Evaluation of the Activity of Choline Acetyltransferase From Different Synaptosomal Fractions at the Distinct Stages of Spatial Learning in the Morris Water Maze

Accumulated data have evidenced that brain cholinergic circuits play a crucial role in learning and memory; however, our knowledge about the participation of neocortical and hippocampal cholinergic systems in spatial learning needs to be refined. The aim of this study was to evaluate the association of the activity of membrane-bound and soluble choline acetyltransferase (ChAT) in the synaptosomal sub-fractions of the neocortex and hippocampus with performance of the spatial navigation task in the Morris water maze at different temporal stages of memory trace formation. To identify distinct stages of memory formation, rats were trained using a 5-day protocol with four trials per day. The mean escape latency for each trial was collected, and the entire dataset was subjected to principal component analysis. Based on the Morris water maze protocol, there were three relatively distinct stages of memory formation: days 1-2, day 3, and days 4-5. The remotely stored memory trace tested in repeated and reversal learning beginning on day 19 (14 days after the end of initial learning) was associated at the individual level mainly with performance during the second trial on day 21 (the third day or repeated or reversal learning). The ChAT activity data suggest the participation of cortical cholinergic projections mainly in the first stage of spatial learning (automatic sensory processing) and the involvement of hippocampal interneurons in the second stage (error-corrected learning). Cholinergic cortical interneurons participated mainly in the stage of asymptotic performance (days 4-5). It is advisable to evaluate other signalling pathways at the identified stages of memory formation.

Opposite Pathways of Cholinergic Mechanisms of Hypoxic Preconditioning in the Hippocampus: Participation of Nicotinic α7 Receptors and Their Association with the Baseline Level of Startle Prepulse Inhibition

(1) Background. A one-time moderate hypobaric hypoxia (HBH) has a preconditioning effect whose neuronal mechanisms are not studied well. Previously, we found a stable correlation between the HBH efficiency and acoustic startle prepulse inhibition (PPI). This makes it possible to predict the individual efficiency of HBH in animals and to study its potential adaptive mechanisms. We revealed a bi-directional action of nicotinic α7 receptor agonist PNU-282987 and its solvent dimethyl sulfoxide on HBH efficiency with the level of PPI > or < 40%. (2) The aim of the present study was to estimate cholinergic mechanisms of HBH effects in different brain regions. (3) Methods: in rats pretested for PPI, we evaluated the activity of synaptic membrane-bound and water-soluble choline acetyltransferase (ChAT) in the sub-fractions of ‘light’ and ‘heavy’ synaptosomes of the neocortex, hippocampus and caudal brainstem in the intact brain and after HBH. We tested the dose-dependent influence of PNU-282987 on the HBH efficiency. (4) Results: PPI level and ChAT activity correlated negatively in all brain structures of the intact animals, so that the values of the latter were higher in rats with PPI < 40% compared to those with PPI > 40%. After HBH, this ChAT activity difference was leveled in the neocortex and caudal brainstem, while for membrane-bound ChAT in the ‘light’ synaptosomal fraction of hippocampus, it was reversed to the opposite. In addition, a pharmacological study revealed that PNU-282987 in all used doses and its solvent displayed corresponding opposite effects on HBH efficiency in rats with different levels of PPI. (5) Conclusion: We substantiate that in rats with low and high PPI two opposite hippocampal cholinergic mechanisms are involved in hypoxic preconditioning, and both are implemented by forebrain projections via nicotinic α7 receptors. Possible causes of association between general protective adaptation, HBH, PPI, forebrain cholinergic system and hippocampus are discussed.

Chronic cerebral ischaemia forms new cholinergic mechanisms of learning and memory

The purpose of this research was a comparative analysis of cholinergic synaptic organization following learning and memory in normal and chronic cerebral ischaemic rats in the Morris water maze model. Choline acetyltransferase and protein content were determined in subpopulations of presynapses of “light” and “heavy” synaptosomal fractions of the cortex and the hippocampus, and the cholinergic projective and intrinsic systems of the brain structures were taken into consideration. We found a strong involvement of cholinergic systems, both projective and intrinsic, in all forms of cognition. Each form of cognition had an individual cholinergic molecular profile and the cholinergic synaptic compositions in the ischaemic rat brains differed significantly from normal ones. Our data demonstrated that under ischaemic conditions, instead of damaged connections new key synaptic relationships, which were stable against pathological influences and able to restore damaged cognitive functions, arose. The plasticity of neurochemical links in the individual organization of certain types of cognition gave a new input into brain pathology and can be used in the future for alternative corrections of vascular and other degenerative dementias.