Naltrexone attenuates plasma nitric oxide release following acute heat stress

Involvement of nitridergic and opioidergic pathways in the antinociception of gabapentin in the orofacial formalin test in mice

BACKGROUND

Pain is one of the most common problems in clinical medicine. There is considerable evidence that pharmacologic approaches are the most widely used therapeutic options to ameliorate persistent or chronic pain. In this study it was evaluated the effect of l-NAME and naltrexone in the antinociception induced by administration of gabapentin in the orofacial formalin test of mice.

METHODS

The algesiometer assay was performed by the administration of 20 μl of 2% formalin solution injected into the upper right lip of each mouse.

RESULTS

The dose of gabapentin that produces the 50% of the maximum possible effect (ED50) was significantly increased by the pretreatment with l-NAME or naltrexone.

CONCLUSIONS

These results suggest that gabapentin produce antinociception partly via the activation nitridergic pathways and opioid system.

Mu-opioid receptor blockade protects against circulatory shock and cerebral ischemia during heatstroke

Naltrexone, a nonselective antagonist of opioid receptors, is found to be beneficial in protecting against heatstroke. Further investigation using selective mu, delta, and kappa opioid receptor antagonists are needed to prove the involvement of specific receptors in heatstroke. Rats under sodium pentobarbital anesthesia were exposed to high ambient temperature of 43 degrees C to induce heatstroke. Control rats were exposed to 24 degrees C. In rats treated with normal saline 20 minutes before heat stress, the values for survival time were found to be 89-101 minutes. Intravenous administration of CTAP (a selective mu-opioid receptor antagonist; 50-200 microg/kg), but not nor-binaltorphimine (20-200 microg/kg; a kappa-opioid receptor antagonist) or ICI-174864 (50-500 microg/kg; a delta-opioid receptor antagonist), significantly increased the survival time to new values of 180-212 minutes. In vehicle-treated rats after heatstroke onset, the values for core temperature, intracranial pressure, and the extracellular markers for ischemia (eg, glutamate and lactate/pyruvate ratio) or damage (eg, glycerol) and neuronal damage scores in striatum were significantly higher than those of normothermic controls. In contrast, the values for mean arterial pressure, cerebral perfusion pressure, cerebral blood flow, and brain partial pressure of O2 were significantly lower than those of normothermic controls. The heatstroke-induced hyperthermia, arterial hypotension, intracranial hypertension, cerebral hypoperfusion and hypoxia, and increased levels of cellular ischemia and damage markers in striatum were all significantly attenuated by prior administration of CTAP. The data indicate that prior antagonism of mu-opioid receptors protects against circulatory shock and cerebral ischemia during heatstroke.

Naltrexone Protects Against Hypotension, Hyperthermia, and β-Endorphin Overproduction During Heatstroke in the Rat

Heat stroke is characterized by hyperthermia, arterial hypotension, decreased baroreflex sensitivity, and increased serum levels of beta-endorphin. Whereas naltrexone may have therapeutic potential in heat stroke, the underlying mechanism remains unclear. We tested the hypothesis that naltrexone may attenuate heat stroke by reducing hyperthermia, hypotension, decreased baroreceptor sensitivity, and/or increased serum levels of beta-endorphin. Heat stroke was induced by exposing the anesthetized adult Sprague-Dawley rats in an incubator at 43 degrees C. The moment in which the mean arterial pressure dropped irreversibly from the peak level was taken as the onset of heat stroke. Control rats were exposed to 24 degrees C. Mean arterial pressure, baroreceptor sensitivity, and maximal reflex bradycardia, after the onset of heat stroke, were all significantly lower than in control rats. However, rectal temperature and serum levels of beta-endorphin were all greater after the onset of heat stroke. Intravenous delivery of naltrexone (10 mg/kg) 20 min before the initiation of heat stress, but not immediately at the onset of heat stroke, significantly attenuated the above-mentioned reactions. Accordingly, naltrexone improved survival during heat stroke. These results suggest that naltrexone protects against hypotension and decrement of both baroreceptor sensitivity and maximal reflex bradycardia during heat stroke by reducing both hyperthermia and increment of serum beta-endorphin and thus improves survival.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).