Circadian regulation of retinomotor movements: II. The role of GABA in the regulation of cone position

Cone photoreceptor movements in lower vertebrates are regulated by the interaction of the light-dark cycle and an endogenous circadian clock. We have suggested that melatonin and dopamine interact to regulate dark- and light-adaptive movements, respectively, and that melatonin affects cones indirectly by inhibiting dopamine release. In fact, any factor modulating dopaminergic neurons in the retina may have effects on either cone elongation or contraction. We have utilized an in vitro eyecup preparation from the African clawed frog, Xenopus laevis, to evaluate a possible role of the neurotransmitter GABA, which is thought to tonically suppress dopamine release. GABA agonists mimic the effects of darkness and induce cone elongation; the effects of GABA agonists are blocked by dopamine. Muscimol-induced cone elongation occurs at low light intensity but is inhibited by bright light in eyecups prepared from cyclic-light-maintained animals. Although neither melatonin nor muscimol stimulates cone elongation in bright light, simultaneous application of both drugs induces elongation. The GABA antagonist picrotoxin induces cone contraction which is blocked by the dopamine receptor antagonist spiroperidol, which suggests that GABA may affect cone movement in Xenopus by regulating dopamine neurons. Consistent with this, picrotoxin-induced cone contraction is Ca+2 dependent and is blocked by high Mg+2 or the Ca+2 antagonist nifedipine. Pharmacological analysis suggests that the effects of GABA may result from its action at more than one receptor subtype. Our results support the hypothesis that dopamine is part of the light signal for cone contraction and that its suppression is part of the signal for cone elongation.     

Circadian activity of the GABAergic system in the golden hamster retina

Daily changes in gamma-aminobutyric acid (GABA) turnover rate were studied in the golden hamster retina. This parameter showed significant variations throughout the light-dark cycle, with minimal values during the day. Retinal glutamic acid decarboxylase (GAD) activity was higher at midnight than at noon. Moreover, [3H]GABA binding significantly varied throughout the 24-h cycle, with maximal values during the day. Saturation studies performed at 12:00 and 24:00 h indicated that the maximal concentration of [3H]GABA binding sites (Bmax) was significantly higher at noon, whereas the dissociation constant (Kd) remained unchanged. High K+-induced GABA release was significantly higher at midnight than at midday. Daily variations in retinal GABA turnover rate, GABA release, and in its specific binding persisted in golden hamsters exposed to constant darkness. In summary, these results support the idea of a circadian clock-controlled GABAergic activity in the hamster retina.     

Circadian changes in the response of the rabbits retina to flashes

It was previously shown that in rabbits the electroretinogram (ERG) and the visually evoked potential show a circadian rhythm consisting of a stable phase during which the animals respond to flashes with day time potentials and another phase with night time potentials (NTPs) and that the occurrence of the sharp phase transitions is programmed by the time course of the preceding light-dark schedule. From the present investigation it follows that photic sensitivity to flashes is markedly higher during the NTP-phase than in the other phase, whereas dark adaptation runs the same time course in both phases. With respect to the ERG it appears that, over a wide range of flash intensities its b wave only is affected by phase of the rhythm. The differences in photic responses between the 2 phases are discussed in the light of possibly synchronous changes in the influence of sympathetic nerve fibres on eye structures.     

Circadian rhythm in the light response of rat retinal disk-shedding and autophagy

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.     

Photic and circadian regulation of retinal melatonin in mammals

Several studies have established that melatonin synthesis occurs in the retina of vertebrates, including mammals. In mammals, a subpopulation of photoreceptors (probably the cones) synthesize melatonin. Melatonin synthesis in the retina is elevated at night and reduced during the day in a fashion similar to events in the pineal gland. Both the MT1 and MT2 melatonin receptors are present in the retina and retinal melatonin does not contribute to circulating levels, suggesting that retinal melatonin acts locally as a neurohormone and/or neuromodulator. Melatonin synthesis in the retina of mammals is under the control of a circadian oscillator, and circadian rhythms in melatonin synthesis and/or release have been described for several species of mammals. These rhythms are present in vivo, persist in vitro, are entrained by light and are temperature compensated. The cloning of the gene responsible for the synthesis of the enzyme arylalkylamine N-acetyltransferase (the key enzyme in the melatonin biosynthetic pathway) has allowed studies of the molecular mechanisms responsible for the generation of retinal melatonin rhythmicity. The present review focuses on the cellular and molecular mechanisms that regulate melatonin synthesis. In particular, we discuss how the photic environment and the circadian clock interact in determining melatonin levels, in addition to the role that melatonin plays in retinal physiology.     

