The effects of tryptophan loading on the 5-hydroxyindole levels of dog blood, brain and CSF

Many pieces o. evidence have accumulated recently which suLgest that 5-hydroxytryptamine (5HT) plays an important role in the normal functioning of the central nervous system. It is quite possible that 5HT may be shown to be a central neuro-transmitter substance in the near future. It is believed that studies of the metabolism or 5HT may be able to throw light on the etiology of mental illness. One reason for this belief is the fact that several important drugs have profound effects on the brain Levels of 5HT. Among the drugs in therapeutic use one finds agents which interfere with synthesis or breakdown of 5HT, others have been shown to interfere with its storage in brain or to be powerful antagonists or its effects on certain tissues.Most of the work described here makes use of the technique of tryptophan loading to study the normal and drug influenced 5-hydroxyindole metabolism of dogs. Udenfriend et al. (1956) first showed that the administration of 5-hydroxytryptoPhan (5HTP), the precursor of 5HT, could cause up to a twenty-fold rise in rat Drain 5ET. Paasonen and Gidian (1958) and later Hess et al. (1959) observed much smaller changes after giving tryptophan itself, the precursor of 5HTP. Green et al, (1962) and Gulley et al. (1963) have shown that brain 5HT Level in the rat are dependent on the tryptophan content of the diet. In the experiments described here a technique devised by Ashcroft et al, (1965) is used which combine's tryptophan loading with the simultaneous estimation of 5HTP, 5HT and its principal metabolite 5-hydroxyindole acetic acid (5HIAA) in a single specimen or brain tissue. This enables one ta distinguish between several possible ways in which a drug can alter the metabolism of the 5-hydroxyindoles. For example, blockage or one or the enzymes in the pathway causes an accumulation or it substrate :and a depletion of the product or its action; irreversible blockage or the central storage mechanism for 5HT causes a prolonged depletion of 5HT, The tryptophan load temporarily increases the level, and probably the rate or turnover, of the 5-hydroxyindoles so that drug induced changes may be more easily detected. The aromatic amino acid decarooxyiating enzyme is widespread in brain and has a high activity so that 5HTP never normally reacts detectable concentrations, but 0: combining a tryptophan load with the administration of an inhibitor or this enzyme Asheroft et al. (1965) were able to saturate it and thus raise 5HTP to detectable levels.Here the method of Ashcroft et al. (1965) has been used to determine what changes in the 5-hydroxyindole concentrations occur in various different areas or dog brain after a standard tryptophan load. The changes in the 5-hydroxyindoles of cereorospinal fluid (CSF) and the concentrations of tryptophan in these tissues and in blood which paralleled them were also measured. All measurements were made at four different times - control and at 1, 2, and 4 hours after the tryptophan was administered. This provides a set of standard profiles which can be compared with the profiles that result from combining the standard load with a drug treatment. Although the tryptophan load used can at least double the brain 5HT levels, it is not unduly unphysiological as the normal dietary intake of tryptophan is between three and four times as much this per day