Glutamatergic Excitatory Postsynaptic Currents Research Articles

Auditory hair cells and spiral ganglion neurons regenerate synapses with refined release properties in vitro

Ribbon synapses between inner hair cells (IHCs) and type I spiral ganglion neurons (SGNs) in the inner ear are damaged by noise trauma and with aging, causing "synaptopathy" and hearing loss. Cocultures of neonatal denervated organs of Corti and newly introduced SGNs have been developed to find strategies for improving IHC synapse regeneration, but evidence of the physiological normality of regenerated synapses is missing. This study utilizes IHC optogenetic stimulation and SGN recordings, showing that, when P3-5 denervated organs of Corti are cocultured with SGNs, newly formed IHC/SGN synapses are indeed functional, exhibiting glutamatergic excitatory postsynaptic currents. When using older organs of Corti at P10-11, synaptic activity probed by deconvolution showed more mature release properties, closer to the specialized mode of IHC synaptic transmission crucial for coding the sound signal. This functional assessment of newly formed IHC synapses developed here, provides a powerful tool for testing approaches to improve synapse regeneration.

Thermoregulatory pathway underlying the pyrogenic effects of prostaglandin E2 in the lateral parabrachial nucleus of male rats.

It has been shown that prostaglandin (PG) E2 synthesized in the lateral parabrachial nucleus (LPBN) is involved in lipopolysaccharide-induced fever. But the neural mechanisms of how intra-LPBN PGE2 induces fever remain unclear. In this study, we investigated whether the LPBN-preoptic area (POA) pathway, the thermoafferent pathway for feed-forward thermoregulatory responses, mediates fever induced by intra-LPBN PGE2 in male rats. The core temperature (Tcore) was monitored using a temperature radiotelemetry transponder implanted in rat abdomen. We showed that microinjection of PGE2 (0.28 nmol) into the LPBN significantly enhanced the density of c-Fos-positive neurons in the median preoptic area (MnPO). The chemical lesioning of MnPO with ibotenate or selective genetic lesioning or inhibition of the LPBN-MnPO pathway significantly attenuated fever induced by intra-LPBN injection of PGE2. We demonstrated that EP3 receptor was a pivotal receptor for PGE2-induced fever, since microinjection of EP3 receptor agonist sulprostone (0.2 nmol) or EP3 receptor antagonist L-798106 (2 nmol) into the LPBN mimicked or weakened the pyrogenic action of LPBN PGE2, respectively, but this was not the case for EP4 and EP1 receptors. Whole-cell recording from acute LPBN slices revealed that the majority of MnPO-projecting neurons originating from the external lateral (el) and dorsal (d) LPBN were excited and inhibited, respectively, by PGE2 perfusion, initiating heat-gain and heat-loss mechanisms. The amplitude but not the frequency of spontaneous and miniature glutamatergic excitatory postsynaptic currents (sEPSCs and mEPSCs) in MnPO-projecting LPBel neurons increased after perfusion with PGE2; whereas the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) and the A-type potassium (IA) current density did not change. In MnPO-projecting LPBd neurons, neither sEPSCs nor sIPSCs responded to PGE2; however, the IA current density was significantly increased by PGE2 perfusion. These electrophysiological responses and the thermoeffector reactions to intra-LPBN PGE2 injection, including increased brown adipose tissue thermogenesis, shivering, and decreased heat dissipation, were all abolished by L-798106, and mimicked by sulprostone. These results suggest that the pyrogenic effects of intra-LPBN PGE2 are mediated by both the inhibition of the LPBd-POA pathway through the EP3 receptor-mediated activation of IA currents and the activation of the LPBel-POA pathway through the selective enhancement of glutamatergic synaptic transmission via EP3 receptors.

Functional Impact of nTS Glutamate Stress on Respiration in an Alzheimer’s Disease Model

