Effects of stimulus frequency and intensity on c-fos mRNA expression in the adult rat auditory brainstem

Induction of the cellular fos gene (c-fos) is one of the earliest transcriptional changes observed following neuronal excitation. Although not an activity marker in the strict electrophysiological sense, many neurons in the central nervous system increase their c-fos expression after periods of sustained stimulation at physiological levels of intensity. In the present study, induction of c-fos mRNA expression was examined in the auditory brainstem after 1 hour of continuous free-field acoustic stimulation. Sprague-Dawley rats were exposed to pure tones of 2, 8, 16, or 32 kHz or half-octave noise bands centered on 2, 8, or 32 kHz at 80-120 dB SPL. Stimulation-induced c-fos mRNA expression was evident at all levels of the auditory brainstem, and this expression was intensity dependent. In some brain areas, induced expression manifested a clear tonotopic organization, i.e., in dorsal, posteroventral, and anteroventral cochlear nuclei, and in the medial nucleus of the trapezoid body. The inferior colliculus exhibited multiple tonotopic representations. The dorsal nucleus of the lateral lemniscus had a crude tonotopy. Although expression was present, tonotopy was not evident in periolivary nuclei or in the ventral or intermediate nuclei of the lateral lemniscus. Free-field diotic stimulation did not induce c-fos mRNA expression in the medial or lateral superior olivary nuclei. Expression was induced in the lateral superior olive by dichotic stimulation (after a unilateral cochlear ablation), and that expression was tonotopically organized. The results suggest that stimulation-induced c-fos mRNA expression can be an effective way of mapping neuronal activity in the central auditory system under both normal and pathological conditions.