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.

1,8-Cineole blocks voltage-gated L-type calcium channels in tracheal smooth muscle.

1,8-Cineole is a cyclic monoterpenoid used in folk medicine for treatment of numerous respiratory diseases and other infections. 1,8-Cineole has anti-inflammatory, antioxidant, and myorelaxant effects, as well as low toxicity. In the present study, the effects of 1,8-cineole on contractility and voltage-gated calcium channels (VGCC) in tracheal smooth muscle were investigated. Intact and dissociated tracheal smooth muscle were used for muscle contraction and patch-clamp recordings, respectively. In experiments involving muscle contraction, 1,8-cineole potentiated contractions at low concentrations and relaxed contractions induced by isotonic K+ at high concentrations. AMTB (a TRPM8 channel blocker) reduced the potentiation induced by 1,8-cineole while indomethacin (a COX inhibitor) did not block this effect. In dissociated myocytes, 1,8-cineole partially blocked Ba2+ currents through VGCC in a concentration-dependent manner. 1,8-Cineole shifted the steady-state activation and inactivation curves to the left and also reduced the current decay time constant. In conclusion, 1,8-cineole has a dual effect on tracheal smooth muscle contraction resulting in a biphasic effect. Our data suggest that the potentiation effect is mediated by activation of TRPM8 channels and the relaxation effect is mediated by the blockage of L-type VGCC.

Hydroxyl Group and Vasorelaxant Effects of Perillyl Alcohol, Carveol, Limonene on Aorta Smooth Muscle of Rats.

The present study used isometric tension recording to investigate the vasorelaxant effect of limonene (LM), carveol (CV), and perillyl alcohol (POH) on contractility parameters of the rat aorta, focusing in particular on the structure-activity relationship. LM, CV, and POH showed a reversible inhibitory effect on the contraction induced by electromechanical and pharmacomechanical coupling. In the case of LM, but not CV and POH, this effect was influenced by preservation of the endothelium. POH and CV but not LM exhibited greater pharmacological potency on BayK-8644-induced contraction and on electromechanical coupling than on pharmacomechanical coupling. In endothelium-denuded preparations, the order of pharmacological potency on electrochemical coupling was LM < CV < POH. These compounds inhibited also, with grossly similar pharmacological potency, the contraction induced by phorbol ester dibutyrate. The present results suggest that LM, CV and POH induced relaxant effect on vascular smooth muscle by means of different mechanisms likely to include inhibition of PKC and IP3 pathway. For CV and POH, hydroxylated compounds, it was in electromechanical coupling that the greater pharmacological potency was observed, thus suggesting a relative specificity for a mechanism likely to be important in electromechanical coupling, for example, blockade of voltage-dependent calcium channel.

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.

Essential oil of Croton argyrophylloides: toxicological aspects and vasorelaxant activity in rats.

Croton argyrophylloides Muell. Arg. is widely used in Brazilian folk medicine to treat diabetes and venereal diseases. This study examined the acute toxicity and cytotoxicity of the essential oil of C. argyrophylloides (EOCA). In addition, vascular effects of the EOCA have been examined. In mice, an oral acute toxicity test revealed that EOCA could be considered as a non toxic essential oil since it showed a very high LD50 (9.84 +/- 0.01 g/kg). In the brine shrimp (Artemia salina) cytotoxic assay, the LC50 value of EOCA was 275 [165-534] microg/mL. EOCA (1-1000 microg/mL) relaxed isolated endothelium-intact aortic rings precontracted with phenylephrine with an IC50 value of 126.7 [89.8-163.7] microg/mL. In rat mesenteric bed preparations precontracted with phenylephrine, EOCA (1-300 microg/mL) also induced a reversible, vasodilator effect with an IC50 value of 46.0 [33.3-58.7] micro/mL. It is concluded that EOCA is a very interesting agent from the point of view of the possibility of therapeutic application. This is because, whilst showing a very small acute toxicity, EOCA also showed maximal efficacy as a vascular antispasmodic agent with a pharmacological potency similar to that of other Croton species essential oils.