Forewing Stretch Receptor Research Articles

Locust wing stretch receptor adaptation and recovery from adaptation

Adaptation of action potential frequency is a common feature of neurons from various parts of the nervous system. Adaptation is thought to contribute to information coding, integration, input filtering and saving of metabolic resources. Despite inten sive investigation, the kinetics of adaptation and recovery from adaptation are not fully described. The locust forewing stretch receptor (fSR) was used to investigate the kinetics of adaptation and recovery from adaptation in response to wing elevation. We have found that adaptation can be well described by a sum of two exponential decays with time constants 5.04 ± 0.6 s and 24.86 ± 2.45 s. The dependence of recovery on adaptation time was biphasic in 70% of the fSRs investigated. In these fSRs, the break point (10.54 ± 1.09 s) separates the initial linear phase from the following exponential one. These findings suggest that at least two mechanisms with different kinetics are involved in the adaptation of locust wing stretch receptor.

Skėrio sparno vyrio tempimo receptoriaus atsako adaptacija esant dinaminiam režimui

* Corresponding author: E-mail: osvaldas.ruksenas@gf.vu.lt Locust wing stretch receptor is known to exhibit fi ring frequency adaptation when the forewing is lift ed to a constant position. However, during fl ight, the wing periodically moves and the receptor’s response properties are in this dynamic mode unknown. We recorded extracellularly the response from the Locusta migratoria forewing stretch receptor while the wing was periodically lift ed and found that the adaptation of the stretch receptor is expressed in a dynamic mode as well; however, this adaptation is weaker than in the static mode.

Identification of the neurotransmitters involved in modulation of transmitter release from the central terminals of the locust wing hinge stretch receptor

The flight motor system of the locust represents a model preparation for the investigation of neuromodulation. At the wing hinges are stretch receptors important in generating and controlling the flight motor pattern. The forewing stretch receptor (fSR) makes direct cholinergic synapses with depressor motor neurons (MN) controlling that wing, including the first basalar MN (BA1). The fSR/BA1 synapse is modulated by muscarinic cholinergic receptors located on gamma-aminobutyric acid (GABA)-ergic interneurons (Judge and Leitch [1999a] J. Comp. Neurol. 407:103-114; Judge and Leitch [1999b] J. Neurobiol. 40:420-431). However, electrophysiology has shown that fSR/BA is also modulated by biogenic amines (Leitch et al. [2003] J. Comp. Neurol. 462:55-70). We have used electron microscopic immunocytochemistry (ICC) to identify the neurotransmitters in neurons presynaptic to the fSR and to determine the relative proportion of these different classes of modulatory inputs. Approximately 55% of all inputs to the fSR are glutamate-IR, indicating that glutamatergic neurons may also play an important role in presynaptically modulating the fSR terminals. Anti-GABA ICC confirmed that over 40% of inputs to the fSR are GABA-IR (Judge and Leitch [1999a] J. Comp. Neurol. 407:103-114). Labelling sections with an antioctopamine antibody revealed neurons containing distinctive large, electron-dense granules, which could reliably be used to identify them. Aminergic neurons that modulate the synapse may have very few morphologically recognizable synaptic outputs. Although putative octopaminergic processes were found in close contact to horseradish peroxidase-filled fSR profiles, no morphologically recognizable synaptic inputs to the fSR were evident. Collectively, these data suggest that most inputs to the fSR are from either glutamatergic or GABAergic neurons.

Octopaminergic modulation of synaptic transmission between an identified sensory afferent and flight motoneuron in the locust

