Recognition of the muscarinic receptor by its endogenous neurotransmitter: binding of [3H]acetylcholine and its modulation by transition metal ions and guanine nucleotides

Opposing effects on muscarinic acetylcholine receptors in the piriform cortex of odor-trained rats

We combined pharmacological studies and electrophysiological recordings to investigate modifications in muscarinic acetylcholine (ACh) receptors (mAChR) in the rat olfactory (piriform) cortex, following odor-discrimination rule learning. Rats were trained to discriminate between positive and negative cues in pairs of odors, until they reached a phase of high capability to learn unfamiliar odors, using the same paradigm ("rule learning"). It has been reported that at 1-3 d after the acquisition of odor-discrimination rule learning, pyramidal neurons in the rat piriform cortex show enhanced excitability, due to a reduction in the spike-activated potassium current I(AHP), which is modulated by ACh. Further, ACh and its analog, carbachol (CCh), lost the ability to reduce the I(AHP) in neurons from trained rats. Here we show that the reduced sensitivity to CCh in the piriform cortex results from a decrease in the number of mAChRs, as well as a reduction in the affinity of the receptors to CCh. Also, it has been reported that 3-8 d after the acquisition of odor-discrimination rule learning, synaptic transmission in the piriform cortex is enhanced, and paired-pulse facilitation (PPF) in response to twin stimulations is reduced. Here, intracellular recordings from pyramidal neurons show that CCh increases PPF in the piriform cortex from odor-trained rats more than in control rats, suggesting enhanced effect of ACh in inhibiting presynaptic glutamate release after odor training.

Action of the muscarinic toxin MT7 on agonist-bound muscarinic M1 receptors

The muscarinic toxin MT7 is the most selective ligand for the muscarinic M(1) receptors. Previous studies have shown that the toxin interacts with the antagonist-receptor complex and slows the antagonist dissociation rate, possibly by binding to an allosteric site and impeding the access to and egress from the orthosteric binding pocket. In the present study, we investigated the action of MT7 on agonist-occupied receptors in functional and radioligand binding assays of the cloned human muscarinic M(1) receptor expressed in Chinese hamster ovary cells. In time-course experiments, the addition of MT7 rapidly blocked the acetylcholine-stimulated guanosine-5'-O-(3-[(35)S]thio)triphosphate binding to membrane G proteins. Similarly, in acetylcholine-treated cells MT7 completely stopped the agonist-stimulated [(3)H]inositol phosphate accumulation. In dissociation experiments using membranes pre-equilibrated with [(3)H]acetylcholine, the addition of MT7 increased the rate of radioligand dissociation. The data indicate that MT7, while partially stabilizing the antagonist-receptor complex, effectively destabilizes the agonist-occupied muscarinic M(1) receptors.

An [3H]oxotremorine binding method reveals regulatory changes by guanine nucleotides in cholinergic muscarinic receptors of cerebral cortex

A rapid, reliable filtration method for [3H]oxotremorine binding to membranes of the cerebral cortex that allows the direct study of regulation by guanine nucleotides of muscarinic receptors was developed. [3H]Oxotremorine binds to cerebral cortex membranes with high affinity (KD, 1.9 nM) and low capacity (Bmax, 187 pmol/g protein). These sites, which represent only about 18% of those labeled with [3H]quinuclidinyl benzilate, constitute a population of GTP-sensitive binding sites. Association and dissociation binding experiments revealed a similar value of KD (2.3 nM). Displacement studies with 1-4000 nM oxotremorine showed the existence of a second binding site of low affinity (KD, 1.2 microM) and large capacity (Bmax, 1904 pmol/g protein). Gpp(NH)p, added in vitro, produced a striking inhibition of [3H]oxotremorine binding with an IC 50 of 0.3 microM. Saturation assays, in the presence of 0.5 microM Gpp(NH)p, revealed a non-competitive inhibition of the binding with little change in affinity. These results are discussed from the viewpoint of conflicting reports in the literature about guanine nucleotide regulation of muscarinic receptors in reconstituted systems and membranes from different tissues.

