- Email: [email protected]
Circadian rhythm in the light response of rat retinal disk-shedding and autophagy
356
Brain Research, 369 (1986) 356-360
Elsevier BRE21447
Circadian rh~hm in the light r e q x m ~ of rat ~ n a l disk-~ and autophagy CHARLOTTE REMI~1, ANNA WIRZ-JUSTICE2, ASTRID RHYNER 1and SILVIA HOFMANN 1 l Department of Ophthalmology, University Ziirich, Ziirich and 2psychiatric University Clinic, Basel (Switzerland)
(Accepted November 19th, 1985) Key words: mammalian retina - - disk-shedding - - autophagy - - light response - - circadian rhythm - - dopamine
Under a light-dark cycle, disk-shedding and autophagy in the rat retina peak in the early and mid light phase, respectively, Under constant conditions, disk-shedding and autophagy responses after light stimulation were elicited at different time points of a 24-h cycle. The greatest magnitude of response occurred in the late dark and early light phase. Thus there is a circadian variation of the light response of important metabolic parameters in the mammalian retina. Inhibition of dopamine synthesis during the early light phase caused a significant dampening of the light responses of both disk-shedding and autophagy.
In the rat retina, disk-shedding from rod outer segments and autophagy in rod inner segments undergo endogenous, circadian rhythms ~8.1s that persist with a dampened peak in the absence of exogenous time cues, removal of hormonal glands and cutting of the optic nerves (for review see ref. 4). Even though disk-shedding and autophagy rhythms can be phase shifted by the light-dark cycle (LD), some input from the central nervous system (CNS) may be necessary for entrainment 16. The majority of observations, however, point toward an independent oscillator within the eye driving retinal rhythms. Recent evidence indicates that the neurohormone melatonin (MT) and the putative retinal neurotransmitter and -modulator dopamine (DA) may participate in retinal rhythm regulationS.7, s.t4.19. Since disk-. shedding and autophagy themselves (besides their rhythmicity) are light sensitive metabolic processes 3.15 it is conceivable that rhythm regulation within the retina requires a changed susceptibility to light input within a 24 h cycle. In earlier studies disk-shedding in the rat retina appeared to be evoked only around the endogenous peak period10. In our studies over the last few years, however, we have consistently elicited peaks of disk-shedding and autophagy after light pulses at different times of an LD cycle. Thus
a detailed examination of the response to light pulses of disk-shedding and autophagy at different circadian phases was carried out. In addition we present preliminary data of pharmacological inhibition of D A synthesis that modulates light responses of these metabolic parameters. Male inbred Fisher rats weighing 200 g at the time of the experiments were kept from birth until the experiment on an L D cycle (12 h light, 12 h dark) at a room illumination of 500 iux and 30-50 lux measured at the top of the cages at 21 °C with water and food ad libitum. At given time points throughout a 24 h cycle, experimental animals received a 30 min light pulse of 400 lux from cool, white, fluorescent light sources mounted at 1.5 m above the cages. During the light exposure, particular care was taken that all animals were moving around and could not hide their heads away from the light source. The light exposures were performed under circadian conditions, i.e. in constant darkness 13-30 h after the last light information. At each time point of light stimulation, a group of animals (n = 5) was sacrificed by decapitation before stimulation (controls), and 4 groups (n = 5 animals per group) were sacrificed immediately after stimulation and in 1.5 h intervals thereafter. Light stimulations were carried out at the following time
Correspondence: Ch. Rem6, Department of Ophthalmology, University of Ztirich, 8091 Ztirich, Switzerland.
