Autophagy mediates the degradation of synaptic vesicles: A potential mechanism of synaptic plasticity injury induced by microwave exposure in rats

To explore how autophagy changes and whether autophagy is involved in the pathophysiological process of synaptic plasticity injury caused by microwave radiation, we established a 30 mW/cm2 microwave-exposure in vivo model, which caused reversible injuries in rat neurons. Microwave radiation induced cognitive impairment in rats and synaptic plasticity injury in rat hippocampal neurons. Autophagy in rat hippocampal neurons was activated following microwave exposure. Additionally, we observed that synaptic vesicles were encapsulated by autophagosomes, a phenomenon more evident in the microwave-exposed group. Colocation of autophagosomes and synaptic vesicles in rat hippocampal neurons increased following microwave exposure.

CONCLUSION

microwave exposure led to the activation of autophagy in rat hippocampal neurons, and excessive activation of autophagy might damage synaptic plasticity by mediating synaptic vesicle degradation.

The GABAA receptor-mediated recurrent inhibition in ventral compared with dorsal CA1 hippocampal region is weaker, decays faster and lasts less

Hippocampal functions appear to be segregated along the dorso-ventral axis of the structure. Differences at the cellular and local neuronal network level may be involved in this functional segregation. In this study the characteristics of CA1 recurrent inhibition (RI) were measured and compared between dorsal (DH, n = 95) and ventral (VH, n = 60) hippocampal slices, using recordings of suprathreshold field potentials. RI strength was estimated as the percentile decrease of the population spike (PS) amplitude evoked with an orthodromic stimulus (at the Schaffer collaterals) when preceded by an antidromic stimulus (at the alveus). Varying the interpulse interval (IPI) between the two stimuli, we estimated RI duration. Alvear stimulation produced significant PS suppression in both VH and DH at every IPI tested, from 10 to 270 ms. Moreover, gradually more oblique DH (but not VH) slices displayed increasing RI, which at IPIs < or = 125 ms was reversibly abolished by the GABAA receptor antagonist picrotoxin (10 microM). The GABAA-mediated RI, measured under the blockade of GABAB receptors, was weaker, decayed faster and lasted less in VH compared to DH slices, regardless of the slice orientation. Specifically, in VH compared to DH, the PS suppression at 20 ms was 34.4 +/- 4.5% versus 69.9 +/- 6.5% (P < 0.001), the time constant of RI decay was 29 +/- 2.4 versus 87.5 +/- 13.6 ms (P < 0.01) and the duration was 50 versus 125 ms (P < 0.001). Thus, GABAA-mediated RI may control the CA1 excitatory output less effectively in VH compared to DH. The observed dorso-ventral differences in RI contribute to the longitudinal diversification of the structure and may underlie to some extent the region-specificity of hippocampal functions.

Actin polymerization and ERK phosphorylation are required for Arc/Arg3.1 mRNA targeting to activated synaptic sites on dendrites

The mRNA for the immediate early gene Arc/Arg3.1 is induced by strong synaptic activation and is rapidly transported into dendrites, where it localizes at active synaptic sites. NMDA receptor activation is critical for mRNA localization at active synapses, but downstream events that mediate localization are not known. The patterns of synaptic activity that induce mRNA localization also trigger a dramatic polymerization of actin in the activated dendritic lamina and phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) throughout the postsynaptic cytoplasm. The local polymerization of actin in the activated dendritic lamina is of particular interest because it occurs in the same dendritic domains in which newly synthesized Arc/Arg3.1 mRNA localizes. Here, we explore the role of activity-induced alterations in the actin network and mitogen-activated protein (MAP) kinase activation in Arc/Arg3.1 mRNA localization. We show that actin polymerization induced by high-frequency stimulation is blocked by local inhibition of Rho kinase, and Arc/Arg3.1 mRNA localization is abrogated in the region of Rho kinase blockade. Local application of latrunculin B, which binds to actin monomers and inhibits actin polymerization, also blocked the targeting of Arc/Arg3.1 mRNA to activated synaptic sites. Local application of the MAP kinase kinase inhibitor U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-amino-phenylthio]butadiene) blocked ERK phosphorylation, and also blocked Arc/Arg3.1 mRNA localization. Our results indicate that the reorganization of the actin cytoskeletal network in conjunction with MAP kinase activation is required for targeting newly synthesized Arc/Arg3.1 mRNA to activated synaptic sites.

