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.

Inactivation of voltage-activated Na(+) currents contributes to different adaptation properties of paired mechanosensory neurons

Voltage-activated sodium current (I(Na)) is primarily responsible for the leading edge of the action potential in many neurons. While I(Na) generally activates rapidly when a neuron is depolarized, its inactivation properties differ significantly between different neurons and even within one neuron, where I(Na) often has slowly and rapidly inactivating components. I(Na) inactivation has been suggested to regulate action potential firing frequency in some cells, but no clear picture of this relationship has emerged. We studied I(Na) in both members of the paired mechanosensory neurons of a spider slit-sense organ, where one neuron adapts rapidly (type A) and the other slowly (type B) in response to a step depolarization. In both neuron types I(Na) activated and inactivated with single time constants of 2--3 ms and 5--10 ms, respectively, varying with the stimulus intensity. However, there was a clear difference in the steady-state inactivation properties of the two neuron types, with the half-maximal inactivation (V(50)) being -40.1 mV in type A neurons and -58.1 mV in type B neurons. Therefore I(Na) inactivated closer to the resting potential in the more slowly adapting neurons. I(Na) also recovered from inactivation significantly faster in type B than type A neurons, and the recovery was dependent on conditioning voltage. These results suggest that while the rate of I(Na) inactivation is not responsible for the difference in the adaptation behavior of these two neuron types, the rate of recovery from inactivation may play a major role. Inactivation at lower potentials could therefore be crucial for more rapid recovery.

Predicting the responses of mechanoreceptor neurons to physiological inputs by nonlinear system identification

The nonlinear dynamic properties of action potential encoding were studied in mechanosensory neurons innervating the slits of a slit-sense organ in the tropical wandering spider, Cupiennius salei. The organ contains two types of neurons that are morphologically similar but have different dynamic properties. Type A neurons produce only one or two action potentials in response to a mechanical or electrical stimulus of any suprathreshold amplitude, while type B neurons can fire prolonged bursts of action potentials in response to similar stimuli. Neurons were stimulated with pseudorandomly modulated intracellular current while recording the resultant fluctuations in membrane potential and action potentials. A parallel cascade method was used to estimate a third-order Volterra series to describe the nonlinear dynamic relationship between membrane potential and action potentials. Kernels measured for the two types of neurons had reproducible forms that showed differences between the two neuron types. The measured kernels were able to predict the responses of the neurons to novel pseudorandomly modulated inputs with reasonable fidelity. However, the Volterra series did not adequately predict the difference in responses to step depolarizations.

Low-voltage-activated calcium current does not regulate the firing behavior in paired mechanosensory neurons with different adaptation properties

Low-voltage-activated Ca(2+) currents (LVA-I(Ca)) are believed to perform several roles in neurons such as lowering the threshold for action potentials, promoting burst firing and oscillatory behavior, and enhancing synaptic excitation. They also may allow rapid increases in intracellular Ca(2+) concentration. We discovered LVA-I(Ca) in both members of paired mechanoreceptor neurons in a spider, where one neuron adapts rapidly (Type A) and the other slowly (Type B) in response to a step stimulus. To learn if I(Ca) contributed to the difference in adaptation behavior, we studied the kinetics of I(Ca) from isolated somata under single-electrode voltage-clamp and tested its physiological function under current clamp. LVA-I(Ca) was large enough to fire single action potentials when all other voltage-activated currents were blocked, but we found no evidence that it regulated firing behavior. LVA-I(Ca) did not lower the action potential threshold or affect firing frequency. Previous experiments have failed to find Ca(2+)-activated K(+) current (I(K(Ca))) in the somata of these neurons, so it is also unlikely that LVA-I(Ca) interacts with I(K(Ca)) to produce oscillatory behavior. We conclude that LVA-Ca(2+) channels in the somata, and possible in the dendrites, of these neurons open in response to the depolarization caused by receptor current and by the voltage-activated Na(+) current (I(Na)) that produces action potential(s). However, the role of the increased intracellular Ca(2+) concentration in neuronal function remains enigmatic.

Primary culture of antennal mechanoreceptor neurons of Manduca sexta

We have developed a primary cell culture system of antennal mechanoreceptor neurons from early-stage pupal sphinx moth Manduca sexta. Dissociated neurons from the moth antennae differentiated, grew and survived for several weeks in a conditioned culture medium. Bipolar neurons with soma diameters of 10-25 microns from the basal portion of the antennae could be positively identified as mechanoreceptor neurons, presumably derived from Johnston's organ, using a monoclonal antibody that recognizes neurofilaments in these neurons. The immunoreactivity was clear and specific from the first day after dissociation and became stronger during several days in culture. These neurons appeared healthy and showed normal whole-cell properties only a few days after plating. We found numerous mechanosensitive ion channels responding to both negative and positive pressures on the somata and neurites of differentiated neurons. This new culture system provides access to mechanoreceptor neurons that has never been possible before, allowing the use of both mechanical and electrical stimuli on neurons that are free from the accessory structures surrounding them in intact preparations.

