Opener Excitor Research Articles

Dual inhibition of the dactyl opener muscle in lobster

The dactyl opener neuromuscular system of crayfish and lobster has long been a popular model system for studies of synaptic physiology and its modulation. Previous studies of its neural innervation in both species have reported that whereas the opener excitor axon (OE) and the specific opener inhibitor (OI) innervate the entire muscle, the common inhibitor (CI) is restricted to a small number of the most proximal muscle fibers and is the physiologically weaker of the two inhibitors. Here, we show in the lobster that, contrary to previous reports, CI innervates fibers along the entire extent of the dactyl opener muscle and thus shares the innervation targets of OE and OI. To characterize the physiological function of CI in the lobster dactyl opener system, we independently stimulated both OI and CI and compared their effects. The physiological impact of each inhibitor was similar: inhibitors elicit inhibitory junction potentials of similar sizes, both trigger muscle relaxations and both inhibit excitatory junction potentials and muscle contractions to a comparable extent. Thus, in the periphery, CI and OI appear to be equally powerful inhibitors with similar physiological roles, both in suppressing contractions triggered by the excitor OE and in directly relaxing muscle tension. In light of these observations, our understanding of the physiological roles of specific and common inhibitors in crustacean motor control requires reevaluation.

Presynaptic inhibition and the participation of GABA(B) receptors at neuromuscular junctions of the crab Eriphia spinifrons.

Presynaptic inhibition exerted by the common inhibitor on the closer and opener muscles and by the specific inhibitor on the opener muscle was investigated in the crab Eriphia spinifrons. In the closer muscle, activation of GABA(B) receptors by baclofen reduced the mean quantal content of excitatory junctional currents by about 25%. Blocking GABA(B) receptors with CGP 55845 diminished presynaptic inhibition at a similar percentage. GABA(B) receptor-mediated presynaptic inhibition is linked to G proteins. Application of pertussis toxin eliminated about 25% of the inhibition exerted by the common inhibitory neuron. GABA(B) receptors participate in presynaptic inhibition at release boutons of the slow and the fast closer excitor at a similar percentage. In the opener muscle, presynaptic inhibition of transmitter release from the same endings of the opener excitor was about 15% stronger with the specific inhibitor than with the common inhibitor. About 10% of the presynaptic inhibition produced by either one of the two inhibitors could be abolished by blocking GABA(B) receptors. The amplitudes of the excitatory junctional currents in the opener were reduced in the presence of baclofen by about 25%, suggesting that synaptic terminals of the opener excitor are endowed with a similar percentage of GABA(B) receptors as terminals of the slow and the fast closer excitors. Baclofen had no effect on postsynaptic inhibition, indicating that GABA(B) receptors are not involved in postsynaptic neuromuscular inhibition.

Dynamic responses of series force receptors innervating the opener muscle apodeme in the blue crab, Callinectes sapidus.

Receptors monitoring muscle force innervate the opener muscle apodeme in the walking legs of the blue crab, Callinectes sapidus. Biocytin backfills reveal 9-15 bipolar neurons with somata as large as 60 micrometer m positioned at the distal end of the apodeme. Sensory endings insert into the apodeme and are in series with the opener muscle. The axons of these neurons form the opener apodeme sensory nerve that merges with the most distal branch of the opener motor nerve. Recordings reveal that the receptors are not spontaneously active nor do they respond to passive muscle stretch. Isometric muscle contraction evoked by stimulating the opener excitor motor neuron is the adequate stimulus for receptor firing. Most significant is the finding that during contraction, over a wide range of forces, the firing rate of individual receptors closely parallels the rate of change of isometric force. The peak instantaneous frequency typically occurs at the force derivative maximum, but not at maximum force development. Thus, receptors of the opener apodeme sensory nerve more closely monitor changes in isometric force rather than the total force achieved.

