Spontaneous Firing Frequency Research Articles

GABA(A) receptor-mediated signaling alters the structure of spontaneous activity in the developing retina.

Ambient GABA modulates firing patterns in adult neural circuits by tonically activating extrasynaptic GABA(A) receptors. Here, we demonstrate that during a developmental period when activation of GABA(A) receptors causes membrane depolarization, tonic activation of GABA(A) receptors blocks all spontaneous activity recorded in retinal ganglion cells (RGCs) and starburst amacrine cells (SACs). Bath application of the GABA(A) receptor agonist muscimol blocked spontaneous correlated increases in intracellular calcium concentration and compound postsynaptic currents in RGCs associated with retinal waves. In addition, GABA(A) receptor agonists activated a tonic current in RGCs that significantly reduced their excitability. Using a transgenic mouse in which green fluorescent protein is expressed under the metabotropic glutamate receptor subtype 2 promoter to target recordings from SACs, we found that GABA(A) receptor agonists blocked compound postsynaptic currents and also activated a tonic current. GABA(A) receptor antagonists reduced the holding current in SACs but not RGCs, indicating that ambient levels of GABA tonically activate GABA(A) receptors in SACs. GABA(A) receptor antagonists did not block retinal waves but did alter the frequency and correlation structure of spontaneous RGC firing. Interestingly, the drug aminophylline, a general adenosine receptor antagonist used to block retinal waves, induced a tonic GABA(A) receptor antagonist-sensitive current in outside-out patches excised from RGCs, indicating that aminophylline exerts its action on retinal waves by direct activation of GABA(A) receptors. These findings have implications for how various neuroactive drugs and neurohormones known to modulate extrasynaptic GABA(A) receptors may influence spontaneous firing patterns that are critical for the establishment of adult neural circuits.

Effects of 17β-Estradiol on Neuronal Cell Excitability and Neurotransmission in the Suprachiasmatic Nucleus of Rat

17beta-Estradiol receptors have been found in several brain nuclei including the suprachiasmatic nucleus (SCN) of mammalian species. The SCN is believed to act as brain clock regulating circadian and circannual biological rhythms, such as body temperature, sleep, and mood. Here, we examined whether 17beta-estradiol (E2) could affect cell excitability and synaptic transmission in the SCN. Bath application of E2 (0.03-3 microM) increased the spontaneous firing frequency and depolarized cell membrane of the SCN neurons significantly. Furthermore, E2 (0.03-3 microM) increased (by about 25-150% of control) frequency of the miniature excitatory postsynaptic currents. Amplitude of the evoked excitatory postsynaptic currents was enhanced (by about 32% of control) after exposure to 1 microM E2. The paired-pulse ratio was reduced by E2. These effects were prevented by the estrogen receptor antagonist, ICI 182780. Exposure to the biologically inactive 17alpha-estradiol did not cause any significant changes in the parameters mentioned above. These findings are in favor of an implication of estrogen in modulation of neuronal activity in SCN and possibly regulating circadian rhythms.

145 Spontaneous pain behaviour is related to spontaneous firing frequency in uninjured nociceptive c-fibre neurons after spinal nerve axotomy

145 Spontaneous pain behaviour is related to spontaneous firing frequency in uninjured nociceptive c-fibre neurons after spinal nerve axotomy

