In the warm-acclimatized group, where impulse propagation was not impaired, MbCD enhanced transmitter release after 5�C 10 min. This effect was consistent with previous observations that MbCD can enhance release if WZ8040 synaptic terminals are excited electrically. Enhancement of release in our trials reversed during subsequent treatment with Ch-HPbCD, suggesting that this effect may involve cholesterol removal. This interpretation, however, remains inconsistent with the bulk of literature indicating that cholesterol loss inhibits the release process. Moreover, we have not ruled out the possibility that removal of MbCD itself may have reversed the enhancement of release in our trials. The apparent increase in transmitter release / quantal content in the warm-acclimatized group, without a concomitant increase in EJP amplitude, could be due to reduced sensitivity of postsynaptic receptors to the transmitter. This is consistent with the partial decline in post-synaptic responsiveness to glutamate in the warm-acclimatized group and a significant reduction in the amplitude of single, evoked quantal currents. Muscle fiber input resistance did not change and, thus, did not contribute to changes in EJP amplitude. Considering the pleiotropic effects of MbCD, additional effects on the pre- and post-synaptic membranes cannot be ruled out. However, a critical role for cholesterol in the post-synaptic response is clear: after the washout of MbCD, reduced cholesterol levels correlate with reduced responsiveness in the cold-acclimatized group. Furthermore, washout of MbCD and supplementation with exogenous cholesterol correlate with the full recovery of responsiveness in the warm-acclimatized group only. Thus, the mechanism underlying changes in postsynaptic sensitivity to glutamate is likely to involve changes in cholesterol and not loss of a critical protein. In contrast with an earlier report indicating that MbCD did not alter the size or shape of miniature end-plate potentials in crayfish dactyl opener muscle, our data indicate that MbCD affects postsynaptic cells. Although it did not alter input resistance in muscle fibers, MbCD dramatically reduced responses to local application of WY 14643 L-glutamate by iontophoresis, suggesting a direct effect on glutamate receptor function. In the cold-acclimatized group, the decreased sensitivity of postsynaptic receptors to glutamate is likely to contribute substantially to the decrease in EJP amplitude. Taken as a whole, the ability of MbCD to alter so many synaptic properties makes it difficult to understand the acute effects of the compound, or to predict how it ultimately alters the overall input/output relation at chemical synapses. As noted above, exposure to MbCD decreased responses to Lglutamate in both experimental groups at times that coincided with reduction in cholesterol levels in muscle cells.