Direct innervation of identified tectothalamic neurons in the inferior colliculus by axons from the cochlear nucleus

The present study sought to identify tectothalamic neurons in the rat infer ior colliculus that receive their innervation directly from the cochlear nu clei and to identify the axons that provide the innervation. A direct proje ction would bypass the binaural interactions of the superior olivary comple x and provide the quickest route to the neocortex. Axons, primarily from th e dorsal cochlear nucleus, were labeled with anterograde transport of dextr an and terminated in the central nucleus of the inferior colliculus in a la minar pattern. Most labeled axons were thin and simply branched. Other axon s were thicker, gnarly, less frequently observed and probably originated fr om the ventral cochlear nucleus. None had concentrated endbulbs or a nest o f endings. Both types of axons terminated primarily in the central nucleus and layer 3 of the external cortex. This pattern suggests that the combinat ion of these subdivisions in the rat are equivalent to the central nucleus as defined in other species. Tectothalamic neurons in the inferior collicul us in the same animals were identified by retrograde transport from the med ial geniculate body and intracellular injection of Lucifer Yellow. A number of different cell types act as tectothalamic neurons and receive contacts from cochlear nucleus axons. These include hat cells (disc-shaped), less-fl at cells and stellate cells. Two innervation patterns were seen: a combinat ion of axosomatic and axodendritic contacts, and predominantly axodendritic contacts. Both patterns were seen in the central nucleus, but axosomatic c ontacts were seen less often in the other subdivisions. This is the first study to show direct connections between cochlear nuclear axons and identified tectothalamic neurons. The layers of axons from cochl ear nuclei may provide convergent inputs to neurons in the inferior collicu lus rather than the heavy inputs from single axons typical of lower auditor y nuclei. Excitatory synapses made by axons from the cochlear nuclei on tec tothalamic neurons may provide a substrate for rapid transmission of monaur al information to the medial geniculate body. (C) 1999 IBRO. Published by E lsevier Science Ltd.