N. M. Dubrovskaya

Effect of Sodium Valproate Administration on Brain Neprilysin Expression and Memory in Rats

Alzheimers disease (AD) is accompanied by memory loss due to neuronal cell death caused by toxic amyloid β-peptide (Aβ) aggregates. In the healthy brain, a group of amyloid-degrading enzymes including neprilysin (NEP) maintain Aβ levels at physiologically low concentrations but, with age and under some pathological conditions, expression and activity of these enzymes decline predisposing to late-onset AD. Hence, up-regulation of NEP might be a viable strategy for prevention of Aβ accumulation and development of the disease. As we have recently shown, inhibitors of histone deacetylases, in particular, valproic acid (VA), were capable of up-regulating NEP expression and activity in human neuroblastoma SH-SY5Y cell lines characterised by very low levels of NEP. In the present study, analysing the effect of i.p. injections of VA to rats, we have observed up-regulation of expression and activity of NEP in rat brain structures, in particular, in the hippocampus. This effect was brain region- and age-specific. Administration of VA has also restored NEP activity and memory deficit in adult rats caused by prenatal hypoxia. This suggests that VA and more specific HDAC inhibitors can be considered as potential pharmaceutical agents for up-regulation of NEP activity and improvement of cognitive functions of ageing brain.

Postnatal physiological development of rats after acute prenatal hypoxia.

The aim of the present work was to identify the characteristics of the physiological development of the brain and the formation of behavior in rats subjected to hypoxia on day 13.5 of embryogenesis. These animals showed delayed development and changes in nerve tissue structure in the sensorimotor cortex, along with disturbances to the processes forming normal movement responses during the first month after birth. These changes were partially compensated with age, though adult animals subjected to acute prenatal hypoxia were less able to learn new complex manipulatory movements. Alterations in nerve tissue structure and changes in the neuronal composition of the sensorimotor cortex correlated with the times of appearance of behavioral impairments at different stages of ontogenesis. Thus, changes in the conditions in which the body is formed during a defined period of embryogenesis lead to abnormalities in the process of ontogenetic development and the ability to learn new movements.The aim of the present work was to identify the characteristics of the physiological development of the brain and the formation of behavior in rats subjected to hypoxia on day 13.5 of embryogenesis. These animals showed delayed development and changes in nerve tissue structure in the sensorimotor cortex, along with disturbances to the processes forming normal movement responses during the first month after birth. These changes were partially compensated with age, though adult animals subjected to acute prenatal hypoxia were less able to learn new complex manipulatory movements. Alterations in nerve tissue structure and changes in the neuronal composition of the sensorimotor cortex correlated with the times of appearance of behavioral impairments at different stages of ontogenesis. Thus, changes in the conditions in which the body is formed during a defined period of embryogenesis lead to abnormalities in the process of ontogenetic development and the ability to learn new movements.

Formation of the structural and ultrastructural organization of the striatum in early postnatal ontogenesis of rats in altered conditions of embryonic development.

Light (Nissl and Golgi methods) and electron microscopy methods were used to study the formation of the structure of the striatum during the first two weeks after birth in rats subjected to acute hypoxia at different times of embryogenesis. The dynamics of the physiological development of the same population of rats were studied in parallel. Hypoxia at day 13.5 of embryogenesis was found to lead to delayed neurogenesis (delayed establishment of elements of the neuropil and differentiation of cells) and abnormalities in the structure of the striatum (degeneration, particularly chromatolysis, of neurons and the appearance of glial nodes). Morphometric analysis demonstrated a decrease in the total number of cells in the striatum; small changes in large neurons were seen. Hypoxia at day 18.5 of embryogenesis produced no significant changes. Structural abnormalities were accompanied by changes in the process of the animals’ physiological development. The data obtained here show that changes in the conditions of embryogenesis (hypoxia) during the period of the most intense proliferation of neuroblasts in the forebrain lead to impairment of the process of formation of striatal nervous tissue and the body as a whole in the period of early postnatal ontogenesis.

Effects of an inhibitor of α-secretase, which metabolizes the amyloid peptide precursor, on memory formation in rats

Intracortical administration of 10−4 M batimastat, a specific inhibitor of α-secretase (a metalloproteinase which cleaves the amyloid peptide precursor), decreased the number of correct runs in a single-level eight-arm maze to 92.78 ± 1.03% compared with baseline (p < 0.01) within 60 min. However, injection of batimastat into the cerebral cortex of animals during the early postnatal period (days 5 and 7 of life) led to impaired orientation in the simple single-level maze when these adults reached adulthood (90.92 ± 2.21% correct runs, p < 0.001) as compared with controls. The data obtained here provide evidence for the important role of α-secretase in memory processes. The possible role of α-secretase in memory processes and the pathogenesis of Alzheimer’s disease is discussed.

