Oxytocin receptor (OTR) activation at the spinal level produces antinociception. Some data suggest that central OTR activation enhances social interaction via an increase of
endocannabinoids (eCB), but we do not know if this could occur at the spinal level, modulating
pain transmission. Considering that
oxytocin via OTR stimulates
diacylglycerol formation, a key intermediate in synthesizing
2-arachidonylglycerol (2-AG), an eCB molecule, we sought to test the role of the eCB system on the spinal
oxytocin-induced antinociception. Behavioral and electrophysiological experiments were conducted in naïve and
formalin-treated (to induce long-term mechanical
hypersensitivity) male Wistar rats. Intrathecal
RHC 80267 injections, an inhibitor of the
enzyme diacylglycerol lipase (thus, decreasing 2-AG formation), produces transient mechanical
hypersensitivity, an effect unaltered by
oxytocin but reversed by
gabapentin. Similarly, in in vivo extracellular recordings of naïve spinal wide dynamic range cells, juxtacellular picoinjection of
RHC 80267 increases the firing of nociceptive Aδ-, C-fibers, and post-discharge, an effect unaltered by
oxytocin. Interestingly, in sensitized rats,
oxytocin picoinjection reverses the RHC 80627-induced hyperactivity of Aδ-fibers (but not C- or post-discharge activity). In contrast, a sub-effective dose of
JZL184 (a
monoacylglycerol lipase inhibitor, thus favoring 2-AG levels), which does not have per se an antinociceptive effect in the
formalin-induced hypernociception, the
oxytocin-induced antinociception is boosted. Similarly, electrophysiological experiments suggest that juxtacellular
JZL184 diminishes the neuronal firing of nociceptive fibers, and co-injection with
oxytocin prolongs and enhances the antinociceptive effect. These data may imply that 2-AG formation may play a role in the spinal antinociception induced by
oxytocin.