Difference in time course of modulation of synaptic transmission by group II versus group III metabotropic glutamate receptors in region CA1 of the hippocampus

We investigated the time course of modulation of synaptic transmission by group II and group III metabotropic glutamate receptors in region CA1 of the hippocampus. In the presence of 50 microM picrotoxin, pressure pulse application of 1 mM glutamate resulted in a fast onset of suppression of synaptic transmission in stratum lacunosum moleculare and a slower onset of suppression in stratum radiatum, with both effects returning to baseline over the course of several minutes. Application of 50 microM of the group II agonist (2R,4R)-APDC in stratum lacunosum moleculare resulted in the same fast onset of suppression while having no effect in stratum radiatum. Pressure pulse application of 100 microM DL-AP4 in stratum lacunosum moleculare and stratum radiatum resulted in a much slower onset of suppression of synaptic transmission than (2R,4R)-APDC. Suppression by (2R,4R)-APDC was accompanied by a rapid enhancement of paired pulse facilitation, indicative of a presynaptic mechanism. This demonstrates that activation of group II mGluRs in the hippocampus causes a fast onset of suppression in stratum lacunosum moleculare, while activation of group III mGluRs causes a slower onset of suppression. The difference in time course for group II vs. group III mGluRs suggests a different functional role, with group II playing a potential role in making synapses act as low pass filters.

Bidirectional redistribution of AMPA but not NMDA receptors after perforant path simulation in the adult rat hippocampus in vivo

Long-term potentiation (LTP) in vitro reveals dynamic regulation of synaptic glutamate receptors. AMPA receptors may be inserted into synapses to increase neurotransmission, whereas NMDA receptors may redistribute within the synapse to alter the probability of subsequent plasticity. To date, the only evidence for these receptor dynamics in the hippocampus is from the studies of dissociated neurons and hippocampal slices taken from young animals. Although synaptic plasticity is induced easily, the extent of AMPA and NMDA receptor mobility after LTP is unknown in the adult, intact hippocampus. To test whether AMPA or NMDAR subunits undergo activity-dependent modifications in adult hippocampal synapses, we induced LTP at perforant path-dentate gyrus (DG) synapses in anesthetized adult rats, using high frequency stimulation (HFS), verified layer-specific Arc induction, and analyzed the distribution of postsynaptic AMPA and NMDAR subunits, using immunogold electron microscopy. The number of synapses with AMPA receptor labeling increased with LTP-inducing HFS in the stimulated region of the dendrite relative to the nonstimulated regions. The opposite trend was noted with low frequency stimulation (LFS). Moreover, HFS increased and LFS decreased the ratio of synaptic to extrasynaptic AMPA receptor labeling in the postsynaptic membrane. In contrast, HFS did not significantly alter NMDAR labeling. Thus, LTP in the adult hippocampus in vivo selectively enhanced AMPA but not NMDAR labeling specifically in synapses undergoing activity-dependent plasticity relative to the remainder of the dendritic tree. The results suggest a mechanism by which rapid adjustments in synaptic strength can occur through localized AMPA receptor mobility and that this process may be competitive across the dendritic tree.

Glutamate exocytosis from astrocytes controls synaptic strength

The release of transmitters from glia influences synaptic functions. The modalities and physiological functions of glial release are poorly understood. Here we show that glutamate exocytosis from astrocytes of the rat hippocampal dentate molecular layer enhances synaptic strength at excitatory synapses between perforant path afferents and granule cells. The effect is mediated by ifenprodil-sensitive NMDA ionotropic glutamate receptors and involves an increase of transmitter release at the synapse. Correspondingly, we identify NMDA receptor 2B subunits on the extrasynaptic portion of excitatory nerve terminals. The receptor distribution is spatially related to glutamate-containing synaptic-like microvesicles in the apposed astrocytic processes. This glial regulatory pathway is endogenously activated by neuronal activity-dependent stimulation of purinergic P2Y1 receptors on the astrocytes. Thus, we provide the first combined functional and ultrastructural evidence for a physiological control of synaptic activity via exocytosis of glutamate from astrocytes.

Spike-timing-dependent plasticity at resting and conditioned lateral perforant path synapses on granule cells in the dentate gyrus: different roles of N-methyl-D-aspartate and group I metabotropic glutamate receptors

