Pharmacological characterization of adenosine A1 receptors and its functional role in brown trout (Salmo trutta) brain

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Adenosine receptor-adenylate cyclase system in the trout testis: involvement in the regulation of germ cell proliferation

To ascertain the presence of adenosine receptors in the trout testis, cells isolated from testes at different spermatogenetic stages were cultured in the presence or absence of adenosine, adenosine receptor agonists, or antagonists and of cAMP analogs, for up to 20 min, or 20 hr, or 4.5 days. Cyclic AMP production was then assayed or 3H-thymidine incorporation was measured. Cellular content of cAMP was enhanced by adenosine, by the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), and by 2-p(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680), an adenosine A2A receptor-selective agonist. The increase in cAMP induced by the adenylate cyclase activator L-858051 was inhibited by the adenosine A1)receptor-selective agonists R-N6-(2-phenylisopropyl)adenosine (R-PIA) and N6-cyclopentyladenosine (CPA). These effects were antagonized by the two adenosine A2)receptor antagonists 3,7-dimethyl-1-propargylxanthine (DMPX) and 8-(3-chlorostyryl)caffeine (CSC), and by the adenosine A1)receptor-selective antagonist 8-cyclopentyl-1,3dipropylxanthine (CPX), respectively. Increase in the cAMP content induced by adenosine was inhibited by the cell permeable adenylate cyclase inhibitor 2',5'-dideoxyadenosine. These data suggest that A(1) and A(2) adenosine receptors which respectively inhibit and stimulate adenylate cyclase activity are present on trout testicular cells (unidentified), while the presence of A3 adenosine receptor subtype was not apparent. 3H-thymidine incorporation decreased in the presence of the adenylate cyclase activator L-858051 and of the cAMP analogs 8-CPT cAMP and Sp-5,6-DCI-cBiMPS, regardless of the presence or absence of the phosphodiesterase inhibitor RO 20-1724. This suggests that an increase in testicular cAMP may act as a negative growth regulator for the mitotic germ cells. In agreement with these data, the activation of A2 stimulatory receptors inhibited short-term (20 hr) DNA synthesis. However, the activation of A1 inhibitory receptors had the same effect. This suggests that events, cAMP-dependent or independent, induced by the activation of testicular adenosine receptors, may participate in the regulation of trout male germ cell proliferation.

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.