Long-term depression at the medial perforant path-granule cell synapse in developing rat dentate gyrus

Plasticity in the Hippocampus, Neurogenesis and Drugs of Abuse

Synaptic plasticity in the hippocampus assists with consolidation and storage of long-lasting memories. Decades of research has provided substantial information on the cellular and molecular mechanisms underlying synaptic plasticity in the hippocampus, and this review discusses these mechanisms in brief. Addiction is a chronic relapsing disorder with loss of control over drug taking and drug seeking that is caused by long-lasting memories of drug experience. Relapse to drug use is caused by exposure to context and cues associated with the drug experience, and is a major clinical problem that contributes to the persistence of addiction. This review also briefly discusses some evidence that drugs of abuse alter plasticity in the hippocampus, and that development of novel treatment strategies that reverse or prevent drug-induced synaptic alterations in the hippocampus may reduce relapse behaviors associated with addiction.

Effects of Ethanol on Synaptic Plasticity and NMDA Currents in the Juvenile Rat Dentate Gyrus

Background and Objectives:

We examined how acute ethanol (EtOH) exposure affects long term depression (LTD) in the dentate gyrus (DG) of the hippocampus in juvenile rats. EtOH is thought to directly modulate n-methyl-D-aspartate receptor (NMDAr) currents, which are believed important for LTD induction. LTD in turn is believed to play an important developmental role in the hippocampus by facilitating synaptic pruning.

Methods:

Hippocampal slices (350μm) were obtained at post-natal day (PND) 14, 21, or 28. Field EPSPs (excitatory post-synaptic potential) or whole-cell EPSCs (excitatory post-synaptic conductance) were recorded from the DG (dentate gyrus) in response to medial perforant path activation. Low-frequency stimulation (LFS; 900 pulses; 120 s pulse) was used to induce LTD.

Results:

Whole-cell recordings indicated that EtOH exposure at 50mM did not significantly impact ensemble NMDAr EPSCs in slices obtained from animals in the PND14 or 21 groups, but it reliably produced a modest inhibition in the PND28 group. Increasing the concentration to 100 mM resulted in a modest inhibition of NMDAr EPSCs in all three groups. LTD induction and maintenance was equivalent in magnitude in all three age groups in control conditions, however, and surprisingly, NMDA antagonist AP5 only reliably blocked LTD in the PND21 and 28 age groups. The application of 50 mM EtOH attenuated LTD in all three age groups, however increasing the concentration to 100 mM did not reliably inhibit LTD.

Conclusions:

These results indicate that the effect of EtOH on NMDAr-EPSCs recorded from DGCs is both age and concentration dependent in juveniles. Low concentrations of EtOH can attenuate, but did not block LTD in the DG. The effects of EtOH on LTD do not align well with it’s effects on NNMDA receptors.

Endocannabinoid receptors contribute significantly to multiple forms of long-term depression in the rat dentate gyrus

Cannabinoid receptors are widely expressed throughout the hippocampal formation, but are particularly dense in the dentate gyrus (DG) subregion. We, and others, have shown in mice that cannabinoid type 1 receptors (CB1Rs) are involved in a long-term depression (LTD) that can be induced by prolonged 10 Hz stimulation of the medial perforant path (MPP)-granule cell synaptic input to the DG. Here, we extend this work to examine the involvement of CB1Rs in other common forms of LTD in the hippocampus of juvenile male and female Sprague–Dawley rats (Rattus norvegicus). We found, as in mice, that prolonged 10 Hz stimulation (6000 pulses) could reliably induce a form of LTD that was dependent upon CB1R activation. In addition, we also discovered a role for both CB1R and mGluR proteins in LTD induced with 1 Hz low-frequency stimulation (1 Hz-LTD; 900 pulses) and in LTD induced by bath application of the group I mGluR agonist (RS)-3,5-Dihydroxyphenylglycine (DHPG; DHPG-LTD). This study elucidates an essential role for endocannabinoid receptors in a number of forms of LTD in the rat DG, and identifies a novel role for CB1Rs as potential therapeutic targets for conditions that involve impaired LTD in the DG.

Impaired Bidirectional Synaptic Plasticity in Juvenile Offspring Following Prenatal Ethanol Exposure

BACKGROUND

The hippocampus is particularly vulnerable to the teratogenic effects of prenatal ethanol exposure (PNEE), and hippocampal structural and functional deficits are thought to contribute to the learning and memory deficits that are a hallmark feature of fetal alcohol spectrum disorders.

