Location analysis of presynaptically active and silent synapses in single-cultured hippocampal neurons.

A morphologically present but non-functioning synapse is termed a silent synapse. Silent synapses are categorized into "postsynaptically silent synapses," where AMPA receptors are either absent or non-functional, and "presynaptically silent synapses," where neurotransmitters cannot be released from nerve terminals. The presence of presynaptically silent synapses remains enigmatic, and their physiological significance is highly intriguing. In this study, we examined the distribution and developmental changes of presynaptically active and silent synapses in individual neurons. Our findings show a gradual increase in the number of excitatory synapses, along with a corresponding decrease in the percentage of presynaptically silent synapses during neuronal development. To pinpoint the distribution of presynaptically active and silent synapses, i.e., their positional information, we employed Sholl analysis. Our results indicate that the distribution of presynaptically silent synapses within a single neuron does not exhibit a distinct pattern during synapse development in different distance from the cell body. However, irrespective of neuronal development, the proportion of presynaptically silent synapses tends to rise as the projection site moves farther from the cell body, suggesting that synapses near the cell body may exhibit higher synaptic transmission efficiency. This study represents the first observation of changes in the distribution of presynaptically active and silent synapses within a single neuron.

Calcineurin regulates synaptic Ca2+-permeable AMPA receptors in hypothalamic presympathetic neurons via α2δ-1-mediated GluA1/GluA2 assembly.

Hypertension is a major adverse effect of calcineurin inhibitors, such as tacrolimus (FK506) and cyclosporine, used clinically as immunosuppressants. Calcineurin inhibitor-induced hypertension (CIH) is linked to augmented sympathetic output from the hypothalamic paraventricular nucleus (PVN). GluA2-lacking, Ca2+-permeable AMPA receptors (CP-AMPARs) are a key feature of glutamatergic synaptic plasticity, yet their role in CIH remains elusive. Here, we found that systemic administration of FK506 in rats significantly increased serine phosphorylation of GluA1 and GluA2 in PVN synaptosomes. Strikingly, FK506 treatment reduced GluA1/GluA2 heteromers in both synaptosomes and endoplasmic reticulum-enriched fractions from the PVN. Blocking CP-AMPARs with IEM-1460 induced a larger reduction of AMPAR-mediated excitatory postsynaptic current (AMPAR-EPSC) amplitudes in retrogradely labelled, spinally projecting PVN neurons in FK506-treated rats than in vehicle-treated rats. Furthermore, FK506 treatment shifted the current-voltage relationship of AMPAR-EPSCs from linear to inward rectification in labelled PVN neurons. FK506 treatment profoundly enhanced physical interactions of α2δ-1 with GluA1 and GluA2 in the PVN. Inhibiting α2δ-1 with gabapentin, α2δ-1 genetic knockout, or disrupting α2δ-1-AMPAR interactions with an α2δ-1 C terminus peptide restored GluA1/GluA2 heteromers in the PVN and diminished inward rectification of AMPAR-EPSCs in labelled PVN neurons induced by FK506 treatment. Additionally, microinjection of IEM-1460 or α2δ-1 C terminus peptide into the PVN reduced renal sympathetic nerve discharges and arterial blood pressure elevated in FK506-treated rats but not in vehicle-treated rats. Thus, calcineurin in the hypothalamus constitutively regulates AMPAR subunit composition and phenotypes by controlling GluA1/GluA2 interactions with α2δ-1. Synaptic CP-AMPARs in PVN presympathetic neurons contribute to augmented sympathetic outflow in CIH. KEY POINTS: Systemic treatment with the calcineurin inhibitor increases serine phosphorylation of synaptic GluA1 and GluA2 in the PVN. Calcineurin inhibition enhances the prevalence of postsynaptic Ca2+-permeable AMPARs in PVN presympathetic neurons. Calcineurin inhibition potentiates α2δ-1 interactions with GluA1 and GluA2, disrupting intracellular assembly of GluA1/GluA2 heterotetramers in the PVN. Blocking Ca2+-permeable AMPARs or α2δ-1-AMPAR interactions in the PVN attenuates sympathetic outflow augmented by the calcineurin inhibitor.

