Larger Afterhyperpolarization Research Articles

Pharmacologic antagonism of CB1 receptors improves electrophysiological alterations in Purkinje cells exposed to 3-AP

IntroductionAlthough ataxia is associated with cerebellar dysfunction, little is known about the effects of 3-AP exposure on Purkinje cell electrophysiological properties. Here, we evaluated these parameters in cerebellar vermis brain slices.MethodsPurkinje cells were exposed to artificial cerebrospinal fluid (aCSF) (control) or to 1 mM 3-acetylpyridine (3-AP) in the recording chamber. The effects of a cannabinoid agonist (WIN; 7.5 nmol) and a cannabinoid antagonist (AM; 20 nmol) were evaluated under both conditions.ResultsExposure to 3-AP induced dramatic changes in cellular excitability that likely would affect Purkinje cell output. In whole-cell current clamp recordings, 3-AP-exposed Purkinje cells demonstrated a significantly higher frequency of action potentials, a larger afterhyperpolarization (AHP), and a larger rebound of action potentials. In addition, 3-AP caused a significant decrease in the interspike interval (ISI), half-width, and first spike latency. Remarkably, the action potential frequency, AHP amplitude, rebound, ISI, action potential halfwidth, and first spike latency were no longer different from controls in 3-AP cells treated with AM. Sag percentage, on the other hand, showed no significant difference under any treatment condition, indicating that cannabinoids' actions on 3-AP-mediated Purkinje cell changes may not include effects on neuronal excitability through changes of Ih.ConclusionsThese data show that cannabinoid antagonists reduce the excitability of Purkinje cells following exposure to 3-AP and suggest their potential as therapeutics in cerebellar dysfunctions.

Serotonin-specific neurons differentiated from human iPSCs form distinct subtypes with synaptic protein assembly

Human induced pluripotent stem cells (hiPSCs) have revolutionized the generation of experimental disease models, but the development of protocols for the differentiation of functionally active neuronal subtypes with defined specification is still in its infancy. While dysfunction of the brain serotonin (5-HT) system has been implicated in the etiology of various neuropsychiatric disorders, investigation of functional human 5-HT specific neurons in vitro has been restricted by technical limitations. We describe an efficient generation of functionally active neurons from hiPSCs displaying 5-HT specification by modification of a previously reported protocol. Furthermore, 5-HT specific neurons were characterized using high-end fluorescence imaging including super-resolution microscopy in combination with electrophysiological techniques. Differentiated hiPSCs synthesize 5-HT, express specific markers, such as tryptophan hydroxylase 2 and 5-HT transporter, and exhibit an electrophysiological signature characteristic of serotonergic neurons, with spontaneous rhythmic activities, broad action potentials and large afterhyperpolarization potentials. 5-HT specific neurons form synapses reflected by the expression of pre- and postsynaptic proteins, such as Bassoon and Homer. The distribution pattern of Bassoon, a marker of the active zone along the soma and extensions of neurons, indicates functionality via volume transmission. Among the high percentage of 5-HT specific neurons (~ 42%), a subpopulation of CDH13 + cells presumably designates dorsal raphe neurons. hiPSC-derived 5-HT specific neuronal cell cultures reflect the heterogeneous nature of dorsal and median raphe nuclei and may facilitate examining the association of serotonergic neuron subpopulations with neuropsychiatric disorders.

Spike Frequency Adaptation in Neurons of the Central Nervous System.

Neuronal firing patterns and frequencies determine the nature of encoded information of the neurons. Here we discuss the molecular identity and cellular mechanisms of spike-frequency adaptation in central nervous system (CNS) neurons. Calcium-activated potassium (KCa) channels such as BKCa and SKCa channels have long been known to be important mediators of spike adaptation via generation of a large afterhyperpolarization when neurons are hyper-activated. However, it has been shown that a strong hyperpolarization via these KCa channels would cease action potential generation rather than reducing the frequency of spike generation. In some types of neurons, the strong hyperpolarization is followed by oscillatory activity in these neurons. Recently, spike-frequency adaptation in thalamocortical (TC) and CA1 hippocampal neurons is shown to be mediated by the Ca2+-activated Cl- channel (CACC), anoctamin-2 (ANO2). Knockdown of ANO2 in these neurons results in significantly reduced spike-frequency adaptation accompanied by increased number of spikes without shifting the firing mode, which suggests that ANO2 mediates a genuine form of spike adaptation, finely tuning the frequency of spikes in these neurons. Based on the finding of a broad expression of this new class of CACC in the brain, it can be proposed that the ANO2-mediated spike-frequency adaptation may be a general mechanism to control information transmission in the CNS neurons.

