Slow Time Constants Research Articles

Local activity in dendrites controls STDP by altering NMDA receptor kinetics

Spike timing-dependent plasticity (STDP) is a temporally asymmetric form of Hebbian plasticity in which the sign and magnitude of synaptic strength is determined by the precise timing between pre- and postsynaptic spikes. Postsynaptic NMDA receptors (NMDARs) play an important role in STDP due to their characteristic magnesium ion (Mg2+) gating process [1]. The Mg2+ gate is opened by near coincident presynaptic glutamate release and postsynaptic spiking. Contrary to previous belief, this gating process is not instantaneous but has distinct fast and slow time constants, which might influence the time window over which spike-EPSP coincidence occurs [2,3]. The aim of this study is to investigate how local activity in dendrites alters NMDAR kinetics and in turn controls the time window over which STDP occurs. Using a detailed biophysical model of an NR2A subunit, we find the NMDAR current to be temporally asymmetric with a faster block than unblock process. This is due to the voltage-dependent fast and slow time constants of Mg2+ gating. These time constants were on the order of 0.8 ms and 5 ms respectively and changed nonlinearly with membrane potential. By varying both amplitude and duration of a model action potential (AP), we find that dendritic Ca2+ spikes cause Mg2+ unblock to be dominated by its fast time constant. Conversely, somatic Na+ spikes elicit a slower unblocking. Since backpropagating APs (BAPs) attenuate along the dendritic arbor, our results suggest that NMDAR kinetics vary depending on synapse location. Next, we incorporated this biophysical model into a compartmentalized model of a neocortical pyramidal cell in order to investigate the effect that dendritic inhibition and synapse location has on STDP. NMDAR kinetics is significantly different in the basal and apical compartments, which gives rise to a heterogeneous set of STDP curves along the length of the dendrite. Distal synapses had a broader potentiation window than apical synapses, suggesting that distal branches are less selective to correlated inputs. This synapse-location dependent STDP learning rule agrees with previous findings [4]. Dendritic inhibition plays an important role in shaping the computations performed by pyramidal neurons in the neocortex. However, the role that inhibition plays in controlling synaptic plasticity is less clear. By incorporating inhibitory synapses in our model, we find that varying the timing of inhibition relative to the EPSP shifted the peak of the STDP curve, resulting in a reversal of the temporal order of depression and potentiation. Inhibition altered NMDAR kinetics by increasing its blocking rate. As a result, changing the balance between excitation and inhibition in the dendritic branch results in a sliding threshold between LTP and LTD. In summary, we show that NMDAR kinetics can be altered in an activity-dependent manner resulting in different STDP learning rules. When taken in context of the BCM theory, our finding implies that dendritic inhibition can dynamically alter the threshold between LTP and LTD by altering the time constants of the NMDAR. Our model provides a novel framework for understanding how STDP in different dendritic compartments selects inputs alters information processing in individual neurons.

Influence of stimulus interval on the habituation of vestibulo-ocular reflex and sensation of rotation in humans

Previous studies in cats revealed that vestibular habituation of the vestibulo-ocular reflex (VOR) only occurs when velocity steps are delivered during the secondary phase nystagmus, suggesting that the presence of anti-compensatory slow phases may trigger the habituation process. We verified this property in humans by comparing vestibular habituation of VOR and sensation of rotation when steps were delivered either immediately after the perception of self-rotation had stopped, which is shortly before the nystagmus reverses direction; or when steps were delivered 60s later, i.e. during the secondary phase. Vestibular habituation of the VOR occurred in both instances. However, the decrease in VOR peak slow phase velocity and time constant was larger when steps were delivered after nystagmus reversal compared to before nystagmus reversal. The duration of the perception of self-rotation habituated equally for both conditions. These results confirm that VOR habituation fully develops only when velocity steps are delivered after the primary phase nystagmus. This finding may be helpful for minimizing the impact of repetitive vestibular stimuli in protocols using crossover design for drug studies, testing recovery in vestibular patients, or training people for different gravitoinertial environments.

