We examined the feasibility of voltage-sensitive dye imaging for detecting the neuronal activity in the neonatal rat corticostriatal projection and analyzed postnatal development of synaptic function in this projection. Coronal slice preparations were dissected from postnatal 3 to 14 day (P3-14) rats and were then stained with the voltage-sensitive absorption dye, NK2761. The transmembrane voltage-related optical changes evoked by cortical stimulation using a bipolar electrode could be recorded simultaneously from many loci in the preparation, using a 464ch hexagonal diode array system. In the striatum, the optical signal was composed of a fast spike-like signal and a slow signal. The slow signal was blocked by DL-2-amino-5-phosphonovaleric acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and enhanced by bicuculline, suggesting that (1) the slow signal includes glutamatergic excitatory postsynaptic potentials (EPSPs) and that (2) inhibitory γ-aminobutyric acid A (GABA(A)) receptor function is expressed in the corticostriatal pathway from the early postnatal stage. We compared the excitatory and inhibitory synaptic responses between the first and second postnatal week preparations, and we showed that (1) in the first postnatal week, excitatory transmission in the corticostriatal pathway is mostly mediated by glutamate, (2) in addition to glutamatergic transmission, other excitatory transmission mechanisms emerge in this pathway until the second postnatal week, and (3) the inhibitory transmission mediated by GABA rapidly develops during the postnatal 2 weeks.
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