Role of protein kinase A in the maintenance of inflammatory pain.
Although the initiation of inflammatory pain (hyperalgesia) has been demonstrated to require the cAMP second messenger signaling cascade, whether this mechanism and/or other mechanisms underlie the continued maintenance of the induced hyperalgesia is unknown. We report that injection of adenylyl cyclase inhibitors before but not after injection of direct-acting hyperalgesic agents (prostaglandin E2 and purine and serotonin receptor agonists) resulted in reduction in hyperalgesia, evaluated by the Randall-Selitto paw-withdrawal test. In contrast, injection of protein kinase A (PKA) inhibitors either before or after these hyperalgesic agents resulted in reduced hyperalgesia, suggesting that hyperalgesia after its activation was maintained by persistent PKA activity but not by adenylyl cyclase activity. To evaluate further the role of PKA activity in the maintenance of hyperalgesia, we injected the catalytic subunit of PKA (PKACS) that resulted in hyperalgesia similar in magnitude to that induced by the direct-acting hyperalgesic agents but much longer in duration (>48 vs 2 hr). Injection of WIPTIDE (a PKA inhibitor) at 24 hr after PKACS reduced hyperalgesia, suggesting that PKACS hyperalgesia is not independently maintained by steps downstream from PKA. In summary, our results indicate that, once established, inflammatory mediator-induced hyperalgesia is no longer maintained by adenylyl cyclase activity but rather is dependent on ongoing PKA activity. An understanding of the mechanism maintaining hyperalgesia may provide important insight into targets for the treatment of persistent pain. (+info
Inhibition by adenosine receptor agonists of synaptic transmission in rat periaqueductal grey neurons.
1. The actions of selective adenosine A1 and A2 receptor agonists were examined on synaptic currents in periaqueductal grey (PAG) neurons using patch-clamp recordings in brain slices. 2. The A1 receptor agonist 2-chloro-N-cyclopentyladenosine (CCPA), but not the A2 agonist, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS21680), inhibited both electrically evoked inhibitory (eIPSCs) and excitatory (eEPSCs) postsynaptic currents. The actions of CCPA were reversed by the A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). 3. In the absence or presence of forskolin, DPCPX had no effect on eIPSCs, suggesting that concentrations of tonically released adenosine are not sufficient to inhibit synaptic transmission in the PAG. 4. CCPA decreased the frequency of spontaneous miniature action potential-independent IPSCs (mIPSCs) but had no effect on their amplitude distributions. Inhibition persisted in nominally Ca2+-free, high Mg2+ solutions and in 4-aminopyridine. 5. The CCPA-induced decrease in mIPSC frequency was partially blocked by the non-selective protein kinase inhibitor staurosporine, the specific protein kinase A inhibitor 8-para-chlorophenylthioadenosine-3',5'-cyclic monophosphorothioate (Rp-8-CPT-cAMPS), and by 8-bromoadenosine cyclic 3',5' monophosphate (8-Br-cAMP). 6. These results suggest that A1 adenosine receptor agonists inhibit both GABAergic and glutamatergic synaptic transmission in the PAG. Inhibition of GABAergic transmission is mediated by presynaptic mechanisms that partly involve protein kinase A. (+info
Role of K+ channels in A2A adenosine receptor-mediated dilation of the pressurized renal arcuate artery.
1. Adenosine A2A receptor-mediated renal vasodilation was investigated by measuring the lumenal diameter of pressurized renal arcuate arteries isolated from the rabbit. 2. The selective A2A receptor agonist CGS21680 dilated the arteries with an EC50 of 130 nM. The CGS21680-induced vasodilation was, on average, 34% less in endothelium-denuded arteries. 3. The maximum response and the EC50 for CGS21680-induced vasodilation in endothelium-intact arteries were not significantly affected by incubation with the K+ channel blockers apamin (100 nM), iberiotoxin (100 nM), 3,4-diaminopyridine (1 mM), glibenclamide (1 microM) or Ba2+ (10 microM). However, a cocktail mixture of these blockers did significantly inhibit the maximum response by almost 40%, and 1 mM Ba2+ alone or 1 mM Ba2+ in addition to the cocktail inhibited the maximum CGS21680-response by 58% and about 75% respectively. 4. CGS21680-induced vasodilation was strongly inhibited when the extracellular K+ level was raised to 20 mM even though the dilator response to 1 microM levcromakalim, a K(ATP) channel opener drug, was unaffected. 5. CGS21680-induced vasodilation was inhibited by 10 microM ouabain, an inhibitor of Na+/K(+)-ATPase, but ouabain had a similar inhibitory effect on vasodilation induced by 30 nM nicardipine (a dihydropyridine Ca2+ antagonist) or 1 microM levcromakalim. 6. The data suggest that K+ channel activation does play a role in A(2A) receptor-mediated renal vasodilation. The inhibitory effect of raised extracellular K+ levels on the A(2A) response may be due to K(+)-induced stimulation of Na+/K(+)-ATPase. (+info
Adenosine induces histamine release from human bronchoalveolar lavage mast cells.
