Temporal difference thresholds and repetitive stimulation: Can click trains improve temporal sensitivity?

Trains of auditory clicks increase subsequent judgements of stimulus duration by approximately 10%. Scalar timing theory suggests this is due to a 10% increase in pacemaker rate, a main component of the internal clock. The effect has been demonstrated in many timing tasks, including verbal estimation, temporal generalisation, and temporal bisection. However, the effect of click trains has yet to be examined on temporal sensitivity, commonly measured by temporal difference thresholds. We sought to investigate this both experimentally; where we found no significant increase in temporal sensitivity, and computationally; by modelling the temporal difference threshold task according to scalar timing theory. Our experimental null result presented three possibilities which we investigated by simulating a 10% increase in pacemaker rate in a newly-created scalar timing theory model of thresholds. We found that a 10% increase in pacemaker rate led to a significant improvement in temporal sensitivity in only 8.66% of 10,000 simulations. When a 74% increase in pacemaker rate was modelled to simulate the filled-duration illusion, temporal sensitivity was significantly improved in 55.36% of simulations. Therefore, scalar timing theory does predict improved temporal sensitivity for a faster pacemaker, but the effect of click trains (a supposed 10% increase) appears to be too small to be reliably found in the temporal difference threshold task.

The endosomal Q-SNARE, Syntaxin 7, defines a rapidly replenishing synaptic vesicle recycling pool in hippocampal neurons

Upon the arrival of repetitive stimulation at the presynaptic terminals of neurons, replenishment of readily releasable synaptic vesicles (SVs) with vesicles in the recycling pool is important for sustained neurotransmitter release. Kinetics of replenishment and the available pool size define synaptic performance. However, whether all SVs in the recycling pool are recruited for release with equal probability and speed is unknown. Here, based on comprehensive optical imaging of various presynaptic endosomal SNARE proteins in cultured hippocampal neurons, all of which are implicated in organellar membrane fusion in non-neuronal cells, we show that part of the recycling pool bearing the endosomal Q-SNARE, syntaxin 7 (Stx7), is preferentially mobilized for release during high-frequency repetitive stimulation. Recruitment of the SV pool marked with an Stx7-reporter requires actin polymerization, as well as activation of the Ca2+/calmodulin signaling pathway, reminiscent of rapidly replenishing SVs characterized previously in calyx of Held synapses. Furthermore, disruption of Stx7 function by overexpressing its N-terminal domain selectively abolished this pool. Thus, our data indicate that endosomal membrane fusion involving Stx7 forms rapidly replenishing vesicles essential for synaptic responses to high-frequency repetitive stimulation, and also highlight functional diversities of endosomal SNAREs in generating distinct exocytic vesicles in the presynaptic terminals.

Temporal properties of painful contrast enhancement using repetitive stimulation

Offset analgesia is characterized by a disproportionately large reduction in pain following a small decrease in a heat stimulus and is based on the phenomenon of temporal pain contrast enhancement (TPCE). The aim of this study is to investigate whether this phenomenon can also be induced by repetitive stimulation, i.e., by stimuli that are clearly separated in time. With this aim, the repetitive TPCE paradigm was induced in healthy, pain-free subjects (n=33) at the volar non-dominant forearm using heat stimuli. This paradigm was performed applying three different interstimulus intervals (ISIs): 5, 15, and 25 seconds. All paradigms were contrasted with a control paradigm without temperature change. Participants continuously rated the perceived pain intensity. In addition, electrodermal activity was recorded as a surrogate measure of autonomic arousal. Temporal pain contrast enhancement was confirmed for both ISI 5 seconds (p < 0.001) and ISI 15 seconds (p = 0.005), but not for ISI 25 seconds (p = 0.07), however the magnitude of TPCE did not differ between ISIs (p = 0.11). Electrodermal activity was consistent previous pain ratings, but showing significantly higher autonomic activity being measured. Thus, the phenomenon of temporal contrast enhancement of pain can also be induced by repetitive stimulation. Both the involvement of the autonomic nervous system and the involvement of habituation processes are conceivable, which consequently points to both central and peripheral mechanisms of TPCE.

Cross-Whisker Adaptation of Neurons in Layer 2/3 of the Rat Barrel Cortex

Neurons in the barrel cortex respond preferentially to stimulation of one principal whisker and weakly to several adjacent whiskers. Such integration exists already in layer 4, the pivotal recipient layer of thalamic inputs. Previous studies show that cortical neurons gradually adapt to repeated whisker stimulations and that layer 4 neurons exhibit whisker specific adaptation and no apparent interactions with other whiskers. This study aimed to study the specificity of adaptation of layer 2/3 cortical cells. Towards this aim, we compared the synaptic response of neurons to either repetitive stimulation of one of two responsive whiskers or when repetitive stimulation of the two whiskers was interleaved. We found that in most layer 2/3 cells adaptation is whisker-specific. These findings indicate that despite the multi-whisker receptive fields in the cortex, the adaptation process for each whisker-pathway is mostly independent of other whiskers. A mechanism allowing high responsiveness in complex environments.

Trauma-induced myasthenia gravis: coincidence or causal relationship?