Circadian and seasonal rhythms in alpha- and beta-adrenergic receptors in the rat brain

Circadian rhythms in the numbers of alpha- and beta-adrenergic receptors exists in the rat forebrain and hypothalamus. These rhythms are endogenous as they persist in the absence of time cues. The alpha- and beta-receptor rhythms differ both in their shapes (wave form) and in the timing (phase) of the peak numbers. In the course of the year, there are shifts in the timing of both alpha- and beta-receptor peaks. Circadian rhythms in synchronizing both the synaptic events and the behaviors related to them to the photoperiod of the natural environment.     

Melatonin: Generation and Modulation of Avian Circadian Rhythms

The pineal organ and its hormone melatonin are significant components of avian circadian pacemaking systems. In songbirds, pinealectomy results in the abolition or destabilization of overt circadian rhythms such as the rhythm of locomotor activity, feeding, or body temperature. A stable rhythmicity can be restored either by reimplanting a pineal organ, by periodic injections or infusions of melatonin, or by applying melatonin rhythmically through the drinking water. Several results suggest that the pineal melatonin rhythm acts on at least one other oscillator within the circadian pacemaking system, presumably the SCN, which in turn, feeds back to the pineal. As described by the “Neuroendocrine Loop” and “Internal Resonance” models, overall pacemaker output thus depends on the relative strengths of the oscillations in the pineal and the SCN. Investigations on migratory birds have shown that the amplitude of the 24-h plasma melatonin rhythm is reduced during the migratory seasons compared with the nonmigratory seasons. According to the models mentioned above, such a reduced melatonin amplitude should result in a reduction in the degree of self-sustainment of the pacemaker as a whole. This, in turn, should facilitate adjustment to the altered Zeitgeber conditions encountered by these birds as a result of their own migratory flights. A seasonal reduction in melatonin amplitude also occurs in some high-latitude birds during midsummer and midwinter. Under such conditions a less self-sustained circadian pacemaker may enhance entrainability to weak zeitgeber conditions. These examples suggest that the properties of the circadian system may be adjusted to match the changing requirements for synchronization, and that this is achieved by altering the melatonin amplitude.     

Dopamine D2 receptor-mediated modulation of rod-cone coupling in the Xenopus retina

We studied the responses of rod photoreceptors that were elicited with light flashes or sinusoidally modulated light by using intracellular recording. Dark-adapted Xenopus rod photoreceptors responded to sinusoidally modulated green lights at temporal frequencies between 1 Hz and 4 Hz. In normal Ringer's solution, 57% of the rods tested could follow red lights that were matched for equal rod absorbance to frequencies >5 Hz, indicating an input from red-sensitive cones. Quinpirole (10 μM), a D2 dopamine agonist, increased rod-cone coupling, whereas spiperone (5 μM), a selective D2 antagonist, completely suppressed it. D1 dopamine ligands were without effect. Neurobiotin that was injected into single rods diffused into neighboring rods and cones in quinpirole-treated retinas but only diffused into rods in spiperone-treated retinas. A subpopulation of rods (ca. 10% total rods) received a very strong cone input, which quickened the kinetics of their responses to red flashes and greatly increased the bandpass of their responses to sinusoidally modulated light. Based on electron microscopic examination, which showed that rod-rod and cone-cone gap junctions are common, whereas rod-cone junctions are relatively rare, we postulate that cone signals enter the rod network through a minority of rods with strong cone connections, from which the cone signal is further distributed in the rod network. A semiquantitative model of coupling, based on measures of gap-junction size and distribution and estimates of their conductance and open times, provides support for this assumption. The same network would permit rod signals to reach cones. J. Comp. Neurol. 398:529–538, 1998. © 1998 Wiley-Liss, Inc.     

