Firing Frequency Of Spontaneous Action Potentials Research Articles

Impaired regulation of heart rate and sinoatrial node function by the parasympathetic nervous system in type 2 diabetic mice

Heart rate (HR) and sinoatrial node (SAN) function are modulated by the autonomic nervous system. HR regulation by the parasympathetic nervous system (PNS) is impaired in diabetes mellitus (DM), which is denoted cardiovascular autonomic neuropathy. Whether blunted PNS effects on HR in type 2 DM are related to impaired responsiveness of the SAN to PNS agonists is unknown. This was investigated in type 2 diabetic db/db mice in vivo and in isolated SAN myocytes. The PNS agonist carbachol (CCh) had a smaller inhibitory effect on HR, while HR recovery time after CCh removal was accelerated in db/db mice. In isolated SAN myocytes CCh reduced spontaneous action potential firing frequency but this effect was reduced in db/db mice due to blunted effects on diastolic depolarization slope and maximum diastolic potential. Impaired effects of CCh occurred due to enhanced desensitization of the acetylcholine-activated K+ current (IKACh) and faster IKACh deactivation. IKACh alterations were reversed by inhibition of regulator of G-protein signaling 4 (RGS4) and by the phospholipid PIP3. SAN expression of RGS4 was increased in db/db mice. Impaired PNS regulation of HR in db/db mice occurs due to reduced responsiveness of SAN myocytes to PNS agonists in association with enhanced RGS4 activity.

The firing frequency of spontaneous action potentials and their corresponding evoked exocytosis are increased in chromaffin cells of CCl4 -induced cirrhotic rats with respect to control rats.

High catecolamine plasma levels because of sympathetic nervous system over-activity contribute to cirrhosis progression. The aim of this study was to investigate whether chromaffin cells of the adrenal gland might potentiate the deleterious effect exerted by this over-activity. Electrophysiological patch-clamp and amperometric experiments with carbon-fibre electrodes were conducted in single chromaffin cells of control and CCl4 -induced cirrhotic rats. The spontaneous action potential firing frequency was increased in chromaffin cells of cirrhotic rats with respect to control rats. The exocytosis evoked by that firing was also increased. However, exocytosis elicited by ACh did not vary between control and cirrhotic rats. Exocytosis triggered by depolarizing pulses was also unchanged. Amperometric recordings confirmed the lack of increased catecholamine charge released in cirrhosis after ACh or depolarization stimuli. However, the amperometric spikes exhibited faster kinetics of release. The overall Ca2+ entry through voltage-dependent Ca2+ channels (VDCC), or in particular through Cav1 channels, did not vary between chromaffin cells of control and cirrhotic rats. The inhibition of VDCC by methionine-enkephaline or ATP was not either altered, but it was increased by adrenaline in cells of cirrhotic rats. When a cocktail composed by the three neurotransmitters was tested in order to approach a situation closer to the physiological condition, the inhibition of VDCC was similar between both types of cells. In summary, chromaffin cells of the adrenal gland might contribute to exacerbate the sympathetic nervous system over-activity in cirrhosis because of an increased exocytosis elicited by an enhanced spontaneous electrical activity.

Kisspeptins modulate the biology of multiple populations of gonadotropin-releasing hormone neurons during embryogenesis and adulthood in zebrafish (Danio rerio).