Alzheimer’s disease (AD) is closely associated with obstructive sleep apnea (OSA). Such hypoxic insults trigger glutamate release of chemoafferents into the nucleus tractus solitarii (nTS), an important upstream center of the chemoreflex. Potential alterations of glutamate handling in the nTS may lead to the respiratory dysfunction seen in AD patients. Using the streptozotocin (STZ)-induced rat model — an effective proxy for human AD — we studied the functional consequences of nTS glutamate stress on respiration. STZ-AD was induced in 6-week-old male Sprague Dawley rats via intracerebroventricular injections of 2 - 2.25 mg/kg STZ. After two weeks, EMG recordings of diaphragm activity served as surrogate for respiratory responses to glutamate microinjections (20 nL of 40 mM) into the caudal nTS. In a subset of rats, chemoafferent terminals were labeled with the fluorescent tracer DiI to permit patch clamp recordings of identified 2nd order nTS neurons participating in the chemoreflex loop. Evoked glutamatergic excitatory postsynaptic currents (TS-EPSCs) were generated via tractus solitarii stimulation at frequencies (10 - 40 Hz) typical for afferent discharge during hypoxia. Acute glutamate injections into the caudal nTS increased respiratory frequency by 15 - 20 breaths per minute. The response magnitude was similar between CTL and STZ-AD animals. Excitatory stress with repeated glutamate injections (5 min. apart) evoked a reliable respiratory response in CTL that slowly declined to ~65% from its maximum over the course of 10 injections. In contrast, the decline of respiratory response in STZ-AD was more pronounced and occurred significantly earlier than in CTL, indicating altered glutamate handling in the caudal nTS of STZ-AD rats. Next, we used patch clamp electrophysiology to analyze increased glutamate release of chemoafferents onto 2nd order nTS neurons. High frequency stimulation of chemoafferents induced a successive depression of TS-EPSC amplitude in nTS neurons. TS-EPSC depression was significantly stronger in STZ-AD and may be due to altered glutamate handling at the synapse. Western Blot analysis of STZ-AD brainstem tissue including the nTS revealed a significant increase in glial fibrillary acidic protein (GFAP) expression and a decrease of excitatory amino acid transporters (EAATs) expression, suggesting astrocyte involvement in compromised nTS glutamate handling in STZ-AD. In summary, the STZ-AD model showed reduced respiratory responses to glutamate stress in the caudal nTS. The altered stress response may come from enhanced depression of TS-EPSC amplitude at nTS neurons in the chemoreflex. Altered astrocytic glutamate removal from the synaptic cleft may contribute to TS-EPSC depression. Together, altered glutamate handling in a respiratory brainstem center may contribute to OSA in AD patients. NIH R15AG065927 (TDO & DO), KCOM Biomedical Graduate Program (RKT &TDO), ATSU Research Support (SKRC). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Prenatal treatment with corticosterone via maternal injection induces learning and memory impairments via delaying postsynaptic development in hippocampal CA1 neurons of rats.

Previously, we reported that prenatal exposure to high corticosterone induced attention-deficit hyperactivity disorder (ADHD)-like behaviors with cognitive deficits after weaning. In the present study, cellular mechanisms underlying cortisol-induced cognitive dysfunction were investigated using rat pups (Corti.Pups) born from rat mothers that were repetitively injected with corticosterone during pregnancy. In results, Corti.Pups exhibited the failure of behavioral memory formation in the Morris water maze (MWM) test and the incomplete long-term potentiation (LTP) of hippocampal CA1 neurons. Additionally, glutamatergic excitatory postsynaptic currents (EPSCs) were remarkably suppressed in Corti.Pups compared to normal rat pups. Incomplete LTP and weaker EPSCs in Corti.Pups were attributed to the delayed postsynaptic development of CA1 neurons, showing a higher expression of NR2B subunits and lower expression of PSD-95 and BDNF. These results indicated that the prenatal treatment with corticosterone to elevate cortisol level might potently downregulate the BDNF-mediated signaling critical for the synaptic development of hippocampal CA1 neurons during brain development, and subsequently, induce learning and memory impairment. Our findings suggest a possibility that the prenatal dysregulation of cortisol triggers the epigenetic pathogenesis of neurodevelopmental psychiatric disorders, such as ADHD and autism.

Insulin modulates the paired-pulse plasticity at glutamatergic synapses of hippocampal neurons under hypoinsulinemia.

Hypoinsulinemia is a pathological consequence of diabetes mellitus that can cause a number of complications of the central and peripheral nervous system. Dysfunction of signaling cascades of insulin receptors under insulin deficiency can contribute to the development of cognitive disorders associated with impaired synaptic plasticity properties. Earlier we have shown that hypoinsulinemia causes a shift of short-term plasticity in glutamatergic hippocampal synapses from facilitation to depression and apparently involves mechanisms of glutamate release probability reduction. Here we used the whole cell patch-clamp recording of evoked glutamatergic excitatory postsynaptic currents (eEPSCs) and the method of local extracellular electrical stimulation of a single presynaptic axon to investigate the effect of insulin (100 nM) on the paired-pulse plasticity at glutamatergic synapses of cultured hippocampal neurons under hypoinsulinemia. Our data indicate that under normoinsulinemia additional insulin enhances the paired-pulse facilitation (PPF) of eEPSCs in hippocampal neurons by stimulating the glutamate release in their synapses. Under hypoinsulinemia, insulin did not have a significant effect on the parameters of paired-pulse plasticity on neurons of PPF subgroup, which may indicate the development of insulin resistance, while the effect of insulin on PPD neurons indicates its ability to recover the form normoinsulinemia, including the increasing probability of plasticity to the control level in of glutamate release in their synapses.