Nyctalopin is essential for synaptic transmission in the cone dominated zebrafish retina

The first synapse in the vertebrate visual system is the photoreceptor synapse between rod and cone photoreceptors and the second-order bipolar cells. Although mutations in the nyctalopin gene (NYX) in humans lead to congenital stationary night blindness (CSNB1), affecting synaptic transmission between both types of photoreceptors and ON-bipolar cells, the function of nyctalopin in cone-dominant animal models has not been studied. Because the larval zebrafish retina is cone-dominant, we isolated the zebrafish nyx ortholog and raised a polyclonal antibody against the protein. Nyctalopin is expressed postsynaptically in both synaptic layers of the retina. Functional disruption via morpholino antisense injection leads to characteristic defects in the electroretinogram and defects in visual contrast sensitivity. We therefore demonstrated that nyctalopin plays a similar role in retinal synapse function in the cone pathway as in the rod pathway, thereby creating a genetic model for CSNB1 and its effects on cone vision.     

Horizontal cell processes in teleost retina

Contacts between horizontal and bipolar cells are described in the retina of the teleost Eugerres plumieri. A single, long expansion observed in the external cone horizontal cells makes contact by means of a terminal button with the cell body of a bipolar. It represents the only contact between this class of horizontal cell and the bipolar soma. On the other hand, the medial and internal cone horizontal cells and the rod horizontal cells, which lack such a single, long expansion, display instead numerous short and fine expansions that terminate by means of a terminal knob on a bipolar cell body. The bipolar-destined, short expansions of the rod horizontal cell make contact with large bipolar cell bodies, whereas corresponding short expansions of cone horizontal cells contact small bipolar cell bodies. It is suggested that the ascending horizontal cell process forms presynaptic terminals in the photoreceptor triad complex, and that the single, long and the multiple, short bipolar-destined expansions are postsynaptic to the bipolar cell body.     

Immunocytochemical localization of the D2-protein in rat retina

The localization of the D2-protein in rat retina was investigated by quantitative immunoelectrophoresis and by immunoperoxidase staining on both the light microscopic and the electron microscopic level. The D2-protein was enriched 1.4 times in retina compared to whole brain and it was located to most presynaptic and some postsynaptic membranes in the inner and outer plexiform layers. Moreover, the D2-protein was located on the cell membrane confining the ciliary neck region connecting the inner and the outer segments of photoreceptor cells. Thus, on the electron microscopic level, the D2-protein is not restricted to the presynaptic membrane, but it is also found in other parts of the neuronal surface membrane.     

Circadian rhythm of plasma melatonin in endogenous depression

1. The circadian rhythm of plasma melatonin was investigated in normal men 18–30 years (N=5), normal men 50–70 years (N=5) and in six patients with endogenous depression.

2. The environmental photoperiod was 11 hours.

3. The subjects and patients were indoors with lights on from 07:00 until 23:00 hours.

4. Blood samples were obtained every 4 hours over a 24 hour period, with additional sampling at 22:00 and 02:00 hours.

5. Plasma melatonin was estimated by radioimmunoassay compared to both groups of controls.

6. In the depressed patients, the levels of melatonin were low throughout the 24 hour period.

7. The depressives had a delayed onset of the dark phase of the rhythm.

8. The patients also showed peak melatonin levels occurring earlier than in the controls.

9. Circadian rhythm of melatonin and therefore of its pacemaker may be altered in endogenous depression.     

Circadian variations in radioligand binding to muscarine receptors in rat brain dependent upon endogenous agonist occupation

Although total levels of muscarine receptors in the rat forebrain and brainstem are constant at different times of day, there are large circadian changes in the numbers and carbachol-affinities of unoccupied receptors, which appear to reflect varying levels of endogenous acetylcholine-receptor complexes and the activity of cholinergic synapses.     

Absence of circadian clock regulation of horizontal cell gap junctional coupling reveals two dopamine systems in the goldfish retina

In fish and other vertebrate retinas, although dopamine release is regulated by both light and an endogenous circadian (24-hour) clock, light increases dopamine release to a greater extent than the clock. The clock increases dopamine release during the subjective day so that D2-like receptors are activated. It is not known, however, whether the retinal clock also activates D1 receptors, which display a much lower sensitivity to dopamine in intact tissue. Because activation of the D1 receptors on fish cone horizontal (H1) cells uncouples the gap junctions between the cells, we studied whether the clock regulates the extent of biocytin tracer coupling in the goldfish retina. Tracer coupling between H1 cells was extensive under dark-adapted conditions (low scotopic range) and similar in the subjective day, subjective night, day, and night. An average of approximately 180 cells were coupled in each dark-adapted condition. However, bright light stimulation or application of the D1 agonist SKF38393 (10 microM) dramatically reduced H1 cell coupling. The D2 agonist quinpirole (1 microM) or application of the D1 antagonist SCH23390 (10 microM) and/or the D2 antagonist spiperone (10 microM) had no effect on H1 cell coupling in dark-adapted retinas. These observations demonstrate that H1 cell gap junctional coupling and thus D1 receptor activity are not affected by endogenous dopamine under dark-adapted conditions. The results suggest that two different dopamine systems are present in the goldfish retina. One system is controlled by an endogenous clock that activates low threshold D2-like receptors in the day, whereas the second system is controlled by light and involves activation of higher threshold D1 receptors.     

