A few studies have appeared examining the effects of heroin. One such study examined the effects of an acute heroin dose upon visual activation using fMRI. Not surprisingly, there was decreased activation after a heroin dose in all the subjects [148]. Another study followed visual drug cues in heroin users with an actual drug dose to evaluate different brain patterns associated with cues for use. The urge to use correlated strongly with increased regional CBF in the inferior frontal and orbitofrontal cortex regions implicated in conditioning and reward [166]. Autoradiographic studies of changes in blood flow in rats after acute doses of heroin showed increased CBF in many different brain regions that could be blocked by naloxone [167,168]. A phMRI study of heroin in rats showed that there was a large difference between spontaneously breathing and artificially ventilated rats with the former showing widespread decreases in BOLD [15]. In the latter case there were systematic increases in BOLD signal regions consistent with the distribution of opiate mu-receptors in rat brain. These increases were blocked by naloxone. These same authors also showed that heroin self-administration in rats led to a decreased number of activated voxels in a number of brain regions, including the nucleus accumbens [169].

The confounds of changes in respiratory gases is pointed out by a number of the studies discussed above. It is clear that simultaneous measurement of blood gases is critical in such studies. However, this introduces the interesting point that changes in global hemodynamic states induced by changes in blood gases may be overridden in selected brain regions by direct action of other vasoactive neurotransmitters. Such a complicated state of affairs means that simple corrections of drift in phMRI signals may not always be a useful, or even feasible proposition.

The use of phMRI for investigations of conditions and systems related to drug abuse can only be described as in its infancy. Nevertheless, MRI lends itself well to multiple longitudinal studies, which will be of great value for studying the effects of withdrawal and therapy. In addition, the high temporal and spatial resolution of MRI may well allow for more fine discrimination of subtle pharmacodynamic effects than is possible with other techniques.

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