Spontaneous Electrical Activity in Sheep Mesenteric Lymphatics

It has recently become apparent that the lymph pump is an electrical entity that rivals the heart in complexity. Many interesting currents have been demonstrated by voltage clamping isolated lymphatic smooth muscle cells, but until now the role of these currents in the intact syncitium has not been studied. Intracellular microelectrode recordings were made from smooth muscle of sheep mesenteric lymphatics to investigate the electrophysiological basis of lymphatic pumping. Approximately 50% of the vessels exhibited spontaneous electrical activity, varying from regular oscillations in membrane potential to spike complexes. Spike complexes generally consisted of one or more action potentials superimposed on a slower depolarization or 'plateau' phase and were often preceded by a slow diastolic depolarization or 'pre-potential'. Norepinephrine (5 microM) induced depolarizing events in quiescent preparations. Both agonist-induced oscillations and spike complexes were attenuated or completely abolished by 2-aminoethoxydiphenyl borate (2-APB); 10-100 microM). Cesium (1 mM) reduced the frequency of spontaneous firing by approximately 30% by flattening the pre-potential phase. In addition to having a negative inotropic effect, 10 mM Cs(+) also caused gradual membrane depolarization and prolonged the plateau. 1 microM nifedipine abolished spontaneous events while tetrodotoxin (TTX; 0.5-1 muM) decreased the amplitude and maximum dV/dt of the spike upstroke or stopped activity completely. Spontaneously active segments of lymphatic vessel were inhibited by the chloride channel blocker, anthracene-9-carboxylic acid (9-AC; 250 microM - 1 mM) suggesting that I(Cl(Ca)) plays a significant role in the generation of spontaneous activity in this tissue. Penitrem-A (0.1 microM) did not affect resting membrane potential but increased action potential amplitude and prolonged the plateau, suggesting that calcium-activated potassium current does not make a significant contribution to resting membrane conductance but is important in membrane repolarization following calcium influx during the action potential. In contrast 4-aminopyridine (4-AP; 5 microM) caused significant membrane depolarization, suggesting the existence of an active 4-AP-sensitive current at rest. These results demonstrate that the currents found in isolated voltage-clamped cells from sheep mesenteric lymphatics do play a significant role in the shaping of spontaneous electrical activity of the intact syncitium.

Effects of Adrenalectomy on the Excitability of Neurosecretory Parvocellular Neurones in the Hypothalamic Paraventricular Nucleus

Glucocorticoids are well known to inhibit the release of hypophysiotrophic hormones from neurones originating in the paraventricular nucleus (PVN), but the cellular mechanisms of the inhibition are not well understood. Here, we examined the effects of adrenalectomy (ADX) on the spontaneous firing activity in the neurosecretory parvocellular PVN neurones of rat brain slices. The neurones were identified by injecting a retrograde dye into the pituitary stalk and classified according to their electrophysiological properties. The intranuclear distribution, electrophysiological properties, and hypophysiotrophic hormone phenotype of the labelled type II PVN neurones were similar to neurosecretory parvocellular PVN neurones. In the neurones of sham-operated rats under the cell-attached recording mode, we observed three spontaneous activity patterns: tonic regular (24%), tonic irregular (36%), and silent (40%). Noradrenaline (100 microM) induced an excitatory or an inhibitory effect on the spontaneous activity. Noradrenergic excitation was blocked by prazosin (2 microM, alpha(1)-adrenoceptor antagonist), and mimicked by phenylephrine (100 microM, alpha(1)-adrenoceptor agonist), whereas noradrenergic inhibition was blocked by yohimbine (2 microM, alpha(2)-adrenoceptor antagonist) and mimicked by clonidine (50 microM, alpha(2)-adrenoceptor agonist). In the neurones of ADX rats, we found burst firing in 35% of neurones tested and an increase in the frequency of spontaneous firing. The burst firing was not observed in the neurones of the sham-operated rats. ADX caused a 1.7-fold increase in the proportion of neurones showing the noradrenergic excitation. Supplementation of the ADX rats with corticosterone (10 mg pellet) reversed the ADX-induced burst firing, and the potentiation of noradrenergic excitation. In summary, our results show that removal of corticosterone by ADX can elevate the neuronal excitability by increasing the spontaneous firing rate and by potentiating the alpha(1)-adrenoceptor-mediated noradrenergic excitation, and it can facilitate hormone release by inducing burst firing. Our results provide new insight to the cellular mechanisms of the feedback inhibition by glucocorticoids in the neurosecretory parvocellular neurones of the PVN.

Electrophysiological Effects of Carvedilol on Rabbit Heart Pacemaker Cells

The electrophysiological effects of carvedilol, a beta-blocking agent with vasodilating actions, have been studied on rabbit pacemaker cells using the whole-cell patch clamp technique. Nystatin-perforated patch recordings from the sinoatrial (SA) and atrioventricular (AV) nodes demonstrated that 1-3 microM of carvedilol caused a decrease in the spontaneous firing frequency, depolarization of the maximal diastolic potential, and prolongation of the action potential duration in both species. Voltage clamp experiments were performed using SA and AV node myocytes to identify and define the carvedilol-induced changes in the Ca(2+) current, I(Ca), delayed rectifier K(+) current, I(K), and hyperpolarization-activated inward current, I(f). In the SA node cells, 1 microM of carvedilol blocked I(K), I(Ca), and I(f) by 72%, 47%, and 22%, respectively. In the AV node cells, the corresponding reductions were 64% (I(K)) and 46% (I(Ca)), respectively. In both the SA and AV nodes the decrease in I(K) appeared to be mainly due to the rapidly activating component of the delayed rectifier, I(Kr), since the high dose of carvedilol blocked I(K) in the SA and AV nodes to a submaximal degree. In conclusion, effective doses of carvedilol have classical class III antiarrhythmic actions and a negative chronotropic effect resulting from the inhibition of I(K) and I(Ca). Both actions may be efficacious for treating supraventricular tachyarrhythmias.

