Block by apomorphine of acetylcholine receptor channels expressed in Xenopus oocytes

Block by phytoestrogens of recombinant human neuronal nicotinic receptors

The effects of phytoestrogens on neuronal nicotinic acetylcholine receptor/channels were examined by expressing recombinant channels in Xenopus oocytes. When functional channels were expressed with human alpha4 and beta2 subunits, daidzein (10 and 100 microM) partially inhibited the ionic current activated by acetylcholine. The current inhibition was also observed when functional channels were expressed with human alpha3 and beta4 subunits or rat homologues. Genistin (100 microM) also inhibited the acetylcholine-activated current. Tamoxifen (100 microM), an antiestrogen did not antagonize the inhibition by daidzein. The results suggest that phytoestrogens, like estrogens and xenoestrogens, block human neuronal acetylcholine receptors through non-genomic mechanisms.

Modulation by estrogens and xenoestrogens of recombinant human neuronal nicotinic receptors

The effects of estrogens and xenoestrogens on human neuronal nicotinic acetylcholine receptor/channels were examined by expressing recombinant channels in Xenopus oocytes. When functional channels were expressed with alpha3 and beta4 subunits, estrogens (17beta-estradiol, 17alpha-estradiol, 17alpha-ethynylestradiol and diethylstilbestrol) and xenoestrogens (bisphenol A, p-nonylphenol and p-octylphenol) inhibited an ionic current activated by acetylcholine at concentrations up to 100 microM. When the subunit combination was changed to alpha4beta2, diethystilbestrol and the xenoestrogens inhibited the acetylcholine-activated current, but 17beta-estradiol or 17alpha-estradiol did not. For 17alpha-ethynylestradiol, the current through the alpha4beta2 receptor/channel was inhibited at 1 microM, but it was markedly enhanced at 10 and 100 microM. Tamoxifen (10 microM), an antiestrogen, itself inhibited the acetylcholine-activated current but did not antagonize the current modulations induced by the estrogens and the xenoestrogens. These and additional results suggest that human neuronal nicotinic acetylcholine receptors are the targets of non-genomic actions of estrogens and xenoestrogens.

Block by 5-hydroxytryptamine and apomorphine of recombinant human neuronal nicotinic receptors

The effects of 5-hydroxytryptamine and apomorphine on human neuronal nicotinic acetylcholine receptor/channels were examined by expressing these channels in Xenopus oocytes. Functional channels were expressed by combining one type of alpha subunits (alpha3 or alpha4) and one type of beta subunits (beta2 or beta4). 5-Hydroxytryptamine (100 microM to 1 mM) and apomorphine (10 to 100 microM) inhibited an inward current activated by acetylcholine in the oocytes expressing the channels. The sensitivity to 5-hydroxytryptamine or apomorphine depended on subunit combinations. When concentration-response relationship was obtained for the acetylcholine-activated current, the maximal response was reduced by these compounds. The inhibition by these compounds exhibited voltage-dependence: the inhibition was augmented at negative potentials. The results suggest that 5-hydroxytryptamine and apomorphine noncompetitively inhibits human recombinant nicotinic acetylcholine receptor/channels, presumably by acting on channel pores.

An asparagine residue regulating conductance through P2X2 receptor/channels

Single channel currents were recorded from Xenopus oocytes expressing wild-type and mutated P2X2 receptors. When 100 mM Na+ was used as the permeant cation, unitary currents of about 80 pS were recorded from the oocyte expressing the wild-type channels. The single channel conductance was roughly halved when Asn333 was replaced by Ile (N333I). A similar decrease in single channel currents was also observed when 100 mM Li+ or Cs+ was used as the permeant cation. With two other mutants, in which Asp315 was replaced by Val (D315V) or Tyr330 was replaced by lie (T333I), single channel conductance was almost the same as that of the wild-type channels. The results suggest that Asn333, which is believed to be involved in the channel pore, plays an essential role in ion transport through P2X2 receptor/channels.

