An analysis of histamine-induced inhibitory response in molluscan neurons

[Demonstration of carcinine synthetase, a new enzyme catalysing the metabolism of histamine in the central nervous system of Carcinus maenas]

Carcinine biosynthesis was induced in vitro from its two components, beta-alanine and histamine. The reaction was catalyzed by muscle, heart, and CNS extracts from Carcinus maenas. The specific activity of the enzyme, carcinine synthetase, was 15 times higher in CNS than in other organs. Only CNS extracts induced biosynthesis of carcinine from histidine, and only in the presence of pyridoxal-5'-phosphate. Hence the seat of carcinine biosynthesis seems to be the CNS. It is highly probable that in the CNS, histidine is transformed into histamine, which is then catabolized into carcinine. The latter would then be transported and accumulated in the cardiac tissue. Thus histamine--the metabolism of which takes place totally within the CNS--would be implicated as a participant in the neuronal activity of Carcinus maenas. Carcinine synthetase is a soluble enzyme that requires the presence of ATP, beta-alanine, and histamine. Mg2+ and dithiothreitol are also essential for activity. Optimum pH is approximately 7.6. Carcinine synthetase differs from carnosine synthetase and gamma-glutamylhistamine synthetase in that it does not catalyze synthesis of beta-alanylhistidine or gamma-glutamylhistamine.

[Biosynthesis and metabolism of histamine in the central nervous system of Carcinus maenas]

The central nervous system of Carcinus maenas synthesizes radioactive histamine when incubated in the presence of [14C] histidine and pyridoxal-5' phosphate. This biosynthesis increases linearly as a function of the amount of enzyme and the incubation time. It is not effected by heart, muscle or hepatopancreas extracts nor by haemolymph. Thus histamine appears to be synthesized mainly in the nervous system. The latter is also the seat of carcinine (beta-alanylhistamine) biosynthesis. Since carcinine seems to be a product of histamine neutralization, histamine metabolism should take place in its entirety in the nervous system. Thus histamine appears to be implicated in the neuronal activity of Carcinus. Different areas of the crustacean central nervous system: brain, eyestalks and thoracic ganglionic mass biosynthesize and metabolize histamine. Thus they all could contain sites of action for histamine. The nervous systems of two other Decapodes, Cancer and Astacus also effect histamine biosynthesis but don't metabolize it into carcinine.

Characterization of long-lasting histaminergic inhibition in a beating pacemaker neuron of Onchidium

A single BPSP (excitatory-inhibitory postsynaptic potential) was monosynaptically produced in an identified Onchidium neuron, Be-1, with a beating rhythm upon stimulation of the cardiac nerve. The BPSPs summated to produce an inhibition of long duration (ILD) upon blockage of the beating rhythm after repeated stimulation, so that the BPSPs seemed to be functionally inhibitory. Ten stimuli (1-2 Hz) applied to the cardiac nerve usually evoked an ILD (0.5-1 min) of about 10 mV. The early and middle phases of this ILD reversed near -80 to -85 mV, but the late phase did not reverse at more negative potentials. None of the phases was significantly affected by low Cl or Na solutions or by high Ca solutions. However, by changing the external K, the shift of the reversal potentials for the early and middle phases reached about 65% of that predicted for the K electrode, although the late phase was insensitive to the external K. Intracellular tetraethylammonium (TEA) attenuated the amplitude of the ILD but did not shorten the duration. These suggest that the ILD has another conductance-independent mechanism simultaneously with the increase in K conductance. Several lines of evidence suggested that a ouabain-sensitive Na pump does not contribute to the ILD. Inhibitors of energy supply, 2,4-dinitrophenol sodium salt (DNP) and cyanide, selectively and reversibly reduced the ILD. Simultaneous applications of intracellular TEA and DNP completely abolished the ILD. As for the ionic basis, the histamine-induced inhibitory response in Be-1 was closely related to the ILD. Cimetidine specifically blocked the ILD and histamine-induced inhibitory response, which were mimicked by 2-methylhistamine, but not by dimaprit. It is concluded that the ILD, mediated by some histamine receptor other than the H1 or H2 type, results from an increase in K conductance and a hyperpolarizing ion pump insensitive to ouabain.

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)

Effects of various cations on the slow K+ conductance increases induced by carbachol, histamine and dopamine in Aplysia neurones

A study was made of the effects of various cations other than K+ on three K+ conductance increases induced by carbachol, histamine and dopamine in an identified group of Aplysia neurones: the 'A' neurones of the cerebral ganglion. The 3 responses were sensitive to alterations of both the extracellular and the intracellular concentrations of Na+ and Ca2+. In particular, they could be reduced markedly by: (a) lowering [Na]0 (replacing NaCl by either Tris-HCl, glucosamine chloride, MgCl2 or sucrose); (b) increasing [Na]i (by intracellular injection of Na+, or by blockade of the Na+-K+ pump); (c) increasing the extracellular divalent cation concentration; or (d) increasing [Ca]i4. Some of the effects of Na+ and divalent cations appear to occur on reaction steps common to the three K+ responses, while others probably imply reaction steps specific to one of the systems, since they differ according to the agonist used. The sensitivity to Na+ and Ca2+ of slow inhibitory responses due entirely to an increase in K+ conductance must be taken into account in the interpretation of some slow hyperpolarizing responses previously assumed to involve changes in Na+ conductance.

Modulation of the histamine-induced inhibitory response in an identified Onchidium neuron by cyclic nucleotides

Intrinsic beating activity in the identified molluscan neuron Be-1 can be inhibited for a relatively long time by a short application of histamine. Cyclic AMP enhanced both the amplitude and duration of this inhibitory histamine response (H2-response), while cyclic GMP depressed the H2-response. However, when only these nucleotides were applied, they produced no significant, change in the beating spike activity.

Excitatory and inhibitory effects of histamine on molluscan neurons

Histamine elicited depolarization (excitation) in some neurons and hyperpolarization (inhibition) in other neurons of the central nervous system of the marine mollusc, Onchidium verruculatum. The histamine sensitive region was along the axon at some distance from the soma. H1-receptor blockers (SA-97 and mepyramine) suppressed the excitatory (H1) response without affecting the inhibitory (H2) response, while H2-receptor blockers (burimamide and metiamide) suppressed the H2-response without affecting the H1-response. The H1-response was associated with a marked increase in membrane conductance and was blocked by removal of the external Na. The H2-response consisted of a hyperpolarization without much change in conductance, compared with the hyperpolarization of same amplitude produced by glutamate in the same neuron. Passive polarization of the membrane and reduction of Cl concentrations to 1/5-1/25 caused no significant change in H2-response. The H2-response was slightly suppressed in K-free saline. Thus, it seems difficult to account for the hyperpolarization only by an increase in K or Cl conductance. Complete removal of Na and addition of ouabain blocked the H2-response, suggesting a contribution of an electrogenic Na-pump to the hyperpolarization. However, in 20 mM Na saline with or without K, histamine still caused clear hyperpolarization. In this solution, the histamine response was not affected by ouabain. Although it is difficult to exclude the possibility that an increase in K conductance may be responsible for the hyperpolarization, it is tentatively proposed as a hypothesis that the H2-response involved some active transport mechanism, different from a ouabain-sensitive electrogenic Na-pump.