Dynamics of donor bound excitons in ZnO

Comprehensive time-resolved photoluminescence measurements are performed on shallow neutral donor bound excitons (D0Xs) in bulk ZnO. It is found that transients of the no-phonon D0X transitions (I6-I9 lines) are largely affected by excitation conditions and change from a bi-exponential decay with characteristic fast (τf) and slow (τs) time constants under above-bandgap excitation to a single exponential one, determined by τs, under two-photon excitation. The slow decay also dominates transients of longitudinal optical phonon-assisted and two-electron-satellite D0X transitions, and is attributed to “bulk” D0X lifetime. The fast component is tentatively suggested to represent effects of surface recombination.

Allocryptopine and benzyltetrahydropalmatine block hERG potassium channels expressed in HEK293 cells

Allocryptopine (ALL) is an alkaloid extracted from Corydalis decumbens (Thunb) Pers. Papaveraceae, whereas benzyltetrahydropalmatine (BTHP) is a derivative of tetrahydropalmatine extracted from Corydalis ambigua (Pall) Cham et Schlecht. The aim of this study was to investigate the effects of ALL and BTHP on the human ether-a-go-go related gene (hERG) current expressed in HEK293 cells. Cultured HEK293 cells were transiently transfected with hERG channel cDNA plasmid pcDNA3.1 using Lipofectamine. The whole-cell current IHERG was evoked and recorded using Axon MultiClamp 700B amplifier. The drugs were applied via supserfusion. Both ALL and BTHP reversibly suppressed the amplitude and density of IHERG in concentration- and voltage-dependent manners (the respective IC50 value was 49.65 and 22.38 μmol/L). BTHP (30 μmol/L) caused a significant negative shift of the steady-state inactivation curve of IHERG, while ALL (30 μmol/L) did not affect the steady-state inactivation of IHERG. Furthermore, BTHP, but not ALL, shortened the time constants of fast inactivation and slow time constants of deactivation of IHERG. But both the drugs markedly lengthened the time constants for recovery of IHERG from inactivation. Using action potential waveform pulses, it was found that both the drugs at 30 μmol/L significantly suppressed the current densities in the late phase of action potential, but did not significantly affect the current densities in the early phase of action potential. Both ALL and BTHP derived from Chinese herbs potently block hERG current.

Photorefractive properties of Fe, Zn co-doped near stoichiometric LiNbO3 crystals at moderate intensities (0.5–6 W/cm2)

Iron and zinc co-doped near stoichiometric lithium niobate (SLN:Fe,Zn) single crystals were grown by the top seeded solution growth technique from Li-rich flux. The Raman scattering analysis confirmed the near stoichiometric composition (Li/Nb∼0.98) whereas the birefringence interferometry revealed the optical homogeneity to be better than 5.2×10−5/mm for the grown crystals. Two beam coupling measurements showed the writing time constant to be 26–3.6s measured in the intensity range 0.5–6W/cm2. The slow and fast erasure time constants as obtained from bi-exponential fit are ∼22–4 and ∼200–24s, respectively. Interestingly, the fast eraser time is found to be nearly the same as the writing time. The improvement in photorefractive sensitivity (∼0.16cm/J) has been observed. The light induced changes in the refractive index (Δn∼5.8×10−5) are found to be in agreement with the estimated value. The intensity dependent photoconductivity estimated from the two beam coupling experiment is found to vary from 1.12×10–11Ω−1m−1 to 6.20×10−11Ω−1m−1 due to primary defects and from 1.24×10−12Ω−1m−1 to 1.04×10−11Ω−1m−1 due to secondary defects.

