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Biomedical subjects

F T Crews

Publications and source records attributed to F T Crews.

At least 109 records · Page 6Linked to original sources
PubMed1983

Biochemical changes of rat brain membranes with aging.

Modification of membrane composition and enzymatic activities both in total brain homogenate and purified synaptic plasma membrane of 3 and 24 month old rats has been investigated. Protein, cholesterol and phospholipid content and (Na+, K+)ATPase and 2',3' cyclic nucleotide phosphohydrolase activities were determined. The major changes occurred in the whole homogenate where a general increase in total protein and cholesterol content with age and a significant increase of the cholesterol/phospholipids molar ratio has been detected. In S.P.M. aging process induced a decrease of protein, cholesterol and phospholipids content associated with an increased membrane viscosity and a decrease of delta E. These data are consistent with a change in the structural organization and in the distribution pattern of different cell population in the aging brain. A possible artifactual effect of freezing on the reported parameter is also discussed.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗
PubMed1983

Rapid down-regulation of serotonin2 receptor binding during combined administration of tricyclic antidepressant drugs and alpha 2 antagonists.

Recent studies have shown that chronic, but not acute, treatment with several different antidepressant drugs decreases the density of serotonin2 (5HT2) receptors in the cerebral cortex (Peroutka and Snyder, 1980a). It was found that combined administration of antidepressant drugs with alpha 2 antagonists could accelerate this down-regulation of 5HT2 receptors induced by antidepressants. Amitriptyline (10 mg/kg, twice daily) in combination with yohimbine (5 mg/kg, twice daily) decreased 5HT2 receptor binding after 4 days of treatment, whereas amitriptyline alone required 10 days to decrease the density of 5HT2 receptors. Other antidepressant--alpha-antagonist combinations which rapidly decreased the density of 5HT2 receptors were desipramine-phenoxybenzamine, amitriptyline-phenoxybenzamine, iprindole-phenoxybenzamine, mianserin-phenoxybenzamine and desipramine-dihydroergotamine. Treatment with desipramine in combination with prazosin, an alpha 1 antagonist, did not accelerate the down-regulation of 5HT2 receptors induced by antidepressants. These results suggest that blockade of central alpha 2 receptors can accelerate the down-regulation of 5HT2 receptors by antidepressants. A combination of antidepressant and alpha antagonist treatment could represent a new, rapid onset antidepressant drug therapy.

Adrenergic alpha-Antagonists↗
PubMed1983

Rapid decrease in rat brain beta adrenergic receptor binding during combined antidepressant alpha-2 antagonist treatment.

The delay in onset of the therapeutic effect of antidepressants is believed to be due to a progressive decrease in the density of central beta adrenergic receptors. The changes in beta adrenergic receptor density could result from an increase in the synaptic concentration of norepinephrine, which is secondary to a decrease in the sensitivity of the alpha-2 adrenergic receptor which normally inhibits release and the firing of the locus ceruleus. We have observed an acceleration of beta receptor desensitization with combined administration of antidepressants and alpha-2 adrenergic antagonists. After one day of administration of desipramine (DMI) with phenoxybenzamine, there was a marked decrease in beta adrenergic receptor density. One day of treatment with DMI alone had no significant effect on beta receptor density. Rapid desensitization occurs not only in rat limbic cortex, but also in hippocampus and mesencephalon. Furthermore, combination therapy of DMI with yohimbine or dihydroergotamine, both alpha-2 adrenergic blockers, is similar to DMI-phenoxybenzamine treatment. Combined administration with prazosin, an alpha-1 antagonist, had no effect on antidepressant-induced desensitization. Combined administration of alpha-2 antagonists accelerated beta receptor desensitization by amitriptyline, mianserin, iprindole, tranylcypromine and pargyline. These observations suggest that pharmacological blockade of alpha-2 adrenergic receptors enhances antidepressant-induced decreases in central nervous system beta-adrenergic receptor density.

Adrenergic alpha-Antagonists↗
PubMed1982

Variants of the rat basophilic leukemia cell line for the study of histamine release.

