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R B Meeker

Publications and source records attributed to R B Meeker.

45 records · Page 3Linked to original sources
PubMed1985

Regulation of cyclic AMP metabolism by muscarinic cholinergic receptors.

The occurrence of muscarinic cholinergic receptor-mediated activation of phosphodiesterase in 1321N1 cells does not represent an isolated phenomenon, since a similar response to cholinergic stimuli is observed in thyroid slices (45) and WI-38 fibroblasts (1,42). Both muscarinic-receptor-mediated inhibition of adenylate cyclase and activation of phosphodiesterase occur in WI-38 fibroblasts (42). Work currently under way in our laboratory is directed toward determining if a guanine nucleotide regulatory protein is involved in the activation of phosphodiesterase in these cells and whether common or separate populations of muscarinic receptors are coupled to these two mechanisms of cyclic AMP metabolism. The analysis of acute hormonal regulation of phosphodiesterase in intact cells is sufficiently complicated to have previously discouraged investigators from pursuing this question in mammalian tissues. The 1321N1 cell line provides a simple model system in which at least one mechanism of hormonal regulation of phosphodiesterase can be examined. In light of the widespread occurrence of muscarinic-receptor-mediated effects on Ca2+ mobilization, it would not be surprising to find that this mechanism represents an important part of cholinergic action in both the peripheral and central nervous systems. Indeed, this system could provide an important regulatory link between Ca2+ -mediated and cyclic-AMP-mediated events in target cells. The potential importance of such a mechanism also need not be restricted to the muscarinic receptor system, since any neurotransmitter or hormone receptor system coupled to events involved in Ca2+ mobilization might produce phenomena similar to that observed for muscarinic receptors in 1321N1 cells. Our studies emphasize that two mechanisms for regulation of cyclic AMP accumulation by muscarinic cholinergic receptors exist. The data to date suggest that separate receptor subtypes are involved in these mechanisms of cholinergic regulation and provide another biochemical basis whereby the well-known interaction of Ca2+ with the cyclic AMP system can be effected. Thus, identification of the molecular events involved in the regulation of PI turnover and its consequences may be crucial in defining the basis of this aspect of cholinergic action. In addition, more extensive analyses of the phosphodiesterase system using cell-free preparations have the potential of providing clues to the molecular basis of this mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Diphosphate Ribose↗
PubMed1983

Muscarinic cholinergic receptor-mediated control of cyclic AMP metabolism. Agonist-induced changes in nucleotide synthesis and degradation.

Activation of muscarinic cholinergic receptors on 1321N1 human astrocytoma cells results in a 40-70% inhibition of isoproterenol- or prostaglandin E1 (PGE1)-stimulated accumulation of cyclic AMP. Previous investigations have demonstrated that this effect is due to a Ca2+-dependent activation of phosphodiesterase in the presence of muscarinic receptor agonists. However, during prolonged exposure of 1321N1 cells to a cholinergic agonist, a series of adaptive changes occurs which culminates in a complete loss of the muscarinic receptor-mediated inhibition of cyclic AMP accumulation. These alterations include: (a) A 50-100% increase in the capacity of isoproterenol and PGE1 to stimulate cyclic AMP accumulation. This phenomenon was rapid in onset, reached a maximum in 15-20 min, and disappeared over the next 2 hr even in the continued presence of carbachol. (b) A loss of the effects of muscarinic receptor stimulation on cyclic AMP accumulation. This phenomenon was apparent within 15 min after addition of carbachol, and complete desensitization was observed after 75 min. The loss of muscarinic receptor-mediated effects on cyclic AMP levels was due to a loss of the Ca2+-dependent stimulation of phosphodiesterase activity by muscarinic receptor agonists. (c) A loss of muscarinic receptors as assessed by [3H]quinuclidinyl benzilate binding. This effect was apparent after 90 min in the presence of carbachol. More than 80% of the receptors were lost after 24 hr, with no change occurring in the KD of [3H]quinuclidinyl benzilate. The concentration-effect curve for carbachol-induced changes in agonist responsiveness of the cyclic AMP system was similar to that for carbachol-induced reductions in cyclic AMP levels. Coincubation of carbachol with a saturating concentration of atropine prevented these adaptive changes from occurring. Although incubation of cells in Ca2+-free buffer or in the presence of 20 mM Co2+ prevented the inhibitory effects of muscarinic receptor stimulation on cyclic AMP accumulation, carbachol preincubations under these conditions still produced the adaptive changes in agonist responsiveness. The divalent cation ionophore, A23187, mimics the effects of muscarinic receptor stimulation on cyclic AMP levels by activating phosphodiesterase. Following complete carbachol-induced loss of responsiveness to muscarinic receptor agonists, A23187 was still capable of inhibiting cyclic AMP accumulation.

