Segmental Ganglia Research Articles

Molecular cloning and functional characterization of a neuronal choline transporter from Trichoplusia ni

A cDNA encoding a high-affinity Na +-dependent choline transporter (TrnCHT) was isolated from the CNS of the cabbage looper Trichoplusia ni using an RT-PCR-based approach. The deduced amino acid sequence of the CHT cDNA predicts a 594 amino acid protein of 64.74 kDa prior to glycosylation. TrnCHT has 80%, 79%, 76%, and 58% amino acid identity to putative CHTs from Anopheles gambiae, Drosophila melanogaster and Apis mellifera, and a cloned CHT from Limulus polyphemus, respectively. In situ hybridization of TrnCHT cRNA in whole-mount preparations of caterpillar CNS revealed that TrnCHT mRNA is expressed by hundreds of presumably cholinergic neurons present in both the brain and cortex of all segmental ganglia. Na +-dependent [ 3H]-choline uptake was induced in Sf9 cells in vitro following infection with a TrnCHT-expressing recombinant baculovirus. Virally induced [ 3H]-choline uptake was found to approximately equal the endogenous rate of choline uptake in insect cells, seen either after infection with a control virus or in TrnCHT-infected cells exposed to [ 3H]-choline in the absence of Na +. The Na +-dependent component of [ 3H]-choline uptake by TrnCHT-infected cells was saturable with a K m for choline transport of 8.4 μM. Several compounds reported to be potent blockers of [ 3H]-choline uptake by cloned vertebrate choline transporters proved to be relatively weak inhibitors of choline uptake by Sf9 cells expressing TrnCHT. Hemicholinium-3 ( K i = 4.1 μ M ) and two oxoquinuclidium analogues of choline, quireston-A ( K i ≈ 10 μ M ) and quireston ( K i ≈ 100 μ M ) inhibited 50% of control uptake only at micromolar concentrations. The endogenous low-affinity Na +-independent uptake of [ 3H]-choline was also inhibited by high micromolar concentrations of hemicholinium-3.

5‐Hydroxytryptamine activates a barium‐sensitive, cAMP‐mediated potassium conductance in the leech giant glial cell

5-Hydroxytryptamine (5-HT), a neurotransmitter and neuromodulator in the central nervous system of the leech Hirudo medicinalis hyperpolarizes the giant glial cell in the neuropil of segmental ganglia at micromolar concentrations. The 5-HT-evoked glial response (EC(50) approximately 2.5 microM) is mediated by a non-desensitizing, G-protein-coupled receptor and due to activation of a Ca(2+)-independent K(+) conductance. The adenylyl cyclase inhibitor SQ22,536 blocks the response to 5-HT; in the presence of 1 mM db-cAMP, but not of 1 mM db-cGMP, the glial response is suppressed. The 5-HT-evoked response is reduced by Ba(2+) with half-maximal inhibition at 50 microM Ba(2+). The results suggest that release of 5-HT from serotonergic neurons, or the maintenance of micromolar levels of extracellular 5-HT in the ganglion, may help to set the glial membrane potential close to the K(+) equilibrium potential.

Sensitization and dishabituation of swim induction in the leech Hirudo medicinalis: role of serotonin and cyclic AMP

In this paper the role of serotonin (5HT) and cyclic AMP (cAMP) in sensitization and dishabituation of swim induction (SI) has been investigated in the leech Hirudo medicinalis. Electrical stimulation of the body wall evokes swimming activity with a constant latency. In animals with a disconnection between head ganglion and segmental ganglia, repetitive stimulation induces habituation of swimming whereas brushing on the dorsal skin provokes sensitization of a naı̈ve response or dishabituation of a previously habituated response. Our findings indicate that 5HT is the neurotransmitter underlying both sensitization and dishabituation of SI. Injection of the 5HT receptor blocking agent methysergide impaires the onset of sensitization and dishabituation induced by brushing. Moreover, injection of 5HT mimics these forms of nonassociative learning, whereas injection of dopamine does not. Finally, the effects of 5HT are mediated by cAMP: (1) after injections of specific adenylate cyclase inhibitors such as MDL 12.330A or SQ22536, brushing becomes ineffective in facilitating the SI in either non-habituated or habituated animals. (2) 8Br-cAMP application mimics both sensitization and dishabituation of SI.

