In vitro ELECTROPHYSIOLOGY OF TWO NOCTURNAL INSECT SPECIES, Periplaneta americana AND Gryllus bimaculatus

In dim light, reliable coding of visual information becomes compromised, unless the sensitivity of the visual system to light is improved by structural and functional adaptations. Thus far, many adaptations for night vision in the compound eyes of nocturnal insects have been described, but little is known about the mechanisms underlying the electrochemical signalling in their photoreceptors. In this thesis, whole-cell patch-clamp and mathematical modelling are utilised to study basic electrical properties and ionic currents in photoreceptors of two nocturnal insects, the American cockroach Periplaneta americana and the field cricket Gryllus bimaculatus. Photoreceptors in both species showed large input resistance, membrane capacitance and phototransduction gain (large single photon responses) compared with most studied diurnal insects, providing improved sensitivity to light. The photoreceptors also expressed two voltage-sensitive outward currents: a transient current and a sustained current. The cricket photoreceptor expressed a dominating transient current, which is a typical characteristic for insects adapted for slow vision in dim light. By contrast, in the majority of cockroach photoreceptors the sustained current dominated, which is more common among fast diurnal species. Model simulations indicated that the sustained current is necessary for improved photoreceptor dynamics. Examination of light-induced currents suggested that the functional variability in cockroach photoreceptors is in part derived from variations in the total area of the photosensitive membrane. Recordings of lightinduced currents also revealed that the cockroach light-gated channels are only moderately Ca 2+ -selective and that the polarisation-sensitive photoreceptors of the cricket may utilise phototransduction machinery in some details different from that in regular photoreceptors. Furthermore, the dynamics and information transfer rates of polarisation-sensitive photoreceptors in the cricket were clearly inferior to their regular counterparts, suggesting that they are not necessary for image formation.