Inhibitory synapses on pacemaker neurons in the heart ganglion of a stomatopod, Squilla oratoria

Characterization of the cardiac ganglion in the crab Neohelice granulata and immunohistochemical evidence of GABA-like extrinsic regulation

The aim of the present work is to provide an anatomical description of the cardiac system in the crab Neohelice granulata and evidence of the presence of GABA by means of immunohistochemistry. The ganglionic trunk was found lying on the inner surface of the heart's dorsal wall. After dissection, this structure appeared as a Y-shaped figure with its major axis perpendicular to the major axis of the heart. Inside the cardiac ganglion, we identified four large neurons of 63.7 μm ± 3.7 in maximum diameter, which were similar to the motor neurons described in other decapods. All the GABA-like immunoreactivity (GABAi) was observed as processes entering mainly the ganglionic trunk and branching in slender varicose fibers, forming a network around the large neurons suggesting that GABAi processes contact them. Our findings strengthen previous results suggesting that the GABAergic system mediates the cardio-inhibitory response upon sensory stimulation.

Neuronal and neurohormonal control of the heart in the stomatopod crustacean,Squilla oratoria

The heart of Squilla oratoria contains a cardiac ganglion that consists of 15 intrinsic neurons, supplied by a pair of inhibitory nerves and two pairs of excitatory nerves, arising from the central nervous system. These comprise the extrinsic cardiac innervation. The paired cardio-inhibitor (CI)nerves run out in the 10th pair of nerve roots emerging from the subesophageal ganglion (SEG). The cell bodies of the CI neurons are found in the hemisphere of the 1st segment of the SEG contralateral to the nerve roots in which the CI axons emerge. The two pairs of 1st and 2nd cardio-accelerator (CA1 and CA2)nerves run out in the 16th and 19th pairs of nerve roots of the SEG. The cell bodies of the CA1 and CA2 neurons are found in the hemispheres of the 3rd and 4th segments of the SEG ipsilateral to the nerve roots in which the CA1 and CA2 axons are found.

The heartbeat was activated by application of glutamate, serotonin,dopamine, octopamine or acetylcholine, which were applied to the heart by perfusion into an organ bath. Joro-spider toxin (JSTX) blocked myocardial excitatory junctional potentials evoked by the cardiac ganglion. Neuronal cell bodies and processes in the heart were examined using immunocytochemical techniques. All 15 neurons of the cardiac ganglion showed glutamate-like immunoreactivity. Glutamate may be a neurotransmitter of the cardiac ganglion neurons.

JSTX also blocked cardiac acceleration by activation of CA1 and CA2 axons. CA1 and CA2 axons showed glutamate-like immunoreactivity. It is likely that glutamate is a neurotransmitter for the cardio-acceleratory neurons.

The heartbeat was inhibited by application of γ-amino-butyric acid(GABA). Cardiac inhibition induced by activation of CI axons was blocked by picrotoxin. CI axons showed GABA-like immunoreactivity. These results may support the identification of GABA as an extrinsic inhibitory neurotransmitter.

Reliable, responsive pacemaking and pattern generation with minimal cell numbers: the crustacean cardiac ganglion

Investigations of the electrophysiology of crustacean cardiac ganglia over the last half-century are reviewed for their contributions to elucidating the cellular mechanisms and interactions by which a small (as few as nine cells) neuronal network accomplishes extremely reliable, rhythmical, patterned activation of muscular activity-in this case, beating of the neurogenic heart. This ganglion is thus a model for pacemaking and central pattern generation. Favorable anatomy has permitted voltage- and space-clamp analyses of voltage-dependent ionic currents that endow each neuron with the intrinsic ability to respond with rhythmical, patterned impulse activity to nonpatterned stimulation. The crustacean soma and initial axon segment do not support impulse generation but integrate input from stretch-sensitive dendrites and electrotonic and chemically mediated synapses on axonal processes in neuropils. The soma and initial axon produce a depolarization-activated, calcium-mediated, sustained potential, the "driver potential," so-called because it drives a train of impulses at the "trigger zone" of the axon. Extreme reliability results from redundancy and the electrotonic coupling and synaptic interaction among all the neurons. Complex modulation by central nervous system inputs and by neurohormones to adjust heart pumping to physiological demands has long been demonstrated, but much remains to be learned about the cellular and molecular mechanisms of action. The continuing relevance of the crustacean cardiac ganglion as a relatively simple model for pacemaking and central pattern generation is confirmed by the rapidly widening documentation of intrinsic potentials such as plateau potentials in neurons of all major animal groups. The suite of ionic currents (a slowly inactivating calcium current and various potassium currents, with variations) observed for the crustacean cardiac ganglion have been implicated in or proven to underlie a majority of the intrinsic potentials of neurons involved in pattern generation.

