Origin and neurochemical properties of bulbospinal neurons projecting to the rat lumbar spinal cord via the medial longitudinal fasciculus and caudal ventrolateral medulla

Abstract

Bulbospinal systems (BS) originate from various regions of the brainstem and influence spinal neurons by classical synaptic and modulatory mechanisms. Our aim was to determine the brainstem locations of cells of origin of BS pathways passing through the medial longitudinal fasciculus (MLF) and the caudal ventrolateral medulla (CVLM). We also examined the transmitter content of spinal terminations of the CVLM pathway. Six adult rats received Fluorogold (FG) injections to the right intermediate gray matter of the lumbar cord (L1–L2) and the b-subunit of cholera toxin (CTb) was injected either into the MLF or the right CVLM (3 animals each). Double-labeled cells were identified within brainstem structures with confocal microscopy and mapped onto brainstem diagrams. An additional 3 rats were injected with CTb in the CVLM to label axon terminals in the lumbar spinal cord. Double-labeled cells projecting via the MLF or CVLM were found principally in reticular regions of the medulla and pons but small numbers of cells were also located within the midbrain. CVLM projections to the lumbar cord were almost exclusively ipsilateral and concentrated within the intermediate gray matter. Most (62%) of terminals were immunoreactive for the vesicular glutamate transporter 2 while 23% contained the vesicular GABA transporter. The inhibitory subpopulation was glycinergic, GABAergic or contained both transmitters. The proportions of excitatory and inhibitory axons projecting via the CVLM to the lumbar cord are similar to those projecting via the MLF. Unlike the MLF pathway, CVLM projections are predominantly ipsilateral and concentrated within intermediate gray but do not extend into motor nuclei or laminia VIII. Terminations of the CVLM pathway are located in a region of the gray matter that is rich in premotor interneurons; thus its primary function may be to coordinate activity of premotor networks.

Corticospinal and Reticulospinal Contacts on Cervical Commissural and Long Descending Propriospinal Neurons in the Adult Rat Spinal Cord; Evidence for Powerful Reticulospinal Connections

“…Two pathways known to have a powerful influence on motor networks are the corticospinal tract (CST) and the reticulospinal tract (RST) [ 1 ]. In the rat the CST projects directly from the sensorimotor cortex and terminates principally in the deep dorsal horn and intermediate grey matter [ 3 – 5 ] whereas the RST originates from medullary and pontine nuclei [ 6 – 9 ] and terminates principally within the intermediate grey matter and lamina (VIII) [ 5 , 10 , 11 ]. Components of this pathway are monosynaptically activated by corticobulbar pathways and form an indirect cortico-reticular pathway to the spinal cord [ 12 , 13 ].…”

Pontine reticulospinal projections in the neonatal mouse: Internal organization and axon trajectories

“…Liang et al () also distinguish the PnV (ventral pontine reticular nucleus), containing a minor population of reticulospinal neurons, which may in fact be the most rostral part of the medullary gigantocellular nucleus, potentially misconstrued as having a pontine location because of mismatch between coronal sections of the adult brain and the planes of rhombomere boundaries (see below). With regard to laterality, Basbaum and Fields (), Auclair et al (), Vanderhorst and Ulfhake (), and Huma et al () all report an ipsilateral predominance in the projection of pRS neurons throughout the PRF, including both the PnO and the PnC. Newman () concurs, with a distinction between the PnC (his RPoC) pars alpha and beta, which project, respectively, with contralateral and ipsilateral predominance, and between the PnO (his RPoO) medialis and lateralis, which project, respectively, with a weak ipsilateral predominance and no lateral bias.…”

“…Several previous retrograde tracing studies have demonstrated projections from the PRF to the spinal cord in the mouse and rat, ranging from embryonic to adult stages (mouse: Auclair et al, ; Vanderhorst and Ulfhake, ; Liang et al, ; rat: Basbaum and Fields, ; Satoh, ; Watkins et al, , 1981; Leong et al, ; Newman, ; Jones and Yang, ; Nudo and Masterton, ; Rye et al, ; Shen et al, ; Masson et al, ; Lakke, ; Auclair et al, ; Reiner et al, ; Huma et al, ). Only three of these studies combined unilateral tracer application with contralateral spinal cord hemisection to ensure unilateral tracing so that the laterality of projections could be unequivocally documented (Basbaum and Fields, ; Auclair et al, ; Vanderhorst and Ulfhake, ), as we have done here.…”

“…Only three of these studies combined unilateral tracer application with contralateral spinal cord hemisection to ensure unilateral tracing so that the laterality of projections could be unequivocally documented (Basbaum and Fields, ; Auclair et al, ; Vanderhorst and Ulfhake, ), as we have done here. In eight other studies, in the adult mouse or rat, unilateral applications were made but not combined with contralateral lesions to ensure unilateral tracing (Watkins et al, , 1981; Newman, ; Nudo and Masterton, ; Reiner et al, ; Liang et al, ; Huma et al, ). These studies will receive additional attention below.…”

“…Huma et al () also aimed to assess, in the adult mouse, the white matter trajectories of bulbospinal neurons, including those in the PRF. They combined unilateral lumbar injection of one retrograde tracer with ipsilateral injection of another retrograde tracer in either the MLF or the caudal ventrolateral medulla.…”