Giant Interneurons Research Articles

Motor program selection and response properties of giant interneurons in the cricket Gryllus bimaculatus

Motor program selection and response properties of giant interneurons in the cricket Gryllus bimaculatus

Actions of snake neurotoxins on an insect nicotinic cholinergic synapse

Here we examine the actions of six snake neurotoxins (alpha-cobratoxin from Naja naja siamensis, erabutoxin-a and b from Laticauda semifasciata; CM12 from N. haje annulifera, toxin III 4 from Notechis scutatus and a long toxin from N. haje) on nicotinic acetylcholine receptors in the cercal afferent, giant interneuron 2 synapse of the cockroach, Periplaneta americana. All toxins tested reduced responses to directly-applied ACh as well as EPSPs evoked by electrical stimulation of nerve XI with similar time courses, suggesting that their action is postsynaptic. Thus, these nicotinic receptors in a well-characterized insect synapse are sensitive to both long and short chain neurotoxins. This considerably expands the range of snake toxins that block insect nicotinic acetylcholine receptors and may enable further pharmacological distinctions between nAChR subtypes.

Changes in response properties of the cricket giant interneurons during postembryonic development

We investigated the directional sensitivity of four wind-sensitive giant interneurons (GIs), 8-1, 9-1, 9-2 and 9-3, in the fourth-instar nymphs of the cricket Gryllus bimaculatus, using a unidirectional air current stimulus. The directional sensitivity curves expressed with threshold velocities revealed that the preferential directions of GIs in the fourth-instar nymphs were basically the same as those of adults. Intensity-response relations for these GIs in the fourth-, sixth-, last-instar nymphs and adults to air current from front and rear directions were also investigated.

Disynaptic and polysynaptic statocyst pathways to an identified set of premotor nonspiking interneurons in the crayfish brain

Nonspiking giant interneurons (NGIs) in the crayfish brain occupy a key position in the neuronal circuit for eyestalk motor control and, hence, play a crucial role in the central compensation process following unilateral deprivation of the statocyst that functions as an equilibrium sensory system. The synaptic organization of their input pathways, however, remains unknown. In the present study we identified 11 local interneurons that were polysynaptically connected to NGIs by making simultaneous intracellular recordings. We also identified three other local interneurons that connected to NGIs monosynaptically. PLNI-2 was a nonspiking interneuron making an excitatory synaptic connection to an NGI that had its cell body on the opposite side. PLSI was a spiking interneuron that made an inhibitory connection to an ipsilateral NGI. These cells were entirely confined to the protocerebrum. Another local spiking interneuron termed UGLI-1 was found to make an excitatory connection with a contralateral NGI, extending dendrites in the anterior and posterior medial protocerebral neuropils and the lateral antenna I neuropil in the deutocerebrum where statocyst afferents project. When a depolarizing current was injected into the UGLI-1, the frequency of discrete excitatory PSPs increased remarkably in the postsynaptic NGI, each PSP following the UGLI-1 spike in one-to-one correspondence. The UGLI was previously reported to be activated monosynaptically by statocyst afferents. The present study thus finally demonstrates the tri-synaptic organization of the statocyst-to-eyestalk motor neuron pathway in its simplest form, suggesting the critical role of the UGLI-1 in the central compensation.

Loss of Escape-Related Giant Neurons in a Spiny Lobster,Panulirus argus

When attacked, many decapod crustaceans perform tailflips, which are triggered by a neural circuit that includes lateral giant interneurons, medial giant interneurons, and fast flexor motor giant neurons (MoGs). Slipper lobsters (Scyllaridae) lack these giant neurons, and it has been hypothesized that behavioral (e.g., digging) and morphological (e.g., flattening and armor) specializations in this group caused the loss of escape-related giant neurons. To test this hypothesis, we examined a species of spiny lobster, Panulirus argus. Spiny lobsters belong to the sister taxon of the scyllarids, but they have a more crayfish-like morphology than scyllarids and were predicted to have escape-related giant neurons. Ventral nerve cords of P. argus were examined using paraffin-embedded sections and cobalt backfills. We found no escape-related giant neurons and no large axon profiles in the dorsal region of the nerve cord of P. argus. Cobalt backfills showed one fewer fast flexor motor neuron than in species with MoGs and none of the fast flexor motor neurons show any of the anatomical specializations of MoGs. This suggests that all palinuran species lack this giant escape circuit, and that the loss of rapid escape behavior preceded, and may have driven, alternative predator avoidance and anti-predator strategies in palinurans.

