Synaptic function of descending projections from the insular cortex to the nucleus solitary tract in the rat

The rostral part of the nucleus of the solitary tract (rNST) receives gustatory inputs via the facial, glossopharyngeal, and vagus nerves. In addition to these ascending pathways, the rNST receives descending projections from higher brain regions, including the insular cortex (IC). Neurons in the dysgranular and granular IC around the middle cerebral artery (MCA) respond to gustatory stimulation; therefore, descending IC projections to the rNST are thought to regulate gustatory information processing. However, little is known about how IC inputs modulate rNST neuronal activity at the synaptic level, which comprises both glutamatergic and GABAergic neurons. In this study, we examined the synaptic strength of IC → rNST projections in glutamatergic and GABAergic neurons using vesicular GABA transporter-Venus transgenic rats. We first confirmed that the IC subregion around the MCA, a region where gustatory information converges, projects axons to the rNST. Whole-cell patch-clamp recordings revealed that rNST neurons could be classified into three groups: regular-spiking (68.2 %), late-spiking (22.7 %), and burst-spiking (9.1 %). Among glutamatergic neurons, 58.8 % were regular-spiking, 23.5 % were late-spiking, and 17.6 % were burst-spiking, whereas GABAergic neurons were predominantly regular-spiking (77.8 %), with fewer late-spiking (18.5 %) and burst-spiking (3.7 %) neurons. In rats injected with AAV5-CAG-ChR2(H134R)-mCherry into the IC subregion, both glutamatergic and GABAergic rNST neurons exhibited photostimulation-induced monosynaptic excitatory postsynaptic currents with comparable amplitudes and latencies. Paired whole-cell recordings further demonstrated that glutamatergic neurons receive inhibitory inputs from rNST GABAergic neurons with a high failure rate. These findings suggest that IC projections to the rNST primarily enhance excitatory output from the rNST neurons. (250/250 words).