Second messenger cascade of glial responses evoked by interneuron activity and by a myomodulin peptide in the leech central nervous system.

The giant glial cell in the neuropil of segmental ganglia of the leech Hirudo medicinalis responds to the activity of the Leydig interneuron and to a peptide of the myomodulin family, the presumed transmitter mediating the Leydig neuron-to-giant glial cell transmission, with a membrane hyperpolarization due to an increased membrane K+ conductance [Britz et al. (2002) Glia, 38, 215-227]. We have now studied the second messenger cascade initiated by Leydig neuron stimulation and by the endogenous myomodulin (MMHir) in the voltage-clamped giant glial cell. Glial responses to both stimuli are mediated by a G-protein-coupled receptor linked to adenylyl cyclase by the following criteria: (i) injection of GDP-beta-S, but not GDP, resulted in an irreversible decrease of the glial responses to both stimuli; (ii) the responses to both stimuli were reversibly inhibited by the adenylyl cyclase inhibitor SQ22,536; and (3) bath-applied di-butyryl-cyclic AMP, but not di-butyryl-cyclic GMP, elicited an outward current, which reduced the responses elicited by neuronal stimulation or myomodulin. A cocktail of protein kinase (PK) inhibitors (H-8, KT5720), the PKA antagonist Rp-cAMPS, or presumed inhibitors of cyclic nucleotide channels, LY83583 and l-cis-diltiazem, had no effect on the glial responses. Our results suggest that Leydig neuron stimulation and MMHir activate a cAMP-mediated K+ conductance in the glial cell, which appeared neither to be due to the activation of PKA nor of known cyclic nucleotide-gated channels directly.

Helicostatins: Brain-gut peptides of the moth,Helicoverpa armigera (Lepidoptera: Noctuidae)

Gene expression and immunolocalisation studies have determined that the helicostatins are brain-gut peptides in larvae of the lepidopteran, Helicoverpa armigera. Mapping of the distribution of these peptides in the nervous system and alimentary canal has provided evidence for multifunctional regulatory roles. In situ hybridisation studies have shown that the helicostatin precursor gene is expressed in neurones of the central and stomatogastric nervous systems, and endocrine cells of the midgut demonstrating that the helicostatins are true brain-gut peptides. Antisera raised against Leu-callatostatin 3 (ANRYGFGL-NH(2)), a peptide isolated from the blowfly, Calliphora vomitoria was used to map the distribution of allatostatin-like immunoreactive (Ast-ir) material in H. armigera to elucidate possible functions of the helicostatins. In situ hybridisation studies verified that the helicostatin precursor gene is expressed in neurones shown to contain Ast-ir, providing strong evidence that the Ast-ir material is helicostatins. Extensive immunoreactive axonal projections into complex regions of neuropile indicate that the helicostatins may have a neuromodulatory role in the brain and segmental ganglia of the ventral nerve cord. The presence of large amounts of immunoreactive material in axons within the corpora cardiaca (CC) and transverse nerves of the perisympathetic nervous system, two known neurohaemal organs, provides evidence for a neurohormonal role. The corpora allata (CA) were innervated only sparsely by Ast-ir axons suggesting that the CA are not a neurohaemal release site or a target. Thus, it is unlikely that the helicostatins regulate juvenile hormone (JH) biosynthesis or release. Ast-ir axons extended from the frontal ganglion through the recurrent nerve and many branches were closely associated with muscles of the foregut, stomodeal valve, and anterior midgut, implicating helicostatins in regulation of foregut motility. Ast-ir material was also present in nerves associated with muscles of the pyloric valve and rectum, and in endocrine cells of the midgut.

