Spider Mechanoreceptor Neurons Research Articles

GABAergic Excitation of Spider Mechanoreceptors Increases Information Capacity by Increasing Entropy Rather than Decreasing Jitter

Neurotransmitter chemicals excite or inhibit a range of sensory afferents and sensory pathways. These changes in firing rate or static sensitivity can also be associated with changes in dynamic sensitivity or membrane noise and thus action potential timing. We measured action potential firing produced by random mechanical stimulation of spider mechanoreceptor neurons during long-duration excitation by the GABAA agonist muscimol. Information capacity was estimated from signal-to-noise ratio by averaging responses to repeated identical stimulation sequences. Information capacity was also estimated from the coherence function between input and output signals. Entropy rate was estimated by a data compression algorithm and maximum entropy rate from the firing rate. Action potential timing variability, or jitter, was measured as normalized interspike interval distance. Muscimol increased firing rate, information capacity, and entropy rate, but jitter was unchanged. We compared these data with the effects of increasing firing rate by current injection. Our results indicate that the major increase in information capacity by neurotransmitter action arose from the increased entropy rate produced by increased firing rate, not from reduction in membrane noise and action potential jitter.

Regional distribution of calcium elevation during sensory transduction in spider mechanoreceptor neurons

Spider mechanosensory VS-3 neurons receive peripheral efferent synaptic modulation, with regional variations in the types of efferent synapses and transmitter receptors. VS-3 somata possess a voltage-activated calcium current, but the levels and time courses of calcium changes in other regions are unknown. The roles of calcium in these neurons are not completely understood, but could include modulation of both mechanosensitivity and response dynamics. Here, we measured calcium concentration rises caused by single, mechanically induced action potentials in VS-3 sensory dendrites, somata and axons, using Oregon Green BAPTA-1 fluorescence. Calcium concentration rose by ∼1 nM following each action potential. Time courses of calcium rise and fall were similar in the three regions but the rise in amplitude was about 50% higher in the sensory dendrite than in the soma. Antibody to the Ca V3.1(α 1g) isotype of T-type calcium channel labeled all three neuronal regions. Some Ca V3.1 labeling colocalized with synapsin labeling, suggesting that calcium channels play some part in efferent modulation. We conclude that mechanically stimulated action potentials start near sensory dendrite tips and pass rapidly through the neurons to the axons, activating low voltage activated calcium channels in all three regions and causing calcium concentration to rise rapidly in each region. These results suggest important roles for calcium in several stages of mechanosensation.

The Power Law of Sensory Adaptation: Simulation by a Model of Excitability in Spider Mechanoreceptor Neurons

The power law of sensory adaptation was introduced more than 50 years ago. It is characterized by action potential adaptation that follows fractional powers of time or frequency, rather than exponential decays and corresponding frequency responses. Power law adaptation describes the responses of a range of vertebrate and invertebrate sensory receptors to deterministic stimuli, such as steps or sinusoids, and to random (white noise) stimulation. Hypotheses about the physical basis of power law adaptation have existed since its discovery. Its cause remains enigmatic, but the site of power law adaptation has been located in the conversion of receptor potentials into action potentials in some preparations. Here, we used pseudorandom noise stimulation and direct spectral estimation to show that simulations containing only two voltage activated currents can reproduce the power law adaptation in two types of spider mechanoreceptors. Identical simulations were previously used to explain the different responses of these two types of sensory neurons to step inputs. We conclude that power law adaptation results during action potential encoding by nonlinear combination of a small number of activation and inactivation processes with different exponential time constants.

Ratiometric calcium concentration estimation using LED excitation during mechanotransduction in single sensory neurons

In a previous study using Oregon Green BAPTA-1 fluorescence we found that intracellular calcium concentration in spider mechanoreceptor neurons rose during mechanical stimulation. We also showed that calcium elevation required the opening of voltage-dependent calcium channels by action potentials, and could not be produced by the receptor potential alone. While evidence for mechanisms of calcium elevation in these neurons was clear, our estimates of actual calcium concentration depended on properties of the fluorescent dye in the neuron cytoplasm that could not be verified. We have now developed a method for ratiometric estimation of calcium concentration in these neurons using Fura Red dye, excitation by two light emitting diodes (LEDs) of different wavelengths, and an avalanche photodiode fluorescence detector. The method is simple and economical to implement, allows concentration changes to be measured in the millisecond time range, and could easily be applied to a wide range of preparations. Resting calcium concentration in these neurons was about 70nM and rose to a maximum of about 400nM at firing rates above 20 action potentials per second.

