Distribution and Structure of Synapses on Medial Vestibular Nuclear Neurons Targeted by Cerebellar Flocculus Purkinje Cells and Vestibular Nerve in Mice: Light and Electron Microscopy Studies.

Hitomi Matsuno,Ryuichi Shigemoto,Soichi Nagao,Akiya Watakabe,Moeko Kudoh,Tetsuo Yamamori,Manuel S Malmierca

doi:10.1371/journal.pone.0164037

Hitomi Matsuno, Ryuichi Shigemoto + Show 5 more

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https://doi.org/10.1371/journal.pone.0164037

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Abstract

Adaptations of vestibulo-ocular and optokinetic response eye movements have been studied as an experimental model of cerebellum-dependent motor learning. Several previous physiological and pharmacological studies have consistently suggested that the cerebellar flocculus (FL) Purkinje cells (P-cells) and the medial vestibular nucleus (MVN) neurons targeted by FL (FL-targeted MVN neurons) may respectively maintain the memory traces of short- and long-term adaptation. To study the basic structures of the FL-MVN synapses by light microscopy (LM) and electron microscopy (EM), we injected green florescence protein (GFP)-expressing lentivirus into FL to anterogradely label the FL P-cell axons in C57BL/6J mice. The FL P-cell axonal boutons were distributed in the magnocellular MVN and in the border region of parvocellular MVN and prepositus hypoglossi (PrH). In the magnocellular MVN, the FL-P cell axons mainly terminated on somata and proximal dendrites. On the other hand, in the parvocellular MVN/PrH, the FL P-cell axonal synaptic boutons mainly terminated on the relatively small-diameter (< 1 μm) distal dendrites of MVN neurons, forming symmetrical synapses. The majority of such parvocellular MVN/PrH neurons were determined to be glutamatergic by immunocytochemistry and in-situ hybridization of GFP expressing transgenic mice. To further examine the spatial relationship between the synapses of FL P-cells and those of vestibular nerve on the neurons of the parvocellular MVN/PrH, we added injections of biotinylated dextran amine into the semicircular canal and anterogradely labeled vestibular nerve axons in some mice. The MVN dendrites receiving the FL P-cell axonal synaptic boutons often closely apposed vestibular nerve synaptic boutons in both LM and EM studies. Such a partial overlap of synaptic boutons of FL P-cell axons with those of vestibular nerve axons in the distal dendrites of MVN neurons suggests that inhibitory synapses of FL P-cells may influence the function of neighboring excitatory synapses of vestibular nerve in the parvocellular MVN/PrH neurons.

Highlights

The horizontal vestibulo-ocular reflex (HVOR) and optokinetic response (HOKR) are respectively compensatory eye movements for the movement of the head and external visual surrounding on the horizontal plane
The results suggest that the inhibitory synapses of FL Purkinje cells (P-cells) axons and the excitatory synapses of vestibular nerve axons are often located close to each other on the distal dendrites of FL-targeted medial vestibular nucleus (MVN) neurons, in the parvocellular MVN/ prepositus hypoglossi (PrH) border region
As previous two studies [36,37] consistently suggest that vesicular glutamate transporter 2 (VGluT2) is expressed abundantly throughout glutamatergic MVN neurons, whereas vesicular transporter 1 (VGluT1) is expressed in a small portion of magnocellular MVN neurons, we examined the colocalization of green florescence protein (GFP) and VGluT2 in the parvocellular MVN/PrH

Summary

Introduction

The horizontal vestibulo-ocular reflex (HVOR) and optokinetic response (HOKR) are respectively compensatory eye movements for the movement of the head and external visual surrounding on the horizontal plane. These two ocular reflexes have been studied as an experimental model of motor learning, because their dynamic characteristics are modifiable by training that induces motor error. When animals are trained to watch the motion of a stripe- or dot-patterned screen, with or without the motion of animals, the mismatch between the screen and eye motion is sensed as retinal slips, which induce adaptation in the magnitude of eye movements evoked by HVOR or HOKR (e.g., [1]). Note that FL Purkinje cells (P-cells) directly inhibit the MVN neurons relaying HVOR and HOKR, not via the cerebellar nuclear neurons. Increasing lines of evidence have been accumulated from experiments of mice [8,9], cats [10], and monkeys [11], suggesting that the memory trace of short-term adaptation that decays within 24 h is maintained in FL, whereas the memory trace of long-term adaptation that remains for more than 24 h is maintained in FL-targeted MVN

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