Role for the skeletal muscle action potential in non‐Hebbian long‐term depression at the amphibian (Bufo marinus) neuromuscular junction

Abstract

Retrograde signaling from skeletal muscle cells to motor nerve terminals is a recognized mechanism for modulating the strength of neuromuscular transmission. We recently described a form of long-term depression of transmitter release at the mature neuromuscular junction that is dependent on the production of nitric oxide, most likely by the muscle cell (Etherington and Everett [2004] J Physiol (Lond) 559:507-517). We now show that the depression is blocked by treating neuromuscular preparations with l-conotoxin G111A, an antagonist of skeletal muscle voltage gated sodium channels, indicating that the depression requires postsynaptic action potential firing. Experiments on dually-innervated sartorius muscles revealed that propagation of action potentials generated by low-frequency stimulation of one nerve branch gives rise to nitric-oxide mediated depression at unstimulated nerve terminals located many millimetres away on the same muscle fiber. The non-Hebbian pattern of expression of the depression, as well as its reliance on postsynaptic action potential firing, distinguish it from forms of synaptic depression described at immature neuromuscular synapses, and may provide a mechanism for coregulation of the strength of motoneurons innervating the same postsynaptic cell. Synapse 00:000-000, 2008. V

Modulation of Synaptic Vesicle Exocytosis in Muscle-Dependent Long-Term Depression at the Amphibian Neuromuscular Junction

“…Our LTD is well suited to such a role, as it provides reversible depression of presynaptic release at all terminals innervating a muscle fiber in response to repetitive muscle AP firing [9]. Furthermore, the LTD acts at a late stage of transmitter release and therefore does not impede other forms of plasticity (e.g.…”

“…Synaptic and action potentials were recorded from iliofibularis muscles with sharp borosilicate glass electrodes (R = 7–20 MΩ) containing 3 M KCl, according to published methods [9], [13]. End-plate potentials (EPPs) were recorded in the presence of d -tubocurarine chloride (0.6–1.6 µM, Sigma) to minimize muscle contraction.…”

“…The d -tubocurarine chloride was only applied after the LFS protocol used to induce LTD; we have shown previously that post-stimulation application of the antagonist does not interfere with expression of LTD, although presence of the antagonist during LFS blocks depression [9], [13]. A labeled nerve terminal was positioned in dye-free medium within the field of view, imaged, and then sciatic nerve stimulation (30 Hz for 10 sec, 1 ms square pulses) was delivered via a suction electrode to destain terminals.…”

“…Similarly, ATP, a known inhibitor of presynaptic secretion [5], [6], is released from skeletal muscles in an activity-dependent manner [7]. Skeletal muscle activity also produces nitric oxide (NO) mediated depression at the amphibian neuromuscular junction in response to postsynaptic metabotropic glutamate receptor (mGluR) activation [8] or skeletal muscle action potential firing [9]. Retrograde signaling pathways involving a complex interplay between cannabinoid, prostanoid and NO signaling [10], [11], as well as distinct mechanisms activated by muscle-derived adenosine [12], have also been implicated in the activity-dependent enhancement of neuromuscular transmission.…”

“…The LTD, which lasts > 2 hours, is NO-dependent, with postsynaptic AP firing both necessary and sufficient to induce reduction in transmitter release. This unqualified dependence on postsynaptic AP firing distinguishes the LTD from most presynaptic forms of plasticity expressed at the vertebrate NMJ and suggests that it may play a novel role in modulating network function [9].…”