DUM Neurons Research Articles

Refined electrophysiological analysis suggests that a depressant toxin is a sodium channel opener rather than a blocker

The effects of a recombinant depressant insect toxin from Leiurus quinquestriatus hebraeus, Lqh IT2-r, have been studied in current and voltageclamp conditions on the isolated axonal and DUM neuron preparations of the cockroach Periplaneta americana. Lqh IT2-r depolarizes the axon, blocks the evoked action potentials, and modifies the amplitude and the kinetics of the sodium current. The inward transient peak current is greatly decreased and is followed by a maintained slow activating-deactivating sodium current. The slow component develops at membrane potentials more negative than the control, and has a time constant of activation of several tens of milliseconds. The flaccid properties of Lqh IT2-r do not correspond to a blockage of the Na + channels, but may be attributed to modified Na + channels which open at more negative potential, activate slowly and do not inactivate normally.

Action of locust neuromodulatory neurons is coupled to specific motor patterns.

1. Many muscles of the locust are supplied by dorsal unpaired median neurons (DUM neurons) that release octopamine and alter the contractions caused by spikes in motor neurons. To determine when these neuromodulatory neurons are normally activated during behaviour, intracellular recordings were made simultaneously from them and from identified motor neurons during the specific motor pattern that underlies kicking. A kick consists of a rapid and powerful extension of the tibia of one or both hind legs that is produced by a defined motor pattern. Only 3 identified DUM neurons of the 20 in the metathoracic ganglion spike during a kick, and they supply muscles involved in generating the kick. Their spikes occur in a distinctive and repeatable pattern that is closely linked to the pattern of spikes in the flexor and extensor tibiae motor neurons. When the extensor and flexor muscles cocontract, these three DUM neurons produce a burst of spikes at frequencies that can rise to 25 Hz, and with the number of spikes (3-15) related to the duration of this phase of the motor pattern. The spikes stop when the flexor muscle is inhibited and therefore before the tibia is extended rapidly. The other DUM neurons which supply muscles that are not directly involved in kicking are either inhibited or spike only sporadically. 2. The activation of a specific subset of DUM neurons during kicking may thus be timed to influence the action of the muscles that participate in this movement and appear to be controlled by the same circuits that determine the actions of the participating motor neurons. These modulatory neurons thus have specific individual actions in the control of movement.

The antennal motor system of crickets: modulation of muscle contractions by a common inhibitor, DUM neurons, and proctolin

In the crickets Gryllus bimaculatus and Gryllus campestris, the two intrinsic antennal muscles in the scape (first antennal segment) control antennal movements in the horizontal plane. Of the 17 excitatory antennal motoneurons, three motoneurons, two fast and one slow, can be stimulated selectively and their effect on muscle contraction, i.e. antennal movement, measured. Simultaneously, either a common inhibitor (CI) neuron or two DUM neurons can be stimulated and the effect on the slow and/or fast muscle contraction measured. The activity of the common inhibitor affected only slow muscle contractions. It decreased contraction rate, increased relaxation rate and suppressed prolonged muscle tension. This effect was blocked by picrotoxin. DUM neuron stimulation affected both slow and fast contractions. It reduced slow and enhanced fast contractions but in only 10% of the experiments could this effect be detected. DUM neuron activity could be mimicked by octopamine application. Proctolin application enhanced both slow and fast contractions but did not increase muscle tension in the absence of motoneuron activity. The results are discussed in relation to the large variability of possible antennal movements during behaviors.

Central nervous sensitization and dishabituation of reflex action in an insect by the neuromodulator octopamine.

Habituation of excitatory synaptic inputs onto identified motor neurons of the locust metathoracic ganglion, driven electrically and by natural stimuli, was examined using intracellular recording. Rapid progressive reduction in amplitude of EPSPs from a variety of inputs onto fast-type motor neurons occurred. The habituated EPSPs were quickly dishabituated by iontophoretic release of octopamine from a microelectrode into the neuropilar region of presumed synaptic action. The zone within which release was effective for a given neuron was narrowly-defined. With larger amounts of octopamine applied at a sensitive site the EPSP became larger than normal, and in many instances action potentials were initiated by the sensitized response. Very small EPSPs onto a motor neuron, which were associated with proprioceptive feedback, and which were originally too small to be detected above the noise, were potentiated to a level of several mV by the iontophoresed octopamine. A DUM neuron (presumed to be octopaminergic) was found, whose direct stimulation was followed by a strong dishabituating and sensitizing action leading to spikes, of inputs to an identified flexor tibiae motor neuron. The action and its time course were closely similar to those evoked by octopamine iontophoresed into the neuropil in the region of synaptic inputs to the motor neuron. It is concluded that DUM (octopaminergic) neurons exert large potentiating actions on central neuronal excitatory synaptic transmission in locusts.

Generation of specific behaviors in a locust by local release into neuropil of the natural neuromodulator octopamine.

The natural insect neuromodulator octopamine (OCT) was released iontophoretically into regions of neuropil in locust metathoracic ganglia. A narrowly-defined site was found on one side of the ganglion at which release caused a prolonged bout of repetitive flex-extend-flex movements of the tibia on the injected side, at a frequency of from 2-3.5 Hz. When a bout had terminated, repetition of the OCT release caused an extremely similar bout to occur, and again with further treatments, indefinitely. OCT iontophoresis at the equivalent site on the contralateral side caused the contralateral flexor to make stepping movements. Two sites were found, in each half of the ganglion, at which similar OCT release evoked a bout of flight motor activity at 10 Hz. The flight bout involved both sides synchronously and nearly equally, except for a slightly greater motor output on the injected side. Evoked bouts lasted from 20 sec to 25 min depending on the preparation and amount of OCT released. At a site in the 6th abdominal ganglion of mature female locusts OCT release suppressed ongoing rhythmic oviposition digging evoked by severing the ventral nerve cord. A number of previously undescribed DUM neurons was encountered and their dendritic patterns, which are distinctive, determined following dye injection. A hypothesis, termed the Orchestration Hypothesis is presented, which considers how modulator neurons such as locust octopaminergic neurons, might be involved in the generation of specific behaviors.

