Proliferative activity and structure-functional organization of chromosomes in cells from the developing brain in respect to genetically determined excitability of the rat nervous system]

Neuroblasts of developing hippocampus of 16-17-day old rat embryo of the line with high threshold excitability are characterised by a high level of proliferative activity and chromosome aberration, as well as high degree of brain chromatin concentration as compared with embryos of a line with low threshold excitability.     

Open-field behavior of rats with different levels of nervous system excitability

The dependence was studied of characteristics of organization of orienting-investigating behaviour in the open field test on the level of nervous system excitability in rats selected by the threshold of excitability of the peripheral nervous system. It is established that the studied rats lines can be divided into groups according to entropy level of their behaviour. Rats of highly excitable line build their behaviour in highly probable stereotypes as compared with the animals of low-excitable line, which organize their behaviour with more plasticity, diversity. Differences in the nervous system excitability influence first of all the organization of animals behaviour.     

Glutamatergic signaling in cellular transformation

The role of the glutamatergic system in cancer cell homeostasis has expanded exponentially over the last decade. Once thought to participate only in synaptic transmission and neuronal excitability, the presence of functional glutamate receptors has since been demonstrated in peripheral tissues. Most notable is the implication of glutamate receptors in the pathophysiology of various human malignancies. We previously described the oncogenic properties of metabotropic glutamate receptor 1 (Grm1), a G-protein-coupled receptor in melanoma development in vivo. TG-3, a transgenic mouse line, developed spontaneous melanoma with 100% penetrance in the absence of any known stimuli. Stable Grm1-mouse melanocytic clones display transformed phenotypes in vitro and were aggressively tumorigenic in vivo. Recent reports from other groups implicate two additional members of the metabotropic glutamate receptor family in melanomagenesis, overexpression of mGluR5 and activating mutations in GRM3. These findings highlight a previously underappreciated link between the glutamate signaling pathway and oncogenesis in melanoma biology, raising exciting possibilities in elucidating mechanisms in melanocyte transformation and exploring glutamate receptors as novel therapeutic targets. Here we further consider the potential mechanisms by which glutamate receptors can function as an oncogene leading to malignant transformation.     

MDA-468, a human breast cancer cell line with a high number of epidermal growth factor (EGF) receptors, has an amplified EGF receptor gene and is growth inhibited by EGF

Epidermal growth factor (EGF) has been noted to stimulate proliferation of a variety of normal and malignant cells including those of human breast epithelium. We report here that MDA-468, a human breast cancer cell line with a very high number of EGF receptors, is growth-inhibited at EGF concentrations that stimulate most other cells. The basis for the elevated receptor level is EGF receptor gene amplification and over-expression. An MDA-468 clone selected for resistance to EGF-induced growth inhibition shows a number of receptors within the normal range. The results are discussed in relation to a threshold model for EGF-induced growth inhibition.     

NGF inhibits M/KCNQ currents and selectively alters neuronal excitability in subsets of sympathetic neurons depending on their M/KCNQ current background

M/KCNQ currents play a critical role in the determination of neuronal excitability. Many neurotransmitters and peptides modulate M/KCNQ current and neuronal excitability through their G protein-coupled receptors. Nerve growth factor (NGF) activates its receptor, a member of receptor tyrosine kinase (RTK) superfamily, and crucially modulates neuronal cell survival, proliferation, and differentiation. In this study, we studied the effect of NGF on the neuronal (rat superior cervical ganglion, SCG) M/KCNQ currents and excitability. As reported before, subpopulation SCG neurons with distinct firing properties could be classified into tonic, phasic-1, and phasic-2 neurons. NGF inhibited M/KCNQ currents by similar proportion in all three classes of SCG neurons but increased the excitability only significantly in tonic SCG neurons. The effect of NGF on excitability correlated with a smaller M-current density in tonic neurons. The present study indicates that NGF is an M/KCNQ channel modulator and the characteristic modulation of the neuronal excitability by NGF may have important physiological implications.     

