Moderate hypercapnia-induced anesthetic effects and endogenous opioids

The purpose of this report is to explore the mechanisms of hypercapnia-induced antinociception. We carried out three experiments, the first to confirm whether moderate hypercapnia induces anesthetic effects, the second to determine whether naloxone reverses the anesthetic effects, and the third to evaluate whether β-endorphin is related to the anesthetic effects. In a pre-test, we determined the optimal CO2 concentration in a chamber which would cause moderate hypercapnia in rats. Eighteen rats were divided into control, hypercapnia, and hypercapnia plus naloxone groups in experiment 1. The naloxone group rats were injected with naloxone (10 mg/kg) intraperitoneally before gas inhalation. After 60 min gas inhalation, 10% formalin was injected into the left rear paw of all rats, and nociceptive behaviors were observed for 1 h. In experiment 2, 11 rats were divided into control and hypercapnia groups. The brain was removed and fixed under pentobarbital anesthesia. Sections were immunostained for c-Fos and β-endorphin (ACTH) with the ABC method. All neurons double-labeled for c-Fos and β-endorphin (ACTH) in the arcuate nucleus were counted by blinded investigators. Moderate hypercapnia (PaCO2 83 ± 7 mmHg) reduced nociceptive behavior in the formalin test and naloxone pre-treatment attenuated this phenomenon. However, β-endorphin-producing neurons were not activated by CO2 inhalation. Endogenous opioids are related to moderate, hypercapnia-induced anesthetic effects, but, β-endorphin-producing neurons in the hypothalamus were not activated by the CO2 inhalation stress.