n(6)-cyclohexyladenosine and 8-(3-chlorostyryl)caffeine

We used the patch-clamp technique to examine the effect of adenosine on the basolateral K channels in the thick ascending limb (TAL) of the rat kidney. A 50-pS inwardly rectifying K channel was detected in the basolateral membrane, and the channel activity was decreased by hyperpolarization. Application of adenosine (10 microM) increased the activity of basolateral 50 pS K channels, defined by NP(o), from 0.21 to 0.41. The effect of adenosine on the 50 pS K channels was mimicked by cyclohexyladenosine (CHA), which increased channel activity by a dose-dependent manner. However, inhibition of the A1 adenosine receptor with 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) failed to block the effect of CHA. In contrast, application of 8-(3-chlorostyryl) caffeine (CSC), an A2 adenosine antagonist, abolished the stimulatory effect of CHA. The possibility that the effect of adenosine and adenosine analog on the basolateral 50 pS K channel was the result of activation of the A2 adenosine receptor was also suggested by the observation that application of CGS-21680, a selected A(2A) adenosine receptor agonist, increased the channel activity. Also, inhibition of PKA with N-[2-(methylamino)ethyl]-5-isoquinoline sulfonamide-2HC1 abolished the stimulatory effect of CHA on the basolateral 50 pS K channel. Moreover, addition of the membrane-permeable cAMP analog increases the activity of 50 pS K channels. We conclude that adenosine activates the 50 pS K channel in the basolateral membrane of the TAL and the stimulatory effect is mainly mediated by a PKA-dependent pathway via the A2 adenosine receptor in the TAL.

Topics: Adenosine; Adenosine A2 Receptor Antagonists; Adenylyl Cyclases; Algorithms; Animals; Bucladesine; Caffeine; Dose-Response Relationship, Drug; Enzyme Activation; Female; Kidney; Male; Patch-Clamp Techniques; Phenethylamines; Potassium Channels; Rats; Rats, Sprague-Dawley; Receptors, Adenosine A2; Stimulation, Chemical; Xanthines

Adenosine reduces opioid withdrawal symptoms by activating A(1) adenosine receptors, probably by inhibiting excitatory amino acid release. Since blockade of A(2A) adenosine receptors seems to enhance dopaminergic striatopallidal transmission, we evaluated the role of the purinergic system in the opiate withdrawal syndrome by using two A(1) receptor agonists [ N(6)-cyclohexyladenosine, CHA and 2-chloro- N(6)-cyclopentyladenosine, CCPA], and two A(2A) receptor antagonists (SCH 58261 and 8-(3-chlorostyryl)caffeine, CSC). Male adult rats received increasing doses of morphine sulphate suspended in 5 ml/kg of a sustained release preparation (40-100 mg/kg s.c.) daily for 4 days and 20 h after the last administration, the withdrawal syndrome was evoked by naloxone (5 mg/kg i.p.). Animals were observed for 30 min for signs of opiate withdrawal. Other groups of rats were implanted with concentric probes for microdialysis and dopamine levels were measured in the nucleus accumbens. CHA and CCPA (0.05, 0.1 or 0.5 mg/kg i.p.) significantly reduced "wet-dog" shakes, diarrhoea, teeth chattering, jumping and writhing. SCH 58261 and CSC (0.1, 0.5 or 1 mg/kg i.p.), given 10 min before naloxone, also reduced signs of opiate withdrawal. CHA plus SCH 58261 and CCPA plus CSC greatly enhanced the reduction of withdrawal signs observed with CHA and CCPA or CSC and SCH 58261 alone. In vivo microdialysis showed that naloxone significantly decreased DA release; this effect was prevented by pretreatment with systemic SCH 58261 and CSC, but not with CHA and CCPA. Our results demonstrate that A(1) and A(2A) adenosine receptors mediate the effect induced by adenosine in opiate withdrawal syndrome and suggest that adenosine A(1) agonists and adenosine A(2A) antagonists may be beneficial in the treatment of this syndrome.

Topics: Adenosine; Adenosine A1 Receptor Agonists; Adenosine A2 Receptor Antagonists; Animals; Caffeine; Chromatography, High Pressure Liquid; Dopamine; Injections, Subcutaneous; Male; Microdialysis; Morphine; Pyrimidines; Rats; Rats, Wistar; Receptors, Dopamine; Substance Withdrawal Syndrome; Triazoles

We have examined the potency of several adenosine receptor antagonists at adenosine A1 and A2A receptors using quantitative autoradiography and have compared the results with those of previous studies using the same radioligands in membrane preparations. The agonists [3H]cyclohexyladenosine and [3H]2-[p-(2-carbonylethyl)-phenylethylamino]-5'-N-ethylcarbo xamido adenosine ([3H]CGS 21680) were used as radioligands for the two receptors. The results show that 1,3-dipropyl-8-cyclopentyl xanthine (DPCPX) is almost 1000-fold and 8-chloro-4-cyclohexyl-amino-1-(trifluoromethyl)[1,2,4]triazolo[4,3-a] quinoxaline (CP-68,247) about 300-fold more potent at adenosine A1 receptors in cortex and striatum than at striatal adenosine A2A receptors. Conversely, 5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo [1,5-c]pyrimidine (SCH 58261) is approximately 1000-fold and 4-(2-[7-amino-2-(2-furyl) [1,2,4]-triazolo[2,3-a][1,3,5]triazin-5-yl amino]ethyl)phenol (ZM 241,385) about 400-fold more potent at adenosine A2A than at A1 receptors. Caffeine and its metabolites did not show any selectivity. Other studied antagonists were non-selective or showed a modest (20- to 40-fold) adenosine A2A receptor selectivity. Thus, only a few of the antagonists show such high selectivity that it is not offset by differences in drug distribution and levels of receptor subtype expression.

Topics: Adenosine; Animals; Autoradiography; Binding, Competitive; Brain; Caffeine; In Vitro Techniques; Male; Phenethylamines; Purinergic P1 Receptor Antagonists; Pyrazines; Quinazolines; Rats; Rats, Sprague-Dawley; Receptor, Adenosine A2A; Receptors, Purinergic P1; Theophylline; Triazines; Triazoles; Tritium; Xanthines