Vagal sensory neurones directly synapse onto parasympathetic motor (DMV) neurones

Autonomic reflexes, controlling unconscious processes such as heart rate and digestion, are essential for homeostasis. Vago-vagal reflexes describe one category of autonomic reflexes in which the sensory input and motor output are both mediated through the vagus nerve. These reflexes are widely considered to rely on the nucleus of the solitary tract (NTS) for the integration of sensory information before regulating the motor output from the dorsal motor nucleus of the vagus (DMV). Here, we electrically activated the solitary tract to evoke sensory input simultaneous to whole cell recordings of DMV neurones. Criteria around measures of synaptic jitter, amplitude variance and paired pulse ratio were used to identify that ∼7% of DMV neurones probably receive direct vagal sensory input. We utilised channelrhodopsin-assisted circuit mapping to definitively confirm sensory to motor synapses. In brain slices where vagal afferents could be activated selectively with optogenetics, we evoked postsynaptic events in some DMV neurones. In some of these, optogenetically evoked input persisted in the presence of tetrodotoxin. These results confirm monosynaptic connections between vagal afferent fibres and DMV motor neurones. This study challenges the canonical circuitry underlying vago-vagal reflexes, confirming some sensory vagal afferents synapse at some DMV motor neurones, bypassing the NTS, to drive parasympathetic motor output directly. KEY POINTS: Vagal afferents terminate at second order neurones in the solitary tract nucleus, paratrigeminal nucleus and area postrema. Neuroanatomical data suggests vagal sensory afferents may also terminate and synapse onto motor neurones within the dorsal motor nucleus of the vagus (DMV) to directly modulate parasympathetic output. We utilised traditional electrical activation and channelrhodopsin-assisted circuit mapping to definitively confirm that vagal sensory afferents synapse onto DMV motor neurones. These findings suggest that some vago-vagal reflexes may be mediated by a direct sensory vagus-to-DMV circuit, enabling precise and uninterrupted drive to increase parasympathetic output.