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

Multiple Biogenic Amine Receptor Types Modulate Spider, Cupiennius salei, Mechanosensory Neurons

The biogenic amines octopamine (OA), tyramine (TA), dopamine (DA), serotonin (5-HT), and histamine (HA) affect diverse physiological and behavioral processes in invertebrates, but recent findings indicate that an additional adrenergic system exists in at least some invertebrates. Transcriptome analysis has made it possible to identify biogenic amine receptor genes in a wide variety of species whose genomes have not yet been sequenced. This approach provides new sequences for research into the evolutionary history of biogenic amine receptors and allows them to be studied in experimentally accessible animal models. The Central American Wandering spider, Cupiennius salei, is an experimental model for neurophysiological, developmental and behavioral research. We identified ten different biogenic amine receptors in C. salei transcriptomes. Phylogenetic analysis indicated that, in addition to the typical receptors for OA, TA, DA, and 5-HT in protostome invertebrates, spiders also have α1- and α2-adrenergic receptors, but lack TAR2 receptors and one invertebrate specific DA receptor type. In situ hybridization revealed four types of biogenic amine receptors expressed in C. salei mechanosensory neurons. We used intracellular electrophysiological experiments and pharmacological tools to determine how each receptor type contributes to modulation of these neurons. We show that arachnids have similar groups of biogenic amine receptors to other protostome invertebrates, but they lack two clades. We also clarify that arachnids and many other invertebrates have both α1- and α2-adrenergic, likely OA receptors. Our results indicate that in addition to an OAβ-receptor that regulates rapid and large changes in sensitivity via a G_s-protein activating a cAMP mediated pathway, the C. salei mechanosensory neurons have a constitutively active TAR1 and/or α2-adrenergic receptor type that adjusts the baseline sensitivity to a level appropriate for the behavioral state of the animal by a G_q-protein that mobilizes Ca²⁺.

GABA and glutamate receptors have different effects on excitability and are differentially regulated by calcium in spider mechanosensory neurons

GABA and glutamate receptors belonging to the ligand-gated chloride-channel family are primary targets of insecticides and antiparasitics, so their molecular structure, pharmacology and biophysical properties have attracted significant attention. However, little is known about the physiological roles of these channels or how they regulate neuronal excitability and animal behavior. Mechanosensory neurons of VS-3 slit sensilla in the patella of the tropical wandering spider, Cupiennius salei, react to the GABA(A)-receptor agonists, GABA and muscimol, with depolarization and an increase in intracellular [Ca(2+)] and, during random noise stimulation, with a mixed inhibitory-excitatory response. We established that the GABA(A)-receptors in all VS-3 neurons are identical, but there are at least two types of glutamate receptors and some neurons do not respond to glutamate at all. Immunohistochemistry with antibodies against Drosophila inhibitory glutamate receptor (GluCls) α-subunit suggests that in addition to VS-3 neurons, these receptors may also be present in the efferent neurons surrounding the sensory neurons. Most VS-3 neurons were inhibited but not depolarized by glutamate during random stimulation, but some depolarized and had a similar excitatory-inhibitory response to glutamate as to muscimol. The membrane-permeable Ca(2+)-chelator BAPTA-AM abolished muscimol effects but potentiated glutamate effects, indicating that GABA and glutamate receptors are differentially modulated by Ca(2+), leading to diverse regulation of neuronal excitability. We hypothesize that this could be achieved by different Ca(2+)-triggered phosphorylation processes at each receptor type. These findings are important for understanding the significance of Ca(2+)-mediated regulation of transmitter receptor molecules and its role in controlling excitability.

Fast negative feedback enables mammalian auditory nerve fibers to encode a wide dynamic range of sound intensities

Mammalian auditory nerve fibers (ANF) are remarkable for being able to encode a 40 dB, or hundred fold, range of sound pressure levels into their firing rate. Most of the fibers are very sensitive and raise their quiescent spike rate by a small amount for a faint sound at auditory threshold. Then as the sound intensity is increased, they slowly increase their spike rate, with some fibers going up as high as ∼300 Hz. In this way mammals are able to combine sensitivity and wide dynamic range. They are also able to discern sounds embedded within background noise. ANF receive efferent feedback, which suggests that the fibers are readjusted according to the background noise in order to maximize the information content of their auditory spike trains. Inner hair cells activate currents in the unmyelinated distal dendrites of ANF where sound intensity is rate-coded into action potentials. We model this spike generator compartment as an attenuator that employs fast negative feedback. Input current induces rapid and proportional leak currents. This way ANF are able to have a linear frequency to input current (f-I) curve that has a wide dynamic range. The ANF spike generator remains very sensitive to threshold currents, but efferent feedback is able to lower its gain in response to noise.

