Distribution of the 5-HT5A serotonin receptor mRNA in the human brain.

The 5-HT5A receptor is a member of a new subfamily of serotonin [5-hydroxytryptamine (5-HT)] receptors recently cloned from the human and rodent brain. The role of this receptor in normal brain functions as well as its possible involvement in pathological states is still to be determined. We therefore studied the regional distribution and cellular localization of 5-HT5A receptor mRNA in human brain sections from autopsy samples by in situ hybridization histochemistry, in order to obtain anatomical information which might be useful in formulating hypotheses on possible functions subserved by this receptor in the central nervous system (CNS). Our results showed that the main sites of 5-HT5A mRNA expression were the cerebral cortex, hippocampus and cerebellum. In the neocortical regions, the 5-HT5A receptor mRNA was mainly distributed in the layers II-III and V-VI. In the hippocampus, the dentate gyrus and the pyramidal cell layer of the CA1 and CA3 fields expressed 5-HT5A mRNA at high levels. The broad distribution in the neocortex and hippocampus supports the view that the 5-HT5A receptor in these areas might be implicated in high cortical and limbic functions. The 5-HT5A mRNA was widely distributed in the cerebellum where it was highly expressed in the Purkinje cells, in the dentate nucleus and, at a lower level, in the granule cells. Since the cerebellum receives diffuse serotonergic afferents, this finding suggests that the 5-HT5A receptor may have an important role in mediating the effects of 5-HT on cerebellar functions.