Sexual dimorphism in vestibular function and dysfunction.

It has been recognized for some time that females appear to be overrepresented in the incidence of many vestibular disorders, and recent epidemiological studies further support this idea. While it is possible that this is due to a reporting bias, another possibility is that there are actual differences in the incidence of vestibular dysfunction between males and females. If this is true, it could be due to a sexual dimorphism in vestibular function and therefore dysfunction, possibly related to the hormonal differences between females and males, although the higher incidence of vestibular dysfunction in females appears to last long after menopause. Many other neurochemical differences exist between males and females, however, that could be implicated in sexual dimorphism. This review critically explores the possibility of sexual dimorphism in vestibular function and dysfunction, and the implications it may have for the treatment of vestibular disorders.

Galvanic vestibular stimulation impairs cell proliferation and neurogenesis in the rat hippocampus but not spatial memory.

Galvanic vestibular stimulation (GVS) is a method of activating the peripheral vestibular system using direct current that is widely employed in clinical neurological testing. Since movement is recognized to stimulate hippocampal neurogenesis and movement is impossible without activation of the vestibular system, we speculated that activating the vestibular system in rats while minimizing movement, by delivering GVS under anesthesia, would affect hippocampal cell proliferation and neurogenesis, and spatial memory. Compared with the sham control group, the number of cells incorporating the DNA replication marker, bromodeoxyuridine (BrdU), was significantly reduced in the bilateral hippocampi in both the cathode left-anode right and cathode right-anode left stimulation groups (P ≤ 0.0001). The majority of the BrdU(+ve) cells co-expressed Ki-67, a marker for the S phase of the cell cycle, suggesting that these BrdU(+ve) cells were still in the cell cycle; however, there was no significant difference in the degree of co-labeling between the two stimulation groups. Single labeling for doublecortin (DCX), a marker of immature neurons, showed that while there was no significant difference between the different groups in the number of DCX(+ve) cells in the right dentate gryus, in the left dentate gyrus there was a significant decrease in the cathode left-anode right group compared with the sham controls (P ≤ 0.03). Nonetheless, when animals were tested in place recognition, object exploration and Morris water maze tasks, there were no significant differences between the GVS groups and the sham controls. These results suggest that GVS can have striking effects on cell proliferation and possibly neurogenesis in the hippocampus, without affecting spatial memory.

Septal elicitation of hippocampal theta rhythm did not repair cognitive and emotional deficits resulting from vestibular lesions.

Bilateral vestibular lesions cause atrophy of the hippocampus in humans and subsequent deficits in spatial memory and the processing of emotional stimuli in both rats and humans. Vestibular lesions also impair hippocampal theta rhythm in rats. The aim of the present study was to investigate whether restoring theta rhythm to the hippocampus of a rat, via stimulation of the medial septum, would repair the deficits caused by vestibular lesions. It was hypothesized that the restoration of theta would repair the deficits and the vestibular rats would exhibit behavior and EEG similar to that of the sham rats. Rats were given either sham surgery or bilateral vestibular deafferentation (BVD) followed in a later operation by electrode implants. Half of the lesioned rats received stimulation. Subjects were tested in open field, elevated T-maze and spatial nonmatching to sample tests. BVD caused a deficit in hippocampal theta rhythm. Stimulation restored theta power at a higher frequency in the vestibular-lesioned rats, however, the stimulation did not repair the cognitive and emotional deficits caused by the lesions. It was concluded that stimulation, at least in the form used here, would not be a viable treatment option for vestibular damaged humans.

Increased BrdU incorporation reflecting DNA repair, neuronal de-differentiation or possible neurogenesis in the adult cochlear nucleus following bilateral cochlear lesions in the rat.

