Pregabalin is not a GABAA or GABAB receptor agonist.
Pregabalin is a gabapentinoid and acts by inhibiting certain calcium channels. Specifically it is a ligand of the auxiliary α2δ subunit site of certain voltage-dependent calcium channels (VDCCs), and thereby acts as an inhibitor of α2δ subunit-containing VDCCs. There are two drug-binding α2δ subunits, α2δ-1 and α2δ-2, and pregabalin shows similar affinity for (and hence lack of selectivity between) these two sites. Pregabalin is selective in its binding to the α2δ VDCC subunit. Despite the fact that pregabalin is a GABA analogue, it does not bind to the GABA receptors, does not convert into GABA or another GABA receptor agonist in vivo, and does not directly modulate GABA transport or metabolism. However, pregabalin has been found to produce a dose-dependent increase in the brain expression of L-glutamic acid decarboxylase (GAD), the enzyme responsible for synthesizing GABA, and hence may have some indirect GABAergic effects by increasing GABA levels in the brain. There is currently no evidence that the effects of pregabalin are mediated by any mechanism other than inhibition of α2δ-containing VDCCs. In accordance, inhibition of α2δ-1-containing VDCCs by pregabalin appears to be responsible for its anticonvulsant, analgesic, and anxiolytic effects.
The endogenous α-amino acids L-leucine and L-isoleucine, which closely resemble pregabalin and the other gabapentinoids in chemical structure, are apparent ligands of the α2δ VDCC subunit with similar affinity as the gabapentinoids (e.g., IC50 = 71 nM for L-isoleucine), and are present in human cerebrospinal fluid at micromolar concentrations (e.g., 12.9 μM for L-leucine, 4.8 μM for L-isoleucine). It has been theorized that they may be the endogenous ligands of the subunit and that they may competitively antagonize the effects of gabapentinoids. In accordance, while gabapentinoids like pregabalin and gabapentin have nanomolar affinities for the α2δ subunit, their potencies in vivo are in the low micromolar range, and competition for binding by endogenous L-amino acids has been said to likely be responsible for this discrepancy.
Pregabalin was found to possess 6-fold higher affinity than gabapentin for α2δ subunit-containing VDCCs in one study. However, another study found that pregabalin and gabapentin had similar affinities for the human recombinant α2δ-1 subunit (Ki = 32 nM and 40 nM, respectively). In any case, pregabalin is 2 to 4 times more potent than gabapentin as an analgesic and, in animals, appears to be 3 to 10 times more potent than gabapentin as an anticonvulsant.