Postactivation inhibition of spontaneously active neurosecretory neurons in the medicinal leech

Spontaneously active neurosecretory neurons in vertebrate and invertebrate nervous systems share similarities in firing frequencies, spike shapes, inhibition by the transmitters they themselves release and postactivation inhibition, an intensity-dependent period of suppressed spontaneous generation of action potentials following phases of high-frequency activity. High-frequency activation of spontaneously active serotonin-containing Retzius cells in isolated ganglia of the leech Hirudo medicinalis induced prolonged membrane hyperpolarisations causing periods of postactivation inhibition of up to 33 s. The duration of the inhibitory periods was directly related to both the number and rate of spikes during activation and was inversely proportional to a cell's spontaneous firing frequency. The periods of postactivation inhibition remained unaffected by both serotonin depletion through repeated injections of 5,7-dihydroxytryptamine and suppressing the afterhyperpolarisation following each action potential with tetraethylammonium (TEA), iberiotoxin or charybdotoxin, suggesting that neither autoinhibition by synaptic release of serotonin nor calcium-activated potassium channels contribute to the underlying mechanism. In contrast, the postactivation inhibitory period was significantly affected both by differential electrical stimulation of the same Retzius cells via microelectrodes filled with molar concentrations of either Na(+)-acetate or K(+)-acetate, and by partial inhibition of Na(+)/K(+)-ATPase with ouabain. Thus, postactivation inhibition in Retzius cells results from prolonged hyperpolarising activity of Na(+)/K(+)-ATPase stimulated by the accumulation of cytosolic Na(+ )during phases of high-frequency spike activity.

Ethanol Inhibition of M-Current and Ethanol-Induced Direct Excitation of Ventral Tegmental Area Dopamine Neurons

Ethanol-induced excitation of ventral tegmental area dopamine (DA VTA) neurons is thought to be critical for the reinforcing effects of ethanol. Although ligand-gated ion channels are known to be the targets of ethanol, ethanol modulation of voltage-dependent ion channels of central neurons has not been well studied. We have demonstrated that ethanol excites DA VTA neurons by the reduction of sustained K(+) currents and recently reported that M-current (I(M)) regulates action potential generation through fast and slow afterhyperpolarization phases. In the present study we thus examined whether ethanol inhibition of I(M) contributes to the excitation of DA VTA neurons using nystatin-perforated patch current- and voltage-clamp recordings. Ethanol (20-120 mM) reduced I(M) in a concentration-dependent manner and increased the spontaneous firing frequency of DA VTA neurons. Ethanol-induced increase in spontaneous firing frequency correlated positively with ethanol inhibition of I(M) with a slope value of 1.3. Specific I(M) inhibition by XE991 (0.3-10 microM) increased spontaneous firing frequency which correlated positively with I(M) inhibition with a slope value of 0.5. In the presence of 10 muM XE991, a concentration that produced maximal inhibition of I(M), ethanol still increased the spontaneous firing frequency of DA VTA neurons in a concentration-dependent manner. Thus we conclude that, although ethanol causes inhibition of I(M) and this results in some increase in the firing frequency of DA VTA neurons, another effect of ethanol is primarily responsible for the ethanol-induced increase in firing rate in these neurons.

Characterization of NO/cGMP-mediated responses in identified motoneurons.

