Two Peptidergic Drugs Increase the Synaptophysin Immunoreactivity in Brains of 24-month-old Rats

The brain-derived peptidergic drug Cerebrolysin has been found to support the survival of neurones in vitro and in vivo. Positive effects on learning and memory have been demonstrated in various animal models and also in clinical trails. In the present study, the effects of Cerebrolysin and its peptide preparation E021 on the synapse density in the hippocampus, the dentate gyrus and in the entorhinal cortex of 24-month-old rats were investigated. Rats received the drugs or saline for control for 19 consecutive days (2.5ml/kg per day). Slices of the brains were immunohistochemically stained with anti-synaptophysin, which is a specific marker of presynaptic terminals. Quantification of the synapse density was done by using light microscopy and a computerised image analysing system. Our results clearly showed that the rats benefit from the administration of both drugs, showing an enhancement in the number of synaptophysin-immunostained presynaptic terminals in the entorhinal cortex, the dentate gyrus, and also in the hippocampal subfields CA1, CA2, CA3 stratum lucidum and CA3 stratum radiatum. It can be assumed that these effects are the reason for improved cognitive performances of rats treated with Cerebrolysin and E021.     

The Drug Cerebrolysin and its Peptide Fraction E021 Increase the Abundance of the Blood–brain Barrier GLUT1 Glucose Transporter in Brains of Young and Old Rats

The brain-derived peptidergic drug Cerebrolysin has been found to support the survival of neurons in vitro and in vivo. In the present study, we investigated the effects of Cerebrolysin and its peptide preparation E021 on spatial learning and memory, as well as on the abundance of the blood–brain barrier GLUT1 glucose transporter (GLUT1) in 2-month-old and 24-month-old rats. Young rats were treated with the drugs or saline (2.5?ml/kg/day) daily on postnatal days 1–7, and old rats for 19 consecutive days. For behavioural testing the Morris water maze was used. The abundance of GLUT1 was determined in brain slices by immunocytochemistry. Quantification of the density of the GLUT1 immunostaining was performed using light microscopy and a computerised image analysing system. All drug-treated rats, young and old, exhibit shorter escape latencies in the water maze, on all testing days (p>0.01), indicating improved cognitive performance. Immunohistochemical data show an age-related decrease of the density of GLUT1 (p>0.05). In young animals, the administration of the drugs led to an increase of the abundance of GLUT1 in all experimental groups (p>0.01). In old rats, the treatment with Cerebrolysin, but not with E021, resulted in an increase in the immunoreactive GLUT1 (p>0.01). The elevated abundance of GLUT1 after the administration of both peptidergic substances might be supportive for the cognitive effects of this drug, by causing an improved nutritional supply of glucose to the neurons.     

Two peptidergic drugs increase the synaptophysin immunoreactivity in brains of 6-week-old rats

An increase of synaptic density has been found in the hippocampus, the dendate gyrus and in the entorhinal cortex of 6-week-old rats after 7 days of treatment with the peptidergic drug Cerebrolysin, its peptide preparation E021 and the diluted peptide preparation E021dil. Rats received drugs on postnatal days 1–7 (2.5ml/kg, each day). Controls received saline. The animals were sacrificed on days 42–48 of their life, after they had undergone behavioural testing in a Morris water maze. Slices of brain were stained immunohistochemically with anti-synaptophysin, a specific marker of presynaptic terminals. The synaptophysin-immunoreactivity of presynaptic terminals was quantified using light microscopy and a computerised image analysis system. Our results showed that rats benefit from the treatment with both drugs. A significant increase in the number of synaptophysin-immunoreactive presynaptic terminals was found in the entorhinal cortex and the hippocampal subfields CA1, CA2, CA3 stratum radiatum and CA3 stratum lucidum. The increased immunoreactive presynaptic terminals found in the present study are in accordance with the positive effects of the drugs on spatial learning and memory in young rats (Gschanes & Windisch 1999).     

