| Presentation rate of sound stimulations (PR) is the parameter representing the energy and information received by animals in limited time. Presbycusis, literally elder hearing, is the general term applied to age-related hearing loss. The disorder is characterized by reduced hearing sensitivity and speech understanding in noisy environments, slowed central processing of acoustic information, and impaired localization of sound sources. In our experiment, to investigate the effect of PR and age on the sound response properties of mouse inferior collicular (IC) neurons and the relationship between the neurons' ability to follow sound presentations and the duration as well as intensity of the sounds, the tones with different durations (10,40 and 120 ms), intensities (10 dB SPL above minimal threshold) at characteristic frequency(CF) of recorded neurons were presented to pentobarbital anesthetized mice (Mus musculus, Km) at different PR (0.5, 1, 3.3, 5, 6.7, 10 and 20 Hz) under free field stimulation conditions.28 healthy mice (male or female, 12 young, 9 adult and 7 old) with natural hearing were used for this study and a total of 109 IC neurons were recorded. The results were as follows: 1) When the intensity of the stimulations was 10 dB SPL above MT and the duration was 40 ms, with the PR increasing, the sound response properties of most recorded neurons were dramatically influenced by changing the PR even within low PR range (< 3.3 Hz). The firing rate of the majority of neurons (young 82.9%, adult 90.9%, and old 81.8%, respectively) monotonically decreased with the PR increasing, 2) In young mouse and adult mouse group, the latency of the majority neurons (62.8%, 22/35 and 66.7%, 22/33, respectively) monotonically prolonged with the PR increasing. On the contrary, in old mouse group, latency of most of the neurons (66.7%, 22/33) varied non-monotonically. 3) When the intensity of the stimulations was 10 dB SPL above MT, in young mouse and adult mouse group, the best duration for firing rate is 10 ms (68.6% and 69.7%, respectively), while for old mouse group, this duration is 120 ms (63.6%). 4) The percentage of phasic pattern in young, adult, and old mouse group is 60%, 84.8% and 60.6%, respectively. For phasic-burst pattern is 37.1%, 12.1% and 15.1%, respectively. And for the tonic pattern is 2.9%, 3% and 24.2%, respectively. Obviously, the tonic pattern in old mouse group is much more than in young and adult mouse group.The results indicated that the sound response properties of IC neurons were closely correlated with the PR of sound stimulation even in relatively low PR range.In the same duration, the number of neurons with monotonically changed latency (31.2%-69.7%) were fewer than the number of neurons with monotonically changed firing rate(81.8%-90.9% ) , suggesting the influence of PR on latency is less than on firing rate. The latency of old mouse group fluctuated much heavier than young and adult mouse group. Our preliminary data suggest that presbycusis in human might be related with stability decrease in spectral representation of central auditory neurons under stimulation condition of larger sound loads. The duration preference of old mouse group provides the proper explanation for the reason why the old people prefer longer and slower sound. We suggest that aging results in the inhibitory neurotransmitter reduction in the IC, and this reduction induced more tonic pattern in old mouse group than in young and adult mouse group. What neuronal circuit is involved in and which inhibitory neurotransmitter is responsive for these changes described above should be further explored.
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