Different states of synaptic vesicle priming explain target cell type-dependent differences in neurotransmitter release.

Pronounced differences in neurotransmitter release from a given presynaptic neuron, depending on the synaptic target, are among the most intriguing features of cortical networks. Hippocampal pyramidal cells (PCs) release glutamate with low probability to somatostatin expressing oriens-lacunosum-moleculare (O-LM) interneurons (INs), and the postsynaptic responses show robust short-term facilitation, whereas the release from the same presynaptic axons onto fast-spiking INs (FSINs) is ~10-fold higher and the excitatory postsynaptic currents (EPSCs) display depression. The mechanisms underlying these vastly different synaptic behaviors have not been conclusively identified. Here, we applied a combined functional, pharmacological, and modeling approach to address whether the main difference lies in the action potential-evoked fusion or else in upstream priming processes of synaptic vesicles (SVs). A sequential two-step SV priming model was fitted to the peak amplitudes of unitary EPSCs recorded in response to complex trains of presynaptic stimuli in acute hippocampal slices of adult mice. At PC-FSIN connections, the fusion probability (Pfusion) of well-primed SVs is 0.6, and 44% of docked SVs are in a fusion-competent state. At PC-O-LM synapses, Pfusion is only 40% lower (0.36), whereas the fraction of well-primed SVs is 6.5-fold smaller. Pharmacological enhancement of fusion by 4-AP and priming by PDBU was recaptured by the model with a selective increase of Pfusion and the fraction of well-primed SVs, respectively. Our results demonstrate that the low fidelity of transmission at PC-O-LM synapses can be explained by a low occupancy of the release sites by well-primed SVs.

Increased glutamatergic neurotransmission between the retinohypothalamic tract and the suprachiasmatic nucleus of old mice.

Circadian rhythms synchronize to light through the retinohypothalamic tract (RHT), which is a bundle of axons coming from melanopsin retinal ganglion cells, whose synaptic terminals release glutamate to the ventral suprachiasmatic nucleus (SCN). Activation of AMPA-kainate and NMDA postsynaptic receptors elicits the increase in intracellular calcium required for triggering the signaling cascade that ends in phase shifts. During aging, there is a decline in the synchronization of circadian rhythms to light. With electrophysiological (whole-cell patch-clamp) and immunohistochemical assays, in this work, we studied pre- and postsynaptic properties between the RHT and ventral SCN neurons in young adult (P90-120) and old (P540-650) C57BL/6J mice. Incremental stimulation intensities (applied on the optic chiasm) induced much lesser AMPA-kainate postsynaptic responses in old animals, implying a lower recruitment of RHT fibers. Conversely, a higher proportion of old SCN neurons exhibited synaptic facilitation, and variance-mean analysis indicated an increase in the probability of release in RHT terminals. Moreover, both spontaneous and miniature postsynaptic events displayed larger amplitudes in neurons from aged mice, whereas analysis of the NMDA and AMPA-kainate components (evoked by RHT electrical stimulation) disclosed no difference between the two ages studied. Immunohistochemistry revealed a bigger size in the puncta of vGluT2, GluN2B, and GluN2A of elderly animals, and the number of immunopositive particles was increased, but that of PSD-95 was reduced. All these synaptic adaptations could be part of compensatory mechanisms in the glutamatergic signaling to ameliorate the loss of RHT terminals in old animals.

Tenuifolin improves learning and memory by regulating long-term potentiation and dendritic structure of hippocampal CA1 area in healthy female mice but not male mice.

