Status of dopamine in bovine pineal glands and the stimulation of N-acetyltransferase activity by D2-dopaminergic receptor agonists in the rat pineal glands in culture

Thyroid hormone and adrenergic signaling interact to control pineal expression of the dopamine receptor D4 gene (Drd4)

Dopamine plays diverse and important roles in vertebrate biology, impacting behavior and physiology through actions mediated by specific G-protein-coupled receptors, one of which is the dopamine receptor D4 (Drd4). Here we present studies on the >100-fold daily rhythm in rat pineal Drd4 expression. Our studies indicate that Drd4 is the dominant dopamine receptor gene expressed in the pineal gland. The gene is expressed in pinealocytes at levels which are approximately 100-fold greater than in other tissues, except the retina, in which transcript levels are similar. Pineal Drd4 expression is circadian in nature and under photoneural control. Whereas most rhythmically expressed genes in the pineal are controlled by adrenergic/cAMP signaling, Drd4 expression also requires thyroid hormone. This advance raises the questions of whether Drd4 expression is regulated by this mechanism in other systems and whether thyroid hormone controls expression of other genes in the pineal gland.

Dopamine transporter immunoreactive terminals in the bovine pineal gland

The dopaminergic system has been proposed to be one of the innervations in controlling the mammalian pineal gland function. The dopamine receptors have been characterized in the pineal and the biphasic effects of dopamine on melatonin production have been demonstrated. Recently, the site of dopamine transporter (DAT), a plasma membrane transport protein of dopaminergic neuron, also has been characterized in the bovine pineal gland. The aim of the present study was to identify the dopaminergic innervation in the bovine pineal gland. The localization and expression of DAT have been performed by using an immunohistochemical method and a reverse transcriptase polymerase chain reaction (RT-PCR) technique. DAT-immunoreactivity was found in the nerve terminals throughout the gland, but not in pinealocytes or neuronal-like cells. Some DAT-immunoreactive nerve fibers were observed along the pineal stalk. DAT mRNA product from RT-PCR was found in the bovine substantia nigra, but not in the pineal gland. The colocalization of DAT with tyrosine hydroxylase (TH) or dopamine beta hydroxylase (DBH) immunoreactivities was observed in nerve terminals. However, no colocalization of DAT with DBH was found in some terminals/fibers. The present results showed the central dopaminergic innervation in the bovine pineal gland distinctively from noradrenergic nerve fibers, and their perikarya origin was located possibly outside of the gland.

Dopamine receptor activation in bovine pinealocyte via a cAMP-dependent transcription pathway

D1- and D2-dopamine receptors in the bovine pineal gland were previously identified and characterized. The data indicate that the density of D1-dopamine receptors far exceeded that of D2-dopamine receptors. In our previous study, the mRNA for both the D1- and D2-dopamine receptors which elucidated the status of dopamine and its possible involvement in the pineal function was identified. A selective D1-agonist enhanced N-acetyltransferase (NAT) activity and increased the melatonin level, whereas, a selective D2-agonist inhibited NAT activity and decreased the melatonin level. An attempt has been made in the present study to clarify the mechanism of dopamine in controlling melatonin production in bovine pineal. The level of intracellular cyclic 3',5'-adenosine monophosphate (cAMP) was determined after a 2-hr incubation of bovine pinealocytes with selected combinations of drugs. SKF 38393, a selective D1-agonist, enhanced intracellular level of cAMP, and its effect was blocked by SCH 23390, a D1-selective antagonist. In contrast quinpirole, a selective D2-agonist, inhibited forskolin-stimulated intracellular level of cAMP while its effect was blocked by a D2-selective antagonist, spiperone. In addition, the dopamine-dependent phosphorylation of the transcription factors, cAMP responsive element-binding protein (CREB) was investigated. Immunoblots showed that SKF 38393 enhanced CREB phosphorylation and the stimulatory effect was abolished by SCH 23390 whereas quinpirole inhibited forskolin-stimulated phosphorylated CREB production and the inhibitory effect was prevented by spiperone. Taken together with our previous data, the results indicate that activation of D1-dopamine receptor in bovine pinealocyte stimulates NAT activity and enhances melatonin level whereas activation of D2-dopamine receptor leads to an inhibitory effect and these stimulatory and inhibitory effects act, in part, via a cAMP-dependent transcription mechanism.

