Phorbol esters mimic alpha-Adrenergic potentiation of serotonin N-acetyltransferase induction in the rat pineal

Adrenergic regulation of mitogen-activated protein kinase in rat pinealocytes: opposing effects of protein kinase A and protein kinase G

The role of adrenergic stimulation in the regulation of mitogen-activated protein kinase (MAPK) in rat pinealocytes was investigated by measuring phosphorylated MAPK using Western blot analysis and a MAPK enzymatic assay. Stimulation with the endogenous neurotransmitter, norepinephrine (NE; a mixed alpha- and beta-adrenergic agonist), concentration dependently increased the phosphorylation of both p44 and p42 isoforms of MAPK. This effect of NE was blocked by PD98059 and U0126 (two inhibitors of MEK). Treatment with prazosin or propranolol significantly reduced the effect of NE on MAPK phosphorylation, suggesting the involvement of both alpha- and beta-adrenergic receptors. Investigation into the intracellular mechanisms of NE action revealed that the increase in MAPK phosphorylation was blocked by KT5823 (a protein kinase G inhibitor), but was enhanced by H89 (a protein kinase A inhibitor). Calphostin C (a protein kinase C inhibitor) and KN93 (a Ca2+/calmodulin-dependent protein kinase inhibitor) also attenuated NE-mediated MAPK activation, but to a lesser degree. Furthermore, inhibition of MAPK phosphorylation by (Bu)2cAMP was effective in reducing MAPK activation by (Bu)2cGMP, an active phorbol ester or ionomycin. These results indicate that the effect of NE on MAPK phosphorylation represents mainly the integration of two signaling mechanisms, protein kinase A and protein kinase G, each having an opposite effect on MAPK phosphorylation.

Down-regulation of pinealocyte protein kinase C: effect on alpha 1-adrenergic potentiation of beta-adrenoceptor stimulation of cyclic AMP accumulation and induction of serotonin N-acetyltransferase activity

Treatment of rat pinealocytes with 4 beta-phorbol 12,13-dibutyrate down-regulated protein kinase C (PKC) activity. Loss of activity was concentration-dependent (50% loss at 8 x 10(-7) M after 18 h of treatment) and time-dependent (50% loss after 2 h with 3 x 10(-6) M). Phenylephrine, an alpha 1-adrenergic agonist, and phorbol esters unable to activate PKC did not down-regulate the enzyme. alpha 1-Adrenergic amplification of beta-adrenergic stimulation of cyclic AMP accumulation, a response previously shown to be mediated by PKC activation, was reduced by only 50% in cells in which PKC activity was down-regulated by approximately 95%. These data suggest that there is not a simple proportional relationship between the degree of activation of pinealocyte PKC and the alpha 1-adrenergic amplification of beta-adrenergic cyclic AMP synthesis. In down-regulated cells, alpha 1-adrenergic amplification of beta-adrenergic induction of serotonin N-acetyltransferase activity, a key cyclic AMP-responsive enzyme involved in the nocturnal synthesis of the pineal hormone melatonin, was unchanged. Thus, even though alpha 1-adrenergic amplification of cyclic AMP synthesis is impaired, sufficient cyclic AMP is generated to allow a full induction of serotonin N-acetyltransferase activity. This finding raises the important question of whether the alpha 1-adrenergic amplification mechanism has a physiological role in regulating melatonin synthesis in vivo.

5-Hydroxytryptamine amplifies beta-adrenergic stimulation of N-acetyltransferase activity in rat pinealocytes

In cultured rat pinealocytes beta-adrenergic induction of N-acetyltransferase, a key enzyme in the synthesis of melatonin, is amplified by addition of 5-hydroxytryptamine (5-HT) to the culture medium. However, 5-HT when added alone has no effect on enzyme activity. Pharmacological experiments with a range of agonists and antagonists suggest that this action is not mediated by 5-HT1, 5-HT2, 5-HT3, or 5-HT4 receptor subtypes but may involve a site similar to the 5-HT1p receptor described in the enteric nervous system. The potential role of 5-HT in modulating adrenergic stimulation of N-acetyltransferase activity is discussed.

Melatonin biosynthesis in the mammalian pineal gland

Rhythmic production of melatonin by the mammalian pineal occurs in response to noradrenergic stimulation which produces a cascade of biochemical events within the pinealocyte. In the rat, massive changes in NAT activity result from an increase in intracellular c-AMP levels produced by a synergistic interaction whereby an alpha 1 activation amplifies beta-adrenergic stimulation. The intracellular events mediating this effect are described. A major aspect of the temporal control of melatonin production is the programmed down-regulation of responses to noradrenergic stimulation once the initial surge of c-AMP is produced. Noradrenergic activation of the gland also influences other enzymic functions, including tryptophan hydroxylase and HIOMT activities, and produces a dramatic increase in intracellular c-GMP levels. Other neurotransmitters and neuropeptides, e.g. VIP, may also influence pineal function and comparisons are made between the rat, the subject of the bulk of experimental studies, and other species.

Evidence for a functional role of alpha-1 adrenoceptors in the regulation of melatonin secretion in man

Nocturnal plasma melatonin concentrations were reduced following administration of the alpha 1 adrenoceptor antagonist, prazosin (2 mg orally at 1900h), to 10 normal men. These data indicate that alpha 1 adrenoceptors are involved in the physiological control of melatonin secretion in man.