Kisspeptin1 (product of the Kiss1 gene) is the key neuropeptide that gates puberty and maintains fertility by regulating the gonadotropin-releasing hormone (GnRH) neuronal system in mammals. Inactivating mutations in Kiss1 and the kisspeptin receptor (GPR54/Kiss1r) are associated with pubertal failure and infertility. Kiss2, a paralogous gene for kiss1, has been recently identified in several vertebrates including zebrafish. Using our transgenic zebrafish model system in which the GnRH3 promoter drives expression of emerald green fluorescent protein, we investigated the effects of kisspeptins on development of the GnRH neuronal system during embryogenesis and on electrical activity during adulthood. Quantitative PCR showed detectable levels of kiss1 and kiss2 mRNA by 1 day post fertilization, increasing throughout embryonic and larval development. Early treatment with Kiss1 or Kiss2 showed that both kisspeptins stimulated proliferation of trigeminal GnRH3 neurons located in the peripheral nervous system. However, only Kiss1, but not Kiss2, stimulated proliferation of terminal nerve and hypothalamic populations of GnRH3 neurons in the central nervous system. Immunohistochemical analysis of synaptic vesicle protein 2 suggested that Kiss1, but not Kiss2, increased synaptic contacts on the cell body and along the terminal nerve-GnRH3 neuronal processes during embryogenesis. In intact brain of adult zebrafish, whole-cell patch clamp recordings of GnRH3 neurons from the preoptic area and hypothalamus revealed opposite effects of Kiss1 and Kiss2 on spontaneous action potential firing frequency and membrane potential. Kiss1 increased spike frequency and depolarized membrane potential, whereas Kiss2 suppressed spike frequency and hyperpolarized membrane potential. We conclude that in zebrafish, Kiss1 is the primary stimulator of GnRH3 neuronal development in the embryo and an activator of stimulating hypophysiotropic neuron activities in the adult, while Kiss2 plays an additional role in stimulating embryonic development of the trigeminal neuronal population, but is an RFamide that inhibits electrical activity of hypophysiotropic GnRH3 neurons in the adult.

Excitation of Rat Cerebellar Golgi Cells by Ethanol: Further Characterization of the Mechanism

Studies with rodents suggest that acute ethanol exposure impairs information flow through the cerebellar cortex, in part, by increasing GABAergic input to granule cells. Experiments suggest that an increase in the excitability of specialized GABAergic interneurons that regulate granule cell activity (i.e., Golgi cells [GoCs]) contributes to this effect. In GoCs, ethanol increases spontaneous action potential firing frequency, decreases the afterhyperpolarization amplitude, and depolarizes the membrane potential. Studies suggest that these effects could be mediated by inhibition of the Na(+)/K(+) ATPase. The purpose of this study was to characterize the potential role of other GoC conductances in the mechanism of action of ethanol. Computer modeling techniques and patch-clamp electrophysiological recordings with acute slices from rat cerebella were used for these studies. Computer modeling suggested that modulation of subthreshold Na(+) channels, hyperpolarization-activated currents, and several K(+) conductances could explain some but not all actions of ethanol on GoCs. Electrophysiological studies did not find evidence consistent with a contribution of these conductances. Quinidine, a nonselective blocker of several types of channels (including several K(+) channels) that also antagonizes the Na(+)/K(+) ATPase, reduced the effect of ethanol on GoC firing. These findings further support that ethanol increases GoC excitability via modulation of the Na(+)/K(+) ATPase and suggest that a quinidine-sensitive K(+) channel may also play a role in the mechanism of action of ethanol.

Ethanol Decreases Purkinje Neuron Excitability by Increasing GABA Release in Rat Cerebellar Slices

Cerebellar Purkinje neurons (PNs) receive inhibitory GABAergic input from stellate and basket cells, which are located in the outer and inner portions of the molecular layer, respectively. Ethanol (EtOH) was recently shown to increase GABAergic transmission at PNs via a mechanism that involves enhanced calcium release from presynaptic internal stores (J Pharmacol Exp Ther 323:356-364, 2007). Here, we further characterized the effect of EtOH on GABA release and assessed its impact on PN excitability. Using whole-cell patch-clamp electrophysiological techniques in cerebellar vermis parasagittal slices, we found that EtOH acutely increases the frequency but not the amplitude or half-width of miniature and spontaneous inhibitory postsynaptic currents (IPSCs). EtOH significantly increased the amplitude and decreased the paired pulse ratio of IPSCs evoked by stimulation in the outer but not inner molecular layer. In current clamp, EtOH decreased both the amplitude of excitatory postsynaptic potentials evoked in PNs by granule cell axon stimulation and the number of action potentials triggered by these events; these effects depended on GABA(A) receptor activation because they were not observed in presence of bicuculline. Loose-patch cell-attached PN recordings revealed that neither the spontaneous action potential firing frequency nor the coefficient of variation of the interspike interval was altered by acute EtOH exposure. These findings suggest that EtOH differentially affects GABAergic transmission at stellate cell- and basket cell-to-PN synapses and that it modulates PN firing triggered by granule cell axonal input. These effects could be in part responsible for the cerebellar impairments associated with acute EtOH intoxication.

