Type 2 metabotropic glutamate receptor (mGluR2) fails to negatively couple to cGMP in stably transfected cells.

The group II metabotropic glutamate receptors 2 and 3 (mGluR2 and mGluR3) share sequence homology, common pharmacology and negative coupling to cAMP. We recently discovered that mGluR3 also is negatively coupled through a G-protein to the cGMP transduction pathway in rat cerebellar granule cells and astrocytes. To test the hypothesis that mGluR2 also has access to the cGMP pathway, C6 glioma cells were stably transfected with mGluR2 and mGluR3 cDNA and their coupling to cGMP levels was characterized. In contrast to many other cell lines, C6 has a robust cGMP response that makes it attractive in the study of receptor coupling to this second messenger pathway. Consistent with prior studies, the mGluR3 receptor was negatively coupled to cGMP and this coupling was blocked by PTX. In contrast, mGluR2 agonists failed to reduce sodium nitroprusside stimulated cGMP levels in transfected cell lines where the receptor was negatively coupled to cAMP. These data provide further support for the functional divergence between these two closely related receptors.

Postovariectomy weight gain in female rats is reversed by estrogen receptor alpha agonist, propylpyrazoletriol.

OBJECTIVE: The objective of this study was to distinguish the role of specific estrogen receptors (ERs), ERalpha and ERbeta, on body weight regulation using a rat model of weight gain subsequent to menopause. STUDY DESIGN: Ovariectomized rats were utilized as the animal model to simulate the postmenopause weight gain. The rats were ovariectomized and subcutaneously injected daily with vehicle, estradiol-17beta (E2), propylpyrazoletriol (PPT; ERalpha agonist) and diarylpropionitrile (DPN; ERbeta agonist). To further control for the possible effect of estrogen secreted from adrenals, a second experiment was conducted during which the rats were adrenalectomized and ovariectomized. RESULTS: Ovariectomy significantly increased (P < .05) body weight, whereas treatment of ovariectomized rats with E2 and PPT, but DPN decreased (P < .05) body weight. The results from the second study with ovariectomized/adrenalectomized rats were consistent with the first experiment. CONCLUSION: These results suggest that the activation of ERalpha is important in regulating body weight.

Luteinizing hormone-releasing hormone I (LHRH-I) and its metabolite in peripheral tissues.

Luteinizing hormone-releasing hormone (LHRH) was first isolated in the mammalian hypothalamus and shown to be the primary regulator of the reproductive system through its initiation of pituitary gonadotropin release. Since its discovery, this form of LHRH (LHRH-I) has been shown to be one of many structural variants with a variety of roles in both the brain and peripheral tissues. Enormous interest has been focused on LHRH-I and LHRH-II and their cognate receptors as targets for designing therapies to treat cancers of the reproductive system. LHRH-I is processed by a zinc metalloendopeptidase EC 3.4.24.15 (EP24.15) that cleaves the hormone at the fifth and sixth bond of the decapeptide (Tyr(5)-Gly(6)) to form LHRH-(1-5). We have previously reported that the autoregulation of LHRH gene expression can also be mediated by its processed peptide, LHRH-(1-5). Furthermore, LHRH-(1-5) has also been shown to be involved in cell proliferation. This review will focus on the possible roles of LHRH and its processed peptide, LHRH-(1-5), in non-hypothalamic tissues.

Regulation of type II luteinizing hormone-releasing hormone (LHRH-II) gene expression by the processed peptide of LHRH-I, LHRH-(1-5) in endometrial cells.

Luteinizing hormone-releasing hormone (LHRH) was first isolated in the mammalian hypothalamus and shown to be the primary regulator of the reproductive system through its initiation of pituitary gonadotropin release. Since its discovery, this form of LHRH (LHRH-I) has been shown to be one of many structural variants with a variety of roles in both the brain and peripheral tissues. Enormous interest has been focused on LHRH-I, LHRH-II, and their cognate receptors as targets for designing therapies to treat cancers of the reproductive system. LHRH-I is processed by a zinc metalloendopeptidase EC 3.4.24.15 (EP24.15) that cleaves the hormone at the Tyr(5)-Gly(6) bond. We have previously reported that the autoregulation of LHRH gene expression can also be mediated by its processed peptide, LHRH-(1-5). Given its importance in the brain, we have investigated the role of the specific processed peptide of LHRH-I, LHRH-(1-5), within Ishikawa cells, a human endometrial cell line. Using real-time polymerase chain reaction, we observed that LHRH-(1-5) upregulates LHRH-II mRNA expression in Ishikawa cells but does not exert any influence on LHRH-I mRNA levels. This is in contrast to the effects of LHRH-I, which affects the expression of LHRH-I mRNA. Our findings support a potential role for LHRH-(1-5) as a processed metabolite in the endometrium. Further investigations are needed to determine the role of this processed metabolite and to identify specific pathways involved in LHRH-(1-5) signaling.

Differential negative coupling of type 3 metabotropic glutamate receptor to cyclic GMP levels in neurons and astrocytes.

Metabotropic receptors may couple to different G proteins in different cells or perhaps even in different regions of the same cell. To date, direct studies of group II and group III metabotropic glutamate receptors' (mGluRs) relationships to second messenger cascades have reported negative coupling of these receptors to cyclic AMP (cAMP) levels in neurons, astrocytes and transfected cells. In the present study, we found that the peptide neurotransmitter N-acetylaspartylglutamate (NAAG), an mGluR3-selective agonist, decreased sodium nitroprusside (SNP)-stimulated cyclic GMP (cGMP) levels in cerebellar granule cells and cerebellar astrocytes. The mGluR3 and group II agonists FN6 and LY354740 had similar effects on cGMP levels. The mGluR3 and group II antagonists beta-NAAG and LY341495 blocked these actions. Treatment with pertussis toxin inhibited the effects of NAAG on SNP-stimulated cGMP levels in rat cerebellar astrocytes but not in cerebellar neurons. These data support the conclusion that mGluR3 is also coupled to cGMP levels and that this mGluR3-induced reduction of cGMP levels is mediated by different G proteins in cerebellar astrocytes and neurons. We previously reported that this receptor is coupled to a cAMP cascade via a pertussis toxin-sensitive G protein in cerebellar neurons, astrocytes and transfected cells. Taken together with the present data, we propose that mGluR3 is coupled to two different G proteins in granule cell neurons. These data greatly expand knowledge of the range of second messenger cascades induced by mGluR3, and have implications for clinical conditions affected by NAAG and other group II mGluR agonists.