The drug cerebrolysin and its peptide fraction E021 increase the abundance of the blood-brain barrier GLUT1 glucose transporter in brains of young and old rats

The brain-derived peptidergic drug Cerebrolysin has been found to support the survival of neurons in vitro and in vivo. In the present study, we investigated the effects of Cerebrolysin and its peptide preparation E021 on spatial learning and memory, as well as on the abundance of the blood–brain barrier GLUT1 glucose transporter (GLUT1) in 2-month-old and 24-month-old rats. Young rats were treated with the drugs or saline (2.5ml/kg/day) daily on postnatal days 1–7, and old rats for 19 consecutive days. For behavioural testing the Morris water maze was used. The abundance of GLUT1 was determined in brain slices by immunocytochemistry. Quantification of the density of the GLUT1 immunostaining was performed using light microscopy and a computerised image analysing system. All drug-treated rats, young and old, exhibit shorter escape latencies in the water maze, on all testing days (p>0.01), indicating improved cognitive performance. Immunohistochemical data show an age-related decrease of the density of GLUT1 (p>0.05). In young animals, the administration of the drugs led to an increase of the abundance of GLUT1 in all experimental groups (p>0.01). In old rats, the treatment with Cerebrolysin, but not with E021, resulted in an increase in the immunoreactive GLUT1 (p>0.01).The elevated abundance of GLUT1 after the administration of both peptidergic substances might be supportive for the cognitive effects of this drug, by causing an improved nutritional supply of glucose to the neurons.     

Opiate withdrawal induces dynamic expressions of AMPA receptors and its regulatory molecule CaMKIIalpha in hippocampal synapses

Adaptive changes in brain areas following drug withdrawal are believed to contribute to drug seeking and relapse. Cocaine withdrawal alters the expression of GluR1 and GluR2/3 subunits of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in nucleus accumbens or amygdala, but the influence of drug withdrawal on hippocampus is little known. Here, we have examined the expression of GluR1 and GluR2/3 in hippocampal membrane and synaptic fractions following repeated morphine exposure and subsequent withdrawal. Repeated morphine exposure for 12 d increased GluR1 and GluR2/3 in synaptosome but not in membrane fraction. Interestingly, CaMKIIalpha, known to be able to regulate the function of AMPA receptors, was decreased in synaptosome but not in membrane fraction; pCaMKIIalpha, the phosphorylated form of CaMKIIalpha, was increased in both fractions. However, during opiate withdrawal, GluR1 was generally reduced while GluR2/3 was prominently increased in both fractions; pCaMKIIalpha was strongly decreased immediately after withdrawal, but detectably increased in late phase of morphine withdrawal in both fractions. Importantly, the opiate withdrawal-induced increase in GluR2/3 was dependent on the activation of glucocorticoid receptors and NMDA receptors, as it was prevented by the glucocorticoid receptor antagonist RU38486, or intrahippocampal injection of the NMDA receptor antagonist AP-5 or the antagonist to NR2B-containing NMDA receptors, Ro25-6981. These findings indicate that opiate withdrawal induces dynamic expression of GluR1 and GluR2/3 subunits of AMPA receptors in hippocampal synapses, possibly revealing an adaptive process of the hippocampal functions following opiate withdrawal.     

Rapid and Transient Learning-Associated Increase in NMDA NR1 Subunit in the Rat Hippocampus

