Effect Of Melatonin On Outward Delayed Rectifier Potassium Currents In Hippocampal CA1 Neurons And The Mechanisms

Effect of melatonin on outward delayed rectifier potassium currents in hippocampal CA1 neurons and the mechanisms Melatonin (N-acetyl-5-methoxytryptamine), the major endocrine product of the pineal gland, has been demonstrated to have significant neuroprotection. Its neuroprotection is mainly attributed to its antioxidant and free radical-scavenging abilities. Voltage-gated potassium currents participate in the regulation of neuronal excitability, activity, and synaptic plasticity. Recently, much of the accumulated data has indicated that outward potassium currents, especially outward delayed potassium current (IK), are involved in neuronal cell death and potassium channel blockers can prevent such an ischemic neuronal cell death. It is well known that hippocampus is essential for memory formation and cognition, and that it is also highly vulnerable to ischemic brain injury and many other pathological insults. Previous electrophysiological experiments have revealed that melatonin modulates hippocampal neuronal excitability and plasticity. However, whether melatonin has any effect on potassium currents in hippocampus is still unknown. Therefore, in the present study, we observed the effect of melatonin on IK recorded by whole-cell patch clamp in CA1 neurons in hippocampal brain slices of SD rats aged 14-18 days. Besides, the mechanisms mediating this effect were analyzed. The results were showed as follows:(1) To observe the effect of melatonin on IK, we clamped membrane potentials at –50 mV. After a prepulse of –110mV followed by a 50 ms interpulse of –50mV, the membrane potential was depolarized to +60mV to evoke IK. Melatonin dose-dependently inhibited IK with the half-maximal inhibitory concentration (IC50) of 3.75 mM. The time course of the effect of melatonin at the concentration of 2.5 mM showed that the inhibition had a rapid onset and was readily reversible. The inhibition was observed within 1 min after melatonin application and completely reversed within 6 min after washing.(2)To examine the effect of melatonin on the steady-state activation and inactivation of IK, normalized conductance or current amplitudes of IK were plotted as a function of membrane potential, then the resulting curves were fitted by Boltzmann equations. The values of VH were 11.6 ± 1.5 mV and 9.6 ± 1.2 mV in the absence and presence of melatonin, respectively. There was no significant difference between them. While, the values of Vh were – 52.8 ± 1.1 mV and – 61.0 ± 1.4 mV in the absence and presence of melatonin, respectively (P < 0.05), indicating that mealtonin does not affect the activation of IK but shifted steady-state inactivation of IK to the hyperpolarizing direction.(3)To investigate whether the melatonin receptors are involved in the melatonin-induced inhibition on IK, we observed the effects of luzindole, a MT1/MT2 antagonist, and prazosin, a MT3 antagonist, on the melatonin-induced IK inhibition. We found that luzindole at 40, 100,or 250 μM did not antagonize the inhibition of IK by melatonin. However, luzindole itself dose-dependently inhibited IK. Prazosin at the concentration of 100 μM or 300 μM did not affect the IK when they were applied alone, neither did they block the effect of melatonin. The results indicate that specific melatonin receptors are not involved in the melatonin-induced IK inhibition.(4)Based on the fact that melatonin in high concentrations can also activate 5-HT receptors, we further examine whether 5-HT receptors is involved in the effect of melatonin. In the present study, we observed the effect of 5-HT, a serotonin receptor agonist, on IK. The results showed that 5-HT at 10, 100, or 500 μM, did not affect the IK , neither did they affect the IK inhibition by melatonin (2.5 mM). These results indirectly indicate that the effect of melatonin is not related to serotonin receptors.(5)To investigate whether the effect of melatonin was related to its free radical-scavenging ability, we observed the effect of H2O2 on IK. The results showed that H2O2 had a rapid and do...