0006-8993/86/$03.50© 1986 Elsevier Science Publishers B.V. (Biomedical Division)
357 formed as described in the previous sections. Peaks of disk-shedding were elicited at all of the above time points (Fig. 1). The magnitude of the response was largest after stimulation around the endogenous peak period, i.e. at the transition from dark to light. An increase to maximal values was already seen after stimulation during the late dark phase. The latency of the response was shortest after stimulation at the dark-light transition. Stimulations around the endogenous peak period revealed high phagosome counts for prestimulation controls, a sharp drop immediately after stimulation and a second rise 2 h later. High prestimulation values represent the endogenous peak of disk-shedding whereas the following drop in phagosome numbers may reflect an inhibitory effect of bright light on disk-shedding Ins. Twoway analysis of variance (ANOVA) was applied to test for the statistical significance of the differences in light response of disk-shedding at different times of the 24 h cycle. The statistical evaluation (Table I) indicated a significant effect of the circadian phase on
points since the last light information was given: L +13, +14, +22 h; D +11, +26, + 3 9 h . At the end of the light exposure, animal cages were carefully protected from any further bright light with black darkroom cloth until after the sacrifice and the dissecting out of the eyes, which was performed in dim red light. After enucleation, the posterior half of the globes was removed and placed in 2.5% glutaraldehyde in 0.1 M cacodylate buffer, a central 4 - 5 mm 2 piece around the optic nerve head was dissected, fixed overnight and processed for electron microscopy the following day. Quantitative analysis of phagosomes was performed in semithin sections Is and of autophagic vacuoles in thin sections 13. In one preliminary experiment, D A synthesis was inhibited by intraperitoneal injections of a-methyl-1tyrosine (AMPT, 250 mg/kg). Injections were performed before and after light stimulation to assure persistence of drug effect until the end of the experiments. Injections were done in dim red illumination, further treatment of animals and tissues was per-
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Fig. 2. Light responses of autophagic vacuoles at difl erent times of a 24-h cycle. Conditions are the same as described in Fig. 1 and quantifications are derived from the same animals.
phagosome response to a light pulse. Similar to disk-shedding, responses of autophagy were elicited at all of the above time points (Fig. 2). The magnitude of the response was largest after stimulation at the late dark phase and gradually dropped to lowest values after stimulations at the late light phase. A particularly steep increase (short latency) of the response occurred after stimulation at mid to late dark phase. Even though the endogenous peak of autophagy occurs later than the phagosome peak (around L +5 h), the elicited autophagy response shows an earlier maximum. In previous studies, night-time stimulation resulted in high autophagic responses with high light intensities, whereas daytime stimulation dit not produce larger responses at higher intensities. Instead, after preceding weak illumina-
tion of 30 lux greater daytime values were obtained15. Our present data confirm this refractory period at midday. Two-way ANOVA (Table 1) indicated a significant effect of the circadian phase on autophagy response to a light pulse. The inhibition of DA synthesis in the early light phase produced significant effects on both phagosome and autophagy responses. Phagosome numbers were reduced in prestimulation controls as well as in the peak after light stimulation (Fig. 3a). Two-way ANOVA revealed a significant drug effect and interaction of the drug with light stimulation (Fd = 14.3, P < 0.01; Fi = 5.3, P < 0.0I). Numbers of autophagic vacuoles were not reduced in prestimulation controis, but the peak after stimulation was significantly dampened (Fig. 3b). Two-way ANOVA indicated a
TABLE I
Statistical evaluation of circadian phase effects on light responses of disk-shedding and autophagy (two-way ANO VA ) Effect of circadian phase
Effect of light
(modification of light response)
Disk-shedding Autophagy
Interaction term (different pattern of rcs~ms¢ to fight atdifferent Circadiah ph :~es}
F
P
F
P
F
P
35.9 27.4
< 0.001 < 0.001
12.2 56.5
< 0.001 < 0.001
12.8 2.2
< 0~001 < 0.01
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Fig. 3. Light responses of phagosome and autophagic vacuole numbers in control animals and after inhibition of DA-synthesis by a-methyl-l-tyrosine (AMPT). Experimental animals received i.p. injections in dim red light at the indicated time points (arrows). The black bar represents DD. The abscissa indicates the hours after the last light information. D and [L] indicate the beginning of the subjective dark and light period, respectively, n = 5 control and 5 experimental animals per time point. Error bar: 1 S.D. The light stumulation was the same as described in Fig. 1.
