Enhancement of tetrodotoxin-induced axonal blockade by adenosine, adenosine analogues, dibutyryl cyclic AMP and methylxanthines in the frog sciatic nerve

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Abstract
1 The effects of adenosine, adenosine analogues (N6-cyclohexyladenosine (CHA), L-N6-phenylisopropyladenosine (L-PIA), D-N6-phenylisopropyladenosine (D-PIA), N6-methyladenosine and 2-chloroadenosine), adenine, inosine, hypoxanthine, cyclic AMP and its analogue the dibutyryl cyclic AMP (db cyclic AMP), and methylxanthines (theophylline, caffeine and isobutylmethylxanthine (Ibmx) on compound action potentials were investigated in de-sheathed sciatic nerve preparations of the frog. 2 Adenosine and its analogues enhanced, in a concentration-dependent manner, the inhibitory action of tetrodotoxin (TTX) on nerve conduction. The order of potencies was: CHA ⋝ D-PIA ⋝ L-PIA ≫ N6-methyladenosine ⋝ 2-chloroadenosine ≫ adenosine. 3 The adenosine metabolites, inosine and hypoxanthine, were inactive on TTX-induced axonal blockade. Adenine enhanced the inhibitory action of TTX on nerve conduction, but was less effective than adenosine. 4 db Cyclic AMP, but not cyclic AMP, mimicked the inhibitory effect of adenosine on nerve conduction. 5 Methylxanthines did not antagonize the effect of adenosine on TTX-induced axonal block and in high concentrations also mimicked the effect of adenosine on nerve conduction. 6 The possibility of adenosine acting on TTX-induced axonal block through an adenosine receptor positively coupled to adenylate cyclase is discussed.