Fibre Heterogeneity in the Closer and Opener Muscles of Crayfish Walking Legs

ABSTRACT The closer and opener muscles in the third walking legs of the three crayfish Pacifastacus leniusculus, Procambarus clarkii and Astacus leptodactylus are composed of fibres which differ in histochemistry, electrophysiology and morphology. Three major groups of fibres (A, B and C) were distinguished. Group A fibres react weakly to histochemical stains for myofibrillar ATPase (mATPase) activity characteristic of fibres with slow shortening speeds. In the opener muscle, they are innervated by the opener excitor (OE) and the specific opener inhibitor (OI). In the closer muscle, group A fibres are innervated by the common inhibitory neurone (CI) in addition to single (slow closer excitor, SCE) or double excitatory (SCE and fast closer excitor, FCE) innervation. Group A fibres have the largest excitatory junction potentials (EJPs), the longest membrane time constants (τ) and the longest sarcomeres. They are located at the very distal and proximal ends of both muscles. Group B fibres show higher mATPase activity than group A fibres. In the opener muscle, they are innervated by OE and OI; in the closer muscle, they receive double excitatory (SCE and FCE) and CI innervation. Single SCE and OE EJPs are small; those caused by FCE are larger. τ is shorter than in the other two fibre groups. Sarcomere lengths lie between those of group A and C fibres. Group B fibres are found along the entire lengths of both muscles. Group C fibres exhibit the highest mATPase activity (characteristic of fibres with fast shortening velocity) which, in contrast to the ATPase of group B fibres, is not resistant to alkaline preincubation at pH10.05. In the closer, these fibres lack innervation by CI, otherwise the innervation pattern is identical to that of group B fibres. EJP size is similar to that of group B fibres; τ ranges between values for group A and B fibres. Sarcomere lengths are the shortest of all the fibre types. Group C fibres constitute the majority of the fibres in the two muscles and mainly occupy the central regions.

Effects of ethanol and other drugs on excitatory and inhibitory neurotransmission in the crayfish.

1. Crayfish exposed to 434 mM ethanol (EtOH) showed signs of hyperactivity within 0.5-2 h, at which times crayfish hemolymph EtOH concentration had reached 60-90 mM. 2. A 10-min exposure to 60-90 mM EtOH reduced presynaptic inhibition of excitatory postsynaptic currents (EPSCs) at the crayfish opener neuromuscular junction (NMJ) in vitro but did not significantly alter excitatory neurotransmission. The same concentrations of EtOH did not alter other potentials or currents associated with inhibition at this synapse, such as presynaptic inhibitory potentials (PIPs), inhibitory postsynaptic potentials (IPSPs), and inhibitory postsynaptic currents (IPSCs). 3. Intermediate EtOH concentrations (120-180 mM) applied for 10 min in vitro reduced the amplitude of excitatory postsynaptic potentials (EPSPs) by decreasing the membrane resistance of opener muscle fibers and by reducing the amplitude of EPSCs. 4. High EtOH concentrations (434 mM) applied for 10 min in vitro had yet greater depressive effects on measures of postsynaptic properties described above. The time course of EPSCs was also significantly reduced. In addition, presynaptic properties such as action-potential (AP) amplitude and frequency of spontaneous release of neurotransmitter were reduced by 434 mM EtOH. 5. Presynaptic inhibition, gamma-aminobutyric acid (GABA; 250-500 microM), muscimol (50 microM), and baclofen (75 microM) all reduced the depolarizing afterpotential of APs in the excitor axon and reduced EPSPs in opener muscle fibers. GABA (500 microM) and baclofen (75 microM) significantly reduced presynaptic AP amplitudes, whereas presynaptic inhibition, GABA (250 microM), and muscimol (50 microM) had no effect on AP amplitude. Bicuculline (250-500 microM), a GABAA antagonist, did not entirely eliminate presynaptic inhibition, whereas picrotoxin (50 microM), another GABAA antagonist, completely removed presynaptic inhibition. Thus presynaptic inhibitory mechanisms may involve both GABAA and GABAB receptors on the opener excitor axon. 6. Our data suggest that the behavioral hyperactivity seen at hemolymph EtOH concentrations of 60-90 mM is not accompanied by a change in excitatory synaptic transmission observed at the opener NMJ. Rather, crayfish hyperactivity may be due to depressive effects of EtOH on inhibitory synapses in the CNS similar to the disinhibition evoked by EtOH at the opener NMJ.

