Biomedical subjects

Scott H Chandler

Publications and source records attributed to Scott H Chandler.

9 recordsLinked to original sources

PubMed2006

Serotonergic modulation of persistent sodium currents and membrane excitability via cyclic AMP-protein kinase A cascade in mesencephalic V neurons.

In rat mesencephalic trigeminal (Mes V) neurons, persistent sodium currents in conjunction with low-threshold potassium currents are critical for generation of subthreshold membrane oscillations and onset of burst behavior. Here we demonstrate that the cAMP/protein kinase A (PKA) signaling pathway modulates persistent sodium currents. In particular, we show that elevation of cAMP suppresses a low-threshold I(NaP) via a PKA intracellular pathway. Bath application of forskolin (20 microM), a stimulant for the production of cAMP, reduced the peak I(NaP). 1,9-Dideoxy-forskolin (20 microM), an inactive form of forskolin, produced minimal effects on I(NaP), and the membrane-permeable cAMP analogue 8-bromo-cAMP (500 microM) mimicked the effect of forskolin. Additionally, preapplication of H89 (2 microM), a specific PKA inhibitor, suppressed the effect of forskolin, suggesting the involvement of the cAMP/PKA intracellular signaling pathway in this modulation. 5-HT receptor stimulation (20 microM) also mimicked the modulation of I(NaP) by forskolin via the cAMP/PKA-dependent signaling pathway. Current clamp analysis demonstrated that voltage-dependent membrane resonance in response to a ZAP input current at depolarized holding potentials (approximately -50 mV) was specifically suppressed by forskolin or 5-HT. Moreover, drug application enhanced frequency adaptation in response to a 1-sec current pulse. These results indicate that modulation of persistent sodium currents by a cAMP/PKA pathway can significantly alter the membrane excitability and discharge characteristics of Mes V neurons.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed2006

Participation of sodium currents in burst generation and control of membrane excitability in mesencephalic trigeminal neurons.

Subthreshold sodium currents are important in sculpting neuronal discharge and have been implicated in production and/or maintenance of subthreshold membrane oscillations and burst generation in mesencephalic trigeminal neurons (Mes V). Moreover, recent data suggest that, in some CNS neurons, resurgent sodium currents contribute to production of high-frequency burst discharge. In the present study, we sought to determine more directly the participation of these currents during Mes V electrogenesis using the action potential-clamp method. In postnatal day 8-14 rats, the whole-cell patch-clamp method was used to record sodium currents by subtraction in response to application of TTX in voltage-clamp mode using the action potential waveform as the command protocol. We found that TTX-sensitive sodium current is the main inward current flowing during the interspike interval, compared with the h-current (Ih) and calcium currents. Furthermore, in addition to the transient sodium current that flows during the upstroke of action potential, we show that resurgent sodium current flows at the peak of afterhyperpolarization and persistent sodium current flows in the middle of the interspike interval to drive high-frequency firing. Additionally, transient, resurgent, and persistent sodium current components showed voltage- and time-dependent slow inactivation, suggesting that slow inactivation of these currents can contribute to burst termination. The data suggest an important role for these components of the sodium current in Mes V neuron electrogenesis.

Action Potentials↗

PubMed2005

Alterations in N-methyl-D-aspartate receptor sensitivity and magnesium blockade occur early in development in the R6/2 mouse model of Huntington's disease.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that affects primarily the striatum and cerebral cortex. A search for the factors that increase the vulnerability of striatal neurons will lead to a better understanding of the pathological cascades of this disease. A current hypothesis for neurodegeneration of striatal medium-sized spiny neurons in HD is an alteration in N-methyl-D-aspartate (NMDA) receptor function. In the present study we examined electrophysiological properties of NMDA receptors in the R6/2 transgenic mouse model. These animals express exon 1 of the human HD gene and present an overt behavioral phenotype at about 5 weeks of age. Whole-cell voltage clamp recordings from acutely dissociated striatal neurons were obtained from three different age groups of transgenic mice (15, 21, and 40 days old) and their littermate controls (WT). In transgenic animals, two groups of neurons were found with respect to NMDA and Mg2+ sensitivity. One group of R6/2 cells displayed responses similar to those of WT, whereas the other showed increased responses to NMDA and decreased Mg2+ sensitivity. These cells were encountered in all age groups. The abnormal sensitivity to NMDA and Mg2+ indicates that NMDA receptor alterations occur very early in development and suggest the presence of constitutively abnormal NMDA receptors. These alterations may contribute to an enhancement of NMDA responses at hyperpolarized membrane potentials that may be a key factor in striatal neuronal dysfunction.

Animals↗

PubMed2005

Voltage-dependent calcium currents in trigeminal motoneurons of early postnatal rats: modulation by 5-HT receptors.

