Competition regulates the efficacy of an identified synapse in crickets.

The efficacy of the synaptic contact between an identified sensory neuron and an identified interneuron in crickets is increased when neighboring afferent synapses are removed early in postembryonic life. The physiological changes are correlated with changes in the structure of the presynaptic neuron's axonal arborizations: When neighboring axons are destroyed, there is a shift of the remaining axonal arbors into deafferented regions and an increase in the number of putative contacts with the postsynaptic neuron. Changes in the structure of the presynaptic neuron also directly affect the probability of formation of this synaptic connection. The connection was found in 67% of the control specimens, but it was present in 100% of the partially deafferented specimens. The results demonstrate that interactions between growing sensory neurons can influence both the probability of synapse formation and the strength of those connections. This is the first case in which the effects of competition on the structure of a single, identified, presynaptic neuron can be directly related to its synaptic efficacy.