Activity-dependent induction of facilitation,depression, and post-tetanic potentiation at an insect central synapse

Summary In Manduca sexta larvae, sensory neurons innervating planta hairs on the tips of the prolegs make monosynaptic excitatory connections with motoneurons innervating proleg retractor muscles. Tactile stimulation of the hairs evokes reflex retraction of the proleg. In this study we examined activity-dependent changes in the amplitude of the excitatory postsynaptic potentials (EPSPs) evoked in a proleg motoneuron by stimulation of individual planta hair sensory neurons. Deflection of a planta hair caused a phasic-tonic response in the sensory neuron, with a mean peak instantaneous firing frequency of >300 Hz, and a tonic firing rate of 10–20 Hz. Direct electrical stimulation was used to activate individual sensory neurons to fire at a range of frequencies including those observed during natural stimulation of the hair. At relatively low firing rates (e.g., 1 Hz), EPSP amplitude was stable indefinitely. At higher instantaneous firing frequencies (>10 Hz), EPSPs were initially facilitated, but continuous stimulation led rapidly to synaptic depression. High-frequency activation of a sensory neuron could also produce post-tetanic potentiation, in which EPSP amplitude remained elevated for several min following a stimulus train. Facilitation, depression, and post-tetanic potentiation all appeared to be presynaptic phenomena. These activity-dependent changes in sensory transmission may contribute to the behavioral plasticity of the proleg withdrawal reflex observed in intact insects.Abbreviations ACh acetylcholine - AChE acetylcholine esterase - CNS central nervous system - EPSP excitatory postsynaptic potential - I _h injected hyperpolarizing current - LTP long-term potentiation - PPR principal planta retractor motoneuron - PTP post-tetanic potentiation - R _in input resistance - V _h hyperpolarized potential - V _m membrane potential - VN ventral nerve - VNA anterior branch of the ventral nerve - V _r resting potential.     

Anoxic block of GABAergic IPSPs

In rat hippocampal slices GABAergic IPSPs are very rapidly suppressed by anoxia (in<2 min). Both early (GABA_A) and late (GABA_B) components are affected. After reoxygenation, the IPSPs recover, but only slowly and not always completely. Iontophoretic applications of GABA or baclofen indicated no major depression of responses during anoxia. It is therefore unlikely that the anoxic suppression of IPSPs is caused by desensitizations of GABA receptors. A more probable explanation is a failure of GABAergic neurons to release GABA from inhibitory nerve terminals.Special issue dedicated to Dr. Eugene Roberts.     

Identification of an insertion sequence, IS1081, in Mycobacterium bovis

Abstract: An insertion sequence, IS1081, in the genome of Mycobacterium bovis has been identified and sequenced. It is 1324 bp long with 15 bp inverted repeat ends and contains a large ORF. There are six copies of IS1081 in the genome of M. bovis and the element is also present in Mycobacterium tuberculosis . IS1081 is not closely related to other DNA elements described in actinomycetes but its putative transposase bears some resemblance to that of IS256 from Staphylococcus aureus . IS1081 may be useful for genetic manipulations and for developing a diagnostic test for bovine tuberculosis based on the polymerase chain reaction.     

Saponins can cause the agglutination of phospholipid vesicles

The interaction of saponins with phospholipid vesicles was investigated by means of liposomal agglutination or a precipitation assay. Ginsenoside-Rc, which has an α-l-arabinofuranose residue at the non-reducing terminus, exhibited remarkable agglutinability toward egg yolk phosphatidylcholine vesicles, while other saponins lacking this characteristic sugar residue showed less or no agglutinability. The molar ratio of ginsenoside-Rc to egg phosphatidylcholine in the aggregates was estimated to be 0.4–0.5 by a precipitation assay using ¹⁴C-labeled egg phosphatidylcholine vesicles. The agglutination was inhibited by p-nitrophenyl α-l-arabinofuranoside but not by p-nitrophenyl β-d-glucopyranoside or arabinogalactan. The results indicated that the α-l-arabinofuranose residue in ginsenoside-Rc should be important for the expression of the agglutinability. The agglutinability of ginsenoside-Rc toward lipid vesicles depended on both the polar head groups and fatty acyl chains of phospholipids. Egg yolk phosphatidylcholine vesicles were strongly agglutinated by ginsenoside-Rc, although sphingomyelin, phosphatidylethanolamine, phosphatidic acid and phosphatidylserine were less agglutinated. The agglutinability of ginsenoside-Rc was effective for phosphatidylcholines with short or unsaturated fatty acyl chains. The results suggested that the interaction of ginsenoside-Rc with phospholipid membranes should be affected not only by the chemical structure of the phospholipid but also by the membrane fluidity.     