The role of the biogenic amine octopamine in modulating cholinergic synaptic transmission between the locust forewing stretch receptor neuron (fSR) and the first basalar motoneuron (BA1) was investigated. The amines 5-hydroxytryptamine (5-HT, serotonin) and dopamine were also studied. Bath application of octopamine, 5-HT, and dopamine at concentrations of 10(-4) M reversibly decreased the amplitude of monosynaptic excitatory postsynaptic potentials (EPSPs) evoked in BA1 by electrically stimulating the fSR axon. These effects occurred without any detectable change in either input resistance or membrane potential of BA1. The amines also reversibly decreased the amplitude of responses to acetylcholine (ACh) pressure-applied to the soma of BA1. The muscarinic antagonist scopolamine (10(-6) M) had no significant effect on the octopamine-induced decrease in ACh responses. These observations suggest that these amines potentially could physiologically depress cholinergic transmission between fSR and BA1, at least in part, by altering nicotinic rather than muscarinic cholinergic receptor function. Although the octopaminergic agonists naphazoline and tolazoline both mimicked the actions of octopamine, the receptor responsible for octopamine-mediated modulation could not be characterized since amine receptor antagonists tested on the preparation had complex actions. Confocal immunocytochemistry revealed intense octopamine immunoreactivity in the anterior lateral association center, thus confirming the presence of octopamine in neuropil regions containing fSR/BA1 synapses and therefore supporting a role for this amine in the modulation of synaptic transmission between the fSR and BA1. 5-HT-immunoreactivity, conversely, was concentrated within the ventral association centers; very little staining was observed in the dorsal neuropil regions in which fSR/BA1 synapses are located.

Intrinsic noise at synapses between a wing hinge stretch receptor and flight motor neurons in the locust.

Variability in postsynaptic potential (PSP) amplitude due to intrinsic noise limits the reliability of communication between neurons. I measured PSP variability at synapses between a forewing stretch receptor and wing depressor motor neurons in locusts, a pathway that is important in the control of flying. The intrinsic noise in the stretch receptor output synapse was measured by subtracting the background noise, originating in other synaptic pathways onto the motor neuron, from the variability in the amplitudes of PSPs evoked by the stretch receptor. Intrinsic synaptic noise caused successive PSPs to vary by 4-10 % in basalar and subalar flight motor neurons. Recordings from pairs of these wing depressor motor neurons showed that the amount of transmitter released varied independently between different output sites from the stretch receptor. Histograms of excitatory postsynaptic potential amplitude were normal distributions that lacked separate peaks. I estimate that quantal amplitude is significantly less than 0.1 mV and that several hundred quanta are released for each presynaptic spike. This accords well with a previous estimate of the number of discrete anatomical synapses and would facilitate modulation of output from the stretch receptor.

Modulation of transmitter release from the locust forewing stretch receptor neuron by GABAergic interneurons activated via muscarinic receptors

Modulation of transmitter release from the locust forewing stretch receptor neuron by GABAergic interneurons activated via muscarinic receptors

Modulation of transmitter release from the locust forewing stretch receptor neuron by GABAergic interneurons activated via muscarinic receptors

The role of muscarinic receptors in the down-regulation of acetylcholine (ACh) release from the locust forewing stretch receptor neuron (fSR) terminals has been investigated. Electrical stimulation of the fSR evokes monosynaptic excitatory postsynaptic potentials (EPSPs) in the first basalar motoneuron (BA1), produced mainly by the activation of postsynaptic nicotinic cholinergic receptors. The general muscarinic antagonists scopolamine (10(-6) M) and atropine (10(-8) to 10(-6) M) caused a reversible increase in the amplitude of electrically evoked EPSPs. However, scopolamine (10(-6) M) caused a slight depression in the amplitude of responses to ACh pressure-applied to the soma of BA1. These observations indicate that the EPSP amplitude enhancement is due to the blockade of muscarinic receptors on neurons presynaptic to BA1. The muscarinic receptors may be located on the fSR itself and act as autoreceptors, and/or they may be located on GABAergic interneurons which inhibit ACh release from the fSR. Electron microscopical immunocytochemistry has revealed that GABA-immunoreactive neurons make presynaptic inputs to the fSR. The GABA antagonist picrotoxin (10(-6) M) caused a reversible increase in the EPSP amplitude, which does not appear to be due to an increase in sensitivity of BA1 to ACh, as picrotoxin (10(-6) M) slightly decreased ACh responses recorded from BA1. Application of scopolamine (10(-6) M) to a preparation preincubated with picrotoxin did not cause the EPSP amplitude enhancement normally seen in control experiments; in fact, it caused a slight depression. This indicates that at least some of the presynaptic muscarinic receptors are located on GABAergic interneurons that modulate transmission at the fSR/BA1 synapse.

