Ventral tegmental analgesia in two strains of rats: effects of amphetamine, naloxone and parachlorophenylalanine.

Pain sensitivity and analgesia induced by the stimulation of the ventral tegmentum (VT) were studied in 72 male rats of two lines, LC2-Hi and LC2-Lo, genetically selected for high and low rates of lateral hypothalamic self-stimulation, respectively. LC2-Lo rats were more sensitive to acute peripheral pain and developed a stronger analgesia than their LC2-Hi counterparts. In order to assess the pharmacological substrate of ventral tegmental stimulation-induced analgesia (VT-SIA), the effects of amphetamine (AMP, 21 animals), naloxone (NX, 24 animals) and parachlorophenylalanine (PCPA, 27 animals) injections were studied. VT-SIA was found to be clearly decreased by PCPA, slightly decreased by AMP and not significantly affected by NX. Ventral tegmental self-stimulation ( VTSS ) was increased by PCPA treatment. The comparison of VTSS and VT-SIA did not reveal any correlation between both phenomena. These data suggest that VT-SIA may be mediated by serotonin while catecholamines may have a modulatory role in this analgesia and that VTSS and VT-SIA seem to be governed by different neuronal systems.