| Presbycusis is one of the very serious problems in old people with difficult communication. It is believed that temporal representation of auditory neurons in the auditory system play an important role in the generation of this disease. Presentation rate (PR) of sound stimulations is a parameter representing the energy and information received in limitative time. PR can affect brain's advanced functions such as receiving and comprehension of words and sentences both in the psychological and pathological conditions.This study is undertaken to clarify the effect of PR on the sound response properties of young,adult and eldly mice inferior collicular (IC) neurons, and the relationships among age,PR and intensity. Under free field stimulation conditions, sound stimulation with different intensities (10, 20, 30, and 40 dB SPL above minimal threshold) at characteristic frequency (CF) of each recorded neuron. The PR was changed irregularly among 1,3.3,5,10 and 20 Hz.28 healthy mice (Mus musculus Km, male or female) with natural hearing were used for this study: 12 young mice (21 days～1 month,10～15g. b. wt.); 9 adult mice (2～3 months, 30～35g. b. wt.); 7 eldly mice (>12 months, >50g. b. wt.). All these mice were pentobarbital anesthetized during the whole process of recording.A total of 103 IC neurons were recorded, including 35 in young group,35 in adult group and 33 in eldly group. The experiment results show: (1) Most IC neurons' firing rate decreased with increasing PR in each age group. There was significant correlation between the firing rate and the PR (p < 0.01) ; (2) The inhibition periods of neuronal firing were distinguished as early-inhibition,late-inhibition and proportional -inhibition. As the PR increased, the proportional-inhibition type was in the majority in adult group; while in young and eldly group, the early-inhibition decreased from 48.6% (17/35),57.6% (19/33) (PR = 1 Hz) to 14.3% (5/35),12.1% (4/33) (PR = 20 Hz) respectively. At the same time, neurons with other type were inclined to transform to the proportional-inhibition; (3) There were four types of rate-intensity function (RIF) of IC neurons when sound PR changed. They were monotonic (M),saturated(S),non-monotonic(N) and irregular. As PR increased, neurons with saturated RIF decreased, while neurons with non-monotonic RIF increased. Neurons with monotonic RIF changed lightly. Neuron with irregular RIF was sparse. In the eldly group, the neurons with M,S,N type changed, e.g. 27.3% (9/33), 72.7% (24/33),0% (0/33) (PR = 1 Hz) and 42.4% (14/33), 24.2% (8/33),21.2% (7/33) (PR = 20 Hz). It showed that the distribution of the neurons with different RIF in the eldly group were relatively average; (4) The duration of PSTH was divided into four phases such as 0 -10,10.5 - 20,20.5 - 30,30.5 - 40 ms. PR increasing leaded to shortening of each IC neuron group. This shortening occurred primarily in posterior portion of the PSTHs. The neuronal firing duration of the neurons' PSTH of the eldly group was rather longer. When PR = 1 and 20 Hz, the number of neuron whose fire duration was during 0-10,10.5 - 20,20.5 - 30 ms were 24.2% (8/33),39.4% (13/33),36.4% (12/33) and 63.6% (21/33),15.2% (5/33),18.2% (6/33) respectively.The results of our research show that the firing patterns of the neurons in the adult group relatively keep consistent to a certain extent, while it fluctuates dramatically in the young and eldly groups when the PR increased. Therefore, we speculate that fluctuation of temporal representation of central auditory neurons may be correlated with presbycusis.
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