Here we describe, for the first time, the occurrence of behavioural sensiti
zation after chronic exposure to Delta (9)-tetrahydrocannabinol. Rats were
treated twice a day, for five days, with increasing doses (5, 10, 20, 40, 4
0 mg/kg i.p.) of Delta (9)-tetrahydrocannabinol or its vehicle and after 20
days of withdrawal, animals were challenged with 5 mg/kg (i.p.) of the dru
g and their behaviour was assessed. Contrary to the motor inhibition induce
d in control rats, challenge with Delta (9)-tetrahydrocannabinol in pre-exp
osed animals elicited a complex behavioural syndrome mainly characterized b
y oral stereotyped items. Due to the relevance of behavioural sensitization
in drug-seeking behaviour that persists long after discontinuation of drug
use, our findings suggest that cannabinoids could trigger neurobiological
alteration not dissimilar from those observed with more harmful abused drug
s.