Phosphorylation of tau protein based on the activity of kinases and phosphatases in various forms of synaptic plasticity.

The aim of this study is to show the relationship between the change in the strengthening of synaptic plasticity and tau phosphorylation and tau-kinases and phosphatase. The averages of the field excitatory-postsynaptic potential (fEPSP) and population spike (PS) in the last 5 min were used as a measure of LTP, LTD and MP. Total and phosphorylated levels of tau, kinases and phosphatases were evaluated by western blot and mRNA levels were evaluated by RT-qPCR. The stimulation of synapses by HFS and LFS+HFS increased the phosphorylation of total-tau and phospho-tau at the Thr181, Ser202/Thr205, Ser396 and Ser416 residues, and these were accompanied by increased enzymatic activity of Akt, ERK1/2. The increased phosphorylation of tau may mediate maintenance of LTP. If the increase in phosphorylation of tau cannot be prevented, together with inhibition of the subsequent LTP, this may indicate that the physiological role of hyperphosphorylated tau in synaptic plasticity may extend to pathological processes.

Hyperthyroidism-Induced Upregulation of Neurodegeneration-Related Gene Expression in Metaplasticity-Induced Hippocampus.

INTRODUCTION: Thyroid hormones, which produce critical changes in our bodies even when their physiological levels alter slightly, are crucial hormones that influence gene transcription. Neuronal plasticity, on the other hand, requires both the activation of local proteins as well as protein translation and transcription in response to external signals. So far, no study has examined metaplastic long-term potentiation (LTP) and related gene expression levels in a hyperthyroid experimental model. METHODS: The Wistar male rats were administered 0.2 mg/kg/day of l-thyroxine for 21 days to induce hyperthyroidism. Perforant path was primed with 1-Hz low-frequency stimuli (LFS) for 900 s to investigate metaplasticity responses. The LFS was followed by high-frequency stimuli (HFS, 100 Hz) after 5 min. Excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude were recorded from the granule cell layer of the dentate gyrus. The mRNA levels of genes related to neurodegeneration (Gsk-3ß, Cdk5, Akt1, Mapt, p35, Capn1, Bace1, and Psen2) were measured using the RT-PCR method for the stimulated hippocampus. RESULTS: Similar to euthyroid rats, hyperthyroid animals had a lower EPSP slope and PS after LFS. Depression of EPSP prevented subsequently induced EPSP-LTP, although HFS was able to elicit PS-LTP despite depression of PS amplitude in both groups. Despite similarities in metaplastic LTP responses, these electrophysiological findings were accompanied by increased Akt, Bace1, Cdk5, and p35-mRNA expressions and decreased Gsk-3ß mRNA expression in hyperthyroid rats' hippocampus. CONCLUSION: These data support the view that in thyroid hormone excess, the mechanism that keeps synaptic efficacy within a dynamic range occurs concurrently with increased mRNA expression of neurodegeneration-related genes. Our study encourages further examination of the increased risk of neurodegenerative disease in hyperthyroidism.

Insulin-induced long-term potentiation in the dentate gyrus of hippocampal formation.

The discovery that brain areas involving in learning and memory express receptors for insulin hormone, led to the idea that insulin signaling may have a role in regulating cognitive function. Although previous studies have shown a role for insulin in regulation of the threshold of plasticity induction, no study has addressed whether insulin can induce a chemical plasticity per se. Young-adult male rats that are fed with standard diets with or without carbohydrate syrup (sucrose or high-fructose corn syrups) were enrolled in this study. Extracellular field potentials were recorded from the dentate gyrus in response to perforant pathway stimulation at 0.033 Hz in anesthetized rats. The slope of field excitatory postsynaptic potentials (fEPSPs) and the amplitude of population spike (PS) were measured 15 min after a 60-min infusion of insulin (500 nM), NT157 (an IRS inhibitor, 6 µM), alone or together, or physiological saline. mRNA expressions of insulin signaling proteins were measured by rt-PCR in the whole hippocampus. We did not observe any appreciable change in the fEPSP slope and the PS amplitude before and after saline infusion. However, intra-hippocampal insulin application results in the induction of LTP of fEPSP and of PS in the dentate gyrus. Insulin infusion together with NT157 inhibited fEPSP-LTP, but not PS-LTP, and rats that are fed with carbohydrate syrup did not express synaptic LTP. In rats that additional carbohydrate syrup is not given, insulin-induced LTP was accompanied with an increase in PI3K-mRNA, AKT-mRNA, and GSK-3ß-mRNA which was not observed when co-administered with NT157. The GSK-3ß-mRNA and IRS1-mRNA levels were found to be lower in rats that received supplemental carbohydrate and that not express insulin-induced synaptic LTP, compared to the rats expressing synaptic LTP and fed by standard diet. The results obtained provide a mechanistic link between insulin and synaptic plasticity. We concluded that insulin not only functions as a modulator of synaptic plasticity but also acts as a chemical inducer of LTP.