METHODS

Sprague Dawley dams were exposed to a liquid diet that contained EtOH (35.5% EtOH-derived calories) throughout gestation, and then, PNEE juvenile (P21-28) male and female offspring were used for in vitro electrophysiological recordings. We examined long-term potentiation (LTP), long-term depression (LTD), and depotentiation in the medial perforant path input to the dentate gyrus (DG) to determine the impact of PNEE on the dynamic range of bidirectional synaptic plasticity in both sexes.

RESULTS

PNEE reduced the responsiveness of the DGs of male but not in female offspring, and this effect was no longer apparent when GABAergic signaling was inhibited. There was also a sex-specific LTD impairment in males, but increasing the duration of the conditioning stimulus could overcome this deficit. The magnitude of LTP was also reduced, but in both sexes following PNEE. This appears to be an increase in the threshold for induction, not in capacity, as the level of LTP induced in PNEE animals was increased to control levels when additional conditioning stimuli were administered.

CONCLUSIONS

These data are the first to describe, in a single study, the impact of PNEE on the dynamic range of bidirectional synaptic plasticity in the juvenile DG in both males and in females. The data suggest that PNEE increases the threshold for LTP in the DG in both sexes, but produces a sex-specific increase in the threshold for LTD in males These alterations reduce the dynamic range for synaptic plasticity in both sexes.

GluN2A-/- Mice Lack Bidirectional Synaptic Plasticity in the Dentate Gyrus and Perform Poorly on Spatial Pattern Separation Tasks

The different secondary subunits of the N-methyl-d-aspartate (NMDA) receptor each convey unique biophysical properties to the receptor complex, and may be key in determining the functional role played by NMDA receptors. In the hippocampus, the GluN2A and GluN2B subunits are particularly abundant; however, their exact roles in synaptic plasticity and behavior remain controversial. Here, we show that mice carrying a deletion for the GluN2A subunit (GluN2A(-/-)) demonstrate a severely compromised NMDA to AMPA receptor current ratio in granule cells from the dentate gyrus (DG), while granule cell morphology is unaltered. This deficit is accompanied by significant impairments in both LTP and LTD in the DG, whereas only minor impairments are observed in the CA1. In accordance with these hippocampal region-specific deficits, GluN2A(-/-) mice show impaired performance on the DG-associated task of spatial pattern separation. In contrast, GluN2A(-/-) mice show no deficit in temporal pattern separation, a process associated with CA1 functioning. Thus, our results establish the GluN2A subunit as a significant contributor to both bidirectional synaptic plasticity and spatial pattern separation in the DG.

Effects of ceftriaxone on the acquisition and maintenance of ethanol drinking in peri-adolescent and adult female alcohol-preferring (P) rats

Increased glutamatergic neurotransmission appears to mediate the reinforcing properties of drugs of abuse, including ethanol (EtOH). We recently reported that the administration of ceftriaxone (CEF), a β-lactam antibiotic known to upregulate glutamate transporter 1 (GLT1) levels/activity, decreased the maintenance of EtOH intake in adult male alcohol-preferring (P) rats. In the present study, we tested whether CEF administration would reduce the acquisition and maintenance of EtOH drinking in adolescent and adult female P rats. The rats were treated with saline or 200mg/kg ceftriaxone for 7 days (starting at 35 or 75 days old, respectively) followed by the EtOH acquisition test. Five weeks later the effects of CEF were examined regarding the maintenance of EtOH intake. For the maintenance test, half of the animals that received CEF during acquisition received CEF for 7 days and the other half received saline for 7 days. Saline-treated acquisition animals were treated similarly. The results indicated that pretreatment with ceftriaxone reduced the maintenance of EtOH intake in both animals that started as adolescents and those that started as adults. However, the beneficial effect of CEF was more pronounced in rats pretreated with CEF as adults compared with rats pretreated as adolescents. Reductions in EtOH intake by ceftriaxone were paralleled by an upregulation of GLT1 protein levels in both the nucleus accumbens (∼25% in rats starting at both ages) and prefrontal cortex (∼50% in rats starting as peri-adolescents and ∼65% in those starting as adults). These findings provide further support for GLT1-associated mechanisms in high alcohol-consuming behavior, and hold promise for the development of effective treatments targeting alcohol abuse and dependence.

