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At least 217 records · Page 12Linked to original sources
PubMed1997

Use-dependent increases in glutamate concentration activate presynaptic metabotropic glutamate receptors.

The classical view of fast chemical synaptic transmission is that released neurotransmitter acts locally on postsynaptic receptors and is cleared from the synaptic cleft within a few milliseconds by diffusion and by specific reuptake mechanisms. This rapid clearance restricts the spread of neurotransmitter and, combined with the low affinities of many ionotropic receptors, ensures that synaptic transmission occurs in a point-to-point fashion. We now show, however, that when transmitter release is enhanced at hippocampal mossy fibre synapses, the concentration of glutamate increases and its clearance is delayed; this allows it to spread away from the synapse and to activate presynaptic inhibitory metabotropic glutamate receptors (mGluRs). At normal levels of glutamate release during low-frequency activity, these presynaptic receptors are not activated. When glutamate concentration is increased by higher-frequency activity or by blocking glutamate uptake, however, these receptors become activated, leading to a rapid inhibition of transmitter release. This effect may be related to the long-term depression of mossy fibre synaptic responses that has recently been shown after prolonged activation of presynaptic mGluRs (refs 2, 3). The use-dependent activation of presynaptic mGluRs that we describe here thus represents a negative feedback mechanism for controlling the strength of synaptic transmission.

Animals↗
PubMed1974

Unidirectional growth and branch formation of a morphological mutant, Agrobacterium tumefaciens.

Morphological characteristics of thermoconditional mutant Agrobacterium tumefaciens F-502 were investigated in relation to growth, division, and synthesis of cellular components. As a result of a shift from 27 to 37 C, mutant cells altered their morphology from short rods to elongated and branched forms; in addition, division and deoxyribonucleic acid synthesis were inhibited at 37 C. At 37 C unidirectional cell growth and branch formation occurred at one end of a cell, and the elongation rate of a cell was proportional to cell length. A hypothetical model for branch formation is presented in which the maximal elongation rate, 1.8 mum/h, at one end of a cell is an essential factor for initiation of branch formation.

Amino Acids↗
PubMed1968

Nutritional factors influencing the development of competence in the Bacillus subtilis transformation system.

Cultures of Bacillus subtilis developed competence for the uptake of deoxyribonucleic acid in a chemically defined medium with a predictable, reproducible pattern. The gross effects of individual amino acids were determined. Seven amino acids, most of which are reported to be major components of the cell wall, were shown to impair the development of maximal levels of competence. When the synthetic growth medium was supplemented with a mixture of the nine amino acids which we found to stimulate the development of competence, the level of transfection was increased to 10 to 15% of the population. The actual level of competence in these populations was assayed by transformation of unlinked bacterial markers and by two different transfection assays. The results indicate that calculations from cotransfer of unlinked markers overestimates the degree of competence in highly competent populations of B. subtilis, whereas the number of plaques obtained in transfection is an under-estimate of the actual level of competence. The results are interpreted to indicate that neither method of analysis gives a true estimate of the competent population, but that more than 80% of the cells may be competent.

Alanine↗