Sex-related differences in somatic plasticity and possible role of ERK1/2: An in-vivo study of young-adult rats.

The present study investigates sex differences in hippocampal functions in the context of synaptic plasticity, which is the cellular basis of learning and memory, and differences in the mitogen-activated protein kinase (MAPK) pathway that accompanies plasticity in young-adult rats. The long-term potentiation (LTP) and long-term depression (LTD) were induced by stimulating the perforant pathway (PP) and field potentials composed of the field excitatory post-synaptic potential (fEPSP) and population spike (PS) were recorded from the dentate gyrus (DG). Following the completion of the electrophysiological recordings, the hippocampi were removed bilaterally, and the protein and gene expression levels of the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK) and P38-MAPK were determined by Western blot analysis and real-time PCR, respectively. No significant difference was found in synaptic and neuronal function before (basal) and after high-frequency stimulation between male and female rats. Nevertheless, female, but not male, rats were able to express long term depression at the PP - DG synapses, suggesting that sex differences in plasticity are stimulation paradigm specific. MAPK1 expression was higher in males and MAPK3 expression was higher in females, but these differences disappeared after induction of plasticity in both sexes. While the expression of MAPK8 is influenced by sex, independent of the induction of plasticity, MAPK14 expression was down regulated by plasticity induction in females, but not males. No effect of sex, HFS and LFS on total and phosphorylated levels of MAPKs was found except phosphorylated ERK1/2. Phosphorylation of ERK1/2 was up regulated after LFS in male rats but did not change in female rats. These findings indicate that LFS-induced plasticity is differentially modulated between sexes, probably as a result of increased activation of ERK1/2 in male rats.

Tau protein is differentially phosphorylated in young- and old-aged rats with experimentally induced hyperthyroidism.

AIM: Aging involves progressive physiological changes, including thyroid dysfunction; thus, changes in plasma thyroid hormone (TH) level may affect neuronal function such as synaptic plasticity and Tau phosphorylation. However, how Tau protein is modulated in hyperthyroidism with aging is not clear. To clarify this issue, long-term potentiation (LTP) and accompanying phosphorylation of Tau protein in different residues were investigated in the hippocampus of young and old rats with experimentally induced hyperthyroidism. MATERIALS AND METHODS: The study was performed in vivo under urethane anesthesia on 2- and 12-month-old Wistar albino male rats. Field potentials, composed of a field of excitatory postsynaptic potential (fEPSP) and a population spike (PS), occurring in the hippocampal dentate gyrus region, were recorded by applying high-frequency stimulation (HFS) to the perforant pathway (100 Hz, four times at 5-min intervals) to induce LTP. Total-Tau and phosphorylated-Tau were measured in HFS-induced hippocampus by using western blotting. RESULTS: The TH suppressed hippocampal somatic LTP (PS) was suppressed with aging, and treatment improved this suppression in aged rats without any changes in synaptic LTP (fEPSP). The phosphorylation of Tau at Ser202/Thr205 and Thr231 residues was increased in aged control rats. Treatment of aged rats with l-thyroxine reduced the phosphorylation of Tau at these residues to the young control condition. CONCLUSION: Impaired LTP that occurs with aging may be among the underlying causes of dementia in relatively older ages, and l-thyroxine treatment restores this impaired LTP. In addition, the phosphorylation level of Tau epitopes known to play a role in the pathogenesis of Alzheimer's disease may support a critical role in the modulation of synaptic plasticity in hyperthyroidism.

Neurodegeneration-related genes are differentially expressed in middle-aged rats compared to young-adult rats having equal performance on long-term memory and synaptic plasticity.