Polygala tenuifolia Wild is an ancient traditional Chinese medicine. Its main component, tenuifolin (TEN), has been proven to improve cognitive impairment caused by neurodegenerative diseases and ovariectomy. However, there was hardly any pharmacological research about TEN and its potential gender differences. Considering the reduction of TEN on learning and memory dysfunction in ovariectomized animals, therefore, we focused on the impact of TEN in different mice genders in the current study. Spontaneous alternation behavior (SAB), light-dark discrimination, and Morris water maze (MWM) tests were used to evaluate the mice's learning and memory abilities. The field excitatory postsynaptic potential (fEPSP) of the hippocampal CA1 region was recorded using an electrophysiological method, and the morphology of the dendritic structure was examined using Golgi staining. In the behavioral experiments, TEN improved the correct rate in female mice in the SAB test, the correct rate in the light-dark discrimination test, and the number of crossing platforms in the MWM test. Additionally, TEN reduced the latency of female mice rather than male mice in light-dark discrimination and MWM tests. Moreover, TEN could significantly increase the slope of fEPSP in hippocampal Schaffer-CA1 and enhance the total length and the number of intersections of dendrites in the hippocampal CA1 area in female mice but not in male mice. Collectively, the results of the current study showed that TEN improved learning and memory by regulating long-term potentiation (LTP) and dendritic structure of hippocampal CA1 area in female mice but not in males. These findings would help to explore the improvement mechanism of TEN on cognition and expand the knowledge of the potential therapeutic value of TEN in the treatment of cognitive impairment.

Sorbs2 regulates seizure activity by influencing AMPAR-mediated excitatory synaptic transmission in temporal lobe epilepsy.

Temporal lobe epilepsy (TLE), the most common type of drug-resistant epilepsy, severely affects quality of life. However, the underlying mechanism of TLE remains unclear and deserves further exploration. Sorbs2, a key synaptic regulatory protein, plays an important role in the regulation of synaptic transmission in the mammalian brain. In this study, we aimed to investigate the expression pattern of Sorbs2 in a kainic acid (KA)-induced TLE mouse model and in patients with TLE to further determine whether Sorbs2 is involved in seizure activity and to explore the potential mechanism by which Sorbs2 affects seizures in this TLE mouse model. First, we found that the expression of Sorbs2 was obviously increased in the hippocampus and cortex of a TLE mouse model and in the temporal cortex of TLE patients, indicating an abnormal expression pattern of Sorbs2 in TLE. Importantly, subsequent behavioral analyses and local field potential (LFP) analyses of a TLE mouse model demonstrated that the downregulation of hippocampal Sorbs2 could prolong the latency to spontaneous recurrent seizures (SRSs) and protect against SRSs. We also found that the knockdown of Sorbs2 in the hippocampus could decrease excitatory synaptic transmission in pyramidal neurons (PNs) in the hippocampal CA1 region and reduce the expression levels of the AMPAR subunits GluA1 and GluA2. Thus, we speculated that Sorbs2 may promote epileptogenesis and the development of TLE by affecting AMPAR-mediated excitatory synaptic transmission in PNs in the CA1 region. Therefore, reducing the expression of hippocampal Sorbs2 could restrain epileptogenesis and the development of TLE.

Repeated Morphine Exposure Alters Temporoamonic-CA1 Synaptic Plasticity in Male Rat Hippocampus.

In this study, the electrophysiological and biochemical consequences of repeated exposure to morphine in male rats on glutamatergic synaptic transmission, synaptic plasticity, the expression of GABA receptors and glutamate receptors at the temporoammonic-CA1 synapse along the longitudinal axis of the hippocampus (dorsal, intermediate, ventral, DH, IH, VH, respectively) were investigated. Slice electrophysiological methods, qRT-PCR, and western blotting techniques were used to characterize synaptic plasticity properties. We showed that repeated morphine exposure (RME) reduced excitatory synaptic transmission and ability for long-term potentiation (LTP) in the VH as well as eliminated the dorsoventral difference in paired-pulse responses. A decreased expression of NR2B subunit in the VH and an increased expression GABAA receptor of α1 and α5 subunits in the DH were observed following RME. Furthermore, RME did not affect the expression of NR2A, AMPA receptor subunits, and γ2GABAA and GABAB receptors in either segment of the hippocampus. In sum, the impact of morphine may differ depending on the region of the hippocampus studied. A distinct change in the short- and long-term synaptic plasticity along the hippocampus long axis due to repeated morphine exposure, partially mediated by a change in the expression profile of glutamatergic receptor subunits. These findings can be useful in further understanding the cellular mechanism underlying deficits in information storage and, more generally, cognitive processes resulting from chronic opioid abuse.

LC-derived excitatory synaptic transmission to dorsal raphe serotonin neurons is inhibited by activation of alpha2-adrenergic receptors.

