NMDA receptors promote survival in somatosensory relay nuclei by inhibiting Bax-dependent developmental cell death

Excitotoxic stimulation activates distinct pathogenic and protective expression signatures in the hippocampus

Excitotoxic events underlying ischaemic and traumatic brain injuries activate degenerative and protective pathways, particularly in the hippocampus. To understand opposing pathways that determine the brain's response to excitotoxicity, we used hippocampal explants, thereby eliminating systemic variables during a precise protocol of excitatory stimulation. N‐methyl‐d‐aspartate (NMDA) was applied for 20 min and total RNA isolated one and 24 h later for neurobiology‐specific microarrays. Distinct groups of genes exhibited early vs. delayed induction, with 63 genes exclusively reduced 24‐h post‐insult. Egr‐1 and NOR‐1 displayed biphasic transcriptional modulation: early induction followed by delayed suppression. Opposing events of NMDA‐induced genes linked to pathogenesis and cell survival constituted the early expression signature. Delayed degenerative indicators (up‐regulated pathogenic genes, down‐regulated pro‐survival genes) and opposing compensatory responses (down‐regulated pathogenic genes, up‐regulated pro‐survival genes) generated networks with temporal gene profiles mirroring coexpression network clustering. We then used the expression profiles to test whether NF‐κB, a potent transcription factor implicated in both degenerative and protective pathways, is involved in the opposing responses. The NF‐κB inhibitor MG‐132 indeed altered NMDA‐mediated transcriptional changes, revealing components of opposing expression signatures that converge on the single response element. Overall, this study identified counteracting avenues among the distinct responses to excitotoxicity, thereby suggesting multi‐target treatment strategies and implications for predictive medicine.

Anatomical characterization of acupoint large intestine 4

Large intestine 4 (LI4) is a major acupoint used in various treatments in acupuncture and Traditional Chinese Medicine. There are structures associated within the region of LI4 that have three-dimensional anatomical relationship that needs further characterization. The aims of this study were: (a) to observe the anatomical variation of structures around LI4; (b) to observe specific overlap of structures around LI4. A 1256 mm² area was dissected in 25 cadaveric hands around LI4. Nondissected areas were marked with pins as reference points. Dissections were photographed with a fixed camera. Subsequently, images were imported to Adobe Photoshop 2020 and analyzed. Descriptive statistics and graphs were compiled using Graphpad Prism 2020. The tributaries of the dorsal venous plexus (22.3%), branches of superficial radial nerve (18.9%), first dorsal interosseous muscle (52.4%), arterial branches in the first interosseous space (10.2%), and deep ulnar nerve (4.0%) were observed in the area of LI4. One branch of the superficial radial nerve passed through LI4. The deep ulnar nerve was found in the bulk of the first dorsal interosseous muscle. Several structures observed intersected at LI4. The superficial radial nerve interweaved with the dorsal venous plexus superficially. The deep ulnar nerve passed anterior to the second palmar metacarpal artery before entering into the first dorsal interosseous muscle. These results provide anatomical evidence and variation into the vascular contributions at LI4.

Sensory Activity-Dependent and Sensory Activity-Independent Properties of the Developing Rodent Trigeminal Principal Nucleus

The whisker-sensory trigeminal central pathway of rodents is an established model for studies of activity-dependent neural plasticity. The first relay station of the pathway is the trigeminal principal nucleus (PrV), the ventral part of which receives sensory inputs mainly from the infraorbital branch of the maxillary trigeminal nerve (ION). Whisker-sensory afferents play an important role in the development of the morphological and physiological properties of PrV neurons. In neonates, deafferentation by ION transection leads to the disruption of whisker-related neural patterns (barrelettes) and cell death within a specific time window (critical period), as revealed by morphological studies. Whisker-sensory inputs control synaptic elimination, postsynaptic AMPA receptor trafficking, astrocyte-mediated synaptogenesis, and receptive-field characteristics of PrV cells, without a postnatal critical period. Sensory activity-dependent synaptic plasticity requires the activation of NMDA receptors and involves the participation of glia. However, the basic physiological properties of PrV neurons, such as cell type-specific ion channels, presynaptic terminal function, postsynaptic NMDA receptor subunit composition, and formation of the inhibitory circuitry, are independent of sensory inputs. Therefore, the first relay station of the whisker sensation is largely mature-like and functional at birth. Delineation of activity-dependent and activity-independent features of the postnatal PrV is important for understanding the development and functional characteristics of downstream trigeminal stations in the thalamus and neocortex. This mini review focuses on such features of the developing rodent PrV.

