Acetylcholinesterase and NADH-diaphorase chemoarchitectonic subdivisions in the rabbit medial geniculate body

The distribution of acetylcholinesterase and NADH-diaphorase activities was studied histochemically in the rabbit medial geniculate body, yielding new data useful for the definition of the common structural pattern of this thalamic complex in mammals. Four chemoarchitectonic subdivisions could be detected in transversal, horizontal and sagittal sections that corresponded to the previously described ventral, dorsal and internal nuclei, and to a fourth subdivision, defined as the mediorostral nucleus of the medial geniculate complex in the rabbit. The topography and cellular typology of the mediorostral nucleus suggest its homology with the so-called magnocellular nucleus of other mammals, an identity that was previously assigned to the internal nucleus. The relative position of the rabbit internal and dorsal nuclei and comparative connectional data are combined to suggest their correspondence with the anterodorsal and posterodorsal subnuclei, respectively, of the cat and the monkey. Global functional interpretations of these nuclei as sites of visuoacoustic and somatoacoustic polymodal integration support the notion of a shell region of the medial geniculate, surrounding the principal cochleotopic ventral nucleus and interconnected to the cortical acoustic belt around the primary auditory area. Acetylcholinesterase and NADH-diaphorase chemoarchitectony may be useful for the detection of similar partitions in species where cytoarchitectonic differentiation of the medial geniculate is less clear.