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Understanding Drugs and Behaviour phần 4

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(ví dụ, aspartate cholecystokinin-8S, glutamate trong neurotensin và glycine trong methionine-enkephalin). Up cho đến khi phát hiện ra các neuropeptides, các nhà khoa học tin rằng một tế bào thần kinh duy nhất chỉ có thể chứa một chất dẫn truyền thần kinh - Dale của pháp luật. Tuy nhiên, | 88 Part II Non-medical Use of Psychoactive Drugs How does THC get to the brain Cannabis is usually smoked in a joint which is approximately the size of a cigarette and tobacco may be included to assist burning and or to reduce potency. Smokers tend to inhale the smoke deeply and hold it in the lungs for long periods in order to maximise absorption of the active compounds. Cannabis can be eaten. However the amount of drug required to produce psychoactive effects is approximately 2-3 times more than when it is smoked this is because the lungs are more efficient at transporting the airborne THC to the blood bioavailability b of 20 than the gastrointestinal tract is at absorbing the fat-soluble THC across its membrane bioavailability b of 6 . Furthermore when ingested orally some of the THC is degraded in the stomach and more is metabolised by the liver before it reaches the brain. When smoked the effects are rapid in onset with THC entering the circulation almost immediately and reaching peak concentrations between 20 and 45 of total THC content within 10 minutes. When taken orally the time to peak is around 1-3 hours and duration is prolonged due to continued slow absorption from the gut Chapter 3 . As a consequence smoking provides greater control it is quicker and easier to selftitrate to the individual s required level of psychoactive sensation when smoking compared with when the drug is eaten Hall et al. 1994 . Interestingly however the peak in blood concentration does not always represent the period when users report experiencing the greatest high this is unusual when compared with other drugs and the answer appears to lie in the body s metabolism of THC. Some of the breakdown products of the cannabinoids e.g. 11-hydroxy-D-9-THC are also psychoactive. As a consequence these breakdown products and THC may act together to produce the greatest effect some 20 to 30 minutes after smoking when serum THC levels have started to decline Figure 7.1 . Figure 7.1. Time .