Characterization of muscarinic receptor subtypes in pig airways: radioligand binding and northern blotting studies

Social Isolation: How Can the Effects on the Cholinergic System Be Isolated?

Social species form organizations that support individuals because the consequent social behaviors help these organisms survive. The isolation of these individuals may be a stressor. We reviewed the potential mechanisms of the effects of social isolation on cholinergic signaling and vice versa how changes in cholinergic signaling affect changes due to social isolation.There are two important problems regarding this topic. First, isolation schemes differ in their duration (1–165 days) and initiation (immediately after birth to adulthood). Second, there is an important problem that is generally not considered when studying the role of the cholinergic system in neurobehavioral correlates: muscarinic and nicotinic receptor subtypes do not differ sufficiently in their affinity for orthosteric site agonists and antagonists. Some potential cholinesterase inhibitors also affect other targets, such as receptors or other neurotransmitter systems. Therefore, the role of the cholinergic system in social isolation should be carefully considered, and multiple receptor systems may be involved in the central nervous system response, although some subtypes are involved in specific functions. To determine the role of a specific receptor subtype, the presence of a specific subtype in the central nervous system should be determined using search in knockout studies with the careful application of specific agonists/antagonists.

M2 muscarinic receptors induce airway smooth muscle activation via a dual, Gbetagamma-mediated inhibition of large conductance Ca2+-activated K+ channel activity

Airway smooth muscle is richly endowed with muscarinic receptors of the M(2) and M(3) subtype. Stimulation of these receptors inhibits large conductance calcium-activated K(+) (BK) channels, a negative feed back regulator, in a pertussis toxin-sensitive manner and thus facilitates contraction. The underlying mechanism, however, is unknown. We therefore studied the activity of bovine trachea BK channels in HEK293 cells expressing the M(2) or M(3) receptor (M(2)R or M(3)R). In M(2)R- but not M(3)R-expressing cells, maximal effective concentrations of carbamoylcholine (CCh) inhibited whole cell BK currents by 53%. This M(2)R-induced inhibition was abolished by pertussis toxin treatment or overexpression of the Gbetagamma scavenger transducin-alpha. In inside-out patches, direct application of 300 nm purified Gbetagamma decreased channel open probability by 55%. The physical interaction of Gbetagamma with BK channels was confirmed by co-immunoprecipitation. Interestingly, inhibition of phospholipase C as well as protein kinase C activities also reversed the CCh effect but to a smaller (approximately 20%) extent. Mouse tracheal cells responded similarly to CCh, purified Gbetagamma and phospholipase C/protein kinase C inhibition as M(2)R-expressing HEK293 cells. Our results demonstrate that airway M(2)Rs inhibit BK channels by a dual, Gbetagamma-mediated mechanism, a direct membrane-delimited interaction, and the activation of the phospholipase C/protein kinase C pathway.

Isolated porcine bronchi provide a reliable model for development of bronchodilator anti-muscarinic agents for human use

BACKGROUND AND PURPOSE

In human airways, muscarinic acetylcholine receptors (mAChRs) exert a predominant role in the control of airways resistance and anti-muscarinic agents are currently included in the pharmacological treatment of chronic obstructive pulmonary disease (COPD). However, the development of more effective mAChR antagonists is hampered by considerable species variability in the ultrastrucural and functional control of airway smooth muscle, making extrapolation of any particular animal model questionable. This study was designed to characterize the mAChRs in a bronchial preparation from pigs, animals considered to provide close models of human biology.

EXPERIMENTAL APPROACH

Smooth muscle bronchial strips were examined by electron microscopy in order to compare their neuromuscular structure with that of human bronchi and used to study the affinity of a series of selective mAChR antagonists, estimated as pKis in competition binding assays with NMS and pA2, by Schild analysis, in contractile experiments.