Several lines of evidence indicate that glutamate NMDA receptors are critically involved in long-term potentiation (LTP) and in certain forms of learning. It was previously demonstrated that memory formation of an inhibitory avoidance task in chick is specifically associated with an increase in the density of NMDA receptor in selected brain regions. Here we report on the effect of a one trial inhibitory avoidance training in rats, a hippocampal-dependent learning task, on the levels of different subunits of the glutamate NMDA receptor in synaptic plasma membranes (SPM) isolated from the hippocampus. Training rats on a one trial inhibitory avoidance task results in a rapid, transient and selective increase (+33 %, p < 0.05) in NMDA NR1 subunit expression in hippocampal SPM of rats sacrificed 30 min posttraining. No changes were observed at 0 or 120 min after training or in shocked animals in comparison to naive control rats. In addition, no training-associated increase in the levels of NMDA NR2A and NR2B or AMPA GluR 2/3 subunits was observed at any timepoint tested. In conclusion, the present findings support the hypothesis that alterations in expression of synaptic NMDA NR1 subunits in the hippocampus are specifically associated with memory formation of an inhibitory avoidance task and strongly suggest that hippocampal NMDA receptors are crucially involved in the neural mechanisms underlying certain forms of learning.These authors contributed equally to this work     

Learning-induced glutamate receptor phosphorylation resembles that induced by long term potentiation

Long term potentiation and long term depression of synaptic responses in the hippocampus are thought to be critical for certain forms of learning and memory, although until recently it has been difficult to demonstrate that long term potentiation or long term depression occurs during hippocampus-dependent learning. Induction of long term potentiation or long term depression in hippocampal slices in vitro modulates phosphorylation of the alpha-amino-3-hydrozy-5-methylisoxazole-4-propionic acid subtype of glutamate receptor subunit GluR1 at distinct phosphorylation sites. In long term potentiation, GluR1 phosphorylation is increased at the Ca2+/calmodulin-dependent protein kinase and protein kinase C site serine 831, whereas in long term depression, phosphorylation of the protein kinase A site serine 845 is decreased. Indeed, phosphorylation of one or both of these sites is required for long term synaptic plasticity and for certain forms of learning and memory. Here we demonstrate that training in a hippocampus-dependent learning task, contextual fear conditioning is associated with increased phosphorylation of GluR1 at serine 831 in the hippocampal formation. This increased phosphorylation is specific to learning, has a similar time course to that in long term potentiation, and like memory and long term potentiation, is dependent on N-methyl-D-aspartate receptor activation during training. Furthermore, the learning-induced increase in serine 831 phosphorylation is present at synapses and is in heteromeric complexes with the glutamate receptor subunit GluR2. These data indicate that a biochemical correlate of long term potentiation occurs at synapses in receptor complexes in a final, downstream, postsynaptic effector of long term potentiation during learning in vivo, further strengthening the link between long term potentiation and memory.     

Spatial learning and memory deficits after whole-brain irradiation are associated with changes in NMDA receptor subunits in the hippocampus

Whole-brain irradiation is used for the treatment of brain tumors, but can it also induce neural changes, with progressive dementia occurring in 20-50% of long-term survivors. The present study investigated whether 45 Gy of whole-brain irradiation delivered to 12-month-old Fischer 344 x Brown Norway rats as nine fractions over 4.5 weeks leads to impaired Morris water maze (MWM) performance 12 months later. Compared to sham-irradiated rats, the irradiated rats demonstrated impaired MWM performance. The relative levels of the NR1 and NR2A but not the NR2B subunits of the NMDA receptor were significantly higher in hippocampal CA1 of irradiated rats compared to control rats. No significant differences were detected for these NMDA subunits in CA3 or dentate gyrus. Further analysis of CA1 revealed that the relative levels of the GluR1 and GluR2 subunits of the AMPA receptor and synaptophysin were not altered by whole-brain irradiation. In summary, a clinically relevant regimen of fractionated whole-brain irradiation led to significant impairments in spatial learning and reference memory and alterations in the relative levels of subunits of the NMDA, but not the AMPA, receptors in hippocampal CA1. These findings suggest for the first time that radiation-induced cognitive impairments may be associated with alterations in glutamate receptor composition.     