An aspartic acid residue near the second transmembrane segment of ATP receptor/channel regulates agonist sensitivity

Charged or polarized amino acid residues near or within the second transmembrane (M2) segment of neuronal ATP receptor/channels (P2X2 receptors) were neutralized by site-directed mutagenesis, and the properties of the mutants were electrophysiologically characterized using Xenopus oocytes. When Asp315 was substituted with Val (D315V), the sensitivity to ATP was reduced by about 60-fold. The sensitivity to ATP was not affected by the neutralization of Lys324, which is involved in a Walker type A ATP-binding sequence, Lys366, Tyr330, or Asn333. With D315V channels, the sensitivities to other agonists (ADP, ATP gamma S, and 2-methylthio ATP) were also reduced. The sensitivities to antagonists (suramin and Cibacron Blue F3GA) were, however, not affected by this neutralization. The results suggest that Asp315, which is assumed to be present in the extracellular region near the M2 segment of P2X2 receptor/channels, serves to maintain agonist sensitivity.

pH dependence of facilitation by neurotransmitters and divalent cations of P2X2 purinoceptor/channels

The pH dependence of the facilitation by dopamine (10 microM), 5-hydroxytryptamine (10 microM), adenosine (1 and 100 microM), Zn2+ (10 microM) and Cd2+ (1 mM) of P2X2 purinoceptor/channels was tested by expressing these channels in Xenopus oocytes. In a pH range between 6.0 and 8.5, concentration-response curves for an inward current activated by ATP were shifted toward a lower concentration range at a more acidic pH, indicating that the sensitivity to ATP is pH-dependent. Comparison of the effects of the neurotransmitters and the divalent cations on the ATP-activated current was made using a concentration of ATP which activated 40-50% of the maximal current at each pH value. The current facilitation by dopamine was obvious at pH 7.1 and 7.7, but was not observed at pH 8.5. At pH 6.0, the current was inhibited upon first trials of dopamine, but it was facilitated upon second trials. With 5-hydroxytryptamine and adenosine, the current facilitation was most remarkable at pH 6.0, less remarkable at pH 7.1 and 7.7, and the facilitation was almost abolished at pH 8.5. On the other hand, the current facilitation by Zn2+ and Cd2+ was more remarkable at alkaline pH values (7.7 and 8.5), and the facilitation was almost abolished at pH 6.0. The results suggest that the facilitation of P2X2 purinoceptors depends on pH, and the pH dependence was different between the neurotransmitters and the divalent cations.

Potent inhibition by trivalent cations of ATP-gated channels

The effects of La3+ and other trivalent cations on ATP-gated channels (P2X purinoceptor/channels) were investigated using rat pheochromocytoma PC12 cells and Xenopus oocytes expressing these channels. La3+, Gd3+, Ce3+ and Nd3+ (30-300 microM) inhibited an inward current activated by 30 microM ATP in PC12 cells. The concentration-response curve for the ATP-activated current was shifted by La3+ or Gd3+ toward a higher concentration range, and the slope of the curve became steeper, suggesting the inhibition is non-competitive. La3+ or Gd3+ did not affect the current component that was slowly activated upon hyperpolarization, and selectively inhibited the remaining 'voltage-independent' component. La3+ and Gd3+ also inhibited currents mediated through P2X1 and P2X2 purinoceptors expressed in Xenopus oocytes. The results suggest that La3+ and other trivalent cations inhibit P2X purinoceptors at low concentrations. The inhibition may at least partly be attributed to an allosteric inhibition.

