Endogenous adenosine inhibits CNS terminal Ca²⁺ currents and exocytosis

Abstract

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.

Caffeine inhibits hypothalamic A1R to excite oxytocin neuron and ameliorate dietary obesity in mice

“…For example, activation of A 1 R by selective agonist, 2-chloroadenosine, suppressed KCl evoked release of Dynorphin A(1-8) from the hippocampal synaptosomes45. Endogenous adenosine was able to cause tonic inhibition of transient Ca 2+ current and evoked exocytosis in neurohypophysial terminals46. In addition, stimulation of A 1 R in the spinal cord inhibits the exocytosis of calcitonin gene-related peptide47.…”

Characterization of purinergic P2X4receptor channels expressed in anterior pituitary cells

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