Calcium-binding protein distributions and fiber connections of the nucleus accumbens in the pigeon (Columba livia).

Until recently, the exact location of the avian nucleus accumbens within the basal forebrain had not been well established (Reiner et al. [2004] J Comp Neurol 473:377-414). While a number of previous studies have shown afferents and efferents of the presumptive "nucleus accumbens," detailed and accurate connection patterns of this newly recognized area are still lacking. We set out to clarify these connections using small, localized injections of cholera toxin subunit B and biotinylated dextran amine directly into the nucleus. In order to increase the accuracy of tracer injections into target sites, we first conducted a systematic comparison of three calcium-binding proteins, namely, parvalbumin, calretinin, and calbindin, to characterize the nucleus accumbens and ascertain its boundaries. The results showed that the avian and mammalian nucleus accumbens had remarkable hodological similarities, including the connections with the hippocampus, amygdala, ventral pallidum, lateral hypothalamus, and ventral tegmental area. However, the most significant aspect of the present study is that the avian nucleus accumbens had extensive reciprocal connections with medial pallial structures, the mammalian counterparts of which are unclear. Three implications of this finding are discussed. First, the avian medial pallium may correspond to part of the mammalian prefrontal cortex based on the connections with the nucleus accumbens. Second, the avian brain has a "limbic loop" involving the medial pallium, which also receives input from the avian equivalent of the mediodorsal thalamus. Third, the extensive connections between the accumbens and medial pallium just dorsal to it suggest a column-like organization of limbic-associated areas in the avian telencephalon.