Compensational recovery of response rate after unilateral cercal ablation and the functional change of cricket giant interneurons

Compensational recovery of response rate after unilateral cercal ablation and the functional change of cricket giant interneurons

Short-term plasticity in wind sensitivity of the cricket giant interneurons

Short-term plasticity in wind sensitivity of the cricket giant interneurons

Physiological changes of premotor nonspiking interneurons in the central compensation of eyestalk posture following unilateral sensory ablation in crayfish

We investigated how the physiological characteristics and synaptic activities of nonspiking giant interneurons (NGIs), which integrate sensory inputs in the brain and send synaptic outputs to oculomotor neurons innervating eyestalk muscles, changed after unilateral ablation of the statocyst in order to clarify neuronal mechanisms underlying the central compensation process in crayfish. The input resistance and membrane time constant in recovered animals that restored the original symmetrical eyestalk posture 2 weeks after operation were significantly greater than those immediately after operation on the operated side whereas in non-recovered animals only the membrane time constant showed a significant increase. On the intact side, both recovered and non-recovered animals showed no difference. The frequency of synaptic activity showed a complex pattern of change on both sides depending on the polarity of the synaptic potential. The synaptic activity returned to the bilaterally symmetrical level in recovered animals while bilateral asymmetry remained in non-recovered ones. These results suggest that the central compensation of eyestalk posture following unilateral impairment of the statocyst is subserved by not only changes in the physiological characteristics of the NGI membrane but also the activity of neuronal circuits presynaptic to NGIs.

Cercal Sensory System and Giant Interneurons in Gryllodes sigillatus

The external morphologies of two cricket species, Gryllodes sigillatus and Gryllus bimaculatus, were investigated. Despite its small body length, G. sigillatus possessed longer cerci and longer cercal filiform hairs than G. bimaculatus. The estimated number of filiform hairs on a cercus was also larger in G. sigillatus than in G. bimaculatus. Wind-sensitive interneurons receiving sensory inputs from cercal filiform hairs and running in the ventral nerve cord (VNC) were investigated in G. sigillatus both morphologically and physiologically. By intracellular staining, these interneurons were proved to be morphologically homologous with previously identified giant interneurons (GIs 8-1, 9-1, 9-2, 9-3, 10-2, and 10-3) in G. bimaculatus and Acheta domesticus. In G. sigillatus, the intensity-response relationship (I-R curve) for each GI was investigated using a unidirectional air current stimulus. The stimulus was applied from 12 different directions, and an I-R curve was obtained for each stimulus direction. Each GI showed a characteristic I-R curve depending on stimulus direction. The directionality curve expressed in terms of threshold velocity showed that each GI had a distinctive directional characteristic. The functional properties of GIs in G. sigillatus, such as I-R curve, threshold velocity, and directional characteristics, were compared with those of homologous GIs in G. bimaculatus in Discussion.

Expression and Function of Variants of Slob, Slowpoke Channel Binding Protein, inDrosophila

Slob binds to and modulates the Drosophila Slowpoke (dSlo) calcium-activated potassium channel and also recruits the ubiquitous signaling protein 14-3-3 to the channel regulatory complex. RT-PCR reveals the presence of multiple slob transcripts in Drosophila heads. The transcripts are predicted to encode proteins that we call Slob51 (kDa), Slob57, Slob65, and Slob71. Slob51 and Slob65 are splice variants that lack a motif important for the binding of 14-3-3. Previous microarray analyses demonstrated the circadian cycling of slob mRNA, and we show by quantitative PCR that more than one transcript cycles in fly heads. Using in situ hybridization, we observe differences in the expression patterns of the different transcripts. Immunohistochemistry on Drosophila heads reveals Slob71/65 protein to be enriched in the lateral neurons, in contrast to Slob57/51 protein, which is expressed most prominently in the pars intercerebralis neurons and dorsal giant interneurons. Using a heterologous expression system, we show that different Slobs bind to different extents to dSlo and 14-3-3. These data reveal an unexpected diversity of the dSlo/Slob/14-3-3 dynamic regulatory complex.

