Deciphering the role of brainstem vestibular-related inhibitory networks in shaping postural reflexes in the Xenopus tadpole

Brainstem vestibulospinal (VS) nuclei generate excitatory commands in response to multi-modality sensory integration, to activate specific spinal networks in order to generate adapted postural reflexes. Comparably organized in bilateral nuclei with both ipsi- and contralateral pathways in all species, excitatory VS projections alone fail to explain the mostly unilateral reflex responses typically observed. In the Xenopus laevis tadpole, we describe secondary vestibular neurons of inhibitory nature, and the synaptic contacts they make on VS neurons. Then, using a brainstem/spinal cord in vitro preparation we show that the spinal responses evoked by galvanic vestibular stimulation are shaped by both commissural and local inhibitory brainstem networks. We further show that a complex interaction between GABAergic and glycinergic inhibitory networks regulate VS neuron excitability and, consequently, the expression of the spinal response. Our data reveal that while excitatory VS neurons execute the neural score, inhibitory neurons in the central vestibular system coordinate and modulate the overall performance.