In the central nervous system, noradrenaline transmission controls the degree to which we are awake, alert, and attentive. Aberrant noradrenaline transmission is associated with pathological forms of hyper- and hypo-arousal that present in numerous neuropsychiatric disorders often associated with dysfunction in serotonin transmission. In vivo, noradrenaline regulates the release of serotonin because noradrenergic input drives the serotonin neurons to fire action potentials via activation of excitatory α1-adrenergic receptors (α1-AR). Despite the critical influence of noradrenaline on the activity of dorsal raphe serotonin neurons, the source of noradrenergic afferents has not been resolved and the presynaptic mechanisms that regulate noradrenaline-dependent synaptic transmission have not been described. Using an acute brain slice preparation from male and female mice and electrophysiological recordings from dorsal raphe serotonin neurons, we found that selective optogenetic activation of locus coeruleus terminals in the dorsal raphe was sufficient to produce an α1-AR-mediated excitatory postsynaptic current (α1-AR-EPSC). Activation of inhibitory α2-adrenergic receptors (α2-AR) with UK-14,304 eliminated the α1-AR-EPSC via presynaptic inhibition of noradrenaline release, likely via inhibition of voltage-gated calcium channels. In a subset of serotonin neurons, activation of postsynaptic α2-AR produced an outward current through activation of GIRK potassium conductance. Further, in vivo activation of α2-AR by systemic administration of clonidine reduced the expression of c-fos in the dorsal raphe serotonin neurons, indicating reduced neural activity. Thus, α2-AR are critical regulators of serotonin neuron excitability.

Pre- versus Post-synaptic Forms of LTP in Two Branches of the Same Hippocampal Afferent.

There has been considerable controversy about pre- versus postsynaptic expression of memory-related long-term potentiation (LTP), with corresponding disputes about underlying mechanisms. We report here an instance in male mice, in which both types of potentiation are expressed but in separate branches of the same hippocampal afferent. Induction of LTP in the dentate gyrus (DG) branch of the lateral perforant path (LPP) reduces paired-pulse facilitation, is blocked by antagonism of cannabinoid receptor type 1, and is not affected by suppression of postsynaptic actin polymerization. These observations are consistent with presynaptic expression. The opposite pattern of results was obtained in the LPP branch that innervates the distal dendrites of CA3: LTP did not reduce paired-pulse facilitation, was unaffected by the cannabinoid receptor blocker, and required postsynaptic actin filament assembly. Differences in the two LPP termination sites were also noted for frequency facilitation of synaptic responses, an effect that was reproduced in a two-step simulation by small adjustments to vesicle release dynamics. These results indicate that different types of glutamatergic neurons impose different forms of filtering and synaptic plasticity on their afferents. They also suggest that inputs are routed to, and encoded by, different sites within the hippocampus depending upon the pattern of activity arriving over the parent axon.

TAAR1 agonist ulotaront modulates striatal and hippocampal glutamate function in a state-dependent manner.

Aberrant dopaminergic and glutamatergic function, particularly within the striatum and hippocampus, has repeatedly been associated with the pathophysiology of schizophrenia. Supported by preclinical and recent clinical data, trace amine-associated receptor 1 (TAAR1) agonism has emerged as a potential new treatment approach for schizophrenia. While current evidence implicates TAAR1-mediated regulation of dopaminergic tone as the primary circuit mechanism, little is known about the effects of TAAR1 agonists on the glutamatergic system and excitation-inhibition balance. Here we assessed the impact of ulotaront (SEP-363856), a TAAR1 agonist in Phase III clinical development for schizophrenia, on glutamate function in the mouse striatum and hippocampus. Ulotaront reduced spontaneous glutamatergic synaptic transmission and neuronal firing in striatal and hippocampal brain slices, respectively. Interestingly, ulotaront potentiated electrically-evoked excitatory synaptic transmission in both brain regions, suggesting the ability to modulate glutamatergic signaling in a state-dependent manner. Similar striatal effects were also observed with the TAAR1 agonist, RO5166017. Furthermore, we show that ulotaront regulates excitation-inhibition balance in the striatum by specifically modulating glutamatergic, but not GABAergic, spontaneous synaptic events. These findings expand the mechanistic circuit hypothesis of ulotaront and TAAR1 agonists, which may be uniquely positioned to normalize both the excessive dopaminergic tone and regulate abnormal glutamatergic function associated with schizophrenia.

