Basal Neuronal Excitability Research Articles

AMPK in the brain: its roles in glucose and neural metabolism.

The adenosine monophosphate-activated protein kinase (AMPK) is an integrative metabolic sensor that maintains energy balance at the cellular level and plays an important role in orchestrating intertissue metabolic signaling. AMPK regulates cell survival, metabolism, and cellular homeostasis basally as well as in response to various metabolic stresses. Studies so far show that the AMPK pathway is associated with neurodegeneration and CNS pathology, but the mechanisms involved remain unclear. AMPK dysregulation has been reported in neurodegenerative diseases such as amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's disease, Parkinson's disease, Huntington's disease, and other neuropathies. AMPK activation appears to be both neuroprotective and pro-apoptotic, possibly dependent upon neural cell types, the nature of insults, and the intensity and duration of AMPK activation. While embryonic brain development in AMPK null mice appears to proceed normally without any overt structural abnormalities, our recent study confirmed the full impact of AMPK loss in the postnatal and aging brain. Our studies revealed that Ampk deletion in neurons increased basal neuronal excitability and reduced latency to seizure upon stimulation. Three major pathways, glycolysis, pentose phosphate shunt, and glycogen turnover, contribute to utilization of glucose in the brain. AMPK's regulation of aerobic glycolysis in astrocytic metabolism warrants further deliberation, particularly glycogen turnover and shuttling of glucose- and glycogen-derived lactate from astrocytes to neurons during activation. In this minireview, we focus on recent advances in AMPK and energy-sensing in the brain.

Potentiating KCC2 activity is sufficient to limit the onset and severity of seizures

The type 2 K+/Cl- cotransporter (KCC2) allows neurons to maintain low intracellular levels of Cl-, a prerequisite for efficient synaptic inhibition. Reductions in KCC2 activity are evident in epilepsy; however, whether these deficits directly contribute to the underlying pathophysiology remains controversial. To address this issue, we created knock-in mice in which threonines 906 and 1007 within KCC2 have been mutated to alanines (KCC2-T906A/T1007A), which prevents its phospho-dependent inactivation. The respective mice appeared normal and did not show any overt phenotypes, and basal neuronal excitability was unaffected. KCC2-T906A/T1007A mice exhibited increased basal neuronal Cl- extrusion, without altering total or plasma membrane accumulation of KCC2. Critically, activity-induced deficits in synaptic inhibition were reduced in the mutant mice. Consistent with this, enhanced KCC2 was sufficient to limit chemoconvulsant-induced epileptiform activity. Furthermore, this increase in KCC2 function mitigated induction of aberrant high-frequency activity during seizures, highlighting depolarizing GABA as a key contributor to the pathological neuronal synchronization seen in epilepsy. Thus, our results demonstrate that potentiating KCC2 represents a therapeutic strategy to alleviate seizures.

Characterization and reversal of Doxorubicin-mediated biphasic activation of ERK and persistent excitability in sensory neurons of Aplysia californica

Doxorubicin (DOX), a common chemotherapeutic agent, impairs synaptic plasticity. DOX also causes a persistent increase in basal neuronal excitability, which occludes serotonin-induced enhanced excitability. Therefore, we sought to characterize and reverse DOX-induced physiological changes and modulation of molecules implicated in memory induction using sensory neurons from the marine mollusk Aplysia californica. DOX produced two mechanistically distinct phases of extracellular signal-regulated kinase (ERK) activation, an early and a late phase. Inhibition of MEK (mitogen-activated protein kinase (MAPK)/ERK kinase) after DOX treatment reversed the late ERK activation. MEK inhibition during treatment enhanced the late ERK activation possibly through prolonged downregulation of MAPK phosphatase-1 (MKP-1). Unexpectedly, the late ERK activation negatively correlated with excitability. MEK inhibition during DOX treatment simultaneously enhanced the late activation of ERK and blocked the increase in basal excitability. In summary, we report DOX-mediated biphasic activation of ERK and the reversal of the associated changes in neurons, a potential strategy for reversing the deleterious effects of DOX treatment.

