Choline acetyltransferase, acetylcholinesterase and aromatic L-amino acid decarboxylase in single identified nerve cell bodies from snail Helix aspersa

Retroviral transfer of a human tyrosine hydroxylase cDNA in various cell lines: regulated release of dopamine in mouse anterior pituitary AtT-20 cells

Little is known about the molecular events mediating neurotransmitter release, a crucial step in synaptic transmission. In this paper, the biosynthesis and release of L-beta-3,4-dihydroxyphenylalanine (L-DOPA) and dopamine were analyzed in three heterologous cell lines after retroviral-mediated gene transfer of tyrosine hydroxylase (EC 1.14.16.2), the rate-limiting enzyme in catecholamine synthesis. A recombinant retrovirus encoding human tyrosine hydroxylase type I as well as neomycin-resistance gene was used to infect a fibroblast (NIH 3T3), a neuroblastoma (NS20 Y), and a neuroendocrine (AtT-20) cell line. After selection in the presence of neomycin and in tyrosine-free medium, high levels of exogenous tyrosine hydroxylase activity were detected in extracts of the three cell lines. High-performance liquid chromatography of cell extracts and culture supernatants confirmed that the three cell lines hydroxylated tyrosine to form L-DOPA and released this metabolite into the culture medium. Interestingly, the neuroendocrine cell line AtT-20 synthesized not only L-DOPA but also dopamine. Evoked secretion studies established that AtT-20 cells released the transmitter upon depolarization in a regulated, calcium-dependent way. We discuss the implication of this approach for the analyses of neurotransmitter release as well as in the context of degenerative disorders such as Parkinson disease.

Long-term regulation of tyrosine hydroxylase activity in the superior cervical ganglion in organ culture: effects of nerve stimulation and dexamethasone

The rat superior cervical ganglion was stimulated in vitro via its preganglionic trunk and then maintained in organ culture for 2-3 days. Following nerve stimulation, the specific activity of tyrosine hydroxylase (TH) increased with a lag period of at least 12 h, and this increased enzyme activity was maintained throughout the culture period. The magnitude of the increase in TH activity depended on the frequency and duration of preganglionic nerve stimulation. This transsynaptic increase in enzyme activity could be completely blocked by the nicotinic antagonist, hexamethonium. The synthetic glucocorticoid dexamethasone, at concentrations above 1 nM, increased TH activity in unstimulated ganglia with a lag period of at least 24 h. The increase in enzyme activity produced by dexamethasone was not affected by nicotinic antagonists or by prior decentralization of the superior cervical ganglion, in contrast to the findings of previous workers. These data indicate that dexamethasone can stimulate ganglionic TH activity in the absence of cholinergic stimulation. In addition, experiments combining dexamethasone with nerve stimulation demonstrate that the steroid can potentiate the effects on TH activity of brief periods of nerve stimulation.

[In vitro biosynthesis of octopamine by the nervous system and the heart of the mollusc gastropode Helix pomatia (author's transl)]

The tyrosine is decarboxylated in vitro by the central nervous tissue and by the intracardiac nervous tissue of Helix pomatia by aromatic amino-acid decarboxylase. The tyramine obtained is then partially transformed into octopamine by tyramine-beta-hydroxylase. The inhibition of monoamine oxidase favours apparition of the 2 amines. The monoamine oxidase seems able to regulate their synthese and to assure their inactivation.

Monoamine oxidase activity in Helix pomatia

The distribution and characterization of MAO in various tissues of the snail were analyzed. Only low amounts of the enzyme exist in the different tissues and data suggest that there is more than one type of MAO.

Dopamine metabolism in characterised neurones of Planorbis 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.