Long-term control of neuronal excitability by corticosteroid hormones

Hippocampal CA1 neurons express both mineralocorticoid and glucocorticoid receptors. Due to the difference in affinity of the two receptor types for corticosterone and variations in endogenous steroid levels, occupation of the receptors will range between a situation of predominant mineralocorticoid receptor activation and conditions where both receptor types are occupied. It was observed that local signal transduction is regulated by activation of the corticosteroid receptors. Particularly, transmission mediated by biogenic amines appears to be sensitive to steroid control. The data indicate that cholinergic and serotonergic responses are small with predominant mineralocorticoid receptor activation, while additional glucocorticoid receptor activation results in large responses; the reverse has been found for noradrenalin. The steroid-dependent control over transmission by biogenic amines will influence local excitability and therefore functional processes in which the hippocampal system is involved.     

Dopamine Modulates the Excitability of Projection Neurons in the Robust Nucleus of the Arcopallium in Adult Zebra Finches

Background

The nervous system in songbirds is an accessible system for studying vocal learning and memory in vertebrates. In the song system, the anterior forebrain pathway (AFP) is essential for song learning and the vocal motor pathway (VMP) is necessary for song production. The premotor robust nucleus of the arcopallium (RA) located in the VMP receives input from the AFP. The RA receives dopaminergic innervations from the periaqueductal gray and ventral tegmental area–substantia nigra pars compacta, but the physiological functions of this projection remain unclear. In this study, we investigated the effects of dopamine (DA) on the excitability of projection neurons (PNs) in the RA.

Methodology

We recorded the electrophysiological changes from neurons in brain slices of male adult zebra finches using a whole-cell recording technique.

Conclusions/Significance

We found that DA significantly increased the excitability of RA PNs. Furthermore, a D1-like receptor agonist increased the excitability of RA PNs, and a D1-like receptor antagonist suppressed the excitability induced by DA. However, a D2-like receptor agonist had no effect on the excitability of RA PNs. Moreover, the D2-like receptor agonist did not change the excitability induced by the D1 receptor agonist. These findings suggest that DA can significantly increase the excitability of RA PNs and that D1 receptors play the main role in regulating the excitability of RA PNs in response to DA, thereby providing direct evidence toward understanding the mechanism of DA signal mediation by its receptors to modulate the excitability of RA PNs.     

A-to-I RNA editing modulates the pharmacology of neuronal ion channels and receptors

The regulation of neuronal excitability is complex, as ion channels and neurotransmitter receptors are underlying a large variety of modulating effects. Alterations in the expression patterns of receptors or channel subunits as well as differential splicing contribute to the regulation of neuronal excitability. RNA editing is another and more recently explored mechanism to increase protein diversity, as the genomic recoding leads to new gene products with novel functional and pharmacological properties. In humans A-to-I RNA editing targets several neuronal receptors and channels, including GluR2/5/6 subunits, the Kv1.1 channel, and the 5-HT_2C receptor. Our review summarizes that RNA editing of these proteins does not only change protein function, but also the pharmacology and presumably the drug therapy in human diseases.     

GABAA receptors at hippocampal mossy fibers

Presynaptic GABAA receptors modulate synaptic transmission in several areas of the CNS but are not known to have this action in the cerebral cortex. We report that GABAA receptor activation reduces hippocampal mossy fibers excitability but has the opposite effect when intracellular Cl- is experimentally elevated. Synaptically released GABA mimics the effect of exogenous agonists. GABAA receptors modulating axonal excitability are tonically active in the absence of evoked GABA release or exogenous agonist application. Presynaptic action potential-dependent Ca2+ transients in individual mossy fiber varicosities exhibit a biphasic dependence on membrane potential and are altered by GABAA receptors. Antibodies against the alpha2 subunit of GABAA receptors stain mossy fibers. Axonal GABAA receptors thus play a potentially important role in tonic and activity-dependent heterosynaptic modulation of information flow to the hippocampus.     