Deubiquitinase CYLD regulates excitatory synaptic transmission and short-term plasticity in the hippocampus.

The fate of proteins is determined by the addition of various forms of polyubiquitin during ubiquitin-mediated proteasomal degradation. Cylindromatosis (CYLD), a K63-specific deubiquitinase, is enriched in postsynaptic density fractions of the rodent central nervous system (CNS), but the synaptic role of CYLD in the CNS is poorly understand. Here we show that CYLD deficiency (Cyld-/-) results in reduced intrinsic hippocampal neuronal firing, a decrease in the frequency of spontaneous excitatory postsynaptic currents and a decrease in the amplitude of field excitatory postsynaptic potentials. Moreover, Cyld-/- hippocampus shows downregulated levels of presynaptic vesicular glutamate transporter 1 (vGlut1) and upregulated levels of postsynaptic GluA1, a subunit of the AMPA receptor, together with an altered paired-pulse ratio (PPR). We also found increased activation of astrocytes and microglia in the hippocampus of Cyld-/- mice. The present study suggests a critical role for CYLD in mediating hippocampal neuronal and synaptic activity.

Rapid astrocyte-dependent facilitation amplifies multi-vesicular release in hippocampal synapses.

Synaptic facilitation is a major form of short-term plasticity typically driven by an increase in residual presynaptic calcium. Using near-total internal reflection fluorescence (near-TIRF) imaging of single vesicle release in cultured hippocampal synapses, we demonstrate a distinctive, release-dependent form of facilitation in which probability of vesicle release is higher following a successful glutamate release event than following a failure. This phenomenon has an onset of ≤500 ms and lasts several seconds, resulting in clusters of successful release events. The release-dependent facilitation requires neuronal contact with astrocytes and astrocytic glutamate uptake by EAAT1. It is not observed in neurons grown alone or in the presence of astrocyte-conditioned media. This form of facilitation dynamically amplifies multi-vesicular release. Facilitation-evoked release events exhibit spatial clustering and have a preferential localization toward the active zone center. These results uncover a rapid astrocyte-dependent form of facilitation acting via modulation of multi-vesicular release and displaying distinctive spatiotemporal properties.

A novel machine learning-based approach for the detection and analysis of spontaneous synaptic currents.

Spontaneous synaptic activity is a hallmark of biological neural networks. A thorough description of these synaptic signals is essential for understanding neurotransmitter release and the generation of a postsynaptic response. However, the complexity of synaptic current trajectories has either precluded an in-depth analysis or it has forced human observers to resort to manual or semi-automated approaches based on subjective amplitude and area threshold settings. Both procedures are time-consuming, error-prone and likely affected by human bias. Here, we present three complimentary methods for a fully automated analysis of spontaneous excitatory postsynaptic currents measured in major cell types of the mouse retina and in a primary culture of mouse auditory cortex. Two approaches rely on classical threshold methods, while the third represents a novel machine learning-based algorithm. Comparison with frequently used existing methods demonstrates the suitability of our algorithms for an unbiased and efficient analysis of synaptic signals in the central nervous system.

Spinal cord injury is associated with changes in synaptic properties of the mouse major pelvic ganglion.