The present study is concerned with assessing differences in plasticity-induced neurodegeneration-related gene expressions and tau phosphorylation between young-aged and middle-aged rats. The experiments were carried out in vivo under urethane anesthesia on adult male Wistar rats between the ages of 2-3 months and 11-12 months. Field potentials, composed of a field of excitatory-postsynaptic potential (fEPSP) and a population-spike (PS), were recorded from granule cells of the dentate gyrus. Plasticity was induced by high-frequency (HFS) or low frequency stimulation (LFS). mRNA of neurodegeneration-related genes and total-and phosphorylated-tau were measured in HFS-and LFS-induced hippocampus by using quantitative rt-PCR and Western blotting. In addition, naive rats (unstimulated) were tested for spatial learning and memory with a 5-day Morris water maze (MWM). HFS-induced LTP of PS had attenuated in middle-aged rats, but there were no gross differences in baseline synaptic function, HFS-induced fEPSP and LFS-induced fEPSP, and PS plasticity between young-aged and middle-aged rats. Relative to young-aged rats, in middle-aged rats, HFS-induced MAPT, CDK5, and AKT1 genes were more up regulated, while LFS-induced Bace1, PSEN2, CAPN1, ANXA, CDK5, and GSK-3ß genes were more down-regulated. Tau and p-tauThr231 were increased by HFS/LFS in the hippocampus of middle-aged rats compared to those of young-aged rats. In MWM, despite the difference in searching strategy of both age groups of rats, memory was not affected by age. Impaired long-term potentiation (LTP) and accompanying changes in intracellular biological markers may underlie in neurodegenerative disease characterized by dementia that occurs gradually later ages. However, these changes were not reflected in behavioral spatial memory.

Comparison of the subsequent LTP in hippocampal synapses primed by low frequency stimulations ranging from 0.5 to 5 Hz: An in vivo study.

According to the Bienenstock,Cooper, andMunro's (BCM) model, the level of afferent activity regulates the point of crossover from long-term depression (LTD) to long-term potentiation (LTP) of the active synapses. Although experimental results from the hippocampus and visual cortex have supported the BCM theory, it remains unclear whether previous activity of synapses regulates the output of neuron populations in vivo, as expected from the theory. In the present study, we studied the effects of priming stimulations at different frequencies (LFS, 0.5, 1, 2 and 5 Hz) on the magnitude of LTP at synaptic and somatic levels in the dentate gyrus of hippocampal formation. LTP in the dentate gyrus (DG) of LFS-primed or unprimed hippocampal formation was induced by delivering of tetanic stimulation to the perforant pathway (PP) in anesthetized rats. The field excitatory postsynaptic potential (fEPSP) slope and the population spike (PS) amplitude were evaluated to measure the magnitude of LTP. 1 Hz- and 5 Hz- (not 0.5 Hz and 2 Hz) stimulation of the PP led to an early LTD of fEPSP. The LTP of fEPSP was completely inhibited by previously delivering 0.5 Hz and 2 Hz LFS, but instead converted to LTD by 1 Hz LFS. However, none of the frequencies used was able to inhibit the LTP of PS. These results suggest that temporal dynamics which are critical to determine the direction of synaptic plasticity has no impact on the plasticity of neuronal output. We concluded that it is needed to explain why neuronal output does not behave within the framework of the BCM theory.

Effect of sodium selenite on synaptic plasticity and neurogenesis impaired by hypothyroidism.

