Temperature sensitivity of transduction and action potential conduction in a spider mechanoreceptor

Walking bumblebees see faster

The behavioural state of animals has profound effects on neuronal information processing. Locomotion changes the response properties of visual interneurons in the insect brain, but it is still unknown if it also alters the response properties of photoreceptors. Photoreceptor responses become faster at higher temperatures. It has therefore been suggested that thermoregulation in insects could improve temporal resolution in vision, but direct evidence for this idea has so far been missing. Here, we compared electroretinograms from the compound eyes of tethered bumblebees that were either sitting or walking on an air-supported ball. We found that the visual processing speed strongly increased when the bumblebees were walking. By monitoring the eye temperature during recording, we saw that the increase in response speed was in synchrony with a rise in eye temperature. By artificially heating the head, we show that the walking-induced temperature increase of the visual system is sufficient to explain the rise in processing speed. We also show that walking accelerates the visual system to the equivalent of a 14-fold increase in light intensity. We conclude that the walking-induced rise in temperature accelerates the processing of visual information-an ideal strategy to process the increased information flow during locomotion.

Temperature dependence of rapidly adapting mechanically activated currents in rat dorsal root ganglion neurons

Rapidly adapting mechanically activated channels (RA) are expressed on somatosensory neurons and thought to play a role in mechanical transduction. Because mechanical sensations can be significantly affected by temperatures, we examined thermal sensitivity of RA currents in cultured dorsal root ganglion (DRG) neurons to see if RA channel activity is highly temperature-dependent. RA currents were evoked from DRG neurons by membrane displacements and recorded by the whole-cell patch-clamp recording technique. We found that RA currents were significantly enhanced by warming temperatures from 22 to 32 °C and reduced by cooling temperatures from 24 to 14 °C. RA channel activation exhibited steep temperature-dependence with a large temperature coefficient (Q10>5) and a high activation energy (Ea>30 kcal/mol). We further showed that RA channel activation by mechanical stimulation led to membrane depolarization, which could result in action potential firing at 22 °C or 32 °C but not at 14 °C. Taken together, our results provide the measurements of thermal dynamics and activation energy of RA channels, and suggest that a high energy barrier is present for RA channels to open. These findings are in agreement with temperature sensitivity of mechanical sensations in mammals.

Multiple mechanisms generate the resting activity of filiform sensilla in the firebug (Pyrrhocoris apterus L.; Heteroptera)

The resting activity was studied in filiform sensilla of the firebug (Pyrrhocoris apterus). Three functional types (T(1), T(2) and T(3)) were detected on the abdomen. A resting discharge of nerve impulses is present in all-always in types T(1) and T(2) and occasionally in type T(3). In T(1) the mean rate is 57, in T(2) 3.3 and in T(3) 0.5 imp/s. Shortening the hair length had a negligible effect on the resting discharge, which indicates an intrinsic origin. The resting activity is highly temperature dependent. In T(1), the activation energy was 56.8, in T(2) 84 and in T(3) 61.4 kJ/mol (Q (10): 2.27, 5.6 and 5.5, respectively). Such values are typical for mechano-transduction, suggesting the involvement of the transduction mechanism itself. The destruction of the hair base in T(1) caused halving of the original discharge rate and shifted the discharge to a regular interval mode. The activation energy decreased to 38 kJ/mol. The destruction of the hair bases in T(2) and T(3) completely abolished the discharge. It appears that at least two mechanisms are involved in the generation of the resting activity in T(1) units while only one can be assumed in case of T(2) and T(3).

The effects of temperature on peripheral neuronal function in eurythermal and stenothermal crustaceans

To determine whether neuronal function in Antarctic crustaceans is adapted to the low and narrow range of temperatures at which these animals live, we have compared conduction velocities in the peripheral nervous systems of two temperate species, the decapod Carcinus maenas and the isopod Ligia oceanica, and two Antarctic species, the isopod Glyptonotus antarcticus and the amphipod Paraceradocus gibber.

Neuronal conduction velocity differs among the species in the order C. maenas > G. antarcticus > P. gibber > L. oceanica. When measured at the normal environmental temperatures characteristic of each species, conduction velocity of the Antarctic peracarid P. gibber is greater than that of its similar sized temperate relativeL. oceanica, demonstrating complete thermal compensation.

The temperate decapod C. maenas has a higher thermal dependence of neuronal conduction velocity than either of the Antarctic species, G. antarcticus and P. gibber, but the temperate L. oceanica does not. These data, when collated with published values,indicate that peracarid crustaceans (L. oceanica, G. antarcticus and P. gibber) have lower neuronal conduction velocities and a lower thermal dependence of neuronal conduction velocity than do other arthropods, irrespective of habitat. There is a linear dependence of conduction velocity on temperature down to –1.8°C in all three species. Our data extend by more than 10° the lower range of temperatures at which conduction velocities have been tested systematically in previous studies.

