Effects of the blockade of high voltage‐activated calcium channels on in vitro pineal melatonin synthesis

Abstract

The presence of high voltage-activated calcium channels in the rat pineal gland is well known. However, their role in pineal metabolism is not completely understood and is even controversial. Better to understand this matter, we investigated the effects of L-, N- or P/Q-type calcium channel blockers (nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, respectively) on melatonin content and arylalkylamine-N-acetyltransferase activity of denervated rat pineal glands kept for 48 h in culture and stimulated with norepinephrine. Melatonin was measured by high performance liquid chromatography with electrochemical detection and arylalkylamine-N-acetyltransferase activity was quantified by radiometric assay. Pre-incubation with any of these high voltage-activated calcium channel blockers reduced the melatonin production induced by norepinephrine although arylalkylamine-N-acetyltransferase activity was reduced only by the N-type calcium channel antagonist, omega-conotoxin GVIA. The results indicate that calcium influx through L-, N- or P/Q-type of high voltage-activated calcium channels is necessary for the full expression of the metabolic process leading to melatonin synthesis in the rat pineal glands. However, the mechanisms involved in this process are different for the L- or P/Q- and N-type calcium channels.

Norepinephrine Causes a Biphasic Change in Mammalian Pinealocye Membrane Potential: Role of α1B-Adrenoreceptors, Phospholipase C, and Ca2+

“…Second, tetrodotoxin, a blocker of Na v channels, does not abolish spon- taneous firing of AP in rat pinealocytes (27), suggesting a role of Ca v channels in spike depolarization. Consistent with this hypothesis, it has been reported that rat and chicken pineal cells express L-, P/Q, and N-type Ca v channels (17,18,35,36). Unpublished RNA transcriptome profiling by RNA-Seq indicates that relative expression of mRNA encoding these channels in rat pinealocytes is L-type Ͼ T-type Ͼ Ͼ P/Q-type and Ϸ R-type channels (Klein, D. C., unpublished data).…”

“…These cells express numerous plasma membrane channels as revealed by electrophysiological studies, including voltage-gated Ca 2ϩ (Ca v ) and voltage-gated Na ϩ (Na v ) channels, two types of Ca 2ϩ -controlled K ϩ (K Ca ) channels, and at least two types of voltage-activated K ϩ channels (7,(15)(16)(17)(18)(19)(20)(21). Rat pinealocytes fire action potentials (AP) spontaneously, with the firing rate being low during the day time and high during the night time (22)(23)(24)(25).…”

Insulin modulates norepinephrine-mediated melatonin synthesis in cultured rat pineal gland

“…AANAT activity was measured by a radiometric assay (Afeche et al, 2006;Deguchi and Axelrod, 1972;Parfitt et al, 1975). Briefly, 100μl of 0.1M sodium phosphate buffer, pH 6.8, containing 40mM tryptamine and [ 3 H]-acetyl coenzyme A (2mM, final specific activity = 4mCi/mmol) were added to a microcentrifuge tube containing one gland kept at 4°C.…”

“…Experiment 1: Standard pineal gland culture Pineal glands were cultured as described previously (Afeche et al, 2006). Briefly after decapitation, rat pineal glands were isolated and immediately placed in ice-cold BGJb (FittonJackson Modification) medium with phenol red, modified by the addition of bovine serum albumin (BSA; 1mg/mL), 2mM glutamine, 0.1mg/mL ascorbic acid and penicillin (100U/mL)-streptomycin (100μg/mL) (Gibco.…”

Tryptophan hydroxylase is modulated by L-type calcium channels in the rat pineal gland

“…Moreover, high voltage-activated calcium channels are also present in the rat pineal gland (Afeche et al, 2006;Chik et al, 1995Chik et al, , 1997Chin et al, 1992) and these seem to be activated either by acetylcholine or norepinephrine (Afeche et al, 2006;Darvish and Russell, 1998;Letz et al, 1997). We have recently reported that the blockade of L-type calcium channel with nifedipine, a specific L-type calcium channel antagonist belonging to dihydropyridine class, reduced melatonin synthesis (Afeche et al, 2006), but the mechanisms involved were not resolved.…”