Photoreceptor Cells of the Pike Pineal Organ as Cellular Circadian Oscillators

In the pike pineal, the rhythm of melatonin (MEL) secretion is driven by a population of cellular circadian oscillators, synchronized by the 24 h light/dark (LD) cycle. Because the pineal photoreceptor contains both the input and output pathways of the clock, this cell is likely to be a cellular circadian system by itself. To support this idea, we have dissociated and cultured pike pineal cells as well as purified photoreceptors. In culture, the pineal cells reassociated in follicles, surrounded by collagen fibres. At the electron microscopic level, they appeared well preserved. Total cells consisted mainly of photoreceptors and glia. Purified cells corresponded exclusively to photoreceptors. Under LD, MEL production was rhythmic. Under constant darkness (DD), the rhythm was well sustained for at least six 24 h cycles (τ= 24/27 h) with 1 × 10⁶ total cells/well or below; with 2 × 10⁶ total cells/well, a strong damping occurred towards high levels as soon as after the second cycle. At the density of 0.5 × 10⁶ cells/well, purified photoreceptors produced less MEL than an equivalent amount of total cells. However, the pattern of the oscillations was similar to that observed with 2 × 10⁶ total cells, i.e. a damping occurred rapidly. Decreasing the density to 0.125 × 10⁶ photoreceptors/well resulted in a loss of homogeneity among replicates. The production of melatonin by single photoreceptors was monitored by means of the reverse haemolytic plaque assay. Both under LD and under DD, the number of photoreceptors releasing melatonin was higher during the (subjective) dark than during the (subjective) light. The results provide strong support to the idea that the pike pineal photoreceptor is a cellular circadian system. Expression of the oscillations seemed to depend on several factors, including cell to cell contacts between photoreceptors. There is indication that also MEL and glia might be involved.     

Localization of the glutamate transporter protein GLAST in rat retina

Glutamate is a neurotransmitter in retina. Glutamate transporter proteins keep the resting extracellular glutamate concentration low. This is required for normal neurotransmission and prevents the extracellular concentration of glutamate from reaching toxic levels. Here we describe the light and electron microscopic localization of the glutamate transporter protein GLAST in rat retina using an antibody raised and affinity purified against a peptide corresponding to amino acid residues 522–541. The strongest immunocytochemical labelling was observed in the outer plexiform layer, ganglion cell layer, and optic disc. GLAST was found in Müller cell processes in all retinal layers, notably ensheathing the photoreceptor terminals in the outer plexiform layer, and in astrocytes close to vessels in the inner retina and optic disc. No labelling was observed in neurons. The electrophoretic mobility of GLAST in retina was similar to that in cerebellum. In conclusion, the findings are in agreement with those reported by Derouiche and Rauen [7], except that we did not detect any GLAST in the retinal pigment epithelium.     

Circadian rhythm in bipolar disorder: A review of the literature

Sleep disturbances and circadian rhythm dysfunction have been widely demonstrated in patients with bipolar disorder (BD). Irregularity of the sleep–wake rhythm, eveningness chronotype, abnormality of melatonin secretion, vulnerability of clock genes, and the irregularity of social time cues have also been well‐documented in BD. Circadian rhythm dysfunction is prominent in BD compared with that in major depressive disorders, implying that circadian rhythm dysfunction is a trait marker of BD. In the clinical course of BD, the circadian rhythm dysfunctions may act as predictors for the first onset of BD and the relapse of mood episodes. Treatments focusing on sleep disturbances and circadian rhythm dysfunction in combination with pharmacological, psychosocial, and chronobiological treatments are believed to be useful for relapse prevention. Further studies are therefore warranted to clarify the relation between circadian rhythm dysfunction and the pathophysiology of BD to develop treatment strategies for achieving recovery in BD patients.