A receptor autoradiographic and in situ hybridization analysis of the distribution of the 5-ht(7) receptor in rat brain

1 Receptor autoradiography and in situ hybridization histochemistry have been used to delineate the distribution of the 5-ht(7) receptor and its mRNA in rat brain. Receptor autoradiographic studies were performed using [H-3]-5-carboxamidotryptamine (5-CT) as the radioligand. The binding characteristics of the masking compounds were determined in Cos-7 cells transfected with a panel of 5-HT receptor subtype cDNAs, including the rat 5-ht(7) cDNA. In situ hybridization studies were carried out with S-35-labelled oligonucleotide probes to the rat 5-ht(7) mRNA. 2 Specific binding of [H-3]-5-CT was observed in many areas of the rat brain. Following co-incubation with 1 mu M ergotamine, this binding was completely eliminated. After addition of the masking ligands, [H-3]-5-CT binding remained in layers 1-3 of cortex, septum, globus pallidus, thalamus, hypothalamus, centromedial amygdala, substantia nigra, periaquaductal gray, and superior colliculus. Addition of the antagonist, methiothepin, to the incubation regimen eliminated most of the remaining [H-3]-5-CT binding in the brain, with the exception of the globus pallidus and substantia nigra. 3 The 5-ht(7) mRNA was discretely localized in rat brain. The most intense hybridization signals were observed over the thalamus, the anterior hippocampal rudiment, and over the CA3 region of the hippocampus. Other regions containing hybridization signals included the septum, the hypothalamus, the centromedial amygdala and the periaquaductal gray. The regions exhibiting a modest receptor binding signal after methiothepin incubation, the globus pallidus and the substantia nigra, contained no 5-ht(7) hybridization signals, suggesting a non-5-ht(7) subtype in these two related structures. 4 The distribution of the 5-ht(7) receptor and its mRNA is suggestive of multiple roles for this novel 5-HT receptor, within several brain systems. The limbic system (centromedial amygdala, anterior hippocampal rudiment, hypothalamus) is particularly well-represented, indicating a potential role for the 5-ht(7) receptor in affective processes.