Clozapine and other competitive antagonists reactivate risperidone-inactivated h5-HT7 receptors: radioligand binding and functional evidence for GPCR homodimer protomer interactions.
BACKGROUND
The h5-HT(7) receptor is subject to inactivation by risperidone and 9-OH-risperidone, apparently through a pseudo-irreversible complex formed between these drugs and the receptor. Although risperidone and 9-OH-risperidone ("inactivating antagonists") completely inactivate the receptor, only 50% of the receptors form a pseudo-irreversible complex with these drugs.


OBJECTIVE
This study aims to more fully determine the mechanism(s) responsible for the novel effects of risperidone and 9-OH-risperidone and to determine if the inactivation can be reversed (reactivation).


METHODS
The ability of non-inactivating drugs (competitive antagonists) to dissociate wash-resistant [(3)H]risperidone binding from h5-HT(7) receptors was investigated. Also, the ability of non-inactivating drugs to reactivate inactivated h5-HT(7) receptors was investigated, using cAMP accumulation as a functional endpoint.


RESULTS
The competitive (non-inactivating) antagonists clozapine and mesulergine released the wash-resistant [(3)H]risperidone binding to the h5-HT(7) receptor. The competitive antagonists clozapine, SB269970, mianserin, cyproheptadine, mesulergine, and ICI169369 reactivated the risperidone-inactivated h5-HT(7) receptors in a concentration-dependent manner. The potencies for reactivation closely match the affinities of these drugs for the h5-HT(7) receptor (r(2) = 0.95), indicating that the reactivating antagonists are binding to and producing their effects through the orthosteric binding site of the h5-HT(7) receptor. Bioluminescence resonance energy transfer analyses indicate that the h5-HT(7) receptor forms homodimers.


CONCLUSIONS
The ability of the non-inactivating drugs to bind h5-HT(7) orthosteric sites and reverse the wash-resistant effects of risperidone or 9-OH-risperidone, also bound to h5-HT(7) orthosteric sites, is evidence for protomer-protomer interactions between h5-HT(7) homodimers. This is the first demonstration of a non-mutated G-protein-coupled receptor homodimer engaging in protomer-protomer interactions in an intact cell preparation.
Teitler
Milt
M
Center for Neuropharmacology & Neuroscience, Albany Medical College, 47 New Scotland Avenue, Albany, NY 12208, USA. teitlem@mail.amc.edu
Toohey
Nicole
N
Knight
Jessica A
JA
Klein
Michael T
MT
Smith
Carol
C
D001667:Binding, Competitive; D015374:Biosensing Techniques; D002460:Cell Line; D003024:Clozapine / Q000378:metabolism / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007555:Isoxazoles / Q000494:pharmacology; D000068882:Paliperidone Palmitate; D011485:Protein Binding; D055503:Protein Multimerization; D021122:Protein Subunits; D011743:Pyrimidines / Q000494:pharmacology; D011869:Radioligand Assay; D011985:Receptors, Serotonin / Q000737:chemistry / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D018967:Risperidone / Q000378:metabolism / Q000494:pharmacology*; D012702:Serotonin Antagonists / Q000378:metabolism / Q000494:pharmacology*; D014162:Transfection
D007555:Isoxazoles; D021122:Protein Subunits; D011743:Pyrimidines; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; C104425:serotonin 7 receptor; D000242:Cyclic AMP; D003024:Clozapine; D018967:Risperidone; D000068882:Paliperidone Palmitate
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