Activity of phasic motor neurons partially transforms the neuronal and muscle phenotype to a tonic-like state

Bacterial endotoxin lipopolysaccharide enhances synaptic transmission at low-output glutamatergic synapses

The endotoxin lipopolysaccharides (LPS), secreted from gram-negative bacteria, has direct effects on synaptic transmission independent of systemic secondary cytokine responses. High concentration of LPS (500 μg/mL) from Serratia marcescens increased synaptic efficacy at glutamatergic low-output synapses more than for high-output synapses. Over an hour of exposure was not toxic to the preparation and continued to enhance synaptic transmission. A small but significant rapid hyperpolarization of the post-synaptic cells occurred, in addition to a slower enhancement of in the amplitude of evoked excitatory junction potentials. LPS may promote reserve pool vesicles to the readily releasable pool for low-output synapses. The action of LPS at the glutamatergic synapses of the crayfish neuromuscular junction is unique in promoting synaptic transmission as compared to other glutamatergic synapses in Drosophila and mammals, where synaptic transmission is depressed.

Effect of temperature change on synaptic transmission at crayfish neuromuscular junctions

Ectothermic animals in areas characterised by seasonal changes are susceptible to extreme fluctuations in temperature. To survive through varied temperatures, ectotherms have developed unique strategies. This study focuses on synaptic transmission function at cold temperatures, as it is a vital component of ectothermic animals' survival. For determining how synaptic transmission is influenced by an acute change in temperature (20°C to 10°C within a minute) and chronic cold (10°C), the crayfish (Procambarus clarkii) neuromuscular junction (NMJ) was used as a model. To simulate chronic cold conditions, crayfish were acclimated to 15°C for 1 week and then to 10°C for 1 week. They were then used to examine the synaptic properties associated with the low output nerve terminals on the opener muscle in the walking legs and high output innervation on the abdominal deep extensor muscle. The excitatory postsynaptic potentials (EPSPs) of the opener NMJs increased in amplitude with acute warming (20°C) after being acclimated to cold; however, the deep extensor muscles showed varied changes in EPSP amplitude. Synaptic transmission at both NMJs was enhanced with exposure to the modulators serotonin or octopamine. The membrane resistance of the muscles decreased 33% and the resting membrane potential hyperpolarised upon warm exposure. Analysis of haemolymph indicated that octopamine increases during cold exposure. These results suggest bioamine modulation as a possible mechanism for ensuring that synaptic transmission remains functional at low temperatures.

Calcium, calpain, and calcineurin in low-frequency depression of transmitter release

Low-frequency depression (LFD) of transmitter release occurs at phasic synapses with stimulation at 0.2 Hz in both isolated crayfish (Procambarus clarkii) neuromuscular junction (NMJ) preparations and in intact animals. LFD is regulated by presynaptic activity of the Ca(2+)-dependent phosphatase calcineurin (Silverman-Gavrila and Charlton, 2009). Since the fast Ca(2+) chelator BAPTA-AM inhibits LFD but the slow chelator EGTA-AM does not, the Ca(2+) sensor for LFD may be close to a Ca(2+) source at active zones. Calcineurin can be activated by the Ca(2+)-activated protease calpain, and immunostaining showed that both proteins are present at nerve terminals. Three calpain inhibitors, calpain inhibitor I, MDL-28170, and PD150606, but not the control compound PD145305, inhibit LFD both in the intact animal as shown by electromyograms and by intracellular recordings at neuromuscular junctions. Analysis of mini-EPSPs indicated that these inhibitors had minimal postsynaptic effects. Proteolytic activity in CNS extract, detected by a fluorescent calpain substrate, was modulated by Ca(2+) and calpain inhibitors. Western blot analysis of CNS extract showed that proteolysis of calcineurin to a fragment consistent with the constitutively active form required Ca(2+) and was blocked by calpain inhibitors. Inhibition of LFD by calpain inhibition blocks the reduction in phosphoactin and the depolymerization of tubulin that normally occurs in LFD, probably by blocking the dephosphorylation of cytoskeletal proteins by calcineurin. In contrast, high-frequency depression does not involve protein phosphorylation- or calpain-dependent mechanisms. LFD may involve a specific pathway in which local Ca(2+) signaling activates presynaptic calpain and calcineurin at active zones and causes changes of tubulin cytoskeleton.

