Long-term in vivo pineal microdialysis

Chronophysiology of domestic animals

This review highlights recent findings on biological rhythms and discusses their implications for the management and production of domestic animals. Biological rhythms provide temporal coordination between organs and tissues in order to anticipate environmental changes, orchestrating biochemical, physiological and behavioural processes as the right process may occur at the right time. This allows animals to adapt their internal physiological functions, such as sleep-wake cycles, body temperature, hormone secretion, food intake and regulation of physical performance to environmental stimuli that constantly change. The study and evaluation of biological rhythms of various physiological parameters allows the assessment of the welfare status of animals. Alteration of biological rhythms represents an imbalance of the state of homeostasis that can be found in different management conditions.

Serotonin modulates melatonin synthesis as an autocrine neurotransmitter in the pineal gland

The pineal gland secretes melatonin principally at night. Regulated by norepinephrine released from sympathetic nerve terminals, adrenergic receptors on pinealocytes activate aralkylamine N-acetyltransferase that converts 5-hydroxytryptamine (5-HT, serotonin) to N-acetylserotonin, the precursor of melatonin. Previous studies from our group and others reveal significant constitutive secretion of 5-HT from pinealocytes. Here, using mass spectrometry, we demonstrated that the 5-HT is secreted primarily via a decynium-22-sensitive equilibrative plasma membrane monoamine transporter instead of by typical exocytotic quantal secretion. Activation of the endogenous 5-HT receptors on pinealocytes evoked an intracellular Ca²⁺ rise that was blocked by RS-102221, an antagonist of 5-HT_2C receptors. Applied 5-HT did not evoke melatonin secretion by itself, but it did potentiate melatonin secretion evoked by submaximal norepinephrine. In addition, RS-102221 reduced the norepinephrine-induced melatonin secretion in strips of pineal gland, even when no exogenous 5-HT was added, suggesting that the 5-HT that is constitutively released from pinealocytes accumulates enough in the tissue to act as an autocrine feedback signal sensitizing melatonin release.

Clock Genes Expression in Peripheral Leukocytes and Plasma Melatonin Daily Rhythm in Horses

In mammals, behavioral and physiological processes display 24-hour rhythms that are regulated by the circadian system. In the present study, we investigated clock gene expression in peripheral leukocytes in horses. For this purpose, 10 Italian Saddle gelding horses (9-11 years old; 475 ± 28 Kg) were housed in individual boxes under natural photoperiod and natural environmental temperature. Blood samples were collected at 4-hour intervals over a 48-hour period. The day before the start of sampling, left jugular furrow of each horse was cannulated for the blood sample collection performed in heparinized tubes, for the assessment of melatonin concentration by means of radioimmunoassay and into PAX gene Blood RNA Tube for the assessment of clock genes by real-time RT-quantitative polymerase chain reaction (RTqPCR). Well-established melatonin showed a daily rhythm with nocturnal acrophase (day 1-21:30; day 2-21:40). All genes tested (Bmal1, Cry 1, Per 1, Per 2, and Per 3) except Clock showed daily rhythmicity of their expression in peripheral blood. Oscillations of Bmal1 and Per 2 were correlated with the oscillation of melatonin, which anticipated the acrophase of Bmal1 (day 1-01:29; day 2-01:00) and Per 2 (day 1-01:00; day 2-00:32) of about 3 hours. Our results support the presence of a cyclic transcription of clock genes in peripheral leukocytes in horses.

Brain Microdialysate Monoamines in Relation to Circadian Rhythms, Sleep, and Sleep Deprivation - a Systematic Review, Network Meta-analysis, and New Primary Data

