http://if-pan.krakow.pl/pjp/pdf/2009/3_383.pdf

Physiology and pharmacology of melatonin in relation to biological rhythms.

Objective:
To present an updated, comprehensive review on clinical and pre-clinical studies on agomelatine.

A systematic, updated review on the antidepressant agomelatine focusing on its melatonergic modulation.

Inner retinal circadian clocks and non-visual photoreceptors: novel players in the circadian system.

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Role of dopamine in distal retina.

According to current knowledge, disturbances in brain monoamine transmission play a major role in many psychiatric disorders, and many of the radioligands used for investigating these disorders bind to targets within the brain monoamine systems However, a phylogenetically ancient and prevailing function of monoamines is to mediate the adaptation of organisms and cells to rhythmical changes in light conditions, and to other environmental rhythms, such as changes in temperature, or the availability of energy resources throughout the seasons The physiological systems mediating these changes are highly conserved throughout species, including humans Here we review the literature on seasonal changes in binding of monoaminergic ligands in the human brain Moreover, we argue for the importance of considering possible effects of season when investigating brain monoamines in healthy subjects and subjects with psychiatric disorders

Imaging of seasonal affective disorder and seasonality effects on serotonin and dopamine function in the human brain.

Substantial evidence suggests that dopamine and melatonin are mutually inhibitory factors that act in the retina as chemical analogs of day and night. Here, we show an impact of environmental light, biological clock, and melatonin on retinal levels of dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the turkey. In turkeys held under different light (L) to dark (D) cycles (16L:8D, 12L:12D, 8L:16D), retinal levels of dopamine and DOPAC fluctuated with daily rhythms. High levels of dopamine and DOPAC were observed during light hours and low during dark hours. Under the three photoperiodic regimes, rhythms of dopamine and DOPAC were out of phase with daily oscillation in retinal melatonin content. In constant darkness, dopamine and DOPAC levels oscillated in circadian rhythms. Light deprivation resulted, however, in a significant decline in amplitudes of both rhythms. Injections of melatonin (0.1–1 μmol/eye) during daytime significantly reduced retinal levels of DOPAC. This suppressive effect of melatonin was more pronounced in the dark-adapted than light-exposed turkeys. Quinpirole (a D2/D4-dopamine receptor agonist; 0.1–10 nmol/eye) injected to dark-adapted turkeys significantly decreased retinal melatonin. Our results indicate that in the turkey retina: (1) environmental light is the major factor regulating dopamine synthesis and metabolism; (2) dopaminergic neurones are controlled, in part, by intrinsic circadian clock; and (3) dopamine and melatonin are components of the mutually inhibitory loop.

Dopamine in the Turkey retina-an impact of environmental light, circadian clock, and melatonin.

Dynamics of rhythmic oscillations in the activity of arylalkylamine N-acetyltransferase (AA-NAT, the penultimate and key regulatory enzyme in melatonin biosynthesis) were examined in the retina and pineal gland of turkeys maintained for 7 days in the environment without daily light–dark (LD) changes, namely constant darkness (DD) or continuous light (LL). The two tissues differentially responded to constant environment. In the retina, a circadian AA-NAT activity rhythm disappeared after 5 days of DD, while in the pineal gland it persisted for the whole experiment. No circadian rhythm was observed in the retinas of turkeys exposed to LL, although rhythmic oscillations in both AA-NAT and melatonin content were found in the pineal glands. Both tissues required one or two cycles of the re-installed LD for the full recovery of the high-amplitude AA-NAT rhythm suppressed under constant conditions. It is suggested that the retina of turkey is less able to maintain rhythmicity in constant environment and is more sensitive to changes in the environmental lighting conditions than the pineal gland. Our results indicate that, in contrast to mammals, pineal glands of light-exposed galliformes maintain the limited capacity to rhythmically produce melatonin.

Anna Lorenc-Duda

Papers

Dopamine in the Turkey retina-an impact of environmental light, circadian clock, and melatonin.

Turkey retina and pineal gland differentially respond to constant environment