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 bee n postulated to follow a longitudinal pattern, each cortical area proj ecting on a longitudinal strip stretching along the whole rostro-cauda l axis of the striatum. However, compared to the rostral striatal regi on, the caudal striatum exhibits distinct features in terms of connect ivity 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 prese nt study, we examined the effects of cortical activation by local appl ication of picrotoxin on the Fos-immunoreactivity (Fos-IR) in the stri atum of the mouse, with special reference to the caudal part of the st riatum. Activation of the auditory cortex induced a dense ipsilateral Fos-IR restricted to the caudal striatum, i.e., in the caudo-medial st riatum and in the caudal part of fundus striati, and a very sparse lab elling in the medial region of the rostral striatum. Conversely, activ ation of both sensori-motor and visual cortices only resulted in Fos-I R in the main rostral part of the striatum, without response in the ca udal 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 observation s suggest that, despite a more widespread cortico-striatal connection pattern deduced from tracing experiments, the strongest functional pro jections from the auditory system mainly converge onto a restricted pa rt of the caudal striatum, according to a connection pattern that is r eminiscent of the transverse segmentation proposed in early lesioning studies of corticostriatal projections.