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At least 145 records · Page 8Linked to original sources
PubMed1975

Effects of thiamine antagonists on nerve conduction. I. Actions of antimetabolites and fern extract on propagated action potentials.

To assess the hypothesis that thiamine is directly involved in the permeability changes at the sodium channel during nerve conduction, the effects of thiamine antagonists on lobster giant axon resting and action potentials were determined. Thiamine antimetabolites, in millimolar concentrations, reversibly decreased the maximum rate of rise and amplitude of the action potential while increasing its duration. In particular, thiamine tert-butyl disulfide (TTBD) elicited the formation of pronounced shoulders during repolarization, lengthening the action potential by 2-50 times, depending on dose. Antimetabolites also depolarized the resting membrane, but this change was poorly reversible and may indicate a dual mechanism for antimetabolite action. An extract of the fern, Pteris aquilina, reversibly decreased the maximum rate of rise of the action potential and depolarized the resting potential. It also elevated and prolonged the action potential after-depolarization, sometimes causing repetitive activity. The strength of these actions was correlated with the antithiamine potency of the extract, and was diminished by addition of thiamine to the extract.

Action Potentials↗
PubMed1976

Agonistic and antagonistic activity of glutamate analogs on neuromuscular excitation in the walking limbs of lobsters.

Forty-six analogs of L-glutamate were tested for activity on muscle fibers in the walking limbs of lobsters. Effects on the membrane potential, input resistance, and amplitude of neurally evoked EPSPs and IPSPs were studied as well as effects on applied L-glutamate. Seventeen of the compounds studied depolarized the muscle fibers in a manner indicative of an agonistic action on receptors in the neuromuscular excitatory membrane. Six analogs selectively reduced the amplitude of evoked EPSPs, and at least three of these (kainic acid, D-glutamate, and D-aspartate) antagonized the excitatory action of applied L-glutamate. Kainic acid was the most potent of the blockers of neuromuscular excitation, but even it was relatively weak since a concentration of 1 mM was required for an apparent effect. Generally those analogs in the L-configuration which possessed activity, had agonistic actions, whereas those in the D-configuration were usually antagonistic. These observations provide pharmacological evidence for the concept that L-glutamate is the transmitter agent which mediates neuromuscular excitation in the walking limbs of lobsters. In addition, our results are consistent with recent studies which indicate that L-aspartate may also function in this neuromuscular excitatory process.

Animals↗
PubMed1979

Lobster claw motorneurons respond to contralateral sensory stimuli.

Unilateral sensory stimuli in an isolated lobster claw--ganglion preparation elicits soma and axon spikes in homologous motorneurons in the ipsilateral as well as the contralateral hemiganglion. This cross excitation of the motorneurons provides a basis for bilateral reflexes in the claw and is likely mediated through interneuron(s).

Animals↗
PubMed1979

Mg++-sensitivity of neuromuscular transmission in two crustaceans: correlation with blood Mg++ levels.

Neuromuscular transmission was measured in muscles of spider crabs (Hyas areneus) and lobsters (Homarus americanus). Solutions containing 40 and 10 mM/1 Mg++, which were approximately the same as those measured in the blood of Hyas and Homarus, respectively, were used to soak the preparations prior to testing. In Homarus, neuromuscular transmission was severely depressed by 40 mM Mg++. In spider crabs, neuromuscular transmission was not severely depressed. Although the amount of transmitter released by nerve impulses was reduced, total membrane depolarization during trains of impulses was not reduced because a compensating increase in muscle fiber membrane resistance occurred in Hyas preparations exposed to M Mg++. Hyas, but not Homarus, is physiologically adapted to function at relatively high blood Mg++ concentrations.

Animals↗
PubMed1980

Heterogeneity of excitatory synapses at the ends of single muscle fibers in lobster, Homarus americanus.