Inhibitory synaptic plasticity regulates pyramidal neuron spiking in the rodent hippocampus

Spike-timing modifies the efficacy of both excitatory and inhibitory synapses onto CA1 pyramidal neurons in the rodent hippocampus. Repetitively spiking the presynaptic neuron before the postsynaptic neuron induces inhibitory synaptic plasticity, which results in a depolarization of the reversal potential for GABA (E GABA). Our goal was to determine how inhibitory synaptic plasticity regulates CA1 pyramidal neuron spiking in the rat hippocampus. We demonstrate electrophysiologically that depolarizing E GABA by 24.7 mV increased the spontaneous action potential firing frequency of cultured hippocampal neurons 254% from 0.12±0.07 Hz to 0.44±0.13 Hz ( n=11; P<0.05). Next we used a single compartment model of a CA1 pyramidal neuron to explore in detail how inhibitory synaptic plasticity of feedforward and feedback inhibition regulates the generation of action potentials, spike latency, and the minimum excitatory conductance required to generate an action potential; plasticity was modeled as a depolarization of E GABA, which effectively weakens inhibition. Depolarization of E GABA at feedforward and feedback inhibitory synapses decreased the latency to the 1st spike by 2.27 ms, which was greater that the sum of the decreases produced by depolarizing E GABA at feedforward (0.85 ms) or feedback inhibitory synapses (0.02 ms) alone. In response to a train of synaptic inputs, depolarizing E GABA decreased the inter-spike interval and increased the number of output spikes in a frequency dependent manner, improving the reliability of input–output transmission. Moreover, a depolarizing shift in E GABA at feedforward and feedback synapses triggered by spike trains recorded from CA1 pyramidal layer neurons during field theta from anesthetized rats, significantly increased spiking on the up- and down-strokes of the first half of the theta rhythm ( P<0.05), without changing the preferred phase of firing ( P=0.783). This study provides the first explanation of how depolarizing E GABA affects pyramidal cell output within the hippocampus.

Circadian- and Light-Dependent Regulation of Resting Membrane Potential and Spontaneous Action Potential Firing of Drosophila Circadian Pacemaker Neurons

The ventral lateral neurons (LNvs) of adult Drosophila brain express oscillating clock proteins and regulate circadian behavior. Whole cell current-clamp recordings of large LNvs in freshly dissected Drosophila whole brain preparations reveal two spontaneous activity patterns that correlate with two underlying patterns of oscillating membrane potential: tonic and burst firing of sodium-dependent action potentials. Resting membrane potential and spontaneous action potential firing are rapidly and reversibly regulated by acute changes in light intensity. The LNv electrophysiological light response is attenuated, but not abolished, in cry(b) mutant flies hypomorphic for the cell-autonomous light-sensing protein CRYPTOCHROME. The electrical activity of the large LNv is circadian regulated, as shown by significantly higher resting membrane potential and frequency of spontaneous action potential firing rate and burst firing pattern during circadian subjective day relative to subjective night. The circadian regulation of membrane potential, spontaneous action potential firing frequency, and pattern of Drosophila large LNvs closely resemble mammalian circadian neuron electrical characteristics, suggesting a general evolutionary conservation of both physiological and molecular oscillator mechanisms in pacemaker neurons.