Intravenous administration of propofol causes an increase in trigeminal spinal nucleus neuronal activity in rats: Fos analysis

Propofol is one of the most widely used sedative agents in the clinic. It has a strong sedative effect but no analgesic effect. However, the neuronal mechanism underlying the lack of analgesic effect during propofol administration is not known. Fos expression in the trigeminal spinal nucleus neurons was precisely analyzed following intravenous injection of propofol at different sedative levels. Sprague-Dawley rats were divided into two groups (light and deep sedative levels) based on the electroencephalogram (EEG) analysis. The rats were perfused, their brainstems were removed, and Fos immunohistochemistry was performed. We also infused lidocaine into the jugular vein to test whether propofol directly activates the nociceptors innervated in the vein. Many Fos protein-LI cells were expressed in the trigeminal spinal nucleus interpolaris and caudalis transition zone (Vi/Vc zone) and caudal Vc with two peaks. The number of Fos protein-LI cells was significantly greater in Vi/Vc zone at the deep level compared with that of light level. The Fos expression in Vi/Vc zone was significantly depressed following pretreatment with iv infusion of lidocaine before propofol administration. These results suggest that intravenous injection of propofol is involved in the increased activity of trigeminal spinal nucleus neurons, resulting in the central sensitization of the trigeminal pain pathways. It is also suggested that the intravenous nociceptors would be involved in an increment of the Vc neuronal activity.