Collateral innervation of the inferior colliculus in the North American opossum: a study using fluorescent markers in a double-labeling paradigm.

The innervation of the opossum inferior colliculus was investigated using the retrograde transport of fluorescent markers in a double-labeling paradigm. True Blue was injected into one inferior colliculus while Nuclear Yellow was placed into the other. Many single-labeled neurons were found in all of the brainstem and cortical areas previously labeled by comparable injections of HRP. When the two injections of fluorescent markers were bilaterally symmetrical within the inferior colliculi, double-labeled neurons were numerous in the medial and lateral superior olivary nuclei. Only a few double-labeled neurons were found in the auditory cortex (AC), the dorsal nucleus of the lateral lemniscus (DNLL), the nucleus reticularis gigantocellularis pars ventralis (RGcv), and the dorsal column nuclei (DColN) and the spinal trigeminal complex (TrS). These data suggest that two patterns of innervation are seen for afferent fibers from regions with bilateral connections to the IC. The first pattern, typified by projections from AC, DNLL, RGcv, the DColN and TrS, primarily consists of neurons with unilaterally directed axons, extending to either the ipsilateral or contralateral IC. The second pattern, typified by projections from LSO and MSO, displays a great number of neurons whose axons apparently innervate both inferior colliculi, presumably through a process of axon collateralization. The bilateral projection of axons from individual neurons in these two nuclei possibly reflects their involvement in the processing of sensory information from the two cochleas and may represent at least one pathway whereby binaural information is relayed to both inferior colliculi.