Guanine nucleotide- and muscarinic agonist-dependent phosphoinositide metabolism in synaptoneurosomes from cerebral cortex of immature rats

Depolarization affects the binding properties of muscarinic acetylcholine receptors and their interaction with proteins of the exocytic apparatus

Membrane depolarization is the signal that triggers release of neurotransmitter from nerve terminals. As a result of depolarization, voltage-dependent Ca(2+) channels open, level of intracellular Ca(2+) increases. and release of neurotransmitter commences. Previous study had shown that in rat brain synaptosomes, muscarinic acetylcholine (ACh) receptors (mAChRs) interact with soluble NSF attachment protein receptor proteins of the exocytic machinery in a voltage-dependent manner. It was suggested that this interaction might control the rapid, synchronous release of acetylcholine. The present study investigates the mechanism for such a voltage-dependent interaction. Here we show that depolarization shifts mAChRs, specifically the m2 receptor subtype, to a low affinity state toward its agonists. At resting potential, mAChRs are in a high affinity state (K(d) of approximately 20 nM) and they shift to a low affinity state (K(d) of tens of microM) upon membrane depolarization. In addition, interaction between m2 receptor subtype and the exocytic machinery increases with receptor occupancy. Both phenomena are independent of Ca(2+) influx. We propose that these results may explain control of ACh release from nerve terminals. At resting potential the exocytic machinery is clamped due to its interaction with the occupied mAChR and depolarization relieves this interaction. This, together with Ca(2+) influx, enables release of ACh to commence.

Postnatal development of muscarinic receptor-stimulated phosphoinositide metabolism in mouse cerebral cortex: sensitivity to ethanol

An enhanced coupling of cholinergic muscarinic receptors to phosphoinositide metabolism had been previously observed in brain from immature rat. This study reports that the postnatal development of muscarinic receptor-stimulated phosphoinositide metabolism is also enhanced in cerebral cortex slices from immature Swiss-Webster and Balb-c mice, as compared to adults. Response to the agonist carbachol was lower on postnatal day 3, peaked between days 5 and 12 and then declined to adult levels. Density of muscarinic binding sites, measured with the M1 ligand [3H]telenzepine on postnatal day 7, was, on the other hand, only half of the adult value. Phosphoinositide hydrolysis stimulated by glutamate decreased with age, while that elicited by norepinephrine increased. These results are also similar to those previously reported in the rat. Ethanol has been found to inhibit muscarinic receptor-stimulated phosphoinositide metabolism in rat brain in an age-dependent manner. This was confirmed in mouse brain, where ethanol inhibited this response in cerebral cortex of immature but not adult animals. These results indicate that the enhanced muscarinic receptor-stimulated phosphoinositide metabolism, which coincides with the brain growth spurt, is similar in rats and mice. Mice may be a useful species in which to genetically manipulate muscarinic receptors to gain a better understanding of their potential role in brain development.

Heterogeneity of cholinergic muscarinic receptors coupled to phosphoinositide metabolism in immature rat brain

The effects of muscarinic agonists and antagonists on phosphoinositide (PtdIns) metabolism were examined in the cerebral cortex and brainstem of 7-day-old rats, in order to evaluate the role of muscarinic receptor subtypes in this process. Additionally, comparative experiments were performed in cortices from adult animals. Accumulation of [3H]inositol phosphates ([3H]InsPs) in [3H]inositol pre-labeled brain slices was taken as an index of PtdIns hydrolysis. In neonatal cortex, maximal stimulation induced by the full agonists acetylcholine, carbachol and methacholine was 8-10 fold over basal [3H]InsPs accumulation. The effect of the partial agonists bethanechol, pilocarpine and oxotremorine varied from 3 to 4 fold over basal. Smaller responses to cholinergic stimulation were found in the brainstem and in the adult cortex. In neonatal cortex, muscarinic antagonists inhibited the stimulatory responses with the following order of potency: 4-DAMP > pirenzepine > AF-DX 116 approximately p-F-HHSiD. Pirenzepine inhibition of full agonist-induced [3H]InsPs accumulation showed biphasic curves, with two thirds of the response being inhibited with high affinity. When partial agonists were used, the resulting pirenzepine curves were better described by interaction at one high affinity site. No differences were found between immature and adult rats in the effect of pirenzepine on [3H]InsPs accumulation induced by carbachol, methacholine, or bethanechol. Inhibition by pirenzepine of PtdIns hydrolysis induced by carbachol or methacholine showed biphasic curves also in the brainstem. In this area, only one third of the response was inhibited with high affinity, and p-F-HHSiD was more potent as an antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)