title: Pontine reticulospinal projections in the neonatal mouse: Internal organization and axon trajectories.
abstract: We recently characterized physiologically a pontine reticulospinal (pRS) projection in the neonatal mouse that mediates synaptic effects on spinal motoneurons via parallel uncrossed and crossed pathways (Sivertsen et al. [2014] J Neurophysiol 112:1628-1643). Here we characterize the origins, anatomical organization, and supraspinal axon trajectories of these pathways via retrograde tracing from the high cervical spinal cord. The two pathways derive from segregated populations of ipsilaterally and contralaterally projecting pRS neurons with characteristic locations within the pontine reticular formation (PRF). We obtained estimates of relative neuron numbers by counting from sections, digitally generated neuron position maps, and 3D reconstructions. Ipsilateral pRS neurons outnumber contralateral pRS neurons by threefold and are distributed about equally in rostral and caudal regions of the PRF, whereas contralateral pRS neurons are concentrated in the rostral PRF. Ipsilateral pRS neuron somata are on average larger than contralateral. No pRS neurons are positive in transgenic mice that report the expression of GAD, suggesting that they are predominantly excitatory. Putative GABAergic interneurons are interspersed among the pRS neurons, however. Ipsilateral and contralateral pRS axons have distinctly different trajectories within the brainstem. Their initial spinal funicular trajectories also differ, with ipsilateral and contralateral pRS axons more highly concentrated medially and laterally, respectively. The larger size and greater number of ipsilateral vs. contralateral pRS neurons is compatible with our previous finding that the uncrossed projection transmits more reliably to spinal motoneurons. The information about supraspinal and initial spinal pRS axon trajectories should facilitate future physiological assessment of synaptic connections between pRS neurons and spinal neurons.
authors.affiliation: Laboratory of Neural Development and Optical Recording (NDEVOR), Department of Physiology, Institute of Basic Medical Sciences, Faculty of Medicine, University of Oslo, 0316, Oslo, Norway.
authors.forename: Magne S
authors.initials: MS
authors.lastname: Sivertsen
authors.affiliation: Department of Physiology, Emory University School of Medicine, Atlanta, Georgia, 30307.
authors.forename: Marie-Claude
authors.initials: MC
authors.lastname: Perreault
authors.affiliation: Laboratory of Neural Development and Optical Recording (NDEVOR), Department of Physiology, Institute of Basic Medical Sciences, Faculty of Medicine, University of Oslo, 0316, Oslo, Norway.|Norwegian Center for Stem Cell Research, Oslo University Hospital, 0317, Oslo, Norway.
authors.forename: Joel C
authors.initials: JC
authors.lastname: Glover
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001369:Axons / Q000737:chemistry / Q000502:physiology*; D001933:Brain Stem / Q000737:chemistry / Q000166:cytology / Q000502:physiology; D051379:Mice; D008813:Mice, Inbred ICR; D009046:Motor Neurons / Q000737:chemistry / Q000502:physiology*; D009434:Neural Pathways / Q000737:chemistry / Q000502:physiology; D011149:Pons / Q000737:chemistry / Q000166:cytology / Q000502:physiology; D065821:Pontine Tegmentum / Q000737:chemistry / Q000166:cytology / Q000502:physiology*; D013116:Spinal Cord / Q000737:chemistry / Q000166:cytology / Q000502:physiology*
keywords: AB_10143907; AB_141784; AB_1502299; brainstem; bulbospinal; descending pathways; locomotion; motor control; posture; spinal cord; startle reflex