Maturation of glutamatergic neurotransmission in dentate gyrus granule cells

We studied the development of glutamatergic neurotransmission in dentate gy rus granule cells (GCs) in hippocampal slices from 5 to 12-day-old rats. Th e active postnatal neuronogenesis in dentate permits GCs with staggered bir thdates to be studied in situ in a single preparation. We recorded evoked r esponses to medial perforant path stimulation using visually-guided whole-c ell patch clamping to select immature GCs, and biocytin filling to correlat e electrophysiologic responses with maturational stage. Even within this im mature cell population we found four distinct electrophysiologic patterns. Type 1 cells had no glutamatergic current; Type 2 cells had only N-methyl-D -aspartate receptor (NMDA) current; Type 3 cells had both NMDA and alpha -a mino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) current although t he NMDA component could be isolated at low stimulus intensity (NMDA thresho ld<AMPA threshold): Type 4 cells had both AMPA and NMDA currents with NMDA threshold<greater than or equal to>AMPA threshold. Type 1 cells were least mature, and Type 4 cells most mature as assessed by cell properties, dendri tic arborization, and penetration of dendrites into the molecular layer. Th us NMDA-mediated currents predominate early in GC development as is consist ent with their role in processes that determine dentate architecture - neur onal migration, dendritic outgrowth and regression. and synapse stabilizati on. By analogy with 'silent synapses' (i.e. synapses that contain only NMDA receptors), Type 2 cells are candidate 'silent cells' that may undergo act ivity-dependent acquisition of functional fast-conducting AMPA receptors wi th maturation. (C) 2000 Elsevier Science B.V. All rights reserved.