NMDA Receptor-Dependent Synaptic Potentiation via APPL1 Signaling Is Required for the Accessibility of a Prefrontal Neuronal Assembly in Retrieving Fear Extinction.

BACKGROUND: The ventromedial prefrontal cortex has been viewed as a locus for storage and recall of extinction memory. However, the synaptic and cellular mechanisms underlying these processes remain elusive. METHODS: We combined transgenic mice, electrophysiological recording, activity-dependent cell labeling, and chemogenetic manipulation to analyze the role of adaptor protein APPL1 in the ventromedial prefrontal cortex in fear extinction retrieval. RESULTS: We found that both constitutive and conditional APPL1 knockout decreased NMDA receptor (NMDAR) function in the ventromedial prefrontal cortex and impaired fear extinction retrieval. Moreover, APPL1 undergoes nuclear translocation during extinction retrieval. Blocking APPL1 nucleocytoplasmic translocation reduced NMDAR currents and disrupted extinction retrieval. We also identified a prefrontal neuronal ensemble that is both necessary and sufficient for the storage of extinction memory. Inducible APPL1 knockout in this ensemble abolished NMDAR-dependent synaptic potentiation and disrupted extinction retrieval, while chemogenetic activation of this ensemble simultaneously rescued the impaired behaviors. CONCLUSIONS: Our results indicate that a prefrontal neuronal ensemble stores extinction memory, and APPL1 signaling supports these neurons in retrieving extinction memory by controlling NMDAR-dependent potentiation.

Pb exposure induces an imbalance of excitatory and inhibitory synaptic transmission in cultured rat hippocampal neurons.

An appropriate balance of excitatory and inhibitory synapse maintains the network stability of the central nervous system. Our recent work showed lead (Pb) exposure can inhibit synaptic transmission in cultured hippocampal neurons. However, it is not clear whether Pb exposure disrupt the balance of excitatory and inhibitory synaptic transmission. Here, primary cultured hippocampal neurons from Sprague-Dawley (SD) rats were exposed to Pb (0.2 µM, 1 µM, 5 µM, respectively) from Days in Vitro (DIV) 7 to DIV 12 for 5 days and the excitatory and inhibitory synaptic transmission was examined. Patch clamp recording results showed that distinct from exposures of 0.2 µM and 5 µM, 1 µM Pb exposure significantly increased the mIPSC frequency and decreased the mEPSC frequency, leading to a uniform inhibitory outcome. Further, the number of inhibitory presynaptic puncta was significantly increased after 1 µM Pb exposure, while the number of excitatory presynaptic terminals was decreased. In addition 1 µM Pb increased the glutamic acid decarboxylase (GAD65) expression and the surface GABAA receptor (GABAAR) clusters. This shift might potentiate the synthesis of GABA and enhance the surface distribution of postsynaptic GABAAR clusters in hippocampus neurons. Together, these data showed that Pb exposure disrupted the balance of excitatory and inhibitory synaptic transmission via abnormal GABAergic neurotransmission.

Pb inhibits hippocampal synaptic transmission via cyclin-dependent kinase-5 dependent Synapsin 1 phosphorylation.

Lead (Pb) exposure impairs the nervous system, of which the injury of cognitive development is obvious. But the mechanism of Pb induced disorders of neuro-transmission remain elusive. In this study, primary hippocampal neurons were exposed to Pb at the dosage of 5 µM from days in vitro (DIV) 3 to DIV14 and the electrophysiological recordings were performed at DIV14. Sprague-Dawley (SD) rat pups were exposed to Pb from parturition to weaning indirectly from their mothers whose drinking water containing 250 ppm Pb, then directly exposed to Pb at the dosage of 250 ppm from postnatal day (PND) 21 to PND30. The results showed that Pb significantly decreased the frequency of both miniature excitatory postsynaptic current (mEPSC) and miniature inhibitory postsynaptic current (mIPSC) in cultured hippocampal neurons. Paird-pulse facilitation (PPF) recordings showed there was significant increase in Pb-exposed group. The increase of the magnitude of PPF (the ratio of second to first response amplitude) further confirmed that Pb reduced presynaptic neuro-transmission. By transmission electron microscope, it found that Pb disarranged presynaptic vesicles distribution and decreased the density of presynaptic vesicles. Moreover, it was interestingly found that phosphorylation of Synapsin1, which was phosphorylated by CDK5, has been decreased upon Pb exposure. With the treatment of R-Roscovitine (Ro), an inhibitor of CDK5, it was detected that Pb induced mEPSC and mIPSC frequency reduction have been reversed. Together, our results suggested that Pb disrupted the distribution of synaptic vesicles and impaired the neurotransmitter release, which was dependent on the phosphorylation level of Synapsin 1 via CDK5. This study will help for elucidation of environmental Pb-induced neuronal disorders.

