Increase in Blood Pressure by Local Injection of Ketamine into the Amygdala in Rats

To confirm that an increase in blood pressure induced by ketamine is mediated through the central nervous system, we examined the effect of ketamine, applied directly to the amygdala, on blood pressure. Six male Sprague-Dawley rats were used in the study. Under head-restrained and unanesthetized condition, 0.2 µL (5 mg/mL) of ketamine was injected in and around the amygdala at a flow rate of 0.2 µL/min through a glass pipette, and the blood pressure was recorded while monitoring the state of the animals by electroencephalogram and electromyogram. After ketamine injection, the injection site was marked by Pontamine Sky Blue infusion. Blood pressure was increased by ketamine injection into the basolateral and central nuclei of the amygdala, endopiriform nucleus and piriform cortex. In a total of 11 responses, an increase in blood pressure started with a mean latency of 193.5 ± 43.0 s, reached its peak 180.2 ± 23.3 s after the response onset, then gradually returned to the baseline with mean duration of 706.7 ± 113.5 s. The mean fluctuation was 17.1 ± 2.5 mmHg. We revealed that blood pressure fluctuations induced by ketamine are associated with the amygdala. Elucidation of the mechanism of ketamine-induced blood pressure increase will lead to understanding of the mechanism of side effects of ketamine, and will contribute to its appropriate use.