Modulation Of Descending Auditory System For The Auditory Information Processing In The Cochlear Nucleus

The cochlear nucleus(CN) is the first central relay of the auditory pathway. It receives input from all afferents innervating the cochlea and projects to upper auditory nuclei. It is responsible for processing and relaying the acoustic signal to higher auditory nuclei via a complex array of neuronal circuits and ascending pathways. Besides ascending input from the cochlea, the CN receives descending auditory input from a number of sources including the contralateral CN and primary auditory cortex, the bilateral trapezoid body, the ventral and dorsal nuclei of the lateral lemniscus, superior olivary complex and inferior colliculus. Recent studies indicate that the descending system plays important roles in modulation of auditory information processing in the medial geniculate body and the inferior colliculus but little to known about CN. Therefore, the understanding of the neural mechanisms for auditory signal processing in CN is incomplete without the exploration of the functional roles of the descending system. To investigate the roles of the descending system for auditory information processing in CN, we recorded the single-unit discharges of CN neurons evoked by pure tone before and after focal electric stimulation of auditory cortex(AC) or behavior conditioning, under anaesthesia and nonanaesthesia conditions respectively, and we observed the frequency-response curves, the best frequency (BF), the response magnitude and mean time courses of the changes in these parameters. The results are described as follows: 1. The frequency-response curves and BF were changed after focal electric stimulation of the AC. When the difference of their BF between cortex and CN neurons was less than±0.5KHz, the responses intensity of CN neurons at BFc increased (9.5±3.3%) without changes in BF. When the difference of their BF was more than ±0.5KHz, the frequency-response curves and BF of CN neurons shifted to the BF of cortical neurons (0.86±0.62KHz) accompanied by a decreasing in response intensity at BFc(14.9±11.1%) and increasing in response intensity at BFs(9.4±9.1%). Such changes started to recover about 30min after electric stimulation and completely recovered in 2h. Cortical neurons facilitated the auditory response of physiological "matched" CN neurons and inhibited the response of "unmatched" neurons through the descending auditory system. These results indicate that the descending auditory system have feedback modulations on the auditory information processing in CN. 2. The modulation of descending auditory system was determined by the difference of BF between cortical and CN neurons. When the BF of CN neurons was lower than those of cortical neurons, the frequency-response curves and BF of CN neurons shifted upward to the BF of cortical neurons(0.90±0.60KHz) accompanied by a decreasing in response intensity at BFc(14.4±11.7%) and increasing in response intensity at BFs(9.3±9.2%). When the BF of CN neurons was higher than those of cortical neurons, the frequency-response curves and BF of CN neurons shifted downward to the BF of cortical neurons(0.83±0.64KHz) accompanied by a decreasing in response intensity at BFc(15.3±10.7%) and increasing in response intensity at BFs(9.6±9.3%). The descending modulation occurred only when the BF difference between AC and CN neurons was less than ±10KHz and the maximal effects occurred when the difference was about ±4.5KHz. These results show that the modulation of descending auditory system is frequency-dependent, that is the modes and levels of modulation are determined by the difference of BF between cortical and CN neurons. 3. ASr could evoke a BF shifts (0.20±0.13KHz) accompanied by a decreasing in response intensity at BFc(6.3±5.4%) and increasing in response intensity at BFs(4.1±4.2%). But the changes occurred only when the BF difference between AC and CN neurons was less than ±6KHz and recovered within 30min. 4. ASt alone couldn't evoke BF shifts and response intensity changes of CN neurons, but ASt + ES1 can. After 30min behavior conditioning,the CN neurons with a BF just near the frequency of ASt (±0.5KHz) increased their response intensity at BFc(15.8±7.8%) without BF shifts. If the difference between its BF and the frequency of ASt was more than±0.5KHz, the BF of CN neurons shifted to the frequency of the conditioned tones (0.99±0.82KHz) accompanied by a decreasing in response intensity at BFc(19.4±17.4%) and increasing in response intensity at BFs(12.6±13.3%). The BF shifts were centripetal andsymmetry, occurring only when the difference of their frequencies were within ±12KHz and the maximal effects occurring at about ±5.6KHz. These changes maintained for 3h, longer than those evoked by ASr or ESat. The results suggest that the descending auditory system can modulate the auditory information processing in CN in nature, and the associative learning can augment the BF shifts and the response intensity changes evoked by ASr or ESat through the descending auditory system. Conclusions: the descending auditory system has feedback modulations on the auditory information processing in CN and the modes and levels of the modulations are frequency-dependent. Behavior conditioning can augment the modulations of the descending system.