Correlation of Miniature Synaptic Activity and Evoked Release Probability in Cultures of Cortical Neurons

Abstract

Spontaneous miniature synaptic activity is caused by action potential (AP)-independent release of transmitter vesicles and is regulated at the level of single synapses. In cultured cortical neurons we have used this spontaneous vesicle turnover to load the styryl dye FM1-43 into synapses with high rates of miniature synaptic activity. Automated selection procedures restricted analysis to synapses with sufficient levels of miniature activity-mediated FM1-43 uptake. After FM1-43 loading, vesicular FM1-43 release in response to AP stimulation was recorded at single synapses as a measure of release probability. We find that synapses with high rates of miniature activity possess significantly enhanced evoked release rates compared with a control population. Because the difference in release rates between the two populations is [Ca 2ϩ ] o -dependent, it is most likely caused by a difference in release probability. Within the subpopulation of synapses with high miniature activity, we find that the probabilities for miniature and AP-evoked release are correlated at single synaptic sites. Furthermore, the degree of miniature synaptic activity is correlated with the vesicle pool size. These findings suggest that both evoked and miniature vesicular release are regulated in parallel and that the frequency of miniature synaptic activity can be used as an indicator for evoked release efficacy.Key words: miniature; evoked; synapse; vesicle; release; probability; pool size; calcium Spontaneous miniature synaptic activity exists throughout the vertebrate nervous system. Miniature activity is attributed to spontaneous AP-independent presynaptic release of one (Frerking et al., 1997) or more (Vautrin and Barker, 1995;Wall and Usowicz, 1998) transmitter quanta. Findings from our laboratory using postsynaptic imaging of miniature activity indicate that the probability for miniature release is highly variant between synapses even on the same dendrite (Murphy et al., 1994(Murphy et al., , 1995Wang et al., 1999). This variability among synapses indicates that miniature release can be regulated on the level of individual synapses. It has been shown that [C a 2ϩ ] i influences the frequency of miniature activity in C NS (Minota et al., 1991;Doze et al., 1995;Scanziani et al., 1995;C apogna et al., 1996aPoisbeau et al., 1996;Schoppa and Westbrook, 1997;Bao et al., 1998;Li et al., 1998) and peripheral nervous system (PNS) (Matthews and Wickelgren, 1977;Marcus et al., 1992;Katz et al., 1995). Furthermore, downstream of C a 2ϩ influx second messenger systems such as protein kinase C (PKC) (Ghiradi et al., 1992;Parfitt and Madison, 1993;C apogna et al., 1995; C arroll et al., 1998;Stevens and Sullivan, 1998) and cAM P-dependent protein kinase A (PKA) (Chavez-Noriega and Stevens, 1994; Kondo and Marty, 1997; Chen and Regehr, 1997) have been implicated in the regulation of miniature activity. Both PK A (Ghiradi et al., 1992;Hell et al., 1995;Tong et al., 1996) and PKC (for review, see Ramakers et al., 1997;Majewski and Iannazzo, 1998) have ...

A balance between excitatory and inhibitory synapses is controlled by PSD-95 and neuroligin

“…We compared changes in excitatory and inhibitory currents in hippocampal neurons transfected with either GFP alone, HA-NLG and GFP, PSD-95 GFP, or PSD-95 GFP and HA-NLG by using whole-cell voltage-clamp recordings of mEPSCs and mIPSCs. Miniature frequency reflects both the size and number of presynaptic contacts, whereas the average amplitude of miniature currents correlates with postsynaptic parameters (33). Ectopic expression of HA-NLG significantly increased the frequency of mEPSCs (2.1-Ϯ 0.3-fold) and IPSCs (1.9-Ϯ 0.4-fold).…”

MEF2C, a transcription factor that facilitates learning and memory by negative regulation of synapse numbers and function

“…The increase in mEPSC frequency could be due to an increase in the number or density of synapses (19) or to an increase in probability of release at the presynaptic site (20). We investigated whether MEF2C was required for presynaptic function by measuring paired-pulse facilitation (PPF), a short-term enhancement of synaptic activity in response to the second of two paired stimuli due to residual Ca ϩ2 in the presynaptic terminal after the initial stimulus.…”

Vesicle number does not predict postsynaptic measures of miniature synaptic activity frequency in cultured cortical neurons

“…A conceivable explanation for the discrepancy of the FM1-43 data with the present data is that MSCT imaging provides a more sensitive measure of release frequency than FM1-43 imaging since it can detect synapses that only show single miniature release event. In contrast, our FM1-43 experiments 37 were only able to detect synapses with relatively higher levels of activity (10s-100 release events within Vescicle number does not predict miniature activity 5 synaptic activity, release events may not always successfully activate postsynaptic receptors. A lack of postsynaptic receptor activation would be surprising given that vesicle release is thought to be both rapid and complete.…”

“…38 Results from our laboratory, using imaging of presynaptic vesicular turnover with FM1-43, indicate a significant correlation between miniature frequency and evoked release probability at single cortical neuron synapses. 37 Furthermore these FM 1-43 experiments indicate that the vesicle pool size (a measure of synapse size) as determined by FM1-43 does correlate, albeit weakly (r 0.59), with the probability of miniature synaptic activity. The reasons for this discrepancy (in relation to the current study) may be due to the fixation conditions used, as we did not attempt to fix preparations rapidly.…”