Inhibition of hyperpolarization-activated cation currents by phencyclidine and some sigma ligands in rat hippocampal CA1 pyramidal neurons in vitro

Using whole-cell voltage-clamp recordings, hyperpolarization-activated cation currents (Ih) were elicited with hyperpolarizing voltage jumps in CA1 pyramidal neurons of rat hippocampal slices, and the effects of phencyclidine (PCP) and some sigma ligands on Ih were studied. PCP concentration-dependently (0.1-100 microM) suppressed Ih and shifted the activation curve of Ih to the negative direction. D-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP, 20 microM) and MK-801 (30 microM), competitive and non-competitive NMDA blockers, respectively, failed to mimic the inhibitory effect of PCP on Ih, and suppression of Ih by PCP was unaffected in the presence of these blockers. To explore the involvement of sigma1 receptors in the reduction of Ih, the effects of representative sigma1 ligands were studied. SKF10047 (100 microM), a sigma1 agonist, attenuated the maximal Ih and shifted the half-activation potential of Ih to the hyperpolarized direction. In the presence of the sigma1 antagonist NE-100 (1 microM), which alone did not affect Ih, the effect of SKF10047 on Ih was unaltered. By contrast, a higher concentration of NE-100 (10 microM) mimicked the effect of SKF10047. Again, no antagonism of Ih suppression by SKF10047 was obtained with rimcazole (100 microM), a sigma1 receptor antagonist that is structurally distinct from NE-100. This concentration of rimcazole alone resulted in a slight but significant reduction of Ih. Thus these major sigma1 ligands appear to suppress Ih independently of their agonistic or antagonistic properties. The results of this study suggest that PCP and some sigma ligands could modulate cell excitability partly through their action on Ih.     

No effects of enhanced central norepinephrine on finger-sequence learning and attention

Rationale When paired with training, substances that increase monoaminergic transmission in the brain support motor and language learning in healthy subjects and in rehabilitation after brain lesions.Objectives To test the hypotheses that enhancement of central norepinephrine by the selective norepinephrine reuptake inhibitor reboxetine (1) improves skilled motor performance, (2) promotes skilled motor learning, and (3) does not exert these effects by modulation of attention.Methods In a double blind, placebo-controlled, crossover study in healthy, adult subjects (n=16), finger-sequence performance and learning was measured after the stimulation of the central noradrenergic system with a single dose (8 mg) of reboxetine and placebo. Effects on attention were assessed by the standardized continuous performance test “CPT-M”.Results No differential effects of reboxetine or placebo on finger-sequence performance, learning and parameters of attention were found.Conclusion Selective stimulation of the central noradrenergic system did not promote skilled motor learning or performance as assessed by finger-sequences. The plasticity-enhancing effect of reboxetine, documented in other studies, appears to be dependent on specific neurophysiological and neuropsychological characteristics of the task, and cannot be generalized to other behavioral paradigms.     

Inhibition of Qi site of mitochondrial complex III with antimycin A decreases persistent and transient sodium currents via reactive oxygen species and protein kinase C in rat hippocampal CA1 cells

Hypoxia-induced inhibition of Qi site of mitochondrial complex III under hypoxia has received attention, but its downstream pathways remain unclear. In this paper, we used Qi site inhibitor antimycin A to mimic the inhibition of the Qi site of mitochondrial complex III and studied the effects of the inhibition of this site on persistent sodium currents, transient sodium currents, and neuronal excitability in rat hippocampal CA1 cells with whole cell patch-clamp methods. The results showed that antimycin A decreased the amplitude of both persistent and transient sodium currents; antioxidant 2-mercaptopropionylglycine or 1,10 phenanthroline abolished the effect of antimycin A; the complex III Qo site inhibitor stigmatellin, the protein kinase C inhibitor chelerythrine, but not the protein kinase A inhibitor H89, canceled the effect of antimycin A; antimycin A decreased the amplitude of both persistent and transient sodium currents only at more depolarized membrane potentials and the decrease percentage of both persistent and transient sodium currents after antimycin A at potentials above -50 mV increased with the change in potentials toward more depolarized direction; exogenous application of H2O2 inhibited the amplitude of both persistent and transient sodium currents; the amount of current required to trigger spikes was increased and the number of spikes produced by varying levels of currents was decreased by antimycin A. These results suggest that the inhibition of Qi site of mitochondrial complex III decreases both persistent and transient sodium currents via reactive oxygen species and protein kinase C in rat hippocampal CA1 cells.     