Postnatal alterations in induction threshold and expression magnitude of long-term potentiation and long-term depression at hippocampal synapses

Activity-dependent synaptic plasticity refines neural networks during development and subserves information processing in adulthood. Previous research has revealed postnatal alterations in synaptic plasticity at nearly all forebrain synapses, suggesting different forms of synaptic plasticity may contribute to network development and information processing. To assess possible relationships between modifications in synaptic plasticity and maturation of cognitive ability, we examined excitatory synaptic function in area CA1 of the mouse hippocampus ∼3 weeks of age, when hippocampal-dependent learning and memory abilities first emerge. Long-term potentiation (LTP) and depression (LTD) of synaptic efficacy were observed in slices from juvenile animals younger than 3 weeks of age. Both pre- and postsynaptic mechanisms supported LTP and LTD in juveniles. After the third postnatal week, the magnitude of LTP was reduced and the threshold for postsynaptic induction was reduced, but the threshold for presynaptic induction was increased. The reduced threshold for postsynaptic LTP appeared to be due, partly, to an increase in baseline excitatory synaptic strength, which likely permitted greater postsynaptic depolarization during induction. Low frequency stimulation did not induce LTD at this more mature stage, but it blocked subsequent induction of LTP, suggesting metaplastic differences across age groups. Late postnatal modifications in activity-dependent synaptic plasticity might reflect attenuation of mechanisms more closely tied to network formation (presynaptic potentiation and pre- and postsynaptic depression) and unmasking of mechanisms underlying information processing and storage (associative postsynaptic potentiation), which likely impact the integrative capacity of the network and regulate the emergence of adult-like cognitive abilities.

Reversal of hippocampal neuronal maturation by serotonergic antidepressants

Serotonergic antidepressant drugs have been commonly used to treat mood and anxiety disorders, and increasing evidence suggests potential use of these drugs beyond current antidepressant therapeutics. Facilitation of adult neurogenesis in the hippocampal dentate gyrus has been suggested to be a candidate mechanism of action of antidepressant drugs, but this mechanism may be only one of the broad effects of antidepressants. Here we show a distinct unique action of the serotonergic antidepressant fluoxetine in transforming the phenotype of mature dentate granule cells. Chronic treatments of adult mice with fluoxetine strongly reduced expression of the mature granule cell marker calbindin. The fluoxetine treatment induced active somatic membrane properties resembling immature granule cells and markedly reduced synaptic facilitation that characterizes the mature dentate-to-CA3 signal transmission. These changes cannot be explained simply by an increase in newly generated immature neurons, but best characterized as "dematuration" of mature granule cells. This granule cell dematuration developed along with increases in the efficacy of serotonin in 5-HT(4) receptor-dependent neuromodulation and was attenuated in mice lacking the 5-HT(4) receptor. Our results suggest that serotonergic antidepressants can reverse the established state of neuronal maturation in the adult hippocampus, and up-regulation of 5-HT(4) receptor-mediated signaling may play a critical role in this distinct action of antidepressants. Such reversal of neuronal maturation could affect proper functioning of the mature hippocampal circuit, but may also cause some beneficial effects by reinstating neuronal functions that are lost during development.

mGluR-mediated and endocannabinoid-dependent long-term depression in the hilar region of the rat dentate gyrus

We report that bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) causes acute inhibition of evoked IPSCs recorded from hilar mossy cells, and that significant long-term depression (LTD) of synaptic transmission remains following washout of DHPG. Subsequent experiments using minimal stimulation techniques revealed that expression of both acute and long-term effects of DHPG are restricted to a subset of GABAergic afferents that are also sensitive to depolarization-induced suppression of inhibition (DSI). Experiments with a selective CB1 antagonist and with transgenic animals lacking CB1 receptors indicate that all effects of DHPG, like DSI, depend on activation of CB1 receptors. Further work with selective mGluR antagonists suggests a direct involvement of mGluR1 receptors. Interestingly, we also report that induction of LTD under our experimental conditions does not require prior direct somatic depolarization via the patch pipette and does not appear to depend critically on the level of activity in incoming GABAergic afferents. Collectively, these results represent the first characterization of mGluR-mediated and endocannabinoid-dependent LTD in the hilar region of the dentate gyrus. The dentate gyrus is thus one of relatively few areas where this mechanism has clearly been demonstrated to induce long-term modulation of inhibitory synaptic transmission.

The CB1 receptor antagonist, SR141716A, prevents high-frequency stimulation-induced reduction of feedback inhibition in the rat dentate gyrus following perforant path stimulation in vivo

