Functional organization of the dorsal cochlear nucleus of the horseshoe bat (Rhinolophus rouxi) studied by GABA and glycine immunocytochemistry and electron microscopy.

Unique among mammals, the dorsal cochlear nucleus (DCN) of horseshoe bats consists of two functionally and anatomically distinct subdivisions: a laminated ventral portion that processes the frequency range below the constant frequency (CF) component of the echolocation signal and a nonlaminated dorsal portion that is specialized for processing the CF-signal range (76 kHz and higher). Using conventional transmission electron microscopy and postembedding immunocytochemistry for the inhibitory neurotransmitters GABA and glycine on semithin-alternating sections, we present further evidence that the ventral laminated subdivision of DCN conserves the main elements of microcircuitry and GABA/glycine labeling patterns typical for the mammalian DCN: (i) the main cell types and synaptic inventory of the granule cell/cartwheel cell system of the superficial layers are present as well as (ii) the tuberculoventral cell system of the deep layers. The nonlaminated dorsal subdivision lacks the granule cell/cartwheel cell system and is composed of a mixture of fusiform projection neurons with tuberculoventral cell analogues. Thus the inhibitory tuberculoventral system known to play an important role in temporal and spectral processing in VCN is conserved throughout the DCN of horseshoe bats, whereas functional components of cerebellar-like circuits are reduced in a specialized region that processes the dominant biosonar component.