Association of nicotinic acetylcholine receptors with central respiratory control in isolated brainstem-spinal cord preparation of neonatal rats

Nicotine exposure is a risk factor in several breathing disorders Nicotinic acetylcholine receptors (nAChRs) exist in the ventrolateral medulla, an important site for respiratory control. We examined the effects of nicotinic acetylcholine neurotransmission on central respiratory control by addition of a nAChR agonist or one of various antagonists into superfusion medium in the isolated brainstem-spinal cord from neonatal rats. Ventral C4 neuronal activity was monitored as central respiratory output, and activities of respiratory neurons in the ventrolateral medulla were recorded in whole-cell configuration. RJR-2403 (0.1-10mM), a4b2 nAChR agonist induced dose-dependent increases in respiratory frequency. Non-selective nAChR antagonist mecamylamine (0.1-100mM), a4b2 antagonist dihydro-b-erythroidine (0.1-100mM), a7 antagonist methyllycaconitine (0.1-100mM), and a-bungarotoxin (0.01-10mM) all induced dose-dependent reductions in C4 respiratory rate. We next examined effects of 20mM dihydro-b-erythroidine and 20mM methyllycaconitine on respiratory neurons. Dihydro-b-erythroidine induces hyperpolarization and decreases intraburst firing frequency of inspiratory and preinspiratory neurons. In contrast, methyllycaconitine has no effect on the membrane potential of inspiratory neurons, but does decrease their intraburst firing frequency while inducing hyperpolarization and decreasing intraburst firing frequency in preinspiratory neurons. These findings indicate that a4b2 nAChR is involved in both inspiratory and preinspiratory neurons, whereas a7 nAChR functions only in preinspiratory neurons to modulate C4 respiratory rate.

Alkaloids from frog skins: selective probes for ion channels and nicotinic receptors

Amphibian skin has provided a wide range of biologically active alkaloids, many of wich have unique profiles of pharmacological activity and therapeutic potential. Over three hundred alkaloids have been identified and structures of over a dozen different classes of alkaloids have been elucidated. These iclude the batrachotoxins, wich were shown to be potent and selective activators and ligands for sodium channels, the histrionicotoxins, wich were shoen to be potent non-competitive blockers and ligands for nicotine receptor channel complexes, the pumiliotoxins and related allo-and homo-pumiliotoxins, wich were shown to have myotonic and cardiotonic activity due to effects on sodium channels, and epibatidine, wich was shown to have potent antinociceptive activity due to selective agonist activity at nicotinic receptors. These alkaloids are known in nature only in amphibian skin, except for homobatrachotoxin, wich was recently identified in feathers and skin of a bird. Further classes of alkaloids from amphibian skin include monocyclic pyrrolidines and piperidines, bicyclic decahydroquinolines, pyrrolizidines, and quinolozidines and tricyclic gephyrotoxins, pyrrolizidine oximes, etc..

Phentonium induces a transient increase of acetylcholine efflux from motor nerve terminals

Phenthonium (10-50 µM), a quaternary derivative of 1-hyoscyamine, increases the frequency of miniature end-plate potentials (205 fold) and blocks the nicotinic receptor-ionic channel in skeletal muscles. When tested on rat diaphragms previously incubated with [3H] choline, phenthonium (50µM) increased the spontaneous release of radiolabelled acetylcholine (ACh) from 11.6 ñ 6.4 to 110.5 ñ 40.2 x 10**3 dpm/g within 15 min. The effect was transient, declining to 24.6 ñ 14.7 after 50 min. Subsequent electrical stimulation still in the prsence of phenthonium increased the efflux to 164.7 ñ 45.3. The fractional release relative to the level before stimulation did not differ from controls. Phenthonium (20 µM) did not increase the spontaneous ACh release but doubled the efflux induced by nerve stimulation. The present results, compared to previous electrophysiological findeings, indicate that quantal and nonquantal release are increased by phenthonium. They also show that the transient effect is not due to ACh depletion in nerve terminals

The nicotinic acetylcholine receptor channel in embryonic muscle of xenopus laevis. A program to separate channel types

En este trabajo se emplea la técnica de control de voltaje en pequeños parches de membrana, para evaluar la actividad eléctrica unitaria del receptor-canal nicotínico para acetilcolina (AChR) en miotomos extraídos de embriones de Xenopus laevis. Dos patrones de actividad unitaria del receptor nicotínico para la acetilcolina, se distinguen por su conductancia, 60 y 40 pS. Regularmente, la actividad de ambos tipos de canales aparece en los registros y las complicaciones del análisis cinético estacionario producidas por la mezcla de los dos tipos de actividad se superaron a través del uso del programa de separación "CLASS"