Activity in neural populations related to threshold sensitivity and frequency resolution in the rat auditory brainstem

It is difficult to measure the coordinated activity of populations of neurons in the central nervous system. Here, we use immunocytochemical localization of Fos protein, a nuclear transcription factor, as a metric for sound activation to examine the populational response in the rat auditory brainstem. Fos protein expression is intimately related to the rate and pattern of neuronal firing. ^ The first issue addressed is the threshold sensitivity of granule and non-granule neuronal cell types in the cochlear nucleus (CN). Our results show that most CN neuron types can upregulate Fos expression when sound activated, and the number of Fos-expressing neurons is directly related to sound level. The threshold for Fos activation in granule cells is lower than that for non-granule cells. The number of Fos activated granule cells saturates at high sound levels, while the number of Fos activated non-granule cells is a monotonic function. ^ The second issue addressed is the frequency resolution in the auditory brainstem. Fos expression was used to investigate the separation of neural populations in the dorsal cochlear nucleus (DCN) and the inferior colliculus (IC) activated by alternating tones at two different frequencies. We show that tones a quarter-octave apart will activate a single population of neurons, while tones of one-half octave separation or more will activate largely separate populations of neurons in the IC and the DCN. The width of the bands broadens with increased sound intensities, more rapidly in the DCN than in the IC. ^ Consequently, the last issue addressed the basis for lateral inhibition in the IC. To study intrinsic ipsilateral connections, small injections of a fluorogold and biotinylated dextran mixture were made into the IC to identify local circuits. Immunocytochemistry was then used to identify GABAergic neurons. Similar methods with larger injections identified the commissural projections from the contralateral IC. Our results show that 93% of the ipsilateral local circuits in the IC are non-GABAergic and presumably excitatory. In contrast, 22% of the commisural connections are GABA-positive. Our results suggest that lateral inhibition in the IC may result primarily from inhibitory afferent axons from the lower auditory brainstem or contralateral IC.