Effects of fluoxetine on cloned Kv4.3 potassium channels

Fluoxetine is widely used for the treatment of depression. We examined the action of fluoxetine on cloned Kv4.3 stably expressed in CHO cells using the whole-cell patch-clamp technique. Fluoxetine did not significantly produce a reduction in the peak amplitude of Kv4.3, but increased the rate of current inactivation in a concentration-dependent manner. Thus, the effect of fluoxetine on Kv4.3 was measured from the integral of the current during the depolarizing pulse. The integral of Kv4.3 was reduced by fluoxetine in a concentration-dependent manner with an IC50 of 11.8μM. Using first-order kinetics analysis, the apparent association and dissociation rate constants were 1.5μM−1s−1 and 22.2s−1, respectively, with a KD of 14.2μM, similar to the IC50 value calculated from the concentration-response curve. Under control conditions, the inactivation of Kv4.3 was best fit by a biexponential function. The fast and slow time constants were significantly decreased in the presence of fluoxetine. Time-to-peak and activation kinetics were significantly accelerated by fluoxetine. The block of Kv4.3 by fluoxetine became more prominent as the membrane potential became more depolarized, displaying a shallow voltage dependence (δ=0.29) in the full activation voltage range. Fluoxetine did not affect the steady-state inactivation curves, but significantly accelerated the closed-state inactivation of Kv4.3. The block of Kv4.3 by fluoxetine was use-dependent during repetitive stimulation, which explained the slowing of the recovery from inactivation of Kv4.3. Our results indicate that fluoxetine blocks Kv4.3 by preferentially interacting with the open and accelerating closed-state inactivation of the channel.

Myoplasmic Calcium (Ca) Movements during Relaxation and Recovery of Superfast Muscle Fibers of the Toadfish Swimbladder (TSB)

Tsb fibers produce high-frequency contractions (80-100 Hz at 16 oC) that generate the fish's cyclical mating call (∼500 ms duration, ∼10 s intercall interval). Our aim was to understand Ca movements during the intercall interval by measuring Ca transients elicited by action potentials (APs). Fluo-4 was micro-injected into one fiber within a dissected bundle. With one AP, the peak, time of peak, and temporal half-width of ΔF/FR (change in indicator fluorescence divided by resting fluorescence) were ∼20, ∼9 ms, and ∼14 ms, respectively. The remaining analysis considered the late time course of ΔF. From 1 to 60 s after cessation of stimulation, ΔF was well fitted by a double-exponential decay plus a small offset. With a 40-AP train at 83 Hz, the two exponential components had approximately equal amplitudes; the fast and slow time constants were ∼2 and ∼10 s. ΔF was well simulated with our kinetic model of Ca movements in tsb fibers (model 3; Rome et al., J. Physiol., 2011) extended to the 60 s time scale. With one AP, the estimated amount of SR Ca released is ∼750 μM (concentration referred to the myoplasmic water volume) and fM(1) (the amount of the released Ca still in the myoplasm 1 s after cessation of stimulation) is ∼630 μM, 97% of which is bound to parvalbumin. With 40 APs and a Ca release of ∼5.5 mM, fM(1) is ∼3.2 mM, with 92% bound to parvalbumin. The calling cycle thus appears to be constrained by Ca accumulation on parvalbumin and the slow rate of Ca pumping that ensues when parvalbumin lowers free [Ca] close to the resting level. Supported by NIH (GM 086167) and NSF (IOS-1145981).

Observing the Temperature Dependent Transition of the GP2 Peptide Using Terahertz Spectroscopy

The GP2 peptide is derived from the Human Epidermal growth factor Receptor 2 (HER2/nue), a marker protein for breast cancer present in saliva. In this paper we study the temperature dependent behavior of hydrated GP2 at terahertz frequencies and find that the peptide undergoes a dynamic transition between 200 and 220 K. By fitting suitable molecular models to the frequency response we determine the molecular processes involved above and below the transition temperature (T D). In particular, we show that below T D the dynamic transition is dominated by a simple harmonic vibration with a slow and temperature dependent relaxation time constant and that above T D, the dynamic behavior is governed by two oscillators, one of which has a fast and temperature independent relaxation time constant and the other of which is a heavily damped oscillator with a slow and temperature dependent time constant. Furthermore a red shifting of the characteristic frequency of the damped oscillator was observed, confirming the presence of a non-harmonic vibration potential. Our measurements and modeling of GP2 highlight the unique capabilities of THz spectroscopy for protein characterization.

