Postsynaptic production of nitric oxide implicated in long‐term depression at the mature amphibian (Bufo marinus) neuromuscular junction

Abstract

We report here evidence for endogenous NO signalling in long-term (> 1 h) synaptic Q1 depression at the neuromuscular junction induced by 20 min of 1 Hz nerve stimulation. Synaptic depression was characterized by a 46% reduction in the end-plate potential (EPP) amplitude and a 21% decrease in miniature EPP (MEPP) frequency, but no change to MEPP amplitude, indicating a reduction in evoked quantal release. Both the membrane-impermeant NO scavenger cPTIO and the NOS inhibitor L-NAME blocked depression, suggesting that it is induced by NO originating from a source outside the terminal. The depression was dependent on activation of muscle-type, but not neuronaL-type, nAChRs and was still observed when Ca 2+ release from the sarcoplasmic reticulum and muscle contraction were blocked with dantrolene. These data suggest that the depression depends on transmission, but not muscle contraction. The calcineurin inhibitors cyclosporin A and FK506, as well as ODQ, an inhibitor of NO-sensitive soluble guanylyl cyclase, Rp-8-pCPT-cGMPS, an inhibitor of cGMP-dependent protein kinase, and the calmodulin antagonist phenoxybenzamine also blocked depression. We propose that low frequency synaptic transmission leads to production of NO at the synapse and depression of transmitter release via a cGMP-dependent mechanism. The NO could be generated either directly from the muscle, or possibly from the Schwann cell in response to an unidentified muscle-derived messenger. We showed that the long-lasting depression of transmitter release was due to sustained activity of the NO signalling pathway, and suggest dephosphorylation of NOS by calcineurin as the basis for continued NO production.

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

“…Interestingly, the depression of transmitter (a presynaptic phenomenon) was blocked by an antagonist of postsynaptic skeletal muscle ACh receptors (Etherington and Everett, 2004). In the present study we show that, unlike forms of depression previously identified at this synapse, this form of depression dependent on skeletal muscle AP firing and can therefore spread between widely spaced nerve terminals coinnervating the same muscle fibers.…”

“…We recently reported a form of depression at synapses of singly innervated fibers in mature amphibian skeletal muscle induced by 20 min of low-frequency nerve stimulation (Etherington and Everett, 2004). Our results implicated postsynaptic calcium elevation in the production of nitric oxide, believed to act as a retrograde messenger to induce depression at the nerve terminal.…”

“…Prolonged low-frequency nerve stimulation at the amphibian neuromuscular junction (1 Hz for 20 min) induces lasting (>1 h) depression of transmitter release from motor nerve terminals, manifest as a decrease in EPP amplitudes and in the frequency, but not the amplitude, of spontaneous synaptic potentials (Etherington and Everett, 2004). Interestingly, the depression of transmitter (a presynaptic phenomenon) was blocked by an antagonist of postsynaptic skeletal muscle ACh receptors (Etherington and Everett, 2004).…”

“…Methods for endplate potential (EPP) recordings were the same as have been described previously (Etherington and Everett, 2004). Briefly, both iliofibularis-sciatic nerve preparations from an animal were mounted in organ baths and the nerve to one muscle was stimulated at 1 Hz for 20 min, while the other preparation was used as a control and not stimulated during this period.…”

“…Skeletal muscle fibers have the capacity to modulate the strength of the synaptic inputs they receive in an activity-dependent manner (Connor and Smith, 1994;Etherington and Everett, 2004;Fitzsimonds and Poo, 1998). This can be seen during development and re-innervation of muscle when individual fibers receive transient innervation from two or more motoneurons (Wyatt and Balice-Gordon, 2003).…”