A chemoarchitectonically similar internal extension connects the rabbit intergeniculate leaflet to midline dorsal thalamaic nuclei.

The adscription of the intergeniculate leaflet to the ventral thalamus or to the dorsal thalamus has remained undecided due to the absence of specific evidence supporting either possibility. This report describes a thin retroreticular band of dorsal thalamic neurons, innervated selectively by Substance-P and L-enkephalin immunoreactive fibers, that extends from the medial end of the intergeniculate leaflet to the lateral border of the midline dorsal thalamic nuclei and abuts on the caudal aspect of the reticular nucleus. The area of the band contains small basophile fusiform neurons, but no immunoreactive cell bodies were observed. NADH-diaphorase histochemistry shows that this band is sharply distinct from ventral thalamic grisea, as well as from the neighboring ventrobasal complex of the dorsal thalamus. Our data suggest that, in the rabbit, the substance-P and L-enkephalin immunoreactive band forms together with the intergeniculate leaflet a chemoarchitectonically distinct structural unit of the dorsal thalamus, constituting a thin mediolateral lamina that bounds other dorsal thalamic nuclei from the ventral thalamus.