Temperature-sensitive gating in a descending visual interneuron, DCMD

Food deprivation and prior anoxic coma have opposite effects on the activity of a visual interneuron in the locust

We compared how different metabolic stressors, anoxic coma and food deprivation, affected signaling in neural tissue. We used the locust's Descending Contralateral Movement Detector (DCMD) interneuron because its large axon, high firing frequencies, and rapid conduction velocity make it energetically expensive. We exposed locusts to a 30min anoxic coma or 1day of food deprivation and found contrasting effects on signaling within the axon. After a prior anoxic coma, the DCMD fired fewer high-frequency (>200Hz) action potentials (APs) (Control: 12.4±1.6; Coma: 6.3±0.9) with a reduction in axonal conduction velocity (CV) at all frequencies (∼4-8%) when presented with a standard looming visual stimulus. Prior anoxic coma was also associated with a loss of supernormal conduction by reducing both the number of supernormal APs and the firing frequency with the highest CV. Initially, food deprivation caused a significant increase in the number of low- and high-frequency APs with no differences observed in CV. After controlling for isolation, food deprivation resulted in an increase in high-frequency APs (>200Hz: Control: 17.1±1.7; Food-deprived: 19.9±1.3) and an increase in relative conduction velocity for frequencies >150Hz (∼2%). Action potentials of food-deprived animals had a smaller half-width (Control: 0.45±0.02ms; Food-deprived: 0.40±0.01ms) and decay time (Control: 0.62±0.03ms; Food-deprived: 0.54±0.02ms). Our data indicate that the effects of metabolic stress on neural signaling can be stressor-dependent.

Octopamine stabilizes conduction reliability of an unmyelinated axon during hypoxic stress

Mechanisms that could mitigate the effects of hypoxia on neuronal signaling are incompletely understood. We show that axonal performance of a locust visual interneuron varied depending on oxygen availability. To induce hypoxia, tracheae supplying the thoracic nervous system were surgically lesioned and action potentials in the axon of the descending contralateral movement detector (DCMD) neuron passing through this region were monitored extracellularly. The conduction velocity and fidelity of action potentials decreased throughout a 45-min experiment in hypoxic preparations, whereas conduction reliability remained constant when the tracheae were left intact. The reduction in conduction velocity was exacerbated for action potentials firing at high instantaneous frequencies. Bath application of octopamine mitigated the loss of conduction velocity and fidelity. Action potential conduction was more vulnerable in portions of the axon passing through the mesothoracic ganglion than in the connectives between ganglia, indicating that hypoxic modulation of the extracellular environment of the neuropil has an important role to play. In intact locusts, octopamine and its antagonist, epinastine, had effects on the entry to, and recovery from, anoxic coma consistent with octopamine increasing overall neural performance during hypoxia. These effects could have functional relevance for the animal during periods of environmental or activity-induced hypoxia.

Burst Firing in a Motion-Sensitive Neural Pathway Correlates with Expansion Properties of Looming Objects that Evoke Avoidance Behaviors

The locust visual system contains a well-defined motion-sensitive pathway that transfers visual input to motor centers involved in predator evasion and collision avoidance. One interneuron in this pathway, the descending contralateral movement detector (DCMD), is typically described as using rate coding; edge expansion of approaching objects causes an increased rate of neuronal firing that peaks after a certain retinal threshold angle is exceeded. However, evidence of intrinsic DCMD bursting properties combined with observable oscillations in mean firing rates and tight clustering of spikes in raw traces, suggest that bursting may be important for motion detection. Sensory neuron bursting provides important timing information about dynamic stimuli in many model systems, yet no studies have rigorously investigated if bursting occurs in the locust DCMD during object approach. We presented repetitions of 30 looming stimuli known to generate behavioral responses to each of 20 locusts in order to identify and quantify putative bursting activity in the DCMD. Overall, we found a bimodal distribution of inter-spike intervals (ISI) with peaks of more frequent and shorter ISIs occurring from 1–8 ms and longer less frequent ISIs occurring from 40–50 ms. Subsequent analysis identified bursts and isolated single spikes from the responses. Bursting frequency increased in the latter phase of an approach and peaked at the time of collision, while isolated spiking was predominant during the beginning of stimulus approach. We also found that the majority of inter-burst intervals (IBIs) occurred at 40–50 ms (or 20–25 bursts/s). Bursting also occurred across varied stimulus parameters and suggests that burst timing may be a key component of looming detection. Our findings suggest that the DCMD uses two modes of coding to transmit information about looming stimuli and that these modes change dynamically with a changing stimulus at a behaviorally-relevant time.

