The connections of the nucleus reuniens thalami: Evidence for a direct thalamo‐hippocampal pathway in the rat

Abstract

The afferent and efferent connections of the rat's midline nucleus reunions thalami (reuniens) were studied by experiments using the methods of retrograde cell marking by horseradish peroxidase (HRP) and anterograde fiber tracing by autoradiography. A microelectrophoretic deposit of tritiated amino acids in reuniens provided the first evidence of a direct thalamo‐hippocampal connection. Labeled reuniens efferents ascend to the genu of the corpus callosum and turn caudally in the cingulate fasciculus, from which fibers distribute to layer I of the anterior medial, cingulate, and retrosplenial cortices. A longer component of this system curves around the callosal splenium and forms a massive rostrally directed fiber sheet that innervates entorhinal and parahippocampal areas and Ammon's horn. Entorhinal afferents are localized to layers I and III, whereas the hippocampal afferent plexus is remarkably restricted to the stratum lacunosum‐moleculare of the CA1 field and the corresponding stratum of the ventral subiculum. Reuniens projects more sparsely and diffusely to many subcortical structures, a number of which lie in the limbic domain: the anterior olfactory nucleus, nucleus accumbens, olfactory tubercle, amygdala, claustrum, septum, preoptic area, medial and lateral hypothalamic regions, deep portions of the pretectum and superior colliculus, rostral levels of the ventral tegmental area and central gray substance and, perhaps, the median eminence. The efferent connections of reuniens were examined with HRP. HRP deposited in the nucleus labeled small to moderate numbers of neurons in many structures extending from the frontal cortex to caudal midbrain levels. The appearance of cell labeling in regions projected upon by reuniens suggests a reciprocity of connections between it and the medial cortex, septum, preoptic area, amygdala, medial and lateral hypothalamic regions, ventral tegmental area, central gray substance, pretectum, superior colliculus and the subiculum. Cell labeling in regions not receiving its efferents – the ventral thalamus, midbrain tegmentum, mesencephalic raphe, and parabrachial nuclei – may hold another clue to the future understanding of the role of the nucleus reuniens in limbic functions.