Aging and stiction dynamics in confined films of a star polymer melt

The stiction properties of a star polyisoprene (PIP) melt (having 22 arms and an arm molecular weight of around 5000, M(w) ≈ 110,000) confined between mica surfaces were investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P; the time constants of the phase transitions in the stiction dynamics (freezing on stopping and melting on starting) were obtained from the force relaxation behaviors. The results were compared with those of a confined linear-PIP melt (M(w) ≈ 48,000) and other confined fluid systems; the effect of star architecture on the phase transitions in confinement during aging is discussed. Estimation of the molecular size gives that the confined star-PIP films consist of three molecular layers; a non-adsorbed layer sandwiched between two layers adsorbed on opposed mica surfaces. There are (at least) four time constants in the freezing transition of the confined star-PIP melt; fast (τ(1)) and slow (τ(2)) time constants for lateral force relaxation on stopping, critical aging time for freezing (τ(f)), and the logarithmic increase of the spike height against t. The three time constants on stopping, τ(1), τ(2), and τ(f), increase with the increase of P (decrease of the thickness D). As regards the melting transition on starting, spike force decay was fitted by a single exponential function and one time constant was obtained, which is insensitive to P (D). Comparison of the time constants between freezing and melting, and also with the results of linear-PIP reveals that the stiction dynamics of the star-PIP system involves the relaxation and rearrangement of segmental-level and whole molecular motions. Lateral force relaxation on stopping is governed by the individual and cooperative rearrangements of local PIP segments and chain ends of the star, which do not directly lead to the freezing of the system. Instead, geometrical rearrangements of the soft star-PIP spheres into dense packing between surfaces (analogous to the concept of a colloidal glass transition) are the major mechanism of the freezing transition (stiction) after aging. Interdigitation of PIP segments/chain ends between neighboring star molecules also contributes to the spike growth along with aging, and the melting transition on starting.

Variation in sodium current amplitude between vasopressin and oxytocin hypothalamic supraoptic neurons

Biophysical characteristics of tetrodotoxin-sensitive sodium (Na(+)) currents were studied in vasopressin (VP) and oxytocin (OT) supraoptic neurons acutely isolated from rat hypothalamus. Na(+) current density (pA/pF) was significantly greater in VP neurons than in OT neurons. No significant difference between VP and OT neurons was detected regarding the voltage dependence of activation and steady-state inactivation, or rate of recovery from inactivation of Na(+) currents. In both VP and OT neurons, the macroscopic inactivation of the Na(+) currents was best fitted with a double-exponential expression suggesting two rates of inactivation. Also in both types, the time course of recovery from inactivation proceeded with fast and slow time constants averaging around 8 and 350 ms, respectively, suggesting the presence of multiple pathways of recovery from inactivation. The slower time constant of recovery of inactivation may be involved in the decrease in action potential (AP) amplitude that occurs after the first spike during burst firing in both neuronal types. The larger amplitude of Na(+) currents in VP vs. OT neurons may explain the previous observations that VP neurons exhibit a lower AP threshold and greater AP amplitude than OT neurons, and may serve to differently tune the firing properties and responses to neuromodulators of the respective neuronal types.

Dynamics of regional lung aeration determined by electrical impedance tomography in patients with acute respiratory distress syndrome

Background: Lung tissue of patients with acute respiratory distress syndrome (ARDS) is heterogeneously damaged and prone to develop atelectasis. During inflation, atelectatic regions may exhibit alveolar recruitment accompanied by prolonged filling with air in contrast to regions with already open alveoli with a fast increase in regional aeration. During deflation, derecruitment of injured regions is possible with ongoing loss in regional aeration. The aim of our study was to assess the dynamics of regional lung aeration in mechanically ventilated patients with ARDS and its dependency on positive end-expiratory pressure (PEEP) using electrical impedance tomography (EIT). Methods: Twelve lung healthy and twenty ARDS patients were examined by EIT during sustained step increases in airway pressure from 0, 8 and 15 cm H2O to 35 cm H2O and during subsequent step decrease to the corresponding PEEP. Regional EIT waveforms in the ventral and dorsal lung regions were fitted to bi-exponential equations. Regional fast and slow respiratory time constants and the sizes of the fast and slow compartments were subsequently calculated. Results: ARDS patients exhibited significantly lower fast and slow time constants than the lung healthy patients in ventral and dorsal regions. The time constants were significantly affected by PEEP and differed between the regions. The size of the fast compartment was significantly lower in ARDS patients than in patients with healthy lung under all studied conditions. Conclusion: These results show that regional lung mechanics can be assessed by EIT. They reflect the lower respiratory system compliance of injured lungs and imply more pronounced regional recruitment and derecruitment in ARDS patients.

