Endogenous adenosine inhibits CNS terminal Ca(2+) currents and exocytosis.
Bursts of action potentials (APs) are crucial for the release of neurotransmitters from dense core granules. This has been most definitively shown for neuropeptide release in the hypothalamic neurohypophysial system (HNS). Why such bursts are necessary, however, is not well understood. Thus far, biophysical characterization of channels involved in depolarization-secretion coupling cannot completely explain this phenomenon at HNS terminals, so purinergic feedback mechanisms have been proposed. We have previously shown that ATP, acting via P2X receptors, potentiates release from HNS terminals, but that its metabolite adenosine, via A(1) receptors acting on transient Ca(2+) currents, inhibit neuropeptide secretion. We now show that endogenous adenosine levels are sufficient to cause tonic inhibition of transient Ca(2+) currents and of stimulated exocytosis in HNS terminals. Initial non-detectable adenosine levels in the static bath increased to 2.9 microM after 40 min. These terminals exhibit an inhibition (39%) of their transient inward Ca(2+) current in a static bath when compared to a constant perfusion stream. CPT, an A(1) adenosine receptor antagonist, greatly reduced this tonic inhibition. An ecto-ATPase antagonist, ARL-67156, similarly reduced tonic inhibition, but CPT had no further effect, suggesting that endogenous adenosine is due to breakdown of released ATP. Finally, stimulated capacitance changes were greatly enhanced (600%) by adding CPT to the static bath. Thus, endogenous adenosine functions at terminals in a negative-feedback mechanism and, therefore, could help terminate peptide release by bursts of APs initiated in HNS cell bodies. This could be a general mechanism for controlling transmitter release in these and other CNS terminals.
Knott
Thomas K
TK
Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA.
Marrero
Héctor G
HG
Fenton
Richard A
RA
Custer
Edward E
EE
Dobson
James G
JG
Lemos
José R
JR
D000241:Adenosine / Q000378:metabolism / Q000502:physiology*; D058916:Adenosine A1 Receptor Antagonists; D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005246:Feedback / Q000502:physiology; D025461:Feedback, Physiological / Q000187:drug effects / Q000502:physiology; D007031:Hypothalamus / Q000187:drug effects / Q000378:metabolism; D008297:Male; D009479:Neuropeptides / Q000378:metabolism*; D010904:Pituitary Gland, Posterior / Q000187:drug effects / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D043682:Receptor, Adenosine A1 / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
C092431:6-N,N-diethyl-beta,gamma-dibromomethylene-D-ATP; D058916:Adenosine A1 Receptor Antagonists; D004791:Enzyme Inhibitors; D009479:Neuropeptides; D043682:Receptor, Adenosine A1; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; C027201:ectoATPase; D000241:Adenosine; D002118:Calcium