AIM OF THE STUDY: We investigated protective effect of sodium selenite (Se) on hypothyroidism-induced impairments in, Morris water maze (MWM), long-term potentiation (LTP) and hippocampal neurogenesis male Wistar rats aged of 2 months. MATERIALS AND METHODS: Hypothyroidism was induced by administration of propylthiouracil (Ptu, 1 mg/kg/d) solution to the rats from postnatal day 60 for 81 days with or without Se (0.5mg/kg/d). Neurogenesis was examined by Ki-67 immunohistochemical staining. Se values on plasma and hippocampus were measured with inductively coupled plasma-mass spectrometry (ICP-MS). RESULTS: Measurement of fT3 and fT4 levels confirmed that the fT3 levels, but not fT4, in Ptu-treated rats (5435.44±816.05 fg/ml, p < 0.05) has returned to control values (8721.66±2567.68 fg/ml) by Se treatment (8661.65±711.43 fg/ml). Analysis of learning performance in water escape learning task showed that Se supplementation disappeared memory deficit in Ptu-treated rats as shown by significantly decreased time spent in the target quadrant (33.7±0.24% in control group; 26.1±0.48% in Ptu-group, p < 0.05; 33.9±0.44 in Ptu+Se group), although there was no significant difference among groups in any measurement of learning performance on the last day. Considering LTP, Se supplementation improved the deficit in synaptic plasticity in Ptu-treated rats, as shown by significant increase in the excitatory postsynaptic potential slope (% 243±31 in control group; 172±49 in Ptu-group, p < 0.05; 222±65 in Ptu+Se group) without affecting of the impairment in somatic plasticity. Se supplementation did not improve the decrease in the number of progenitor cells in the subgranular layer (SGL) of dentate gyrus (DG) of Ptu treated rats. CONCLUSIONS: These findings suggest that selenium supplementation in hypothyroid patients may improve learning and memory disorders with different physiological mechanisms.HighlightsSe increased serum fT3 levels and hippocampus Se levels in hypothyroid rats.Se attenuated impairment of population spike-LTP in hypothyroid ratsHypothyroidism disrupts neurogenesis process in the dentate gyrus of hippocampus.Se supplementation could not increase new born cells in hypothyroid rats.

Rho-associated kinases contribute to the regulation of tau phosphorylation and amyloid metabolism during neuronal plasticity.

BACKGROUND: Neural plasticity under physiological condition develops together with normal tau phosphorylation and amyloid precursor protein (APP) processing. Since restoration of PI3-kinase signaling has therapeutic potential in Alzheimer's disease, we investigated plasticity-related changes in tau and APP metabolism by the selective Rho-kinase inhibitor fasudil. METHODS: Field potentials composed of a field excitatory post-synaptic potential (fEPSP) and a population spike (PS) were recorded from a granule cell layer of the dentate gyrus. Plasticity of synaptic strength and neuronal function was induced by strong tetanic stimulation (HFS) and low-frequency stimulation (LFS) patterns. Infusions of saline or fasudil were given for 1 h starting from the application of the induction protocols. Total and phosphorylated tau levels and soluble APPα levels were measured in the hippocampus, which was removed after at least 1 h post-induction period. RESULTS: Fasudil infusion resulted in attenuation of fEPSP slope and PS amplitude in response to both HFS and LFS. Fasudil reduced total tau and phosphorylated tau at residue Thr181 in the HFS-stimulated hippocampus, while Thr231 phosphorylation was reduced by fasudil treatment in the LFS-stimulated hippocampus. Ser416 phosphorylation was increased by fasudil treatment in both HFS- and LFS-stimulated hippocampus. Fasudil significantly increased soluble APPα in LFS-stimulated hippocampus, but not in HFS-stimulated hippocampus. CONCLUSION: In light of our findings, we suggest that increased activity of Rho kinase could trigger a mechanism that goes awry during synaptic plasticity which is reversed by a Rho-kinase inhibitor. Thus, Rho-kinase inhibition might be a therapeutic target in cognitive disorders.

Long-term depression-related tau phosphorylation is enhanced by methylene blue in healthy rat hippocampus.

BACKGROUND: The present study examined whether inhibition of guanylate cyclase (GC) is associated with the plasticity-related microtubule-stabilizing protein tau phosphorylation in the dentate gyrus (DG) of hippocampal formation. METHODS: To address this issue, methylene blue (MB 50 µM) or saline was infused into the DG starting from the induction of long-term potentiation (LTP) or depression (LTD) for 1 h. Then, protein phosphatase 1 alpha (PP1α), glycogen synthase kinase 3 beta (GSK3ß), and tau total and phosphorylated protein levels were measured in these hippocampi using western blotting. LTP and LTD were induced by application of high- and low-frequency stimulation protocols (HFS and LFS), respectively. 5-min averages of the excitatory postsynaptic potential (EPSP) slopes and population spike amplitudes at the end of recording were averaged to measure the magnitude of LTP or LTD. RESULTS: Low-frequency stimulation protocols was unable to phosphorylate thr181 and thr231epitopes of tau, but possessed kinase activity similar to the HFS in phosphorylation of ser396 and ser416 epitopes. MB infusion during LTD induction attenuated LTD, prevented EPSP/spike dissociation and increased tau phosphorylation at ser396 and ser416 epitopes, without changing tau phosphorylation at thr181 and thr231 epitopes. Neither LTP nor LTP-related tau phosphorylation state was changed by MB infusion. CONCLUSION: Although MB can benefit to stabilize the balance between LTP and LTD, and to fix the increased spike wave discharges, it might trigger deregulation of tau phosphorylation, leading to the development of Alzheimer's disease by a mechanism that goes awry during induction of LTD. Thereby detailed studies to reveal more precise evidence for the use of MB in this disease are needed.

