Muscimol suppression of the dorsal cochlear nucleus modifies frequency tuning in rats

The cochlear nucleus is composed of three sub-nuclei: the dorsal (DCN), anteroventral (AVCN) and posteroventral cochlear nucleus (PVCN). Intrinsic connections from the DCN to the AVCN are inhibitory and organised tonotopically. In this investigation, this pathway and its possible role in frequency tuning was examined using in vivo extracellular recordings. Extracellular recordings were made from 191 units in the AVCN, 69 of which were recorded after suppression of DCN by application of the GABA agonist Muscimol (15 ng, 0.26 mM). Tuning curves were plotted and characteristic frequency (CF) and response threshold (measured in dB SPL) were determined for each unit. Units recorded post-Muscimol showed significantly broader tuning characteristics and lower thresholds. Primary-like and transient chopper neurons contributed to this decrease in threshold suggesting that they receive 'on' CF inhibitory drive from the DCN. Sustained chopper units did not show a significant decrease in response threshold after Muscimol; however, there was a tendency for broader tuning and a significant increase in CF tone evoked maximum discharge rate and chopping frequency suggesting that the DCN may play a role in regulating the temporal firing of these units in addition to providing lateral inhibition. These results suggest that the DCN to AVCN pathway may aid in fine tuning frequency information.