Mean Burst Length Research Articles

Neural activity of the subthalamic nucleus during voluntary movements in patients with Parkinson’s disease

Microelectrode recording (MER) during electrode implantation for deep brain stimulation (DBS) can determine the boundaries of the subthalamic nucleus (STN), and track neural activity during rest and voluntary movement in Parkinson’s disease (PD) patients. However, the functional role of STN in motor control remains unclear, despite its effectiveness in stimulation. Single unit activity of STN was recorded in 16 patients with PD during implantation of deep brain stimulation electrodes. Electromyograms of forearm muscles and a phonogram featuring verbal commands were simultaneously recorded with MER. The patients were instructed to perform motor tests involving clenching their hand into a fist. Out of the 560 neurons studied, 93 (16.6%) responded to motor tasks. The registered neurons were classified into three patterns using the hierarchical clustering method of histograms of interspike interval density — namely, tonic, burst, and pause patterns. Sensitive neurons were classified according to their responses, with activation (76.9%) and inhibition (23.1%) being the two identified types. In 90% of cases, neuron activation preceded movement. Of the responses, 53.8% were classified as tonic and 46.2% as phasic. Two thirds of inhibitory responses were advanced, occurring 0.2–0.3 seconds before movement onset. One third of neurons were activated after movement initiation with a delay of 0.05–0.2 seconds. 83.3% of the inhibitory neurons responded tonically, while the remaining 16.7% responded phasically. Notably, all phasic reactions occurred before the onset of movement. A comparison of the parameters of sensitive and non-sensitive neurons revealed several differences. The findings suggest a potential physiological distinction between the two neuron types. All three patterns of activity were present in both types of neurons, but sensitive neurons exhibited a wider representation of pause neurons (57.9% vs. 49.5%). Additionally, burst and pause neurons responded to movements that featured significantly lower variance of multiple activity parameters, such as firing rate, burst index, mean burst length, and oscillation scores in the 8–12 and 12–20 Hz range. The distribution of sensitive neurons along the electrode trajectory through the subthalamic nucleus was analyzed. Sensitive neurons were located significantly more dorsally compared to non-sensitive neurons, and no sensitive pause cells were observed in the ventral half of the STN. Based on our findings, it is presumed that the pause pattern plays a critical role in both motor control and its related disorders in Parkinson’s disease. A diverse range of neuronal responses suggests a high degree of heterogeneity among STN neurons implicated in motor control. The presence of both delayed and advanced neural responses suggests that STN involvement in motor control encompasses both the preparation and initiation of movement, as well as the regulation of ongoing movements via afferent feedback. The range of neural responses aligns with the dynamic model of basal ganglia, which suggests that STN can perform distinct functional roles during different stages of movement, including initiation, control, and completion.

DbOBS: dual buffered switch for variable optical bursts in future datacenters

Modern data-driven applications pose stringent requirements of high bandwidth, ultra-low-latency, low-powered, and scalable interconnections among switches and routers in data-centers. To address these demands, electronic switching is not a viable choice due to bandwidth and computing bottlenecks. Thus, researchers explored effective optical switch design principles for next-generation data-centers. In optical switches, data aggregates in the form of optical bursts (OB) at ultra-high speeds. In the case of OB contention, solutions are proposed by researchers to store OB as recirculating patterns in fiber delay lines (FDL) with induced optical delay. However, due to variable burst length, it is not possible to measure slot delay length, thus storage of contending bursts is not possible at intermediate core switches. Motivated from the aforementioned discussions, in this paper, we propose a switch design DbOBS, that is capable to store variable OB during contention slots. DbOBS estimates mean burst length, and possible deviation from mean length to minimize burst loss. The considered switch design is validated through parameters like-burst length estimation, over-reservation, and waiting time. For network-layer simulations, poison arrivals of data bursts are considered as packetized units. The packets are sent through Monte-Carlo arrivals and burst loss probability (BLP) is estimated at various input load conditions and buffer sizes. DbOBS achieves a BLP in order of $$10^{-4}$$ at load $$\approx$$ 0.8, and buffer-size of 50, and burst length of $$L=5$$ , that outperforms the traditional switch designs.

