Sunday, December 06, 2015

LSD exhibits a low transport number in glass pipettes

A iontophoretic pipette is a thin metal wire in a glass cylinder, that is filled with a solution of drug. A "retaining" current is passed through the micropipette via the wire to suppress the spontaneous release of drug. A large retaining current will greatly depress the release of drug from the pipette tip, although no value of retaining current will prevent completely the diffusional efflux of drug from the ionotophoretic pipette. When the pipette is in place and the experimenter is ready to release the drug, an "ejection" current is applied. This stimulus is applied as a brief current pulse, not a steady current. There is a theoretical relationship between the ejection current and release of drug ions. The drug solution will be expelled from the tip of the pipette, depending on the amount of current passed and the drug's "transport number," which is the ratio of drug released to charge passed.
LSD has an exceptionally low transport number (t=0.02) for a chemical substance, thus most of the LSD molecules tend to stay in the pipette, even with large ejection currents. In comparison, acetylcholine and 5-HT have high transport numbers (t=0.2-0.4), and require short bursts of current to be released from the pipette.

When dealing with iontophoretic release of drugs from pipettes, we usually want to know the amount of drug released, but the value known with confidence is the ejection current. To calculate the transport number of each drug, the electrophysiologist must use pipettes that are filled with a radioactive isotope of the drug and apply different ejection currents. Then, the electrical charge (ucoulomb) in the pipette is plotted versus iontophoretic release (pmol) of drug, as shown for [3H]-LSD in the figure below. LSD-25 release from pipettes was directly proportional to the charge passed through the pipette. The transport number of LSD was obtained from this data by multiplying the slope (expressed in mol/coulomb) by Faraday's number.


In 1974, Haigler and colleagues reported that the transport number of LSD (0.0023) was much smaller than that of 5-HT (0.219). According to these researchers, 100 nA of ejection current of LSD would be equivalent to 1 nA ejection current of 5-HT in terms of the number of drug molecules ejected.
“... since equal currents of both agents inhibit the raphe, LSD emerges as being more potent than 5-HT, molecule for molecule, on the raphe neurons." (Haigler,H.J. 1974)
Some researchers found it difficult to pass current through LSD-containing glass electrodes.
“It was often impossible to pass current through barrels containing 2% LSD 25 solution for long periods. Longer applications of LSD 25 from a 0.5% solution had a depressant action on 22 out of 35 neurons tested.” (Boakes,R.J. 1970)
Chlorpromazine and Levallorphan, a drug similar to naloxone, have low transport numbers too. The transport numbers of chlorpromazine and Levallorphan are 0.0858 and 0.0737 (Zieglgansberger et al., 1974).

A molecule's transport number is easy to measure, and it gives information about the electronic properties of a drug. There was more similarity between chlorpromazine and LSD than LSD and 5-HT. The similarity of physical properties of chlorpromazine and LSD may be due to more conjugation of the rings compared to 5-HT.

Reference

Bradley P. B. and J. M. Candy (1970). Iontophoretic release of acetylcholine, noradrenaline, 5-hydroxytryptamine and D-lysergic acid diethylamide from micropipettes. British Journal of Pharmacology 40, 194-201. 

Boakes R. J., P. B. Bradley, I. Briggs and A. Dray (1970). Antagonism of 5-hydroxytryptamine by LSD 25 in the central nervous system: a possible neuronal basis for the actions of LSD 25. British Journal of Pharmacology 40, 202-218.

Haigler H. J. and G. K. Aghajanian (1974). Lysergic acid diethylamide and serotonin: a comparison of effects on serotonergic neurons and neurons receiving a serotonergic input. The Journal of Pharmacology and Experimental Therapeutics 188, 688-699.

Zieglgansberger W., G. Sothmann and A. Herz (1974). Iontophoretic release of substances from micropipettes in vitro. Neuropharmacology 13, 417-422. 10.1016/0028-3908(74)90129-4

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