Previous studies have shown that in vitro adenosine enhances histamine release from activated human lung mast cells obtained by enzymic dispersion of lung parenchyma. However, adenosine alone has no effect on histamine release from these cells. Given the evidence for direct activation of mast cells after endobronchial challenge with adenosine and previous studies indicating that mast cells obtained at bronchoalveolar lavage are a better model for asthma studies than those obtained by enzymic dispersion of lung tissue, the histamine-releasing effect of adenosine was examined on lavage mast cells. Bronchoalveolar lavage fluid was obtained from patients attending hospital for routine bronchoscopy (n=54). Lavage cells were challenged with adenosine or adenosine receptor agonists (20 min, 37 degrees C) and histamine release determined using an automated fluorometric assay. Endogenous adenosine levels were also measured in lavage fluid (n=9) via an HPLC method. Adenosine alone caused histamine release from lavage mast cells in 37 of 54 patients with a maximal histamine release of 20.56+/-2.52% (range 5.2-61%). The adenosine receptor agonists (R)-N6-(2-phenylisopropyl)adenosine, 5'-N-ethylcarboxamidoadenosine and CGS21680 also induced histamine release from lavage mast cells. Preincubation of lavage mast cells with the adenosine receptor antagonist xanthine amine congener caused significant inhibition of the response to adenosine (P=0.007). There was an inverse correlation between endogenous adenosine levels in the lavage fluid and the maximal response to in vitro adenosine challenge of the lavage cells. The findings of the present study indicate a means by which adenosine challenge of the airways can induce bronchoconstriction and support a role for adenosine in the pathophysiology of asthma. The results also suggest that cells obtained from bronchoalveolar lavage fluid may provide the ideal model for the testing of novel, adenosine receptor, targeted therapies for asthma. (+info
Cerebral cortical blood flow maps are reorganized in MAOB-deficient mice.
Cerebral cortical blood flow (CBF) was measured autoradiographically in conscious mice without the monoamine oxidase B (MAOB) gene (KO, n=11) and the corresponding wild-type animals (WILD, n=11). Subgroups of animals of each genotype received a continuous intravenous infusion over 30 min of phenylethylamine (PEA), an endogenous substrate of MAOB, (8 nmol g-1 min-1 in normal saline at a volume rate of 0.11 microl g-1 min-1) or saline at the same volume rate. Maps of relative CBF distribution showed predominance of midline motor and sensory area CBF in KO mice over WILD mice that received saline. PEA enhanced CBF in lateral frontal and piriform cortex in both KO and WILD mice. These changes may reflect a differential activation due to chronic and acute PEA elevations on motor and olfactory function, as well as on the anxiogenic effects of this amine. In addition to its effects on regional CBF distribution, PEA decreased CBF globally in KO mice (range -31% to -41% decrease from control levels) with a lesser effect in WILD mice. It is concluded that MAOB may normally regulate CBF distribution and its response to blood PEA. (+info
Kinetics of rate-dependent shortening of action potential duration in guinea-pig ventricle; effects of IK1 and IKr blockade.