Serotonin and epilepsy

In recent years, there has been increasing evidence that serotonergic neurotransmission modulates a wide variety of experimentally induced seizures. Generally, agents that elevate extracellular serotonin (5-HT) levels, such as 5-hydroxytryptophan and serotonin reuptake blockers, inhibit both focal and generalized seizures, although exceptions have been described, too. Conversely, depletion of brain 5-HT lowers the threshold to audiogenically, chemically and electrically evoked convulsions. Furthermore, it has been shown that several anti-epileptic drugs increase endogenous extracellular 5-HT concentration. 5-HT receptors are expressed in almost all networks involved in epilepsies. Currently, the role of at least 5-HT(1A), 5-HT(2C), 5-HT(3) and 5-HT(7) receptor subtypes in epileptogenesis and/or propagation has been described. Mutant mice lacking 5-HT(1A) or 5-HT(2C) receptors show increased seizure activity and/or lower threshold. In general, hyperpolarization of glutamatergic neurons by 5-HT(1A) receptors and depolarization of GABAergic neurons by 5-HT(2C) receptors as well as antagonists of 5-HT(3) and 5-HT(7) receptors decrease the excitability in most, but not all, networks involved in epilepsies. Imaging data and analysis of resected tissue of epileptic patients, and studies in animal models all provide evidence that endogenous 5-HT, the activity of its receptors, and pharmaceuticals with serotonin agonist and/or antagonist properties play a significant role in the pathogenesis of epilepsies.     

Evidence for a protein tether involved in somatic touch

The gating of ion channels by mechanical force underlies the sense of touch and pain. The mode of gating of mechanosensitive ion channels in vertebrate touch receptors is unknown. Here we show that the presence of a protein link is necessary for the gating of mechanosensitive currents in all low‐threshold mechanoreceptors and some nociceptors of the dorsal root ganglia (DRG). Using TEM, we demonstrate that a protein filament with of length ～100 nm is synthesized by sensory neurons and may link mechanosensitive ion channels in sensory neurons to the extracellular matrix. Brief treatment of sensory neurons with non‐specific and site‐specific endopeptidases destroys the protein tether and abolishes mechanosensitive currents in sensory neurons without affecting electrical excitability. Protease‐sensitive tethers are also required for touch‐receptor function in vivo. Thus, unlike the majority of nociceptors, cutaneous mechanoreceptors require a distinct protein tether to transduce mechanical stimuli.     

Pre & postsynaptic tuning of action potential timing by spontaneous GABAergic activity

Frequency and timing of action potential discharge are key elements for coding and transfer of information between neurons. The nature and location of the synaptic contacts, the biophysical parameters of the receptor-operated channels and their kinetics of activation are major determinants of the firing behaviour of each individual neuron. Ultimately the intrinsic excitability of each neuron determines the input-output function. Here we evaluate the influence of spontaneous GABAergic synaptic activity on the timing of action potentials in Layer 2/3 pyramidal neurones in acute brain slices from the somatosensory cortex of young rats. Somatic dynamic current injection to mimic synaptic input events was employed, together with a simple computational model that reproduce subthreshold membrane properties. Besides the well-documented control of neuronal excitability, spontaneous background GABAergic activity has a major detrimental effect on spike timing. In fact, GABA(A) receptors tune the relationship between the excitability and fidelity of pyramidal neurons via a postsynaptic (the reversal potential for GABA(A) activity) and a presynaptic (the frequency of spontaneous activity) mechanism. GABAergic activity can decrease or increase the excitability of pyramidal neurones, depending on the difference between the reversal potential for GABA(A) receptors and the threshold for action potential. In contrast, spike time jitter can only be increased proportionally to the difference between these two membrane potentials. Changes in excitability by background GABAergic activity can therefore only be associated with deterioration of the reliability of spike timing.     

