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—The effects of sensory stimulation on the model of paired Mauthner neurons of monocularly deprived goldfish have been studied by light- and electron microscopy. Three-dimensional reconstruction was performed using serial semi-thin sections in order to determine the volumes of soma and dendrites for the right and left Mauthner neurons. These data suggested that long-term stimulation of some fish caused a decrease in the volume of the lateral dendrites of neurons contralateral with respect to the side of the enucleated eye. As a result, the morphological homeostasis in the dendrites of Mauthner neurons, which was observed in the control fish, was disturbed after the effect of stimulation upon this group of fish. The main mechanism of this effect of stimulation, according to ultrastructural analysis, is the compaction of the cytoskeleton in the lateral dendrite of a neuron and a reduction in the number of synaptic vesicles in the region of afferent synapses. In contrast, stimulation of the remaining fish induced the resistance of the contralateral Mauthner neuron to prolonged stimulation, as well as the hypertrophy of its lateral and shortening of the ventral dendrites. These results suggest that the homeostasis of the dendrites of Mauthner neurons essentially depends on the state of the visual inputs, the load on which increased as a result of the doubled impact.

The effects of haloperidol, an antagonist of D2 dopamine receptors, on the functioning of Ca2+, K+, and Cl− ion channels in the membrane of Chara corallina cells and on the functional properties of their cytoskeleton was studied. Haloperidol blocked Ca2+ channels of the plasmalemma. In addition to bringing about a decrease in the amplitude of the calcium current, exposure to haloperidol decelerated the activation and inactivation of calcium channels. The effect of haloperidol was reversible; after it was removed, the characteristics of calcium current were restored. Haloperidol did not affect Ca2+-activated chloride channels. Haloperidol also inhibited microfilament-dependent motion of the cytoplasm. Cytoplasmic streaming was restored after haloperidol was removed from the extracellular solution. These results suggest that the concentration of free Ca2+ ions in the cytoplasm increases in the presence of haloperidol, and that Ca2+ channels of C. corallina plasmalemma possess specific binding sites both for dopamine receptors and for their antagonists.

The ultrastructure of motoneurons and synapses in the somatic zone of the oculomotor nerve nuclei was studied in C57 Black/6 mice. Axodendritic and axosomatic synapses were characterized by round densities (round subjunctional densities) located in the postsynaptic part of the synapse some distance from the postsynaptic density. The synaptic cleft close to subjunctional densities was widened to up to 30 μm. Other parts of the cleft in the same synapse could be narrowed almost twofold. Axosomatic and axodendritic spiny synapses were found. Motoneuron bodies and dendrites bore synapses of the en passant type. Gap junctions were not seen.

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.

Morphometric methods were used to study the ultrastructure of afferent synapses on the ventral dendrites of Mauthner neurons (MN) during adaptation of goldfish to prolonged, fatiguing, sensory (visual) stimulation inducing increased resistance of MN. The extent of active zones (AZ) in synapses located on the MN ventral dendrite was found to decrease significantly, by 23%, after adaptation. The extent of the AZ of excitatory visual synapses decreased by 29% as compared with controls, with a simultaneous 71% increase in the extent of desmosome-like contacts (DLC) bordering AZ. There was also a 19% reduction in the extent of AZ of inhibitory synapses after adaptation, which is consistent with the important role of inhibitory processes in sensory pathways during memory formation. Considering the actin nature of DLC, it can be suggested that adaptation to visual stimulation is based on a synaptic mechanism of regulation of neurotransmitter secretion by actin.

Immunogold labeling was used to identify serotoninergic synapses of the chemical and mixed types on the ventral dendrite (VD) of Mauthner neurons (MN) in goldfish. These had the characteristic feature that they contained several mitochondria in the bouton and, apart from an active zone (AZ), had an obligatory desmosome-like contact (DlC). The AZ of these synapses generally contacted unlabeled chemical crested synapses, which in turn interacted directly with the VD. These latter synapses had virtually no mitochondria and lacked a DlC, such that they could be identified as inhibitory. This “two-level” organization of excitatory serotoninergic and inhibitory synapses appears to be directly related to the reciprocal mechanism regulating the functional activity of MN via the visual apparatus.

The effects of training with adaptive vestibular stimulation on the function and three-dimensional structure of Mauthner neurons (MN) were studied in goldfish with experimental amyloidosis induced by application of aggregated β-amyloid protein (Aβ25–35). In comparison with controls, adapted MN acquired significant resistance to Aβ25–35. Considering the key role of dopamine in the adaptation of MN to sensory stimuli, its actions on the development of an amyloidosis model were studied; the results showed that application of dopamine to MN, like increases in the dopamine concentration induced by L-DOPA, was followed by protection of MN structure and function from the pathogenic actions of Aβ25–35. Electron microscopic studies showed that the protective action of dopamine on neurons can be explained in terms of its ability to dissociate β-amyloid polymers into short inactive fragments.