Neurogenesis is known to occur in response to injury in the brain, for example, as a result of neurodegenerative diseases. However, there have been few investigations into how the brain responds to damage to peripheral sensory nerves, in other areas such as the brainstem. Here, we report that bilateral surgical lesions of the cochlea result in increased incorporation of the DNA replication marker, bromodeoxyuridine (BrdU), in cells of the brainstem cochlear nucleus (CN) of the adult rat, suggesting either cell proliferation or DNA repair. Some of the BrdU-labelled cells colabelled for the mature neuron marker, NeuN and the GABAergic enzyme GAD-65, suggesting the possibility that neurogenesis might have occurred and resulted in the generation of new neurons with a GABAergic phenotype. However, some of the mature neurons also re-expressed immature neuronal intermediate filament and microtuble-associated proteins, without apoptotic neuronal death, which suggests that the colabelling of BrdU with NeuN and GAD-65 may not be a true reflection of neurogenesis, but injury-stimulated neuronal dedifferentiation. These results suggest the possibility that DNA repair, neuronal de-differentiation or possible neurogenesis occurs in the cochlear nucleus, in response to damage to the peripheral auditory system.

Move it or lose it--is stimulation of the vestibular system necessary for normal spatial memory?

Studies in both experimental animals and human patients have demonstrated that peripheral vestibular lesions, especially bilateral lesions, are associated with spatial memory impairment that is long-lasting and may even be permanent. Electrophysiological evidence from animals indicates that bilateral vestibular loss causes place cells and theta activity to become dysfunctional; the most recent human evidence suggests that the hippocampus may cause atrophy in patients with bilateral vestibular lesions. Taken together, these studies suggest that self-motion information provided by the vestibular system is important for the development of spatial memory by areas of the brain such as the hippocampus, and when it is lost, spatial memory is impaired. This naturally suggests the converse possibility that activation of the vestibular system may enhance memory. Surprisingly, there is some human evidence that this may be the case. This review considers the relationship between the vestibular system and memory and suggests that the evolutionary age of this primitive sensory system as well as how it detects self-motion (i.e., detection of acceleration vs. velocity) may be the reasons for its unique contribution to spatial memory.

Hippocampal synaptic transmission and LTP in vivo are intact following bilateral vestibular deafferentation in the rat.

Numerous studies in animals and humans have shown that damage to the vestibular system in the inner ear results in spatial memory deficits, presumably because areas of the brain such as the hippocampus require vestibular input to accurately represent the spatial environment. Consistent with this hypothesis, studies in animals have demonstrated that complete bilateral vestibular deafferentation (BVD) causes a disruption of place cell firing as well as theta activity. The aim of this study was to investigate whether BVD in rats affects baseline field potentials (field excitatory postsynaptic potentials and population spikes) and long-term potentiation (LTP) in CA1 and the dentate gyrus (DG) of awake freely moving rats up to 43 days post-BVD and of anesthetized rats at 7 months post-BVD. Compared to sham controls, BVD had no significant effect on either baseline field potentials or LTP in either condition. These results suggest that although BVD interferes with the encoding, consolidation, and/or retrieval of spatial memories and the function of place cells, these changes are not related to detectable in vivo decrements in basal synaptic transmission or LTP, at least in the investigated pathways.

Evidence that spatial memory deficits following bilateral vestibular deafferentation in rats are probably permanent.

Previous studies of rats with bilateral vestibular deafferentation (BVD) have demonstrated spatial memory deficits, suggesting adverse effects on the hippocampus. However, the longest post-operative time interval that has been studied was approx. 5-7 months post-surgery. In this study, we investigated whether rats exhibited spatial memory deficits at 14 months following BVD and whether these deficits could be exacerbated by administration of cannabinoid (CB) drugs. Twenty-eight adult rats were divided into four groups: (1) sham surgery+vehicle; (2) sham surgery+the CB1/CB(2) receptor agonist WIN55,212-2 ('WIN'); (3) BVD+vehicle; and (4) BVD+WIN. WIN (1.0 or 2.0 mg/kg/day) or vehicle, was administered (s.c.) on days 1-10 and 11-20 (respectively), 30 min before the rats performed in a foraging task. On day 21, the CB receptor inverse agonist, AM251 (3.0 mg/kg, s.c.), was administered before WIN or vehicle. To our surprise, BVD animals were impaired in using the visual cues during the probe test in light. In the dark trials, when visual cues were unavailable, BVD animals were unable to use self-movement cues in homing. However, WIN at 2 mg/kg, significantly improved BVD animals' homing time and number of errors in the dark through strategies other than the improvement in using self-movement cues. Furthermore, AM251 significantly improved heading angle in vehicle-treated animals and the first home choice in WIN-treated animals. These results suggest that at 14 months post-BVD, the animals are not only impaired in path integration, but also piloting and that the spatial memory deficits may be permanent. The involvement of the cannabinoid system is more complicated than expected.

