Loading of
GABA and
glycine into synaptic vesicles via the
vesicular GABA transporter (VGAT) is an essential step in inhibitory neurotransmission. As a result of the evidence linking alterations in GABAergic and/or glycinergic neurotransmission to various
pain disorders, we investigated the possible influence of down-regulation of VGAT on pain threshold and behavioral responses in mice. The phenotypes of heterozygous VGAT knockout [VGAT(+/-)] mice were compared with wild-type (WT) mice using behavioral assays. In addition, GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs) were recorded in dorsal horn neurons. Western blot analysis confirmed significant reduction of VGAT
protein levels in VGAT(+/-) mice. However, high-performance liquid chromatography revealed that
glutamate,
GABA, and
glycine contents in the whole brain and spinal cord were normal in VGAT(+/-) mice. Behavioral analysis of VGAT(+/-) mice showed unchanged motor coordination, anxiety, memory performance, and
anesthetic sensitivity to
propofol and
ketamine, although thermal nociception and inflammatory
pain were enhanced. Patch-clamp recordings revealed that the frequency and amplitude of glycinergic mIPSCs in lamina II neurons were reduced in VGAT(+/-) mice. Genotype differences in glycinergic mIPSCs were more evident during sustained stimulation by solutions with high
potassium levels, suggesting that the estimated size of the readily releasable pool of
glycine-containing vesicles was reduced in VGAT(+/-) mice. These results provide genetic, behavioral, and electrophysiological evidence that VGAT-mediated inhibitory drive alters very specific forms of sensory processing: those related to
pain processing. More close examination will be needed to verify the possibility of VGAT as a new therapeutic target for the treatment of inflammatory
pain.