Melatonin attenuates light-at-night effects on systolic blood pressure and body temperature but does not affect diastolic blood pressure and heart rate circadian rhythms

Forty-six young adults (YA), 17–24 years old of both genders were studied under amodified
CR protocol for 26 h. Initially, participants were investigated
under constant light (CR-LL) and 2 weeks later under the same
conditions though 1.5mg melatonin (Melaxen) was given orally at 22:30.

Systolic blood pressure (SBP) and diastolic blood pressure
(DBP), heart rate (HR) and body temperature (BT) were measured
every 2 h. To verify the effect of constant light, formerly published
results obtained under light-dark conditions (CR-LD) were reanalyzed.

Administration of 1.5 mg of exogenous melatonin modified the
24 h patterns of BT and SBP within short 3.5 h time window but
did not influence DBP and HR. A short-term reduction of SBP and
BT for 1.5–3.5 hours was observed. The values in the CR-LL+M
group were significantly lower than in CR-LL at 2:00 h. Hence,
exogenous melatonin did mimic the scotophase.

Though this effect was gender-specific and found only in female YA.
Results of this study prompt further research to qualify and
quantify dosage-, duration- and time-dependent differences of
melatonin effects, to discern between short-term (acute) and longterm
(chronic) melatonin administration, and to clarify its underlying
mechanisms.

  Introduction Regular and predictable external cycles justify evolutionary emergence of the built-in
genetic clocks (Bhadra et al. 2017). Hence, biological rhythmicity is a fundamental
property of living beings that constitute multi-oscillatory systems (Aschoff 1965,
Pittendrigh 1993, Bell-Pedersen et al. 2005, Laje et al. 2018).

Circadian rhythms, in particular, are a ubiquitous feature of the single cell, though need to be synchronized and orchestrated both internally, with each other; and externally, with environmental time cues (Reppert and Weaver 2002, Merrow et al. 2005, Lamont et al. 2007, Golombek and Rosenstein 2010, Gubin 2013).

 The light-dark cycle is the main zeitgeber, which entrains the endogenous rhythms to the 24-h environment. The non-image forming photic information is perceived by a subpopulation of retinal ganglion cells, the so-called intrinsically photosensitive retinal ganglion cells (ipRGC). The axons of these neurons form the retinohypothalamic tract (RHT), the main afferent pathway to the site of the central circadian clock, the paired suprachiasmatic nuclei (SCN) which are located in the hypothalamus (Weaver 1998, Freedman et al. 1999, Golombek and Rosenstein 2010, Markwell et al. 2010).

 Human SCN is tiny clusters of approximately 100,000 neurons, that orchestrates myriads of
downstream functions in a rhythmic fashion (Bollinger and Schibler 2014). Nocturnal pineal hormone melatonin assists SCN providing signaling to the peripheral cells and back to SCN. Loss of integrity between rhythms either intrinsically within the body or extrinsically with external time cues provoke so-called circadian disruption that is potentially harmful for health and may cause certain pathologies (Karatsoreos et al. 2011, Salgado-Delgado et al. 2011, Gubin et al. 2013, Touitou et al. 2016).