Endocannabinoids acting through CB(1) receptors are thought to regulate GABAergic and glutamatergic neurotransmission and may modulate long-term potentiation (LTP). High-frequency stimulation (HFS) of the medial perforant path to induce LTP was studied in the dentate gyrus with or without the selective CB(1) receptor antagonist, SR141716A in isoflurane-anaesthetised rats. HFS significantly increased the slope of the field excitatory post-synaptic potential (fEPSP) and the amplitude of the population spike (PS; P<0.001 in each case; n=6). Following administration of SR141716A, HFS no longer increased fEPSP slope, whereas PS amplitude potentiation remained significant (P<0.0001; n=6). Paired-stimuli revealed that HFS significantly reduced inhibition observed at intervals of 10 ms (P<0.01; n=6), and produced a leftward shift of the interval-inhibition curve (P<0.05; n=6). Following administration of SR141716A, HFS no longer reduced inhibition at the 10 ms interval, but a leftward shift in the interval-inhibition curve was still observed (P<0.05, n=6). These results indicate that LTP in the dentate gyrus reduces local circuit inhibition, consistent with a reduction of GABA release and/or duration of the post-synaptic GABA-receptor mediated response. Selective effects of SR141716A on the degree, but not the timecourse, of paired-pulse inhibition suggest that the reduction in GABA release following LTP induction is due to CB(1) activation. Results also suggest that CB(1) receptors contribute to HFS-induced potentiation of the fEPSP, but not to the mechanism underlying potentiation of PS amplitude. We suggest that CB(1) activation during HFS of the medial perforant path increases glutamate release from perforant path synapses, but inhibits release of GABA from local circuit interneurons.

Effects of carbachol on lead-induced impairment of the long-term potentiation/depotentiation in rat dentate gyrus in vivo

The present study aims at evaluating the impairment of LTP and depotentiation (DP) of LTP induced by acute lead exposure, and the effects of peripheral carbachol (CCh) application on LTP/DP of acute and chronic lead-exposed rats in dentate gyrus in vivo. Rats (80-100 days) were acutely exposed to lead by intraperitoneal injection of 0.2% lead acetate (PbAc) solution (1.5mg/100g) and/or CCh (1 micro g/100g). Rats were chronically exposed to lead from parturition through adulthood (80-100 days) by the drinking of 0.2% PbAc solution and/or CCh (1 micro g/100g) chronic intraperitoneal injection one week. The input-output (I/O) function, paired-pulse reaction (PPR), excitatory postsynaptic potential (EPSP) and population spike (PS) amplitude were measured in response to stimulation applied to the lateral perforant path. Results showed that: first, acute lead exposure significantly depressed the amplitudes of LTP/DP of both EPSP slope and PS amplitude. Second, CCh significantly increased the amplitudes of both EPSP LTP/DP and PS LTP of acute Pb-exposed rats. After CCh treatment, the magnitudes of EPSP LTP/DP and PS LTP of acute Pb-exposed rats showed no significant difference with controls. Third, Chronic CCh application also reversed chronic Pb-induced impairment of PS LTP and EPSP DP of LTP. As CCh does not cross blood-brain barrier in healthy animals, the data suggest that CCh may traverse BBB in Pb-exposed animals and cure Pb-induced dysfunction of learning and memory.

Comparison of cellular mechanisms of long-term depression of synaptic strength at perforant path-granule cell and Schaffer collateral-CA1 synapses

This chapter compares the cellular mechanisms that have been implicated in the induction and expression of long-term depression (LTD) at Schaffer collateral-CA1 synapses to perforant path-dentate gyrus (DG) synapses. In general, Schaffer collateral LTD and long-term potentiation (LTP) both appear to be a complex combination of many alterations in synaptic transmission that occur at both presynaptic and postsynaptic sites, while at perforant path synapses, most evidence has focused on postsynaptic long-term alterations. Within the DG, the medial perforant path is far more studied than lateral perforant path synapses, where most evidence relates to the induction of heterosynaptic LTD at lateral perforant path synapses when LTP is induced in the medial perforant path. Of course, there remain many other classes of synapses in the DG where synaptic plasticity, including LTD, have been largely neglected. It is clear that a better understanding of the range of DG loci where long-lasting activity-dependent plasticity, both LTD and LTP, are expressed will be essential to improve our understanding of the cognitive roles of such DG plasticity.

Persistent (>24h) long-term depression in the dentate gyrus of freely moving rats is not dependent on activation of NMDA receptors, L-type voltage-gated calcium channels or protein synthesis

Hippocampal long-term depression (LTD) comprises a persistent reduction of synaptic strength that is typically induced by low frequency stimulation (LFS). Although LTD has been described for the dentate gyrus in vitro, this phenomenon in the dentate gyrus of the intact animal is less well understood. In the current study, we investigated the contribution of NMDA receptors, L-type voltage gated calcium channels and protein synthesis to LFS-induced LTD in the dentate gyrus of freely moving rats. Animals were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. LTD persisted for at least 24h, and was unaffected by prior treatment with the NMDA receptor antagonists AP5 or ifenprodil, which, in contrast, prevented LTP. Neither the L-type voltage-gated calcium channel antagonist, methoxyverapamil, nor the protein translation inhibitors, anisomycin or emetine had an effect on the profile of LTD. Our results suggest that NMDA receptors and L-type voltage-gated calcium channels are not involved in the induction of LTD in the dentate gyrus in vivo. Intriguingly, persistent LTD can be established without the synthesis of new proteins, suggesting that in the dentate gyrus, alternative mechanisms exist for the sustainment of enduring LTD.

