Potentiation by cadmium ion of ATP-evoked dopamine release in rat phaeochromocytoma cells

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Inhibition of transmitter release from rat sympathetic neurons via presynaptic M(1) muscarinic acetylcholine receptors

BACKGROUND AND PURPOSE

M(2), M(3) and/or M(4) muscarinic acetylcholine receptors have been reported to mediate presynaptic inhibition in sympathetic neurons. M(1) receptors mediate an inhibition of K(v)7, Ca(V)1 and Ca(V)2.2 channels. These effects cause increases and decreases in transmitter release, respectively, but presynaptic M(1) receptors are generally considered facilitatory. Here, we searched for inhibitory presynaptic M(1) receptors.

EXPERIMENTAL APPROACH

In primary cultures of rat superior cervical ganglion neurons, Ca(2+) currents were recorded via the perforated patch-clamp technique, and the release of [(3)H]-noradrenaline was determined.

KEY RESULTS

The muscarinic agonist oxotremorine M (OxoM) transiently enhanced (3)H outflow and reduced electrically evoked release, once the stimulant effect had faded. The stimulant effect was enhanced by pertussis toxin (PTX) and was abolished by blocking M(1) receptors, by opening K(v)7 channels and by preventing action potential propagation. The inhibitory effect was not altered by preventing action potentials or by opening K(v)7 channels, but was reduced by PTX and omega-conotoxin GVIA. The inhibition remaining after PTX treatment was abolished by blockage of M(1) receptors or inhibition of phospholipase C. When [(3)H]-noradrenaline release was triggered independently of voltage-activated Ca(2+) channels (VACCs), OxoM failed to cause any inhibition. The inhibition of Ca(2+) currents by OxoM was also reduced by omega-conotoxin and PTX and was abolished by M(1) antagonism in PTX-treated neurons.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that M(1), in addition to M(2), M(3) and M(4), receptors mediate presynaptic inhibition in sympathetic neurons using phospholipase C to close VACCs.

Dehydroepiandrosterone sulfate (DHEAS) suppresses P2X purinoceptor-coupled responses in PC12 cells

Some steroids rapidly alter neuronal excitability through interaction with neurotransmitter-gated ion channels in addition to their well-known genomic effects via intracellular steroid receptors. Such effects were found in GABA receptor, nicotinic receptors, yet not investigated in P2X purinoceptors. In this study, the effects of dehydroepiandrosterone sulfate on the P2 purinoceptor was investigated. Results show that dehydroepiandrosterone sulfate acutely inhibits P2X purinoceptor functions in PC12 cells. Dehydroepiandrosterone sulfate suppressed ATP-induced cytosolic free calcium concentration ([Ca(2+)](i)) rise, cytosolic free sodium concentration ([Na(+)](i)) rise, and dopamine secretion in the presence of external calcium, but had no effect on ATP-induced [Ca(2+)](i) rise in the absence of external calcium or on UTP-induced [Ca(2+)](i) rise in the absence or presence of external calcium. Our data show that dehydroepiandrosterone sulfate exerted its effect on P2X, but not on the P2Y purinoceptors found in PC12 cells. Estradiol and estrone have similar effects on P2X purinoceptor, but dehydroepiandrosterone and progesterone do not.

Antagonists of P2 receptor prevent NGF-dependent neuritogenesis in PC12 cells

The pheochromocytoma PC12 cell line that develops neuronal characteristics of sympathetic cells after treatment with nerve growth factor (NGF) represents a well-established cellular model system for studying NGF signalling. Interesting information on the different mechanistic pathways of NGF can be obtained by adopting the pharmacological approach of inhibiting P2 receptors, expressed in naive PC12 cells and recognised as important biological mediators of neurotransmitters and growth factors. We show here that Basilen Blue, an antagonist of P2 receptor, reversibly prevents NGF-dependent neurite outgrowth with an IC(50) in the 5-10 microM range. Suramin, oxidised-ATP and diisothiocyanatostilbene-disulfonic acid, differently from other purinoceptor ligands, are also effective in this regard. NGF-dependent regeneration and stability of neurites, selected NGF-dependent extracellular and intracellular protein phosphorylations, binding of [(3)H] ATP to PC12 cell membranes are also modulated by Basilen Blue. On the contrary, cell adhesion, cellular duplication, 5'-nucleotidase activity, NGF-induced tyrosine autophosphorylation of TrkA receptors are not affected. NGF furthermore directly modulates the extracellular release of ATP and especially the levels of P2X(2) receptor protein in PC12 cells. In addition, extracellular ATP improves the neuritogenic effect of sub-optimal concentrations of NGF. Our study identifies P2 receptor ligands, particularly Basilen Blue, as useful tools to dissect different NGF-evoked functions, suggesting a mechanistic role for P2 receptors in the signalling pathways of NGF.

Increase of striatal dopamine release by cadmium in nursing rats and its prevention by dexamethasone-induced metallothionein

Repeated daily intraperitoneal (i.p.) administrations of cadmium (CdCl2, 1 mg/kg per day for 5 days) increased striatal dopamine (DA) release (180% of controls) and turnover (150% of controls) in 13-day-old rats. Cd treatment also increased striatal metallothionein (MT) content (161%), Cd (127%) and lipid peroxidation (LPO, 190%). In addition, Cd treatment decreased striatal tyrosine hydroxylase (TH) activity (-28%), and such an effect may result from D-2 receptor blockade as a consequence of excessive dopamine release, since sulpiride (a specific D-2 receptor antagonist) administration to Cd-treated rats abolished the effect of Cd on TH. No effect was observed on striatal monoamine oxidase (MAO) activity. Dexamethasone (Dx) treatment increased striatal MT content and caused no effect on either DA release or turnover. However, Dx administration prevented the effects caused by Cd, including the increased DA release and enhanced striatal lipid peroxidation. These results indicate that toxic effects on the brain are to be expected as a result of Cd exposure and that Dx administration can attenuate them.

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.