GABA immunoreactivity in processes presynaptic to the locust wing stretch receptor neuron

Studies on the locust Locusta migratoria (Leitch et al. [1998] Soc. Neurosci. Abstr. 624.10) suggest that gamma-aminobutyric acid (GABA)ergic interneurons, activated via muscarinic receptors, may be involved in modulation of transmitter release at synapses between the forewing stretch receptor (fSR) and wing depressor motoneurons. To help elucidate the role GABAergic interneurons may play in modulation of transmitter release from fSR terminals, the proportion and distribution of GABA-immunoreactive (GABA-IR) inputs to the fSR were analysed using double-labelling (horseradish peroxidase and GABA immunocytochemistry) and electron microscopy. Forty-three percent of synaptic inputs to the fSR were from GABA-IR profiles, the majority of which were located on lateral branches. The highest proportion (57%) of inputs to the fSR, however, were from non-IR processes containing either clear spherical vesicles or mixed clear and dense-cored vesicles. Outputs from the fSR to GABA-IR profiles were also found, although their number was relatively low (7%). Networks were identified in which both the fSR and its non-IR postsynaptic partner received input from the same GABA-IR neuron. Such an arrangement would allow both pre- and postsynaptic inhibition of fSR afferent outputs, for example at fSR/motoneuron synapses. These observations demonstrate that the fSR does receive presynaptic inputs from GABA-IR neurons, thus providing morphological support for pharmacological and electrophysiological findings that GABAergic neurons are involved in the presynaptic modulation of the fSR. Nevertheless, modulation at this synapse may be more intricate and involve other, as yet unidentified, neurotransmitters released from non GABA-IR presynaptic processes and also muscarinic receptors located on the fSR itself.

Effects of heat stress on axonal conduction in the locust flight system

Pretreatment of tissues or whole organisms with high, sublethal temperatures (heat shock) induces thermotolerance to normally lethal temperatures. It is of interest whether heat shock induces protection of neuronal function at normally lethal temperatures by investigating effects of heat shock on the temperature sensitivity of neuronal parameters in the locust flight system. The rhythm frequency of the deafferented flight motor was measured as well as the conduction velocity and amplitude of extracellularly recorded action potentials conveyed along the forewing stretch receptor axon. Measurements were made at temperatures ranging from 10 to 50°C in heat shocked and control animals. The deafferented rhythm was less sensitive to temperature changes above 35°C in heat shocked animals. The conduction velocity and relative amplitude of action potentials conveyed along the stretch receptor axon were less sensitive to temperature increases above 20°C in heat shocked animals. These data suggest that heat shock conserves the operation of the flight system at high temperatures. This may be accomplished by a decrease in the thermosensitivity of the conduction velocity and amplitude of action potentials within the central flight circuitry. The latter effect may serve to protect synaptic interactions and thus allow the circuitry to operate within optimal parameters.

Co-ordination of the Flight Motor Pattern with Forewing Stretch Receptor Stimulation in Immature and Mature Adult Locusts

Abstract We investigated the effect of proprioceptive feedback on the flight motor pattern during maturation of the locust flight system by stimulating the forewing stretch receptors (fSRs) in immature and mature adults and recording the frequency of wing depressor muscle activity. Stimulation of both fSRs produced co-ordination with the motor pattern in 4 of 10 immature animals and in 6 of 16 mature animals. The strength of co-ordination was measured as the variability of the phase of hindwing depressor muscle activity within the stimulus period. Co-ordination was stronger and the phase values higher in mature animals presented with a stimulus that mimics fSR activity in response to rhythmic wing elevation. We conclude that maturation of the flight motor is due, in part, to a change in the response of the central circuitry to a change in the timing of proprioceptive input.

Structure of the forewing stretch receptor axon in immature and mature adult locusts.

