Prenatal expression of 5-HT1C and 5-HT2 receptors in the rat central nervous system

The expression of mRNAs for two 5-HT receptors (5-HT1C, 5-HT2) has been investigated by evaluating in situ hybridization in the prenatal rat CNS. At Embryonic Day 14 (E14), the highest signal for 5-HT1C was found in the choroid plexus, while the marginal/intermediate (m/i) zones of the midbrain, brain stem (including monoaminergic groups), and spinal cord also displayed label. By E18-21 a number of more rostral regions contained transcript, including the hippocampus (CA1), in addition to more intense signal in midbrain, brain stem, and spinal cord. Expression in the choroid plexus appeared to peak between E16-E18, although considerable hybridization signal remained at E21. 5-HT2 transcripts were also detected at E14. Label was present in m/i zones of the midbrain and in a number of other areas. In comparison to 5-HT1C, 5-HT2 mRNA was distributed over a wider rostral-caudal extent at this age. As with 5-HT1C mRNA, signal increased over rostral and brain stem areas at late gestational ages with significant labeling appearing in the olfactory bulb, cerebellum, cortical plate and subplate, hippocampus (dentate gyrus), and monoaminergic nuclei. 5-HT1C and 5-HT2 receptor transcripts were also present over the meninges at E16 and may represent transient expression of these receptors. These expression patterns in the embryonic rat brain, in conjunction with previous evidence indicating that 5-HT can act as a differentiation signal for target neurons, suggests that prenatal 5-HT receptors are positioned to play a role in the prenatal development of the CNS.