Identification of two types of synaptic activity in the earthworm nervous system during locomotion

In the ventral nervous system of the earthworm, a central pattern generator and motor neurons are activated during locomotion. We have previously reported that bath application of octopamine (OA) induces fictive locomotion in the earthworm, and the burst frequency of electrical activity from the first lateral nerves increases with OA concentration. However, there are no reports concerning locomotor neural networks in the earthworm. To identify neural networks involved in fictive locomotion, we optically monitored activity-dependent fluorescent staining in the earthworm ventral nerve cord (VNC) with a styryl dye, N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide (FM1–43), and a confocal laser scanning microscope. OA induces FM1–43 fluorescence in a dose-dependent manner, with bright fluorescent spots of 3–10 μm in diameter observed to be localized around specified neurons in the segmental ganglion of the VNC. We compared OA dose-response curves for FM1–43 fluorescence with the bursting frequency for fictive locomotion, and found that two types of curves could be identified: one fluorescence response shows a similar dose-dependency to that of the burst frequency, while another response has a higher sensitivity to OA. From these results, we suggest that OA acts as one of the neuromodulators for the earthworm locomotion. This is the first attempt to record motor and inter-neuronal activities simultaneously in a locomotor network in the earthworm.

Architectonics of crayfish ganglia.

The central nervous system of crayfish consists of a chain of segmental ganglia that are linked by cables of intersegmental axons. Each ganglion contains a highly-ordered core of longitudinal tracts, vertical tracts, commissures, and synaptic neuropils. We review from a technical perspective the history of the description of these ganglia, and recognize four episodes of progress. Each major innovation in anatomical methods has led to new insight into the structure and function of this nervous system, and new awareness of the structural patterns that are common to the CNS of all arthropods. Ganglia in different segments of the body differ in size, and appear to differ in anatomy. From a comparison of the structures of the cores of abdominal, thoracic, and subesophageal ganglia, we argue that this apparent difference is illusory. Rather, each of these ganglia is organized on the same plan, a plan also found in insect segmental ganglia. The apparent differences follow from longitudinal compression during development and from allometric growth of particular neuropils associated with innervation of the walking legs. Different authors have described the internal organization of ganglia in different segments, so we provide a cross-reference to the nomenclatures they have introduced. We compare the locations of cell bodies of motor neurons and accessory neurons that innervate different peripheral structures, and demonstrate double-labeling of certain GABAergic peripheral inhibitory neurons. Finally, we describe the construction of digital movies of serial sections of these ganglia, and discuss their utility.

Presynaptic modulation of sensory neurons in the segmental ganglia of arthropods.

The afferent terminals of arthropod sensory neurones receive abundant input synapses, usually closely intermingled with the sites of synaptic output. The majority of the input synapses use the neurotransmitter GABA, but in some afferents there is a significant glutamatergic or histaminergic component. GABA and histamine shunt afferent action potentials by increasing chloride conductance. Though glutamate can also have this effect in the arthropod central nervous system, its action on afferent terminals appears to be mediated by increases in potassium conductance or by the action of metabotropic receptors. The action of the presynaptic synapses on the afferents are many and varied. Even on the same afferent, they may have several distinct roles that can involve both tonic and phasic patterns of primary afferent depolarisation. Despite the ubiquity and importance of their effects however, the populations of neurones from which the presynaptic synapses are made, remain largely unidentified.

A Functional Asymmetry in the Leech Heartbeat Timing Network Is Revealed by Driving the Network across Various Cycle Periods

We tested predictions of a computational model (Hill et al., 2002) of the leech heartbeat timing network. The timing network consists of two segmental oscillators located in the third (G3) and fourth (G4) segmental ganglia. Each oscillator consists of two reciprocally inhibitory oscillator interneurons along with the coordinating interneuron fibers that link them. In the model, the network was driven to cycle periods around the normal period of the network by repeatedly stimulating one of the paired oscillator interneurons in G3 or G4. Here we replicate these experiments in the biological system. The model predicts that the G3 and G4 oscillators can entrain the timing network to periods faster but not slower than the inherent period of the nondriven (“follower”) oscillator and that they can do so symmetrically. The biological system can be driven to periods both faster (such that the driven oscillator leads in phase) and slower (such that the driven oscillator lags in phase) than the inherent period of the timing network. Although both oscillators can entrain the network, the G4 oscillator does so over a narrower range of periods. Two differences between the assumptions of the model and the properties of the biological network, spike frequency adaptation in coordinating interneurons and asymmetry in the connections from the oscillator interneurons to the coordinating interneurons, may account for these discrepancies. Individual coordinating interneurons were also able to entrain the oscillators but with little effect of the phase relationship between the oscillators, suggesting that phase relations are determined by properties inherent to the oscillator interneurons.

