C-fos immunocytochemical evidence for acoustic pathway mapping in rats.

Principles of the functional organization of brain structures are manifested in the response characteristics of the composing neuronal elements. In order to visualize and map some of the fundamental organization principles in the mammalian auditory system, namely tonotopy, ipsilateral versus contralateral inputs, and excitatory and inhibitory interactions, the present study employed the c-fos immunocytochemistry technique for imaging changes in acoustically evoked synaptic activity. Young rats were either monaurally or binaurally stimulated with pure tone pulses, and the distribution of immunoreactive neurons was analyzed in auditory brainstem nuclei. In a quantitative approach, i.e. by counting the number of labeled neurons in a given nucleus, monaural nuclei could be distinguished from binaural nuclei. Moreover, by relating areas with increased or decreased labeling to the stimulus conditions, further information was obtained on the nature of the inputs, i.e. whether ipsilateral and contralateral stimuli elicit predominantly excitatory and/or inhibitory responses. Taken together, the present data demonstrate the suitability of the c-fos immunocytochemistry technique to identify tone-evoked excitation (and inhibition) in auditory brainstem neurons and to visualize fundamental principles of organization.