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L T Potter

Publications and source records attributed to L T Potter.

At least 19 recordsLinked to original sources
PubMed1991

Coupling of m1 muscarinic receptors to G protein in Alzheimer disease.

The potential usefulness of cholinergic replacement therapy for Alzheimer disease (AD) is dependent upon retention of postsynaptic muscarinic receptors and their transduction mechanisms long after the degeneration of cholinergic nerves. The receptors most clearly associated with cholinergic cerebral excitation are m1 muscarinic receptors, which work via a G protein to activate phospholipase C. The ability of these receptors to couple to their associated G protein was assessed in the middle temporal gyrus of postmortem brains from persons with and without AD. A low concentration of [3H]-pirenzepine (1 nM) was used to label m1 receptors preferentially. The affinity of the agonist, oxotremorine-M, for labeled receptors and the ability of these receptors to couple with G protein were assessed by competition between the agonist and 1 nM [3H]-pirenzepine in the presence and absence of guanine nucleotide. Brain tissue from 7 patients with AD and five age-matched controls showed very similar levels of labeled receptors, agonist affinities for the high- and low-affinity states of m1 receptors, and guanine nucleotide-sensitive high-affinity binding. It is concluded that the coupling of m1 receptors to G protein is adequate to permit responses to exogenous muscarinic drugs in AD.

Aged↗
PubMed1991

Evidence of paired M2 muscarinic receptors.

Binding assays involving various antagonists, including N-[3H] methylscopolamine, [3H]quinuclidinyl benzilate, AFDX-116, pirenzepine, and propylbenzilylcholine mustard, disclosed only a single population of M2 muscarinic receptors in membranes from the rat "brainstem" (medulla, pons, and colliculi). However, competition curves between N-[3H]methylscopolamine and various agonists, including oxotremorine, cis-dioxolane, and acetylethylcholine mustard, showed approximately equal numbers of guanine nucleotide-sensitive high affinity (H) sites and guanine nucleotide-insensitive low affinity (L) sites. This 50% H phenomenon persisted in different buffers, at different temperatures, after the number of receptors was halved (and, thus, the remaining receptor to guanine nucleotide-binding protein ratio was doubled), after membrane solubilization with digitonin, and when rabbit cardiac membranes were used instead of rat brainstem membranes. Preferential occupation of H sites with acetylethylcholine mustard, and of L sites with quinuclidinyl benzilate or either mustard, yielded residual free receptor populations showing predominantly L and H sites, respectively. Low concentrations of [3H]-oxotremorine-M labeled only H sites, and the Bmax for these sites was 49% of the Bmax found with [3H]quinuclidinyl benzilate plus guanine nucleotide. These and other results are most consistent with the idea that H and L receptor sites exist on separate but dimeric receptor molecules and with the hypothesis that only the H receptors cycle between high and low affinity, depending upon interactions between this receptor molecule and a guanine nucleotide-binding protein.

Animals↗
PubMed1990

Effects of benzoyltropine and tropacocaine on several cholinergic processes in the rat brain.

Benzoyltropine and tropacocaine are two contaminants of street-cocaine reported to have parasympatholytic activity. Because the mechanism underlying this activity is obscure, we investigated the effects of these compounds on several cholinergic processes: sodium-dependent choline uptake, sodium-independent choline uptake, acetylcholine synthesis, acetylcholine release (spontaneous and veratridine-induced) and binding of [3H]quinuclidinyl benzylate to muscarinic receptors. These studies used rat cerebral cortical synaptosomes, except for the receptor-binding studies, which used whole brain plasma membranes. Benzoyltropine and tropacocaine each inhibited sodium-dependent choline uptake and acetylcholine synthesis in a dose-related manner that was competitive with extracellular choline. Benzoyltropine was 4 to 5 times more potent in both actions than tropacocaine. Sodium-independent choline uptake was not affected by either compound. Benzoyltropine (30 microM) had no effect on the sodium-dependent uptake of norepinephrine, gamma-amino-butyric acid, glutamate or serotonin; tropacocaine (30 microM) inhibited only norepinephrine uptake at this concentration. Benzoyltropine and tropacocaine each inhibited the spontaneous and veratridine-induced release of newly synthesized acetylcholine, but not via activation of presynaptic muscarinic receptors. Instead, each compound was able to attenuate the oxotremorine-induced inhibition of the release of acetylcholine, suggesting antimuscarinic activity. Binding experiments showed that benzoyltropine and tropacocaine were, respectively, about 1,000- and 10,000-fold less potent than scopolamine as receptor antagonists. Finally, we demonstrated that benzoyltropine accumulates in the rat brain after its peripheral injection (10 mg/kg i.p.) and remains there with a half-life similar to that of cocaine.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed1989

Affinities of different cholinergic agonists for the high and low affinity states of hippocampal M1 muscarine receptors.

