Single Evoked Action Potential Research Articles

Expression of hyperpolarization-activated current (Ih) in zonally defined vestibular calyx terminals of the crista.

Calyx terminals make afferent synapses with type I hair cells in vestibular epithelia and express diverse ionic conductances that influence action potential generation and discharge regularity in vestibular afferent neurons. Here we investigated the expression of hyperpolarization-activated current (Ih) in calyx terminals in central and peripheral zones of mature gerbil crista slices, using whole cell patch-clamp recordings. Slowly activating Ih was present in >80% calyces tested in both zones. Peak Ih and half-activation voltages were not significantly different; however, Ih activated with a faster time course in peripheral compared with central zone calyces. Calyx Ih in both zones was blocked by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288; 100 µM), and the resting membrane potential became more hyperpolarized. In the presence of dibutyryl-cAMP (dB-cAMP), peak Ih was increased, activation kinetics became faster, and the voltage of half-activation was more depolarized compared with control calyces. In current clamp, calyces from both zones showed three different categories of firing: spontaneous firing, phasic firing where a single action potential was evoked after a hyperpolarizing pulse, or a single evoked action potential followed by membrane potential oscillations. In the absence of Ih, the latency to peak of the action potential increased; Ih produces a small depolarizing current that facilitates firing by driving the membrane potential closer to threshold. Immunostaining showed the expression of HCN2 subunits in calyx terminals. We conclude that Ih is found in calyx terminals across the crista and could influence conventional and novel forms of synaptic transmission at the type I hair cell-calyx synapse.NEW & NOTEWORTHY Calyx afferent terminals make synapses with vestibular hair cells and express diverse conductances that impact action potential firing in vestibular primary afferents. Conventional and nonconventional synaptic transmission modes are influenced by hyperpolarization-activated current (Ih), but regional differences were previously unexplored. We show that Ih is present in both central and peripheral calyces of the mammalian crista. Ih produces a small depolarizing resting current that facilitates firing by driving the membrane potential closer to threshold.

Local knockdown of the NaV1.6 sodium channel reduces pain behaviors, sensory neuron excitability, and sympathetic sprouting in rat models of neuropathic pain

In the spinal nerve ligation (SNL) model of neuropathic pain, as in other pain models, abnormal spontaneous activity of myelinated sensory neurons occurs early and is essential for establishing pain behaviors and other pathologies. Sympathetic sprouting into the dorsal root ganglion (DRG) is observed after SNL, and sympathectomy reduces pain behavior. Sprouting and spontaneous activity may be mutually reinforcing: blocking neuronal activity reduces sympathetic sprouting, and sympathetic spouts functionally increase spontaneous activity in vitro. However, most studies in this field have used nonspecific methods to block spontaneous activity, methods that also block evoked and normal activity. In this study, we injected small inhibitory (si) RNA directed against the NaV1.6 sodium channel isoform into the DRG before SNL. This isoform can mediate high-frequency repetitive firing, like that seen in spontaneously active neurons. Local knockdown of NaV1.6 markedly reduced mechanical pain behaviors induced by SNL, reduced sympathetic sprouting into the ligated sensory ganglion, and blocked abnormal spontaneous activity and other measures of hyperexcitability in myelinated neurons in the ligated sensory ganglion. Immunohistochemical experiments showed that sympathetic sprouting preferentially targeted NaV1.6-positive neurons. Under these experimental conditions, NaV1.6 knockdown did not prevent or strongly alter single evoked action potentials, unlike previous less specific methods used to block spontaneous activity. NaV1.6 knockdown also reduced pain behaviors in another pain model, chronic constriction of the sciatic nerve, provided the model was modified so that the lesion site was relatively close to the siRNA-injected lumbar DRGs. The results highlight the relative importance of abnormal spontaneous activity in establishing both pain behaviors and sympathetic sprouting, and suggest that the NaV1.6 isoform may have value as a therapeutic target.

Direct detection of a single evoked action potential with MRS in Lumbricus terrestris

Functional MRI (fMRI) measures neural activity indirectly by detecting the signal change associated with the hemodynamic response following brain activation. In order to alleviate the temporal and spatial specificity problems associated with fMRI, a number of attempts have been made to detect neural magnetic fields (NMFs) with MRI directly, but have thus far provided conflicting results. In this study, we used MR to detect axonal NMFs in the median giant fiber of the earthworm, Lumbricus terrestris, by examining the free induction decay (FID) with a sampling interval of 0.32 ms. The earthworm nerve cords were isolated from the vasculature and stimulated at the threshold of action potential generation. FIDs were acquired shortly after the stimulation, and simultaneous field potential recordings identified the presence or absence of single evoked action potentials. FIDs acquired when the stimulus did not evoke an action potential were summed as background. The phase of the background‐subtracted FID exhibited a systematic change, with a peak phase difference of (−1.2 ± 0.3) × 10–5 radians occurring at a time corresponding to the timing of the action potential. In addition, we calculated the possible changes in the FID magnitude and phase caused by a simulated action potential using a volume conductor model. The measured phase difference matched the theoretical prediction well in both amplitude and temporal characteristics. This study provides the first evidence for the direct detection of a magnetic field from an evoked action potential using MR. Copyright © 2011 John Wiley & Sons, Ltd.

Statistical independence and neural computation in the leech ganglion.

