Regulation of calcium influx and phospholipase C activity by indoleamines in dinoflagellate Crypthecodiniurn cohnii

Abstract

Exogenous indoleamines such as melatonin and 5-methoxytryptamine have been shown to induce cyst formation (encystment) in many species of dinoflagellate. Induction of inositol phosphates formation by indoleamine has previously been demonstrated in Crypthecodinium cohnii. In addition, depletion of extracellular Ca2+ blocks the indoleamine-induced encystment. In the present study, 12 indoleamines (including melatonin and related compounds) were examined for their abilities to induce Ca2+ influx, inositol phosphates formation, and encystment in C. cohnii. The results showed that melatonin, 5-methoxytryptamine, and the peptide toxin mastoparan stimulated 45Ca2+ influxes in dose- and time-dependent manners. The EC50 values of 5-methoxytrypramine and mastoparan to stimulate 45Ca2+ uptake were 2 mM and 35 microM, respectively. The 5-methoxytryptamine- and mastoparan-induced 45Ca2+ influx were partially attenuated by the calcium channel blockers, verapamil and ruthenium red. A series of indoleamines were examined for their structure-activity relationship on the induction of encystment and formation of inositol phosphates. Melatonin-induced inositol phosphates formation was completely blocked by U73122, indicating the possible involvement of phospholipase C. Taken together, we conclude that indoleamines may induce encystment of the dinoflagellate C. cohnii via parallel activation of phospholipase C and Ca2+ influx signaling pathways. However, activation of phospholipase C and Ca2+ influx are not always necessary or sufficient for inducing encystment. Also, these data provided the first direct evidence of a Ca2+ influx regulating mechanism in dinoflagellate C. cohnii.

Melatonin regulates neuronal plasticity in the hippocampus

“…In contrast to the case for LTP, the majority of studies (Creager et al, 1980; Christie and Abraham, 1994; Schulz et al, 1994; Carter et al, 2002) and a recently proposed model for a short‐term plasticity (Dittman et al, 2000) implicate a transient elevation in the presynaptic calcium concentration as a major mechanism of PPF (but see Wang and Kelly, 1996, but see 1997). Therefore, since the basic properties of LTP were not influenced by melatonin, one can assume that this hormone modifies neuronal excitability, modulating, as in other systems, presynaptic calcium concentration (Zisapel and Laundon, 1983; Vacas et al, 1984; Rosenstein et al, 1991; Faillace et al, 1996; Tsim et al, 1998). The reported influence of melatonin on presynaptic mechanisms of glutamergic neurotransmission (Zhang et al, 1999; Escames et al, 2001) supports this conclusion.…”

Melatonin and 5-methoxytryptamine in non-metazoans

“…Moreover, inhibitors of monoamine oxidase, which cause the accumulation of 5MT to cyst-inducing concentrations [12,15], also elicited the two responses [13][14][15]17]. Therefore, the question is how findings on rises in 5MT, signalled by proton transfer and effects on calcium and inositol phosphates, as found in Crypthecodinium [38,39], can be matched; that is, in which temporal and functional sequence such events will interact.…”

“…Recent findings in Crypthecodinium cohnii [38,39], showing increases in calcium influx and in the formation of inositol phosphates, are affected by the problem that, apart from the extremely high concentrations required, the dose-responses to melatonin and 5MT are relatively similar, despite the strong difference in cyst-inducing efficiency. [24].…”

“…Since [31,33], and in the angiosperm Chenopodium rubrum [23]. [38,39]). Moreover, the effects can be conditional, as in Gonyaulax polyedra, in which the encystment response occurs only at a lower temperature [2,13,14]; such a treatment leads to enormous accumulations of melatonin anyway (transiently more than 1 mM) and finally to high concentrations of 5-methoxytryptamine (= 5MT) [ 12,15].…”

Proliferation of dinoflagellates: blooming or bleaching

“…(45) The signal transduction system linking indoleamines to the encystment process involves G proteins, nuclear receptors and inositol phosphates. (47)(48)(49) The intracellular concentration of melatonin can reach levels higher than that in the mammalian pineal gland. Subencystment levels of 5-MT can dose-dependently inhibit cell proliferation and exogenous dibutyl cAMP is able to counter melatonin-induced temporary encystment in dinoflagellates.…”