Afferent projections to the dorsal thalamus of the rat as shown by retrograde lectin transport. II. The midline nuclei.

Afferent projections to midline thalamic cell groups which innervate nucleus accumbens, were identified by the retrograde transport of unconjugated wheat germ agglutinin followed by the identification of labelled cell groups with immunocytochemistry. Large numbers of neurones were labelled in a variety of hypothalamic nuclei; principally in the medial preoptic area, anterior hypothalamic area, ventromedial, periventricular, arcuate and posterior hypothalamic nuclei, and in the supramammillary and lateral hypothalamic areas. Following rostral thalamic injections labelled cells were also found in the lateral septum, bed nucleus of the stria terminalis, and zona incerta. Prominent and localised label was found in the nucleus reticularis of thalamus at its most rostral medial tip. Cortical label was found in the ventral subiculum following rostral injections, and in the perirhinal cortex following mid-thalamic injections. In the brainstem label was found in central grey, laterodorsal tegmental nucleus, raphe, dorsal and ventral parabranchial nuclei and nucleus of the tractus solitarius. The results are discussed in the context of striatal function, particularly the nucleus accumbens, which is a component of the ventral striatum. Thus, the midline thalamic nuclei may provide an interface where a variety of inputs from many limbic regions and hypothalamic nuclei can influence nucleus accumbens function. Comparison of afferents to several thalamic nuclei directly related to striatal function and the prefrontal cortex show, that forebrain thalamic afferents from pallidal and hypothalamic sites, are organised with a clear topography. Some afferents suggest specific routes which may allow the reticular activating system to participate in the regulation of basal ganglia function.