Glutathione and glutathione S-transferase in the rainbow trout olfactory mucosa during retrograde degeneration and regeneration of the olfactory nerve

Molecular characterization and histochemical demonstration of salmon olfactory marker protein in the olfactory epithelium of lacustrine sockeye salmon (Oncorhynchus nerka)

Despite the importance of olfactory receptor neurons (ORNs) for homing migration, the expression of olfactory marker protein (OMP) is not well understood in ORNs of Pacific salmon (genus Oncorhynchus). In this study, salmon OMP was characterized in the olfactory epithelia of lacustrine sockeye salmon (O. nerka) by molecular biological and histochemical techniques. Two cDNAs encoding salmon OMP were isolated and sequenced. These cDNAs both contained a coding region encoding 173 amino acid residues, and the molecular mass of the two proteins was calculated to be 19,581.17 and 19,387.11Da, respectively. Both amino acid sequences showed marked homology (90%). The protein and nucleotide sequencing demonstrates the existence of high-level homology between salmon OMPs and those of other teleosts. By in situ hybridization using a digoxigenin-labeled salmon OMP cRNA probe, signals for salmon OMP mRNA were observed preferentially in the perinuclear regions of the ORNs. By immunohistochemistry using a specific antibody to salmon OMP, OMP-immunoreactivities were noted in the cytosol of those neurons. The present study is the first to describe cDNA cloning of OMP in salmon olfactory epithelium, and indicate that OMP is a useful molecular marker for the detection of the ORNs in Pacific salmon.

Salmon olfaction is impaired by an environmentally realistic pesticide mixture

Many of the salmon-producing waterways of the world contain pesticides known to harm olfactory sensory neurons (OSNs) that are critically important throughout the salmon lifecycle. The ability of OSNs to retain functionality after exposure to complex pesticide mixtures remains unknown. Here we show that a 96-h exposure to an environmentally realistic concentration of a mixture made from the ten most frequently occurring pesticides in British Columbia's Nicomekl River reduced the OSN responses of rainbow troutto a behaviorally relevant odorant. Odor-evoked responses were not altered by exposure to one-fifth of the realistic concentration, and this may have been due an upregulation in detoxification enzymes, since glutathione-S-transferase activity reached a maximum (> 32% above control) at this concentration. Mixture exposure did not help to prevent OSN impairment from a second, brief (5 min) exposure to a higher (20 x) concentration of the mixture, suggesting longer-term, low-concentration exposures may not prevent damage from brief, high-concentration pulse exposures. This study demonstrates that environmentally observed pesticide mixtures can injure salmon olfactory tissue, and by extension, contribute to the threatened and endangered status of many salmon stocks.

Calcium-signaling networks in olfactory receptor neurons

The olfactory neuroepithelium represents a unique interface between the brain and the external environment. Olfactory function comprises a distinct set of molecular tasks: sensory signal transduction, cytoprotection and adult neurogenesis. A multitude of biochemical studies has revealed the central role of Ca(2+) signaling in the function of olfactory receptor neurons (ORNs). We set out to establish Ca(2+)-dependent signaling networks in ORN cilia by proteomic analysis. We subjected a ciliary membrane preparation to Ca(2+)/calmodulin-affinity chromatography using mild detergent conditions in order to maintain functional protein complexes involved in olfactory Ca(2+) signaling. Thus, calmodulin serves as a valuable tool to gain access to novel Ca(2+)-regulated protein complexes. Tandem mass spectrometry (nanoscale liquid-chromatography-electrospray injection) identified 123 distinct proteins. Ninety-seven proteins (79%) could be assigned to specific olfactory functions, including 32 to sensory signal transduction and 40 to cytoprotection. We point out novel perspectives for research on the Ca(2+)-signaling networks in the olfactory system of the rat.

Mitral cells in the olfactory bulb of adult zebrafish (Danio rerio): morphology and distribution

The mitral cell is the primary output neuron and central relay in the olfactory bulb of vertebrates. The morphology of these cells has been studied extensively in mammalian systems and to a lesser degree in teleosts. This study uses retrograde tract tracing and other techniques to characterize the morphology and distribution of mitral cells in the olfactory bulb of adult zebrafish, Danio rerio. These output neurons, located primarily in the glomerular layer and superficial internal cell layer, had variable-shaped somata that ranged in size from 4-18 microm in diameter and 31-96 microm2 in cross-sectional area. The mitral cells exhibited two main types of morphologies with regard to their dendrites: the unidendritic morphology was a single primary dendrite with one or more tufts, but multidendritic cells with several dendritic projections also were seen. The axons of these cells projected to either the medial or the lateral olfactory tract and, in general, the location of the cell on the medial or lateral side of the bulb was indicative of the tract to which it would project. Further, this study shows that the majority of zebrafish mitral cells likely innervate a single glomerulus rather than multiple glomeruli. This information is contrary to the multiple innervation pattern suggested for all teleost mitral cells. Our findings suggest that mitral cells in zebrafish may be more similar to mammalian mitral cells than previously believed, despite variation in size and structure. This information provides a revised anatomical framework for olfactory processing studies in this key model system.

