Effect of hindlimb unloading on motor activity in adult rats: impact of prenatal stress.

Environmental changes that occur in daily life or, in particular, in situations like actual or simulated microgravity require neuronal adaptation of sensory and motor functions. Such conditions can exert long-lasting disturbances on an individual's adaptive ability. Additionally, prenatal stress also leads to behavioral and physiological abnormalities in adulthood. Therefore, the aims of the present study were (a) to evaluate in adult rats the behavioral motor adaptation that follows 14 days of exposure to simulated microgravity (hindlimb unloading) and (b) to determine whether restraint prenatal stress influences this motor adaptation. For this purpose, the authors assessed rats' motor reactivity to novelty, their skilled walking on a ladder, and their swimming performance. Results showed that unloading severely impaired motor activity and skilled walking. By contrast, it had no effect on swimming performance. Moreover, results demonstrated for the first time that restraint prenatal stress exacerbates the effects of unloading. These results are consistent with the role of a steady prenatal environment in allowing an adequate development and maturation of sensorimotor systems to generate adapted responses to environmental challenges during adulthood.

Variations in amino acid neurotransmitters in the rat ventral spinal cord after hindlimb unloading.

We have measured by HPLC the neurotransmitter content in L(4) and L(5) spinal segmental levels in CONT rats, after 7 (HU7) and after 14 days (HU14) of hindlimb unloading. These segments are known to contain the hindlimb muscle motoneurons. The main result is the increase of two neuroexcitators (glutamate and aspartate) and two neuroinhibitors (glycine and GABA) at the L(5) spinal segmental level in HU7 group. Our data indicated that the neurotransmitter changes are restricted to spinal segmental level containing motoneurons from muscles which are strongly modified by HU condition.

A 14-day period of hindpaw sensory deprivation enhances the responsiveness of rat cortical neurons.

Hypodynamia-hypokinesia (HH) is a model of hindpaw sensory deprivation. It is obtained by unloading of the hindquarters during 14 days. In this situation, the feet are not in contact with the ground and as a consequence, the cutaneous receptors are not activated; the sensory input to the primary somatosensory cortex (SmI) is thus reduced. In a previous study, we have shown that HH induced a cortical reorganisation of the hindlimb representation. The understanding of the mechanisms involved in cortical map plasticity requires a close examination of the changes in response properties of cortical neurons during HH. The aim of the present study was thus to study the characteristics of neurons recorded from granular and infragranular layers in hindlimb representation of SmI. A total of 289 cortical neurons were recorded (158 from control rats and 131 from HH rats) in pentobarbital-anaesthetized rats. Cutaneous threshold, cutaneous receptive fields, spontaneous activity (discharge rate and instantaneous frequency) and activity evoked by air-jet stimulation (response latency and duration, amplitude) were analysed. The present study suggests that activity-dependent changes occur in the cortex. The duration of the spike waveform presented two populations of spikes: thin-spike cells (<1 ms, supposed to be inhibitory interneurons) and regular cells (>1 ms). Thin-spike cells were less frequently encountered in HH than in control rats. The analysis of regular cells revealed that after HH (1) spontaneous activity was unchanged and (2) cortical somatosensory neurons were more responsive: the cutaneous threshold was reduced and the response magnitude increased. Taken together, these results suggest a down-regulation of GABAergic function.

Contractile properties and myosin expression in rats born and reared in hypergravity.

The effects of hypergravity (HG) on soleus and plantaris muscles were studied in Long Evans rats aged 100 days, born and reared in 2-g conditions (HG group). The morphological and contractile properties and the myosin heavy chain (MHC) content were examined in whole muscles and compared with terrestrial control (Cont) age-paired rats. The growth of HG rats was slowed compared with Cont rats. A decrease in absolute muscle weight was observed. An increase in fiber cross-sectional area/muscle wet weight was demonstrated, associated with an increase in relative maximal tension. The soleus muscle changed into a slower type both in contractile parameters and in MHC content, since HG soleus contained only the MHC I isoform. The HG plantaris muscle presented a faster contractile behavior. Moreover, the diversity of hybrid fiber types expressing multiple MHC isoforms (including MHC IIB and MHC IIX isoforms) was increased in plantaris muscle after HG. Thus the HG environment appears as an important inductor of muscular plasticity both in slow and fast muscle types.

Fictive motor activity in rat after 14 days of hindlimb unloading.

