The perception of odors is influenced by a variety of neuromodulators, and there is growing evidence that modulation already takes place in the olfactory epithelium. Here we report on cannabinergic actions in the olfactory epithelium of Xenopus laevis tadpoles. First we show that CB1 receptor-specific antagonists AM251, AM281, and LY320135 modulate odor-evoked calcium changes in olfactory receptor neurons. Second, we localize CB1-like immunoreactivity on dendrites of olfactory receptor neurons. Finally, we describe the cannabinergic influence on odor-induced spike-associated currents in individual olfactory receptor neurons. Here we demonstrate that the cannabinergic system has a profound impact on peripheral odor processing and discuss its possible function.CB1 antagonist ͉ dendrite ͉ modulation ͉ odor processing ͉ olfactory receptor neuron
Using fluorophore-conjugated phalloidin, we show that filamentous (F)-actin is strongly aggregated in olfactory glomeruli within primary olfactory centers of vertebrates and insects. Our comparative study demonstrates that aggregation of F-actin is a common feature of glomeruli across phyla, and is independent of glomerular architecture and/or the presence or absence of cellular borders around glomeruli formed by neurons or glial cells. The distribution of F-actin in axonal and dendritic compartments within glomeruli, however, appears different between vertebrates and insects. The potential role of the actin-based cytoskeleton in synaptic and structural plasticity within glomeruli is discussed.
Nucleotides and amino acids are acknowledged categories of water-borne olfactory stimuli. In previous studies it has been shown that larvae of Xenopus laevis are able to sense amino acids. Here we report on the effect of ATP in the olfactory epithelium (OE) of Xenopus laevis tadpoles. First, ATP activates a subpopulation of cells in the OE. The ATP-sensitive subset of cells is almost perfectly disjoint from the subset of amino acid-activated cells. Both responses are not mediated by the well-described cAMP transduction pathway as the two subpopulations of cells do not overlap with a third, forskolin-activated subpopulation. We further show that, in contrast to amino acids, which act exclusively as olfactory stimuli, ATP appears to feature a second role. Surprisingly it activated a large number of sustentacular supporting cells (SCs) and, to a much lower extent, olfactory receptor neurons. The cells of the amino acid- and ATP-responding subsets featured differences in shape, size and position in the OE. The latencies to activation upon stimulus application differed markedly in these subsets. To obtain these results two technical points were important. We used a novel dextran-tetramethylrhodamine-backfilled slice preparation of the OE and we found out that an antibody to calnexin, a known molecular chaperone, also labels SCs. Our findings thus show a strong effect of ATP in the OE and we discuss some of the possible physiological functions of nucleotides in the OE.
Changes in glycolytic flux have been observed in liver under conditions where effects of cAMP seem unlikely. We have, therefore, studied the phosphorylation of four enzymes involved in the regulation of glycolysis and gluconeogenesis (6-phosphofructo-1-kinase from rat liver and rabbit muscle; pyruvate kinase, 6-phosphofructo-2-kinase and fructose-l,6-bisphosphatase from rat liver) by defined concentrations of two CAMP-independent protein kinases: Ca2 +/calmodulin-dependent protein kinase and Ca2 +/phospholipid-dependent protein kinase (protein kinase C). The results were compared with those obtained with the catalytic subunit of CAMP-dependent protein kinase. The following results were obtained.1. Ca2 +/calmodulin-dependent protein kinase phosphorylates 6-phosphofructo-1-kinase and L-type pyruvate kinase at a slightly lower rate as compared to CAMP-dependent protein kinase.2. 6-Phosphofructo-1-kinase is phosphorylated by the two kinases at a single identical position. There is no additive phosphorylation. The final stoichiometry is 2 mol phosphate/mol tetramer. The same holds for L-type pyruvate kinase except that the stoichiometry with either kinase or both kinases together is 4 mol phosphate/mol tetramer .3. Rabbit muscle 6-phosphofructo-1-kinase is phosphorylated by CAMP-dependent protein kinase but not by Ca2 +/calmodulin-dependent protein kinase.4. Fructose-1,6-bisphosphatase from rat but not from rabbit liver is phosphorylated at the same position but at a markedly lower rate by Ca2+/calmodulin-dependent protein kinase when compared to the phosphorylation by CAMP-dependent protein kinase.5. 6-Phosphofructo-2-kinase is phosphorylated by Ca2 +/calmodulin-dependent protein kinase only at a negligible rate.6. Protein kinase C does not seem to be involved in the regulation of the enzymes examined: only 6-phosphofructo-2-kinase became phosphorylated to a significant degree. In contrast to the phosphorylation by CAMP-dependent protein kinase, this phosphorylation is not associated with a change of enzyme activity. This agrees with our observation that the sites of phosphorylation by the two kinases are different.The results indicate that Ca2 +/calmodulin-dependent protein kinase but not protein kinase C could be involved in the regulation of hepatic glycolytic flux under conditions where changes in the activity of CAMP-dependent protein kinase seem unlikely.One possible mechanism for the regulation of glycolysis/ gluconeogenesis is the covalent modification of rate-limiting enzymes by phosphorylation/dephosphorylation. For pyruvate kinase and the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (rat liver), the regulatory role of reversible phosphorylation/dephosphorylation is now well established [l -31. This contrasts with 6-phosphofructo-1-kinase and fructose-l,6-bisphosphatase for which the physiological role of this modification remains largely unclear [4 -61.
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