Disruption of actin impedes transmitter release in snake motor terminals

1. To investigate the role of actin in vertebrate nerve terminals, nerve-mu scle preparations from garter snake (Thamnophis sirtalis) were treated with the actin-depolymerizing agent latrunculin A. Immunostaining revealed that actin filaments within presynaptic motor terminal. boutons were disrupted by the drug. 2. In preparations loaded with the optical probe FM1-43, destaining was red uced by latrunculin treatment, suggesting that transmitter release was part ially blocked. 3. Latrunculin treatment did not influence the amplitude or time course of spontaneous miniature endplate potentials (MEPPs). Similarly, endplate pote ntials (EPPs) evoked at low frequency were comparable in control and latrun culin-treated curarized preparations. 4. Brief tetanic stimulation of the muscle nerve (25 Hz, 90 a) depressed EP P amplitudes in both control and latrunculin-treated preparations. After te tanus, EPPs elicited at 0.2 Hz in control preparations recovered rapidly (0 -5 min) and completely (usually potentiating to above pre-tetanus levels; 1 30 +/- 11%, mean +/- S.E.M). In contrast, EPPs evoked in latrunculin-treate d preparations recovered slowly (8-10 min) and incompletely (84 +/- 8 %). 5. The influence of latrunculin on post-tetanic EPPs depended on its concen tration in the bath (K-D = 3.1 mu M) and on time of incubation. 6. These observations argue that actin filaments facilitate transmitter rel ease rather than impede it. Specifically, actin may facilitate mobilization of vesicles towards the releasable pools.