significant drug effect (Fd = 28.8, P < 0.01) and interaction (Fi = 7.8, P < 0.01). Earlier studies had shown that DA content and synthetic rate are high during the light phase and low during the dark phase 19. Thus, blocking DA synthesis when it is normally high dampens the light responses of retinal metabolic parameters. Further experiments inhibiting DA synthesis prior to light stimulation at differ-, ent time points throughout a 24 h cycle may show circadian phase dependence in DA actions. The finding that DA agonist drugs also modify retinal metabolic parameters provides evidence for an important functional relationship in rhythm regulation 14. This is the first demonstration of a circadian rhythm in response to light of important metabolic processes in the mammalian retina. Although it appeared from earlier experiments that disk-shedding could only be elicited around the endogenous peak period 10, we were able to evoke a disk-shedding response at all of the investigated time points. The experimental conditions in our study, however, were significantly different from earlier work in that those studies applied a pretreatment of 22 h of continuous light (LL) followed by 2 h of darkness before light stimulation. The existence of a self-sustaining oscillator in the retina exclusive of but modulated by the CNS is well documented for invertebrate eyes2 and has also been postulated for the mammalian eye (for review see ref. 4). The recent finding that psychophysically tested visual sensitivity rhythms in the rat eye show all the characteristics of a self-sustaining oscillator (free running period, phase-response curve, phase shifts, skeleton photoperiod as well as persistence without input from the suprachiasmatic nucleus (SCN), the putative circadian pacemaker in the hypothalamus), provides the first direct evidence of an ocular osciUator 17. No study has as yet investigated disk-shedding and autophagy rhythms in SCN-lessioned rats. Our study adds to the psychophysical evidence another criterion of retinal 'sensitivity' changes: the metabolic processes of disk-shedding and autophagy in visual cells vary in their light response according to circadian phase. Conceivably, other parameters in the mammalian retina will prove to vary in their light responses as well. A diurnal rhythm in rod electroretinogram (ERG) a- and b-wave amplitudes has been
360 d e m o n s t r a t e d in the human 6. By changing retinal 'sensitivity', the eye may a u t o n o m o u s l y regulate the circadian rhythms of disk-shedding and autophagy. In this context the question as to the functional role of melatonin and D A remains. D o p a m i n e r g i c amacrine cells have been suggested to be involved in functions of the rod system H, perhaps by serving some basic regulatory function such as dark/light adaptation that is not easily observed with standard neurophysiological test systems 9. The putative circadian p a c e m a k e r in the SCN re-
1 Baker, B.N., Morija, M. and Williams, T.P., Inhibition of light-induced shedding by above normal stimulating intensity, Invest. Ophthalmol. Vis. Sci. Suppl. 25 (1984) 241. 2 Barlow, R.B. and Chamberlain, S.C., Light and circadian clock modulate structure and function in Limulus photoreceptors. In T.P. Williams and B.N. Baker, (Eds.), The Effect of Constant Light on Visual Processes, Plenum Press, New York, 1980, pp. 247-269. 3 Besharse, J.C., Light and membrane biogenesis in rod photoreceptors of vertebrates. In T.P. Williams and B.N. Baker, (Eds.), The Effect of Constant Light on Visual Processes, Plenum Press, New York, 1980, pp. 409-431. 4 Besharse, J.C., The daily light-dark cycle and rhythmic metabolism in the photoreceptor-pigment epithelial complex. In N. Osborne and G. Chader, (Eds.), Progress in Retinal Research, Vol. 1, Pergamon Press, Oxford, 1982, pp. 81-124. 5 Besharse, J.C. and Dunis, D.A., Methoxyindoles and photoreceptor metabolism: activation of rod shedding, Science, 219 (1983) 1341-1343. 6 Birch, D.G., Berson, E.L. and Sandberg, M.D., Diurnal rhythm in the human rod ERG, Invest. Ophthalmol. Vis. Sci., 25 (1984) 236-238. 7 Dearry, A. and Burnside, B., Dopamine inhibits forskolinand 3-Isobutyl-l-methylxanthine-induced dark-adaptive retinomotor movements in isolated teleost retinas, J. Neurochem., in press. 8 Dubocovic, M., Melatonin is a potent modulator of dopamine release in the retina, Nature (London), 306 (1983) 782-784. 9 Ehinger, B., Functional role of dopamine in the retina. In N. Osborne and G. Chader, (Eds.), Progress in Retinal Research, Vol. 2, Pergamon Press, Oxford, 1983, pp. 213-232.
