Title: Patterning of Somatosympathetic Reflexes Reveals Non-uniform Organization of Presympathetic Drive from C1 and Non-c1 Rvlm Neurons Abbreviated Title / Running Title: Organization of Presympathetic Drive from C1 and Non-c1 Rvlm Neurons

Manuscript: Number of words: 5715 (excluding figure legends and references) Number of figures: 8 (includes 1 table) Number of pages: 23 Abstract To determine the organization of presympathetic vasomotor drive by phenotypic populations of RVLM neurons, we examined the somatosympathetic reflex (SSR) evoked in four sympathetic nerves together with selective lesions of RVLM presympathetic neurons. Urethane-anesthetized (1.3g/kg ip), paralyzed, vagotomized and artificially ventilated Sprague Dawley rats (n = 41) were used. First, we determined the afferent inputs activated by sciatic nerve (SN) stimulation at graded stimulus intensities (50 sweeps at 0.5-1Hz, 1-80V). Second, we recorded sympathetic nerve responses (cervical, renal, splanchnic and lumbar) to intensities of SN stimulation that activated A-fiber afferents (low) or both A-/C-fiber afferents (high). Third, with low intensity SN stimulation, we examined the cervical SSR following RVLM microinjection of somatostatin (SST), and we determined the splanchnic SSR in rats in which presympathetic C1 neurons were lesioned following intraspinal injections of anti-dopamine-beta-hydroxylase-saporin (anti-DβH-SAP). Low intensity SN stimulation activated A-fiber afferents and evoked biphasic responses in the renal, splanchnic and lumbar nerves and a single peak in the cervical nerve. Depletion of presympathetic C1 neurons (59 ± 4% TH-ir profiles lesioned) eliminated the 2 nd peak of the splanchnic SSR and attenuated the 1 st peak suggesting that only RVLM neurons with fast axonal conduction were spared. RVLM injections of SST abolished the single early peak of cervical SSR confirming that RVLM neurons with fast axonal conduction were inhibited by SST. It is concluded that unmyelinated RVLM presympathetic neurons, presumed to be all C1, innervate splanchnic, renal and lumbar but not cervical sympathetic outflows, whereas myelinated C1 and non-C1 RVLM neurons innervate all sympathetic outflows examined. These findings suggest that multiple levels of neural control of vasomotor tone exist: myelinated populations may set baseline tone whilst unmyelinated neurons may be recruited to provide actions at specific vascular beds in response to distinct stressors.