Reversal of pathological pain through specific spinal GABA_A receptor subtypes

Inflammatory diseases and neuropathic insults are frequently accompanied by severe and debilitating pain, which can become chronic and often unresponsive to conventional analgesic treatment. A loss of synaptic inhibition in the spinal dorsal horn is considered to contribute significantly to this pain pathology. Facilitation of spinal γ-aminobutyric acid (GABA)ergic neurotransmission through modulation of GABA_A receptors should be able to compensate for this loss. With the use of GABA_A-receptor point-mutated knock-in mice in which specific GABA_A receptor subtypes have been selectively rendered insensitive to benzodiazepine-site ligands, we show here that pronounced analgesia can be achieved by specifically targeting spinal GABA _A receptors containing the α2 and/or α3 subunits. We show that their selective activation by the non-sedative ('α1-sparing') benzodiazepine-site ligand L-838,417 (ref. 13) is highly effective against inflammatory and neuropathic pain yet devoid of unwanted sedation, motor impairment and tolerance development. L-838,417 not only diminished the nociceptive input to the brain but also reduced the activity of brain areas related to the associative-emotional components of pain, as shown by functional magnetic resonance imaging in rats. These results provide a rational basis for the development of subtype-selective GABAergic drugs for the treatment of chronic pain, which is often refractory to classical analgesics.