Potential Afterhyperpolarization Research Articles

Spontaneous firing properties of rat medial vestibular nucleus neurons in brain slices by infrared visual patch clamp technique

Domestic application of infrared patch clamp techniques on brain slices is limited. The key of the technique is to prepare high-quality brain slices. The present paper describes the preparation procedure of brainstem slices and the spontaneous firing properties of rat medial vestibular nucleus (MVN) neurons. By infrared differential interference contrast technique, neurons of rat MVN were visualized directly at the depth of 50–100 μm underneath the surface of slices. Firing activities of MVN neurons were recorded by the whole-cell patch clamp technique in artificial cerebrospinal fluid (ACSF) and low Ca2+-high Mg2+ fluid. The firing mode was more irregular and depressive in low Ca2+-high Mg2+ fluid than in ACSF. According to the averaged waveform of action potentials, cells were classified as the neurons with monophasic after-hyperpolarization potential (AHP), and the neurons with biphasic AHP. The resting membrane potential (RMP), input resistance (Rin) and membrane capacitance (Cm) of neurons were recorded and compared between groups. With infrared videomicroscopy, patch clamp recordings could be made under direct observation in freshly prepared brainstem slices. The discharge activities of MVN neurons were spontaneous and the firing mode was modulated by extracellular calcium concentration. The basic membrane properties of two types of neurons were not significantly different, while the differences in waveform might play a role in the segregation between tonic and kinetic cells.

Systematic selection of model parameter values matching biological behavior under different simulation scenarios

In this work, we systematically explore a 12-dimensional parameter space of a 2-compartment model of the AB (anterior burster) neuron, which is one of the two cells that form the pacemaker kernel in the pyloric network in the lobster stomatogastric ganglion (STG). Our computational exploration started with a hand-tuned AB model [1] and systematically varied maximal conductances of membrane currents. Every parameter set for an individual model neuron was simulated and classified as functional if it produced biologically realistic bursting activity. Specifically, we were looking at the period, burst duration, spike and slow wave amplitude, number of spikes per burst, spike frequency, and after-hyperpolarization potential, which all had to be within limits determined in our physiological experiments. Furthermore, in order to be classified as good, the models had to exhibit proper responses to STG deafferentation (i.e., neuromodulator deprivation) as well as current injections both in the presence and in the absence of neuromodulation. The above selection criteria were applied in a step-by-step fashion, meaning that only the models deemed good in the previous step were tested in the next. After applying all the criteria, not only have we determined that many different parameter sets performed successfully under all tested conditions, but we have also found that application of just a subset of the selection criteria was often enough to almost fully constrain the model parameters and application of additional criteria did not significantly reduce the extent of the model's solution space. In order to further analyze this saturation effect, rather than utilizing the step-by-step approach, we have re-simulated models in our database for all testing criteria. This allowed us to draw conclusions about the relative selectiveness of particular criteria as well as to analyze overlapping regions between subsets of models that simultaneously fulfilled some, but not all, conditions. Due to this approach, we were able to pinpoint parameter values that are crucial for a proper functioning of a model under each of the simulation scenarios separately, which helps explain which maximal conductances of particular membrane currents are important for a given type of activity.

Muscarinic M 1 modulation of N and L types of calcium channels is mediated by protein kinase C in neostriatal neurons

In some neurons, muscarinic M 1-class receptors control L-type (Ca V1) Ca 2+-channels via protein kinase C (PKC) or calcineurin (phosphatase 2B; PP-2B) signaling pathways. Both PKC and PP-2B pathways start with phospholipase C (PLC) activation. In contrast, P/Q- and N-type (Ca V2.1, 2.2, respectively) Ca 2+-channels are controlled by M 2-class receptors via G proteins that may act, directly, to modulate these channels. The hypothesis of this work is that this description is not enough to explain muscarinic modulation of Ca 2+ channels in rat neostriatal projection neurons. Thus, we took advantage of the specific muscarinic toxin 3 (MT-3) to block M 4-type receptors in neostriatal neurons, and leave in isolation the M 1-type receptors to study them separately. We then asked what Ca 2+ channels are modulated by M 1-type receptors only. We found that M 1-receptors do modulate L, N and P/Q-types Ca 2+ channels. This modulation is blocked by the M 1-class receptor antagonist (muscarinic toxin 7, MT-7) and is voltage-independent. Thereafter, we asked what signaling pathways, activated by M 1-receptors would control these channels. We found that inactivation of PLC abolishes the modulation of all three channel types. PKC activators (phorbol esters) mimic muscarinic actions, whereas reduction of intracellular calcium virtually abolishes all modulation. As expected, PKC inhibitors prevented the muscarinic reduction of the afterhyperpolarizing potential (AHP), an event known to be dependent on Ca 2+ entry via N- and P/Q-type Ca 2+ channels. However, PKC inhibitors (bisindolylmaleimide I and PKC-1936) only block modulation of currents through N and L types Ca 2+ channels; while the modulation of P/Q-type Ca 2+ channels remains unaffected. These results show that different branches of the same signaling cascade can be used to modulate different Ca 2+ channels. Finally, we found no evidence of calcineurin modulating these Ca 2+ channels during M 1-receptor activation, although, in the same cells, we demonstrate functional PP-2B by activating dopaminergic D 2-receptor modulation.