Neurons evaluate both the amplitude and the meaning of signals

The neuronal threshold, which can be determined by the level of depolarization immediately prior to spike generation, is different for responses to conditioned and discriminated stimuli after conditioning. However, it is impossible to determine excitability within a response to stimuli that failed to generate a spike. In the present study we examined the role of the AP threshold of two related Helix defensive neurons in the initiation of an AP during elaboration of the neuronal analog of a classical conditional reflex. These neurons fire in a synchronous manner though spikes sometimes are not generated simultaneously. We compared the change in spike threshold within the responses with other parameters of intracellular activity that also affect the neuronal response, but which can be measured without an analysis of the spike waveform (spike latency, slope of excitatory postsynaptic potentials, etc.). We found that the change in slope is not the reason for the change in the threshold during pairing. The thresholds within conditioned and discriminated responses affected spike generation and their values correlated with the spike presence or absence in related neurons. The excitability changed transiently within the responses, since these alterations were selective for the conditioned and discriminated stimuli and applied primarily to the first spike of the response. The firing threshold seems to be a dynamic property of excitable membrane. Neurons appear to evaluate the significance of the input signal, transiently change their own excitability and only after that compare the magnitude of this signal and threshold.     

Extracellular potassium alters frequency and profile of retinal spreading depression waves

The phenomenon of spreading depression (SD) was observed in chicken retina by means of optical registration via a microscope and a CCD camera applying modern methods of image processing for optimized evaluation of the wave profiles. The propagation dynamics of SD waves was investigated as a function of extracellular potassium. Two main findings were obtained. Firstly, the frequency of spontaneous wave generation increased with the increase of K⁺ concentration. Secondly, there was an effect of potassium on the wave profile. In particular, the recovery zone of SD waves was shortened at increased K⁺. This effect was not only due to the dispersion relation of waves in excitable media as shown by the result of the mechanically induced wave trains. Applying the basic principles of chemical excitability for the interpretation of the data led us to the conclusion that these potassium effects are due to perturbations of an autocatalytic reaction to be further explored.     

Altered synaptic and electrical properties of lumbar motoneurons in the neurological glial mutant<Emphasis Type="Italic"> taiep</Emphasis> rat

Maturation and differentiation of electrical properties of neurons and synaptic transmission are modulated by neuronal interaction. In vitro experiments have shown that these processes also seem to be regulated by signals from non-neuronal elements such as glial cells. It is not known, however, whether glial alterations in intact neural networks may also affect the maturation of electrical properties and synaptic transmission during development. We used the taiep rat, a neurological mutant with a progressive demyelination and astrogliosis, as an experimental model to study the postnatal development of motoneurons in an altered glial environment. Using the patch-clamp technique, we made intracellular recording from motoneurons of Rexeds lamina IX in spinal cord slices of neonatal rats (postnatal day P4–P10). The electrical properties of normal motoneurons changed significantly with age, showing decreasing input resistance (R_in) and increasing membrane capacity (C_m). The rheobase increased with age, accompanied by an increase of the amplitude and a decrease of the duration of action potentials (APs). In contrast, mutant neurons showed no age-dependent changes of R_in, C_m, or AP characteristics. After blocking inhibitory transmission, intralaminar bipolar stimulation elicited, in both control and taiep motoneurons, fast glutamatergic excitatory postsynaptic potentials (EPSPs). Two types of taiep motoneurons were identified according to the temporal patterns of synaptic responses; (1) taiep _SYN neurons, which showed no significant differences to control motoneurons, and (2) taiep _ASYN neurons, in which the initial EPSP was followed by a variable number of delayed, asynchronous EPSP responses (for up to 300 ms). All these electrophysiological findings suggest that the mutation in taiep rats interfered with the development of the electrical properties of neurons and with the maturation of synaptic transmission, probably due to alterations in the neuron-glia interactions.This work was presented in part at the Fifth European Meeting on Glial Cell Function in Health and Disease (Euroglia 2002, Rome, Italy)     

Biophysical alterations of hippocampal pyramidal neurons in learning, ageing and Alzheimer's disease

A series of behavioral, electrophysiological, and molecular biochemical experiments are reviewed indicating that when animals learn hippocampus-dependent tasks, output neurons in the CA1 and CA3 hippocampal subfields show reductions in the slow, post-burst afterhyperpolarization (AHP). The slow AHP is mediated by an apamin-insensitive calcium-activated potassium current. A reduction in the slow AHP makes hippocampal neurons more excitable and facilitates NMDA receptor-mediated response and temporal summation. During normal aging and in a mouse model of Alzheimer's disease (AD), the slow AHP is increased, making neurons less excitable and making learning more difficult. The subgroup of aging animals that are able to learn demonstrates the capacity to increase neuronal excitability by reducing the size of the slow AHP. Similarly, in a mouse model of AD, mice that are able to learn normally after a genetic alteration have a normal capacity for increasing hippocampal neuron excitability by reducing their slow AHP. We suggest that reduction in the slow AHP is basic to learning in young and aging animals. Inability to modulate the slow AHP contributes to learning deficits that occur during aging and early stages of AD.     

