<p class="first" id="P2">Eph-ephrin interactions guide topographic mapping and pattern
formation in a variety
of systems. In contrast to other sensory pathways, their precise role in the assembly
of central auditory circuits remains poorly understood. The auditory midbrain, or
inferior colliculus (IC) is an intriguing structure for exploring guidance of patterned
projections as adjacent subdivisions exhibit distinct organizational features. The
central nucleus of the IC (CNIC) and deep aspects of its neighboring lateral cortex
(LCIC, Layer 3) are tonotopically-organized and receive layered inputs from primarily
downstream auditory sources. While less is known about more superficial aspects of
the LCIC, its inputs are multimodal, lack a clear tonotopic order, and appear discontinuous,
terminating in modular, patch/matrix-like distributions. Here we utilize X-Gal staining
approaches in
<i>lacZ</i> mutant mice (ephrin-B2, -B3, and EphA4) to reveal EphA-ephrinB expression
patterns
in the nascent IC during the period of projection shaping that precedes hearing onset.
We also report early postnatal protein expression in the cochlear nuclei, the superior
olivary complex, the nuclei of the lateral lemniscus, and relevant midline structures.
Continuous ephrin-B2 and EphA4 expression gradients exist along frequency axes of
the CNIC and LCIC Layer 3. In contrast, more superficial LCIC localization is not
graded, but confined to a series of discrete ephrin-B2 and EphA4-positive Layer 2
modules. While heavily expressed in the midline, much of the auditory brainstem is
devoid of ephrin-B3, including the CNIC, LCIC Layer 2 modular fields, the dorsal nucleus
of the lateral lemniscus (DNLL), as well as much of the superior olivary complex and
cochlear nuclei. Ephrin-B3 LCIC expression appears complementary to that of ephrin-B2
and EphA4, with protein most concentrated in presumptive extramodular zones. Described
tonotopic gradients and seemingly complementary modular/extramodular patterns suggest
Eph-ephrin guidance in establishing juxtaposed continuous and discrete neural maps
in the developing IC prior to experience.
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