Evidence for prejunctional M₂ muscarinic receptors in pulmonary cholinergic nerves in the rat

Abstract

1 The effects of muscarinic antagonists considered to be selective for M1 receptors (pirenzepine) and for M2 receptors (gallamine and methoctramine) were used to investigate the existence of prejunctional muscarinic receptors on cholinergic nerves in the rat lung. The tracheal tube preparation was used in vitro, and contraction of the trachealis muscle was induced by electrical field stimulation (EFS) and by application of an exogenous muscarinic agonist (pilocarpine), and measured as an increase in intraluminal pressure in the tube. 2 The muscarinic antagonists, gallamine and methoctramine, enhanced the contractions induced by nerve stimulation, while contractions elicited by exogenous application of pilocarpine were inhibited by the antagonists. 3 In contrast, pirenzepine blocked contractions induced by both EFS and pilocarpine in a dosedependent manner (EC50 O.1 pM) due to blockade of the postjunctional muscarinic receptors on airway smooth muscle. Potentiation of the response to EFS was never seen with this antagonist. 4 The muscarinic agonist, pilocarpine, caused a slow maintained increase in tone of the tracheal tube and at the same time reduced the contractions induced by EFS. This inhibitory effect was blocked by gallamine and methoctramine. 5 The results suggest that prejunctional inhibitory muscarinic receptors may be localised on the parasympathetic cholinergic nerve terminals innervating tracheal smooth muscle in the rat. This confirms previous findings obtained by measuring transmitter release in this species. The present results suggest that these receptors are of the M2 subtype. Blockade of these autoreceptors with gallamine or methoctramine would increase the output of acetylcholine (ACh) and thereby enhance the nerve-induced contraction of tracheal smooth muscle.

Hyperinsulinemia Potentiates Airway Responsiveness to Parasympathetic Nerve Stimulation in Obese Rats

“…Release of ACh from parasympathetic nerves is normally locally controlled by inhibitory neuronal M 2 muscarinic receptors (16)(17)(18)(19)(20), which were initially described on nerves supplying lungs in guinea pigs (19) and have subsequently been described in the parasympathetic nerves supplying the lungs of all species studied thus far (22,(33)(34)(35)(36)(37)(38), including humans. Loss of M 2 receptor function has been described in asthma (21,23) and is a common feature of many different animal models of airway hyperresponsiveness, including acute infection with parainfluenza virus (24), sensitization and challenge with antigen (26,33), acute exposure to ozone (25), and acute exposure to organophosphate pesticides (39).…”

Modulatory role of tachykinin NK1receptor in cholinergic contraction of mouse trachea

“…These substances were added to prevent the re-uptake of choline, the hydrolysis of acetylcholine and the presynaptic autoinhibition of acetylcholine release. Presynaptic muscarinic autoreceptors have indeed been demonstrated in the trachea of different species [18][19][20] and preliminary experiments in the absence or presence of atropine during stimulation phase 3 (S 3 , see below) showed that no acetylcholine release could be detected in the mouse trachea without atropine. After this 60-min washout period, the content of the organ baths (1 mL from now on) was collected and replaced every 3 min for a total of 30 collections.…”

In Vivo Pharmacological Characterization of TD-4208, a Novel Lung-Selective Inhaled Muscarinic Antagonist with Sustained Bronchoprotective Effect in Experimental Animal Models

“…Each data point represents an individual animal. Maclagan, 1990;Celli, 2004). Although it is likely that postjunctional M 3 receptor antagonism drives the bronchoprotective activity of the three compounds in rats and dogs, the possible involvement of postjunctional M 2 receptors cannot be excluded given that this receptor can also cause contraction of airway tissues through direct and indirect mechanisms (Hirshman et al, 1999;Sarria et al, 2002).…”