Central to the linkage of pain circuitry with the limbic system is its initial NAα2-mediated antinociceptive effect in acute pain models, followed by contradictory pronociceptive activation by the locus coeruleus seen in chronic pain models. Rats with a stable, long-term (>10 weeks) inflammatory compression of the trigeminal infraorbital nerve (FRICT-ION) preclinical model were given daily doxazosin, a slow-release NAα1 receptor antagonist, in weeks 8–10. Facial hypersensitivity was reversed back to baseline in male and female rats, but anxiety was only reduced in male animals. Doxazosin-decreased astrocytic activation was indicated by a decrease in both intracranial cathepsin B imaging in vivo and GFAP immunostaining in the somatosensory cortex and hippocampus. Doxazosin reduction in NAα1 receptor activation diminished glial-neuronal interactions, resulting in downstream reduction in pain-related behaviors. Other significant differences by sex included improved elevated zero maze anxiety measures only in males, and improved novel recognition scores only in females. Elevated thymus chemokine CXCL7 levels were reduced by doxazosin but only in male rats. These sexually dimorphic contradictions further complicate the understanding of the noradrenergic system’s involvement in nociception. The findings indicate that by reducing NAα1 receptor drive with doxazosin, the role of the locus coeruleus can be shifted back to NAα2-receptor-mediated pain inhibition.
Westlund et al. (Wed,) studied this question.