Development of synaptic currents in immobilized muscle of Xenopus laevis.

Abstract

SUMMARY1. The effect ofchronic immobilization on the development ofsynaptic currents was studied in myotomal muscle of Xenopus laevis. Embryos and tadpoles were immobilized by rearing them in the presence of tetrodotoxin (TTX) after removal of the egg membranes.2. Immobilization did not affect the developmental change in duration of miniature end-plate currents (m.e.p.c.s). Rise times decreased from about 3 to 0-6 ms in both immobilized and control muscle, and decay constants decreased from about 7 to 1-2 ms in both conditions. 3. M.e.p.c.s with double exponential decays were recorded in both immobilized and control muscle at intermediate and late developmental stages. The fast and slow decay constants were 0 7 ms and slightly less than 3 ms in older muscle ofboth groups. These values are comparable to the apparent open times of fast and slow ACh receptors present on Xenopus muscle.4. Application ofan anticholinesterase (methanesulphonyl fluoride) lengthened the duration of m.e.p.c.s comparably in immobilized and control muscle.5. These data indicate that the deposition of junctional acetylcholinesterase and the reduction in open time of acetylcholine receptor channels in developing Xenopus myotomal muscle are independent ofcontractile activity ofmuscle and TTX-blockable action potentials in muscle or motoneurones.

Three conductance classes of nicotinic acetylcholine receptors are expressed in developing amphibian skeletal muscle

“…Assuming resting potentials of about 80 mV (Kullberg et al, 1985), this corresponds to a reversal potential of about 0 mV. This value is comparable to that found for the 40 and 60 pS AChR channels (Brehm et al, 1984a, b;Owens and Kullberg, 1989).…”

“…The interhyoideus, a flat muscle of the lower jaw, begins to develop at about the same time. In other studies, we have shown that these muscles differ in their development of endplate currents and probably have 3 distinct patterns of AChR development (Kullberg et al, 1985;Kullberg and Owens, 1986).…”

Activation of the primary kinetic modes of large‐ and small‐conductance cholinergic ion channels in Xenopus myocytes.

“…It is well documented that during development of Xenopus neuromuscular synapses, short-lived channels replace long-lived channels and that both the m.e.p.c. decay times (Brehm et al 1982) and rise times (Kullberg, Owens & Vickers, 1985) decrease. It is not clear, however, if the differences in activation kinetics between 760 and 740 channels are sufficient to account for all of the differences in m.e.p.c.…”

Two types of acetylcholine receptor channels in developing Xenopus muscle cells in culture: further kinetic analyses.

“…These two types of receptor channels are distributed at the extrajunctional region (Sakmann, Patlak & Neher, 1980; Kullberg, Brehm & Steinbach, 1981) as well as at the junction (Kullberg, Owens & Vickers, 1985). In the adult frog after denervation ACh receptors appear in the extrajunctional region.…”