Alprostadil↗
PubMed1983

Interaction of a radiolabeled agonist with cardiac muscarinic cholinergic receptors.

The interaction of a radiolabeled muscarinic cholinergic receptor agonist, [methyl-3H]oxotremorine acetate [( 3H]OXO), with a washed membrane preparation derived from rat heart, has been studied. In binding assays at 4 degrees C, the rate constants for association and dissociation of [3H]OXO were 2 X 10(7) M-1 min-1 and 5 X 10(-3) min-1, respectively, Saturation binding isotherms indicated that binding was to a single population of sites with a Kd of approximately 300 pM. The density of [3H]OXO binding sites (90-100 fmol/mg of protein) was approximately 75% of that determined for the radiolabeled receptor antagonist [3H]quinuclidinyl benzilate. Both muscarinic receptor agonists and antagonists inhibited the binding of [3H]OXO with high affinity and Hill slopes of approximately one. Guanine nucleotides completely inhibited the binding of [3H]OXO. This effect was on the maximum binding (Bmax) of [3H]OXO with no change occurring in the Kd; the order of potency for five nucleotides was guanosine 5'-O-(3-thio-triphosphate) greater than 5'-guanylylimidodiphosphate greater than GTP greater than or equal to guanosine/diphosphate greater than GMP. The [3H]OXO-induced interaction of muscarinic receptors with a guanine nucleotide binding protein was stable to solubilization. That is, membrane receptors that were prelabeled with [3H]OXO could be solubilized with digitonin, and the addition of guanine nucleotides to the soluble, [3H]OXO-labeled complex resulted in dissociation of [3H]OXO from the receptor. Pretreatment of membranes with relatively low concentrations of N-ethylmaleimide inhibited [3H]OXO binding by 85% with no change in the Kd of [3H]OXO, and with no effect on [3H]quinuclidinyl benzilate binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1979

GABA-mediated behavioral inhibition during ontogeny in the mouse.

Although immature rats and mice generally demonstrate poor behavioral inhibitory capacities, some recent evidence may indicate the presence of substantial inhibitory control. The present experiment investigated the possibility that gamma-aminobutyric acid (GABA) systems may mediate some behavioral inhibition during early development. Mice 9-100 days old were injected with the GABA-elevating agent amino-oxyacetic acid (AOAA) and tested for behavioral activity. High levels of locomotor activity characteristic of immature control mice were attenuated following AOAA injection, whereas AOAA had little effect on the activity of adult mice. Moreover, AOAA produced a period of rebound hyperactivity for young but not for adult mice. These findings suggest that although GABA systems may mediate early behavioral inhibition, coordination between excitatory and inhibitory capacities matures slowly. In a second experiment the dopamine-beta-hydroxylase inhibitor FLA-63 prevented rebound hyperactivity in young mice pretreated with AOAA, suggesting that the excitatory component may be mediated by noradrenergic systems.

Aminooxyacetic Acid↗
PubMed1977

(-)-Hydroxycitrate and conditioned aversions.

The capacity of various salts of (-)-hydroxycitrate to produce conditioned rejection of a 0.25% saccharin solution was evaluated. The ethylenediamine salt of (-)-hydroxycitrate produced strong conditioned rejection of saccharin under both deprivation and nondeprivation conditions, but this effect was less than produced by equimolar doses of lithium chloride. The sodium salt of hydroxycitrate produced no conditioned rejection of saccharin in water deprived rats but did so in nondeprived animals. In these experiments, food intake was reduced by (-)-hydroxycitrate only during the first hour following administration of the drug. The magnitude of appetite rejection did not correspond to the degree of conditioned rejection, lending support to the conclusion that the food intake reduction was not merely a consequence of aversive effects of the drug.

Animals↗
PubMed1976

Effects of alpha-MSH on motivation, vigilance and brain respiration.

In a series of experiments designed to assess the effects of alpha-MSH on various motivational processes, it was observed that the hormone can slightly decrease food intake and increase water consumption during the first hr after administration in rats. alpha-MSH also modified avoidance behavior in 1- and 3-day-old chicks, but there were no reliable effects on activity, distress vocalizations and the tonic immobility response. alpha-MSH appeared to modulate the sleep-waking activity of rats, and the most prominent effect was an increase of slow wave sleep during the 2-3 rd hr after treatment. A possible second effect was a homogenization of sleep patterns--with poor sleepers exhibiting increases of activated sleep and good sleepers a reduction. Measurement of in vitro brain oxygen consumption indicated that mice treated with alpha-MSH exhibit an 18% reduction in respiration of the brain stem section which includes the locus coeruleus, but did not reliably change respiration in forebrain cortices. alpha-MSH also produced a modest 14% increase of plasma glucose. These results are discussed in terms of possible modulation by alpha-MSH of activity in central autonomic cell groups such as the locus coeruleus.

Animals↗