Disruption of the monoaminergic system, e.g. by sleep deprivation (SD), seems to promote certain diseases. Assessment of monoamine levels over the circadian cycle, during different sleep stages and during SD is instrumental to understand the molecular dynamics during and after SD. To provide a complete overview of all available evidence, we performed a systematic review. A comprehensive search was performed for microdialysis and certain monoamines (dopamine, serotonin, noradrenaline, adrenaline), certain monoamine metabolites (3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA)) and a precursor (5-hydroxytryptophan (5-HTP)) in PubMed and EMBASE. After screening of the search results by two independent reviewers, 94 publications were included. All results were tabulated and described qualitatively. Network-meta analyses (NMAs) were performed to compare noradrenaline and serotonin concentrations between sleep stages. We further present experimental monoamine data from the medial prefrontal cortical (mPFC). Monoamine levels varied with brain region and circadian cycle. During sleep, monoamine levels generally decreased compared to wake. These qualitative observations were supported by the NMAs: noradrenaline and serotonin levels decreased from wakefulness to slow wave sleep and decreased further during Rapid Eye Movement sleep. In contrast, monoamine levels generally increased during SD, and sometimes remained high even during subsequent recovery. Decreases during or after SD were only reported for serotonin. In our experiment, SD did not affect any of the mPFC monoamine levels. Concluding, monoamine levels vary over the light-dark cycle and between sleep stages. SD modifies the patterns, with effects sometimes lasting beyond the SD period.

Peripheral reproductive organ health and melatonin: ready for prime time

Melatonin has a wide variety of beneficial actions at the level of the gonads and their adnexa. Some actions are mediated via its classic membrane melatonin receptors while others seem to be receptor-independent. This review summarizes many of the published reports which confirm that melatonin, which is produced in the ovary, aids in advancing follicular maturation and preserving the integrity of the ovum prior to and at the time of ovulation. Likewise, when ova are collected for in vitro fertilization-embryo transfer, treating them with melatonin improves implantation and pregnancy rates. Melatonin synthesis as well as its receptors have also been identified in the placenta. In this organ, melatonin seems to be of particular importance for the maintenance of the optimal turnover of cells in the villous trophoblast via its ability to regulate apoptosis. For male gametes, melatonin has also proven useful in protecting them from oxidative damage and preserving their viability. Incubation of ejaculated animal sperm improves their motility and prolongs their viability. For human sperm as well, melatonin is also a valuable agent for protecting them from free radical damage. In general, the direct actions of melatonin on the gonads and adnexa of mammals indicate it is an important agent for maintaining optimal reproductive physiology.

Circadian regulation of pineal gland rhythmicity

The pineal gland is a neuroendocrine organ of the brain. Its main task is to synthesize and secrete melatonin, a nocturnal hormone with diverse physiological functions. This review will focus on the central and pineal mechanisms in generation of mammalian pineal rhythmicity including melatonin production. In particular, this review covers the following topics: (1) local control of serotonin and melatonin rhythms; (2) neurotransmitters involved in central control of melatonin; (3) plasticity of the neural circuit controlling melatonin production; (4) role of clock genes in melatonin formation; (5) phase control of pineal rhythmicity; (6) impact of light at night on pineal rhythms; and (7) physiological function of the pineal rhythmicity.

Posttranslational regulation of TPH1 is responsible for the nightly surge of 5-HT output in the rat pineal gland

Serotonin (5-hydroxytryptamine, 5-HT), a precursor for melatonin production, is produced abundantly in the pineal gland of all vertebrate animals. The synthesis of 5-HT in the pineal gland is rate limited by tryptophan hydroxylase 1 (TPH1) whose activity displays a twofold increase at night. Earlier studies from our laboratory demonstrate that pineal 5-HT secretion exhibits dynamic circadian rhythms with elevated levels during the early night, and that the increase is controlled by adrenergic signaling at night. In this study, we report that (a) 5-HT total output from the pineal gland and TPH1 protein levels both display diurnal rhythms with a twofold increase at night; (b) stimulation of cAMP signaling elevates 5-HT output in vivo; (c) 5-HT total output and TPH1 protein content in rat pineal gland are both acutely inhibited by light exposure at night. Consistent with these findings, molecular analysis of TPH1 protein revealed that (a) TPH1 is phosphorylated at the serine 58 in vitro and in the night pineal gland; and (b) phosphorylation of TPH1 at this residue is required for cAMP-enhanced TPH1 protein stability. These data support the model that increased nocturnal 5-HT synthesis in the pineal gland is mediated by the phosphorylation of TPH1 at the serine 58, which elevates the TPH1 protein content and activity at night.

