Slowing of human arm movements during weightlessness: the role of vision.

The human motor system responds to weightlessness by the slowing of movement. It has been suggested that deficits in visuo-motor co-ordination cause this effect. We studied the mechanisms of the slowing of movement in three long-term missions to the Russian space station Mir. In particular, the role of vision in the control of movement in microgravity has been studied in these experiments on seven cosmonauts, pre-, in-, and post-flight. The cosmonauts made arm movements to visual targets under the following conditions of visual control: no visual control, interrupted visual control, and undisturbed visual control. The results showed that the slowing of movement during weightlessness was manifested by decreases of peak velocity and peak acceleration, was not associated with a prolongation of the movement phase of deceleration, and was not affected by manipulation of the conditions of visual control. The slowing of movement tended to subside after the months of the flight and completely disappeared within days after the landing. Accuracy of the movements strictly depended on the constraints imposed on the vision and remained unaffected in-flight. The data presented demonstrate that the slowing of movement in microgravity is not directly related to deficits in sensori-motor co-ordination and is not associated with a reduction of the accuracy of movement. The strategy for motor control in microgravity seems to be directed towards the generation of smooth movements and the maintenance of their accuracy.

Modulation of cyclic-nucleotide-gated channels and regulation of vertebrate phototransduction.

Cyclic-nucleotide-gated (CNG) channels are crucial for sensory transduction in the photoreceptors (rods and cones) of the vertebrate retina. Light triggers a decrease in the cytoplasmic concentration of cyclic GMP in the outer segments of these cells, leading to closure of CNG channels and hyperpolarization of the membrane potential. Hence, CNG channels translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through the photoreceptor cell and be transmitted to the rest of the visual system. The sensitivity of phototransduction can be altered by exposing the cells to light, through adaptation processes intrinsic to photoreceptors. Intracellular Ca(2+) is a major signal in light adaptation and, in conjunction with Ca(2+)-binding proteins, one of its targets for modulation is the CNG channel itself. However, other intracellular signals may be involved in the fine-tuning of light sensitivity in response to cues internal to organisms. Several intracellular signals are candidates for mediating changes in cyclic GMP sensitivity including transition metals, such as Ni(2+) and Zn(2+), and lipid metabolites, such as diacylglycerol. Moreover, CNG channels are associated with protein kinases and phosphatases that catalyze changes in phosphorylation state and allosterically modulate channel activity. Recent studies suggest that the effects of circadian rhythms and retinal transmitters on CNG channels may be mediated by such changes in phosphorylation. The goal of this paper is to review the molecular mechanisms underlying modulation of CNG channels and to relate these forms of modulation to the regulation of light sensitivity.

Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue.

Illumination of vertebrate rod photoreceptors leads to a decrease in the cytoplasmic cGMP concentration and closure of cyclic nucleotide-gated (CNG) channels. Except for Ca(2+), which plays a negative feedback role in adaptation, and 11-cis-retinal, supplied by the retinal pigment epithelium, all of the biochemical machinery of phototransduction is thought to be contained within rod outer segments without involvement of extrinsic regulatory molecules. Here we show that insulin-like growth factor-I (IGF-I), a paracrine factor released from the retinal pigment epithelium, alters phototransduction by rapidly increasing the cGMP sensitivity of CNG channels. The IGF-I-signaling pathway ultimately involves a protein tyrosine phosphatase that catalyzes dephosphorylation of a specific residue in the alpha-subunit of the rod CNG channel protein. IGF-I conjointly accelerates the kinetics and increases the amplitude of the light response, distinct from events that accompany adaptation. These effects of IGF-I could result from the enhancement of the cGMP sensitivity of CNG channels. Hence, in addition to long-term control of development and survival of rods, growth factors regulate phototransduction in the short term by modulating CNG channels.

Mechanism of inhibition of cyclic nucleotide-gated channel by protein tyrosine kinase probed with genistein.