Two Novel DEG/ENaC Channel Subunits Expressed in Glia Are Needed for Nose-Touch Sensitivity inCaenorhabditis elegans

Neuronal DEG/ENaC (degenerin and epithelial Na(+) channel) Na(+) channels have been implicated in touch sensation. For example, MEC-4 is expressed in touch neurons in Caenorhabditis elegans and mediates gentle-touch response. Similarly, homologous mammalian ASIC2 and ASIC3 are expressed in sensory neurons and produce touch phenotypes when knocked out in mice. Here, we show that novel DEG/ENaC subunits DELM-1 and DELM-2 (degenerin-like channel mechanosensory linked-1 and degenerin-like channel mechanosensory linked-2) are expressed in glia associated with touch neurons in C. elegans and that their knock-out causes defects in mechanosensory behaviors related to nose touch and foraging, which are mediated by OLQ and IL1 sensory neurons. Cell-specific rescue supports that DELM-1 and DELM-2 are required cell-autonomously in glia to orchestrate mechanosensory behaviors. Electron microscopy reveals that in delm-1 knock-outs, OLQ and IL1 sensory neurons and associated glia are structurally normal. Furthermore, we show that knock-out of DELM-1 and DELM-2 does not disrupt the expression or cellular localization of TRPA-1, a TRP channel needed in OLQ and IL1 neurons for touch behaviors. Rather, rescue of the delm-1 nose-touch-insensitive phenotype by expression of a K(+) channel in socket glia and of a cationic channel in OLQ neurons suggests that DELM channels set basal neuronal excitability. Together, our data show that DELM-1 and DELM-2 are expressed in glia associated with touch neurons where they are not needed for neuronal structural integrity or cellular distribution of neuronal sensory channels, but rather for their function.

Insulin Reduces Neuronal Excitability by Turning on GABAA Channels that Generate Tonic Current

Insulin signaling to the brain is important not only for metabolic homeostasis but also for higher brain functions such as cognition. GABA (γ-aminobutyric acid) decreases neuronal excitability by activating GABAA channels that generate phasic and tonic currents. The level of tonic inhibition in neurons varies. In the hippocampus, interneurons and dentate gyrus granule cells normally have significant tonic currents under basal conditions in contrast to the CA1 pyramidal neurons where it is minimal. Here we show in acute rat hippocamal slices that insulin (1 nM) “turns on” new extrasynaptic GABAA channels in CA1 pyramidal neurons resulting in decreased frequency of action potential firing. The channels are activated by more than million times lower GABA concentrations than synaptic channels, generate tonic currents and show outward rectification. The single-channel current amplitude is related to the GABA concentration resulting in a single-channel GABA affinity (EC50) in intact CA1 neurons of 17 pM with the maximal current amplitude reached with 1 nM GABA. They are inhibited by GABAA antagonists but have novel pharmacology as the benzodiazepine flumazenil and zolpidem are inverse agonists. The results show that tonic rather than synaptic conductances regulate basal neuronal excitability when significant tonic conductance is expressed and demonstrate an unexpected hormonal control of the inhibitory channel subtypes and excitability of hippocampal neurons. The insulin-induced new channels provide a specific target for rescuing cognition in health and disease.

Mechanisms underlying basal and learning-related intrinsic excitability in a mouse model of Alzheimer's disease

Accumulations of β-amyloid (Aβ) contribute to neurological deficits associated with Alzheimer's disease (AD). The effects of Aβ on basal neuronal excitability and learning-related AHP plasticity were examined using whole-cell recordings from hippocampal neurons in the 5XFAD mouse model of AD. A robust increase in Aβ42 (and elevated levels of Aβ38-40) in naïve 5XFAD mice was associated with decreased basal neuronal excitability, evidenced by a select increase in Ca(2+)-sensitive afterhyperpolarization (AHP). Moreover, trace fear deficits observed in a subset of 5XFAD weak-learner mice were associated with a greater enhancement of the AHP in neurons, as compared to age-matched 5XFAD learner and 5XFAD naïve mice. Importantly, learning-related plasticity of the AHP remained intact in a subset of 5XFAD mice that learned trace fear conditioning to a set criterion. We show that APP-PS1 mutations enhance Aβ and disrupt basal excitability via a Ca(2+)-dependent enhancement of the AHP, and suggest disruption to learning-related modulation of intrinsic excitability resulted, in part, from altered cholinergic modulation of the AHP in the 5XFAD mouse model of AD (170 of 170).

