Chronic Exposure to Adenosine Receptor Agonists and Antagonists Reciprocally Regulates the A1 Adenosine Receptor-Adenylyl Cyclase System in Cerebellar Granule Cells

Abstract: Chronic treatment with the adenosine receptor antagonist caffeine evokes an up-regulation of A₁ adenosine receptors and increased coupling of the receptor to G proteins in rat brain membranes. However, chronic agonist exposure has not been explored. Primary cultures of cerebellar granule cells were exposed chronically to A₁ adenosine receptor agonists and antagonists. Exposure to the A₁ adenosine receptor agonist N ⁶-cyclopentyladenosine resulted in (1) a time- and concentration-dependent reduction in the density of receptors labeled by 1,3-[³H]dipropyl-8-cyclopentylxanthine, (2) an enhanced ability of guanyl nucleotides to decrease the fraction of A₁ adenosine receptor sites displaying high affinity for 2-chloroadenosine, and (3) a functional uncoupling of receptors from adenylyl cyclase (EC 4.6.1.1). The adenosine antagonists caffeine and 8- p -sulfophenyltheophylline produced alterations in A₁ adenosine receptor homeostasis that were antipodal to those associated with agonist treatment. Antagonist exposure (1) increased the density of A₁ adenosine receptors in cerebellar granule cell membranes, (2) blunted the effect of guanyl nucleotides on receptor coupling to G proteins, and (3) increased the functional coupling of receptors to adenylyl cyclase inhibition. Forskolin treatment of cerebellar granule cells did not affect receptor density, suggesting that cyclic AMP is not involved in the regulation of A₁ adenosine receptor expression.     

EVOLUTIONARY POTENTIAL AND LOCAL GENETIC DIFFERENTIATION IN A PHENOTYPICALLY PLASTIC TRAIT OF A CYCLICAL PARTHENOGEN,DAPHNIA MAGNA

The frequent use of neutral markers to quantify genetic variation in natural populations emphasizes the role of stochastic events in explaining genetic differentiation between populations. Complementary studies on ecologically relevant traits are needed to assess the role of natural selection acting on adaptive variation in the development of local genetic differentiation. To test the hypothesis of local adaptation in the cyclical parthenogenetic species Daphnia magna, the phototactic behavior in the absence and presence of fish kairomone was assayed for 30 clones derived from resting eggs isolated from three habitats characterized by a different predation pressure by fish. Clones derived from populations in which fish are present were, on average, more negatively phototactic in and more responsive to the presence of fish kairomone than clones derived from a fishless habitat. In addition, the results show a high genetic diversity for the traits studied in all three gene pools investigated, indicating a high potential for microevolutionary changes in behavior of these Daphnia populations in the face of changes in predation pressure. The results of the present study indicate that working with large samples at the expense of having less precise estimates of genotypic values for specific genotypes may result in a loss of information with regard to the evolutionary potential of local populations.     

Distribution and hydraulic significance of large woody debris in a lowland Australian river

The line-intersect technique was used to measure the loading of large woody debris in a 1.8 km reach of the Thomson River, Victoria (catchment area of 3540 km²). A debris census (measuring every item present) was done over 0.775 km of this reach. The transect technique over-estimated the actual loading revealed by the census. The loading of debris 0.01 m in diameter for the total 1.8 km reach was 0.0172 m³ m^–2, which is higher than that measured in many headwater streams in other parts of the world. The volume loading of debris measured from low level aerial photographs was only 4.8% of the value estimated by the line-intersect technique. The line-intersect estimates were biased due to non-random orientation of debris in the stream (causing estimated errors of +8% for volume loading and +16% for surface area loading). It is recommended that to avoid this problem, when using the line-intersect transect technique in lowland rivers, each line should comprise at least two obliquely-angled transects across the channel. The mean item of debris (0.1 m in diameter) had a trunk basal diameter of 0.45 m, a length of 7.4 m, and volume of 0.7 m³. The riparian trees and the in-channel debris were of similar dimensions. The debris tended to be close to the bed and banks and was oriented downstream by the flow at a median angle of 27°. Because of this orientation, most debris had a small projected cross-sectional area, with the median value being only 1 m². Thus, the blockage ratio (proportion of projected area of debris to channel cross-sectional area) was also low, ranging from 0.0002 to 0.1, with a median value of 0.004. The average item of debris, which occupied only 0.4% of the cross-section, would have minimal influence on banktop flow hydraulics, but the largest items, which occupied around 10%, could be significant. Judicious re-introduction of debris into previously cleared rivers is unlikely to result in a large loss of conveyance, or a detectable increase in flooding frequency.     

