Metabotropic P2Y receptors inhibit P2X3 receptor-channels via G protein-dependent facilitation of their desensitization

BACKGROUND AND PURPOSE

The aim of the present study was to investigate whether the endogenous metabotropic P2Y receptors modulate ionotropic P2X(3) receptor-channels.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp experiments were carried out on HEK293 cells permanently transfected with human P2X(3) receptors (HEK293-hP2X(3) cells) and rat dorsal root ganglion (DRG) neurons.

KEY RESULTS

In both cell types, the P2Y(1,12,13) receptor agonist, ADP-beta-S, inhibited P2X(3) currents evoked by the selective agonist, alpha,beta-methylene ATP (alpha,beta-meATP). This inhibition could be markedly counteracted by replacing in the pipette solution the usual GTP with GDP-beta-S, a procedure known to block all G protein heterotrimers. P2X(3) currents evoked by ATP, activating both P2Y and P2X receptors, caused a smaller peak amplitude and desensitized faster than those currents evoked by the selective P2X(3) receptor agonist alpha,beta-meATP. In the presence of intracellular GDP-beta-S, ATP- and alpha,beta-meATP-induced currents were identical. Recovery from P2X(3) receptor desensitization induced by repetitive ATP application was slower than the recovery from alpha,beta-meATP-induced desensitization. When G proteins were blocked by intracellular GDP-beta-S, the recovery from the ATP- and alpha,beta-meATP-induced desensitization were of comparable speed.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that the activation of P2Y receptors G protein-dependently facilitates the desensitization of P2X(3) receptors and suppresses the recovery from the desensitized state. Hence, the concomitant stimulation of P2X(3) and P2Y receptors of DRG neurons by ATP may result both in an algesic effect and a partly counterbalancing analgesic activity.

Distinct mechanisms underlying alpha1-adrenoceptor and P2x purinoceptor operated ATP release and contraction in the guinea-pig vas deferens

The temperature-dependence of ATP release and contraction response evoked by different agonists were investigated in superfused guinea-pig vas deferens. Alpha-adrenoceptor agonists, i.e. noradrenaline (300 microM), and alpha-methyl-noradrenaline (300 microM), increased the basal ATP outflow, measured by the luciferin-luciferase assay, and induced biphasic contractile response. Cooling the bath temperature to 12 degrees C almost completely inhibited ATP release and twitch contraction evoked by alpha-adrenoceptor agonists, whereas the phasic contraction remained unaffected. In contrast, twitch contraction and subsequent ATP release induced by beta,gamma-methylene-ATP, a selective P2 receptor agonist (100 microM), was not reduced by low temperature. The ectoATPase activity, measured by HPLC technique was not significantly different at 37 degrees C and 12 degrees C. Nifedipine (1 microM), the voltage sensitive Ca2+ channel blocker eliminated beta,gamma-methylene-ATP evoked twitch contraction but not ATP release. In conclusion, alpha-adrenoceptor and P2 receptor agonists utilize distinct mechanisms to elicit ATP release and contraction: alpha-adrenoceptor-mediated ATP release and contraction is temperature-dependent, indicating the involvement of a carrier-mediated process in it, whereas P2x purinoceptor evoked ATP release and twitch is mediated by a different mechanism.

Analysis of high intracellular [Na+]-induced release of [3H]noradrenaline in rat hippocampal slices

Our aim was to investigate the mechanisms involved in the high intracellular sodium-induced transmitter release in the CNS through the characterisation of the veratridine-evoked (40 microM) noradrenaline release from rat hippocampal slices. The response to veratridine was completely inhibited by tetrodotoxin (1 microM), indicating that the effect is due to the activation of sodium channels. Omission of Ca2+ from the superfusion fluid inhibited the veratridine-evoked release by 72%, showing that the majority of release results from external Ca2+-dependent exocytosis. The residual Ca2+-independent release was not blocked by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (100 microM) suggesting that intracellular Ca2+ stores are not involved in this component of veratridine effect. The noradrenaline uptake blockers, desipramine (10 microM) and nisoxetine (10 microM), inhibited the external Ca2+-independent release by 50 and 46%, respectively, indicating that the release partly originates from the reversal of transporters (carrier-mediated release). In contrast to uptake blockers, lowering the temperature, another possibility to inhibit transporter function, completely inhibited the effect of veratridine in the absence of Ca2+. Further experiments revealed that low temperature (20 and 12 degrees C) reduces the veratridine-induced increase of intracellular sodium concentration ([Na+]i) in rat cortical synaptosomes (68 and 78% inhibition, respectively). The clinical relevance of our data is that during ischemia a massive release of transmitters occurs mainly due to the elevation of [Na+]i, which contributes to the development of ischemic brain injury. Our results show that low temperature may be a better therapeutic approach to the treatment of ischemia because it has a dual action on this process. Firstly, it inhibits the function of uptake transporters and hence reduces the carrier-mediated outflow of transmitters. Secondly, it inhibits the sodium influx and therefore prevents the unwanted elevation of [Na+]i. Our data also suggest that veratridine stimulation can be a suitable model for ischemic conditions.

