GIANT, TTX-INSENSITIVE, INHIBITORY POSTSYNAPTIC CURRENTS IN CULTURED RAT SPINAL-CORD AND MEDULLARY NEURONS

1. In whole cell patch-clamp studies on cultured rat embryonic spinal cord and medullary neurons bathed in tetrodotoxin, DL-2-amino-5-phosph onovaleric acid, and 6-cyano-7-nitroquinoxaline-2,3-dione, large and l ong-lasting spontaneous inhibitory postsynaptic currents were occasion ally recorded. The amplitudes of these events were 1 order of magnitud e larger than those of spontaneous miniature inhibitory postsynaptic c urrents. Because these large currents had reduced amplitudes in calciu m-free saline and in solutions containing glycinergic or GABAergic ant agonists, we conclude that they were probably produced by large and pr olonged release of glycine and/or 4-amino-n-butyric acid (GABA), which subsequently bind to their postsynaptic receptors. 2. The frequency o f spontaneous miniature postsynaptic currents increased dramatically d uring the long, slow decay phase of these large postsynaptic currents. Considering the requirement for extracellular calcium for the occurre nce of these large responses, we hypothesize that this increased frequ ency reflected an increased intracellular calcium concentration in the presynaptic terminal.3. Similar evidence for large inhibitory postsyn aptic currents and prolonged transmitter release was observed in cell- attached patches, which also exhibited the smaller, spontaneous miniat ure inhibitory postsynaptic currents, suggesting that these large even ts are properties of single synaptic terminals. 4. A comparison of the properties of these large inhibitory postsynaptic currents recorded i n whole cell mode or cc:ll-attached patches showed no statistically si gnificant differences. The overall mean values, then, are 13.9 +/- 1.6 (SE) ms and 4.5 +/- 0.5 s for the 10-90% rise time and duration, resp ectively. Furthermore, these large events had amplitudes that were 11- fold larger than the mean amplitude of the miniatures (i.e., mean ampl itude ratio of 10.8 +/- 0.5). 5. Periodic large increases in the frequ ency of spontaneous miniature inhibitory postsynaptic currents occurre d in both cell-attached patches and in the whole cell mode, and these increases were only sometimes associated with the large inhibitory pos tsynaptic currents. The rhythmicity in both recording configurations h ad similar temporal characteristics, with average interburst intervals of 5 and 12-14 s. Presumably these bursts of spontaneous miniature po stsynaptic currents reflected periodic oscillations in the Ca2+ concen tration in presynaptic terminals. 6. Both the probability and the freq uency of occurrence of large inhibitory postsynaptic currents doubled during the 7-day period of time in culture when experiments were perfo rmed, suggesting that these large currents may play a role during deve lopment.