Stimulation of the rat dorsal raphe in vivo releases labeled serotonin from the parietal cortex

In vivo release of labeled serotonin ([³H]5-HT) from the parietal cortex was investigated by cortical cup technique and electrical stimulation of midbrain raphe in rats anesthetized with pentobarbital sodium. The spontaneous efflux of tritium from the parietal cortex preloaded with [³H]5-HT followed a multiphasic exponential course. After 120 min, the rate of efflux appeared to fit the single exponential function (slow phase). Imipramine (10⁻⁶−10⁻³ M) produced a dose-dependent increase in the spontaneous release. When pargyline in concentrations ranging from 10⁻⁴ to 10⁻³ M were added to the medium in the cup, the unchanged [³H]5-HT significantly increased in a dose-dependent manner and the slow declining coefficient of tritium efflux significantly decreased in the presence of 10⁻⁴ pargyline. Stimulation of the rostral two-thirds of the dorsal raphe and the lateral 5-HT bundle originating from the dorsal raphe significantly increased the release of [³H]5-HT and its metabolites while stimulation of the caudal one-third of the dorsal raphe did not produce a significant increase in the release of [³H]5-HT and its metabolites. Stimulation of the median raphe produced no or only a slight increase in the release of [³H]5-HT and its metabolites. These findings are a direct demonstration of the in vivo release of [³H]5-HT from the parietal cortex with stimulation of the dorsal raphe, particularly the rostral two-thirds of the nucleus and provide the neurochemical evidence for the dorsal raphe-cortical 5-HT pathway via the lateral 5-HT bundle.