We examined the mechanisms underlying spike-timing-dependent plasticity induction at resting and conditioned lateral perforant pathway (LPP) synapses in the rat dentate gyrus. Two stimulating electrodes were placed in the outer third of the molecular layer and in the granule cell layer in hippocampal slices to evoke field excitatory postsynaptic potentials (fEPSPs) and antidromic field somatic spikes (afSSs), respectively. Long-term potentiation (LTP) of LPP synapses was induced by paired stimulation with fEPSP preceding afSS. Reversal of the temporal order of fEPSP and afSS stimulation resulted in long-term depression (LTD). Induction of LTP or LTD was blocked by D,L-2-amino-5-phosphonopentanoic acid (AP5), showing that both effects were N-methyl-D-aspartate receptor (NMDAR)-dependent. Induction of LTP was also blocked by inhibitors of calcium-calmodulin kinase II, protein kinase C or mitogen-activated/extracellular-signal regulated kinase, suggesting that these are downstream effectors of NMDAR activation, whereas induction of LTD was blocked by inhibitors of protein kinase C and protein phosphatase 2B. At LPP synapses previously potentiated by high-frequency stimulation or depressed by low-frequency stimulation, paired fEPSP-afSS stimulation resulted in 'de-depression' at depressed LPP synapses but had no effect on potentiated synapses, whereas reversal of the temporal order of fEPSP-afSS stimulation resulted in 'de-potentiation' at potentiated synapses but had no effect on depressed synapses. Induction of de-depression and de-potentiation was unaffected by ap5 but was blocked by 2-methyl-6-(phenylethynyl) pyridine hydrochloride, a group I metabotropic glutamate receptor blocker, showing that both were NMDAR-independent but group I metabotropic glutamate receptor-dependent. In conclusion, our results show that spike-timing-dependent plasticity can occur at both resting and conditioned LPP synapses, its induction in the former case being NMDAR-dependent and, in the latter, group I metabotropic glutamate receptor-dependent.

Excitotoxic hippocampal neuron loss following sustained electrical stimulation of the perforant pathway in the mouse

Prolonged electrical stimulation of the perforant pathway in the rat evokes epileptiform discharges in dentate granule cells and irreversibly damages hilar neurons. In this in vivo study, we demonstrate that similar perforant path stimulation in C57Bl/6 mice causes the same pattern of hippocampal neuron loss. Therefore, this mouse model of seizure-induced hippocampal injury can be used for a wide variety of studies in genetically altered conditions not available in rats.

Facilitation of granule cell epileptiform activity by mossy fiber-released zinc in the pilocarpine model of temporal lobe epilepsy

Recurrent mossy fiber synapses in the dentate gyrus of epileptic brain facilitate the synchronous firing of granule cells and may promote seizure propagation. Mossy fiber terminals contain and release zinc. Released zinc inhibits the activation of NMDA receptors and may therefore oppose the development of granule cell epileptiform activity. Hippocampal slices from rats that had experienced pilocarpine-induced status epilepticus and developed a recurrent mossy fiber pathway were used to investigate this possibility. Actions of released zinc were inferred from the effects of chelation with 1 mM calcium disodium EDTA (CaEDTA). When granule cell population bursts were evoked by mossy fiber stimulation in the presence of 6 mM K(+) and 30 microM bicuculline, CaEDTA slowed the rate at which evoked bursting developed, but did not change the magnitude of the bursts once they had developed fully. The effects of CaEDTA were then studied on the pharmacologically isolated NMDA receptor- and AMPA/kainate receptor-mediated components of the fully developed bursts. CaEDTA increased the magnitude of NMDA receptor-mediated bursts and reduced the magnitude of AMPA/kainate receptor-mediated bursts. CaEDTA did not affect the granule cell bursts evoked in slices from untreated rats by stimulating the perforant path in the presence of bicuculline and 6 mM K(+). These results suggest that zinc released from the recurrent mossy fibers serves mainly to facilitate the recruitment of dentate granule cells into population bursts.

Studies of the synaptic plasticity of field CA3 of the hippocampus during tetanization of the perforant path

Studies on living slices of hippocampus-entorhinal cortex formation from adult rats were performed to investigate changes in responses in field CA3 to stimulation of mossy fibers in conditions of perforant path tetanization with different parameters. Tetanization of the perforant path at frequencies of 10 and 100 Hz induced depression of responses in CA3 on testing of this same path. Tetanization of the perforant path at a frequency of 10 Hz and an amplitude subthreshold for potentiating mossy fiber synapses in CA3 became threshold if preceded by tetanization of the perforant path at a frequency of 100 Hz. Tetanization of mossy fibers at 10 Hz resulted in potentiation of the input to CA3, while tetanization at 100 Hz induced depression. High-frequency tetanization of the perforant path (100 Hz) delivered in trains following at the frequency of the theta rhythm, led mainly to depression of field CA3 responses to stimulation of mossy fibers.