The goal of the present study was to examine the effects of chronic hindlimb unloading on fictive motor patterns which can be developed in hindlimb nerves of adult rats. The animals were divided into two groups. The first group was submitted to hindlimb unloading for 2 weeks by tail suspension. The second group served as controls. After this initial phase, the animals of both groups were acutely decorticated, paralysed and electroneurographic efferent activity was recorded from hindlimb muscle nerves under conditions of "fictive locomotion" in order to evaluate variations in central locomotor command. Fictive rhythmic motor episodes were either spontaneous or evoked by electrical stimulation of the mesencephalic locomotor region. Only the second ones were recognised as locomotor-like activities. The motor pattern was not fundamentally affected by unloading except that, after the unloading period, extensor muscle nerves were significantly more frequently activated and their burst durations were increased compared to activity in control animals, despite the fact that the phasic sensory afferent inputs were suppressed. This suggests that unloading induces plastic modifications of the central networks of neurons implicated in the locomotor command. The origin of this extensor hyperactivity is discussed. It is proposed that it could be the consequence of either changes in motoneuronal properties or of an increase in afferent input to motoneurones.

Time course of recovery of the somatosensory map following hindpaw sensory deprivation in the rat.

Hindlimb sensory deprivation is known to induce a decrease in the cortical representation of hindpaw, and an increase in the size of the cutaneous receptive fields. The aim of the present study was to determine (i) the time-course of recovery when the rat retrieves a normal use of its limbs after a 14-day period of sensory disruption and (ii) whether a 1-day period of sensory deprivation is sufficient to induce a plasticity. Our results indicate that the remodelling of the cortical map was not observed after 1 day of sensory deprivation. On the other hand, the recovery was achieved after 6 h. These findings suggest that a procedure reducing sensory function resulted in reversible changes in the somatosensory cortex. The recovery was more rapid than the induction of plasticity.

Hypodynamia--hypokinesia induced variations in expression of fos protein in structures related to somatosensory system in the rat.

There have been many reports describing modifications of the sensory and motor cortex following various types of disuse. Hypodynamia--hypokinesia is characterized by the absence of weight-bearing and by a decrease in motor activity. We have shown a reorganization of the cortical cartography after hypodynamia--hypokinesia. In order to give an anatomical account for this cortical plasticity, we set out to determine whether cerebral and spinal structures exhibited variations of their neuronal activation. For this purpose, immunocytochemical detection of Fos protein was performed in the rat brain and spinal cord. Following stimulation of the sciatic nerve, Fos protein was detected in the primary and secondary somatosensory cortex in control rats and in rats submitted to an episode of 14 days of hypodynamia--hypokinesia. Results showed that the stimulation of the sciatic nerve induced an increase in the number of Fos-immunoreactive neurons in all these structures. Moreover, after hypodynamia--hypokinesia, the number of Fos-immunoreactive neurons was increased in the primary and secondary somatosensory cortex and in the spinal cord. These results provide evidence for a higher activation of cortical cells after hypodynamia--hypokinesia in comparison to controls. These data support the hypothesis that hypodynamia--hypokinesia contributes to the development of functional plasticity.

Phenotypic changes in the composition of muscular fibres in rat soleus motor units after 14 days of hindlimb unloading.

The main goal of this study was to identify the different fibre types of the motor units (MUs) contained in the soleus muscles from control (CONT) rats and from rats submitted to 14 days of hindlimb unloading (HU). The MU types were classified according to their contractile properties and also using glycogen depletion followed by adenosine triphosphatase (ATPase) staining. In CONT rats, the soleus muscle contained two MU types identified as slow and fast types. After HU, the MU distribution showed three populations: slow, intermediate and fast. All the MUs from HU soleus were heterogeneous in terms of fibre type composition, indicating a complex remodelling of the muscle.

Effects of beta(2)-agonist clenbuterol on biochemical and contractile properties of unloaded soleus fibers of rat.

The effects of clenbuterol beta(2)-agonist administration were investigated in normal and atrophied [15-day hindlimb-unloaded (HU)] rat soleus muscles. We showed that clenbuterol had a specific effect on muscle tissue, since it reduces soleus atrophy induced by HU. The study of Ca(2+) activation properties of single skinned fibers revealed that clenbuterol partly prevented the decrease in maximal tension after HU, with a preferential effect on fast-twitch fibers. Clenbuterol improved the Ca(2+) sensitivity in slow- and fast-twitch fibers by shifting the tension-pCa relationship toward lower Ca(2+) concentrations, but this effect was more marked after HU than in normal conditions. Whole muscle electrophoresis indicated slow-to-fast transitions of the myosin heavy chain isoforms for unloaded and for clenbuterol-treated soleus. The coupling of the two latter conditions did not, however, increase these phenotypical transformations. Our findings indicated that clenbuterol had an anabolic action and a beta(2)-adrenergic effect on muscle fibers and appeared to counteract some effects of unloading disuse conditions.