Long-term deficits on a foraging task after bilateral vestibular deafferentation in rats.

Animal studies have shown that bilateral vestibular deafferentation (BVD) causes deficits in spatial memory that may be related to electrophysiological and neurochemical changes in the hippocampus. Recently, human studies have also indicated that human patients can exhibit spatial memory impairment and hippocampal atrophy even 8-10 yr following BVD. Our previous studies have shown that rats with unilateral vestibular deafferentation (UVD) showed an impairment at 3 months after the surgery on a food foraging task that relies on hippocampal integration of egocentric cues, such as vestibular information; however, by 6 months postop, they showed a recovery of function. By contrast, the long-term effects of BVD on spatial navigation have never been well studied. In this study, we tested BVD or sham rats on a food foraging task at 5 months postop. Under light conditions, BVD rats were able to use visual cues to guide themselves home, but did so with a significantly longer homing time. However, in darkness, BVD rats were severely impaired in the foraging task, as indicated by a significantly longer homing distance and homing time, with more errors and larger heading angles when compared with sham rats. These results suggest that, unlike UVD, BVD causes long-term deficits in spatial navigation that are unlikely to recover, even with repeated T-maze training.

Synaptic protein expression in the medial temporal lobe and frontal cortex following chronic bilateral vestibular loss.

Several studies have reported that bilateral vestibular deafferentation (BVD) results in the disruption of place cell function and theta activity in the hippocampus. Recent magnetic resonance imaging (MRI) studies in humans demonstrated that bilateral but not unilateral vestibular loss is associated with a bilateral atrophy of the hippocampus. In this study we investigated whether BVD in rats resulted in changes in the expression of four proteins related to neuronal plasticity, synaptophysin, SNAP-25, drebrin and neurofilament-L, in the hippocampal subregions (CA1, CA2/3, the DG) and the entorhinal (EC), perirhinal (PRC) and frontal cortices (FC), using western blotting. At 6 months following BVD, there were no significant differences in the expression of synaptophysin in any region. There were also no significant differences in SNAP-25 expression in CA1, CA2/3, EC, PRC, or the FC; however, there was a significant increase in SNAP-25 expression in the DG compared to sham controls. Drebrin A and E expression was significantly reduced in the EC and drebrin A was significantly reduced in the FC of BVD animals. NF-L expression was not significantly different in CA1, CA2/3, DG, EC, or the PRC. However, its expression was significantly reduced in the FC of BVD animals. These data suggest that circumscribed neurochemical changes in SNAP-25, drebrin and NF-L expression occur in the DG, EC, and the FC over 6 months following BVD.

Effects of aging on agmatine levels in memory-associated brain structures.

Agmatine is a metabolite of L-arginine by arginine decarboxylase. Recent evidence suggests that it exists in mammalian brain and is a novel neurotransmitter. The present study measured agmatine levels in several memory-associated brain structures in aged (24-month-old), middle-aged (12-month-old), and young (4-month-old) male Sprague Dawley rats using liquid chromatography/mass spectrometry. Agmatine levels were significantly decreased in the CA1, but increased in the CA2/3 and dentate gyrus, subregions of the hippocampus in aged and middle-aged rats relative to the young adults. In the prefrontal cortex, a dramatic decrease in agmatine level was found in aged rats as compared with middle-aged and young rats. There were significantly increased levels of agmatine in the entorhinal and perirhinal cortices in aged relative to middle-aged and young rats. In the postrhinal and temporal cortices, agmatine levels were significantly increased in aged and middle-aged rats as compared with young adults. The present findings, for the first time, demonstrate age-related changes in agmatine levels in memory-associated brain structures and raise a novel issue of the potential involvement of agmatine in the aging process.

Locomotor and exploratory behavior in the rat following bilateral vestibular deafferentation.

