Limited collateralization of neurons in the rat prefrontal cortex that project to the nucleus accumbens

The specificity and selectiveness of a neuronal message depends in part on the number of recipient neurons that simultaneously receive this message. Hence, projections involved in higher order cognitive processes might be expected to exhibit a lower degree of collateralization than projections that mediate more basic brain functions. This study sought to determine the degree to which neurons projecting from the prefrontal cortex to the nucleus accumbens collateralize to major cortical and subcortical regions: the contralateral prefrontal cortex, the basolateral amygdala or the ventral tegmental area. Fluoro-Gold and cholera toxin-b were used to label prefrontal cortex neurons that project to these targets, and the proportion of neurons singly and dually labeled by immunofluorescence for these tracers was determined. The prefrontal cortex neurons projecting to these regions exhibited a partially complementary laminar distribution. Furthermore, of the neurons projecting to the nucleus accumbens, 13% sent a collateralized projection to the contralateral prefrontal cortex, 7% collateralized to the basolateral amygdala, and 3% sent a branched projection to the ventral tegmental area. No differences were observed in the degree of collateralization of neurons in superficial versus deep layers. Thus, the degree of collateralization of corticoaccumbens neurons was overall limited, but significantly greater to a cortical target than to subcortical regions. These branching patterns provide anatomical substrates for temporal and spatial coordination of activity in limbic circuits. (C) 2000 IBRO. Published by Elsevier Science Ltd.