Effects of osmotic conditions on ultrastructure of rat mammary epithelium

Summary Effects of osmotic conditions on secretion of milk serum were examined using standard transmission electron microscopy. Rat mammary glands were infused with hyper-, iso-, and hypo-osmotic solutions. The intramammary infusion of these agents elicited distinct and repeatable morphological responses from lactating epithelial cells. The response to hyperosmolarity was an increase in compound exocytotic figures and an increase in secretory vesicle size (¯x=1.65 m in diameter). Glands infused with hypo-osmolar solutions exhibited the opposite response; reduction in compound exocytotic figures and reduced vesicle size (¯x=0.34 n in diameter). The response to iso-osmotic solutions was indistinguishable from untreated control tissue. The ratio of vesicular projections to depressions (vesicle membrane/plasma membrane interactions) could be experimentally altered through the intramammary infusion of solutions with different osmotic potentials. These observations support the suggestion that osmotic conditions may influence exocytosis of milk serum.     

The localization and rate of disappearance of a synaptic vesicle antigen following denervation

Summary A proteoglycan-specific antiserum has been used to monitor the effects of denervation in the electric organ of Torpedo marmorata. The antiserum was produced by injecting a highly purified synaptic vesicle fraction prepared from the electric organs of Torpedo marmorata. Following absorption the serum appears to be specific towards synaptic vesicles. The ultrastructural localization of the antigen determined by immuno-electron microscopy confirmed the specificity of the antiserum and showed that it did not crossreact with the proteoglycans of the basal lamina. The rate of disappearance of the vesicle proteoglycans following denervation was evaluated by means of the antiserum and was compared to the rate of disappearance of other vesicular and nerve terminal-associated markers. The results suggest that degeneration affects the vesicular constituents at varying rates resulting in a progressive disappearance of the entire functional capacity of the synaptic vesicles.     

Bluelight-induced,flavin-mediated transport of redox equivalents across artificial bilayer membranes

Summary This paper continues our studies of physico-chemical properties of vesicle-bound flavins. Based on previous results, an advanced model system was designed in order to study the mechanisms underlying bluelight-induced redox transport across artificial membranes. The lumen of single-shelled vesicles was charged with cytochromec, and amphiphilic flavin (AF1 3, AF1 10) was bound to the membrane. Upon bluelight irradiation redox equivalents are translocated from exogeneous 1e ^–(EDTA)-and 2e ^–(BH₃CN^–) donors across the membrane finally reducing the trapped cytochromec both under aerobic and anaerobic conditions. The mechanisms involved are explored and evidence for the involvement of various redox states of oxygen, dihydroflavin and flavosemiquinone is presented.     

Desensitization to Nicotinic Cholinergic Agonists and K+, Agents That Stimulate Catecholamine Secretion, in Isolated Adrenal Chromaffin Cells

Desensitization of catecholamine (CA) release from cultured bovine adrenal chromaffin cells was studied to characterize the phenomenon of desensitization and to attempt an elucidation of the mechanism(s) involved in this phenomenon at the level of the isolated chromaffin cell. Prior exposure of chromaffin cells to nicotinic cholinergic agonists [acetylcholine (ACh) or nicotine] caused a subsequent depression or desensitization of CA release during restimulation of the cells with the same agonists. Rates of development of and recovery from nicotinic desensitization were in the minute time range and the magnitude of nicotinic desensitization of CA release was greater at 37 degrees C than at 23 degrees C. ACh- (or nicotine)-induced desensitization was shown to be the result of two processes: (1) a Ca2+-dependent component of desensitization, possibly due to a depletion of intracellular CA stores and (2) a Ca2+-independent, depletion-independent component of desensitization. Prior exposure of cultured chromaffin cells to an elevated concentration of K+ also resulted in desensitization of K+-induced CA release in these cells. K+-induced desensitization was completely Ca2+-dependent and was shown to be the result, at least in part, of a mechanism that is independent of depletion of CA stores.     

Characterization of a membrane-specific tubulin isoform by peptide mapping

A membrane-specific tubulin-like protein, found in preparations of synaptic plasma membranes and brain mitochondria, was analyzed by chemical and proteolytic peptide mapping to determine which part of the molecule was different from cytoplasmic tubulin. The membrane polypeptide was identical to alpha tubulin in the first two-thirds of the molecule containing the amino terminal, as found by peptide mapping. However, some differences were observed in the peptide maps of the carboxy terminal one third of the molecule which includes a domain that is important in the regulation of tubulin self-assembly.