Illustration of the distribution pattern of 5-HT1A–FGFR1 heteroreceptor and 5-HT1A–5-HT2A isoreceptor complexes visualized with PLA in the rat dorsal hippocampus. The 5-HT1A–FGFR1 heteroreceptor complexes, in red, have a more widespread distribution, mainly located within the pyramidal cell layer of the CA1–CA4 and in the plexiform cell layer of the dentate gyrus. However, they exist in low densities in the stratum oriens and radiatum of the CA1–CA4 regions and the subgranular layer. Instead, the 5-HT1A–5-HT2A isoreceptor complexes, shown in yellow, are concentrated to the pyramidal cell layer of the CA1–CA3 regions, with some located in the subgranular layer of the dentate gyrus.