Biomedical subjects

S Sombati

Publications and source records attributed to S Sombati.

22 records · Page 2Linked to original sources

PubMed1986

Membrane currents in identified lactotrophs of rat anterior pituitary.

Qualitative features of the primary inward and outward current components of identified lactotrophs of the rat anterior pituitary were examined. Identification of lactotrophs in heterogeneous dissociated anterior pituitary cultures was accomplished by application of the reverse hemolytic plaque assay. Currents in lactotrophs were subsequently examined using whole-cell or patch recording techniques. Two components of inward calcium current were observed: a transient component and a sustained component. The transient component activated at voltages as negative as -50 mV and was the major contributor to total lactotroph calcium current. The sustained component activated at voltages above about -10 mV. The 2 currents could be qualitatively separated by differences in inactivation properties and in sensitivity to cadmium. At least 3 components of outward current were distinguished. Either 30 mM TEA or 0 calcium eliminated a major portion of sustained outward current. This is likely to represent primarily calcium- and voltage-activated potassium current. The remaining current could be further differentiated into a transient current component that could be inactivated with conditioning potentials above -60 mV. A slowly activating and deactivating potassium current remained following inactivation of the transient current. Although the time course of the transient current is reminiscent of "A" current, activation of this current required potentials above -30 mV. Candidates for the single-channel currents that underlie the whole-cell outward currents were observed in cell-attached recordings. When combined with patch-clamp electrophysiological methods, the reverse hemolytic plaque assay promises to be a powerful technique for the electrophysiological characterization of specific cell subtypes in heterogeneous dissociated cell populations.

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PubMed1984

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.

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PubMed1984

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.

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PubMed1984

Glutamatergic central nervous transmission in locusts.

It is generally believed that neural transmission in the central nervous systems of insects is cholinergic, on the basis of secondary evidence: the presence of cholinesterase, and sensitivity of a nonsynaptic region of the neuron, its cell body, to iontophoresed acetylcholine. In the present work a preparation has been developed which takes advantage of the availability of identified motor neurons in the locust metathoracic ganglion with known 3-dimensional geometry of dendritic fields. These neurons transmit at their peripheral neuromuscular junctions with glutamate. The fast extensor tibiae motor neuron also makes excitatory central connections onto its functional antagonists the flexor tibiae motor neurons. Unless Dale's principle is contravened, transmission at these central synapses should also be glutamatergic. This transmission onto flexor motor neurons was found to be attenuated 70% by a glutamatergic blocker. Glutamate iontophoresed into selected areas of neuropil into which the motor neurons have dendritic branches caused the neurons to be depolarized, in a dose-dependent manner. Individual motor neurons were directly excited to spike with suprathreshold iontophoretic current. With long durations of release they were desensitized, but recovered quickly with rest. The data provide evidence that central transmission onto motor neurons in the locust metathoracic ganglion is glutamatergic.

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