and autonomic effects of MH stimulation were significantly reduced by conditioning stimulation of dorsal APtN. However, the very similar escape and autonomic effects of NRM stimulation were unaffected by APtN stimulation. These results suggest that electrical stimulation of the dorsal parts of the APtN has positive rewarding properties as well as the well-known antinociceptive effects. The antiaversive effects of dorsal APtN stimulation may be due in part to the inhibition of central substrates of aversion as well as inhibition of sensory neurones.