NEURITE GROWTH-PATTERNS LEADING TO FUNCTIONAL SYNAPSES IN AN IDENTIFIED EMBRYONIC NEURON

We explored the relationship between neurite outgrowth and the onset o f synaptic activity in the central neuropil of the leech embryo in viv o. To follow changes in early morphology and the onset of synaptic act ivity in the same identified neuron, we obtained whole-cell patch-clam p recordings and fluorescent dye fills from dorsal pressure-sensitive (P) cells, the first neurons that could be reliably identified in the early embryo. We followed the development of the P cell from the first extension of neurites to the elaboration of an adult-like arbor. Afte r the growth of primary neurites, we observed a profuse outgrowth of t ransient neurites within the neuropil. Retraction of the transient neu rites left the primary branches studded with spurs. After a dormant pe riod, stable secondary branches grew apparently from the spurs and bec ame tipped with terminals. At this time, neurites of the Retzius (R) c ell, a known presynaptic partner in the adult, were observed to appare ntly contact the terminals. Although voltage-dependent currents were s een in the P cell at the earliest stage, spontaneous synaptic activity was only observed when terminals had formed. Spontaneous release was observed before evoked release could be detected from the R cell. Our results suggest that transient neurites are formed during an explorato ry phase of development, whereas the more precisely timed outgrowth of stable neurites from the spurs signals functional differentiation dur ing synaptogenesis. Because spurs have also been observed in neurons o f the mammalian brain, they may constitute a primordial synaptic organ izer.