Persistence of circadian ERG rhythm in the cricket with optic tract severed

Abstract

We report here that the cricket Gryllus bimaculatus showed a circadian ERG rhythm in constant darkness (DD) and temperature (26 ~ even after the optic tracts (OT) were severed. This is the first detection, in insects, of circadian rhythm from the optic lobe (OL)-compound eye (CE) system neurally isolated from the central nervous system (CNS). The significance of this finding is not confined to the physiology of ERG rhythm but extends potentially to the physiology of locomotor rhythm. This cricket is diurnaIly active in the nymphal stage but becomes nocturnal 4 to 5 days after the imaginal molt [11]. Adult males which had become fully nocturnal were obtained from a laboratory stock kept in LD 12:12 (L: 0600-1800) and at a constant temperature of 26 ~ the standard environmental condition. The crickets, whose legs were cut off, were fixed to a supporting rod and Ag-AgC1 wire electrodes were chronically implanted into the immediate vicinity of the receptor layer of the compound eyes. The apparatus was so arranged that ERGs elicited by a 50 ms flash of green light at intervals of 1 h were recorded automatically. The stimulus intensities were always held below saturation for the ERG. The OT is relatively long in the cricket, connecting medulla to lobula [4]. A small square piece of head capsule was removed to make a window, through which the OT was cut with microscissors. Strict attention had to be paid to minimize damage to tracheae running into the OL. Finally, the piece of head capsule was replaced to close the window, and the wound was sealed with vaseline. The ERG was diphasic, composed of onand off-components. Amplitudes of the two components showed circadian rhythm in phase with each other. Hereafter, we refer to the peak-to-trough amplitude of the diphasic wave, in other words, the summation of amplitudes of onand off-components. In intact animals, the ERG amplitude changed synchronously with LD 12:12, peaking in the dark fraction. This rhythm freeran in the ensuing DD with a period (tau) a little shorter than 24 h. When the OT was cut unilaterally, the 4 crickets all exhibited this rhythm in both intact and operated eyes (Fig. 1). Moreover, in 2 of 3 animals in which the OT were bilaterally severed, the rhythm was obvious in both eyes under DD (Fig. 2). The remaining animal showed the rhythm only in one eye; the other eye was not rhythmic probably due to damage to tracheae during the operation. In both cases of unilateral and bilateral cuts, there was a subtle difference between taus of the two eyes obtained by the periodogram. However, since data are limited, it is premature to discuss bilateral organization of the rhythm of the two eyes as was done elegantly in the beetle [5].