Abstract
The neuromodulator adenosine is known to decrease neurotransmitter release at the neuromuscular junction by activation of an A1 adenosine receptor coupled to a pertussis toxin-sensitive G protein. Among the mechanisms that could contribute to the depression of neurotransmitter release is reduced entry of calcium through channels located in the presynaptic terminal. In the present study, we have examined the effects of adenosine on high-voltage-activated (HVA) calcium currents in motoneurons, the presynaptic cells of the neuromuscular junction. The motoneurons were isolated from embryonic mice, placed in primary tissue culture for 16 hr, and analyzed by means of the whole-cell patch- clamp technique. Adenosine (40 microM) reduced both transient and sustained components of HVA calcium current. This effect was blocked by the A1 antagonist 8-cyclopentyltheophylline (CPT; 100 nM) and was mimicked by the A1 agonist N6-cyclohexyladenosine (CHA; 50 nM to 10 microM) but not by the A2a agonist 2-p-(2-carboxyethyl)phenethylamino- 5′-N-ethylcarboxamido adenosine (CGS-21680; 1 micron). Pretreatment with pertussis toxin (200 ng/ml, > 16 hr) abolished the depression of HVA calcium current by adenosine receptor activation. Brief (3 min) exposure of the cells to 10 microM omega-conotoxin GVIA irreversibly blocked a part of the HVA current, which can therefore be attributed to N-type channels; the remaining current was unaffected by adenosine receptor activation. Hence, it appears that adenosine decreases only the N-current portion of HVA current and that this inhibition occurs via an A1 receptor linked to a pertussis toxin-sensitive G protein. Other investigators have shown that N-type channels do not play a primary role in eliciting transmitter release at the mammalian neuromuscular junction. Thus, it is uncertain what motoneuronal functions are influenced by adenosine modulation of N-type channels.