The spike generator in the labellar taste receptors of the blowfly is differently affected by 4-aminopyridine and 5-hydroxytryptamine

In taste chemoreception of invertebrates the interaction of taste stimuli with specific membrane receptors and/or ion channels located in the apical membrane of taste receptor cells results in the generation of a receptor potential which, in turn, activates the ‘encoder’ region to produce action potentials which propagate to the CNS. This study investigates, in the labellar chemosensilla of the blowfly, Protophormia terraenovae, the voltage- gated K+ currents involved in the action potential repolarization and repetitive firing of the neurons by way of the Kv channel inhibitors, 4-aminopyridine and 5-hydroxytryptamine. The receptor potential and the spike activity were simultaneously recorded from the ‘salt’, ‘sugar’ and ‘deterrent’ cells, by means of the extracellular side-wall technique, in response to 150 mM NaCl, 100 mM sucrose and 1 mM quinine HCl, before, 010 min after apical administration of 4-AP (0.01–10 mM) or 5- HT (0.1–100 mM). The results show that the receptor potential in all three cells is neither affected by 4-AP nor by 5-HT. Instead, spike activity is significantly decreased, by way of blocking different Kv channel types: an inactivating A-type K+ current (KA) modulating repetitive firing of the cells and responsible for the after hyperpolarization, and a sustained K+ current that resembles the delayed rectifier (DKR) and contributes to action potential repolarization