Quantal secretion at release sites of nerve terminals in toad (Bufo marinus) muscle during formation of topographical maps.

1. The number of quanta secreted from selected sites along terminal branches at suppressed synapses in the developing toad (Bufo marinus) gluteus muscle has been determined. The topographical projection from segmental nerves 8 and 9 to the ventral surface of this muscle matures slowly as toads develop in size from 12 to 40 g. Terminal branches of nerves 8 and 9 were visualized by prior staining with the fluorescent dye, 3-3-diethyloxardicarbocyanine iodide (DiOC2(5]. 2. The evoked quantal release recorded with an extracellular electrode (m(e) at different positions along the length of terminal branches at synaptic sites innervated either by nerve 8 (me,8) or nerve 9 (me,9) was determined in an external Ca2+ concentration, [Ca2+]o, of 0.35-0.45 mM. For over 90% of branches longer than 80 microns, me declined along exponential curves from a relatively large value at the proximal end of branches for both nerve 8 and nerve 9 terminals; the exponent for these exponential curves gave quantal length constants that varied from 26 to 80 microns (48 +/- 4 microns, mean +/- S.E.M.) depending on the length of the branch. 3. The evoked quantal release recorded with an intracellular electrode (m) at synaptic sites dually innervated by nerve 8 and nerve 9 was nearly always (greater than 90%) greater for nerve 8 terminals than for nerve 9 terminals. At singly innervated sites the value of m per 100 microns length of terminal declined approximately exponentially with an increase in total terminal length (length constant 400 microns). However, at dually innervated sites the value of m per 100 microns length of nerve 9 terminal was very low at all total terminal lengths compared with singly innervated sites; this indicates that nerve 9 terminals were suppressed at dually innervated sites. 4. At five dually innervated sites, seven out of nine terminal branches of nerve 8 showed an exponential decline in me,8 along their length, from a relatively large value near the proximal end of the branches (length constant 35 +/- 3 microns, mean +/- S.E.M.). In contrast, all the terminal branches of nerve 9 greater than 80 microns showed a uniformly low value of me,9 along their length. 5. It is suggested that the suppression of nerve 9 terminals at dually innervated sites is primarily due to a decrease in the probability of secretion of normally highly secreting release sites at the proximal end of terminal branches.