Supraspinal facilitation of painful stimuli by glutamatergic innervation from the retrosplenial to the anterior cingulate cortex.

The anterior cingulate cortex (ACC) is recognized as a pivotal cortical region involved in the perception of pain. The retrosplenial cortex (RSC), located posterior to the ACC, is known to play a significant role in navigation and memory processes. Although the projections from the RSC to the ACC have been found, the specifics of the synaptic connections and the functional implications of the RSC-ACC projections remain less understood. In this study, we employed a combination of whole-brain imaging, in vitro electrophysiology, and two-photon calcium imaging techniques to confirm the presence of direct excitatory glutamatergic projections from the RSC to the ACC in mice. This excitatory transmission is predominantly mediated by the postsynaptic AMPA receptors. Furthermore, the activation of the RSC-ACC projections through opto-/chemogenetics significantly facilitated the behavioral responses to both mechanical and thermal nociceptive stimuli in adult mice. Notably, this activation did not influence spinal nociceptive responses in the tail-flick test, nor did it affect anxiety-like or aversive behaviors. These findings indicate that the RSC-ACC glutamatergic pathway modulates nociceptive perception primarily at the supraspinal cortical level. We have identified a novel cortico-cortical facilitatory pathway that contributes to nociceptive processing in the cingulate cortex. The RSC-ACC pathway probably serves to integrate memory engrams with pain perception in both humans and animals.