Minimum effective brain level (MEBL) demonstrates high potency of tryptamine

Vogel and colleagues assigned a value called the minimum effective brain level (MEBL) to several hallucinogenic drugs. These researchers wanted to know the true potency of drugs, without the confounding effects of drug uptake, elimination, distribution, and biotransformation.

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.

Vogel wrote:

"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)

Reference

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

Effect of Cyanide and Sodium azide on fish surfacing reaction

Weiss and colleagues tested the hypothesis that LSD acts by anoxia and asphyxia. They added Siamese fighting fish to water containing the oxidase inhibitors, potassium cyanide (KCN) and sodium azide. KCN and sodium azide affected Siamese fighting fish in a way which was very similar to LSD, suggesting that LSD might normally work by preventing oxygen uptake. Figure 2 (below) shows the fish surfacing reaction for 1 ug LSD (left) and 1 ug KCN (middle). There is no surfacing reaction in the H2O control (right).

From LSD fish

It was interesting that KCN was nearly as potent as LSD, since I am unaware of any other drugs, with the exception of other hallucinogens, that cause LSD-like behavior in the same concentration range as LSD in a biological assay. There were some differences between KCN and LSD, for example fish recovered more quickly from 1 ug/mL KCN than 1 ug/mL LSD. The figure on the right is the KCN surfacing curve plotted on the LSD surfacing curve.

From LSD fish

The authors observed that CO2 poisoning itself causes a nose up-tail down reactions in fish. When carbon dioxide was permitted to accumulate in the absence of renewed oxygen, all the fish assumed the nose up-tail down position in several hours. Thus inhibition of O2 oxidation was a general feature of the fish intoxication produced by LSD, KCN, and sodium azide.

"A decreased oxygen supply with simultaneous prevention of accumulation of carbon dioxide resulted in the nose up-tail down position similar to that for LSD in several hours, the fish remaining alive." (B. Weiss, 1958)

A concentration of methylene blue of 100 ug/mL also caused a nose up-tail down reaction. There were similar results with gentian violet and Bindschedler's green.



Reference

WEISS B., H. A. ABRAMSON and M. O. BARON. (1958). Lysergic acid diethylamide (LSD-25). XXV. Effect of potassium cyanide and other oxidase and respiratory inhibitors on the Siamese fighting fish. AMA Arch.Neurol.Psychiatry. 80, 345-350.

pH 5.0 prevents fish surfacing reaction

Betta fish show a "surfacing reaction" when LSD is added to the tank water. About 10 minutes after 1 ug/mL LSD is added to water, almost 100 per cent of fish ascend to the surface of the tank and swim very slowly or not at all.

Abramson and Gettner attempted to block the LSD surfacing reaction in goldfish by lowering the water pH. In a bowl at pH 5.0, only 30 percent of the fish demonstrated the fish surfacing reaction. When shifted to pH 5.8, nearly all of the fish began the surfacing reaction.

From LSD fish

The authors suspected that pH affects the absorption of LSD across the gills. They wrote:

“Alkaloids are not absorbed to any extent from the stomach when the reaction of the gastric juice is strongly acid. If the gastric juice is rendered alkaline, however, alkaloids are rapidly absorbed from the ligated stomach. It is of interest that absorption slows at about pH 5.0 and is fairly rapid at pH 6.0.” (H.H. Gettner, 1973)

Using a different methodological approach, the researchers dipped goldfish for 30 seconds in LSD-containing liquid, and then placed them in normal tank water. Drug concentrations in the dipping tank ranged from 5 to 50 ug/mL. About 70% of the fish that were briefly dunked in LSD-containing water still demonstrated the fish surfacing reaction, but recovered more quickly than fish who were immersed in LSD-containing water for hours. The dotted lines in the figure below represent the die-away curves when fish were dipped for 30 s only, which demonstrated that LSD can be rapidly absorbed by the animal body. The surfacing reaction was more persistent when the fish were immersed in LSD solutions (solid black lines).

From LSD fish

Similar curves were obtained by dipping goldfish for 30 seconds into solutions of methysergide.


Reference

Gettner H. H., P. A. Carone and H. A. Abramson. (1973). Lysergic acid diethylamide (LSD 25). XXXX. Effect of pH on transport of methysergide and LSD 25 across gill membrane. J.Psychol. 84, 111-118.

HOMO calculations of phenothiazines

Molecular orbital calculations of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies reflect the electron-donating and electron-accepting properties of a molecule.

In the work described below, the HOMO energies of several tricyclic antidepressants and neuroleptics were calculated with 3 different methods, referred to as the RHF, LSD, and SE-AM1 methods. The drugs studied were promazine, chlorprothixene, chlorpromazine, dibenzepine, amitriptyline, imipramine, clomipramine and opipramol. These eight drugs have a phenothiazine-like structure in common.

From Chlorpromazine HOMO LEMO

When the HOMO energies of these 8 drugs were calculated using the RHF method, all values were between 7.4-8.87 electron volts (eV). HOMO values for these drugs ranged from 7.36-8.34 eV using the SE-AM1 method. These HOMO energies, obtained theoretically, are similar to the ionization energies of phenothiazine drugs obtained experimentally with photoelectron spectroscopy.

From Chlorpromazine HOMO LEMO

Most of the drugs studied have Nitrogen or Sulfur on the main ring. Out of this series of 8, promazine consistently had the smallest absolute value for HOMO energy.

“The contribution to the HOMO and LUMO from the atomic orbitals of the Nitrogen in the central ring is remarkable.” (J.A. Cogordan, 1999)

Reference

Cogordan J. A., M. Mayoral, E. Angeles, R. A. Toscano and R. Martinez. (1999). Neuroleptic and antidepressant tricyclic compounds: Theoretical study for predicting their biological activity by semiempirical, density functional, and hartree-fock methods. Int J Quant Chem. 71, 415-432.

Mescaline interferes with axoplasmic transport

Microtubules are involved in axonal transport, the movement of lipid-protein cargo from the cell body to the axon. In 1973, Paulson and McClure showed that colchicine and mescaline inhibit axoplasmic transport in the cat optic nerve preparation, probably by interfering with microtubules.

In Figure 1 (below), the authors compared the effects of mescaline, TMA, and TMA-2. TMA (middle panel) was more effective than mescaline (top panel), while the most potent inhibitor of axonal transport was TMA-2 (lower panel), which has a 2,4,5-trimethoxy configuration. This is interesting because TMA-2 is considered to be the most potent psychotomimetic drug among the TMA, e.g. more potent than TMA or mescaline. The authors proposed a correlation between the relative effectiveness of several trimethoxyphenylalkylamines as hallucinogens and as inhibitors of fast axoplasmic transport.

From Hallucinogen microtubule

The authors wrote,

“Inhibition of axoplasmic transport is also observed with two classes of anti-mitotic drugs, typified by colchicine and vinblastine. These drugs are thought to inhibit cell division through disruption of the microtubules of the mitotic spindle. Isolated microtubule subunits, designated as tubulin or microtubule protein, bind colchicine and vinblastine at different sites. Inhibition of axoplasmic transport by colchicine and vinblastine has been cited as support for the hypothesis that the microtubules which are always present in axons are involved in the transport process.” (J.C. Paulson, 1973)

In 1976, Harrison and colleagues showed that mescaline and colchicine bind purified microtubule protein, in support of the hypothesis that hallucinogens interfere with microtubules.

Reference

Paulson J. C. and W. O. McClure. (1973). Inhibition of axoplasmic transport by mescaline and other trimethoxyphenylalkylamines. Mol.Pharmacol. 9, 41-50.