The effect of zinc on glycinergic inhibitory postsynaptic currents in rat spinal dorsal horn neurons.

The effect of zinc on glycinergic spontaneous inhibitory postsynaptic currents (IPSCs) was investigated using the whole-cell patch-clamp technique in mechanically dissociated rat spinal dorsal horn neurons. Zinc at a concentration of 10 microM reversibly increased the spontaneous IPSC frequency without changing the current amplitudes, suggesting that zinc increases spontaneous glycine release from presynaptic nerve terminals. At a low concentration of 1 microM, on the other hand, zinc potentiated the amplitude of spontaneous IPSCs but had no effect on the frequency. At a high concentration of 100 microM, zinc increased the spontaneous IPSC frequency while it inhibited the IPSC amplitude. The current evoked by exogenously applied glycine was potentiated and inhibited by low and high concentrations of zinc, respectively. The increase in spontaneous IPSC frequency by 10 microM zinc was inhibited by blocking the voltage-dependent Ca(2+) channels in the presence of both omega-conotoxin-MVIIC and nifedipine. The facilitatory effect of zinc on spontaneous IPSC frequency was also inhibited in the presence of tetrodotoxin. In the slice preparation, 30 microM zinc potentiated the evoked IPSC amplitude and decreased the paired pulse ratio. These results suggest that, in addition to an action on the postsynaptic glycine receptors, zinc may depolarize the presynaptic nerve terminals, leading to an activation of voltage-dependent Na(+) and Ca(2+) channels that in turn increases glycine release. Since dorsal horn neurons receive nociceptive inputs, zinc may play an important role in the regulation of sensory transmission.

Modulation of ATP-induced inward currents by docosahexaenoic acid and other fatty acids in rat nodose ganglion neurons.

The effects of docosahexaenoic acid (DHA) and other fatty acids on P2X-receptor-mediated inward currents in rat nodose ganglion neurons were studied using the nystatin perforated patch-clamp technique. DHA accelerated the desensitization rate of the ATP-induced current. DHA showed use-dependent inhibition of the peak ATP-induced current. Other polyunsaturated fatty acids, such as arachidonic acid and eicosapentaenoic acid, displayed a similar use-dependent inhibition. The inhibitory effects of saturated fatty acids including palmitic acid and arachidic acid were weaker than those of polyunsaturated fatty acids. The results suggest that fatty acids may modulate the P2X receptor-mediated response when the channel is in the open-state.

Parkin potentiates ATP-induced currents due to activation of P2X receptors in PC12 cells.

Loss-of-function mutations of the parkin gene causes an autosomal recessive juvenile-onset form of Parkinson's disease (AR-JP). Parkin was shown to function as a RING-type E3 ubiquitin protein ligase. However, the function of parkin in neuronal cells remains elusive. Here, we show that expression of parkin-potentiated adenosine triphosphate (ATP)-induced currents that result from activation of the P2X receptors which are widely distributed in the brain and involved in neurotransmission. ATP-induced inward currents were measured in mock-, wild-type or mutant (T415N)-parkin-transfected PC12 cells under the conventional whole-cell patch clamp configuration. The amplitude of ATP-induced currents was significantly greater in wild-type parkin-transfected cells. However, the immunocytochemical study showed no apparent increase in the number of P2X receptors or in ubiquitin levels. The increased currents were attenuated by inhibition of cAMP-dependent protein kinase (PKA) but not protein kinase C (PKC) or Ca2+ and calmodulin-dependent protein kinase (CaMKII). ATP-induced currents were also regulated by phosphatases and cyclin-dependent protein kinase 5 (CDK5) via dopamine and cyclic AMP-regulated phosphoprotein (DARPP-32), though the phosphorylation at Thr-34 and Thr-75 were unchanged or rather attenuated. We also tried to investigate the effect of alpha-synuclein, a substrate of parkin and also forming Lysine 63-linked multiubiquitin chains. Expression of alpha-synuclein did not affect the amplitude of ATP-induced currents. Our finding provides the evidence for a relationship between parkin and a neurotransmitter receptor, suggesting that parkin may play an important role in synaptic activity.

Inhibition of the serotonin-induced inward current by dextromethorphan in rat nodose ganglion neurons.

Dextromethorphan is one of the most widely used antitussives for the treatment of cough. In the present study, we investigated the effect of dextromethorphan on 5-hydroxytryptamine (5-HT)-induced currents in acutely dissociated rat nodose ganglion neurons using nystatin-perforated patch-clamp recording configuration. The 5-HT-induced current was inhibited by the 5-HT(3) receptor antagonist tropisetron, while the selective 5-HT(3) receptor agonist 1-(m-chlorophenyl)-biguanide hydrochloride (mCPBG) induced a similar current. Dextromethorphan reversibly and concentration-dependently inhibited the 5-HT-induced inward current. The inhibition did not appear to be voltage-dependent. Both the peak and steady-state 5-HT-induced currents were inhibited by dextromethorphan, although the peak current was more sensitive to dextromethorphan block. The IC(50) values for the inhibition of peak and steady currents evoked by 3 muM 5-HT were 16.4 and 34.4 muM, respectively. In the presence of 10 muM dextromethorphan, the concentration-response curve for 5-HT was shifted to the right without changing the maximum response, while high concentrations reduced the maximum current. The 5-HT EC(50) values in the presence of 0, 10, 30 and 60 muM dextromethorphan were 4.3, 6.8, 15.5 and 40.6 muM, respectively. The results indicate that dextromethorphan inhibits the 5-HT-induced current of rat nodose ganglion neurons, and further suggest that dextromethorphan at a low concentration acts as a competitive inhibitor of 5-HT(3) receptors.