Principal dynamic mode analysis of action potential firing in a spider mechanoreceptor

The encoding of mechanical stimuli into action potentials in two types of spider mechanoreceptor neurons is modeled by use of the principal dynamic modes (PDM) methodology. The PDM model is equivalent to the general Wiener-Bose model and consists of a minimum set of linear dynamic filters (PDMs), followed by a multivariate static nonlinearity and a threshold function. The PDMs are obtained by performing eigen-decomposition of a matrix constructed using the first-order and second-order Volterra kernels of the system, which are estimated by means of the Laguerre expansion technique, utilizing measurements of pseudorandom mechanical stimulation (input signal) and the resulting action potentials (output signal). The static nonlinearity, which can be viewed as a measure of the probability of action potential firing as a function of the PDM output values, is computed as the locus of points of the latter that correspond to output action potentials. The performance of the model is assessed by computing receiver operating characteristic (ROC) curves, akin to the ones used in decision theory and quantified by computing the area under the ROC curve. Three PDMs are revealed by the analysis. The first PDM exhibits a high-pass characteristic, illustrating the importance of the velocity of slit displacement in the generation of action potentials at the mechanoreceptor output, while the second and third PDMs exhibit band-pass and low-pass characteristics, respectively. The corresponding three-input nonlinearity exhibits asymmetric behavior with respect to its arguments, suggesting directional dependence of the mechanoreceptor response on the mechanical stimulation and the PDM outputs, in agreement to our findings from a previous study (Ann Biomed Eng 27:391-402, 1999). Differences between the Type A and B neurons are observed in the zeroth-order Volterra kernels (related to the average firing), as well as in the magnitudes of the second and third PDMs that perform band-pass and low-pass processing of the input signal, respectively.

Calcium concentration changes during sensory transduction in spider mechanoreceptor neurons

Most mechanoreceptor neurons encode mechanical signals into action potential trains within the same cell. Evidence suggests that intracellular calcium ion concentration, [Ca2+], increases during mechanotransduction, either by direct entry through mechanically activated channels or indirectly through voltage-activated calcium channels. However, little is known about the amounts of calcium involved or its roles in mechanotransduction. We estimated [Ca2+] in mechanoreceptor neurons of the spider, Cupiennius salei, during mechanical stimulation using Oregon Green BAPTA-1, and a single-compartment model of [Ca2+] as a function of action potential firing rate. Resting [Ca2+] was approximately 400 nM and increased to up to 2 microM at 30 action potentials/s. Similar levels of resting and stimulated [Ca2+] were obtained in the cell soma, axon and two parts of the sensory dendrite, including the region immediately adjacent to the site of sensory transduction. The time constant of rise and fall of [Ca2+] was 1-5 s in the dendrite and axon, but up to 15 s in the soma. Calcium elevation was dependent on action potentials and could not be induced by the receptor potential alone. Blockade of voltage-activated calcium channels by nickel ions prevented calcium increase, but thapsigargin, which empties intracellular calcium stores, had no effect. Estimates of calcium entry per action potential from fluorescence changes agreed approximately with estimates based on action potential voltage-time profile and previous reports of calcium channel properties. This first report of calcium levels during transduction in spiking mechanoreceptors suggests that calcium signaling plays important roles in primary somatosensory neurons.

Slow adaptation in spider mechanoreceptor neurons

Slow adaptation of action potential firing is a common but poorly understood property of sensory neurons. We quantified slow adaptation in a cuticular mechanoreceptor organ of the spider, Cupiennius salei, by stimulating with continuous pseudorandom mechanical displacements while recording action potentials intracellularly from the cell bodies. Firing rate declined over a period of several minutes before reaching a steady level at about half the initial rate. This slow adaptation was fitted by an exponential decay with mean time constant of 18.5 s. Recovery from slow adaptation was also fitted by an exponential process, but with a longer time constant of 167 s. The receptor potential produced by the same stimulation protocol did not change its amplitude or dynamics, showing that slow adaptation occurs during action potential encoding from the receptor potential. Experiments with chemical blockers of calcium entry or the known potassium currents failed to reduce the slow adaptation. The Na+/K+ pump blocker Ouabain decreased the time constant of slow adaptation, suggesting that ion accumulation is involved. In some experiments, a second class of small action potentials were observed, which were tentatively attributed to failed conduction from the sensory dendrite through the soma to the axon.