The effects of γ-aminobutyric acid on voltage-clamped motoneurons of the lobster cardiac ganglion

1. We examined the electrophysiological and pharmacological effects of GABA on voltage-clamped motoneurons of the lobster cardiac ganglion. 2. GABA caused a dose-dependent current (EC50 = 0.7 mM), which reversed at the estimated Cl- equilibrium potential. 3. The conductance activated by GABA was voltage-dependent, increasing as a non-linear function of depolarization. 4. A Na(+)-dependent GABA uptake mechanism was only weakly sensitive to nipecotic acid. 5. Picrotoxin inhibited the GABA response, but bicuculline had no effect. 6. We conclude that the effect of GABA in the lobster cardiac ganglion is similar to its effect on other crustacean neuromuscular tissues and on vertebrate GABAA receptors. 7. There appear to be differences among species with respect to the physiology and pharmacology of the Na(+)-dependent GABA transporter. 8. The effect of GABA is also similar to the ionic mechanism underlying the action of histamine in the cardiac ganglion.

Structure and localization of synaptic complexes in the cardiac ganglion of a portunid crab

The cardiac ganglion of Portunus sanguinolentus exhibits spontaneous rhythmic activity when isolated. The ganglion contains five large and four small intrinsic neurons and is innervated by three pairs of fibres originating in the thoracic ganglia. We have identified the processes of the large neurons in electron micrographs by injecting these cells with two electron-dense markers, horseradish peroxidase (HRP) and Procion Rubine (PR). In addition we have studied the processes of the four smaller neurons by light microscopy serial reconstructions and by electron microscopy of selected regions. Both markers were found only in neuronal processes and not in glial cells nor in the extracellular space, except close to the soma of the injected cell. We found contacts between the small secondary (collateral) processes of the large cells but not between their somata or their primary processes (axons and dendrites). Two specialized structures present at the contacts between the collateral processes were small membrane close appositions, possibly the site of electrotonic junctions, and chemical synapses. Contacts between processes marked by HRP and those marked by PR were common, as were contacts between processes marked by either HRP or PR and those of the other intrinsic neurons. Adjacent processes stained by PR could contain PR deposits of different densities, but it is unclear whether this finding was due to intercellular diffusion of the dye or to its diffusing at different rates into branches of the same process. Identified processes of all the intrinsic neurons contained the same type of vesicles, which were different from those found in processes of the extrinsic fibres. Chemical synapses were present at contacts between processes of the extrinsic and intrinsic neurons, as well as at contacts between processes of the intrinsic neurons. The axons of three small cells made a series of contacts at which extensive arrays of membrane close appositions, but not chemical synapses, were found. These three axons also formed contacts, either directly or through their collateral branches, with processes of the large cells, at which both membrane close appositions and chemical synapses were present. The axon of the fourth small cell could not be followed in our series.

Acceleratory synapses on pacemaker neurons in the heart ganglion of a stomatopod, Squilla oratoria

The pacemaker neurons of the heart ganglion are innervated from the CNS through two pairs of acceleratory nerves. The effect of acceleratory nerve stimulation was examined with intracellular electrodes from the pacemaker cells. The major effects on the pacemaker potential were an increase in the rate of rise of the spontaneous depolarization and in the duration of the plateau. The aftereffect of stimulation could last for minutes. No clear excitatory postsynaptic potential (EPSP) was observed, however. On high frequency stimulation, a small depolarizing response (the initial response) was sometimes observed, but the major postsynaptic event was the following slow depolarization, or the enhancement of the pacemaker potential (the late response). With hyperpolarization the initial response did not significantly change its amplitude, but the late response disappeared, showing that the latter has the property of the local response. The membrane conductance did not increase with acceleratory stimulation. The injection of depolarizing current increased the rate of rise of the spontaneous depolarization, but only slightly in comparison with acceleratory stimulation, and did not increase the burst duration. It is concluded that the acceleratory effect is not mediated by the EPSP but is due to a direct action of the transmitter on the pacemaker membrane.