Regulation of Melatonin Production by Pineal Photoreceptor Cells: Role of Cyclic Nucleotides in the Trout (Oncorhynchus mykiss)

Abstract

The light/dark cycle influences the rhythmic production of melatonin by the trout pineal organ through a modulation of the serotonin N-acetyltransferase (NAT) activity. In static organ culture, cyclic AMP (cAMP) levels (in darkness) and NAT activity (in darkness or light) were stimulated in the presence of forskolin, isobutylmethylxanthine, or theophylline. Analogues of cAMP, but not of cyclic GMP, induced an increase in NAT activity. Light, applied after dark adaptation, inhibited NAT activity. This inhibitory effect was partially prevented in the presence of drugs stimulating cAMP accumulation. In addition, cAMP accumulation and NAT activity increase, induced by forskolin, were temperature dependent. Finally, melatonin release, determined in superfused organs under normal conditions of illumination, was stimulated during the light period of a light/dark cycle by adding an analogue of cAMP or a phosphodiesterase inhibitor. However, no further increase in melatonin release was observed during the dark phase of this cycle in the presence of the drugs. This report shows for the first time that cAMP is a candidate as intracellular second messenger participating in the control of NAT activity and melatonin production by light and temperature.

cAMP Regulation of ArylalkylamineN-Acetyltransferase (AANAT, EC 2.3.1.87)

“…A similar situation may exist with control versus FSK-treated 1E7 cells. Data on the pike pineal gland in culture are also in agreement with this hypothesis, because it has been found the DBcAMP treatment increases melatonin production Ͼ10-fold, without markedly increasing AANAT activity measured in broken cell preparations (15).…”

“…Although most data are consistent with the view that AANAT activity is regulated by controlling the steady-state levels of AANAT protein, limited observations in the literature point to the existence of another regulatory mechanism through which melatonin production might change without altering AANAT protein and AANAT activity as estimated in broken cell preparations (13)(14)(15). This hypothetical activation/ inactivation mechanism is of special interest, because it may explain the very rapid physiological increase in circulating melatonin seen at the onset of the dark period in humans as well as other species (1, 16 -18).…”

Unique arylalkylamine N-acetyltransferase-2 polymorphism in Salmonids and profound variations in thermal stability and catalytic efficiency conferred by two residues

“…in addition to these molecular adaptations some cellular mechanisms are probably involved, which transduce the temperature effect and regulate melatonin secretion. One target might be the cyclic AMP-dependent pathway (Thibault et al, 1993). Previous studies have shown that the dark induced rise in cyclic AMP induces AANAT phosphorylation through the RRNH and RRNS sequences at both ends of the molecule (present in the two AANAT2 Arctic charr isoforms); this allows binding of the enzyme to a chaperone protein (14-3-3), providing higher stability and preventing degradation through the proteasome (Falcón et (saAANAT2_IA, light blue; saAANAT2_VV, blue; AANAT2_VA mutant, green; omAANAT2_IV, purple).…”

Stress inhibition of melatonin synthesis in the pineal organ of rainbow trout (Oncorhynchus mykiss) is mediated by cortisol.

“…Only salmonids, including rainbow trout, are an exception to this rule. In all salmonid species investigated to date it has been demonstrated that pineal melatonin synthesis does not involve an endogenous clock, so that lack of melatonin oscillation has been described under constant conditions (Thibault et al, 1993;Gern and Greenhouse, 1988;Mizusawa et al, 2000;Migaud et al, 2007). However even under constant darkness several core circadian genes continue to cycle in other trout neural regions (retina and hypothalamus) (López-Patiño et al, 2011a) that are involved in the regulation of daily rhythms of several parameters such as feeding behavior and locomotor activity (Cuenca and de la Higuera, 1994;Sánchez-Vázquez and Tabata, 1998).…”