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.

Regulated spacing of synapses and presynaptic active zones at larval neuromuscular junctions in different genotypes of the fliesDrosophila andSarcophaga

“…The number of active zones is regulated, apparently in accordance with the functional demands of neurotransmission. At the crayfish neuromuscular junction, for example, high-output terminals have more active zones than low-output terminals (Cooper et al, 1995a(Cooper et al, , 1996b.…”

Structure/function assessment of synapses at motor nerve terminals

“…Evidence supports that the structural makeup of the synapse, in relation to AZs in multiple vertebrate and invertebrate models, is a component that is correlated with the amount of evoked transmitter release (Cooper et al,1996a,b; Rheuben,1985; Walrond and Reese,1985). One advantage of the invertebrate crustacean model for synaptic transmission is that physiological recordings can be directly associated with synaptic ultrastructure.…”

Active zone structure‐function relationships at the neuromuscular junction

“…(a2) A crayfish motoneuron stained with 4‐Di‐2‐Asp to label varicosities. Adapted from Cooper, Harrington, Marin, and Atwood, (). (a3) Quantal content calculated from charge transfer measurements recorded from a single bouton.…”

“…Neurotransmitter release at the crayfish NMJ was demonstrated to be primarily triggered by P/Q type VGCCs (Allana & Lin, ; Araque, Clarac, & Buño, ; Wright, Brodwick, & Bittner, ) and ultrastructural studies estimated that the total number of VGCCs to be on the order of 13 channels per release site, while the number of docked synaptic vesicles per release site was estimated to average ~6–7 (Cooper, Harrington et al, ; Cooper, Winslow, Govind, & Atwood, ). Therefore, each synaptic vesicle has ~1–2 associated VGCCs.…”

“…The magnitude of neurotransmitter release across a single motoneuron axon was found to be quite variable at the crayfish NMJ, with proximal regions releasing more neurotransmitter than distal regions (Cooper, Harrington et al, ; Cooper, Marin, & Atwood, ; Parnas, Parnas, & Dudel, ). This variability in synaptic strength appeared to correlate with the number of dense bodies or bars, with proximal synapses having a greater number and longer dense bars than distal regions (Govind et al, ).…”