Effects of D1- and D2-dopamine receptor activation on melatonin synthesis in bovine pinealocytes

Previous studies have identified and characterized D1- and D2-dopamine receptors in bovine pineal glands. The data indicate that the density of D1-dopamine receptors (974 fmol/mg protein) far exceed that of D2-dopamine receptors (37 fmol/mg protein). The objective of this study was to identify the mRNAs for both D1- and D2-dopamine receptors and to elucidate the status of dopamine and its possible involvement in the pineal function, particularly on melatonin synthesis. The expression of these dopamine receptor subtypes were determined by using a reverse transcriptase-polymerase chain reaction technique with specific pairs of primers to amplify D1- and D2-dopamine receptor mRNAs. Amplification of RNAs from bovine striatum (positive control) and bovine pineal gland resulted in products of the predicted lengths of 231 bp for D1- and 333 bp for D2-dopamine receptors. The results indicate that both D1- and D2-dopamine receptor mRNAs are present in the bovine pineal gland. The role of dopamine receptors was investigated by studying the effects of selective D1- and D2-dopamine agonists and antagonists on the N-acetyltransferase (NAT) activity of cultured bovine pinealocytes. The data showed that SKF-38393, a selective D1-agonist, enhanced NAT activity, and increased melatonin level, and the stimulatory effect was blocked by SCH-23390, a D1-selective antagonist, whereas quinpirole, a selective D2-agonist, inhibited NAT basal activity and decreased the melatonin basal level. Furthermore the inhibitory effect was blocked by D2-selective antagonists, spiperone, haloperidol, and domperidone. The present results indicate that the pineal dopamine receptors have a distinct effect on pineal function. The precise mechanism whereby activation of dopamine receptors altered the NAT activity and melatonin level needs to be further delineated.

Identification of dopamine transporter in bovine pineal gland using [3H]GBR 12935

The mammalian pineal gland contains several neurotransmitters and receptors for amino acids, biogenic amines, and peptides. Some of these, such as D1 and D2 dopamine receptors, have been previously identified and characterized in the bovine pineal gland by our group. As a matter of fact, the density of D1 dopamine receptors in the pineal gland is higher than that of corpus striatum, suggesting that this organ must possess a high affinity dopamine transporter, which has been identified in this study by using [3H]GBR 12935 as a radiological ligand and nomifensine to determine non-specific binding. The association rate of [3H]GBR 12935 binding to the pineal membrane was examined as a function of time. The binding reached equilibrium within 45 min of incubation at 25 degrees C. The specific binding was reversible and saturable. The dissociation time course of the specific [3H]GBR 12935 binding from the bovine pineal membrane was also studied. A half-life (t1/2) of 14-min was obtained. The saturation analysis of the [3H]GBR 12935 binding revealed a dissociation equilibrium constant (Kd) of 6.0 +/- 0.9 nm and a receptor density (Bmax) of 6.9 +/- 0.3 pmol/mg protein, which were comparable with those values obtained from bovine striatum and frontal cortex. In competitive experiments, the concentrations of drugs required to inhibit 50% of the binding (IC50) were in descending order GBR 12909 > GBR 12935 > trans-flupenthixol > nomifensine > cis-flupenthixol > amitriptyline > imipramine > desipramine > dopamine > fluoxetine > fuvoxamine > d-amphetamine. However, nisoxetine, SCH 23390, norepinephrine, and serotonin were unable to displace [3H]GBR binding. These results show that drugs capable of blocking dopamine transporters were effective in displacing [3H]GBR binding; whereas specific norepinephrine and serotonin transporter inhibitors were less effective or ineffective. In addition, the dopamine transporter is ion-dependent as sodium increased [3H]GBR binding in a concentration related manner. These results indicate that a high affinity dopamine transporter exists in the bovine pineal, which may exhibit circadian periodicity, and whose physiological functions need to be delineated and characterized in future investigations.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Dopamine induces intracellular Ca2+ signals mediated by alpha1B-adrenoceptors in rat pineal cells

We have studied the functional interaction of dopamine with alpha1-adrenoceptor subtypes by measuring intracellular Ca2+ levels in pineal cells, a cell type where adrenoceptors are well characterized. We show that dopamine induces transient intracellular Ca2+ signals in only 70% of cells responding to phenylephrine. Dopamine-induced Ca2+ signals desensitise faster than Ca2+ transients elicited with phenylephrine and are selectively blocked by desipramine, imipramine, and alpha1B-adrenoceptor antagonists. These results suggest that dopamine induced Ca2+ signals are mainly due to the activation of one subtype of alpha-adrenoceptor, the alpha1B.

