The calcium-dependent [3H]acetylcholine release from synaptosomes of brown trout (Salmo trutta) optic tectum is inhibited by adenosine A1 receptors: effects of enucleation on A1 receptor density and cholinergic markers.
Presynaptic inhibition is one of the major control mechanisms in the CNS. Previously we reported that A1 adenosine receptors are highly concentrated in the brain, including optic tectum, of trout and that they inhibited the release of glutamate. The optic tectum is heavily innervated by cholinergic nerve terminals. We have investigated whether A1 receptors inhibit the presynaptic release of acetylcholine and whether the inhibition is triggered by calcium. The release of [3H]ACh evoked by 30 mM KCl was Ca2+ dependent and it was dose-dependently inhibited by the A1 adenosine receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA) ranging between 10 nM to 100 microM. The maximum of inhibition was reached at 10 microM. The A1 receptor antagonist 8-cyclopentyltheopylline (CPT, 10 microM), reversed almost completely the inhibition induced by CCPA 10 microM. In Fura-2/AM loaded synaptosomes, K(+) depolarization raised [Ca2+](i) by about 64%. CCPA (10 microM) reduced the K(+)-evoked Ca2+ influx increase by about 48% and this effect was completely antagonised by CPT 10 microM. Synaptosome pretreatment with different Ca2+ channel blockers differently affected K(+)-evoked Ca2+ influx. This was not significantly modified by nifedipine (1 microM, L-type blocker) nor by omega-agatoxin IVA (0.3 microM, P/Q-type blocker), whereas about 50% reduction was shown by 0.5 microMomega-conotoxin GVIA (N-type blocker). Neurochemical parameters associated with cholinergic transmission and the density of A(1) adenosine receptors were measured in the trout optic tectum 12 days after unilateral eye ablation. A significant drop of both acetylcholinesterase (AChE) activity (24%) and choline acetyltransferase (CAT) activity (32%) was observed in deafferentated optic tectum, whereas the high affinity choline uptake did not parallel the decrease in enzyme activity. Eye ablation caused a marked decrease (43%) of A1 receptor density without changing the affinity. The K(+)-evoked release of [3H]ACh from synaptosomes of deafferentated was not modify as well as the efficacy of 10 microMCCPA in decreasing [3H]ACh release was not apparently modified.
Poli
A
A
Department of Biology, University of Bologna, Via Selmi 3, 40126 Bologna, Italy. a.poli@alma.unibo.it
Di Iorio
P
P
Beraudi
A
A
Notari
S
S
Zaccanti
F
F
Villani
L
L
Traversa
U
U
D000109:Acetylcholine / Q000378:metabolism*; D000110:Acetylcholinesterase / Q000378:metabolism; D000241:Adenosine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D000344:Afferent Pathways / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000502:physiology*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D002795:Choline O-Acetyltransferase / Q000378:metabolism; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009543:Nifedipine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D058906:Purinergic P1 Receptor Agonists; D018047:Receptors, Purinergic P1 / Q000502:physiology*; D013477:Superior Colliculi / Q000502:physiology*; D013574:Synaptosomes / Q000187:drug effects / Q000502:physiology*; D013806:Theophylline / Q000031:analogs & derivatives* / Q000494:pharmacology*; D014316:Tritium; D014337:Trout; D020867:omega-Agatoxin IVA / Q000494:pharmacology; D020866:omega-Conotoxin GVIA / Q000494:pharmacology
D002121:Calcium Channel Blockers; D058906:Purinergic P1 Receptor Agonists; D018047:Receptors, Purinergic P1; D020867:omega-Agatoxin IVA; D014316:Tritium; C053907:8-cyclopentyl-1,3-dimethylxanthine; C058394:2-chloro-N(6)cyclopentyladenosine; D011189:Potassium Chloride; D020866:omega-Conotoxin GVIA; D013806:Theophylline; D002795:Choline O-Acetyltransferase; D000110:Acetylcholinesterase; D009543:Nifedipine; D000241:Adenosine; D000109:Acetylcholine; D002118:Calcium