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THE ROLE OF INTRANEURONAL 5‐HT AND OF TRYPTOPHAN HYDROXYLASE ACTIVATION IN THE CONTROL OF 5‐HT SYNTHESIS IN RAT BRAIN SLICES INCUBATED IN K + ‐ENRICHED MEDIUM
Author(s) -
Hamon M.,
Bourgoin S.,
Artaud F.,
Glowinski J.
Publication year - 1979
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1979.tb05239.x
Subject(s) - pargyline , tryptophan hydroxylase , chemistry , tryptophan , serotonin , stimulation , reserpine , 5 ht receptor , incubation , enzyme , biochemistry , medicine , endocrinology , receptor , serotonergic , biology , amino acid
— The incubation of brain stem slices from adult rats in a K + ‐enriched medium containing a 5‐HT uptake inhibitor (fluoxetine) significantly increased their capacity to synthesize 5‐HT from tryptophan. The K+‐induced stimulation of 5‐HT synthesis was at least partly dependent on the depletion of the indoleamine in tissues since: (1) a good correlation was found between the respective changes in 5‐HT release and synthesis evoked by high K + concentrations in the presence of various 5‐HT uptake inhibitors; (2) the modifications in endogenous 5‐HT levels produced by in vim treatments with drugs (reserpine, pargyline) or by incubating slices with 5‐HT altered the stimulating effect of high K + concentrations and fluoxetine on 5‐HT synthesis; (3) the replacement of Ca 2+ by Co 2+ (4 mM) or EGTA (0.1 mM) in the incubating medium completely prevented the increased 5‐HT release and synthesis evoked by high K + concentrations and fluoxetine. The extraction of tryptophan hydroxylase from incubated tissues revealed that the increased 5‐HT synthesis occurring in K + ‐enriched medium was associated with an activation of this enzyme. Kinetic analyses indicated that this activation resulted from an increase in the V max of tryptophan hydroxylase, its apparent affinities for both tryptophan and 6‐MPH 4 being not significantly affected. In contrast to the tryptophan hydroxylase from tissues incubated in normal physiological medium, the activated enzyme from tissues depolarized by K + was hardly stimulated by Ca 2+ ‐mediated phosphorylating conditions. This led to the proposition of a hypothetical model by which the Ca 2+ influx produced by the neuronal depolarization would trigger the activity of a Ca 2+ ‐dependent protein kinase capable of activating tryptophan hydroxylase. Although this sequence is still largely speculative it must be emphasized that, as expected from such a model, the regional differences in the K + ‐evoked activation of tryptophan hydroxylase in slices (cerebral cortex > brain stem > spinal cord) were parallel to those of the Ca 2+ ‐dependent protein phosphorylation ( r = 0.92) and those of the activating effect of phosphorylating conditions on soluble tryptophan hydroxylase ( r = 0.96).