N.N. Osborne

THE NEUROBIOLOGY OF A SEROTONERGIC NEURON

Publisher Summary This chapter examines a specific giant neuron that occurs in each metacerebral ganglion of the snail Helix pomatia. Homologous neurons exist in all other pulmonate and opisthobranch molluscs studied. The only transmitter candidate present in appreciable amounts in these neurons is serotonin and, for this reason, the cell has come to be termed as giant serotonin cell (GSC). The GSCs can accumulate tryptophan and can metabolize it to form 5-HTP and serotonin; the GSCs also metabolize serotonin directly from 5-HTP. Also, it has been observed that serotonin within the GSC is primarily associated with small granular vesicles. From a number of electrophysiological and morphological experiments, it has been determined that the GSCs make direct monosynaptic links with neurons in the buccal ganglia. Moreover, analysis of the serotonergic receptors in the buccal neurons showed both morphine and 5, 7-DHT to be serotonergic receptor blockers. Stimulation of the GSCs reveals that glucose is metabolized within the cell to form two substances, both of which are labile.

Dopamine metabolism in characterised neurones ofPlanorbis corneus

A sensitive chromatographic procedure was used to study the metabolism of [14C]tyrosine, [3H]DOPA and [3H]dopamine in 3 defined cell-types situated in the nervous system of Planorbis corneus. One of the cell-types contains dopamine (GDC), the other serotonin (GSC) and the other neither amine (GC). The GDCs metabolise [14C]tyrosine to form DOPA and dopamine while the other two cells lack this ability. In contrast, the GDCs and the GSC, but not the GCs, metabolise [3H]DOPA to form dopamine. In addition the GDCs incorporate radioactivity from [3H]DOPA into DOPAC, homovanillic acid and methoxytyramine. After incubation of cells in [3H]dopamine, only the GDCs metabolise it to form DOPAC, homovanillic acid and methoxytyramine. In no instance did the GDCs form significant amounts of noradrenaline from the incorporated radioactive substances. These results, together with data on the amine histochemistry of the individual cell-types following pretretment of animals with drugs known to affect specific enzymes in the synthesis of amine transmitter substances, clearly demonstrate that the GDCs alone have the enzymes requisite for the biosynthesis and catabolism of dopamine, but not noradrenaline.

Amino acid and serotonin content in the nervous system, muscle and blood of the cockroach Periplaneta americana.

Abstract The levels of different free amino acids and related compounds in the nervous tissue, muscle and blood of the cockroach Periplaneta americana were measured using a micro dansyl procedure. More than 33 dansyl derivatives were identified in the various cockroach tissues. Of these only the nervous tissue contains serotonin but lacks an unknown substance (spot No. 24) which occurs in muscle and blood. The composition of the amino acid pool in the 3 tissue types shows a number of dissimilarities, although the predominant amino acids in all 3 tissue types include proline, alanine, glycine, glutamic acid and glutamine. The nervous tissue also contains a relatively high amount of GABA and aspartic acid. Studies on the accumulation of the 4 major nervous tissue amino acids (proline, GABA, glycine and glutamic acid) into the nervous tissue are also reported. When nervous tissue was incubated at 24 °C in a medium containing 10−5M of either [14C]-proline, [14C]GABA, [14C]glycine or [14C]glutamic acid, tissue-medium ratios of 10:1, 14:1, 8.5:1 and 13.5:1 respectively were obtained after 30 min incubation. The processes responsible for the accumulation of [14C]GABA and [14C]glutamic acid were most sensitive to temperature and were partly inhibited by 2,4-dinitrophenol and ouabain.

Putative neurotransmitters in the annelid central nervous system: Presence of 5-Hydroxytryptamine and octopamine-stimulated adenylate cyclases

Abstract 1. Annelid (earthworm, Lumbricus terrestris ) nervous tissue was analysed for putative neurotransmitter substances using the microdansyl and radiochemical-enzymatic techniques. 2. Glutamate, glycine, aspartate, alanine and taurine were the predominant amino acid. 5-Hydroxytrptamine (5-HT) was also demonstrated by the micro-dansyl technique. 3. The concentrations of octopamine, dopamine and noradrenaline were 3.6 μg/g, 1.8 μg/g and 0.8 μg/g respectively. 4. Earthworm nervous tissue contains both 5-HT- and octopamine-sensitive adenylate cyclases. 5. Lysergic acid diethylamide (LSD) in low concentration (1 μM) stimulated adenylate cyclase activity and partially blocked both 5-HT and octopamine.