Electrophysiological and morphological properties of rat supratrigeminal premotor neurons targeting the trigeminal motor nucleus

The electrophysiological and morphological characteristics of premotor neurons in the supratrigeminal region (SupV) targeting the trigeminal motor nucleus (MoV) were examined in neonatal rat brain stem slice preparations with Ca(2+) imaging, whole cell recordings, and intracellular biocytin labeling. First, we screened SupV neurons that showed a rapid rise in intracellular free Ca(2+) concentration ([Ca(2+)]i) after single-pulse electrical stimulation of the ipsilateral MoV. Subsequent whole cell recordings were generated from the screened SupV neurons, and their antidromic responses to MoV stimulation were confirmed. We divided the antidromically activated premotor neurons into two groups according to their discharge patterns during the steady state in response to 1-s depolarizing current pulses: those firing at a frequency higher (HF neurons, n = 19) or lower (LF neurons, n = 17) than 33 Hz. In addition, HF neurons had a narrower action potential and a larger afterhyperpolarization than LF neurons. Intracellular labeling revealed that the axons of all HF neurons (6/6) and half of the LF neurons (4/9) entered the MoV from its dorsomedial aspect, whereas the axons of the remaining LF neurons (5/9) entered the MoV from its dorsolateral aspect. Furthermore, the dendrites of three HF neurons penetrated into the principal sensory trigeminal nucleus (Vp), whereas the dendrites of all LF neurons were confined within the SupV. These results suggest that the types of SupV premotor neurons targeting the MoV with different firing properties have different dendritic and axonal morphologies, and these SupV neuron classes may play unique roles in diverse oral motor behaviors, such as suckling and mastication.

Excitability and Synaptic Alterations in the Cerebellum of APP/PS1 Mice

In Alzheimer's disease (AD), the severity of cognitive symptoms is better correlated with the levels of soluble amyloid-beta (Aβ) rather than with the deposition of fibrillar Aβ in amyloid plaques. In APP/PS1 mice, a murine model of AD, at 8 months of age the cerebellum is devoid of fibrillar Aβ, but dosage of soluble Aβ1–42, the form which is more prone to aggregation, showed higher levels in this structure than in the forebrain. Aim of this study was to investigate the alterations of intrinsic membrane properties and of synaptic inputs in Purkinje cells (PCs) of the cerebellum, where only soluble Aβ is present. PCs were recorded by whole-cell patch-clamp in cerebellar slices from wild-type and APP/PS1 mice. In APP/PS1 PCs, evoked action potential discharge showed enhanced frequency adaptation and larger afterhyperpolarizations, indicating a reduction of the intrinsic membrane excitability. In the miniature GABAergic postsynaptic currents, the largest events were absent in APP/PS1 mice and the interspike intervals distribution was shifted to the left, but the mean amplitude and frequency were normal. The ryanodine-sensitive multivescicular release was not altered and the postsynaptic responsiveness to a GABAA agonist was intact. Climbing fiber postsynaptic currents were normal but their short-term plasticity was reduced in a time window of 100–800 ms. Parallel fiber postsynaptic currents and their short-term plasticity were normal. These results indicate that, in the cerebellar cortex, chronically elevated levels of soluble Aβ1–42 are associated with alterations of the intrinsic excitability of PCs and with alterations of the release of GABA from interneurons and of glutamate from climbing fibers, while the release of glutamate from parallel fibers and all postsynaptic mechanisms are preserved. Thus, soluble Aβ1–42 causes, in PCs, multiple functional alterations, including an impairment of intrinsic membrane properties and synapse-specific deficits, with differential consequences even in different subtypes of glutamatergic synapses.