The amino-terminal domain of glutamate receptor δ2 triggers presynaptic differentiation

Glutamate receptor (GluR) δ2 selectively expressed in cerebellar Purkinje cells plays key roles in synapse formation, long-term depression and motor learning. We propose that GluRδ2 regulates synapse formation by making a physical linkage between the active zone and postsynaptic density. To examine the issue, GluRδ2-transfected 293T cells were cultured with cerebellar neurons. We found numerous punctate signals for presynaptic markers on the surface of 293T cells expressing GluRδ2. The presynaptic specializations induced by GluRδ2 were capable of exo- and endocytosis as indicated by FM1-43 dye labeling. Replacement of the extracellular N-terminal domain (NTD) of GluRδ2 with that of the AMPA receptor GluRα1 abolished the inducing activity. The NTD of GluRδ2 fused to the immunoglobulin constant region successfully induced the accumulation of presynaptic specializations on the surface of beads bearing the fusion protein. These results suggest that GluRδ2 triggers presynaptic differentiation by direct interaction with presynaptic components through the NTD.     

人参皂甙Rb1对阿尔茨海默病大鼠海马结构β-淀粉样蛋白表达的影响

目的观察人参皂甙Rb1对阿尔茨海默病（AD）模型大鼠学习记忆能力及海马结构β-淀粉样蛋白表达的影响。方法动物分3组：对照组、模型组及治疗组,用D半乳糖联合三氯化铝建立AD大鼠模型,治疗组在造模后给予人参皂甙Rb1腹腔注射4周;采用Morris水迷宫测试大鼠的空间学习记忆能力,用免疫组织化学方法观察海马结构β-淀粉样蛋白的表达。结果与对照组相比,模型组大鼠各时间段的逃避潜伏期均显著延长（P〈0．01）,海马CA1、CA3区及齿状回β-淀粉样蛋白表达的阳性细胞数明显增多（P〈0．01）;治疗组大鼠的逃避潜伏期较模型组明显缩短（P〈0．01）,海马CA1、CA3区及齿状回的β-淀粉样蛋白阳性细胞数显著减少（P〈0．01）。结论人参皂甙Rb1对AD模型大鼠学习记忆损害具有明显改善作用,其机制可能与人参皂甙Rb1减少海马结构β-淀粉样蛋白的表达有关。     

寡聚体Aβ1-42对大鼠海马突触可塑性的慢性影响

突触传递的长时程抑制(long-term depression,LTD)和长时程增强(longterm-potentiation,LTP)是突触可塑性的两种重要形式,并且与学习记忆密切相关.本文探讨Sprague-Dawley(SD)大鼠在海马齿状回区(dentate gyrus,DG)注射36 h孵育形成的寡聚体Aβ...     

Regulated delivery of AMPA receptor subunits to the presynaptic membrane

In recent years, a role for AMPA receptors as modulators of presynaptic functions has emerged. We have investigated the presence of AMPA receptor subunits and the possible dynamic control of their surface exposure at the presynaptic membrane. We demonstrate that the AMPA receptor subunits GluR1 and GluR2 are expressed and organized in functional receptors in axonal growth cones of hippocampal neurons. AMPA receptors are actively internalized upon activation and recruited to the surface upon depolarization. Pretreatment of cultures with botulinum toxin E or tetanus toxin prevents the receptor insertion into the plasma membrane, whereas treatment with alpha-latrotoxin enhances the surface exposure of GluR2, both in growth cones of cultured neurons and in brain synaptosomes. Purification of small synaptic vesicles through controlled-pore glass chromatography, revealed that both GluR2 and GluR1, but not the GluR2 interacting protein GRIP, copurify with synaptic vesicles. These data indicate that, at steady state, a major pool of AMPA receptor subunits reside in synaptic vesicle membranes and can be recruited to the presynaptic membrane as functional receptors in response to depolarization.     

