Localized adaptation within the rhabdomeral lobe of Limulus ventral photoreceptors

Recent papers published in this journal (Stern et al., 1983; Calman and Chamberlain, 1983) have modified our understanding of the structure of Limulus ventral photoreceptors. The receptor's light-sensitive rhabdom has been found to be restricted to one or more specialized lobes termed R (rhabdomeral) lobes. In the light microscope, the R lobes can be distinguished from the A (arhabdomeral) lobe only after glial tissue has been removed from around the photoreceptor (Stern et al., 1983). Following these discoveries, previous studies of photoreception by the ventral eye may require revision. In particular, Stern et al. (1983) discuss the work of Fein and Charlton (1975) in which adaptat ion of the photoreceptor was shown to be localized within the cell. In this work, two spots of light were focused onto a cell having intact glia. Prolonged stimulation at one spot did not adapt the response to the other spot when the spots were separated by >50 #m. Stern et al. point out that the two spots may have been placed in separate R lobes within the same cell. This possibility leaves unresolved the question of whether adaptat ion is localized within a single R lobe. We have therefore repeated Fein and Charlton's (1975) experiment, using cells stripped of their glia so that the R lobes were clearly visible (Stern et al., 1983). After the glia and connective tissue had been removed from a ventral photoreceptor cell body (Stern et al., 1983), the axon remained the only part of the cell still a t tached to the ventral nerve. The free-floating cell body was held in place for intracellular recording by the use of a suction electrode (see Fig. 1A). The cell shown in Fig. 1A has a single R lobe and was chosen to illustrate our findings because the separation between the A lobe and the R lobe of the photoreceptor is clearly delineated by a prominent indentation (Stern et al., 1983). The R lobe is located to the right of the indentation. The region of this cell having the highest sensitivity to light is indicated in Fig. 1B and corresponds to the location of the R lobe (Stern et al., 1983). Figs. 1C and D show that each of the two regions that are illuminated by small spots in Fig. 1A can be independently desensitized. Thus, adaptat ion within an R lobe is localized over a distance less than the 60-#m separation of the spots in the experiment of Fig. 1. Extension of the time of adaptat ion from 9 to 70 s did