An autoradiographic study of subcortical forebrain projections from mediodorsal and adjacent midline thalamic nuclei in the rabbit

Conclusions from lesion studies on the subcortical efferent connections of the thalamic mediodorsal nucleus have been varied and conflicting. In this autoradiographic study, tritiated proline was injected into the mediodorsal nucleus and adjacent midline nuclear areas (intermediodorsal nucleus and paraventricular nucleus of the thalamus) of 18 rabbits. Terminal labeling from injections that included parts of the paraventricular nucleus was seen in the following ipsilateral areas: central amygdaloid nucleus, lateral septum, a restricted portion of the nucleus of the stria terminalis, entorhinal cortex and the entire extent of the medial (periventricular) hypothalamus. Injections that encroached into the intermediodorsal nucleus produced ipsilateral labeling in most of the nucleus accumbens, layers II and III of the olfactory tubercle, the magnocellular portion of the posterolateral basal amygdaloid nucleus and the caudate nucleus. Injections restricted to the mediodorsal nucleus produced labeling only in the ipsilateral caudate nucleus.It is concluded that medial portions of the mediodorsal nucleus only project to the caudate nucleus; whereas the midline nuclei project to the other eight subcortical areas in addition to the caudate nucleus. By attributing to the midline nuclei many of the projections previously associated with the mediodorsal nucleus, many discrepancies from earlier reports can be explained by lesion of fibers of passage. Efferent functional relationships of the mediodorsal nucleus with subcortical centers may be established through the caudate projections or projections to other thalamic structures, such as the adjacent midline nuclei.