ceives visual input via the retinohypothalamic tract t2. By altering retinal 'sensitivity' to light, the photoreceptive input to the circadian system may be changed as a consequence. Thus the eye may be an autonomous oscillator coupled to that in the SCN within the m a m m a l i a n multi-oscillatory system. F u r t h e r m o r e retinal dopaminergic neurons may subserve this special task of luminance coding at dawn and dusk to the circadian system ~9 as well as the oscillator in the retina itself,
10 Goldman, A.I., Teirstein, P.S. and O'Brien, P.. The role of ambient lighting m circadian disk-shedding in the rod outer segment of the rat retina, Invest. Ophthalmol. Vis. Sci.. 19 (1980) 1257-I267. 11 Madani, A.P., Kolb, H. and Nelson, R., Dopamine-containing amacrine cells of rhesus monkey retina parallel rods in spatial distribution. Brain Research, 322 (1984) 1-7 12 Moore, R.Y.. Retino-hypothalamic projections in mammals: a comparative study, Brain Research. 49 f1973) 403-409. 13 Rem6. Ch.. Autophagy in rods and cones of the vertebrate retina, Dev. Ophthalmol., 4 (1981) 101-148. 14 Rein6, Ch., Wirz-Justice. A.. DaPrada, M., Retinal rhythms in photoreceptors and dopamine synthetic rate and the effect of a monoamiaeoxidase inhibitor. Trans. Ophthalmol. Soc. U.K.. 103 (1984) 405-410. 15 Rem~, Ch.. Aeberhard, B. and Schoch. M.. Circadian rhythm of autophagy and light responses of autophagy and disk-shedding in the rat retina, J. Comp. Physiol.. 155 (1985) 669-677. t6 Teirstein, P.S., Goldmann, A.I. and O'Brien, P., Evidence for both local and central regulation of rat rod outer segment disk-shedding, Invest. Ophthalmol. Vis. Sci. 19 (1980) 1268-1273. 17 Terman, M. and Terman, J., A circadian pacemaker for visual sensitivity? Ann. N. Y. Acad. Sci., in press. 18 LaVail, M.M., Rod outer segment disk-shedding in rat retina: relationship to cyclic lighting, Science, 194 (1976) 1071-1073. 19 Wirz-Justice, A., DaPrada, M. and Rem6, Ch., Circadian rhythm in rat retinal dopamine, Neurosci. Lett., 45 (1984) 21-25.
Brain Research, 369 (1986) 356-360
Elsevier BRE21447
Circadian rh~hm in the light r e q x m ~ of rat ~ n a l disk-~ and autophagy CHARLOTTE REMI~1, ANNA WIRZ-JUSTICE2, ASTRID RHYNER 1and SILVIA HOFMANN 1 l Department of Ophthalmology, University Ziirich, Ziirich and 2psychiatric University Clinic, Basel (Switzerland)
(Accepted November 19th, 1985) Key words: mammalian retina - - disk-shedding - - autophagy - - light response - - circadian rhythm - - dopamine
Under a light-dark cycle, disk-shedding and autophagy in the rat retina peak in the early and mid light phase, respectively, Under constant conditions, disk-shedding and autophagy responses after light stimulation were elicited at different time points of a 24-h cycle. The greatest magnitude of response occurred in the late dark and early light phase. Thus there is a circadian variation of the light response of important metabolic parameters in the mammalian retina. Inhibition of dopamine synthesis during the early light phase caused a significant dampening of the light responses of both disk-shedding and autophagy.