A Neuromodulatory System that Links Opioid‐Induced Sedation and Respiratory Depression

The main cause of death from opioid overdose is respiratory depression, mediated by mu‐opioid‐receptors located in the respiratory brainstem. Opioids also cause sedation, which further reduces respiratory drive through largely unknown mechanisms. The Kölliker‐Fuse nucleus (KF) in the pons is highly susceptible to inhibition by opioids and significantly contributes to opioid‐induced respiratory rate depression and associated changes in breathing pattern. Taking advantage of cell‐type specific retrograde neural tracing, we characterized anatomical projections between the main noradrenergic arousal center of the brain, the locus coeruleus (LC) and the KF, which represent a potential link between arousal/sedation and breathing. Our results show that 30% of noradrenergic LC neurons send projections to the KF. To begin to understand the functional role and to define the neurochemical identity of LC to KF projections, we used whole‐cell patch clamp recordings from the KF combined with optogenetic stimulation of LC neurons. Optical stimulation of LC axon terminals induces light‐evoked glutamatergic excitatory postsynaptic currents in KF neurons. Pharmacological isolation of LC to KF synapses suggests that this pathway is monosynaptic. The glutamatergic excitatory postsynaptic currents are dependent on vesicular glutamate transporter 2 expression in noradrenergic LC neurons. Additionally, prolonged optical stimulations lead to noradrenergic excitatory postsynaptic currents that are blocked by alpha1 receptor antagonist. Thus, LC may drive KF neuron activity via glutamate and/or noradrenaline release. This direct excitatory drive in the LC‐KF circuit is sensitive to opioids, which can directly inhibit KF neuron somas, as well as inhibit neurotransmitter release from presynaptic LC terminals. This work provides exciting evidence for an excitatory pathway between the LC and the KF, that may play an important link between opioid‐induced sedation and respiratory depression.

NTS Glutamate Signaling in the Chemoreflex Axis of an Alzheimer's Disease Rat Model

Respiratory dysfunction is commonly observed in Alzheimer’s disease (AD) and likely due to neuronal alterations of the respiratory network in the brain. The streptozotocin (STZ) ‐induced AD model mimics respiratory problems seen in humans and exhibits neuronal hyperactivity in the nucleus tractus solitarii (nTS), the central integration site for respiratory afferents in the brainstem. This ongoing study looks at the changes of glutamate signaling within the chemoreflex axis as a possible mechanism for nTS hyperactivity and respiratory dysfunction in STZ‐AD.AD was induced by intracerebroventricular injection of 2 mg/kg STZ in 6‐week old male Sprague Dawley rats. After 14 days, caudal nTS neurons within close proximity of DiI‐labeled chemoafferent terminals were recorded using the patch clamp technique. Electrical stimulation of the tractus solitarii (TS) generated glutamatergic excitatory postsynaptic currents (EPSCs) that were compared between experimental groups. Consistent with enhanced glutamatergic signaling, TS‐EPSC amplitude was increased in STZ‐AD. High TS stimulation frequency induced a time and frequency dependent depression that was similar between groups, indicating no change in presynaptic vesicle turnover or postsynaptic receptor desensitization. On the other hand, asynchronous EPSCs following high frequency stimulation seemed enhanced in STZ‐AD, supporting an increased excitatory tone in the nTS. Immunohistochemical analysis of the caudal nTS revealed no change in neuron number but showed a reduced synaptic density in the AD model. Astrocyte number and morphology were unchanged between groups. However, microglia, which are able to influence glutamate‐handling of astrocytes, showed an activated phenotype.Overall, our data show enhanced glutamatergic signaling in the chemoreflex axis contributing to nTS hyperactivity of the STZ‐AD model. nTS hyperactivity may lead to the observed respiratory dysfunction in this model and potentially constitutes a mechanism for sleep disordered breathing in AD patients.

