Influence of veratridine on [3H]-L-quinuclidinyl benzilate ([3H]QNB) binding in mouse hindbrain

Binding characteristics of the muscarinic receptor subtype in rabbit pancreas

The muscarinic receptor in the rabbit pancreas was characterized with the use of the labeled ligand (3H)-(-)-quinuclidinyl-benzylate ((3H)-(-)-QNB). Specific binding of (3H)-(-)-QNB to pancreatic acini was found to be reversible and of high affinity, with an equilibrium dissociation constant (KD) of 68 pmol/l and a receptor density (RT) of 170 fmol/mg protein. Agonist binding behaviour was investigated by displacement of (3H)-(-)-QNB binding by eight agonists like arecoline, arecaïdine-propargylester (APE) and carbachol, yielding only low affinity binding sites. The inhibition of (3H)-(-)-QNB binding by the selective antagonists pirenzepine, hexahydrosiladifenidol (HHSiD) and (11-[2-[diethyl-amino)-methyl)-1-piperidinyl]acetyl)-5,11-dihydro-6H-pyr ido (2,3-b) (1,4) benzodiazepin-6-one) (AF-DX 116) confirmed the M3 nature of the rabbit pancreatic receptor.

Muscarinic receptor activation attenuates D2 dopamine receptor mediated inhibition of acetylcholine release in rat striatum: indications for a common signal transduction pathway

In the present investigations, we used a superfusion system to study the effect of simultaneous activation of D2 dopamine receptors and so-called muscarinic "autoreceptors" on the K(+)-evoked in vitro release of [3H]acetylcholine from rat striatal tissue slices. Activation of D2 receptors with the selective agonist LY 171555 (0.01-1 microM) clearly decreased the evoked release of [3H]acetylcholine. This effect was markedly attenuated in the presence of either the selective muscarinic receptor agonist oxotremorine (3 microM) or the cholinesterase inhibitor physostigmine (1 microM). Conversely, D2 receptor activation with LY 171555 (1 microM) completely abolished the muscarinic receptor mediated inhibition of evoked [3H]acetylcholine release induced by oxotremorine (0.03-10 microM). These results show that the inhibitory effects of D2 dopamine receptor and muscarinic receptor activation on striatal acetylcholine release are non-additive and therefore are interdependent processes. In addition, we investigated some aspects of the signal transduction mechanism by which the muscarinic receptor mediates inhibition of K(+)-evoked in vitro release of [3H]acetylcholine from rat striatal tissue slices. It appeared that the effect of muscarinic receptor activation was not significantly influenced either by a lowering of the extracellular Ca2+ concentration from the usual 1.2-0.12 mM or by an increase of the intracellular cyclic adenosine-3',5'-monophosphate content. However, increasing extracellular K+ strongly decreased the inhibition of evoked [3H]acetylcholine release mediated by activation of muscarinic receptors. This set of results indicates that the muscarinic "autoreceptor" mediates the decrease of depolarization induced [3H]acetylcholine release from rat striatum to a large extent through stimulation of K+ efflux (opening of K+ channels) in a cyclic adenosine-3',5'-monophosphate independent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement by potassium of carbachol-stimulated inositol phospholipid breakdown in rat cerebral cortical miniprisms: comparison with other depolarising agents

Increasing the [K+] in the assay medium from 5.7 to 17.8 mM produces a large enhancement of the inositol phospholipid breakdown response to the muscarinic agonist carbachol in rat cerebral cortical miniprisms, with minor effects on basal inositol phospholipid breakdown. This effect is also found with Rb+. The enhancement by a raised [K+] is not accompanied by a change in the composition of the labelled polyphosphoinositides. The carbachol-stimulated inositol phospholipid breakdown at 17.8 and 42.7 mM K+ was antagonised by veratrine (5-80 microM), 4-aminopyridine (5 mM), and tetraethylammonium (20 mM). These compounds, however, also inhibited the binding of [3H]quinuclidinyl benzilate to cortical membranes. BRL 34915 (0.2-20 microM) was without significant effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+.Mg2+ (10 mM) considerably reduced the carbachol-stimulated inositol phospholipid breakdown at 17.8, but not 42.7, mM K+. Inositol phospholipid breakdown was also stimulated, albeit to a small extent, by L-glutamate (100-3,000 microM) and quisqualate (1-100 microM), with the stimulation being additive to that produced by carbachol at both 5.7 and 17.8 mM K+. N-Methyl-D-aspartate (10-1,000 microM in Mg2+-free medium) had no significant effect on basal inositol phospholipid breakdown and had little or no effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+. It is concluded that it may not be correct to ascribe wholly the enhancement by K+ of carbachol-stimulated inositol phospholipid breakdown to the tissue-depolarising actions of this ion and that other actions of K+ may be involved.

