Muscarinic receptors on intact, cultured neurons. Characterization by [3H]quinuclidinylbenzilate binding

Cultured neurons from mouse brain reproduce the muscarinic receptor profile of their tissue of origin

These studies investigate the regional variations in the muscarinic acetylcholine receptor (mAChR) profiles in neuron populations of the CNS using primary neuron cultures derived from three areas of the mouse brain--the cerebral hemispheres, the mesencephalon and the medulla-pons--that have distinct mAChR systems. We first assessed the extent to which neurons reproduced their in vivo properties in culture by monitoring the binding capacity, the pharmacological profiles and the levels of mAChR transcripts in neuron cultures and their tissues of origin. We showed that the primary neuron cultures accumulated mAChRs with initial rates similar to those in vivo, had pharmacological profiles very close to those of their area of origin, and accumulated m1, m2, m3, m4 and m5 receptor transcripts according to patterns resembling those in the tissues. We conclude that most of the characteristics of the mAChRs in a given area are proper to the neuron population of that area, that the pattern is established early in ontogenesis, and that it is reproduced in vitro. We also show that the stimulation of phosphoinositide turnover is mediated by mAChRs with distinct pharmacological profiles in neuron cultures from the three brain areas.

Stimulation of phosphatidylinositol hydrolysis by brain-derived neurotrophic factor and neurotrophin-3 in rat cerebral cortical neurons developing in culture

Phosphatidylinositol (PI) breakdown represents a powerful system participating in the transduction mechanism of some neurotransmitters and growth factors and producing two second messengers, diacylglycerol and inositol trisphosphate. The transformation of PC12 neuroblastoma cells into neuron-like cells induced by nerve growth factor (NGF) is preceded by a rapid stimulation of PI breakdown; however, it was not known whether PI breakdown mediates actions of other members of the neurotrophin family. The present study analyzed the effects of NGF, brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) on PI breakdown in primary cultures of embryonic rat brain cells. Cultures were grown for 7 days; PI was then labeled by incubating cultures with myo-[3H]inositol, which then were exposed acutely to growth factors. BDNF and NT-3, but not NGF, elevated the levels of labeled inositol phosphates within 10-15 min after addition to the cultures in a dose-dependent manner. ED50 values for BDNF and NT-3 were 12.4 and 64.5 ng/ml, respectively. Comparable effects were found in cultures of cortical, striatal, and septal cells. The actions of BDNF and NT-3 probably reflect actions on neurons, because no effects were seen in cultures of nonneuronal cells. In contrast, basic fibroblast growth factor induced a marked stimulation of PI breakdown in cultures of nonneuronal cells. K252b, which selectively blocks neurotrophin actions by inhibiting trk-type receptor proteins, prevented the PI breakdown mediated by BDNF and NT-3. The findings suggest that rapid and specific induction of PI breakdown is involved in the signal transduction of BDNF and NT-3, and they provide evidence that cortical neurons are functionally responsive to BDNF and NT-3 during development.

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 acetylcholine receptor: thermodynamic analysis of the interaction of agonists and antagonists

The thermodynamic parameters of the interaction of agonists and antagonists with heart and brain muscarinic receptors were determined. The binding of quinuclidinyl [3H]benzilate and the inhibition of quinuclidinyl benzilate (QNB) binding by agonists and antagonists were examined at temperatures between 2 degrees C and 27 degrees C. The density of specific binding sites and the relative proportions of high- and low-affinity binding components of drugs were unaffected by the temperature changes. The binding of atropine was entropy driven in brain and heart membranes. In contrast, net values of these thermodynamic parameters for QNB binding and for the high-affinity binding component of pirenzepine to brain membranes were decreased with the enhancement of the temperature. The low-affinity binding component of the agonists carbachol, oxotremorine and pilocarpine was enthalpy driven. Their high-affinity binding component was entropy driven at 2 degrees C and became enthalpy driven when the incubation temperature was increased. The guanine nucleotide Gpp[NH]p partly prevented the temperature-dependent decrease of net entropy and enthalpy values. Considering that the net changes of thermodynamic parameters are relevant of the interactions between the ligand, the receptor protein and the adjoining membranous molecules, a three-state conformational model is proposed for the muscarinic receptor protein. The receptor selectivity is reappreciated owing to these three states of the receptor protein and the different components of the muscarinic receptor complexes.

Muscarinic receptors in slice cultures of rat brain

Muscarinic acetylcholine receptors in organotypic slice cultures of hippocampus of the rat, have been examined using the tritiated muscarinic antagonist quinuclidinylbenzilate [( 3H]QNB) as a as a marker. Maximum specific binding of [3H]QNB in mature explants of hippocampus amounted to 316 fmol/mg protein and a dissociation constant (KD) of 185 pM was determined. Scatchard analysis suggested binding to one single binding site. In younger cultures smaller KDs were registered. This decrease in ligand affinity in maturer cultures possibly reflects a decrease in the turnover of acetylcholine. Muscarinic antagonists inhibited the total binding of [3H]QNB significantly, whereas muscarinic agonist, nicotinic antagonists and cholinesterase inhibitors had no influence whatsoever on the total binding of [3H]QNB. The content of muscarinic acetylcholine receptors varied between cultures with explants from different brain areas: hippocampus greater than striatum greater than septum greater than spinal cord greater than cerebellum. These in vitro results are generally in good agreement with results obtained in situ by other investigators and suggest that the binding of [3H]QNB observed in these cultures is indeed correlated to specific muscarinic receptor sites.

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

Intact brain cell aggregates were dissociated from adult rat brains, by a simple sieving technique, and were used to study the binding characteristics of [3H]N-methylscopolamine to muscarinic acetylcholine receptors. The magnitude of binding of this ligand was related linearly to the amount of cell protein in the binding assay, with a high ratio of total to nonspecific binding. In addition, specific binding showed saturability and high affinity. Muscarinic receptor antagonists displaced specific [3H]N-methylscopolamine binding according to the law of mass-action, while it was possible to resolve displacement curves using receptor agonists into high- and low-affinity components. The results are discussed in terms of the usefulness of dissociated intact rat brain cells in studying muscarinic acetylcholine receptors in the central nervous system.