Molecular cloning and tissue distribution of an avian D2 dopamine receptor mRNA from the domestic turkey (Maleagris gallopavo)

The reverse transcriptase-polymerase chain reaction (RT-PCR), in combination with 5' and 3' rapid amplification of cDNA ends (RACE), was used to clone a G protein-coupled receptor from turkey brain mRNA. This cDNA clone has an open reading frame of 1,311 base pairs encoding a 436-residue protein with seven transmembrane-spanning domains and exhibits high homology with previously cloned mammalian D2 dopamine receptors. Northern blot analysis of turkey brain mRNA detected an approximate 2.4-kb transcript. RT-PCR and subsequent nucleotide sequence analysis of turkey brain and peripheral tissue mRNA also demonstrated the presence of an alternatively spliced mRNA corresponding to the predicted D2 short isoform. RT-PCR experiments demonstrated a widespread distribution of alternatively spliced D2 dopamine receptor transcripts throughout the turkey brain and in select peripheral tissues as well. In situ hybridization experiments detected strong autoradiographic signals over much of the turkey telencephalon, diencephalon, mesencephalon, cerebellum, pituitary, and pineal gland. Dopamine has several important functions as a neurotransmitter and hormone in mammals and may have similar actions in avian species. The cloning and tissue distribution of the D2 receptor subtype should enable the investigation of any functional role dopamine and dopamine receptors exert on the physiology and behavior of birds.

Diurnal changes in the content of indoleamines, catecholamines, and methoxyindoles in the pineal gland of the Djungarian hamster (Phodopus sungorus): effect of photoperiod

Previous studies in Syrian hamster have shown that the correlations between the daily fluctuations in the contents of pineal indoleamines and methoxyindoles are influenced by the photoperiod, and that dopamine may play a role in the regulation of pineal function. The present study investigated the 24 hour changes in the content of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), N-acetylserotonin (NAS), melatonin, 5-methoxytryptophol (5-MT), noradrenaline (NA), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC) in the pineal gland of female Djungarian hamsters exposed to long (LP; LD 16:8) or short (SP; LD 10:14) photoperiods for 10 weeks. Pronounced nocturnal increases of N-acetylserotonin and melatonin content were observed irrespective of the photoperiod regime. The content of 5-HT was markedly decreased during the first hour of the night in LP, which contrasted with the lack of changes in NAS and melatonin content at this time. In SP, an increased 5-HTP content and a less obvious decrease in 5-HT content was observed during the night, although melatonin and NAS content were even higher than in LP. Similar daily patterns as for 5-HT were observed in the 5-HT oxidative metabolites (5-HIAA and 5-MT). When considering values throughout the day, a poor correlation for 5-HT vs.. NAS and melatonin content was observed, which was particularly evident during the nighttime. These data indicate that the daily variation of pineal indoles may not only be dependent on changes in the N-acetyltransferase activity but also in other mechanisms regulating pinealocyte 5-HT availability. As previously reported in other species of hamster, pineal NA content did not show daily variations in LP conditions, although a nocturnal increase was detected in SP. In contrast, pronounced nocturnal increases were observed in the content of DA and its acid metabolite, DOPAC, irrespective of the photoperiod. These data indicate the existence of an increased dopaminergic turnover during the night and further support a role for DA in the regulation of melatonin synthesis and in the synchronization of the pineal functions.

Reduced sympathetic innervation after alteration of target cell neurotransmitter phenotype in transgenic mice

Neurotransmitters play a variety of important roles during nervous system development. In the present study, we hypothesized that neurotransmitter phenotype of both projecting and target cells is an important factor for the final synaptic linkage and its specificity. To test this hypothesis, we used transgenic techniques to convert serotonin/melatonin-producing cells of the pineal gland into cells that also produce dopamine and investigated the innervation of the phenotypically altered target cells. This phenotypic alteration markedly reduced the noradrenergic innervation originating from the superior cervical ganglia. Although the mechanism by which the reduction occurs is presently unknown, quantitative enzyme-linked immunoassay showed the presence of the equivalent amounts of nerve growth factor (NGF) in the control and transgenic pineal glands, suggesting that it occurred in a NGF-independent manner. The results suggest that target neurotransmitter phenotype influences the formation of afferent connections during development.