Cannabinoid CB2 receptors are upregulated via bivalent histone modifications and control primary afferent input to the spinal cord in neuropathic pain

Type-2 cannabinoid receptors (CB2, encoded by the Cnr2 gene) are mainly expressed in immune cells, and CB2 agonists normally have no analgesic effect. However, nerve injury upregulates CB2 in the dorsal root ganglion (DRG), following which CB2 stimulation reduces neuropathic pain. It is unclear how nerve injury increases CB2 expression or how CB2 activity is transformed in neuropathic pain. In this study, immunoblotting showed that spinal nerve ligation (SNL) induced a delayed and sustained increase in CB2 expression in the DRG and dorsal spinal cord synaptosomes. RNAscope in situ hybridization also showed that SNL substantially increased CB2 mRNA levels, mostly in medium and large DRG neurons. Furthermore, we found that the specific CB2 agonist JWH-133 significantly inhibits the amplitude of dorsal root–evoked glutamatergic excitatory postsynaptic currents in spinal dorsal horn neurons in SNL rats, but not in sham control rats; intrathecal injection of JWH-133 reversed pain hypersensitivity in SNL rats, but had no effect in sham control rats. In addition, chromatin immunoprecipitation–qPCR analysis showed that SNL increased enrichment of two activating histone marks (H3K4me3 and H3K9ac) and diminished occupancy of two repressive histone marks (H3K9me2 and H3K27me3) at the Cnr2 promoter in the DRG. In contrast, SNL had no effect on DNA methylation levels around the Cnr2 promoter. Our findings suggest that peripheral nerve injury promotes CB2 expression in primary sensory neurons via epigenetic bivalent histone modifications and that CB2 activation reduces neuropathic pain by attenuating nociceptive transmission from primary afferent nerves to the spinal cord.

Вплив штучної гіпоінсулінемії на синаптичну активність та пластичність глутаматергічної нейропередачі в культурі нейронів гіпокампа

We investigated the effect of chronic hypoinsulinemia on the level of synaptic activity and short-term plasticity in cultured hippocampal neurons. Hypoinsulinemia was induced by culturing mature (16-20 days in vitro) rat’s hippocampal neurons without insulin for 1, 2, and 4 days. The control insulin concentration was 100 nM. Spontaneous and evoked glutamatergic excitatory postsynaptic currents (sEPSC and eEPSC, respectively) in these neurons were analyzed using the whole-cell patch-clamp method and the method of local electrical stimulation of individual axon. Hypoinsulinemia during the 1st, 2nd and 4th days led to significantly reduction of the mean sEPSC’s frequency to 49.9 ± 15.8% (n = 6), 8.5 ± 7.7% (n = 6) and 16.6 ± 5.2% (n = 8) respectively, relative to control. Also, there was a decrease of the average sEPSC’s amplitudes to 52.6 ± 5.5% (n = 6), 36.6 ± 5.8% (n = 6) and 43.9 ± 8.4% (n = 8), respectively, relative to control. Quantal analysis of the sEPSC’s amplitudes showed a decrease of multivesicular glutamate release at the synapses under such conditions. Hypoinsulinemia caused a shift in the direction of short-term plasticity in glutamatergic hippocampal synapses from potentiation to depression. The paired-pulse ratio decreased from 1.83 ± 0.25 in the control to 0.59 ± 0.07, 0.77 ± 0.07, and 0.80 ± 0.06 after the 1st, 2nd, and 4th days under cultivation without insulin. Accordingly, the ratio of the coefficients of variation of eEPSC’s amplitudes (CV2/ CV1) increased from 0.82 ± 0.07 to 1.30 ± 0.28, 1.52 ± 0.27, and 1.61 ± 0.24. The presented results indicate a significant reduction of synaptic activity and decrease in the probability of multivesicular release of glutamate at the synapses of cultured hippocampal neurons under hypoinsulinemia.

Purinergic P2 and glutamate NMDA receptor coupling contributes to osmotically driven excitability in hypothalamic magnocellular neurosecretory neurons.