A functional asymmetry in the Leech Heartbeat Timing Network is revealed by driving the network across various cycle periods.

We tested predictions of a computational model (Hill et al., 2002) of the leech heartbeat timing network. The timing network consists of two segmental oscillators located in the third (G3) and fourth (G4) segmental ganglia. Each oscillator consists of two reciprocally inhibitory oscillator interneurons along with the coordinating interneuron fibers that link them. In the model, the network was driven to cycle periods around the normal period of the network by repeatedly stimulating one of the paired oscillator interneurons in G3 or G4. Here we replicate these experiments in the biological system. The model predicts that the G3 and G4 oscillators can entrain the timing network to periods faster but not slower than the inherent period of the nondriven ("follower") oscillator and that they can do so symmetrically. The biological system can be driven to periods both faster (such that the driven oscillator leads in phase) and slower (such that the driven oscillator lags in phase) than the inherent period of the timing network. Although both oscillators can entrain the network, the G4 oscillator does so over a narrower range of periods. Two differences between the assumptions of the model and the properties of the biological network, spike frequency adaptation in coordinating interneurons and asymmetry in the connections from the oscillator interneurons to the coordinating interneurons, may account for these discrepancies. Individual coordinating interneurons were also able to entrain the oscillators but with little effect of the phase relationship between the oscillators, suggesting that phase relations are determined by properties inherent to the oscillator interneurons.

Experimental small bowel obstruction in chick embryos: Effects on the developing enteric nervous system

Background/Purpose: After surgical repair of congenital small bowel atresias, intestinal motility disorders often are observed. These may be caused by changes in the enteric nervous system (ENS) secondary to obstruction. To assess these changes, small bowel atresias were induced experimentally in chick embryos. Methods: On day 11, the small intestines of 90 chicken embryos were ligated microsurgically in ovo. Breeding of the eggs was continued until day 19. The small bowel was removed, fixed, and embedded for silver-staining, semithin serial sections, and transmission electron microscopy. Additionally, acetyl-cholinesterase (AChE)-staining was performed. Normal chick embryos of the same age served as controls. Results: Macroscopically, experimentally induced small bowel atresias had the same characteristics as human newborns. Microscopically, the wall structure was preserved; however, the ENS differed markedly from controls. Both proximal and distal to the obstruction, the submucosal plexus was almost completely absent, whereas the myententeric plexus was diminished only in the proximal dilated blind pouch. The axonal net was disrupted additionally. Ganglion cells of the myenteric plexus in the proximal segment were arranged in longitudinal clusters of densely packed cells. In the distal segment ganglion cells formed round clusters. The cells of Cajal, which normally surround the myenteric ganglia, were absent in the proximal and distal segments. Conclusions: In our experiments, structural changes in the ENS could be observed secondary to experimentally induced small bowel atresias in the chick. Because of the lack of ischemia in this model, the main cause of these ENS changes seems to be the dilatation oft the proximal gut. Dilatations are common features in intestinal atresias, anorectal malformations, and Hirschsprung's disease. Our observations, thus, explain motility disorders after the surgical repair of these diseases. J Pediatr Surg 37:735-740. Copyright 2002, Elsevier Science (USA). All rights reserved.

Characterization of a synaptiform transmission between a neuron and a glial cell in the leech central nervous system.