Excessive release of [3H] noradrenaline by veratridine and ischemia in spinal cord

In this study, the properties of ischemic condition-induced and veratridine-evoked [3H]noradrenaline ([3H]NA) release from rat spinal cord slices were compared. It was expected that ischemia mimicked by oxygen and glucose deprivation results in the impairment of Na+/K+ -ATPase with a consequent elevation of the intracellular Na+ -level which reverses the NA carrier and promotes excessive NA release, and veratridine, by the activation of Na+ channels, releases NA both carrier-mediated and Ca2+ -dependent, i.e. vesicular manner. In our experiments, veratridine (1-100 microM) dose-dependently increased the resting [3H]NA release, and its effect was only partially blocked by low temperature or the lack of external calcium, whereas the sodium channel inhibitor tetrodotoxin (TTX, 1 microM) completely prevented it, indicating that veratridine induces NA release via axonal depolarization and reversing the transporters by eliciting Na+ -influx. In contrast to TTX, the local anesthetic lidocaine (100 microM) only partially blocked the veratridine-induced [3H]NA release due to its inhibitory action on K+ channels. The ischemia-induced [3H]NA release was abolished at 12 degrees C, a temperature known to block only the transporter-mediated release of transmitters. However, lidocaine was also partially effective to reverse the action of ischemia on the NA release, indicating that lidocaine is not a useful compound in the treatment of spinal cord-injured patients against the excessive excytotoxic NA release.

Low temperature prevents potentiation of norepinephrine release by phenylephrine

The effect of 1-phenylephrine (1-PE), an alpha(1)-receptor agonist, was investigated on the release of tritiated norepinephrine ([3H]NE). Pairs of guinea pig vasa deferentia were loaded with [3H]NE, superfused continuously, and stimulated electrically. 1-PE (10, 100 microM) enhanced the basal release of tritium in concentration-dependent manner. The stimulation-evoked release of radioactivity was significantly increased by 100 microM 1-PE. Both basal and stimulation-evoked release by 1-PE were reduced by desipramine (10 microM), a monoamine uptake inhibitor. The effect of 1-PE on basal release was independent on extracellular Ca(2+) concentration ([Ca(2+)](o)) and alpha(1)-adrenoceptor blockade. However, the 1-PE-induced release was temperature dependent: at low temperature 1-PE failed to increase either basal or stimulation-evoked release of NE. Using three different temperatures (7, 12, 17 degrees C, respectively), it was found that basal release was blocked at all three temperature values but the stimulation-evoked release was inhibited only at the lower values. The effect of 1-PE on the NE release appears to involve a desipramine-, and temperature-sensitive process. These results suggest that a non-receptorial and direct carrier-mediated mechanism is involved in NE releasing effect of 1-PE.

Effect of bimoclomol (N-[2-hydroxy-3-(1-piperidinyl) propoxy]-3 pyridine-carboximidoyl-chloride) on iminodipropionitrile-induced central effects

Dyskinesia is frequently seen in neurological disorders affecting the basal ganglia. Iminodipropionitrile (IDPN) produces a somewhat similar motor syndrome in rodents, one that is a possible model for dyskinesia. Because in previous studies the compound (N-[2-hydroxy-3-(1-piperidinyl) propoxy]-3 pyridine-carboximidoyl-chloride) (Bimoclomol, BRLP-42) was shown to provide protection against IDPN-induced retinopathy; we investigated the effect of BRLP-42 on IDPN-induced motor changes and on IDPN-induced cerebral amino acid level changes in rats and mice. IDPN had a biphasic effect on motor activity in C57BL/6 mice: it was a depressant for 24 days and a stimulant after 30 days. Bimoclomol inhibited the motor depressant effect and enhanced the stimulatory effect of IDPN in this mouse strain. In BALB/cBy mice and Sprague Dawley rats IDPN produced persistent vertical head movements and changes in the level of glutamic acid in brain. Bimoclomol reduced the effect of IDPN on head movements and blocked the effect on cerebral glutamate; by itself it had no effect on motor activity in either species. Bimoclomol inhibited ischemia-induced [3H]norepinephrine release from rat hippocampal slices. Our findings indicate that Bimoclomol could have a beneficial effect on some dyskinesias, and on drug-induced vertical head movements.