ABSTRACT
We have studied the effects of group I, II, and III metabotropic glutamate receptor (mGluR) agonists on rhythm generation by the gastric circuit of the stomatogastric ganglion (STG) of the Caribbean spiny lobster Panulirus argus. All mGluR agonists and some antagonists we tested in this study had clear and distinct effects on gastric rhythm generation when superfused over combined oscillating or blocked silent STG preparations. A consistent difference between group I agonists and group II and III agonists was that group I agonists acted excitatory. The group I-specific agonists L-quisqualic acid and (S)-3,5-dihydroxyphenylglycine, as well as the nonspecific agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid accelerated ongoing rhythms and could induce gastric rhythms in silent preparations. The group II agonist (2S,1'S, 2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I) and the group III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) slowed down or completely blocked ongoing gastric rhythms and were without detectable effect on silent preparations. The action of L-CCG-I was blocked partially by the group-II-specific antagonist, (RS)-1-amino-5-phosphonoindan-1-carboxylic acid [(RS)APICA], and the group-III-specific antagonist (RS)-alpha-methyl-4-phosphonophenylglycine completely blocked the action of L-AP4. Besides its antagonistic action, the group-II-specific antagonist (RS)APICA had a remarkably strong apparent inverse agonist action when applied alone on oscillating preparations. The action of all drugs was dose dependent and reversible, although recovery was not always complete. In our experiments, the effects of none of the mGluR-specific agonists were antagonized or amplified by the N-methyl-D-aspartate (NMDA)-receptor-specific antagonist D(-)-2-amino-5-phosphonopentanoic acid, excluding the contamination of responses to mGluR agonists by nonspecific cross-reactivity with NMDA receptors. Picrotoxin did not prevent the inhibitory action of L-CCG-I and L-AP4. We conclude that mGluRs, probably similar to those belonging to groups I, II, and III described in mammals, may play a role as modulators of gastric circuit rhythm generation in vivo.
Subject(s)
Ganglia, Invertebrate/drug effects , Nephropidae/physiology , Nerve Net/physiology , Receptors, Metabotropic Glutamate/drug effects , Stomach/innervation , Alanine/analogs & derivatives , Alanine/pharmacology , Amino Acids, Dicarboxylic/pharmacology , Animals , Cycloleucine/analogs & derivatives , Cycloleucine/pharmacology , Dose-Response Relationship, Drug , Excitatory Amino Acid Agonists/pharmacology , Excitatory Amino Acid Antagonists/pharmacology , Female , In Vitro Techniques , Male , Methoxyhydroxyphenylglycol/analogs & derivatives , Methoxyhydroxyphenylglycol/pharmacology , Motor Neurons/drug effects , Motor Neurons/physiology , Quisqualic Acid/pharmacology , Receptors, Metabotropic Glutamate/agonists , Receptors, Metabotropic Glutamate/antagonists & inhibitorsABSTRACT
We have characterized in crustacean neurones in culture a receptor for gamma-aminobutyric acid (GABA) which conforms to the pharmacological profile of the proposed type-C GABA receptor (GABAC) found in the vertebrate retina. It is associated with a chloride-selective ion channel and is blocked by picrotoxin. It is neither inhibited by bicuculline nor activated by baclofen, while diazepam and phenobarbital are without modulatory effect. Like the GABAC-like receptor of the vertebrate retina it is activated by the folded GABA analogue cis-4-aminocrotonic acid (CACA). Desensitization is moderate allowing for a more sustained action of GABA. Single channel recordings revealed a bicuculline-resistant GABA- and CACA-activated chloride channel with a conductance about eight times higher than that described for the bicuculline-resistant GABA receptor channel from the rat retina.
Subject(s)
Neurons/metabolism , Receptors, GABA/drug effects , Animals , Bicuculline/pharmacology , Cell Membrane/drug effects , Cell Membrane/metabolism , Cells, Cultured , Chloride Channels/drug effects , Chloride Channels/metabolism , Crotonates/pharmacology , Ganglia, Invertebrate/drug effects , Ganglia, Invertebrate/metabolism , Kinetics , Nephropidae , Neurons/drug effects , gamma-Aminobutyric Acid/pharmacologyABSTRACT
The morphology of the adult electromotoneurones of the electric ray Torpedo marmorata has been investigated by intracellular injection of horseradish peroxidase. The results show particularly small dendritic fields, poor dendritic branching, and the existence of two distinct types of dendrites. The electrophysiological data demonstrate that the electromotoneurones behave similarly in vivo and in in vitro brain slices. Observations on spontaneous and stimulus-evoked postsynaptic potentials suggest quantal release of transmitter in the Torpedo central nervous system.
Subject(s)
Brain/physiology , Motor Neurons/physiology , Animals , Axonal Transport , Brain/cytology , Dendrites/physiology , Dendrites/ultrastructure , Electric Conductivity , Female , Horseradish Peroxidase , In Vitro Techniques , Membrane Potentials , Motor Neurons/cytology , TorpedoABSTRACT
Miniature and stimulus-evoked electroplaque potentials were recorded in Torpedo electrocytes intracellularly and extracellularly and analysed quantitatively. Tetrodotoxin reversibly blocked stimulus evoked potentials but hardly affected spontaneous miniature potentials in amplitude and frequency. The quantum content of stimulus-evoked potentials varied between 150 and 400 in normal saline and decreased in low Ca2+ high Mg2+ solution. Quantal release conformed to binomial statistics and allowed determination of the release parameters p and n. Analysis of the time constant of decay of spontaneous miniature electroplaque currents showed variation around a mean of 0.75 +/- 0.16 msec (SD) which was greatly prolonged by application of neostigmine.