Rod cyclic nucleotide-gated (CNG) channels are modulated by changes in tyrosine phosphorylation catalyzed by protein tyrosine kinases (PTKs) and phosphatases (PTPs). We used genistein, a PTK inhibitor, to probe the interaction between the channel and PTKs. Previously, we found that in addition to inhibiting tyrosine phosphorylation of the rod CNG channel alpha-subunit (RETalpha), genistein triggers a noncatalytic inhibitory interaction between the PTK and the channel. These studies suggest that PTKs affects RETalpha channels in two ways: (1) by catalyzing phosphorylation of the channel protein, and (2) by allosterically regulating channel activation. Here, we study the mechanism of noncatalytic inhibition. We find that noncatalytic inhibition follows the same activity dependence pattern as catalytic modulation (phosphorylation): the efficacy and apparent affinity of genistein inhibition are much higher for closed than for fully activated channels. Association rates with the genistein-PTK complex were similar for closed and fully activated channels and independent of genistein concentration. Dissociation rates were 100 times slower for closed channels, which is consistent with a much higher affinity for genistein-PTK. Genistein-PTK affects channel gating, but not single channel conductance or the number of active channels. By analyzing single channel gating during genistein-PTK dissociation, we determined the maximal open probability for normal and genistein-PTK-bound channels. genistein-PTK decreases open probability by increasing the free energy required for opening, making opening dramatically less favorable. Ni(2+), which potentiates RETalpha channel gating, partially relieves genistein inhibition, possibly by disrupting the association between the genistein-PTK and the channel. Studies on chimeric channels containing portions of RETalpha, which exhibits genistein inhibition, and the rat olfactory CNG channel alpha-subunit, which does not, reveals that a domain containing S6 and flanking regions is the crucial for genistein inhibition and may constitute the genistein-PTK binding site. Thus, genistein-PTK stabilizes the closed state of the channel by interacting with portions of the channel that participate in gating.

Interactions of cyclic nucleotide-gated channel subunits and protein tyrosine kinase probed with genistein.

The cGMP sensitivity of cyclic nucleotide-gated (CNG) channels can be modulated by changes in phosphorylation catalyzed by protein tyrosine kinases (PTKs) and protein tyrosine phosphatases. Previously, we used genistein, a PTK inhibitor, to probe the interaction between PTKs and homomeric channels comprised of alpha subunits (RETalpha) of rod photoreceptor CNG channels expressed in Xenopus oocytes. We showed that in addition to inhibiting phosphorylation, genistein triggers a noncatalytic interaction between PTKs and homomeric RETalpha channels that allosterically inhibits channel gating. Here, we show that native CNG channels from rods, cones, and olfactory receptor neurons also exhibit noncatalytic inhibition induced by genistein, suggesting that in each of these sensory cells, CNG channels are part of a regulatory complex that contains PTKs. Native CNG channels are heteromers, containing beta as well as alpha subunits. To determine the contributions of alpha and beta subunits to genistein inhibition, we compared the effect of genistein on native, homomeric (RETalpha and OLFalpha), and heteromeric (RETalpha+beta, OLFalpha+beta, and OLFalpha+RETbeta) CNG channels. We found that genistein only inhibits channels that contain either the RETalpha or the OLFbeta subunits. This finding, along with other observations about the maximal effect of genistein and the Hill coefficient of genistein inhibition, suggests that the RETalpha and OLFbeta subunits contain binding sites for the PTK, whereas RETbeta and OLFalpha subunits do not.

Activity-dependent modulation of rod photoreceptor cyclic nucleotide-gated channels mediated by phosphorylation of a specific tyrosine residue.

Cyclic nucleotide-gated (CNG) channels are crucial for phototransduction in vertebrate rod photoreceptors. The cGMP sensitivity of these channels is modulated by diffusible intracellular messengers, including Ca2+/calmodulin, contributing to negative feedback during sensory adaptation. Membrane-associated protein tyrosine kinases and phosphatases also modulate rod CNG channels, but whether this results from direct changes in the phosphorylation state of the channel protein has been unclear. Here, we show that bovine rod CNG channel alpha-subunits (bRET) contain a tyrosine phosphorylation site crucial for modulation. bRET channels expressed in Xenopus oocytes exhibit modulation, whereas rat olfactory CNG channels (rOLF) do not. Chimeric channels reveal that differences in the C terminus, containing the cyclic nucleotide-binding domain, account for this difference. One specific tyrosine in bRET (Y498) appears to be crucial; replacement of this tyrosine in bRET curtails modulation, whereas installation into rOLF confers modulability. As the channel becomes dephosphorylated, there is an increase in the rate of spontaneous openings in the absence of ligand, indicating that changes in the phosphorylation state affect the allosteric gating equilibrium. Moreover, we find that dephosphorylation, which favors channel opening, requires open channels, whereas phosphorylation, which promotes channel closing, requires closed channels. Hence, modulation by changes in tyrosine phosphorylation is activity-dependent and may constitute a positive feedback mechanism, contrasting with negative feedback systems underlying adaptation.