Embryonic MGE Precursor Cells Grafted into Adult Rat Striatum Integrate and Ameliorate Motor Symptoms in 6-OHDA-Lesioned Rats

We investigated a strategy to ameliorate the motor symptoms of rats that received 6-hydroxydopamine (6-OHDA) lesions, a rodent model of Parkinson's disease, through transplantation of embryonic medial ganglionic eminence (MGE) cells into the striatum. During brain development, embryonic MGE cells migrate into the striatum and neocortex where they mature into GABAergic interneurons and play a key role in establishing the balance between excitation and inhibition. Unlike most other embryonic neurons, MGE cells retain the capacity for migration and integration when transplanted into the postnatal and adult brain. We performed MGE cell transplantation into the basal ganglia of control and 6-OHDA-lesioned rats. Transplanted MGE cells survived, differentiated into GABA(+) neurons, integrated into host circuitry, and modified motor behavior in both lesioned and control rats. Our data suggest that MGE cell transplantation into the striatum is a promising approach to investigate the potential benefits of remodeling basal ganglia circuitry in neurodegenerative diseases.

Galanin Alters GABAergic Neurotransmission in the Dorsal Raphe Nucleus

The neuropeptide galanin and its three receptor subtypes (Gal R1-3) are highly expressed in the dorsal raphe nucleus (DRN), a region of the brain that contains a large population of serotonergic neurons. Galanin is co-expressed with serotonin in approximately 40% of the DRN neurons, and galanin and GALR2 expression are elevated by antidepressants like the SSRI fluoxetine, suggesting an interaction between serotonin and galanin. The present study examines the effect of galanin (Gal 1-29), a pan ligand for GalR (1-3) and the GalR2/GalR3-selective ligand, Gal 2-11, on the electrophysiological properties of DRN serotonergic neurons in a slice preparation. We recorded from cells in the DRN with electrophysiological characteristics consistent with those of serotonergic neurons that exhibit high input resistance, large after-hyperpolarizations and long spike duration as defined by Aghajanian and Vandermaelen. Both Gal 1-29 and Gal 2-11 decreased the amplitudes pharmacologically-isolated GABAergic inhibitory postsynaptic potentials (IPSPs) in these putative serotonergic neurons. Furthermore, based on paired pulse facilitation studies, we show that Gal 1-29 likely decreases GABA release through a presynaptic mechanism, whereas Gal 2-11 may act postsynaptically. These findings may enhance understanding of the cellular mechanisms underlying the effects of antidepressant treatments on galanin and galanin receptors in DRN.

Electrophysiologic Changes in Dorsal Root Ganglion Neurons and Behavioral Changes in a Lumbar Radiculopathy Model

The DRG neuron was electrophysiologically investigated using a rat model with constriction of the proximal site of the DRG. To investigate the pathomechanisms of lumbar radiculopathy, we established a rat model with constriction of the proximal site of the DRG. And to characterize the DRG neurons in the rat model of lumbar radiculopathy, the physiologic properties regarding action potential, Na, and K current of the DRG neurons were analyzed through the use of patch clamp recordings. In lumbar root constriction models, properties of secondary afferent neurons in the dorsal horn have been investigated. However, the electrical properties of DRG neuron have not been well investigated. To compare the excitability of DRG neurons between root constriction models and sham, we examined the threshold current, action potential (AP) threshold, resting membrane potential (RMP), afterhyperpolarization (AHP), action potential duration 50 (APD50), action potential amplitude, maximum rise time of AP, and pattern of discharges evoked by depolarizing current. We also examined the peak Na current and steady-state Na and K currents with the voltage clamp technique. The rats in the root constriction group demonstrated mechanical allodynia and thermal hyperalgesia. In measurement of the action potential, lower threshold current, more depolarized RMP, larger AHP, and prolonged APD50 were measured in the root constriction neurons compared with the sham group. The incidence of sustained burst was significantly higher in root constriction neurons. The Na current in root constriction neurons was markedly larger. There were no significant differences in K current density and voltage dependency. The constriction of lumbar root increased excitability and Na current amplitude of DRG neurons. These findings indicate that lumbar radicular pain may be associated with increased excitability of involved DRG neurons.

Acetyl- l-carnitine induces a sustained potentiation of the afterhyperpolarization

