We studied swimming of goldfish fries about 3 cm long in a narrow channel by calculating the numbers of spontaneous turns on different sides. The ratio of fishes preferring to turn to the right vs to the left was 1.5: 1.0, respectively; two-thirds of the fishes demonstrated an ambilateral behavior. Experiments with compulsory 10-min-long rotation of the fishes (clockwise around the longitudinal body axis for fishes preferring right-side turns and anticlockwise for fishes preferring left-side turns) showed that the behavioral asymmetry smoothed somewhat after such a procedure, and a greater number of the fishes became ambilateral in their preference to turn to one side or another. After a one-or two-day-long test, the initial asymmetry of motor behavior completely recovered. Compulsory rotation of similar fishes in the opposite direction exerted no influence on the asymmetry in the choice of the turning direction. Adaptation-induced training of the fishes (using fatiguing long-lasting vestibular stimulation) resulted in some smoothing of motor asymmetry but did not change its general pattern. Thus, our findings allow us to believe that a noticeable proportion of the goldfish individuals (similarly to other animals and humans) is characterized by an innate asymmetry of the motor function with a clear preference for either right-or left-side turnings. These relations can be smoothed under experimental influences but are recovered later on, i.e., they are stable and are not fundamentally transformed. We assume that the asymmetry of motor behavior of fishes in a narrow channel can be an adequate pre-requisite for further examination of the asymmetry of the brain and motor centers controlling changes in locomotion (body turnings)

We studied changes in the motor asymmetry of the goldfish induced by single-session long-lasting vestibular stimulations (clockwise and counter clockwise rotations around the rostro-caudal body axis) and repetitive everyday short sessions of such stimulation (training); the latter mode led to the development of adaptation (resistance to fatigue). Rotational stimulation of different durations and directions elicited effects of different patterns and intensities. Such stimulation enhanced or, vice versa, smoothed the motor asymmetry in "dextral" and "sinistral" fishes, up to full symmetry or even a change of the preferred turning direction. Adaptation to unilateral rotational stimulation allows an experimenter to selectively and gradually induce the resistivity of the left-or right-ward asymmetry to fatigue effects. Earlier, we found that the motor asymmetry in the goldfish, which is determined by the functional asymmetry of the brain, correlates with the morphological asymmetry of Mauthner neurons localized in the medulla in a mirror manner and playing a crucial role in the control of turnings in the course of locomotion (swimming). Experimental rotational stimulation-induced gradual modification of the motor asymmetry in the goldfish can serve as a physiological model for more detailed studies of the structural base of the functional brain asymmetry and some mechanisms of adaptation on the level of single neurons.

We examined the morphological peculiarities of Mauthner neurons, MNs, in goldfishes with a phenotypically different or an experimentally modified preference to perform rightward vs leftward turnings in the course of motor behavior; this preference was characterized by values of the motor asymmetry coefficient (MAC). 3D reconstruction of MNs was performed based on several histological sections; volumes of the soma, lateral and ventral dendrites (LD and VD, respectively), initial segment of the axon, as well as full volumes of the right and left neurons, were calculated. Differences between the above parameters were expressed as structural asymmetry coefficients (SACs). It was shown that clear orientation asymmetry of motor behavior of the fish is accompanied by differences in the dimensions of MNs and their compartments; MNs localized contralaterally with respect to the preferred turning side were considerably bigger than ipsilateral neurons. Experimental influences inducing inversion of the motor asymmetry of fishes inverted structural asymmetry of their MNs. In fishes with no phenotypical preference of the turning side and in individuals whose motor asymmetry was smoothed due to experimental influences (rotational stimulations), structural asymmetry of the MNs was also smoothed. Changes of the structural proportions developed, as a rule, due to decreases in the dimensions of one or both MNs and their compartments. The MAC value was in direct correlation with the value of SAC of the MNs and with values of this coefficient for the soma and the sum soma + LD. At the same time, reciprocal relations were found for the MAC and structural asymmetry of the VD; the decrease in the volume of VD was related to an increase in the preference of the contralateral turning side by the fish, and vice versa. In general, the results of our study demonstrate that both morphological and functional peculiarities of MNs correlate to a significant extent with such a form of motor behavior of fishes as realization of spontaneous turnings.