1. Nitric oxide (NO) is thought to play a neuromodulatory role in the nervous system of vertebrate and invertebrate species. In the hornworm Manduca sexta, NO-mediated signaling has been implicated in behavioral and developmental processes, but its exact function in neurons is unknown. In this study, we identify specific neurons in the CNS of Manduca larvae that accumulate cGMP in response to treatment with NO donors in the presence of cGMP-phosphodiesterase inhibitors. Subsets of these neurons were identified as motoneuron-12 (MN12) and intersegmental motoneurons (ISMs), which innervate dorsal oblique muscles of the larvae. 2. To investigate the physiological role of NO-evoked increases in cGMP in these motoneurons we performed intracellular recordings; we found that application of NO donors caused an increase in neuronal excitability that was characterized by an increase in the spontaneous firing frequency. When action potentials and EPSPs were blocked, NO treatment evoked a depolarization of the resting membrane potential and a decrease in the measured input resistance in both MN12 and the ISMs. 3. Additional experiments with MN12 showed that treatment with the cGMP analogue, 8-Br-cGMP mimicked the NO effect on the resting potential and the input resistance. Furthermore, MN12 incubation with the NOS inhibitor, L-NNA, resulted in a small hyperpolarization of the resting potential and an increase in the input resistance, and incubation with the sGC inhibitor, ODQ blocked the NO-evoked depolarization of MN12. Finally, NO treatment during voltage clamping of MN12 evoked an inward positive current. 4. Taken together, these results suggest that NO can act as a "gain control" of neuronal excitability, which might have an important role in insect behavior.

The Effects of Long Chain-Lengthn-Alcohols on the Firing Frequency of Dopaminergic Neurons of the Ventral Tegmental Area

The dopaminergic neurons of the ventral tegmental area (DA VTA neurons) have been implicated in the reinforcing properties of drugs of abuse, including ethanol (ethyl alcohol). Ethanol increases the spontaneous firing frequency of DA VTA neurons in vitro, in both brain slices and acutely dissociated neurons, and also in vivo. In many systems, longer n-alkyl alcohols have a more potent effect than ethanol, and the potency is a function of the number of carbons in the alkyl chain. We studied n-alcohols of chain length 1 (methanol) to 5 (pentanol) on the firing rate of DA VTA neurons in brain slice preparations. All of the alcohols studied produced increases in the spontaneous firing frequency in DA VTA neurons; as the chain length increased, lower concentrations of the alcohols were needed to produce the same percentage increase in firing. With very high concentrations of all the alcohols except methanol, we observed apparent depolarization block of firing. In addition, trichloroethanol (TCE), the active metabolite of chloral hydrate, increased the firing frequency of DA VTA neurons, and the EC(40) (concentration to produce a 40% increase in firing rate) of TCE was below that of ethanol. These studies indicate that excitation of VTA dopamine neurons by n-alcohols is related to the chain length of the carbons. This is likely to be a characteristic of the ethanol-sensitive element of DA VTA neurons and may be useful in identifying the element of the membrane that is responsible for ethanol-induced excitation.

Spontaneous Pain, Both Neuropathic and Inflammatory, Is Related to Frequency of Spontaneous Firing in Intact C-Fiber Nociceptors

Spontaneous pain, a poorly understood aspect of human neuropathic pain, is indicated in animals by spontaneous foot lifting (SFL). To determine whether SFL is caused by spontaneous firing in nociceptive neurons, we studied the following groups of rats: (1) untreated; (2) spinal nerve axotomy (SNA), L5 SNA 1 week earlier; (3) mSNA (modified SNA), SNA plus loose ligation of the adjacent L4 spinal nerve with inflammation-inducing chromic gut; and (4) CFA (complete Freund's adjuvant), intradermal complete Freund's adjuvant-induced hindlimb inflammation 1 and 4 d earlier. In all groups, recordings of SFL and of spontaneous activity (SA) in ipsilateral dorsal root ganglion (DRG) neurons (intracellularly) were made. Evoked pain behaviors were measured in nerve injury (SNA/mSNA) groups. Percentages of nociceptive-type C-fiber neurons (C-nociceptors) with SA increased in intact L4 but not axotomized L5 DRGs in SNA and mSNA (to 35%), and in L4/L5 DRGs 1-4 d after CFA (to 38-25%). SFL occurred in mSNA but not SNA rats. It was not correlated with mechanical allodynia, extent of L4 fiber damage [ATF3 (activation transcription factor 3) immunostaining], or percentage of L4 C-nociceptors with SA. However, L4 C-nociceptors with SA fired faster after mSNA (1.8 Hz) than SNA (0.02 Hz); estimated L4 total firing rates were approximately 5.0 and approximately 0.6 kHz, respectively. Similarly, after CFA, faster L4 C-nociceptor SA after 1 d was associated with SFL, whereas slower SA after 4 d was not. Thus, inflammation causes L4 C-nociceptor SA and SFL. Overall, SFL was related to SA rate in intact C-nociceptors. Both L5 degeneration and chromic gut cause inflammation. Therefore, both SA and SFL/spontaneous pain after nerve injury (mSNA) may result from cumulative neuroinflammation.

