PROPERTIES OF SPONTANEOUS INHIBITORY SYNAPTIC CURRENTS IN CULTURED RAT SPINAL CORD AND MEDULLARY NEURONS. Lewis, C. A. and D. S. Faber. Department of Anatomy and Neurobiology, Medical College of Pennsylvania and Hahnemann University, 3200 Henry Avenue, Philadelphia, PA 19129.
APStracts 3:0030N, 1996.
1. To identify the type(s) and properties of inhibitory postsynaptic receptor(s) involved in synaptic transmission in cultured rat embryonic spinal cord and medullary neurons, we have used whole-cell patch clamp techniques to record miniature inhibitory postsynaptic currents in the presence of tetrodotoxin, DL-2-amino-5-phosphonovaleric acid and 6-cyano-7- nitroquinoxaline-2,3-dione. 2. The mIPSCs recorded from both spinal cord and medullary neurons had skewed amplitude distributions. 3. The glycinergic antagonist strychnine and the GABAergic antagonist bicuculline each decreased both the frequency and mean peak amplitudes of mIPSCs. We conclude that both glycine and GABA are neurotransmitters at inhibitory synapses in our cultured cells. 4. Most (96-97%) mIPSCs decay with single exponential time constants, and decay time distributions were consistently best fitted by the sum of four Gaussians with decay constants of: D1 = 5.8 +/- 0.1 (n=63), D2 = 12.2 +/- 0.2 (n=61), D3 = 23.2 +/- 0.4 (n=54), and D4 = 44.7 +/- 1.0 (n=57) ms. We conclude that the four classes of decay times represent kinetically different inhibitory postsynaptic receptor populations. 5. Strychnine and bicuculline usually had one of two different effects on the mIPSC decay time constant distributions; either selective decreases in the frequency of mIPSCs with decay times in certain classes (i.e. the D1 class was reduced by bicuculline, the D2 class by strychnine, and the D3 and D4 by both antagonists) or a nonselective depression in the frequency of mIPSCs with decay times in all four classes. The particular effect observed in a given neuron was correlated with the presence or absence of ATP and GTP in the patch pipette. Namely, in 71% of the antagonist applications where the pipette contained ATP and GTP, the result was a nonselective decrease in mIPSCs in all decay time constant classes. Conversely, in 54% of the antagonist applications in their absence, the result was a selective decrease in the frequency of mIPSCs in specific decay time constant classes. 6. In some experiments, mIPSCs reappeared in antagonist solution after an essentially complete block. Recovery from block in the continued presence of antagonist was never observed in the absence of ATP and GTP (8 neurons), and, at the same time, 5 out of 9 neurons patched with ATP and GTP in the pipette did show recovery (56%).

Received 14 November 1994; accepted in final form 22 January 1996.
APS Manuscript Number J719-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 8 February 96