Nitric oxide modulates the firing rate of the rat supraoptic magnocellular neurons

In vitro, nitric oxide (NO) inhibits the firing rate of magnocellular neurosecretory cells (MNCs) of hypothalamic supraoptic and paraventricular nuclei and this effect has been attributed to GABAergic activation. However, little is known about the direct effects of NO in MNCs. We used the patch-clamp technique to verify the effect of l-arginine, a precursor for NO synthesis, and Nω-nitro-l-arginine methyl ester hydrochloride (l-NAME), an inhibitor of NOS, on spontaneous electrical activity of MNCs after glutamatergic and GABAergic blockade in Wistar rat brain slices. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 μM) and dl-2-amino-5-phosphonovaleric acid (dl-AP5) (30 μM) were used to block postsynaptic glutamatergic currents, and picrotoxin (30 μM) and saclofen (30 μM) to block ionotropic and metabotropic postsynaptic GABAergic currents. Under these conditions, 500 μM l-arginine decreased the firing rate from 3.7±0.6 Hz to 1.3±0.3 Hz. Conversely, 100 μM l-NAME increased the firing rate from 3.0±0.3 Hz to 5.8±0.4 Hz. All points histogram analysis showed changes in resting potential from −58.1±0.8 mV to −62.2±1.1 mV in the presence of l-arginine and from −59.8±0.7 mV to −56.9±0.8 mV by l-NAME. Despite the nitrergic modulator effect on firing rate, some MNCs had no significant changes in their resting potential. In those neurons, hyperpolarizing after-potential (HAP) amplitude increased from 12.4±1.2 mV to 16.8±0.7 mV by l-arginine, but without significant changes by l-NAME treatment. To our knowledge, this is the first demonstration that NO can inhibit MNCs independent of GABAergic inputs. Further, our results point to HAP as a potential site for nitrergic modulation.

Evidence that TASK1 channels contribute to the background current in AH/type II neurons of the guinea-pig intestine

Neurons that have AH (designation of neurons with a prominent and prolonged afterhyperpolarizing potential that follows the action potential) electrophysiological characteristics and type II morphology (AH/type II neurons) are the first neurons in reflex circuits in the small intestine. Thus, the state of excitation of these neurons strongly influences the properties of enteric reflexes. The resting outward current in the type II neurons is reduced, causing depolarization and increased excitability, when protein kinase C (PKC) or synaptic inputs are activated, suggesting that regulation of background channels is an important determinant of the state of excitability of these neurons. However, the channels that carry the background current are not yet identified. We used intracellular microelectrodes to record from myenteric AH/type II neurons of the guinea-pig ileum, immunohistochemistry to localize channels and reverse transcriptase–polymerase chain reaction (RT-PCR) to characterize channel transcripts. The blockers of TASK1 channels, bupivacaine (1 mM) and methanandamide (10 μM), depolarized AH/type II neurons by 11.6 mV and 7.9 mV, respectively, and increased resting input resistance by about 30%. The reversal potential determined for the effect of bupivacaine was −92 mV, indicating that bupivacaine acts at K + channels, without significant action on other channel types that are open at rest. The membrane potential of type II neurons was depolarized by acidification to pH 6.4, but this depolarization was associated with decreased input resistance and was not reduced by bupivacaine. Thus an unidentified current that is activated by reduced pH masks effects on TASK channels. Slow excitatory post-synaptic potentials in the neurons were reduced in amplitude by methanandamide, suggesting that they are generated in part by closure of TASK1 channels. TASK1 immunoreactivity occurred in all type II neurons (determined by double labeling for IB4 and NeuN), but no type II neurons were immunoreactive for TASK2 or TASK3. These latter channels were localized to non-type II neurons. Transcripts for TASK1, TASK2, TASK3 and other two-pore-domain potassium channels were found in ganglion extracts. It is concluded that TASK1 channels contribute to the resting outward current in AH/type II neurons, and that neurotransmitters that evoke slow depolarizations in these neurons do so through the closure of resting K + channels that include TASK1 channels.

