Expression mapping of 5-HT1 serotonin receptor subtypes during fetal and early postnatal mouse forebrain development

Serotonin (5-HT) is implicated in several aspects of brain development, yet the ontogenetic expression patterns of 5-HT receptors responsible for transducing specific effects have largely not been characterized. Fifteen different 5-HT receptor genes have been cloned; therefore any spatial and/or temporal combination of their developmental expression could mediate a wide array of 5-HT effects. We undertook a detailed analysis of expression mapping of the Gi/o-coupled 5-HT1 (5-HT1A, 1B, 1D and 1F) receptor subtypes in the fetal and early postnatal mouse forebrain. Using receptor subtype-specific riboprobes and in situ hybridization, we observed that all 5-HT1 receptor subtypes are expressed as early as embryonic day (E) 14.5 in the forebrain, typically in gradients within specific structures. Among 5-HT1 receptors, the 5-HT1A receptor transcript is expressed densely in E14.5–16.5 thalamus, in hippocampus, and in a medial to lateral gradient in cortex, whereas the 5-HT1B receptor mRNA is expressed in more lateral parts of the dorsal thalamus and in the striatum at these ages. The 5-HT1D receptor transcript, which also is expressed heavily in E14.5–E16.5 thalamus, appears to be down-regulated at birth. The 5-HT1F receptor transcript is present in proliferative regions such as the cortical ventricular zone, ganglionic eminences, and medial aspects of the thalamus at E14.5–16.5, and otherwise presents similarities to the expression patterns of 5-HT1B and 1D receptor transcripts. Overall, the 5-HT1 subfamily of Gi/o-coupled 5-HT receptors displays specific and dynamic expression patterns during embryonic forebrain development. Moreover, all members of the 5-HT1 receptor class are strongly and transiently expressed in the embryonic dorsal thalamus, which suggests a potential role for serotonin in early thalamic development.