Autoreceptor-mediated purinergic and cholinergic inhibition of motor nerve terminal calcium currents in the rat.
1. After blocking K+ currents with 10 mM-tetraethylammonium (TEA) or TEA plus 250 microM-3,4-diaminopyridine (3,4-DAP). motor nerve terminal Ca2+ currents were recorded using focal extracellular electrodes. Two transmitters released from the terminal. ATP and acetylcholine (ACh), were then applied, and the effects on the nerve terminal Ca2+ current were measured. 2. ATP (50 microM) reduced the Ca2+ current by 34%, but this action is prevented when hydrolysis to adenosine is blocked by alpha,beta-methyladenosine 5'-diphosphate (200 microM). Thus, inhibition by ATP presumably occurs subsequent to ATP hydrolysis to adenosine. 3. Adenosine (50 microM) inhibited the terminal Ca2+ current by 29%. This was mimicked by the adenosine analogue L-phenylisopropyl adenosine (L-PIA) and blocked by theophylline (100 microM), which antagonizes adenosine receptors at micromolar concentrations. 4. ACh (100 microM) or the anticholinesterase methane sulphonyl fluoride (MSF; 1 mM) also depressed the terminal Ca2+ current. This response was mimicked by muscarine (100 microM) and antagonized by atropine (100 microM) or pirenzipine (4 microM), which is generally specific for M1 receptors. 5. Addition of Ba2+, which blocks adenosine-mediated K+ currents, had no effect on the inhibitory effects of either adenosine or ACh; similarly, neither adenosine nor ACh in the bath affected K+ current records obtained after blocking all inward currents with 10 mM-Co2+ and focal application of tetrodotoxin. 6. Incubation of the muscle for 4 h in pertussis toxin (10(-5) g ml-1) eliminated both adenosine- and ACh-induced inhibition of the terminal Ca2+ current. This result indicates the possible involvement of a G protein in the transduction of the feedback pathway. 7. Neither cyclic AMP analogues, the adenylate cyclase activator forskolin (10 microM), the phorbol ester phorbol 12-myristate 13-acetate (PMA; 3 microM) nor the diacylglycerol analogue 1,2-oleoylacetylglycerol (OAG; 3 microM) had any effect on adenosine- or ACh-induced depression of the terminal Ca2+ current. Therefore, pathways involving these particular second messengers are most probably not involved. 8. The effects of adenosine and ACh are non-additive. 9. These results indicate that ATP and ACh, which are released during exocytosis, may inhibit their own release through attenuation of the terminal Ca2+ current via autoreceptors coupled to a G protein.
Hamilton
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Department of Physiology, University of Wisconsin, Madison 53706.
Smith
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D000109:Acetylcholine / Q000502:physiology*; D000255:Adenosine Triphosphate / Q000502:physiology*; D037361:Adenylate Cyclase Toxin; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000242:Cyclic AMP / Q000502:physiology; D004594:Electrophysiology; D006706:Homeostasis / Q000502:physiology; D009046:Motor Neurons / Q000378:metabolism*; D009411:Nerve Endings / Q000378:metabolism*; D037342:Pertussis Toxin; D011188:Potassium / Q000378:metabolism; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D037361:Adenylate Cyclase Toxin; D010566:Virulence Factors, Bordetella; D000255:Adenosine Triphosphate; D000242:Cyclic AMP; D037342:Pertussis Toxin; D011493:Protein Kinase C; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium