Changes In Membrane Properties Of Neurons Research Articles

P.1.007 - How does fractalkine modulate synaptic transmission in the rat basolateral amygdala? Insights from electrophysiology

Background Convincing evidence has implicated chemokines in many neurobiological processes potentially relevant to psychiatric disorders, beyond their traditional chemotactic functions. These may include neuromodulator and neurotransmitter-like effects [1]. Yet, recognition and characterization of chemokine effects on neurophysiology are still lacking. The chemokine fractalkine (CX3CL1) is mostly expressed in neurons, whereas its cognate receptor, CX3CR1, is mainly expressed in microglia; however, some reports also demonstrate its neuronal localization [2]. They are constitutively and diffusively expressed in the brain, mostly in structures such as the hippocampus and amygdala [3]. The amygdala plays critical roles in a variety of behavioral responses, including fear and anxiety. The main input structure of the amygdala, the basolateral nuclei (BLA), receives sensory information from thalamus and cortex. Dysregulation in this region contributes to the pathophysiology of anxiety disorders. Therefore, this study aimed to elucidate the neurophysiological effects of fractalkine in the rat BLA. Method Whole-cell patch clamp recordings were performed from principal neurons in acute brain slices (300 μm) containing the BLA. After recording a baseline, fractalkine (2 nM) was bath-applied. Both inhibitory and excitatory synaptic transmission were measured by recording spontaneous synaptic currents (sIPSC/sEPSC). Additionally, synaptic responses were electrically evoked using concentric bipolar stimulating electrodes placed in either the external capsule or the BLA, inducing AMPA receptor-mediated excitatory synaptic currents (eEPSC), NMDA receptor-mediated currents or GABAA receptor-mediated inhibitory synaptic currents (eIPSC). Results Our data indicate that application of fractalkine results in the decreased amplitude of AMPA-mediated eEPSC (paired t-test; p=0.0081, t=3.379, df=9, n=10). On the contrary, the amplitude of NMDA currents was elevated (paired t-test; p=0.0056, t=5.425, df=4, n=5). Regarding inhibitory transmission, fractalkine increased the frequency (paired t-test; p=0.0434, t=2.917, df=4, n=5) and reduced the amplitude of recorded spontaneous currents (paired t-test; p=0.0346, t=3.149, df=4, n=5). These results were consistent with eIPSC data, as both amplitude (paired t-test; p=0.04, t=2.9, df=4, n=5) and paired-pulse ratio were altered (paired t-test; p=0.0275, t=3.4, df=4, n=5). Conclusions Our results suggest multifaceted effects of fractalkine application. The decreased amplitude of AMPA currents and the increased amplitude of NMDA currents result in a reduced AMPA/NMDA ratio and indicate a change in either expression or function of postsynaptic channels. Similar effects after fractalkine application were observed in the hippocampus, and they might be dependent on microglia [4,5]. Interestingly, changes in the inhibitory transmission involve both pre- and postsynaptic mechanisms and possibly homeostatic plasticity mechanisms affecting neural output. The actions of fractalkine in the BLA may be due to their ability to activate its only receptor CX3CR1 localized on neurons and microglia leading to changes in neuronal membrane properties and synaptic transmission. In conclusion, our data show that fractalkine has a profound effect on BLA synaptic transmission, indicating that this protein can be an active modulator of neuronal activity in the fear-related response circuitry, which may have significant scientific and therapeutic implications.

Suppression of epileptiform activity by a single short-duration electric field in rat hippocampus in vitro

The mechanisms behind the therapeutic effects of electrical stimulation of the brain in epilepsy and other disorders are poorly understood. Previous studies in vitro have shown that uniform electric fields can suppress epileptiform activity through a direct polarizing effect on neuronal membranes. Such an effect depends on continuous DC stimulation with unbalanced charge. Here we describe a suppressive effect of a brief (10 ms) DC field on stimulus-evoked epileptiform activity in rat hippocampal brain slices exposed to Cs(+) (3.5 mM). This effect was independent of field polarity, was uncorrelated to changes in synchronized population activity, and persisted during blockade of synaptic transmission with Cd(2+) (500 μM). Antagonists of A(1), P(2X), or P(2Y) receptors were without effect. The suppressive effect depended on the alignment of the external field with the somato-dendritic axis of CA1 pyramidal cells; however, temporal coincidence with the epileptiform activity was not essential, as suppression was detectable for up to 1 s after the field. Pyramidal cells, recorded during epileptiform activity, showed decreased discharge duration and truncation of depolarizing plateau potentials in response to field application. In the absence of hyperactivity, the applied field was followed by slow membrane potential changes, accompanied by decreased input resistance and attenuation of the depolarizing afterpotential following action potentials. These effects recovered over a 1-s period. The study suggests that a brief electric field induces a prolonged suppression of epileptiform activity, which can be related to changes in neuronal membrane properties, including attenuation of signals depending on the persisting Na(+) current.