Acetyl- l-carnitine is known to improve many aspects of the neural activity even if its exact role in neurotransmission is still unknown. This study investigates the effects of acetyl- l-carnitine in T segmental sensory neurons of the leech Hirudo medicinalis. These neurons are involved in some forms of neural plasticity associated with learning processes. Their physiological firing is accompanied by a large afterhyperpolarization that is mainly due to the Na +/K + ATPase activity and partially to a Ca 2+-dependent K + current. A clear-cut hyperpolarization and a significant increase of the afterhyperpolarization have been recorded in T neurons of leeches injected with 2 mM acetyl- l-carnitine some days before. Acute treatments of 50 μM acetyl- l-carnitine induced similar effects in T cells of naive animals. In the presence of apamin, a pharmacological blocker of Ca 2+-dependent K + channel, acetyl- l-carnitine still enhanced the residual afterhyperpolarization, suggesting an effect of the drug on the Na +/K +ATPase. Acetyl- l-carnitine also increased the hyperpolarization induced by intracellular injection of Na + ions. Therefore, acetyl- l-carnitine seems to be able to exert a positive sustained effect on the Na +/K + ATPase activity in leech T sensory neurons. Moreover, in these cells, widely arborized, the afterhyperpolarization seems to play an important role in determining the action potential transmission at neuritic bifurcations. A computational model of a T cell has been previously developed considering detailed data for geometry and the modulation of the pump current. Herein, we showed that to a larger afterhyperpolarization, due to the acetyl- l-carnitine-induced effects, corresponds a decrement in the number of action potentials reaching synaptic terminals.

Growth hormone-releasing hormone (GHRH) neurons in GHRH-enhanced green fluorescent protein transgenic mice: a ventral hypothalamic network.

The hypothalamic GHRH neurons secrete pulses of GHRH to generate episodic GH secretion, but little is known about the mechanisms involved. We have made transgenic mice expressing enhanced green fluorescent protein (eGFP) specifically targeted to the secretory vesicles in GHRH neurons. GHRH cells transported eGFP from cell bodies in the arcuate nucleus to extensively arborized varicose fiber terminals in the median eminence. Patch clamp recordings from visually identified GHRH cells in mature animals showed spontaneous action potentials, often firing in short bursts up to 10 Hz. GHRH neurons received frequent synaptic inputs, as demonstrated by the recording of abundant inward postsynaptic currents, but spikes were followed by large after-hyperpolarizations, which limited their firing rate. Because many GHRH neurons lie close to the ventral hypothalamic surface, this was examined by wide-field binocular epifluorescence stereomicroscopy. This approach revealed an extensive horizontal network of GHRH cells at low power and individual fiber projections at higher power in the intact brain. It also showed the dense terminal projections of the GHRH cell population in the intact median eminence. This model will enable us to characterize the properties of individual GHRH neurons and their structural and functional connections with other neurons and to study directly the role of the GHRH neuronal network in generating episodic secretion of GH.

Distinguishing characteristics of serotonin and non-serotonin-containing cells in the dorsal raphe nucleus: electrophysiological and immunohistochemical studies

The membrane properties and receptor-mediated responses of rat dorsal raphe nucleus neurons were measured using intracellular recording techniques in a slice preparation. After each experiment, the recorded neuron was filled with neurobiotin and immunohistochemically identified as 5-hydroxytryptamine (5-HT)-immunopositive or 5-HT-immunonegative. The cellular characteristics of all recorded neurons conformed to previously determined classic properties of serotonergic dorsal raphe nucleus neurons: slow, rhythmic activity in spontaneously active cells, broad action potential and large afterhyperpolarization potential. Two electrophysiological characteristics were identified that distinguished 5-HT from non-5-HT-containing cells in this study. In 5-HT-immunopositive cells, the initial phase of the afterhyperpolarization potential was gradual (tau=7.3±1.9) and in 5-HT-immunonegative cells it was abrupt (tau=1.8±0.6). In addition, 5-HT-immunopositive cells had a shorter membrane time constant (tau=21.4±4.4) than 5-HT-immunonegative cells (tau=33.5±4.2). Interestingly, almost all recorded neurons were hyperpolarized in response to stimulation of the inhibitory 5-HT 1A receptor. These results suggested that 5-HT 1A receptors are present on non-5-HT as well as 5-HT neurons. This was confirmed by immunohistochemistry showing that although the majority of 5-HT-immunopositive cells in the dorsal raphe nucleus were double-labeled for 5-HT 1A receptor-IR, a small but significant population of 5-HT-immunonegative cells expressed the 5-HT 1A receptor. These results underscore the heterogeneous nature of the dorsal raphe nucleus and highlight two membrane properties that may better distinguish 5-HT from non-5-HT cells than those typically reported in the literature. In addition, these results present electrophysiological and anatomical evidence for the presence of 5-HT 1A receptors on non-5-HT neurons in the dorsal raphe nucleus.

D-tubocurarine reduces GABA responses in rat substantia nigra dopamine neurons.

