Polyamines in the lateral vestibular nuclei of the squirrel monkey and their potential role in vestibular compensation

Nitric oxide synthase and arginase expression in the vestibular nucleus and hippocampus following unilateral vestibular deafferentation in the rat

The aim of this study was to investigate the possible relationship between changes in neuronal and endothelial nitric oxide synthase (nNOS and eNOS) and arginase expression in the vestibular nucleus complex and the hippocampus (CA1, CA2/3 and the dentate gyrus (DG) at 10 h or 2 weeks following a unilateral vestibular deafferentation (UVD) in rats. There were no significant differences in nNOS or arginase II expression in the ipsilateral or contralateral VNC at either 10 h or 2 weeks post-UVD. For eNOS, there was only a significant decrease in expression in the ipsilateral VNC at 2 weeks post-UVD (P<0.01). In the hippocampus, the only significant difference in nNOS expression was a decrease in the ipsilateral DG at 2 weeks post-UVD (P<0.05). There was a significant decrease in eNOS expression in the contralateral CA2/3 region at 10 h post-UVD (P<0.01). The only other significant change in eNOS was an increase in the contralateral DG at 10 h post-UVD (P<0.01). Although arginase II was expressed in all regions of the hippocampus, there were no significant differences in arginase II expression at any time point following UVD. These results suggest that the changes in NOS expression that occur in the VNC and hippocampus following UVD are not correlated with one another or with changes in arginase II.

Evidence for a microglial reaction within the vestibular and cochlear nuclei following inner ear lesion in the rat

Following unilateral inner ear lesion, astrocytes undergo hypertrophy in the deafferented vestibular and cochlear nuclei as shown by an increase in the level of glial fibrillary acid. The present study extends our understanding of vestibular and cochlear system plasticity by examining microglial changes in these deafferented nuclei. The microglial reaction was studied 1, 2, 4, 8, 14, 21, 28 and 42 days following the lesion with a monoclonal OX-42 antibody and lectins (Griffonia simplicifolia, B4 isolectin) labelled with horseradish peroxidase or fluorescein. The deafferented nuclei were also examined for apoptotic cells by terminal transferase-mediated nick end labelling of nuclear DNA fragments. In control and sham-operated rats, the distribution of the resting microglial cells was uniform in both the vestibular and cochlear nuclei. In the deafferented vestibular complex, the microglial cells increased in number, became hypertrophied and were distributed in the medial, lateral, superior and inferior vestibular nuclei. Reactive microglial cells were also detected in the ipsilateral cochlear nuclei. Some of the immunostained cells were hypertrophic whereas others presented an ameboid morphology with few short and stout processes. The microglial reaction was confined to the antero- and posteroventral cochlear nuclei. Finally, reactive microglia was also observed in the prepositus hypoglossi ipsilateral to the lesion. The microglial reactions within the prepositus hypoglossi, the vestibular and the cochlear nuclei were detectable as early as one day after the lesion and persisted several weeks in both the vestibular and cochlear nuclei. Apoptotic cells were not detected in the vestibular nuclei at any stage following the lesion. In contrast, terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were first detected in the deafferented cochlear nuclei on the 3rd day following the lesion. They reached an apparent maximum by day 8 and then declined until day 24. Double labelling experiments demonstrate that these cochlear terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were also lectin-positive suggesting that reactive cochlear lectin-positive microglia cells were eliminated by a programmed cell death. Our results establish the two experimental models as reliable tools to understand the role of microglia in adult brain plasticity. The cochlear microglial reaction was probably induced by the degeneration of the acoustic nerve which follows the acoustic ganglion destruction. Interestingly, the same reasoning cannot apply to the vestibular microglial reaction following unilateral labyrinthectomy: the vestibular ganglion was spared and the primary vestibular neurons did not degenerate, at least during the first week following the lesion.

