Interactions between dopamine and melatonin organize circadian rhythmicity in the retina of the green iguana

Circadian rhythm regulation in the sea cucumber Apostichopus japonicus: Insights into clock gene expression, photoperiod susceptibility, and neurohormone signaling

Sea cucumber Apostichopus japonicus displays the typical circadian rhythms. This present study investigated the molecular regulation of clock genes, as well as monoamines and melatonin, in multiple tissues of A. japonicus, responding to the photoperiod. In order to determine their pivotal role in circadian rhythms, the crucial clock genes, namely AjClock, AjArnt1, AjCry1, and AjTimeless, were identified and a comprehensive analysis of their expressions across various tissues in adult A. japonicus was conducted, revealing the potential existence of central and peripheral oscillators. Results demonstrated that the tissues of polian vesicle and nerve ring exhibited significant clock gene expression associated with the orchestration of circadian regulation, and that environmental light fluctuations exerted influence on the expression of these clock genes. However, a number of genes, such as AjArnt1 and AjCry1, maintained their circadian rhythmicity even under continuous light conditions. Moreover, we further investigated the circadian patterns of melatonin (MT), serotonin (5-HT), and dopamine (DA) secretion in A. japonicus, data that underscored the tissue-specific regulatory differences and the inherent adaptability to dynamic light environments. Collectively, these findings will provide the molecular mechanisms controlling the circadian rhythm in echinoderms and the candidate tissues playing the role of central oscillators in sea cucumbers.

Therapeutic Effects of Melatonin on Ocular Diseases: Knowledge Map and Perspective

Melatonin plays a critical role in the pathophysiological process including circadian rhythm, apoptosis, and oxidative stress. It can be synthesized in ocular tissues, and its receptors are also found in the eye, triggering more investigations concentrated on the role of melatonin in the eye. In the past decades, the protective and therapeutic potentials of melatonin for ocular diseases have been widely revealed in animal models. Herein, we construct a knowledge map of melatonin in treating ocular diseases through bibliometric analysis and review its current understanding and clinical evidence. The overall field could be divided into twelve topics through keywords co-occurrence analysis, in which the glaucoma, myopia, and retinal diseases were of greatest research interests according to the keywords burst detection. The existing clinical trials of melatonin in ocular diseases mainly focused on the glaucoma, and more research should be promoted, especially for various diseases and drug administration. We also discuss its bioavailability and further research topics including developing melatonin sensors for personalized medication, acting as stem cell therapy assistant drug, and consuming food-derived melatonin for facilitating its clinical transformation.

Bioinformatics analysis and identification of genes and molecular pathways involved in Parkinson's disease in patients with mutations in the glucocerebrosidase gene

Glucocerebrosidase (GBA) mutations occur frequently in Parkinson's disease (PD) patients. This study aims to identify potential crucial genes and pathways associated with GBA mutations in patients with PD and to further analyze new molecular mechanisms related to the occurrence of gene mutations from the perspective of bioinformatics. Gene expression profiles of datasets GSE53424 and GSE99142 were acquired from the Gene Expression Ominibus database. Differentially expressed genes (DEGs) were detected, using the 'limma' package in R, comparing IDI-PD 1 (idiopathic PD patients) and GBA-PD 1 [PD patients with heterozygous GBA mutations (GBA N370S)] group samples. The functions of top modules were assessed using the DAVID, whereas gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed. Protein-protein interaction networks were assembled with Cytoscape software and separated into subnetworks using the Molecular Complex Detection Algorithm. Data from GSE53424 and GSE99142 were also extracted to verify our findings. There were 283 DEGs identified in PD patients heterozygous for GBA mutations. Module analysis revealed that GBA mutations in PD patients were associated with significant pathways, including Calcium signaling pathway, Rap1 signaling pathway and Cytokine-cytokine receptor interaction. Hub genes of the two modules were corticotropin-releasing hormone (CRH) and Melatonin receptor 1B (MTNR1B). The expression of CRH was downregulated, whereas that of MTNR1B was upregulated in PD patients with GBA mutations. The expression of CRH and MTNR1B has diagnostic value for PD patients with heterozygous GBA mutations. Novel DEGs and pathways identified herein might provide new insights into the underlying molecular mechanisms of heterozygous GBA mutations in PD patients.