Synaptic plasticity at crayfish neuromuscular junctions: Presynaptic inhibition

Intracellular recordings at sites electronically near terminals of the opener excitor axon in the claw of crayfish (Procambarus simulans) show that stimulation of the inhibitor neuron produces hyperpolarizing or depolarizing presynaptic inhibitory potentials (PIPs). GABA applied anywhere along the length of the opener excitor or inhibitor axons also produces hyperpolarizing or depolarizing potentials. The amplitude of action potentials (APs) at recording sites near some excitor terminals is reduced by an average of 6 mV during presynaptic inhibition, which also reduces excitatory postsynaptic potentials (EPSPs) by 50-70%. The time course of AP reduction equals the time course of EPSP reduction and the amount of AP reduction is independent of the sign or amplitude of the PIPs. All these data are consistent with a hypothesis that a conductance increase produced by GABA in these presynaptic terminals of the excitor axon is responsible for presynaptic inhibition. However, the effect of presynaptic inhibition upon the accumulation of short-term facilitation of excitatory transmitter release is not the same in all muscle fibers. In some terminals, the accumulation of short-term facilitation during short, high-frequency trains of action potentials which are presynaptically inhibited often equals the accumulation of facilitation without inhibition. In other terminals, short-term facilitation accumulated during presynaptic inhibition often does not equal facilitation accumulated in the absence of presynaptic inhibition. These data suggest that some other factor which may contribute to presynaptic inhibition, such as a direct effect to decrease calcium currents, may also affect short-term facilitation in some terminals.

Innervation and control of tension in abdominal muscles of Blaberus discoidalis and Gromphadorhina portentosa

In Blaberus discoidalis and Gromphadorhina portentosa, the distribution of motor axons to the muscles which control movements of the spiracular valves at both respiratory and non-respiratory spiracles is identical. Both fast and slowly contracting heads of the opener muscles are innervated by an excitatory motor axon. Physiological properties of the opener excitor axon correlate with valve function. The slowly contracting head of the opener muscle is, in addition, innervated by a common inhibitor which also occasionally innervates closer muscle fibers. Activation of the common inhibitor terminates contraction of slowly contracting opener muscle fibres and initiates a rapid relaxation of these fibres.

Paired impulses in lobster claw motoneurons: in vitro and in vivo production

The somata of the claw opener excitor (OE), the slow closer excitor (SCE), and the fast closer excitor (FCE) motoneurons were physiologically identified in the lobster, Homarus americanus, using an isolated claw ganglion preparation. Depolarization of the soma produced paired impulses with an interpulse interval of 10–12 ms, which demonstrates that doublet production is an intrinsic motoneuron property. Similar paired impulses were found for the FCE axon in intact lobsters where they occurred with a shorter interpulse interval for the cutter than for the crusher claw.

Differences in motor output and fiber composition of the opener muscle in lobster dimorphic claws

AbstractThe structural and biochemical properties of the opener muscle and the impulse pattern of its single excitatory motoneuron differed between the paired cutter and crusher claws in the lobster, Homarus americanus. The soma of the opener excitor (OE) motoneuron was physiologically identified in an isolated claw‐ganglion preparation and its responsiveness to sensory stimulation of the first and second nerve roots monitored. The OE soma to the crusher claw had a higher spike frequency and burst duration than its counterpart to the cutter claw. The OE axon to the cutter claw fatigues much more readily than that to the crusher claw at 50 and 100 Hz stimulation in isolated claws. The mean sarcomere length of 8 and 10μm, in fibers of the cutter and crusher opener muscles was significantly different. The electrophoretic pattern of myosin extracted from the cutter opener muscle revealed four bands not found in the crusher opener muscle and a single band in the crusher not seen in the cutter. Thus properties of the opener muscle and its motoneuron differ on the two sides in keeping with the dimorphic nature of the claws.

Intracellular recordings from crustacean motor axons during presynaptic inhibition

Action potentials intracellularly recorded from near the nerve terminals of the opener excitor motor axon in crayfish are reduced in amplitude during presynaptic inhibition. The amplitude and sign (hyperpolarizing or depolarizing) of presynaptic inhibitor potentials (PIPs) depends upon the relationship between the resting membrane potential of each excitor axon and its PIP equilibrium potential (which equals the equilibrium potential for γ-aminobutyric acid).

Activity of Excitor and Inhibitor Claw Motor Neurones During Habituation and Dishabituation of the Crayfish Defence Response

