Top-down descending modulation of dorsal spinal excitatory transmission from the insular cortex.

The insular cortex (IC), a critical hub for nociception, emotion, and cognition processing, has emerged as a key role in the descending modulation of spinal cord excitability. Although previous studies have suggested that IC may influence spinal nociceptive reflexes through direct or indirect top-down pathways, the specific effects of IC stimulation on spinal nociceptive transmission remain unclear. In this study, by combining in vivo whole-cell patch-clamp, behavioral and morphological approaches, we identified a direct projection from the IC to the contralateral dorsal spinal cord. To determine whether IC activation affect the spinal nociceptive reflex, we measured the spinal nociceptive tail-flick (TF) reflex during IC stimulation. We found that activating the IC by electric stimulation did not significantly alter the spinal TF reflex. Furthermore, in vivo whole-cell patch-clamp recordings from spinal dorsal horn neurons revealed that IC stimulation produced delayed inhibition of spontaneous excitatory transmission in some neurons, while exciting or having no significant effect on others. These results indicate that the top-down modulation from the IC to the spinal cord is not uniformly facilitatory, distinguishing it from the consistently facilitatory effects observed in the anterior cingulate cortex (ACC)-spinal cord projection.