Effect of microgravity on the electromyographic activity of two upperlimb muscles during a goal-directed movement and during locomotion.

It is well known that both neuromuscular and perceptual properties are affected during spaceflight. These modificaitons can therefore induce dramatic alterations in the mechanical basis of movements and locomotion disturbances. The main objectives of this study were: 1) to examine whether the nervous control of muscular activity in the upper limbs of the rhesus monkey (Macaca mulatta) was modified in a microgravity environment; and 2) to quantify the electromyographic (EMG) pattern of biceps (BI) and triceps (TRI) muscles pre-, in-, and postflight during performance of goal-directed movements and locomotion.

Short-term reorganization of the rat somatosensory cortex following hypodynamia-hypokinesia.

This study was performed to determine if hypodynamia-hypokinesia (HH) could induce a reorganization of the rat somatosensory cortex. The cortical hindpaw representation was determined by stimulating the limb and recording multi-unit cortical activity. The size of the cutaneous receptive fields was also measured. After 14 days of HH, the size of the cortical hindpaw representation was decreased. The proportion of small cutaneous receptive fields decreased while the large ones increased. After 7 days of HH, no change in the two studied parameters was noticed in five animals. In the other rats, a number of sites unresponsive to cutaneous stimulation or with high thresholds was observed. This study provides evidence of a plasticity of the somatosensory cortex induced by a situation that reduces both sensory and motor functions. The cortical reorganization occurs in two stages.

Short-term plasticity in primary somatosensory cortex of the rat after hindlimb suspension.

Since the last 25 years, the cortex is considered as a dynamic entity, susceptible of changes. Various types of modifications in stimuli may lead to the plasticity of the target neurons. These include immobilisation, denervation, amputation, deafferentation... In the somatosensory system, the most important changes are a substantial reorganisation of the cortical somatotopic representation, and an enlargement of the receptive fields (RF) of cortical neurons. Hindlimb suspension (HS) is characterized by the absence of weight-bearing and a reduced motor activity. In these conditions, the cutaneous receptors located on the foot sole are deactivated. Our hypothesis is that this condition of HS can produce a reorganisation of the somatosensory cortex (SmI) and a modification in the size of the cutaneous RF.

Effect of hindlimb unloading on interlimb coordination during treadmill locomotion in the rat.

Effects of hindlimb unloading on interlimb coordination were examined in adult rats walking on a treadmill at moderate speed. In the first group of animals, the electromyographic activity (EMG) of soleus muscle of both hindlimbs was recorded after 7 and 14 days of unloading. In the second group, the EMG was recorded daily until the 14th day of unloading. The general organization of locomotion was preserved in the two groups whatever the duration of the unloading. The step cycles of the two hindlimbs were always strictly alternating. However, the locomotor pattern was very irregular. A lateral instability was observed. It was accompanied by an abduction of the hindlimbs, and frequent hyperextensions of the ankle when walking. The EMG analysis showed an increase in step cycle duration and in coactivation duration of the soleus muscles (i.e. in the double stance duration). In the rats recorded daily, mean EMG was dramatically reduced the 1st day of unloading, suggesting a decrease in the neural drive. Taken together, these data indicate that 14 days of hindlimb unloading can alter the neuromuscular pattern during locomotion. It is proposed that these changes are related to changes in the peripheral sensory information.

Effect of hindlimb unloading on two hindlimb muscles during treadmill locomotion in rats.

The purpose of the study was to examine the pattern of electromyographic (EMG) activity of the rat soleus (SOL) and tibialis anterior (TA) muscles during treadmill locomotion at various speeds after 7 days of hindlimb unloading (HU). Raw EMG signals were processed to determine cycle duration, burst duration and mean EMG (burst surface divided by its duration). Cycle duration and SOL burst duration increased after HU (+7% and +5%, respectively) while TA burst duration decreased (-16%). After HU, the alternating pattern of activity between extensor and flexor muscles was maintained. Nevertheless, a co-activation of the two muscles was sometimes observed. The EMG pattern revealed no difference in the timing of the coordination between flexor and extensor muscles after HU. The delay between TA offset and SOL onset was increased (+12 ms), but this increase could be explained by the decrease in TA burst duration. Neither TA burst duration nor TA mean EMG were changed with increased treadmill speed, so that the flexor muscle activity was not related to speed of locomotion. These results would suggest that SOL activity is centrally programmed. Moreover, it is proposed that a decline in afferent feedback from SOL in rats which are suspended has an effect upon the locomotor pattern, leading to an hyperexcitability of SOL motoneurons and, via reciprocal inhibition, to a reduction in TA activity.