KEY RESULTS

Pharmacodynamic binding parameters and affinity profiles of a series of antagonists were consistent with the presence of a majority of M2 mAChRs along with a minor population of M3 mAChRs. Functionally, the highly significant correlation between postjunctional pA2 affinities and corresponding affinity constants at human recombinant M1-M5 subtypes indicated that smooth muscle contraction in porcine bronchi, as in human bronchi, was dependent on the M3 subtype.

CONCLUSION AND IMPLICATIONS

Based on the characterization of mAChRs, isolated porcine bronchi provide an additional experimental model for development of mAChR antagonists for the treatment of human airway dysfunctions.

Segment-dependent expression of muscarinic acetylcholine receptors and G-protein coupling in the equine respiratory tract

Muscarinic receptors are considered to be of comparable clinical importance in chronic obstructive pulmonary disease (COPD) in equines and in humans. At present, data are scarce on the expression and distribution of probable subtypes of these receptors and their signalling pathways in airway segments, including lung parenchyma and bronchial and tracheal epithelium with the underlying smooth muscle in horses. Specific [N-methyl-3H]scopolamine chloride ([3H]NMS) binding to all three tissues was saturable and of high affinity, with KD values ranging between 1.6+/-0.7 and 1.9+/-0.3 nmol/L. [3H]NMS binding identified a higher density of total muscarinic receptors (fmol/mg protein) in the trachea (720+/-59 nmol/L) than in bronchi (438+/-48 nmol/L) or lung (22 +/- 3 nmol/L). Competitive binding studies using [3H]NMS and the unlabelled subtype-selective antagonists pirenzepine and telenzepine (M1), methoctramine and himbacine (M2), 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) (M3), tropicamide (M4) and mamba toxin (MT-3) (M4) indicated the presence of at least three muscarinic receptor subtypes in peripheral lung tissue (50:40:24-28%: M2>M3>M1), whereas in bronchus and trachea M2 subtypes (87-90%) predominated over M3 (14-22%), and M1 subtypes were lacking. No differences were found between tissues in high-affinity binding sites for carbachol in the absence (31-36%) or presence of guanosine 5'-triphosphate (GTP) (approximately 100%). Western blotting for G-protein alpha-subunits showed a much more robust expression of G(alphai1/2) in the trachea (with highest receptor density) than in the lung or bronchi, whereas G(alphas)-protein was dominantly expressed in bronchus. Concomitantly, carbachol inhibited isoproterenol- and GTP-stimulated adenylyl cyclase activity with increasing muscarinic receptor expression (trachea > bronchi > lung). We conclude that the expression and signalling pathways of muscarinic receptors in the equine respiratory tract are segment-dependent. These receptors might contribute to the pathogenesis of COPD in the horse and could provide potential drug targets for the therapeutic use of anticholinergics in this species.

Pharmacological characterization of the muscarinic receptor antagonist, glycopyrrolate, in human and guinea-pig airways

1. In this study we have evaluated the pharmacological profile of the muscarinic antagonist glycopyrrolate in guinea-pig and human airways in comparison with the commonly used antagonist ipratropium bromide. 2. Glycopyrrolate and ipratropium bromide inhibited EFS-induced contraction of guinea-pig trachea and human airways in a concentration-dependent manner. Glycopyrrolate was more potent than ipratropium bromide. 3. The onset of action (time to attainment of 50% of maximum response) of glycopyrrolate was similar to that obtained with ipratropium bromide in both preparations. In guinea-pig trachea, the offset of action (time taken for response to return to 50% recovery after wash out of the test antagonist) for glycopyrrolate (t1/2 [offset]=26.4+/-0.5 min) was less than that obtained with ipratropium bromide (81.2+/-3.7 min). In human airways, however, the duration of action of glycopyrrolate (t1/2 [offset]>96 min) was significantly more prolonged compared to ipratropium bromide (t1/2 [offset]= 59.2+/-17.8 min). 4. In competition studies, glycopyrrolate and ipratropium bromide bind human peripheral lung and human airway smooth muscle (HASM) muscarinic receptors with affinities in the nanomolar range (K1 values 0.5-3.6 nM). Similar to ipratropium bromide, glycopyrrolate showed no selectivity in its binding to the M1-M3 receptors. Kinetics studies, however, showed that glycopyrrolate dissociates slowly from HASM muscarinic receptors (60% protection against [3H]-NMS binding at 30 nM) compared to ipratropium bromide. 5. These results suggest that glycopyrrolate bind human and guinea-pig airway muscarinic receptors with high affinity. Furthermore, we suggest that the slow dissociation profile of glycopyrrolate might be the underlying mechanism by which this drug accomplishes its long duration of action.

