Opioid and non-opioid central cardiovascular effects of ketamine

The cardiovascular responses to ketamine injected intracisternally were examined in chloralose anaesthetized cats. Blood pressure and heart rate were recorded at different time intervals after intracisternal injection of drug or saline vehicle. The low doses of ketamine (0.5 or 1.0 mg) elicited dose dependent increase in blood pressure and heart rate. In contrast the high dose of ketamine (4 mg), produced a fall in blood pressure and heart rate. The cardiovascular response elicited by the low dose was naloxone insensitive and completely blocked by haloperidol, but not by dopamine antagonist pimozide. The vasodepressor and bradycardiac effect of the 4 mg dose was naloxone antagonizable. These data show that excitatory cardiovascular effects of the low dose result from a naloxone resistant site while in high doses an inhibitory effect is elicited by action at naloxone sensitive opiate receptors.