Induction and expression mechanisms of postsynaptic NMDA receptor-independent homosynaptic long-term depression

The induction of long-term depression (LTD) can be divided into two main forms, one dependent upon activation of postsynaptic NMDAR, and another independent of postsynaptic NMDAR. Non-postsynaptic NMDAR-LTD (non-NMDAR-LTD) occurs in many regions of the brain, and encompasses a wide variety of induction and expression mechanisms. In this article, the induction and expression mechanisms of such LTD in over 10 brain regions are described, with a number of common mechanisms compared across a large range of types of LTD. The article describes the involvement of different presynaptic or postsynaptic receptors in the induction of non-NMDAR-LTD, especially metabotropic glutamate receptors, cannabinoid receptors and dopamine receptors. An increase in presynaptic or postsynaptic intracellular Ca concentration is a key event in induction, commonly followed by activation of certain kinases, especially PKC, p38 MAPK and ERK. Expression mechanisms are either presynaptic via a reduction in release probability, or postsynaptic involving a decrease in AMPAR via phosphorylation of a glutamate receptor subunit, especially GluR2, followed by clathrin-mediated endocytosis. Retrograde signalling from postsynaptic to presynaptic occurs when induction is postsynaptic and expression is presynaptic.

Group II mGluR-induced long term depression in the dentate gyrus in vivo is NMDA receptor-independent and does not require protein synthesis

Long term depression (LTD) can be induced by low frequency stimulation (LFS) as well as by agonist activation of neurotransmitter receptors. Group II metabotropic glutamate receptors (mGluRs) play an essential role in the regulation of electrically-induced LTD in the hippocampus in vivo: LTD is inhibited by antagonists, and enhanced by agonists of group II mGluRs. Here we investigated induction of LTD by activation of group II mGluRs as well as the cellular mechanisms which might mediate group II mGluR-induced LTD. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Drug application was made through a cannula implanted into the ipsilateral cerebral ventricle. LTD could be induced by agonist activation of either group II mGluRs, or the group II mGluR subtype, mGluR3. Both, group II mGluR-induced LTD and mGluR3-induced LTD were not abolished by mRNA/protein synthesis inhibition. Furthermore, mGluR3-induced LTD was not inhibited by NMDA receptor antagonists or altered by L-type voltage-gated calcium channel blockers. Our data suggest that sole activation of group II mGluRs can mediate LTD in vivo. Intriguingly, this form of LTD is not dependent on protein synthesis or activation of NMDA receptors.

In vivo recordings of long-term potentiation and long-term depression in the dentate gyrus of the neonatal rat

Previous in vitro studies demonstrated that long-term potentiation (LTP) could be elicited at medial perforant path (MPP) synapses onto hippocampal granule cells in slices from 7-day-old rats. In contrast, in vivo studies suggested that LTP at perforant path synapses could not be induced until at least days 9 or 10 and then in only a small percentage of animals. Because several characteristics of the oldest granule cells are adult-like on day 7, we re-examined the possibility of eliciting LTP in 7-day-old rats in vivo. We also recorded from 8- and 9-day-old rats to further elucidate the occurrence and magnitude of LTP in neonates. With halothane anesthesia, all animals in each age group exhibited synaptic plasticity of the excitatory postsynaptic potential following high-frequency stimulation of the MPP. In 7-day-old rats, LTP was elicited in 40% of the animals and had an average magnitude of 143%. Long-term depression (LTD) alone (magnitude of 84%) was induced in 40% of the animals, while short-term potentiation (STP) alone (magnitude of 123%) was induced in 10%. STP followed by LTD was elicited in the remaining 10%. Data were similar for all ages combined. In addition, the N-methyl-d-aspartate (NMDA) antagonist (R,S)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) blocked the occurrence of LTP at each age and doubled the percentage of animals expressing LTD alone for all ages combined. These results demonstrate that tetanic stimulation can elicit LTP or LTD at MPP synapses in 7-day-old rats, supporting our premise that at least a portion of the dentate gyrus is functional at this early age.