Spinal cord injury (SCI) has substantial impacts on autonomic function. In part, SCI results in loss of normal autonomic activity that contributes to injury-associated pathology such as neurogenic bladder, bowel, and sexual dysfunction. Yet little is known of the impacts of SCI on peripheral autonomic neurons that directly innervate these target organs. In this study, we measured changes in synaptic properties of neurons of the mouse major pelvic ganglion (MPG) associated with acute and chronic SCI. Our data show that functional and physiological properties of synapses onto MPG neurons are altered after SCI and differ between acute and chronic injury. After acute injury excitatory postsynaptic potentials (EPSPs) show increased rise and decay time constants leading to overall broader and longer EPSPs, whereas in chronic-injured animals EPSPs are reduced in amplitude and show faster rise and decay leading to shorter EPSPs. Synaptic depression and low-pass filtering are also altered in injured animals. Finally, cholinergic currents are smaller in acute-injured animals but larger in chronic-injured animals relative to control animals. These changes in synaptic properties are associated with differences in nicotinic receptor subunit expression as well. MPG CHRNA3 mRNA levels decreased after injury, whereas CHRNA4 mRNAs increased. Furthermore, changes in the correlations of α- and ß-subunit mRNAs suggest that nicotinic receptor subtype composition is altered after injury. Taken together, our data demonstrate that peripheral autonomic neurons are fundamentally altered after SCI, suggesting that longer-term therapeutic approaches could target these neurons directly to potentially help ameliorate neurogenic target organ dysfunction.NEW & NOTEWORTHY Spinal cord injury (SCI) has substantial impacts on autonomic function, yet little is known of the impacts of SCI on autonomic neurons that directly innervate effectors impacted by injury. Here we investigated changes at the cellular level associated with SCI in neurons that are "downstream" of the central injury. An understanding of these off-target impacts of SCI ultimately will be critical in the context of effective restoration of function through neuromodulation of pharmacological therapeutic approaches.

Ca2+ -independent transmission at the central synapse formed between dorsal root ganglion and dorsal horn neurons.

A central principle of synaptic transmission is that action potential-induced presynaptic neurotransmitter release occurs exclusively via Ca2+ -dependent secretion (CDS). The discovery and mechanistic investigations of Ca2+ -independent but voltage-dependent secretion (CiVDS) have demonstrated that the action potential per se is sufficient to trigger neurotransmission in the somata of primary sensory and sympathetic neurons in mammals. One key question remains, however, whether CiVDS contributes to central synaptic transmission. Here, we report, in the central transmission from presynaptic (dorsal root ganglion) to postsynaptic (spinal dorsal horn) neurons in vitro, (i) excitatory postsynaptic currents (EPSCs) are mediated by glutamate transmission through both CiVDS (up to 87%) and CDS; (ii) CiVDS-mediated EPSCs are independent of extracellular and intracellular Ca2+ ; (iii) CiVDS is faster than CDS in vesicle recycling with much less short-term depression; (iv) the fusion machinery of CiVDS includes Cav2.2 (voltage sensor) and SNARE (fusion pore). Together, an essential component of activity-induced EPSCs is mediated by CiVDS in a central synapse.

eEF2 in the prefrontal cortex promotes excitatory synaptic transmission and social novelty behavior.

Regulation of mRNA translation is essential for brain development and function. Translation elongation factor eEF2 acts as a molecular hub orchestrating various synaptic signals to protein synthesis control and participates in hippocampus-dependent cognitive functions. However, whether eEF2 regulates other behaviors in different brain regions has been unknown. Here, we construct a line of Eef2 heterozygous (HET) mice, which show a reduction in eEF2 and protein synthesis mainly in excitatory neurons of the prefrontal cortex. The mice also show lower spine density, reduced excitability, and AMPAR-mediated synaptic transmission in pyramidal neurons of the medial prefrontal cortex (mPFC). While HET mice exhibit normal learning and memory, they show defective social behavior and elevated anxiety. Knockdown of Eef2 in excitatory neurons of the mPFC specifically is sufficient to impair social novelty preference. Either chemogenetic activation of excitatory neurons in the mPFC or mPFC local infusion of the AMPAR potentiator PF-4778574 corrects the social novelty deficit of HET mice. Collectively, we identify a novel role for eEF2 in promoting prefrontal AMPAR-mediated synaptic transmission underlying social novelty behavior.

NX210c Peptide Promotes Glutamatergic Receptor-Mediated Synaptic Transmission and Signaling in the Mouse Central Nervous System.

NX210c is a disease-modifying dodecapeptide derived from the subcommissural organ-spondin that is under preclinical and clinical development for the treatment of neurological disorders. Here, using whole-cell patch-clamp recordings, we demonstrate that NX210c increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)- and GluN2A-containing N-methyl-D-aspartate receptor (GluN2A-NMDAR)-mediated excitatory postsynaptic currents in the brain. Accordingly, using extracellular field excitatory postsynaptic potential recordings, an enhancement of synaptic transmission was shown in the presence of NX210c in two different neuronal circuits. Furthermore, the modulation of synaptic transmission and GluN2A-NMDAR-driven signaling by NX210c restored memory in mice chronically treated with the NMDAR antagonist phencyclidine. Overall, by promoting glutamatergic receptor-related neurotransmission and signaling, NX210c represents an innovative therapeutic opportunity for patients suffering from CNS disorders, injuries, and states with crippling synaptic dysfunctions.

