DIFFUSION, NOT UPTAKE, LIMITS GLYCINE CONCENTRATION IN THE SYNAPTIC CLEFT. Titmus, Margaret J., Henri Korn and Donald S. Faber. Department of Anatomy and Neurobiology, Medical College of PA and Hahnemann University, 3200 Henry Avenue, Philadelphia, PA 19129.
APStracts 2:0326N, 1995.
SUMMARY AND CONCLUSIONS
1. The question of whether active uptake limits the duration of action of the inhibitory transmitter glycine has been addressed in vivo at inhibitory synapses on the goldfish Mauthner (M-) cell. The kinetics of inhibitory postsynaptic potentials and currents (IPSPs and IPSCs) evoked antidromically and by eighth nerve stimulation were recorded in control and in conditions expected to block glycine uptake or slow its diffusion. Theoretical considerations, based on simulated quantal currents, predicted that if diffusion was slow, rapid uptake of glycine would be required and its block would prolong the synaptic responses. 2. Q10 values for IPSC decay time constants (t) are in the range of 2.0 for temperatures between 15 to 23oC, suggesting diffusion is not the rate-limiting step. 3. Li+, Ch+ or N-methyl-D- glucamine (NMG) were substituted for 80% of the Na+ in the extracellular fluid, to analyze the effects of blocking the Na+-dependent glycine uptake. These procedures enhanced the maximum inhibitory shunt produced by glycine iontophoresis, leading to the suggestion that uptake may buffer the concentration of the transmitter in the cleft. In contrast, the Na+ substitutes had no effect on the t of the recurrent collateral IPSC, which involves synchronous activation of a pool of interneurons and has a mono- exponential decay (tÿ7E 10-11msec). 4. The decay phase of the disynaptic IPSCs produced by stimulating the contralateral VIIIn has fast and slow components, with a prolonged tail lasting up to 100msec, particularly in the case of repetitive nerve stimulation. The tail is inhibitory, as revealed by its shunt of the antidromic action potential, and it is at least partially Cl-- dependent. However, it can be accelerated by superfusion with the glutamate receptor antagonists, CNQX and APV. In the presence of these blockers, the IPSC decay remains bi-exponential (tfast = 5.2 and 5.9msec, tslow = 94 and 130msec, for single and burst stimuli, respectively). Blocking uptake in this condition did not modify tfast or tslow. 5. We conclude that an active uptake mechanism does not shape glycinergic IPSCs, including the longer lasting components that might include a contribution due to persistence of the transmitter. Rather, diffusion alone is sufficient to remove glycine at a rate faster than channel unbinding. 6. To test if glycine might diffuse to adjacent excitatory synapses and enhance activation of N-methyl-d-aspartate (NMDA) receptors, CNQX and APV were applied locally, by pressure, to the M-cell soma, but they had no effect on the prolonged decay of eighth nerve evoked responses. Thus, the effect of the antagonists when added to the superfusate are exerted at the network level. 

Received  10 July 1995; accepted in final form 24 October 1995.
APS Manuscript Number J442-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95