Purinergic and glutamatergic signalling pathways play a key role in regulating the activity of hypothalamic magnocellular neurosecretory neurons (MNNs). However, the precise cellular mechanisms by which ATP and glutamate act in concert to regulate osmotically driven MNN neuronal excitability remains unknown. Here, we report that ATP acts on purinergic P2 receptors in MNNs to potentiate in a Ca2+ -dependent manner extrasynaptic NMDAR function. The P2-NMDAR coupling is engaged in response to an acute hyperosmotic stimulation, contributing to osmotically driven firing activity in MNNs. These results help us to better understand the precise mechanisms contributing to the osmotic regulation of firing activity and hormone release from MNNs. The firing activity of hypothalamic magnocellular neurosecretory neurons (MNNs) located in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) is coordinated by the combined, fine-tuned action of intrinsic membrane properties, synaptic and extrasynaptic signalling. Among these, purinergic and glutamatergic signalling pathways have been shown to play a key role regulating the activity of MNNs. However, the precise cellular mechanisms by which ATP and glutamate act in concert to regulate osmotically driven MNN neuronal excitability remains unknown. Whole-cell patch-clamp recordings obtained from MNNs showed that ATP (100μM) induced an increase in firing rate, an effect that was blocked by either 4-[[4-formyl-5-hydroxy-6-methyl-3-[(phosphonooxy)methyl]2-pyridinyl]azo]1,3-benzenedisulfonic acid tetrasodium salt (PPADS) (10μM) or kynurenic acid (1mm). While ATP did not affect the frequency or magnitude of glutamatergic excitatory postsynaptic currents (EPSCs), it induced an inward shift in the holding current that was prevented by PPADS or kynurenic acid treatment, suggesting that ATP enhances a tonic extrasynaptic glutamatergic excitatory current. We observed that ATP-potentiated glutamatergic receptor-mediated currents were evoked by focal application of L-glu (1mm) and NMDA (50μM), but not AMPA (50μM). ATP potentiation of NMDA-evoked currents was blocked by PPADS (10μM) and by chelation of intracellular Ca2+ with BAPTA (10mm). Finally, we report that a hyperosmotic stimulus (mannitol 1%, +55mOsm/kgH2 O) potentiated NMDA-evoked currents and increased MNN firing activity, effects that were blocked by PPADS. Taken together, our data support a functional excitatory coupling between P2 and extrasynaptic NMDA receptors in MNNs, which is engaged in response to an acute hyperosmotic stimulus.

026 Vasoactive Intestinal Polypeptide Directly Excites Neurons of the Subparaventricular Zone

Abstract Introduction The suprachiasmatic nucleus (SCN) is responsible for generating the circadian rhythmicity in mammals. The ventral region or core of the SCN contains neurons that express the neuropeptide vasoactive intestinal polypeptide (VIP). VIP signaling is central for coherency and synchrony of SCN activity. VIP-expressing neurons in the SCN densely project to the ventral subregions of the subparaventricular zone (vSPZ). We studied the effects of VIP on vSPZ neurons in brain slices of mice with a combined calcium imaging and whole-cell patch-clamp recording techniques. We used calcium imaging to assess the effects of VIP on vSPZ neurons as a population and we acquired patch-clamp recordings to explore the effects of VIP on the electrical properties and the synaptic inputs to vSPZ neurons. Methods We expressed GCamp6 in vSPZ neurons by stereotaxically injecting AAV10-DIO-Ef1a-GCamp6 into the vSPZ of vGAT-IRES-Cre mice. Brain slices were prepared two weeks later and images were captured using a standard GFP filter set. We performed whole-cell recordings of the vSPZ neurons of wild-type mice. We assessed the effects of VIP on the membrane potential and the on excitatory synaptic input in vSPZ neurons. Results Using GCamp6-based in vitro calcium imaging we found that VIP excites 17% of vSPZ neurons and this effect was maintained in the presence of tetrodotoxin (TTX) and synaptic blockers for AMPA/NMDA and GABAA transmissions suggesting a direct effect of VIP on vSPZ neurons. We confirmed this result with patch-clamp recordings. We found that 29% of vSPZ neurons were excited by VIP. VIP produced a membrane depolarization of vSPZ neurons in the presence of antagonists for AMPA/NMDA and GABAA receptors. In addition, we found that in a small percentage of vSPZ neurons VIP increased the frequency of the glutamatergic excitatory postsynaptic currents, suggesting an additional excitatory mechanism. Conclusion Our results demonstrate that exogenous VIP directly excites the vSPZ neurons producing an increase in intracellular calcium and membrane depolarization. In addition, VIP increases glutamatergic afferent inputs to vSPZ neurons indicating an additional synergistic excitation. We conclude that when VIP is released from the SCN VIP fibers it can activate vSPZ neurons. Support (if any) NS091126 and HL149630.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H+] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca2+, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H+ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

5-HT1A Receptor Agonist Promotes Retinal Ganglion Cell Function by Inhibiting OFF-Type Presynaptic Glutamatergic Activity in a Chronic Glaucoma Model.