Fusaric acid in Fusarium moniliforme cultures, corn, and feeds toxic to livestock and the neurochemical effects in the brain and pineal gland of rats

Fusaric acid is produced by several species of Fusarium, which commonly infect corn and other agricultural commodities. Since this mycotoxin may augment the effects of other Fusarium toxins, a gas chromatography/mass spectrometry method of analysis in feeds was developed. Fusaric acid was analyzed as the trimethylsilyl-ester from F. moniliforme-cultures, -contaminated corn screenings, and feeds toxic to livestock. The mycotoxin was found in all samples and ranged from 0.43 to 12.39 micrograms/g sample. Also, fusaric acid was tested for its neurochemical effects in the brain and pineal gland of rats. Animals were dosed intraperitoneally (100 mg/kg body weight) 30 min prior to the onset of the dark phase (lights out) and the effects were studied at 1.5, 3.5, and 5.5 h after treatment. Brain serotonin (5HT), 5-hydroxyindoleacetic acid (5HIAA), tyrosine (TYRO), and dopamine (DA) were increased (P < 0.05) by fusaric acid, and norepinephrine (NEpi) was decreased (P < 0.05). Analogously, DA in the pineal gland increased and NEpi decreased (P < 0.05). Pineal N-acetylserotonin (NAc5HT) was increased (P < 0.05), whereas pineal 5HT and its two major metabolites 5HIAA and 5-hydroxytryptophol (5HTOL) decreased (P < 0.05). Elevated brain TYRO and brain and pineal DA, with decreased NEpi, may be consistent with fusaric acid's partial inhibitory effect on tyrosine-hydroxylase and its inhibitory effect on dopamine-beta-hydroxylase, respectively. Elevated pineal Nac5HT is consistent with decreased pineal 5HT and the increased pineal DA, and support the dopaminergic stimulatory activity of the enzyme responsible for the conversion of 5HT to NAc5HT. This is the first report of fusaric acid's in vivo effect on pineal DA, NEpi, 5HT, and NAc5HT in rats, and a relation for the effects on TYRO, 5HT, and 5HIAA in brain tissue. The results indicate fusaric acid alters brain and pineal neurotransmitters and may contribute to the toxic effects of Fusarium-contaminated feeds.

Tyrosine hydroxylase activity in peripherally denervated rat pineal gland

The presence of tyrosine hydroxylase (TH) in the rat pineal gland was studied using a combination of immunochemical and biochemical methods. In superior cervical ganglionectomized (SCGx) animals and in isolated pineals incubated for 72 h, both TH immunoreactive (TH-IR) fibers and TH biochemical activity were still present but reduced. Conversely, in dispersed pinealocytes incubated for only 24 h we were unable to detect either TH activity or TH-positive cells. Since in the pineal gland of intact rats total 3-methoxy-4-hydroxy phenylglycol (MHPG) was undetectable, and only traces of norepinephrine (NE) were present in the pineal of ganglionectomized animals, the results suggest a central pinealopetal catecholaminergic pathway which could use dopamine as a neurotransmitter.

Distribution of cholinergic and dopaminergic receptors in rainbow trout pineal gland

The involvement of multiple receptors in modulating the function of the pineal gland was investigated by searching for dopaminergic and cholinergic receptors in trout pineal gland. Dopamine D1 and D2 receptors were measured using [3H]SCH23390 and [3H]spiperone, respectively. Muscarinic and nicotinic cholinergic receptors were measured using quinuclidinyl benzilate ([3H]QNB) and [3H]methylcarbamyl choline, respectively. High-affinity choline uptake sites were measured using [3H]hemicholinium-3. The distribution of dopaminergic receptors varied throughout the pineal gland in that the density of D2 receptors, which was higher than that of D1 receptors, was most abundant in the distal region, exhibiting a value of 112 +/- 17 fmol/mg tissue. The distribution of both muscarinic and nicotinic receptors was uniform throughout the pineal gland. However, the highest value for the high-affinity choline transporter (106 +/- 17 fmol/mg tissue) occurred in the proximal portion of the trout pineal gland. The results of these studies indicate that the pineal gland should not be viewed as a homogeneous tissue possessing identical density of various receptors. Furthermore, these results, along with previous data, are interpreted to suggest that different regions of pineal gland may indeed possess unique functions.

Autoradiographic localization of dopaminergic and noradrenergic receptors in the bovine pineal gland

Dopamine and norepinephrine are involved in regulation of melatonin synthesis in the pineal gland. In bovine pineal gland, D1- and D2-dopaminergic and alpha 1-adrenergic receptors have been characterized pharmacologically in several laboratories, while beta 1-adrenergic receptors have been studied using physiological technique. The current study presents a quantitative autoradiographic analysis of these four dopaminergic and noradrenergic receptors in bovine pineal gland. The density order of the receptors is D1 greater than alpha 1 greater than D2 greater than or equal to beta 1. The Bmax of dopamine D1 receptors is about 5 to 6 times higher than the Bmax for alpha 1-adrenergic receptors and about 20 times higher than the Bmax values for beta 1-adrenergic and D2-dopaminergic receptors. Dopamine D1 receptors are significantly denser in the pineal cortex than in the medulla. Both dopamine receptors are more concentrated in the distal area than in the proximal area (close to the habenula), whereas both noradrenergic receptors are homogeneously distributed along the longitudinal axis. Only D1-dopaminergic receptors display a heterogeneous distribution between the superior and the inferior areas, being denser in the inferior area. The observation of a much higher concentration of D1-dopaminergic receptors relative to the other receptors suggests an important role for dopamine in the regulation of bovine pineal physiology.