Dynamics of regional lung aeration determined by electrical impedance tomography in patients with acute respiratory distress syndrome

BackgroundLung tissue of patients with acute respiratory distress syndrome (ARDS) is heterogeneously damaged and prone to develop atelectasis. During inflation, atelectatic regions may exhibit alveolar recruitment accompanied by prolonged filling with air in contrast to regions with already open alveoli with a fast increase in regional aeration. During deflation, derecruitment of injured regions is possible with ongoing loss in regional aeration. The aim of our study was to assess the dynamics of regional lung aeration in mechanically ventilated patients with ARDS and its dependency on positive end-expiratory pressure (PEEP) using electrical impedance tomography (EIT).MethodsTwelve lung healthy and twenty ARDS patients were examined by EIT during sustained step increases in airway pressure from 0, 8 and 15 cm H2O to 35 cm H2O and during subsequent step decrease to the corresponding PEEP. Regional EIT waveforms in the ventral and dorsal lung regions were fitted to bi-exponential equations. Regional fast and slow respiratory time constants and the sizes of the fast and slow compartments were subsequently calculated.ResultsARDS patients exhibited significantly lower fast and slow time constants than the lung healthy patients in ventral and dorsal regions. The time constants were significantly affected by PEEP and differed between the regions. The size of the fast compartment was significantly lower in ARDS patients than in patients with healthy lung under all studied conditions.ConclusionThese results show that regional lung mechanics can be assessed by EIT. They reflect the lower respiratory system compliance of injured lungs and imply more pronounced regional recruitment and derecruitment in ARDS patients.

Expression and properties of hyperpolarization-activated current in rat dorsal root ganglion neurons with known sensory function.

The hyperpolarization-activated current (Ih) has been implicated in nociception/pain, but its expression levels in nociceptors remained unknown. We recorded Ih magnitude and properties by voltage clamp from dorsal root ganglion (DRG) neurons in vivo, after classifying them as nociceptive or low-threshold-mechanoreceptors (LTMs) and as having C-, Aδ- or Aα/β-conduction velocities (CVs). For both nociceptors and LTMs, Ih amplitude and Ih density (at −100 mV) were significantly positively correlated with CV. Median Ih magnitudes and Ih density in neuronal subgroups were respectively: muscle spindle afferents (MSAs): −4.6 nA, −33 pA pF−1; cutaneous Aα/β LTMs: −2.2 nA, −20 pA pF−1; Aβ-nociceptors: −2.6 nA, −21 pA pF−1; both Aδ-LTMs and nociceptors: −1.3 nA, ∼−14 pA pF−1; C-LTMs: −0.4 nA, −7.6 pA pF−1; and C-nociceptors: −0.26 nA, −5 pA pF−1. Ih activation slow time constants (slow τ values) were strongly correlated with fast τ values; both were shortest in MSAs. Most neurons had τ values consistent with HCN1-related Ih; others had τ values closer to HCN1+HCN2 channels, or HCN2 in the presence of cAMP. In contrast, median half-activation voltages (V0.5) of −80 to −86 mV for neuronal subgroups suggest contributions of HCN2 to Ih. τ values were unrelated to CV but were inversely correlated with Ih and Ih density for all non-MSA LTMs, and for Aδ-nociceptors. From activation curves ∼2–7% of Ih would be activated at normal membrane potentials. The high Ih may be important for excitability of A-nociceptors (responsible for sharp/pricking-type pain) and Aα/β-LTMs (tactile sensations and proprioception). Underlying HCN expression in these subgroups therefore needs to be determined. Altered Ih expression and/or properties (e.g. in chronic/pathological pain states) may influence both nociceptor and LTM excitability.

