The calcium-dependent [H-3]acetylcholine release from synaptosomes of brown trout (Salmo trutta) optic tectum is inhibited by adenosine A(1) receptors - Effects of enucleation on A(1) receptor density and cholinergic markers

Presynaptic inhibition is one of the major control mechanisms in the CNS. P reviously we reported that A(1) 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 cholinergi c nerve terminals. We have investigated whether A. receptors inhibit the pr esynaptic release of acetylcholine and whether the inhibition is triggered by calcium. The release of [H-3]ACh evoked by 30 mM KCl was Ca2+ dependent and it was dose-dependently inhibited by the A, adenosine receptor agonist 2-chloro-N-6-cyclopentyladenosine (CCPA) ranging between 10 nM to 100 muM. The maximum of inhibition was reached at 10 muM. The A, receptor antagonist 8-cyclopentyltheopylline (CPT, 10 muM), reversed almost completely the inh ibition induced by CCPA 10 muM. In Fura-2/AM loaded synaptosomes, K+ depola rization raised [Ca2+](i) by about 64%. CCPA (10 muM) reduced the K+-evoked Ca2+ influx increase by about 48% and this effect was completely antagonis ed by CPT 10 muM. Synaptosome pretreatment with different Ca2+ channel bloc kers differently affected K+-evoked Ca2+ influx. This was not significantly modified by nifedipine (1 muM, L-type blocker) nor by omega -agatoxin IVA (0.3 muM, P/Q-type blocker), whereas about 50% reduction was shown by 0.5 m uM omega -conotoxin GVIA (N-type blocker). Neurochemical parameters associa ted with cholinergic transmission and the density of A(1) adenosine recepto rs were measured in the trout optic tectum 12 days after unilateral eye abl ation. A significant drop of both acetylcholinesterase (AChE) activity (24% ) and choline acetyltransferase (CAT) activity (32%) was observed in deaffe rentated optic tectum, whereas the high affinity choline uptake did not par allel the decrease in enzyme activity. Eye ablation caused a marked decreas e (43%) of A(1) receptor density without changing the affinity. The K+-evok ed release of [H-3]ACh from synaptosomes of deafferentated was not modify a s well as the efficacy of 10 muM CCPA in decreasing [H-3]ACh release was no t apparently modified. (C) 2001 Elsevier Science B.V. All rights reserved.