Changes in ambient temperature alter the blood pressure response to endotoxin and the effectiveness of naloxone

At room temperature, naloxone, a competitive opiate antagonist, ameliorates the hypotensive effect of endotoxin, suggesting that endotoxin increases the secretion of endogenous opioids that have a cardiodepressor action. It was previously observed in our laboratory that reducing the ambient temperature from 24°C to 19°C blocked this protective effect of naloxone in dogs. This suggested that activation of peripheral cold receptors might also increase endogenous opioid activity and together with the opioid activity induced by endotoxin might be sufficient to override the competitive blockage by naloxone. In support of this, it was found in the present study that an increased dose of naloxone was effective at 19°C. Studies done at 30°C revealed that the hypotensive effect of endotoxin is inversely related to the ambient temperature, and naloxone is effective in low doses at the higher temperature. Core temperature was not altered significantly by the ambient temperatures used by naloxone, by endotoxin, or by any combination thereof. These findings suggest that, at least within moderate ranges, acute changes in ambient temperature induce inversely related changes in endogenous opioid activity, representing a specific thermal rather than a generalized stress response.