Ca²(+) /calmodulin-dependent protein kinase II mediates the octopamine-induced increase in sensitivity in spider VS-3 mechanosensory neurons

G-protein-coupled octopamine (OA) receptors mediate their effects by Ca²(+) signaling or adjusting intracellular cAMP levels. Depending on OA concentration and cell type, activation of OA receptors in excitable cells triggers excitatory or inhibitory effects, but the mechanisms by which Ca²(+) or cAMP mediates these effects are not well understood. We investigated signaling mechanisms that are potentially activated by OA, and OA effects on excitability and frequency sensitivity in mechanosensory neurons innervating the VS-3 slit sensilla on the patella of the spider Cupiennius salei. These neurons are directly innervated by octopaminergic efferents, and possess OA receptors that were immunoreactive to an antibody against an OA receptor highly expressed in mushroom bodies. OA application enhanced VS-3 neuron sensitivity, especially at high stimulation frequencies. This enhancement lasted for at least 1 h after OA application. Changes in sensitivity were also detected when the Ca²(+) ionophore ionomycin or the cAMP analog 8-Br-cAMP was applied. However, the cAMP pathway was unlikely to mediate the OA effect, as the protein kinase A inhibitor RP-cAMPS did not diminish this effect. In contrast, the OA-induced sensitivity enhancement was significantly reduced by KN-62, an inhibitor of Ca²(+) /calmodulin-dependent protein kinase II (CaMKII), and by the Ca²(+) chelator BAPTA-AM. OA depolarized the neurons by 3.8 mV from resting potential, well below the threshold for opening of voltage-activated Ca²(+) channels. OA also reduced the amplitudes of voltage-activated K(+) currents. We propose that OA receptors in VS-3 neurons activate inositol 1,4,5-trisphosphate, leading to Ca²(+) release from intracellular stores. The Ca²(+) surge switches on CaMKII, which modulates voltage-activated K(+) channels, resulting in persistent enhancement in excitability.

Secondhand smoke exposure alters K+ channel function and intrinsic cell excitability in a subset of second-order airway neurons in the nucleus tractus solitarius of young guinea pigs

Extended exposure to secondhand smoke (SHS) in infants and young children increases the incidence of cough, wheeze, airway hyper-reactivity and the prevalence and earlier onset of asthma. The adverse effects may result from environmentally-induced plasticity in the neural network regulating cough and airway function. Using whole-cell patch-clamp recordings in brainstem slices containing anatomically identified second-order lung afferent neurons in the nucleus tractus solitarius (NTS), we determined the effects of extended SHS exposure in young guinea pigs for a duration equivalent to human childhood on the intrinsic excitability of NTS neurons. SHS exposure resulted in marked decreases in the intrinsic excitability of a subset of lung afferent second-order NTS neurons. The neurons exhibited a decreased spiking capacity, prolonged action potential duration, reduced afterhyperpolarization, decrease in peak and steady-state outward currents, and membrane depolarization. SHS exposure effects were mimicked by low concentrations of the K+ channel blockers 4-aminopyridine and/or tetraethyl ammonium. The data suggest that SHS exposure downregulates K+ channel function in a subset of NTS neurons, resulting in reduced cell excitability. The changes may help to explain the exaggerated neural reflex responses in children exposed to SHS.

Random Stimulation of Spider Mechanosensory Neurons Reveals Long-Lasting Excitation by GABA and Muscimol

γ-Aminobutyric acid type A (GABAA) receptor activation inhibits many primary afferent neurons by depolarization and increased membrane conductance. Deterministic (step and sinusoidal) functions are commonly used as stimuli to test such inhibition. We found that when the VS-3 mechanosensory neurons innervating the spider lyriform slit-sense organ were stimulated by randomly varying white-noise mechanical or electrical signals, their responses to GABAA receptor agonists were more complex than the inhibition observed during deterministic stimulation. Instead, there was rapid excitation, then brief inhibition, followed by long-lasting excitation. During the final excitatory phase, VS-3 neuron sensitivity to high-frequency signals increased selectively and their linear information capacity also increased. Using experimental and simulation approaches we found that the excitatory effect could also be achieved by depolarizing the neurons without GABA application and that excitation could override the inhibitory effect produced by increased membrane conductance (shunting). When the VS-3 neurons were exposed to bumetanide, an antagonist of the Cl− transporter NKCC1, the GABA-induced depolarization decreased without any change in firing rate, suggesting that the effects of GABA can be maintained for a long time without additional Cl− influx. Our results show that the VS-3 neuron's response to GABA depends profoundly on the type of signals the neuron is conveying while the transmitter binds to its receptors.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