The cross-talk between neurons and glial cells is receiving increased attention because of its potential role in information processing in nervous systems. Stimulation of a single identifiable neuron, the neurosecretory Leydig interneuron in segmental ganglia of the leech Hirudo medicinalis, which modulates specific behaviors in the leech, evokes membrane hyperpolarization directly in the giant glial cell (Schmidt and Deitmer. Eur J Neurosci 11:3125-3133, 1999). We have studied the neuron-to-glia signal transmission in the voltage-clamped giant glial cell to determine whether this interaction exhibits properties of a chemical synapse. The glial response had a mean latency of 4.9 s and was dependent on the action potential frequency; the glial cell responded to as few as five Leydig neuron action potentials in 50% of the trials. The glial current was sustained for minutes during repetitive Leydig neuron activity without any sign of desensitization. The current was sensitive to tetraethylammonium, and its reversal potential of -78 mV shifted with the external K+ concentration. The glial response increased with the duration of the neuronal action potentials and was sensitive to the external Ca2+/Mg2+ concentration ratio. The results suggest that Leydig neuron activity leads to a Ca2+-dependent release of transmitter from the neuronal dendrites, evoking an K+ outward current in the giant glial cell, implying a synapse-like transmission between a neuron and a glial cell.

Phase relationships between segmentally organized oscillators in the leech heartbeat pattern generating network.

Motor pattern generating networks that produce segmentally distributed motor outflow are often portrayed as a series of coupled segmental oscillators that produce a regular progression (constant phase differences) in their rhythmic activity. The leech heartbeat central pattern generator is paced by a core timing network, which consists of two coupled segmental oscillators in segmental ganglia 3 and 4. The segmental oscillators comprise paired mutually inhibitory oscillator interneurons and the processes of intersegmental coordinating interneurons. As a first step in understanding the coordination of segmental motor outflow by this pattern generator, we describe the functional synaptic interactions, and activity and phase relationships of the heart interneurons of the timing network, in isolated nerve cord preparations. In the timing network, most (approximately 75%) of the coordinating interneuron action potentials were generated at a primary spike initiation site located in ganglion 4 (G4). A secondary spike initiation site in ganglion 3 (G3) became active in the absence of activity at the primary site. Generally, the secondary site was characterized by a reluctance to burst and a lower spike frequency, when compared with the primary site. Oscillator interneurons in G3 inhibited spike activity at both initiation sites, whereas oscillator interneurons in G4 inhibited spike activity only at the primary initiation site. This asymmetry in the control of spike activity in the coordinating interneurons may account for the observation that the phase of the coordinating interneurons is more tightly linked to the G3 than G4 oscillator interneurons. The cycle period of the timing network and the phase difference between the ipsilateral G3 and G4 oscillator interneurons were regular within individual preparations, but varied among preparations. This variation in phase differences observed across preparations implies that modulated intrinsic membrane and synaptic properties, rather than the pattern of synaptic connections, are instrumental in determining phase within the timing network.

Fictive locomotion induced by octopamine in the earthworm.

We investigated the function of octopamine (OA) as a motor pattern inducer in the earthworm Eisenia fetida. We used semi-intact preparations, consisting of 20 sequential segmental ganglia of the ventral nerve cord (VNC) together with the body wall from the middle of the animal. Bath-application of 10(-4) mol l(-1) OA to the semi-intact preparation induced phasic muscle contractions, which are consistent with crawling. In the isolated VNC, OA induced bursts of motor neuron activity in the first lateral nerves. Burst frequency increased with OA concentration, with a large increase in activity observed in the range 10(-6)-10(-4) mol l(-1). At 10(-4) mol l(-1), the coefficient of variation of burst periods (BPs) was smaller than that seen upon application of OA at other concentrations, which is indicative of rhythmic bursts. These rhythmic bursts propagated along the VNC from the anterior to posterior, with a propagation velocity ranging from 60 to 110 mm s(-1). This velocity is consistent with the propagation velocity of muscle contraction during crawling behavior in the intact earthworm. From these results, we conclude that fictive crawling motor patterns are observed at 10(-4) mol l(-1) OA, and that OA can induce rhythmic bursts in the isolated VNC of the earthworm.