Dorsal unpaired median neurons of the cockroach metathoracic ganglion.

On the dorsal surface in the metathoracic ganglion of the cockroach Periplaneta americana, eight large and a number of small somata described by Crossman et al., (1971) were reexamined. These eight large cells (40-60 micron in diameter) could be divided into three different types by Lucifer yellow or nickel injection, depending on the branching of their axons. No neuron was found which sent axons into either the anterior or posterior connectives. In the mesothoracic ganglion an interganglionic H-shaped DUM neuron was found which had four axons, one into each anterior and posterior connective.

Fine structure of an octopaminergic neuron and its terminals.

The large octopaminergic dorsal unpaired median neuron of the locust that innervates the extensor tibiae muscle, DUMETi, was examined electronmicroscopically. Its soma contains many Golgi complexes apparently making dense-core vesicles similar to those found in peripheral branches and terminals. There are also larger stores of the dense material in the soma, especially near the exit of the principal neurite, that are not in vesicular form. Since the neurons can be penetrated and stimulated by microelectrodes, they form favorable subjects for direct studies of the control of neurosecretion. Preterminal fine branches of the neuron were located in proximal outer bundles of muscle fibers into which they had been traced electrophysiologically. They contain numerous large dense-core vesicles arrayed in rows near microtubules. These fine branches have a thick layer of collagenous connective tissue between the axon and the muscle fiber. Final terminals have varicosities containing many vesicles, lying inside the outer layers of the sarcolemmal complex of muscle fibers. They do not form synaptic structures. Terminals of another DUM neuron, one that innervates the dorsal longitudinal flight muscles (DUMDL), were similar in detail to those of DUMETi. DUMETi swelled about 20-fold in cross-sectional area above a ligature, in a 12-hr period, indicating that there is an extensive centrifugal flow of material in it, and sprouted a branch.

Isogenic locusts and genetic variability in the effects of temperature on neuronal threshold

Physiological variability was previously encountered while studying the effects of temperature on identified motorneurons. Of 22 locusts examined from within the breeding colony, 3 deviant animals were found in which the current threshold of the fast extensor tibiae (FETi) motorneuron did not decrease as was expected with increasing temperature (Fig. 1). We have used clones of isogenic locusts to study the genetic basis, physiological extent, and behavioral effects of a similar abnormal threshold response of FETi to increases in temperature. Over 30 clones, raised by parthenogenetic breeding, were used to isolate naturally-occurring genotypic variability. In this study we have focused our attention on the behaviorally abnormal clone 7 animals that have a low probability of jumping which is not altered by heating; and the behaviorally normal clone 8 animals that have a higher probability of jumping which increases with increasing temperature (Table 1). In clone 8 animals, the physiological properties of FETi show the normal steady-state responses to increases in temperature: the current threshold decreases (Fig. 1), the voltage threshold decreases, and the membrane resistance remains relatively unchanged. In clone 7 animals, the physiological properties of FETi show abnormal steady-state responses to increases in temperature: the current threshold increases (Fig. 1), the voltage threshold remains relatively unchanged (Fig. 2), and the membrane resistance decreases (Fig. 3), all of which are similar to the abnormal responses observed occasionally from the heterogenic breeding colony. These abnormalities in FETi of clone 7 animals are regarded as a neurophysiological correlate of the absence of an increase in jumpiness with increasing temperature. An abnormal response to heating in clone 7 animals is also found in AAdC (Fig. 6), ASFlTi, and the first basalar motorneuron. A normal response to heating in clone 7 animals is found in the spiracle closer motorneurons (Fig. 7), the DUM neurons, and the CI neuron.

THE RESPONSE OF SINGLE AUDITORY-NERVE FIBERS TO ACOUSTIC STIMULATION

THE AIM OF the experiments described here was to study the response of single auditory-nerve fibers to acoustic stimulation delivered to the intact ear. Similar studies on other single afferent fibers have remarkably extended knowledge of the mode of action of diverse sensory mechanisms, while at the same time emphasizing their fundamental similarities. The present report describes the behavior of single auditory fibers and stresses similarities to other sensory nerves; in a subsequent paper we hope to relate these findings to a specific theory of action of the mammalian cochlea. METHOD Young cats anesthetized with dial (0.75 cc. per kilo) were used in these experiments. The postero-dorsal aspect of the auditory nerve was exposed by removing the lateral portion of the occipital bone where it meets the petrous bone. Bleeding from the sinus petrosus inferior was stopped by judicious cauterization or by Clotting Globulin. * A Ringer-filled glass micropipette with Ag-AgCl wire inserted as close as possible to the tip served as the active lead from the nerve. It was early established that pipettes with openings greater than 5~ do not allow isolation of the action potentials of single auditory fibers. The 3 to 5~ electrodes used have an impedance of about 1 megohm when tested on a resistance-capacity bridge between 0.6 and 2.5 kc. The microelectrode and the indifferent electrode (a silver plate in the neck muscles) led to a capacity-coupled amplifier (Grass) with an input done photographically