Figure 3.
Differential synaptic dynamics of feedback and feedforward connections. A, An example of converging MSN–MSN connectivity. A postsynaptic MSN (M3) received direct connections from two neighboring MSNs (M1 and M2). The connection was tested in different train frequencies (10, 20, 40 Hz trains of 8 action potentials), showing frequency-dependent depression and facilitation. Note the increase in amplitude of the recovery test response (denoted by blue arrows), revealing the underlying facilitatory component. B, Examples of divergent synaptic connectivity from FS to two neighboring MSNs as recorded in voltage clamp (red traces, bottom). Note the depression of the recovery test response after 0.55 s after a 20 Hz train. The top traces are of the same connections as in A, acquired in voltage-clamp mode. C, MSNs receive different types of input from neighboring MSNs and FS cells. Two examples of synaptic responses to 20 Hz trains (green) and a recovery response are depicted. Note the facilitation of the recovery response in the MSN→MSN connection (black) compared with the depression of the FS→MSN response (red). D, Average responses of all analyzed connections normalized to the amplitude of the first PSP (FS–MSN connections in red, n = 23; MSN–MSN in black, n = 31). E, Facilitation and depression time constants of all analyzed connections are plotted against each other in a logarithmic plot. The dashed line represents the F = D curve, showing that all FS→MSN, but not all MSN→MSN connections, were depressing. F, The synaptic dynamics of the two connection types are significantly different, as seen by the values of the time constants for facilitation (F) and depression (D), and the ratio (right bar graph). ***p < 0.001, Student's t test. Error bars indicate SEM.