Acetylcholine iontophoretic

FIGURE 7.14 Time-course of G-protein-mediated activation of GIRK potassium channels in rabbit sinoatrial node cells, (a). Outward current evoked by a 33-msec, 50-nA iontophoretic pulse of acetylcholine (between arrows), (b). Response of the unclamped cell to an iontophoretic pulse of acetylcholine (ACh). (Record (a) is adapted with permission from Trautwein et al., in Drug Receptors and Their Effectors, Birdsall, N. J. M., Ed., Macmillan, New York, 1980, pp. 5-22 record (b) is adapted with permission from Noma, in Electrophysiology of Single Cardiac Cells, Noble, D. and Powell, T., Eds., Academic Press, San Diego, CA, 1987, pp. 223-246.)... 

Figure 6. Iontophoretic application of acetylcholine (Ach) and pyrantel (a) Comparison of inward currents (upper trace) generated by the iontophoresis of Ach and pyrantel from a double-barrelled micropipette, b) Interaction of Ach and pyrantel showing reversible block of Ach currents following pyrantel application. The micropipette was placed within a distance of 50 urn from the bag membrane. Ejection currents of 250 nA for 1s were used. Vertical calibration 20 nA, membrane current. 400 nA, iontophoretic current (lower traces). (Adapted with permission from Ref. 2 Copyright 1985, Society of Chemical Industry).

Pitman, R. M. and Kerkut, G. A. (1970) Comparison of the actions of iontophoretically applied acetylcholine and gamma aminobutyric acid with the EPSP and IPSP in cockroach central neurones. Comp. gen. Pharmacol., 1,221-231. 

Fig. 1. The effects of acetylcholine (ACh), noradrenaline (NA) and 5-hydroxytryptamine (5-HT), applied iontophoretically to the same neurone in the brain stem of the unanaesthetised decerebrate cat. The frequency of discharge (f) is plotted against time and the black bars indicate the period of iontophoretic application (with currents of 50 nA) in each case.

Bradley, P. B. and Candy, J. M. (1970) Iontophoretic release of acetylcholine, noradrenaline, 5-hydroxytryptamine and D-lysergic acid diethylamide from micropipettes. Brit. J. Pharmacol., 40, 194-201. 

The possibility cannot be excluded that the actions of noradrenaline, as revealed by iontophoretic studies, reflect its physiological function in some areas but not in others. The large amounts of noradrenaline in the hypothalamus and mid-brain suggest that the observed stimulation of reticular neurones by noradrenaline points to its having an excitatory function in the brain stem. If it has a transmitter function elsewhere, it may well be inhibitory in nature. It is clear that even the evidence that noradrenaline is a transmitter in the reticular system is not nearly so convincing as that which supports the argument that acetylcholine is a mediator of transmission at central synapses. 

Bolton, T. B., 1976, On the latency and form of the membrane responses of smooth muscle to the iontophoretic application of acetylcholine or carbachol, Proc. R. Soc. London, Ser. B 194 99-119. 

Dodd, J., Dingledine, R., and Kelly, J. S., 1978, Intracellular recording from CA3 pyramidal neurones of hippocampal slices and the action of iontophoretic acetylcholine, in Iontophoresis and Transmitter Mechanisms in the Mammalian Central Nervous System (R. W. Ryall and J. S. Kelly, eds.), Elsevier/North-Holland, Amsterdam. 

The only substance reasonably satisfying these conditions is acetylcholine as it affects spinal cord Renshaw cells. At a recent symposium, however, the existence of Renshaw cells as discrete units was challenged on electrophysiological and histochemical grounds but also staunchly defended Little evidence has been presented to support a widespread central transmitter function for acetylcholine. Krnjevi suggested that the slow, prolonged cortical iontophoretic effects of acetylcholine jure consistent with a general facilitatory rather than a direct transmitter action. 

The experiments were carried out on the frog sartorius muscle, perfused with Ringer solution. The end-plate region was conventionally voltage clamped with two intracellular microelectrodes (3M KCl, resistance 8-15 MQ). Acetylcholine (ACh) was applied by a third microelectrode (2M acetylcholine chloride, resistance 15-20 M ) to the chemosensitive area of the muscle fibre. The iontophoretic micropipette tip was moved towards the point of maximum response as to achieve the maximum receptor density in the epicenter of the micropipette. 

Ionic activities were measured in R2, R15 and the left upper quadrant cells Li 4 and L6. Measurements were often made upon several cells from the same animal. Cells Li L4 and have been grouped together for the following reasons 1) they are pacemaker cells, 2) they receive a common inhibitory input from interneuron L q, and 3) they respond similarly to iontophoretic application of acetylcholine (ACh.). 

Histrionicotoxins antagonize both acetylcholine and glutamate-elicited excitation of central neurons (755), and have been cited as antagonizing glutamate-responses in invertebrate muscles (245). Histrionicotoxin also has a depressant effect on spontaneous activity of cortical and spinal neurons (114, 155). Perhydrohistrionicotoxin at very low concentrations (<0.1 pM) blocks endplate currents elicited by iontophoretic acetylcholine in rat neuromuscular preparations, while having no effect on spontaneous miniature endplate currents or endplate currents evoked by nerve stimulation (6, 14). Further studies will be required to clarify the reason for the remarkable potency of perhydrohistrionicotoxin versus responses to ionto-phoretically applied acetylcholine. 

The 2,7-epimer of perhydrohistrionicotoxin has been synthesized (75). While no biological data were available for this racemic compound, the corresponding dioxa -2,7 epimer (see Scheme XXXI, reaction G) was stated to have ca one fourth the biological activity of the naturally derived perhydrohistrionicotoxin . It was stated that the activity was ascertained using murine nerve/diaphragm preparation by Dr. E. X. Albuquerque and associates . A synthetic 2-pentenyl-7-butyl analog of histrionicotoxin caused a time-dependent inhibition of responses to iontophoretic acetylcholine in neuromuscular preparations (27). 

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