Effect of hindlimb unloading on locomotor strategy during treadmill locomotion in the rat.

Electromyographic activity (EMG) was recorded from the soleus muscles of adult rats during treadmill locomotion after 7 and 14 days of hindlimb unloading, and after 7 days of recovery. Observation of the rats indicated that treadmill locomotion was disrupted after unloading since the animals had some difficulty in moving. Soleus muscle EMG analysis was performed. Onset and offset of bursts of activity were determined, and the relationships between step duration and cycle duration were analysed. Our main results were as follows: firstly, mean cycle duration was increased after 14 days of hindlimb unloading when walking at low speed. At high speed, no difference was observed. Secondly, linear regression analysis indicated that the relationships between step duration and cycle duration were altered after 7 days of unloading. Thirdly, adaptation occurred, since the normal slope and correlation coefficient were restored after 14 days of unloading. Fourthly, when speed increased, no variation of mean EMG was demonstrated after hindlimb unloading whereas an increase occurred in normal rats. Fifthly, video analysis showed that the rats presented frequent hyperextension of the hindlimb after unloading. These abnormal steps were more numerous when walking at low speed. These data would indicate that a transitory disruption of the soleus muscle motor pattern occurred after 7 days of unloading. This disruption depended on the treadmill belt speed. Possible origins of these modifications are discussed.

Relationship between posterior thalamic nucleus unit activity and parietal cortical rhythms (beta) in the waking cat.

Cat behavioural states of attentive fixation on a target are associated with episodes of electrocortical rhythms at 40 Hz ("beta rhythms") in the parietal cortex. Previous field potential studies indicate that the nucleus posterior pars medialis of the thalamus displays this particular rhythmic activity. We investigated single units of the nucleus posterior pars medialis and its surrounding nuclei to assess their participation in the cortical beta rhythms. Only a small proportion of thalamic cells underwent changes in their firing pattern during beta episodes. "Beta-related cells" were localized in the nucleus posterior pars medialis or its immediate vicinity; no such beta-related cells were found in other regions of the lateral thalamus. Some beta-related cells showed a one-spike to one-wave relationship ("homorhythmicity"), while others displayed a prolonged decrease or a suppression of their firing throughout each beta episode ("pause cells"). For comparison, neurons in the same thalamic area were also recorded during sleep episodes with slow waves and spindles: there was no correlation between spindles and cell firing. Thus, the nucleus posterior pars medialis thalamic nucleus contains cells whose firing is correlated with the beta rhythms. No such correlation was found with sleep spindles.

Effects of locus coeruleus lesions on vigilance and attentive behaviour in cat.

Previous data have suggested that in the cat, expectancy behaviour (waiting for a target to appear) and associated electrocortical, focal, synchronized activity ('mu' rhythms) are modulated by a noradrenergic system possibly originating from the locus coeruleus (LC). To test the latter hypothesis, we have examined the behavioural and ECoG changes induced after bilateral LC lesions. Our results demonstrated that destruction of the anterior 3/4th of the LC (A6 noradrenergic cell group) resulted in a considerable increase of mu rhythms and expectancy behaviour, without episodes of drowsiness that normally occur. Destruction of the posterior fourth of LC (A4 noradrenergic group) only increased the duration of slow sleep. Extending the A6 lesion to include the dorsal ascending noradrenergic bundle also increased the expectancy behaviour and mu rhythms. Finally, when the nucleus subcoeruleus was also involved, the duration of slow sleep and the frequency of paradoxical sleep episodes increased. These findings indicate that the LC exerts an inhibitory effect on structures involved in the induction and persistence of expectancy behaviour with accompanying mu rhythms.

Nucleus reticularis thalami participates in sleep spindles, not in beta rhythms concomitant with attention in cat.

In the awake cat, characteristic electrocortical synchronized oscillations (beta rhythms) can develop on the parietal cortex in specific behavioural situations such as during attentive fixation of a target. These activities differ from other synchronized rhythms which accompany slow sleep ("spindles") and are more widespread. It is shown here through single unit recording from the thalamic nucleus reticularis (RET), that the latter structure participates in spindles (as already postulated by other authors), but not in the beta rhythms which seem to depend on a more restricted thalamic focus in the posterior thalamic nucleus. These data thus support the idea that RET plays a role in slow-wave sleep but not in the cognitive operations involved in focussing attention.