NMDA Neurotransmission Dysfunction in Behavioral and Psychological Symptoms of Alzheimer's Disease

Dementia has become an all-important disease because the population is aging rapidly and the cost of health care associated with dementia is ever increasing. In addition to cognitive function impairment, associated behavioral and psychological symptoms of dementia (BPSD) worsen patient’s quality of life and increase caregiver’s burden. Alzheimer’s disease is the most common type of dementia and both behavioral disturbance and cognitive impairment of Alzheimer’s disease are thought to be associated with the N-methyl-D-aspartate (NMDA) dysfunction as increasing evidence of dysfunctional glutamatergic neurotransmission had been reported in behavioral changes and cognitive decline in Alzheimer’s disease. We review the literature regarding dementia (especially Alzheimer’s disease), BPSD and relevant findings on glutamatergic and NMDA neurotransmission, including the effects of memantine, a NMDA receptor antagonist, and NMDA-enhancing agents, such as D-serine and D-cycloserine. Literatures suggest that behavioral disturbance and cognitive impairment of Alzheimer’s disease may be associated with excitatory neurotoxic effects which result in impairment of neuronal plasticity and degenerative processes. Memantine shows benefits in improving cognition, function, agitation/aggression and delusion in Alzheimer’s disease. On the other hand, some NMDA modulators which enhance NMDA function through the co-agonist binding site can also improve cognitive function and psychotic symptoms. We propose that modulating NMDA neurotransmission is effective in treating behavioral and psychological symptoms of Alzheimer’s disease. Prospective study using NMDA enhancers in patients with Alzheimer’s disease and associated behavioral disturbance is needed to verify this hypothesis.

Differential hippocampal gene expression is associated with climate-related natural variation in memory and the hippocampus in food-caching chickadees

There is significant and often heritable variation in cognition and its underlying neural mechanisms, yet specific genetic contributions to such variation are not well characterized. Black-capped chickadees present a good model to investigate the genetic basis of cognition because they exhibit tremendous climate-related variation in memory, hippocampal morphology and neurogenesis rates throughout the North American continent, and these cognitive traits appear to have a heritable basis. We examined the hippocampal transcriptome profiles of laboratory-reared chickadees from the two most divergent populations to test whether differential gene expression in the hippocampus is associated with population differences in spatial memory, hippocampal morphology and adult hippocampal neurogenesis rates. Using high-resolution mRNA sequencing coupled to a de novo transcriptome assembly, we generated 23 295 consensus sequences, which predicted 16 206 protein sequences with 13 982 showing high similarity to known protein sequences or conserved hypothetical proteins in other species. Of these, we identified differential expression in nearly 380 genes, with 47 genes specifically linked to neurogenesis, apoptosis, synaptic function, and learning and memory processes. Many of the other differentially expressed genes, however, may be associated with other functions. Our study presents the first avian hippocampal transcriptome, and it is the first study identifying differential gene expression associated with natural variation in cognition and the hippocampus. Our results provide additional support to the hypothesis that population differences in memory, hippocampal morphology and neurogenesis in chickadees have likely resulted from natural selection that appears to act on memory and its underlying neural mechanisms.

Activity-dependent survival of developing neocortical neurons depends on PI3K signalling

