The effect of vestibular lesions on the hippocampal cholinergic system
Bilateral vestibular loss (BVL) produces profound behavioural changes, some of which do not appear to be directly associated with the acute loss of vestibular information, such as hyperlocomotion and spatial memory dysfunction. These behaviours appear to be due to long-term changes within the basal ganglia and limbic system. Both of these areas of the brain are associated with locomotion and memory. The hippocampus, in particular, has been extensively studied in relation to vestibular function. However, the exact mechanism which produces locomotor hyperactivity and cognitive dysfunction following BVL has yet to be discovered. Cholinergic signalling within the hippocampus is strongly associated with learning, memory, theta rhythm, and long-term potentiation (LTP). In the striatum, acetylcholine (ACh) release is correlated with procedural memory and locomotor activity. The aim of this study was to examine the effect of vestibular loss on behaviour, as well as examining how vestibular loss alters cholinergic signalling within the theta-generating pathway, hippocampus, and striatum. The behaviour of animals that underwent BVL or sham lesions was assessed in the open field to determine and distinguish the specific behavioural profile produced by BVL caused by intratympanic sodium arsanilate injections. BVL rats demonstrated reduced thigmotaxis, increased locomotor activity, an increase in transitions between each zone of the open field, and increased whole body rotations. The measured behavioural effects of BVL could correctly predict whether animals had received a BVL with a high degree of accuracy at both day 3 and day 23 post-BVL (83% and 100%, respectively). Further experiments aimed to determine whether BVL produces changes to hippocampal and striatal cholinergic signalling. Initially, the distribution and density of muscarinic M1 and M2 receptors in the striatum and hippocampus were examined 7 and 30 days post-BVL. The density and distribution of M1 receptors was measured with beta-imaging autoradiography. The number of neuronal and non-neuronal cells expressing M2 receptors was measured with flow cytometry. Thirty days following BVL in both the hippocampus and striatum, the density of M1 receptors decreased (P ≤ 0.0001) and the number of neurons expressing M2 receptors increased (P ≤ 0.05). However, at the earlier 7 day time point, there was no change in muscarinic receptors in either area (P > 0.05). Another experiment recorded local field potentials in the hippocampus using electrophysiology. Ninety days post-BVL or -unilateral vestibular loss (UVL), rats were anaesthetised with urethane and the theta rhythm generated from a tail pinch was recorded, analysed, and compared to sham-lesioned animals. The generation of type 2 theta was potentiated following BVL (P ≤ 0.05). The cholinergic neurons in the pedunculopontine tegmental nucleus (PPT) of these rats were counted using stereology. Choline acetyltransferase (ChAT) immunohistochemistry selectively stained the cholinergic cells in the PPT. PPT cholinergic neurons increased in number contralateral to the lesion in UVL rats (P ≤ 0.05), with BVL rats exhibiting an increase in ChAT-positive neurons on both sides (P ≤ 0.05). Finally, the release of ACh within the hippocampus following BVL was measured using microdialysis and high-performance liquid chromatography with electrochemical detection (HPLC-ECD). Microdialysis samples were collected under urethane anaesthesia. Baseline ACh release was unchanged between BVL and sham animals (P > 0.05). Neither a tail pinch stimulus nor a rotational vestibular stimulus produced detectable changes in ACh release (P > 0.05). The results suggest that the hippocampal and striatal cholinergic systems are significantly altered following BVL. The potentiation of type 2 theta rhythm induced by somatosensory stimulation and the increase in cholinergic PPT neurons indicate potential changes to memory consolidation. Whether these changes are the cause of, or an adaptation to, the behavioural and cognitive symptoms which occur following BVL remains unknown.
Advisor: Smith, Paul; Zheng, Yiwen
Degree Name: Doctor of Philosophy
Degree Discipline: Pharmacology and Toxicology
Publisher: University of Otago
Keywords: Vestibular; BVL; Cholinergic; Hippocampus; Pedunculopontine; Theta
Research Type: Thesis