The Contributions of Mu-Opioid Receptors on Glutamatergic and GABAergic Neurons to Analgesia Induced by Various Stress Intensities

The endogenous opioid system plays a crucial role in stress-induced analgesia. Mu-opioid receptors (MORs), one of the major opioid receptors, are expressed widely in subpopulations of cells throughout the CNS. However, the potential roles of MORs expressed in glutamatergic (MOR(Glut)) and γ-aminobutyric acidergic (MOR(GABA)) neurons in stress-induced analgesia remain unclear. By examining tail-flick latencies to noxious radiant heat of male mice, here we investigated the contributions of MOR(GABA) and MOR(Glut) to behavioral analgesia and activities of neurons projecting from periaqueductal gray (PAG) to rostral ventromedial medulla (RVM) induced by a range of time courses of forced swim exposure. The moderate but not transitory or prolonged swim exposure induced a MOR-dependent analgesia, although all of these three stresses enhanced β-endorphin release. Selective deletion of MOR(GABA) but not MOR(Glut) clearly attenuated analgesia and blocked the enhancement of activities of PAG-RVM neurons induced by moderate swim exposure. Under transitory swim exposure, in contrast, selective deletion of MOR(Glut) elicited an analgesia behavior via strengthening the activities of PAG-RVM neurons. These results indicate that MOR-dependent endogenous opioid signaling participates in nociceptive modulation in a wide range, not limited to moderate, of stress intensities. Endogenous activation of MOR(GABA) exerts analgesia, whereas MOR(Glut) produces antianalgesia. More importantly, with an increase of stress intensities, the efficiencies of MORs on nociception shifts from balance between MOR(Glut) and MOR(GABA) to biasing toward MOR(GABA)-mediated processes. Our results point to the cellular dynamic characteristics of MORs expressed in excitatory and inhibitory neurons in pain modulation under various stress intensities.