Despite many studies of the postural and ocular reflex deficits caused by chronic bilateral vestibular loss in rats and guinea pigs, there have been few systematic studies of the effects of vestibular loss on locomotor activity and exploratory behavior over a period of several months following the lesion. In this study, the authors quantified locomotor and exploratory behavior in an open field maze at 3 weeks, 3 months, and 5 months following bilateral vestibular loss in rats. As a result of bilateral surgical vestibular lesions, rats exhibited a persistent increase in locomotor velocity, duration, and distance traveled, with a marked tendency for increased inner field activity and reduced thigmotaxis. Rats without balance-sense were also found to spend less time exploring the environment, as indicated by a decreased frequency and duration of wall-supported rearings. These results suggest that sudden and complete loss of balance-sense has persistent and complex effects on the way that rats navigate through and explore the environment.

Glutamate receptor subunits expression in memory-associated brain structures: regional variations and effects of aging.

We investigated regional variations and the effects of aging on the expression of the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptor subunits in several memory-associated structures using Western blotting. In young adult rats, NR1, NR2A, and GluR2 levels varied between the hippocampus and parahippocampal region and between the subregions of the hippocampus. When a comparison was made between young (4-month-old) and aged (24-month-old) rats, significant decreases in NR1 expression were found in the aged ventral hippocampus and the entorhinal and postrhinal cortices. There were significant decreases in NR2A expression in the aged parahippocampal region, but not in the hippocampus. The expression of the GluR2 subunit was significantly reduced in the ventral hippocampus and the postrhinal cortex. A dramatic decrease in NR1 and GluR2 expression was found in the aged CA2/3 and CA1, respectively, but there were no significant age-related changes in NR2A expression. All three subunits were expressed at a similar level in the two age groups in the prefrontal cortex. These results suggest differential expression and effects of aging on NMDA and AMPA receptor subunits in memory-associated brain structures.

Effects of bilateral vestibular deafferentation on anxiety-related behaviours in Wistar rats.

Despite frequent reports that patients with vestibular dysfunction exhibit an unusually high incidence of anxiety disorders, few studies have investigated the emotional effects of vestibular damage in animals. In this study we investigated the effects of a permanent surgical bilateral vestibular deafferentation (BVD) on the performance of rats in a series of anxiety tests at 3 weeks (3-W), 3 months (3-M) and 5 months (5-M) following the lesion. We used the elevated plus maze (EPM), elevated T maze (ETM), hyponeophagia and social interaction tests. Contrary to expectation, we found that, at 3 and 5M post-op, BVD rats spent more rather than less time on the open arms of the EPM compared to sham controls, and they displayed a lack of learned inhibitory avoidance in the ETM. Compared to sham controls, BVD rats showed no significant difference over the 3 time points in their latencies to eat in a novel situation; however, they did engage in social interaction to a significantly lesser extent. Finally, blood corticosterone levels were not significantly different between BVD and sham rats at 6 months post-op. These results suggest that BVD causes changes in the performance of rats in the EPM and ETM that might reflect emotional changes, or could be due to the cognitive impairment and hyperactivity caused by BVD.

Monoamine transporter and enzyme expression in the medial temporal lobe and frontal cortex following chronic bilateral vestibular loss.

Peripheral vestibular damage has been reported to be associated with a high incidence of anxiety disorders and depression. In this study we investigated whether chronic bilateral vestibular deafferentation (BVD) would affect the expression of several biogenic amine enzymes and transporters in the medial temporal lobe (CA1, CA2/3, dentate gyrus (DG), entorhinal (EC) and perirhinal cortices (PRC)) and frontal lobes (FL) of rats. BVD was not associated with any significant differences in dopamine beta-hydroxylase or the dopamine transporter in any brain region studied. There was a significant decrease in tyrosine hydroxylase expression only in the FL and a significant decrease in the expression of the serotonin transporter in the FL and CA1 in BVD rats. Tryptophan hydroxylase showed a significant increase in expression in the FL, CA2/3, and DG and a significant decrease in the EC. These results suggest that biogenic amine pathways in the medial temporal lobe and FL undergo changes following BVD.

The effect of delta 9-tetrahydrocannabinol on the extinction of an adverse associative memory.