Spontaneous electrical network activity plays a major role in the control of cell survival in the developing brain. Several intracellular pathways are implicated in transducing electrical activity into gene expression dependent and independent survival signals. These include activation of phosphatidylinositol 3-kinase (PI3K) and its downstream effector Akt, activation of Ras and subsequently MAPK/extracellular signal-regulated kinase (MEK) and extracellular signal-regulated kinase and signalling via calcium/calmodulin-dependent protein kinase (CaMK). In the present study, we analyzed the role of these pathways for the control of neuronal survival in different extracellular potassium concentrations ([K(+) ](ex) ). Organotypic neocortical slice cultures prepared from newborn mice were kept in 5.3, 8.0 and 25.0mM [K(+) ](ex) and treated with specific inhibitors of PI3K, MEK1, CaMKK and a broad spectrum CaMK inhibitor. After 6h of incubation, slices were immunostained for activated caspase 3 (a-caspase 3) and the number of apoptotic cells was quantified by computer based analysis. We found that in 5.3 and 8.0mM [K(+) ](ex) only PI3K was important for neuronal survival. When [K(+) ](ex) was raised to 25.0mM, a concentration above the depolarization block, we found no influence of PI3K on neuronal survival. Our data demonstrate that only the PI3K pathway, and not the MEK1, CaMKK or CaMKs pathway, plays a central role in the regulation of activity-dependent neuronal survival in the developing cerebral cortex.

The regulation and role of neuronal gap junctions during development

Coupling of neurons by electrical synapses (gap junctions) transiently increases in the mammalian CNS during development and plays a role in a number of developmental events, including neuronal death. The coupling subsequently decreases and remains low in the adult, confined to specific subsets of neurons. In a recent study we have demonstrated that the developmental increase in neuronal gap junction coupling is regulated by the balance between the activity of two neurotransmitter receptors, group II metabotropic glutamate receptors (mGluR) and GABA(A) receptors. Specifically, we found that activation of group II mGluRs induces the developmental increases in neuronal gap junction coupling and expression of connexin 36 (Cx36; neuronal gap junction protein) and activation of GABA(A) receptors counteracts to these increases. We also established that the regulation by both neurotransmitter receptors is via a neuron-restrictive silencer element in the Cx36 gene promoter and the 3'-untranslated region of the Cx36 mRNA. Importantly, we demonstrated that mechanisms for the developmental increase in neuronal gap junction coupling directly control the death/survival mechanisms in developing neurons.

Corticospinal motor neurons and related subcerebral projection neurons undergo early and specific neurodegeneration in hSOD1G⁹³A transgenic ALS mice

Amyotrophic lateral sclerosis (ALS) is characterized by predominant vulnerability and central degeneration of both corticospinal/corticobulbar motor neurons (CSMN; "upper motor neurons") in cerebral cortex, and spinal/bulbar motor neurons (SMN; "lower motor neurons") in spinal cord and brainstem. Increasing evidence indicates broader cerebral cortex pathology in cognitive, sensory, and association systems in select cases. It remains unclear whether widely accepted transgenic ALS models, in particular hSOD1(G93A) mice, undergo degeneration of CSMN and molecularly/developmentally closely related populations of nonmotor projection neurons [e.g., other subcerebral projection neurons (SCPN)], and whether potential CSMN/SCPN degeneration is specific and early. This relative lack of knowledge regarding upper motor neuron pathology in these ALS model mice has hindered both molecular-pathophysiologic understanding of ALS and their use toward potential CSMN therapeutic approaches. Here, using a combination of anatomic, cellular, transgenic labeling, and newly available neuronal subtype-specific molecular analyses, we identify that CSMN and related nonmotor SCPN specifically and progressively degenerate in hSOD1(G93A) mice. Degeneration starts quite early and presymptomatically, by postnatal day 30. Other neocortical layers, cortical interneurons, and other projection neuron populations, even within layer V, are not similarly affected. Nonneuronal pathology in neocortex (activated astroglia and microglia) is consistent with findings in human ALS cortex and in affected mouse and human spinal cord. These results indicate previously unknown neuron type-specific vulnerability of CSMN/sensory and association SCPN, and identify that characteristic dual CSMN and SMN degeneration is conserved in hSOD1(G93A) mice. These results provide a foundation for detailed investigation of CSMN/SCPN vulnerability and toward potential CSMN therapeutics in ALS.

