Free tryptophan/large neutral amino acids ratios in blood plasma do not predict cerebral spinal fluid tryptophan concentrations in interleukin-1-induced anorexia

Peripheral administration of interleukin-1 (IL-1) reduces food intake and affects brain serotonergic activity, suggesting a causal relationship. Furthermore, IL-1 increases the brain concentrations of the serotonin precursor, tryptophan (TRP), by unclear mechanism(s). We aimed at confirming the link between IL-1 administration, raised brain TRP concentrations and the development of anorexia, and at investigating the mechanisms of TRP entry into the brain. Thirty adult, overnight fasted Sprague-Dawley rats were randomly assigned to i.p. injections of 1 μg/kg BW of IL-1α (n = 10) or vehicle (n = 10), or to pair-feeding with IL-1 animals (n = 10). After 2 h, food intake, blood plasma concentrations of total TRP, free TRP, large neutral amino acids (LNAA; competing with TRP for brain entry) were measured. Cerebral spinal fluid (CSF) TRP concentrations were also measured. TRP brain availability was assessed by calculating the plasma ratio free TRP/LNAA. Following IL-1 injection, food intake significantly declined in IL-1 rats, which was paralleled by decreased plasma free TRP and increased plasma LNAA. Despite a decrease in the free TRP/LNAA ratios in plasma, IL-1 significantly increased concentrations of TRP in CSF. These data show that the acute peripheral administration of IL-1 induces anorexia and raises CSF TRP levels. Considering the possible role of the raised CSF TRP in influencing brain serotonin activity, it is postulated that increased serotonergic neurotransmission could be involved in IL-1 induced anorexia.