Science Cannabis

The Effects of Cannabis on the Central Nervous System 99Positron Emission Tomography (PET) a small amount of radioactivelylabelled water is injected and the radioactive tracer is then imaged byplacing the subject in a special camera that detects the low levels ofradioactivity in the brain. Alternatively subjects may inhale a smallamount of radioactively labelled gas (Xenon). The tracer enters the brainwithin a few seconds after administration, and the amount present indifferent brain regions reflects differences in their regional blood flow.The camera images are analyzed by a computerized method known astomogaphy to yield a three-dimensional image of blood flow throughoutthe brain. When THC was administered to volunteers with a history ofmarijuana use there was an increased blood flow in most brain regions,both in the cerebral cortex and in deeper brain structures. These changesreached a peak 30-60 minutes after drug administration, correspondingto the peak period of intoxication. The regions in which the changes inblood flow were greatest and in which there was the best correlation withsubjective reports of intoxication were in the frontal cortex (Matthew etal., 1997). This is interesting because it fits well with the concept thatfrontal cortex is important in the control of "executive" brain functions—whichare particularly sensitive to disruption by marijuana (seeprevious paragraph). Other studies of regional blood flow have reachedsimilar conclusions, with increases in the frontal cortex and temporalcortex reported as the most prominent changes, consistent with the druginducedimpairment of working memory.Another way of asking which brain regions are involved in the actionsof marijuana is to examine the anatomical distribution of the CB-1receptor in the brain. This has been studied in detail in both animal andhuman brains. Most of the published reports have used the technique ofautomdiogmphy to produce images of the CB-1 receptors in thin sectionsof brain tissue. The tissue sections are incubated with a radioactive cannabinoid(usually CP55,940 containing a radioactive hydrogen atom), asdescribed in Chapter 2, this tracer binds selectively to the CB-1 receptors.Excess radioactive tracer is washed away, and the tissue sections arethen covered with a photographic emulsion that is sensitive to radioactivity.After some time the emulsion layer can be processed with a photographicdeveloper and silver grains will become visible in regions that