Injected tryptophan increases brain but not plasma tryptophan levels more in ethanol treated rats

Inhibition of restraint-induced anorexia by injected tryptophan

Tryptophan injected at doses of 50mg/kg did not alter 24 h cumulative food intake and growth rate in rats. A single episode of 2 h restraint stress decreased food intake and growth rate of saline and tryptophan injected rats. The decreases of both food intake and growth rate were smaller in tryptophan injected (food intake 23.9% p<0.05; growth rate 2.9% p<0.05) than saline injected (food intake 78.5% p<0.01; growth rate 6.1% p<0.01) rats suggesting that tryptophan administration inhibits restraint-induced anorexia. Following an acute challenge with 2h restraint increases of 5-hydroxytryptamine (5-HT; serotonin) and 5-hydroxyindoleacetic acid (5-HIAA) but not tryptophan were greater in tryptophan injected than saline injected rats. The findings imply that tryptophan-induced increases of brain 5-HT and 5-HIAA have little effect on functional serotoninergic activity under basal conditions but a facilitatory effect on functional response occurs in conditions of increased serotoninergic neuronal activity such as during stress.

Effects of acute, chronic ethanol and withdrawal on dorsal raphe neurons: electrophysiological studies

The effect of a single intravenous administration of ethanol (0.25-1.0 g/kg) on the spontaneous activity of putative serotonin neurons of the dorsal raphe nucleus was studied in unanesthetized rats. Ethanol produced a slight but progressive decline in neuronal activity in 67% (six of nine) of all neurons tested. The remaining 33% (three of nine) were unresponsive. Upon withdrawal of chronic ethanol treatment (1-5 g/kg every 6 h for six consecutive days, 12 h from last ethanol administration), the mean firine rate of dorsal raphe neurons was found to be significantly reduced, by about 30% (n=71), as compared with the control group (n=83), whereas the cells/track index was unaltered. Under these conditions, ethanol administration further reduced firing rate in 67% (four of six) of all the neurons tested. In the remaining 33% (two of six), no response was observed. At 72 h after the last ethanol administration, the mean firing rate of dorsal raphe neurons was found to be within control values (n=90). Further, to evaluate the functional status of the autoreceptors under control conditions and after withdrawal from chronic ethanol, the selective serotonin-1A receptor agonist 8-hydroxy-(2-di-n-propylamino)tetralin was administered intravenously in cumulative doses (1-16 microg/kg) and dose-response curves were generated for both groups. Autoreceptor sensitivity of dorsal raphe neurons was found to be not statistically different in control and ethanol withdrawn rats (n=6 for both groups) as indexed by a similar potency displayed by 8-hydroxy-(2-di-n-propylamino)tetralin in reducing the spontaneous activity of dorsal raphe neurons. The results indicate that, in spite of the widespread use of serotonin transmission potentiating agents in the treatment of alcoholism, neither acute nor withdrawal from chronic ethanol administration produces drastic effects on dorsal raphe neurons. However, the inhibition of dorsal raphe neuronal activity after acute ethanol may be due to the reported ability of ethanol to increase serotonin release from terminal areas. This increased serotonin tone could, at the level of recurrent axon collaterals in the dorsal raphe nucleus, reduce the spontaneous activity of the cells. On the other hand, a similar reduction in spontaneous activity after withdrawal from ethanol correlates well with the reduction in serotonin levels observed under these conditions in microdialysis studies.

24h withdrawal following repeated administration of caffeine attenuates brain serotonin but not tryptophan in rat brain: implications for caffeine-induced depression

Caffeine injected at doses of 20, 40 and 80 mg/kg increased brain levels of tryptophan, 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) in rat brain. In view of a possible role of 5-HT in caffeine-induced depression the effects of repeated administration of high doses of caffeine on brain 5-HT metabolism are investigated in rats. Caffeine was injected at doses of 80 mg/kg daily for five days. Control animals were injected with saline daily for five days. On the 6th day caffeine (80 mg/kg) injected to 5 day saline injected rats increased brain levels of tryptophan, 5-HT and 5-HIAA. Plasma total tryptophan levels were not affected and free tryptophan increased. Brain levels of 5-HT and 5-HIAA but not tryptophan decreased in 5 day caffeine injected rats injected with saline on the 6th day. Plasma total and free tryptophan were not altered in these rats. Caffeine-induced increases of brain tryptophan but not 5-HT and 5-HIAA were greater in 5 day caffeine than 5 day saline injected rats. The findings are discussed as repeated caffeine administration producing adaptive changes in the serotonergic neurons to decrease the conversion of tryptophan to 5-HT and this may precipitate depression particularly in conditions of caffeine withdrawal.

Serotonin and alcohol intake, abuse, and dependence: findings of animal studies

Despite a relatively large body of literature on the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake, the functional significance of serotonergic neurotransmission and its relationship to alcohol intake, abuse, and dependence remains to be fully elucidated. In part two of this review, the experimental (animal) data is summarized along two lines: the effects of serotonergic manipulations on the intake of alcohol, and the effects of acute and chronic alcohol intake, as well as the withdrawal of chronic alcohol, on the serotonergic system. It is concluded that serotonin mediates ethanol intake as a part of its larger role in behavior modulation, such that increases in serotonergic functioning decrease ethanol intake, and decreased serotonergic functioning increases ethanol intake. Ethanol produces transient increases in serotonergic functioning that activate the mesolimbic dopaminergic reward system. The results are discussed in light of recent theories describing the regulatory role of serotonin in general behavior.

Enhancement of hepatic tryptophan pyrrolase activity and decreases of open field locomotion following single and repeated administration of high doses of caffeine in rats

In view of a possible role of kynurenine in caffeine-induced anxiety syndrome, the effects of single and repeated administration of caffeine on hepatic tryptophan (T)-pyrrolase activity are investigated. Single administration of caffeine at doses of 80 mg/kg decreased open field locomotion and increased hepatic T-pyrrolase activity. Locomotor stimulating effects of 80 mg/kg caffeine, monitored in the home cages of rats, were attenuated following daily administration of caffeine for 5 days. Open field locomotor activity of rats and its caffeine-induced decrement were also attenuated following 5 daily administrations of caffeine on the 6th day. Basal levels of hepatic T-pyrrolase activity increased after 5 daily administrations of caffeine on the 6th day. Acute administration of caffeine did not further elevate hepatic T-pyrrolase activity in 5 day caffeine injected rats. Drug adjuvants decreasing hepatic T-pyrrolase activity may prove valuable for extending the clinical utility of caffeine.

Decreases of plasma tryptophan concentrations following restricted feeding do not decrease serotonin and its metabolite in rat brain

Rats trained on a restricted feeding (RF) schedule of 4 h/day were killed before (starved) and after (fed) the presentation of food to compare hepatic and brain tryptophan metabolism with feeding (FF) controls. The RF rats exhibited greater holo-tryptophan pyrrolase activity in the liver than FF controls. Holoenzyme activity of starved rats was greater than fed animals. Plasma levels of tryptophan higher in the FF rats were comparable in the starved and fed groups of RF rats. Plasma levels of glucose comparable in the FF and RF fed rats were smaller in the starved animals. Brain levels of tryptophan comparable in the RF fed rats and FF controls were smaller in RF starved rats. Brain concentrations of serotonin (5-hydroxytryptamine) and its metabolite 5-hydroxyindoleacetic acid comparable in the RF starved rats and FF controls were higher in the RF fed rats. Possible implication of the findings in the pathogenesis of food deprivation/starvation related disease, anorexia nervosa is discussed.