So this thread has taught me a good bit about relevant details of GABAnergic agonism. I have two main questions:
1. How wide is the gulf between GABA-A and GABA-B mediated inhibition? To the extent that the relevant processing pathways are entirely separate or profoundly downstream, we should expect concurrent GABA-A and GABA-B agonism (or positive allosteric modulation thereof, as is usually the case with GABA-A 'agonism') to be additive in effect. But to what extent is it synergistic? How do we account for such synergy?
2. To what extent is neural adaptation to exogenous GABAnergic agonists specific to these ligands' effects? That is, to what extent will there be cross-tolerance between GABA-A and GABA-B agonists? To the extent that tolerance is limited to receptor downregulation, I would expect tolerance to be highly specific. But to what extent are other mechanisms, eg downregulation of the endogenous conversion of glutamate to GABA, involved?
To partially answer your questions, first- there is a huge gulf between GABA-a & GABA-b exogenous-positive-modulators & agonists, respectively. GABA-b is a
metabotropic receptor complex for the endogenously occurring GABA NT'ers. GABA-b is also linked to G-proteins via potassium channels. Second, GABA-a is a alternatively an
ionotropic receptor, along w/ being a ligand-gated ion channel, not a G protein receptor. (the GABAergic system is
ABSOLUTELY THE MOST INTRICATE receptor-complex). Both GABA-a & GABA-b receptors & their related subunits have endogenous GABA as their ligands of course.
As far as GABA-a/GABA-b positive allosteric modulators/agonists impacting decarboxylation of glutamate to GABA, I don't think this occurs to any major effect--or at least, i haven't read anything on this. I am sure a small amount of this does occur--but most downregulation is specific to each ligand's respective receptor subunit AFAIK. There is certainly synergetic effects between both GABA-a & GABA-b ligands, i suspect however this occurs via a deeper mechanism rather than through GABAergic cross tolerance.. B/c there is no cross tolerance between GABA-a positive allosteric modulators & GABA-b agonists. This can be obviously seen, in that, withdrawal from even
moderate dependencies on potent & short acting GABA-b agonists like GHB do not respond with even extremely high doses of benzodiazepines (however, barbiturates.. which are GABA-a PAM's but do not bind at the GABA-a benzodiazepine sites--at the GABA-a beta binding sites I believe--),
While GABA-a PAM's aren't terribly effective in replacing GABA-b dependencies... early research is showing that GABA-b agonists (particularly baclofen, a very purely acting GABA-b agonist) are more effective in replacing GABA-a PAM's; especially ethanol--albeit these results are preliminary and
MAY be reversed in future university research, if some attempt to do so; b/c there's absolutely
ZERO interest by big PhRMA here in the US for GABA-b meds!
Hope this helps.