Recent progress has seen the development of new radiotracers for the 5-hydroxytryptamine ( 5-HT) system; hypothesized to be dysfunctional in affective illness and to be the prime target for many antidepressant treatments.(5) In particular, radioligands for 5-HT 1A and 5-HT2A receptors, the 5-HT transporter, and radiotracers of 5-HT synthesis are being developed (Table 1).
One recent success is the radioligand [ (11)C]WAY 100635 for imaging 5-HT1A receptors in the human brain.(22) As many different antidepressant treatments alter 5-HT1A receptor function in rodents, this ligand will be useful investigating 5-HT 1A receptor populations in depressed patients before and after treatment especially.
[(11)C]M-methylspiperone, [(18)F]altanserin, [(18)F]ethylspiperone, [(18)F]setoperone or [(18)F]altanserin, and the SPET tracer [(123)I]ketanserin have been used to measure 5-HT2 receptor number; a receptor implicated in depressive illness, suicidal behaviour, and psychosis. Many of these 5-hT 2 ligands have been hampered by either the lack of selectivity, or the relatively low ratio of specific to non-specific signal obtained in the human brain, (23> although a few reports have appeared reporting reduced 5-HT2 receptor number in drug-free depressed patients. Further studies are needed using more selective ligands with higher signal to noise ratios, such as [ (11)C]MDL 100907, a promising selective ligand for 5-HT2A receptors (Table 1).
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