Spatial pattern of evoked synaptic excitation in the mouse neostriatum in vitro

The spatial distribution of stimulus-evoked excitation in the mouse neostriatum was investigated in vitro by using voltage-sensitive dyes and an optical multi-site recording system (laser scanning microscopy). The scanning area (880 x 830 microns) was positioned in the center of coronal neostriatal slices and records were taken simultaneously from up to 20 detection sites. Stimulus-induced optical signals were blocked by tetrodotoxin (TTX) and disappeared following removal of Ca2+ from the extracellular medium. Furthermore, these responses were inhibited by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) indicating that the evoked signals reflected mainly glutamatergic synaptic activity. Electrical stimulation at defined positions elicited characteristic spatial patterns of activity within the neostriatum. Stimulation of the medial subcortical white matter or stimulation at the dorsomedial corner or at the midpoint of the scanning area evoked synaptic activity at all recording sites. However, the largest response amplitudes were invariably observed in the ventrolateral part of the scanning area. In contrast, stimulation at the dorsolateral, ventrolateral or at the ventromedial corner induced synaptic responses which remained restricted to a relatively small area in close vicinity to the site of stimulation. The GABAA receptor antagonist bicuculline did not influence the pattern of activity distribution. However, in the presence of bicuculline, a N-methyl-D-aspartate (NMDA) receptor-mediated delayed signal component was observed which again was most pronounced in the ventrolateral part of the scanning area. These results, obtained in an in vitro slice preparation, demonstrate that spatially defined afferent activation of neostriatal neuronal circuits leads to a characteristic pattern of activity distribution within the neostriatum. Thus, our data complement observations from morphological investigations as well as from electrophysiological studies in vivo that suggest a functional compartmentalization of this brain area.