Muscarinic depression of synaptic transmission at the hippocampal mossy fiber synapse.

1. The action of muscarine was studied in the CA3 region of the rat hippocampal slice with single-electrode voltage-clamp techniques. 2. Bath application of 1 or 10 microM muscarine produced an increase in the input resistance of these cells and reduced the slow afterhyperpolarization (sAHP) response. Changes in input resistance were more pronounced around the resting potential of the cell (-50 to -60 mV), but in many cells an effect was also seen at -80 mV. These effects were absent when cesium chloride-containing microelectrodes were used. 3. At 1 microM, muscarine had little effect on synaptic transmission, causing a 0 +/- 7% (mean +/- SE, n = 19) change in excitatory postsynaptic potential (EPSP) and decreasing the excitatory postsynaptic current (EPSC) by 11 +/- 6% (n = 14); neither change was statistically significant. 4. In contrast, 10 microM muscarine produced a reliable depression of both the EPSP and EPSC. This effect was independent of the electrolyte used: with KCl the EPSP was depressed 23 +/- 4% (n = 5) and the EPSC 35 +/- 5% (n = 4); for CsCl the EPSP was depressed 23 +/- 10% (n = 7) and the EPSC 34 +/- 5% (n = 7). 5. Muscarine did not alter the reversal potential of the synaptic current but merely produced a decrease in slope conductance (37 +/- 5%, n = 6). 6. Muscarine did not significantly alter the shape of the EPSC waveform. This was assessed by comparing the 10-90% rise time and the half decay time of the current before and after muscarine.(ABSTRACT TRUNCATED AT 250 WORDS)