N-methyl-D-aspartate receptor blockade reduces plasticity-related tau expression and phosphorylation of tau at Ser416 residue but not Thr231 residue.

The molecular mechanisms regulating N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity are complex, and the contribution of Tau protein in the physiological process is not fully understood. Herein, we investigated whether the blockade of NMDA receptor activation might change Tau phosphorylation during long-term potentiation (LTP) and long-term depression (LTD) via contribution of GSK3ß as a major Tau kinase. For this, we recorded two components (synaptic and population spike components) of hippocampal field potential, which is evoked by the stimulation of the perforant pathway with high- and low-frequency stimulation (HFS and LFS). We found under a 20-µl volume of D-AP5 infusion lasting 1 h that,HFS caused significant synaptic depression, whereas LFS induced a synaptic potentiation. Both the HFS and LFS protocols resulted in a significant increase in population spike component but were characterized by a slow increase in amplitude that occurred with the LFS. D-AP5 attenuated HFS-induced population spike potentiation, but augmented LFS-induced population spike potentiation. The enzymatic activity of GSK-3ß was decreased by D-AP5 infusion in the hippocampus, indicating that NMDA receptor activity modulates the enzymatic activity of GSK-3ß. In addition, NMDA receptor blockade reduced tau expression and phosphorylation of tau at Ser416 residue, but not Thr231 residue. These findings confirm previous studies that D-AP5 applied to the DG in vivo blocks HFS-induced LTP, but we further also showed that the same dose of D-AP5 resulted in a slowly rising LFS-induced LTP and HFS-induced LTD. The formation of such an LTP, together with reduced enzymatic activity of GSK-3ß and tau phosphorylation at Ser416 epitope, can make it a candidate mechanism for prevention of taupathies.

Effect of L-thyroxine administration on long-term potentiation and accompanying mitogen-activated protein kinases in rats.

The present study investigated the differences in the activation of c-Jun NH2-terminal kinases (JNK), p38 mitogen-activated protein kinases (p38MAPK ), and extracellular signal-regulated kinases 1/2 (Erk1/2) 1 hr after the induction of long-term potentiation (LTP) between rats with hyperthyroidism that was produced at two different stages of development. Hyperthyroidism was produced in rats by daily injections of L-thyroxine (T4, ip., 0.2 mg/kg) to their dams for lactation period or to the rats itself during the young adult period. LTP was induced by application of high-frequency stimulation protocol. Five-min averages of the excitatory postsynaptic potential (EPSP) slopes and population spike (PS) amplitudes at the end of recording were averaged to measure the magnitude of LTP. Total and phosphorylated levels of Erk1/2, JNK, and P38-MAPK were assessed via western blotting in these hippocampi. LTP was found to be impaired in both groups of hyperthyroidisms, but this impairment observed together with increased expression and phosphorylation of ERK1/2, and increased phosphorylation of JNK in rats treated maternally with T4 compared to those treated adultly. These results suggest that excessiveness of thyroid hormone has longstanding effects on hippocampal function and may account for failed LTP in both early and relatively late stage of development depending on various molecular pathways, such as ERK1/2 and JNK.

Maternal L-thyroxine treatment during lactation affects learning and anxiety-like behaviors but not spatial memory in adult rat progeny.