Serotonin receptors are potential neuroprotective agents in degenerative diseases of the central nervous system. The protective effects of serotonin receptor (5-HT1A) agonists on the survival and function of retinal ganglion cells (RGCs) by regulating the release of the presynaptic neurotransmitter γ-aminobutyric acid (GABA) were confirmed in our previous study of a chronic glaucoma rat model. However, the roles of excitatory amino acids and their interactions with the 5-HT1A receptor in glaucoma remain unknown. Here, we found that ocular hypertension increased glutamine synthetase (GS) and excitatory amino acid transporter 2 (EAAT2) expression in rat retinas. In addition, the high expression of GS and EAAT2 induced by glaucoma was downregulated by the 5-HT1A receptor agonist 8-OH-DPAT and the 5-HT1A receptor antagonist WAY-100635, respectively. Patch-clamp techniques were used to record glutamate receptor-mediated spontaneous and miniature glutamatergic excitatory post-synaptic currents (sEPSCs and mEPSCs) as well as L-glutamate-induced current in OFF-type and ON-type RGCs in rat retinal slices. Although there were no significant differences in the frequency and amplitude of sEPSC and mEPSC release between normal and glaucoma OFF- and ON-type RGCs, exogenous 8-OH-DPAT administration specifically reduced the frequency, but not the amplitude, of sEPSC and mEPSC release in glaucoma OFF-type rather than ON-type RGCs; these effects were completely blocked by WAY-100635. In summary, 8-OH-DPAT decreases and increases GS and EAAT2 expression of glaucomatous retina, respectively, while decreasing sEPSC and mEPSC frequency. In contrast, WAY-100635 increases and decreases GS and EAAT2 expression of glaucomatous retina, respectively, while increasing sEPSC and mEPSC frequency. The reduction of glutamatergic presynaptic transmission by 8-OH-DPAT deactivates RGCs at the neural network level and reduces the excitotoxic damage in the pathological process of chronic glaucoma.

Modulation of acid sensing ion channel dependent protonergic neurotransmission at the mouse calyx of Held

Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in neurodegenerative diseases. It has been reported that homomeric ASIC-1a channels are expressed in neurons of the medial nucleus of the trapezoid body (MNTB) of the auditory system in the CNS. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H+ from synaptic vesicles activates postsynaptic ASIC-1a channels in mice up to 3 weeks old. This generates synaptic currents (ASIC1a-SCs) that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that neuromodulators like histamine and natural products like lactate and spermine potentiate ASIC1a-SCs in an additive form such that excitatory ASIC synaptic currents as well as the associated calcium influx become significantly large and physiologically relevant. We show that ASIC1a-SCs enhanced by endogenous neuromodulators are capable of supporting synaptic transmission in the absence of glutamatergic EPSCs. Furthermore, at high frequency stimulation (HFS), ASIC1a-SCs contribute to diminish short term depression (STD) and their contribution is even more relevant at early stages of development. Since ASIC channels are present in almost all types of neurons and synaptic vesicles content is acid, the participation of protons in synaptic transmission and its potentiation by endogenous substances could be a general phenomenon across the central nervous system. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Calcium channel subtypes on glutamatergic mossy fiber terminals synapsing onto rat hippocampal CA3 neurons.

The current electrophysiological study investigated the functional roles of high- and low-voltage-activated Ca2+ channel subtypes on glutamatergic small mossy fiber nerve terminals (SMFTs) that synapse onto rat hippocampal CA3 neurons. Experiments combining both the "synapse bouton" preparation and single-pulse focal stimulation technique were performed using the conventional whole cell patch configuration under voltage-clamp conditions. Nifedipine, at a high concentration, and BAY K 8644 inhibited and facilitated the glutamatergic excitatory postsynaptic currents (eEPSCs) that were evoked by 0.2-Hz stimulation, respectively. However, these drugs had no effects on spontaneous EPSCs (sEPSCs). Following the use of a high stimulation frequency of 3 Hz, however, nifedipine markedly inhibited eEPSCs at the low concentration of 0.3 µM. Moreover, ω-conotoxin GVIA and ω-agatoxin IVA significantly inhibited both sEPSCs and eEPSCs. Furthermore, SNX-482 slightly inhibited eEPSCs. R(-)-efonidipine had no effects on either sEPSCs or eEPSCs. It was concluded that glutamate release from SMFTs depends largely on Ca2+ entry through N- and P/Q-type Ca2+ channels and, to a lesser extent, on R-type Ca2+ channels. The contribution of L-type Ca2+ channels to eEPSCs was small at low-firing SMFTs but more significant at high-firing SMFTs. T-type Ca2+ channels did not appear to be involved in neurotransmission at SMFTs. NEW & NOTEWORTHY Action potential-evoked glutamate release from small mossy fiber nerve terminals (SMFTs) that synapse onto rat hippocampal CA3 neurons is regulated by high-threshold but not low-threshold Ca2+ channel subtypes. The functional contribution mainly depends on N- and P/Q-type Ca2+ channels and, to a lesser extent, on R-type Ca2+ channels. However, in SMFTs stimulated at a high 3-Hz frequency, L-type Ca2+ channels contributed significantly to the currents. The present results are consistent with previous findings from fluorometric studies of large mossy fiber boutons.