Activation of ionotropic γ-aminobutyric acid type A (GABAA) receptors depolarizes neurons that have high intracellular [Cl−], causing inhibition or excitation in different cell types. The depolarization often leads to inactivation of voltage-gated Na channels, but additional ionic mechanisms may also be affected. Previously, a simulated model of spider VS-3 mechanosensory neurons suggested that although voltage-activated Na+current is partially inactivated during GABA-induced depolarization, a slowly activating and inactivating component remains and may contribute to the depolarization. Here, we confirmed experimentally, by blocking Na channels prior to GABA application, that Na+current contributes to GABA-induced depolarization in VS-3 neurons. Ratiometric Ca2+imaging experiments combined with intracellular recordings revealed a significant increase in intracellular [Ca2+] when GABAAreceptors were activated, synchronous with the depolarization and probably due to Ca2+influx via low-voltage–activated (LVA) Ca channels. In contrast, GABAB-receptor activation in these neurons was previously shown to inhibit LVA current. Blockade of voltage-gated K channels delayed membrane repolarization, extending GABA-induced depolarization. However, inhibition of Ca channels significantly increased the amplitude of GABA-induced depolarization, indicating that Ca2+-activated K+current has an even stronger repolarizing effect. Regulation of intracellular [Ca2+] is important for many cellular processes and Ca2+control of K+currents may be particularly important for some functions of mechanosensory neurons, such as frequency tuning. These data show that GABAA-receptor activation participates in this regulation.

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.

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.

Shunting versus inactivation: simulation of GABAergic inhibition in spider mechanoreceptors suggests that either is sufficient

Afferent neurons entering the central nervous systems of vertebrates and invertebrates receive presynaptic inhibition on their axon terminals. This usually involves an increase in membrane conductance (shunting) and depolarization (primary afferent depolarization, PAD). In arachnids and crustaceans the peripherally located parts of afferent neurons also receive efferent synapses. GABA (gamma-aminobutyric acid) plays a major role in both central and peripheral inhibition, activating chloride channels that depolarize the membrane and increase its conductance. Although both central and peripheral inhibition have been widely investigated, debate continues about the mechanisms involved, especially concerning the relative contributions of shunting versus inactivation of sodium channels by depolarization. Sensory neurons innervating spider VS-3 slit sensilla are accessible to intracellular recordings during mechanical or electrical stimulation. These neurons are inhibited by GABA, and both the electrophysiology and pharmacology of this inhibition have been studied previously. Here, we developed a Hodgkin-Huxley style model to simulate VS-3 neuron activity before and after GABA treatment. The model indicates that GABA-activated chloride current can entirely account for action potential suppression, and that either shunting or inactivation are sufficient to produce inhibition. This model also demonstrates that slowing of sodium current contributes to inhibition.

Intracellular recording from a spider vibration receptor

The present study introduces a new preparation of a spider vibration receptor that allows intracellular recording of responses to natural mechanical or electrical stimulation of the associated mechanoreceptor cells. The spider vibration receptor is a lyriform slit sense organ made up of 21 cuticular slits located on the distal end of the metatarsus of each walking leg. The organ is stimulated when the tarsus receives substrate vibrations, which it transmits to the organ's cuticular structures, reducing the displacement to about one tenth due to geometrical reasons. Current clamp recording was used to record action potentials generated by electrical or mechanical stimuli. Square pulse stimulation identified two groups of sensory cells, the first being single-spike cells which generated only one or two action potentials and the second being multi-spike cells which produced bursts of action potentials. When the more natural mechanical sinusoidal stimulation was applied, differences in adaptation rate between the two cell types remained. In agreement with prior extracellular recordings, both cell types showed a decrease in the threshold tarsus deflection with increasing stimulus frequency. Off-responses to mechanical stimuli have also been seen in the metatarsal organ for the first time.

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.

