Serotonin enhances central olfactory neuron responses to female sex pheromone in the male sphinx moth Manduca sexta

In the brain of the sphinx moth Manduca sexta, sex-pheromonal information i s processed in a prominent male-specific area of the antennal lobe called t he macroglomerular complex (MGC). Whole-cell patch-clamp recordings from id entified projection (output) neurons in the MGC have shown that serotonin [ 5-hydroxytryptamine (5-HT)] increases both the excitability of MGC projecti on neurons and their responses to stimulation with pheromone. At least two types of voltage-activated potassium currents in these cells are modulated by 5-HT. 5-HT decreases the maximal conductance of a transient potassium cu rrent (I-A) and shifts its voltage for half-maximal inactivation to more ne gative potentials without affecting the half-maximal voltage for activation . This reduces the "window current" between the voltage activation and inac tivation curves, decreasing the tonically active I-A near the resting poten tial and causing the cell to depolarize. 5-HT's effect in this case is to d ecrease both the transient and resting K+ conductance by modulating the sam e channel (I-A). 5-HT also decreases the maximal conductance of a sustained potassium current [I-K(V)] without affecting its voltage dependence. Using HPLC, we show also that levels of 5-HT in the antennal lobes fluctuate sig nificantly over a 24 hr period. Interestingly, 5-HT levels are highest at t imes when the moths are most active. We suggest that by controlling the res ponsiveness of antennal-lobe projection neurons to olfactory stimuli, 5-HT will have significant impact on the performance of odor-dependent behaviors .