Contribution of Dopamine D1/5 Receptor Modulation of Post-Spike/Burst Afterhyperpolarization to Enhance Neuronal Excitability of Layer V Pyramidal Neurons in Prepubertal Rat Prefrontal Cortex

Dopamine (DA) receptors in the prefrontal cortex (PFC) modulate both synaptic and intrinsic plasticity that may contribute to cognitive processing. However, the ionic basis underlying DA actions to enhance neuronal plasticity in PFC remains ill-defined. Using whole-cell patch-clamp recordings in layer V-VI pyramidal cells in prepubertal rat PFC, we showed that DA, via activation of D1/5, but not D2/3/4, receptors suppress a Ca²⁺-dependent, apamin-sensitive K⁺ channel that mediates post-spike/burst afterhyperpolarization (AHP) to enhance neuronal excitability of PFC neurons. This inhibition is not dependent on HCN channels. The D1/5 receptor activation also enhanced an afterdepolarizing potential (ADP) that follows the AHP. Additional single-spike analyses revealed that DA or D1/5 receptor activation suppressed the apamin-sensitive post-spike mAHP, further contributing to the increase in evoked spike firing to enhance the neuronal excitability. Taken together, the D1/5 receptor modulates intrinsic mechanisms that amplify a long depolarizing input to sustain spike firing outputs in pyramidal PFC neurons.     

Endogenous ion channel complexes: the NMDA receptor

Ionotropic receptors, including the NMDAR (N-methyl-D-aspartate receptor) mediate fast neurotransmission, neurodevelopment, neuronal excitability and learning. In the present article, the structure and function of the NMDAR is reviewed with the aim to condense our current understanding and highlight frontiers where important questions regarding the biology of this receptor remain unanswered. In the second part of the present review, new biochemical and genetic approaches for the investigation of ion channel receptor complexes will be discussed.     

Localization of mineralocorticoid receptors at mammalian synapses

In the brain, membrane associated nongenomic steroid receptors can induce fast-acting responses to ion conductance and second messenger systems of neurons. Emerging data suggest that membrane associated glucocorticoid and mineralocorticoid receptors may directly regulate synaptic excitability during times of stress when adrenal hormones are elevated. As the key neuron signaling interface, the synapse is involved in learning and memory, including traumatic memories during times of stress. The lateral amygdala is a key site for synaptic plasticity underlying conditioned fear, which can both trigger and be coincident with the stress response. A large body of electrophysiological data shows rapid regulation of neuronal excitability by steroid hormone receptors. Despite the importance of these receptors, to date, only the glucocorticoid receptor has been anatomically localized to the membrane. We investigated the subcellular sites of mineralocorticoid receptors in the lateral amygdala of the Sprague-Dawley rat. Immunoblot analysis revealed the presence of mineralocorticoid receptors in the amygdala. Using electron microscopy, we found mineralocorticoid receptors expressed at both nuclear including: glutamatergic and GABAergic neurons and extra nuclear sites including: presynaptic terminals, neuronal dendrites, and dendritic spines. Importantly we also observed mineralocorticoid receptors at postsynaptic membrane densities of excitatory synapses. These data provide direct anatomical evidence supporting the concept that, at some synapses, synaptic transmission is regulated by mineralocorticoid receptors. Thus part of the stress signaling response in the brain is a direct modulation of the synapse itself by adrenal steroids.     

Effect of catecholamines on excitability of single mechanoreceptors of Pacinian corpuscles

A method of continuous external perfusion was used to study the effect of adrenalin and noradrenalin on the excitability of single isolated mechanoreceptors of Pacinian corpuscles. On the addition of adrenalin and noradrenalin in concentrations of 1·10^–5–1·10^–7 g/ml to the solution the excitability of the receptors was increased by 10–15% and the amplitude of the receptor potential rose on the average by 10–20%. It is postulated that the change in excitability is due to an increase in the receptor membrane potential. The results are discussed in connection with the possible adrenergic innervation of the Pacinian corpuscles.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 6, No. 3, pp. 312–317, May–June, 1974.     