In the goldfish, we studied the effects of intramedullar applications of glutamate (Glu), dopamine (DA), and of long-lasting rotational stimulation on the functional activity, dimensional characteristics, and ultrastructure of Mauthner neurons (MNs). Applications of Glu, especially when combined with rotational stimulation, were found to result in suppression of the function of MNs, in a decrease in their dimensions and lengths of desmosome-like contacts (DLCs, whose structure is determined by filamentous actin) in afferent mixed and chemical synapses, and in destruction of actin microfilaments in the cytoskeleton of MNs. Applications of DA, vice versa, induced an increase in the resistance to the effects of long-lasting stimulation and stabilized the dimensions of MNs; the length of DLCs increased in afferent synapses of both the above types, and the number of fibrillar actin bridges in the DLC cleft of mixed synapses also increased. Bundles of the actin filaments, which were preserved after stimulation, appeared in the cytoskeleton of MNs. Testing of the action of neurotransmitters on actin preparations in vitro showed that Glu entirely depolymerizes filamentous actin, while DA, vice versa, polymerizes monomeric actin. Thus, the Glu-and DA-induced reactions are similar in their types and are of a reciprocal nature both in the actin cytoskeleton of MNs in situ and in purified actin in vitro; these effects correlate with suppression of the functional state of MNs under the influence of Glu and with stabilization of this state under the influence of DA. These results agree with the concept on the roles of depolymerization and polymerization of actin in changes of the morphofunctional state of MNs and show that actin of the cytoskeleton of MNs is a cellular target for the actions of Glu and DA. The similarity between the effects of tested neurotransmitters on actin in MNs in situ and in cell-free preparations in vitro allows us to hypothesize that these transmitters can penetrate into the neuron.

In goldfishes, we studied (i) manifestations of functional activity of Mauthner neurons (MNs) reflected in motor behavior and (ii) changes in 3-D morphometry (ratio of volumes) and ultrastructure of MNs after applications of an actin-polymerizing peptide obtained from scorpion venom and after vestibular rotational stimulations (trainings) inducing natural modification of functions of the MNs (adaptation). In MNs subjected to application of the peptide, the increase in the functional resistance and morphological stability caused by long-lasting stimulation directly depended on the dose of the applied peptide or on the effectiveness of trainings, whereas in intact and control MNs such stimulation resulted in significant decreases in the activity and volumes of these cells. At the ultrastructure level, both applications of the peptide and trainings caused the formation of extensive bundles of actin filaments ("stress-fibers") in the cytoplasm of MNs and led to an increase in the dimension of desmosome-like contacts (DLCs) in afferent synapses. At chemical synapses, the effect looked like a reciprocal decrease in the length of the active zones (a structural sign of long-term depression, LTD), while at mixed synapses this was manifested in an increase in the number of fibrillar bridges in the gaps of DLCs (a structural sign of long-term potentiation, LTP). The data obtained allow us to hypothesize that LTD of the efficacy of transmission through chemical synapses is involved in the formation of the adaptation state of the MNs and that polymerization of actin in the cytoplasm and DLCs underlies the mechanism of LTD and adaptation. The development of ultrastructural manifestations of LTP at mixed synapses after polymerization of actin by the peptide, which is related to a reciprocal increase in the efficacy of "mixed" afferent inputs, explains the maintenance of a high integral level of activity of the MNs, despite a drop in the functional activity of "chemical" afferent inputs. Therefore, the actin cytoskeleton plays a clearly significant role in the maintenance of the balance of excitation at the neuronal level.