[3H]N-Methylscopolamine binding to muscarinic receptors in intact adult rat brain cell aggregates

Effect of mazindol administration on (3H)-N-methylscopolamine binding to rat cerebral cortex

Mazindol has been shown to produce anorexia in several animal species including humans, and its pharmacological effects may be elicited by blockade of neuronal reuptake of dopamine and norepinephrine. Chronic treatment of rats with mazindol (10 mg/kg daily, p.o.) caused an increase in muscarinic receptor density (Bmax = 874.7 femtomoles/mg protein) as compared to controls (Bmax = 629.1 femtomoles/mg protein) in the cerebral motor cortex. An increase in dissociation constant (Kd) values was also observed which changes from 0.57 nM (control) to 1.17 nM after mazindol treatment. The mechanism underlying the observed effect of mazindol on receptors could be related to processes involving other neurotransmitters which also regulate cholinergic activity in the CNS. However, the implications of the eventual dopaminergic control on muscarinic receptors in the cortex is still an open question.

[3H]N-methylscopolamine binding to muscarinic receptors in human peripheral blood lymphocytes: characterization, localization on T-lymphocyte subsets and age-dependent changes

The properties of the binding of the muscarinic receptor ligands, [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]N-methylscopolamine ([3H]NMS) in human mononuclear cells were compared. The binding of [3H]QNB showed a high, non-specific component and lack of saturability in both intact mononuclear cells and preparations of lysed mononuclear cell membranes. Conversely the specific binding of [3H]NMS had a high affinity and was saturable at concentrations greater than 30 nM in both intact and broken cells. Classical muscarinic receptor antagonists displaced specific binding of [3H]NMS binding according to the law of mass action, while displacement curves for pirenzepine and muscarinic agonists were very shallow (nH less than 1), suggesting the presence of more than one subtype of muscarinic receptor on mononuclear cell membranes. Binding studies with [3H]NMS to purified mononuclear cell subpopulations demonstrated that muscarinic binding sites were mainly localized on thymus-derived (T) lymphocytes and large granule lymphocytes. Moreover evidence is presented of an age-dependent increase of the density of muscarinic binding sites on T-lymphocytes. The results are discussed in terms of the usefulness of the binding of [3H]NMS in studying the physiological function of muscarinic receptors on human T-lymphocytes and their possible changes in patients with neurological diseases.

Muscarinic receptors on cultured cells of rat hippocampus: cholinergic regulation and presence of subtypes

Muscarinic acetylcholine receptors in intact, cultured explants of rat hippocampus were investigated in binding experiments with tritiated quinuclidinyl benzilate ([3H]QNB) as ligand. Dissociation constants (Kd) were determined to 320-575 pM and maximal binding capacity (Bmax) to 67-87 fmol/explant. The KdS obtained in kinetic experiments were very similar. Hippocampal explants cultured alone contained more muscarinic receptors than hippocampal explants reinnervated by cholinergic fibers from co-cultured septal explants. Pretreatment of hippocampal explants with carbachol resulted in a down-regulation of receptor number which was counteracted by the simultaneous addition of atropine. Atropine added alone had no effect on receptor number in hippocampal explants cultured alone whereas it occasionally caused an up-regulation in co-cultured hippocampus. Displacement experiments with scopolamine and oxotremorine as competitors, showed that hippocampal explants cultured alone contain multiple types of muscarinic receptors. With atropine, pirenzepine and AF-DX 116, only one class of receptors could be detected.

Muscarinic M2 receptor-mediated cyclic AMP reduction in mechanically dissociated rat cortex

The coupling of the muscarinic receptor to the inhibition of the adenylate cyclase system was studied in adult rat cortical tissue dissociated by teasing tissue minces through finely meshed Nitex filters. The intracellular ATP stores in the final preparation were metabolically prelabeled with [3H]adenine and the [3H]cAMP formed in the tissue was isolated by ion exchange chromatography. Forskolin (3-30 microM) elevated [3H]cAMP levels 5- to 9-fold over basal in the preparation, with maximum stimulation achieved by 10-15 min. In the dissociated cortex, carbachol inhibited forskolin-elevated [3H]cAMP levels with an EC50 value of 1.4 microM; maximal inhibition was in the range of 20-30%. Atropine completely blocked the response (Ki = 1.8 nM), which showed that carbachol stimulates a muscarinic receptor to inhibit [3H]cAMP levels in this preparation. Pirenzepine, an M1-selective antagonist, blocked the response to carbachol with low potency (Ki = 467 nM), which indicated that an M2 muscarinic receptor subtype mediates [3H]cAMP inhibition in the cortex. The response to 10 microM carbachol was not affected by 10 mM EGTA, 50 microM D-tubocurarine, or 100 nM tetrodotoxin; thus, activation of nicotinic receptors or a neuronal release process was not involved. [3H]cAMP reduction in response to muscarinic stimulation was also observed in dissociated tissue prepared from other brain regions. A robust response was encountered in striatal preparations (maximal inhibition 40%), while hippocampal responses were smaller and less reproducible than in the cortex. The striatal response was shown to be pharmacologically similar to the cortical response.

Agonist-induced muscarinic acetylcholine receptor down-regulation in intact rat brain cells

Intact brain cells were prepared by dissociating whole adult rat brains without the cerebellum using a sieving technique. It has been found that preincubation of these cells with the muscarinic acetylcholine receptor agonist, carbamylcholine, results in a significant reduction in the specific binding of [3H]N-methylscopolamine to the receptors after the agonist was washed away. This agonist-mediated receptor down-regulation increased with prolongation of the exposure period to the agonist, and a steady state was achieved after 3 h at 37 degrees C. This effect of agonist was concentration-dependent, reaching a 30-35% decline in subsequent ligand binding upon preincubation with 1 mM carbamylcholine for 3 h. Carbamylcholine-induced receptor down-regulation was not apparent when exposure to the agonist was performed at 15 degrees C. In addition, it was abolished when the receptors were blocked by atropine. The decline in [3H]N-methylscopolamine binding induced by agonist was reflected as a significant reduction in the receptor density with no change in receptor affinity, suggesting that 'true' receptor down-regulation takes place. Moreover, when the receptors were labeled with the lipophilic antagonist [3H]quinuclidinyl benzilate instead of the hydrophilic ligand [3H]N-methylscopolamine, the magnitude of the observed receptor down-regulation was significantly lower in case of the former than the latter. This suggests that exposure of intact brain cells to muscarinic agonists might induce a slight degree of accumulation of receptors in intracellular sites before the receptors are actually degraded. These results are discussed in relation to previous findings regarding muscarinic receptor regulation in clonal cell lines.