ABSTRACT The activity of the opener excitor and inhibitor motor neurones was recorded during habituation and dishabituation of the defensive claw opening response in partially restrained crayfish (Astacus sp.). Claw opening evoked by brief tactile stimulation of the thorax undergoes habituation if the stimulation is repeated at 30 s or 1 min intervals. This habituation is accompanied by a progressive decrease in evoked excitor activity, while the activity of the inhibitor remains unchanged. The habituated claw opening response can be dishabituated by tactile stimulation of the claw or head, or by visual stimulation. Dishabituation is accompanied by both an increase in evoked excitor activity and a decrease in evoked inhibitor activity. Dishabituation is also accompanied by potentiation of claw opening: that is, the same number of excitor and inhibitor spikes produce greater claw opening following dishabituating stimulation. The potentiation of claw opening following dishabituation is due in part to post-tetanic potentiation (PTP) at the excitor neuromuscular junction. PTP was demonstrated with physiological parameters of stimulation in isolated claw preparations, and is of sufficient magnitude and duration to account for the observed potentiation. These results contradict the conclusion from an earlier study (see Schone, 1961) that habituation of the claw opening response is due to an increase in inhibitor activity. They also provide new evidence for a role in dishabituation of both disinhibition and PTP.

Ionic basis of presynaptic inhibitory potentials at crayfish claw opener.

1. Intracellular recordings from the claw opener excitor axon of the crayfish, Procambarus clarkii, were obtained near the terminal arborizations of the axon on the surface of the opener muscle. Rest potential in the excitor axon averaged --80 mV over 20 cells. Action-potential amplitude and duration averaged 100 mV and 2 ms, respectively. 2. A single action potential in the opener inhibitor axon produces a hyperpolarizing synaptic potential (average amplitude 0.3 mV) in the excitor axon. The apparent reversal potential of this inhibitory synaptic potential is approximately 5 mV more negative than rest in control saline. No excitor axons were observed to have depolarizing synaptic potentials at rest. 3. A decrease in external chloride concentration from 240 to 24 mM causes the apparent reversal potential to depolarize an average of 12 mV, with no change in rest potential. In low-chloride saline, the synaptic potential evoked by stimulation of the inhibitor axon becomes depolarizing. 4. An increase in external potassium concentration from 5 to 10 mM causes the apparent reversal potential to depolarize by 16 mV; however, rest potential depolarizes by 10 mV. Low external potassium has the opposite effects, causing both rest potential and the apparent reversal potential to hyperpolarize. 5. Presynaptic inhibition at the Procambarus claw opener neuromuscular junction appears to be mediated by a hyperpolarizing synaptic potential. The results of these experiments suggest that chloride serves as the charge for the presynaptic potential. The evidence for a direct involvement of potassium as a charge carrier is equivocal due to Donnan equilibrium effects involving Cl.

Motoneuron interactions in crayfish claw control: Evidence from intracellular recording

1. Intracellular recordings from crayfish claw motoneuron somata revealed synaptic potentials concurrent with spikes in other specific claw motoneurons when the latter were activated by: a) sensory stimulation; b) antidromic axon stimulation; or c) current injection. 2. These synaptic potentials were studied mainly by signal-averaging one cell's recorded response to antidromic stimulation of another. The interactions thus revealed were: a) central inhibition of the opener excitor (OE) by the opener inhibitor (OI); b) excitation of OI by OE; and c) mutual excitation among all of OI, SCE, and FCE (the slow and fast closer excitors). 3. Central latencies of the connecting pathways lie between 0.5 and 2.0 ms, suggesting monosynaptic cross-connections among these motoneurons. 4. These conclusions are consistent both with earlier data from spike-train analysis (Wiens and Gerstein, 1975) and with the requirements of the control system.

Electrical and structural properties of crayfish claw motoneurons in an isolated claw-ganglion preparation

1. An isolated claw-ganglion preparation of the crayfish is described in which reflex activity is maintained for eight hours or more. 2. Intracellular recording and cobalt injection have made it possible to locate and characterize the claw motoneurons. Soma recordings show attenuated axon spikes of 5–10 mV amplitude and subthreshold potentials of the same magnitude. 3. The fast closer excitor (FCE) receives subthreshold excitation in parallel with the slow closer excitor (SCE) and the opener inhibitor (OI) even though it seldom fires. 4. IPSP's are recorded in the opener excitor (OE) concurrent with OI spikes. 5. Cobalt injection reveals a parallel dendritic structure among the three synergists FCE, SCE and OI, and a distinctly different dendritic distribution for OE. The soma siza ranking: OI< OE≦FCE = SCE is apparent. 6. The results are discussed and compared with other arthropod systems with regard to the relation between soma size and electrical functions, distribution of somata and dendrites, and effects of cobalt on electrical functioning. A conflict with previous work on this system is discussed.

Autogenic production of paired impulses by the opener excitor neuron of the crayfish claw

Autogenic production of paired impulses by the opener excitor neuron of the crayfish claw