Inhibitory glutamate receptors in spider peripheral mechanosensory neurons

Most mechanosensory neurons are inhibited by GABAergic efferent neurons. This inhibition is often presynaptic and mediated by ionotropic GABA receptors at the axon terminals. GABA receptor activation opens Cl- channels, leading to membrane depolarization and an increase in membrane conductance. In many invertebrate preparations, efferent neurons that innervate mechanosensory afferents contain glutamate in addition to GABA, suggesting that the sensory neurons are also modulated by glutamate. However, the effects of glutamate on these neurons are not well understood. Peripheral parts of the spider (Cupiennius salei) mechanosensory neurons are surrounded by efferent fibers immunoreactive to antibodies against GABA and glutamate. GABA and its analogue muscimol were shown to effectively inhibit spider mechanosensory neurons innervating lyriform slit sensilla VS-3 that detects cuticular strains in the leg. Here, we show that glutamate also inhibits the VS-3 neurons, but its effects are different from those of GABA or muscimol, suggesting that it acts on a different group of receptors. GABA and muscimol always depolarized these neurons and the inhibitory effect was strongly correlated with the amount of depolarization. In contrast, glutamate inhibited the VS-3 neurons even when it did not depolarize them. In addition, while glutamate inhibited both the axonal action potentials elicited with electrical stimulation and dendritic action potentials produced by mechanical stimulation, muscimol only inhibited the axonal action potentials. Therefore, the inhibitory glutamate receptors in the VS-3 neurons are distinct from and differently distributed than the GABA receptors, providing a subtle control of the neurons' sensitivity in varying behavioural situations.

Spider peripheral mechanosensory neurons are directly innervated and modulated by octopaminergic efferents

Octopamine is a chemical relative of noradrenaline providing analogous neurohumoral control of diverse invertebrate physiological processes. There is also evidence for direct octopaminergic innervation of some insect peripheral tissues. Here, we show that spider peripheral mechanoreceptors are innervated by octopamine-containing efferents. The mechanosensory neurons have octopamine receptors colocalized with synapsin labeling in the efferent fibers. In addition, octopamine enhances the electrical response of the sensory neurons to mechanical stimulation. Spider peripheral mechanosensilla receive extensive efferent innervation. Many efferent fibers in the legs of Cupiennius salei are GABAergic, providing inhibitory control of sensory neurons, but there is also evidence for other neurotransmitters. We used antibody labeling to show that some efferents contain octopamine and that octopamine receptors are concentrated on the axon hillocks and proximal soma regions of all mechanosensory neurons in the spider leg. Synaptic vesicles in efferent neurons were concentrated in similar areas. Octopamine, or its precursor tyramine, increased responses of mechanically stimulated filiform (trichobothria) leg hairs. This effect was blocked by the octopamine antagonist phentolamine. The octopamine-induced modulation was mimicked by 8-Br-cAMP, a cAMP analog, and blocked by Rp-cAMPS, a protein kinase A inhibitor, indicating that spider octopamine receptors activate adenylate cyclase and increase cAMP concentration. Frequency response analysis showed that octopamine increased the sensitivity of the trichobothria neurons over a broad frequency range. Thus, the major effect of octopamine is to increase its overall sensitivity to wind-borne signals from sources such as flying insect prey or predators.

Acetylcholine receptors in spider peripheral mechanosensilla

Peripherally located parts of spider mechanosensory neurons are modulated by several neurotransmitters released from apposed efferent fibers. Activities of acetylcholine (ACh) synthesizing enzyme choline acetyltransferase (ChAT) and ACh degrading enzyme acetylcholine esterase (AChE) were previously found in some efferent fibers. ChAT activity was also present in all the mechanosensory neurons, while AChE activity was only found in some. We show that spider mechanosensory neurons and probably some efferent neurons are immunoreactive to a monoclonal antibody against muscarinic ACh receptors (mAChRs). However, application of muscarinic agonists did not change the physiological responses or membrane potentials of neurons in the lyriform organ VS-3. Similarly, the sensitivities of the neurons of trichobothria (filiform hairs) remained unchanged after application of these agonists. Therefore, activation of mAChRs may only modulate the function of spider mechanosensory neurons indirectly, for example, by affecting the release of other transmitter(s). However, a subgroup of VS-3 neurons was inhibited by ACh, which also depolarized the membrane similar to these neurons' responses to GABA, suggesting that ACh activates anion channels in these neurons. Interestingly, all of the neurons responding to ACh were the rapidly adapting Type A neurons that were previously shown to express AChE activity.

