Resistance changes associated with the response of insect monopolar neurons.

The large monopolar cells (LMC’s) of the dra­ gonfly lamina are second order interneurons that respond to input from photoreceptors with a tri­ phasic graded hyperpolarisation, consisting of an initial ‘on’ transient, a sustained plateau and a depolarising ‘off’ transient1. One of two general types of membrane process could generate the LMC hyperpolarisation which can attain amplitudes of up to 50 m V 1. As in vertebrate cones2, and probably horizontal cells and bipolar cells3, the hyperpolarisation could be associated with a cell input resistance increase that is indicative of a de­ creased sodium ion membrane conductance. Alter­ natively the response could be linked with a re­ sistance decrease, the response being generated by increased potassium and/or chloride ion con­ ductances, as in the hyperpolarising photoreceptors of scallop 4. Although the propagation of the LMC signal has been studied in fly 5 the resistance change associated with lamina hyperpolarisations has only been examined in locust 6 where they are associated with a resistance decrease and a reversal potential compatible with a process dependant upon potas­ sium and chloride ions. The resistance changes as­ sociated with the dragonfly LMC intracellular re­ sponse were measured to show that they are probably generated by a conductance increase and to confirm that locust lamina hyperpolarisations resemble the responses of LMC’s in yet another detail of their response. The preparation of the lamina of the dragonfly Hemicordulia tau has already been described, together with the intracellular recording techni­ ques 1. However, instead of using 150 Mi? fibrefilled microelectrodes, the electrodes used here had resistances of 80 —100 Mi? and were filled by the