Simulation of different firing patterns in paired spider mechanoreceptor neurons: the role of Na(+) channel inactivation.

The spider VS-3 slit-sense organ contains two types of primary mechanoreceptor neurons that are morphologically similar but have different electrical behavior. Type A neurons fire only one or two action potentials in response to a mechanical or electrical step of any amplitude above the threshold, whereas type B neurons fire prolonged bursts of action potentials in response to similar stimuli. Voltage-clamp studies have shown that two voltage-activated ion currents, a noninactivating potassium current and an inactivating sodium current, dominate the firing behavior. We simulated the electrical behavior of the two neuron types, using a simplified form of Hodgkin-Huxley model based on published voltage-clamp and current-clamp recordings. Changing only two parameters of sodium inactivation, the slope of the h(infinity) curve and the time constant of recovery from inactivation, allowed a complete switch between the two firing patterns. Our simulations support previous evidence that sodium inactivation controls the firing properties of these neurons and indicate that two parameter changes are needed to achieve complete transformation between the two neuron types.

Frequency response functions and information capacities of paired spider mechanoreceptor neurons.

Pseudorandom white-noise stimulation followed by direct spectral estimation was used to obtain linear frequency response and coherence functions from paired, but dynamically different, spider mechanosensory neurons. The dynamic properties of the two neuron types were similar with either mechanical or electrical stimulation, showing that action potential encoding dominates the dynamics. Phase-lag data indicated that action potential initiation occurs more rapidly during mechanical stimulation, probably in the distal sensory dendrites. Total information capacity, calculated from coherence, as well as information per action potential, were both similar in the two types of neurons, and similar to the few available estimates from other spiking neurons. However, information capacity and information per action potential both depended strongly on neuronal firing rate, which has not been reported before.

Voltage-activated potassium outward currents in two types of spider mechanoreceptor neurons.

We studied the properties of voltage-activated outward currents in two types of spider cuticular mechanoreceptor neurons to learn if these currents contribute to the differences in their adaptation properties. Both types of neurons adapt rapidly to sustained stimuli, but type A neurons usually only fire one or two action potentials, whereas type B neurons can fire bursts lasting several hundred milliseconds. We found that both neurons had two outward current components, 1) a transient current that activated rapidly when stimulated from resting potential and inactivated with maintained stimuli and 2) a noninactivating outward current. The transient outward current could be blocked by 5 mM tetraethylammonium chloride, 5 mM 4-aminopyridine, or 100 microM quinidine, but these blockers also reduced the amplitude of the noninactivating outward current. Charybdotoxin or apamin did not have any effect on the outward currents, indicating that Ca2+-activated K+ currents were not present or not inhibited by these toxins. The only significant differences between type A and type B neurons were found in the half-maximal activation (V50) values of both currents. The transient current had a V50 value of 9. 6 mV in type A neurons and -13.1 mV in type B neurons, whereas the V50 values of noninactivating outward currents were -48.9 mV for type A neurons and -56.7 mV for type B neurons. We conclude that, although differences in the activation kinetics of the voltage-activated K+ currents could contribute to the difference in the adaptation behavior of type A and type B neurons, they are not major factors.

Ionic selectivity of mechanically activated channels in spider mechanoreceptor neurons.

The lyriform slit-sense organ on the patella of the spider, Cupiennius salei, consists of seven or eight slits, with each slit innervated by a pair of mechanically sensitive neurons. Mechanotransduction is believed to occur at the tips of the dendrites, which are surrounded by a Na+-rich receptor lymph. We studied the ionic basis of sensory transduction in these neurons by voltage-clamp measurement of the receptor current, replacement of extracellular cations, and application of specific blocking agents. The relationship between mechanically activated current and membrane potential could be approximated by the Goldman-Hodgkin-Katz current equation, with an asymptotic inward conductance of approximately 4.6 nS, indicating that 50-230 channels of 20-80 pS each would suffice to produce the receptor current. Amiloride and gadolinium, which are known to block mechanically activated ion channels, also blocked the receptor current. Ionic replacement showed that the channels are not permeable to choline or Rb+, but are partly permeable to Li+. The receptor current was inward at all membrane potentials (-200 to +200 mV) and never reversed, indicating high selectivity for Na+ over K+. This situation contrasts strongly with insect mechanoreceptors, vertebrate hair cells, and mechanically activated ion channels in nonsensory cells, most of which are either unselective for monovalent cations or selective for K+.