Membrane potentials, synaptic responses, neuronal circuitry, neuromodulation and muscle histology using the crayfish: student laboratory exercises

The purpose of this report is to help develop an understanding of the effects caused by ion gradients across a biological membrane. Two aspects that influence a cell's membrane potential and which we address in these experiments are: (1) Ion concentration of K+ on the outside of the membrane, and (2) the permeability of the membrane to specific ions. The crayfish abdominal extensor muscles are in groupings with some being tonic (slow) and others phasic (fast) in their biochemical and physiological phenotypes, as well as in their structure; the motor neurons that innervate these muscles are correspondingly different in functional characteristics. We use these muscles as well as the superficial, tonic abdominal flexor muscle to demonstrate properties in synaptic transmission. In addition, we introduce a sensory-CNS-motor neuron-muscle circuit to demonstrate the effect of cuticular sensory stimulation as well as the influence of neuromodulators on certain aspects of the circuit. With the techniques obtained in this exercise, one can begin to answer many questions remaining in other experimental preparations as well as in physiological applications related to medicine and health. We have demonstrated the usefulness of model invertebrate preparations to address fundamental questions pertinent to all animals.

Migration-related changes in gene expression in leg muscle of the Christmas Island red crab Gecarcoidea natalis: seasonal preparation for long-distance walking

During their annual breeding migration the Christmas Island land crab Gecarcoidea natalis sustains locomotion aerobically for up to 12 h per day compared with just 10 min during the dry season when their muscles quickly become anaerobic. A seasonal transition to an endurance-muscle phenotype would thus seem essential for migrating crabs. The current study employed a gene discovery approach comparing two expressed sequence tag (EST) libraries, one each for leg muscle from dry (non-migrating) and wet season (migrating) crabs. The 14 most abundant transcripts differed in their representation between the two libraries. The abundances of transcripts of genes predicted to code for different proteins forming contractile muscle components, including actin, troponin and tropomyosin, were significantly different between seasons and thus between physiological states. The shift in the isoform composition of the contractile elements provided evidence for a switch from slow phasic (S1) to slow tonic (S2) fatigue-resistant muscle fibres. A tropomyosin (tm) transcript aligned with a tm isoform of lobster (tmS2), and semi-quantitative RT-PCR confirmed this isoform to be more abundant in the migrating crab muscle. Two LIM protein coding genes, a paxillin-like transcript (pax) and a muscle LIM protein (mlp), were relatively up-regulated in muscle of wet season crabs. These proteins have a fundamental role in muscle development and reconstruction, and their comparative up-regulation is consistent with a remodelling of leg muscle for migration in the wet season. Such a transition would result in an increased representation of aerobic endurance-type fibres concomitant with the greater aerobic exercise capacity of the migrating red crabs.

In vivo long-term synaptic plasticity of glial cells

Evidence showing the ability of glial cells to detect, respond to and modulate synaptic transmission and plasticity has contributed to the notion of glial cells as active synaptic partners. However, synaptically induced plasticity of glia themselves remains ill defined. Here we used the amphibian neuromuscular junction (NMJ) to study plasticity of perisynaptic Schwann cells (PSCs), glial cells at this synapse, following long-term in vivo modifications of synaptic activity. We used two models that altered synaptic activity in different manners. First, chronic blockade of postsynaptic nicotinic receptors using alpha-bungarotoxin (alpha-BTx) decreased facilitation, increased synaptic depression and decreased post-tetanic potentiation (PTP). Second, chronic nerve stimulation increased facilitation and resistance to synaptic depression, while leaving PTP unaltered. Our results indicate that there is no direct relationship between transmitter release and PSC calcium responses. Indeed, despite changes in transmitter release and plasticity in stimulated NMJs, nerve-evoked PSC calcium responses were similar to control. Similarly, PSC calcium responses in alpha-BTx treated NMJs were delayed and smaller in amplitude, even though basal level of transmitter release was increased. Also, when isolating purinergic and muscarinic components of PSC calcium responses, we found an increased sensitivity to ATP and a decreased sensitivity to muscarine in chronically stimulated NMJs. Conversely, in alpha-BTx treated NMJs, PSC sensitivity remained unaffected, but ATP- and muscarine-induced calcium responses were prolonged. Thus, our results reveal complex modifications of PSC properties, with differential modulation of signalling pathways that might underlie receptor regulation or changes in Ca(2+) handling. Importantly, similar to neurons, perisynaptic glial cells undergo plastic changes induced by altered synaptic activity.

