Fos expression in the rat brain after exposure to gravito-inertial force changes.

The immediate-early genes constitute useful neurobiological tools for mapping brain functional activity after sensory stimulation. We immunohistochemically investigated Fos protein expression in the brain of rats exposed to gravito-inertial force changes. Experiments were performed in hypergravity rats born and housed for 60 days in terrestrian gravity (1xg) and thereafter exposed for 90 min to 2xg or 4xg in a centrifuge, and in hypogravity rats born and housed for 60 days at 2xg and submitted for 90 min to 1xg. Data from these two experimental groups were quantified by light microscopy and compared to those from two groups of control rats born and permanently housed in either 1xg or 2xg environments that never had to adapt to novel gravito-inertial environments. Results showed a low basal Fos expression in the controls and a strong Fos staining in the experimental rats. Only the hypergravity rats displayed Fos-positive cells in vestibular-related brainstem regions (medial, inferior, and superior vestibular nuclei (VN); group y; dorsomedial cell column (DMCC) of the inferior olive (IO)). By contrast, many suprabulbar areas were strongly labeled in both the hyper- and hypogravity rats, as shown by the numerous Fos-positive cells in mesencephalic (colliculus, laterodorsal periaqueductal gray, autonomic nuclei), diencephalic (hypothalamic and thalamic nuclei), and telencephalic (parietal, temporal, entorhinal and visual cortices) structures. These spatial patterns of Fos expression suggest that an increase in gravito-inertial force activates otolith-vestibulo-olivar pathways and various suprabulbar structures underlying the corticovestibular interactions, which govern the multiple representations of vestibular information in the cortex. A decrease in gravito-inertial force has the opposite effects on the vestibulo-olivar structures as a result of otolith system disfacilitation which, in turn, modifies the activity of complex neural pathways. Exposure to both hyper- and hypogravity environments likely induces neurovegetative and/or stress effects that could account for Fos labeling in autonomic nuclei and in nervous structures involved in the hypothalamo-pituitary-adrenal axis.

Fos expression in the cat brainstem after unilateral vestibular neurectomy.

Immediate early genes are generally expressed in response to sensory stimulation or deprivation and can be used for mapping brain functional activity and studying the molecular events underlying CNS plasticity. We immunohistochemically investigated Fos protein induction in the cat brainstem after unilateral vestibular neurectomy (UVN), with special reference to the vestibular nuclei (VN) and related structures. Fos-like immunoreactivity was analyzed at 2, 8, and 24 h, and 1 and 3 weeks after UVN. Data from these subgroups of cats were quantified in light microscopy and compared to those recorded in control and sham-operated animals submitted to anesthesia and anesthesia plus surgery, respectively. Results showed a very low level of Fos expression in the control and sham conditions. By contrast, Fos was consistently induced in the UVN cats. Asymmetrical labeling was found in the medial, inferior, and superior VN (ipsilateral predominance) and in the prepositus hypoglossi (PH) nuclei and the beta subnuclei of the inferior olive (betaIO) (contralateral predominance). Symmetrical staining was observed in the autonomic, tegmentum pontine, pontine gray, locus coeruleus and other reticular-related nuclei. As a rule, Fos expression peaked early (2 h) and declined progressively. However, some brainstem structures including the ipsilateral inferior VN and the bilateral pontine gray nuclei displayed a second peak of Fos expression (24 h-1 week). By comparing these data to the behavioral recovery process, we conclude that the early Fos expression likely reflects the activation of neural pathways in response to UVN whereas the delayed Fos expression might underlie long-term plastic changes involved in the recovery process.

Short-term changes in neck muscle and eye movement responses following unilateral vestibular neurectomy in the cat.

The purpose of this study was to investigate changes in neck muscle and eye movement responses during the early stages of vestibular compensation (first 3 weeks after unilateral vestibular neurectomy, UVN). Electromyographic (EMG) activity from antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles and eye movements were recorded during sinusoidal visual and/or otolith vertical linear stimulations in the 0.05-1 Hz frequency range (corresponding acceleration range 0.003-1.16 g) in the head-fixed alert cat. Preoperative EMG activity from the splenius and longus capitis muscles showed a pattern of alternate activation of the antagonist neck muscles in all the cats. After UVN, two motor strategies were observed. For three of the seven cats, the temporal activation of the individual neck muscles was the same as that recorded before UVN. For the other four cats, UVN resulted in a pattern of coactivation of the flexor and extensor neck muscles because of a phase change of the splenius capitis. In both subgroups, the response patterns of the antagonist neck muscles were consistent for each cat independently of the experimental conditions, throughout the 3 weeks of testing. Cats displaying alternate activation of antagonist neck muscles showed an enhanced gain of the visually induced neck responses, particularly in the high range of stimulus frequency, and a gain decrease in the otolith-induced neck responses at the lowest frequency (0.25 Hz) only. By contrast, for cats with neck muscle coactivation, the gain of the visually induced neck responses was basically unaffected relative to preoperative values, whereas otolith-induced neck responses were considerably decreased in the whole range of stimulation. As concerns oculomotor responses, results in the two subgroups of cats were similar. The optokinetic responses were not affected by the vestibular lesion. On the contrary, otolith-induced eye responses showed a gain reduction and a phase lead. Deficits and short-term changes after UVN of otolith- and semicircular canal-evoked collic and ocular responses are compared.