Mechanotransduction in spider slit sensilla

Mechanoreception is a vital constituent of several sensory modalities and a wide range of internal regulatory processes, but fundamental mechanisms for neural detection of mechanical stimuli have been difficult to characterize because of the morphological properties of most mechanoreceptors and the nature of the stimulus itself. An invertebrate preparation, the VS-3 lyriform slit sense organ of the spider, Cupiennius salei, has proved useful because it possesses large mechanosensory neurons, whose cell bodies are close to the sites of sensory transduction, and accessible to intracellular recording during mechanotransduction. This has made it possible to observe and experiment with all the major stages of mechanosensation. Here, we describe several important findings from this preparation, including the estimated number, conductance and ionic selectivity of the ion channels responsible for mechanotransduction, the major voltage-activated ion channels responsible for action potential encoding and control of the dynamic properties of the neurons, the location of action potential initiation following mechanical stimulation, and the efferent control of mechanoreception. While many details of mechanosensation remain to be discovered, the VS-3 system continues to offer important opportunities to advance our understanding of this crucial physiological process.

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.

Distribution and function of GABAB receptors in spider peripheral mechanosensilla

The mechanosensilla in spider exoskeleton are innervated by bipolar neurons with their cell bodies close to the cuticle and dendrites attached to it. Numerous efferent fibers synapse with peripheral parts of the mechanosensory neurons, with glial cells surrounding the neurons, and with each other. Most of these efferent fibers are immunoreactive to gamma-aminobutyric acid (GABA), and the sensory neurons respond to agonists of ionotropic GABA receptors with a rapid and complete inhibition. In contrast, little is known about metabotropic GABAB receptors that may mediate long-term effects. We investigated the distribution of GABAB receptors on spider leg mechanosensilla using specific antibodies against 2 proteins needed to form functional receptors and an antibody that labels the synaptic vesicles on presynaptic sites. Both anti-GABAB receptor antibodies labeled the distal parts of the sensory cell bodies and dendrites but anti-GABABR1 immunoreactivity was also found in the axons and proximal parts of the cell bodies and some glial cells. The fine efferent fibers that branch on top of the sensory neurons did not show GABAB receptor immunoreactivity but were densely labeled with anti-synapsin and indicated synaptic vesicles on presynaptic locations to the GABAB receptors. Intracellular recordings from sensory neurons innervating the slit sensilla of the spider legs revealed that application of GABAB receptor agonists attenuated voltage-activated Ca2+ current and enhanced voltage-activated outward K+ current, providing 2 possible mechanisms for controlling the neurons' excitability. These findings support the hypothesis that GABAB receptors are present in the spider mechanosensilla where their activation may modulate information transmission.

Active signal conduction through the sensory dendrite of a spider mechanoreceptor neuron

Rapid responses to sensory stimulation are crucial for survival. This must be especially true for mechanical stimuli containing temporal information, such as vibration. Sensory transduction occurs at the tips of relatively long sensory dendrites in many mechanoreceptors of both vertebrates and invertebrates, but little is known about the electrical properties of these crucial links between transduction and action potential generation. The VS-3 slit-sense organ of the spider Cupiennius salei contains bipolar mechanosensory neurons that allow voltage-clamp recording from the somata, whereas mechanotransduction occurs at the tips of 100- to 200-microm-long sensory dendrites. We studied the properties of VS-3 sensory dendrites using three approaches. Voltage-jump experiments measured the spread of voltage outward from the soma by observing total mechanically transduced charge recovered at the soma as a function of time after a voltage jump. Frequency-response measurements between pseudorandom mechanical stimulation and somatic membrane potential estimated the passive cable properties of the dendrite for voltage spread in the opposite direction. Both of these sets of data indicated that the dendritic cable would significantly attenuate and retard a passively propagated receptor potential. Finally, current-clamp observations of receptor potentials and action potentials indicated that action potentials normally start at the distal dendrites and propagate regeneratively to the soma, reducing the temporal delay of passive conduction.

Extracellular acid increases the open probability of transduction channels in spider mechanoreceptors

Ion channels of the epithelial sodium channel, degenerin and acid-sensitive channel (ENaC/DEG/ASIC) family share a number of structural and functional homologies. Several members of this group have been linked to mechanoreception and nociception, but there is no direct evidence that these molecules cause the transduction of mechanical stimuli in any mechanoreceptor. The receptor channels of a spider mechanoreceptor, the VS-3 slit-sense organ of Cupiennius salei, show several similarities to ENaC/DEG/ASIC channels, including Na+ selectivity and amiloride blockade. We recorded the receptor current under voltage clamp in VS-3 neurons at different extracellular pH values. Acid pH partially blocked the delayed rectifier K+ current and increased the receptor current in these cells. Noise analysis of the receptor current showed that low pH increased the open probability of the receptor channels. Therefore, acid sensitivity is a further similarity between these mechanoreceptor channels and the ENaC/DEG/ASIC family.

