J. Vautrin et Me. Kriebel, HIGH PERCENTAGE OF SKEW-DISTRIBUTED MINIATURE END-PLATE CURRENTS IN OLD MICE, Canadian journal of physiology and pharmacology, 71(2), 1993, pp. 165-174
Muscle fibers from diaphragms of old (14 to 24 months) and young adult
(I to 2 months) inbred (strain C57BL/6) mice were voltage clamped at
- 140 mV with two microelectrodes near the neuromuscular junction. Min
iature endplate currents (MEPCs) were digitized so that peak amplitude
s and rise times could be determined. MEPC amplitude distributions fro
m old mice varied greatly between fibers from the same diaphragm, and
the mean MEPC amplitude (2.1 +/- 0.83 nA, mean +/-SD) was smaller than
in young mice (5.2 +/- 0.59 nA). In old mice, some (50%) amplitude di
stributions were bell shaped, composed of mainly bell-MEPCs with a 2-
to 5-nA mode. whereas others (30%) were skewed with a 0.5- to 2-nA mod
e, and some (20%) showed two peaks, representing both skew- and bell-M
EPC classes. MEPC rise-time distributions from old mice varied between
fibers, although they all had similar modes. Some (30%) were bell sha
ped (similar to those in young mice) with a mode between 0.5 and 1 ms
(coefficient of variation, 40%), but most distributions were skewed. E
ndplates with smaller mean MEPC amplitudes showed a longer mean rise t
ime, and for a given junction, MEPC amplitudes were correlated positiv
ely to the corresponding rise times. This observation, together with a
nalyses of the rising phases, indicates that MEPCs with long rise time
s were not generated at remote sites. We discuss our results with rega
rd to the hypothesis of a dynamic formation of transmitter packets, an
d we attribute long rise time, skew-MEPCs to a prolonged release proce
ss. During aging, the state of release that generates the skew-MEPC cl
ass appears more dominant than the state generating the bell-MEPC clas
s.