Cloning of the rat basophilic leukemia (RBL) cell lines demonstrates variability in cell chromosome number (approximately 44-70) and in their capacity to release histamine following an IgE- or Ca2+-ionophore stimulus. After IgE activation there is increased phospholipid methylation, Ca2+ influx, arachidonic acid, and histamine release. On Ca2+ ionophore A23187 stimulation, phospholipid methylation is not increased, but Ca2+ influx, arachidonic acid, and histamine release occur. Variants of the RBL-cloned sublines defective at different stages in the release process were obtained and used to sequence the different events in the release process: IgE activation is followed by methylation, Ca2+ influx, arachidonic acid, and histamine release. However, there are other variants with defects in intermediate steps in the pathway, e.g., increased phospholipid methylation that is not followed by Ca2+ influx or arachidonic acid release not followed by histamine release. Isolating variants carrying drug-resistance markers made hybridization (reconstitution) experiments possible. Two variants were recognized, each of which was deficient in one of the two phospholipid methyltransferase enzymes. Neither of these two variants released histamine; hybrids formed by fusion of these two cell lines have both phospholipid methyltransferase enzymes and release histamine. By other complementation experiments, groups of variants with defects at different steps in the histamine release sequence were recognized. Clearly, these basophilic leukemia cell lines provide a unique system for the study of the mechanism of histamine release.

Animals↗
PubMed1981

Age dependent changes in the methylation of rat brain phospholipids.

Phospholipid methyltransferase I and II enzymes methylate phosphatidylethanolamine three times to form phosphatidylcholine. The activity of these two enzymes was determined in synaptosome-enriched fractions from rats 1, 3, 7, 15 and 21 months old. The activity of phospholipid methyltransferase I was significantly greater in 7-, 15- and 21-month-old rats than in 1- and 3-month-old rats. In contrast, the activity of phospholipid methyltransferase II did not change with age. These changes in methyltransferase activity with increasing age may be related to changes in beta-receptor function with increasing age.

Aging↗
PubMed1981

Acceleration of beta-receptor desensitization in combined administration of antidepressants and phenoxybenzamine.

The delayed therapeutic effects of antidepressants (usually between 10 and 14 days) and of the tricyclic antidepressants in particular, are believed (on the basis of animal experiments) to lie in a progressive decrease of the sensitivity of cortical beta-adrenergic receptors. This is thought to be due to an increase in the synaptic concentration of noradrenaline, in turn accomplished by a decrease in the sensitivity of the presynaptic alpha 2 receptors which normally regulate noradrenaline secretion by a negative feedback mechanism. This model suggests that the desensitization of postsynaptic beta-receptors by antidepressants should be accelerated by the inhibition of the presynaptic alpha 2- adrenergic system, and we have indeed observed such an effect in preliminary studies with desipramine and phenoxybenzamine (PBZ) combined. We now show that the administration of either tricyclic or monoamine oxidase inhibitor antidepressants in combination with PBZ, an irreversible alpha-adrenergic blocker, accelerates and intensifies the desensitization of beta-adrenergic receptors. Our observations may have therapeutic implications.

Animals↗
PubMed1981

Rat basophilic leukemia cell lines defective in phospholipid methyltransferase enzymes, Ca2+ influx, and histamine release: reconstitution by hybridization.

Variants of the rat basophilic leukemia (RBL) cell line were isolated and screened for phospholipid methyltransferase I and II activities, enzymes that convert phosphatidylethanolamine to phosphatidylcholine. Two variants were found that had decreased phospholipid methyltransferase enzyme levels and were unable to cause an influx of Ca2+ or release histamine in an IgE-mediated reaction. However, these cells were able to release histamine through an ionophore-induced reaction, indicating that the releasing mechanism distal to the Ca2+ channel was intact. One cell line, 1C1.B1, had low specific activity for phospholipid methyltransferase I. A second variant, 2H3.B6, had reduced phospholipid methyltransferase II activity. Although both variants were unable to incorporate label from [methyl-3H]methionine or [3H]serine into phosphatidylcholine, they were able to incorporate [methyl-3H]choline and myo-[2-3H(N)]inositol into phospholipids. Fusion of the two cell lines and isolation on selective media resulted in the growth of eight independent hybrids. All eight had an increased number of chromosomes and normal phospholipid methyltransferase activities. Stimulation of the hybrids with IgE resulted in CA2+ influx and histamine release. These results indicate that phospholipid methylation precedes and is necessary for Ca2+ influx, and they further support the hypothesis that methylation is a necessary early step in the IgE-mediated histamine release reaction in RBL cells.

Animals↗
PubMed1980

Phospholipid methyltransferase asymmetry in synaptosomal membranes.