During the first 2 weeks following imaginal ecdysis, the wingbeat frequency of Locusta migratoria doubles, and the activity of the forewing stretch receptor (fSR), in response to wing elevation, increases. We examined the three-dimensional structure of the centrally projecting axon of the fSR during adult maturation to determine if there are changes in the branching geometry. We found that changes occur in the mesothoracic projection (IISR Meso). Here, there was a significant increase in the volume of the projection from 2.3 x 10(4) +/- 0.2 x 10(4) microns 3 in immature locusts to 6.0 x 10(4) +/- 1.2 x 10(4) microns 3 in mature locusts. There were also significant increases in the total length, the number of branch points, the number of axonal swellings, and the diameters of first- and second-order branches of the projection. No significant changes were observed in the prothoracic projection (IISR Pro), and the only significant change observed in IISR Meta was negative allometric growth relative to IISR Meso. These results demonstrate that during adult maturation, growth of the fSR axon is heteromorphic between different ganglionic projections and that there is a potential increase in the connectivity of IISR Meso to other flight neurons in the mesothoracic ganglion. We suggest that this may be a mechanism for maintaining the efficacy of afferent input to flight interneurons that are also growing during maturation.

Modulation of transmitter release from the terminals of the locust wing stretch receptor neuron by muscarinic antagonists

The forewing stretch receptor (SR) neuron makes monosynaptic connections with wing depressor motoneurons; in this article the pharmacology of its output onto the first basalar motoneuron (BA1) has been investigated. The SR, like other insect afferents that have been studied so far, appears to be cholinergic; transmission was suppressed reversibly by the nicotinic antagonist gallamine (10(-4) M) and irreversibly by alpha-bungarotoxin (10(-6) M). The choline reuptake blocker hemicholinium-3 (10(-4) M) also caused a reversible reduction in the amplitude of SR excitatory postsynaptic potentials (EPSPs) recorded in BA1. The receptor subtype nonselective muscarinic antagonists atropine (10(-4) M), scopolamine (10(-4) M), and quinuclidinyl benzilate (10(-5) M), unlike nicotinic antagonists, caused an augmentation in EPSP amplitude. This effect does not appear to be caused by an increase in sensitivity of the motoneuron to acetylcholine (ACh), since atropine produced a marked reduction rather than an increase in the amplitude of responses to ACh pressure applied to the soma of BA1. Scopolamine only caused a modest reduction in the amplitude of ACh somatic responses. The simplest explanation for these observations is that muscarinic antagonists bring about an increase in EPSP amplitude by blockade of presynaptic autoreceptors that normally down-regulate the release of ACh from SR terminals. The effects of muscarinic receptor subtype-selective antagonists indicate that presynaptic receptors in this preparation may have a pharmacological profile more similar to that of vertebrate M2 receptors than to that of M1 or M3 subtypes. The functional significance of autoreceptors in this preparation are discussed.

Effects of maturation on synaptic potentials in the locust flight system

We have measured parameters of identified excitatory postsynaptic potentials from flight interneurons in immature and mature adult locusts (Locusta migratoria) to determine whether parameters change during imaginal maturation. The presynaptic cell was the forewing stretch receptor. The postsynaptic cells were flight interneurons that were filled with Lucifer Yellow and identified by their morphology. Excitatory postsynaptic potentials from different postsynaptic cells had characteristic amplitudes. The amplitude, time to peak, duration at half amplitude and the area above the baseline of excitatory postsynaptic potentials did not change with maturation. The latency from action potentials in the forewing stretch receptor to onset of excitatory postsynaptic potentials decreased significantly with maturation. We suggest this was due to an increase in conduction velocity of the forewing stretch receptor. We also measured morphological parameters of the postsynaptic cells and found that they increased in size with maturation. Growth of the postsynaptic cell should cause excitatory postsynaptic potential amplitude to decrease as a result of a decrease in input resistance, however, this was not the case. Excitatory postsynaptic potentials in immature locusts depress more than in mature locusts at high frequencies of presynaptic action potentials. This difference in frequency sensitivity of the immature excitatory postsynaptic potentials may account in part for maturation of the locust flight rhythm generator.