The ability of 16 well-known cholinergic agonists to compete with 1 nM [3H]pirenzepine for the high (KH) and low (KL) affinity states of M1 muscarine receptors was studied, using rabbit hippocampal membranes suspended in 20 mM Tris buffer containing 1 mM MnCl2, at pH 7.4 and 25 degrees C. The hippocampus, like the cerebral cortex, has primarily m1 and m3 subtypes of M1 receptors; these receptors have very similar affinities for pirenzepine and for agonists, and both receptors are coupled to the activation of phospholipase C. KH values varied more than 10,000-fold, and did not correlate with prior measurements of the ability of agonists to promote cerebral excitation or phosphoinositide turnover in cortical tissue. KL values varied more than 1,000-fold, and also did not correlate with agonist activities. In contrast, KL/KH ratios for individual agonists varied from 66 for cis-dioxolane to near 1 for oxotremorine, and correlated closely with data concerning the relative physiological and biochemical effectiveness of the agonists. Each agonist showed only a single affinity in 0.2 mM guanyl-5'-yl imidodiphosphate. Thus binding measurements of KL/KH appear to be a new way to screen cholinergic agonists for relative efficacy at m1 or m3 receptors coupled to their native G proteins. Whereas both quaternary and tertiary agonists are known to activate M2 receptors, only the quaternary agents tested were highly effective M1 agonists. The five agonists which have been tested for improving human memory all appear to have low efficacy at M1 receptors.

Animals↗
PubMed1989

Tetrahydroaminoacridine and other allosteric antagonists of hippocampal M1 muscarine receptors.

Tetrahydroaminoacridine (THA) and a variety of other nonclassical antagonists of muscarine receptors were studied for their ability to bind to primary and allosteric sites on muscarine receptors in rabbit hippocampal membranes. Competition curves between 13 antagonists and 1 nM [3H]pirenzepine (Kd = 3 nM) were simple mass action curves, but THA produced steeper curves, indicating positive cooperativity. Nonetheless, THA inhibited the binding of low concentrations of [3H]pirenzepine, [3H]N-methylscopolamine, and [3H]oxotremorine-M to M1 receptors with similar IC50 values, indicating competition for primary sites. Antagonists were also compared for their ability to bind to allosteric sites and to slow the dissociation of [3H]pirenzepine from primary sites. THA was 6-8-fold more potent than verapamil, d-tubocurare, quinidine, and secoverine, the next most effective allosteric agents, and THA was more effective. McN-A-343, gallamine, pancuronium, and pirenzepine showed weaker allosteric effects. The large size and considerable rigidity of these compounds suggest large allosteric sites. The Hill coefficient for the allosteric effects of THA was 1.7, indicating more than one allosteric site. Solubilization of receptors did not alter steep inhibition curves between THA and [3H]quinuclidinyl benzilate or THA-induced slowing of the dissociation of this ligand. Hence, cooperative allosteric effects of THA are probably exerted on receptor monomers. Inhibition curves between THA and [3H]oxotremorine-M were not steep, and THA had no (allosteric) effect on the dissociation of this ligand from M1 or M2 receptors. Thus, the high affinity agonist conformation of muscarine receptors, once formed, may not bind THA readily. The present results indicate that compounds that can act allosterically may compete with acetylcholine for primary receptor sites but that allosteric effects of these drugs on muscarine receptors are not likely to be important clinically.

Allosteric Regulation↗
PubMed1988

Effects of tetrahydroaminoacridine on M1 and M2 muscarine receptors.

Tetrahydroaminoacridine (THA) has been reported to improve the memory of persons with Alzheimer's disease, but its mechanism of action is uncertain. We found that clinically effective concentrations, 0.03-0.3 microM, readily inhibit acetylcholinesterase and butyrylcholinesterase from rabbit hippocampal tissue in artificial cerebrospinal fluid (CSF) at 37 degrees C with physiological levels of substrate Above 1 microM, THA was found to act at primary and allosteric sites on M1 and M2 muscarine receptors as an antagonist. This is not clinically important, and low levels of THA do not improve the binding of the agonist, oxotremorine-M. Only 10-1000 microM THA has been shown to block K+ channels. Thus THA probably acts as an esterase inhibitor.

Acetylcholinesterase↗
PubMed1988

Two affinity states of M1 muscarine receptors.