BACKGROUND: The present study compared behavioral and molecular indicators of hippocampal function in L-thyroxine treated rats to determine whether thyroid hormone excessiveness produces relatively stable lifelong changes. METHODS: Hyperthyroidism was induced in rats by daily injections of L-thyroxine (0.2 mg/kg) to their dams for lactation period (MOH: maternal-onset hyperthyroidism) or to the rats itself during the young adult period (AOH: adult-onset hyperthyroidism; between the day 39-60). Spatial learning was assessed in the Morris Water Maze (MWM). Levels of type 2 and type 3 deiodinases, Erk1/2, JNK and P38MAPK were assessed via western blotting in the hippocampus of trained rats. Measurements were all done in rats aged 60-66 days. RESULTS: In MWM, maternally treated rats with L-thyroxine swam more away from the hidden platform, with showing more anxiety-like behavior, as compared to the rats treated or no treated with L-thyroxine in young adulthood. In spite of impaired acquisition, MOH group was not significantly different from the other groups in probe trial. In Western blot of the hippocampus, a decreased the expression of P38MAPK was found in rats treated with L-thyroxine in young adulthood period. However, maternal treatment with L-thyroxine resulted in an increased expression of Type 2 deiodinase and a tendency toward decreased expression of total and phosphorylated ERK1/2. No detectable band for type 3 deiodinase, p-JNK and p-P38MAPK was observed in all three groups. CONCLUSION: These results suggest that perinatal excessiveness of thyroid hormone has longstanding effects on hippocampal function and may account for memory problems experienced by adolescents with lactational hyperthyroidism.

Differential Effects of Pentoxifylline on Learning and Memory Impairment Induced by Hypoxic-ischemic Brain Injury in Rats.

OBJECTIVE: Hypoxic-ischemic (HI) brain injury in the human perinatal period often leads to significant long-term neurobehavioral dysfunction in the cognitive and sensory-motor domains. Using a neonatal HI injury model (unilateral carotid ligation followed by hypoxia) in postnatal day seven rats, the present study investigated the long-term effects of HI and potential behavioral protective effect of pentoxifylline. METHODS: Seven-day-old rats underwent right carotid ligation, followed by hypoxia (FiO2 = 0.08). Rats received pentoxifylline immediately after and again 2 hours after hypoxia (two doses, 60‒100 mg/kg/dose), or serum physiologic. Another set of seven-day-old rats was included to sham group exposed to surgical stress but not ligated. These rats were tested for spatial learning and memory on the simple place task in the Morris water maze from postnatal days 77 to 85. RESULTS: HI rats displayed significant tissue loss in the right hippocampus, as well as severe spatial memory deficits. Low-dose treatment with pentoxifylline resulted in significant protection against both HI-induced hippocampus tissue losses and spatial memory impairments. Beneficial effects are, however, negated if pentoxifylline is administered at high dose. CONCLUSION: These findings indicate that unilateral HI brain injury in a neonatal rodent model is associated with cognitive deficits, and that low dose pentoxifylline treatment is protective against spatial memory impairment.

Age-dependent evaluation of long-term depression responses in hyperthyroid rats: Possible roles of oxidative intracellular redox status.

BACKGROUND: According to the free radical theory, a gradual accumulation of the free radicals normally produced in the body underlies the changes associated with aging. Thyroid hormones (THs) are related to oxidative stress not only due to their stimulation of metabolism but also due to their effects on antioxidant mechanisms. Thyroid dysfunction increases with age; thus, changes in TH levels in elderly individuals could be a factor affecting the development of neurodegenerative diseases. However, the relationship is not always clear, based on current evidence regarding synaptic plasticity. METHODS: Hippocampal long-term depression (LTD) and oxidative status in the hippocampus were evaluated at two different ages (2-3 and 12-14 months) in male rats. Rats were administered 0.2 mg/kg/day of l-thyroxine for 21 days starting at postnatal day 40 to induce hyperthyroidism. LTD was induced in the dentate gyrus using low frequency stimulation of the perforant pathway. Spectrophotometry was performed to measure catalase (CAT), superoxide dismutase (SOD), and malondialdehyde (MDA) levels, glutathione peroxidase (GPx) activity, and total nitrite/nitrate (tNOx) and nitric oxide synthase (NOS) levels. RESULTS: A reliable LTD was elicited in young rats with hyperthyroidism, while the same protocol could induce a small magnitude of synaptic LTD in the absence of spike-LTD in control rats. In aged rats, controls did not express LTD, but a significant LTP of spike was induced in the absence of synaptic LTD in hyperthyroid rats. While CAT levels were significantly decreased, MDA levels were increased in the aged groups compared to the corresponding young groups. Young rats with euthyroidism had significantly lower GPx activity than each of the hyperthyroid groups. There was no significant difference in SOD levels among the groups. Compared with aged rats, young rats exhibited a hyperthyroidism-induced decrease in NOS levels. Nevertheless, neither the main effects of age and thyroxine administration nor the interaction between these factors reached significance for tNOx. CONCLUSION: These results indicate that hyperthyroidism-related changes in synaptic plasticity are modulated by aging. This modulation may explain the increased cognitive impairment in this disease at older ages, which probably depends on alterations in NOS levels.