The cannabinoid Delta(9)-tetrahydrocannabinol (Delta(9)-THC) is currently being employed or tested for a number of therapeutic uses. Cannabinoid CB1 receptors are critically involved in adverse memory extinction. It is not known whether an unintended effect of Delta(9)-THC is increased extinction rates of adverse memories. We therefore tested the effect of Delta(9)-THC on adverse memory extinction using an auditory-stimulus/foot shock paradigm. We found that repeated administration of a high dose of Delta(9)-THC over 6 days retarded extinction compared with controls. This is in contrast to other reports and might be a result of endocannabinoid signalling dysfunction due to cannabinoid receptor densensitization.

Cannabinoid CB2 receptor expression in the rat brainstem cochlear and vestibular nuclei.

CONCLUSION: This evidence suggests that both CB1 and CB2 receptors are important in the control of balance and hearing. OBJECTIVE: Although the cannabinoid CB1 receptor has been identified in the brainstem vestibular and cochlear nuclei, the existence of the second cannabinoid receptor subtype, the CB2 receptor, has been more controversial. The aim of this study was to determine whether or not CB2 receptors are expressed in the vestibular and cochlear nuclei. MATERIALS AND METHODS: Data were obtained from four young male Wistar rats In analyzing the presence of CB2 receptors in the vestibular and cochlear nuclei, the immunohistochemical complex was visualized by exposure to diaminobenzidine for 20 min. Positive immunoreactivity to CB2 was expressed as brown staining in the cytoplasm, nucleus, nuclear membrane and cell membrane. RESULTS: We confirmed the existence of the CB2 receptor in the vestibular and cochlear nuclei in the brainstem of Wistar rats.

Carbamazepine reduces the behavioural manifestations of tinnitus following salicylate treatment in rats.

CONCLUSION: The results are consistent with the hypothesis that carbarmazepine (CBZ) has efficacy against tinnitus in humans. OBJECTIVE: CBZ is an anti-epileptic drug that is widely used for the treatment of tinnitus. Despite this, there are relatively few clinical trials or preclinical studies supporting its efficacy. In an effort to increase the amount of information available on CBZ, the aim of this study was to investigate the efficacy of CBZ in salicylate-induced tinnitus in rats MATERIALS AND METHODS: We investigated the effects of CBZ in an animal model of tinnitus induced by the injection of salicylate using a conditioned lick suppression paradigm. RESULTS: We found that CBZ, at a dose of 15 mg/kg i.p., but not at 5 mg/kg or 30 mg/ kg, significantly suppressed the behavioural manifestations of tinnitus.

Drug treatments for tinnitus.

Many of the drug treatments that are presently in use for tinnitus are aimed at either the cochlea, e.g. using intratympanic injections of gentamicin, dexamethasone or lidocaine, or the CNS using systemic delivery. Earlier benzodiazepines and anticonvulsants have been used and more recently, antidepressants have been introduced, partly in an attempt to treat the emotional aspect of tinnitus. The fact that there are many different forms of tinnitus with different and often multiple causes and that the pathophysiology is poorly understood, are obstacles to finding effective treatments. This situation has been exacerbated by the lack of clinical trials to formally test even some of the most commonly used drugs, as well as a lack of preclinical studies to investigate novel agents. It is suggested that the animal models of tinnitus that have been developed could be used to screen potential anti-tinnitus drugs as a preliminary step before conducting clinical trials.

MBP-8298, a synthetic peptide analog of myelin basic protein for the treatment of multiple sclerosis.

BioMS Medical Corp, under license from the University of Alberta, is developing MBP-8298, a synthetic peptide analog of myelin basic protein, for the potential treatment of multiple sclerosis. Phase II and III clinical trials of MBP-8298 are underway.

Cannabinoid receptor down-regulation in the ventral cochlear nucleus in a salicylate model of tinnitus.

Cannabinoid CB1 receptors have not been systematically investigated in the brainstem cochlear nucleus, nor have they been investigated in relation to tinnitus. Using immunohistochemistry and cell counting, we showed that a large number of neurons in the rat cochlear nucleus possess cannabinoid CB1 receptors. Following salicylate injections that induced the behavioural manifestations of tinnitus, the number of principal neurons in the ventral cochlear nucleus expressing CB1 receptors significantly decreased, while the number of CB1-positive principal neurons in the dorsal cochlear nucleus did not change significantly. These results suggest that CB1 receptors in the cochlear nucleus may be important for auditory function and that a down-regulation of CB1 receptors in the ventral cochlear nucleus may be related to the development of tinnitus.