Perceptual Quality Maximization for Video Calls With Packet Losses by Optimizing FEC, Frame Rate, and Quantization

We consider video calls affected by bursty packet losses, where frame-level forward error correction (FEC) is employed due to delay constraints, and damaged frames and others predicted from them are discarded. Here, a high frame rate (FR) at low bitrates leads to large quantization step sizes (QS), small frames, and suboptimal FEC, whereas a low FR at high bitrates reduces the perceptual quality. To mitigate frame losses and freezing, hierarchical-P (hierP) temporal layering can be used with lower coding efficiency than IPPP coding. We study the received quality maximization for hierP and IPPP, by jointly optimizing the encoding FR, QS, and the FEC redundancy, under sending bitrate constraints. Building upon Q-STAR perceptual quality and R-STAR bitrate models, which depend on QS, and the decoded and encoding FR, respectively, we cast the problem as a combinatorial optimization problem. We solve for the encoding FR and the bitrate using exhaustive search, hill-climbing, and a greedy FEC distribution algorithm to determine the FEC redundancies. We show that, for random losses, the FEC bitrate ratio is an affine function of the packet-loss rate; low encoding FR is preferred at wider sending bitrate ranges with higher packet loss; layer protection is more even at higher bitrates; and IPPP, while achieving higher Q-STAR scores, is prone to freezing. For bursty losses, we show that layer redundancies are higher, rising with the mean burst length, reaching 80%; and hierP achieves higher Q-STAR scores than IPPP for longer bursts, and a smaller mean and variance of decoded frame distances.

A review on a new conservation law in optical burst switching networks

A major challenge in optical burst switching (OBS) networks is how to provide quality of service (QoS). One scheme is burst preemption, in which a high priority burst can preempt a low priority one if there is no available output channel. To analyze the performance of prioritized preemption scheme, a conservation law is widely used without proof in the literature. In this paper, we show that the original conservation law does not hold if the mean burst length of different classes is unequal. We then present an exact model for burst blocking probabilities of multiple priority classes with unequal mean burst lengths by considering the effects of blocking probabilities and preemption probabilities on the mean overall service time. We also propose an approximate model. Our extensive simulation results show that the approximation model can yield accurate solutions under different traffic conditions.

TARP‐associated AMPA receptors display an increased maximum channel conductance and multiple kinetically distinct open states

Fast excitatory synaptic transmission in the CNS is mediated mainly by AMPA-type glutamate receptors (AMPARs), whose biophysical properties are dramatically modulated by the presence of transmembrane AMPAR regulatory proteins (TARPs). To help construct a kinetic model that will realistically describe native AMPAR/TARP function, we have examined the single-channel properties of homomeric GluA1 AMPARs in combination with the TARPs, γ-2, γ-4 and γ-5. In a saturating concentration of agonist, each of these AMPAR/TARP combinations gave rise to single-channel currents with multiple conductance levels that appeared intrinsic to the receptor-channel complex, and showed long-lived subconductance states. The open time and burst length distributions of the receptor complexes displayed multiple dwell-time components. In the case of γ-2- and γ-4-associated receptors, these distributions included a long-lived component lasting tens of milliseconds that was absent from both GluA1 alone and γ-5-associated receptors. The open time distributions for each conductance level required two dwell-time components, indicating that at each conductance level the channel occupies a minimum of two kinetically distinct open states. We have explored how these data place novel constraints on possible kinetic models of TARP-associated AMPARs that may be used to define AMPAR-mediated synaptic transmission.

Activation of α7 Nicotinic Receptors by Orthosteric and Allosteric Agonists: Influence on Single-Channel Kinetics and Conductance

