QUANTAL RELEASE AT VISUALIZED TERMINALS OF A CRAYFISH MOTOR AXON - INTRATERMINAL AND REGIONAL DIFFERENCES

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 Girar d). Two questions were addressed: 1) How uniform is quantal emission a t different locations along terminals innervating a single muscle fibe r, and 2) can differences in quantal emission account for the differen t excitatory postsynaptic potential (EPSP) amplitudes generated by ter minals localized in defined regions of the muscle? Extracellular ''mac ropatch'' 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: Varico sities closer to the point of origin of the terminal branch released m ore transmitter than those located more distally. Quantal content was higher for varicosities of the muscle's proximal region (where large E PSPs occur) than for varicosities of the central region (where small E PSPs occur). The probability of transmitter release per synapse is est imated 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 hi gher mean input resistance for the proximal muscle fibers, the differe nce 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. (C) 1996 Wiley-Liss, Inc.