Abstract
The serotonin (5-HT)2A and 5-HT2C receptors share a high degree of sequence homology and have very similar pharmacological profiles. Although it is generally believed that the cellular signal transduction mechanisms activated by these receptors are indistinguishable, recent data suggest significant differences in their signaling cascades. In this study we explored differences in the characteristics and mechanisms of rapid desensitization between the 5-HT2A and 5-HT2C receptor systems. For both receptor systems, pretreatment with 5-HT reduced the ability of a maximal concentration of 5-HT to stimulate phospholipase C-mediated inositol phosphate accumulation by about 65%, although the 5-HT2C receptor system was more sensitive to the desensitizing stimulus. Differences in the concentration dependence of the rate constant for desensitization (kdes) suggested different mechanisms of desensitization for the 5-HT2A and 5-HT2C receptor systems. At very high receptor occupancy (>99%), the responsiveness of the 5-HT2A, but not the 5-HT2C, receptor system returned to control levels despite the continued presence of the agonist. This resensitization was dependent upon the activity of protein kinase C (PKC). Agonist-induced desensitization of the 5-HT2A, but not the 5-HT2C, receptor system was reduced by the PKC inhibitors staurosporine and bisindolylmaleimide, and by down-regulation of PKC. In addition, inhibitors of calmodulin (W-7) or of calmodulin-dependent protein kinase II, reduced 5-HT2A, but not 5-HT2C, desensitization. Desensitization of the 5-HT2C, but not the 5-HT2A, receptor system was dependent on G protein receptor kinase activity. These data further emphasize the major differences in the signaling systems coupled to 5-HT2A/2C receptors.
Footnotes
-
This work was supported by U.S. Public Health Service Grants DA 09094 (to K.A.B. and S.M.) and GM 58652 (to W.P.C.).
- Abbreviations:
- 5-HT
- serotonin
- 7-TMS
- seven transmembrane spanning
- PLC
- phospholipase C
- GRK
- G protein receptor kinase
- CaM
- calcium/calmodulin
- PKC
- protein kinase C
- PMA
- phorbol-12-myristate-13-acetate
- CHO
- Chinese hamster ovary
- MEM
- minimal essential medium
- FBS
- fetal bovine serum
- dnGRK2
- dominant-negative mutant of GRK2 (GRK2-K220R)
- IP
- inositol phosphate
- [Ca2+]i
- intracellular calcium levels
- PBZ
- phenoxybenzamine
- DOI
- (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane
- RGS
- regulator of G protein signaling
- Received April 18, 2001.
- Accepted July 17, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|