Stress-induced antinociception to noxious heat requires α 1A -adrenaline receptors of spinal inhibitory neurons in mice

It is well known that acute exposure to physical stress produces a transient antinociceptive effect (called stress-induced analgesia [SIA]). One proposed mechanism for SIA involves noradrenaline (NA) in the central nervous system. NA has been reported to activate inhibitory neurons in the spinal dorsal horn (SDH), but its in vivo role in SIA remains unknown. In this study, we found that an antinociceptive effect on noxious heat after acute exposure to restraint stress was impaired in mice with a conditional knockout of α 1A -adrenaline receptors (α 1A -ARs) in inhibitory neurons ( Vgat-Cre ; Adra1a flox/flox mice). A similar reduction was also observed in mice treated with N -(2-chloroethyl)- N -ethyl-2-bromobenzylamine, a selective neurotoxin for NAergic neurons in the locus coeruleus (LC). Furthermore, whole-cell patch-clamp recordings using spinal cord slices revealed that NA-induced increase in the frequency of spontaneous inhibitory postsynaptic currents in the substantia gelatinosa neurons was suppressed by silodosin, an α 1A -AR antagonist, and by conditional knockout of α 1A -ARs in inhibitory neurons. Moreover, under unstressed conditions, the antinociceptive effects of intrathecal NA and phenylephrine on noxious heat were lost in Vgat-Cre ; Adra1a flox/flox mice. Our findings suggest that activation of α 1A -ARs in SDH inhibitory neurons, presumably via LC-NAergic neurons, is necessary for SIA to noxious heat.