Phenylethanolamine: a new putative neurotransmitter in Aplysia

Catecholamine neurons in Aplysia: improved light-microscopic resolution and ultrastructural study using paraformaldehyde and glutaraldehyde (FaGlu) cytochemistry

We have modified the formaldehyde-glutaraldehyde (FaGlu) histofluorescence method of Furness, Costa, and Blessing (1977a) and Furness, Costa, and Wilson (1977b) to examine wholemounts and sections of both juvenile and adult ganglia as well as peripheral tissues of Aplysia californica. FaGlu fluorescence is the result of a reaction between formaldehyde and tissue catecholamines to produce water-insoluble (fixed) fluorophores. In serially sectioned cerebral ganglia, 70-80 positive neurons were observed (many in clusters of 10-20 cells), many more than were found using the glyoxylic acid technique. Catecholamine-containing varicosities were densely packed in localized portions of the neuropil of all central ganglia. Exclusive localization in the neuropil of presumed dopamine release sites is similar to that previously found for the neuropeptide SCP but differs from the widespread ramification of varicose neurites containing 5-HT, FMRFamide, and ELH. The FaGlu technique also enabled us to study the ultrastructure of catecholamine-containing neurons. In contrast to the larger vesicles found in serotonergic and histaminergic neurons, these dopaminergic neurons contain 70 nm dense-cored vesicles.

Localization of catecholamines in the buccal ganglia of Aplysia californica

The distribution of catecholamines and serotonin in the buccal ganglia and buccal nerves of Aplysia californica was examined using glyoxylic acid-induced histofluorescence. Three identifiable, medium-sized cell bodies, two paired and one unpaired, and 5 smaller cells fluoresced blue-green indicating the presence of catecholamines. Numerous fluorescent axons were observed in the neuropil and peripheral nerves, including a network of catecholaminergic processes emerging from the esophageal nerve and surrounding the base of the esophagus. The presence of catecholaminergic cells and processes in the buccal system suggests that these transmitters are used in the control of feeding behavior and digestion.

Modulation of dopamine receptors in the Tapes clam by dextroamphetamine and phenylethanolamine

The mechanism underlying the modulation, by dextroamphetamine and compounds related to phenylethanolamine, of responses to dopamine and serotonin has been studied in the isolated ventricle and aortic bulb of the clam Tapes watlingi. Dextroamphetamine and phenylethanolamine but not cocaine and benztropine have the ability to unmask inhibitory responses to both dopamine and serotonin in the ventricle. Chlordimeform but not clozapine attenuates the inhibitory response to both dextroamphetamine and phenylethanolamine in concentrations which have little or no effect on the inhibitory response to dopamine in the ventricle. Phenylethanolamine, dextroamphetamine, phenylpropylolamine and p-chloro-phenylethanolamine but not octopamine or noradrenaline attenuate the contractile responses to both dopamine and serotonin in preparations of the quiescent aortic bulb. These data show that there are specific receptors for phenylethanolamine in the Tapes heart capable of modulating responses to dopamine and serotonin, and suggests that this biogenic phenethylamine can act as an environmental and physiological factor which may determine how the mollusc heart responds to dopamine.

Effects of catecholamines, monophenolamines and phenylamines on identifiable giant neurons of an African giant snail (Achatina fulica Férussac)

The effects of catecholamines, monophenolamines and phenylamines on eight identifiable giant neurons of an African giant snail (Achatina fulica Férussac) were examined to classify these neurons into several categories according to their sensitivities to the various substances. The five neurons, PON (periodically oscillating neuron), TAN (tonically autoactive neuron), 1-VMN (left-visceral multiple spike neuron), d-RPeAN (dorsal-right pedal autoactive neuron) and VIN (visceral intermittent firing neuron), were sensitive to catecholamines. Of these neurons, PON was excited most markedly by dopamine (dopamine-sensitive); TAN, 1-VMN and d-RPeAN were inhibited most markedly by epinine (epinine-sensitive); and VIN was excited equally by the four catecholamines, dopamine, epinine, L-noradrenaline and L-adrenaline (widely sensitive). The three other neurons, FAN (frequently autoactive neuron), d-LPeLN (dorsal-left pedal large neuron) and d-LCDN (dorsal-left cerebral distinct neuron) were sensitive to monophenolamines. DL-Octopamine was the most inhibitory on FAN and d-LCDN, but was the most excitatory on d-LPeLN. DL-Synephrine had the same but somewhat weaker effects on the three neurons as did DL-octopamine. The three phenylamines, L-phenylalanine, beta-phenylethylamine and DL-beta-phenylethanolamine, had no effect on any of the eight neurons examined.

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)

Receptors for gamma-aminobutyric acid (GABA) on Aplysia neurons

Aplysia neurons show 5 different types of response (three excitatory and two inhibitory) to iontophoretic application of gamma-aminobutyric acid (GABA). Four of these are associated with a membrane conductance increase, but one is associated with a conductance decrease. The most common response is a fast hyperpolarization which reverses at about--58 mV and is sensitive to manipulation of external Cl- concentration, and thus is due to a specific increase in Cl- conductance. There is an infrequent, slower hyperpolarizing response which does not reverse above about--80 mV and is insensitive to external Cl-. This response appears to result from a conductance increase to K+. Two types of depolarizing responses are associated with conductance increases. These responses differ in their latency, duration and sensitivity to curare. The more frequent is relatively rapid (peak at 1-2 sec) and is depressed by curare at high concentrations. In other neurons, GABA causes a slower response, peaking at 6-10 sec, which is not curare-sensitive. Usually for both types of response, the voltage and conductance changes are completely abolished by perfusion with Na+-free seawater, and the responses cannot be reversed with depolarization. In other neurons such as L11, the response can be reversed with depolarization, and appears to result from a conductance increase to both Na+ and Cl-. In neuron R15, GABA causes a slow depolarizing response (peak at about 9 sec) which is associated with a decreased membrane conductance, probably to K+. The classical GABA antagonists, picrotoxin and bicuculline, block Cl- responses but no others, while the fast Na+ and Cl- responses are depressed by curare. Strychnine does not affect any GABA response. The multiplicity of GABA responses, the specificity of their organization and the fact that only some neurons have receptors for GABA, argue that GABA may have a role as a neurotransmitter in Aplysia. Furthermore, the existence of several types of excitatory GABA response suggests that GABA may function both as an inhibitory and excitatory neurotransmitter.