Antagonism of Supraspinal Histamine H3 Receptors Modulates Spinal Neuronal Activity in Neuropathic Rats

There is growing evidence supporting a role for histamine H3 receptors in the modulation of pathological pain. To further our understanding of this modulation, we examined the effects of a selective H3 receptor antagonist, 6-((3-cyclobutyl-2,3,4,5-tetrahydro-1H-3-benzazepin-7-yl)oxy)-N-methyl-3-pyridinecarboxamide (GSK189254), on spinal neuronal activity in neuropathic (L5 and L6 ligations) and sham rats. Systemic administration of GSK189254 (0.03–1 mg/kg i.v.) dose-dependently decreased both evoked (10-g von Frey hair for 15 s) and spontaneous firing of wide dynamic range (WDR) neurons in neuropathic, but not sham-operated, animals. The effects on spontaneous firing suggest that H3 receptors may have a role in central sensitization and/or modulating non-evoked pain. Transection of the spinal cord (T9-T10) completely eliminated the effects (both evoked and spontaneous) of systemic GSK189254 (1 mg/kg, i.v.) on WDR neuronal firing in neuropathic rats, indicating that the descending modulatory system has an important role in the H3-related dampening of spinal neuronal activity. Subsequently, lesions of the locus coeruleus, or direct GSK189254 (3 and 10 nmol/0.5 μl) injections into this site, demonstrate that the locus coeruleus is a key component of the H3 descending modulatory pathway. In summary, blockade of H3 receptors reduces spontaneous firing as well as the responses of spinal nociceptive neurons to mechanical stimulation. This effect is in large part mediated via supraspinal sites, including the locus coeruleus, that send descending projections to modulate spinal neuronal activity.