Reduction of long-term potentiation at Schaffer collateral-CA1 synapses in the rat hippocampus at the acute stage of vestibular compensation

Long-lasting spatial memory deficits and impaired hippocampal plasticity following unilateral vestibular loss

Unilateral vestibular loss (UVL) induces a characteristic vestibular syndrome composed of various posturo-locomotor, oculomotor, vegetative and perceptivo-cognitive symptoms. Functional deficits are progressively recovered over time during vestibular compensation, that is supported by the expression of multiscale plasticity mechanisms. While the dynamic of post-UVL posturo-locomotor and oculomotor deficits is well characterized, the expression over time of the cognitive deficits, and in particular spatial memory deficits, is still debated. In this study we aimed at investigating spatial memory deficits and their recovery in a rat model of unilateral vestibular neurectomy (UVN), using a wide spectrum of behavioral tasks. In parallel, we analyzed markers of hippocampal plasticity involved in learning and memory. Our results indicate the UVN affects all domains of spatial memory, from working memory to reference memory and object-in-place recognition. These deficits are associated with long-lasting impaired plasticity in the ipsilesional hippocampus. These results highlight the crucial role of symmetrical vestibular information in spatial memory and contribute to a better understanding of the cognitive disorders observed in vestibular patients.

The pattern of hippocampal neuronal, LTP and unilateral labyrinthectomy in male rats: the role of exercise and curcumin

A wide range of cognitive disorders, long-term potentiation (LTP) disorders, and biochemical changes have been reported in labyrinthectomy situations in the hippocampus of rodents. In order to investigate whether exercise and/or curcumin induce LTP and increase neurons in the hippocampus of unilateral labyrinthectomy male rats, after undergoing labyrinthectomy, animals were subjected to treadmill exercises after the intraperitoneal injection of curcumin five days per week, for 30 days. An increase of LTP and neuron count of the hippocampus was observed in unilateral labyrinthectomy rats. Interestingly the combination of exercise and curcumin did not enhance LTP in unilateral labyrinthectomy rats. These findings demonstrated that treadmill exercise and curcumin individually, significantly affect reinforcement of many of the pathological processes playing a role in increasing memory in unilateral labyrinthectomy situations.

The Differential Effects of Acute Right- vs. Left-Sided Vestibular Deafferentation on Spatial Cognition in Unilateral Labyrinthectomized Mice

This study aimed to investigate the disparity in locomotor and spatial memory deficits caused by left- or right-sided unilateral vestibular deafferentation (UVD) using a mouse model of unilateral labyrinthectomy (UL) and to examine the effects of galvanic vestibular stimulation (GVS) on the deficits over 14 days. Five experimental groups were established: the left-sided and right-sided UL (Lt.-UL and Rt.-UL) groups, left-sided and right-sided UL with bipolar GVS with the cathode on the lesion side (Lt.-GVS and Rt.-GVS) groups, and a control group with sham surgery. We assessed the locomotor and cognitive-behavioral functions using the open field (OF), Y maze, and Morris water maze (MWM) tests before (baseline) and 3, 7, and 14 days after surgical UL in each group. On postoperative day (POD) 3, locomotion and spatial working memory were more impaired in the Lt.-UL group compared with the Rt.-UL group (p < 0.01, Tamhane test). On POD 7, there was a substantial difference between the groups; the locomotion and spatial navigation of the Lt.-UL group recovered significantly more slowly compared with those of the Rt.-UL group. Although the differences in the short-term spatial cognition and motor coordination were resolved by POD 14, the long-term spatial navigation deficits assessed by the MWM were significantly worse in the Lt.-UL group compared with the Rt.-UL group. GVS intervention accelerated the vestibular compensation in both the Lt.-GVS and Rt.-GVS groups in terms of improvement of locomotion and spatial cognition. The current data imply that right- and left-sided UVD impair spatial cognition and locomotion differently and result in different compensatory patterns. Sequential bipolar GVS when the cathode (stimulating) was assigned to the lesion side accelerated recovery for UVD-induced spatial cognition, which may have implications for managing the patients with spatial cognitive impairment, especially that induced by unilateral peripheral vestibular damage on the dominant side.

Galvanic Vestibular Stimulation Improves Spatial Cognition After Unilateral Labyrinthectomy in Mice

Objectives: To investigate the deficits of spatial memory and navigation from unilateral vestibular deafferentation (UVD) and to determine the efficacy of galvanic vestibular stimulation (GVS) for recovery from these deficits using a mouse model of unilateral labyrinthectomy (UL).