1. The kinetics of shortening of action potential duration (APD) following an increase in pacing rate, from 2 to 3.3 Hz, was characterized in guinea-pig ventricular preparations. Terikalant (RP62719), an inhibitor of the inwardly rectifying K+ current (IK1), and dofetilide, a specific inhibitor of the rapidly activating delayed-rectifier current (IKr), were applied to determine the effect of inhibition of these ion currents on slow APD shortening. 2. Action potentials were recorded from isolated guinea-pig ventricular myocytes using the perforated-patch patch-clamp technique, and monophasic action potentials were recorded from Langendorff-perfused guinea-pig ventricles using a contact epicardial probe. 3. Under control conditions, after an increase in pacing rate, APD immediately decreased, and then shortened slowly with an exponential time course. In ventricular myocytes, the time constant of this exponential shortening was 28+/-4 s and the amount of slow shortening was 21.9+/-0.9 ms (n=8) for an increase in rate from 2 to 3.3 Hz. Similar values were observed in Langendorff-perfused ventricles. 4. Terikalant dose-dependently increased APD and the increase was enhanced by rapid pacing ('positive' rate-dependence). The drug dose-dependently decreased the time constant of shortening and amount of slow APD shortening. In contrast, dofetilide, an inhibitor of IKr, which shows 'reverse' rate-dependent APD widening, had no significant effect on the time constant or amount of slow shortening. 5. These observations suggest that IK1 plays a role in rate-dependent shortening of APD. The results appear to support the hypothesis that 'reverse' rate-dependent effects of IKr blockers are due to these drugs not affecting the ion current(s) mediating intrinsic rate-dependent slow shortening of APD. (+info
Thermogenic effects of sibutramine and its metabolites.
1. The thermogenic activity of the serotonin and noradrenaline reuptake inhibitor sibutramine (BTS 54524; Reductil) was investigated by measuring oxygen consumption (VO2) in rats treated with sibutramine or its two pharmacologically-active metabolites. 2. Sibutramine caused a dose-dependent rise in VO2, with a dose of 10 mg kg(-1) of sibutramine or its metabolites producing increases of up to 30% that were sustained for at least 6 h, and accompanied by significant increases (0.5-1.0 degrees C) in body temperature. 3. Based on the accumulation in vivo of radiolabelled 2-deoxy-[3H]-glucose, sibutramine had little or no effect on glucose utilization in most tissues, but caused an 18 fold increase in brown adipose tissue (BAT). 4. Combined high, non-selective doses (20 mg kg(-1)) of the beta-adrenoceptor antagonists, atenolol and ICI 118551, inhibited completely the VO2 response to sibutramine, but the response was unaffected by low, beta1-adrenoceptor-selective (atenolol) or beta2-adrenoceptor-selective (ICI 118551) doses (1 mg kg(-1)). 5. The ganglionic blocking agent, chlorisondamine (15 mg kg(-1)), inhibited completely the VO2 response to the metabolites of sibutramine, but had no effect on the thermogenic response to the beta3-adrenoceptor-selective agonist BRL 35135. 6. Similar thermogenic responses were produced by simultaneous injection of nisoxetine and fluoxetine at doses (30 mg kg(-1)) that had no effect on VO2 when injected individually. 7. It is concluded that stimulation of thermogenesis by sibutramine requires central reuptake inhibition of both serotonin and noradrenaline, resulting in increased efferent sympathetic activation of BAT thermogenesis via beta3-adrenoceptor, and that this contributes to the compound's activity as an anti-obesity agent. (+info
Effects of ractopamine HCl stereoisomers on growth, nitrogen retention, and carcass composition in rats.
The objectives of this study were to determine the effects of ractopamine HCl (RAC) stereoisomers (RR, RS, SR, and SS) on performance, carcass composition, and nitrogen retention in growing female rats. Forty-eight rats (eight rats/treatment) were treated with 0 or 320 microg/d of RAC or with 80 microg/d of the RR, RS, SR, or SS stereoisomers of ractopamine. Rats had free access to feed and water before and during the experiment. Ractopamine and stereoisomers were delivered via i.p. implanted osmotic pumps for 14 d, and rats were then slaughtered. Control rats were fitted with osmotic pumps containing saline. Ractopamine increased (P < .05) feed intake (d 1 to 6); body weight; carcass CP; and intake, apparent absorption, retention, and retained:intake ratio of CP on d 1 to 6 of the study. Ractopamine decreased (P < .05) carcass lipid and visceral lipid. Rats dosed with the RR stereoisomer responded similarly to rats dosed with RAC, except for carcass lipid. Carcass lipid was decreased (P < .01) by RAC relative to controls, but it was not different from controls in rats treated with the RR isomer. Compared with controls, BW, carcass CP, and CP retention were increased by the RR stereoisomer, and visceral lipid was decreased. The RS isomer also decreased visceral lipid (P < .10), but variables measured in rats dosed with the RS, SR, and SS isomers generally did not differ from controls. Results of this study indicate that the RR isomer of RAC is responsible for a majority of the leanness-enhancing effects of RAC in rats. (+info