SynapseVolume 47, Issue 3 p. 236-239 D-Tubocurarine reduces GABA responses in rat substantia nigra dopamine neurons Luigi Caputi, Luigi Caputi IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, Italy Clinica Neurologica Università di Roma – Tor Vergata, Rome, ItalySearch for more papers by this authorC. Peter Bengtson, C. Peter Bengtson IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, ItalySearch for more papers by this authorEzia Guatteo, Ezia Guatteo IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, ItalySearch for more papers by this authorGiorgio Bernardi, Giorgio Bernardi IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, Italy Clinica Neurologica Università di Roma – Tor Vergata, Rome, ItalySearch for more papers by this authorNicola B. Mercuri, Corresponding Author Nicola B. Mercuri [email protected] IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, Italy Clinica Neurologica Università di Roma – Tor Vergata, Rome, ItalyFondazione IRCCS S. Lucia, Experimental Neurology Lab, Via Ardeatina 306, 00179, Rome, ItalySearch for more papers by this author Luigi Caputi, Luigi Caputi IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, Italy Clinica Neurologica Università di Roma – Tor Vergata, Rome, ItalySearch for more papers by this authorC. Peter Bengtson, C. Peter Bengtson IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, ItalySearch for more papers by this authorEzia Guatteo, Ezia Guatteo IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, ItalySearch for more papers by this authorGiorgio Bernardi, Giorgio Bernardi IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, Italy Clinica Neurologica Università di Roma – Tor Vergata, Rome, ItalySearch for more papers by this authorNicola B. Mercuri, Corresponding Author Nicola B. Mercuri [email protected] IRCCS – Fondazione Santa Lucia, Via Ardeatina 306, 00179, Rome, Italy Clinica Neurologica Università di Roma – Tor Vergata, Rome, ItalyFondazione IRCCS S. Lucia, Experimental Neurology Lab, Via Ardeatina 306, 00179, Rome, ItalySearch for more papers by this author First published: 09 December 2002 https://doi.org/10.1002/syn.10164Citations: 5AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume47, Issue31 March 2003Pages 236-239 RelatedInformation

Age-related enhancement of the slow outward calcium-activated potassium current in hippocampal CA1 pyramidal neurons in vitro.

Aging is associated with learning deficits and a decrease in neuronal excitability, reflected by an enhanced post-burst afterhyperpolarization (AHP), in CA1 hippocampal pyramidal neurons. To identify the current(s) underlying the AHP altered in aging neurons, whole-cell voltage-clamp recording experiments were performed in hippocampal slices from young and aging rabbits. Similar to previous reports, aging neurons were found to rest at more hyperpolarized potentials and have larger AHPs than young neurons. Given that compounds that reduce the slow outward calcium-activated potassium current (sI(AHP)), a major constituent of the AHP, also facilitate learning in aging animals, the sI(AHP) was pharmacologically isolated and characterized. Aging neurons were found to have an enhanced sI(AHP,) the amplitude of which was significantly correlated to the amplitude of the AHP (r = 0.63; p < 0.001). Thus, an enhanced sI(AHP) contributes to the enhanced AHP in aging. No differences were found in the membrane resistance, capacitance, or kinetic and voltage-dependent properties of the sI(AHP). Because enhanced AHP in aging neurons has been hypothesized to be secondary to an enhanced Ca2+ influx via the voltage-gated L-type Ca2+ channels, we further examined the sI(AHP) in the presence of an L-type Ca2+ channel blocker, nimodipine (10 microm). Nimodipine caused quantitatively greater reductions in the sI(AHP) in aging neurons than in young neurons; however, the residual sI(AHP) was still significantly larger in aging neurons than in young neurons. Our data, in conjunction with previous studies showing a correlation between the AHP and learning, suggest that the enhancement of the sI(AHP) in aging is a mechanism that contributes to age-related learning deficits.

