GABA transporter type 1 (GAT-1) uptake inhibition reduces stimulated aspartate and glutamate release in the dorsal spinal cord in vivo via different GABAergic mechanisms

Mechanisms through which the reported antinociceptive activity of GABA re-uptake inhibitors is mediated (and where on the sensory neuraxis) have not been defined. Here, microdialysis in the anaesthetised rat was used to examine the effect of selective GABA transporter type 1 (GAT-1) inhibition on basal and evoked amino acid release in the dorsal spinal cord. Reverse dialysis of the selective GAT-1 inhibitor NO-711 (10–300 μM) induced a concentration-related increase in extracellular GABA (maximal threefold of basal levels) without affecting other amino acids. Employing an S2/S1 paradigm, release evoked by brief high (45 mM) K⁺-induced depolarisation of aspartate and glutamate, but not GABA or glycine, was found to be significantly reduced by reverse dialysis of NO-711 (300 μM). Co-administration of selective antagonists for GABA_A or GABA_B receptors ((+)-bicuculline (100 μM) or SCH 50911 (100 μM), respectively) prevented the GAT-1 inhibition-induced reduction of evoked aspartate. In contrast, while (+)-bicuculline also antagonised the reduction of evoked glutamate, SCH 50911 (up to 1 mM) was without effect. Inhibition of GAT-1 re-uptake was further found to play a permissive role in autoinhibitory effects on GABA release mediated through GABA_A and GABA_B receptors. These data demonstrate that augmentation of GABAergic neurotransmission by re-uptake inhibition activates pharmacologically distinguishable inhibitory influences on aspartate and glutamate release in the dorsal spinal cord. Thus, inhibition of spinal pro-nociceptive neurotransmitter release may contribute to the analgesic action of this drug class.