1. The binding of oxotremorine-M to M1 muscarine receptors was examined by measuring competition between the agonist and 3H-pirenzepine, using rabbit hippocampal membranes suspended in 20 mM Tris buffer containing 1 mM Mn2+. 2. Both ligands interacted with a single class of receptors. The receptors could assume two affinity states for oxotremorine-M, with equal numbers of high-affinity (KH) and low-affinity (KL) sites. 3. KH interconverted reversibly to KL in the absence of divalent cations and interconverted reversibly to a state similar to KL in the presence of guanyl 5'-yl imidodiphosphate. 4. The results are compatible with a model in which a pair of receptor molecules can be stabilized by a guanine nucleotide-binding "G protein" and have one site each of KH and KL affinity.

Animals↗
PubMed1987

Dopamine D2 receptors in the striatum and frontal cortex following chronic administration of haloperidol.

The effects of chronic treatment with a neuroleptic on D2 dopamine receptors in the striatum and frontal cortex were studied. Exposure to haloperidol for 21 days caused an upregulation in the striatum but not in the cortex of D2 receptors. These results indicate that dopamine-regulating mechanisms in the cortex may differ from those in the striatum and suggest that the anti-psychotic action of neuroleptics may be due in part to blockade of receptors in the cortex.

Animals↗
PubMed1986

Autoradiographic localization of M1 and M2 muscarine receptors in the rat brain.

The distribution of M1 and M2 muscarine receptors in the rat brain was investigated by in vitro autoradiography. Muscarine receptors were visualized after complete receptor uncoupling in ethylenediaminetetraacetic acid buffer containing 1 mM N-ethyl maleimide and saturation with the ligand [3H]quinuclidinyl benzilate. Pirenzepine, an M1-selective antagonist, was used in our assays as a counter ligand to occlude M1 sites, allowing the primary ligand, [3H]quinuclidinyl benzilate, to label the remaining M2 muscarine receptors. In adjacent section, M1 muscarine receptors were labelled with [3H]quinuclidinyl benzilate in the presence of sufficient carbachol, and M2-selective agonist, to inhibit the binding to M2 sites. Our results reveal a heterogeneous distribution of M1 and M2 receptors. Increased densities of carbachol-resistant and pirenzepine-sensitive sites (M1 receptor subtype) were apparent over many forebrain structures including the olfactory tubercle, caudate-putamen, nucleus accumbens, hippocampus, amygdala and cerebral cortex. In contrast, pirenzepine-resistant and carbachol-sensitive sites (M2 receptor subtype) were distributed throughout the brain with increased densities apparent over regions known to contain large numbers of cholinergic cell bodies. M2 receptor localization patterns were largely coincident with the regional distribution and intensity of acetylcholinesterase positive sites. Since the M2 receptor pattern appears to parallel regional innervation densities, we conclude that the M2 receptor may serve as a marker for cholinergic pathways. The findings also suggest that M1 muscarine receptors are involved in the presumptive postsynaptic actions of acetylcholine in many forebrain structures.

Acetylcholine↗
PubMed1986

Effect of solubilization on the distinct binding properties of muscarine receptors from rabbit hippocampus and brainstem.

The binding of carbachol, quinuclidinyl benzilate, pirenzepine, and scopolamine to muscarine receptors from the rabbit hippocampus and brainstem was examined in membranes and in digitonin solution, in order to determine whether the dispersion of receptor molecules altered the distinct ligand-binding profiles of the receptors of these tissues. The modification of receptor-effector complexes showing high affinity for carbachol in membranes, with guanyl 5'-yl imidodiphosphate (GppNHp), N-ethyl-maleimide (NEM), and ethylenediaminetetraacetate (EDTA), did not remove receptor heterogeneity. In addition to receptor heterogeneity, GppNHp-sensitive high affinity agonist binding to membranes was NEM sensitive in the brainstem but NEM insensitive in the hippocampus. Solubilization with digitonin in the presence of Mn2+ ions yielded GppNHp-sensitive, high affinity agonist-binding complexes from the brainstem but not the hippocampus. More extensive dissociation was achieved with solubilization in EDTA and NEM, and resulted in receptor populations which showed markedly lower and slightly different affinities for carbachol. However, the selectivities and affinities for three antagonists were little changed in solution from those found in membranes. These results support the view that there are differences in muscarine receptors as well as different receptor complexes with guanine nucleotide-binding regulatory proteins. The selective binding of carbachol appears largely dependent upon the association of receptors with other molecules, including effector molecules, whereas antagonists appear to recognize receptors irrespective of associated molecules.

Animals↗
PubMed1985

Muscarine-binding sites localized to cortical dopamine terminals.