Small-Conductance Calcium-Activated Potassium Channels Control Excitability and Firing Dynamics in Gonadotropin-Releasing Hormone (GnRH) Neurons

The cellular mechanisms determining the firing patterns of GnRH neurons are presently under intense investigation. In this study, we used GnRH-green fluorescent protein transgenic mice and perforated-patch electrophysiology to examine the role of small conductance calcium-activated potassium (SK) channels in determining the electrical excitability and burst-firing characteristics of adult GnRH neurons. After establishing an appropriate protocol for examining the afterhyperpolarization potential (AHP) currents in GnRH neurons, the highly selective SK channel blocker apamin was used to demonstrate that all GnRH neurons express functional SK channels (35.7 +/- 2.7 pA, mean decay time constant = 2167 msec, apamin IC(50) = 9.6 nm) and that this channel underlies approximately 90% of the AHP in these cells. Current-clamp experiments showed that apamin-sensitive SK channels were tonically active in the majority (74%) of GnRH neurons, with apamin (100 nm) administration resulting in a mean 6.9 +/- 0.5 mV membrane depolarization. Apamin also elevated the firing rate of GnRH neurons, including increased burst frequency and duration in spontaneously bursting cells as well as the ability of GnRH neurons to fire action potentials in response to current injection. In GnRH neurons activated by current injection, apamin significantly enhanced the amplitude of the afterdepolarization potential after a single action potential and eliminated spike frequency adaptation. Together, these studies show that apamin-sensitive SK channels play a key role in restraining GnRH neuron excitability. Through direct modulation of the AHP and indirect actions on the afterdepolarization potential, the SK channel exerts a powerful tonic influence upon the firing dynamics of GnRH neurons.

Effects of Levetiracetam on granule cells and fast spikeing basket cells in the dentate gyrus

Event Abstract Back to Event Effects of Levetiracetam on granule cells and fast spikeing basket cells in the dentate gyrus Stefan Hefft1*, Stefanie Heigele1, Gerd Münzner1, Andreas Schulze-Bonhage1 and Ad Aertsen2 1 Freiburg University, Germany 2 BCCN, Germany Levetiracetam (LEV) is one of the most important antiepileptic drugs commonly used in clinical practice. Although binding studies have shown that LEV interacts with the synaptic vesicle proteine 2A (SV2A), it's mechanism of action has largely remained unknown. Furthermore, experimental data have also shown small effects on voltage-gated ion channels. Here we used mainly transverse hippocampal slices of juvenile rats (P17-P22) in order to study the electrophysiological effects of the active enantiomer of LEV (100 μM) on granule cells (GCs) and fast spiking basket cells (BCs) in the dentate gyrus (DG)at room temperature. Current clamp recordings in both cell types show no effect on resting membrane conductance, de- and re-polarization kinetics of action potential waveforms, afterde- or afterhyper-polarization potentials, input-output curves, constructed by 1 second lasting constant current injections ( -0,1 to 1-1,2 nA) or bursting patterns induced by short, depolarizing current pulses in both GCs and BCs. These data argue against a significant physiological effect of LEV through voltage-gated channels. However, preliminary data show a reduction of spontaneous IPSC amplitudes recorded in GCs. Importantly, this effect was reversible, and its time course was compatible with the onset of the effect of LEV seen upon intravenously applied drug in epileptic patients. Thus, fine-tuning of GABAergic inhibitory transmission may be the main effect of LEV. In the next step, we will now repeat these experiments in human tissue and thereafter integrate the data in a simple neuronal network in order to perform a computational analysis of the effects of LEV on ictogenesis. Conference: Bernstein Symposium 2008, Munich, Germany, 8 Oct - 10 Oct, 2008. Presentation Type: Poster Presentation Topic: All Abstracts Citation: Hefft S, Heigele S, Münzner G, Schulze-Bonhage A and Aertsen A (2008). Effects of Levetiracetam on granule cells and fast spikeing basket cells in the dentate gyrus. Front. Comput. Neurosci. Conference Abstract: Bernstein Symposium 2008. doi: 10.3389/conf.neuro.10.2008.01.058 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 16 Nov 2008; Published Online: 16 Nov 2008. * Correspondence: Stefan Hefft, Freiburg University, Freiburg, Germany, stefan.hefft@uniklinik-freiburg.de Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Stefan Hefft Stefanie Heigele Gerd Münzner Andreas Schulze-Bonhage Ad Aertsen Google Stefan Hefft Stefanie Heigele Gerd Münzner Andreas Schulze-Bonhage Ad Aertsen Google Scholar Stefan Hefft Stefanie Heigele Gerd Münzner Andreas Schulze-Bonhage Ad Aertsen PubMed Stefan Hefft Stefanie Heigele Gerd Münzner Andreas Schulze-Bonhage Ad Aertsen Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Phenotypic changes of morphologically identified guinea‐pig myenteric neurons following intestinal inflammation

