DEVELOPMENTAL CHANGES IN MEMBRANE PROPERTIES AND POSTSYNAPTIC CURRENTS OF GRANULE CELLS IN RAT DENTATE GYRUS. Liu, Ying-Bing, Peter A. Lio, Joseph F. Pasternak, and Barbara L. Trommer. Division of Pediatric Neurology, Evanston Hospital, Evanston, Illinois, 60201 and the Departments of Pediatrics and Neurology, Northwestern University Medical School, Chicago, Illinois, 60611.
APStracts 3:0056N, 1996.
1. Whole cell patch-clamp recordings were used to study dentate gyrus granule cells in hippocampal slices from juvenile rats (postnatal days 8-32). Membrane properties were measured using current clamp recordings and were correlated with morphology of a subgroup of neurons filled with biocytin. The components of the postsynaptic currents (PSCs) induced by medial perforant path stimulation were characterized using specific receptor antagonists in voltage clamp recordings. 2. Granule cells located in the middle third of the superior blade of stratum granulosum from the rostral third of hippocampus were divided into three groups according to their input resistance (IR). Neurons with low IR (206 +/- 182 M[omega]) had hyperpolarized resting membrane potentials (-82 +/- 7 mV) and high amplitude action potentials (108 +/- 23 mV). Neurons with high IR (1259 +/- 204 M[omega]) had more depolarized resting membrane potentials (-54 +/- 6 mV) and lower amplitude action potentials (71 +/- 10 mV). Neurons with intermediate IR (619+/-166 M[omega]) also had intermediate resting membrane potentials (-63+/-7 mV) and action potential amplitudes (86+/-14 mV). Low input resistance neurons became increasingly prevalent with advancing postnatal age, but neurons from each group could be found throughout the entire period under study. 3. Morphologic studies of low IR neurons revealed an extensive dendritic arborization that traversed the entire molecular layer and was characteristic of mature granule cells. High IR cells had smaller somata and short, simple dendritic arborization that incompletely penetrated the molecular layer and were classified as immature. Intermediate IR cells had morphologic features of intermediate maturity. 4. The initial phase of the PSC evoked at -80 mV was a fast inward current that was comparable with respect to latency to peak, latency to onset, and 10-90% rise time in neurons of all maturities held at -80 mV. This current was CNQX sensitive. 5. The decay phase of PSCs at -80 mV varied with neuronal maturity. Mature neurons had monoexponential decays (_ = 8.9+/-3.6). Intermediate and immature neurons had prominent later inward currents that resulted in slower decays . In the case of the immature neurons the inward current during the decay phase could be separated from the initial fast inward peak. The later inward currents in intermediate and immature neurons were bicuculline sensitive. 6. Using uniform ionic conditions of the extracellular and patch solutions, I-V relations and reversal potentials for pharmacologically isolated AMPA, NMDA, and GABA A currents were comparable across all cell maturities. Calculated ratios for peak GABA A /NMDA/AMPA currents decreased significantly with maturation as follows: 9.4+/-2.9/1.4+/-0.5/1.0 for immature cells, 7.2+/-2.5/1.5+/-0.7/1.0 for intermediate cells, and 2.0+/-1.2/0.9+/- 0.4/1.0 for mature cells. 7. GABA current was mediated both by polysynaptic activation of interneurons and by direct activation of interneurons with monosynaptic input onto granule cells. The proportional contributions of mono- and polysynaptic GABA to total GABA were comparable across all cell maturities; latency to peak GABA current decreased with increasing cell maturity for both mono and polysynaptic components. 8. We conclude that PSCs evoked in granule cells by medial perforant path activation in neurons of all maturities consist of both glutamatergic and GABAergic components. PSCs are dominatedb y GABA-ergic neurotransmission in immature granule cells, and the contribution of glutamatergic neurotransmission increases with neuronal maturation. The greater ratio of peak GABA A to glutamate currents and the longer time interval between their respective peaks combine to produce a distinctive PSC shape in the immature compared with mature granule cells.

Received 1 December 1995; accepted in final form 8 March 1996.
APS Manuscript Number J812-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 27 March 96