Complementary multimodal compartments in the developing inferior colliculus

The auditory system is responsible for detecting, encoding, and deciphering hearing. The inferior colliculus (IC) is a major relay hub situated in the midbrain, that is subdivided into a central nucleus, and surrounding dorsal and lateral cortices. The central nucleus of the inferior colliculus (CNIC) is organized tonotopically based on a frequency gradient and strictly processes auditory information. In contrast, recent studies show that the lateral cortex of the inferior colliculus (LCIC) is actually multimodal, receiving inputs from not just auditory sources, but also somatosensory and visual structures. The precise organization of patterned inputs to the LCIC and their development has yet to be fully established. Mounting evidence suggests a modular LCIC framework with surrounding extramodular zones that provide an anatomical substrate for input-output arrays. Previously, a series of histochemical and immunocytochemical stains including acetylcholinesterase (AChE), cytochrome oxidase (CO), glutamic acid decarboxylase (GAD), nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), and parvalbumin (PV) were identified as discrete markers of LCIC layer 2 modular fields. The present study builds upon these findings and establishes calretinin (CR), a calcium binding protein, as a complementary extramodular marker. CR-specific labeling was observed in LCIC zones surrounding presumptive layer 2 modules at all ages, yet became increasingly more distinct at later developmental stages. This finding somewhat contrasts previous results in developing rat in which LCIC CR patterns were more evident prior to hearing onset (Lohmann and Friauf, 1996). NADPH-d and CR double-labeling confirms a complementary modular/extramodular LCIC substrate that is established during the early postnatal period. Similarly organized Eph-ephrin guidance expression patterns and developing multimodal projection patterns suggest that this arrangement is functionally important. Understanding the neuronal development of the modular/extramodular architecture of the LCIC is crucial in future development of therapeutic treatments regarding brain plasticity and tinnitus.