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.

Information transmission at 500 bits/s by action potentials in a mechanosensory neuron of the cockroach

Action potentials are widely used to transmit information within nervous systems but information encoding and transmission rates by action potentials are poorly understood. In the absence of knowledge about encoding, most previous work has used signal-to-noise ratios to estimate information capacities. We used a mechanosensory neuron to transmit information by a simple encoding scheme that allowed us to measure the transmission rate directly. Using either mechanical or electrical stimulation, information was transmitted at rates up to 500 bits/s, higher than ever reported before for real action potentials. However, the maximum possible message length decreased strongly with transmission rate, from approximately infinite at 100 bits/s to approximately 100 ms at 500 bits/ s, probably due to ionic adaptation processes within the neuronal membrane.

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+.

Patterns of cell death during gastrulation in chick and mouse embryos

We have examined the distribution of cells at an early stage of the cell death process in gastrulating chick and mouse embryos, using a DNA nick end-labelling technique to label nuclei that are undergoing DNA fragmentation in situ. In the chick embryo, the incidence of nuclei showing DNA fragmentation was mapped by digitizing the occurrence of these nuclei from sections, and reconstructing the three separate layers of the entire embryo at several stages of gastrulation. In the chick, DNA fragmentation was found in nuclei throughout the embryo, in cells of all three germ layers, but most especially in the epiblast in the rostral germinal crescent and in the lateral marginal zones. This region of greatest cell death formed an arc rostrally and laterally in the epiblast, and was consistent through gastrulation and into the early neurulation stage. While the extensive cell death in the chick embryo may be due to cell redundancy, it is also possible that the pattern of death observed could be related to the compression of the embryo against the barrier of yolk at the periphery of the area pellucida during expansion. In a number of cases in the chick, local regions of elevated cell death were also observed in the primitive streak. This may be associated with the changing cell-cell and cell-matrix interactions experienced by cells traversing the primitive streak. In the gastrulating mouse embryo, by contrast, nuclei undergoing DNA fragmentation showed no consistent regions of elevated incidence, in any of the embryonic layers. DNA fragmentation in these embryos was, however, observed in nuclei of cells in the visceral endoderm and in the epiblast. The lack of any clear pattern of DNA fragmentation in the mouse embryo at this stage of development leaves the roles of the dying cells enigmatic. The death may, however, be lineage-related or be a reflection of a cellular redundancy necessary in a developing system that is undergoing extensive cell rearrangement and cellular adhesive change.

Slowly inactivating outward currents in a cuticular mechanoreceptor neuron of the cockroach (Periplaneta americana)

1. Although rapid adaptation is a widespread feature of sensory receptors, its ionic basis has not been clearly established in any touch receptor, because their small sizes have severely restricted the range of experiments tat can be performed. In the cockroach tactile spine, intracellular voltage-clamp recordings are now possible. 2. The basic electrophysiological properties of the cockroach femoral tactile spine neuron were studied using discontinuous (switching) single-electrode current- and voltage-clamp recordings. A slowly inactivating voltage-sensitive K+ outward current was detected after the major inward currents were blocked with tetrodotoxin. 3. The total outward current activated in < 1 ms at voltages above 0 mV. At moderate depolarizations it did not inactivate, but at higher depolarizations an inactivation time constant of approximately 260 ms was measured. Some recordings also revealed an additional, slower inactivation time constant of approximately 2.5 s. 4. More than half of the voltage-sensitive K+ outward current could be blocked with the Ca2+ channel blockers Co2+ and Cd2+. Tetraethylammonium chloride (TEA) also reduced the amplitude of the outward current to about half of its original amplitude. The actions of both blockers were reversible and probably reflect overlapping blockades of two separate outward currents. 5. The reversal potentials of the currents that remained after block with Co2+ (-91.7 mV) or TEA (-86.8 mV) were both near the K+ equilibrium potential expected for the tactile spine neuron. The voltage dependencies of activation of the Co(2+)- and TEA-resistant currents were both well fitted by Boltzmann distributions, giving values of half maximal activation (V50) equal to -34.5 mV for the Co(2+)-resistant current and -51.3 mV for the TEA-resistant current. 6. Current-clamp recordings revealed that the TEA-sensitive K+ current was the major component of action potential repolarization but that it did not effect the frequency of action potentials evoked by steady depolarization. On the other hand, blockers of Ca(2+)-sensitive K+ currents (Cd2+, Co2+, or charybdotoxin) reduced adaptation and increased the frequency of action potentials significantly but did not effect the duration or amplitude of individual action potentials.

