Regulation of corticotropin releasing hormone receptor (CRH-R) in the rat anterior pituitary as assessed by radioimmunoassay.

Since the cloning of corticotropin releasing hormone receptor type 1 (CRH-R1), an essential component of the hypothalamo-pituitary-adrenal (HPA) axis, numerous studies have been conducted to monitor its changes in transcription levels under various conditions. However, the precise dynamics at the protein levels are yet to be elucidated. In the present study we aimed at establishing an RIA system for CRH-R1 protein, with an antiserum against the C-terminal fragment of human/rat CRH-R1. The generated antiserum showed a moderate cross-reactivity with CRH-R2. We examined the in vivo effect of adrenalectomy (ADX) on immunoreactive CRH-R (irCRH-R) levels in the rat AP, and the in vitro profile of irCRH-R levels in cultured rat AP cells after administration of CRH. The irCRH-R in the AP membrane of intact rats was 51.8 +/- 6.8 fmol/mg protein, which is comparable to those reported in binding studies. ADX elicited a significant decrease of irCRH-R to approximately 50% of the control level one day after ADX, which returned to the baseline level the following day. Addition of CRH to cultured AP cells resulted in a significant decrease of irCRH-R in the membrane fraction to 18% of the control level at 4 h, and it returned rapidly to 70% at 8 h. These experiments together with our previous study implicate that irCRH-R makes a different profile, with an earlier recovery than that of mRNA. Although this system cannot precisely discriminate between CRH-R1 and CRH-R2, our findings may serve to demonstrate differing CRH receptor regulations at the synthesis level and at the protein level in the rat AP.