Nicotinic acetylcholine receptors (nAChRs) are oligomeric transmembrane proteins in which five subunits coassemble to form a central ion channel pore. Conventional agonists, such as acetylcholine (ACh), bind to an orthosteric site, located at subunit interfaces in the extracellular domain. More recently, it has been demonstrated that nAChRs can also be activated by ligands binding to an allosteric transmembrane site. In the case of α7 nAChRs, ACh causes rapid activation and almost complete desensitization. In contrast, allosteric agonists such as 4-(4-bromophenyl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c] quin oline-8-sulfonamide (4BP-TQS) activate α7 nAChRs more slowly and cause only low levels of apparent desensitization. In the present study, single-channel patch-clamp recording has been used to investigate differences in the mechanism of activation of α7 nAChRs by ACh and 4BP-TQS. The most striking difference between activation by ACh and 4BP-TQS is in single-channel kinetics. In comparison with activation by ACh, single-channel open times and burst lengths are substantially longer (~160-800-fold, respectively), and shut times are shorter (~8-fold) when activated by 4BP-TQS. In addition, coapplication of ACh and 4BP-TQS results in a further increase in single-channel burst lengths. Mean burst lengths seen when the two agonists are coapplied (3099 ± 754 ms) are ~2.5-fold longer than with 4BP-TQS alone and ∼370-fold longer than with ACh alone. Intriguingly, the main single-channel conductance of α7 nAChRs, was significantly larger when activated by 4BP-TQS (100.3 ± 2.4 pS) than when activated by ACh (90.0 ± 2.7 pS), providing evidence that activation by allosteric and orthosteric agonists results in different α7 nAChRs open-channel conformations.

QoE Prediction Model and its Application in Video Quality Adaptation Over UMTS Networks

The primary aim of this paper is to present a new content-based, non-intrusive quality of experience (QoE) prediction model for low bitrate and resolution (QCIF) H.264 encoded videos and to illustrate its application in video quality adaptation over Universal Mobile Telecommunication Systems (UMTS) networks. The success of video applications over UMTS networks very much depends on meeting the QoE requirements of users. Thus, it is highly desirable to be able to predict and, if appropriate, to control video quality to meet such QoE requirements. Video quality is affected by distortions caused both by the encoder and the UMTS access network. The impact of these distortions is content dependent, but this feature is not widely used in non-intrusive video quality prediction models. In the new model, we chose four key parameters that can impact video quality and hence the QoE-content type, sender bitrate, block error rate and mean burst length. The video quality was predicted in terms of the mean opinion score (MOS). Subjective quality tests were carried out to develop and evaluate the model. The performance of the model was evaluated with unseen dataset with good prediction accuracy ( ~ 93%). The model also performed well with the LIVE database which was recently made available to the research community. We illustrate the application of the new model in a novel QoE-driven adaptation scheme at the pre-encoding stage in a UMTS network. Simulation results in NS2 demonstrate the effectiveness of the proposed adaptation scheme, especially at the UMTS access network which is a bottleneck. An advantage of the model is that it is light weight (and so it can be implemented for real-time monitoring), and it provides a measure of user-perceived quality, but without requiring time-consuming subjective tests. The model has potential applications in several other areas, including QoE control and optimization in network planning and content provisioning for network/service providers.

Performance Comparison of Packet-Level Multiplexing Algorithms with Bursty Traffic

Packet-based networks have opened the possibility of carrying a great variety of applications with different traffic profiles. Traffic profiles such as bursty traffic is essential for the analysis and evaluation of network performance. This paper is a simulation study devoted to comparing the performance of a range of packet-level multiplexing algorithms with bursty traffic. Four multiplexing algorithms are studied: round robin, random selection, weighted random selection and longest queue. Bursty traffic in this study is specified using two parameters: average packet rate and mean burst length. The algorithms performance has been measured in terms of three performance measures: average delay, delay variation and packet loss. The performance of multiplexing algorithms deteriorated when traffic intensity and burstiness of traffic increase. The algorithms competed between each other. Each algorithm showed better performance for a certain measure, however, with the expense of deterioration in other performance measures.

Self-organized dendritic sidebranching in directional solidification: Sidebranch coherence within uncorrelated bursts

We experimentally study the level of organization of dendritic sidebranching in directional solidification. For this, we extract successive interface positions at a fixed distance from the dendrite tips and we perform various correlation analyses. The sidebranching signals appear composed of randomly distributed bursts in which sidebranching coherence is surprisingly large and robust. This is attested by the large autocorrelation found in single bursts and the large cross-correlation found in any couple of bursts, even belonging to different sides of a dendrite or to different dendrites. However, the phase coherence of sidebranching breaks down at the transition between bursts. This restricts the coherence of extended sidebranching signals to a mean burst length and prevents the occurrence of large scale cross-correlation between them. This balanced view on sidebranching coherence stresses the capability of self-organization of dendrites in material science and sheds light on the nature of sidebranching on curved growing forms.

