Ventral mesopontine projections of the caudomedial shell of the nucleus accumbens and extended amygdala in the rat: Double dissociation by organization and development

The shell of the nucleus accumbens and central division of the extended amy gdala are telencephalic structures that influence motor activity and lately have been regarded by some as components of a single functional-anatomic c ontinuum. Each has a highly differentiated internal organization and output system and distinct pharmacologic responses however, and it is thus likely that each subserves distinct contributions to behavior. In this investigat ion, nucleus accumbens and extended amygdala outputs were compared by using retrograde tracing in adult and postnatal rats. Fluoro-Gold, when injected into the ventral tegmental area, produced substantial retrograde labeling in the adult nucleus accumbens shell, but only trivial amounts in the centr al division of the extended amygdala. Injection sites in the lateral mesopo ntine tegmentum produced robust labeling in the central extended amygdala b ut little in the nucleus accumbens. The projections of extended amygdala we re substantially developed by postnatal day 1, whereas those of the caudome dial shell of the nucleus accumbens only reached the ventral tegmental area by approximately postnatal day 6. Few neurons projecting from the caudomed ial shell of the accumbens to the ventral tegmental area were observed even at postnatal day 21. In consideration of the reported importance of the nu cleus accumbens, particularly the caudomedial shell, in neural processing r elated to reward and motivation and the central nervous system response to antipsychotic drugs, it may be important to determine whether processes occ urring during the protracted postnatal development of the caudomedial shell are vulnerable to destructive circumstances, such as drug intoxication, ma ternal separation, or social isolation. J. Comp. Neurol. 436: 111-125, 2001 . (C) 2001 Wiley-Liss, Inc.