Memory deficits are associated with impaired ability to modulate neuronal excitability in middle-aged mice

Normal aging disrupts hippocampal neuroplasticity and learning and memory. Aging deficits were exposed in a subset (30%) of middle-aged mice that performed below criterion on a hippocampal-dependent contextual fear conditioning task. Basal neuronal excitability was comparable in middle-aged and young mice, but learning-related modulation of the post-burst afterhyperpolarization (AHP)--a general mechanism engaged during learning--was impaired in CA1 neurons from middle-aged weak learners. Thus, modulation of neuronal excitability is critical for retention of context fear in middle-aged mice. Disruption of AHP plasticity may contribute to contextual fear deficits in middle-aged mice--a model of age-associated cognitive decline (AACD).

Separate Paths to the Surface

AMPA glutamate receptors (GluR) are hetero-oligomeric proteins and multiple combinations of subunits are expressed in the same neuron. The different combinations have been implicated in different types of neuronal activity, for example, receptors formed from GluR1/GluR2 subunits increase in response to neuronal activity and are important for mediating long-term potentiation, a process implicated in learning and memory. Receptors formed from GluR2/GluR3 subunits are involved in controlling basal neuronal excitability. Shi et al. used cultured neurons and organotypic slice preparations to express various AMPA GluR subunits and modified subunits or portions thereof. The delivery of the proteins was monitored by fusion with green fluorescent protein, the inclusion of a point mutation that altered their electrophysiological properties, or both. Based on the results from expression of the various combinations of proteins, two independent pathways for delivery of the receptors to the cell surface were identified. The GluR1-containing receptors, including GluR1/GluR2 receptors, were not inserted in the plasma membrane unless the synapses were stimulated or unless activated calcium-calmodulin-dependent protein kinase II was also expressed. GluR2 homo-oligomers or GluR2/GluR3 hetero-oligomers were constitutively inserted into the membrane, and the endogenous receptors were continuously endocytosed. Trafficking of GluR-containing receptors depended on the binding domain for PDZ group 1 proteins, and trafficking of the GluR2/GluR2 or GluR2/GluR3 receptors depended on the GluR2 PDZ group II binding domain and interaction with N-ethylmaleimide-sensitive factor (NSF). The independence of the trafficking pathways was confirmed by expression of the COOH-domains of GluR1 or GluR2, which showed only selective inhibition of the cognate receptor's delivery. These two independently regulated pathways contribute to the control of synaptic activity.S.-H. Shi, Y. Hayashi, J. A. Esteban, R. Malinow, Subunit-specific rules governing AMPA receptor trafficking to synapses in hippocampal pyramidal neurons. Cell 105, 331-343 (2001). [Online Journal]

Separate Paths to the Surface

AMPA glutamate receptors (GluR) are hetero-oligomeric proteins and multiple combinations of subunits are expressed in the same neuron. The different combinations have been implicated in different types of neuronal activity, for example, receptors formed from GluR1/GluR2 subunits increase in response to neuronal activity and are important for mediating long-term potentiation, a process implicated in learning and memory. Receptors formed from GluR2/GluR3 subunits are involved in controlling basal neuronal excitability. Shi et al. used cultured neurons and organotypic slice preparations to express various AMPA GluR subunits and modified subunits or portions thereof. The delivery of the proteins was monitored by fusion with green fluorescent protein, the inclusion of a point mutation that altered their electrophysiological properties, or both. Based on the results from expression of the various combinations of proteins, two independent pathways for delivery of the receptors to the cell surface were identified. The GluR1-containing receptors, including GluR1/GluR2 receptors, were not inserted in the plasma membrane unless the synapses were stimulated or unless activated calcium-calmodulin-dependent protein kinase II was also expressed. GluR2 homo-oligomers or GluR2/GluR3 hetero-oligomers were constitutively inserted into the membrane, and the endogenous receptors were continuously endocytosed. Trafficking of GluR-containing receptors depended on the binding domain for PDZ group 1 proteins, and trafficking of the GluR2/GluR2 or GluR2/GluR3 receptors depended on the GluR2 PDZ group II binding domain and interaction with N -ethylmaleimide-sensitive factor (NSF). The independence of the trafficking pathways was confirmed by expression of the COOH-domains of GluR1 or GluR2, which showed only selective inhibition of the cognate receptor's delivery. These two independently regulated pathways contribute to the control of synaptic activity. S.-H. Shi, Y. Hayashi, J. A. Esteban, R. Malinow, Subunit-specific rules governing AMPA receptor trafficking to synapses in hippocampal pyramidal neurons. Cell 105 , 331-343 (2001). [Online Journal]