Crustacean neuromuscular synapses arising from a single excitor axon are known to be well differentiated among different muscle fibers but little is known about their condition along single fibers. Focal recording techniques were used to examine the quantal transmitter release and facilitation properties of synapses in the single excitatory innervated distal accessory flexor muscle of the lobster, Homarus americanus. Synapses were reliably differentiated with respect to quantal output so that those located near the tendon end were 1.15--4.12 times greater than those at the opposite, exoskeletal end (p less than 0.01, paired t-test). Regional differences were also seen in the amount of facilitation determined from twin pulse experiments. The fine structural basis for these differences was determined by serial section electron microscopy of 10-micrometer segments at each end to ensure that the area of focal recording was sampled. No quantitative differences were found in the terminals or synapses in the two regions. Instead, the physiological diversity was correlated with number and size of presynaptic dense bars. Thus, the tendon end had a greater number and larger mean surface area of dense bars compared to the exoskeletal end. This heterogeneity of excitatory multiterminal innervation is correlated with the axonal branching pattern. Thus, the main axon and the larger primary axon branches lie in close proximity to the tendon end of the muscle fibers, whereas the exoskeletal end is innervated by smaller secondary and tertiary axonal branches. This proximity to the large axonal branches of the higher quantal output synapses at the tendon end may be regulated by some neural influence including a timing of innervation and/or access to greater amounts of metabolites in the larger branches which may be conducive to forming high-output synapses.

Animals↗
PubMed1980

Spontaneous and evoked postsynaptic potentials in an embryonic neuromuscular system of the lobster, Homarus americanus.

The embryonic motor innervation to the deep extensor abdominal muscles was studied in lobster eggs in which reflex twitches and tail flips could be evoked by mechanical stimulation in early embryos. Recordings from impaled fibers during early and later stages of embryonic development revealed spontaneous depolarizing and hyperpolarizing potentials, suggesting the presence of excitatory and inhibitory axons. Stimulation of the extensor motor innervation produced a variety of EPSPs and IPSPs. The depolarizing responses included small and large EPSPs and nonovershooting spikes. Although moderate facilitation of the EPSP was sometimes observed, defacilitation was observed in the majority of fibers of all stages. Spiking could not be evoked by motor axon stimulation in embryos of early stages. These findings indicate that from the outset the deep abdominal extensor neuromuscular system of the lobster is phasic in its response to nerve stimulation and is functional as part of the tail flip reflex at least six months before hatching.

Action Potentials↗
PubMed1980

A peptide action in a lobster neuromuscular preparation.

The neuropeptide proctolin causes a sustained contraction of the opener muscle of the dactyl of the lobster walking leg. This substance acts directly on the muscle at concentrations as low as 10(-10)M. The contraction is dependent on extracellular calcium. Neither a significant depolarization nor a detectable change in the input resistance accompanies the response. No presynaptic action of proctolin is indicated; excitatory and inhibitory junctional potential sizes and the frequency of spontaneous miniature excitatory junctional potentials are unaffected.

Animals↗
PubMed1981

Biochemistry and ultrastructure of serotonergic nerve endings in the lobster: serotonin and octopamine are contained in different nerve endings.

In this article we report that the distribution of serotonin in the lobster nervous system parallels the distribution of octopamine and that the same tissues that contain endogenous serotonin can synthesize it from tryptophan. Octopamine and serotonin are highly concentrated in a neurosecretory region of the second thoracic roots in association with a group of neurosecretory cells. The roots possess separate high-affinity uptake systems for both serotonin and tryptophan. Radioactive serotonin, accumulated in tissues during incubations with either tritiated serotonin or tritiated tryptophan, can be released, in a calcium-dependent manner, by depolarization with potassium. A detailed morphological examination of the second thoracic roots shows four distinct categories of nerve endings in the vicinity of the neurosecretory cells. Octopamine is synthesized in one of these types of endings and serotonin in another. The high-affinity uptake systems for serotonin and tryptophan are found only in association with the endings that make serotonin. These endings and all the biochemical parameters of serotonin metabolism in the roots are selectively destroyed by previous injection of animals with the neurotoxin 5,7-dihydroxytryptamine.

