Circadian regulation of teleost retinal cone movements in vitro.

In the retinas of many species of lower vertebrates, retinal photoreceptors and pigment epithelium pigment granules undergo daily movements in response to both diurnal, and in the case of teleost cone photoreceptors, endogenous circadian signals. Typically, these cone movements take place at dawn and at dusk when teleosts are maintained on a cyclic light (LD) regime, and at expected dawn and expected dusk when animals are maintained in continuous darkness (DD). Because these movements are so strictly controlled, they provide an overt indicator of the stage of the underlying clock mechanism. In this study we report that both light-induced and circadian-driven cone myoid movements in the Midas cichlid (Cichlasoma citrinellum), occur normally in vitro. Many of the features of retinomotor movements found in vivo also occur in our culture conditions, including responses to light and circadian stimuli and dopamine. Circadian induced predawn contraction and maintenance of expected day position in response to circadian modulation, are also normal. Our studies suggest that circadian regulation of cone myoid movement in vitro is mediated locally by dopamine, acting via a D2 receptor. Cone myoid contraction can be induced at midnight and expected mid-day by dark culture with dopamine or the D2 receptor agonist LY171555. Further, circadian induced predawn contraction can be increased with either dopamine or LY171555, or may be reversed with the dopamine D2 antagonist, sulpiride. Sulpiride will also induce cone myoid elongation in retinal cultures at expected mid-day, but will not induce cone myoid elongation at dusk. In contrast, circadian cone myoid movements in vitro were unaffected by the D1 receptor agonist SCH23390, or the D1 receptor antagonist SKF38393. Our short-term culture experiments indicate that circadian regulation of immediate cone myoid movement does not require humoral control but is regulated locally within the retina. The inclusion of dopamine, or dopamine receptor agonists and antagonists in our cultures, has indicated that retinal circadian regulation may be mediated by endogenously produced dopamine, which acts via a D2 mechanism.

Light and circadian modulation of teleost retinal tyrosine hydroxylase activity.

PURPOSE: The authors examined the effects of both light and circadian phase on the synthetic activity of the dopamine producing enzyme, tyrosine hydroxylase (TH), in the retina of the Midas cichlid (Cichlasoma citrinellum). METHODS: TH activity was assayed in the retina-retinal pigment epithelium (RPE) complex of fish entrained to cyclic light regimens. The animals were killed throughout cyclic light and continuous dark cycles and after light exposure following short-term dark adaptation to assess the effects of both diurnal and circadian phase and light exposure on TH activity. Variations in retina-RPE TH activity were compared with cone retinomotor position. RESULTS: The authors report that TH activity was influenced both by light and by circadian phase. In both cyclic light and continuous darkness, TH activity fluctuated in cyclic fashion, increasing to peak values at night, declining through dawn, and reaching minimal values at midday. In continuous dark-adapted retinae, the peak activity at night was approximately sixfold greater than the minimum activity during the day. In cyclic light-adapted retinae at midday, the TH activity was 2.4-fold higher than that determined at the same time point in continuous dark-adapted retinae. Light onset enhanced the TH activity in previously dark-adapted animals by 2.2-fold at midday and 1.7-fold at dusk. Thus, retinal TH activity exhibits fluctuations of approximately sixfold in response to circadian signals and is stimulated approximately twofold by light. CONCLUSIONS: These observations indicate that dopamine production in the retina is regulated both by light and by circadian phase and that there are cyclic changes in dopamine production during the 24-hr cycle both in normal light and dark cycles and under conditions of constant darkness.

A role for endogenous dopamine in circadian regulation of retinal cone movement.

Cone movements in the retina of the Midas cichlid (Cichlasoma citrinellum) take place in response both to light and endogenous circadian signals. In the normal light/dark cycle (LD) cone myoids are long at night (50-55 microns), begin to contract before expected dawn, and with light onset contract to their fully contracted positions (5 microns) which are retained throughout the day. In continuous darkness (DD) cone myoids are fully elongate at night, but undergo pre-dawn contractions to partially contracted positions which they retain throughout expected day (20-25 microns). To investigate the mechanisms by which circadian signals modulate cone myoid movements in teleost retinas, we have tested the effects on circadian cone movements of optic nerve section, intraocular injection of dopamine agonists or antagonists, and intraocular injection of melatonin. We report here that both light-induced and circadian-driven cone myoid movements can occur in the absence of efferent input from higher centres: both are retained with full amplitude after optic nerve section in vivo. Intraocular injection studies suggest that circadian regulation of cone myoid movement is mediated locally within the eye by dopamine acting via a dopaminergic D2-receptor. Cone myoid contraction can be induced at midnight in LD or DD animals by intraocular injection of dopamine or the D2-receptor agonist LY171555. The partially contracted cones of DD animals at expected mid-day can be induced to fully contract by intraocular injection of dopamine or the D2-receptor agonist, or to elongate by intraocular injection of the dopamine D2-antagonist sulpiride. Furthermore, the pre-dawn cone myoid contraction observed in both LD and DD animals in response to circadian signals can be completely blocked in DD animals by intraocular injection of the D2-antagonist sulpiride shortly before the time of expected light onset. In contrast, circadian cone myoid movements were unaffected by intraocular injection of the D1-receptor agonist SCH23390, or the D1-receptor antagonist SKF38393. In addition, we report that intraocularly injected melatonin had no effect on cone position when injected in the light at mid-day, in darkness at midnight or in darkness just before expected light onset at dawn. However, both melatonin and iodomelatonin induced cone myoid contraction (the light-adaptive movement) when injected in darkness at expected mid-day in DD animals. This paradoxical result is not consistent with observations from other species in which melatonin induces dark-adaptive photoreceptor responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of circadian phase on cone retinomotor movements in the Midas cichlid.

In retinas of the Midas cichlid (Cichlasoma citrinellum), cone myoid lengths change in response to both light and circadian signals. Cone myoids are short in day (or in the light), and long at night (or in darkness). During the normal light/dark cycle, cone myoids elongate to 58 microns after light offset at dusk, and remain long until just before dawn, at which time they begin to contract before the time of expected light onset and eventually (after light onset) assume daytime lengths of 5 microns. In continuous light, cone myoids remain fully contracted regardless of the time in the cycle. In continuous darkness, cone myoids undergo circadian movements; they elongate fully at night and contract partly during expected day to 23 microns (65% of the contraction seen at dawn in a normal light/dark cycle). To clarify further the regulation of these retinomotor movements, we have investigated the effects of circadian phase on cone movements induced by light or dark onset. Circadian phase had no significant effect on either the initial rate or the final extent of light-induced cone myoid contraction: at mid-dark cones began to contract immediately after light onset and contracted fully at 3.0 +/- 0.5 microns min-1; at mid-night cones contracted immediately and fully at 4.0 +/- 0.2 microns min-1. At all of the relatively high intensities of light tested (30-5000 lx) cones contracted at similar rates and to similar final extents.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenesis of diurnal rhythms of cAMP concentration, outer segment disc shedding and retinomotor movements in the eye of the brown trout, Salmo trutta.

Histological analyses of the retina of Salmo trutta post-embryos exhibit rhythmic disc shedding at 6 weeks, followed by cyclic retinomotor movements at 7 weeks. Diurnal variations in cAMP concentration, as established by a radioreceptor assay, develop simultaneously with retinomotor movements and are fully expressed at 9.5 weeks. In continuous darkness, neither retinomotor movements nor intra-ocular peaks of cAMP occur, indicating that the endogenous oscillator for retinomotor movements and cAMP levels is not yet developed.