e cannabinoids also produce a loss of motor coordination, the researchers
had to test the hypothesis that turning off the RVM simply reversed the motor
deficits and not the analgesic effects of the cannabinoid drug. To test this
possibility, they gave the rats a motor coordination test called the retard
treadmill, in which the rats are placed on a rod and the time that it takes for
them to fall off is measured. Rats that received WIN55, 212-2 showed a severe
loss of motor coordination. However, after the RVN was shut down, a condition
that reversed the analgesic effects, rats continued to show a loss of motor
coordination.
"This experiment demonstrates that the rats given the cannabinoid did not leave
their tails on the heat source simply because they had lost their motor
coordination," said Meng. "The cannabinoid is also having an analgesic effect."
In a second set of experiments, performed in anesthetized rats, the researchers
examined the effect of cannabinoids on the activity of the brain cells called
neurons in the RVM. Using a method called electrophysiology, which measures the
electrical impulses of neurons, the researchers were able to detect the activity
of single neurons in the RVM after several different drug treatments.
The researchers determined that WIN55,212-2 produced changes in the activity of
neurons in the RVM that were identical to that of morphine. However, they were
also able to show that these two drugs act on the RVM in different ways. A drug
that blocks the effect of morphine was unable to prevent the effect of
WIN55,212-2. Likewise, a drug that blocks the effect of the WIN55,212-2 was
unable to prevent the changes produced by morphine.
"These results indicate that the marijuana-like drug can reduce pain by
affecting the same pain modulating neurons as morphine, but through separate
mechanisms," said Meng.
Other co-authors of the UCSF study were Barton H. Manning, PhD, a postdoctoral
fellow in Howard Fields lab, and
'"/>
Contact: Jennifer O'Brien
jobrien@itsa.ucsf.edu
(415) 476-2557
University of California - San Francisco
23-Sep-1998
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