5-HT5 receptors.

The 5-HT(5) receptor family consists of two members designated as 5-HT(5A) and 5-HT(5B). To date the 5-HT(5A) receptor has been identified in the mouse, rat, and human. The 5-HT(5B) receptor also is expressed in the mouse and rat, but not in the human where the coding sequence is interrupted by stop codons. Both receptors are essentially limited in distribution to the central nervous system (CNS), although the 5-HT(5A) receptor has also been found on neurons and neuronal-like cells of the carotid body. Within the CNS the 5-HT(5A) receptor shows a relatively broad distribution, while the 5-HT(5B) receptor has a very restricted distribution. The 5-HT(5A) receptor has been demonstrated to couple to G proteins, and the primary coupling appears to be through Gi/o to inhibit adenylyl cyclase activity. The 5-HT(5) receptors have not been extensively characterized pharmacologically. Both receptors show their highest affinity for LSD, which appears to act as a partial agonist at the 5-HT(5A) receptor. Amongst agonist-like molecules, 5-CT (5-carboxamidotryptamine) also has high affinity and has greater potency and affinity at the 5-HT(5A) receptor than does 5-HT itself. Both [(125)I]LSD and [(5)H]5-CT have been used as radioligands to study the receptors in vitro. Nothing is known about the role of the 5-HT(5B) receptor in vivo. A mouse line has been developed where the 5-HT(5A) receptor has been knocked out and these animals have been shown to have a diminished response to LSD-induced increases in locomotion. The 5-HT(5) receptors remain as two of the least studied and understood of the 5-HT receptor subtypes.