Previous studies have shown that contralateral (to the preferred turning side) optokinetic stimulation and ipsilateral enucleation of the eye lead to significant (by a factor of 2–4) decreases in the volume of the ventral dendrite (VD) in one of the two Mauthner neurons (MN) of the goldfish, which becomes functionally more active. We report here our studies of MN ultrastructure after these unilateral influences from the visual system. In both cases, the whole length of the shrunken VD showed emptying of synapses and compaction of its cytoskeleton as compared with the cytoskeleton of the VD of the contralateral MN and the cytoskeleton of the lateral dendrites and bodies of both neurons. It is suggested that emptied synapses are part of the excitatory visual input and that the control of the functional activity of MN via VD involves both cytoskeletal and synaptic mechanisms.

Three-dimensional reconstruction was used to study the structural correlates of significant increases in the activity of Mauthner neurons (MN) deafferented after unilateral enucleation of the eye evidenced by an irreversible shift in motor asymmetry in goldfish towards the “blind” side. The results showed that in some cases, the dominance of the MN correlated significantly with a decrease in the volume of its ventral dendrite, while in others, it correlated with an increase in the sizes of the body and the lateral dendrite. Both structural signs appeared to result from local redistribution of the neurotransmitter balance due to the tension of the undamaged sensory inputs. Overall, the results demonstrated that prolonged adaptive changes in the behavior of the fish may be regulated by specific morphological rearrangements of MN at the level of individual dendrites acting via feedforward and feedback mechanisms.

We studied the electrophysiological characteristics of Mauthner neurons (MN) in in vitro preparations of the medulla fragments of goldfish fries. The characteristics of extracellularly recorded responses of MN were found to be close to those usually recorded in vivo. It was demonstrated that in vitro intracellular microelectrode recording of MN activity in goldfish fries is, in principle, possible. The main experimental approaches for successful intracellular recording from such objects have been developed, and the possible artifacts met in the course of the experiments, as well as the parameters of stimulation, have been identified.

Using the morphometric techniques, the ultrastructural changes of the afferent synapses on the ventral dendrite of the Mauthner neurons (MNs) were studied after the adaptation of goldfish to long-term fatiguing sensory (visual) stimulation, characterized by the growth of MN resistance. It was shown that after the adaptation, the length of active zones (AZs) in the synapses located on the MN ventral dendrite was significantly reduced by 23%. At the same time, the length the AZs of the excitatory visual synapses was reduced by 29% in comparison with the control, while the length of desmosome-like contacts (DLCs) bordering AZs was increased by 71%. It was also found that the length of AZs in the inhibitory synapses was decreased by 19% after the adaptation, which is consistent with the important role of inhibitory processes in the sensory pathways during the memory formation. Taking into account the actin nature of the DLCs, the basis of the adaptation to the visual stimulation is suggested to be in the presynaptic mechanism of neurotransmitter secretion regulation by actin.

Using immunogold labeling, excitatory serotoninergic synapses of both chemical and mixed types, were found on the ventral dendrite (VD) of goldfish Mauthner neuron (MN).They are characterized by the presence of several mitochondria in the bouton and by an obligatory desmosome-like contact (DLC) besides the active zone (AZ). Their AZs were commonly found to make contact with the unlabeled chemical crested synapses, which, in turn, directly interacted with VD. These synapses were practically devoid of mitochondria and had no DLCs, thus allowing to identify them as the inhibitory ones. This "two-level" organization of excitatory serotoninergic and inhibitory synapses appears to be related to the reciprocal mechanism of the regulation of MN functional activity by visual input.

The effect of training adaptive vestibular stimulations on goldfish Mauthner neurons (MN) function and three-dimensional morphology was studied in experimental amyloidosis caused by application of aggregated beta-amyloid protein (Abeta25-35). It was found that as compared with control, adapted (trained) MN gained significant resistance to Abeta25-35. Taking into consideration the key role of dopamine in MN adaptation to sensory stimulations, its effect on the development of model amyloidosis was studied. It was shown that the application of dopamine onto MN as well as the increase of its concentration in brain by means of L-dopa protected MN structure and function against pathogenic influence of Abeta25-35. Using electron microscopy it was shown that dopamine protective action on neurons was due to its ability to dissociate polymer amyloid molecules into short inactive fragments.

It was previously shown that the contralateral (relative to preferred side of turns) optokinetic stimulation and ipsilateral eye enucleation cause a significant, 2- to 4-fold reduction of the ventral dendrite (VD) volume in one of two goldfish Mauthner neurons (MN) that becomes more active functionally. In this study, we investigated the MN ultrastructure after mentioned unilateral visual effects. In both cases, devastation of the afferent synapses was detected along the full length of the reduced VD, with simultaneous compaction of its cytoskeleton, in contrast to those of VD of the contralateral MN and of lateral dendrites and cell bodies of both neurons. It is suggested that the depleted synapses belong to the excitatory visual afferent input, and both cytoskeletal and synaptic mechanisms are involved in the regulation of MN functional activity through VD.