The Effects of Experimental Neurolysis on Ectopic Firing in a Rat Chronic Constriction Nerve Injury Model

We studied the effects of experimental neurolysis on ectopic firing in a rat chronic constriction nerve injury (CCI) model. Sixteen Wistar rats were used as CCI models; 8 in a neurolysis group and 8 in a sham operation group. Eight additional Wistar rats were used in a normal control group. The CCI model was created by loosely ligating the sciatic nerve with 4-0 chromic gut sutures. Seven days after surgery the same portion of the sciatic nerve, now embedded in scar tissue, was exposed. For the neurolysis group all sutures and scar tissue around the nerve were removed, and for the sham operation group only exposure of the sciatic nerve embedded in scar tissue was performed. Fourteen days after the first surgery the spinal cords of all animals in the 3 groups were transeated at the most rostral level of the spinal cord and ectopic firing was recorded antidromically from the sural nerve. The spontaneous ectopic firing frequency and the firing pattern at rest and the ectopic firing frequency under a hypoxic condition were analyzed. The ectopic firing frequency in the neurolysis group was significantly lower than that of the sham operation group but higher than that of the normal control group. Also the appearance rate of an on-off pattern was 0% in the neurolysis and normal control groups and 38% in the sham operation group. Finally the accumulated number of ectopic firings under a hypoxic condition was significantly smaller in the neurolysis and normal control groups than that of the sham operation group. Experimental neurolysis significantly improved the abnormal ectopic firing frequency and on-off firing pattern in the injured nerve of the CCI model. These findings may explain the efficacy of neurolysis on sensory symptoms such as numbness or spontaneous pain in chronic compression neuropathy.

Hyperpolarization-Activated Cation Current (Ih) Is an Ethanol Target in Midbrain Dopamine Neurons of Mice

Ethanol stimulates the firing activity of midbrain dopamine (DA) neurons, leading to enhanced dopaminergic transmission in the mesolimbic system. This effect is thought to underlie the behavioral reinforcement of alcohol intake. Ethanol has been shown to directly enhance the intrinsic pacemaker activity of DA neurons, yet the cellular mechanism mediating this excitation remains poorly understood. The hyperpolarization-activated cation current, Ih, is known to contribute to the pacemaker firing of DA neurons. To determine the role of Ih in ethanol excitation of DA neurons, we performed patch-clamp recordings in acutely prepared mouse midbrain slices. Superfusion of ethanol increased the spontaneous firing frequency of DA neurons in a reversible fashion. Treatment with ZD7288, a blocker of Ih, irreversibly depressed basal firing frequency and significantly attenuated the stimulatory effect of ethanol on firing. Furthermore, ethanol reversibly augmented Ih amplitude and accelerated its activation kinetics. This effect of ethanol was accompanied by a shift in the voltage dependence of Ih activation to more depolarized potentials and an increase in the maximum Ih conductance. Cyclic AMP mediated the depolarizing shift in Ih activation but not the increase in the maximum conductance. Finally, repeated ethanol treatment in vivo induced downregulation of Ih density in DA neurons and an accompanying reduction in the magnitude of ethanol stimulation of firing. These results suggest an important role of Ih in the reinforcing actions of ethanol and in the neuroadaptations underlying escalation of alcohol consumption associated with alcoholism.

The Molecular Basis of Odor Coding in the Drosophila Larva

We have analyzed the molecular basis of odor coding in the Drosophila larva. A subset of Or genes is found to be expressed in larval olfactory receptor neurons (ORNs). Using an in vivo expression system and electrophysiology, we demonstrate that these genes encode functional odor receptors and determine their response spectra with 27 odors. The receptors vary in their breadth of tuning, exhibit both excitation and inhibition, and show different onset and termination kinetics. An individual receptor appears to transmit signals via a single ORN to a single glomerulus in the larval antennal lobe. We provide a spatial map of odor information in the larval brain and find that ORNs with related functional specificity map to related spatial positions. The results show how one family of receptors underlies odor coding in two markedly different olfactory systems; they also provide a molecular mechanism to explain longstanding observations of larval odor discrimination.

