Optogenetic activation of A18a in 6mM Sr2+ saline stimulates asynchronous release of minis.
(A) Representative traces show synaptic currents recorded in aCC following optogenetic stimulation (1s LED) of A18a in control (Ca2+) or 6mM Sr2+ saline. Recordings made in control saline feature large synaptic events (∼30pA), indicative of normal vesicular release from the premotor. By contrast, the large current response to optogenetic stimulation is absent in 6mM Sr2+. (B) Analysis of peak currents recorded in aCC, elicited by optogenetic stimulation of A18a are significantly larger in control (Ca2+) than 6mM Sr2+ saline (28.96 ± 4.56s, Ca2+; 4.4 ± 0.85s, Sr2+; n = 8, 8; p < 0.0001), thus, the changes we observed in current amplitude are consistent with the asynchronous mini release reported when using Sr2+ saline in mammals (Xu-Friedman and Regehr, 1999; Bekkers and Clements, 1999). (C) Similarly, there is a significant increase in the frequency of asynchronous synaptic events (minis) recorded in aCC in 6mM Sr2+ saline, in the 3s following stimulation of A18a (Evoked) versus the 3s prior (4.25 ± 0.59Hz, Spontaneous; 10.83 ± 1.12Hz, Evoked; n = 8, 8; p < 0.0001). In combination, these results suggest recording current in aCC following optogenetic stimulation of A18a in 6mM Sr2+ saline, enables quantifying the frequency and amplitude of minis elicited by this premotor interneuron.