The thalamotectal system: An ancient projection for modulating the auditory midbrain

Descending projections from the thalamus and related structures to the tectum are evolutionarily conserved. Here we characterize basic properties of the mouse auditory thalamotectal pathway and compare them to a homologous pathway in the frog: a species that does not contain a neocortex or an additional corticocollicular pathway. Mouse auditory thalamotectal neurons were found to not stain for the calcium-binding markers typically found in the mammalian thalamus. However, many tectal-projecting cells from the brachium of the inferior colliculus were observed to be GABAergic. In addition, mouse thalamotectal neurons in vitro were found to not demonstrate the low-threshold bursting that is commonly found in thalamocortical neurons. The latter two properties - descending inhibition and lack of bursting - are also found in the frog thalamotectal system. However, these properties are not found in mammalian neocortically-projecting thalamic neurons. Given the divergence of amphibians and mammals hundreds of millions years ago, these data suggest that the thalamotectal projection represents an ancient pathway for providing top-down modulation of acoustic responses in the auditory midbrain. This work is presented in homage to Professor Albert Feng, who pioneered a comparative approach to understanding the auditory system, particularly with respect to the auditory midbrain in amphibians and mammals.