Application of long-term microdialysis in circadian rhythm research

Our laboratory has pioneered long-term microdialysis to monitor pineal melatonin secretion in living animals across multiple circadian cycles. There are numerous advantages of this approach for rhythm analysis: (1) we can precisely define melatonin onset and offset phases; (2) melatonin is a reliable and stable neuroendocrine output of the circadian clock (versus behavioral output which is sensitive to stress or other factors); (3) melatonin measurements can be performed extremely frequently, permitting high temporal resolution (10 min sampling intervals), which allows detection of slight changes in phase; (4) the measurements can be performed for more than four weeks, allowing perturbations of the circadian clock to be followed long-term in the same animals; (5) this is an automated process (microdialysis coupled with on-line HPLC analysis), which increases accuracy and bypasses the labor-intensive and error-prone manual handling of dialysis samples; and (6) our approach allows real-time investigation of circadian rhythm function and permits appropriate timely adjustments of experimental conditions. The longevity of microdialysis probes, the key to the success of this approach, depends at least in part on the methods of the construction and implantation of dialysis probes. In this article, we have detailed the procedures of construction and surgical implantation of microdialysis probes used currently in our laboratory, which are significantly improved from our previous methods.

Relationship between nocturnal serotonin surge and melatonin onset in rodent pineal gland

Background

We have recently reported dynamic circadian rhythms of serotonin (5-HT, 5-hydroxytryptamine) output in the pineal gland of rat, which precedes the onset of N-acetylserotonin (NAS) and melatonin secretion at night. The present study was aimed at investigating in detail the relationship between 5-HT onset (5HT-on) and melatonin onset (MT-on) in multiple strains of rats and comparing them with those of hamsters.

Methods

Animals were maintained in chambers equipped with light (250 lux at cage levels) and ventilation in a temperature-controlled room. Following surgical implantation of a microdialysis probe in the pineal gland, animals were individually housed for on-line pineal microdialysis and for automated HPLC analysis of 5-HT and melatonin. Animals were under a light-dark cycle of 12:12 h for the duration of the experiments.

Results

All animals displayed dynamic 5-HT and melatonin rhythms at night. In all cases, 5HT-on (taken at 80% of the daily maximum levels) preceded MT-on (taken at 20% of the daily maximum levels). Within the same animals, 5HT-on as well as MT-on across multiple circadian cycles exhibited minimum variations under entrained conditions. Large inter-individual variations of both 5HT-on and MT-on were found in outbred rats and hamsters under entrained conditions. In comparison, inbred rats displayed very small individual variations of 5HT-on and MT-on. Importantly, we have uncovered a species-specific relationship of 5HT-on and MT-on. 5HT-on of rats, regardless of the strain, preceded MT-on of the same rats by 50 min. In contrast, 5HT-on of hamsters led MT-on by as much as 240 min. Thus, while a constant relationship of 5HT-on and MT-on exists for animals of the same species, the relative timings of 5HT-on and MT-on differ between animals of different species.

Conclusion

These results suggest that both 5-HT and melatonin could serve as reliable markers of the circadian clock because of their day-to-day precision of onset timings within the same animals or within individuals of the same strain or same species. The results also demonstrate that data for MT-on cannot be compared directly between different species, and that 5HT-on may be a more reliable circadian marker when data from animals of different species are compared.

An improved liquid chromatographic method with electrochemical detection for direct determination of serotonin in microdialysates from Caudate-putamen and pineal gland regions of rat brain

A liquid chromatography method coupled with electrochemical detection has been developed for the direct measurement 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in brain microdialysates. The separation conditions have been optimized to detect only the 5-HT and 5-HIAA in dialysates and elute the other monoamines and their metabolites in the void. Linear regression analysis of chromatographic peak area as a function of concentration in the range 5-1000 pg/mL gave correlation coefficients over 0.995. Sample stability and an assay validation for precision and accuracy were also performed. The limit of detection (S/N = 3) for 5-HT was 12 femtomol/mL. The method has been applied to simultaneously measure extracellular 5-HT and 5-HIAA in brain microdialysates from the pineal gland and caudate-putamen of awake and freely-moving rats under basal conditions.

