Descending projections from the insular cortex to the lateral parabrachial nucleus facilitate nocifensive behaviors in rats

The lateral parabrachial nucleus (LPBN) receives orofacial nociceptive inputs from the trigeminal spinal subnucleus caudalis and transmits them to higher brain regions, including the central amygdala (CeA), intralaminar thalamic nucleus, and insular cortex (IC). The IC processes orofacial nociception and sends descending axons to the LPBN. This study aimed to investigate the physiological functions of IC→LPBN projections using optogenetics. We classified LPBN neurons into glutamatergic and noncholinergic (Glu-nonAch), GABAergic and noncholinergic (GABA-nonAch), glutamatergic and cholinergic (Glu-Ach), and GABAergic and cholinergic (GABA-Ach) neurons. Only Glu-nonAch and Glu-Ach neurons projected to the CeA. Fos proto-oncogene (c-Fos) immunoreactivity in response to capsaicin injection into the whisker pad was observed in all types of neurons. Photostimulation-induced evoked excitatory postsynaptic currents (pEPSCs) by depolarization of axons arising from the spinal subnucleus caudalis were primarily observed in Glu-nonAch neurons, whereas activation of IC axons evoked pEPSCs in all neuronal subtypes. Interestingly, in the IC→LPBN projections, Glu-Ach neurons exhibited greater charge transfer with slower temporal kinetics of pEPSCs than Glu-nonAch neurons. Conversely, GABA-nonAch neurons presented less charge transfer and shorter temporal kinetics of pEPSCs than Glu-Ach and GABA-Ach neurons. Selective activation of IC→LPBN inputs under depolarized membrane potential often induced phase-locked action potentials in Glu-nonAch neurons projecting to the CeA. Behavioral tests demonstrated that the activation of IC→LPBN projections increased face-wiping behavior responding to capsaicin injection into the whisker pad and decreased the threshold of the head-withdrawal reflex. These results suggest that IC projections evoke larger EPSCs in LPBN excitatory projection neurons than in GABAergic neurons, contributing to the facilitation of facial pain.