The X-linked intellectual disability protein TSPAN7 regulates excitatory synapse development and AMPAR trafficking

Mutations in TSPAN7--a member of the tetraspanin protein superfamily--are implicated in some forms of X-linked intellectual disability. Here we show that TSPAN7 overexpression promotes the formation of filopodia and dendritic spines in cultured hippocampal neurons from embryonic rats, whereas TSPAN7 silencing reduces head size and stability of spines and AMPA receptor currents. Via its C terminus, TSPAN7 interacts with the PDZ domain of protein interacting with C kinase 1 (PICK1), to regulate PICK1 and GluR2/3 association and AMPA receptor trafficking. These findings indicate that, in hippocampal neurons, TSPAN7 regulates AMPA receptor trafficking by limiting PICK1 accessibility to AMPA receptors and suggest an additional mechanism for the functional maturation of glutamatergic synapses, whose impairment is implicated in intellectual disability.     

Synaptic neurotransmission depression in ventral tegmental dopamine neurons and cannabinoid-associated addictive learning

Drug addiction is an association of compulsive drug use with long-term associative learning/memory. Multiple forms of learning/memory are primarily subserved by activity- or experience-dependent synaptic long-term potentiation (LTP) and long-term depression (LTD). Recent studies suggest LTP expression in locally activated glutamate synapses onto dopamine neurons (local Glu-DA synapses) of the midbrain ventral tegmental area (VTA) following a single or chronic exposure to many drugs of abuse, whereas a single exposure to cannabinoid did not significantly affect synaptic plasticity at these synapses. It is unknown whether chronic exposure of cannabis (marijuana or cannabinoids), the most commonly used illicit drug worldwide, induce LTP or LTD at these synapses. More importantly, whether such alterations in VTA synaptic plasticity causatively contribute to drug addictive behavior has not previously been addressed. Here we show in rats that chronic cannabinoid exposure activates VTA cannabinoid CB1 receptors to induce transient neurotransmission depression at VTA local Glu-DA synapses through activation of NMDA receptors and subsequent endocytosis of AMPA receptor GluR2 subunits. A GluR2-derived peptide blocks cannabinoid-induced VTA synaptic depression and conditioned place preference, i.e., learning to associate drug exposure with environmental cues. These data not only provide the first evidence, to our knowledge, that NMDA receptor-dependent synaptic depression at VTA dopamine circuitry requires GluR2 endocytosis, but also suggest an essential contribution of such synaptic depression to cannabinoid-associated addictive learning, in addition to pointing to novel pharmacological strategies for the treatment of cannabis addiction.     

Extracellular interactions between GluR2 and N-cadherin in spine regulation

Via its extracellular N-terminal domain (NTD), the AMPA receptor subunit GluR2 promotes the formation and growth of dendritic spines in cultured hippocampal neurons. Here we show that the first N-terminal 92 amino acids of the extracellular domain are necessary and sufficient for GluR2's spine-promoting activity. Moreover, overexpression of this extracellular domain increases the frequency of miniature excitatory postsynaptic currents (mEPSCs). Biochemically, the NTD of GluR2 can interact directly with the cell adhesion molecule N-cadherin, in cis or in trans. N-cadherin-coated beads recruit GluR2 on the surface of hippocampal neurons, and N-cadherin immobilization decreases GluR2 lateral diffusion on the neuronal surface. RNAi knockdown of N-cadherin prevents the enhancing effect of GluR2 on spine morphogenesis and mEPSC frequency. Our data indicate that in hippocampal neurons N-cadherin and GluR2 form a synaptic complex that stimulates presynaptic development and function as well as promoting dendritic spine formation.     

Muscarinic Cholinergic Receptor Subtypes in Hippocampus in Human Cognitive Disorders