Castration reduces the nocturnal rise of pineal melatonin levels in the male rat by impairing its noradrenergic input

Abstract The effects of castration and testosterone treatment on pineal day-night rhythms were studied in male rats. Bilateral gonadectomy was performed at 21 days of age. Testosterone propionate was given subcutaneously to castrated animals in a dose of 10 mug/100 g body weight during two consecutive days before sacrifice. Animals were killed 40 days after gonadectomy at four different times of a 12:12 h light-dark cycle (1600, 2400, 0400 and 0800h). Tyrosine hydroxylase activity was measured in individual pineals by means of high-performance liquid chromatography determination of L-DOPA formed. Pineal levels of norepinephrine, dopamine, 5-hydroxytryptamine and 5-hydroxyindole acetic acid were determined by high-performance liquid chromatography with amperometric detection, while pineal melatonin content was measured by radioimmunoassay. Castration abolished the day-night rhythms of pineal tyrosine hydroxylase activity and norepinephrine content, both by elevating their daytime levels and by blocking their nocturnal rise. In addition, gonadectomy drastically modified pineal indoleamine metabolism by increasing daytime levels of both 5-hydroxytryptamine and 5-hydroxyindole acetic acid, and by reducing the nocturnal elevation of pineal melatonin content. Testosterone treatment was unable to prevent the effect of orchidectomy on pineal rhythms of tyrosine hydroxylase activity, 5-hydroxytryptamine or 5-hydroxyindole acetic acid content, however it partially restored the day-night pineal rhythms of both norepinephrine and melatonin content. These results are indicative of a possible participation of reproductive hormones in the control of pineal rhythmic activity in the male rat. Apparently, since gonadectomy abolished the nocturnal rise of both pineal tyrosine hydroxylase activity and norepinephrine content, the primary site of action of reproductive hormones could be at the level of the superior cervical ganglion.

Effects of constant light and darkness on the intrapineal neurons of golden hamsters, stained for tyrosine hydroxylase. A morphometric analysis

Tyrosine hydroxylase (TH)-positive neurons (TH neurons) were found in the pineal gland of golden hamsters. To examine possible relations between TH neurons and environmental light, we kept male animals under constant light (LL) and darkness (DD) for a week, and morphometrically compared the number, size, and immunoreactivity of TH neurons with those of control animals kept under 12L/12D (LD), using an image processor, Nexus 6400. In LL animals, the number of TH neurons/mm2 of pineal tissue and each cell area were decreased, and immunoreactivity to TH was less than in LD animals. In DD animals, the number of TH neurons and each cell area were increased, and immunoreactivity decreased slightly. These data suggested that environmental light affected the TH neurons, and the amount of TH in the neurons would be decreased by LL, but increased by DD.

Regional differences in norepinephrine and dopamine concentration and effect on serotonin uptake and release in bovine pineal gland

In order to analyze whether the bovine pineal gland is a homogeneous or a heterogeneous structure as far as monoamine content, the regional differences in norepinephrine (NE), dopamine (DA), serotonin (5HT), and 5-hydroxyindoleacetic acid (5HIAA) contents were assessed by high-pressure liquid chromatography. NE content was maximal in the proximal (close to the recessus pinealis) region and decreased in a rostral-caudal direction to achieve minimal values at the distal region. DA exhibited an opposite trend to NE, NE/DA ratios varying from 3.2 (proximal region) to 1.4 (distal region). Significantly lower NE content was found at the inferior as compared to the superior pineal region, and at the cortex as compared to the medulla. No significant differences were detected in DA concentration of these latter pineal regions, or in 5HT or 5HIAA concentration as a function of the region examined. 3H-5HT and 3H-NE uptake were maximal at the proximal zone in a rostral-caudal direction, at the superior as compared to the inferior region, and at the medulla as compared to the cortex. Unlabeled NE was equally effective to compete with 3H-5HT uptake in the several pineal regions studied. While NE increased maximally 3H-5HT release in a rostral-caudal direction, DA exhibited an opposite trend, displaying maximal 5HT release activity at the distal pineal region. DA and NE 5HT-releasing activity were greater in the pineal medulla than in the cortex, and did not exhibit differences in the superior as compared to the inferior pineal aspects. Excess (55 mM) K+ released 3H-5HT to a similar extent regardless of the pineal region examined.