Adult-Onset Hypothyroidism Alters the Metaplastic Properties of Dentate Granule Cells by Decreasing Akt Phosphorylation.

The expression of homosynaptic long-term depression (LTD) governs the subsequent induction of long-term potentiation (LTP) at hippocampal synapses. This process, called metaplasticity, is associated with a transient increase in the levels of several kinases, such as extracellular signal-regulated protein kinases 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), and Akt kinase. It has been increasingly realized that the chemical changes in the hippocampus caused by hypothyroidism may be the key underlying causes of the learning deficits, memory loss, and impaired LTP associated with this disease. However, the functional role of thyroid hormones in the "plasticity of synaptic plasticity" has only begun to be elucidated. To address this issue, we sought to determine whether the administration of 6-n-propyl-2-thiouracil (PTU) alters the relationship between priming and the induction of subsequent LTP and related signaling molecules. The activation of ERK1/2, JNK, and Akt was measured in the hippocampus at least 95 min after priming onset. We found that priming stimulation at 5 Hz for 3 s negatively impacted the induction of LTP by subsequent tetanic stimulation in hypothyroid animals, as manifested by a more rapid decrease in the fEPSP slope and population spike amplitude. This phenomenon was accompanied by lower levels of phosphorylated Akt in the surgically removed hippocampus of the hypothyroid rats compared to the euthyroid rats. The metaplastic response and the expression of these proteins in the 1-Hz-primed hippocampus were not different between the two groups. These observations suggest that decreased PI3K/Akt signaling may be involved in the compromised metaplastic regulation of LTP observed in hypothyroidism, which may account for the learning difficulties/cognitive impairments associated with this condition.

Effects of Chronic and Acute Lithium Treatment on the Long-term Potentiation and Spatial Memory in Adult Rats.

OBJECTIVE: Although, accumulating evidence is delineating a neuroprotective and neurotrophic role for lithium (Li), inconsistent findings have also been reported in human studies especially. Moreover, the effects of Li infusion into the hippocampus are still unknown. The aims of this work were (a) to assess whether basal synaptic activity and long-term potentiation (LTP) in the hippocampus are different in regard to intrahippocampal Li infusion; (b) to assess spatial learning and memory in rats chronically treated with LiCO3 in the Morris water maze. METHODS: Field potentials were recorded form the dentate gyrus, stimulating perforant pathways, in rats chronically (20 mg/kg for 40 days) or acutely treated with LiCO3 and their corresponding control rats. In addition, performance of rats in a Morris water maze was measured to link behaviour of rats to electrophysiological findings. RESULTS: LiCO3 infusion into the hippocampus resulted in enhanced LTP, especially in the late phases, but attenuated LTP was observed in rats chronically treated with Li as compared to controls. Li-treated rats equally performed a spatial learning task, but did spend less time in target quadrant than saline-treated rats in Morris water maze. CONCLUSION: Despite most data suggest that Li always yields neuroprotective effects against neuropathological conditions; we concluded that a 40-day treatment of Li disrupts hippocampal synaptic plasticity underlying memory processes, and that these effects of prolonged treatment are not associated with its direct chemical effect, but are likely to be associated with the molecular actions of Li at genetic levels, because its short-term effect preserves synaptic plasticity.

Deficiency but Not Supplementation of Selenium Impairs the Hippocampal Long-Term Potentiation and Hippocampus-Dependent Learning.