Swelling of the Intratemporal Facial Nerve in Ramsay Hunt Syndrome

Although Ramsay Hunt syndrome is one of the most important diseases causing peripheral facial palsy, the detailed pathology of the disease in the intratemporal facial nerve remains unclear. The purpose of this study was to increase knowledge of the pathogenesis of the syndrome by means of surgical findings. Between April 1976 and March 1997 we performed subtotal decompression of the facial nerve in 74 patients with severe Ramsay Hunt syndrome. The grade of nerve swelling was assessed using a microscope and recorded in a standardized form. The relationships between nerve swelling, the timing of surgery and the swelling of each segment were analyzed. Pronounced neural swelling, involving the geniculate ganglion and the horizontal segment, was consistent finding in the acute phase. Although the incidence of pronounced swelling of the horizontal segment gradually declined with time after onset, in most cases nerve swelling persisted even beyond the 16th week after onset. These data suggest that diffuse viral neuritis occurs throughout the intratemporal facial nerve. We assume that the viral inflammatory swelling involving the geniculate ganglion and horizontal segment is mostly responsible for the acute facial palsy in the acute phase.

Efectos de la axotomía en neuronas nociceptivas de la sanguijuela

It has been described several similarities between mammalian and invertebrate nociceptors. Lateral N neuron, in the segmental ganglion of the leech Hirudo medicinalis, behave like polimodal nociceptors in mammals, meanwhile medial N cells is similar to mechano nociceptors (Pastor, Soria, Belmonte 1996). The effects of axotomy in sensory neurons in leeches are poorly understood. We have focused on the electrophysiological properties of leech nociceptive neurons after axotomy. In N lateral neurons, axotomy induced an increase in the total membrane capacitance, membrane time constant, decrease in the steady state/maximum hyperpolarization rate, increment in the time constant of voltage relaxation of inward rectifier, increase in action potential duration, dV/dtmax, dV/dtmin, and maximum amplitude and duration of after hyperpolarization; firing threshold (always in not axotomized) and an increment in adaptation spike frequency. In N medial cells, axotomy produced a slight depolarization and a decrease in Rinput, a rise in the time constant of voltage relaxation of inward rectifier, a decrement of action potential maximum amplitude, dV/dtmax, dV/dtmin, and after hyperpolarization maximum amplitude, with an increase in action potential and after hyperpolarization duration. Finally, firing threshold increased both in axotomized and not axotomized after axotomy. Axotomy not only induced changes in injured cells, but modified neurons whose axons did not run into the cutting nerves. axotomy induces different changes in N lateral and N medial nociceptive cells, and it is suggested some kind of communication between axotomized and not axotomized cells

Limb Movements during Locomotion: Tests of a Model of an Intersegmental Coordinating Circuit

During normal forward swimming, the swimmerets on neighboring segments of the crayfish abdomen make periodic power-stroke movements that have a characteristic intersegmental difference in phase. Three types of intersegmental interneurons that originate in each abdominal ganglion are necessary and sufficient to maintain this phase relationship. A cellular model of the intersegmental coordinating circuit that also produces the same intersegmental phase has been proposed. In this model, coordinating axons synapse with local interneurons in their target ganglion and form a concatenated circuit that links neighboring segmental ganglia. This model assumed that coordinating axons projected to their nearest-neighboring ganglion but not farther. We tested this assumption in two sets of experiments. If the assumption is correct, then blocking synaptic transmission in an intermediate ganglion should uncouple swimmeret activity on opposite sides of the block. We bathed individual ganglia in a low Ca(2+)-high Mg(2+) saline that effectively silenced both motor output from the ganglion and the coordinating interneurons that originated in it. With this block in place, other ganglia on opposite sides of the block could nonetheless maintain their normal phase difference. Simultaneous recordings of spikes in coordinating axons on opposite sides of the blocked ganglion showed that these axons projected beyond the neighboring ganglion. Selective bilateral ablation of the tracts in which these axons ran showed that they were necessary and usually sufficient to maintain coordination across a blocked ganglion. We discuss revisions of the cellular model of the coordinating circuit that would incorporate these new results.

