Exogenous melatonin abolishes mechanical allodynia but not thermal hyperalgesia in neuropathic pain. The role of the opioid system and benzodiazepine-gabaergic mechanism

Melatonin (MT) is a neurohormone synthesized and secreted by the pineal gland. MT plays an important role in the regulation of physiological and neuroendocrine functions. The purpose of this study was to assess the overall effect of melatonin on neuropathic pain, the type of melatonin receptor involved, and potential role of the opioid system and GABA(A) receptors. The experiments were conducted by using the animal neuropathic pain model (CCI). The rats with CCI showed the characteristic for the mechanical allodynia and thermal hyperalgesia signs that were calculated by using the von Frey's and Hargreaves' tests. The conducted studies measured the effects of intraperitoneal administration of naloxone (opioid antagonist), prazosin (MT3 antagonist), luzindole (MT1/MT2 receptor antagonist), picrotoxin (GABA(A) antagonist) and flumazenil (benzodiazepine antagonist) on the antinociceptive effects caused by melatonin. Melatonin caused the increase in the pain threshold of the mechanical allodynia and the slight increase in the threshold of the thermal hyperalgesia. The pre-treatment with naloxone completely abolished the antinociceptive effects of melatonin in von Frey's test, but not thermal sensation in the Hargreaves's test. Prazosin did not have any effects, while administration of luzindole significantly suppressed the antinociceptive effect of melatonin. The antiallodynic effect of MT was also abolished by flumazenil and picrotoxin. Melatonin influences the mechanical allodynia but not thermal hyperalgesia via activation of opioid system and benzodiazepine-GABAergic pathway. Antinociceptive effects of melatonin are mostly related to the MT1/MT2 receptors interaction.

Magnetically induced vagus nerve stimulation and feeding behavior in rats

Vagus nerve (VN) contribute to the bidirectional communication between the gastrointestinal tract and the central nervous system. Stimulation of the VN by a magnetically-driven solenoid with parameters similar to those during food-induced stomach distension has been thought to mimic short-term signaling of satiety and suppress food intake. In this study, the determination of optimal parameters of vagal neuro-modulation to achieve decreased food intake with a resulting reduction in body mass of rats is explored as therapy to treat obesity. The experimental design consisted of three groups of obese adult male Wistar rats: Group 1: VEMF - with solenoid's electrodes placed on the left VN in the magnetic field exposure (MFE); Group 2: EMF - without solenoid's electrodes on the VN in MFE; Group 3: CON - without solenoid's electrodes on the VN outside the MFE. This study suggests that the rats with solenoid's electrodes placed on the left VN significantly decreased their food intake, weight gain and serum leptin concentrations when compared to that of the CON group. PP levels were found to be higher in the VEMF group when compared to the controls groups. It was found that the most effective parameters of vagal stimulation on eating behavior were 3631, 7861, 14523 A(2) x h/m(2). The magnetic field by unknown mechanisms also influences feeding behavior. This study suggests that vago-vagal reflexes are involved in the feeding homeostasis and that neuromodulation might be an effective method for managing obesity. Further studies are required to confirm these effects in humans.

The role of female and male sex hormones in the healing process of preexisting lingual and gastric ulcerations

Studies in different animal species and in humans have suggested that sex hormones influence gastric acid secretion and contribute to the integrity of the oral and gastroduodenal mucosa but the effect of male and female sex hormones on the healing of the preexisting ulcers in the oral cavity and stomach have not been studied. We compared the effects of major male hormone, testosterone, and female hormone, progesterone, on the healing of lingual and gastric ulcers induced by acetic acid technique in male rats with intact or removed testicles (testectomy) and female rats with intact or removed ovaries (ovariectomy). The gastric acid secretion was determined in rats with gastric ulcers equipped with chronic gastric fistula (GF). Rats were sacrificed at day 7 upon ulcer induction; the ulcer area was measured by planimetry and the lingual and gastric blood flow (GBF) was determined by H(2)-gas clearance method and venous blood was collected for determination of plasma gastrin and plasma proinflammatory cytokine interleukin (IL)-1 beta levels. Gastric acid output from GF rats was significantly reduced while plasma gastrin was significantly enhanced in testectomized animals as compared to those in intact control rats and these effects were reversed by supplementation of testectomized animals with testosterone. The area of lingual and gastric ulcers in placebo-control rats decreased significantly at day 7 and this effect was significantly accelerated by testectomy or ovariectomy. In contrast, testosterone significantly delayed ulcer healing while producing a significant fall in the gastric blood flow and lingual blood flow determined at the margin of these ulcers. Treatment with progesterone significantly accelerated ulcer healing and increased the gastric and lingual blood flow at margin of these ulcers. Testosterone applied alone or supplemented in testectomized animals produced the significant increment in plasma IL-1 beta levels as compared to the respective levels of this cytokine in placebo-control animals. We conclude that: 1) major male (testosterone) and female (progesterone) sex hormones exhibit opposite effect on healing of preexisting ulcers in the oral cavity and stomach because testosterone markedly delayed while progesterone significantly accelerated this healing; 2) testosterone-induced delay in ulcer healing involves the fall in the gastric microcirculation at the margin of lingual and gastric ulcers and the excessive production and release of proinflammatory cytokine IL-1 beta; and 3) testectomy improves the gastric ulcer healing due to inhibition of gastric acid secretion and the rise in plasma gastrin, which exerts gastroprotective, trophic and ulcer healing action on the gastric mucosa.