In this report, the input/output relations in an isolated ganglion of the leech Hirudo medicinalis were studied by simultaneously using six or eight suction pipettes and two intracellular electrodes. Sensory input was mimicked by eliciting action potentials in mechanosensory neurons with intracellular electrodes. The integrated neural output was measured by recording extracellular voltage signals with pipettes sucking the roots and the connectives. A single evoked action potential activated electrical activity in at least a dozen different neurons, some of which were identified. This electrical activity was characterized by a high degree of temporal and spatial variability. The action potentials of coactivated neurons, i.e. activated by the same mechanosensory neuron, did not show any significant pairwise correlation. Indeed, the analysis of evoked action potentials indicates clear statistical independence among coactivated neurons, presumably originating from the independence of synaptic transmission at distinct synapses. This statistical independence may be used to increase reliability when neuronal activity is averaged or pooled. It is suggested that statistical independence among coactivated neurons may be a usual property of distributed processing of neuronal networks and a basic feature of neural computation.

PGE2 modulates the tetrodotoxin-resistant sodium current in neonatal rat dorsal root ganglion neurones via the cyclic AMP-protein kinase A cascade.

1. In current-clamp recordings, 1 microM prostaglandin E2 (PGE2) increased the excitability of neonatal rat dorsal root ganglion neurones. The current threshold for firing was reduced, and the response to a constant suprathreshold stimulation was modified such that a single evoked action potential was converted to a train of action potentials. The excitatory action of PGE2 was still apparent when action potentials were evoked in the presence of 500 nM tetrodotoxin. 2. In voltage-clamp experiments 1 microM PGE2 frequently increased the magnitude of the peak currents recorded, and caused a hyperpolarizing shift (of approximately 6 mV) in the activation curve for the tetrodotoxin-resistant sodium current (TTX-R INa). In some cells, the hyperpolarizing shift in the activation curve was accompanied by a decrease in peak conductance. PGE2 also caused a hyperpolarizing shift in the steady-state inactivation curve for the sodium current. 3. Extracellular application of the cAMP analogue dibutyryl cAMP (dbcAMP) at a concentration of 1 mM produced effects on both the current-voltage relationship and the steady-state inactivation curve for the TTX-R INa which were indistinguishable from those observed with PGE2. Prior exposure of the neurones to dbcAMP occluded the effect of a subsequent treatment with PGE2. 4. Forskolin (10 microM), a direct activator of adenylate cyclase, mimicked the effects of PGE2 and dbcAMP on TTX-R INa. The inactive congener of forskolin, 1, 9-dideoxyforskolin (10 microM), reduced the amplitude of TTX-R INa, but did not evoke a hyperpolarizing shift in the activation curve. 5. Intracellular perfusion of the neurones with an inhibitor of protein kinase A inhibited the effect of PGE2 on TTX-R INa. 6. PGE2 also reduced the amplitude of voltage-gated potassium currents (IK), which will contribute to the excitatory action. The mechanisms underlying the changes in IK have yet to be elucidated. 7. We propose that the PGE2-mediated increase in excitability in sensory neurones may be due, at least in part, to the cAMP-protein kinase A-dependent modulation of the tetrodotoxin-resistant sodium channel.

Post‐inhibitory excitation and inhibition in layer V pyramidal neurones from cat sensorimotor cortex.

1. The effect of conditioning pre-pulses on repetitive firing evoked by intracellular current injection was studied in layer V pyramidal neurones in a brain slice preparation of cat sensorimotor cortex. Most cells displayed spike frequency adaptation (monotonic decline of firing rate to a tonic value) for several hundred milliseconds when depolarized from resting potential, but the cells differed in their response when pre-pulses to other potentials were employed. In one group of cells, the initial firing rate increased as the pre-pulse potential was made more negative (post-hyperpolarization excitation). Adaptation was abolished by depolarizing prepulses. In a second group, the initial firing rate decreased as the pre-pulse potential was made more negative (post-hyperpolarization inhibition). Hyperpolarizing pre-pulses caused the initial firing to fall below and accelerate to the tonic rate over a period of several seconds. A third group displayed a mixture of these two responses: the first three to seven interspike intervals became progressively shorter and subsequent intervals became progressively longer as the conditioning pre-pulse was made more negative (post-hyperpolarization mixed response). 2. Cells were filled with horseradish peroxidase or biocytin after the effect of pre-pulses was determined. All cells whose firing patterns were altered by pre-pulses were large layer V pyramidal neurones. Cells showing post-hyperpolarization excitation or a mixed response had tap root dendrites, fewer spines on the apical dendrite and larger soma diameters than cells showing post-hyperpolarization inhibition. 3. Other electrophysiological parameters varied systematically with the response to conditioning pre-pulses. Both the mean action potential duration and the input resistance of cells showing post-hyperpolarization excitation were about half the values measured in cells showing post-hyperpolarization inhibition. Values were intermediate in cells showing a post-hyperpolarization mixed response. The after-hyperpolarization following a single evoked action potential was 20% briefer in cells showing post-hyperpolarization excitation compared to those showing inhibition. 4. Membrane current measured during voltage clamp suggested that two ionic mechanisms accounted for the three response patterns. Post-hyperpolarization excitation was caused by deactivation of the inward rectifier current (Ih). Selective reduction of Ih with extracellular caesium diminished post-hyperpolarization excitation, whereas blockade of calcium influx had no effect. Post-hyperpolarization inhibition was caused by enhanced activation of a slowly inactivating potassium current. Selective reduction of this current with 4-aminopyridine diminished the post-hyperpolarization inhibition. 5. Chord conductances underlying both Ih and the slow-transient potassium current were measured and divided by leakage conductance to control for differences in cell size.(ABSTRACT TRUNCATED AT 400 WORDS)