Efferent Connections and Nigral Afferents of the Nucleus Accumbens Septi in the Rat
The results of this study by the methods of autoradiographic fiber-tracing and retrograde cell-labeling confirm earlier reports of accumbens projections to the globus pallidus and to dorsal strata of the medial half of the substantia nigra. Also in accord with previous autoradiographic evidence, sparser projections could be traced to a variety of subcortical structures implicated in the circuitry of the limbic system: bed nucleus of the stria terminalis, septum, preoptic region, hypothalamus, ventral tegmental area, nuclei paratenialis and mediodorsalis thalami, and lateral habenular nucleus. Contrary to earlier reports, striatopallidal fibers from the accumbens were found to be distributed largely to the subcommissural part of the external pallidal segment (ventral pallidum of Heimer & Wilson, 1975) and to avoid almost entirely the internal pallidal segment (entopeduncular nucleus). Mesencephalic projections from the accumbens largely coincide with those from the preoptic region and hypothalamus; like the latter they prominently involve the region of the out-lying nigral cell groups A10 and A8 and extend caudally beyond the nigral complex to the cuneiform and parabrachial regions of the tegmentum as well as to caudoventral parts of the central grey substance.
Horseradish peroxidase injected into the nucleus accumbens labels numerous neurons in the region of cell group A10 and in the supralemniscal ‘retrorubral nucleus’, but only sporadic cells in the pars compacta of the substantia nigra proper. It thus appears that the accumbens projects to a region of the nigral complex considerably larger than that from which it receives nigrostriatal fibers, and hence, that the nigro-striato-nigral circuit associated with the accumbens is not organized in a mode of simple point-for-point reciprocity.
The problem of delimiting and accumbens from the rest of the striatum is examined by comparing cases of tracer injection into various discrete loci within the ventral zone of the striatum.
KeywordsSubstantia Nigra Nucleus Accumbens Ventral Tegmental Area Globus Pallidus Medial Forebrain Bundle
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