Interplay of chemical neurotransmitters regulates developmental increase in electrical synapses

Coupling of neurons by electrical synapses (gap junctions) transiently increases in the mammalian CNS during development. We report here that the developmental increase in neuronal gap junction coupling and expression of connexin 36 (Cx36; neuronal gap junction protein) are regulated by an interplay between the activity of group II metabotropic glutamate receptors (mGluRs) and GABA(A) receptors. Specifically, using dye coupling, electrotonic coupling, Western blots and small interfering RNA in the rat and mouse hypothalamus and cortex in vivo and in vitro, we demonstrate that activation of group II mGluRs augments, and inactivation prevents, the developmental increase in neuronal gap junction coupling and Cx36 expression. However, changes in GABA(A) receptor activity have the opposite effects. The regulation by group II mGluRs is via cAMP/PKA-dependent signaling, and regulation by GABA(A) receptors is via Ca(2+)/PKC-dependent signaling. Furthermore, the receptor-mediated upregulation of Cx36 requires a neuron-restrictive silencer element in the Cx36 gene promoter, and the downregulation involves the 3'-untranslated region of the Cx36 mRNA, as shown using reverse-transcription quantitative real-time PCR and luciferase reporter activity analysis. In addition, the methyl thiazolyl tetrazolium analysis indicates that mechanisms for the developmental increase in neuronal gap junction coupling directly control the death/survival mechanisms in developing neurons. Together, the results suggest a multitiered strategy for chemical synapses in developmental regulation of electrical synapses.

Glutamate transporters regulate lesion-induced plasticity in the developing somatosensory cortex

Glutamate transporters are involved in neural differentiation, neuronal survival, and synaptic transmission. In the present study, we examined glutamate transporter 1 (GLT1) expression in the neonatal somatosensory cortex of C57BL/6 mice, and pursued its role in somatosensory development by comparing barrel development between GLT1 knock-out and control mice. During the first few neonatal days, a critical period for barrels, GLT1 expression is strikingly upregulated in cortical astrocytes, whereas it was downregulated in neuronal elements to below the detection threshold. GLT1 knock-out neonates developed normally in terms of body growth, cortical histoarchitecture, barrel formation, and critical period termination. However, when row C whiskers were lesioned during the critical period, reduction of lesioned row C barrels and reciprocal expansion of intact row B/D barrels were both milder in GLT1 knock-out mice than in control littermates. Accordingly, the map plasticity index, calculated as (B + D)/2C, was significantly lowered in GLT1 knock-out mice. We also found that extracellular glutamate levels in the neonatal somatosensory cortex were significantly elevated in GLT1 knock-out mice. Diminished lesion-induced plasticity was further found in mutant mice lacking glutamate-aspartate transporter (GLAST), an astrocyte-specific glutamate transporter throughout development. Therefore, glutamate transporters regulate critical period plasticity by enhancing expansion of active barrels and shrinkage of inactive barrels. Because cortical contents of glutamate receptors and GLAST were unaltered in GLT1 knock-out mice, this action appears to be mediated, at least partly, by keeping the ambient glutamate level low. Considering an essential role of glutamate receptors in the formation of whisker-related thalamocortical synapse patterning, glutamate transporters thus facilitate their activity-dependent remodeling.

The role of Akt-GSK-3beta signaling and synaptic strength in phencyclidine-induced neurodegeneration

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP) can induce positive and negative symptoms of schizophrenia in humans and related effects in rodents. PCP treatment of developing rats induces apoptotic neurodegeneration and behavioral deficits later in life that mimic some symptoms of schizophrenia. The precise mechanism of PCP-induced neural degeneration is unknown. This study used selective antagonists, siRNA, and Western analysis to investigate the role of the Akt-glycogen synthase kinase-3beta (GSK-3beta) pathway in PCP-induced neuronal apoptosis in both neuronal culture and postnatal day 7 rats. PCP administration in vivo and in vitro reduced the phosphorylation of Akt Ser427 and GSK-3beta Ser9, decreasing Akt activity and increasing GSK-3beta activity. The alteration of Akt-GSK-3beta signaling parallels the temporal profile of caspase-3 activation by PCP. Reducing GSK-3beta activity by application of selective inhibitors or depletion of GSK-3beta by siRNA attenuates caspase-3 activity and blocks PCP-induced neurotoxicity. Moreover, increasing synaptic strength by either activation of L-type calcium channels with BAY K8644 or potentiation of synaptic NMDA receptors with either a low concentration of NMDA or bicuculline plus 4-aminopyridine completely blocks PCP-induced cell death by increasing Akt phosphorylation. These neuroprotective effects are associated with activation of phosphoinositide-3-kinase-Akt signaling, and to a lesser extent, the MAPK signaling pathway. Overall, these data suggest that PCP-induced hypofunction of synaptic NMDA receptors impairs the Akt-GSK-3beta cascade, which is necessary for neuronal survival during development, and that interference with this cascade by PCP or natural factors may contribute to neural pathologies, perhaps including schizophrenia.