Distinct actions of etomidate and propofol at β3-containing γ-aminobutyric acid type A receptors

Etomidate and propofol have clearly distinguishable effects on the central nervous system. However, studies in knock-in mice provided evidence that these agents produce anesthesia via largely overlapping molecular targets, namely GABA A receptors containing β3 subunits. Here the authors address the question as to whether etomidate and propofol are targeting different subpopulations of β3 subunit containing GABA A receptors. The effects of etomidate and propofol (0.5 μM and 1.0 μM) on spontaneous activity of neocortical neurons were investigated in organotypic slice cultures from wild-type and β3(N265M) knock-in mice. Firing patterns were characterized by mean burst length and number of action potentials per burst. Additionally, etomidate and propofol actions on GABA A receptor-mediated currents were investigated by whole-cell voltage clamp recordings. On the network level, the duration of spontaneously occurring bursts of action potentials was decreased by etomidate but increased by propofol in the wild-type. The effects of etomidate were abolished in β3(N265M) mutant slices while those of propofol were qualitatively inverted. On the receptor level, GABA A receptor-mediated inhibition of cortical neurons was modulated by etomidate and propofol in different ways. Again, drug-specific actions of etomidate and propofol were largely attenuated by the β3(N265M) mutation. Etomidate and propofol alter the firing patterns and GABA A receptor-mediated inhibition of neocortical neurons in different ways. This suggests that etomidate and propofol act via non-uniform molecular targets. Because the major effects induced by these anesthetics were attenuated by the β3(N265M) mutation, different subpopulations of β3-containing GABA A receptors are likely to be involved.

Functional Characteristics of alpha3beta3gamma2 and alpha1beta2gamma2 GABA-A receptors

The GABAA receptor is a member of the cys-loop family of ligand gated ion channels and the major mediator of neuronal inhibition. Each receptor is a pentameric protein complex composed of homologous subunits, the combination of which gives rise to numerous GABAA receptor subtypes. Within the thalamus, two synaptic GABAA receptor isoforms predominate; the α1β2γ2 channels, which are found at the synapses of relay neurons, and the α3β3γ2 channels that are located at synapses in the reticular nucleus. We endeavoured to characterize the kinetic properties to these two GABAA receptors. Our preliminary data suggest that both receptors open to a single channel conductance level of 26 pS, irrespective of the GABA concentration used to elicit activity. α3β3γ2 GABAA receptors, however, exhibit longer active periods (bursts) at saturating (5 mM) and low (2 μM) concentrations of GABA compared to α1β2γ2 GABAA receptors. The mean burst length and intra-burst open probability (Po) for the α3β3γ2 channels was 136.7 ± 16.8 ms and 0.84 ± 0.05 (n = 4), respectively and for the α1β2γ2 channels was 87.4 ± 12.0 ms and 0.82 ± 0.02 (n = 3), respectively, when exposed to 5 mM GABA. At 2 μM GABA, the mean burst length and Po for α3β3γ2 channels was 25.8 ± 4.7 ms and 0.78 ± 0.03 (n = 3), respectively and for α1β2γ2 channels was 8.4 ± 1.9 ms and 0.73 ± 0.02 (n = 3), respectively. These measurements are consistent with the reported slower deactivation phase of ensemble, α3β3γ2 mediated synaptic currents and suggest that GABA has a longer occupancy at α3β3γ2 channels than at α1β2γ2 channels.

Duty Time of Rabbit Jaw Muscles Varies with the Number of Activity Bursts

The relative duration of muscle activity during a specified period (duty time) varies depending on activity level and time of the day. Since both the number and the length of activity bursts contribute to the duty time, it was hypothesized that these variables would show intra-day variations similar to those of the duty time. To test this, we determined duty times, burst numbers, and burst lengths per hour, in relation to multiple activity levels, in a 24-hour period of concurrent radio-telemetric long-term electromyograms of various rabbit jaw muscles. The marked intra-day variation of the burst number resembled that of the duty time in all muscles, and was in contrast to the relatively invariable mean burst length. Furthermore, the duty times were more highly correlated with the number than with the length of bursts at all activity levels. Thus, the variation of the duty time in rabbit jaw muscles is caused mainly by changes in burst numbers.