Neuroprotective effects of DHA in Alzheimer’s disease models

Alzheimer’s disease (AD) is a major public health concern in all developped countries. Although the precise cause of AD is still unknown, a growing body of evidence supports the notion that soluble oligomers of amyloid b-peptide (Aβ) may be the proximate effectors of synaptic injuries and neuronal death in the early stages of AD. AD patients display lower levels of docosahexaenoic acid (DHA, C22:6; n-3) in plasma and brain tissues as compared to control subjects of same age. Furthermore, epidemiological studies suggest that high DHA intake might have protective properties against neurodegenerative diseases. These observations are supported by in vivo studies showing that DHA-rich diets limit the synaptic loss and cognitive defects induced by Aβ peptide. Although the molecular basis underlying these neuroprotective effects remains unknown, several mechanisms have been proposed such as (i) regulation of the expression of potentially protective genes, (ii) activation of antiinflammatory pathways, (iii) modulation of functional properties of the synaptic membranes along with changes in their physicochemical and structural features. We recently demonstrated that DHA protects neurons from soluble Aβ oligomer-induced apoptosis. Indeed, DHA pretreatment was observed to significantly increase neuronal survival upon Aβ treatment by preventing cytoskeleton perturbations, caspase activation and apoptosis, as well as by promoting ERK-related survival pathways. These data suggest that DHA enrichment most likely induces changes in neuronal membrane properties with functional outcomes, thereby increasing protection from soluble Aβ oligomers. Such neuroprotective effects could be of major interest in the prevention of AD and other neurodegenerative diseases.

Ketones inhibit mitochondrial production of reactive oxygen species production following glutamate excitotoxicity by increasing NADH oxidation

Dietary protocols that increase serum levels of ketones, such as calorie restriction and the ketogenic diet, offer robust protection against a multitude of acute and chronic neurological diseases. The underlying mechanisms, however, remain unclear. Previous studies have suggested that the ketogenic diet may reduce free radical levels in the brain. Thus, one possibility is that ketones may mediate neuroprotection through antioxidant activity. In the present study, we examined the effects of the ketones β-hydroxybutyrate and acetoacetate on acutely dissociated rat neocortical neurons subjected to glutamate excitotoxicity using cellular electrophysiological and single-cell fluorescence imaging techniques. Further, we explored the effects of ketones on acutely isolated mitochondria exposed to high levels of calcium. A combination of β-hydroxybutyrate and acetoacetate (1 mM each) decreased neuronal death and prevented changes in neuronal membrane properties induced by 10 μM glutamate. Ketones also significantly decreased mitochondrial production of reactive oxygen species and the associated excitotoxic changes by increasing NADH oxidation in the mitochondrial respiratory chain, but did not affect levels of the endogenous antioxidant glutathione. In conclusion, we demonstrate that ketones reduce glutamate-induced free radical formation by increasing the NAD+/NADH ratio and enhancing mitochondrial respiration in neocortical neurons. This mechanism may, in part, contribute to the neuroprotective activity of ketones by restoring normal bioenergetic function in the face of oxidative stress.

Docosahexaenoic acid prevents neuronal apoptosis induced by soluble amyloid‐β oligomers

A growing body of evidence supports the notion that soluble oligomers of amyloid-beta (Abeta) peptide interact with the neuronal plasma membrane, leading to cell injury and inducing death-signalling pathways that could account for the increased neurodegeneration occurring in Alzheimer's disease (AD). Docosahexaenoic acid (DHA, C22:6, n-3) is an essential polyunsaturated fatty acid in the CNS and has been shown in several epidemiological and in vivo studies to have protective effects against AD and cognitive alterations. However, the molecular mechanisms involved remain unknown. We hypothesized that DHA enrichment of plasma membranes could protect neurones from apoptosis induced by soluble Abeta oligomers. DHA pre-treatment was observed to significantly increase neuronal survival upon Abeta treatment by preventing cytoskeleton perturbations, caspase activation and apoptosis, as well as by promoting extracellular signal-related kinase (ERK)-related survival pathways. These data suggest that DHA enrichment probably induces changes in neuronal membrane properties with functional outcomes, thereby increasing protection from soluble Abeta oligomers. Such neuroprotective effects could be of major interest in the prevention of AD and other neurodegenerative diseases.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14–P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10μM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II–III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Cholinergic stimulation of hippocampal pyramidal cells is inhibited by increasing membrane cholesterol

Carbachol is known to stimulate pyramidal cells in the CA1 region of hippocampal slices. The incorporation of cholesterol by perfusion of slices with cholesterol-containing buffer abolishes the carbachol response. The response returns upon removal of cholesterol. Changes in neuronal membrane properties could contribute to alterations in cholinergic transmission thought to occur in senile dementia and chronic alcoholism.

Receptor and membrane function in the alcohol tolerant/dependent animal.

Neurochemical changes which are associated with the development or expression of tolerance to or physical dependence on ethanol may be expected to display a time course of appearance and disappearance which correlates positively with the time course for tolerance or dependence. Previous studies of striatal dopaminergic receptor function indicated that ethanol-withdrawn mice displayed decreased physiological and biochemical responses to dopamine (DA) agonists, which could be best explained by postulating an inefficient coupling between DA receptors and various receptor-mediated processes, possibly as a result of ethanol-induced changes in neuronal membrane properties. The membrane-bound enzyme, (Na+-K+)ATPase, obtained from ethanol-withdrawn animals, displays an altered transition temperature and resistance to the effects of ethanol on enzyme activity. These changes also suggest compensatory alterations in neuronal membrane properties. All of these alterations show a time course of disappearance which corresponds to that for the disappearance of tolerance to the hypothermic and sedative effects of ethanol. Ethanol-withdrawn mice also display increased numbers of hippocampal muscarinic cholinergic receptors; however, the time course for the increase in receptor number appears to correlate with that of withdrawal symptomatology. Thus, compensatory changes in neuronal membrane properties in response to ethanol may be expressed via diverse functional changes.