Analysis of myofibrillar proteins and transcripts in adult skeletal muscles of the American lobster Homarus americanus: variable expression of myosins, actin and troponins in fast, slow-twitch and slow-tonic fibres

Skeletal muscles are diverse in their contractile properties, with many of these differences being directly related to the assemblages of myofibrillar isoforms characteristic of different fibers. Crustacean muscles are similar to other muscles in this respect, although the majority of information about differences in muscle organization comes from vertebrate species. In the present study, we examined the correlation between myofibrillar protein isoforms and the patterns of myofibrillar gene expression in fast, slow-phasic (S(1)) and slow-tonic (S(2)) fibers of the American lobster Homarus americanus. SDS-PAGE and western blotting were used to identify isoform assemblages of myosin heavy chain (MHC), P75, troponin T (TnT) and troponin I (TnI). RT-PCR was used to monitor expression of fast and slow (S(1)) MHC, P75 and actin in different fiber types, and the MHC and actin levels were quantified by real-time PCR. Fast and slow fibers from the claw closers predominantly expressed fast and S(1) MHC, respectively, but also lower levels of the alternate MHC. By contrast, fast fibers from the deep abdominal muscle expressed fast MHC exclusively. In addition, slow muscles expressed significantly higher levels of actin than fast fibers. A distal bundle of fibers in the cutter claw closer muscle was found to be composed of a mixture of S(1) and S(2) fibers, many of which possessed a mixture of S(1) and S(2) MHC isoforms. This pattern supports the idea that S(1) and S(2) fibers represent extremes in a continuum of slow muscle phenotype. Overall, these patterns demonstrate that crustacean skeletal muscles cannot be strictly categorized into discrete fiber types, but a muscle's properties probably represent a point on a continuum of fiber types. This trend may result from differences in innervation pattern, as each muscle is controlled by a unique combination of phasic, tonic or both phasic and tonic motor nerves. In this respect, future studies examining how muscle phenotype correlates with innervation pattern may help account for variation in crustacean fiber types.

An implantable electrode design for both chronic in vivo nerve recording and axon stimulation in freely behaving crayfish

A chronically implantable electrode design permitting alternate extracellular nerve recording and axon stimulation in freely behaving crayfish was developed. The electrode consists of a double hook made from 20 microm thin platinum wire that can be fitted to various nerve diameters, and is easily implantable. A fast curing, flexible two-component silicone was used for insulation. The double hook was connected to plugs and fixed on the carapace of a crayfish allowing the animals to roam freely. The setup also allows for repeated dis- and re-connection of the crayfish for alternating recording and stimulation. Two channel recordings were used to determine directionality and to discriminate between afferent activity of the two stretch receptor neurons and efferent activity of several motor neurons. In addition, they were also used to determine the conduction velocity of the recorded efferent activity. Stable two-channel recordings could be obtained for up to 5 months and 15 days without apparent effects on the animal. In vivo stimulation could be performed for at least 3 1/2 weeks. The implantable double hook is suitable for widespread use in invertebrate neurobiology.

Phenotype plasticity in postural muscles of the crayfish Orconectes limosus Raf.: correlation of myofibrillar ATPase-based fiber typing with electrophysiological fiber properties and the effect of chronic nerve stimulation

The characteristics of the medial and lateral superficial extensor muscles (sem and sel) in the crayfish Orconectes limosus abdomen and their developmental and activity-dependent plasticity were studied. It was shown that both muscles are innervated by at least five excitatory and one inhibitory motor neuron in a nonuniform pattern. The muscles are composed of at least three different mATPase histochemistry-based fiber types that are all different from a fourth type in the uniform deep extensor muscles. sem and sel are composed of different ratios of these fiber types but do not show a constant fiber type pattern between segments and even between hemisegments. The three histochemically defined superficial extensor-fiber types have characteristic electrophysiological properties. The fiber types were shown to develop successively during the first postembryonic stages of development without a change in the number of muscle fibers. Based on histochemical ATPase staining after 21 days of chronic stimulation by means of an implantable, double-hook electrode, we show preliminary evidence that the fiber composition in the sem can switch from the presumably fast fiber type III to an intermediate type II. Repeated axotomy up to 53 days had no effect on the fiber type composition of the muscles.