Neurochemical characteristics of the rat neostriatum and motor cortex after the development of a unilateral manipulatory reflex

Indicators of the activity of acetylcholinesterase (ACE), 5′-nucleotidase (NT), adenylate cyclase (AC) in the sensorimotor cortex and the neostriatum (NS) of the right and left cerebral hemispheres of control rats and rats trained to perform a food-procuring movement by pressing against an obstacle with the forelimb. An identical level of the averaged bilateral values of the activity of NT and AC in both of the structures in question and an increased ACE activity in the NS were found in the control animals. After the development of a manipulatory skill, the activity of AC decreased in the cortex and the NS in the presence of unchanged ACE activity, while NT activity decreased in the cortex and increased in the NS. The bilateral values of the activity of the enzymes differed significantly in well and poorly trained rats. At the same time, the activity of the enzymes was similar in character in the dominant and subdominant hemispheres for each group of animals. Overall the neurochemical changes obtained can be regarded as specific correlates of the developed unilateral manipulatory reactions that are characteristic for the structures in question of both cerebral hemispheres.

Role of caspase-3 in development of neuronal plasticity and memory

Caspases are known to play an important role in apoptosis and their increased activity in pre- and postsynaptic terminals leads to proteolysis of synapse-associated proteins, resulting in disruption of synaptic functions. Moreover, caspases degrade the C-terminal fragment of amyloid-precursor protein intracellular domain (AICD) which regulates expression of a variety of genes including an amyloid-degrading enzyme neprilysin (NEP). As such, inhibition of caspases is considered as a tool for prevention and compensation of various synaptic pathologies leading to cognitive deficit and Alzheimer’s disease pathogenesis. In this study we have evaluated the role of prenatal hypoxia on the activity of caspases and neuronal network characteristics in rat brain and the effect of caspase inhibitors on these parameters. We have found that the brain of rats subjected to prenatal hypoxia (Е14, О2 7%, 3 h) is characterised by an increased number of caspase-3-positive neurones and higher activity of this enzyme in the neocortex and hippocampus in the period of intensive synaptogenesis (Р20-30) compared to controls. Subsequently, in later life these animals had a reduced number of synaptopodin-positive dendritic spines and reduced activity of NEP accompanied by disruption of cognitive functions. Single i.v. injection of caspase-3 inhibitors (Ac-DEVD-CHO) to hypoxic rats on Р18-23 led to a decrease in caspase-3 activity and increased NEP expression. In these animals we have also observed restoration of synaptopodin levels and distribution of the labile synaptic spines in the neocortex and hippocampus which were accompanied by improved memory. The effects of inhibitors on memory was observed within one month after administration but not detected 2.5 months later. These data testify to the involvement of caspase-3 in normal brain development and indicate an important role of this enzyme in neuronal plasticity and regulation of cognitive functions. Supported by RFBR (13-04-00388), ARUK.

Activity of adenylate cyclase and 5'-nucleotidase in the sensorimotor and limbic structures of the brain in rats after manipulatory training.

Adenylate cyclase (AC) and 5′-nucleotidase (NT) activities were measured in the limbic (amygdala, hippocampus) and sensorimotor (cortex, striatum) structures of the brain in three groups of rats: untrained rats and rats which were good and poor learners in training to perform movements involving pushing against an obstruction. After training, AC activity decreased in all structures studied Activity decreased in the cortex and striatum to a greater extent in good learners, and in the amygdala in poor learners. NT activity decreased in all brain structures apart from the striatum, to a greater extent in rats which were less able to learn to produce movements involving prolonged pushing. The striatum was the only structure in which increases in NT activity occurred, from the lowest initial level in the control group, 1.0±0.04 μg Pi/mg protein/min, to 1.3±0.1 μg Pi/mg protein/min in poor learners and to 2.0±0.1 μg Pi/mg protein/min in good learners. Interhemisphere asymmetries in AC activity in the cortex and hippocampus were seen, along with an interhemisphere difference in NT activity in the amygdala. Thus, the activity of enzymes involved in adenine and cAMP biosynthesis changed in different ways in the limbic and sensorimotor structures of the brain, depending on the ability of rats to learn. The increase in NT activity after training of rats, which was limited to the striatum, may reflect a special role for the purinergic system in these structures in mediating sensation-regulated movements.