Effects Of MDA On Learning & Memory Capability And Hippocampal Neurons Ca²⁺ Homeostasis Of Rats

The present study was performed in the whole animal level and cell molecular level to investigate the influences of carbonyl stress induced by malondialdehyde(MDA),a typical intermediate of lipid peroxidation,on the learning and memory capability and intracellular Ca²⁺ hemostatsis in cultured hippocampal neurons of the SD rats.At the whole animal level,the place navigation test(PNT) and spatial probe test(SPT) were conducted using Morrise water maze test system after lateral cerebral ventricular injection of MDA into SD rats. Comparing to the physiological saline treated animals and/or blank control,it was found that the escape latent period(ELP) for searching the underwater platform was significant increased(P＜0.01) during the PNT and the incidence of crossing the underwater platform within 120s was decreased(P＜0.05) during the SPT.The experimental results demonstrated that the learning and memory capability were impaired underby different concentration of MDA.And also,the transmission electronic microscope(TEM) was applied for studying the hippocampal CA1 area neuronal ultra-structures of rats following treatment of different concentration of MDA.It was observed that the architectures of mitochondria in CA1 area neurons were deformed and their cristae were decreased or disappeared.The ultra-microstructures of synaptic junction including the thickness of postsynaptic density material(PSD),the width of synaptic cleft(WSC),the curvature of synaptic interface(CSI) and the length of active zone(LAZ) were also altered in different severity.These results suggested that remarkable decrease of learning and memory capability were resulted from the alteration of hippocampal neuronal architectures and their synptic junction after MDA stress.At the cellular and molecular levels,the morphological alterations were conducted using the light microscope and the Ca²⁺ hemostasis was performed in cultured hippocampal neurons.The microphotographic study clearly demonstrated that the hippocampal neurons became gradually damaged following exposure to different concentrations of MDA.Further study indicated that the plasma membrane Ca²⁺-ATPase (PMCA) activity was inhibited by MDA in a concentration(1.0～1000μMol/L) and time(30 min) dependent manner.The supplementation of 100μMol/L MDA was found to cause a notable early phase increase of [Ca²⁺]_i in hippocampal neuron cultures followed by a more pronounced late phase elevation of[Ca²⁺]_i.Such effect of MDA was prevented by the addition of nimodipine,an inhibitor of L-type calcium channel or by an extracellular Ca²⁺ chelator EGTA.The identification of the calcium signalling pathways were studied by applying U73122,an inhibitor of PL-C,and H-89,an inhibitor of protein kinase A(PKA),showing the involvement of PL-C/IP3 pathway but not the PKA/cAMP pathway. These results suggested that MDA-related carbonyl stress caused damages of rat hippocampal neurons by triggering Ca²⁺ influx and influencing Ca²⁺ homeostasis in cultured neurons,and also MDA may act as a signalling molecule regulating Ca²⁺ release from intracellular stores.In 1995,the aging theory of "carbonyl stress"was brought out by Professor Yin Dazhong and Brunk.This theory suggests that unstatured aldehydes and ketons(such as MDA and 4-HNE),which were originated from metabolic processes of orgnisms,were the critical processes and crucial materials of biological aging and different types of degenerative diseases.The present study probed the possible"carbonyl stress" mechnisms resulting in impairment of learning and memory capability of SD rats under carbonyl stress at whole animal level.A significant damage was displayed of the structure of mitochondira and synaptic junction at cellular and sub-cellular levels.And also,the possible mechanisms of intercellular Ca²⁺ homeostasis disrupted following MDA stress were investigated at molecular level.