The pharmacology of molluscan neurons.
It is commonly accepted that the basic physiological properties of the neurons as well as the nature of transmitter substances have remained relatively unchanged through evolution, while brain size and neuron number have greatly increased. Among invertebrates the molluscs, due to the large size of their neurons and lesser complexity of the neural networks controlling specific behavior, have proved to be especially useful for studying elementary properties of single neurons, network organization as well as various forms of learning and memory. The study of putative neurotransmitters has indicated that molluscs use the same low molecular-weight substances and peptides or their metabolites and cyclic nucleotides as transmitters and second messengers as the other species of various phyla. At the same time the receptors of neurotransmitters were found to have certain characteristic properties in the molluscs. The large molluscan neurons have permitted the isolation of individual identifiable nerve cells, and the subsequent analysis of quantities of the transmitters and their metabolic enzymes. These studies have demonstrated that single neurons frequently can contain more than one putative neurotransmitter. It can be expected that this model will contribute to an understanding of the role of multiple transmitters within a single neuron assuring the plasticity of the nervous system. The cellular mechanisms of plasticity have been demonstrated first in molluscan nervous systems. It was proved in identified Aplysia neurons that the same transmitter (ACh) can be released from an interneuron onto two or more follower neurons and can excite one and inhibit another or evoke a biphasic response on a third type of cell. The biphasic response of the molluscan neurons to neurotransmitters was the first demonstration of the plastic synaptic changes. The discovery of individual neurons with their groups of follower cells acting as chemical units has provided an insight into the organization of various behavioral acts. Study of the gastropod molluscs has also shown that the giant serotonergic cells can act as peripheral modulator neurons, as well as interneurons, and in this way they can affect their target organs at more than one level. The molluscan studies have provided more information on transmitter receptors as it was shown that molluscan neurons have at least six different 5HT receptors, three Ach receptors which can be separated pharmacologically. This type of study has led to the discovery of numerous new antagonists and poisons.(ABSTRACT TRUNCATED AT 400 WORDS)
D000109:Acetylcholine / Q000378:metabolism / Q000502:physiology; D000596:Amino Acids / Q000378:metabolism / Q000502:physiology; D000818:Animals; D002395:Catecholamines / Q000378:metabolism / Q000502:physiology; D002490:Central Nervous System / Q000378:metabolism; D004594:Electrophysiology; D005247:Feeding Behavior / Q000502:physiology; D005724:Ganglia / Q000502:physiology; D006321:Heart / Q000502:physiology; D006632:Histamine / Q000502:physiology; D007668:Kidney / Q000502:physiology; D007858:Learning / Q000502:physiology; D008568:Memory / Q000502:physiology; D008974:Mollusca / Q000378:metabolism*; D009474:Neurons / Q000166:cytology / Q000378:metabolism* / Q000502:physiology; D018377:Neurotransmitter Agents / Q000378:metabolism / Q000502:physiology; D009711:Nucleotides / Q000502:physiology; D010455:Peptides / Q000378:metabolism; D011941:Receptors, Adrenergic / Q000378:metabolism; D017459:Receptors, Amino Acid; D017983:Receptors, Catecholamine; D011956:Receptors, Cell Surface / Q000378:metabolism; D011950:Receptors, Cholinergic / Q000378:metabolism; D011985:Receptors, Serotonin / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism / Q000502:physiology; D013569:Synapses / Q000378:metabolism; D013654:Taurine / Q000502:physiology; D014018:Tissue Distribution
D000596:Amino Acids; D002395:Catecholamines; D018377:Neurotransmitter Agents; D009711:Nucleotides; D010455:Peptides; D011941:Receptors, Adrenergic; D017459:Receptors, Amino Acid; D017983:Receptors, Catecholamine; D011956:Receptors, Cell Surface; D011950:Receptors, Cholinergic; D011985:Receptors, Serotonin; D013654:Taurine; D012701:Serotonin; D006632:Histamine; D000109:Acetylcholine