Modeling and prediction of conduction delay in an unmyelinated axon

Conduction delay in axons is a function of the axon morphology, passive membrane properties and voltage-gated ionic currents. Although conduction delay is usually assumed to be constant, indicating perfect temporal fidelity, recent work has shown that the delay of each action potential depends on the prior short- and long-term history of activity in the axon [1]. Violation of temporal fidelity leads to substantial variations in inter-spike intervals which has potential impact on temporal neural activity. In experiments using the motor axon of the pyloric dilator (PD) neuron of the lobster, H. americanus, we measured conduction delay using Poisson random stimuli applied at different mean rates [2]. These measurements showed that the mean value and the variance of conduction delay increases as a function of stimulus time until these values reach a steady state at about 5 minutes post-stimulation. Even at steady state, conduction delay showed a non-monotonic relationship with stimulus frequency (finst): it was higher at small or large values of finst but has a minimum at finst~45 Hz. Moreover, bath application of dopamine drastically improved the temporal fidelity of conduction delay. We have constructed a conductance-based model of the PD axon to examine the role of different ionic currents in shaping the history dependence of conduction delay. In addition to the standard Hodgkin-Huxley currents, this model includes a transient potassium current IA and a hyperpolarization activated inward current Ih, both of which have been characterized in this axon. The main effect of dopamine in the biological axon is an increase of Ih which counteracts the activity-dependent hyperpolarization [3], mimicked by adding a Na+/K+ pump in the model. We find that the Na+/K+ pump enables the model to reproduce the long-term history dependence observed in the biological PD axon. A pump current with a slow time constant (τ=90 sec) results in a slow increase of the mean and CV of conduction delay with stimulus time (Fig. ​(Fig.1A),1A), and a non-monotonic relationship between conduction delay and finst (Fig. ​(Fig.1B),1B), as observed experimentally [2]. Additionally, as in the experiments, temporal fidelity is improved as Ih is increased. Figure 1 Changes in action potential delay in the model axon in response to 5 mins of Poisson stimulation (mean rate 10 Hz). Compare with experimental data [2]. A. Both mean and CV of conduction delay increase slowly in response to activity. B. Delay (fifth minute) ... Action potential conduction velocity can be approximated by (d=axon diam, R*=total memb resistance/unit area at peak, Ra=axial resistivity, Cm=capacitance/unit area [4]. Although this equation accurately predicts the conduction velocity of a single action potential in our model, it fails to the variations in conduction velocity in the presence of the history-dependent effects. This discrepancy is partly due to non-uniform conduction velocity along the axon when active at high rates. We revise the formula to predict the relationship between the conduction delay and total membrane resistance at the action potential peak.

Mechanisms Shaping the Slow Nicotinic Synaptic Current at the Motoneuron-Renshaw Cell Synapse

In spinal cord slices from newborn mice we have analyzed the kinetics of the EPSCs mediated by heteromeric nicotinic receptors at the motoneuron-Renshaw cell (MN-RC) synapse. The miniature EPSCs decay with a time constant of 13.0 ± 1.1 ms whereas the decay of the evoked EPSCs (eEPSCs) is biphasic, with time constants of 15.6 ± 0.8 and 124.8 ± 9.0 ms. The slow component becomes prominent during a repetitive stimulation, but its time constant is unchanged. It is selectively reduced by the addition of acetylcholinesterase (AChE), and thus appears to involve ACh spillover. The constancy of the slow time constant during a train is best explained by a local spillover activating high-affinity receptors. In many cells a fraction of the eEPSC originates in neighboring RCs and is transmitted by the low-pass filter of the gap junctions. The component transmitted electrically can be eliminated by meclofenamic acid, a blocker of gap junctions. The local spillover produced by a repetitive stimulation was compared with the long-range spillover produced by inactivation of AChE. The pharmacological inactivation of AChE by neostigmine caused the appearance of an ultra-slow (second range) decay component in eEPSCs and also a continuous inward current interpreted as resulting from a continuous ACh presence. In animals lacking functional AChE in the CNS (PRiMA(-/-) mice) the EPSCs resembled those observed in neostigmine but the steady inward current was much smaller, suggesting an adaptation to the absence of AChE.