Noncatalytic inhibition of cyclic nucleotide-gated channels by tyrosine kinase induced by genistein.

Rod photoreceptor cyclic nucleotide-gated (CNG) channels are modulated by tyrosine phosphorylation. Rod CNG channels expressed in Xenopus oocytes are associated with constitutively active protein tyrosine kinases (PTKs) and protein tyrosine phosphatases that decrease and increase, respectively, the apparent affinity of the channels for cGMP. Here, we examine the effects of genistein, a competitive inhibitor of the ATP binding site, on PTKs. Like other PTK inhibitors (lavendustin A and erbstatin), cytoplasmic application of genistein prevents changes in the cGMP sensitivity that are attributable to tyrosine phosphorylation of the CNG channels. However, unlike these other inhibitors, genistein also slows the activation kinetics and reduces the maximal current through CNG channels at saturating cGMP. These effects occur in the absence of ATP, indicating that they do not involve inhibition of a phosphorylation event, but rather involve an allosteric effect of genistein on CNG channel gating. This could result from direct binding of genistein to the channel; however, the time course of inhibition is surprisingly slow (>30 s), raising the possibility that genistein exerts its effects indirectly. In support of this hypothesis, we find that ligands that selectively bind to PTKs without directly binding to the CNG channel can nonetheless decrease the effect of genistein. Thus, ATP and a nonhydrolyzable ATP derivative competitively inhibit the effect of genistein on the channel. Moreover, erbstatin, an inhibitor of PTKs, can noncompetitively inhibit the effect of genistein. Taken together, these results suggest that in addition to inhibiting tyrosine phosphorylation of the rod CNG channel catalyzed by PTKs, genistein triggers a noncatalytic interaction between the PTK and the channel that allosterically inhibits gating.

Modulation of rod photoreceptor cyclic nucleotide-gated channels by tyrosine phosphorylation.

Cyclic nucleotide-gated (CNG) channels in vertebrate photoreceptors are crucial for transducing light-induced changes in cGMP concentration into electrical signals. In this study, we show that both native and exogenously expressed CNG channels from rods are modulated by tyrosine phosphorylation. The cGMP sensitivity of CNG channels, composed of rod alpha-subunits expressed in Xenopus oocytes, gradually increases after excision of inside-out patches from the oocyte membrane. This increase in sensitivity is inhibited by a protein tyrosine phosphatase (PTP) inhibitor and is unaffected by three different Ser/Thr phosphatase inhibitors. Moreover, it is suppressed or reversed by application of ATP but not by a nonhydrolyzable ATP analog. Application of protein tyrosine kinase (PTK) inhibitors causes an increase in cGMP sensitivity, but only in the presence of ATP. Taken together, these results suggest that CNG channels expressed in oocytes are associated with active PTK(s) and PTP(s) that regulate their cGMP sensitivity by changing phosphorylation state. The cGMP sensitivity of native CNG channels from salamander rod outer segments also increases and decreases after incubation with inhibitors of PTP(s) and PTK(s), respectively. These results suggest that rod CNG channels are modulated by tyrosine phosphorylation, which may function as a novel mechanism for regulating the sensitivity of rods to light.

Pointing arm movements in short- and long-term spaceflights.

Accuracy and kinematics of horizontal arm pointing movements to visual targets were studied on three cosmonauts in 10-, 140-, and 172-d spaceflights in order to investigate mechanisms of the sensorimotor adaptation to microgravity. The Austrian equipment MONIMIR was mounted on board the Russian space station MIR and used for three-dimensional recording of the arm position and presentation of the targets. It was found that movement accuracy remained constant whereby movement durations significantly increased in all inflight sessions compared to the preflight baseline values. Inflight, movement peak velocities as well as acceleration and deceleration peak values decreased significantly. Analysis of the velocity-time profiles showed that the ratio between acceleration and deceleration phases decreased slightly for one cosmonaut and increased insignificantly for the other two cosmonauts. All phases of the acceleration-time profiles increased inflight by the same factor. These data fail to support the assumption of an increased role of the direct visual guidance in movement execution in microgravity. This suggests that the movement slowing in microgravity may be caused by a control strategy employed by the CNS to avoid the specific disadvantage of the absence of gravity. It is hypothesized that intra-movement control mechanisms play an important role in the movement coordination in the altered gravity environment.

Grip reorganization during wrist transport: the influence of an altered aperture.