Trigeminal motoneurons relay the final output signals generated within the oral-motor pattern generating circuit(s) to muscles for execution of various motor patterns. In recent years, these motoneurons were shown to possess voltage dependent nonlinear membrane properties that allow them to actively participate in sculpting their final output. A complete understanding of the factors controlling trigeminal motoneuronal (TMN) discharge during oral-motor activity requires, at a minimum, a detailed understanding of the palette of ion channels responsible for membrane excitability and a determination of whether these ion channels are targets for modulation. Toward that end, we studied in detail the properties of calcium channels in TMNs and their susceptibility to modulation by 5-HT in rat brain slices. We found that based on pharmacological and voltage-dependent properties, high-voltage-activated (HVA) N-type [omega-conotoxin GVIA (omega-CgTX)]-sensitive, and to a lesser extent P/Q-type [omega-agatoxin IVA (omega-Aga IVA)]-sensitive, calcium channels make up the majority of the whole cell calcium current. 5-HT (5.0 microM) decreased HVA current by 31.3 +/- 2.2%, and the majority of this suppression resulted from reduction of current flow through N- and P/Q-type calcium channels. In contrast, 5-HT had no effect on low-voltage-activated (LVA) current amplitude in TMNs. HVA calcium current inhibition was mimicked by 5-CT, a 5-HT1 receptor agonist, and by R(+)-8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT), a specific 5-HT1A agonist. The effects of 5-HT were blocked by the 5-HT1A antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine hydrobromide (NAN-190) but not by ketanserin, a 5-HT(2/1C) antagonist. Under current clamp, omega-CgTX and 5-HT were most effective in suppressing the mAHP and both increased the spike frequency and input/output gain in response to current injection. Calcium current modulation by 5-HT1A receptors likely is an important mechanism to fine tune the input/output gain of TMNs in response to small incoming synaptic inputs and accounts for some of the previously reported effects of 5-HT on TMN excitability during tonic and burst activity during oral-motor behavior.

8-Hydroxy-2-(di-n-propylamino)tetralin↗

PubMed2004

Persistent sodium currents in mesencephalic v neurons participate in burst generation and control of membrane excitability.

The functional and biophysical properties of a persistent sodium current (I(NaP)) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7-p12) using whole cell patch-clamp methods. I(NaP) activated around -76 mV and peaked at -48 mV, with V1/2 of -58.7 mV. Ramp voltage-clamp protocols showed that I(NaP) undergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (tau(onset) = 2.04 s, tau(recov) = 2.21 s). Riluzole (< or =5 microM) substantially reduced I(NaP), membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na+ current (I(NaT)). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the I(NaT) were obtained and revealed that the amplitude and voltage dependence of the resulting "window current" were not consistent with those of the observed I(NaP) recorded in the same neurons. This suggests an additional mechanism for the origin of I(NaP). A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na+ and K+ currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model g(NaP) parameters indicate that I(NaP) is critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.

Action Potentials↗

PubMed2004

Striatal potassium channel dysfunction in Huntington's disease transgenic mice.

Huntington's disease (HD) is a neurodegenerative disorder that mainly affects the projection neurons of the striatum and cerebral cortex. Genetic mouse models of HD have shown that neurons susceptible to the mutation exhibit morphological and electrophysiological dysfunctions before and during development of the behavioral phenotype. We used HD transgenic mouse models to examine inwardly and outwardly rectifying K+ conductances, as well as expression of some related K+ channel subunits. Experiments were conducted in slices and dissociated cells from two mouse models, the R6/2 and TgCAG100, at the beginning and after full development of overt behavioral phenotypes. Striatal medium-sized spiny neurons (MSNs) from symptomatic transgenic mice had increased input resistances, depolarized resting membrane potentials, and reductions in both inwardly and outwardly rectifying K+ currents. These changes were more dramatic in the R6/2 model than in the TgCAG100. Parallel immunofluorescence studies detected decreases in the expression of K+ channel subunit proteins, Kir2.1, Kir2.3, and Kv2.1 in MSNs, which contribute to the formation of the channel ionophores for these currents. Attenuation in K+ conductances and channel subunit expression contribute to altered electrophysiological properties of MSNs and may partially account for selective cellular vulnerability in the striatum.

Age Factors↗

PubMed2002

Development of inward rectification and control of membrane excitability in mesencephalic v neurons.