Age-Related Enhancement of the Slow Outward Calcium-Activated Potassium Current in Hippocampal CA1 Pyramidal NeuronsIn Vitro

Aging is associated with learning deficits and a decrease in neuronal excitability, reflected by an enhanced post-burst afterhyperpolarization (AHP), in CA1 hippocampal pyramidal neurons. To identify the current(s) underlying the AHP altered in aging neurons, whole-cell voltage-clamp recording experiments were performed in hippocampal slices from young and aging rabbits. Similar to previous reports, aging neurons were found to rest at more hyperpolarized potentials and have larger AHPs than young neurons. Given that compounds that reduce the slow outward calcium-activated potassium current (s<i>I</i>_AHP), a major constituent of the AHP, also facilitate learning in aging animals, the s<i>I</i>_AHP was pharmacologically isolated and characterized. Aging neurons were found to have an enhanced s<i>I</i>_AHP, the amplitude of which was significantly correlated to the amplitude of the AHP (<i>r</i> = 0.63; <i>p</i> &lt; 0.001). Thus, an enhanced s<i>I</i>_AHP contributes to the enhanced AHP in aging. No differences were found in the membrane resistance, capacitance, or kinetic and voltage-dependent properties of the s<i>I</i>_AHP. Because enhanced AHP in aging neurons has been hypothesized to be secondary to an enhanced Ca²⁺ influx via the voltage-gated L-type Ca²⁺ channels, we further examined the s<i>I</i>_AHP in the presence of an L-type Ca²⁺ channel blocker, nimodipine (10 μm). Nimodipine caused quantitatively greater reductions in the s<i>I</i>_AHP in aging neurons than in young neurons; however, the residual s<i>I</i>_AHP was still significantly larger in aging neurons than in young neurons. Our data, in conjunction with previous studies showing a correlation between the AHP and learning, suggest that the enhancement of the s<i>I</i>_AHP in aging is a mechanism that contributes to age-related learning deficits.

Development of membrane conductance improves coincidence detection in the nucleus laminaris of the chicken.

Coincidence detection at the nucleus laminaris (NL) of a chicken was improved between embryos (embryonic days (E) 16 and 17) and chicks (post-hatch days (P) 2-7) in slice preparations. Electrical stimuli were applied bilaterally to the projection fibres to the NL at various intervals. The response window corresponding to the temporal separation of electrical stimuli that resulted in half-maximal firing probability was adopted as the measure of coincidence detection, and was narrower in chicks (1.4 ms) than in embryos (3.9 ms). Between these two ages, the membrane time constant of NL neurons was reduced from 18.4 to 3.2 ms and the membrane conductance was increased 5-fold, while no difference was measured in the input capacitance. Evoked EPSCs decayed slightly faster in chicks, while the size and the time course of miniature EPSCs were unchanged. Action potentials had lower thresholds and larger after-hyperpolarization in chicks than in embryos. Dendrotoxin-I depolarized cells and increased their input resistance significantly at both ages, eliminated the after-hyperpolarization, and delayed the decay phase of action potentials, indicative of the expression of low-threshold K(+) channels. Cs(+) hyperpolarized the cells, increased the input resistance and eliminated sags during hyperpolarization at both ages, while the hyperpolarization sag was affected by neither Ba(2+) nor TEA. These data indicate the expression of hyperpolarization-activated cation channels. Between these two ages, the maximum conductance of low-threshold K(+) channels increased 4-fold to about 16 nS, and hyperpolarization-activated channels increased 6-fold to about 10 nS. Improvement of coincidence detection correlated with the acceleration of the EPSP time course as a result of the increase of these conductances.

The characteristics of slowly adapting mechanical sensory neurons in the trigeminal ganglia of crotaline snakes

Mechanical pressure (P) and touch (M) neurons, two types of slowly adapting neurons, can be distinguished in the trigeminal ganglion of crotaline snakes, Trimeresurus flavoviridis, by their electrophysiological membrane properties. Intracellular stimulation and the recording of the responses from P and M neurons in vivo was performed with microelectrodes. The characteristics of the receptive field (RF) of both types of neurons were examined. P neurons had a much larger RF size and a higher mechanical threshold than those of M neurons. Both of them responded with discharges to mechanical stimulation. The active and passive electrophysiological membrane properties were measured from nine P and 18 M neurons. The active membrane properties of P neurons showed a larger amplitude and longer duration of action potential, a larger after-hyperpolarization with a longer duration to half-decay, and a higher electric threshold in response to intracellularly injected depolarizing current than M neurons. The passive membrane properties of P neurons showed a higher input resistance, much longer time constant, and larger capacitance than M neurons. The rebound spike which responded to injecting sufficient hyperpolarizing current was sometimes observed, and had a longer latency in P neurons than M neurons. These results indicated that some electrophysiological membrane properties of primary sensory neurons are dependent on their sensory modalities.