Past studies have examined the coupling of reach and grasp components during prehensile movements. Many of these studies have supported the view that these components reflect the output of two parallel, though temporally coupled, motor programs. When the grip aperture is Altered prior to the onset of prehension from its usual, normally flexed position to one of maximal finger extension, our previous work has shown that the grasp component appears to reorganize itself during the reach. This reorganization, consisting of a brief closing and reopening of the grip aperture, only slightly influenced the temporal components of the wrist transport. The present experiment continues this research theme by examining the characteristics of grip aperture reorganization through the comparison of the kinematics of prehension components during movements to two different size objects under normal and Altered grip aperture conditions. It was hypothesized that if the grip reorganization is task dependent it should be related to object size. The experiment found that in the Altered grip condition reorganization did occur, as indicated by a slight closing and reopening of the aperture without influencing the transport of the wrist. The amplitude of and the time to the observed inflection point in the aperture time course were related to object size. The velocity of grip closing for the large object showed double peaks, with the first substantially smaller than the second. Moreover, for the small object, the velocity of grip aperture closing also was double peaked, but the difference between peaks was less pronounced. These changes in grip velocity suggest that the grip reorganization is related to object size. No effect of Altered aperture was observed on the transport component. For both object sizes in the Altered condition, the final peak velocity of grip aperture was statistically significantly correlated with transport time and time to peak deceleration. In contrast, such correlations were not observed for the initial peak velocity of the grip aperture. Furthermore, time to maximum grip aperture was correlated with both time to peak wrist velocity and time peak to wrist deceleration. Thus, as the reach progressed toward the object, the grip and transport components became more interdependent. The results are consistent with the notion that, when a well-practiced, coordinated act such as prehension is confronted with an Altered grip posture at the onset of the reach, the grip can be reorganized during the transport to preserve the relative timing between them. Thus these data add to the growing awareness that not only is there temporal coupling between the reach and grasp components but that these components may be integrated by higher-order control mechanism.

A new method of fitting and analysis of simple uni-joint arm movements.

A new method for fitting and analysis of simple uni-joint arm movements is proposed. The method is based on a model which postulates that the acceleration-time profile of the movement can be described by a linear combination of two Gaussian functions (positive for acceleration and negative for deceleration). The method was tested on more than 21000 arm movements performed under different control conditions and showed high fitting precision. It allows to completely describe a movement using only five parameters of two Gaussian functions. The method is sensitive to differences between the acceleration and deceleration phases of movement as well as between subsequent movements because of the independent calculation of the Gaussian functions for accelerative and decelerative movement parts. Relationships between conventional kinematic and model parameters as well as areas of application of the method are discussed.

The effects of dopamine and serotonin on rat dorsal root ganglion neurons: an intracellular study.

The effects of bath application of dopamine and serotonin (10(-10)-10(-8) M) were studied in the superfused dorsal root ganglia of 30-36-day-old rats by means of the intracellular technique. In the majority of cells, dopamine and serotonin caused depolarization (60% and 64% of the tested cells, respectively). In other cells hyperpolarization, biphasic reactions or absence of responses have been observed. All reactions were dose dependent and reversible. Depolarization was accompanied by a decrease of input membrane resistance and hyperpolarization by its increase. Some cells did not show these alterations. Monoamines were also capable of modulating spikes. In some cases dopamine (10(-8)-10(-7) M) decreased the amplitude of the action potential and increased its duration, but the same concentration of serotonin produced the opposite effect on these parameters. The correlation between the electrophysiological properties of the dorsal root ganglion neurons and their responses to monoamines were discovered. Neurons with high input membrane resistance, prolonged action potential and slow conduction velocity (small cells) were influenced much more by monoamines than neurons with low input membrane resistance, "fast" action potential and rapid conduction velocity (large cells). (1) Small cells had lower threshold to monoamines (10(-8)-10(7) M) than large ones, some of which did not respond even to 10(-6) M. (2) The amplitude and duration of monoamine-induced depolarization in small cells were on average about two to three times higher than those in large cells. These data provide evidence for the modulatory role of monoamines in spinal afferent sensory functions.

Different action of ethosuximide on low- and high-threshold calcium currents in rat sensory neurons.