Myofibrillar protein isoform expression is correlated with synaptic efficacy in slow fibres of the claw and leg opener muscles of crayfish and lobster

In the crayfish and lobster opener neuromuscular preparations of the walking legs and claws, there are regional differences in synaptic transmission even though the entire muscle is innervated by a single excitatory tonic motor neuron. The innervation of the proximal fibres produced larger excitatory postsynaptic potentials (EPSPs) than those of the central fibres. The amplitudes of the EPSPs in the distal fibres were intermediate between those of the proximal and central regions. These differences in EPSP amplitudes were correlated with differences in short-term facilitation between the three regions. When given a 10- or 20-pulse train of stimuli, the proximal fibres showed greater short-term facilitation initially, often followed by a maximization of short-term facilitation towards the end of a train. In contrast, the central fibres showed a linear increase in short-term facilitation throughout a stimulus train. The distal fibres showed intermediate short-term facilitation compared with the other two regions. Analysis of myofibrillar isoforms showed that levels of troponin-T1 (TnT1), a 55 kDa isoform expressed in slow-tonic (S2) fibres, were correlated with synaptic properties. Proximal fibres had the highest levels of TnT1, with lower levels in distal fibres; central fibres lacked TnT1, which is characteristic of slow-twitch (S1) fibres. In addition, differences in troponin-I isoforms correlated with TnT1 levels between the proximal, central and distal regions. The correlation between slow fibre phenotype and strength of innervation suggests a relationship between synaptic structure and expression of troponin isoforms.

Altered responsiveness to 5-HT at the crayfish neuromuscular junction due to chronic p-CPA and m-CPP treatment

Serotonin (5-HT) levels in the hemolymph of crustaceans has been implied to alter aggressiveness which influences social interactions. The activation of IP3 as a second messenger cascade within crayfish motor neurons in response to application of 5-HT, suggests that the 5-HT receptor subtypes on the motor neurons are analogous to the vertebrate 5-HT2A receptors. Based on evidence in other systems, it would be expected that chronically sustained 5-HT levels in aggressive individuals would result in a compensatory negative feed-back regulation and/or that target tissues would diminish their sensitivity to high levels of circulating, free 5-HT. We addressed the issue of up- and down-regulation in the sensitivity of the responsiveness to exogenously applied 5-HT at the NMJs of crayfish in which the animals have altered endogenous 5-HT levels. Injections of the 5-HT1 and 5-HT2 vertebrate receptor agonist, 1-(3-Chlorophenyl) piperazine dihydrochloride (m-CPP), for 1 week resulted in a decreased responsiveness to application of 5-HT. The compound p-chlorophenylalanine (p-CPA) blocks the enzymatic synthesis of 5-HT and following 7 days of p-CPA injections, a super-sensitivity to exogenous application of 5-HT for both tonic and phasic neuromuscular junctions (NMJs) was observed. However, acute applications of p-CPA and m-CPP, followed by extensive saline washing, did not reveal any altered receptivity to 5-HT application.

Muscle phenotype remains unaltered after limb autotomy and unloading

Loss of chelipeds in crustaceans results in severe atrophy of the major muscle responsible for lifting the limb, the anterior levator. We decided to test if this loss of mechanical load altered muscle phenotype as measured by SDS-PAGE analysis of levator total protein and actomyosin fractions. Levator muscles of adult crayfish, Procambarus clarkii, with either functional regenerate limbs or lack of limb buds (papilla stage) were compared with those from normal contralateral limbs and those from pristine animals. We find that there is no difference in protein profiles among the three conditions. However, the total protein profile for the dually excited levator muscle is unique compared to those of fast or slow muscles of the abdomen (L and SEL, respectively), which receive only phasic or tonic excitatory innervation. The levator myosin heavy chain profile is similar to that of slow phenotype muscles such as the SEL and opener. We conclude that load does not influence levator phenotype. This is likely due either to the intact innervation and continued activation of the levator during atrophy or to the maintenance of passive tension on the muscle. J. Exp. Zool. 289:10-22, 2001.

