Effects of long-term exposure to Δ9-THC on expression of cannabinoid receptor (CB1) mRNA in different rat brain regions

The time course of changes across 21 days of continuous exposure to Delta(9)-tetrahydrocannabinol (Delta(9)-THC) was assessed for the level of cannabinoid receptor (CB1) mRNA expression in three different rat brain regions: cerebellum, hippocampus and corpus striatum. Expression levels of CB1 mRNA were determined using semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) following a protocol which included a gene standard, 28S ribonucleic acid protein (rRNA), for normalization of levels of RNA in the three different brain regions. The levels of CB1 mRNA were assessed in four different rats at each of seven time points (6 h, and 1, 2, 3, 7, 14 and 21 days) during a 21-day Delta(9)-THC one dose day(-1) (10 mg kg(-1)) treatment regimen. In the cerebellum and hippocampus, CBI mRNA levels were increased above vehicle control animals at 7 and 14 days of treatment. In the striatum the levels of CB1 transcripts were severely reduced from days 2-14. CB1 message expression in all three brain areas returned to vehicle control levels by day 21 of Delta(9)-THC treatment, a time at which behavioral tolerance has been previously reported. An additional measure, receptor stimulated GTP gamma S binding, performed over the same time period revealed differential desensitization within the 3 brain areas as a function of chronic exposure to Delta(9)-THC. Hippocampus was the earliest to desensitize decreasing to 35% of control by treatment day 7, followed by a decrease in the cerebellum to that same level on day 14 of treatment. The striatum showed only half that degree of desensitization (65%) over the entire 21-day treatment period. Comparisons suggests that CB1 message may be regulated by different effector systems in each of the three areas during chronic Delta(9)-THC exposure. (C) 1998 ElIsevier Science B.V. All rights reserved.