The physiology, pharmacology, and trophic effectiveness of synapses formed by autonomic preganglionic nerves on frog skeletal muscle.

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SUMMARY1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back.2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2-3 months; tetanic stimulation of the nerve elicited 90-100 % of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60-100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80-100% of the maximal tension to direct stimulation. If one nerve was implanted 2-3 months before the other, the second, although usually less effective than the first, normally innervated 50-100% of the fibres, with approximately the same time course of innervation as a single s.m.n.4. Mutual synaptic repression was seen on examination of twitch tensions. With either simultaneous or staggered innervation, stimulation of each s.m.n. resulted in a twitch of 30-50 % of the total direct twitch tension, with little overlap between the fields driven by the two nerves. Intracellular recordings showed that the distribution of subthreshold and spike-producing e.p.p.s reflected the existence of separate twitch fields. Even if one s.m.n. was implanted several months before the other and had time to establish suprathreshold junctions on most muscle fibres, an s.m.n.

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Competitive interaction between foreign nerves innervating frog skeletal muscle.

Grinnell¹,

Letinsky²,

Rheuben³

1979

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“…Functioning neuromuscular junctions are formed by cholinergic neurones only (Grinell & Rheuben, 1979;Cohen Anderson, Zorychta & Weldon, 1979). With the channel gating at the vagus junctions being similar to that at normal end-plates, the question arises whether all cholinergic neurones, i.e.…”

Section: P Breitschmid and H R Brenner Discussionmentioning

confidence: 99%

“…It is well known that preganglionic autonomic neurones transplanted onto denervated skeletal muscle will form functioning neuromuscular synapses (Langley & Anderson, 1904;Brown & Satinsky, 1951;Landmesser, 1971Landmesser, , 1972Ramirez & Luco, 1973;Bennett, McLachlan & Taylor, 1973;Grinell & Rheuben, 1979). These neurones normally make synapses with autonomic ganglion cells whose nicotinic ACh receptors differ from those at the neuromuscular junction because they respond differently to certain stimulating and blocking agents.…”

Section: Introductionmentioning

confidence: 99%

Channel gating at frog neuromuscular junctions formed by different cholinergic neurones.

Breitschmid¹,

Brenner²

1981

SUMMARY1. Drug-induced membrane current fluctuations were analysed at frog ectopic neuromuscular junctions formed de novo by somatic motoneurones and by preganglionic autonomic neurones of the vagus nerve, and at denervated end-plates reinnervated by the vagus nerve.2. At ectopic motor end-plates, the mean open time of the ion channels activated by ACh is TACh = 1P8+0'1 msec (S.E.) at -70 to -90 mV and 15-18 'C. Carbacholand suberyldicholine-induced channels have average open times TCarb = 1P0 + 0-1 msec (-80 mV, 6-8 TC) and TSub = 3-3+041 msec (-80 mV, 16-18 TC).3. The conductances y of single channels induced by ACh, carbachol and suberyldicholine are: YACh = 210+ 11 pS (+S.E.), YCarb 146+12 pS and YSub = 22 1 + 1-2 pS.4. The mean open times T of the channels is prolonged as the membrane is hyperpolarized. Their voltage dependence is similar to that of normal end-plate channels.5. The average open time of ACh-induced channels at ectopic vagus junctions is r = 1-6 + 0-2 msec (S.E.) at -70 to -80 mV and 14-15 'C. At denervated motor end-plates reinnervated by vagal neurones, the mean channel open time is T = 1P5 + 0-1 msec at -80 mV and 14-18 'C.6. At all vagus junctions, the synaptic currents are blocked by c-bungarotoxin.7. Somatic motoneurones and vagal neurones induce junctional folds in the muscle membrane where they contact it to form an ectopic end-plate. Staining the

“…Skeletal muscle fibres will also accept innervation by various other types of cholinergic neurones. Thus, functional neuromuscular contacts are formed by preganglionic autonomic neurones of sympathetic and parasympathetic origin (Elsberg, 1917;Landmesser, 1971;Grinell & Rheuben, 1979;Breitschmid & Brenner, 1981). The nicotinic ACh receptors in autonomic ganglia differ pharmacologically from those at the neuromuscular junction because they respond differently to certain stimulating and blocking agents.…”

Section: Introductionmentioning

confidence: 99%

On the neurotrophic control of acetylcholine receptors at frog end‐plates reinnervated by the vagus nerve.

Brenner¹,

Micheroli²

1985

SUMMARY1. To test whether the properties of subsynaptic acetylcholine (ACh) receptors in skeletal muscle fibre are influenced by the type of the innervating neurone some pharmacological properties of ACh receptor in normal end-plates and in denervated end-plates reinnervated by the vagus nerve in the frog were compared.2. Blockade of nerve-evoked synaptic currents by 200 /M-hexamethonium was stronger at vagus-reinnervated than at normal end-plates. Blockade at both types of junctions was voltage dependent. The effect of hexamethonium on equilibrium currents induced by bath-applied ACh and carbamylcholine was similar at the two types of junctions. 3. At both normal and vagus-reinnervated junctions, decamethonium had similar partial agonist properties.4. Following a step in membrane potential, the relaxations of ACh-induced conductance changes at the two types of junctions were affected in a similar fashion by heximethonium: hyperpolarization first produced a fast decrease and then a slow exponential increase in conductance. Upon depolarization, a fast increase was followed by an exponential decline to its original level. The time constant of the slow relaxation was slightly prolonged compared to control. These findings are consistent with a fast blocking action of open channels by hexamethonium.5. The effectiveness of hexamethonium in blocking end-plate currents was reduced in the presence of (+ )-tubocurarine, indicating that hexamethonium has a competitive blocking action on the receptors.6. These results do not indicate that the pharmacological properties of the ACh receptors are changed after an end-plate is reinnervated by a preganglionic neurone. The differential effect of hexamethonium on transmission at normal and vagusreinnervated end-plates is discussed as a consequence of different transmitter release characteristics at the two types of junctions.