PATTERNS OF CONVERGENCE AND SEGREGATION IN THE MEDIAL NUCLEUS-ACCUMBENS OF THE RAT - RELATIONSHIPS OF PREFRONTAL CORTICAL, MIDLINE THALAMIC, AND BASAL AMYGDALOID AFFERENTS

In the rat, fibers from the prelimbic cortex terminate in the medial n ucleus accumbens. Anterior paraventricular thalamic and parvicellular basal amygdaloid fibers reach both the prelimbic cortex and the medial nucleus accumbens. All three afferent systems have an inhomogeneous d istribution within the nucleus accumbens, and whether or not these pro jections actually reach the same areas is unknown. Our aim was to eval uate the relationships of the three afferents with respect to the shel l, the core, and the cell clusters of the nucleus accumbens. Double an terograde tracing and single anterograde tracing combined with immunoh istochemistry for calbindin (D28k) or Nissl stain was used. Following tracer injections in the prelimbic cortex and the anterior paraventric ular thalamus, a complementary (i.e., nonoverlapping) pattern of fiber s was found in the shell. Thus, afferents from the prelimbic cortex ar e associated with cell clusters, whereas those from the anterior parav entricular thalamus avoid these cells but are affiliated with regions exhibiting weak homogeneous calbindin immunoreactivity. In the calbind in-poor patches of the core, the situation is reversed as both sets of fibers overlap. In cases with injections in the prelimbic cortex and the parvicellular basal amygdala, a pattern of overlap was seen in the shell and core. Thus, the fibers in the shell were found together in association with cell clusters, whereas regions of weak homogeneous ca lbindin immunoreactivity were avoided. In the core, overlap was seen i n the patch compartment. Finally, with parvicellular basal amygdala/pa raventricular thalamus injections, a complementary fiber organization was present in the shell, but overlap was prominent in the patches of the core. The results demonstrate that the relationships of prelimbic cortical, paraventricular thalamic, and parvicellular basal amygdaloid afferents in the nucleus accumbens vary according to their compartmen tal (immunohistochemical and cellular) affiliation. Compartmentalizati on is therefore a possible anatomical substrate for condensation or se gregation of neuronal signals passing through the nucleus accumbens. ( C) 1995 Wiley-Liss, Inc.