Habituation of the Proleg Withdrawal Reflex in Manduca sexta Does Not Involve Changes in Motoneuron Properties or Depression at the Sensorimotor Synapse

Larvae of the hawkmoth, Manduca sexta, exhibit a defensive proleg withdrawal reflex in which deflection of mechanosensory hairs on the proleg tip (the planta) evokes retraction of the proleg. A previous behavioral study showed that this reflex habituates in response to repeated planta hair deflection and exhibits several other defining features of habituation. In a semi-intact preparation consisting of a proleg and its associated segmental ganglion, repeated deflection of a planta hair or electrical stimulation of its sensory neuron causes a neural correlate of habituation, manifested as a decrease in the number of action potentials evoked in the proleg motor nerve. Monosynaptic connections from planta hair sensory neurons to the principal planta retractor motoneuron exhibit several forms of activity-dependent plasticity. In the present study we recorded intracellularly from this motoneuron during repetitive electrical stimulation of a planta hair sensory neuron. The number of action potentials evoked in the motoneuron decreased significantly, representing a neural correlate of habituation. The motoneuron's resting membrane potential, input resistance, and spike threshold measured before and after repetitive stimulation did not differ between the stimulated group and a control group. Furthermore, the amplitude of the monosynaptic excitatory postsynaptic potential, as well as the magnitude of paired-pulse facilitation, evoked in the motoneuron by the sensory neuron did not change after repetitive stimulation. These results suggest that depression at the sensorimotor synapse does not contribute to reflex habituation. Rather, other mechanisms in the ganglion of the stimulated segment, such as changes in polysynaptic reflex pathways, appear to be responsible.

The effects of 5-HT on sensory, central and motor neurons driving the abdominal superficial flexor muscles in the crayfish

Serotonin (5-HT) induces a variety of physiological and behavioral effects in crustaceans. However, the mechanisms employed by 5-HT to effect behavorial changes are not fully understood. Among the mechanisms by which these changes might occur are alterations in synaptic drive and efficacy of sensory, interneurons and motor neurons, as well as direct effects on muscles. We investigated these aspects with the use of a defined sensory-motor system, which is entirely contained within a single abdominal segment and consists of a ‘cuticular sensory neurons–segmental ganglia–abdominal superficial flexor motor neurons–muscles’ circuit. Our studies address the role of 5-HT in altering (1) the activity of motor neurons induced by sensory stimulation; (2) the inherent excitability of superficial flexor motor neurons; (3) transmitter release properties of the motor nerve terminal and (4) input resistance of the muscle. Using en passant recordings from the motor nerve, with and without sensory stimulation, and intracellular recordings from the muscle, we show that 5-HT enhances sensory drive and output from the ventral nerve cord resulting in an increase in the firing frequency of the motor neurons. Also, 5-HT increases transmitter release at the neuromuscular junction, and alters input resistance of the muscle fibers

Functionally heterogeneous segmental oscillators generate swimming in the medical leech.

Swimming behavior in the leech Hirudo medicinalis arises from neuronal circuits within the ventral nerve cord. Although the ventral nerve cord comprises a series of homologous segmental ganglia, it remains unresolved whether the swim oscillator circuits within individual ganglia are functionally equivalent. We have extended previous studies on pairs of ganglia to test whether individual ganglia throughout the nerve cord are capable of generating swim oscillations and to measure the cycle periods of local oscillations. We found that the swim-generating function of individual ganglia is broadly distributed, but not uniform. The swim-like oscillations in isolated ganglia from the anterior ganglia nerve cord were less robust than those from mid-cord. Swimming activity in posterior cord ganglia is even weaker we were unable to obtain swim-like oscillations from individual ganglia of the nerve cord posterior to segment 12. Swim-cycle periods exhibited a U-shaped function: those recorded in the most anterior individual ganglia (2.3 s for ganglion M2) and short chains of posterior ganglia (up to 4.0 s) were two to four times longer than those obtained from mid-cord ganglia (near 1.0 s). We conclude that the leech swim system comprises a functionally heterogeneous set of local oscillator units.