We investigated the responses of morphologically identified myenteric neurons of the guinea-pig ileum to inflammation that was induced by the intraluminal injection of trinitrobenzene sulphonate, 6 or 7 days previously. Electrophysiological properties were examined with intracellular microelectrodes using in vitro preparations from the inflamed or control ileum. The neurons were injected with marker dyes during recording and later they were recovered for morphological examination. A proportion of neurons with Dogiel type I morphology, 45% (32/71), from the inflamed ileum had a changed phenotype. These neurons exhibited an action potential with a tetrodotoxin-resistant component, and a prolonged after-hyperpolarizing potential followed the action potential. Of the other 39 Dogiel type I neurons, no changes were observed in 36 and 3 had increased excitability. The afterhyperpolarizing potential (AHP) in Dogiel type I neurons was blocked by the intermediate conductance, Ca(2+)-dependent K(+) channel blocker TRAM-34. Neurons which showed these phenotypic changes had anally directed axonal projections. Neither a tetrodotoxin-resistant action potential nor an AHP was seen in Dogiel type I neurons from control preparations. Dogiel type II neurons retained their distinguishing AH phenotype, including an inflection on the falling phase of the action potential, an AHP and, in over 90% of neurons, an absence of fast excitatory transmission. However, they became hyperexcitable and exhibited anodal break action potentials, which, unlike control Dogiel type II neurons, were not all blocked by the h current (I(h)) antagonist Cs(+). It is concluded that inflammation selectively affects different classes of myenteric neurons and causes specific changes in their electrophysiological properties.

Effect of intestinal inflammation on the afterhyperpolarizing potential (AHP) in myenteric neurons

The inflammation-induced increases in the excitability of neurons might contribute to disorders of motility, secretion and hypersensitivity during and following gastrointestinal inflammation. The late AHP following an action potential, which in the non-inflamed intestine is a distinctive feature of Dogiel type II (DII) neurons, controls the excitability of neurons. Dogiel type I (DI) neurons in non-inflamed intestine do not exhibit late AHPs.

Effects of modulators of Ca2+‐activated, intermediate‐conductance potassium channels on motility of the rat small intestine, in vivo

The movements of the intestine shift between different motor patterns, including between propulsion and mixing, but there is little information concerning mechanisms that may lead to changes in the patterns of motility. We have investigated the influence on intestinal motility of drugs that affect the after-hyperpolarization potential (AHP) of intrinsic primary afferent neurons (IPANs). The current of the AHP is carried by the intermediate conductance, calcium-activated, potassium (IK) channel. In anaesthetized rats, the IK channel blocker, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (0.05-1 mg kg(-1), i.v.) disrupted the regular propulsive pressure waves that occur in the small intestine and reduced propulsion of the contents (after 1 mg kg(-1), the fluid propelled was <25% of control). If the propulsion in the intestine was regular, the IK channel opener, 5,6-dichloro-1-ethyl-2-benzimidazolinone (DC-EBIO, 0.1 mg kg(-1) h(-1)) had no effect. DC-EBIO (0.1 mg kg(-1) h(-1)) restored propulsive activity after the nitric oxide synthase inhibitor, Nomega-nitro-l-arginine had changed motility to a mixing pattern. We suggest that the AHP determines the synchrony of action potential firing in synaptically coupled IPANs, and that this synchrony influences the patterns of firing of muscle motor neurons, and hence the pattern of contraction of the muscle and whether the pattern is predominantly propulsive or predominantly mixing.

