PROPERTIES OF SPONTANEOUS INHIBITORY SYNAPTIC CURRENTS IN CULTURED RAT SPINAL-CORD AND MEDULLARY NEURONS

1. To identify the type(s) and properties of inhibitory postsynaptic r eceptor(s) involved in synaptic transmission in cultured rat embryonic spinal cord and medullary neurons, we have used whole cell patch-clam p techniques to record miniature inhibitory postsynaptic currents (mIP SCs) in the presence of tetrodotoxin, DL-2-amino-5-phosphonovaleric ac id, and 6-cyano-7-nitroquinoxaline-2,3-dione. 2. The mIPSCs recorded f rom both spinal cord and medullary neurons had skewed amplitude distri butions. 3. The glycinergic antagonist strychnine and the GABAergic an tagonist bicuculline each decreased both the frequency and mean peak a mplitudes of mIPSCs. We conclude that both glycine and gamma-aminobuty ric acid (GABA) are neurotransmitters at inhibitory synapses in our cu ltured cells. 4. Most (similar to 96-97%) mIPSCs decay with single-exp onential time constants, and decay time distributions were consistentl y best fitted by the sum of four Gaussians with decay constants as fol lows: D1 = 5.8 +/- 0.1 (SE) ms (n = 63), D2 = 12.2 +/- 0.2 ms (n = 61) , D3 = 23.2 +/- 0.4 ms (n = 54), and D4 = 44.7 +/- 1.0 ms (n = 57). We conclude that the four classes of decay times represent kinetically d ifferent inhibitory postsynaptic receptor populations. 5. Strychnine a nd bicuculline usually had one of two different effects on the mIPSC d ecay time constant distributions; either selective decreases in the fr equency of mIPSCs with decay times in certain classes (i.e., the D1 cl ass was reduced by bicuculline, the D2 class by strychnine, and the D3 and D4 classes by both antagonists) or a nonselective depression in t he frequency of mIPSCs with decay times in all four classes. The parti cular effect observed in a given neuron was correlated with the presen ce or absence of ATP and guanosine 5'-triphosphate (GTP) in the patch pipette. Namely, in 71% of the antagonist applications where the pipet te contained ATP and GTP, the result was a nonselective decrease in mI PSCs in all decay time constant classes. Conversely, in 54% of the ant agonist applications in their absence, the result was a selective decr ease in the frequency of mIPSCs in specific decay time constant classe s. 6. In some experiments, mIPSCs reappeared in antagonist solution af ter an essentially complete block. Recovery from block in the continue d presence of antagonist was never observed in the absence of ATP and GTP (8 neurons), and, at the same time, 5 of 9 neurons patched with AT P and GTP in the pipette did show recovery (56%).