In the rat retina, disk-shedding from rod outer segments and autophagy in rod inner segments undergo endogenous, circadian rhythms ~8.1s that persist with a dampened peak in the absence of exogenous time cues, removal of hormonal glands and cutting of the optic nerves (for review see ref. 4). Even though disk-shedding and autophagy rhythms can be phase shifted by the light-dark cycle (LD), some input from the central nervous system (CNS) may be necessary for entrainment 16. The majority of observations, however, point toward an independent oscillator within the eye driving retinal rhythms. Recent evidence indicates that the neurohormone melatonin (MT) and the putative retinal neurotransmitter and -modulator dopamine (DA) may participate in retinal rhythm regulationS.7, s.t4.19. Since disk-. shedding and autophagy themselves (besides their rhythmicity) are light sensitive metabolic processes 3.15 it is conceivable that rhythm regulation within the retina requires a changed susceptibility to light input within a 24 h cycle. In earlier studies disk-shedding in the rat retina appeared to be evoked only around the endogenous peak period10. In our studies over the last few years, however, we have consistently elicited peaks of disk-shedding and autophagy after light pulses at different times of an LD cycle. Thus
a detailed examination of the response to light pulses of disk-shedding and autophagy at different circadian phases was carried out. In addition we present preliminary data of pharmacological inhibition of D A synthesis that modulates light responses of these metabolic parameters. Male inbred Fisher rats weighing 200 g at the time of the experiments were kept from birth until the experiment on an L D cycle (12 h light, 12 h dark) at a room illumination of 500 iux and 30-50 lux measured at the top of the cages at 21 °C with water and food ad libitum. At given time points throughout a 24 h cycle, experimental animals received a 30 min light pulse of 400 lux from cool, white, fluorescent light sources mounted at 1.5 m above the cages. During the light exposure, particular care was taken that all animals were moving around and could not hide their heads away from the light source. The light exposures were performed under circadian conditions, i.e. in constant darkness 13-30 h after the last light information. At each time point of light stimulation, a group of animals (n = 5) was sacrificed by decapitation before stimulation (controls), and 4 groups (n = 5 animals per group) were sacrificed immediately after stimulation and in 1.5 h intervals thereafter. Light stimulations were carried out at the following time
Correspondence: Ch. Rem6, Department of Ophthalmology, University of Ztirich, 8091 Ztirich, Switzerland.
0006-8993/86/$03.50© 1986 Elsevier Science Publishers B.V. (Biomedical Division)
357 formed as described in the previous sections. Peaks of disk-shedding were elicited at all of the above time points (Fig. 1). The magnitude of the response was largest after stimulation around the endogenous peak period, i.e. at the transition from dark to light. An increase to maximal values was already seen after stimulation during the late dark phase. The latency of the response was shortest after stimulation at the dark-light transition. Stimulations around the endogenous peak period revealed high phagosome counts for prestimulation controls, a sharp drop immediately after stimulation and a second rise 2 h later. High prestimulation values represent the endogenous peak of disk-shedding whereas the following drop in phagosome numbers may reflect an inhibitory effect of bright light on disk-shedding Ins. Twoway analysis of variance (ANOVA) was applied to test for the statistical significance of the differences in light response of disk-shedding at different times of the 24 h cycle. The statistical evaluation (Table I) indicated a significant effect of the circadian phase on
points since the last light information was given: L +13, +14, +22 h; D +11, +26, + 3 9 h . At the end of the light exposure, animal cages were carefully protected from any further bright light with black darkroom cloth until after the sacrifice and the dissecting out of the eyes, which was performed in dim red light. After enucleation, the posterior half of the globes was removed and placed in 2.5% glutaraldehyde in 0.1 M cacodylate buffer, a central 4 - 5 mm 2 piece around the optic nerve head was dissected, fixed overnight and processed for electron microscopy the following day. Quantitative analysis of phagosomes was performed in semithin sections Is and of autophagic vacuoles in thin sections 13. In one preliminary experiment, D A synthesis was inhibited by intraperitoneal injections of a-methyl-1tyrosine (AMPT, 250 mg/kg). Injections were performed before and after light stimulation to assure persistence of drug effect until the end of the experiments. Injections were done in dim red illumination, further treatment of animals and tissues was per-
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Fig. 1. Light responses of phagosome numbers at different times of a 24-h cycle. The black bar indicates the period of constant darkness (DD) to which animals were exposed. The abscissa indicates hours since the last light information (i.e. in DD). D and ILl indicate subjective dark and light period. Animals received 30 min light pulses of 400 lux at the indicated hours. Each point represents the mean from 5 animals. Error bar: 1 S.D.