Electrophysiological study of nucleus gigantocellularis neurons in guinea-pig brainstem slices.

Gigantocellular neurons of the medullary nucleus gigantocellularis represent a major source of reticulospinal pathways. Among other roles, they have been involved in the processing of vestibular information. The aim of the present study was to describe the major intrinsic membrane properties of these cells in guinea-pig brainstem slices. We found nucleus gigantocellularis neurons to be segregated in two cell types. Type A nucleus gigantocellularis neurons were characterized by the presence of a single large afterhyperpolarization and a potent transient 4-aminopyridine-sensitive rectification likely due to the presence of a transient outward potassium current. In contrast, type B nucleus gigantocellularis neurons had a narrower and faster rising action potential followed by an early fast and a delayed slower after-hyperpolarization. In contrast to type A neurons, type B neurons were, in addition, endowed with subthreshold tetrodotoxin-sensitive sodium-dependent plateau potentials. Whereas both cell types were endowed with high-threshold calcium-dependent action potentials, only type B nucleus gigantocellularis neurons also displayed long-lasting calcium-dependent plateau potentials. These results show that nucleus gigantocellularis neurons can be segregated by their intrinsic membrane properties it two cell types which are very similar to those that we have previously described in the medial vestibular nucleus. The possibility that these differences between type A and B neurons might play a role in the segregation between tonic and kinetic cells is discussed.

Electrophysiological diversity of pyramidal-shaped neurons at the granule cell layer/hilus border of the rat dentate gyrus recorded in vitro.

In the rat dentate gyrus, pyramidal-shaped cells located on the border of the granule cell layer and the hilus are one of the most common types of gamma-aminobutyric acid (GABA)-immunoreactive neurons. This study describes their electrophysiological characteristics. Membrane properties, patterns of discharge, and synaptic responses were recorded intracellularly from these cells in hippocampal slices. Each cell was identified as pyramidal-shaped by injecting the marker Neurobiotin intracellularly (n = 17). In several respects the membrane properties of the sampled cells were similar to "fast-spiking" cells (putative inhibitory interneurons) that have been described in other areas of the hippocampus. For example, input resistance was high (mean 91.3 megohms), the membrane time constant was short (mean 7.7 ms), and there was a large afterhyperpolarization following a single action potential (mean 10.5 mV at resting potential). However, the action potentials of most pyramidal-shaped cells were not as brief (mean 1.2 ms total duration) as those of most previously described fast-spiking cells. Many pyramidal-shaped neurons had strong spike frequency adaptation relative to other fast-spiking cells. Although these latter two characteristics were apparent in the majority of the sampled cells, there were exceptional pyramidal-shaped neurons with fast action potentials and weak adaptation, demonstrating the electrophysiological variability of pyramidal-shaped cells. Responses to outer molecular layer stimulation were composed primarily of excitatory postsynaptic potentials (EPSPs) rather than inhibitory postsynaptic potentials (IPSPs), and were usually small (EPSPs evoked at threshold were often less than 2 mV), and brief (less than 30 ms). There was variability, because in a few cells EPSPs evoked at threshold were much larger. However, regardless of EPSP amplitude, suprathreshold stimulation (up to 4 times the threshold stimulus strength) rarely evoked more than one action potential in any cell. The results suggest that stimulation of perforant path axons produces limited excitatory synaptic responses in pyramidal-shaped neurons. This may be one of the reasons why they are relatively resistant to prolonged perforant path stimulation. The pyramidal-shaped neurons located at the base of the granule cell layer have been associated historically with a basket plexus around granule cell somata, and have been called pyramidal "basket" cells. However, basket-like endings were rare and axon collaterals outside the granule cell layer as the outer molecular layer and the central hilus, and antidromic action potentials could be recorded in some cells in response to weak stimulation of these areas. Taken together with the electrophysiological variability, the results indicate that these cells are physiologically heterogeneous.