Effect of sodium channel blocker (mexiletine) on pathological ectopic firing pattern in a rat chronic constriction nerve injury model

We studied the efficacy of mexiletine as a sodium channel blocker for neuropathic pain by investigating the effect of mexiletine on the pathological ectopic firing pattern in a chronic constriction nerve injury (CCI) model. The experiment was conducted with 60 male Wistar rats. The CCI model was created by loosely ligating the sciatic nerve. After breeding 7 days, the frequency and pattern of ectopic firing antidromically recorded from the sural nerve and the amplitude of antidromic sensory nerve-evoked potential were analyzed. The CCI rats were given an intravenous injection of normal saline and mexiletine (5 or 15 mg/kg). Mexiletine significantly suppressed spontaneous firing frequency, an on-off firing pattern that consisted of cyclic bursting spikes and ectopic firing generation under the hypoxic condition. Mexiletine did not influence the amplitude of Α-δ component in the antidromic sensory nerve-evoked potential. Mexiletine suppressed ectopic firing by blocking activity of the abnormal sodium channel at the nerve-injured site and dorsal root ganglion without blocking nerve conduction. This study suggests that mexiletine is useful for treating neuropathic pain in peripheral neuropathy.

Electrical Synapses between Dopaminergic Neurons of the Substantia Nigra Pars Compacta

Spatiotemporal properties of dopamine release play a major role both in striatal and nigral physiology because dopamine is released from nerve terminals and dendrites of nigrostriatal dopaminergic (DA) neurons. Pioneering work revealed gap junctional communication (assessed by dye-coupling experiments) between DA cells in the substantia nigra pars compacta (SNc). However, direct evidence of functional electrical synapses between DA neurons is still lacking. In this study, gap junctional communication between DA neurons was investigated in rat brain slices. Tracer coupling was observed in postnatal day 5 (P5) to P10 and P15-P25 rats. Dual whole-cell patch-clamp recordings revealed that 96% of DA neurons were coupled by electrical synapses in P7-P10 rats, and 20% were coupled in P17-P21 rats. These electrical synapses were mainly symmetrical and displayed strong low-pass filtering properties. When spontaneous firing activity was monitored, no significant synchronization was observed. Nevertheless, an efficient modulation of the spontaneous firing frequency of the postsynaptic cell occurred during injection of hyperpolarizing and depolarizing currents in the coupled presynaptic cell. Together, these observations demonstrate the existence of a fast communication between SNc DA neurons through electrical synapses.

Activity Dependence of Cortical Axon Branch Formation: A Morphological and Electrophysiological Study Using Organotypic Slice Cultures

The influence of neuronal activity on cortical axon branching was studied by imaging axons of layer 2/3 neurons in organotypic slice cultures of rat visual cortex. Upper layer neurons labeled by electroporation of plasmid encoding yellow fluorescent protein were observed by confocal microscopy. Time-lapse observation of single-labeled axons showed that axons started to branch after 8-10 d in vitro. Over the succeeding 7-10 d, branch complexity gradually increased by both growth and retraction of branches, resulting in axon arbors that morphologically resembled those observed in 2- to 3-week-old animals. Electrophysiological recordings of neuronal activity in the upper layers, made using multielectrode dishes, showed that the frequency of spontaneous firing increased dramatically approximately 10 d in vitro and remained elevated at later stages. To examine the involvement of spontaneous firing and synaptic activity in branch formation, various blockers were applied to the culture medium. Cultures were silenced by TTX or by a combination of APV and DNQX but exhibited a homeostatic recovery of spontaneous activity over several days in the presence of blockers of either NMDA-type or non-NMDA-type glutamate receptors alone. Axonal branching was suppressed by TTX and AMPA receptor blockade but not by NMDA receptor blockade. We conclude that cortical axon branching is highly dynamic and that neural activity regulates the early developmental branching of upper layer cortical neurons through the activation of AMPA-type glutamate receptors.