Dynamics of Protein Kinase A Signaling at the Membrane, in the Cytosol, and in the Nucleus of Neurons in Mouse Brain Slices

The cAMP-dependent protein kinase A (PKA) plays a ubiquitous role in the regulation of neuronal activity, but the dynamics of its activation have been difficult to investigate. We used the genetically encoded fluorescent probe AKAR2 to record PKA activation in the cytosol and the nucleus of neurons in mouse brain slice preparations, whereas the potassium current underlying the slow afterhyperpolarization potential (sAHP) in thalamic intralaminar neurons was used to monitor PKA activation at the membrane. Adenylyl cyclase was stimulated either directly using forskolin or via activation of 5-HT7 receptors. Both stimulations produced a maximal effect on sAHP, whereas in the cytosol, the amplitude of the 5-HT7 receptor-mediated response was half of that after direct adenylyl cyclase stimulation with forskolin. 5-HT7-mediated PKA responses were obtained in 30 s at the membrane, in 2.5 min in the cytosol, and in 13 min in the nucleus. Our results show in morphologically intact mammalian neurons the potential physiological relevance of PKA signal integration at the subcellular level: neuromodulators produce fast and powerful effects on membrane excitability, consistent with a highly efficient functional coupling between adenylyl cyclases, PKA, and target channels. Phosphorylation in the cytosol is slower and of graded amplitude, showing a differential integration of the PKA signal between the membrane and the cytosol. The nucleus integrates these cytosolic signals over periods of tens of minutes, consistent with passive diffusion of the free catalytic subunit of PKA into the nucleus, eventually resulting in a graded modulation of gene expression.

Effects of Compounds That Influence IK (KCNN4) Channels on Afterhyperpolarizing Potentials, and Determination of IK Channel Sequence, in Guinea Pig Enteric Neurons

The late afterhyperpolarizing potential (AHP) that follows the action potential in intrinsic primary afferent neurons of the gastrointestinal tract has a profound influence on their firing patterns. There has been uncertainty about the identity of the channels that carry the late AHP current, especially in guinea pigs, where the majority of the physiological studies have been made. In the present work, the late AHP was recorded with intracellular microelectrodes from myenteric neurons in the guinea pig small intestine. mRNA was extracted from the ganglia to determine the identity of the guinea pig intermediate conductance potassium (I(K)) channel gene transcript. The late AHP was inhibited by two blockers of I(K) channels, TRAM34 (0.1-1 microM) and clotrimazole (10 microM), and was enhanced by the potentiator of the opening of these channels, DC-EBIO (100 nM). Action potential characteristics were unchanged by TRAM34 or DC-EBIO. The full sequence of the gene transcript and the deduced amino acid sequence were determined from extracts including myenteric ganglia and from bladder urothelium, which is a rich source of I(K) channel mRNA. This showed that the guinea pig sequence has a high degree of homology with other mammalian sequences but that the guinea pig channel lacks a phosphorylation site that was thought to be critical for channel regulation. It is concluded that the channels that carry the current of the late afterhyperpolarizing potential in guinea pig enteric neurons are I(K) channels.

Comparative effects of lidocaine and tramadol on injured peripheral nerves

The aim of the present study was to investigate the action of lidocaine and tramadol on the abnormal impulse characteristics of injured peripheral nerves. The ultrastructure of nerves was studied with electron microscopy and the action of lidocaine and tramadol on intact and injured rat sciatic nerves was examined by using the sucrose gap recording technique. Tramadol and lidocaine caused concentration- and frequency-dependent decreases in the amplitude of the compound action potential. Injured nerves were more sensitive to lidocaine than to tramadol. Lidocaine suppressed the delayed depolarization and decreased the hyperpolarizing afterpotentials to a greater extent than did tramadol. A low concentration of lidocaine may restore the abnormal impulse characteristics of injured nerves without changing the normal impulse pattern. The efficacy of lidocaine and inefficacy of tramadol on abnormal impulse characteristics may contribute, at least in part, to our understanding of the mechanisms of action of these drugs in neuropathic pain therapy.