Expression of the mu-opioid receptor is induced in dentate gyrus granule cells after focal cerebrocortical ischaemia and stimulation of entorhinal afferents

Focal ischaemia in the cerebral cortex affects the inducibility of long-term potentiation (LTP) in the hippocampus. This impairment of hippocampal function may result from excessive activation of cortico-hippocampal afferents and subsequent perturbation of hippocampal LTP-relevant transmitter systems, which include opioids. Here, we tested if permanent focal ischaemia and electrical afferent stimulation influence the expression of the mu-opioid receptor (MOR) in the rat hippocampus. In the applied ischaemia model, the entire ipsilateral cortical hemisphere and hippocampus experienced sustained excitation as indicated by a long-lasting increase in the expression of arg 3.1/arc (ARG) mRNA, a marker for neuronal activity. Expression of MOR mRNA and protein was strongly increased in granule cells, which contain very low MOR levels under normal conditions, but not in gamma-aminobutyric acid (GABA)ergic neurons, which express the MOR constitutively. In the molecular layer, which contains the dendrites of granule cells, focal ischaemia caused a redistribution of MOR-like immunoreactivity. In contrast to the dentate gyrus, MOR expression was unaltered in the hippocampus proper and in non-infarcted cortical areas. Repetitive high-frequency stimulation of cortico-hippocampal perforant path afferents induced strong MOR mRNA expression throughout the granular layer. However, weak tetanization sufficient to induce LTP and ARG expression did not influence MOR mRNA levels. Taken together, we provide direct evidence for the induction of MOR expression in granule cells experiencing sustained excitation by cortical afferents. In activated, MOR-expressing granule cells, inhibitory opioids may counter-regulate glutamatergic excitation by the perforant path.

Patterns of dentate granule cell responses to perforant path stimulation in epileptic mice with granule cell dispersion

In adult mice, intrahippocampal administration of kainic acid induces a structural modification of the granule cell layer reminiscent of granule cell dispersion (GCD) seen in humans with temporal lobe epilepsy. We tested that GCD might be involved in the patterns of granule cell responses to perforant path stimulation by recording field potentials in vivo after kainic acid-induced status epilepticus until the phase of chronic seizure activity in presence of GCD or after its alteration by K252a co-treatment, an inhibitor of tyrosine kinase activities. Stimulation triggered bursts of multiple population spikes, the number of which progressively increased with time whereas their amplitude decreased in parallel with the progressive decrease in granule cell density. The population spike threshold was reached for a lower excitatory synaptic drive than in controls, as assessed by the initial slope of the field excitatory post-synaptic potential. This indicates that, for identical synaptic responses, granule cells were closer to the firing threshold. Fast inhibition, assessed by paired pulse stimulation, was compromised immediately after the initial status epilepticus, consistent with the rapid loss of most hilar cells. Neither the epileptic course nor the epileptiform responses of the granule cells were modified and manipulation by alteration following GCD manipulation while granule cell neuropeptide-Y immunostaining was substantially decreased. In this mouse model of TLE, granule cells display a progressive increase in epileptiform responses to afferent input until the occurrence of spontaneous seizures. The population spike amplitude decreases in parallel with GCD while the granule cell excitability is enhanced. Consequently, data from field potentials in epilepsy experiments should be interpreted with care, taking into account the possible variations in the neuronal density in the recorded area.

Morpho-functional characterization of neuronal cells at different stages of maturation in granule cell layer of adult rat dentate gyrus

Neurogenesis occurs throughout adult life in dentate gyrus of mammal hippocampus. Therefore, neurons at different stages of electrophysiological and morphological maturation and showing various, if any, synaptic inputs co-exist in the adult granule cell layer, as occurs during dentate gyrus development. The knowledge of functional properties of new neurons throughout their maturation can contribute to understanding their role in the hippocampal function. In this study electrophysiological and morphological features of granule layer cells, characterized as immature or mature neurons, without and with synaptic input, were comparatively described in adult rats. The patch-clamp technique was used to perform electrophysiological recordings, the occurrence of synaptic input evoked by medial perforant pathway stimulation was investigated and synaptic input was characterized. Cells were then identified and morphologically described via detection of biocytin injected through the patch pipette. The neuronal phenotype of recorded cells was assessed by immunohistochemistry and single-cell RT-PCR. Cells with very low capacitance, high input resistance, depolarized resting membrane potential and without synaptic activity were found exclusively at the border of the GCL facing hilus; this type of cell expressed the class III beta-tubulin neuronal marker (mRNA and protein) and did not express a glial marker. Immature neuronal cells with progressively increasing capacitance, decreasing input resistance and resting membrane potential getting more hyperpolarized showed only depolarizing GABAergic synaptic input at first and then also glutamatergic synaptic input. Finally, cells showing electrophysiological, synaptic, and morphological features of mature granule, expressing the mature neuron marker NeuN, were identified.