The effects of ethosuximide on calcium channels were studied on dorsal root ganglion neurons from one-day-old rats using the patch-clamp technique. Bath application of ethosuximide induced dose-dependent and reversible suppression of calcium currents without affecting their time-course. Substantial differences between the effects of ethosuximide on the low-threshold and high-threshold (T- and L-) currents were observed. Ethosuximide reduced the T-current with greater potency than the L-current (Kd for T-current is 7 microM vs 15 microM for L-current). This relative specificity of its action still remained if applied at concentrations up to 1 mM. These data support the hypothesis according to which the anti-epileptic action of ethosuximide is related to reduction of the low-threshold calcium currents in sensory neurons.

[The mechanism of milk contamination in the territory of Gomel Oblast]. / Zakonomernosti zagriazneniia moloka na territorii Gomel'skoi oblasti.

The authors provide the results of studying 137Cs and 90Sr pollution of soil, plants and milk in the Gomel region in 1986-1989. Describe the dynamics of the ratios soil-grass and milk-soil during that period. The content of 137Cs in milk ranged from n.10(-10) to n.10(-7) Ci/l in polluted regions and from n.10(-10)-n.10(-9) Ci/l in the so-called pure regions. In all the regions examined, the concentration of 90Sr in milk was far lower than that of 137Cs. After the accident the levels of 137Cs in milk rose 18-350-fold and those of 90Sr 3-16-fold on the average in the regions. One can observe a progressive decrease of the ratios of accumulation in the grass-soil and milk-soil systems and a decline of the concentration of 137Cs and 90Sr in milk. However, the percentage of samples with an increase of the temporarily permissible in 1988 levels of cesium radionuclides was fairly high in 1989 in certain regions: in the Vetkovsk region, it was 35.3%, in the Chechersky region, it was 28.0%. During 1987-1989, the concentration of 90Sr in milk did not exceed 1 x 10(-9) Ci/l in the areas examined. The broad range of the indicators of milk pollution with 137Cs and 90Sr is specified by varying density of the area pollution, differences in the physicochemical status of radionuclides in accident fall-outs, and geochemical characteristics of the soil sheet.

[The action of dopamine and serotonin on the cells of an isolated spinal cord ganglion of the rat]. / Deistvie dofamina i serotonina na kletki izolirovannogo spinno-mozgovogo gangliia krys.

The effect of bath application of dopamine (DA) and serotonin (HT) was studied in the isolated perfused dorsal root ganglia (DRG) of 30-36 days old rats by means of intracellular technique. 92% of investigated cells responded to the application of DA and 87%--to HT. DA and HT evoked depolarization in most of cells (64.6 and 73.7% of cells, respectively). Responses were dose-dependent and reversible. Depolarization caused by the DA and HT application was accompanied by a decrease in the input resistance (Rm) and alteration of the action potential shape. Hyperpolarization was followed by an increase of Rm. It was demonstrated that both DA and HT influenced predominantly DRG neurons with membrane biophysical characteristics inherent in small cells. Possibility of modulation of the afferent impulsation on the level of the primary sensory neurons is supposed.

[Effect of oxytocin on rat dorsal root ganglia in vitro]. / Vliianie oksitotsina na neirony spinno-mozgovogo gangliia krysy in vitro.

The influence of bath application of oxytocin (OT) was investigated on the isolated dorsal root ganglion (DRG) cells by means of intracellular recording. The results showed that OT evoked depolarization in most cells and affected predominantly the DRG neurons with biophysical+ characteristics inherent in small neurons.

[Effect of vasopressin on the somatic membrane of spinal ganglion neurons]. / Vliianie vazopressina na somaticheskuiu membranu neironov spinal'nykh gangliev.

The application of vasopressin (VP) in the isolated perfused dorsal root ganglia of 22-36 days old rats was studied by means of intracellular technique. 86.76% of cells have responded to the VP application. Depolarization was observed in 67.8% responded cells, the mixed response--in 16.95% cells, hyperpolarization--in 15.25% cells. All responses were dose-dependent and reversible. Input resistance (Rm) of the cell membrane decreased during depolarization and increased during hyperpolarization. The VP-evoked depolarization was accompanied by an increase in the action potential (AP) duration and decrease in the AP amplitude and after-hyperpolarization. Neurons with slow conduction velocity, high Rm and prolonged AP (small cells) had the lowest threshold of the sensitivity to VP (1.10(-11) M) and prolonged high-amplitude responses. Cells with the rapid conduction velocity, low Rm and rapid AP (large cells) responded to 1.10(-8) M, but sometimes even 1.10(-6) M had no effect. Depolarization in these neurons had smaller duration and low amplitude: sometimes hyperpolarization was observed. These results confirm the possibility that VP has effect on small neurons predominantly.