Cyclic AMP Mediates Serotonin-Induced Synaptic Enhancement of Lateral Giant Interneuron of the Crayfish

The lateral giant (LG)-mediated escape behavior of the crayfish habituates readily on repetitive sensory stimulation. Recent studies suggested that the biogenic amines serotonin and octopamine modulate the time course of recovery and/or re-depression of the LG response after habituation. However, little is known of how serotonin and octopamine effect LG habituation and what second-messenger cascades they may activate. To investigate the effect of biogenic amines on LG habituation, serotonin and octopamine were superfused before presenting repetitive sensory stimulation. Serotonin and octopamine increased the number of stimuli needed to habituate the LG response. Their effects were mimicked by mixed application of a cAMP analogue [8-(4-chlorophenylthio)-cAMP (CPT-cAMP)] and a phosphodiesterase inhibitor [3-isobutyl-1-methylxanthine (IBMX)] but not by a cGMP analogue (8-bromoguanosine 3',5'-cyclic monophosphate). Perfusion of the adenylate cyclase inhibitor (SQ22536) abolished the effect of serotonin but not that of octopamine. To investigate the site of action of each biogenic amines in the neural circuit meditating LG escape, the effect of drugs on directly and indirectly elicited postsynaptic potentials in LG was investigated. Serotonin, octopamine, and a mixture of CPT-cAMP and IBMX increased both the direct and indirect synaptic inputs. Simultaneous application of SQ22536 abolished the effect of serotonin on both inputs but did not block the effect of octopamine. Direct injection of the cAMP analogue (Sp-isomer of adenosine-3',5'-cyclic monophosphorothioate) into LG increased both the direct and indirect inputs to LG. These results indicate that serotonin mediates an increase in cAMP levels in LG, but octopamine acts independently of cAMP and cGMP.

Response properties of wind-sensitive giant interneurons in the fourth-instar nymphs of the cricket, Gryllus bimaculatus

The response properties of four wind-sensitive giant interneurons (GIs) 8-1, 9-1, 9-2 and 9-3 in the fourth-instar nymphs of the cricket Gryllus bimaculatus were investigated to clarify the differences and/or similarities of the escape eliciting neural system between nymphs and adults. Air current was presented to the animal from 12 different directions in the horizontal plane, and the intensity–response curves for each GI were obtained at each stimulus direction. The intensity–response curves showed that the response magnitudes of GI 8-1 in the fourth-instar crickets increased with stimulus velocity up to 300 mm/s regardless of the stimulus direction. The response magnitudes of GI 9-1 in the nymphs reached a plateau at a stimulus velocity of 30 mm/s in most stimulus directions. The response magnitudes of GIs 9-2 and 9-3 increased with stimulus velocity up to 300 mm/s regardless of the stimulus direction. The directional sensitivity curves plotted on the basis of threshold velocities revealed that the preferential directions of the GIs were the ipsilateral-side in GI 8-1, the ipsilateral-front and contralateral-rear in GI 9-1, the ipsilateral-rear in GI 9-2 and the ipsilateral-front in GI 9-3, designated with respect to the side of the ventral nerve cord containing the axons. Although the GIs in nymphs occasionally showed higher threshold velocities and larger response magnitudes, the directional sensitivities, i.e., the preferred directions, of the GIs were basically the same with those of adults.

Mas-allatotropin/Lom-AG-myotropin I immunostaining in the brain of the locust, Schistocerca gregaria.

Mas-allatotropin (Mas-AT) and Lom-accessory gland-myotropin I (Lom-AG-MTI) are two members of a conserved family of insect neuropeptides, collectively termed allatotropins, which have diverse functions, ranging from stimulation of juvenile hormone secretion to myotropic effects on heart and hindgut. In addition, allatotropins appear to be abundant within the nervous system, suggesting neuroactive roles. To identify neurons in the insect brain suitable for a neurophysiological analysis of the roles of allatotropins, we used antisera against Mas-AT and Lom-AG-MTI to map allatotropin-immunoreactive neurons in the brain of a suitable insect, the locust Schistocerca gregaria. Both antisera revealed basically identical staining patterns throughout the locust brain with more than 12,500 immunostained interneurons per brain hemisphere. Neurosecretory cells were not labeled, and the retrocerebral complex was devoid of immunostaining. Prominent immunoreactive cell types include about 9,600 lamina monopolar neurons, medulla to lobula interneurons, local neurons of the antennal lobe, a giant interneuron of the mushroom body, projection neurons of the glomerular lobe to the mushroom body, and three systems of tangential neurons of the central complex. Several groups of neurons showed colocalization of Mas-AT- and gamma-aminobutyric acid immunostaining. Mass spectrometric analysis identified a peptide with a molecular mass identical to Lom-AG-MTI in all major parts of the locust brain but not in the retrocerebral complex. This study strongly suggests that Lom-AG-MTI is highly abundant in the locust brain, and is likely to play a neuroactive role in many brain circuits including all stages of sensory processing, learning and memory, and higher levels of motor control.

