SUMMARY Oxytocin (OT) is a neuropeptide elaborated by the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei. Magnocellular OT neurons of these nuclei innervate numerous forebrain regions and release OT into the blood from the posterior pituitary. The PVN also harbors parvocellular OT cells that project to the brainstem and spinal cord, but their function has not been directly assessed. Here, we identified a subset of approximately 30 parvocellular OT neurons, with collateral projections onto magnocellular OT neurons and neurons of deep layers of the spinal cord. Evoked OT release from these OT neurons suppresses nociception and promotes analgesia in an animal model of inflammatory pain. Our findings identify a new population of OT neurons that modulates nociception in a two tier process: (1) directly by release of OT from axons onto sensory spinal cord neurons and inhibiting their activity and (2) indirectly by stimulating OT release from SON neurons into the periphery.

In humans, gamma-band oscillations in the primary somatosensory cortex (S1) correlate with subjective pain perception. However, functional contributions to pain and the nature of underlying circuits are unclear. Here we report that gamma oscillations, but not other rhythms, are specifically strengthened independently of any motor component in the S1 cortex of mice during nociception. Moreover, mice with inflammatory pain show elevated resting gamma and alpha activity and increased gamma power in response to sub-threshold stimuli, in association with behavioral nociceptive hypersensitivity. Inducing gamma oscillations via optogenetic activation of parvalbumin-expressing inhibitory interneurons in the S1 cortex enhances nociceptive sensitivity and induces aversive avoidance behavior. Activity mapping identified a network of prefrontal cortical and subcortical centers whilst morphological tracing and pharmacological studies demonstrate the requirement of descending serotonergic facilitatory pathways in these pain-related behaviors. This study thus describes a mechanistic framework for modulation of pain by specific activity patterns in the S1 cortex.

The identity of cortical circuits mediating nociception and pain is largely unclear. The cingulate cortex is consistently activated during pain, but the functional specificity of cingulate divisions, the roles at distinct temporal phases of central plasticity and the underlying circuitry are unknown. Here we show in mice that the midcingulate division of the cingulate cortex (MCC) does not mediate acute pain sensation and pain affect, but gates sensory hypersensitivity by acting in a wide cortical and subcortical network. Within this complex network, we identified an afferent MCC-posterior insula pathway that can induce and maintain nociceptive hypersensitivity in the absence of conditioned peripheral noxious drive. This facilitation of nociception is brought about by recruitment of descending serotonergic facilitatory projections to the spinal cord. These results have implications for our understanding of neuronal mechanisms facilitating the transition from acute to long-lasting pain.