Comparison of basal level metabolic enzyme activities of freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and rat

Biotransformation plays a key role in detoxification, bioactivation and bioaccumulation of xenobiotics in fish. Biotransformation capabilities in fish, however, are not as thoroughly characterized as in mammals. In this study, basal level activities of 7-ethoxyresorufin O-dealkylase (EROD), 7-methoxyresorufin O-dealkylase (MROD), 7-pentoxyresorufin O-dealkylase, chlorzoxazone 6-hydroxylase, testosterone 6beta-hydroxylase, lauric acid 11-hydroxylase, and glutathione S-transferase in freshly isolated hepatocytes from rainbow trout (Oncorhynchus mykiss) and rat were obtained and compared. All the activities, when normalized to cellular protein concentrations, were significantly lower in rainbow trout hepatocytes than those in rat hepatocytes. Rainbow trout cytochrome P450 (CYP) 2B-, 2E1-, and 3A-like activities were respectively 14-, 18.4-, and 11.8-fold lower than rat. The smallest difference between the two species was CYP1A-type activities (EROD and MROD, 3.4- and 3.7-fold lower in rainbow trout, respectively). Our results suggest that the relative importance of CYP1A enzymes (among CYP subfamilies) in the biotransformation of xenobiotics in rainbow trout is likely much greater than that in mammals.

Characterization of the mouse olfactory glutathione S-transferases during the acute phase response

The acute phase response (APR) has been shown to alter expression and activity of biotransformation enzymes, such as the phase I cytochromes p450 and phase II glutathione S-transferases (GSTs). The cytochromes p450 and GSTs are expressed abundantly and colocalized to non-neuronal cells of the olfactory mucosa. Previous studies indicate that olfactory cytochromes p450 expression and activity is altered during periods of localized inflammation and infection. Little is understood, however, about the influence of the APR on olfactory GST enzymes. This study investigated effects of the APR on olfactory GST isozymes expression and activity in mouse olfactory mucosa after 24-hr treatment with the acute phase inducer, polyinosinic: polycytidylic acid (polyIC). Western blot analysis using antibodies directed against specific GST isoforms alpha (A1-1), micro (M1-1), and pi (P1-1) demonstrated that their expression was unaltered by polyIC treatment. In contrast, olfactory p450 2E1 expression was significantly decreased. Enzymatic activity of the olfactory GSTs toward the general substrate, 1-chloro-2,4-dinitrobenzene (CDNB) was unchanged during the APR. Analysis of olfactory glutathione content during the APR showed that it was also unaffected by polyIC. The insensitivity of these olfactory GST isoforms during the APR may play a significant role toward limiting the impact of infection and inflammation on the olfactory system.

Sensory deafferentation and olfactory bulb morphology in the zebrafish and related species

The zebrafish, Danio rerio, has become an important model species for examining olfactory system structure and function, yet little is known about developmental changes in olfactory bulb morphology from embryo to adult. The present study examined both normal growth and the effects of deafferentation on the bulb from hatching to adulthood. In young animals, the bulb is small relative to body size and has a higher percentage of its volume occupied by incoming olfactory nerve fibers. Young animals are also more affected by sensory deafferentation. Olfactory rosette removal resulted in more than 50% reductions in laminar volumes, indicating that sensory input is important during periods of rapid development. In addition, three closely related species were examined to compare how differing bulb morphology might influence the effects of bulb manipulation. The cherry barb, Barbus (=Puntius) titteya, and giant danio, Danio aequipinnatus, have larger bulbs and laminar volumes relative to body size than the zebrafish or scissortail rasbora, Rasbora trilineata. Both are also more affected by deafferentation, with at least a 35% reduction in laminar sizes in many of the bulb layers. The studies are discussed in terms of the importance of the olfactory system to each species and are also compared to the effects of sensory manipulations in other animals.

Salmonid olfactory system-specific protein (N24) exhibits glutathione S-transferase class pi-like structure

A salmonid olfactory system-specific protein (N24) that has been identified in lacustrine sockeye salmon (Oncorhynchus nerka) was characterized by biochemical and molecular biological techniques. N24 is a homodimer, and the intact molecular mass is estimated as approximately 43.3 kDa by gel filtration. Furthermore, N24 was located only in the cytosolic fraction of the olfactory tissues as determined by subcellular fractionation. cDNA encoding the lacustrine sockeye salmon N24 was isolated and sequenced. This cDNA contained a coding region encoding 216 amino acid residues and the molecular mass of this protein is calculated to be 242,224.77. The protein and nucleotide sequencing demonstrates the existence of a remarkable homology between N24 and glutathione S-transferase (GST; EC 2.5.1.18) class pi enzymes. Northern analysis showed that N24 mRNA with a length of 950 bases is expressed in lacustrine sockeye salmon olfactory epithelium. Olfactory receptor cells showed strong hybridization signals for N24 mRNA in the olfactory epithelium. N24 demonstrated glutathione binding activity in affinity-purified GST column experiments. The present study describes for the first time cDNA cloning of GST in fish olfactory epithelium.