Muscarinic receptor binding sites of the M4 subtype in porcine lung parenchyma

Muscarinic acetylcholine receptors regulate distal airway resistance and secretion. The subtype expressed in the lung in different species remains uncertain. It has recently become possible to identify the M4 subtype by careful comparison of antagonist affinities. We characterized the binding of [3H]quinuclidinyl benzilate ([3H]QNB) to muscarinic receptors in cell membranes from lung parenchyma of 2-8 week old pigs in comparison to cloned human M3 and M4 receptors expressed in COS cells, to M2 in rat atria and to M4 in bovine adrenal medulla. In porcine lung, [3H]QNB bound with high affinity (Kd = 95 +/- 9 pM) to a single homogeneous population of muscarinic receptor sites (Bmax = 340 +/- 10 fmol/mg protein). Competition studies showed that the affinity (expressed as pKi) of 3 selective blockers was in close agreement between pig lung and cloned human m4 (r = 0.996). A series of 10 blockers showed affinities closely matching reported values for M4 receptors of the adrenal medulla (r = 0.965). Conversely, affinity values in porcine lung differed significantly (P < 0.05, t-test) from those determined in parallel with either human cloned M3 or with rat atria expressing the M2 subtype. We conclude that pig lung muscarinic receptor binding sites most closely resemble the M4 subtype, in contrast to the M3 subtype typical of large airways in this species.

On muscarinic control of neurogenic mucus secretion in ferret trachea

1. Muscarinic receptor subtypes mediating neurogenic mucus secretion in ferret trachea were characterized in vitro and in vivo using 35SO4 as a label for secreted mucus, and the muscarinic receptor antagonists telenzepine for the M1 receptor subtype, methoctramine for the M2 subtype and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) for the M3 receptor. We also performed receptor binding and mapping studies. 2. Each muscarinic antagonist displaced [N-methyl-3H]scopolamine binding with high-affinity binding constant (KH) values of 1.9, 2.7 and 5.0 nM for telenzepine, methoctramine and 4-DAMP, respectively. Muscarinic M1 and M3 receptors localized to submucosal glands, whereas M2 receptors did not. 3. In vitro, electrical stimulation (50 V, 10 Hz, 0.5 ms for 5 min) increased 35SO4 output by 160%. Telenzepine did not inhibit the neurogenic secretory response at concentrations two-or twentyfold its KH value, nor did it inhibit secretion induced by acetylcholine (ACh). 4-DAMP inhibited neurogenic secretion by 80 and 95%, respectively, at concentrations two-and twentyfold its KH value, and also inhibited ACh-induced secretion. Methoctramine potentiated neurogenic secretion induced at 2.5 Hz (50 V, 0.5 ms for 5 min) in a dose-related (5.4-100 nM) manner with increases of 33-451% above electrically stimulated values. Methoctramine did not potentiate secretion induced at 10 Hz and did not have any effect on ACh-induced secretion. 4. In vivo, vagal stimulation (10 V, 10 Hz, 2 ms for 8 min) increased output of 35SO4 by approximately 120%. Telenzepine had no significant effect on neurogenic secretion. Methoctramine approximately doubled the stimulated response, whereas 4-DAMP abolished the stimulated secretory response. 5. We conclude that in ferret trachea, cholinergic nerve stimulation increases mucus secretion via muscarinic M3 receptors on the submucosal glands. The magnitude of the secretory response is regulated by neuronal M2 muscarinic receptors. The muscarinic M1 receptors localized to the submucosal glands do not appear to be involved with mucus secretion.