Lithium inhibits adrenergic and cholinergic increases in GTP binding in rat cortex

Lithium is a unique drug with therapeutic as well as prophylactic value for both manic and depressive phases of manic-depressive illness. The precise mechanisms of its clinical efficacy remain unknown, but there are two main theories of its biochemical action. One proposes that lithium inhibits adrenergically activated adenylate cyclase function whereas the other suggests that it inhibits phosphatidyl inositol turnover, which is known to be activated by cholinergic agonists. Neither mechanism alone, however, can explain both the antimanic and antidepressant effects of lithium. Because of the pivotal role of G proteins in post-receptor information transduction, we have investigated the interaction of lithium with G protein function. Lithium at therapeutically efficacious concentrations completely blocked both adrenergic and cholinergic agonist-induced increases in [3H]GTP binding to membranes from rat cerebral cortex, in both in vitro and ex vivo experiments. The same lithium treatments also abolished guanine nucleotide modulation of agonist binding. Our findings suggest G proteins (Gs and Gi or Go) as the molecular site of action for both the antimanic and antidepressant effects of lithium.

Binding studies with [3H]cis-methyldioxolane in different tissues. Under certain conditions [3H]cis-methyldioxolane labels preferentially but not exclusively agonist high affinity states of muscarinic M2 receptors

Special conditions--tricine buffer containing Ca2+ and Mg2+, 22 degrees C (TCM)--allow to label a much higher proportion of muscarinic receptors by [3H]cis-methyldioxolane (CD) than hitherto described (Vickroy et al. 1984a). Taking the maximum number of binding sites, Bmax, of [3H]QNB as 100%, Bmax of [3H]CD amounts to 83% in the rat heart instead of the reported 17%, 33% in the cerebral cortex instead of 6%, 20% in hippocampus and 55% in pons/medulla. In the salivary glands specific binding was negligible. The affinities of a number of muscarinic agonists and antagonists to [3H]CD and [3H]QNB binding sites in different tissues of the rat are compared. Apparent affinities of agonists are much higher in the [3H]CD system, affinities of antagonists are slightly higher in the [3H]QNB system. In both assay systems receptors of heart and pons/medulla membranes seem to have similar drug specificity. They differ somewhat from those in the cortex. Receptors in the salivary glands, however, seem to be completely different from those in the other three tissues. In the heart [3H]CD binding can be abolished almost completely by GppNHp. In the cortex about half of the [3H]CD binding is susceptible to GppNHp. The reduction of binding in the cortex is due to a change in Bmax and not in the dissociation constant KD. Competition of unlabelled pirenzepine with [3H]CD: In heart and pons/medulla only low affinity sites for pirenzepine (M2-receptors) are labelled by [3H]CD.(ABSTRACT TRUNCATED AT 250 WORDS)

Batrachotoxin changes the properties of the muscarinic receptor in rat brain and heart: possible interaction(s) between muscarinic receptors and sodium channels

The effects of Na+-channel activator batrachotoxin (BTX) on the binding properties of muscarinic receptors in homogenates of rat brain and heart were studied. BTX enhanced the affinity for the binding of the agonists carbamoylcholine and acetylcholine to the muscarinic receptors in brainstem and ventricle, but not in the cerebral cortex. Analysis of the data according to a two-site model for agonist binding indicated that the effect of BTX was to increase the affinity of the agonists to the high-affinity site. Guanyl nucleotides, known to induce interconversion of high-affinity agonist binding sites to the low-affinity state, canceled the effect of BTX on carbamoylcholine and acetylcholine binding. BTX had no effect on the binding of the agonist oxotremorine or on the binding of the antagonist [3H]-N-methyl-4-piperidyl benzilate. The local anesthetics dibucaine and tetracaine antagonized the effect of BTX on the binding of muscarinic agonists at concentrations known to inhibit the activation of Na+ channels by BTX. On the basis of these findings, we propose that in specific tissues the muscarinic receptors may interact with the BTX binding site (Na+ channels).