A role for potassium channels in the regulation of cortical muscarinic acetylcholine receptors in an in vitro slice preparation

The rules underlying muscarinic acetylcholine receptor (mAChR) regulation in an in vitro cortical slice preparation of adult rats were examined following various alterations of bioelectric activity and following agonist stimulation. Muscarinic ACh antagonists [3H]N-methyl scopolamine ([3H]NMS) or [3H]quinuclidinyl benzylate ([3H]QNB) were used to label cell surface vs total (i.e. surface and internal) receptors, respectively. Depolarization of neural membranes for 4 h at 22-37 degrees C using veratridine or high external potassium (K+) led to a temperature-dependent down-regulation of surface mAChR of 26.2% and 11.3%. Total mAChRs decreased by 37.6% and 8.1%. Addition of picrotoxin and glutamic acid also led to decreases in mAChRs. Increases in inward chloride ion current induced by gamma-aminobutyric acid (GABA) or gold chloride had no significant effect on mAChRs. Blockade of calcium channels and synaptic transmission by magnesium or cobalt and postsynaptic calcium channels with nifedipine showed a significant effect on mAChRs only in the latter case. In contrast, agonist stimulation using carbachol led to a large down-regulation for both [3H]NMS and [3H]QNB (26.1%, 35.9%). ACh decreased [3H]QNB binding by 33.9%, but had little effect on [3H]NMS binding (6.3%). For [3H]QNB binding sites the effects of carbachol appeared to summate with those of veratridine. Down-regulation of [3H]NMS labelled mAChRs by carbachol and veratridine had an estimated half-time of 30 min and 2 h, respectively. Neither the effects of veratridine nor carbachol could be antagonized by tetrodotoxin (TTX), showing that the effects were not due to an increase in sodium ion currents. However, a common thread linking the various agents which induce mAChR down-regulation appears to involve changes in potassium (K+) current. Potassium channel blockers tetraethylammonium chloride (TEA), 4-aminopyridine (4-AP) and apamin had little independent effect on mAChR number, but prevented veratridine-induced down-regulation, presumably through a blockade of K+- and Ca2+-dependent K+-channels. Only TEA and 4-AP diminished carbachol-induced down-regulation suggesting that this effect involves only the non Ca2+-dependent K+-channels. It thus appears that mAChR regulation in the rat cerebral cortex is linked to changes in active K+-channel currents: activation of the K+-channel by depolarization-induced changes in K+ current or by agonist stimulation leading to changes in the selective K+ currents stimulate mAChR down-regulation; blockage of the K+-channels prevents this down-regulation.

Sodium channel toxins veratrine and veratridine modify opioid and muscarinic but not beta-adrenergic binding sites in brain slices

We have examined the influence of the sodium channel toxins veratrine and veratridine on mu-opioid ([3H]-DAGO), muscarinic ([3H] NMS) and beta-adrenergic ([3H] CGP) receptors in rat brain slices. These drugs reduce opioid and muscarinic binding while leaving beta-receptors unaffected. Veratrine is inhibitory at 0 degree or at 30 degrees C whereas veratridine is without effect at 0 degree C. These data suggest that some factor contained in the mixture of drugs (veratrine) can block opioid and muscarinic receptors independently of depolarization. Veratridine does not affect muscarinic receptors at ice temperature. Similar observations were made in thin sections of cat brain at 0 degree C. The concentrations of the toxins which cause 50% inhibition of binding are well within the range (5 x 10(-5) M-10(-4) M) routinely used for depolarization experiments. We suggest that caution be used in the interpretation of results obtained from veratrum alkaloid-induced depolarizations. It would not be surprising if the binding of other ligands to their receptors was also affected.