Weighted Viterbi decoding strategies for distributed speech recognition over IP networks

The presence of burst-like packet losses may produce a serious degradation of the performance of Distributed Speech Recognition systems over IP networks. Several strategies that exploit high speech-signal correlation have already been proposed to overcome this problem. One of the most common mechanisms is to fill in the gap using the correctly received frames, as in nearest frame repetition (NFR), the mechanism proposed in the ETSI Aurora standard. Other strategies involve the reconstruction of the lost segment using interpolation algorithms or even more complex mechanisms. However, the effectiveness of these strategies may be seriously compromised if losses appear in long bursts. Interpolation may simply become unfeasible if the lost segment is a lengthy one. In repetition algorithms, on the other hand, the unreliability of the substituted frame produces a high insertion rate that can reduce the word error rate to the point where the mechanism becomes counterproductive. A feasible strategy for palliating this effect is to use a soft-decoding algorithm. In this kind of strategy, the reliability of the frame is taken into account at the decoding stage, aiming to mitigate the effect of the incorrect vectors in the recognition stage. In this paper we present a comprehensive study of the performance of some soft-decoding weighted Viterbi algorithms in a burst-like network environment. Tests will be conducted using the Aurora 3 framework working over three simulated network conditions, with the NFR algorithm as the baseline approach. Three techniques will be thoroughly explored. The first one uses a fixed weighting coefficient along the burst. This improves the results obtained by the basic NFR algorithm, but with the drawback, however, of showing a strong dependence on mean burst length. The application of a varying weighting coefficient is proposed next as a way to solve this problem. Two strategies—one heuristic and one data-driven—are proposed to obtain the variation in the weighting coefficient. Finally, a third method that exploits the different degree of correlation of individual components of the feature vector is presented.

Single Channel Analysis Reveals Different Modes of Kv1.5 Gating Behavior Regulated by Changes of External pH

In the voltage-gated potassium channel Kv1.5, extracellular acidification decreases the peak macroscopic conductance and accelerates slow inactivation. To better understand the mechanistic basis for these two effects, we recorded unitary currents of Kv1.5 expressed in a mouse cell line ( ltk −) using the voltage clamp technique both in cell-attached and excised outside-out patches. Single channel current amplitude at 100 mV (1.7 ± 0.2 pA at pH 7.4, 1.7 ± 0.2 pA at pH 6.4) and the single channel conductance between 0 and 100 mV (11.8 ± 0.6 pS at pH 7.4 and 11.3 ± 0.8 pS at pH 6.4) did not change significantly with pH. External acidification significantly decreased the number of active sweeps, and this reduction in channel availability accounted for most of the reduction of the peak macroscopic current. The results of runs analyses suggested the null sweeps occur in clusters, and the rate constants for the transition between clusters of null and active sweeps at pH 6.4 were slow (0.12 and 0.18 s −1, to and from the active clusters, respectively). We propose that low pH facilitates a shift from an available mode (mode A) into an unavailable mode of gating (mode U). In addition to promoting mode U gating, external acidification accelerates depolarization-induced inactivation, which is manifest at the single channel level as a reduction of the mean burst length and an apparent increase of the interburst interval. These effects of external acidification, which are thought to reflect the protonation of a histidine residue in the turret (H-463), point to an important role for the turret in the regulation of channel availability and inactivation.

Burst characteristics of daily jaw muscle activity in juvenile rabbits

Muscle activation varies with different behaviors and can be quantified by the level and duration of activity bursts. Jaw muscles undergo large anatomical changes during maturation, which are presumably associated with changes in daily muscle function. Our aim was to examine the daily burst number, burst length distribution and duty time (fraction of the day during which a muscle was active) of the jaw muscles of juvenile male rabbits (Oryctolagus cuniculus). A radio-telemetric device was implanted to record muscle activity continuously from the digastric, superficial and deep masseter, medial pterygoid and temporalis during maturation week 9-14. Daily burst characteristics and duty times were determined for activations, including both powerful and non-powerful motor behavior. All muscles showed constant burst numbers, mean burst lengths and duty times during the recording period. Including all behavior, the temporalis showed significantly larger daily burst numbers (205,000) and duty times (18.2%) than the superficial and deep masseter (90,000; 7.5%). Burst numbers and duty times were similar for the digastric (120,000; 11.1%) and medial pterygoid (115,000; 10.4%). The temporalis and deep masseter showed many short low activity bursts (0.05 s), the digastric showed many long bursts (0.09 s). For activations during powerful behaviors the superficial masseter and medial pterygoid had the largest burst numbers and duty times. Both muscles showed similar burst characteristics for all activation levels. It was concluded that activation of the jaw muscles is differently controlled during powerful and non-powerful motor behaviors and the functional organization of motor control patterns does not vary from 9 to 14 weeks of age.

