Roles Of Norepinephrine In Hippocampal Dentate Gyrus In Learning And Memory

Learning and memory are major functions of the higher levels of the brain,and the long-term potentiation(LTP)is widely accepted as the best-studied model for neuro-physiological mechanisms that could underlie learning and memory formation.The hippocampus is the key structure for learning and memory in mammals,and hippocampal LTP is believed to underlie several learning and memory processes.In the hippocampus,not only high-frequency electrical stimulation can induce LTP,learning and memory behavior can also induce LTP-like response,we consider it as learning-dependent LTP(LD-LTP).In the structure,mammalian hippocampus is mainly divided into three sub-regions,CA1,CA3 and dentate gyrus(DG),among them,as the main entry point of information into the hippocampus,the DG plays a key role in the coding and processing of new information and learning and memory.In the hippocampus,especially in CA1 and DG region,LTP is typically dependent on the activation of N-methyl-D-aspartate(NMDA)-type glutamate receptors on the postsynaptic membrane,leading to a significant increase in Ca2+ influx into the postsynaptic cells,and then trigger a series of Ca2+-activated physiological and biochemical reactions.In addition,the induction and persistence of LTP in the hippocampus has been shown to require a heterosynaptic interplay of glutanatergic input and modulatory afferent influences,such as noradrenergic neurotransmission.The majority of central noradrenergic projections originate in the brainstem nucleus locus coeruleus(LC),and in the hippocampus,DG receives a greater density of LC noradrenergic fiber projections than otlher hippocampal subregions.Despite a number of studies indicating that norepinephrine(NE)in the hippocampal DG is important in the modulation of learning and memory and the related LTP,but the studies are inconsistent.Therefore,in part ? of present study,we measured extracellular concentrations of NE and amplitudes of field excitatory postsynaptic potential(fEPSP)in the hippocampal DG during the acquisition and extinction of active-avoidance behavior by using in vivo brain microdialysis,electrophysiological reeording and HPLC techniques in freely-ruoving conscious rats.Next,we examined the role of NE and its receptors in the DG on active avoidance learning and related LD-LTP by using microinjection of the antagonists or agonists of adrenoceptors into the DG.Chronic stress is ubiquitous in the life.Studies have shown that learning and memory processes carried out within the hippocampus are influenced by various stresses,however,how does stress affect synaptic plasticity in the DG during learning and memory process remains unclear.It has been shown that the LC-NE system is activated by various stress,and the DG receives strong modulation via direct NE projections from the LC,and it also contains a high concentration of adrenoceptors.In in part ? of present study,we measured NE levels and fEPSP amplitudes in the hippocampal DG during Morris water maze(MWM)test in chronic restraint stress(CRS)rats.In addition,the activation of cyclic AMP-response element binding protein(CREB)and the expression of brain derived neurotrophic factor(BDNF)protein in the DG was detected in CRS rats.Following experiment were carried out:1.Part ?:(1)The expression of ?1,?2-and ?-adrenoceptors in the hippocampal DG were observed by immunohistochemical staining.(2)In the process of active avoidance learning,the extracellular concentrations of NE and fEPSP amplitude in the DG were examined by brain microdialysis,electrophysiological recording and HPLC techniques.(3)The effects of antagonist or agonist of ?1-adrenoceptors on amplitudes of fEPSP were measured in the DG region during the acquisition and extinction of active-avoidance behavior in freely-moving conscious rats.(4)The effects of antagonist or agonist of ?-adrenoceptors on extracellular Glu concentrations and amplitudes of fEPSP were measured in the DG region during the acquisition and extinction of active-avoidance behavior in freely-moving conscious rats.2.Part ?:(1)The levels of corticosterone(CORT)and epinephrine(EPI)in serum were examined by Elisa assay in CRS rats.(2)The extracellular concentrations of NE were examined by in vivo microdialysis and HPLC methods in CRS rats.(3)The extracellular concentrations of NE and fEPSP amplitude in the DG in CRS rats were examined during Morris water maze(MWM)test.(4)The expression of p-CREB?CREB and BDNF in the DG were measured by Western blot methods.The experimental results are as follows:1.Part ?:(1)In the hippocampal DG,?1-adrenoceptors mainly expressed in the polymorphic layer and p-adrenoceptors expressed in both granular and polymorphic layers,in contrast,there was no significant expression of ?2-adrenoceptors.(2)Extracellular levels of NE and fEPSP amplitudes in the DG were significantly increased during the acquisition of the active avoidance behavior and gradually returned following extinction training.The changes of NE in the DG were accompanied by changes in fEPSP,and that both changes correspond roughly to the avoidance rate.(3)Local microinjection of prazosin(antagonist of ?1-adrenoceptors)into the DG significantly accelerated the acquisition of the active-avoidance behavior,whereas local micrioinjection of phenylephrine(aonist of cci-adrenoceptors)retarded the acquisition of the active-avoidance behavior.Furthermore,in all groups,the changes in fEPSP amplitude were accompanied by corresponding changes in active-avoidance behavior.(4)In propranolol(antagonist of ?-adrenoceptors)group,antagonism of?-adrenoceptor in the DG significantly reduced the change in Glu concentration and fEPSP amplitude,as well as the acquisition of the active-avoidance behavior.(5)The change in Glu concentration was significantly accelerated by isoproterenol(agonist of ?-adrenoceptors),as well as the acquisition of the active-avoidance behavior.Furthermore,the changes in extracellular Glu were accompanied by corresponding changes in fEPSP amplitude and active-avoidance behavior.2.Part ?:(1)The levels of CORT and EPI in the serum,and the extracellular concentration of NE in DG were significantly increased in CRS rats.(2)In CRS rats,the spatial learning and memory ability was significantly enllanced,and the changes of NE level and fEPSP amplitude in the DG during MWM test were also significantly increased compared with control group.(3)The expression of p-CREB and BDNF in the DG significantly increased in CRS rats,while there was no significant difference in CREB protein expression between the two groups.Conclusions:1.NE in the hippocampal DG modulates the active-avoidance learning and the LD-LTP in two-way by different adrenoceptors.2.Activation of ?1-adrenoceptors in the hippocampal DG inhibits active avoidance learning via modulation of synaptic efficiency in rats.3.Activation of ?-adrenoceptors in the hippocampal DG facilitates active avoidance learning via modulations of glutamate levels and synaptic efficiency in rats.4.Chronic restraint stress enhances spatial learning and memory and the LD-LTP via increases of NE level and its downstream protein expression,including p-CREB and BDNF,in the DG.