Development of inhibitory and excitatory synaptic transmission in the rat dentate gyrus.

We studied the ontogeny of inhibitory and excitatory processes in the rat dentate gyrus by examining paired-pulse plasticity in the hippocampal slice preparation. The mature dentate gyrus produces characteristic paired-pulse responses across a wide range of interpulse intervals (IPI). Paired-pulse effects on population excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude were analyzed at postnatal day 6 (PN6), PN7/8, PN9/10, PN15/16, and PN > 60. The synaptic paired-pulse profile (10-5,000 ms IPI) matured by PN7/8. The triphasic pattern of short-latency depression, a relative facilitation at intermediate intervals, and long-latency depression was present at all ages tested. Paired-pulse effects on granule cell discharge indicated the presence of weak short-latency (20 ms IPI) inhibition at PN6, the earliest day that a population spike could be evoked. By PN7/8, short-latency inhibition was statistically equivalent to the mature dentate gyrus. Long-latency (500-2,000 ms IPI) PS inhibition was present, and equal to the mature dentate gyrus by PN6. The most consistent difference between the mature and developing dentate gyrus occurred at intermediate IPIs (40-120 ms) where spike facilitation was significantly depressed in the development groups. The studies indicate that short-term plasticity matures rapidly in the dentate gyrus and suggest that the inhibitory circuitry can function at a surprisingly early age.