Endogenous ligand(s) of benzodiazepine receptors?

Neurochemistry International, Vol. 5, No. 6. pp. 673 674, 1983 Printed in Great Britain. All rights reserved

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0197-0186/83-S3.00+0.00 1983 Pergamon Press ktd

CRITIQUE ENDOGENOUS LIGAND(S) OF BENZODIAZEPINE RECEPTORS? MANFRED KAROBATH Sandoz Ltd., Preclinical Research, CH-4002 Basle, Switzerland

In the preceding review by M. Hamon an attempt has can lead to a physiological modulation of diazepam been made to evaluate current evidence for the binding. Additional hypotheses have to be formuexistence of endogenous ligands for benzodiazepine lated, like compartmentation with high local concenreceptors. Initially, before the functional association trations at GABA-ergic synapses. Thus, 1 agree with of benzodiazepine receptors as constituents of certain Hamon that it is highly questionable that an endotypes of GABA-receptors was known, such attempts genous compound has been isolated from biological appeared straightforward and resembled in their material which has the properties of an endogenous strategy the search for an endogenous ligand for ligand, and it appears therefore premature to specuopiate receptors. The successful isolation of en- late on the chemical nature of this compound. kephalines has greatly been aided by the existence of Whereas initially (due to the presence of endoa bioassay for compounds with morphine like activ- genous GABA in the membrane preparations which ity. Unfortunately a bioassay for benzodiazepines were used in binding experiments) no functional link and related drugs does not exist and potential endo- between GABA and benzodiazepine receptors could genous ligands must be detected by binding assays. be observed, subsequent investigations have provided As pointed out by Hamon binding assays are very ample evidence for an association of benzodiazepine sensitive to perturbation by various factors unrelated receptors with certain types of GABA receptors. to endogenous ligands. For example, Gfinter Sperk Thus benzodiazepine receptors appear to be phylogeand I were somehow surprised to observe that repeat- netically young additional constituents of GABA edly washed Dowex 50 or XAD-2 columns to which receptors and they may even reside on the same no biological material had been applied, eluted quite peptides as GABA recognition sites. Accordingly it high amounts of material which inhibited has been suggested that the occupancy of benzo[~H]diazepam binding. Similarly, in their studies diazepine receptors with its ligands may modulate the which led to the isolation (respectively synthesis) of signal transfer from GABA-recognition sites to the /3-carbolines from urine, Braestrup and his colleagues chloride conductance mechanism by facilitating cerobserved fractions from urine which inhibited bind- tain configurations of the involved proteins. In this ing, most likely by the presence of surface active concept benzodiazepine receptors are considered as compounds which perturb membranes (Nielson et al., drug receptors of certain GABA receptors, (M6hler 1979). Whereas these artefacts can be detected by 1981), similar to the local anaesthetic binding sites of suitable controls they nevertheless point to the prob- nicotine receptors, and an endogenous ligand for lems which are encountered when binding assays are benzodiazepine (drug) receptors is not required. used to detect biological activity. Whereas the possibility of the existence of an A number of low molecular compounds, men- endogenous ligand for benzodiazepine receptors cantioned in M. Hamon's review, have been identified not be excluded by this concept, such a postulated which (with the exception of the ~q-carbolines which endogenous ligand would serve as a co-transmitter or appear to have been formed during the extraction co-modulator in addition to GABA, the endogenous procedure) have in common that they are recognized ligand of this receptor. Indeed, a peptide termed DBI with quite low affinity by benzodiazepine receptors. It (diazepam binding inhibitor) with a molecular weight is difficult to imagine bow compounds, like several of of approximately 9 K daltons has been isolated to the purines which inhibit binding in concentrations apparent homogeneity from brain (Massotti et al., close to their limits of solubility in aqueous solutions, 1981). This protein has been reported to inhibit 673

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diazepam binding at 0 C by 50",, m 1 4 it M concentrations ( - 1 0 4 0 t l g D B I m l ) . This rather low affinity interaction of DBI with benzodiazepinc receptors implies again that either high concentrations of this protein occur in brain and:or that it is highly compartmentalized in GABA-ergic synapses. More investigations are therefore required before the specilicity and functional meaning of DBI for G A B A receptors is established.

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Massotti M., Ouidotti A. and ('osta E. (1981) Characterization of benzodiazepine and 7-aminobutyric acid recognition sites tlnd their endogenous modulators. J..\uurovc/.

I, 409 418. M(:~lller ft. (I 981) Bcnzodiazepmc receptors: tire Ihere undogenous ligands m the brain. T I P S 2 , 116 119. Nielsen M., Grcndal (). aim Braestrup ('. (1979) Some properties of [~tl]diazcpam displacing actixil5 from human urine I.i/i, .S'ci. 25, 679 6~fl