Using the method of 3D reconstruction, the structural correlates of significantly increased functional activity of denervated Mauthner neuron (MN) were studied after the unilateral eye enucleation, that resulted in the irreversible shift of the goldfish motor asymmetry to a "blind" side. It was established that in some cases the functional dominance of MN was significantly correlated with the reduction of the volume of its ventral dendrite, while in the other cases it was correlated with the increase in sizes of its soma and the lateral dendrite. Both structural features, probably, were caused by local redistribution of the neurotransmitters due to the stress of sensory inputs that remained undamaged. Thus, it was demonstrated that the prolonged adaptive changes in goldfish behavior could be regulated by means of specific morphological reorganizations of MN at the level of their individual dendrites by the principle of feedback or feedforward mechanisms.

Previously we have demonstrated, that the contralateral optokinetic (visual) stimulation (COS) induces an inversion of goldfish motor asymmetry and three-fold decrease of the ventral dendrite of ipsilateral Mauthner neuron (MN) volume, whereas repetitive in training mode daily COS sessions induced a motor behavior resistance to this influence in fish. In the present investigation we have examined the effect of the training COS sessions on the structure of MN and their parts. It was found that daily visual training stabilizes the size of the ventral dendrites, significantly increasing their resistance to single prolonged COS. Thus, the adapted morpho-functional state was induced in the individual dendrite and in the whole neuron by repetitive stimulation of that dendrite. These results make it possible to investigate in more detail the role of an individual dendrite in the modification of functional activity of the whole neuron and in adaptation and memory mechanisms at the cellular level.

The effects of applying aggregated beta-amyloid peptide fragment 25-35 on the three-dimensional structure and volume of Mauthner neurons (MN) and on motor asymmetry were assessed in goldfish using reconstructions based on serial histological sections. These experiments showed that the motor asymmetry of the fish was stable in the intact state and in controls and correlated tightly with structural asymmetry of neurons. beta-Amyloid produced large changes or inversion in motor asymmetry, which did not coincide with or even contradicted the structural asymmetry of MN. This occurred as a result of marked dystrophy or, conversely, hypertrophy of individual neurons and their individual dendrites, with changes in their proportions. It is suggested that the harmful action of beta-amyloid on MN structure and the discordant ("incorrect") behavior of the fish may result from mechanical deformation evoked by its tape-like fibrils. Overall, the results lead to the conclusion that MN provide a suitable system for studying the structural aspects of amyloidosis.

We have previously shown that contralateral optokinetic (visual) stimulation (COS) evokes inversion of motor asymmetry in goldfish and three-fold reductions in the volume of the ventral dendrite of the ipsilateral Mauthner neuron (MN). A training regime consisting of repeated daily sessions of COS induced resistance of the motor behavior of the fish to this treatment. We report here our studies of the effects of training sessions of COS on the structure of MN and their components. Daily visual training was found to stabilize the sizes of the dorsal dendrites of MN, significantly increasing their resistance to single prolonged sessions of COS. Thus, repeated stimulation of an individual dendrite induces an adaptive morphological state in the dendrite and in the neuron as a whole. This allows more detailed studies of the role of the individual dendrite in modifying the functional activity of the whole neuron in the mechanisms of adaptation and memory at the cellular level to be performed.

It was recently demonstrated that following the optomotor drum, rotating contralaterally to the turn side, spontaneously preferred by the goldfish fry, inverted their motor asymmetry. The study of Mauthner neuron (MN) structure using the histological method of 3D reconstruction, and the measurements of right and left MN in these fishes, has detected the 3-fold decrease of the volume of the ventral dendrite (VD) of ipsilateral MN, which was reciprocally related to MN functional activity. At the same time, total volume of contralateral MN remained larger than that of ipsilateral one by one quarter, which was the case in intact fishes. It is suggested that the decrease in the size of VD, which receives the afferentation from the contralateral eye, that follows the moving stripes of the optomotor drum, is the result of a specific contralateral visual stimulation and is the first evidence of the possibility of MN natural stimulation through VD.

Recent studies have established that following an optomotor drum rotating in the direction contralateral to that spontaneously preferred by goldfish fry inverts their motor asymmetry. Studies of the structure of Mauthner neurons (MN) using the histological technique of three-dimensional reconstruction along with measurement of the volumes of the right and left MN in these fish showed that the ipsilateral MN underwent a three-fold reduction in the volume of the ventral dendrite (VD), which was in inverse proportion to the functional activity of the MN. The total volume of the contralateral MN was 25% larger than the volume of the ipsilateral MN, the situation typical of intact fish. It is suggested that the decreases in the size of the VD receiving afferentation from the contralateral eye, which follows the moving band of the optomotor drum, results from the specific contralateral visual stimulation, and is the first evidence of the possible natural stimulation of the MN via the VD.