Age-related deterioration of cortical responses to slow FM sounds in the auditory belt region of adult C57BL/6 mice.

To compare age-related deterioration of neural responses in each subfield of the auditory cortex in C57BL/6 mice, we evaluated amplitudes of tonal responses in young (5-11 weeks old) and adult (16-23 weeks old) groups using transcranial flavoprotein fluorescence imaging. Cortical responses to 20-kHz amplitude-modulated (AM) sounds, which were mainly found in the anterior auditory field (AAF) and the primary auditory cortex (AI) of the core region, were not markedly different between the two groups. In contrast, cortical responses to direction reversal of slow frequency-modulated (FM) sounds, which were mainly found in the ultrasonic field (UF), were significantly disrupted in the adult group compared with those in the young group. To investigate the mechanisms underlying such age-related deterioration, biotinylated dextran amine (BDA) was injected into UF. The number of retrograde labeled neurons in the dorsal division of the medial geniculate body (MGd) was markedly reduced in the adult group compared with that in the young group. These results strongly suggest that cortical responses to FM direction reversal in UF of adult C57BL/6 mice are mainly deteriorated by loss of non-lemniscal thalamic inputs from MGd to UF due to aging.