Barium, glibenclamide and CGS21680 prevent adenosine A1 receptor changes of ES coupling and spike threshold

Activation of adenosine A1 receptors raised spike thresholds and induced a dissociation of excitatory postsynaptic potential (EPSP) spike coupling in hippocampal pyramidal neurones. This effect could be prevented by activation of A2A adenosine receptors. The A1 receptor agonist N6-cyclopentyladenosine caused a dissociation of the EPSP spike coupling recorded extracellularly and increased the threshold for spike generation measured intracellularly. These effects were prevented by the A2A receptor agonist CGS21680. A series of agents interfering with adenylate cyclase activity, protein kinase A or C, or nitric oxide synthase had no effect on these responses to N6-cyclopentyladenosine. Superfusion with barium or glibenclamide prevented both the dissociation of EPSP spike coupling and the increase of spike threshold. It is concluded that a barium- and glibenclamide-sensitive potassium current may be involved in the postsynaptic effects of A1 receptors on spike threshold, and it is suggested that a similar suppression of a potassium current by A2A receptors could underlie the inhibition of A1 receptor responses.     

SK-N-BE: A Human Neuroblastoma Cell Line Containing Two Subtypes of δ-Opioid Receptors

Abstract: A human neuroblastoma cell line, SK-N-BE, was shown to express a substantial amount of opioid receptors (200–300 fmol/mg of protein). A ligand binding profile of these receptors revealed that they could belong to two distinct subtypes of δ-opioid receptors. Results from sucrose-gradient sedimentation experiments were compared with similar data obtained with the μ-opioid receptor of the rabbit cerebellum and the δ-opioid receptor of the hybrid NG108–15 cell line and have shown that the opioid receptor of the SK-N-BE cell line behaved hydrodynamically as an intermediate between μ-and δ-opioid receptors. Taken together, pharmacological and hydrodynamic studies suggest that the opioid receptors present in the SK-N-BE cell membranes could belong to two δ-opioid receptor subtypes interacting allosterically. Functional experiments suggest that at least one of these subtypes of δ-opioid receptor is negatively coupled to the adenylate cyclase via a G_i protein and that the opiate receptors of the SK-N-BE neuroblastoma cell line undergo a rapid down-regulation when preincubated in the presence of the high-affinity opioid agonist, etorphine.     

Clustering of extrasynaptic GABA(A) receptors modulates tonic inhibition in cultured hippocampal neurons

Tonic inhibition plays a crucial role in regulating neuronal excitability because it sets the threshold for action potential generation and integrates excitatory signals. Tonic currents are known to be largely mediated by extrasynaptic gamma-aminobutyric acid type A (GABA(A)) receptors that are persistently activated by submicromolar concentrations of ambient GABA. We recently reported that, in cultured hippocampal neurons, the clustering of synaptic GABA(A) receptors significantly affects synaptic transmission. In this work, we demonstrated that the clustering of extrasynaptic GABA(A) receptors modulated tonic inhibition. Depolymerization of the cytoskeleton with nocodazole promoted the disassembly of extrasynaptic clusters of delta and gamma(2) subunit-containing GABA(A) receptors. This effect was associated with a reduction in the amplitude of tonic currents and diminished shunting inhibition. Moreover, diffuse GABA(A) receptors were less sensitive to the GAT-1 inhibitor NO-711 and to flurazepam. Quantitative analysis of GABA-evoked currents after prolonged exposure to submicromolar concentrations of GABA and model simulations suggest that clustering affects the gating properties of extrasynaptic GABA(A) receptors. In particular, a larger occupancy of the singly and doubly bound desensitized states can account for the modulation of tonic inhibition recorded after nocodazole treatment. Moreover, comparison of tonic currents recorded during spontaneous activity and those elicited by exogenously applied low agonist concentrations allows estimation of the concentration of ambient GABA. In conclusion, receptor clustering appears to be an additional regulating factor for tonic inhibition.