Dedicated C-fibre viscerosensory pathways to central nucleus of the amygdala

Emotional state is impacted by changes in visceral function, including blood pressure, breathing and digestion. A main line of viscerosensory information processing occurs first in the NTS. Here, in rats, we examined the synaptic characteristics of visceral afferent pathways to the central nucleus of the amygdala (CeA) in brainstem slices by recording from retrogradely labelled NTS projection neurons. We simultaneously recorded neuron pairs: one dye positive (i.e. NTS-CeA) and a second unlabelled neighbor. Graded shocks to the solitary tract (ST) always (93%) triggered EPSCs at CeA projecting NTS neurons. Half of the NTS-CeA neurons received at least one primary afferent input (classed 'second order') indicating that viscerosensory information arrives at the CeA conveyed by a pathway involving as few as two synapses. The remaining NTS-CeA neurons received viscerosensory input only via polysynaptic pathways. In contrast, ∼3/4 of unlabelled neighboring neurons were directly connected to ST. NTS-CeA neurons received greater numbers of ST related inputs compared to unlabelled NTS neurons indicating that highly convergent viscerosensory signals reach the CeA. Remarkably, despite multi-fibre convergence, all single NTS-CeA neurons received inputs derived from only unmyelinated afferents (TRPV1 expressing C-fibres) or only non-TRPV1 ST afferent inputs – never a combination of both. Such segregation means that visceral afferent information followed separate lines to reach CeA. Their very different physiological activation profiles mean that these parallel visceral afferent pathways encode viscerosensory signals to the amygdala that may provide interoceptive assessments to impact behaviours.

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