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Sensorimotor restriction by a 14-day period of hindlimb unloading (HU) in the adult rat induces a reorganization of topographic maps and receptive fields. However, the underlying mechanisms are still unclear. Interest was turned towards a possible implication of intracellular MAPK signaling pathway since Extracellular-signal-Regulated Kinase 1/2 (ERK1/2) is known to play a significant role in the control of synaptic plasticity. In order to better understand the mechanisms underlying cortical plasticity in adult rats submitted to a sensorimotor restriction, we analyzed the time-course of ERK1/2 activation by immunoblot and of cortical reorganization by electrophysiological recordings, on rats submitted to hindlimb unloading over four weeks. Immunohistochemistry analysis provided evidence that ERK1/2 phosphorylation was increased in layer III neurons of the somatosensory cortex. This increase was transient, and parallel to the changes in hindpaw cortical map area (layer IV). By contrast, receptive fields were progressively enlarged from 7 to 28 days of hindlimb unloading. To determine whether ERK1/2 was involved in cortical remapping, we administered a specific ERK1/2 inhibitor (PD-98059) through osmotic mini-pump in rats hindlimb unloaded for 14 days. Results demonstrate that focal inhibition of ERK1/2 pathway prevents cortical reorganization, but had no effect on receptive fields. These results suggest that ERK1/2 plays a role in the induction of cortical plasticity during hindlimb unloading.

Picquet, F., V. Bouet, L. Cochon, M. Lacour, and M. Falempin. Changes in rat soleus muscle phenotype consecutive to a growth in hypergravity followed by normogravity. Am J Physiol Regul Integr Comp Physiol 289: R217-R224, 2005. First published March 17, 2005; doi:10.1152/ajpregu.00596.2004.-It has been demonstrated that a long-term stay in hypergravity (HG: 2G) modified the phenotype and the contractile properties of rat soleus muscle. The ability of this muscle to contract was drastically reduced, which is a sign of anticipated aging. Consequently, our aim was to determine whether rats conceived, born, and reared in hypergravity showed adaptative capacities in normogravity (NG: 1G). This study was performed on rats divided into two series: the first was reared in HG until 100 days and was submitted to normogravity until 115 to 220 postnatal days (HG-NG rats); the second was made up of age paired groups reared in normogravity (NG rats). The contractile, morphological, and phenotypical properties of soleus muscle were studied. Our results showed that the NG rats were characterized by coexpressions of slow and fast myosin, respectively, 76.5 and 23.5% at 115 days. During their postnatal maturation, the fast isoform was gradually replaced by slow myosin. At 220 days, the relative proportions were respectively 91.05% and 8.95%. From 115 to 220 days, the HG-NG rats expressed 100% of slow myosin isoform and they presented a slower contractile behavior compared with their age-matched groups; at 115 days, the whole muscle contraction time was increased by 35%, and by 15%, at 220 days. Our study underlined the importance of gravity in the muscular development and suggested the existence of critical periods in muscle phenotype installation. muscular properties; myosin transition; gravity change; hindlimb muscle ALL LIVING ORGANISMS ARE DIRECTLY under the influence of a common and constant factor, gravity. It is well known that the development and the maturation of numerous life systems such as cell division, axonal growth, posture, and body movements are directly regulated by this factor (6,14,15,16).The effects of real (spaceflight) and simulated (hypodynamia-hypokinesia model, HH) microgravity on the neuromuscular system are well documented. In hindlimb muscles, an exposure to a HH period induces more marked effects in slow extensor muscles such as the soleus than in fast muscles (for a review, see Refs. 23 and 24). Most studies have been carried out in adult rats. Indeed, in soleus muscle from rats submitted to real and simulated microgravity, a muscular atrophy correlated to a decrease both in fiber and muscle cross-sectional area and to a decrease in muscle strength, which has been commonly described (for review, see Ref. 13). A slow-to-fast phenotype transition, characterized by changes in contractile protein isoforms such as myosin heavy and light chains (3, 32) and regulatory proteins (2), has also been reported. Moreover, the proprioceptive information could be disturbed (9), and a cortical reorganization has b...