Quantal release at visualized terminals of a crayfish motor axon: Intraterminal and regional differences

Abstract

Synaptic transmission was measured at visualized terminal varicosities of the motor axon providing the sole excitatory innervation of the "opener" muscle in walking legs of crayfish (Procambarus clarkii Girard). Two questions were addressed: 1) How uniform is quantal emission at different locations along terminals innervating a single muscle fiber, and 2) can differences in quantal emission account for the different excitatory postsynaptic potential (EPSP) amplitudes generated by terminals localized in defined regions of the muscle? Extracellular "macropatch" electrodes were placed over individual varicosities, viewed after brief exposure to a fluorescent dye, and synaptic currents were recorded to determine quantal content of transmission. Along terminals supplying a single muscle fiber, nonuniform release was found: Varicosities closer to the point of origin of the terminal branch released more transmitter than those located more distally. Quantal content was higher for varicosities of the muscle's proximal region (where large EPSPs occur) than for varicosities of the central region (where small EPSPs occur). The probability of transmitter release per synapse is estimated to be greater for the proximal varicosities. At low frequencies of stimulation, quantal content per muscle fiber is two to four times larger in the proximal region. Taken in conjunction with a twofold higher mean input resistance for the proximal muscle fibers, the difference in quantal content can account for a four- to eightfold difference in EPSP amplitude. The observed mean EPSP amplitude is at least eight times larger in the proximal region. We discuss factors contributing to differences in EPSP amplitudes.

Characterization of development, behavior and neuromuscular physiology in the phorid fly, Megaselia scalaris

“…Active zones are structural entities on the presynaptic membrane where vesicles dock to fuse with the membrane. Such differences in synaptic complexity account for the differences between the Ib and Is varicosities in D. melanogaster (Atwood et al, 1993;Kurdyak et al, 1994) and for high and low output synapses in crustaceans Cooper, 1995, 1996a,b;Cooper et al, 1995aCooper et al, , 1996aGovind and Chiang, 1979;King et al, 1996). To resolve this issue in relation to synaptic efficacy in M. scalaris, electron micrographic studies of the motor nerve terminals will need to be undertaken.…”

Presynaptic Depression in Phasic Motor Nerve Terminals and Influence of 5-HT on Vesicle Dynamics

“…In the crayfish opener NMJ preparation, the dense bodies of the excitatory terminals are viewed as hemispheres of about 60 nm in diameter and sit within the cross-section of a hemisphere that faces the synapse [15]. This is documented by the occasional face on view in parallel sectioning of synapses [16]. Thus, in cross-sections of 75 nm thickness a dense body can be contained within a single section.…”

Actions of MDMA at glutamatergic neuromuscular junctions

“…Since multiple analysis protocols are utilized in the synaptic transmission field (Del Castillo and Katz, 1954;Cooper et al, 1995bCooper et al, , 1996Viele et al, 2003) to estimate the number of release sites and the probability of release, we provide the original values for further computational assessment by other investigators using their analysis protocol of choice.…”

“…The macropatch electrode is lightly placed over the varicosity to form a loose seal (100-200 k ) which measures the potentials preferentially within the electrode lumen. The number of quantal events within the fEPSPs were recorded and analyzed to determine the mean quantal content (m) for the crayfish NMJ (Cooper et al, 1995b(Cooper et al, , 1996. Since the evoked release recorded at the Drosophila NMJ are multiquantal the average area of mfEPSPs and the average area of fEPSPs were used to determine mean quantal content (Cooper et al, 1996).…”