Point-Counterpoint: Heat-induced membrane depolarization of hypothalamic neurons: a putative/an unlikely mechanism of central thermosensitivity

heating and cooling various sites of the preoptic area and anterior hypothalamus (PO/AH) of mammals with fine thermodes reflexively evoke thermoregulatory responses, indicating that many central thermostats regulate temperature (T) of the brain ([16][1]). In PO/AH, heating- and cooling-activated

Effects of Sodium Pump Activity on Spontaneous Firing in Neurons of the Rat Suprachiasmatic Nucleus

Cell-attached and whole cell recording techniques were used to study the effects of electrogenic sodium pump on the excitability of rat suprachiasmatic nucleus (SCN) neurons. Blocking the sodium pump with the cardiac steroid strophanthidin or zero K+ increased the spontaneous firing of SCN neurons to different degrees with different recording modes, whereas turning the sodium pump into a nonselective cation channel with the marine toxin palytoxin invariably increased the spontaneous firing to the point of total blockade. Current-clamp recordings indicated that strophanthidin increased the rate of membrane depolarization and reduced the peak afterhyperpolarization potential (AHP), whereas zero K+ also increased the rate of depolarization, but enhanced the peak AHP. The dual effect of zero K+ was reflected by the biphasic time course of voltage responses to zero K+: an inhibitory phase with enhanced peak AHP and slower firing, followed by a delayed excitatory phase with faster rate of membrane depolarization and faster firing. In the presence of strophanthidin to block the sodium pump, zero K+ consistently decreased firing by enhancing the peak AHP. Repetitive applications of K+ -free solution gradually turned the biphasic inhibitory-followed-by-excitatory voltage response into a monophasic inhibitory response in cells recorded with the whole cell (but not the cell-attached) mode, suggesting rundown of sodium pump activity. Taken together, the results suggest that spontaneous firing of SCN neurons is regulated by sodium pump activity as well as the AHP, and that sodium pump activity is modulated by intracellular soluble substances subject to rundown under the whole cell conditions.

Role of protein kinase C in modulation of excitability of CA1 pyramidal neurons in the rat

Biochemical and in situ hybridization studies demonstrated that the levels of protein kinase C variants were significantly increased in the hippocampus of the experimental models of epilepsy in rats. In addition it has been demonstrated that protein kinase C plays an important role in modulating synaptic transmission in the hippocampus. We examined the effects of activating of protein kinase C on the excitability of CA1 pyramidal neurons and synaptic transmission, using whole-cell current-clamp and extracellular field potential recording techniques. Indolactam V (1 μM) a novel protein kinase C activator, increased the excitability of CA1 neurons acting at both pre- and post-synaptic sites. Indolactam V, acting postsynaptically, significantly reduced the threshold for initiation of action potential from −42±3.8 mV to −51±3.1 mV and selectively inhibited the slow afterhyperpolarizing potential. Indolactam V also altered the neuronal firing properties in response to prolonged depolarizing pulse by eliminating the spike frequency accommodation. Our data indicate that indolactam V potentiated both amplitudes of Shaffer-collateral stimulation evoked excitatory postsynaptic currents and disynaptically evoked inhibitory evoked postsynaptic currents. However, the potentiation of inhibitory evoked postsynaptic currents amplitudes was not observed after blockade of NMDA and AMPA/kainate currents suggesting it was due to excitatory activity driving inhibitory neurons. The results indicate that the potentiation of pharmacologically isolated excitatory postsynaptic currents (215% of control) and amplitudes of population spikes (290% of control) was due to action of indolactam V presynaptically since the agonist reduced the paired-pulse ratio and the potentiating effect was not blocked by dialyzing the postsynaptic neuron through the recording electrode with a specific protein kinase C inactivator calphostin C. These findings suggest that protein kinase C increases the amplitude of epileptiform activity by causing potentiation of excitatory synaptic transmission, increasing the excitability of postsynaptic neurons and reducing negative feed back provided by slow afterhyperpolarizing potential.