Among the chemical factors that have been implicated in the etiology of dementia, recent concern has focused on both increased and decreased exposure to the metalloid selenium (Se). This report describes the molecular, behavioral, and electrophysiological analysis of rats that were fed with Se-free chow and Se-enriched tap water for 21 days. Three groups were produced, feeding them on a deficient diet with different Selenium content. Hippocampus-dependent spatial learning was measured using the water maze. Long-term potentiation (LTP) was recorded in the hippocampal dentate gyrus to assess how memory is formed at the cellular level. Hippocampal Se levels were measured in trained rats by using inductively coupled plasma mass spectrometry. Phosphorylated and total tau levels were measured in whole hippocampus by Western blot. An impairment of learning of rats feeding with Se-deficient diet was accompanied by attenuated LTP, and increased ratio of p231Tau-to- and decreased ratio of p416Tau-to-Tau in the non-stimulated hippocampus, despite no significant change was observed in Se levels of hippocampus and plasma. Se supplementation resulted in an increase in both tissues and an increase in the ratio of p231Tau-to-Tau in the non-stimulated hippocampus but did not change learning performance and LTP. Despite impaired learning and LTP, no group differed in probe trial and in the fraction of phosphorylated tau in LTP-induced hippocampus. Reduced level of selenium would probably result in reduced synaptic plasticity as well as impairment of learning ability, suggesting requirement of Se for normal synaptic function.

Depotentiation of Long-Term Potentiation Is Associated with Epitope-Specific Tau Hyper-/Hypophosphorylation in the Hippocampus of Adult Rats.

It is well-known that some kinases which are involved in the induction of synaptic plasticity probably modulate tau phosphorylation. However, how depression of potentiated synaptic strength contributes to tau phosphorylation is unclear because of the lack of experiments in which depotentiation of LTP was induced. Field excitatory postsynaptic potential (fEPSP) and population spike (PS) were recorded from the dentate gyrus in response to the perforant pathway stimulation. To induce LTP, high-frequency stimulation (HFS) was used, while, for depotentiation of LTP, low-frequency stimulation (LFS) consisting of 900 pulses at 1 Hz was applied 5 min after tetanization. In some experiments, a neutral protocol at 0.033 Hz was applied throughout the experiment without any induction of synaptic plasticity. One-hertz depotentiation protocol was able to decrease fEPSP slope which was previously increased by HFS, whereas no significant change in fEPSP slope and PS amplitude was observed in neutral protocol experiments. Relative to saline infusion, LTP was lower in magnitude and was more reversed by subsequent LFS in the presence of ERK1/2 inhibitor. Western blot experiments indicated that tau protein was hyperphosphorylated at ser416 epitope but rather hypophosphorylated at thr231 epitope in the whole hippocampus upon depotentiation of LTP. These changes concomitantly occurred with a notable increase in the levels of total tau and in the levels of phosphorylated form of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). ERK1/2 inhibition resulted in a decrease in phosphorylation of tau at p416Tau when ERK1/2 was inhibited. These findings indicate that some forms of long-term plastic changes might be related with epitope-specific tau phosphorylation and ERK1/2 activation in the hippocampus. Therefore, we emphasize that tau may be crucial for physiological learning as well as Alzheimer's disease pathology.

The effects of intra-hippocampal L-thyroxine infusion on long-term potentiation and long-term depression: A possible role for the αvß3 integrin receptor.

Although the effects of long-term experimental dysthyroidism on long-term potentiation (LTP) and long-term depression (LTD) have been documented, the relationship between LTP/LTD and acute administration of L-thyroxine (T4) has not been described. Here, we investigated the effects of intra-hippocampal administration of T4 on synaptic plasticity in the dentate gyrus of the hippocampal formation. After a 15-minute baseline recording, LTP and LTD were induced by application of high- and low-frequency stimulation protocols, respectively. Infusions of saline or T4 and tetraiodothyroacetic acid (tetrac), a T4 analog that inhibits binding of iodothyronines to the integrin αvß3 receptor, either alone or together, were made during the stimulation protocols. The averages of the excitatory postsynaptic potential (EPSP) slopes and population spike (PS) amplitudes, between 55 to 60 minutes, were used as a measure of the LTP/LTD magnitude and were analyzed by two-way univariate ANOVA with T4 and tetrac as between-subjects factors. The input-output curves of the infusion groups were comparable to each other, as shown by the non significant interaction observed between stimulus intensity and infused drug. The magnitude of the LTP in T4-infused rats was significantly lower as compared to saline-infused rats. Both the PS amplitude and the EPSP slope were depressed more markedly with T4 infusion than with saline, tetrac, and T4 + tetrac infusion. Data of this study provide in vivo evidence that T4 can promote LTD over LTP via the integrin αvß3 receptor, and that the effect of endogenous T4 on this receptor can be suppressed by tetrac in the hippocampus. © 2016 Wiley Periodicals, Inc.