Somatic Ca 2+ dynamics in response to choline-mediated excitation in histaminergic tuberomammillary neurons

Histaminergic tuberomammillary (TM) neurons of the posterior hypothalamus have been implicated in cognition, alertness and sleep-wakefulness cycles. Spontaneous firing of TM neurons has been associated with histamine release and wakefulness. The expression of α7 nicotinic acetylcholine receptors (nAChRs) in TM neurons suggests a role for endogenous choline and for nicotinic drugs in the regulation of intracellular Ca 2+ metabolism, normal TM neuronal activity and histamine release. First, we established the link between TM neuronal spontaneous firing frequency and cytosolic free Ca 2+ concentration ([Ca 2+] i). A strong correlation was observed: an onset of spontaneous firing (3–4Hz) was accompanied by a 20-fold increase in [Ca 2+] i from 56±18nM to 1.0±0.6μM. The same range of firing frequencies has been observed in TM neurons in vivo and is associated with wakefulness. Secondly, choline-induced activation of α7 nAChRs did not elevate [Ca 2+] i directly, i.e. in the absence of high-threshold voltage-gated Ca 2+ channel (HVGCC) activation. Cd 2+ (200μM) completely blocked all Ca 2+ signals, but inhibited only 37±16% of α7 nAChR-mediated currents. Thirdly, the responsiveness of [Ca 2+] i to choline-mediated excitation was inhibited by hyperpolarization and enhanced by depolarization, sensitizing [Ca 2+] i at membrane voltages associated with normal TM neuronal activity. These properties of [Ca 2+] i define the ability of TM neurons to translate cholinergic stimuli of identical strengths into different cytosolic Ca 2+ effects, providing the physiological substrate for state-specific modulation of incoming cholinergic information and would be expected to play a very important role in determining activity profiles of TM neurons exposed to elevated concentrations of cholinergic agents, such as choline and nicotine.

Voltage-clamp analysis and computational model of dopaminergic neurons from mouse retina

Isolated dopaminergic amacrine (DA) cells in mouse retina fire rhythmic, spontaneous action potentials and respond to depolarizing current with trains of low-frequency action potentials. To investigate the roles of voltage-gated ion channels in these processes, the transient A-type K+ current (I(K,A)) and Ca2+ current (I(Ca)) in isolated mouse DA cells were analyzed by voltage clamp. The I(K,A) activated at -60 mV and inactivated rapidly. I(Ca) activated at around -30 mV and reached a peak at 10 mV without apparent inactivation. We also extended our previous computational model of the mouse DA cell to include the new electrophysiological data. The model consisted of a membrane capacitance in parallel with eight currents: Na+ transient (I(Na,T)), Na+ persistent (I(Na,P)), delayed rectifier potassium (I(Kdr)), I(K,A), calcium-dependent potassium (I(K,Ca)), L-type Ca2+ I(Ca), hyperpolarization-activated cation current (I(h)), and a leak current (I(L)). Hodgkin-Huxley type equations were used to define the voltage- and time-dependent activation and inactivation. The simulations were implemented using the neurosimulator SNNAP. The model DA cell was spontaneously active from a wide range of initial membrane potentials. The spontaneous action potentials reached 35 mV at the peak and hyperpolarized to -76 mV between spikes. The spontaneous firing frequency in the model was 6 Hz. The model DA cell responded to prolonged depolarizing current injection by increasing its spiking frequency and eventually reaching a depolarization block at membrane potentials greater than -10 mV. The most important current for determining the firing rate was I(K,A). When the amplitude of I(K,A) was decreased, the firing rate increased. I(Ca) and I(K,Ca) also affected the width of action potentials but had only minor effects on the firing rate. Ih affected the firing rate slightly but did not change the waveform of the action potentials.

Electrophysiological study on primary afferent properties of a chronic constriction nerve injury model in spinal rats

AbstractA chronic constriction nerve injury (CCI) model of the rat sciatic nerve is known to exhibit neuropathic pain behavior. The authors conducted electrophysiological analysis for the primary afferent properties of this model in a decerebrate spinal preparation. In the CCI model, electrical transcutaneous stimulation for A-δ and C-fibers showed a low current threshold to elicit the flexion withdrawal reflex. The antidromic ectopic firing activity recorded from the sural nerve showed abnormal firing patterns, which were not seen in normal rats, as follows: (1) an increase of spontaneous firing frequency, (2) development of an on–off pattern that consisted of cyclic burst spikes, and (3) increased firing number under the hypoxic condition. The amplitude of the A-δ component in the antidromic sensory nerve-evoked potential was lower than that in normal rats. The current study clarified the electrophysiological parameters reflecting pathological hypersensivity and excitability of primary afferents in the CCI model, which could not found by behavioral analysis. These results may be useful in future studies evaluating possible treatments of neuropathic pain.