Melatonin Regulates the Neurotransmitter Secretion Disorder Induced by Caffeine Through the Microbiota-Gut-Brain Axis in Zebrafish (Danio rerio)

Melatonin has been widely used as a “probiotic agent” capable of producing strong neurotransmitter secretion regulatory effects, and the microbiota-gut-brain axis-related studies have also highlighted the role of the gut microbiota in neuromodulation. In the present study, a zebrafish neural hyperactivity model was established using caffeine induction to explore the regulatory effects of melatonin and probiotic on neurotransmitter secretion disorder in zebrafish. Disorders of brain neurotransmitter secretion (dopamine, γ-aminobutyric acid, and 5-hydroxytryptamine) caused by caffeine were improved after interference treatment with melatonin or probiotic. Shotgun metagenomic sequencing demonstrated that the melatonin-treated zebrafish gradually restored their normal intestinal microbiota and metabolic pathways. Germ-free (GF) zebrafish were used to verify the essential role of intestinal microbes in the regulation of neurotransmitter secretion. The results of the neurotransmitter and short-chain fatty acid determination revealed that the effect on the zebrafish in the GF group could not achieve that on the zebrafish in the melatonin group after adding the same dose of melatonin. The present research revealed the potential mode of action of melatonin through the microbiota-gut-brain axis to regulate the disruption of neurotransmitter secretion, supporting the future development of psychotropic drugs targeting the intestinal microbiota.

Illuminating and Sniffing Out the Neuromodulatory Roles of Dopamine in the Retina and Olfactory Bulb

In the central nervous system, dopamine is well-known as the neuromodulator that is involved with regulating reward, addiction, motivation, and fine motor control. Yet, decades of findings are revealing another crucial function of dopamine: modulating sensory systems. Dopamine is endogenous to subsets of neurons in the retina and olfactory bulb (OB), where it sharpens sensory processing of visual and olfactory information. For example, dopamine modulation allows the neural circuity in the retina to transition from processing dim light to daylight and the neural circuity in the OB to regulate odor discrimination and detection. Dopamine accomplishes these tasks through numerous, complex mechanisms in both neural structures. In this review, we provide an overview of the established and emerging research on these mechanisms and describe similarities and differences in dopamine expression and modulation of synaptic transmission in the retinas and OBs of various vertebrate organisms. This includes discussion of dopamine neurons’ morphologies, potential identities, and biophysical properties along with their contributions to circadian rhythms and stimulus-driven synthesis, activation, and release of dopamine. As dysregulation of some of these mechanisms may occur in patients with Parkinson’s disease, these symptoms are also discussed. The exploration and comparison of these two separate dopamine populations shows just how remarkably similar the retina and OB are, even though they are functionally distinct. It also shows that the modulatory properties of dopamine neurons are just as important to vision and olfaction as they are to motor coordination and neuropsychiatric/neurodegenerative conditions, thus, we hope this review encourages further research to elucidate these mechanisms.

The timing of Timezyme diversification in vertebrates

All biological functions in vertebrates are synchronized with daily and seasonal changes in the environment by the time keeping hormone melatonin. Its nocturnal surge is primarily due to the rhythmic activity of the arylalkylamine N-acetyl transferase AANAT, which thus became the focus of many investigations regarding its evolution and function. Various vertebrate isoforms have been reported from cartilaginous fish to mammals but their origin has not been clearly established. Using phylogeny and synteny, we took advantage of the increasing number of available genomes in order to test whether the various rounds of vertebrate whole genome duplications were responsible for the diversification of AANAT. We highlight a gene secondary loss of the AANAT2 in the Sarcopterygii, revealing for the first time that the AAANAT1/2 duplication occurred before the divergence between Actinopterygii (bony fish) and Sarcopterygii (tetrapods, lobe-finned fish, and lungfish). We hypothesize the teleost-specific whole genome duplication (WDG) generated the appearance of the AANAT1a/1b and the AANAT2/2′paralogs, the 2′ isoform being rapidly lost in the teleost common ancestor (ray-finned fish). We also demonstrate the secondary loss of the AANAT1a in a Paracantopterygii (Atlantic cod) and of the 1b in some Ostariophysi (zebrafish and cave fish). Salmonids present an even more diverse set of AANATs that may be due to their specific WGD followed by secondary losses. We propose that vertebrate AANAT diversity resulted from 3 rounds of WGD followed by previously uncharacterized secondary losses. Extant isoforms show subfunctionalized localizations, enzyme activities and affinities that have increased with time since their emergence.

Role of dopamine in distal retina

Dopamine is the most abundant catecholamine in the vertebrate retina. Despite the description of retinal dopaminergic cells three decades ago, many aspects of their function in the retina remain unclear. There is no consensus among the authors about the stimulus conditions for dopamine release (darkness, steady or flickering light) as well as about its action upon the various types of retinal cells. Many contradictory results exist concerning the dopamine effect on the gross electrical activity of the retina [reflected in electroretinogram (ERG)] and the receptors involved in its action. This review summarized current knowledge about the types of the dopaminergic neurons and receptors in the retina as well as the effects of dopamine receptor agonists and antagonists on the light responses of photoreceptors, horizontal and bipolar cells in both nonmammalian and mammalian retina. Special focus of interest concerns their effects upon the diffuse ERG as a useful tool for assessment of the overall function of the distal retina. An attempt is made to reveal some differences between the dopamine actions upon the activity of the ON versus OFF channel in the distal retina. The author has included her own results demonstrating such differences.

