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Notes: Summary TheAplysia eye is capable of both photoreception and circadian oscillation. We have tried to find out if either of these ocular functions plays an important role in controlling the circadian rhythm of locomotor behavior. Eyeless, sham-operated and intactAplysia were tested in LD, DD and pseudo-LD. The behavioral rhythm was measured and analyzed by event recorder techniques supplemented with periodogram analysis. 1. In agreement with others we found that (i) intactAplysia were diurnal, (ii) activity onsets frequently occurred slightly before dawn inLD 12∶12 and (iii) a majority of intact animals gave convincing freeruns for up to 2 weeks following release into DD or pseudo-LD (Figs. 1 and 2). 2. In intact animals the phase angle difference was more positive in LD 8∶16 than in LD 12∶12 (compare Figs. 1 and 5). 3. EyelessAplysia reliably responded to light onset (Figs. 3, 4, and 5). The response persisted for more than 90 days after eye removal (Figs. 3 and 8) and at lighttime intensities of 3 lux or lower (Figs. 7, 8, and 10). Photic responses in eyelessAplysia did not depend on social communication with intact animals (Figs. 9 and 10). 4. Even though eye removal did not prevent photic responses, it could have several effects on the daily pattern of activity in LD. These could include (i) an increase in scattered activity during the darktime, (ii) a decrease in activity during the final two-thirds of the lighttime, and (iii) attenuation or elimination of predawn anticipatory activity (Figs. 3, 4, and 5). The third of these effects was by far the most reliable consequence of eye removal in LD. 5. Only a minority of eyelessAplysia gave convincing freeruns in DD or pseudo-LD. Nonetheless a few eyeless animals freeran as vigorously as the most vigorous intact ones (compare Fig. 2 with Figs. 3, 4, and 6). We saw eyeless animals in neighboring apparatuses which freeran with distinctly different periods (Fig. 4). These facts indicate that the eye is neither the only photoreceptor nor the only circadian oscillator coupled to the locomotor rhythm. The eye, however, does participate in the generation of temporal patterns of activity in the absence of h-to-h sensory guidance. It is therefore reasonable to believe that the eye is one among several endogenous oscillators coupled to locomotion. The only role so far demonstrated for the ocular photoreceptors is their previously known capacity to entrain the ocular clock.

Notes: Summary The eye ofAplysia contains a circadian oscillator which can be entrained by cycles of white light (Jacklet, 1969b). Photoreceptors sufficient for entrainment of this oscillator reside within the eye itself (Eskin, 1971). We now report that extraocular photoreceptors can also entrain the ocular oscillator. Our evidence is: (1) Although the eye gives only a weak sensory response to red light (Fig. 2), red light cycles will entrain the ocular oscillator in an intactAplysia (Fig. 3). (2) Ocular entrainment by red light cycles, but not by white, requires that the optic nerve be intact (Fig. 4a, b); denervation of the eye allows the ocular oscillator to freerun in red light cycles (Fig. 4c). (3) The procedure of denervation does not prevent ocular entrainment by white light cycles in denervated eyes (Fig. 8). TheAplysia nervous system contains several circadian oscillators and several photoreceptors. Functional integrity of the organism requires that these elements be synchronized with one another. A neural, as opposed to hormonal, link is required for coupling the circadian oscillator in the eye to other neural sources of temporal information.

Notes: Summary We recorded the circadian rhythms of locomotion in vivo and then recorded the rhythm of optic nerve activity in vitro inAplysia maintained in constant conditions (CC). Some of the animals were intact, some had one optic nerve cut (1-nerve-x group) and others had both optic nerves cut (2-nerve-x group). We compared the two eye rhythms with each other and with the foregoing locomotor rhythms. 1. The procedures of eye removal and setting up for recording shifted the phase of the eye rhythm by a maximum of ±2.5 h. The largest phase shifts occurred when the eyes were removed during the middle of the subjective night. The phase shifts were similar in magnitude to those caused by 1 h pulses of light and were not large enough to invalidate the estimation of the approximate phase of the eye rhythm in vivo from data obtained subsequently in vitro. 2. After three or more weeks of constant conditions, the two eye rhythms of intactAplysia were usually out of phase with each other. There was a preference for very large interocular phase differences near 180 ° at the expense of medium interocular phase differences near 90 °. The large phase differences developed as soon or sooner after the beginning of CC than medium phase differences (Fig.3A). 3. InAplysia with one or both optic nerves cut, the two eye rhythms also got out of phase, but large phase differences were not more numerous than medium ones and medium phase differences developed sooner than large ones. Ocular desynchrony developed earlier in 1-nerve-x animals than in either intact or 2-nerve-x animals. Ocular desynchrony developed no sooner and perhaps later in the 2-nerve-x animals than in the intact animals (Fig. 3). Findings 2 and 3 mean that neural coupling does not assist at all in keeping the two eye clocks in phase with each other as they are in LD 12∶12. The rate of development of ocular desynchrony and the preferred interocular phase difference, however, may be weakly influenced by some type of efferent regulation of the eye clocks. 4. IntactAplysia that had medium or large interocular phase differences seldom expressed strong rhythmicity in their locomotor behavior, and animals that had strong behavioral rhythmicity had small interocular phase differences (Fig. 4). 5. The locomotor rhythmicity of 1-nerve-x animals was as strong as that of intact animals, but unlike intact animals, the strength of rhythmicity in the 1-nerve-x animals was unrelated to the size of the interocular phase difference (Figs. 5, 6). 6. In most 2-nerve-x animals, locomotor circadian rhythmicity was weak or nonexistent (Fig. 6). Nonetheless, there were a few 2-nerve-x animals that had convincing locomotor freeruns, indicating that a circadian oscillator can be coupled to locomotion even after both optic nerves are cut (Fig. 7). 7. In intactAplysia with strong behavioral rhythmicity and small interocular phase angles, the rising phase of the eye rhythm usually occurred at, or slightly before, the rising phase of the locomotor rhythm. There were exceptions to this rule, however (Fig. 8). 8. In 1-nerve-x animals the phase of the rhythm of the attached eye, but not of the detached eye, corresponded to the phase of the locomotor rhythm (Fig. 9). Results 4 through 8 strengthen the conclusion that the eyes participate in the control of the circadian locomotor rhythm. Further, the eyes are neurally, not hormonally, coupled to the locomotor system. The eyes, however, may not be the driver oscillators for the locomotor rhythm.