Total muscarinic receptor levels, the levels of the subtypes exhibiting high and low affinity for pirenzepine, and the high- and low-affinity agonist states of the receptor were investigated in hippocampal tissue obtained at autopsy from mentally normal individuals and the following pathological groups: Alzheimer's disease, Parkinson's disease, Down's syndrome, alcoholic dementia, Huntington's chorea, and motor-neurone disease. A moderate decrease in the density of both high-affinity pirenzepine and high-affinity agonist subtypes was found in Alzheimer's disease, whereas a trend towards an increase in the overall muscarinic receptor density was apparent in the parkinsonian patients without dementia, mainly due to an increase in the low-affinity agonist state; the differences between the Alzheimer's disease and nondemented parkinsonian cases were highly significant. As previously reported, the levels of both choline acetyltransferase and acetylcholinesterase were markedly reduced in both Alzheimer's disease and Parkinson's disease--with a greater loss of both enzymes in the demented subgroup of parkinsonian patients. Activities of the cholinergic enzymes were also extensively reduced in Down's syndrome, accompanied by a loss of high-affinity pirenzepine binding. There were no significant receptor or enzyme alterations in the other groups studied. These observations suggest that in the human brain, extensive degeneration of cholinergic axons to the hippocampus, as indicated by a loss of cholinergic enzymes, is not necessarily accompanied by extensive muscarinic receptor abnormalities (as might be expected if a major subpopulation were presynaptic). Moreover, the opposite changes in muscarinic binding in Parkinson's and Alzheimer's diseases may be related to the greater severity of dementia in the latter disease.     

Glutamate transporter type 3 regulates mouse hippocampal GluR1 trafficking

Background

Rapid trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) to the plasma membrane is considered a fundamental biological process for learning and memory. GluR1 is an AMPAR subunit. We have shown that mice with knockout of excitatory amino acid transporter type 3 (EAAT3), a neuronal glutamate transporter, have impaired learning and memory. The mechanisms for this impairment are not known and may be via regulation of AMPAR trafficking.

Methods

Freshly prepared 300 μm coronal hippocampal slices from wild-type or EAAT3 knockout mice were incubated with or without 25 mM tetraethylammonium for 10 min. The trafficking of GluR1, an AMPAR subunit, to the plasma membrane and its phosphorylation were measured.

Results

Tetraethylammonium increased the trafficking of GluR1 and EAAT3 to the plasma membrane in the wild-type mouse hippocampal slices but did not cause GluR1 trafficking in the EAAT3 knockout mice. Tetraethylammonium also increased the phosphorylation of GluR1 at S845, a protein kinase A (PKA) site, in the wild-type mice but not in the EAAT3 knockout mice. The PKA antagonist KT5720 attenuated tetraethylammonium-induced GluR1 phosphorylation and trafficking in the wild-type mice. The PKA agonist 6-BNz-cAMP caused GluR1 trafficking to the plasma membrane in the EAAT3 knockout mice. In addition, EAAT3 was co-immunoprecipitated with PKA.

Conclusions

These results suggest that EAAT3 is upstream of PKA in a pathway to regulate GluR1 trafficking.

General significance

Our results provide initial evidence for the involvement of EAAT3 in the biochemical cascade of learning and memory.     

Long-term alterations in glutamate receptor and transporter expression following early-life seizures are associated with increased seizure susceptibility

Prolonged seizures in early childhood are associated with an increased risk of development of epilepsy in later life. The mechanism(s) behind this susceptibility to later development of epilepsy is unclear. Increased synaptic activity during development has been shown to permanently alter excitatory neurotransmission and could be one of the mechanisms involved in this increased susceptibility to the development of epilepsy. In the present study we determine the effect of status-epilepticus induced by lithium/pilocarpine at postnatal day 10 (P10 SE) on the expression of glutamate receptor and transporter mRNAs in hippocampal dentate granule cells and protein levels in dentate gyrus of these animals in adulthood. The results revealed a decrease in glutamate receptor 2 (GluR2) mRNA expression and protein levels as well as an increase in protein levels for the excitatory amino acid carrier 1 (EAAC1) in P10 SE rats compared to controls. Expression of glutamate receptor 1 (GluR1) mRNA was decreased in both P10 SE rats and identically handled, lithium-injected littermate controls compared to naive animals, and GluR1 protein levels were significantly lower in lithium-controls than in naive rats, suggesting an effect of either the handling or the lithium on GluR1 expression. These changes in EAA receptors and transporters were accompanied by an increased susceptibility to kainic acid induced seizures in P10 SE rats compared to controls. The current data suggest that early-life status-epilepticus can result in permanent alterations in glutamate receptor and transporter gene expression, which may contribute to a lower seizure threshold.     

