2 DISTINCT GLUTAMATERGIC SYNAPTIC INPUTS TO STRIATAL MEDIUM SPINY NEURONS OF NEONATAL RATS AND PAIRED-PULSE DEPRESSION

1. Excitatory postsynaptic currents (EPSCs) were recorded from the medium spiny neurones of neonatal rat striatal slices using the whole-cell patch clamp method. EPSCs were selectively elicited in the presence of picrotoxin with a glass stimulating pipette placed in the striatum. 2. We found two distinct unitary EPSCs that were evoked by stimulation of single presynaptic fibres. The major type of EPSC, termed 'S-type', failed frequently and had a small mean amplitude (2. 05 pA). They probably represented cortical afferents. 3. The other type of unitary EPSC, the 'H-type', seldom failed and was 13 times larger than the S-type. Spontaneous EPSCs with amplitudes similar to those of H-type EPSCs could be induced. 4. H-type EPSCs were mediated by both non-NMDA and NMDA receptors. 5. The two types of EPSCs could be evoked in the same neurones. The intensity of stimulation for H-type EPSCs was higher than that for S-type EPSCs. 6. H-type EPSCs could be polysynaptically activated, suggesting the presence of glutamatergic interneurones in the striatum that generated H-type EPSCs. 7. H-type EPSCs displayed particularly long-lasting paired-pulse depression, while that displayed by the S-type EPSCs was short. The paired-pulse depression of both EPSCs was Ca2+ dependent and involved presynaptic mechanisms. 8. We have demonstrated that the medium spiny neurones of neonatal rats receive two different glutamatergic input systems having different amplitudes, origins and paired pulse depression, reminiscent of cerebellar Purkinje cells. This suggests that the two types of EPSCs also play distinctive roles in striatal neuronal circuitry.