The effects of lesions to the mesocortical dopaminergic system on D2 dopamine receptors and muscarine receptors in the frontal cortex of the rat was examined. Four weeks following 6-hydroxydopamine lesioning of the ventral tegmental area, there was a 26% increase in the number of [3H]spiroperidol sites, and a 13% decrease in the number of [3H]oxotremorine-M sites in the frontal cortex, indicating a development of D2 receptor supersensitivity, as a result of deafferentation, and a loss of acetylcholine sites, as result of terminal degeneration. This demonstrates that in the frontal cortex, as in the striatum and nucleus accumbens, the activity of dopaminergic terminals may be partially modulated by cholinergic inputs.

Afferent Pathways↗
PubMed1985

Loss of M2 muscarine receptors in the cerebral cortex in Alzheimer's disease and experimental cholinergic denervation.

Cerebral cortex samples from patients with Alzheimer's disease and from rats after experimental cholinergic denervation of the cerebral cortex exhibited reductions in the presynaptic marker choline acetyltransferase activity and in the number of M2 muscarine receptors, with no change in the number of M1 receptors. These results are in keeping with evidence that M2 receptors function in cholinergic nerve terminals to regulate the release of acetylcholine, whereas M1 receptors are located on postsynaptic cells and facilitate cellular excitation. New M1-selective agonists and M2-selective antagonists directed at post- or presynaptic sites deserve consideration as potential agents for the treatment of the disease.

Aged↗
PubMed1985

D-2 dopamine receptors in the frontal cortex of rat and human.

D-2 dopamine receptors and serotonin receptors in the frontal cortex of rat and human were labelled with 3H-spiroperidol. The D-2 receptors were then distinguished in 4 ways. Dissociation of spiroperidol was biphasic, indicating two populations of sites. Cinanserin in competition with 3H-spiroperidol exhibited high (75%) and low (25%) affinity sites. Dopamine and LY 141865 in competition with 1.25 nM 3H-spiroperidol exhibited high (20-25%) and low (80-75%) affinity sites in the absence of cinanserin, while in the presence of 300 nM cinanserin only the high affinity sites remained. Lesioning of the dopaminergic meso-cortical pathway increased the number of cinanserin-resistant sites by 26%. Thus 3H-spiroperidol binding in the presence of cinanserin can be used to selectively label D-2 receptors in the frontal cortex.

Animals↗
PubMed1985

A pre-positron emission tomography study of L-3,4-dihydroxy-[3H]phenylalanine distribution in the rat.

The distribution of L-3,4-dihydroxy-[3H]phenylalanine (L-[3H]DOPA) was examined in rats following i.v. injection, to ascertain the possible usefulness of using this ligand to image dopaminergic systems using positron emission tomography. It was found that L-DOPA and its metabolites were preferentially localized in the basal ganglia as compared to other brain regions, and that this preferential localization could be abolished by lesioning of the nigro-striatal tract. The parameters of the L-DOPA uptake and the sensitivity of this uptake to alterations in dopaminergic pathways indicate that this ligand may be useful in visualizing aberrations in dopaminergic pathways in various pathological conditions.

Animals↗
PubMed1985

Alpha-adrenoreceptors and muscarine receptors in human pial arteries and microvessels: a receptor binding study.

Human pial arteries and intraparenchymal microvessels were isolated for enzyme assays and radioligand binding studies of receptors. Special attention was paid to contamination with brain tissue, which was assessed by luxol staining and cerebroside assays for myelin and by scanning electron microscopy. The amount of contamination was approximately 1% for pial vessels and 14% for microvessel preparations. Significant levels of alpha 1-adrenoreceptors (binding sites for [3H]prazosin) and alpha 2-adrenoreceptors (sites labeled by [3H]azidoclonidine) were found in both types of vessels, suggesting that each receptor can modify contractility in these human vessels. Levels of muscarine receptors (sites labeled with [3H]quinuclidinyl benzilate) and choline acetyltransferase activity were considered significant only in pial vessels.

Adult↗
PubMed1985

Different effects of N-ethylmaleimide on M1 and M2 muscarine receptors in rat brain.

N-Ethylmaleimide (MalNEt) disclosed three differences between M2 muscarine receptors in membranes from the rat brainstem and M1 receptors in the hippocampus. At 0.1 mM, MalNEt completely interconverted the higher affinity state of M2 receptors for carbachol to a lower affinity state, while having no effect on the two affinity states of M1. This "uncoupling" effect is similar to that produced by guanine nucleotides and appears to be due to separation of an agonist-receptor complex from a guanine nucleotide-binding protein. Higher MalNEt concentrations (1-5 mM) increased the affinity of uncoupled M2 receptors, again without effect on M1 states. Finally, in MalNEt, the affinity of M2 receptors for carbachol was different from values for M1 receptors. Thus, MalNEt is an excellent agent for distinguishing M1 and M2 receptors and the two states of M2 receptors. MalNEt had no effect on the affinity or M1-selectivity of the antagonist pirenzepine.

Animals↗