Hyperconnectivity and Slow Synapses during Early Development of Medial Prefrontal Cortex in a Mouse Model for Mental Retardation and Autism

Neuronal theories of neurodevelopmental disorders (NDDs) of autism and mental retardation propose that abnormal connectivity underlies deficits in attentional processing. We tested this theory by studying unitary synaptic connections between layer 5 pyramidal neurons within medial prefrontal cortex (mPFC) networks in the Fmr1-KO mouse model for mental retardation and autism. In line with predictions from neurocognitive theory, we found that neighboring pyramidal neurons were hyperconnected during a critical period in early mPFC development. Surprisingly, excitatory synaptic connections between Fmr1-KO pyramidal neurons were significantly slower and failed to recover from short-term depression as quickly as wild type (WT) synapses. By 4-5 weeks of mPFC development, connectivity rates were identical for both KO and WT pyramidal neurons and synapse dynamics changed from depressing to facilitating responses with similar properties in both groups. We propose that the early alteration in connectivity and synaptic recovery are tightly linked: using a network model, we show that slower synapses are essential to counterbalance hyperconnectivity in order to maintain a dynamic range of excitatory activity. However, the slow synaptic time constants induce decreased responsiveness to low-frequency stimulation, which may explain deficits in integration and early information processing in attentional neuronal networks in NDDs.

The Dose-Dependent Effect of S(+)-Ketamine on Cardiac Output in Healthy Volunteers and Complex Regional Pain Syndrome Type 1 Chronic Pain Patients

Ketamine is used as an analgesic for treatment of acute and chronic pain. While ketamine has a stimulatory effect on the cardiovascular system, little is known about the concentration-effect relationship. We examined the effect of S(+)-ketamine on cardiac output in healthy volunteers and chronic pain patients using a pharmacokinetic-pharmacodynamic modeling approach. In 10 chronic pain patients (diagnosed with complex regional pain syndrome type 1 [CRPS1] with a mean age 43.2 ± 13 years, disease duration 8.4 years, range 1.1 to 21.7 years) and 12 healthy volunteers (21.3 ± 1.6 years), 7 increasing IV doses of S(+)-ketamine were given over 5 minutes at 20-minute intervals starting with 1.5 mg with 1.5-mg increments. Cardiac output (CO) was calculated from the arterial pressure curve obtained from an arterial catheter in the radial artery. Ketamine and norketamine plasma concentrations were measured. A novel pharmacokinetic-pharmacodynamic model was constructed to quantify the direct stimulatory effect of ketamine on CO and the following adaptation/inhibition. Significant differences in pharmacokinetic estimates were observed between study groups with 15% and 40% larger S(+)-ketamine S(+)-norketamine concentrations in healthy volunteers compared to CRPS1 patients. S(+)-ketamine had a dose-dependent stimulatory effect on CO in patients and volunteers. After infusion an inhibitory effect on CO was observed. Pharmacodynamic model parameters did not differ between CRPS1 patients and healthy volunteers. The concentration of S(+)-ketamine causing a 1 L/min increase in CO was 243 ± 54 ng/mL with an onset/offset half-life of 1.3 ± 0.21 minutes. The inhibitory component was slow (time constant of 67.2 ± 17.0 minutes). S(+)-ketamine pharmacokinetics but not pharmacodynamics differed between study populations, related to differences in disease state (CRPS1 or not) or age. The dose-dependent effect of S(+)-ketamine on CO was well described by the biphasic dynamic model. The effect of S(+)-ketamine on CO was similar between study groups with respect to its stimulatory and inhibitory components, despite group differences in age and health.

Diffusion of Chlorin-p6 Across Phosphatidyl Choline Liposome Bilayer Probed by Second Harmonic Generation

