Ultrastructure of the neuropil of the brain oculomotor nuclei was studied in mice after 30-day exposure to microgravity on Bion-M1 biosatellite and after 13-h exposure to Earth gravity. The number of axo-dendritic synapses in the neuropil of the oculomotor nucleus significantly decreased after the flight. Degenerated axon terminals containing conglomerates of presynaptic vesicles appeared. The number of synapses with high functional activity increased and the length of active zones of the axo-dendritic synapses significantly increased. The observed ultrastructural changes of the neuropil of the oculomotor nuclei of mice exposed to microgravity reflect the development of long-term deafferentation of the analyzed brain structures. These changes in the neuropil ultrastructure can determine the disturbances in the oculomotor system, e.g. development of atypical nystagmus under conditions of microgravity.

No abstract

During spaceflight and immediately after it, adaptive neuroplastic changes occur in the sensorimotor structures of the central nervous system, which are associated with changes of mainly vestibular and visual signals. It is known that the movement of the eyeball in the vertical direction is carried out by muscles that are innervated by the trochlear nerve (CN IV) and the oculomotor nerve (CN III). To elucidate the cellular processes underlying the atypical vertical nystagmus that occurs under microgravity conditions, it seems necessary to study the state of these nuclei in animals in more detail after prolonged space flights. In this work, we carried out a qualitative and quantitative light-optical and ultrastructural analysis of the nuclei of the trochlear nerve in mice after a 30-day flight on the Bion-M1 biosatellite and followed by a stay for 13-14 hours under the influence of the Earth's gravity after landing. As a result, it was shown that the motoneurons in the nucleus of the trochlear nerve changed their morphology under the influence of microgravity. Cell nuclei of the motoneurons had a more simplified rounded shape than in the control. In addition, the dendrites of these motoneurons significantly reorganized geometry and orientation under microgravity conditions; the number of dendritic branches has been increased to enhance the reduced signal flow. Apparently, to ensure such plastic changes, the number and size of mitochondria in the soma of motoneurons and in axons coming from the vestibular structures increased. In addition, the experimental animals showed an increase in the size of the cisterns of the rough and smooth endoplasmic reticulum in comparison with the control group of animals left on Earth, for which the environmental conditions in the spacecraft were reproduced.Thus, the main role in the adaptation of the trochlear nucleus to microgravity conditions, apparently, belongs to the dendrites of motoneurons, which rearrange their structure and function to enhance the flow of sensory information. These results are useful for the development of new, more effective means to facilitate the stay and work of space travelers in a long spaceflight..

Goldfish are known to exhibit motor asymmetry due to functional asymmetry of their Mauthner neurons that induce the turns to the right or left during free swimming. It has been previously found that if the less active neuron is subjected to prolonged aimed visual stimulation via its ventral dendrite, the motor asymmetry of goldfish is inverted, testifying that this neuron becomes functionally dominant, while the size of the ventral dendrite under these conditions is reduced 2-3 times compared to its counterpart in mirror neuron. Earlier it has been also revealed that training optokinetic stimulation induces adaptation, a substantial resistance of both fish motor asymmetry and morphofunctional state of Mauthner neurons against prolonged optokinetic stimulation. The aim of this work was to study the cellular mechanisms of the effect of an unusual visual afferent input on goldfish motor asymmetry and Mauthner neuron function in norm and under adaptation. It was shown that serotonin applied onto Mauthner neurons greatly reduces their activity whereas its antagonist ondansetron increases it. Against the background of visual stimulation, serotonin strengthens functional asymmetry between neurons whereas ondansetron smoothes it. Taken together these data suggest the involvement of serotonergic excitatory synaptic transmission in the regulation of Mauthner neurons by vision. Ultrastructural study of the ventral dendrites after prolonged optokinetic stimulation has revealed depletions of numeral axo-axonal synapses with specific morphology, identified by means of immunogold label as serotonergic ones. These latter in turn are situated mainly on shaft boutons, which according to specific ultrastructural features are assigned to axo-dendritic inhibitory synapses. Thus, the excitatory serotonergic synapses seem to affect Mauthner neuron indirectly through inhibitory synapses. Further, it was morphometrically established that adaptation is accompanied by the significant decrease of active zones dimensions in both serotonergic and inhibitory synapses. Finally, it was determined in model experiments that the interaction of globular actin with glycine, a main inhibitory neurotransmitter supposedly directly and chronically affecting the ventral dendrite, results in actin filaments formation. It is assumed that glycine-induced cytosolic actin polymerization is a cause of reduction in the ventral dendrite size under stimulation. Thus, it was established that a rather small group of synapses situated on an individual dendrite of the neuron determines the execution of the important form of animal behavior.

The structural mechanisms that control the neuronal functional activity maintaining the brain functional asymmetry were studied using the relationship between the function and structure of goldfish Mauthner neurons (MNs) responsible for fish motor asymmetry as a model. It was shown for the first time that the dominant activity in one of the two counter neurons symmetrically situated in the medulla oblongata directly correlates with changes in its integral volume and is inversely regulated by the size of its ventral dendrite. It is known that the variability of the neuron dimensions is due to changes in the actin component of the cytoskeleton. The experimental data presented are discussed in terms of the involvement of cytosol actin in the control of the volume of somata and the main dendrites of MNs with the participation of dopamine and glutamate, two major neurotransmitters that are known to regulate the function of MNs.