Gating of vagal inputs by sciatic afferents in nonspinally projecting neurons in the rat rostral ventrolateral medulla oblongata

Integration and coordination of somato-visceral sensory information is crucial to achieve adaptive behavioural responses. We have recently shown that sensory vagal and somato-sensory (sciatic nerve) inputs converge in neurons of the rostral ventrolateral medulla oblongata, which was implicated in adjusting visceral activities to changing somatic performances. In the present study, the neuronal mechanism of interaction between sciatic and vagal sensory inputs was examined in the rostral ventrolateral medulla oblongata using in vivo intracellular recording and labelling. Conditioning stimulation of the contralateral sciatic nerve (2 V) led to a time-dependent inhibition of responses to vagal stimulation (100 muA) in each RVLM neuron that received convergent sciatic and vagal sensory inputs (n = 50). None of these neurons had direct spinal projections, and only 8% of them exhibited a visible response to stimulation of the aortic depressor nerve. A significant attenuation of the amplitude of vagal test responses was present for up to 800 ms of conditioning delay, although the duration of this sciatico-vagal inhibition was greatly dependent on the intensity of both stimuli. The electrophysiological data indicated that sciatico-vagal inhibition is mediated presynaptically, via activation of GABA(B) receptors. Morphological evidence of axo-axonic interactions that may underlie sciatico-vagal inhibition was subsequently found in the electron microscope. It is suggested that during movements of the hindleg, activation of sciatic sensory fibres leads to re-patterning of neuronal activity in RVLM neurons via inhibition of visceral sensory inputs. Sciatico-vagal inhibition is likely to affect the activity of those RVLM neurons that modulate higher neuronal activities via ascending projections.