Involvement of the caudal striatum in auditory processing: c-fos response to cortical application of picrotoxin and to auditory stimulation.

The topographical organization of corticostriatal connections have been postulated to follow a longitudinal pattern, each cortical area projecting on a longitudinal strip stretching along the whole rostro-caudal axis of the striatum. However, compared to the rostral striatal region, the caudal striatum exhibits distinct features in terms of connectivity and neuronal phenotype. The induction of c-fos expression in the striatum by cortical activation or sensory stimulation may throw more light on these functional corticostriatal relationships. In the present study, we examined the effects of cortical activation by local application of picrotoxin on the Fos-immunoreactivity (Fos-IR) in the striatum of the mouse, with special reference to the caudal part of the striatum. Activation of the auditory cortex induced a dense ipsilateral Fos-IR restricted to the caudal striatum i.e., in the caudo-medial striatum and in the caudal part of fundus striati, and a very sparse labelling in the medial region of the rostral striatum. Conversely, activation of both sensori-motor and visual cortices only resulted in Fos-IR in the main rostral part of the striatum, without response in the caudal extremity of the striatum. On the other hand, visual or auditory stimulation in awake animals failed to induce c-fos expression in the striatum. However, using quantitative in-situ hybridization for c-fos mRNA, we found that auditory, but not visual stimulation significantly potentiated the c-fos response to the D1 agonist SKF 38393 (2 mg/kg, i.p.) in the caudal part of the striatum. These functional observations suggest that, despite a more widespread cortico-striatal connection pattern deduced from tracing experiments, the strongest functional projections from the auditory system mainly converge onto a restricted part of the caudal striatum, according to a connection pattern that is reminiscent of the transverse segmentation proposed in early lesioning studies of corticostriatal projections.