The axonal projections and synaptic input of the C1 adrenergic neurons of the rostral ventrolateral medulla (VLM) were examined using transgenic dopamine-beta hydroxylase Cre mice and modified rabies virus. Cre-dependent viral vectors expressing TVA (receptor for envelopeA) and rabies glycoprotein were injected into the left VLM. EnvelopeA-pseudotyped rabies-EGFP glycoprotein-deficient virus (rabies-EGFP) was injected 4-6 weeks later in either thoracic spinal cord (SC) or hypothalamus. TVA immunoreactivity was detected almost exclusively (95 %) in VLM C1 neurons. In mice with SC injections of rabies-EGFP, starter cells (expressing TVA + EGFP) were found at the rostral end of the VLM; in mice with hypothalamic injections starter C1 cells were located more caudally. C1 neurons innervating SC or hypothalamus had other terminal fields in common (e.g., dorsal vagal complex, locus coeruleus, raphe pallidus and periaqueductal gray matter). Putative inputs to C1 cells with SC or hypothalamic projections originated from the same brain regions, especially the lower brainstem reticular core from spinomedullary border to rostral pons. Putative input neurons to C1 cells were also observed in the nucleus of the solitary tract, caudal VLM, caudal spinal trigeminal nucleus, cerebellum, periaqueductal gray matter and inferior and superior colliculi. In sum, regardless of whether they innervate SC or hypothalamus, VLM C1 neurons receive input from the same general brain regions. One interpretation is that many types of somatic or internal stimuli recruit these neurons en bloc to produce a stereotyped acute stress response with sympathetic, parasympathetic, vigilance and neuroendocrine components.
Keywords: C1 neurons; Connectome; HPA axis; Modified rabies virus; Parasympathetic nervous system; Sympathetic nervous system.