The sequential methylation of phosphatidylethanolamine to form phosphatidylcholine is carried out by two methyltransferases in rat brain synaptosomes. The first enzyme methylates phosphatidylethanolamine to form phosphatidylmonomethylethanolamine. The second enzyme methylates the monomethylated phospholipid two additional times, forming phosphatidylcholine. Experiments comparing the rate of methylation between intact and lysed synaptosomes indicate that synaptosomes accumulate S-adenosyl-L-methionine and that the first methylation takes place on the cytoplasmic side of the membrane. Studies comparing trypsin digestion of proteins in intact and lysed synaptosomes indicate that the first enzyme is localized on the cytoplasmic side of the membrane and the second enzyme faces the external surface. Phospholipase C hydrolyzed phosphatidylcholine formed by methylation, suggesting its localization in the external layer of the phospholipid bilayer. A mechanism for an enzyme-mediated flip-flop of phospholipids from the cytoplasmic side to the outer surface of the synaptosomal plasma membrane is presented.

Animals↗
PubMed1980

Potentiation of responses to adrenergic nerve stimulation in isolated rat atria during chronic tricyclic antidepressant administration.

Several weeks of tricyclic antidepressant administration are required to effectively reverse depression. To determine whether there are adaptive changes in andrenergic nerve function which correspond to the clinical onset of antidepressant action, the endogenous norepinephrine content, [3H]norepinephrine uptake and retention, responses to exogenous norepinephrine and the release of norepinephrine during field stimulation were studied using left atrial strips isolated from rats treated with either acutely or chronically with tricyclic antidepressants. Desipramine, nortriptyline, chlorimipramine and iprindole were administered to rats, 10 mg/kg i.p., twice daily. After 14 days of drug administration, the responses to field stimulation were potentiated markedly by all four tricyclics. In contrast, 1 day of tricyclic treatment had only slight potentiating effects. When phenoxybenzamine, 10(-7) M, was added to the organ bath in order to block the inhibitory presynaptic alpha receptor, the responses of control atria and atria from rats treated for 1 day with desipramine were potentiated but those of atria treated for 21 days with desipramine were not potentiated. The development of presynaptic alpha receptor subsensitivity during chronic tricyclic administration would explain these findings. Other possible explanations were also investigated. The uptake and retention of [3H]norepinephrine was markedly inhibited to a similar degree of either 1 or 14 days of desipramine or nortriptyline administration. One day of chlorimipramine treatment decreased the amount of [3H]norepinephrine taken up and retained by left atrial strips, and after 14 days of treatment decreased the amount further. In contrast, neither 1 nor 14 days of iprindole administration had any effect on the uptake and retention of [3H]norepinephrine. These data indicate that the potentiation of the responses to field stimulation cannot be explained by the inhibition of norepinephrine uptake. The inotropic response to exogenous norepinephrine was not altered by any duration of administration of any of the four tricyclics studied. Furthermore, the endogenous norepinephrine content of atria did not change after as many as 21 days of desipramine administration. The present results indicate that the potentiation of the effects of adrenergic nerve transmission during chronic tricyclic administration is the result of an increase in norepinephrine release which occurs due to the development of presynaptic alpha receptor subsensitivity. The time course of development of presynaptic receptor subsensitivity corresponds well with the onset of clinical activity of these drugs.

Animals↗
PubMed1979

Concanavalin A stimulates phospholipid methylation and phosphatidylserine decarboxylation in rat mast cells.

When histamine release from rat peritoneal mast cells is stimulated by concanavalin A, membrane phospholipids are methylated in the early stage of this process. Exogenously added phosphatidylserine enhances the concanavalin A-induced histamine release, and at the same time the lectin markedly stimulates the decarboxylation and methylation of phosphatidylserine. Within minutes after the addition of concanavalin A to rat mast cells, the newly methylated phospholipids begin to disappear and an increased formation of lysophosphatidylcholine is observed. When rat mast cells are treated with concanavalin A in the absence of Ca2+, phospholipid methylation is stimulated but no significant release of histamine is detected. The subsequent exposure of the pretreated cells to Ca2+ causes increased release of histamine and degradation of methylated phospholipids. The inhibition of either synthesis or degradation of methylated phospholipids results in the inhibition of histamine release. These observations suggest that the synthesis and degradation of methylated lipids are an intrinsic part of the biochemical mechanism that modulate histamine release from mast cells.

Animals↗