Bidirectional modulation of evoked synaptic transmission by pulsed infrared light.

Infrared (IR) neuromodulation (INM) has been demonstrated as a novel modulation modality of neuronal excitability. However, the effects of pulsed IR light on synaptic transmission have not been investigated systematically. In this report, the IR light (2 µm) is used to directly modulate evoked synaptic transmission at the crayfish opener neuromuscular junction. The extracellularly recorded terminal action potentials (tAPs) and evoked excitatory postsynaptic currents (EPSCs) modulated by localized IR light illumination (500 ms, 3-13 mW) aimed at the synapses are analyzed. The impact of a single IR light pulse on the presynaptic Ca2+ influx is monitored with Ca2+ indicators. The EPSC amplitude is enhanced, and its rising phase is accelerated under relatively low IR light power levels and localized temperature rises. Increasing the IR light power reversibly suppresses and eventually blocks the EPSCs. Meanwhile, the synaptic delay, tAP amplitude, and presynaptic Ca2+ influx decrease monotonously with higher IR light power. It is demonstrated for the first time that IR light illumination has bidirectional effects on evoked synaptic transmission. These results highlight the efficacy and flexibility of using pulsed IR light to directly control synaptic transmission and advance our understanding of INM of neural networks.

A direct excitatory projection from entorhinal layer 6b neurons to the hippocampus contributes to spatial coding and memory.

The mammalian hippocampal formation (HF) plays a key role in several higher brain functions, such as spatial coding, learning and memory. Its simple circuit architecture is often viewed as a trisynaptic loop, processing input originating from the superficial layers of the entorhinal cortex (EC) and sending it back to its deeper layers. Here, we show that excitatory neurons in layer 6b of the mouse EC project to all sub-regions comprising the HF and receive input from the CA1, thalamus and claustrum. Furthermore, their output is characterized by unique slow-decaying excitatory postsynaptic currents capable of driving plateau-like potentials in their postsynaptic targets. Optogenetic inhibition of the EC-6b pathway affects spatial coding in CA1 pyramidal neurons, while cell ablation impairs not only acquisition of new spatial memories, but also degradation of previously acquired ones. Our results provide evidence of a functional role for cortical layer 6b neurons in the adult brain.

Impaired learning and memory generated by hyperthyroidism is rescued by restoration of AMPA and NMDA receptors function.

Hyperthyroidism has been identified as a risk factor for cognitive disorders. The hippocampus is a key brain region associated with cognitive function, among which excitatory synapse transmission plays an important role in the process of learning and memory. However, the mechanism by which hyperthyroidism leads to cognitive dysfunction through a synaptic mechanism remains unknown. We investigated the synaptic mechanisms in the effects of hyperthyroidism in an animal model that involved repeated injection of triiodothyronine (T3). These mice displayed impaired learning and memory in the Novel object recognition test, Y-maze test, and Morris Water Maze test, as well as elevated anxiety in the elevated plus maze. Mature dendritic spines in the hippocampal CA1 region of hyperthyroid mice were significantly decreased, accompanied by decreased level of AMPA- and NMDA-type glutamate receptors in the hippocampus. In primary cultured hippocampal neurons, levels of AMPA- and NMDA-type glutamate receptors also decreased and whole-cell patch-clamp recording revealed that excitatory synaptic function was obviously attenuated after T3 treatment. Notably, pharmacological activation of AMPAR or NMDAR by intraperitoneal injection of CX546, an AMPAR agonist, or NMDA, an NMDAR agonist can restore excitatory synaptic function and corrected impaired learning and memory deficit in hyperthyroid mice. Together, our findings uncovered a previously unrecognized AMPAR and NMDAR-dependent mechanism involved in regulating hippocampal excitatory synaptic transmission and learning and memory disorders in hyperthyroidism.