Ketamine exposure in adult mice leads to increased cell death in C3H, DBA2 and FVB inbred mouse strains

BACKGROUND

Drug abuse is common among adolescents and young adults. Although the consequences of intoxication are known, sequelae of drugs emerging on campuses and in clubs nationwide are not. We previously demonstrated that ketamine exposure results in lasting physiological abnormalities in mice. However, the extent to which these deficits reflect neuropathologic changes is not known.

METHODS

The current study examines neuropathologic changes following sub-anesthetic ketamine administration (5mg/kg i.p. x 5) to three inbred mouse strains. Stereologic quantification of silver stained nuclear and linear profiles as well as activated caspase-3 labeling was used to address: (1) whether or not ketamine increases excitotoxic and apoptotic cell death in hippocampal CA3 and (2) whether or not ketamine-induced cell death varies by genetic background.

RESULTS

Ketamine increased cell death in hippocampal CA3 of adult C3H, DBA2 and FVB mice. Neither silver staining nor activated caspase-3 labeling varied by strain, nor was there an interaction between ketamine-induced cell death and strain.

CONCLUSIONS

Ketamine exposure among young adults, even in limited amounts, may lead to irreversible changes in both brain function and structure. Loss of CA3 hippocampal cells may underlie persistent ERP changes previously shown in mice and possibly contribute to lasting cognitive deficits among ketamine abusers.

Gap junctions are required for NMDA receptor dependent cell death in developing neurons

A number of studies have indicated an important role for N-methyl-D-aspartate (NMDA) receptors in cell survival versus cell death decisions during neuronal development, trauma, and ischemia. Coupling of neurons by electrical synapses (gap junctions) is high or increases in neuronal networks during all three of these conditions. However, whether neuronal gap junctions contribute to NMDA receptor-regulated cell death is not known. Here we address the role of neuronal gap junction coupling in NMDA receptor-regulated cell death in developing neurons. We report that inactivation or hyperactivation of NMDA receptors induces neuronal cell death in primary hypothalamic cultures, specifically during the peak of developmental gap junction coupling. In contrast, increasing or decreasing NMDA receptor function when gap junction coupling is low has no or greatly reduced impact on cell survival. Pharmacological inactivation of gap junctions or knockout of neuronal connexin 36 prevents the cell death caused by NMDA receptor hypofunction or hyperfunction. The results indicate the critical role of neuronal gap junctions in cell death caused by increased or decreased NMDA receptor function in developing neurons. Based on these data, we propose the novel hypothesis that NMDA receptors and gap junctions work in concert to regulate neuronal survival.

Conversion of functional synapses into silent synapses in the trigeminal brainstem after neonatal peripheral nerve transection

One of the major consequences of neonatal infraorbital nerve damage is irreversible morphological reorganization in the principal sensory nucleus (PrV) of the trigeminal nerve in the brainstem. We used the voltage-clamp technique to study synaptic transmission in the normal and the denervated PrV of neonatal rats in an in vitro brainstem preparation. Most of the synapses in the PrV are already functional at birth. Three days after peripheral deafferentation, functional synapses become silent, lacking AMPA receptor-mediated currents. Without sensory inputs from the whiskers, silent synapses persist through the second postnatal week, indicating that the maintenance of AMPA receptor function depends on sensory inputs. High-frequency (50 Hz) electrical stimulation of the afferent pathway, which mimics sensory input, restores synaptic function, whereas low-frequency (1 Hz) stimulation has no effect. Application of glycine, which promotes AMPA receptor exocytosis, also restores synaptic function. Therefore, normal synaptic function in the developing PrV requires incoming activity via sensory afferents and/or enhanced AMPA receptor exocytosis. Sensory deprivation most likely results in AMPA receptor endocytosis and/or lateral diffusion to the extrasynaptic membrane.