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Abstract. The prolonged depolarizing after potential (PDA) in the R1-6 receptors of the fly was used to isolate intermediate processes in phototransduction which are not manifested directly in the voltage response. It is first demonstrated that a pigment shift by light from metarhodopsin to rhodopsin in four species of the flies: Drosophila, Calliphora, Chrysomya and Musca induces an independent antagonistic process to the PDA, which is manifested in a strong inhibitory effect on PDA induction and is called the anti-PDA. By using mutants of Drosophila the existence of processes underlying the PDA were examined. The norpA(H52) and the trp mutant were used in which the voltage response of the photoreceptors could be reversibly abolished by elavated temperature and long intense light respectively. It is shown that the excitatory process underlying the PDA could be induced and depressed in conditions that block the voltage response of the photoreceptors, thus indicating the existance of intermediate processes which link the pigment activation by light to the PDA voltage response.

The formation of metarhodopsin in fly photoreceptor no. 1--6 occurs at room temperature with a time constant of 125 microseconds (Q10 approximately 2.5). The formation of rhodopsin is faster by factor of 1/10 to 1/100 at least.

Bleaching adaptation is the reduction in sensitivity of photoreceptors to light, following large photoconversion of rhodopsin molecules. It is accompanied by a prolonged dark excitation, and the two processes may be causally linked. There are remarkable similarities in the prolonged dark excitation of vertebrate rods and invertebrate photoreceptors. There is much evidence to indicate that prolonged dark excitation is tightly linked to processes that occur at the photo-pigment level. A molecular mechanism is suggested to explain the prolonged dark excitation. According to this model, quenching of photoreceptor excitation operates by phosphorylation of metarhodopsin (M) molecules. When unphosphorylated M molecules exist in the dark, they give rise to prolonged dark excitation. This prolonged dark excitation seems to be coupled to photoreceptors' desensitization by an unknown mechanism.

Red illumination of a Balanus amphitrite photoreceptor that has been adapted to blue light leads to prolonged depolarization in the late receptor potential. This depolarization can be switched off by further exposure to a blue stimulus. The early receptor potential in this cell is purely depolarizing or largely hyperpolarizing; the former is true if the cell has been adapted to red light, and the latter, if blue light has been used. The color-adaptation "memories" for both early and late receptor potentials appear to be permanent. The existence of two stable states for the early receptor potential directly implies a pigment with two stable states, and these apparently contribute antagonistically to the late receptor potential.