Effects of neuroamines and divalent cations on cloned and mutated ATP-gated channels

Sensitivities to dopamine, 5-hydroxytryptamine, Zn2+ and Cd2+ were studied in P2X1, P2X2, P2X3 and P2X4 purinoceptors and mutants of P2X2 purinoceptors expressed in Xenopus oocytes. Dopamine (10 and 100 microM) and 5-hydroxytryptamine (1 to 100 microM) enhanced the inward current activated by extracellular ATP through P2X2 and P2X4 purinoceptors. Zn2+ (1 to 100 microM) and Cd2+ (10 microM to 1 microM) enhanced the current through P2X2 purinoceptors. As for P2X4 purinoceptors, the ATP-activated current was, however, enhanced after the washout of Zn2+ (100 microM) or Cd2+ (1 mM). Three mutants of P2X2 purinoceptors were constructed by substituting negatively charged amino-acid residues. The magnitude of the enhancement by Zn2+, Cd2+ and dopamine was attenuated when Asp221 was replaced by histidine. The results suggest that dopamine, Zn2+ and Cd2+ require some common motif for the current enhancement.

Dopamine and 5-hydroxytryptamine selectively potentiate neuronal type ATP receptor channels

Dopamine and 5-hydroxytryptamine have been shown to facilitate a cationic current activated by extracellular ATP in rat pheochromocytoma PC12 cells. Effects of these and other modulators were examined by expressing ATP receptor channels in Xenopus oocytes using cDNAs from rat vas deferens ('P2x1-purinoceptor channels') and PC12 cells ('P2x2-purinoceptor channels'). Dopamine and 5-hydroxytryptamine (10 and 100 mu M) facilitated the ATP-activated current mediated through P2x2-purinoceptor channels, but not the current through P2x1-purinoceptor channels. Adenosine (1 mu M) facilitated the current through both P2x1- and P2x2-purinoceptor channels. Cd2+ (1 mM) as well as Zn2+ (10 mu M) selectively potentiated the current through P2x2-purinoceptor channels. The results suggest that (1) the facilitation by dopamine and other modulators also occurs in recombinant ATP-receptor channels, and (2) the selective facilitation by dopamine, 5-hydroxytryptamine and divalent cations of P2x2-purinoceptor channels is attributed to some structural difference of the channels from P2x1-purinoceptor channels.

Reduction of acetylcholine-activated current by low concentrations of extracellular adenosine 5'-triphosphate

Effects of adenosine 5'-triphosphate (ATP) on ionic currents activated by acetylcholine (ACh) were investigated using rat pheochromocytoma PC12 cells and Xenopus oocytes expressing nicotinic receptors. In PC12 cells, ATP (10 nM to 1 microM) inhibited an inward current activated by ACh in not all but about 60% of cell batches. The ACh-activated current was also inhibited by ATP in Xenopus oocytes and, with a subunit combination of alpha 3 plus beta 4, the inhibition was observed at concentration as low as 100 fM. Uridine 5'-triphosphate (UTP) induced a similar inhibition of the ACh-activated current both in PC12 cells and Xenopus oocytes. These and other properties suggest that the current reduction by ATP is not mediated through conventional P2-purinoceptors.

Block by 5-hydroxytryptamine of neuronal acetylcholine receptor channels expressed in Xenopus oocytes

1. Effects of 5-hydroxytryptamine (5-HT) on neuronal nicotinic acetylcholine (ACh) receptor channels were investigated by expressing cloned channel subunits in Xenopus oocytes. 2. When channels were expressed with a combination of alpha 3 and beta 4 subunits, 5-HT (10 to 300 microM) reversibly inhibited an inward current activated by 100 microM ACh in a concentration-dependent manner. The inhibition was also observed when alpha 3 subunit was combined with beta 2 subunit instead of beta 4 subunit, or beta 4 subunit was combined with alpha 2 or alpha 4-1 subunit instead of alpha 3 subunit to express channels. 3. Compounds known to antagonize at 5-HT receptors (LY53857, metoclopramide and propranolol) exhibited an agonistic effect: they inhibited the ACh-activated current. 4. The results suggest that 5-HT inhibits recombinant neuronal nicotinic receptor channels through a binding-site distinct from conventional 5-HT receptors. The binding-site may not be attributed to a unique type of channel subunits.