Stuart Sealfon, Jay Gingrich, and colleagues published their findings in the February 1, 2007 issue of the journal Neuron, published by Cell Press.

Researchers have long known that hallucinogens activate specific receptors in the brain, called 5-HT2A receptors (2ARs), that are normally triggered by the neurotransmitter serotonin. Neurotransmitters are chemicals that one brain cell launches at receptors on another to trigger a nerve impulse in the receiving cell. However, a fundamental mystery has been why other compounds that activate the same receptors are not hallucinogenic.

In their studies, the researchers compared the differences between the effects of LSD and a nonhallucinogenic chemical that also activates 2AR receptors on the mouse neural machinery. Since the animals could not report the kinds of perception-altering effects that humans experience on hallucinogens, the researchers determined hallucinogenic properties by measuring a head twitch response the mice characteristically showed when under hallucinogens but not when under nonhallucinogens.

The scientists concentrated their studies on the cortex of the brain, which earlier studies had shown to be the center for action of the hallucinogens. Their analysis revealed that LSD produced genetic, electrophysiological, and internal cellular signaling responses that were distinctively different from those induced by a nonhallucinogenic compound.

They also explored whether 2ARs were central to the hallucinogenic effect of LSD by producing mice lacking the receptors, but in which receptor activity could be selectively restored in the cortex. The researchers found that mice without functioning receptors showed no hallucinogenic response to LSD, but restoring the receptors rendered LSD hallucinogenic in the animals.

The researchers wrote that "These studies identify the long-elusive neural and signaling mechanisms responsible for the unique effects of hallucinogens."

They also concluded that "The strategy we developed to elucidate [hallucinogen] action should be applicable to [central nervous system]-active compounds, with therapeutic potential in other

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