Pharmacological characterization of presynaptic alpha 2-autoreceptors in rat submaxillary gland and heart atrium.

1. The pharmacological properties of presynaptic alpha 2-autoreceptors were studied in rat isolated submaxillary glands and atria. Tissue pieces were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine, and stimulated electrically. In one series of experiments, pEC30 values of 12 alpha-adrenoceptor antagonists were determined, i.e., negative logarithms of concentrations that increased the electrically evoked overflow of tritium by 30%. In another series, pKD values of 9 alpha-adrenoceptor antagonists against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), and of 3 antagonists against the release-inhibiting effect of methoxamine, were determined. 2. In submaxillary glands, the pEC30 values of the antagonists correlated well with their pKD values against UK 14304 (r = 0.93). The same was true for atria (r = 0.92). 3. In submaxillary glands, the pKD values of 3 antagonists against UK14304 were very similar to their pKD values against methoxamine, with a maximal difference of 0.4. The same was true for atria where the maximal difference was 0.3. 4. The pEC30 values obtained in submaxillary glands correlated significantly with those obtained in atria (r = 0.81). The same was true for the pKD values (r = 0.79). However, the pEC30 and pKD values also indicated consistent differences between the two tissues. 5. It is concluded that the sites of action of the imidazoline UK 14304 (alpha 2-selective), the phenylethylamine noradrenaline, and the phenylethylamine methoxamine (alpha 1-selective) are exclusively alpha 2-adrenoceptors. There is no indication for presynaptic alpha 1-adrenoceptors or for an effect of UK 14304 mediated by presynaptic imidazoline receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Only activated but not non-activated presynaptic α2-autoreceptors interfere with neighbouring presynaptic receptor mechanisms

Summary Experiments were carried out in rabbit cerebrocortical slices in order to find out whether the attenuation by presynaptic ₂-autoreceptors of effects mediated by presynaptic opioid - and adenosine A₁-receptors requires activation of the ₂-receptors. The slices were preincubated with ³H-noradrenaline and then superfused with medium containing desipramine 1 mol/l. They were stimulated electrically either with single pulses or with trains of 32 pulses at 1 Hz.The overflow of tritium elicited by a single pulse amounted to 0.21% of the tritium content of the tissue. It was Ca²⁺-dependent and tetrodotoxin-sensitive and not changed by rauwolscine 1 mol/l or yohimbine 0.3 mol/l. Ethylketocyclazocine (EK; 0.1–10 nmol/l) and R-(–)-N⁶-phenylisopropyladenosine (PIA; 1–1,000 nmol/1) potently inhibited the overflow evoked by a single pulse, and their effects were not changed by yohimbine. — The overflow of tritium elicited by trains of 32 pulses at 1 Hz amounted to 0.92% of the tritium content of the tissue and was increased approximately fourfold by yohimbine 0.3 mol/l. EK and PIA were less potent inhibitors than in the one pulse experiments. Yohimbine greatly enhanced the effects of EK and PIA. The enhancement was even more pronounced when the Ca²⁺ concentration in the medium was reduced in order to obtain a control tritium overflow similar to that evoked by 32 pulses in the absence of yohimbine.The results demonstrate that there is no ₂-adrenergic autoinhibition when noradrenaline release is elicited by a single pulse. Under these conditions, the non-activated presynaptic ₂-adrenoceptor does not interfere with presynaptic opioid - and adenosine A₁-receptor mechanisms. It is only when the autoreceptor is activated by released noradrenaline that it attenuates neighbouring presynaptic receptor mechanisms, and this attenuation is removed by ₂-adrenoceptor antagonists.Send offprint requests to N. Limberger at the above address     

Competitive control of the self-renewing T cell repertoire

We develop a mathematical model for the self-renewing part of the T cell repertoire. Assuming that self-renewing T cells have to be stimulated by immunogenic MHC-peptide complexes presented on the surfaces of antigen-presenting cells, we derive a model of T cell growth in which competition for MHC-peptide complexes limits T cell clone sizes and regulates the total number of self-renewing T cells in the animal. We show that for a sufficient diversity and/or degree of cross-reactivity, the total T cell number hardly depends upon the diversity of the T cell repertoire or the diversity of the set of presented peptides. Conversely, for repertoires of lower diversity and/or cross-reactivity, steady-state total T cell numbers may be limited by the diversity of the T cells. This provides a possible explanation for the limited repertoire expansion in some, but not all, mouse T cell re-constitution experiments. We suggest that the competitive interactions described by our model underlie the normal T cells numbers observed in transgenic mice, germ-free mice and various knockout mice.      

Localization of a gene for otosclerosis to chromosome 15q25-q26

Among white adults otosclerosis is the single most common cause of hearing impairment. Although the genetics of this disease are controversial, the majority of studies indicate autosomal dominant inheritance with reduced penetrance. We studied a large multi- generational family in which otosclerosis has been inherited in an autosomal dominant pattern. Five of16 affected persons have surgically confirmed otosclerosis; the remaining nine have a conductive hearing loss but have not undergone corrective surgery. To locate the disease- causing gene we completed genetic linkage analysis using short tandem repeat polymorphisms (STRPs) distributed over the entire genome. Multipoint linkage analysis showed that only one genomic region, on chromosome 15q, generated a lod score >2.0. Additional STRPs were typed in this area, resulting in a lod score of 3.4. STRPs FES (centromeric) and D15S657 (telomeric) flank the 14. 5 cM region that contains an otosclerosis gene.      

IMPT1, an imprinted gene similar to polyspecific transporter and multi- drug resistance genes

Human chromosome 11p15.5 and distal mouse chromosome 7 include a megabase-scale chromosomal domain with multiple genes subject to parental imprinting. Here we describe mouse and human versions of a novel imprinted gene, IMPT1 , which lies between IPL and p57 KIP2 and which encodes a predicted multi-membrane-spanning protein similar to bacterial and eukaryotic polyspecific metabolite transporter and multi- drug resistance pumps. Mouse Impt1 and human IMPT1 mRNAs are highly expressed in tissues with metabolite transport functions, including liver, kidney, intestine, extra-embryonic membranes and placenta, and there is strongly preferential expression of the maternal allele in various mouse tissues at fetal stages. In post-natal tissues there is persistent expression, but the allelic bias attenuates. An allelic expression bias is also observed in human fetal and post-natal tissues, but there is significant interindividual variation and rare somatic allele switching. The fact that Impt1 is relatively repressed on the paternal allele, together with data from other imprinted genes, allows a statistical conclusion that the primary effect of human chromosome 11p15.5/mouse distal chromosome 7 imprinting is domain-wide relative repression of genes on the paternal homolog. Dosage regulation of the metabolite transporter gene(s) by imprinting might regulate placental and fetal growth.