The differential projections from the dorsal raphe and median raphe nuclei of the midbrain were autoradiographically traced in the rat brain after 3H‐proline micro‐injections. Six ascending fiber tracts were identified, the dorsal raphe nucleus being the sole source of four tracts and sharing one with the median raphe nucleus. The tracts can be classified as those lying within the medial forebrain bundle (dorsal raphe forebrain tract and the median raphe forebrain tract) and those lying entirely outside (dorsal raphe arcuate tract, dorsal raphe periventricular tract, dorsal raphe cortical tract, and raphe medial tract). The dorsal raphe forebrain tract lies in the ventrolateral aspect of the medial forebrain bundle (MFB) and projects mainly to lateral forebrain areas (e.g., basal ganglion, amygdala, and the pyriform cortex). The median raphe forebrain tract lies in the ventromedial aspect of the MFB and projects to medial forebrain areas (e.g., cingulate cortex, medial septum, and hippocampus). The dorsal raphe cortical tract lies ventrolaterally to the medial longitudinal fasciculus and projects to the caudate‐putamen and the parieto‐temporal cortex. The dorsal raphe periventricular tract lies immediately below the midbrain aqueduct and projects rostrally to the periventricular region of the thalamus and hypothalamus. The dorsal raphe arcuate tract curves laterally from the dorsal raphe nucleus to reach the ventrolateral edge of the midbrain and projects to ventrolateral geniculate body nuclei and the hypothalamic suprachiasmatic nuclei. Finally, the raphe medial tract receives fibers from both the median and dorsal raphe nuclei and runs ventrally between the fasciculus retroflexus and projects to the interpeduncular nucleus and the midline mammillary body. Further studies were done to test whether the fiber tracts travelling in the MFB contained 5‐HT. Unilateral (left) injections of 5,7‐dihydroxytryptamine (5 μgm/400 nl) 18 days before midbrain raphe microinjections of 3H‐proline produced a reduction in the grain concentrations in all the ascending fibers within the MFB. Furthermore, pharmacological and behavioural evidence was obtained to show that the 5‐HT system had been unilaterally damaged; these animals displayed preferential ipsilateral turning in a rotameter which was strongly reversed to contralateral turning after 5‐hydroxytryptophan administration. The results show that DR and MR nuclei have numerous ascending projections whose axons contain the transmitter 5‐HT. The results agree with the neuroanatomical distribution of the 5‐HT system previously determined biochemically, histochemically, and neurophysiologically. The midbrain serotonin system seems to be organized by a series of fiber pathways. The fast transport rate in these fibers was found to be about 108 mm/day.
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