Animals↗
PubMed1981

Some effects of proctolin on the cardiac ganglion of the Maine Lobster, Homarus americanus (Milne Edwards).

The neuropeptide proctolin has excitatory effects on the isolated lobster cardiac ganglion. Selective application to the anterior cell body region produces a dose-dependent (10(-8)--10(-5) M) prolonged depolarization of large anterior cells as well as marked increases in burst frequency and/or duration. In ganglia which have been silenced with tetrodotoxin, proctolin application to anterior cells elicits long-lasting depolarizing responses which are accompanied by a 10-30% increase of the apparent membrane input resistance. Higher proctolin concentrations produce high-frequency trains of driver potentials. It is proposed that a proctolin like peptide may serve a neurohumoral role in the lobster cardiac ganglion and that the anterior motor neurons exhibit endogenous rhythmicity in its presence.

Animals↗
PubMed1982

The ionic dependence of black widow spider venom action at the stretch receptor neuron and neuromuscular junction of crustaceans.

The effects of black widow spider venom (BWSV) on the crayfish stretch receptor and the lobster neuromuscular junction were examined. In crayfish stretch receptor neurons, BWSV caused a slight hyperpolarization followed by a large depolarization. The venom-induced depolarization of the stretch receptor was caused by an increase in membrane conductance to Na+ and Ca2+. Black widow spider venom also caused an increase in the frequency of miniature inhibitory postsynaptic potentials recorded in the stretch receptor. The ability of BWSV to increase the frequency of miniature excitatory postsynaptic potentials (MEPSPs) at the lobster neuromuscular junction was dependent on the divalent cation composition of the bathing medium. Ringer solutions containing Ca2+ supported the greatest venom-induced increase in MEPSP frequency, Mg2+ and Mn2+ supported a moderate increase in MEPSP frequency, while Co2+ and Zn2+ blocked this venom effect entirely. Black widow spider venom did not block axonal conduction in lobster walking leg axons or in the axon of the crayfish stretch receptor. The results suggest that in crustaceans, BWSV interacts specifically with membrane of the soma-dendritic region of the stretch receptor and with nerve terminal membrane, causing an increase in Na+ and Ca2+ conductance.

Animals↗
PubMed1982

All-or-none control of the bursting properties of the pacemaker neurons of the lobster pyloric pattern generator.

In Crustacea the central pattern generator for the pyloric motor rhythm (filtration to the midgut) is known to be located within the stomatogastric ganglion (STG); its cycling activity is known to be organized by three endogenous burster neurons acting as pacemakers and driving 11 follower neurons. In Homarus, recordings from the isolated stomatogastric nervous system (Fig. 1) indicate that (1) the pyloric output can be generated only when the STG is afferented (i.e., connected to the more rostral oesophageal and commissural ganglia) (Fig. 2) and (2) the deafferentation of the STG results in a complete loss of the bursting properties of the pacemaker neurons (Fig. 4). Manipulation of the STG inputs responsible for unmasking the properties of the pacemakers strongly suggests that (1) they are not phasic inputs (Fig. 5) and (2) they are long-term acting inputs (Fig. 6). These results provide evidence for a neural all-or-none control of the bursting properties of the pacemaker neurons of a motor pattern generator.

Afferent Pathways↗
PubMed1983

A neurochemical description of the dopaminergic innervation of the stomatogastric ganglion of the spiny lobster.

The spiny lobster stomatogastric ganglion has been shown to be innervated by catecholaminergic processes which derive from cells of large central ganglia (Kushner and Maynard, 1977). Biochemical evidence had indicated that the stomatogastric system synthesizes dopamine and not norepinephrine from tritiated tyrosine (Barker, Kushner, and Hooper, 1979). Studies reported here document that the stomatogastric ganglion itself contains dopamine, as measured with a sensitive endogenous assay. Moreover, the ganglion can synthesize dopamine from tritiated tyrosine or DOPA. Additionally, when incubated in tritiated dopamine, the ganglion takes up dopamine and protects it from degradation; this process is inhibited by cocaine. When incubated with 3H-tyrosine, small but measurable amounts of tritiated dopamine were detected in the medium surrounding the ganglion.