Activity-dependent sensitivity of proprioceptive sensory neurons in the stick insect femoral chordotonal organ

Mechanosensory neurons exhibit a wide range of dynamic changes in response, including rapid and slow adaptation. In addition to mechanical factors, electrical processes may also contribute to sensory adaptation. We have investigated adaptation of afferent neurons in the stick insect femoral chordotonal organ (fCO). The fCO contains sensory neurons that respond to position, velocity, and acceleration of the tibia. We describe the influence of random mechanical stimulation of the fCO on the response of fCO afferent neurons. The activity of individual sensory neurons was recorded intracellularly from their axons in the main leg nerve. Most fCO afferents (93%) exhibited a marked decrease in response to trapezoidal stimuli following sustained white noise stimulation (bandwidth = 60 Hz, amplitudes from +/-5 to +/-30 degrees ). Concurrent decreases in the synaptic drive to leg motoneurons and interneurons were also observed. Electrical stimulation of spike activity in individual fCO afferents in the absence of mechanical stimulation also led to a dramatic decrease in response in 15 of 19 afferents tested. This indicated that electrical processes are involved in the regulation of the generator potential or encoding of action potentials and partially responsible for the decreased response of the afferents. Replacing Ca(2+) with Ba(2+) in the saline surrounding the fCO greatly reduced or blocked the decrease in response elicited by electrically induced activity or mechanical stimulation when compared with control responses. Our results indicate that activity of fCO sensory neurons strongly affects their sensitivity, most likely via Ca(2+)-dependent processes.

Neuromodulation of arthropod mechanosensory neurons

Arthropod mechanosensory afferents have long been known to receive efferent synaptic connections onto their centrally located axon terminals. These connections cause presynaptic inhibition by attenuating the action potentials arriving at the axon terminals, thus reducing the synaptic potentials in the postsynaptic neurons. This type of inhibition can specifically reduce the excitation of selected postsynaptic neurons while leaving others unaffected. However, recent research has demonstrated that sensory signals detected by arthropod mechanosensory neurons can also be synaptically modulated before they ever arrive at the axon terminals. In arachnids and crustaceans, wide and complex networks of synapses on all parts of the afferent neurons, including the somata and dendrites, provide mechanisms to inhibit or enhance the responses to mechanical stimuli as they are being detected. This modulation will affect the signal transmission to all axonal branches and postsynaptic cells of the affected receptor neuron. In addition to the increased complexity of mechanosensory information transmission produced by these synapses, a variety of circulating neuroactive substances also modulate these neurons by acting on their postsynaptic receptors.

Peripheral GABAergic inhibition of spider mechanosensory afferents

Spider mechanosensory neurons receive an extensive network of efferent synapses onto their sensory dendrites, somata and distal axonal regions. The function of these synapses is unknown. Peripheral synapses are also found on crustacean stretch-receptor neurons but not on mechanosensory afferents of other species, although inhibitory GABAergic synapses are a common feature of centrally located axon terminals. Here we investigated the effects of GABA receptor agonists and antagonists on one group of spider mechanosensory neurons, the slit sense organ VS-3, which are accessible to current- and voltage-clamp recordings. Bath application of GABA activated an inward current that depolarized the membrane and increased the membrane conductance leading to impulse inhibition. VS-3 neuron GABA receptors were activated by muscimol and inhibited by picrotoxin but not bicuculline, and their dose-response relationship had an EC(50) of 103.4 microm, features typical for insect ionotropic GABA receptors. Voltage- and current-clamp analysis confirmed that, while the Na(+) channel inhibition resulting from depolarization can lead to impulse inhibition, the increase in membrane conductance (i.e. 'shunting') completely inhibited impulse propagation. This result argues against previous findings from other preparations that GABA-mediated inhibition is caused by a depolarization that inactivates Na(+) conductance, and it supports those findings that assign this role to membrane shunting. Our results show that GABA can rapidly and selectively inhibit specific mechanoreceptors in the periphery. This type of peripheral inhibition may provide spiders with a mechanism for distinguishing between signals from potential prey, predators or mates, and responding with appropriate behaviour to each signal.

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.

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.

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.