Effects of cytochalasin treatment on short‐term synaptic plasticity at developing neuromuscular junctions in frogs.

Abstract

1. The role of actin microfilaments in synaptic transmission was tested by monitoring spontaneous and evoked transmitter release from developing neuromuscular synapses in Xenopus nerve-muscle cultures, using whole-cell recording of synaptic currents in the absence and presence of microfilament-disrupting agents cytochalasins B and D. 2. Treatment with cytochalasins resulted in disruption of microfilament networks in the growth cone and the presynaptic nerve terminal of spinal neurons in Xenopus nerve-muscle cultures, as revealed by rhodamine-phalloidin staining. 3. The same cytochalasin treatment did not significantly affect the spontaneous or evoked synaptic currents during low-frequency stimulation at 005 Hz in these Xenopus cultures. Synaptic depression induced by high-frequency (5 Hz) stimulation, however, was reduced by this treatment. Paired-pulse facilitation for short interpulse intervals was also increased by the treatment. 4. These results indicate that disruption of microfilaments alters short-term changes in transmitter release induced by repetitive activity, without affecting normal synaptic transmission at low frequency. 5. Our results support the notion that actin microfilaments impose a barrier for mobilization of synaptic vesicles from the reserve pool, but do not affect the exocytosis of immediately available synaptic vesicles at the active zone.

Dual pools of actin at presynaptic terminals

“…We also observed a slight enhancement in neurotransmitter release during high-frequency stimulation. While this latter effect is not profound, similar effects have also been recorded in other systems (Bernstein and Bamburg 1989; Cole et al 2000;Richards et al 2004;Wang et al 1996), although under conditions of prolonged high-frequency stimulation blockade of actin polymerization may inhibit synaptic transmission (Kuromi and Kidokoro 1998;Richards et al 2004) or prolong recovery of synaptic transmission (Cole et al 2000). It is difficult to reconcile our results with those seen in previous studies utilizing latrunculin and other stronger actin depolymerizing agents, although the greater effects seen with depolymerizing agents known to sever actin filaments may have additional effects on cortical actin not previously examined (see below).…”

“…However, G-actin has also been shown to incorporate into presynaptic F-actin clusters at rest, suggesting additional activity-independent reorganization (Bourne et al 2006). Indeed, actin turnover is necessary for synaptic transmission because stabilization of F-actin by phalloidin (Vandekerckhove et al 1985) inhibits neurotransmitter release (Bernstein and Bamburg 1989;Photowala et al 2005), while agents that prevent actin polymerization or induce depolymerization result in only minimal enhancing effects on exocytosis (Kuromi and Kidokoro 1998;Sankaranarayanan et al 2003) and synaptic transmission (Cole et al 2000;Kuromi and Kidokoro 1998;Morales et al 2000;Richards et al 2004;Wang et al 1996). Actin may also act as a barrier to exocytosis (Dillon and Goda 2005), for blockade of actin polymerization can enhance evoked release (Morales et al 2000;Wang et al 1996), However, this may not be true for all synapses (Sakaba and Neher 2003).…”

Structural Domains Involved in the Regulation of Transmitter Release by Synapsins

“…Various studies have led to proposals that actin has no presynaptic function (Job and Lagnado, 1998) or that actin plays negative (Wang et al, 1996;Bernstein et al, 1998;Morales et al, 2000;Sankaranarayanan et al, 2003) or positive (Kuromi and Kidokoro, 1998;Cole et al, 2000;Sakaba and Neher, 2003) roles in synaptic vesicle mobilization. It has also been proposed that actin is involved in other aspects of vesicle recycling (Shupliakov et al, 2002).…”

Synaptic Vesicle Mobility and Presynaptic F-Actin Are Disrupted in aN-ethylmaleimide–sensitive Factor Allele ofDrosophila

“…Differing reports ascribe inhibitory (Wang et al, 1996;Bernstein et al, 1998;Morales et al, 2000), positive (Kuromi and Kidokoro, 1998;Cole et al, 2000;Sakaba and Neher, 2003), or supportive (Sankaranarayanan et al, 2003) roles in synaptic vesicle mobilization. The data herein support the idea that actin filaments promote intrabouton mobility of synaptic vesicles.…”