The contribution of N-methyl-D-aspartate receptors to lesion-induced plasticity in the vestibular nucleus

The aim of this paper is to: i) review the behavioural, electrophysiological, pharmacological and biochemical evidence relating to the involvement of N-methyl-D-aspartate (NMDA) receptors in the vestibular compensation process which follows unilateral peripheral vestibular deafferentation (UVD); and ii) suggest a unifying hypothesis based on this literature and recent studies of long-term depression (LTD)-like phenomena in the brainstem vestibular nucleus complex (VNC). It is suggested that NMDA receptors may induce a form of heterosynaptic LTD in the ipsilateral VNC, which is partly responsible for the extent of the hypoactivity which occurs immediately following UVD, and the severity of the associated vestibular syndrome. It is also suggested that vestibular compensation may develop as this LTD dissipates, allowing remaining synaptic inputs and the intrinsic properties of ipsilateral VNC neurons to re-establish the resting activity which is responsible for static vestibular compensation. It is argued that this hypothesis accounts for the majority of the available data on NMDA receptors in relation to vestibular compensation, and may serve as a useful working hypothesis, in order to formulate further experiments to investigate the contribution of NMDA receptors to the compensation process.

Activation of ornithine decarboxylase and accumulation of putrescine after traumatic brain injury

Activation of ornithine decarboxylase (ODC), the initial enzyme in polyamine synthesis, and accumulation of putrescine are thought to mediate pathological processes in the ischemic and traumatized brain. Past studies have separately investigated either ODC or polyamines after head injury. The purpose of the present study was to quantify both ODC activity and polyamines in the rat parietal cortex before and after controlled cortical impact injury. Adult, male rats underwent a right craniectomy and were subjected to a 5 m/sec, 2-mm deformation impact injury. Rats were sacrificed 1, 4, 8, and 24 h postimpact and tissues from the injured (right) and contralateral (left) hemisphere were analyzed for ODC and polyamines. ODC activity was determined by measuring the decarboxylation of [14C]ornithine to putrescine. Putrescine, spermidine, and spermine were determined by high performance liquid chromatography. Cortical impact induced a 10- to 20-fold increase in ODC activity and a 4- to 5-fold increase in putrescine in the ipsilateral cortex. Spermidine and spermine did not significantly increase in the ipsilateral (right) cortex compared to controls (right cortex). In contrast, there was a slight increase in spermidine content in the contralateral (left) cortex after injury. The delayed increase in ODC activity and accumulation of putrescine may mediate pathophysiological changes observed after head injury.

alpha-Difluoromethylornithine delays behavioral recovery and induces decompensation after unilateral labyrinthectomy

Biochemical and pharmacologic studies suggest a role for the ornithine decarboxylase-polyamine system as a modulator of behavioral changes during vestibular compensation. alpha-Difluoromethylornithine specifically blocks the rate-limiting step of polyamine biosynthesis. To assess the effects of alpha-difluoromethylornithine on the acute phase of postural compensation, guinea pigs were divided into groups subjected to either unilateral labyrinthectomy only (n = 7), alpha-difluoromethylornithine (500 mg/kg/day) for 4 days before labyrinthectomy (n = 10), equivalent volumes of saline for 4 days before labyrinthectomy (n = 8), and sham operations (n = 5). Yaw head tilt and roll head tilt, trunk curvature, and air-righting reflex were measured at baseline and at regular intervals up to 4 weeks. alpha-Difluoromethylornithine significantly delayed recovery of normal air-righting but had no effect on yaw head tilt, roll head tilt, and trunk curvature. We also evaluated effects of alpha-difluoromethylornithine in compensated guinea pigs. Fully compensated animals from phase 1 were randomly assigned to receive alpha-difluoromethylornithine (500 mg/kg/day) or saline once daily for 4 days. Only 33% of alpha-difluoromethylornithine animals maintained air-righting, compared with 100% of saline-treated animals (p = 0.003). Maximum trunk curvature was greater in the alpha-difluoromethylornithine group (p = 0.02). Thus alpha-difluoromethylornithine not only delayed the time course for postural recovery after unilateral labyrinthectomy, it also transiently disrupted the maintenance of the compensated state.