Differential in Vivo Regulation of the Pituitary Growth Hormone-Releasing Hormone (GHRH) Receptor by GHRH in Young and Aged Rats

Abstract

In aging, alterations of pituitary GH-releasing hormone (GHRH) receptor (GHRH-R)-binding sites have been proposed as one of the initiating factors contributing to the loss of somatotroph responsiveness to GHRH. Changes in the characteristics and/or concentration of the functional GHRH-R could take place in the course of aging and reduce the sensitivity of the somatotroph axis to GHRH. Because chronic exposure to GHRH has been proposed to resensitize aged somatotroph cells, better knowledge of its effects on the regulation of the somatotroph axis is required, particularly at the level of GHRH-R. Two- and 18-month-old male Sprague Dawley rats were treated for 14 days with a daily s.c. injection of 0.5 or 1.0 mg/kg BW human GHRH-(1-29)NH2 or saline. In 2-month-old rats, treatment with 0.5 mg/kg GHRH increased the number of high affinity pituitary GHRH-R-binding sites by 2-fold (P < 0.05) and hypothalamic somatostatin (SRIF) content by 45% (P < 0.05). It did not affect hypothalamic GHRH content, serum total insulin-like growth factor I (IGF-I), or body weight gain. Treatment with 1.0 mg/kg GHRH decreased the number of high affinity pituitary GHRH-R-binding sites by 2.4-fold compared with that in rats treated with 0.5 mg/kg BW (P < 0.05) and increased hypothalamic SRIF content by 45% (P < 0.05), but did not affect GHRH content. It also decreased circulating levels of IGF-I by 13% (P < 0.05) and slowed the growth rate by 17% (P < 0.05). In 18-month-old rats, treatment with 0.5 mg/kg GHRH for 14 days was not sufficient to rejuvenate pituitary GHRH binding parameters. However, treatment with 1.0 mg/kg GHRH restored the affinities of high and low affinity classes of GHRH-binding sites to values similar to those found in 2-month-old rats. Binding capacities of the high and low affinity classes of sites were increased by 1.8- and 3-fold, respectively, although significance was only reached for the low affinity site (P < 0.05). These changes were associated with a normalization of the level of 2.5-kb GHRH-R messenger RNA transcript, which was decreased by 31% in aging rats (P < 0.05), and by a trend for an increase in the 4-kb GHRH-R messenger RNA transcript, which was already increased by 49% in 18-month-old rats (P < 0.05). A normalization of serum IGF-I levels, which were decreased by 11% in 18-month-old control rats (P < 0.01), was also observed. No treatment effect was detected on body weight or hypothalamic SRIF and GHRH contents. We conclude that a 14-day administration of GHRH induces a differential GHRH-R-mediated regulation at the level of the pituitary and probably the hypothalamus as a function of age.

International Union of Pharmacology. XXXV. The Glucagon Receptor Family

“…Receptor gene expression is positively regulated by glucocorticoids (Lam et al, 1996;Korytko and Cuttler, 1997;Miller and Mayo, 1997) and thyroid hormone Korytko and Cuttler, 1997) and negatively regulated by estrogen (Lam et al, 1996). GHRH itself may modulate the expression of its receptor, although investigators have reached somewhat different conclusions regarding whether this is a positive or negative regulation, and the effects are likely dependent on the age of the animal and the duration of GHRH exposure (Horikawa et al, 1996;Miki et al, 1996;Aleppo et al, 1997;Girard et al, 1999;Lasko et al, 2001). In transgenic models with varying levels of growth hormone expression, the level of circulating growth hormone may contribute to regulation of GHRH receptor expression in some tissues (Peng et al, 2001).…”

Effects of a high-glucose environment on the pituitary growth hormone-releasing hormone receptor: type 1 diabetes compared with in vitro glucotoxicity

“…The RNA integrity number of all samples was Ͼ9.0. Reverse transcription (RT) of 2 g total RNA was performed with the SuperScript II RT kit and oligo(dT) [12][13][14][15][16][17][18] primers according to the manufacturer's protocol (Invitrogen Canada), including RNase H treatment. Rat GHRH and GAPDH (internal control) mRNA levels were determined by realtime PCR in separate tubes at 1:50 (GHRH) and 1:100 (GAPDH) dilution of the RT product and reagents from the Quantitect SYBR Green PCR kit.…”

“…The presence of GHRH and GHRH-R mRNAs has also been demonstrated in extrapituitary rat tissues (29), and the Henle's loop of the renal medulla contains the highest GHRH-R mRNA levels (5). Pituitary and renal GHRH-R mRNA levels are regulated in a complex manner according to physiological and hormonal status (5,15,20,21,23,32,34,37,42,51).Changes occurring in chronic pathologies, such as diabetes, induce disturbances of the somatotroph axis. In spontaneous diabetic rats and in the streptozotocin (STZ)-induced diabetic rat (STZ rat), a widely used model of type 1 diabetes, pulsatile GH secretion and GHRH-induced GH release are decreased (35,38,53,55), and declines of pituitary GH and hypothalamic GHRH mRNA levels are observed (8).…”

“…The presence of GHRH and GHRH-R mRNAs has also been demonstrated in extrapituitary rat tissues (29), and the Henle's loop of the renal medulla contains the highest GHRH-R mRNA levels (5). Pituitary and renal GHRH-R mRNA levels are regulated in a complex manner according to physiological and hormonal status (5,15,20,21,23,32,34,37,42,51).…”

“…Although the short and most abundant transcript generates the functional 423-amino acid GHRH-R (30,33), the structure of the 4-kb transcript remains to be elucidated. Correlative evidence (15) suggests that an increased ratio of 4/2.5-kb transcript levels may reflect a shift of high-to low-affinity GHRH binding sites. Chemical crosslinking with 125 I-GHRH(1-44)NH 2 identified the presence of specific Ϸ27-and Ϸ42-kDa (16) or Ϸ28-, Ϸ47-, and Ϸ65-kDa-labeled entities in the rat pituitary (6).…”