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Fig. 2. Light responses of autophagic vacuoles at difl erent times of a 24-h cycle. Conditions are the same as described in Fig. 1 and quantifications are derived from the same animals.
phagosome response to a light pulse. Similar to disk-shedding, responses of autophagy were elicited at all of the above time points (Fig. 2). The magnitude of the response was largest after stimulation at the late dark phase and gradually dropped to lowest values after stimulations at the late light phase. A particularly steep increase (short latency) of the response occurred after stimulation at mid to late dark phase. Even though the endogenous peak of autophagy occurs later than the phagosome peak (around L +5 h), the elicited autophagy response shows an earlier maximum. In previous studies, night-time stimulation resulted in high autophagic responses with high light intensities, whereas daytime stimulation dit not produce larger responses at higher intensities. Instead, after preceding weak illumina-
tion of 30 lux greater daytime values were obtained15. Our present data confirm this refractory period at midday. Two-way ANOVA (Table 1) indicated a significant effect of the circadian phase on autophagy response to a light pulse. The inhibition of DA synthesis in the early light phase produced significant effects on both phagosome and autophagy responses. Phagosome numbers were reduced in prestimulation controls as well as in the peak after light stimulation (Fig. 3a). Two-way ANOVA revealed a significant drug effect and interaction of the drug with light stimulation (Fd = 14.3, P < 0.01; Fi = 5.3, P < 0.0I). Numbers of autophagic vacuoles were not reduced in prestimulation controis, but the peak after stimulation was significantly dampened (Fig. 3b). Two-way ANOVA indicated a
TABLE I
Statistical evaluation of circadian phase effects on light responses of disk-shedding and autophagy (two-way ANO VA ) Effect of circadian phase
Effect of light
(modification of light response)
Disk-shedding Autophagy
Interaction term (different pattern of rcs~ms¢ to fight atdifferent Circadiah ph :~es}
F
P
F
P
F
P
35.9 27.4
< 0.001 < 0.001
12.2 56.5
< 0.001 < 0.001
12.8 2.2
< 0~001 < 0.01
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Fig. 3. Light responses of phagosome and autophagic vacuole numbers in control animals and after inhibition of DA-synthesis by a-methyl-l-tyrosine (AMPT). Experimental animals received i.p. injections in dim red light at the indicated time points (arrows). The black bar represents DD. The abscissa indicates the hours after the last light information. D and [L] indicate the beginning of the subjective dark and light period, respectively, n = 5 control and 5 experimental animals per time point. Error bar: 1 S.D. The light stumulation was the same as described in Fig. 1.