Effects of GYKI 52466 and some 2,3-benzodiazepine derivatives on hippocampal in vitro basal neuronal excitability and 4-aminopyridine epileptic activity

In order to determine whether the anticonvulsant effect of 2,3-benzodiazepines is also displayed in a model of in vitro epilepsy, such as the “epileptiform” hippocampal slice, we studied the effects of 2,3-benzodiazepine 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxe-5 H 2,3-benzodiazepine hydrochloride (GYKI 52466) and some new 2,3-benzodiazepine derivatives on CA1 basal neuronal excitability and on CA1 epileptiform burst activity produced by 4-aminopyridine in rat hippocampal slices. The results showed that GYKI 52466 affected basal neuronal excitability as evidenced by its influence on the magnitude of the CA1 orthodromic-evoked field potentials. 2,3-Benzodiazepines showed their antiepileptic effect also in an in vitro model of experimental epilepsy. The effects of the new 2,3-benzodiazepine derivatives suggest that the methylenedioxidation in positions 7 and 8 of the 2,3-benzodiazepine ring is the main structural modification for the antiepileptic effect of 2,3-benzodiazepines to take place.

Mineralocorticoid receptor-mediated enhancement of neuronal excitability and synaptic plasticity in the dentate gyrus in vivo is dependent on the beta-adrenergic activity.

The dentate gyrus neurons in the hippocampus contain a high density of both mineralocorticoid and adrenergic receptors. By in vivo extracellular recording from adrenalectomized rats we investigated the possible relationships between the two systems with regard to neuronal excitability and activity-dependent synaptic plasticity. Pretreatment with aldosterone significantly enhanced both basal neuronal excitability and tetanically evoked synaptic plasticity in adrenalectomized, but not sham-operated rats. The enhancement was blocked by spironolactone, indicating a mineralocorticoid receptor-dependent effect. The adrenomedullary hormone epinephrine also significantly enhanced synaptic plasticity via activation of beta-adrenergic receptors. Beta-adrenergic antagonist propranolol, infused directly into the dentate gyrus granule cell layer, significantly reduced the effect of aldosterone on neuronal excitability and partly canceled the aldosterone-enhanced synaptic plasticity. No effect of propranolol was found after its amygdaloid infusion. The mineralocorticoid receptor antagonist spironolactone did not affect the epinephrine-induced effects. These results indicate that the pretreated adrenal steroids interact with the catecholaminergic system in the dentate gyrus of adrenalectomized rats and that the functional beta-adrenergic pathway is involved in the mechanism of mineralocorticoid-induced cellular effects in vivo.

Glutamate metabotropic receptors modulate the expression of [formula omitted] epileptiform activity in rat hippocampal slices

The effects of the mixed class I and II mGLUR agonist (+) 1S, 3R-trans-amino-cyclopentane-1,3-dicarboxylic acid (ACPD) and antagonists (+) alpha-methyl-4-carboxyphenylglycine (MCPG) and L-2-amino-3-phosphonopropionic acid (L-AP3) on the basal neuronal excitability and on the expression of in vitro epileptiform activity produced by the convulsant drugs picrotoxin and penicillin were investigated in rat hippocampal slices. The duration of the CA1 epileptiform bursting produced by 0.05 mM picrotoxin or 1 mM penicillin or 0.075 mM ACPD was significantly (p < 0.05) and dose-dependently decreased by 0.3–0.5 mM MCPG or L-AP3, but not by 0.05 mM ACPD. The data demostrate an involvement of class I and II mGLURs in the basal neuronal excitability and in the expression of in vitro epileptiform activity produced by some convulsants.