Characterization of purinoceptors mediating depolarization of rat isolated vagus nerve

1. As part of a broader study to characterize neuronal purinoceptors, the effects of adenosine 5'-triphosphate (ATP) and a range of ATP analogues were investigated on the extracellularly recorded membrane potential of the rat isolated vagus nerve, using a 'grease-gap' technique. 2. ATP evoked depolarization of the rat vagus nerve. The concentration-effect curve to ATP was not monophasic: at the lower concentrations (1 x 10(-5)-1 x 10(-3) M) the curve was shallow (< 50% of the near maximal response to 5-hydroxytryptamine (5-HT)) whilst at higher concentrations the relationship between concentration and amplitude of depolarization was steeper (> 135% of the response to 5-HT at the highest concentration tested, 1 x 10(-2) M). On washout of the high drug concentrations large after-hyperpolarizations were often observed. 3. alpha,beta-methylene ATP (1 x 10(-6)-3 x 10(-4) M), beta,gamma-methylene ATP (1 x 10(-6)-1 x 10(-3) M), and 5'-adenylylimidodiphosphate (beta,gamma-imido ATP; 1 x 10(-6)-1 x 10(-3) M) were all more potent than ATP and produced large depolarizations of the rat vagus nerve at the highest concentrations tested (> 150% of the response to 5-HT). The overall rank order of potency was alpha,beta-methylene ATP > beta,gamma-methylene ATP = beta,gamma-imido ATP > ATP. 4. In contrast, 2-methylthio ATP (1 x 10(-6)-1 x 10(-3) M) produced relatively small depolarizations (< 100% of the response to 5-HT). As was the case with low concentrations of ATP, the concentration-effect curve to 2-methylthio ATP was very shallow. 5. Adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP), adenosine and adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-s; all 1 x 10-6 1 x 10-3M) evoked only small depolarizations of the vagus nerve, amounting to 47 +/- 2.5%, 40.8 +/- 7.8%, 33.7 +/- 3.3% and 62.4 +/- 12.7% of the response to 5-HT, respectively. Uridine 5'-triphosphate (UTP; 1 X 10-6 1 X 10-3M) was inactive.6. The P2 purinoceptor antagonist, suramin (1 x 10-5-M-1 X 10-4 M), antagonized responses to alpha-beta-methylene ATP. The nature of this antagonism was not, however, consistent with simple competitive kinetics between agonist and antagonist. Depolarizations produced by beta,gamma-methylene ATP and beta,gamma-imido ATP were also attenuated by suramin (1 x 10-4 M), but in contrast, suramin had no effect on responses to ADP, 2-methylthio ATP, ADP-beta-S or 5-HT.7. In addition to its antagonist effects, suramin (10-4 M) markedly increased the maximum amplitude of the depolarization produced by ATP.8. It is concluded that a heterogeneous receptor population mediates depolarization of the rat vagus nerve by purine nucleotides. Importantly, the large amplitude depolarizations to alpha,beta-methylene ATP,beta,gamma-methylene ATP and beta,gamma-imido ATP are mediated via receptors that share many characteristics of the classical P2, receptor. In contrast, the relatively small depolarizing effects of ADP, ADP-beta-S and 2-methylthio ATP were suramin-resistant. Although it appears that other purinoceptors are present,these data suggest that the rat vagus nerve may serve as a useful preparation for studying the pharmacology of neuronal P2x receptors.

Calcium spikes and calcium plateaux evoked by differential polarization in dendrites of turtle motoneurones in vitro.

1. The ability of dendrites in turtle motoneurones to support calcium spikes and calcium plateaux was investigated using differential polarization by applied electric fields. 2. Electric fields were generated by passing current through transverse slices of the turtle spinal cord between two plate electrodes. The linear extracellular voltage gradient generated by the field implied that the tissue was ohmic and homogeneous. 3. The transmembrane potential at the cell body of motoneurones was measured as the voltage difference between an intracellular and an extracellular microelectrode. 4. In normal medium an applied field induced synaptic activity as well as intrinsic polarization of motoneurones. Synaptic activity was suppressed by tetrodotoxin (TTX, 1 microM). 5. In the presence of TTX and tetraethylammonium (TEA, 1-5 mM), applied fields evoked multicomponent Ca2+ spikes in both the soma-hyperpolarizing and soma-depolarizing direction of the field. The different components of Ca2+ spikes were discrete and additive. High amplitude components had higher threshold and faster time course and were followed by larger after-hyperpolarizations, than low amplitude components. The frequency of field-evoked regenerative responses was relatively insensitive to somatic bias current. 6. TTX-resistant Ca(2+)-mediated plateau potentials promoted by apamin were evoked by differential polarization in both the soma-depolarizing and soma-hyperpolarizing direction. 7. It is concluded that Ca2+ channels responsible for Ca2+ spikes and Ca2+ plateaux are present in dendrites of spinal motoneurones of the turtle.