Animals↗
PubMed1983

The identification and localization of a catecholamine in the motor neurons of the lobster cardiac ganglion.

The cardiac ganglion from Homarus americanus was investigated for the purpose of providing biochemical and histochemical information as to the identity of the neurotransmitter(s) utilized by this system. Three techniques were employed in this study: (1) the glyoxylic acid histofluorescence staining technique (GA), which showed fluorescence characteristic of catecholamines localized in the five motor neurons; (2) high-voltage electrophoresis (HVE) in one dimension followed by ascending chromatography in the second dimension, which indicated incorporation of label from tritiated tyrosine into norepinephrine (NE) and small amounts of dopamine (DA); (3) high-pressure liquid chromatography with electrochemical detection (HPLC/EC), which indicated the presence of endogenous norepinephrine.

Animals↗
PubMed1983

Innervation and motor patterns of the abdominal superficial flexor muscles in larval lobsters.

The pattern of innervation and motor program of the abdominal superficial flexor muscle was investigated electrophysiologically in larval lobsters (Homarus americanus). The muscle receives both excitatory and inhibitory innervation in the larval as well as in the embryonic stages. Individual muscle fibers receive a single inhibitory neuron (f5) and a maximum of three excitors. Based on spike heights these axons belong to either the small (f1 or f2) or large (f3, f4) motoneurons. While the small axons preferentially innervate the medial muscle fibers the large axons innervate medial as well as lateral fibers. This larval pattern of innervation resembles the pattern in the adult lobster. The resemblance extends to the firing patterns as well with both large and small excitors firing spontaneously. Furthermore, evoked activity in the larvae produces reciprocal (and occasionally cyclical) bursts of excitor and inhibitor neurons denoting abdominal extension and flexion and resembling the firing patterns in adults. Consequently motor programs employed in steering the pelagic larvae are reminiscent of the programs for maintaining posture in the benthic adult lobsters.

Animals↗
PubMed1983

Influence of walking on swimmeret beating in the lobster Homarus gammarus.

Influence of walking on swimmeret beating in intact lobsters, Homarus gammarus, has been analyzed using a treadmill experimental device. Belt movement activates both leg stepping and swimmeret beating. The simultaneity of the onset of the two motor systems in this situation is demonstrated to be the result of a startle response initiated when the belt begins to move. This reaction consists of a non-specific motor activity involving several antagonist postural and dynamic muscles. Abdominal extension and vigorous swimmeret beating are the main features of this reaction. The main characteristics of the swimmeret beating as defined by Davis (1969) has been observed here in sequences without walking. However during long walking sequences a very different swimmeret beating pattern occurs. It is suggested that this slow swimmeret beating is completely subordinate to the walking rhythm during sequences of absolute coordination. In more rapid swimmeret beating a relative coordination with leg stepping is very common. The functional meaning of this linkage between legs and swimmerets is discussed.

Animals↗
PubMed1983

Differential backfilling of interneuron populations based upon axon projections in a lobster abdominal ganglion.

Intersegmental interneurons in the lobster second abdominal ganglion were differentially stained by simultaneously backfilling their axons in the anterior and posterior hemiconnectives with nickel and cobalt ions. When precipitated with rubeanic acid the nickel and cobalt formed different colors in the cell bodies depending upon the location of the axon(s) in the connectives. In the most completely stained preparations 55 ascending, 56 descending, and 25 bidirectional interneurons were observed. Soma diameters ranged from 15-120 micron. One of the somata was located in the connective. The differential staining technique has advantages over conventional backfilling techniques for examining the morphological relationships of populations of neurons having different axon projections.

Animals↗