PROPERTIES OF HISTAMINE-ACTIVATED CHLORIDE CHANNELS IN THE LARGE MONOPOLAR CELLS OF THE DIPTERAN COMPOUND EYE - A COMPARATIVE-STUDY

The large monopolar cells (LMCs) of the first optic neuropil (lamina) in insects respond to the photoreceptor neurotransmitter histamine wit h an increase in chloride conductance. We have compared the properties of this conductance from a range of diptera from different visual env ironments: Tipula paludosa (slow flying, crepuscular), Drosophila mela nogaster (slow-flying diurnal), and 3 fast-flying diurnal species Musc a domestica, Calliphora vicina and Lucilia sericata. In whole-cell rec ordings of dissociated LMCs, histamine-induced currents were elicited using a multichannel parallel perfusion device, allowing rapid determi nation of the dose-response function, characterised by affinity (K-d) and Hill coefficient (n). Calliphora, Lucilia and Musca had the steepe st dose response curves (n = 2.8) and the lowest affinity for histamin e (K-d 35-50 mu M); the crepuscular Tipula had a significantly higher affinity (K-d = 16 mu M) and lower Hill coefficient (n = 1.8). Drosoph ila had a high affinity (K-d 24 mu M), and a high Hill coefficient (n = 2.5). In excised inside-out patch recordings all species showed simi lar single channel properties (conductance 40-60 pS, mean open time < 1 ms). The low Hill coefficient in Tipula would be expected to result in lower synaptic gain. We suggest this may be an adaptation to preven t the LMC's response bandwidth being filled with the high levels of ph oton noise typical of photoreceptors adapted for low light levels. The lower affinity for histamine found in the more photopic species sugge sts that the concentration of histamine (and therefore presumably numb er of synaptic vesicles released from the photoreceptors) should be hi gher. This might improve signal-to-noise ratio by decreasing the contr ibution of the shot event noise introduced by stochastic release of sy naptic vesicles.