Evidence for a neurotoxic effect of ascorbic acid after an intranigral injection in the cat

Abstract Injection of 4 μl of 1% ascorbic acid into the cats substantia nigra produces a behavioural effect comparable to that found for a similar treatment with 6-hydroxydopamine, namely the rotational behaviour which manifests itself after amphetamine (5 mg/kg i.p.) injection. However, ascorbic acid produces no change in the dopamine content or acetylcholine acetylase activity in the caudate nucleus on the injected side. These experiments bring into the question the cause of the circling behaviour of animals injected with 6-hydroxydopamine in their substantia nigra and also the mode of action of the injected ascorbic acid.

Putative neurotransmitters in the cerebral ganglia of the tunicate Ciona intestinalis

Abstract 1. A number of sensitive procedures were used to analyse the dopamine, noradrenaline, octopamine, 5-hydroxytryptamine and putative amino acid distribution in the cerebral ganglia of the tunicate Ciona intestinalis . 2. The following putative transmitter substances exist in the nervous tissue: dopamine, noradrenaline, GABA, taurine, glutamate, glycine and aspartate. No trace of either 5-hydroxytryptamine or octopamine was found. 3. An amine-sensitive adenylate cyclase is also present in Ciona nervous tissue. Dopamine, noradrenaline, 5-hydroxytryptamine and octopamine at 100 μM concentrations all stimulated adenylate cyclase activity to the same extent. All the amino acids tested had no effect on the adenylate cyclase. 4. The significance of these findings is discussed.

Mutant spastic Han-Wistar rats: Biochemical abnormalities in their striata

Abstract A comparison between various substances analysed in the striata of mutant spastic Han-Wistar rats and normal rats from the same parents revealed variations in the amounts present. The mutant rats were characterized by significantly greater quantities of dopamine, choline acetyltransferase, glutamate, taurine, glutamine, cystathionine, valine, leucine, isoleucine and urea, but lower levels of tyrosine-hydroxylase and citrulline. From these findings it was concluded that the mutants can be eliminated as a model for studying Parkinsons disease.

Electromyographical and biochemical studies on mutant Han-Wistar rats with progressive spastic paresis

Abstract 1. 1. The motor hyperactivity associated with spastic paresis in mutant Han-Wistar rats can be temporarily reversed with injection of either l -Dopa or apomorphine, but not d -Dopa. 2. 2. Injection of l -Dopa but not d -Dopa caused a similar increase in the dopamine concentrations in the striata of mutant and control animals. 3. 3. Striatal tissues from mutant and control rats have similar capacities for taking up radioactive dopamine. 4. 4. Both mutants and controls show no difference between the spontaneous and the potassium-induced release of radioactive dopamine from striata loaded with the amine. 5. 5. The endogenous dopamine and serotonin contents are greater in the CNS of the mutants than in the control littermates, while the homovanillic acid contents are similar. 6. 6. Both groups of animals have the same adenylate cyclase basal activity and the dopamine-stimulated increase in the enzyme activity.

A preliminary study on the behaviour and biochemical responses subsequent to the injection of 5,6-dihydroxy-tryptamine into the substantia nigra of the rat

Injection of 5,7-dihydroxytryptamine into the substantia nigra of rats produces an increase of dopamine in the ipsilateral striatum, and when these animals are injected with amphetamine they do not exhibit any rotation. The mode of action of this neurotoxin is compared with that of 6-hydroxydopamine.

The accumulation ofdl-glutamate by the central nervous system of the snailHelix pomatia

Summary Isolated snail ganglia are capable of maintaining their free amino acid levels steady for the first 60 min of incubation in physiological saline. Within this time the ganglia also possess an uptake mechanism for dl -glutamate which can be divided into sodium-sensitive and -insensitive components. The accumulation of dl -glutamate showed saturation kinetics typical of a carrier-mediated process. The Vmax value for the uptake is 1.5 × 10−8mole/g/min and theKm value 1.1 × 10−4M. The amino acid accumulation is quite specific towards l -dicar☐ylic acids and insensitive to a number of metabolic inhibitors. It is unlikely to be due to a homoexchange phenomenon because the ganglia are capable of achieving a net uptake of glutamate and the efflux of dl -[3H]glutamate is not increased by the addition of non-radioactive l -glutamate to the incubation medium.