In species representing different levels of vertebrate evolution, olfactory receptor genes have been identified by molecular cloning techniques. Comparing the deduced amino-acid sequences revealed that the olfactory receptor gene family of Rana esculenta resembles that of Xenopus laevis, indicating that amphibians in general may comprise two classes of olfactory receptors. Whereas teleost fish, including the goldfish Carassius auratus, possess only class I receptors, the 'living fossil' Latimeria chalumnae is endowed with both receptor classes; interestingly, most of the class II genes turned out to be pseudogenes. Exploring receptor genes in aquatic mammals led to the discovery of a large array of only class II receptor genes in the dolphin Stenella Coeruleoalba; however, all of these genes were found to be non-functional pseudogenes. These results support the notion that class I receptors may be specialized for detecting water-soluble odorants and class II receptors for recognizing volatile odorants. Comparing the structural features of both receptor classes from various species revealed that they differ mainly in their extracellular loop 3, which may contribute to ligand specificity. Comparing the number and diversity of olfactory receptor genes in different species provides insight into the origin and the evolution of this unique gene family.

No abstract

G protein betagamma (Gbetagamma) complexes are considered to play an important role in second messenger signaling of phospholipase C (PLC). Monitoring the inositol 1,4,5-trisphosphate (IP(3)) response in circumvallate tissue homogenates upon stimulation with denatonium benzoate, it was demonstrated that a glutathione S-transferase-GRK3ct fusion protein-a Gbetagamma scavenger-attenuates the bitter tastant-induced second messenger reaction. Towards an identification of the Gbetagamma complex involved in rat bitter taste transduction, it was found that the G protein beta(3) subtype is specifically expressed in taste receptor cells of circumvallate papillae. Gbeta(3)-specific antibodies blocked the denatonium benzoate-induced IP(3) formation in a dose-dependent manner; the inhibitory effect was reversed by preincubation with the antigenic peptide. A less pronounced inhibition was observed using Gbeta(1)-specific antibodies. Analyzing individual taste cells by single cell reverse transcriptase-polymerase chain reaction approaches, overlapping expression patterns for PLCbeta(2), Galpha(gust), Gbeta(3) and Ggamma(3) could be demonstrated. Furthermore, the co-expression of all profiled signal transduction components in individual taste receptor cells could be detected. These data support the concept that the denatonium benzoate-induced IP(3) response is mediated by an activation of PLCbeta(2) via a Gbetagamma complex, possibly composed of Gbeta(3) as the predominant beta subunit and Ggamma(3), and imply that multiple second messenger pathways may exist in individual taste receptor cells.

Mature olfactory receptor neurons of mammals are characterized by the expression of the highly conserved olfactory marker protein (OMP) encoded by single copy genes. In Xenopus laevis, two homologous genes encoding olfactory marker proteins have been identified that share a sequence identity with mammalian OMPs of about 50%. Sequence comparison revealed significant variability in the N-terminus and C-terminus regions; in contrast, two internal domains were highly conserved between amphibian and mammalian OMPs, suggesting some functional relevance. The two OMP subtypes were regionally expressed in the olfactory nasal epithelium of Xenopus. XOMP1 transcripts were more abundant in the lateral diverticulum and XOMP2 in the medial diverticulum. The lateral location of XOMP1 and medial location of XOMP2 correspond to the suggested locations of olfactory receptor neurons responsive to water-borne and air-borne odorants, respectively.

The present study indicates that male rat urinary components in female rat vomeronasal organ microvillar preparations not only induce a rapid and transient IP(3) signal, but in addition, the level of cAMP decreases with a delayed and sustained time course. This decrease seems to be a consequence of the preceding activation of the phosphoinositol pathway rather than the result of an enhanced phosphodiesterase activity or an inhibition of adenylyl cyclase (AC) via Galpha(i) or Galpha(o). This notion is supported by the finding that activation of the endogenous protein kinase C suppresses basal as well as forskolin-induced cAMP formation. Furthermore, it was observed that elevated levels of calcium inhibit cAMP formation in rat VNO microvillar preparations. These properties of cAMP signaling in the VNO of rats may be mediated by a calcium- and protein kinase C-inhibited AC VI subtype, which is localized in microvillar preparations of the VNO.