Response properties and tonotopical organization in the dorsal cochlear nucleus in rats

Pure tone burst stimulation was used to study discharge properties and tonotopic organization in the dorsal cochlear nucleus (DCN) of rats anesthetized with alpha-chloralose urethane. An animal's head was fixed with a fine nail embedded in the skull in order to prevent the head from moving. Unit discharges of DCN neurons were recorded with micropipettes filled with fast green dye. Free field pure tone bursts were presented using a ribbon tweeter; it delivered stimuli at a maximum SPL of 96 dB. Electrode tracks were reconstructed through subsequent histological examinations. A total of 429 units were sampled in 51 animals. Of these, 230 units (54%) responded to tone bursts higher than 20 kHz with a mean latency of 6.3 ms. Tonotopical organization was clearly evident throughout the mediolateral axis of the DCN; high frequencies were represented medially and low frequencies laterally. Nearly constant frequencies were represented along the dorso-ventral axis and no systematic organization was observed along the rostrocaudal extension of the DCN. When tuning curves were measured in DCN neurons with excitatory and inhibitory responses, we found that inhibitory sidebands were usually located in a frequency range higher or lower than the excitatory response area and sometimes partially overlapped with it.