Thalamic midline cell populations projecting to the nucleus accumbens, amygdala, and hippocampus in the rat.

The organization of the thalamic midline efferents to the amygdaloid complex, hippocampal formation, and nucleus accumbens was investigated in the rat by means of multiple retrograde fluorescent tracing. The present findings indicate that these connections derive from separate cell populations of the thalamic midline, with a low degree of divergent collateralization upon more than one of the targets examined. The neural populations projecting to the amygdala, hippocampus, or accumbens are highly intermingled throughout the thalamic midline, but display some topographical prevalence. Midline thalamo-hippocampal cells are concentrated in the nucleus reuniens; thalamo-accumbens neurons prevail in the ventral portion of the paraventricular nucleus, and in the central medial nucleus. Thalamo-amygdaloid cells display a topographical prevalence in the rostral third of the thalamic midline and are concentrated in the dorsal part of the paraventricular nucleus and in the medial part of the nucleus reuniens. Both dorsally in the paraventricular nucleus and ventrally in the nucleus reuniens, thalamo-amygdaloid cells are located closer to the ependymal lining than the neurons projecting to the hippocampus or nucleus accumbens. Further, thalamo-amygdaloid cells, especially in the paraventricular nucleus, extend their dendritic processes in the vicinity of the ependymal lining, where they arborize profusely. These features indicate a close topographical relationship of neurons projecting to the amygdala with ependymal cells. The fairly discrete origin of midline outputs to the amygdala, hippocampus, and accumbens indicates that the flow of information is conveyed through separate channels from the thalamic midline to limbic and limbic-related targets. Together with the literature on the limbic afferents to the thalamus, these findings emphasize the relationships between the thalamus and the limbic system subserved by parallel input-output routes. However, because of the overlap of the projection cell populations, the thalamic midline may represent a locus of interaction among neurons connected with different parts of the limbic system. The functional implications of these findings are discussed in relation to the "nonspecific" thalamic system, as well as to the circuits involved in memory formation.