Effects of stimulus frequency and age on bidirectional synaptic plasticity in the dentate gyrus of freely moving rats

We investigated the frequency-dependent transition from homosynaptic long-term depression (LTD) to long-term potentiation (LTP) at the lateral perforant pathway/dentate gyrus synapse in adult (90 days of age) and immature (15 days of age) awake, freely moving rats. Dentate-evoked field potentials were recorded and analyzed using the population spike amplitude and the field EPSP slope measures following sustained stimulation (900 pulses) of the lateral perforant pathway at various frequencies (1, 3, 7, 30, 50, or 200 Hz). Our results indicate that both the strength and the direction (LTP or LTD) of synaptic plasticity vary as a function of activation frequency: sustained low-frequency stimulation ranging from 1 to 7 Hz results in depression of activated synapses, whereas high-frequency stimulation (30-200 Hz) produces potentiation. In addition, a significant (P < 0.01) ontogenetic shift in the frequency of transition from LTD to LTP was observed; the transition frequency in immature animals was significantly lower than that obtained in adult animals. These observations agree strongly with the prediction of the Bienenstock-Cooper-Munro theory of synapse modification, indicating perhaps a neurophysiological basis for this theoretical model of learning in the dentate gyrus of awake behaving rats.

Maturation of granule cell dendrites after mossy fiber arrival in hippocampal field CA3

Most granule neurons in the rat dentate gyrus are born over the course of the first 2 postnatal weeks. The resulting heterogeneity has made it difficult to define the relationship between dendritic and axonal maturation and to delineate a time course for the morphological development of the oldest granule neurons. By depositing crystals of the fluorescent label Dil in hippocampal field CA3, we retrogradely labeled granule neurons in fixed tissue slices from rats aged 2-9 days. The results showed that all labeled granule cells, regardless of the age of the animal, exhibited apical dendrites. On day 2, every labeled neuron had rudimentary apical dendrites, and a few dendrites on each cell displayed immature features such as growth cones, varicosities, and filopodia. Some cells displayed basal dendrites. By day 4, the most mature granule neurons had longer and more numerous apical branches, as well as various immature features. Most had basal dendrites. On days 5 and 6, the immature features and the basal dendrites had begun to regress on the oldest cells, and varying numbers of spines were present. On day 7, the first few adult-like neurons were seen: immature features and basal dendrites had disappeared, all dendrites reached the top of the molecular layer, and the entire dendritic tree was covered with spines. These data show that dendritic outgrowth occurs before, or concurrent with, axon arrival in the CA3 target region, and that adult-like granule neurons are present by the end of the first week.

Differential phosphorylation at serine sites in glutamate receptor-1 within neonatal rat hippocampus

We used immunoblot analysis to investigate the phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA)-receptor glutamate receptor-1 (GluR1) and related protein kinases in rat hippocampus on postnatal days (PND) 1-28. Total GluR1 expression increased up to PND 9, and stayed high thereafter. The proportions of the forms of GluR1 phosphorylated at serines (S) 845 and S831, which were both high at birth, decreased at different rates: phosphorylated S831 decreased rapidly after PND 7, and was almost zero after PND 21, while phosphorylated S845 decreased slowly, and after PND 14 stayed at one third of its PND 1 level. The expression patterns of cyclic AMP-dependent protein kinase (PKA) and protein kinase C (PKC), but not that of calcium/calmodulin-dependent protein kinase II, were similar to those of the forms of GluR1 phosphorylated at S845 and S831, respectively. Thus, the status of GluR1 phosphorylation, through PKA and PKC modulation, may contribute to the development of hippocampal synaptic plasticity.

The influence of developmental period of aluminum exposure on synaptic plasticity in the adult rat dentate gyrus in vivo

Previous studies from our group have demonstrated that chronic aluminum exposure from parturition throughout life impairs both long-term potentiation (LTP) and long-term depression (LTD) of the excitatory postsynaptic potential (EPSP) slope and reduces the population spike (PS) amplitude in the rat dentate gyrus in vivo. The present study sought to extend these findings by evaluating the developmental periods critical for aluminum-induced impairment of synaptic plasticity. Rats were exposed to aluminum (gestational, lactational and postlactational) through drinking 0.3% aluminum chloride in water over different developmental intervals: (1) prenatal exposure; (2) beginning from birth and terminating at weaning; (3) beginning at weaning throughout life; (4) beginning at birth and continuing throughout life. As adults (postnatal day 80-100), field potentials were measured in the dentate gyrus of hippocampus in response to stimulation applied to the lateral perforant path. The results showed: (1) Prenatal aluminum exposure had no effect on the magnitude of LTP as measured by the EPSP slope and LTD as measured for the PS amplitude, while it had a small effect on the magnitude of LTP as measured for the PS amplitude and LTD as measured by the EPSP slope. (2) Lactational, postlactational and throughout life exposure to aluminum impaired both LTP and LTD of the EPSP slope and PS amplitude, except that LTD of PS amplitude was not significantly changed in animals postlactationally exposed. (3) Aluminum exposure from parturition throughout life caused the greatest impairment of the range of synaptic plasticity, while prenatal aluminum exposure caused the least. From these results we conclude that the lactational period was the most susceptible to aluminum-induced impairment of synaptic plasticity and that chronic aluminum exposure from parturition throughout life is extremely disruptive to synaptic plasticity and should be avoided.

