Altered expression and functions of serotonin 5‐HT1A and 5‐HT1B receptors in knock‐out mice lacking the 5‐HT transporter

By taking up serotonin (5‐hydroxytryptamine, 5‐HT) released in the extracellular space, the 5‐HT transporter (5‐HTT) regulates central 5‐HT neurotransmission. Possible adaptive changes in 5‐HT neurotransmission in knock‐out mice that do not express the 5‐HT transporter were investigated with special focus on 5‐HT1A and 5‐HT1B receptors. Specific labelling with radioligands and antibodies, and competitive RT‐PCR, showed that 5‐HT1A receptor protein and mRNA levels were significantly decreased in the dorsal raphe nucleus (DRN), increased in the hippocampus and unchanged in other forebrain areas of 5‐HTT–/– vs. 5‐HTT+/+ mice. Such regional differences also concerned 5‐HT1B receptors because a decrease in their density was found in the substantia nigra (−30%) but not the globus pallidus of mutant mice. Intermediate changes were noted in 5‐HTT+/– mice compared with 5‐HTT+/+ and 5‐HTT–/– animals. Quantification of [35S]GTP‐γ‐S binding evoked by potent 5‐HT1 receptor agonists confirmed such changes as a decrease in this parameter was noted in the DRN (−66%) and the substantia nigra (−30%) but not other brain areas in 5‐HTT–/– vs. 5‐HTT+/+ mice. As expected from actions mediated by functional 5‐HT1A and 5‐HT1B autoreceptors, a decrease in brain 5‐HT turnover rate after i.p. administration of ipsapirone (a 5‐HT1A agonist), and an increased 5‐HT outflow in the substantia nigra upon local application of GR 127935 (a 5‐HT1B/1D antagonist) were observed in 5‐HTT+/+ mice. Such effects were not detected in 5‐HTT–/– mice, further confirming the occurrence of marked alterations of 5‐HT1A and 5‐HT1B autoreceptors in these animals.