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 med ium spiny neurones of neonatal rat striatal slices using the whole-cel l 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 stimulatio n of single presynaptic fibres. The major type of EPSC, termed 'S-type ', failed frequently and had a small mean amplitude (2. 05 pA). They p robably represented cortical afferents. 3. The other type of unitary E PSC, the 'H-type', seldom failed and was 13 times larger than the S-ty pe. 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 tha n that for S-type EPSCs. 6. H-type EPSCs could be polysynaptically act ivated, suggesting the presence of glutamatergic interneurones in the striatum that generated H-type EPSCs. 7. H-type EPSCs displayed partic ularly long-lasting paired-pulse depression, while that displayed by t he S-type EPSCs was short. The paired-pulse depression of both EPSCs w as Ca2+ dependent and involved presynaptic mechanisms. 8. We have demo nstrated that the medium spiny neurones of neonatal rats receive two d ifferent glutamatergic input systems having different amplitudes, orig ins and paired pulse depression, reminiscent of cerebellar Purkinje ce lls. This suggests that the two types of EPSCs also play distinctive r oles in striatal neuronal circuitry.