Aggravation of Absence Seizures by Carbamazepine in a Genetic Rat Model Does Not Induce Neuronal c-Fos Activation

The mechanisms underlying carbamazepine aggravation of absence seizures are uncertain but are thought to involve enhancement of neuronal activity within the thalamocortical circuitry. We used c-Fos immunohistochemistry (cFos-ir) to examine patterns of neuronal activation and the relationship to seizure expression following administration of carbamazepine in a rat model of absence epilepsy (Genetic Absence Epilepsy Rats of Strasbourg, GAERS). Female ovariectomized GAERS implanted with extradural EEG electrodes received either 20 mg/kg carbamazepine or vehicle IP. Seizure expression was quantified by measuring the total number and duration of spike-wave discharges (SWD) and with the individual burst discharge lengths over a 90-minute EEG. This was correlated with cFos-ir in thalamocortical slices from rats killed 180 minutes after carbamazepine administration. Carbamazepine-treated rats (n = 5) had a significantly greater total duration of SWD than vehicle-treated rats (17.9% versus 8.8%, P = 0.04). Despite this aggravation of seizures, the level of cFos-ir did not differ between the treatment groups. A positive correlation was found between cFos-ir in the reticularis thalami (Rt) and the total seizure duration (R = 0.66, P = 0.04) and mean burst length (R = 0.68, P = 0.03) but not total number of seizures. The lack of difference in cFos activation patterns between carbamazepine and vehicle-treated animals suggests that the mechanism for carbamazepine aggravation of absence seizures may not involve neuronal activation but rather enhanced neuronal synchronization. The association between increased neuronal activation in the Rt and seizure burden in GAERS provides further support for the critical role of this structure in the maintenance, but not initiation, of absence seizure activity.

An Accurate Model for Evaluating Blocking Probabilities in Multi-Class OBS Systems

This letter presents an analytical model that can be used to evaluate the blocking probability of each service class in a multi-class optical burst-switching network. The model allows, for the first time, the evaluation of blocking probabilities in optical burst switching (OBS) systems with arbitrary burst-length distributions and arbitrary offsets. This includes OBS systems in which the mean burst length of each class is different. Such systems do not follow the conservation law and cannot, therefore, be analyzed using previously published OBS models. For an OBS system with two classes, an offered load of 10/sup -3/, and a 1:5 ratio of high-priority to low-priority traffic, we show that our model accurately predicts the blocking probability for each class, whereas predictions from previously published models that assume conservation disagree with the simulation results by as much as 75%.

The Effects of β3 Subunit Incorporation on the Pharmacology and Single Channel Properties of Oocyte-expressed Human α3β4 Neuronal Nicotinic Receptors

We compared the main properties of human recombinant alpha3beta4beta3 neuronal nicotinic receptors with those of alpha3beta4 receptors, expressed in Xenopus oocytes. beta3 incorporation decreased the channel mean open time (from 5.61 to 1.14 ms, after approximate correction for missed gaps) and burst length. There was also an increase in single channel slope conductance from 28.8 picosiemens (alpha3beta4) to 46.7 picosiemens (alpha3beta4beta3; in low divalent external solution). On the other hand, the calcium permeability (determined by a reversal potential method in chloride-depleted oocytes) and the pharmacological properties of beta3-containing receptors differed little from those of alpha3beta4. The main pharmacological difference in alpha3beta4beta3 "triplet" receptors was a 3-fold decrease in the potency of lobeline relative to acetylcholine. Nevertheless, there was no change in the rank order of potency for agonists (epibatidine >> lobeline > cytisine, 1,1-dimethyl-4-phenylpiperazinium iodide, nicotine > acetylcholine > carbachol for both receptors; measured at low agonist concentrations). Sensitivity to the competitive antagonists trimetaphan (0.2-1 microM) and dihydro-beta-erythroidine (30 microM) was similar for the two combinations, with a Schild KB for trimetaphan of 76 and 66 nM on alpha3beta4 and alpha3beta4beta3, respectively. The change in single channel conductance confirms that beta3 replaces a beta4 subunit in the pentamer. The absence of pronounced differences in the pharmacological profile of the triplet receptor argues against a role for the beta3 subunit in the formation of agonist binding sites, whereas the changes in channel kinetics suggest an important effect on receptor gating. The shortening of the burst length of beta3-containing receptors implies that any synaptic currents mediated by such channels would have faster decay kinetics.

