Antinociceptive effects of brain rewarding system in the developing rabbit: behavioral and electrophysiological analysis

The modulating effects of electrical stimulation (ES) in the reward sites (RSs) of the hypothalamus and adjacent brain areas on the defensive reaction (DR) in response to peripheral noxious stimulation (PNS) and on evoked potentials (EPs) recorded in the thalamic centromedian-parafascicular complex (CM-Pf) to the same PNS have been investigated in 20-40-, 41-60-day old and 3-5-month old rabbits. Previously, RSs were identified by the method of intracranial self-stimulation (ICSS). Behavioral and electrophysiological testings successively performed on each rabbit first awake and then anaesthetized have revealed the ES in all investigated RSs to inhibit DR and EPs. These effects were not observed if the stimulating electrode tip was localized in emotionally neutral brain sites. In behavioral testings, the antinociceptive effect of rewarding stimulation was positively correlated with the ICSS intensity in given brain sites regardless of the age of the rabbits. In electrophysiological testings, a similar dependence of nociceptive EPs inhibition on emotional values of stimulated brain sites (determined as a rate of lever pressings during ICSS) was discovered only in 20-40-day and 3-5-month old rabbits. Stimulation of low emotional value RSs (low rate (LR)-rabbits) exerted a weaker inhibitory influence in 3-5-month old rabbits in comparison with 20-40- and 41-60-day old rabbits both in behavioral and electrophysiological testings. The weakest antinociceptive effect of rewarding stimulation among the rabbits with a high rate of lever pressings (HR-rabbits) was found in 41-60-day olds in electrophysiological testings. In 41-60-day old rabbits, we have observed a discrepancy between the behavioral analgesia and its electrophysiological correlates. It may be suggested that the observed peculiarities of antinociceptive influences produced by RSs ES are determined by the age factors of neurotransmitter brain systems. (C) 1999 Elsevier Science B.V. All rights reserved.