In vivo‐synthesized radioactively labelled α‐methyl serotonin as a selective tracer for visualization of brain serotonin neurons

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Abstract
To investigate the use of α‐[3H]methyl tryptophan (α‐[3H]MTrp) as a tracer for the in vivo study of brain serotonergic neurons, we examined whether α‐[3H]MTrp and its metabolite α‐[3H]methyl serotonin (α‐[3H]M5‐HT) selectively label serotonergic neurons and whether once accumulated in these neurons, the radioactive metabolite behaves like endogenous serotonin. Rats received a systemic injection of 1–5 mCi of α‐[3H]MTrp and 24 h later their brains were immediately removed or fixed by perfusion before removal. Tissue sections in which serotonergic neurons had been immunostained for 5‐HT or its synthesizing enzyme, tryptophan hydroxylase, were processed for radioautography at the light and electron microscopic level. In another group of rats, the release of radioactivity from different brain areas was studied both under basal and depolarizing conditions. In the dorsal raphe nucleus, the light microscopic examination revealed almost complete colocalization between serotonergic neurons and those that accumulated radioactivity, with a heterogeneity in the content of α‐[3H]M5‐HT among the various cells. At the ultrastructural level, immunoidentified serotonergic perikarya and dendritic processes in the dorsal raphe nucleus, as well as nerve terminals in the cerebral cortex were also found to contain α‐[3H]M5‐HT. Under basal conditions, radioactivity was released from the brainstem raphe region and from projection areas such as the striatum and hippocampus. The basal output of α‐[3H]M5‐HT increased approximately twofold after a depolarizing 50 mM KCl solution was added to the perfusion fluid. These findings suggest that newly synthesized α‐[3H]M5‐HT can be released both at somatodendritic and terminal sites. In sum, the present results demonstrate the selectivity of α‐[3H]MTrp as a tracer for serotonergic cells, and further suggest that α‐[3H]MTrp radiolabelling provides for a direct assessment of the in vivo dynamics of brain serotonergic neurons at the cellular level.

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