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Characterisation of an acute anorexigenic action of l-tryptophan: a link with neural mechanisms regulating appetite

Recent evidence suggests that individual amino acids may promote hypophagia, however, our understanding of processes underlying their anorexigenic action is extremely limited. The few human and laboratory animal studies performed to date suggest that a free essential amino acid, l-tryptophan (TRP) reduces food intake. It is unclear, however, whether TRP acutely affects consumption driven by energy needs or by palatability, and whether central mechanisms underlie TRP-induced hypophagia. This thesis provides a detailed characterisation of ingestive behavioural effects induced by TRP administered intragastrically or (in order to bypass the gut) via an intraperitoneal (IP) injection in laboratory rodents prior to a meal as well as relevant neural processes triggered by TRP. The first set of experiments explored whether an intragastric preload of TRP affects energy- and palatability-induced feeding in mice. A conditioned taste aversion (CTA) test was used to assess whether hypophagia is unrelated to sickness. Finally, c-Fos immunohistochemistry was employed to detect changes in activation of feeding-related brain sites induced by an anorexigenic dose of intragastric TRP. TRP generated a short-lasting reduction in the intake of energy-dense standard chow in deprived animals and energy-dense palatable chow in sated mice. Anorexigenic doses of TRP did not cause a CTA. TRP failed to affect intake of palatable yet calorie-dilute or noncaloric solutions (10% sucrose, 4.1% Intralipid or 0.1% saccharin) even for higher TRP doses that decreased water intake in thirsty mice. c-Fos analysis revealed an increase in activation of key feeding-related brain areas, especially in the brainstem and hypothalamus. Overall, intragastric TRP was found to diminish energy-driven food consumption as well as (at high doses) thirst-induced water intake and affect activation of relevant central circuits. In the second set of experiments, I investigated whether anorexigenic and brain activation effects persist when TRP bypasses the gut, i.e., TRP is injected directly in the IP cavity in rats. The effect of IP TRP was examined in both energy- and palatability-induced feeding. 30 and 100 mg/kg IP TRP suppressed chow intake in energy deprived rats. Only a higher 100 mg/kg IP TRP dose reduced consumption of palatable chow and palatable sucrose, saccharin and Intralipid solutions in sated animals. Thirst-driven water intake was reduced after 30 and 100 mg/kg IP TRP. Neither 30 nor 100 mg/kg IP TRP caused a CTA. c-Fos analysis showed the most pronounced effect of IP TRP on hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei and the nucleus of the solitary tract in the brainstem. It can be concluded that IP TRP suppresses energy deprivation-induced intake of chow and thirst-induced water intake within the same dose range, whereas higher doses are necessary to reduce consumption of palatable solutions in sated animals. IP TRP hypophagia is accompanied by activation of central circuitry that encompasses brainstem and hypothalamic sites, though changes are not as prominent as with intragastric TRP. Intragastric and IP TRP administered at hypophagic doses increases c-Fos immunoreactivity in the hypothalamic PVN and SON. As these sites host neurons that synthesise a key satiety mediator, oxytocin (OT), the third and final set of studies explored a hypothesis that TRP’s anorexigenic action relies on enhanced activity of the OT system. By employing double-immunohistochemistry, I determined that the lowest effective dose of intragastric and IP TRP increased the activation of both PVN and SON OT neurons, although the effect was more pronounced after intragastric administration. Intragastric TRP upregulated OT mRNA in the hypothalamus. A blood-brain barrier-penetrant OT receptor antagonist, L-368,899, reversed hypophagia induced by TRP administered via both routes. Unlike the lowest effective dose of intragastric TRP which decreases only feeding, IP TRP at the lowest effective dose diminishes both food and water intake. Hence it was confirmed that L-368,899 pre-treatment does not affect TRP-driven reduction in drinking. The findings suggest therefore a functional relationship between TRP and OT in the regulation of energy-driven food intake. Overall, the findings presented herein shed more light on the nature of anorexigenic properties of TRP. It appears that TRP acutely suppresses ingestive behaviour driven by energy, but it is a very weak inhibitor of palatability-motivated consumption. While both intragastric and IP TRP also decreased water intake, the fact that a low dose of the intragastric amino acid was sufficient to decrease feeding but not drinking, strongly suggests that a direct interaction of TRP with the gastric mucosa is a crucial contributor to reduced energy intake. TRP engages broad brain circuits relevant to feeding, especially the hypothalamus and brainstem. OT appears to be a candidate molecule potentially mediating TRP’s effect on food intake.
Type of thesis
Gartner, S. N. (2018). Characterisation of an acute anorexigenic action of l-tryptophan: a link with neural mechanisms regulating appetite (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12046
The University of Waikato
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