Activity of the forewing stretch receptor in immature and mature adult locusts

Maturation of the flight system of Locusta migratoria occurs during the first two weeks following imaginal ecdysis. One aspect of maturation is an increase in the wingbeat frequency from about 13 Hz to about 23 Hz. We investigated physiological and anatomical mechanisms that may contribute to this process. The difference between the frequencies of the central flight rhythms of immature and mature deafferented preparations was not as great as that between the wingbeat frequencies of immature and mature intact animals. Results from static and dynamic wing elevation showed that the intensity of the forewing stretch receptor response to a given stimulus increased during maturation. The diameter of the main stretch receptor axon was larger and the conduction velocity of signals conveyed along the forewing stretch receptor and the dorsal longitudinal motoneuron was faster in mature than in immature animals. We conclude that during maturation of the flight system the forewing stretch receptor responds to wing elevation with a higher frequency signal that reaches the central circuitry faster. These findings are discussed in the context of a model that describes the influence of stretch receptor input on wingbeat frequency along with other potential mechanisms involved in flight maturation.

Effects of temperature on synaptic potentials in the locust flight system.

1. Neuronal circuitry in the locust flight system operates normally within a temperature range of 24-42 degrees C. I investigated the effects of temperature on parameters of postsynaptic potentials generated in different neurons following action potentials of the forewing stretch receptor. 2. Increases in temperature reduced latency, time-to-peak and duration (Q10s = 0.51, 0.70, and 0.68, respectively; 24-34 degrees C) and increased the slope (Q10 = 1.13; 24-34 degrees) of the excitatory postsynaptic potential (EPSP). However, increases in temperature increased EPSP amplitude below room temperature (Q10 = 1.25; 14-24 degrees C) but decreased EPSP amplitude above room temperature (Q10 = 0.80; 24-34 degrees C). 3. I conclude that neuronal and synaptic function were affected by temperature in ways predictable by well-established thermal effects on channel conductance and kinetics and on membrane properties. Thus temperature compensation of the output of the flight system must be mediated in some way by the operation of the circuitry. 4. I propose that below room temperature EPSP amplitude was increased by predominant effects on channel conductance and membrane time constant, and above room temperature EPSP amplitude was decreased by a predominant effect on the amplitude and duration of the presynaptic action potential. Further, I suggest that the frequency of the output rhythm is unaffected by the amplitude of single EPSPs, within permissive limits.

Sensory adaptation: extracellular recording from locust wing hinge stretch receptor.

Good student laboratory exercises that do not require much manipulative or technical expertise of the student and that have minor equipment demands are hard to find. One experiment that has these desirable characteristics is the description of adaptation of the firing frequency of the locust forewing stretch receptor after elevation of the wing. Unambiguous recordings of the activity of the stretch receptor can be made using a simple monopolar hook electrode inserted into the thoracic cavity of a decapitated locust. Elevation movements of the forewing are simple to perform and measure. The response of the stretch receptor as a function of time and the stimulus history is monitored. Within a relatively short time it is possible to collect enough data to characterize thoroughly the adequate stimulus of a single sensory neuron. There is considerable scope for student innovation, and several important concepts of sensory physiology can be discussed.

Modulation of swimming rhythmicity by 5-hydroxytryptamine during post-embryonic development in Xenopus laevis.

During the first 24 h of post-embryonic development in Xenopus laevis, a rapid change in the neural activity underlying swimming occurs in which the duration of ventral root discharge on each cycle increases from a single compound impulse to discrete bursts of activity. Moreover, this change in motor output progresses rostrocaudally, suggesting that it could result from the influence of a descending neural pathway upon the spinal rhythm-generating circuitry during early post-embryonic development. To begin to examine whether serotonergic neurons of brainstem raphe nuclei might have a role in this swimming development, we have studied the effects of 5-hydroxytryptamine (5HT) on fictive swimming in embryonic and larval animals. As previously demonstrated for other vertebrate locomotor rhythms, we find that bath-applied 5HT enhances the duration of motor activity on each cycle of larval fictive swimming. In addition, our results show that the sensitivity of the swimming rhythm to exogenous 5HT follows a strict rostrocaudal gradient. In young embryos (stages 32-36) 5HT does not affect the duration of ventral root impulses per cycle; by the time of hatching (stage 37/38), rostral but not caudal discharge is enhanced, and by stage 42 (24 h post-hatching) 5HT can increase motor burst durations along most of the length of the animal. These reversible changes induced by bath-applied 5HT closely resemble the normal rostrocaudal development of burst discharge during swimming in animals some 12 h older.(ABSTRACT TRUNCATED AT 250 WORDS)

