Inhibition of RNA synthesis differentially affects in vitro melatonin release from the pineal organs of ayu (Plecoglossus altivelis) and rainbow trout (Oncorhynchus mykiss)

Neural regulation of dark-induced abundance of arylalkylamine N-acetyltransferase (AANAT) and melatonin in the carp (Catla catla) pineal: an in vitro study

In all the vertebrates, synthesis of melatonin and its rhythm-generating enzyme arylalkylamine N-acetyltransferase (AANAT) reaches its peak in the pineal during the night in a daily light-dark cycle, but the role of different neuronal signals in their regulation were unknown for any fish. Hence, the authors used specific agonist and antagonists of receptors for different neuronal signals and regulators of intracellular calcium (Ca(2+)) and adenosine 3',5'-cyclic monophosphate (cAMP) in vitro to study their effects on the abundance of AANAT and titer of melatonin in the carp (Catla catla) pineal. Western blot analysis followed by quantitative analysis of respective immunoblot data for AANAT protein, radioimmunoassay of melatonin, and spectrophotometric analysis of Ca(2+) in the pineal revealed stimulatory effects of both adrenergic (α(1) and β(1)) and dopaminergic (D(1)) agonists and cholinergic (both nicotinic and muscarinic) antagonists, inhibition by both adrenergic and dopaminergic antagonists and cholinergic agonists, but independent of the influence of any agonists or antagonists of α(2)-adrenergic receptors. Band intensity of AANAT and concentration of melatonin in the pineal were also enhanced by the intracellular calcium-releasing agent, activators of both calcium channel and adenylate cyclase, and phophodiesterase inhibitor, but suppressed by inhibitor of calcium channel and adenylate cyclase as well as activator of phophodiesterase. Moreover, an inhibitory effect of light on the pineal AANAT and melatonin was blocked by both cAMP and proteasomal proteolysis inhibitor MG132. Collectively, these data suggest that dark-induced abundance of AANAT and melatonin synthesis in the carp pineal are a multineuronal function, in which both adrenergic (α(1) and β(1), but not α(2)) and dopaminergic signals are stimulatory, whereas cholinergic signals are inhibitory. This study also provides indications, though arguably not conclusive evidence, that in either case the neuronal mechanisms follow a signal-transduction pathway in which Ca(2+) and cAMP may act as the intracellular messengers. It also appears that proteasomal proteolysis is a conserved event in the regulation of AANAT activity in vertebrates.

Effects of Macromolecule Synthesis Inhibitors on Light-Induced Phase Shift of the Circadian Rhythm in Melatonin Release from the Cultured Pineal Organ of a Teleost, Ayu (Plecoglossus altivelis)

Effects of macromolecule synthesis inhibitors on the light-induced phase shift of the circadian clock in the photoreceptive pineal organ of a teleost, ayu (Plecoglosus altivelis) were investigated using melatonin release as an indicator. A single light pulse during the early- and late-subjective night delayed and advanced the phase of the circadian rhythm in melatonin release, respectively. During the late subjective-night, protein synthesis inhibitor cycloheximide (CHX) delayed the rhythm while RNA synthesis inhibitor 5,6-dichlorobenzimidazole riboside (DRB) had little effect. Light-induced phase advance was diminished by the treatment of CHX but not by DRB. During the early subjective-night, DRB, CHX, light and combination of these (DRB+light, CHX+light) all phase-delayed the rhythm. There were no additive effects of light and DRB or CHX. These results indicate that macromolecule synthesis is somehow involved in generation of circadian oscillation, and that de novo protein synthesis is required for light-induced phase shift of the circadian clock in the ayu pineal organ.