Effect of Pb Exposure on Synaptic Scaling Through Regulation of AMPA Receptor Surface Trafficking.

Homeostatic synaptic plasticity (HSP) helps to stabilize the neuronal network activity, which is essential for optimal information coding. Synaptic scaling is a form of homeostatic plasticity that stabilizes neuronal firing in response to activity blockade. Lead (Pb) is a ubiquitous environmental neuro-toxicant and can impair the input-specific Hebbian type synaptic plasticity, but whether Pb exerts effects in HSP remains unknown. We previously reported that blocking L-type calcium channel induces synaptic scaling, which stimulates the synthesis of all-trans retinoic acid (RA) and the expression of GluA2-lacking α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor. Given Pb is a potent blocker of calcium channel, we hypothesized Pb may participate in synaptic scaling accompanied by RA synthesis and AMPA receptor trafficking. In this study, cultured hippocampal neurons were treated with Pb (1 µM 5 min, 15 min, 4 h, 24 h, and 10 µM 24 h) alone or in combination with tetrodotoxin (TTX, 1 µM, 24 h). The results showed that Pb alone, either at 1 µM or 10 µM, cannot induce synaptic scaling. But Pb participated in synaptic scaling when concurrent with TTX (10 µM Pb + 1 µM TTX, 24 h). Further results showed that surface heteromeric GluA1 and GluA2 AMPA receptors were increased in TTX+ Pb-induced synaptic scaling. In addition, RA was proved not to participate in TTX+ Pb-mediated synaptic scaling. Taken together, our work supported that TTX+ Pb could induce synaptic scaling and enhance synaptic accumulation of AMPAR GluA1 and GluA2 during synaptic up scaling. Our study would help for elucidation of the Pb-induced neuronal network instability mechanism.

Early developmental bisphenol-A exposure sex-independently impairs spatial memory by remodeling hippocampal dendritic architecture and synaptic transmission in rats.

Bisphenol-A (BPA, 4, 4'-isopropylidene-2-diphenol), a synthetic xenoestrogen that widely used in the production of polycarbonate plastics, has been reported to impair hippocampal development and function. Our previous study has shown that BPA exposure impairs Sprague-Dawley (SD) male hippocampal dendritic spine outgrowth. In this study, the sex-effect of chronic BPA exposure on spatial memory in SD male and female rats and the related synaptic mechanism were further investigated. We found that chronic BPA exposure impaired spatial memory in both SD male and female rats, suggesting a dysfunction of hippocampus without gender-specific effect. Further investigation indicated that BPA exposure causes significant impairment of dendrite and spine structure, manifested as decreased dendritic complexity, dendritic spine density and percentage of mushroom shaped spines in hippocampal CA1 and dentate gyrus (DG) neurons. Furthermore, a significant reduction in Arc expression was detected upon BPA exposure. Strikingly, BPA exposure significantly increased the mIPSC amplitude without altering the mEPSC amplitude or frequency, accompanied by increased GABAARß2/3 on postsynaptic membrane in cultured CA1 neurons. In summary, our study indicated that Arc, together with the increased surface GABAARß2/3, contributed to BPA induced spatial memory deficits, providing a novel molecular basis for BPA achieved brain impairment.