The development of swimming rhythmicity in post-embryonic Xenopus laevis

The post-embryonic development of 'fictive' swimming in immobilized Xenopus laevis tadpoles has been examined during the first day of larval life. In Xenopus embryos (stage 37-38; Nieuwkoop & Faber 1956), the rhythmic ventral root activity underlying swimming occurs as single brief (ca. 7 ms) compound impulses on each cycle. However, by stage 42 (about 24 h after hatching), ventral root discharge consists of bursts lasting around 20 ms per cycle. In addition to increased burst duration in each cycle of larval swimming, the range of cycle periods within an episode increases, although mean period values (ca. 70-80 ms) remain similar to those of the younger animal. Consequently, motoneurons at developmental stage 42 are active during swimming for a greater percentage (ca. 25%) of cycle time than at stage 37-38 (ca. 10%). Developmental stage 40 (ca. 12 h post-hatching) is an intermediate stage in rhythm development. Ventral root discharge varies from bursts of 10-20 ms at the start of an episode to embryonic (ca. 7 ms) spikes at the end of an episode. Furthermore, discharge varies from bursts of activity in rostral segments of stage 40 larvae to 7 ms spikes more caudally, as in embryos. The data thus suggest that Xenopus swimming rhythmicity develops relatively rapidly, along a rostrocaudal gradient, and may involve acquisition of multiple spiking in spinal neurons.

Octopaminergic modulation of the forewing stretch receptor in the locust Locusta Migratoria

ABSTRACT Modulatory actions of various biogenic amines and peptides on the locust forewing stretch receptor (SR) were examined. The response of the SR to sinusoidal wing movements was unaffected by physiological concentrations (5×10−8mol l−1) of the peptides AKHI, AKHII, proctolin and FMRFamide. The biogenic amine octopamine, however, enhanced the SR response in a dosedependent manner when injected into the haemolymph of an almost intact animal or perfused over an isolated thorax preparation in which head, abdomen, gut and the entire central nervous system were removed (threshold at 5×10−8mol l−1, maximal effect at 5×10−4mol l−1 DL-octopamine). The SR was as sensitive to D-octopamine, the naturally occurring isomer of octopamine, as it was to DL-octopamine. Serotonin was equal to octopamine in effectiveness, followed in order of potency by synephrine, metanephrine and tyramine. Dopamine was ineffective. Phentolamine, but not DL-propranolol, antagonized the action of octopamine. The threshold of the modulatory effect of octopamine on the SR suggests that the increased haemolymph octopamine level which occurs during flight is sufficient to increase the SR activity. Two observations suggest that dorsal unpaired median (DUM) cells are involved in the octopaminergic modulation of the SR during flight: (1) selective stimulation of these cells modulated the SR response and this effect was blocked by phentolamine; and (2) a number of DUM cells were activated during flight. These results suggest that the SR activity is enhanced by octopamine following the onset of flight. Since the SR is involved in the control of wing beat frequency, the modulation of the SR might influence the generation of the motor pattern in flying locusts.

Physiological development of a monosynaptic connection involved in an adult insect behavior.

Locust flight is an exclusively adult behavior whose neural basis has been extensively studied. The coordinated neural pattern underlying this behavior appears rapidly at the end of postembryonic development. This paper examines the ontogeny of elements of the nervous system involved in the behavior. Alternative extreme hypotheses are: 1) the neurons and synapses involved develop concomitant with the behavior, or 2) they are constructed early in development, and are activated at the appropriate time by, for example, the release of inhibition. These hypotheses were evaluated by selecting a synapse that is important in adult flight, and monitoring its physiological features during postembryonic development. The synapse between the forewing Stretch Receptor (SR) and the First Basalar (BA) motor neuron, two uniquely identified neurons, mediates a monosynaptic reflex which operates only in flight. The EPSP, initiated by SR in BA, was recorded intracellularly during the last four of six postembryonic instars. As early as third instar, the monosynaptic EPSP is present and appears to be as effective as in the adult. It also decrements and summates similarly in younger animals and adults. Therefore, some flight system synapses are present and effective throughout most of postembryonic development, and thus do not develop concomitant with the behavior.