Figure 6.
Frequency histograms of EPSCaT amplitudes indicate that increases in both pr and in the number of active release site scan contribute to the tetanus-induced increase in efficacy of individual mossy fiber synapses. A, Amplitude frequency histogram of EPSCaTs recorded at a single mossy fiber synapse before (white bars) and 30 min after (gray bars) LTP induction. In this experiment, induction led to an increase in pCa (note reduction in failures), with no change in the apparently unimodal distribution of EPSCaT amplitudes. This type of change, also observed at six other synapses in independent experiments, suggests that an increase in efficacy at the observed synapse resulted from a simple increase in the probability of transmitter release. B, Amplitude frequency histogram at another mossy fiber synapse. In this case, there were no failures, and EPSCaT amplitudes clustered unimodally around ΔF/F = 50% before LTP induction (white bars). After LTP induction (gray bars), most EPSCaTs had substantially larger amplitudes; a similar emergence of new, high-amplitude responses after LTP was observed at three other synapses in independent experiments and is most easily explained by the recruitment of at least one additional active release site. C, At this MF synapse, LTP induction resulted in both an increase in pCa and the emergence of higher-amplitude EPSCaTs. D, An additional example of an MF synapse at which LTP induction resulted in both an increase in pCa and the emergence of higher-amplitude EPSCaTs. Addition of adenosine to the superfusate after the collection of post-LTP induction data resulted in a reduction of pCa and elimination of the higher-amplitude EPSCaTs (red bars), to values similar to those seen before potentiation. The pattern of change illustrated in C and D was observed at six other synapses in independent experiments and is also consistent with an LTP-associated recruitment of additional active release sites.