Increased melatonin may play dual roles in the striata of a 6-hydroxydopamine model of Parkinson's disease

AIMS

To investigate dynamic changes and roles of melatonin (MLT) in the striata of 6-hydroxydopamine (6-OHDA)-treated rats.

MAIN METHODS

A Parkinson's disease (PD) rat was established by a unilateral injection of 6-OHDA into the right substantia nigra pars compacta (SNc) and the right medial forebrain bundle (MFB) to achieve a complete lesion of the ipsilateral nigrostriatal DA system. Dialysates were collected in the lesioned striatum at different time intervals by in vivo microdialysis. In addition, both contralateral and ipsilateral striatum tissues were collected at two time intervals (10:00 and 22:00 h) at 3 and 6 weeks after lesioning. The levels of DA, 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dialysates, as well as MLT in the dialysates and tissues were determined using HPLC.

KEY FINDINGS

The dialysate contents of DA, DOPAC and HVA in the lesioned striatum were significantly decreased (P<0.001) in comparison with those in the controls or in the unlesioned side 3 weeks after lesioning while the extracellular level of MLT in the lesioned striatum in these corresponding time intervals distinctly increased when compared with those in the controls (P<0.05). The tissue MLT contents increased in the bilateral striata in different degrees at 6 weeks post-lesion (P<0.05). Moreover, increased MLT levels correlate well with rotations or DA changes in the lesioned striatum.

SIGNIFICANCE

These data suggest that 6-OHDA lesion manipulates the MLT secretion pattern. Increased striatal MLT level by a unilateral intracerebral injection of 6-OHDA may play dual roles in the progression of PD in rats.

Dopamine modulates diurnal and circadian rhythms of protein phosphorylation in photoreceptor cells of mouse retina

Many aspects of photoreceptor metabolism are regulated as diurnal or circadian rhythms. The nature of the signals that drive rhythms in mouse photoreceptors is unknown. Dopamine amacrine cells in mouse retina express core circadian clock genes, leading us to test the hypothesis that dopamine regulates rhythms of protein phosphorylation in photoreceptor cells. To this end we investigated the phosphorylation of phosducin, an abundant photoreceptor-specific phosphoprotein. In mice exposed to a daily light-dark cycle, robust daily rhythms of phosducin phosphorylation and retinal dopamine metabolism were observed. Phospho-phosducin levels were low during the daytime and high at night, and correlated negatively with levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid. The effect of light on phospho-phosducin levels was mimicked by pharmacological activation of dopamine D4 receptors. The amplitude of the diurnal rhythm of phospho-phosducin was reduced by > 50% in D4 receptor-knockout mice, due to higher daytime levels of phospho-phosducin. In addition, the daytime level of phospho-phosducin was significantly elevated by L-745,870, a dopamine D4 receptor antagonist. These data indicate that dopamine and other light-dependent processes cooperatively regulate the diurnal rhythm of phosducin phosphorylation. Under conditions of constant darkness a circadian rhythm of phosducin phosphorylation was observed, which correlated negatively with the circadian rhythm of 3,4-dihydroxyphenylacetic acid levels. The circadian fluctuation of phospho-phosducin was completely abolished by constant infusion of L-745,870, indicating that the rhythm of phospho-phosducin level is driven by dopamine. Thus, dopamine release in response to light and circadian clocks drives daily rhythms of protein phosphorylation in photoreceptor cells.

The circadian clock system in the mammalian retina

Daily rhythms are a ubiquitous feature of living systems. Generally, these rhythms are not just passive consequences of cyclic fluctuations in the environment, but instead originate within the organism. In mammals, including humans, the master pacemaker controlling 24-hour rhythms is localized in the suprachiasmatic nuclei of the hypothalamus. This circadian clock is responsible for the temporal organization of a wide variety of functions, ranging from sleep and food intake, to physiological measures such as body temperature, heart rate and hormone release. The retinal circadian clock was the first extra-SCN circadian oscillator to be discovered in mammals and several studies have now demonstrated that many of the physiological, cellular and molecular rhythms that are present within the retina are under the control of a retinal circadian clock, or more likely a network of hierarchically organized circadian clocks that are present within this tissue. BioEssays 30:624-633, 2008. (c) 2008 Wiley Periodicals, Inc.