Increased transmitter release at excitatory synapses produced by direct activation of adenylate cyclase in rat hippocampal slices

Abstract

The field EPSP recorded in the CA1 region of rat hippocampal slices is potentiated by bath application of the direct adenylate cyclase activator forskolin (Chavez-Noriega and Stevens, 1992a). We have now used the whole- cell patch-clamp technique to analyze the effect of forskolin on evoked synaptic currents and on spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) recorded in rat hippocampal slices in order to determine the relative contributions of pre- and postsynaptic mechanisms to this increased synaptic strength. Application of 50 μM forskolin in the presence of 3-isobutyl-1-methylxanthine (IBMX; a phosphodiesterase inhibitor) enhanced the evoked EPSC (eEPSC) peak amplitude to 230 ± 43% of control (n = 13). No significant change in sEPSC or in mEPSC amplitude was detected after forskolin addition (106 ± 7%, n = 9), indicating that postsynaptic receptor sensitivity at synaptic junctions is not greatly affected. In contrast, a large increase in sEPSC and mEPSC frequency was noted in all cells (299 ± 81%). Following forskolin application, the amplitude distribution of evoked synaptic currents shifted to larger values, but more significantly, a sharp decrease in failure rate was produced in all cells tested. Also, a significant correlation was found between the potentiation produced by forskolin in IBMX on the eEPSC and the ratio of the squared coefficient of variation (CV = SD/mean). Finally, a quantal analysis of four cells was consistent with the hypothesis that transmitter release was increased by forskolin/IBMX with, if anything, a concomitant decrease in quantal size. Together, these observations indicate that presynaptic mechanisms significantly contribute to the enhancement produced by this diterpene. Altogether, our data indicate that activation of the cAMP cascade can enhance evoked and spontaneous release of neurotransmitter in Schaffer/commissural terminals in field CA1 of rat hippocampal slices.