The connectome of rostral ventrolateral medulla sympathetic premotor neurons

      Spinally projecting neurons in the rostral ventrolateral medulla (RVLM) are thought to play a critical role in the generation of vasomotor sympathetic tone and represent a major site of convergence for multiple descending and reflex pathways that co-ordinate sympathetic nerve activity. Elucidating the organization of the circuits that drive these neurons is a key research objective. Here we present brain-wide connectomic maps of neurons that provide monosynaptic drive to putative RVLM sympathetic premotor neurons, generated using a two-step restricted trans-synaptic viral tracing strategy. We made focal microinjections of a genetically restricted reporter-expressing rabies vector, SADΔG(EnvA)-RFP, into the RVLM, and restricted its entry to neurons that project to the T2 spinal cord by first transducing them with a cassette required for rabies entry and trans-synapsis, YTB. By precisely targeting rabies injections to the core of the RVLM bulbospinal population we were able to restrict rabies ‘seeding’ to small numbers (as low as one single neuron) of RVLM presympathetic neurons immediately caudal to the facial nucleus, and then map the locations of neurons that provide monosynaptic input to the seed population. We observed reproducible patterns of inputs arising from the dorsal, contralateral, and midline medulla, and local RVLM interneurons including catecholaminergic non-bulbospinal neurons and neurons likely to reside within the ventral respiratory column. Distant inputs were identified in the pons, cerebellum and midbrain, and included previously suspected sites of monosynaptic drive such as the paraventricular nucleus of the hypothalamus.
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