Effects of the fruit essential oil of Cuminum cyminum Linn. (Apiaceae) on pentylenetetrazol-induced epileptiform activity in F1 neurones of Helix aspersa

The effect of the fruit essential oil of Cuminum cyminum Linn. (Apiaceae) (syn. Cuminum odorum Salisb) on the epileptiform activity induced by pentylenetetrazol (PTZ) was evaluated, using intracellular technique. The results demonstrated that extracellular application of the essential oil of Cuminum cyminum (1% and 3%) dramatically decreased the frequency of spontaneous activity induced by PTZ in a time and concentration dependent manner. In addition it showed protection against pentylenetetrazol-induced epileptic activity by increasing the duration, decreasing the amplitude of after hyperpolarization potential (AHP) following the action potential, the peak of action potential, and inhibition of the firing rate. These membrane effects suggest cellular mechanisms by which the essential oil of Cuminum cyminum can inhibit the PTZ-induced epileptic activity.

Exercise Training Differentially Affects Intrinsic Excitability of Autonomic and Neuroendocrine Neurons in the Hypothalamic Paraventricular Nucleus

Oxytocinergic and vasopressinergic brain stem projections have been shown to play an important role in mediating cardiovascular adjustments during exercise training (ET). The aim of the present work was to determine whether the intrinsic excitability of hypothalamic neurons giving rise to brain stem peptidergic projections is altered as a consequence of ET. Whole cell patch-clamp recordings were obtained from nucleus of the solitarii tract (NTS)-projecting paraventricular nucleus of the hypothalamus (PVN) neurons and from supraoptic nucleus (SON) and PVN magnocellular cells (MNCs), in hypothalamic slices obtained from sedentary (S) and ET rats. Our results indicate that intrinsic excitability of PVN neurons that innervate the NTS (PVN-NTS) is enhanced by ET, resulting in a more efficient input-output function (increase number of evoked actions potentials, steeper frequency/current relationships and slower decaying frequency/time relationships). Changes in input-output function were accompanied by smaller hyperpolarizing afterpotentials (HAPs) and afterhyperpolarizing potentials (AHPs), during and after trains of spikes, respectively. On the other hand, a decreased efficacy in the input-output function was observed in SON/PVN MNCs during ET. Altogether, our results indicate that ET differentially affects the intrinsic excitability of autonomic and neurosecretory SON and PVN neurons. Increased excitability in PVN-NTS neurons may contribute to enhanced release of OT and VP peptides in the dorsal brain stem, and cardiovascular fine-tuning during exercise training.

Investigation of PKC isoform‐specific translocation and targeting of the current of the late afterhyperpolarizing potential of myenteric AH neurons

AH neurons in the enteric nervous system play an essential role in initiating intestinal reflexes and factors that control AH neuron excitability therefore influence the state of the digestive system. Prominent afterhyperpolarizations that follow action potentials in these neurons strongly affect their excitability. In the present work, we have investigated the regulation of the afterhyperpolarizing current (I(AHP)) by protein kinase C (PKC). Electrophysiological responses and protein translocation were investigated in AH neurons of freshly dissected preparations of myenteric ganglia from the guinea-pig ileum. The activator of conventional and novel PKCs, phorbol dibutyrate, but not the activator of novel PKCs, ingenol, blocked the I(AHP). Phorbol dibutyrate had no effect on the hyperpolarization-activated current (I(h)) or on the A current (I(A)). Stimulation of synaptic inputs to the neurons also reduced the I(AHP), and had no effect on I(h) or I(A). Phorbol dibutyrate also reduced a background outward current that was present after the I(AHP) current had been blocked by clotrimazole. Both phorbol dibutyrate and ingenol caused translocation of the novel PKC, PKCepsilon, in these neurons. Only phorbol dibutyrate caused translocation of PKCgamma, a conventional PKC. The studies thus indicate that the activation of PKC by phorbol esters and by nerve stimulation affects AH neurons in a similar way, and that PKC activation targets both the I(AHP) and another background K(+) current. The I(AHP) is targeted by a conventional PKC, suggested to be PKCgamma, as this is the only conventional PKC that is prominent in AH neurons.

Cholinergic Control of Firing Pattern and Neurotransmission in Rat Neostriatal Projection Neurons: Role of CaV2.1 and CaV2.2 Ca2+Channels

Besides a reduction of L-type Ca2+-currents (Ca(V)1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through Ca(V)2.1 (P/Q) and Ca(V)2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.