Angiotensin II facilitates GABAergic neurotransmission at postsynaptic sites in rat amygdala neurons

The central nucleus of the amygdala (CeA) is critical in the regulation of sodium appetite. Angiotensin II (Ang II) is important in the generation of sodium appetite and may function as a neurotransmitter or modulator to affect the synaptic transmission and the excitability of neurons. However, the role of Ang II in the CeA remains unclear. In this study, we determined the effects of Ang II on the excitatory and inhibitory synaptic inputs to the CeA neurons in brain slices with whole-cell patch-clamp recordings. Ang II (0.5–5 μM) significantly potentiated the amplitude of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) in a concentration-dependent manner. Ang II (2 μM) significantly increased the amplitude of miniature GABAergic inhibitory postsynaptic currents (mIPSCs) without affecting the frequency. This effect was blocked by Ang II type 1 (AT1) receptor antagonist, losartan. One mM guanosine 5′-O-(-2-thiodiphosphate) (GDP-β-s) in the pipette internal solution eliminated the facilitatory effect of Ang II on GABAergic synaptic transmission. In contrast, Ang II had no effect on the spontaneous glutamatergic excitatory postsynaptic currents (EPSCs) and did not alter the frequency and amplitude of miniature EPSCs at concentrations that facilitated IPSCs. Furthermore, Ang II decreased the firing activity of CeA neurons, and this effect was abolished by losartan and GDP-β-s. In addition, Ang II failed to inhibit CeA neurons in the presence of bicuculline. These data provide substantial new evidence that Ang II inhibits the CeA neurons by facilitation of GABAergic synaptic input efficacy through activation of postsynaptic AT1 receptors.

Differential Modulation of GABAA and NMDA Receptors by an α7-nicotinic Acetylcholine Receptor Agonist in Chronic Glaucoma.

Presynaptic modulation of γ-aminobutyric acid (GABA) release by an alpha7 nicotinic acetylcholine receptor (α7-nAChR) agonist promotes retinal ganglion cell (RGC) survival and function, as suggested by a previous study on a chronic glaucomatous model from our laboratory. However, the role of excitatory and inhibitory amino acid receptors and their interaction with α7-nAChR in physiological and glaucomatous events remains unknown. In this study, we investigated GABAA and N-methyl-D-aspartate (NMDA) receptor activity in control and glaucomatous retinal slices and the regulation of amino acid receptor expression and function by α7-nAChR. Whole-cell patch-clamp recordings from RGCs revealed that the α7-nAChR specific agonist PNU-282987 enhanced the amplitude of currents elicited by GABA and reduced the amplitude of currents elicited by NMDA. The positive modulation of GABAA receptor and the negative modulation of NMDA receptor (NMDAR) by PNU-282987-evoked were prevented by pre-administration of the α7-nAChR antagonist methyllycaconitine (MLA). The frequency and the amplitude of glutamate receptor-mediated miniature glutamatergic excitatory postsynaptic currents (mEPSCs) were not significantly different between the control and glaucomatous RGCs. Additionally, PNU-282987-treated slices showed no alteration in the frequency or amplitude of mEPSCs relative to control RGCs. Moreover, we showed that expression of the α1 subunit of the GABAA receptor was downregulated and the expression of the NMDAR NR2B subunit was upregulated by intraocular pressure (IOP) elevation, and the changes of high IOP were blocked by PNU-282987. In conclusion, retina GABAA and NMDARs are modulated positively and negatively, respectively, by activation of α7-nAChR in in vivo chronic glaucomatous models.

