A Critical Role for Dopamine D5 Receptors in Pain Chronicity in Male Mice.

Dopaminergic modulation of spinal cord plasticity has long been recognized, but circuits affected by this system and the precise receptor subtypes involved in this modulation have not been defined. Dopaminergic modulation from the All nucleus of the hypothalamus contributes to plasticity in a model of chronic pain called hyperalgesic priming. Here we tested the hypothesis that the key receptor subtype mediating this effect is the D5 receptor (D5R). We find that a spinally directed lesion of dopaminergic neurons reverses hyperalgesic priming in both sexes and that a D1/D5 antagonist transiently inhibits neuropathic pain. We used mice lacking D5Rs (DRÜ5KO mice) to show that carrageenan, interleukin 6, as well as BDNF-induced hyperalgesia and priming are reduced specifically in male mice. These male DRD5KO mice also show reduced formalin pain responses and decreased heat pain. To characterize the subtypes of dorsal horn neurons engaged by dopamine signaling in the hyperalgesic priming model, we used c-fos labeling. We find that a mixed D1/D5 agonist given spinally to primed mice activates a subset of neurons in lamina 111 and IV of the dorsal horn that coexpress PAX2, a transcription factor for GABAergic interneurons. In line with this, we show that gabazine, a GABA-A receptor antagonist, is antihyperalgesic in primed mice exposed to spinal administration of a D1/D5 agonist. Therefore, the D5R, in males, and the DIR, in females, exert a powerful influence over spinal cord circuitry in pathological pain likely via modulation of deep dorsal horn GABAergic neurons. [ABSTRACT FROM AUTHOR]