Peripheral and central antinociceptive action of Na+-K+-2Cl- cotransporter blockers on formalin-induced nociception in rats.

The possible local peripheral and spinal (intrathecal) antinociceptive effect of Na(+)-K(+)-2Cl(-) cotransporter (NKCC) inhibitors was investigated in the rat formalin test. Nociceptive flinching behavior induced by formalin (1%) injection in the hind paw was assessed following administration of cotransporter inhibitors. Local peripheral pretreatment in the ipsilateral paw with bumetanide (ED(30), 27.1+/-12.7 microg/paw), piretanide (ED(30), 109.2+/-21.6 microg/paw) or furosemide (ED(30), 34.3+/-5.0 microg/paw), but not vehicle (DMSO 100%), produced dose-dependent antinociception in phase 2 of the test. Local bumetanide had the greatest effect (approximately 70% antinociception). Bumetanide also inhibited formalin-induced flinching behavior during phase 1 (ED(30), 105.6+/-99.1 microg/paw). Spinal intrathecal pretreatment with bumetanide (ED(30), 194.6+/-97.9 microg), piretanide (ED(30), 254.4+/-104.9 microg) or furosemide (ED(30), 32.0+/-6.9 microg), but not vehicle (DMSO 100%), also produced antinociception in phase 2. In this case, only intrathecal furosemide reduced flinching behavior during phase 1 (ED(30), 99.4+/-51.4 microg) and had the maximal antinociceptive effect in phase 2 (approximately 65% antinociception). The opioid receptor-antagonist naloxone (2 mg/kg, s.c.) did not reverse antinociception induced by either peripheral or spinal administration of NKCC blockers. Our data suggest that the Na(+)-K(+)-2Cl(-) cotransporter localized in sensory neurons at intraspinal and peripheral sites is involved in formalin-induced nociception.