Ketamine attenuates the glutamatergic neurotransmission in the ventral posteromedial nucleus slices of rats

BackgroundKetamine is a frequently used intravenous anesthetic, which can reversibly induce loss of consciousness (LOC). Previous studies have demonstrated that thalamocortical system is critical for information transmission and integration in the brain. The ventral posteromedial nucleus (VPM) is a critical component of thalamocortical system. Glutamate is an important excitatory neurotransmitter in the brain and may be involved in ketamine-induced LOC.MethodsThe study used whole-cell patch-clamp to observe the effect of ketamine (30 μM–1000 μM) on glutamatergic neurotransmission in VPM slices.ResultsKetamine significantly decreased the amplitude of glutamatergic spontaneous excitatory postsynaptic currents (sEPSCs), but only higher concentration of ketamine (300 μM and 1000 μM) suppressed the frequency of sEPSCs. Ketamine (100 μM–1000 μM) also decreased the amplitude of glutamatergic miniature excitatory postsynaptic currents (mEPSCs), without altering the frequency.ConclusionsIn VPM neurons, ketamine attenuates the glutamatergic neurotransmission mainly through postsynaptic mechanism and action potential may be involved in the process.

Neuroadaptations of presynaptic and postsynaptic GABAB receptor function in the paraventricular nucleus in response to chronic unpredictable stress.

Chronic stress impairs GABAA (GABA type A) receptor-mediated inhibition in the hypothalamic paraventricular nucleus (PVN). It is not clear whether GABAB receptor function is also altered. We hypothesize that chronic stress alters GABAB receptor function in PVN corticotrophin-releasing hormone (CRH) neurons to control hypothalamus-pituitary-adrenal axis activity. Whole-cell patch clamp recordings were made of PVN-CRH neurons expressing eGFP driven by CRH promoter in brain slices from unstressed rats and rats exposed to chronic unpredictable mild stress (CUMS). CUMS elevated the basal circulating corticosterone levels and increased the basal firing activity of PVN-CRH neurons. Microinjection of GABAB receptor agonist baclofen into the PVN suppressed the increased corticosterone levels in CUMS rats compared with unstressed rats. CUMS blunted the baclofen-induced inhibition on PVN-CRH neurons and outward currents in these neurons. Furthermore, CUMS reduced expression of GABAB1 (GABAB R1) protein in the PVN. Blocking NMDA receptors with AP5 restored the reduced baclofen-induced currents in CUMS rats but had no effect on GABAB1 expression. Furthermore, CUMS treatment augmented the baclofen-induced decrease in the frequency of glutamatergic excitatory postsynaptic currents (EPSCs) and GABAergic inhibitor postsynaptic currents in PVN-CRH neurons. The GABAB receptor antagonist CGP55845 increased the firing activity of PVN-CRH neurons only in CUMS-treated rats and not in unstressed rats. These findings suggest that chronic stress impairs postsynaptic GABAB receptor function but augments presynaptic GABAB receptor function in controlling glutamatergic and GABAergic synaptic inputs in PVN-CRH neurons.

Altered neurobiological function of brainstem hypoglossal neurons in DiGeorge/22q11.2 Deletion Syndrome

DiGeorge/22q11.2 Deletion Syndrome (22q11DS) is a common genetic microdeletion syndrome that underlies several neurodevelopmental disorders including autism, attention deficit/hyperactivity disorder, and schizophrenia. In addition to cognitive impairments, those with 22q11DS have disrupted feeding and swallowing from birth onward. This perinatal dysphagia significantly compromises nutritional status, impairs appropriate weight gain, and can lead to life threatening aspiration-based infections. Appropriately timed excitation and inhibition of brainstem hypoglossal motor neurons, which innervate tongue muscles, is essential for proper feeding and swallowing. In this study we have examined changes in hypoglossal motor neuron function in the LgDel mouse model of 22q11DS. Hypoglossal motor neurons from LgDel mouse pups have action potentials with afterhyperpolarizations, mediated by a large conductance charybdotoxin-sensitive Ca-activated K current, that are significantly shorter in duration and greater in magnitude than those in wild-type pups. In addition, the amplitude, but not frequency, of glutamatergic excitatory glutamatergic postsynaptic currents (EPSCs) is diminished, and GABAergic, but not glycinergic, neurotransmission to hypoglossal motor neurons was reduced in LgDel animals. These observations provide a foundation for understanding the neurological changes in hypoglossal motor neuron function and their contribution to swallowing abnormalities that occur in DiGeorge/22q11.2 Deletion Syndrome.