We report a case of a 55-year-old man presenting with diplopia, masticatory weakness and dysarthria several weeks post multitrauma. The clinical suspicion of myasthenia gravis (MG) was supported with positive acetylcholine receptor antibodies and abnormal repetitive stimulation study. He responded well to pyridostigmine, intravenous immunoglobulin and oral prednisolone. In this report, we describe the timing and progression of MG in our patient, and review the literature pertaining to the relationship between trauma and MG. The search for definitive evidence of causation may be impractical, but should not delay the recognition and management of a treatable condition.

Synaptotagmin 7 switches short-term synaptic plasticity from depression to facilitation by suppressing synaptic transmission

Short-term synaptic plasticity is a fast and robust modification in neuronal presynaptic output that can enhance release strength to drive facilitation or diminish it to promote depression. The mechanisms that determine whether neurons display short-term facilitation or depression are still unclear. Here we show that the Ca2+-binding protein Synaptotagmin 7 (Syt7) determines the sign of short-term synaptic plasticity by controlling the initial probability of synaptic vesicle (SV) fusion. Electrophysiological analysis of Syt7 null mutants at Drosophila embryonic neuromuscular junctions demonstrate loss of the protein converts the normally observed synaptic facilitation response during repetitive stimulation into synaptic depression. In contrast, overexpression of Syt7 dramatically enhanced the magnitude of short-term facilitation. These changes in short-term plasticity were mirrored by corresponding alterations in the initial evoked response, with SV release probability enhanced in Syt7 mutants and suppressed following Syt7 overexpression. Indeed, Syt7 mutants were able to display facilitation in lower [Ca2+] where release was reduced. These data suggest Syt7 does not act by directly sensing residual Ca2+ and argues for the existence of a distinct Ca2+ sensor beyond Syt7 that mediates facilitation. Instead, Syt7 normally suppresses synaptic transmission to maintain an output range where facilitation is available to the neuron.

The Orai1 inhibitor BTP2 has multiple effects on Ca2+ handling in skeletal muscle

BTP2 is an inhibitor of the Ca2+ channel Orai1, which mediates store-operated Ca2+ entry (SOCE). Despite having been extensively used in skeletal muscle, the effects of this inhibitor on Ca2+ handling in muscle cells have not been described. To address this question, we used intra- and extracellular application of BTP2 in mechanically skinned fibers and developed a localized modulator application approach, which provided in-preparation reference and test fiber sections to enhance detection of the effect of Ca2+ handling modulators. In addition to blocking Orai1-dependent SOCE, we found a BTP2-dependent inhibition of resting extracellular Ca2+ flux. Increasing concentrations of BTP2 caused a shift from inducing accumulation of Ca2+ in the t-system due to Orai1 blocking to reducing the resting [Ca2+] in the sealed t-system. This effect was not observed in the absence of functional ryanodine receptors (RYRs), suggesting that higher concentrations of BTP2 impair RYR function. Additionally, we found that BTP2 impaired action potential–induced Ca2+ release from the sarcoplasmic reticulum during repetitive stimulation without compromising the fiber Ca2+ content. BTP2 was found to have an effect on RYR-mediated Ca2+ release, suggesting that RYR is the point of BTP2-induced inhibition during cycles of EC coupling. The effects of BTP2 on the RYR Ca2+ leak and release were abolished by pre-exposure to saponin, indicating that the effects of BTP2 on the RYR are not direct and require a functional t-system. Our results demonstrate the presence of a SOCE channels–mediated basal Ca2+ influx in healthy muscle fibers and indicate that BTP2 has multiple effects on Ca2+ handling, including indirect effects on the activity of the RYR.

Early Responsiveness of Ocular Symptoms of Myasthenia Gravis with Plasma Exchange

75-year-old male presente with ptosis (figure 1), fatigue, slurred speech and dysphagia to the emergency department. Low frequency repetitive stimulation of the spinal acessory nerve with recordong from trapeius showed signifivant decrement. Plama excange was started and three days after treatment there was complete resolution of ptosis (figure 2). Older men show complete responsiveness to plasma exchange including an early and near complete response tp ocular symptoms.

Pre-determined macromolecular leakage sites with dynamic properties establish endothelial intra-vessel heterogeneity

SummaryEndothelial cells display heterogeneous properties based on location and function. How this heterogeneity influences endothelial barrier stability both between and within vessel subtypes is unexplored. We find here that endothelial cells exhibit heterogeneous barrier properties on inter-organ and intra-vessel levels. Using intravital microscopy and sequential stimulation of the ear dermis with vascular endothelial growth factor-A and/or histamine, we observe distinct, reappearing sites, common for both agonists, where leakage preferentially takes place. Further, through repetitive stimulation of the diaphragm and trachea, we find inter-organ conservation of such leakage sites. Qualitatively, pre-determined sites display distinct leakage properties and enhanced barrier breakdown compared to less susceptible regions. Mechanistically, pre-determined sites exhibit lower laminin α5 deposition, which correlates with reduced junctional vascular endothelial (VE)-Cadherin. These data thus highlight functional intra-vessel heterogeneity that defines pre-determined sites which display distinct leakage properties and which may have great impact on pathological vascular leakage and drug delivery.