Maturation of escape circuit function during the early adulthood of cockroaches Periplaneta americana.

During postembryonic development of insects, sensorimotor pathways, which generate specific behaviors, undergo maturational changes. It is less clear whether such pathways are typically stable, or undergo further maturation, during the adult stage. In the present study, we have examined this issue by multilevel analysis of a simple model system, the escape behavior of the cockroach, from identified synapses to behavior. We show that the escape system is highly responsive immediately after the molt to adulthood, but that the latency of escape responses was not at its typical value immediately after the molt to adult. The latency of escape behavior increased over the first 30 days of adult life, perhaps indicating maturational adjustments of the escape sensorimotor pathway. The first station in the escape circuitry is the synaptic connections between the cercal wind receptors and the giant interneurons. We measured unitary excitatory synaptic potentials between single sensory neurons and an identified giant interneuron (GI(2)). We found a decrease in the synaptic strength between identified cercal hairs from a single column and GI(2) over the first month after the adult molt. Consequently, the latency and the number of action potentials of GI(2) in response to natural stimuli increased and decreased respectively during this time. Thus, we show that both behavioral performance and the wind sensitivity of GI(2) decreased over the first month after molt. We conclude that the cockroach escape system undergoes further sensorimotor maturation over a period of 1 month, and that cellular changes correlate with, or predict, some changes in behavioral performance.

Loss of escape responses and giant neurons in the tailflipping circuits of slipper lobsters, Ibacus spp. (Decapoda, Palinura, Scyllaridae)

In many decapod crustaceans, escape tailflips are triggered by lateral giant (LG) and medial giant (MG) interneurons, which connect to motor giant (MoG) abdominal flexor neurons. Several decapods have lost some or all of these giant neurons, however. Because escape-related giant neurons have not been documented in palinurans, I examined tailflipping and abdominal nerve cords for giant neurons in two scyllarid lobster species, Ibacus peronii and Ibacus alticrenatus. Unlike decapods with giant neurons, Ibacus do not tailflip in response to sudden taps. Ibacus can perform non-giant tailflipping: the frequency of tailflips during swimming is adjusted by altering the gap between each individual tailflip. Abdominal nerve cord sections show no LG or MG interneurons. Backfilling nerve 3 of abdominal ganglia revealed no MoG neurons, and the fast flexor motor neuron population is otherwise identical to that described for crayfish. The loss of giant neurons in Ibacus represents an independent deletion of these cells compared to other reptantian decapods known to have lost these giant neurons. This loss is correlated with the normal posture in scyllarids, in which the last two abdominal segments are flexed, and an alternative defensive strategy, concealment by digging into sand.

Directional sensitivity of dendritic calcium responses to wind stimuli in the cricket giant interneuron

We examined directional sensitivities in the dendritic activity of the identified giant interneurons (GIs) in the cricket, using in vivo Ca 2+ imaging during different directional air-current stimuli. Air current stimulus evoked action potential burst and quick Ca 2+ increase in GI. The stimulus direction of the maximal Ca 2+ responses corresponded to that of the maximal voltage response. However, the shapes of the directional tuning curves based on the Ca 2+ responses for each dendritic branch were different from the overall tuning curve based on spike counts for the cell. Moreover, different dendritic branches displayed distinct directional sensitivity profiles to the air-current stimuli. We propose that postsynaptic activities will influence the local Ca 2+ signals in the distal dendrites, and produce the difference in directional sensitivity of the dendritic Ca 2+ response.