Lack of effects of vitamin E on aluminium-induced deficit of synaptic plasticity in rat dentate gyrus in vivo

Aluminium (Al), has the potential to be neurotoxic in humans and animals, and is present in many manufactured foods and medicines and is also added to drinking water for purification purposes. Our previous study demonstrated that chronic Al exposure induced deficits of both long-term potentiation (LTP) and long-term depression (LTD) of excitatory postsynaptic potential (EPSP) and population spike (PS) in rat dentate gyrus (DG) of hippocampus in vivo (Wang et al., 2001). The purpose of the present study was to investigate whether the Al-induced impairment of synaptic plasticity could be reversed by dietary supplementation with vitamin E (Vit E; alpha-tocopherol). Neonatal Wistar rats were exposed to Al from parturition throughout life by drinking 0.3% aluminium chloride (AlCl3) solution or a diet supplemented with Vit E at 500 microg/g/day with 0.3% AlCl3. The input/output (I/O) function, EPSP and PS were measured in DG area of adult rats (80-100 days of age) in response to stimulation applied to the lateral perforant path. The results showed that: (1) chronic Al exposure reduced the amplitudes of both EPSP LTP (control: 130.4+/-3%, n=7; Al-exposed: 110+/-2%, n=9, P<0.001) and PS LTP (control: 241+/-19%, n=7; Al-exposed: 130+/-7%, n=9, P<0.001) significantly. Vit E had no significant effects on the Al-induced deficits of EPSP LTP (Al-exposed: 110+/-2%, n=9; Al-exposed+Vit E: 112+/-2%, n=8, P>0.05) and PS LTP (Al-exposed: 130+/-7%, n=9; Al-exposed+Vit E: 129+/-4%, n=8; P>0.05); (2) the amplitudes of EPSP LTD (control: 84+/-4%, n=7; Al-exposed: 92+/-7%, n=9, P<0.01) and PS LTD (control: 81+/-4%, n=7; Al-exposed: 98+/-5%, n=9, P<0.001) were also decreased by Al treatment. The impaired EPSP LTD (Al-exposed: 92+/-7%, n=9; Al-exposed+Vit E: 93+/-4%, n=8, P>0.05) and PS LTD (Al-exposed: 98+/-5%, n=9; Al-exposed+Vit E: 94+/-6%, n=8, P>0.05) were also not significantly affected by Vit E treatment. It was suggested that dietary supplementation with Vit E did not reverse the impairment of synaptic plasticity induced by Al in DG in vivo.

Soluble oligomers of beta amyloid (1-42) inhibit long-term potentiation but not long-term depression in rat dentate gyrus

The dementia in Alzheimer disease (AD) is usually attributed to widespread neuronal loss in conjunction with the pathologic hallmarks of intracellular neurofibrillary tangles and extracellular plaques containing amyloid (A beta) in fibrillar form. Recently it has been demonstrated that non-fibrillar assemblies of A beta possess electrophysiologic activity, with the corollary that they may produce dementia by disrupting neuronal signaling prior to cell death. We therefore examined the effects of soluble oligomers of A beta(1-42) on long-term potentiation (LTP) and long-term depression (LTD), two cellular models of memory, in the dentate gyrus of rat hippocampal slices. Compared with vehicle controls, slices pre-incubated 60 min in the presence of A beta-derived diffusible ligands (ADDLs) showed no differences in threshold intensity to evoke a synaptic response, slope of field excitatory post-synaptic potentials (EPSPs), or the input/output function. Tetanus-induced LTP and reversal of LTD were strongly inhibited in ADDLs-treated slices whereas LTD was unaffected. These data suggest that soluble non-fibrillar amyloid may contribute to the pathogenesis of AD both by impairing LTP/memory formation at the cellular level and by creating 'neuroplasticity imbalance' manifested by unopposed LTD in the setting of impaired capacity for neural repair via reversal of LTD or LTP.

