Inhibitory gating of thalamocortical inputs onto rat gustatory insular cortex

Abstract

In the rat primary gustatory cortex (GC), a subregion of the larger insular cortex, neurons display time-varying neural responses to gustatory stimuli. GC taste responses are dramatically reduced following inactivation of the gustatory thalamus, the parvicellular region of the ventral posteromedial thalamic nucleus (VPMpc). Pharmacological inactivation of VPMpc also has a profound effect on GC spontaneous activity. This indicates that the projection from VPMpc plays a crucial role in GC taste processing as well as in the control of its state. How VPMpc afferents engage GC circuits and drive neuronal ensembles to effectively code tastant identity, as well as modulate the overall state of the GC network, remains unclear. To investigate the synaptic properties and organization of VPMpc afferents in GC, we employed a circuit-breaking optogenetic approach, stimulating VPMpc terminal fields while performing whole-cell patch clamp recordings from GC neurons in rat acute slices. Informed by previous studies of thalamocortical inputs to other sensory cortices, we hypothesized that VPMpc-GC synapses have laminar- and cell-specific properties that gate sensory input, conferring computationally flexibility to how taste information is processed in GC. We found that VPMpc-GC synapses are strongly gated by the activity regime of VPMpc afferents, as well as by feedforward and feedback inhibition onto VPMpc terminals. These results provide novel insight into the circuit underpinning of GC responsiveness to incoming thalamocortical activity. SIGNIFICANCE STATEMENTWe report that the input from the primary taste thalamus to the primary gustatory cortex (GC) shows distinct properties compared to primary thalamocortical synapses onto other sensory areas. VPMpc afferents in GC make synapses with excitatory neurons distributed across all cortical layers and display frequency-dependent short-term plasticity to repetitive stimulation, thus they do not fit the classic distinction between drivers and modulators typical of other sensory thalamocortical circuits. Feedforward inhibition gates thalamocortical activation and provides local corticothalamic feedback via presynaptic ionotropic and metabotropic GABA receptors. The connectivity and inhibitory control of thalamocortical synapses support the time-varying response dynamics to taste stimuli observed in GC neurons in vivo.