Functional Changes of Cricket Giant Interneurons Caused by Chronic Unilateral Cercal Ablation during Postembryonic Development

Functional Changes of Cricket Giant Interneurons Caused by Chronic Unilateral Cercal Ablation during Postembryonic Development

Postembryonic changes in the response properties of wind-sensitive giant interneurons in cricket

The intensity–response (I–R) relations for four wind-sensitive giant interneurons (GIs 8-1, 9-1, 9-2 and 9-3) in the fourth-, sixth- and last-instar nymphs of the cricket, Gryllus bimaculatus, were investigated using a unidirectional air current stimulus in order to explore the functional changes of GIs during postembryonic development. Contrary to our expectations, the response properties of GIs in nymphs were largely different from those in adults. The response magnitude of GI 8-1 in an intact cricket decreased during development, i.e. the GI in younger insects showed a larger response magnitude. Although the response magnitudes of GIs 9-1 and 9-2 were almost identical during the nymphal period, a significant decrease was observed after the imaginal ecdysis. During the nymphal period, the response magnitude of GI 9-3 increased according to the developmental stage. However, it decreased significantly after the imaginal ecdysis. We also investigated the response magnitudes of the GIs in nymphs after unilateral cercal ablation. From the results of ablation experiments, the changes in excitatory and/or inhibitory connections between filiform hairs and each GI during postembryonic development were revealed.

Axonal regeneration of descending and ascending spinal projection neurons in spinal cord-transected larval lamprey

The distributions of descending and ascending spinal projection neurons (i.e., spinal neurons with moderate to long axons) were compared in normal larval lamprey and in animals that had recovered for 8 weeks following a complete spinal cord transection at 50% body length (BL, normalized distance from the anterior head). In normal animals, application of HRP to the spinal cord at 60% BL (40% BL) labeled an average of 713.8 ± 143.2 descending spinal projection neurons (718.4 ± 108.0 ascending spinal projection neurons) along the rostral (caudal) spinal cord, most of which were unidentified neurons. Some of these neurons project for at least ∼50–60 spinal cord segments (∼36–47 mm in animals with an average length of ∼90 mm used in the present study). At 8 weeks posttransection, the numbers of HRP-labeled descending or ascending spinal neurons that extended their axons through the transection were about 40% of those in similar areas of the spinal cord in normal animals. Thus, in larval lamprey, axonal regeneration of descending and ascending spinal projection neurons is incomplete, similar to that found for descending brain neurons (Davis and McClellan, 1994a). The majority of restored projections were from unidentified spinal neurons that have not been documented previously. In contrast to results from several other lower vertebrates, in the lamprey ascending spinal neurons exhibited substantial axonal regeneration. Identified descending spinal neurons, such as lateral interneurons and crossed contralateral interneurons, and identified ascending spinal neurons, such as giant interneurons and edge cells, regenerated their axons at least 9 mm beyond the transection site in animals with an average length of ∼90 mm, which is appreciably farther than previously reported. In contrast, most dorsal cells, which are centrally located sensory neurons, exhibited very little axonal regeneration.

Calbindin and calretinin immunoreactivities identify different types of neurons in the adult lamprey spinal cord.

The central pattern generator for locomotion in vertebrates is composed of different spinal neuronal populations that generate locomotor movement. In the lamprey spinal cord, several classes of interneurons have been identified based on morphologic and physiological criteria and integrated in the spinal cord circuits implicated in the generation of locomotion. However, the lack of histochemical markers for most of the interneurons makes it difficult to study whole populations along the spinal cord. We have investigated the immunoreactivity with antibodies raised against calbindin and calretinin. Several types of neurons could be classified: (1). strongly immunoreactive neurons located dorsomedially, (2). moderately immunoreactive neurons located laterally, (3). small weakly immunoreactive neurons, d). ventromedial neurons, (4). liquor contacting cells, and (5). motoneurons. The ventromedial group of calbindin-immunoreactive neurons also is immunoreactive for serotonin and, therefore, represents the ventromedial group of dopamine/serotonin spinal neurons. Some of the lateral calbindin-immunoreactive neurons may be CC-type cells (cells with caudal-crossed axons), because they are retrogradely labeled by tracer injections into the contralateral spinal cord. Other well-characterized cell types, such as sensory dorsal cells, lateral interneurons, descending propriospinal edge cells, and spinobulbar giant interneurons are negative for both calbindin and calretinin. Therefore, calbindin and calretinin are useful markers for the study of cell populations that may be integrated in locomotor circuits.