A metabotropic glutamate receptor variant functions as a taste receptor.

Sensory transduction for many taste stimuli such as sugars, some bitter compounds and amino acids is thought to be mediated via G protein-coupled receptors (GPCRs), although no such receptors that respond to taste stimuli are yet identified. Monosodium L-glutamate (L-MSG), a natural component of many foods, is an important gustatory stimulus believed to signal dietary protein. We describe a GPCR cloned from rat taste buds and functionally expressed in CHO cells. The receptor couples negatively to a cAMP cascade and shows an unusual concentration-response relationship. The similarity of its properties to MSG taste suggests that this receptor is a taste receptor for glutamate.

Effects of isoquinolinesulfonamide H-8 on Fundulus heteroclitus ovarian follicles: role of cyclic nucleotide-dependent protein kinases on steroidogenesis and oocyte maturation in vitro.

The possible role of cyclic nucleotide-dependent protein kinases in mediating the stimulatory actions of Fundulus heteroclitus pituitary extract (FPE) during ovarian steroidogenesis and oocyte maturation in vitro was investigated. Follicle-enclosed oocytes were cultured in the presence of FPE and/or N-[2-Methylamino)ethyl]-5-isoquinolinesulfonamide (H-8), a compound that inhibits protein kinase A (PKA) and cGMP-dependent protein kinase. H-8 alone (0.1-1 mM) promoted oocyte germinal vesicle breakdown (GVBD) in a dose-dependent manner. However, the process of GVBD initiated by H-8 was much slower that that triggered by 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (17,20 betaP), the natural inducer of oocyte maturation in F. heteroclitus. Treatment with H-8 also increased 17,20 betaP production by the follicles and the accumulation of this steroid in the media was much slower than that initiated by FPE. However, in contrast to the FPE action on the oocyte, which is mediated by 17,20 betaP, the stimulatory action of H-8 on GVBD appears to be independent of follicular steroid production, since aminoglutethimide (AGI), an inhibitor of steroidogenesis, did not-block H-8-induced GVBD while inhibiting H-8 induced 17, 20 betaP production. Moreover, addition of H-8 to FPE-treated follicles significantly reduced 17,20 betaP secretion and the percentage of GVBD. These results provide further support for the involvement of PKA in the mechanism by which FPE stimulates ovarian steroidogenesis in F. heteroclitus. Furthermore, the fact that H-8 alone increased 17,20 betaP levels may imply that basal follicular production of this steroid could be induced by inactivation of cyclic nucleotide-dependent protein kinases. Data also indicate that inhibition of PKA and/or c-GMP-dependent protein kinase in the oocyte may be involved in the mechanism leading to resumption of meiosis in this species.

Inhibitory action of the gonadopeptide inhibin on amphibian (Rana pipiens) steroidogenesis and oocyte maturation.

In view of recent reports on the production of inhibin- and activin-like proteins in lower vertebrates and their important role during development, we have examined the effects of the gonadopeptide inhibin in the process of oocyte maturation using amphibian (Rana pipiens) fully grown preovulatory ovarian follicles cultured in vitro. In the presence of frog pituitary homogenate (FPH), which stimulates progesterone (P4) levels and the subsequent germinal vesicle breakdown (GVBD), purified porcine inhibin (35-50 IU) inhibited both of these responses in a dose-dependent manner. Inhibin also blocked GVBD initiated by exogenously added P4 in intact as well as denuded oocytes. Thus, inhibin seems to act at the follicle (granulosa) cells because it blocked steroidogenesis and at the oocyte because it altered the steroid-induced oocyte maturation. The P4-treated follicles were susceptible to the inhibin action during the first 3 hr of steroid stimulation, which indicates that inhibin affects some early events during the process of GVBD. Maximum inhibitory effect was observed when P4 and inhibin were added simultaneously at the beginning of the incubations. Moreover, the inhibitory effect on GVBD caused by the gonadopeptide was dependent on the length of exposure of the follicles to inhibin. The continuous presence of inhibin in the culture was required to block GVBD efficiently. Data also indicate that the inhibitory effect of inhibin was reversible. Taken together, results from this study present evidence that inhibin may be a relevant paracrine/autocrine regulator of ovarian functions.