Ontogenetic distribution of 5-HT2C, 5-HT5A, and 5-HT7 receptors in the rat hippocampus

Abstract

It is known that serotonin exerts its different nociceptive and motor functions by interacting with distinct receptors subtypes, which could be either G-protein coupled or ionotropic. Previous reports demonstrated the early activation of serotonin receptor transcripts during rat development, suggesting a potential role of the serotoninergic system during ontogeny. In this study we have compared the cellular distribution of three serotonin receptor subtypes: 5-HT2C, 5-HT5A, and 5-HT7. Immunocytochemical methods were used in slices of rat hippocampus obtained during the postnatal development. 5-HT2c immunoreactivity was strong at all developmental stages in the CA1 region, whereas differences were observed between P0 and P5 in the CA3 region. The 5-HT5A receptor immunosignal in CA1 and CA3 was strong at P0, decreased at P11, and then increased in the adult. The immunoreactivity to 5-HT7 receptors was high in all regions at P0 and then decreased progressively during postnatal development; the signal was stronger for 5-HT2c than for 5-HT5A and 5-HT7 receptors. Changes in the expression level of each receptor may result in differences in functional and pharmacological properties of the cells expressing them as well as in the hippocampal neuronal network. The distribution of the three serotonin receptor subtypes studied varied during the ontogeny, which supports their potential role during development and will help to understand their mechanisms.

Figure 1

Distribution of 5-HT_2C receptors in the rat hippocampus. The ages studied were: newborn (P0), 5 days postnatal (P5), 11 days postnatal (P11), and adult (AD). (A) Immunohistochemistry of coronal sections. (B) Densitometry of immunohistochemical staining in the CA1, CA2, and CA3 regions. Asterisks on the error bars indicate the statistical difference with the previous bar. (C) Coronal section of the CA3 hippocampus at P11. (D) Control section of the CA3 hippocampus at P11 without primary antibody.

Figure 2

Distribution of 5-HT_5A receptors in the rat hippocampus. (A) Immunostaining of coronal sections during development. (B) Semi-quantitation of the 5-HT_5A receptor immunosignal in CA1, CA2, CA3, and dentate gyrus during postnatal development. Asterisks on the error bars indicate the statistical difference with the previous bar. (C) Coronal section of the CA3 hippocampus at P11. (D) Coronal section of the adult CA3 region.

Figure 3

Distribution of 5-HT₇ receptors. (A) Immunohistochemistry of coronal sections during development. (B) Densitometry of CA1, CA2, CA3, and dentate gyrus during development. Asterisks on the error bars indicate the statistical difference with the previous bar. Coronal sections at P0, CA1 (C) and CA3 (D).