Melatonin Reduces Excitability in Dorsal Root Ganglia Neurons with Inflection on the Repolarization Phase of the Action Potential.

Melatonin is a neurohormone produced and secreted at night by pineal gland. Many effects of melatonin have already been described, for example: Activation of potassium channels in the suprachiasmatic nucleus and inhibition of excitability of a sub-population of neurons of the dorsal root ganglia (DRG). The DRG is described as a structure with several neuronal populations. One classification, based on the repolarizing phase of the action potential (AP), divides DRG neurons into two types: Without (N0) and with (Ninf) inflection on the repolarization phase of the action potential. We have previously demonstrated that melatonin inhibits excitability in N0 neurons, and in the present work, we aimed to investigate the melatonin effects on the other neurons (Ninf) of the DRG neuronal population. This investigation was done using sharp microelectrode technique in the current clamp mode. Melatonin (0.01-1000.0 nM) showed inhibitory activity on neuronal excitability, which can be observed by the blockade of the AP and by the increase in rheobase. However, we observed that, while some neurons were sensitive to melatonin effect on excitability (excitability melatonin sensitive-EMS), other neurons were not sensitive to melatonin effect on excitability (excitability melatonin not sensitive-EMNS). Concerning the passive electrophysiological properties of the neurons, melatonin caused a hyperpolarization of the resting membrane potential in both cell types. Regarding the input resistance (Rin), melatonin did not change this parameter in the EMS cells, but increased its values in the EMNS cells. Melatonin also altered several AP parameters in EMS cells, the most conspicuously changed was the (dV/dt)max of AP depolarization, which is in coherence with melatonin effects on excitability. Otherwise, in EMNS cells, melatonin (0.1-1000.0 nM) induced no alteration of (dV/dt)max of AP depolarization. Thus, taking these data together, and the data of previous publication on melatonin effect on N0 neurons shows that this substance has a greater pharmacological potency on Ninf neurons. We suggest that melatonin has important physiological function related to Ninf neurons and this is likely to bear a potential relevant therapeutic use, since Ninf neurons are related to nociception.

Monoterpenoid terpinen-4-ol inhibits voltage-dependent Na+ channels of small dorsal root ganglia rat neurons.

The monoterpenoid terpinen-4-ol (4TERP) is known to inhibit cell excitability, has low toxicity and important pharmacological activities, which are likely related to neural excitability, such as anti-inflammatory, antiepileptic and antinociceptive effects. However, the pharmacological characteristics and mechanisms underlying the effects of 4TERP on blockade of neural action potential are not completely elucidated. Since Na+ current (INa) through voltage-dependent Na+ channels (NaV) is a major mechanism for excitability, the present study investigated the pharmacological characteristics and mechanisms of the action of 4TERP on INa through NaV. For this aim, dissociated small neurons of dorsal root ganglia of adult rats were used for whole cell patch-clamp recordings. 4TERP concentration-dependently inhibits INa (IC50 0.8 ±â€¯0.3 mM; pharmacological efficacy 42.89 ±â€¯5.54%). 4TERP interfered with INa through a mechanism with various components, which includes predominantly channel pore block and sensitivity to frequency of use. In presence of 4TERP (3 mM), decreasing stimulation from 5 Hz to very low frequency (75 s of quiescence previously to stimulation) induced INa decrease to 65.17 ±â€¯5.86% of control. 4TERP also altered (left shift) voltage sensitivity of the steady state activation of NaV. Data are discussed aiming to interpret the importance of blockade of INa through NaV as participant of 4TERP-induced inhibition of membrane excitability.

Diabetes mellitus alters electrophysiological properties in neurons of superior cervical ganglion of rats.

Diabetic neuropathy is the most prevalent complication associated with diabetes mellitus (DM). The superior cervical ganglion (SCG) is an important sympathetic component of the autonomic nervous system. We investigated the changes in cellular electrophysiological properties and on Na+K+-ATPase activity of SCG neurons of rats with DM induced by streptozotocin (STZ). Three types of action potentials (AP) firing pattern were observed in response to a long (1 s) depolarizing pulse. Whilst some neurons fired a single AP (single firing phasic, SFP), others fired few APs (multiple firing phasic, MFP). A third type fired APs during more than 80% of the stimulus duration (tonic-like, TL). The occurrence of SFP, MFP and TL was 84.5, 13.8, and 1.7%, respectively. SFP and MFP differed significantly in their membrane input resistance (Rin). At the end of the 4th week of its time course, DM differently affected most types of neurons: DM induced depolarization of resting membrane potential (RMP), decreased AP amplitude in SFP, and decreased Rin in MFP. DM decreased spike after-hyperpolarization amplitude in MFP and the duration in SFP. Based on the RMP depolarization, we investigated the Na+K+-ATPase action and observed that DM caused a significant decrease in Na+K+-ATPase activity of SCG. In conclusion, we have demonstrated that DM affects several parameters of SCG physiology in a manner likely to have pathophysiological relevance.

Melatonin decreases neuronal excitability in a sub-population of dorsal root ganglion neurons.

Melatonin, a powerful antioxidant, participates in the regulation of important physiological and pathological processes. We investigated the actions of melatonin on neuronal excitability of intact dorsal root ganglions (DRG) from rats using intracellular recording techniques in current clamps. Melatonin blocked the generation of action potentials in a concentration-dependent manner. Bath applied melatonin (1.0-1000.0 nM) hyperpolarized the resting membrane potential, and increased the input resistance and rheobase. Melatonin also altered the active electrophysiological properties of the action potential, amplitude and maximum descendant inclination, in a statistically significant way. In order to provide evidence on the mechanism of action of melatonin in the DRG, quantitative PCR (qPCR) was performed. Analyses were performed for melatonin membrane receptors, MT1 and MT2, and it was observed that the DRG expresses MT1 receptors. In addition, we noted that the melatonin-induced effects were blocked in the presence of luzindole, a melatonin receptor antagonist. The minimal effective concentrations of melatonin (10.0 nM) and the blockade of effects caused by luzindole suggest that the effects of melatonin are hormonal, and are induced when it binds to MT1 receptors.