Suppression of the cardiac conditioned response by delta-9-tetrahydrocannabinol: a comparison with other drugs.

Using classical conditioning procedures, the cardiac conditioned response (CCR) was established by pairing one of two tones with the delivery of a peripheral electric shock in Rhesus monkeys. The other tone had no terminal consequence. Such a procedure results in an anticipatory 'anxiety' or 'fear' response to the impending shock signalled by the reinforced tone. The heart rate before the tone in two of the animals was characterized by tachycardia and by bradycardia in the other animal. The effect of intravenous Delta-9-tetrahydrocannabinol (THC) was compared to various doses of diazepam, chlorpromazine, and morphine. The results indicate that THC blocks the CCR in a does-related manner. The effects of THC were similar to diazepam, an anti-anxiety drug. Chlorpromazine and morphine affected the conditioned response in an unreliable manner, and both drugs would attenuate the response in some cases and potentiate it in other instances.