The early neuronal organization predicts the path followed by some major axonal bundles in the embryonic brainstem

Segmental patterns of vestibular-mediated synaptic inputs to axial and limb motoneurons in the neonatal mouse assessed by optical recording

Proper control of movement and posture occurs partly via descending projections from the vestibular nuclei to spinal motor circuits. Days before birth in rodents, vestibulospinal neurons develop axonal projections that extend to the spinal cord. How functional these projections are just after birth is unknown. Our goal was to assess the overall functional organization of vestibulospinal inputs to spinal motoneurons in a brainstem-spinal cord preparation of the neonatal mouse (postnatal day (P) 0-5). Using calcium imaging, we recorded responses evoked by electrical stimulation of the VIIIth nerve, in many motoneurons simultaneously throughout the spinal cord (C2, C6, T7, L2 and L5 segments), in the medial and lateral motor columns. Selective lesions in the brainstem and/or spinal cord distinguished which tracts contributed to the responses: those in the cervical cord originated primarily from the medial vestibulospinal tracts but with a substantial contribution from the lateral vestibulospinal tract; those in the thoracolumbar cord originated exclusively from the lateral vestibulospinal tract. In the thoracolumbar but not the cervical cord, excitatory commissural connections mediated vestibular responses in contralateral motoneurons. Pharmacological blockade of GABA(A) receptors showed that responses involved a convergence of excitatory and inhibitory inputs which in combination produced temporal response patterns specific for different segmental levels. Our results show that by birth vestibulospinal projections in rodents have already established functional synapses and are organized to differentially regulate activity in neck and limb motoneurons in a tract- and segment-specific pattern similar to that in adult mammals. Thus, this particular set of descending projections develops several key features of connectivity appropriately at prenatal stages. We also present novel information about vestibulospinal inputs to axial motoneurons in mammals, providing a more comprehensive platform for future studies into the overall organization of vestibulospinal inputs and their role in regulating postural stability.

Expression by midbrain dopamine neurons of Sema3A and 3F receptors is associated with chemorepulsion in vitro but a mild in vivo phenotype

Here we explore the role of semaphorin 3A and 3F (Sema3A, Sema3F) in the formation of the mesotelencephalic pathway. We show that Sema3A and 3F are expressed in the ventral mesencephalon (VM) of E13.5 rat embryos; the receptors Neuropilin 1 and Neuropilin 2, and co-receptors L1CAM, NrCAM, and Plexins A1 and A3 but not A4 are expressed by VM dopaminergic neurons; these neurons bind Sema3A and 3F in vitro which induces collapse of their growth cones and elicits, with different potencies, a repulsive response; and this response is absent in axons from Nrp1 and Nrp2 null embryos. Despite these in vitro effects, only very mild anatomical defects were detected in the organization of the mesotelencephalic pathway in embryonic and adult Nrp1 or Nrp2 null mice. However, the dopaminergic meso-habenular pathway and catecholaminergic neurons in the parafascicular and paraventricular nuclei of the thalamus were significantly affected in Nrp2 null mice. These data are consistent with a model whereby Sema3A and 3F, in combination with other guidance molecules, contributes to the navigation of DA axons to their final synaptic targets.

Differential origin of reticulospinal drive to motoneurons innervating trunk and hindlimb muscles in the mouse revealed by optical recording

To better understand how the brainstem reticular formation controls and coordinates trunk and hindlimb muscle activity, we used optical recording to characterize the functional connections between medullary reticulospinal neurons and lumbar motoneurons of the L2 segment in the neonatal mouse. In an isolated brainstem-spinal cord preparation, synaptically induced calcium transients were visualized in individual MNs of the ipsilateral and contralateral medial and lateral motor columns (MMC, LMC) following focal electrical stimulation of the medullary reticular formation (MRF). Stimulation of the MRF elicited differential responses in MMC and LMC, according to a specific spatial organization. Stimulation of the medial MRF elicited responses predominantly in the LMC whereas stimulation of the lateral MRF elicited responses predominantly in the MMC. This reciprocal response pattern was observed on both the ipsilateral and contralateral sides of the spinal cord. To ascertain whether the regions stimulated contained reticulospinal neurons, we retrogradely labelled MRF neurons with axons coursing in different spinal funiculi, and compared the distributions of the labelled neurons to the stimulation sites. We found a large number of retrogradely labelled neurons within regions of the gigantocellularis reticular nucleus (including its pars ventralis and alpha) where most stimulation sites were located. The existence of a mediolateral organization within the MRF, whereby distinct populations of reticulospinal neurons predominantly influence medial or lateral motoneurons, provides an anatomical substrate for the differential control of trunk and hindlimb muscles. Such an organization introduces flexibility in the initiation and coordination of activity in the two sets of muscles that would satisfy many of the functional requirements that arise during postural and non-postural motor control in mammals.