Repeated THC Shifted Heart Rate While Other Effects Persisted in Freely Moving Rats
A 1980 experiment found that repeated THC changed the direction of the heart-rate response while blood-pressure and behavioral effects persisted differently. Adaptation could not be summarized as a uniform fading of drug action.
The animals were measured while freely moving
Rats with arterial and electrode implants received injected THC daily for ten days. On the first day, THC lowered heart rate and raised blood pressure, alongside several abnormal movement-related behaviors. The preparation differs from studies conducted under anesthesia.
The later response was not simply a smaller first-day response
Heart-rate slowing diminished and was replaced by tachycardia by day ten. The pressor effect increased on later treatment days. The observed abnormal behaviors did not show corresponding disappearance. A heart-rate adaptation therefore did not demonstrate normalization of the other outcomes.
Brain-stimulation responses remained sensitive
THC suppressed cardiovascular and behavioral responses to stimulation of selected brain regions, and tolerance did not develop to those suppressive effects. The authors proposed reduced vagal activity as a possible contributor to heart-rate tolerance, rather than proving a complete explanation for every change.
This study is a useful example of changing response direction and persistent endpoint sensitivity during repeated exposure. It cannot establish a human cardiovascular-risk estimate or show that frequent cannabis use protects against adverse effects. Species, route and the particular cardiovascular variable remain essential.
Related: interpreting cannabis tolerance claims.
Research cited
Kawasaki H; Watanabe S; Ueki S. Effects of chronic administration of delta 9-tetrahydrocannabinol on the cardiovascular system, and pressor and behavioral responses to brain stimulation in freely moving rats. European journal of pharmacology (1980). doi:10.1016/0014-2999(80)90209-5. Findings discussed from the study abstract.
