Calcium currents at motor nerve endings: absence of effects of adenosine receptor agonists in the frog.

Abstract

SUMMARY1. The effects of adenosine (50 fIM) and 2-chloroadenosine (1-25 /M) were studied on Ca2+ currents in frog motor nerve endings.2. Ca2+ currents associated with the synchronous, neurally evoked release of acetylcholine (ACh) were measured using either perineural or patch recording methods. Tetraethylammonium and/or 3,4-diaminopyridine were employed to block K+ currents.3. Ca2+ currents were depressed by wo-conotoxin (1-5-2-5 gM), Cd21 (100 fM-2 mM), Co2+ (500 /M-5 mM) or by a reduction of the extracellular calcium concentration. Such currents were also observed when Sr2+ was substituted for Ca2+. Both ACh release and Ca2+ currents at motor nerve endings have been reported to be insensitive to 1,4-dihydropyridine antagonists in this species.4. Adenosine receptor agonists did not affect Ca2+ currents at concentrations that produced maximal inhibition of ACh release. 5. The effects of adenosine receptor agonists were examined on asynchronous K+-dependent ACh release under conditions in which the Ca2+ concentration gradient is likely to be reversed (Ca2+-free Ringer solution containing 1 mm EGTA). ACh release was measured by monitoring the frequency of occurrence of miniature endplate potentials (MEPPs). In Ca2+-free solutions containing 1 mm EGTA, high K+ depolarization caused a decrease in MEPP frequency, presumably because it elicits the efflux of Ca2+ from the nerve ending via membrane Ca2+channels in a reverse Ca2+

Presynaptic inhibition of spontaneous acetylcholine release induced by adenosine at the mouse neuromuscular junction

“…A neuromodulatory action of CCPA via a decrease in Ca 2 þ influx was also observed at the mammalian neuromuscular junction when nerve terminals were stimulated electrically (Hamilton & Smith, 1991). In contrast, at frog neuromuscular junctions, this mechanism was not found to be associated with the inhibitory action of AD (Silinsky & Solsona, 1992;Redman & Silinsky, 1994;Robitaille et al, 1999), suggesting that, in this species, as in other tissues, the nucleoside inhibits tonic secretion by modulating the release machinery downstream of Ca 2 þ influx (Prince & Stevens, 1992;Scanziani et al, 1992;Scholz & Miller, 1992;Capogna et al, 1996;Dittman & Regehr, 1996; for a review see Wu & Saggau, 1997). In order to investigate which of the VDCCs was involved in the presynaptic effect of CCPA, we studied the different blocking effects of specific VDCC blockers and CCPA on mepp frequency.…”

“…At the frog neuromuscular junction, it has been shown that the inhibitory effect of AD upon evoked and spontaneous neurotransmitter release is not related to a reduction in calcium influx through the N-type voltage-dependent calcium channels (VDCCs), suggesting that AD exerts its action at a site distal to the locus of Ca 2 þ entry (Silinsky, 1984;Silinsky & Solsona, 1992;Redman & Silinsky, 1994). On the other hand, Hamilton & Smith (1991) showed that, at rat neuromuscular synapses, AD inhibits the motor nerve terminal Ca 2 þ current in response to nerve stimulation, but the mechanism(s) by which AD induces presynaptic inhibition of spontaneous neurotransmitter release remains unknown.…”

“…An alternative mechanism is that AD might directly impair transmitter exocytosis. Most of the reports of the effect of presynaptic modulators upon spontaneous release show that they modulate the release machinery downstream of Ca 2 þ influx (Silinsky, 1984;Prince & Stevens, 1992;Scanziani et al, 1992;Scholz & Miller, 1992;Silinsky & Solsona, 1992;Capogna et al, 1996;Dittman & Regehr, 1996).…”

Inhibition of spontaneous acetylcholine secretion by 2‐chloroadenosine as revealed by a protein kinase inhibitor at the mouse neuromuscular junction

“…Plausible mechanisms for the prejunctional inhibitory e ect of adenosine analogues on mepp frequency which have been proposed are: a decrease in the ability of Ca 2+ to promote the exocytosis process (e.g. Silinsky, 1984Silinsky, , 1986Silinsky & Solsona, 1992) or a decrease in Ca 2+ entry via Ca 2+ channels (Hamilton & Smith, 1991). However, the latter explanation appears to be less likely in these present experiments because we have restricted this study to spontaneous ACh release which, in the absence of nerve terminal depolarization, is little in¯uenced by extracellular Ca 2+ .…”

Reduction by intracellular calcium chelation of acetylcholine secretion without occluding the effects of adenosine at frog motor nerve endings

“…Previous results suggest that adenosine inhibits ACh secretion in frog without reducing Ca2+ influx through voltagedependent Ca2' channels (Silinsky, 1981(Silinsky, , 1984Silinsky & Solsona, 1992;Hunt & Silinsky, 1993). We have now investigated the possibility that adenosine reduces the amount of cytoplasmic Ca2+ available to trigger secretion by increasing the rate at which Ca2+ is removed from the cytoplasm adjacent to the active zone.…”