Reentrainment of the circadian pacemaker through three distinct stages

Circadian rhythms are endogenously generated by a central pacemaker and are synchronized to the environmental LD cycle. The rhythms can be resynchronized, or reentrained, after a shift of the LD cycle, as in traveling across time zones. The authors have performed high-resolution mapping of the pacemaker to analyze the reentrainment process using rat pineal melatonin onset (MT(on)) and melatonin offset (MT(off)) rhythms as markers. Following LD (12:12) delays of 3, 6, and 12 h, MT(on) was phase locked immediately, whereas MT(off) shifted rapidly during the initial 1 through 3 cycles. In all animals, the MT(off) shifted beyond their expected phase positions in the new LD cycle, which resulted in a transient expansion of melatonin secretion duration for several cycles. It took MT(off) only 1, 2, or 3 cycles to complete most of the required phase shifts after 3, 6, or 12 h of the LD cycle delays, respectively. However, the final stabilization of phase relationships of both MT(on) and MT(off) required at least 6 cycles for rats experiencing a 3-h LD delay and much longer for the rest. These results reaffirmed the notion that both onset and offset phases of melatonin rhythms are important markers for the pacemaker and demonstrated that the reentrainment of the central pacemaker to a delay shift of the LD cycle is a 3-step process: an immediate phase lock of onset and a rapid delay shift of offset rhythms, overshoot of the offset, and, finally, a slow adjustment of both onset and offset phases. This study represents the 1st detailed analysis of the pacemaker behavior during reentrainment using melatonin and supports the notion that the eventual adaptation of the circadian pacemaker to a new time zone is a time-consuming process.

Free-running rhythms of pineal circadian output

Circadian rhythms are self-sustaining oscillations that free-run in constant conditions with a period close to 24 h. Overt circadian rhythms have been studied mostly using onset phase as the marker for the underlying pacemaker. Using in vivo online pineal microdialysis, the authors have performed detailed analysis of free-running profiles of rat pineal secretory products, including N-acetylserotonin (NAS) and melatonin that have precisely defined onsets and offsets. When rats entrained in LD 12:12 were released into constant darkness (DD), both onset and offset phases of melatonin and NAS free-run. However, while onsets free-run with a period closer to a day (FRP(on) = 24-24.17 h) at the beginning, offset phases free-run with significantly larger FRPs (free-running periods) (FRP(off) = 24.24-24.42 h). This asymmetric free-running of onset and offset of NAS and melatonin in DD resulted in a 60- to 120-min increase of secretion duration of both NAS and melatonin. The rate of expansion of melatonin duration was 10 to 15 min per circadian cycle. The expansion of melatonin secretion duration ended for some within 4 days, while others were still expanding by the end of 10th day in DD. These results revealed that upon release into DD, the pacemaker's oscillation is initially driven by 2 forces, free running and decompression, before reaching a stable state of free running, and suggest that the circadian pacemaker may be an elastic structure that can decompress and compress under varying photic conditions. They also illustrate the importance of using both onset and offset of a given rhythm as phase markers, as compression/decompression, and transient disparity between FRP(on) and FRP(off) may be a common phenomenon of the circadian pacemaker.

N-acetyltransferase is not the rate-limiting enzyme of melatonin synthesis at night

Circadian melatonin production in the pineal gland and retina is under the control of serotonin N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase. Because NAT activity varies diurnally, it has been considered both the melatonin rhythm-generating enzyme and the rate-limiting enzyme of melatonin synthesis. In rats with dramatically reduced NAT activity due to a H28Y mutation in NAT, melatonin levels remained the same as in wildtype controls, suggesting that NAT does not determine the rate of melatonin production at night. Using a combination of molecular approaches with a sensitive in vivo measurement of pineal diurnal melatonin production, we demonstrate that (i) N-acetylserotonin (NAS), the enzymatic product of NAT, is present in vast excess in the night pineals compared with melatonin; (ii) the continuous increase in NAT protein levels at late night does not produce a proportional increase in melatonin; and (iii) an increase in NAS in the same animal over several circadian cycles do not result in corresponding increase in melatonin output. These results strongly suggest that NAT is not the rate-limiting enzyme of melatonin formation at night.