Integration and segregation of limbic cortico-striatal loops at the thalamic level: an experimental tracing study in rats

The frontal lobe and the basal ganglia are involved in a number of parallel , functionally segregated circuits. Information is thought to pass from dis tinct parts of the (pre)frontal cortex, via the striatum, the pallidum/subs tantia nigra and the thalamus, back to the premotor/prefrontal cortices. Cu rrently, different views exist as to whether these circuits are to be consi dered as open or closed loops, as well as to the degree of interconnection between different circuits. The main goal of the present study is to answer some of these questions for the limbic corticostriatal circuits. The latte r circuits involve the nucleus accumbens, the ventral pallidum/dorsomedial substantia nigra pars reticulata, the medial parts of the mediodorsal and v entromedial thalamic nuclei and the prefrontal cortex. Within the nucleus a ccumbens, a core and a shell region are recognized on the basis of anatomic al and functional criteria. The shell of the nucleus accumbens projects pre dominantly to the mediodorsal, the midline and the reticular thalamic nucle i via the ventral pallidum, whereas the core reaches primarily the medial p art of the ventromedial thalamic nucleus, the intralaminar and mediodorsal thalamic nuclei via a relay in the dorsomedial substanta nigra pars reticul ata. By means of double labeling experiments with injections of anterograde tracers in both the ventral pallidum and the substantia nigra of rats, we were able to demonstrate that circuits involving the shell and the core of the nucleus accumbens remain largely segregated at the level of the thalamu s. Only restricted areas of overlap of ventral pallidal and reticular nigra l projections occur in the mediodorsal and ventromedial thalamic nuclei, wh ich allows for a limited degree of integration, at the thalamic level, of i nformation passing through the two circuits. (C) 1999 Elsevier Science B.V. All rights reserved.