The upper thermal block of neuronal conduction is similar in C. maenas, G. antarcticus, P. gibber and L. oceanica at 24.5, 19.5, 21.5 and 19.5°C, respectively. This suggests that failure to conduct action potentials is not what determines the mortality of Antarctic invertebrates at approximately 10°C.

The excitability of axons in the leg nerve of G. antarcticus is not affected by temperatures ranging from –1.8 to +18°C. The responses of sensory neurones activated by movements of spines on the leg,however, are strongly modulated by temperature, with maximal responses at 5–10°C; well above the normal environmental temperature range for the species. The responses fail at 20–22°C.

The number of large diameter axons (which produce the fast action potentials recorded in this study) is the same in L. oceanica and G. antarcticus, but the median axon diameter is greater in L. oceanica than G. antarcticus. In G. antarcticus,however, there are glial wrappings around some large (>5 μm diameter)axons that may increase their conduction velocity. Such wrappings are not found in L. oceanica.

Ambient temperature affects mechanosensory host location in a parasitic wasp

Certain parasitic wasps (Ichneumonidae, Pimplinae) use self-produced vibrations transmitted on plant substrate to locate their immobile concealed hosts (i.e. lepidopteran pupae). This mechanosensory mechanism, called the vibrational sounding, depends both on physical cues of the environment and physical activity of the parasitoid and is postulated to depend on ambient temperature. We analysed the influences of temperature on vibrational sounding by choice experiments using plant-stem models with hidden host mimics in the temperate species Pimpla turionellae. The results show a significant effect of temperature on host-location activity and on the success of this process. Outside an optimum range, the performance of the wasps decreased both at low and high temperatures. Below 10 degrees C and beyond 24 degrees C, the wasps displayed (1) substantial reduction in responsiveness, i.e. proportion of females showing ovipositor insertions, (2) reduction of quantitative activity with ovipositor insertions in the individuals, and (3) reduced precision of mechanosensory host location. Nevertheless, female wasps were able to locate their host over a surprisingly broad range of ambient temperatures which indicates that the wasps are able to compensate for temperature effects on vibrational sounding.

Temperature compensation in the escape response of a marine copepod, Calanus finmarchicus (Crustacea)

Calanus finmarchicus, the dominant mesozooplankter of the North Atlantic, is an important food source for many fishes and other planktivores. This species, which has limited diel vertical migration, depends on its fast-start escape response to evade predators. It has myelinated neuronal axons, which contribute to its rapid and powerful escape response. The thermal environment that C. finmarchicus inhabits ranges from below 0 degrees C to 16 degrees C. Previous studies have shown that respiration, growth, and reproductive rates are strongly dependent on temperature, with Q10 > 2.5. A comparable dependence of the escape response could place the animal at higher risk for cold-compensated predators. Our work focused on the temperature dependence of the behavioral response to stimuli that mimic predatory attacks. We found that in contrast to other biological processes, all aspects of the escape response showed a low dependence on temperature, with Q10 values below 2. This low temperature dependence was consistent for escape parameters that involved neural as well as muscle components of the behavioral response. These findings are discussed in the contexts of the predator-prey relations of copepods and the thermal dependence of behavior in other taxa.

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.

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.

Peripheral synaptic contacts at mechanoreceptors in arachnids and crustaceans: morphological and immunocytochemical characteristics

Two types of sensory organs in crustaceans and arachnids, the various mechanoreceptors of spiders and the crustacean muscle receptor organs (MRO), receive extensive efferent synaptic innervation in the periphery. Although the two sensory systems are quite different-the MRO is a muscle stretch receptor while most spider mechanoreceptors are cuticular sensilla-this innervation exhibits marked similarities. Detailed ultrastructural investigations of the synaptic contacts along the mechanosensitive neurons of a spider slit sense organ reveal four important features, all having remarkable resemblances to the synaptic innervation at the MRO: (1) The mechanosensory neurons are accompanied by several fine fibers of central origin, which are presynaptic upon the mechanoreceptors. Efferent control of sensory function has only recently been confirmed electrophysiologically for the peripheral innervation of spider slit sensilla. (2) Different microcircuit configuration types, identified on the basis of the structural organization of their synapses. (3) Synaptic contacts, not only upon the sensory neurons but also between the efferent fibers themselves. (4) Two identified neurotransmitter candidates, GABA and glutamate. Physiological evidence for GABAergic and glutamatergic transmission is incomplete at spider sensilla. Given that the sensory neurons are quite different in their location and origin, these parallels are most likely convergent. Although their significance is only partially understood, mostly from work on the MRO, the close similarities seem to reflect functional constraints on the organization of efferent pathways in the brain and in the periphery.