Retinal dopamine-containing amacrine neurons are rapidly activated by light, as shown by an increase in the rate of dopamine formation in vivo and a concomitant increase in the activity of tyrosine hydroxylase, measured in vitro with a subsaturating concentration of pteridine cofactor. Activation of tyrosine hydroxylase also occurs when isolated eyes from rats killed in the dark are exposed to a strobe light. Studies of amacrine neurons should provide basic data about the biochemical processing of visual information, as well as the physiological presynaptic regulatory mechanisms of dopamine-containing neurons.
Circadian rhythms are controlled by endogenous oscillators or clocks. These clocks exhibit a persistent period of approximately 24 h in constant conditions, a specific phase relationship to a periodic cue (zeitgeber) in the external environment, and plasticity in that the phase of the clock may be altered in response to a phase change in the zeitgeber. Although many processes exhibit circadian rhythmicity, the nature and location of endogenous clocks remain poorly defined. Recent evidence in vertebrates suggests that the mammalian suprachiasmatic nucleus and the avian pineal gland contain clocks that affect the rhythmicity of indoleamine metabolism. The vertebrate retina also exhibits a circadian rhythm of serotonin N-acetytransferase activity (NAT, EC 2.1.1.4), a key enzyme controlling melatonin synthesis, and of photoreceptor disk shedding. The latter process may be regulated by melatonin, and the immediate cellular events seem to be controlled locally within the eye. Although sustained oscillation and entrainment were not demonstrated, data suggesting that an ocular circadian clock influences disk shedding have been reported. We sought evidence for an ocular clock by studying retinal NAT activity in Xenopus eye cups maintained in culture and report here both sustained oscillation and entrainment of the in vitro system. The data indicate that in addition to the suprachiasmatic nucleus and pineal gland, the eye itself must be regarded as the locus of a circadian clock in vertebrates.
The normal postnatal development, the influence of age, and the effects of visual deprivation on the dopamine system in the retina of rhesus monkeys were examined. The lowest level of retinal dopamine was found at birth. By 3-4 weeks of age, the dopamine concentration had more than doubled. This level remained relatively constant in the retinas of older infants and of adult monkeys up to 34 yr of age. The level of the dopamine metabolite 3,4-dihydroxyphenyIacetic acid (DOPAC) and the activity of tyrosine hydroxylase did not significantly change as a function of age during the postnatal life span. Monocular occlusion of newborn or infant monkeys for 1-15 months with opaque contact lenses resulted in decreases in the retinal concentrations of dopamine and DOPAC relative to the concentrations in the same animals' unoccluded eyes. Occlusion also resulted in a lower level of tyrosine hydroxylase activity in the retina. Monocular eyelid suture from birth to 15 months of age resulted in less consistent alterations of retinal dopamine and DOPAC levels; tyrosine hydroxylase activity, however, was consistently reduced by lid suture. Thus, dopamine synthesis and metabolism, and the ontogenetic increase of the retinal dopamine level of rhesus monkey are reduced by light deprivation.
In chicken retinas, melatonin levels and the activity of serotonin N-acetyltransferase (NAT), a key regulatory enzyme of melatonin biosynthesis, are expressed as circadian rhythms with peaks of levels and activity occurring at night. In the present study, NAT activity was examined in retinas of embryonic and posthatch chicks to assess the ontogenic development of regulation of the enzyme by light, circadian oscillators, and the second messenger cyclic AMP. During embryonic development, NAT activity was consistently detectable by embryonic day 6 (E6). Significant light-dark differences were first observed on E20, and increased to a maximum amplitude of sixfold by posthatch day 3 (PH3). Circadian rhythmicity of NAT activity appears to develop at or prior to hatching, as evidenced by day-night differences of activity in constant darkness observed in PH1 chicks that had been exposed to a light-dark cycle in ovo only. NAT activity is regulated by a cyclic AMP-dependent mechanism. Activity was significantly increased by incubating retinas with forskolin or dibutyryl cyclic AMP as early as E7, and seven- to ninefold increases were observed following treatment with these agents on E14. Thus, development of the cyclic AMP-dependent mechanism for increasing NAT activity significantly precedes that of rhythmicity, suggesting that the onset of rhythmicity may be related to the onset of photoreception or development of the circadian oscillator in chick retina.
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