Visual stimulus characteristics that elicit tracking and striking in the Praying Mantises, Parasphendale affinis (Giglio-Tos), Popa spurca (Stål), and Sphodromantis lineola (Burmeister)

We tested three species of praying mantis, Parasphendale affinis (Giglio-Tos), Popa spurca (Stål), and Sphodromantis lineola (Burmeister) with computer generated stimuli that differed in size, contrast, configuration and movement pattern to determine the effects of these parameters on visual tracking and striking behavior. Overall, black disks moving erratically against a white background were strong releasers of both behaviors. When stimulus presentation order was randomized by size, P. affinis and P. spurca struck at progressively higher rates as the stimuli enlarged up to 44 deg; S. lineola struck most at intermediate sized (10-20 deg) disks. When disks were size-ordered from small to large, P. affinis and S. lineola struck at higher rates to the smaller disks; however, when the order was reversed, the early appearance of large disks suppressed subsequent responses to the smaller. Stimulus order did not differentially affect the responses of P. spurca. All species responded at higher rates to black disks moving against a white background versus the reverse. However, only P. spurca and S. lineola responded at higher rates to relatively darker grey disks, only P. affinis responded to mottled grey disks moving against an identically patterned background, and only P. spurca struck more frequently in response to rectangular stimuli oriented parallel (versus perpendicular) to their direction of movement. In conjunction with data on other species, these results support the hypothesis that praying mantises recognize prey based on assessing several category-specific, spatiotemporal features, e.g., size, contrast, speed, movement pattern, and leading edge length.

Coma in response to environmental stress in the locust: a model for cortical spreading depression

Spreading depression (SD) is an interesting and important phenomenon due to its role in mammalian pathologies such as migraine, seizures, and stroke. Until recently investigations of the mechanisms involved in SD have mostly utilized mammalian cortical tissue, however we have discovered that SD-like events occur in the CNS of an invertebrate model, Locusta migratoria. Locusts enter comas in response to stress during which neural and muscular systems shut down until the stress is removed, and this is believed to be an adaptive strategy to survive extreme environmental conditions. During stress-induced comas SD-like events occur in the locust metathoracic ganglion (MTG) that closely resemble cortical SD (CSD) in many respects, including mechanism of induction, extracellular potassium ion changes, and propagation in areas equivalent to mammalian grey matter. In this review we describe the generation of comas and the associated SD-like events in the locust, provide a description of the similarities to CSD, and show how they can be manipulated both by stress preconditioning and pharmacologically. We also suggest that locust SD-like events are adaptive by conserving energy and preventing cellular damage, and we provide a model for the mechanism of SD onset and recovery in the locust nervous system.

Loss of potassium homeostasis underlies hyperthermic conduction failure in control and preconditioned locusts

At extreme temperature, neurons cease to function appropriately. Prior exposure to a heat stress (heat shock [HS]) can extend the temperature range for action potential conduction in the axon, but how this occurs is not well understood. Here we use electrophysiological recordings from the axon of a locust visual interneuron, the descending contralateral movement detector (DCMD), to examine what physiological changes result in conduction failure and what modifications allow for the observed plasticity following HS. We show that at high temperature, conduction failure in the DCMD occurred preferentially where the axon passes through the thoracic ganglia rather than in the connective. Although the membrane potential hyperpolarized with increasing temperature, we observed a modest depolarization (3-6 mV) in the period preceding the failure. Prior to the conduction block, action potential amplitude decreased and half-width increased. Both of these failure-associated effects were attenuated following HS. Extracellular potassium concentration ([K+]o) increased sharply at failure and the failure event could be mimicked by the application of high [K+]o. Surges in [K+]o were muted following HS, suggesting that HS may act to stabilize ion distribution. Indeed, experimentally increased [K+]o lowered failure temperature significantly more in control animals than in HS animals and experimentally maintained [K+]o was found to be protective. We suggest that the more attenuated effects of failure on the membrane properties of the DCMD axon in HS animals is consistent with a decrease in the disruptive nature of the [K+]o-dependent failure event following HS and thus represents an adaptive mechanism to cope with thermal stress.