Protein Kinases Are Involved in Prolonged Acetylcholine Release from Rat Hippocampus Induced by Thyrotropin-Releasing Hormone Analogue NS-3

Abstract: The effects of various protein kinase inhibitors on acetylcholine release from the rat hippocampus induced by the local application of NS-3 (montirelin hydrate, CG-3703), a thyrotropin-releasing hormone analogue, into the medial septum-diagonal band were examined using in vivo microdialysis. Perfusion of NS-3 (1 µ M ) into the medial septum-diagonal band for 20 min produced a pronounced and prolonged increase in the hippocampal acetylcholine efflux. Pretreatment of the medial septum-diagonal band with either K-252a, a nonselective protein kinase inhibitor, or selective protein kinase A inhibitor H-89 almost completely blocked the acetylcholine efflux evoked by NS-3, and selective protein kinase C inhibitor calphostin C inhibited the action of NS-3. On the other hand, NS-3 (0.1–10 µ M ) or TRH (1–100 µ M ) increased the cyclic AMP efflux from the medial septum-diagonal band in a concentration-dependent manner, as measured by microdialysis. These findings suggest that protein kinases A and C in the neurons of the medial septum-diagonal band are involved in the mechanism of the prolonged stimulation of acetylcholine release from the hippocampus induced by thyrotropin-releasing hormone and its analogue, NS-3.     

反义寡核苷酸片段可抑制鲫鱼视网膜电压依赖性钾离子通道的表达

爪蟾卵母细胞经注射鲫鱼视网总ＲＮＡ后,可表达大量电压依赖性钾离子通道。在此基础上,我们进一步发现,一个特定序列的寡核苷酸能专一地抑制该通道的表达。由于我们设计与合成的该片段与果蝇、小鼠中已克隆的钾通道中编码Ｎ端的一个多肽的ｍＲＮＡ完全互补,因此推测：鲫鱼视网膜中的Ｋ这个区域与其它物种的Ｋ通道有高度同源性,这为克隆该基因和研究它的功能提供了重要资料。     

细胞外Ca^2＋内流入胞质的机制

细胞外Ｃａ＾２＋主要是通过塌压依赖性Ｃａ＾２＋通道和钙池耗竭依赖性Ｃａ＾２＋通道而内流的。前者主要见于电兴奋细胞,这一过程比较清楚;后者主要见非兴奋细胞,情况远较复杂：外来信号激活内贮钙池,钙池在释放Ｃａ＾２＋同时通过目前尚不清楚的途径将直接或间接传至质膜Ｃａ＾２＋通道,而诱发Ｃａ＾２＋内流。     

Influence of glycosylation inhibitors on dihydropyridine binding to cardiac cells

In primary cultures of neonatal rat heart cells we found a linear correlation between the number of L-type calcium channel-specific dihydropyridine (DHP) binding sites and spontaneous beating frequency (v).Formation of glycoproteins in tissue culture was suppressed by different inhibitors of N-glycosylation. This inhibition alters to a different extent the binding of the DHP ligand (+)-[methyl-³H]PN 200-110 and v. The most severe but reversible effect occurs at 6 g/ml tunicamycin (B_max 45% and v 6%, resp., of control), a slight increase in B_max at 0.1–0.5 mM castanospermine and 0.05–2.5 mM deoxymannojirimycin. The other inhibitors gave no significant alterations of B_max.