Comparison of response properties of inferior colliculus neurons of two inbred mouse strains differing in susceptibility to audiogenic seizures.

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Abstract
Extracellular response properties of inferior colliculus (IC) neurons were analyzed in mice of 2 inbred strains that differ in their behavioral response to sound: DBA mice, which are innately susceptible to audiogenic seizures (AGS), and C57 mice, which are AGS resistant. Mice aged 21-23 days (when AGS susceptibility is highest in DBA mice) and 28-30 days (when susceptibility has declined in DBA) were employed. Compared to neurons of C57 mice, those of DBA were more likely to have high thresholds. Best frequencies (BF, the frequency for which threshold is lowest) were largely restricted to the range of 10-20 kHz in DBA neurons, while a much broader range of frequencies was well represented by BF of C57 neurons. There was a substantial age-related decline in sensitivity of IC neurons in DBA mice, but not in C57. In DBA neurons, response ranges disproportionally represented the frequencies of 8-22 kHz, and the frequency at which maximal discharge rate was elicited rarely occurred at very high or low frequencies in DBA neurons. Octave ranges of response areas tended to be narrower in DBA neurons compared to those of C57. Less inhibition was seen in DBA neurons than in C57 neurons, as indicated by the incidence of nonmonotonic intensity functions and inhibition of spontaneous activity. Afterdischarges (AD, a continuation of sustained excitatory discharges after termination of a stimulus) were seen in a substantial number of neurons (25%) only in DBA mice aged 21-23 days, the age of peak AGS susceptibility. Typically, AD were driven by only a limited range of frequencies within a neuron''s response area and only at higher intensities (e.g., 80 dB SPL). These genetically determined abnormalities in the responses of IC neurons in DBA mice are apparently related to AGS susceptibility and the most important area of the IC in this behavior is apparently the ventral portion of the central nucleus.