significant drug effect (Fd = 28.8, P < 0.01) and interaction (Fi = 7.8, P < 0.01). Earlier studies had shown that DA content and synthetic rate are high during the light phase and low during the dark phase 19. Thus, blocking DA synthesis when it is normally high dampens the light responses of retinal metabolic parameters. Further experiments inhibiting DA synthesis prior to light stimulation at differ-, ent time points throughout a 24 h cycle may show circadian phase dependence in DA actions. The finding that DA agonist drugs also modify retinal metabolic parameters provides evidence for an important functional relationship in rhythm regulation 14. This is the first demonstration of a circadian rhythm in response to light of important metabolic processes in the mammalian retina. Although it appeared from earlier experiments that disk-shedding could only be elicited around the endogenous peak period 10, we were able to evoke a disk-shedding response at all of the investigated time points. The experimental conditions in our study, however, were significantly different from earlier work in that those studies applied a pretreatment of 22 h of continuous light (LL) followed by 2 h of darkness before light stimulation. The existence of a self-sustaining oscillator in the retina exclusive of but modulated by the CNS is well documented for invertebrate eyes2 and has also been postulated for the mammalian eye (for review see ref. 4). The recent finding that psychophysically tested visual sensitivity rhythms in the rat eye show all the characteristics of a self-sustaining oscillator (free running period, phase-response curve, phase shifts, skeleton photoperiod as well as persistence without input from the suprachiasmatic nucleus (SCN), the putative circadian pacemaker in the hypothalamus), provides the first direct evidence of an ocular osciUator 17. No study has as yet investigated disk-shedding and autophagy rhythms in SCN-lessioned rats. Our study adds to the psychophysical evidence another criterion of retinal 'sensitivity' changes: the metabolic processes of disk-shedding and autophagy in visual cells vary in their light response according to circadian phase. Conceivably, other parameters in the mammalian retina will prove to vary in their light responses as well. A diurnal rhythm in rod electroretinogram (ERG) a- and b-wave amplitudes has been
360 d e m o n s t r a t e d in the human 6. By changing retinal 'sensitivity', the eye may a u t o n o m o u s l y regulate the circadian rhythms of disk-shedding and autophagy. In this context the question as to the functional role of melatonin and D A remains. D o p a m i n e r g i c amacrine cells have been suggested to be involved in functions of the rod system H, perhaps by serving some basic regulatory function such as dark/light adaptation that is not easily observed with standard neurophysiological test systems 9. The putative circadian p a c e m a k e r in the SCN re-
1 Baker, B.N., Morija, M. and Williams, T.P., Inhibition of light-induced shedding by above normal stimulating intensity, Invest. Ophthalmol. Vis. Sci. Suppl. 25 (1984) 241. 2 Barlow, R.B. and Chamberlain, S.C., Light and circadian clock modulate structure and function in Limulus photoreceptors. In T.P. Williams and B.N. Baker, (Eds.), The Effect of Constant Light on Visual Processes, Plenum Press, New York, 1980, pp. 247-269. 3 Besharse, J.C., Light and membrane biogenesis in rod photoreceptors of vertebrates. In T.P. Williams and B.N. Baker, (Eds.), The Effect of Constant Light on Visual Processes, Plenum Press, New York, 1980, pp. 409-431. 4 Besharse, J.C., The daily light-dark cycle and rhythmic metabolism in the photoreceptor-pigment epithelial complex. In N. Osborne and G. Chader, (Eds.), Progress in Retinal Research, Vol. 1, Pergamon Press, Oxford, 1982, pp. 81-124. 5 Besharse, J.C. and Dunis, D.A., Methoxyindoles and photoreceptor metabolism: activation of rod shedding, Science, 219 (1983) 1341-1343. 6 Birch, D.G., Berson, E.L. and Sandberg, M.D., Diurnal rhythm in the human rod ERG, Invest. Ophthalmol. Vis. Sci., 25 (1984) 236-238. 7 Dearry, A. and Burnside, B., Dopamine inhibits forskolinand 3-Isobutyl-l-methylxanthine-induced dark-adaptive retinomotor movements in isolated teleost retinas, J. Neurochem., in press. 8 Dubocovic, M., Melatonin is a potent modulator of dopamine release in the retina, Nature (London), 306 (1983) 782-784. 9 Ehinger, B., Functional role of dopamine in the retina. In N. Osborne and G. Chader, (Eds.), Progress in Retinal Research, Vol. 2, Pergamon Press, Oxford, 1983, pp. 213-232.
ceives visual input via the retinohypothalamic tract t2. By altering retinal 'sensitivity' to light, the photoreceptive input to the circadian system may be changed as a consequence. Thus the eye may be an autonomous oscillator coupled to that in the SCN within the m a m m a l i a n multi-oscillatory system. F u r t h e r m o r e retinal dopaminergic neurons may subserve this special task of luminance coding at dawn and dusk to the circadian system ~9 as well as the oscillator in the retina itself,
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