Chronic olfactory receptor neuron removal increases the ability of excitatory local neurons to depolarize projection neurons. A, We recorded simultaneously from an eLN and a PN, injecting current into the eLN and monitoring the responses of the PN. Antennae were removed either acutely or chronically. Palps were removed acutely to minimize spontaneous activity in the network. Most of the recorded PNs are likely to innervate antennal glomeruli, because PNs were recorded randomly and most PNs are antennal. B, Simultaneous recordings from a presynaptic eLN (single trial) and from a postsynaptic PN (average of 50 trials). The eLN is alternately hyperpolarized and depolarized by step-current injections, and the PN responds to both hyperpolarization and depolarization of the eLN. In a fly from which antennae have been chronically removed, typical PN responses are larger than in flies from which antennae are acutely removed. Changes in the eLN membrane potential in response to current injection are not significantly different. C, The coupling coefficient (postsynaptic V_m change/presynaptic V_m change) of eLN-to-PN synapses is significantly larger after chronic versus acute antennal removal (p < 0.003, repeated-measures two-way ANOVA, n = 36 acute and 22 chronic). Coupling coefficients are shown for both hyperpolarizing and depolarizing current injections into the eLN. Larger coupling coefficients for depolarizing steps may indicate rectification in the gap junctional conductance, and/or better propagation of depolarizing pulses between the soma and the site of the electrical synapse. D, There was no significant change in the input resistance of PNs or eLNs with chronic versus acute antennal removal (p > 0.05, t tests, n = 5 acute vs 5 chronic PNs, 7 acute vs 6 chronic eLNs).