Contrasting Neurochemical Interactions of Tiletamine, a Potent Phencyclidine (PCP) Receptor Ligand, with the N-Methyl-D-Aspartate-Coupled and -Uncoupled PCP Recognition Sites

Neurochemical interactions of tiletamine, a potent phencyclidine (PCP) receptor ligand, with the N-methyl-D-aspartate (NMDA)-coupled and -uncoupled PCP recognition sites were examined. Tiletamine potently displaced the binding of [3H]1-(2-thienyl)cyclohexylpiperidine with an IC50 of 79 nM without affecting sigma-, glycine, glutamate, kainate, quisqualate, or dopamine (DA) receptors. Like other PCP ligands acting via the NMDA-coupled PCP recognition sites, tiletamine decreased basal, harmaline-, and D-serine-mediated increases in cyclic cGMP levels and induced stereotypy and ataxia. Tiletamine was nearly five times more potent than PCP at inhibiting the binding of 3-hydroxy[3H]PCP to its high-affinity NMDA-uncoupled PCP recognition sites. However, following parenteral administration, dizocilpine maleate (MK-801), ketamine, PCP, dexoxadrol, and 1-(2-thienyl)cyclohexylpiperidine HCl, but not tiletamine, increased rat pyriform cortical DA metabolism and/or release, a response modulated by the NMDA-uncoupled PCP recognition sites. Pretreatment with tiletamine did not attenuate the MK-801-induced increases in rat pyriform cortical DA metabolism, a result suggesting that tiletamine is not a partial agonist of the NMDA-uncoupled PCP recognition sites in this region. However, following intracerebroventricular administration (100-500 micrograms/rat), tiletamine increased pyriform cortical DA metabolism with a bell-shaped dose-response curve. These data indicate a differential interaction of tiletamine with the NMDA-coupled and -uncoupled PCP recognition sites. The paradoxical effects of tiletamine suggest that tiletamine might activate receptor(s) or neuronal pathways of unknown pharmacology.     

Patterns of nucleotide substitutions inferred from the phylogenies of the class I major histocompatibility complex genes

Summary Patterns of nucleotide substitutions in human major histocompatibility complex (MHC) class I genes were estimated by using phylogenetic trees of DNA sequences. The pattern is defined as a set of 12 parameters, each of which represents the relative frequency of substitutions from a particular nucleotide to another. The pattern at the antigen recognition sites (ARS) in functional MHC genes was remarkably different from that at the remaining coding region (non-ARS). In particular, the proportion of transitions among all the nucleotide substitutions (P _s) was extremely low at the third codon positions of ARS. In the HLA-A genes, P _s at the third codon positions was only 6% in ARS, whereas it was 69% in non-ARS. In HLA-B, the corresponding values were 30% in ARS and 80% in non-ARS, respectively. On the other hand, P _s in a class I pseudogene (HLA-H) was 57%, which was in good agreement with P _s in other pseudogenes. Because pseudogenes are selectively neutral, the pattern in pseudogenes is regarded as the pattern of spontaneous substitution mutations. In general, the pattern in functional genes that are subject to selective forces deviates from the pattern in pseudogenes. At the third codon positions in coding regions, transitions scarcely cause amino acid replacements, whereas about half of transversions do cause replacements. Accordingly, P _s at the third codon positions decreases if amino acid replacements are accelerated by natural selection but increases if amino acids are conserved by functional constraint. Our observations imply that the ARS region is subject to natural selection favoring amino acid replacements, whereas the non-ARS region is subject to functional constraint. Offprint requests to: T. Gojobori     

Lectin Binding to Radopholus citrophilus and R. similis Proteins

Lectin-binding glycoproteins in seven populations of two burrowing nematode sibling species were probed with five different biotinylated lectins on Western blots, and differences were correlated with nematode ability to parasitize citrus and to overcome citrus rootstock resistance. Banding patterns of molecular weight standards were fit best by an exponential decay function, and a predictive equation was used to estimate molecular weights (r² = 0.999). A band (131 kDa) that labeled with the lectin Concanavalin A (Con A) occurred in extracts from cuticles and egg shells of populations of Radopholus citrophilus that parasitize citrus. Wheat germ agglutin labeled a band (58 kDa) in aqueous homogenates of populations that reproduce in roots of citrus rootstock normally resistant to burrowing nematodes. The two sibling species R. citrophilus and R. similis were distinguished by a high molecular weight Con A-labeled band (608 kDa) from cuticle and egg shells. Probing blots with the lectin Limulus polyphemus agglutinin indicated that each population contained a band (12-16 kDa) specifically inhibited by the addition of 25 mM neuraminic acid, suggesting that glycoproteins with sialic acid moieties are present in burrowing nematodes.     

Dielectric probe of the electric-field-sensitive peptide alamethicin

Dielectric constant and loss of alamethicin in solvents of various degrees of lipophilicity (namely mixtures of n-octanol and dioxane) have been measured at frequencies from 5 kHz to 50 MHz. By means of a conventional Cole-Cole approach, dielectric properties were evaluated to obtain information about the structural and dipolar properties of the peptide in view of its function as a voltage-dependent pore former in membranes. The results for a pure octanol solvent (together with an apparent molecular weight determined by ultracentrifugation) indicate the existence of primarily monomeric particles of quite elongated shape and of comparatively high dipole moment. Adding dioxane was found to yield considerable aggregation and a decrease of polarity.     

Effects of temperature on the size and shape of Escherichia coli cells

Two substrains of Escherichia coli B/r were grown to steady-state in batch cultures at temperatures between 22 and 42° C in different growth media. The size and shape of the cells were measured from light and electron micrographs and with the Coulter channelizer. The results indicate that cells are shorter and somewhat thicker at the lower temperatures, especially in rich growth media; cell volume is then slightly smaller. A lower temperature was further found to increase the relative duration of the constriction period. The shapes of the cell size distributions are indistinguishable, indicating that the pattern of growth of the cells is the same at all temperatures. The adaptation of the cells to a temperature shift lasted several generations, indicating that the morphological effects of temperature are mediated by the cell's physiology.     

Patterned distribution of membrane-associated Ca2+ during pore formation inMicrasterias

Summary During the stage of pore formation developing cells ofMicrasterias denticulata show a patterned distribution of fluorescent dots on the plasma membrane after treatment with chlorotetracycline. The center-to-center spacing of these dots corresponds with the distances between the individual cell wall pores ofMicrasterias. Therefore it is supposed that the patterned distribution of pores and their formation which is mediated by special pore vesicles are related to local accumulations of membrane-associated Ca²⁺. Membrane-associated Ca²⁺ seems not only to be functional in tip growth but to be a general mediator for recognition and fusion processes between various vesicles and the plasma membrane.