Small injections of tritiated leucine and proline confined to the ventral tegmental
area (AVT) were found to label fibers ascending: (a) to the entire ventromedial half
of the striatum, but most massively to the ventral striatal zone that includes the
nucleus accumbens; (b) to the thalamus: lateral habenular nucleus, nuclei reuniens
and centralis medius, and the most medial zone of the mediodorsal nucleus; (c) to
the posterior hypothalamic nucleus and possibly the lateral hypothalamic and preoptic
region; (d) to the nuclei amygdalae centralis, lateralis and medialis; (e) to the
bed nucleus of the stria terminalis, the nucleus of the diagonal band, and the medial
half of the lateral septal nucleus; (f) to the anteromedial (frontocingulate) cortex;
and (g) to the entorhinal area. Further AVT efferents descend to the medial half of
the midbrain tegmentum including an anterior region of the median raphe nucleus, to
the ventral half of the central grey substance including the dorsal raphe nucleus,
to the parabrachial nuclei, and to the locus coeruleus. Similar injections centered
in the pars compacta of the substantia nigra (SNC) label fibers that are distributed
in the striatum in an orderly medial-to-lateral arrangement, and almost entirely avoid
the nucleus accumbens and olfactory tubercle. With the exception of the lateral quarter
of the substantia nigra, which apparently does not project to the extreme rostral
pole of the striatum, each small SNC locus, regardless of its anteroposterior localization,
distributes nigrostriatal fibers throughout the length of the striatum. Descending
SNC efferents are distributed to the same general regions that receive descending
AVT projections, except that no SNC fibers appear to enter the locus coeruleus. Isotope
injections confined to the pars reticulata (SNR) label sparse nigrostriatal fibers,
and numerous nigrothalamic fibers ascending mainly to the nucleus ventromedialis and
in lesser number to the parafascicular nucleus and the paralamellar zone of the nucleus
mediodorsalis. Descending SNR fibers leave the nigra as a voluminous fiber bundle
that bifurcates into a large nigrotectal and a smaller nigrotegmental component, the
latter terminating largely in the pedunculopontine nucleus of the pontomesencephalic
tegmentum.