Openings of the rat recombinant alpha 1 homomeric glycine receptor as a function of the number of agonist molecules bound.

The functional properties of rat homomeric α1 glycine receptors were investigated using whole-cell and outside-out recording from human embryonic kidney cells transfected with rat α1 subunit cDNA. Whole-cell dose-response curves gave EC 50 estimates between 30 and 120 μM and a Hill slope of ∼3.3. Single channel recordings were obtained by steady-state application of glycine (0.3, 1, or 10 μM) to outside-out patches. Single channel conductances were mostly 60–90 pS, but smaller conductances of ∼40 pS were also seen (10% of the events) with a relative frequency that did not depend on agonist concentration. The time constants of the apparent open time distributions did not vary with agonist concentration, but short events were more frequent at low glycine concentrations. There was also evidence of a previously missed short-lived open state that was more common at lower glycine concentrations. The time constants for the different components of the burst length distributions were found to have similar values at different concentrations. Nevertheless, the mean burst length increased with increasing glycine. This was because the relative area of each burst-length component was concentration dependent and short bursts were favored at lower glycine concentrations. Durations of adjacent open and shut times were found to be strongly (negatively) correlated. Additionally, long bursts were made up of longer than average openings separated by short gaps, whereas short bursts usually consisted of single isolated short openings. The most plausible explanation for these findings is that long bursts are generated when a higher proportion of the five potential agonist binding sites on the receptor is occupied by glycine. On the basis of the concentration dependence and the intraburst structure we provide a preliminary kinetic scheme for the activation of the homomeric glycine receptor, in which any number of glycine molecules from one to five can open the channel, although not with equal efficiency.

Single channel properties of P2X ATP receptors in outside-out patches from rat hippocampal granule cells.

The single channel properties of P2X ATP receptors were investigated in outside-out patches from hippocampal granule cells in brain slices from 12-day-old rats. The results demonstrate that functional P2X ATP receptors are expressed in hippocampal granule cells and, combined with previously published information on the P2X subunits expressed in the hippocampus, suggest that the receptors may be heteromers of the P2X4 and P2X6 subunits or P2X1, P2X2, P2X4 and P2X6 subunits. Two distinct types of P2X channel openings were observed. A flickery P2X receptor channel was observed in three patches with a mean chord conductance of 32 +/- 6 pS, a mean open time of 1.0 +/- 0.3 ms and a mean burst length of 11 +/- 5 ms at a membrane potential of -60 mV. A large conductance P2X receptor was observed in 19 out of 98 patches with a mean conductance of 56 +/- 1. 8 pS, a linear current-voltage relationship between -80 and +60 mV with a reversal potential around 0 mV, a mean open time of 2.6 +/- 0. 2 ms and a mean burst length of 8.8 +/- 1.8 ms at -60 mV. At an ATP concentration of 1 mM, these channels exhibited a low steady-state open probability (Popen, 0.07 +/- 0.008; n = 15), little apparent desensitisation and were also activated by alpha,beta-methylene ATP (alpha,beta-meATP, 40 microM; Popen, 0.007 +/- 0.0002; conductance, 57 +/- 1.1 pS; n = 3). No decrease in the single channel conductance was observed on increasing the free extracellular calcium concentration from 0.3 to 0.85 mM. Channel closed time distributions were fitted with five exponential components with time constants (and relative areas) of 90 micros (20 %), 0.77 ms (32 %), 10 ms (15 %), 90 ms (18 %) and 403 ms (15 %) at 1 mM ATP. Of these, the first two components are suggested to represent gaps within single activations of the receptor based on the lack of agonist concentration dependence of these two shut time components between 1 microM and 1 mM ATP. Suramin (40 microM) significantly increased the single channel conductance (19 +/- 7 %; n = 5) and produced a small decrease in Popen (39 +/- 9 %; n = 5) by decreasing mean open time, burst length and total open time per burst. These actions of suramin are not consistent with simple competitive antagonism.