On the neurochemical basis of self-stimulation with midbrain raphe electrode placements

Bar-pressing for electrical stimulation of the median raphe nucleus in rats was not attenuated by 5HT receptor blockade with metergoline or cyproheptadine, by 5HT depletion induced with parachloroamphetamine or by prior destruction of ascending 5HT pathways with intracerebral microinjections of 5,7-dihydroxytryptamine. Furthermore, in a shuttle box paradigm in which rats could both initiate and terminate stimulation, parachlorophenylalanine did not antagonize initiation of stimulation. It is concluded that the rewarding effects of raphe stimulation are not mediated by serotonergic mechanisms. In contrast to these results, alpha-methyl-paratyrosine induced catecholamine depletion exerted an inhibitory effect on initiation behaviour without impairing termination of stimulation. It is concluded that the rewarding component of raphe stimulation is mediated by catecholamines. Termination of (escape from) stimulation was not materially affected by catecholamine or 5HT depletion suggesting the aversive component of raphe stimulation may not be mediated by these monoamines.