The recovery function of paired somatosensory evoked potentials in cerebral ischemic rabbits

In cerebral ischemia of different degrees, the amplitudes of somatosensory evoked potentials (SEPs) are facilitated or depressed. Different mechanisms might be responsible for the explanation of the contradictory phenomena. SEPs obtained using the paired stimulation method were employed in trying to find out the role of the functional changes of the inhibitory system in the brain during ischemia. Eight rabbits were used for recording the amplitude recovery curve of paired SEPs in the normal state, and the other ten were used for recording the recovery curve in different cerebral ischemic degrees. Cerebral ischemia was induced by the iron particle injection method. The attenuation of the depressed recovery function of the paired SEP amplitudes was detected during cerebral ischemia when SEP amplitudes were either facilitated or depressed. The results indicate that the function of the inhibitory system in the brain is selectively vulnerable during cerebral ischemia.     

电刺激强度对豚鼠听神经兴奋性的影响

目的在不同电刺激下观察不同刺激强度的电诱发听性脑干反应(electricalevokedauditorybrainstemresponse,EABR)I波幅值的变化,评估刺激强度对豚鼠听神经兴奋性影响。方法选取健康短毛白色纯种红目豚鼠45只,标准电极分别插入豚鼠两侧鼓阶内大约4mm,靠近蜗尖两个电极作为刺激电极,随机选择一侧耳为刺激耳,另一侧耳为对照耳。选用二种不同的刺激强度[EABR阈上6dB(n=25)和阈上18dB(n=20)]电荷平衡双相脉冲电流,连续刺激2小时,刺激速率分别为200、1000PPS,观察刺激前和刺激后3小时内EABRI波幅值的变化。结果刺激强度为EABR阈上6dB、刺激速率为200PPS时I波幅值同刺激前相比升高20%,刺激速率为1000PPS时I波幅值较刺激者相比升高约7%;刺激强度为EABR阈上18dB时,应用200PPS刺激速率,I波的幅值在刺激后0分钟下降21%(P<0.05)、30分钟时下降11%(P<0.05)、60分钟时下降9%(P<0.05)、90分钟时下降7%(P>0.05)、120分钟时下降约2%(P>0.05)、150分钟时完全恢复正常;应用1000PPS的刺激速率,I波的幅值在刺激后0分钟下降56%(P<0.01)、30分钟时下降47%(P<0.01)、60分钟时下降58%(P<0.01)、90分钟时下降63%(P<0.01)、120分钟时下降约48%(P<0.01)、150分钟时下降43%(P<0.01)、180分钟时下降44%(P<0.01)。结论高速率和高强度连续电刺激导致EABRI波幅值持久下降反应了听神经的兴奋性变化不但受电刺激的速率影响,而且受电刺激的强度影响。     

外淋巴“枯竭“状态对听神经兴奋性影响

目的 通过记录电刺激诱发听觉脑干电位(EABR)阈值变化搞清楚内耳外淋巴“枯竭”状态对听神经兴奋性影响。方法 在手术显微镜帮助下,小心把标准刺激电极沿蜗轴插入鼓阶约4mm,用压碎的肌肉轻轻封住圆窗口。然后用波宽为50μs/phase、刺激速率为30次/秒电荷平衡双相脉冲电流刺激受试动物鼓阶内的靠近蜗尖的电极对(1/2),记录两次EABR阈值,取均值。然后把明胶海绵做成直径约为1mm,长约为4cm小的圆柱形,把一端放入前庭阶持续吸干外淋巴,直到显微镜下见鼓阶外淋巴消失,模拟外淋巴“枯竭”状,用以上电刺激参数再次记录电极对(1/2)的EABR阈值两次,取均值。比较前后两次EABR阈值的变化(t检验)。结果 耳蜗外淋巴“枯竭”状态时测试到的EABABR阈值0.63±0.11mA较充满外淋巴时0.27±0.08mA明显升高(P<0.001)。结论 EABR阈值升高说明耳蜗鼓阶外淋巴呈现“枯竭”状态时听神经兴奋性明显下降,临床上少数重聋或者全聋患者耳蜗外淋巴呈现“枯竭”状态,推测其电子耳蜗植入临床效果可能较耳蜗鼓阶内外淋巴正常患者差。