MEBL is defined as the brain level of a drug expressed as moles/g of brain. After receiving a drug, when a rat begins to show a significant deviation from its normal behavior, the rat is sacrificed and the drug in its brain is quantified at that point. In this case, the rat learned a conditioned avoidance response, and at a certain time after receiving LSD, it was unable to execute the learned response, e.g. to escape from a box. After receiving LSD, the rats apparently forgot how to perform the learned behavior or didn't care.
The Table below lists the effective dose (left) and MEBL (right) of several different drugs. The MEBL data provides a behavioral index of drug potency, and correlates with Domelsmith's ionization potentials, thus facilitating the analysis of structure-activity relationships.
|From Domelsmith ionization potential|
Based on MEBL criteria, diethyltryptamine, DMT, and 5-methoxy-DMT were relatively weak, and phenethanolamines were very weak. LSD and 5-methoxytryptamine were the most potent compounds tested, with MEBL 0.5 ng/g. Tryptamine plus MAO inhibition was close to 5-methoxytryptamine in potency (MEBL = 1 ng/g), thus revealing the true potency of tryptamine when its inactivating enzyme is blocked.
"It is of interest to note that the two compounds with the highest potency, tryptamine and 5-methoxytryptamine, are naturally occurring in the mammalian brain." (Vogel,W.H. 1977)
Vogel W. H. and B. D. Evans. (1977). Structure-activity-relationships of certain hallucinogenic substances based on brain levels. Life Sci. 20, 1629-1635. doi:10.1016/0024-3205(77)90335-6