PKC anchoring to GluR4 AMPA receptor subunit modulates PKC-driven receptor phosphorylation and surface expression

Changes in the synaptic content of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors lead to synaptic efficacy modifications, involved in synaptic plasticity mechanisms believed to underlie learning and memory formation. Early in development, GluR4 is highly expressed in the hippocampus, and GluR4-containing AMPA receptors are inserted into synapses. During synapse maturation, the number of AMPA receptors at the synapse is dynamically regulated, and both addition and removal of receptors from postsynaptic sites occur through regulated mechanisms. GluR4 delivery to synapses in rat hippocampal slices was shown to require protein kinase A (PKA)-mediated phosphorylation of GluR4 at serine 842 (Ser842). Protein kinase C (PKC) can also phosphorylate Ser842, and we have shown that PKCgamma can associate with GluR4. Here we show that activation of PKC in retina neurons, or in human embryonic kidney 293 cells cotransfected with GluR4 and PKCgamma, increases GluR4 surface expression and Ser842 phosphorylation. Moreover, mutation of amino acids R821A, K825A and R826A at the GluR4 C-terminal, within the interacting region of GluR4 with PKCgamma, abolishes the interaction between PKCgamma and GluR4 and prevents the stimulatory effect of PKCgamma on GluR4 Ser842 phosphorylation and surface expression. These data argue for a role of anchored PKCgamma in Ser842 phosphorylation and targeting to the plasma membrane. The triple GluR4 mutant is, however, phosphorylated by PKA, and it is targeted to the synapse in CA1 hippocampal neurons in organotypic rat hippocampal slices. The present findings show that the interaction between PKCgamma and GluR4 is specifically required to assure PKC-driven phosphorylation and surface membrane expression of GluR4.     

Developmental Hypothyroxinemia Caused by Mild Iodine Deficiency Leads to HFS-Induced LTD in Rat Hippocampal CA1 Region: Involvement of AMPA Receptor

Hypothyroidism induced by severe iodine deficiency (ID) during developmental period seriously damages the central nervous system function. In addition to developmental hypothyroidism induced by severe ID, developmental hypothyroxinemia induced by mild ID is potentially damaging for neurodevelopment and learning and memory in children. Wistar rats were treated with iodine-deficient diet or methimazole (MMZ) during pregnancy and lactation to induce developmental hypothyroxinemia or hypothyroidism in the present study. Pups were weaned on postnatal day (PN) 21 and used for electrophysiological recordings on PN80. It is generally accepted that long-term depression (LTD) is induced at low-frequency stimulation (LFS) in hippocampal CA1 region. Surprisingly, we observed developmental hypothyroxinemia as well as developmental hypothyroidism led to high-frequency stimulation (HFS)-induced LTD in hippocampal CA1 region. The abnormal HFS-induced LTD suggests not only developmental hypothyroidism but also developmental hypothyroxinemia impairs learning and memory. To explore the mechanisms responsible for the HFS-induced LTD, the phosphorylation status of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) was investigated. The results showed that developmental hypothyroxinemia as well as developmental hypothyroidism decreased the phosphorylation of AMPAR subunit glutamate receptor 1 (GluR1) at serine 831 and serine 845 in hippocampal CA1 region. Neither developmental hypothyroxinemia nor developmental hypothyroidism altered the phosphorylation of AMPAR subunit glutamate receptor 2 (GluR2) at serine 880. Increased levels of protein phosphatase-1 (PP1) were also observed in hippocampal CA1 regions of pups subjected to developmental hypothyroxinemia or hypothyroidism. Taken together, our results suggest that the increased levels of PP1 caused by developmental hypothyroxinemia or hypothyroidism may account for the dephosphorylation of GluR1 at serine 831 and serine 845, which may contribute to HFS-induced LTD in hippocampal CA1 region.