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 behavioral 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.

Sensitivity of transformed (phasic to tonic) motor neurons to the neuromodulator 5-HT

Long-term adaptation resulting in a 'tonic-like' state can be induced in phasic motor neurons of the crayfish, Procambarus clarkii, by daily low-frequency stimulation [Lnenicka, G.A., Atwood, H.L., 1985b. Long-term facilitation and long-term adaptation at synapses of a crayfish phasic motoneuron. J. Neurobiol. 16, 97-110]. To test the hypothesis that motor neurons undergoing adaptation show increased responses to the neuromodulator serotonin (5-HT), phasic motor neurons innervating the deep abdominal extensor muscles of crayfish were stimulated at 2.5 Hz, 2 h/day, for 7 days. One day after cessation of conditioning, contralateral control and conditioned motor neurons of the same segment were stimulated at 1 Hz and the induced excitatory post-synaptic potentials (EPSPs) were recorded from DEL(1) muscle fibers innervated by each motor neuron type. Recordings were made in saline without and with 100 nM 5-HT. EPSP amplitudes were increased by 5-HT exposure in all cases. Conditioned muscles exposed to 5-HT showed a 2-fold higher percentage of increase in EPSP amplitude than did control muscles. Thus, the conditioned motor neurons behaved like intrinsically tonic motoneurons in their response to 5-HT. While these results show that long-term adaptation (LTA) extends to 5-HT neuromodulation, no phenotype switch could be detected in the postsynaptic muscle. Protein isoform profiles, including the myosin heavy chains, do not change after 1 week of conditioning their innervating motor neurons.

Role of alpha-SNAP in promoting efficient neurotransmission at the crayfish neuromuscular junction

In this manuscript, we address the role of the soluble N-ethylmaleimide sensitive factor attachment protein (alpha-SNAP) in synaptic transmission at the neuromuscular junction of the crayfish opener muscle. Immunochemical methods confirm the presence of alpha-SNAP in these preparations and show that it is concentrated in the synaptic areas. Microinjection and electrophysiological studies show that alpha-SNAP causes an increase in neurotransmitter release yet does not significantly affect the kinetics. More specific quantal analysis, using focal, macropatch, synaptic current recordings, shows that alpha-SNAP increases transmitter release by increasing the probability of exocytosis but not the number of potential release sites. These data demonstrate that the role of alpha-SNAP is to increase the efficiency of neurotransmission by increasing the probability that a stimulus will result in neurotransmitter release. What this suggests is that alpha-SNAP is critical for the formation and maintenance of a "ready release" pool of synaptic vesicles.

Importance of stimulation paradigm in determining facilitation and effects of neuromodulation

Evoked synaptic activity within the CNS and at the neuromuscular junction in most in vivo preparations studied occurs not with single isolated stimuli, but with trains, or bursts, of stimuli. Although for ease in studying the mechanisms of vesicular synaptic transmission one often uses single discrete stimuli, the true mechanisms in the animal may be far more complex. When repetitive stimuli are present at a nerve terminal, often a heightened (i.e., facilitated) postsynaptic potential can be as a result. Facilitation is commonly used as an index of synaptic function and plasticity induced by chronic stimulation or by neuromodulation. The mechanisms that give rise to facilitation are thought to be the same that may underlie short-term learning and memory [C.H. Bailey, E.R. Kandel, Structural changes accompanying memory storage. Annu. Rev. Physiol. 55 (1993) 397-426.]. Differences in short term facilitation (STF) are seen depending on the conventional stimulation paradigm (twin pulse, train, or continuous) used to induce facilitation. Thus, a battery of paradigms should be used to characterize synaptic function to obtain a closer understanding of the possible in vivo conditions.