COMPETITIVE INTERACTIONS BETWEEN SUPERNUMERARY AND NORMAL SENSORY NEURONS IN THE COCKROACH ARE MEDIATED THROUGH A CHANGE IN QUANTAL CONTENTAND NOT QUANTAL SIZE

1. The final steps in synapse formation and stabilization involve the adjustment of strength of connections through competitive interactions between neurons contacting a common target. The mechanisms underlying this competition-driven adjustment of synaptic strength are not well understood. We have studied one aspect of this phenomenon using the ce real sensory system of first instar cockroach nymphs. 2. Quantal analy sis of excitatory postsynaptic potentials (EPSPs) recorded at the syna pse between the lateral filiform hair sensory neuron (L) and giant int erneuron 3 (GI3) was carried out to determine whether the reduction in EPSP amplitude observed in the presence of an additional sensory neur on (''space invader neuron'' or SIN) was due to pre- or postsynaptic c hanges. 3. Mean quantal content, not quantal size, was reduced at the L-GIS synapse in the presence of a SIN. Fitting binomial distributions to the EPSP amplitude histograms gave estimates of the number of rele asable quanta or of synaptic sites (n) and the probability of release at these sites (p). The reduction in quantal content is associated wit h a decrease in the binomial parameter n and not p, suggesting that th ere is a decrease in the number of contacts, release sites, or quanta available for release, rather than a change in probability of release.