Journal
PSYCHOPHARMACOLOGY
Volume 190, Issue 4, Pages 497-506Publisher
SPRINGER
DOI: 10.1007/s00213-006-0635-5
Keywords
tryptophan depletion; serotonin (5-HT); 5-HT1A receptor; 5-HT2A receptor; rats; radioligand binding
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Rationale Tryptophan depletion is used to reduce central serotonergic function and to investigate its role in psychiatric illness. Despite widespread clinical use, its effects on serotonin (5-HT) receptors have not been well characterized. Objective The aim of this study was to examine the effect of acute (ATD) and chronic tryptophan depletion (CTD) on free-plasma tryptophan (TRP), central TRP and 5-HT and brain 5-HT1A and 5-HT2A receptor binding in the rat. Methods TRP and 5-HT were measured by high-performance liquid chromatography and receptor levels determined by homogenate radioligand binding and in-vitro receptor autoradiography. Results Free-plasma TRP, central TRP and central 5-HT levels were significantly and similarly reduced by ATD and 1- and 3-week CTD compared to controls. ATD significantly reduced 5-HT1A binding in the dorsal raphe (14%) but did not significantly alter postsynaptic 5-HT1A binding (frontal cortex, remaining cortex and hippocampus) or 5-HT2A binding (cortex and striatum). One-week CTD did not significantly alter cortical 5-HT2A binding or postsynaptic 5-HT1A binding. Furthermore, 3-week CTD did not significantly alter 5-HT1A binding but significantly increased cortical 5-HT2A binding without affecting striatal or hippocampal levels. In the CTD 1 and 3-week groups, rat body weight was significantly decreased as compared to controls. However, weight loss was not a confounding factor for decreased cortical 5-HT2A-receptor binding. Conclusion ATD-induced reduction in somatodendritic 5-HT1A autoreceptor binding may represent an intrinsic 'homeostatic response' reducing serotonergic feedback in dorsal raphe projection areas. In contrast, the increase in 5-HT2A receptor after CTD may be a compensatory response to a long-term reduction in 5-HT.
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