Compound unknown. GABA is the main inhibitory neurotransmitter in the Central Nervous System and its effect is mediated through GABA receptors. Benzodiazepines are compounds that reinforce the action of GABA in the brain, which gives them antiepileptic properties. Consequently, benzodiazepines are one of the most common groups of antiepileptic drugs used to interrupt acute epileptic seizures. Interestingly, benzodiazepines have their own binding site on the GABA receptor, suggesting that they might actually mimic the effect of another, yet unknown substance that is present in the brain. The identity of this mysterious substance, the endogenous benzodiazepine or endozepine, has been one the romantic mysteries of neuroscience. Now, a recent paper in Neuron provides strong evidence that products of the DBI gene are the long-sought endozepine.
Of NAMS and PAMS. The existence of endozepines was first hypothesized in 1977 and the DBI gene was discovered as a possible candidate in 1991. However, the evidence regarding the DBI gene was conflicting. Some studies suggested that it worked as a so-called negative allosteric modulator (NAM), while other studies suggested that the DBI protein product is a positive allosteric modulator (PAM). Allosteric modulators are chemical compounds acting on a receptor, but not through the actual binding site for the natural ligand. In the case of DBI, this allosteric binding site is the so-called benzodiazepine receptor in the GABA-A receptor. Accordingly, the earlier studies did not produce clear evidence whether the DBI protein activates or inhibits the GABA-A receptor and –most importantly – it remained unknown whether this mechanism is important in vivo.
GABRA3, reticular thalamus. Christian and collaborators used a set of interesting experiments to investigate whether the DBI gene codes for an endozepine that has physiological activity in the rodent brain. First, they looked at two different mouse lines. One mouse line had a mutation in the benzodiazepine binding site (GABRA3-H126R) the other mouse line was a DBI knock-out mouse. Both mouse models had a higher propensity to absence seizures than control rodents. In the GABRA3-H126R mouse, Christian and collaborators discovered briefer inhibitory potentials that were resistant to a benzodiazepine antagonist. In wildtypes, the same antagonist reduced the length of inbitory potential, suggesting that some endogenous mechanism is at play that works through the benzodiazepine binding site of the GABA-A receptor.
The reticular thalamus. Interestingly, this observation was only true for the reticular thalamus (nRT), a brain structure with strong DBI expression. When patches of neuronal membrane from other brain regions were transplanted into the nRT, the same observation was made, suggesting that some substance in the nRT works as a positive allosteric modulator. When Christian and collaborators looked at the DBI knock-out mouse, they made similar observations. The duration of inhibitory potentials in the nRT were resistant to benzodiazepine antagonists and could be rescued if DBI was expressed in the nRT using a viral vector. This additional piece of information strongly suggests that the identified PAM action is carried out by the DBI gene products. Taken together, the publication by Christian and collaborators makes a very strong case of DBI being the long-missing endozepine.
DBI and human epilepsy. Might there be any role for endozepines in human epilepsies, particularly absence epilepsies? The DBI gene on chromosome 2 is part of various pathogenic microdeletions and microduplications, but none of the patients reported in DECIPHER had seizures as a phenotype. However in 2001, Wallace and collaborators identified a mutation in GABRG2 in a family with childhood absence epilepsy and Febrile Seizures. Interestingly, the mutation in GABRG2, the gamma-2 subunit of the GABA-A receptor, was in the benzodiazepine receptor, suggesting that the endogenous action of endozepines might be impaired, resulting in absence seizures. This finding in conjunction with the current study by Christian and collaborators suggests that endozepines are at work in the human brain and that disruption of the endogenous action results in a predisposition to epilepsy. There is still much to be learned about this mechanism. However, these studies already indicate that there is more to GABAergic transmission than GABA alone.