Effect of specific membrane perturbations on alpha 1-adrenergic and muscarinic-cholinergic signal transduction in rat parotid cell aggregates

Ademetionine (S-adenosylmethionine) neuropharmacology: implications for drug therapies in psychiatric and neurological disorders

Ademetionine (S-adenosylmethionine; SAMe) is a ubiquitous metabolite present in all cells and biological fluids, and serves as a methyl donor in a multitude of different methylation reactions involving proteins, phospholipids, catecholamines and DNA. Pharmaceutical preparations of some stable salts of SAMe are available for parenteral and oral use in humans, and have been shown to increase plasma and cerebrospinal fluid SAMe concentrations. In experimental studies administration of SAMe is associated with increases in brain monoamine neurotransmitters and b-adrenergic and muscarinic receptor functions. These neuropharmacological effects are postulated to be involved in the antidepressant activity of SAMe which has been confirmed in numerous controlled studies. Preliminary studies indicate that SAMe has therapeutic potential in the treatment of other CNS disorders including dementia, acquired immune deficiency syndrome (AIDS)-associated myelopathy, and brain ischaemia. This review will focus on recent experimental and clinical aspects of SAMe in the central nervous system, and the therapeutic use in psychiatric and neurological disorders.

Effect of saponin and filipin on antagonist binding to AT 1 receptors in intact cells

In the present study, [ 3H ]-candesartan binding experiments were performed on intact Chinese Hamster Ovary cells transfected with the human AT1 receptor (CHO-AT1 cells). Cells were pre-treated with 0.01mg/ml saponin or filipin. Both pre-treatments resulted in an increased dissociation rate and decreased affinity of the insurmountable non-peptide antagonist [3H ]-candesartan. A similar decrease in affinity was observed for the peptide antagonist Sar1-Ile8 angiotensin II and for other non-peptide antagonists, irrespectively of their degree of insurmountability. A similar discrepancy in [ 3H ]-candesartan binding was earlier observed when comparing intact CHO-AT1 cells and membrane preparations thereof. This similarity is further highlighted by the observations that saponin or filipin no longer affect [ 3H ]-candesartan binding to CHO-AT1 cell membranes and that both agents permeabilise the CHO-AT1 cells. This suggests that the intracellular composition and/or organisation of living cells play an active role with regard to antagonist-AT1 receptor interactions.

Differential effect of aging on cholesterol modulation of carbachol-stimulated low-K(m) GTPase in striatal synaptosomes

Previous research has suggested that age-related decline in physiological functions may be the result of substantial alterations in membrane molecular structure. The purpose of the present experiments was to elucidate the role of cholesterol domains in the age-related decline in receptor-G-protein interactions in striatal synaptosomes. We observed a significant age-related deficit in muscarinic cholinergic stimulated Low-Km GTPase activity and its age-related susceptibility to cholesterol treatment in range of 10(-10)-10(-5) M. Treatment of synaptosomes from old rats with cholesterol in range of 10(-8)-10(-6) M restored the Low-Km GTPase activity up to the level seen in young animals and reached a maximum at 10(-7) M. In synaptosomes from young rats, however, cholesterol treatment did not have any effect on striatal Low-Km GTPase activity. We observed significant alterations in the membrane lipid composition of striatal synaptosomes as a function of age. Our results suggested a significant interaction of age and cholesterol treatment on physical properties of striatal synaptosomes. Thus, the present results of experiments in vitro support our previous results of experiments in vivo and suggested an interaction of cholesterol domains with muscarinic-cholinergic receptor G-protein alpha subunit coupling/uncoupling through regulation of physical properties of striatal synaptosomes.

Effects of age and food restriction on calcium signaling in parotid acinar cells of Fischer 344 rats

In this study, we characterized alpha-adrenergic (alpha AR) and muscarinic induced [Ca2+]i changes in individual parotid acinar cells from male Fischer 344 rats (6-24 month-old) fed ad libitum (AL) or 60% ad libitum intake (FR). Cells were prepared by collagenase/hyaluronidase digestion. [Ca2+]i was measured by video image, fluorescent microscopy in single acinar cells loaded with FURA2. Neither age nor food restriction altered the peak [Ca2+]i achieved in response to carbachol (100 microM). Similar results were obtained for epinephrine (Epi = 100 microM) stimulation in 6- and 12-month-old animals. However, the peak [Ca2+]i response to Epi declined between 12 and 18 months in both dietary groups (e.g., AL: 12 months = 387 +/- 21 nM, 18 months = 253 +/- 10 nM; FR: 12 months = 430 +/- 22 nM, 18 months = 325 +/- 14 nM). The decline in response to Epi seen with age was less in FR than in AL animals at 18 months, but not at 24 months. In addition, db cAMP reduced the carbachol-stimulated [Ca2+]i response to levels comparable to those observed with epinephrine. The results support the view that calcium mobilization in parotid acinar cells from male Fischer 344 rats in response to alpha AR, but not to muscarinic, stimulation is impaired with age. Food restriction may slow down, but does not prevent, the functional decline. Furthermore, cAMP appears to modulate the muscarinic response.

Partial restoration of impaired alpha 1-adrenergic responsiveness in parotid cells of aged rats by S-adenosylmethionine treatment

The age related decrease in alpha 1-adrenergic stimulated inositol 1, 4, 5 trisphosphate (IP3) production in parotid cells of aged rats can be partially restored by treatment with S-adenosylmethionine (SAM). This effect is completely blocked by S-adenosyl homocysteine (SAH) and occurs in association with an increase in the conversion of phosphatidylethanolamine to phosphatidylcholine and a decrease in membrane viscosity. In contrast, SAM treatment actually inhibits stimulated IP3 production in cells of young rats. The membrane viscosity of these cells is lower than that of those from aged rats. Although conversion of phosphatidylethanolamine to phosphatidylcholine is enhanced, no further decrease in membrane viscosity is elicited in young cell preparations. These findings suggest that age changes in the membrane environment may result in impaired alpha 1-adrenergic signal transduction and that such alterations may be at least partially reversible by SAM treatment.