12-Hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-HPETE stimulate melatonin synthesis in rat pineals

The role of arachidonic acid metabolites in norepinephrine (NE)-induced N-acetyltransferase (NAT) activity and melatonin release was examined from 6 h-incubations of rat pineal glands. A cyclooxygenase inhibitor, indomethacin (5 x 10(-8) - 5 x 10(-6) M) was ineffective on melatonin release, in the presence of absence of NE (5 x 10(-6) M) while a lipoxygenase inhibitor, nordihydroguaiaretic acid (5 x 10(-7) -5 x 10(-5) M) had an inhibitory effect. Among the lipoxygenase metabolites, 12-hydroperoxyeicosatetraenoic acid (12-HPETE) and 15-HPETE stimulated both NAT activity and melatonin release in a dose-dependent manner, with a maximal effect occurring at 10(-6) M, while 5-HPETE or hydroxy derivatives of these compounds (12-HETE, 15-HETE and 5-HETE) were ineffective. These results indicate that 12-HPETE and 15-HPETE can be involved in NE-induced melatonin release.

Cellular and molecular mechanisms controlling melatonin release by mammalian pineal glands

1. The pineal gland is regulated primarily by photoperiodic information attaining the organ through a multisynaptic pathway initiated in the retina and the retinohypothalamic tract. 2. Norepinephrine (NE) released from superior cervical ganglion (SCG) neurons that provide sympathetic innervation to the pineal acts through alpha1- and beta 1- adrenoceptors to stimulate melatonin synthesis and release. 3. The increase in cyclic AMP mediated by beta 1-adrenergic activation is potentiated by the increase in Ca2+ flux, inositol phospholipid turnover, and prostaglandin and leukotriene synthesis produced by alpha 1-adrenergic activation. 4. Central pinealopetal connections may also participate in pineal control mechanisms; transmitters and modulators in these pathways include several neuropeptides, amino acids such as gamma-aminobutyric acid (GABA) and glutamate, and biogenic amines such as serotonin, acetylcholine, and dopamine. 5. Secondary regulatory signals for pineal secretory activity are several hormones that act on receptors sites on pineal cells or at any stage of the neuronal pinealopetal pathway.

Translocation of protein kinase C in anterior pituitary tumor cells

Previous studies have shown that phorbol esters and lithium each stimulate the secretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary tumor cell line AtT20/D16-16. Pretreatment with either lithium or phorbol ester desensitizes the cells to subsequent stimulation by phorbol ester. An early consequence of phorbol ester action in other systems is the translocation of protein kinase C from cytosol to membranes. We have assayed protein kinase C activity in cytosol and membranes of AtT20 cells after treatment with phorbol dibutyrate, lithium, or other agents that stimulate secretion of ACTH in these cells. Phorbol dibutyrate clearly induced translocation of protein kinase C, but lithium treatment did not cause translocation itself, nor did pretreatment with lithium affect the translocation induced by phorbol dibutyrate. These results are consistent with a role for translocation of protein kinase C in the stimulatory and desensitizing effects of phorbol esters but fail to implicate translocation in the actions of lithium on AtT20 cells.

Translocation of protein kinase C in rat hippocampal slices

Tumor-promoting phorbol esters specifically activate protein kinase C and mimic the effects of neurotransmitters in certain systems. Treatment of hippocampal slices with phorbol dibutyrate caused translocation of protein kinase C activity from cytoplasm to membranes. Experiments with carbachol, norepinephrine, glutamate, KCl, and LiCl failed to demonstrate a similar translocation. Translocation more readily provides an index of protein kinase C involvement for phorbol esters than for other agents in hippocampus.

Pineal indoles: significance and measurement

Despite intensive investigation, particularly over the past fifteen years, many aspects of pineal function with respect to mammalian physiology remain obscure. Much of this work is reviewed and particular attention focussed on indole metabolism within the pineal gland. Emphasis is placed on the development of new analytical techniques with special reference to high performance liquid chromatography coupled with electrochemical detection. The growth in knowledge regarding pineal indole synthesis which can be attributed to the use of this technique is discussed. The possibility that pineal indoles other than melatonin may function as hormones or neuromodulators is considered. A functional role for 5-hydroxytryptophol as a neuromodulator, possibly associated with diffuse neuroendocrine function (amine precursor, uptake and decarboxylation, APUD) is suggested.

Studies and perspectives of protein kinase C

Protein kinase C, an enzyme that is activated by the receptor-mediated hydrolysis of inositol phospholipids, relays information in the form of a variety of extracellular signals across the membrane to regulate many Ca2+-dependent processes. At an early phase of cellular responses, the enzyme appears to have a dual effect, providing positive forward as well as negative feedback controls over various steps of its own and other signaling pathways, such as the receptors that are coupled to inositol phospholipid hydrolysis and those of some growth factors. In biological systems, a positive signal is frequently followed by immediate negative feedback regulation. Such a novel role of this protein kinase system seems to give a logical basis for clarifying the biochemical mechanism of signal transduction, and to add a new dimension essential to our understanding of cell-to-cell communication.