We have investigated the diffusion of the photosensitizer Chlorin-p(6) (Cp(6)) across a egg lecithin lipid bilayer at different pH by the Second Harmonic Generation (SHG) method. Cp(6) has three ionizable carboxylic acid groups, and consequently, neutral and several ionic forms of Cp(6) are expected to be present in the pH range 3-8. The absorption spectra of Cp(6) get considerably modified in the presence of liposomes as the pH is decreased indicating that the drug liposome binding is pH dependent. The first pK(a) of interconversion (D-C) has been identified at pH ~7.0 by fluorescence measurement in an earlier work. In this work, the second pK(a) of interconversion (C-B) has been identified at pH ~4.8 by the hyper-Rayleigh scattering method. At acidic pH (3, 4, and 5), where species A, B, and C are dominant, the addition of liposomes to a Cp(6) solution generates an instantaneous rise (less than 1 s) in the second harmonic (SH) signal followed by decays whose time constants ranged from ten to hundreds of seconds. The instantaneous rise is attributed to the adsorption of Cp(6) to the outer lipid bilayer, and the decay is attributed to the diffusion of the neutral and charged (A and B) species of the drug. The observed fast and slow time constants for diffusion in the pH range 3-5 are attributed to the neutral (A) and ionic form (B) of Cp(6), respectively. At pH 6, the intensity of the generated SH signals on the addition of liposome reduced, and at physiological pH, it was too weak to be detected. These results are consistent with previous studies that show that the interaction between Cp(6) and egg-PC liposomes is pH dependent. At lower pH due to the presence of the hydrophobic species (A and B) of Cp(6), its interaction with liposomes is strong, and at higher pH, the abundance of the negatively charged hydrophilic species (C and D) decreases the interaction with the like charged liposomes. We have also studied the effect of increasing the bilayer rigidity by decreasing the temperature of the medium or by incorporating 50 mol % cholesterol in the lipid bilayer and observed that lowering of temperature has more profound effect on the diffusion rates. The characteristics of the SH signal changed significantly when liposomes incorporating 50 mol % cholesterol were used at a low (3 °C) temperature. Under these conditions, the SH signal consisted of an instantaneous (<1s) followed by a slower rise (10-90s), and then, it decayed on a much longer time scale. This slow rise of the SH signal at pH 3 and 4 may be attributed to the temperature dependent adsorption of the anionic species (B) of Cp(6) with the liposomes. Further investigations are required in order to understand clearly the pH dependent diffusion of this drug across lipid bilayers.

Neural locus of color afterimages.

After fixating on a colored pattern, observers see a similar pattern in complementary colors when the stimulus is removed [1-6]. Afterimages were important in disproving the theory that visual rays emanate from the eye, in demonstrating interocular interactions, and in revealing the independence of binocular vision from eye movements. Afterimages also prove invaluable in exploring selective attention, filling in, and consciousness. Proposed physiological mechanisms for color afterimages range from bleaching of cone photopigments to cortical adaptation [4-9], but direct neural measurements have not been reported. We introduce a time-varying method for evoking afterimages, which provides precise measurements of adaptation and a direct link between visual percepts and neural responses [10]. We then use in vivo electrophysiological recordings to show that all three classes of primate retinal ganglion cells exhibit subtractive adaptation to prolonged stimuli, with much slower time constants than those expected of photoreceptors. At the cessation of the stimulus, ganglion cells generate rebound responses that can provide afterimage signals for later neurons. Our results indicate that afterimage signals are generated in the retina but may be modified like other retinal signals by cortical processes, so that evidence presented for cortical generation of color afterimages is explainable by spatiotemporal factors that modify all signals.

Fluorescence Correlation Spectroscopy in Drug Discovery: Study of Alexa532-Endothelin 1 Binding to the Endothelin ETAReceptor to Describe the Pharmacological Profile of Natural Products

Fluorescence correlation spectroscopy and the newly synthesized Alexa532-ET1 were used to study the dynamics of the endothelin ETA receptor-ligand complex alone and under the influence of a semisynthetic selective antagonist and a fungal extract on living A10 cells. Dose-dependent increase of inositol phosphate production was seen for Alexa532-ET1, and its binding was reduced to 8% by the selective endothelin ETA antagonist BQ-123, confirming the specific binding of Alexa532-ET1 to the endothelin ETA receptor. Two different lateral mobilities of the receptor-ligand complexes within the cell membrane were found allowing the discrimination of different states for this complex. BQ-123 showed a strong binding affinity to the “inactive” receptor state characterized by the slow diffusion time constant. A similar effect was observed for the fungal extract, which completely displaced Alexa532-ET1 from its binding to the “inactive” receptor state. These findings suggest that both BQ-123 and the fungal extract act as inverse agonists.