Vasopressin reverses aluminum-induced impairment of synaptic plasticity in the rat dentate gyrus in vivo

Aluminum (Al), an important neurotoxin, contributes to a variety of cognitive dysfunction and mental diseases. Previous studies have demonstrated that Al impairs hippocampal long-term potentiation (LTP) in vitro and in vivo. In the present study, both LTP and LTD (long-term depression) were recorded in the same animal to investigate the Al-induced impairment of synaptic plasticity. Another aim of the present research was to verify whether the impairment of synaptic plasticity induced by Al could be reversed by vasopressin (VP) treatment. Neonatal Wistar rats were exposed to Al from parturition through adulthood (pre- and post-weaning) by the drinking of 0.3% aluminum chloride (AlCl(3)) solution. The input-output (I/O) function, paired-pulse reaction (PPR), excitatory postsynaptic potential (EPSP) and population spike (PS) amplitude were measured in the dentate gyrus (DG) of adult rats (60-90 days) in response to stimulation applied to the lateral perforant path. The results showed: (1) Al reduced the amplitudes of both EPSP LTP (control: 132+/-7%, n=7; Al-exposed: 115+/-10%, n=8, P<0.05) and PS LTP (control: 242+/-18%, n=7; Al-exposed: 136+/-7%, n=8, P<0.01) significantly. The amplitudes of EPSP LTD (control: 82+/-6%, n=7; Al-exposed: 92+/-7%, n=8, P<0.05) and PS LTD (control: 81+/-4%, n=7; Al-exposed: 98+/-5%, n=8, P<0.05) were also decreased by Al treatment. The Al-induced impairments of PS LTP and PS LTD were more serious than that of EPSP LTP and EPSP LTD. (2) In control rats, VP had an increase in the PS LTP amplitude (control: 242+/-18%, n=7; control+VP: 358+/-23%, n=6, P<0.01), while it had no significant effects on PS LTD (control: 81+/-4%, n=7; control+VP: 76+/-7%, n=6, P>0.05). (3) In Al-exposed rats, VP had a significant increase in the amplitudes of both PS LTP (Al-exposed: 136+/-7%, n=8, Al-exposed+VP: 255+/-16%, n=6, P<0.01) and PS LTD (Al-exposed: 98+/-5%, n=8; Al-exposed+VP: 81+/-6%, n=6, P<0.05). After the application of VP, the range of synaptic plasticity (PS LTP+PS LTD) in Al-exposed rats increased from 38% to 174%, which surpassed that in control rats (161%). It was suggested that VP could reverse Al-induced impairment of synaptic plasticity and might be an effective medicine to cure Al-induced neurological disorders.

Protein phosphatase-1 regulation in the induction of long-term potentiation: heterogeneous molecular mechanisms

Protein phosphatase inhibitor-1 (I-1) has been proposed as a regulatory element in the signal transduction cascade that couples postsynaptic calcium influx to long-term changes in synaptic strength. We have evaluated this model using mice lacking I-1. Recordings made in slices prepared from mutant animals and also in anesthetized mutant animals indicated that long-term potentiation (LTP) is deficient at perforant path-dentate granule cell synapses. In vitro, this deficit was restricted to synapses of the lateral perforant path. LTP at Schaffer collateral-CA1 pyramidal cell synapses remained normal. Thus, protein phosphatase-1-mediated regulation of NMDA receptor-dependent synaptic plasticity involves heterogeneous molecular mechanisms, in both different dendritic subregions and different neuronal subtypes. Examination of the performance of I-1 mutants in spatial learning tests indicated that intact LTP at lateral perforant path-granule cell synapses is either redundant or is not involved in this form of learning.

Orphanin FQ suppresses NMDA receptor-dependent long-term depression and depotentiation in hippocampal dentate gyrus

We reported previously that orphanin FQ (OFQ) inhibited NMDA receptor-mediated synaptic currents and consequently suppressed induction of long-term potentiation (LTP) in the hippocampal dentate gyrus. This study examines the effect of OFQ on several other forms of long-term synaptic plasticity in the lateral perforant path of mouse hippocampal dentate gyrus. (1) Long-term depression (LTD): a low frequency stimulation (1 Hz, 15 min) applied to the lateral perforant path induced a long-lasting reduction in the dentate field potentials in slices from 22- to 30-day-old mice. This LTD was sensitive to the NMDA receptor blocker D-AP5, and could be significantly attenuated by bath application of OFQ (1 microM, 25 min). (2) Primed LTD: induction of LTD in slices from 50- to 65-day-old mice required a priming procedure consisting of multiple high frequency stimulus trains delivered in the presence of D-AP5 before the low-frequency stimulation. OFQ applied during the low-frequency stimulation, but not during the priming trains, blocked induction of primed LTD. (3) Depotentiation: high-frequency train-induced dentate LTP could be reversed by a subsequent low-frequency stimulation. This depotentiation was also attenuated by either OFQ or D-AP5 applied during low-frequency stimulation. These results, together with our previous findings, suggest that OFQ inhibits bidirectional changes in synaptic strength in the dentate; and its multiple actions on NMDA receptor-dependent, long-term synaptic plasticity might work in tandem to regulate hippocampus-dependent learning and memory.