Characterization of a transient outward current in a rapidly adapting insect mechanosensory neuron

This paper describes the first voltage-clamp recordings from an arthropod cuticular sensory neuron. In the femoral tactile spine neuron of the cockroach Periplaneta americana, a rapidly activating and inactivating outward current, IA, appeared when the neuron was hyperpolarized for a short period before a depolarizing test pulse. IA could be separated from the other outward currents using 5 mM 4-aminopyridine (4-AP), which specifically blocked it. Tetraethylammonium (TEA), (50 mM) did not remove IA, but decreased the steady-state outward current by about 50%. The threshold for IA activation was about -75 mV. The minimum activation and inactivation time constants were approximately 0.2 ms and 15 ms, respectively. The voltage dependencies of activation and inactivation were well fit-ted by Boltzmann distributions, giving values of membrane potential at half-maximal activation (V50) equal to -56.5 mV and an equivalent gating charge of n = 3.9 for activation and V50 = -86.7 mV and n = 3.4 for inactivation. In current-clamp recordings, 4-AP reversibly reduced the cell's normal adaptation by lowering the threshold for action potentials, but did not affect the amplitude or duration of single action potentials. These results indicate that IA plays a role in short-term adaptation by opposing membrane depolarization and reducing the spike frequency during maintained stimulation.

The time course of sensory adaptation in the cockroach tactile spine

Power-law dynamics have been widely used to fit the adaptation of sensory receptors, including the cockroach tactile spine. We used a new log-binning technique to re-examine step responses in the tactile spine. The power-law only fitted responses over a restricted time period, while a sum of three exponential decays gave an accurate fit over the entire response duration.

The Basis of Rapid Adaptation in Mechanoreceptors

Mechanoreceptors often adapt rapidly and completely to a sustained mechanical stimulus. This adaptation is usually assumed to be caused by mechanical structures surrounding sensory endings, but old and new evidence from several preparations indicates that ionic processes in the cell membranes are primarily responsible for such complete adaptation.

Mapping extracellular excitability in an insect mechanoreceptor neuron

The cockroach tactile spine contains a single bipolar mechanosensory neuron. Extracellular stimulation of the neuron is possible by cutting the spine and lowering a microelectrode into the lumen, where the neuron is located, but neither the microelectrode nor the neuron can be visualized during stimulation. The threshold for electrical stimulation of the neuron was measured as a function of spatial position in the lumen. The spine was then fixed and serially sectioned for computer-aided reconstruction. Alignment of threshold measurements with reconstructions produced maps of excitability around the neuron. The lowest threshold was always close to the sensory dendrite or the adjacent soma. These results are discussed in terms of models of action potential initiation in this class of sensory neurons.

Immunocytochemical localization of sodium channels in an insect central nervous system using a site-directed antibody

Antibodies to channel proteins and specific peptide sequences have been previously used to localize voltage-activated sodium channels in the rat brain. Here we describe the first localization of sodium channels in an insect nervous system using a site-directed antibody. The mesothoracic ganglion of the cockroach was stained with an antibody to the highly conserved SP19 sequence. Antibody labelling was visualized by light microscopy using the avidin/biotin method on wax sections, and transmission electron microscopy of immunogold-labelled thin sections. Central ganglia of insects contain clearly separated regions of cell bodies, synaptic neuropil, axon tracts, and nerves. Antibody staining by light microscopy was limited to neurons, and was intense in axons throughout the ganglion and nerves. Staining was also strong in the cytoplasm, but not the nuclei, of many neuronal cell bodies. Neuropil regions were relatively lightly labelled. These findings can be correlated with the known electrophysiology of the ganglion. Electron microscopy detected sodium channels in areas surrounding axons, probably including axon membranes and enveloping glial cell membranes. Axonal mitochondria were also heavily labelled, suggesting a sodium channel transport function for these organelles.

Octopamine selectively modifies the slow component of sensory adaptation in an insect mechanoreceptor

The effects of octopamine were studied on the dynamic behavior of the sensory neuron in the cockroach femoral tactile spine. The neuron is a rapidly adapting mechanoreceptor in which adaptation occurs by elevation of the threshold for action potential encoding. The threshold follows increases or decreases of membrane potential, with a delay that involves two separate exponential components. Previous evidence has associated the slow component with sodium pumping and the fast component with sodium channel inactivation. Octopamine reversibly raised the resting threshold and increased but slowed the slow component. These data indicate that octopamine has specific effects on membrane-ionic processes in insect sensory neurons.