The present study was performed to assess the postnatal development and functional roles of inward rectifying currents in rat mesencephalic trigeminal (Mes V) neurons, which are involved in the genesis and control of oral-motor activities. Whole cell voltage-clamp recordings obtained from Mes V neurons in brain stem slices identified fast (I(KIR)) and slow (I(h)) inward rectifying currents, which were specifically blocked by BaCl(2) (300-500 microM) or 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD 7288, 10 microM), respectively. The whole cell current density for these channels increased between postnatal days 2 to 12 (P2-P12), and the time courses for I(h) activation and deactivation were each well described by two time constants. Application of ZD 7288 produced membrane hyperpolarization in the majority of cells and prolonged afterhyperpolarization repolarization. Additionally, in the presence of ZD 7288, spike frequency was decreased and adaptation was more pronounced. Interestingly, these neurons exhibited a voltage-dependent membrane resonance (<10 Hz) that was prominent around resting potential and more negative to rest and was blocked by ZD 7288. These results suggest that I(h) contributes to stabilizing resting membrane potential and controlling cell excitability. The presence of I(h) imparts the neuron with the unique property of low-frequency membrane resonance; the ability to discriminate between synaptic inputs based on frequency content.

Action Potentials↗

PubMed2002

Differential NR2A and NR2B expression between trigeminal neurons during early postnatal development.

N-methyl-D-aspartate (NMDA) receptors play an important role in the production of rhythmical trigeminal motor activity resembling suckling and chewing. The developmental relationship between the expression of NMDA receptor subunits and the function of neurons comprising brainstem oral-motor circuitry is not clear. We conducted receptor immunohistochemistry studies to demonstrate the expression of NR2A and NR2B subunits in trigeminal motoneurons (Mo5) and mesencephalic trigeminal neurons (Me5) during the first 2 weeks of development. During this time period, rats begin the transition from suckling to chewing, two distinct motor behaviors. In Mo5, NR2A and NR2B immunoreactivity was observed throughout the time frame sampled. A significant increase in the NR2A:NR2B ratio occurred between P3-4 and P11 due to a reduction in the number of NR2B immunoreactive neurons. The temporal and spatial expression of NR2A and NR2B was differentially regulated between caudal and rostral regions of Me5. In contrast to Mo5, the NR2A:NR2B ratio decreased between P0-1 and P11 in caudal Me5 due to a concurrent increase in the number of NR2A and NR2B immunoreactive neurons. In rostral Me5, NR2A and NR2B immunoreactivity emerged at P3 and P11, respectively. Our data provides further insight into the molecular changes of trigeminal neurons during the transition from suckling to chewing behaviors. The differences in the NR2A:NR2B ratio between Mo5 and Me5 suggest functional differences in these neurons during NMDA-mediated neurotransmission.

Animals↗

PubMed2002

Development and serotonergic modulation of NMDA bursting in rat trigeminal motoneurons.

The development of N-methyl-D-aspartate (NMDA)-induced burst discharge in rat trigeminal motoneurons (TMNs) between postnatal days P1 and P10 was examined using whole cell patch-clamp recording methods in brain slices. Bath application of NMDA (50 microM) induced a Mg(2+)-dependent rhythmical bursting activity starting around P8. Prior to the onset of bursting, the membrane potential depolarized and the input resistance increased. Hyperpolarization of the membrane potential with extrinsic current demonstrated a narrow window of membrane potential where maintained rhythmical burst discharge was evident. In P1-P4 neurons, NMDA application produced membrane depolarization and a minimal change in input resistance, but no burst activity at any membrane potential. Voltage-clamp analysis indicated that the bursting activity was related to the presence or absence of a voltage-dependent Mg(2+) block and induction of a negative slope conductance (NSC) region in the I(NMDA)-V relationship. Regardless of age, reduction of extracellular Mg(2+) from 1 mM to 30 microM enhanced I(NMDA) at voltages negative to -60 mV. However, in 1 mM Mg(2+), P1-P4 neurons were devoid of a prominent NSC region compared with P8-P10 neurons, suggesting that the efficacy of depolarization in unblocking the NMDA receptors increased with age. NMDA bursting was not dependent on calcium influx through voltage-gated calcium channels (VGCC) but did require a minimal concentration of Ca(2+) in the bath. Intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid application suppressed burst discharge completely, suggesting that intracellular Ca(2+) directly, or via second-messenger systems, regulates NMDA receptor activity and bursting. Interestingly, NMDA bursting could be induced in P1-P4 neurons by simultaneous bath application of serotonin (5-HT, 10 microM), which by itself did not produce bursting, suggesting an "enabling" role for 5-HT. Voltage-clamp analysis demonstrated that the NMDA/5-HT bursting resulted from induction of an NSC in the I-V relationship of total membrane current. 5-HT by itself produced no such effect. The mechanisms for this effect were due to an enhancement of the NSC region of the I(NMDA)-V relationship and reduction of a presumed leak current by 5-HT. These data indicate that NMDA bursting in trigeminal motoneurons is developmentally regulated and subject to neuromessenger modulation. Control of the Mg(2+) sensitivity of the NMDA receptor and voltage-dependent block by neuromessengers could be an effective means to control the efficacy of glutamatergic synaptic drive to motoneurons during rhythmical oral-motor activity at early postnatal ages.

Action Potentials↗