Methods: Thirty-six male C57BL/6 mice were allocated into three groups that comprise a control group and two experimental groups, UVD with (GVS group) and without GVS intervention (non-GVS group). In the experimental groups, we assessed the locomotor and cognitive behavioral function before (baseline) and 3, 7, and 14 days after surgical UL, using the open field (OF), Y maze, and Morris water maze (MWM) tests. In the GVS group, the stimulations were applied for 30 min daily from postoperative day (POD) 0–4 via the electrodes inserted subcutaneously close to both bony labyrinths.

Results: Locomotion and spatial cognition were significantly impaired in the mice with UVD non-GVS group compared to the control group. GVS significantly accelerated recovery of locomotion compared to the control and non-GVS groups on PODs 3 (p < 0.001) and 7 (p < 0.05, Kruskal–Wallis and Mann–Whitney U tests) in the OF and Y maze tests. The mice in the GVS group were better in spatial working memory assessed with spontaneous alternation performance and spatial reference memory assessed with place recognition during the Y maze test than those in the non-GVS group on POD 3 (p < 0.001). In addition, the recovery of long-term spatial navigation deficits during the MWM, as indicated by the escape latency and the probe trial, was significantly better in the GVS group than in the non-GVS group 2 weeks after UVD (p < 0.01).

Conclusions: UVD impairs spatial memory, navigation, and motor coordination. GVS accelerated recoveries in short- and long-term spatial memory and navigation, as well as locomotor function in mice with UVD, and may be applied to the patients with acute unilateral vestibular failure.

Vestibular Modulation of Long-Term Potentiation and NMDA Receptor Expression in the Hippocampus

Loss of vestibular function is known to cause spatial memory deficits and hippocampal dysfunction, in terms of impaired place cell firing and abnormal theta rhythm. Based on these results, it has been of interest to determine whether vestibular loss also affects the development and maintenance of long-term potentiation (LTP) in the hippocampus. This article summarizes and critically reviews the studies of hippocampal LTP following a vestibular loss and its relationship to NMDA receptor expression, that have been published to date. Although the available in vitro studies indicate that unilateral vestibular loss (UVL) results in reduced hippocampal field potentials in CA1 and the dentate gyrus (DG), the in vivo studies involving bilateral vestibular loss (BVL) do not. This may be due to the differences between UVL and BVL or it could be a result of in vitro/in vivo differences. One in vitro study reported a decrease in LTP in hippocampal slices following UVL; however, the two available in vivo studies have reported different results: either no effect or an increase in EPSP/Population Spike (ES) potentiation. This discrepancy may be due to the different high-frequency stimulation (HFS) paradigms used to induce LTP. The increased ES potentiation following BVL may be related to an increase in synaptic NMDA receptors, possibly increasing the flow of vestibular input coming into CA1, with a loss of selectivity. This might cause increased excitability and synaptic noise, which might lead to a degradation of spatial learning and memory.

Hippocampal LTP modulation and glutamatergic receptors following vestibular loss

Vestibular dysfunction strongly impairs hippocampus-dependent spatial memory performance and place cell function. However, the hippocampal encoding of vestibular information at the synaptic level, remains sparsely explored and controversial. We investigated changes in in vivo long-term potentiation (LTP) and NMDA glutamate receptor (NMDAr) density and distribution after bilateral vestibular lesions (BVL) in adult rats. At day 30 (D₃₀) post-BVL, the LTP of the population spike recorded in the dentate gyrus (DG) was higher in BVL rats, for the entire 3 h of LTP recording, while no difference was observed in the fEPSP slope. However, there was an increase in EPSP-spike (E-S) potentiation in lesioned rats. NMDArs were upregulated at D₇ and D₃₀ predominantly within the DG and CA1. At D₃₀, we observed a higher NMDAr density in the left hippocampus. NMDArs were overexpressed on both neurons and non-neuronal cells, suggesting a decrease of the entorhinal glutamatergic inputs to the hippocampus following BVL. The EPSP-spike (E-S) potentiation increase was consistent with the dorsal hippocampus NMDAr upregulation. Such an increase could reflect a non-specific enhancement of synaptic efficacy, leading to a disruption of memory encoding, and therefore might underlie the memory deficits previously reported in rats and humans following vestibular loss.