Mechanism of the local vascular actions of 1, 1-dimethyl-4-phenylpiperazinium (DMPP), a potent ganglionic stimulant

Silent agonists for α7 nicotinic acetylcholine receptors

We discuss models for the activation and desensitization of α7 nicotinic acetylcholine receptors (nAChRs) and the effects of efficacious type II positive allosteric modulators (PAMs) that destabilize α7 desensitized states. Type II PAMs such as PNU-120596 can be used to distinguish inactive compounds from silent agonists, compounds that produce little or no channel activation but stabilize the non-conducting conformations associated with desensitization. We discuss the effects of α7 nAChRs in cells of the immune system and their roles in modulating inflammation and pain through what has come to be known as the cholinergic anti-inflammatory system (CAS). Cells controlling CAS do not generate ion channel currents but rather respond to α7 drugs by modulating intracellular signaling pathways analogous to the effects of metabotropic receptors. Metabotropic signaling by α7 receptors appears to be mediated by receptors in nonconducting conformations and can be accomplished by silent agonists. We discuss electrophysiological structure-activity relationships for α7 silent agonists and their use in cell-based and in vivo assays for CAS regulation. We discuss the strongly desensitizing partial agonist GTS-21 and its effectiveness in modulation of CAS. We also review the properties of the silent agonist NS6740, which is remarkably effective at maintaining α7 receptors in PAM-sensitive desensitized states. Most silent agonists bind to sites overlapping those for orthosteric agonists, but some appear to bind to allosteric sites. Finally, we discuss α9^⁎ nAChRs and their potential role in CAS, and ligands that will be useful in defining and distinguishing the specific roles of α7 and α9 in CAS.

Blockade of nicotinic receptors of bovine adrenal chromaffin cells by nanomolar concentrations of atropine

Nanomolar concentrations of atropine have been considered up to now to be selective for blockade of muscarinic receptors for acetylcholine. A collateral finding indicated to us that these low concentrations of atropine could also target the neuronal nicotinic receptors. We report here a detailed study on this novel property of atropine. Catecholamine release, measured on-line with amperometry in chromaffin cells stimulated with acetylcholine pulses was blocked by atropine in a competitive manner. To corroborate a direct action of atropine on nicotinic receptors, we have employed N,N-dimethyl-N'-phenyl-piperazinium (DMPP), a pure nicotinic receptor agonist; atropine blocked its secretory responses with an IC50 of 2.04 nM. Nicotinic currents, recorded with the whole cell configuration of the patch-clamp technique were blocked by atropine in a concentration-dependent manner (IC50 of 11 nM), also showing a competitive nature. Nicotinic receptor currents in oocytes expressing bovine alpha7 and alpha3beta4 nicotinic receptors were blocked by atropine with an IC50 of 11.2 and 46.8 nM, respectively. Atropine (30 nM) also decreased the increment of the cytosolic calcium concentrations after stimulation with 30 microM DMPP in bovine chromaffin cells. However, action potentials evoked by DMPP were not modified by atropine. Our results demonstrate that nicotinic currents and their downstream consequences (i.e. cytosolic calcium elevations and catecholamine release) were blocked by nanomolar concentrations of atropine; although the blockade was partial, it must be considered when using atropine to study cholinergic neurotransmission, particularly at synapses where both nicotinic and muscarinic receptors are present i.e., the adrenal medulla and autonomic ganglia.

Leukotriene-C4 enhances mucus production from submucosal glands in canine trachea in vivo

Because leukotrienes have been implicated as putative mediators in upper airways disease, we studied whether leukotriene C4 (LTC4) might also have a mucus enhancing effect on submucosal glands. We anesthetized mongrel dogs with chloralose (100 mg/kg) and urethane (500 mg/kg) and ventilated them on a pump. To help visualize the secretions from submucosal glands, we exposed the mucosa of the upper trachea and coated its surface with powdered tantalum. Secretions from the glands (hillocks) were measured with time: The number of hillocks was measured at four time points on 19 dogs after each treatment in the sequence: no LTC4, LTC4, no LTC4 and LTC4 + blocker. The potential blockers were nerve cutting, atropine, FPL-55,712, and hexamethonium. Each potential blocker was used on 3-5 dogs. LTC4 was injected into the cranial thyroid artery. In 19 dogs with 27 responses, LTC4(8.6-11.0 micrograms) gave a positive response that was significantly different from control (P less than 0.01) at 1-4 min. These effects were not abolished in 5 dogs by cutting the superior laryngeal (SLN) and the vagus nerves (P less than 0.01). Pretreatment of the dogs (n = 5) with atropine, hexamethonium and the specific SRS-A (LTC4) antagonist FPL 55,712 (n = 3) gave a statistically significant (P less than 0.01-0.05) reduction in mucus secretion at all times for atropine, hexamethonium, and at all times except 4 min for (FPL 55,712). These results indicate that leukotriene C4 induces mucus secretion in dogs. This secretion does not depend on an intact reflex pathway but is altered at the individual gland by agents which block ganglionic motor pathways.

Responses of the Bronchial Veins in a Heart-Lung-Bronchial Preparation

Blood collected from the bronchial veins of a heart-lung-bronchial preparation consists of two components: (a) fraction from the bronchial arteries, and (b) pulmonary to bronchial shunt It is estimated that the latter is more than half of total bronchial venous flow. The pulmonary to bronchial shunt is reduced by procedures which increase pulmonary arterial pressure. An intrinsic nervous reflex mechanism is suggested to explain such a response, particularly because the bronchial veins are more sensitive than the bronchial arteries to the vasoconstrictor action of epinephrine and norepinephirine, and to the vasodilator action of isoproterenol. acetylchohine, histamine, and DMPP. Alterations in bronchial venous flow induced by such chemical agents are independent of bronchoconstriction or bronchodilatation.