Herbert Underwood

The pineal and melatonin: Regulators of circadian function in lower vertebrates

The pineal has been identified as a major circadian pacemaker within the circadian system of a number of lower vertebrates although other pacemaking sites have been implicated as well. The rhythmic synthesis and secretion of the pineal hormone, melatonin, is suggested as the mechanism by which the pineal controls circadian oscillators located elsewhere. Both light and temperature cycles can entrain the pineal melatonin rhythm. The pineal, therefore, acts as a photo and thermoendocrine transducer which functions to synchronize internal cycle with cycles in the environment. A model is presented which portrays the pineal as a major component of a ‘multioscillator’ circadian system and which suggests how these multiple circadian clocks are coupled to each other and to cycles of light and temperature in the external world.

Vertebrate Circadian and Photoperiodic Systems: Role of the Pineal Gland and Melatonin:

first of these compounds isolated, and it had the interesting property of causing blanching of frog skin. A more important reason, however, arose from the belief that the terminal enzyme in the synthesis of melatonin, hydroxyindole-O-methyltransferase (HIOMT), was uniquely localized to pineal tissue. Consequently, melatonin and related methoxyindoles were, at least potentially, unique pineal products. Finally, interest in pineal products was generated because the pineal appeared to be involved in controlling important physiological functions, such as reproduction. Of the several related methoxyindoles, only 5-methoxytryptamine and 5-methoxytryptophol exhibit actions that, like those of melatonin, mimic certain influences of photoperiod on mammalian reproduction. Melatonin, however, appears to be at least 10 times more potent than any of the other naturally occurring methoxyindoles with respect to these actions (Reiter et al., 1975; Rollag, 1982; Goldman et al., 1984). This review focuses on melatonin’s role in two systems-the circadian system and the photoperiodic system. These two systems are discussed together because of the interrelationship between the circadian and photoperiodic systems: Many vertebrates utilize a circadian mechanism to measure daylength. A brief description of the structural evolution of the pineal and related organs is given as background for a more detailed discussion of melatonin’s role within the circadian and photoperiodic systems.

Vertebrate circadian rhythms: Retinal and extraretinal photoreception

Both the pineal and the SCN are elements of the vertebrate multioscillator system although the relative importance of these 2 areas probably varies between, and possibly within, the different vertebrate classes. Extraretinal photoreception is a universal feature of submammalian vertebrates, and possibly of neonatal mammals, but is absent in adult mammals. Although the pineal systems of sumammalian vertebrates are photosensitive, the pineal system has been directly implicated as an extraocular site for the perception of entraining light cycles only in amphibians. In all other submammalian vertebrates extraretinal entrainment can occur in the absence of the pineal system although it is certainly conceivable that the pineal system may act as an alternate route of photoreception. These extraretinal-extrapineal receptors are located within the brain but the exact location(s) of these receptors within the brain is unknown. The hypothalamus would be likely area for this extraretinal photoreception, however, for several reasons: 1. Neurophysiological studies have identified light sensitive neurons in the frogs hypothalamus43. 2. The avian hypothalamus is a site of photoperiodic photoreception100–103. 3. The only other light sensitive structures known in vertebrates—the pineal system and the lateral eyes—are all derived embryologically from the hypothalamus. 4. The hypothalamus appears to be the site of a circadian clock and there may be advantages in having the photoreceptors and the clock anatomically close to one another. These considerations, of course, do not exclude the possibility that other brain areas may be involved as well. The reason behind the loss of extraretinal photoreception in mammals is uncertain. The shift to exclusive retinal photoreception in mammals may have been dictated by the extensive reorganization that occurred during the evolution of the mammalian brain. Or, perhaps, the increased size of the mammalian skull and overlying tissue made direct photoreception difficult and necessitated a shift to retinal photoreception. The persistence of extraretinal photoreceptors in submammalian vertebrates, however, underscores their importance in the sensory repertoire of vertebrates.

Pineal melatonin rhythms in the lizardAnolis carolinensis: effects of light and temperature cycles

SummaryPineal and ocular melatonin was assessed, over 24 h periods, in male lizards (Anolis carolinensis) entrained to 24 h light-dark (LD) cycles and a constant 32 ‡C, and in lizards entrained to both 24 h LD cycles and 24 h temperature cycles (32 ‡C/20 ‡C). At a constant temperature, the duration of the photoperiod has a profound effect on the duration, amplitude, and phase of the pineal melatonin rhythm (Fig. 1). The pineal melatonin rhythm under cyclic temperature peaks during the cool (20 ‡C) phase of the cycle regardless of whether or not the cool phase occurs during the light or dark phase of a LD 12∶12 cycle (Fig. 3). Under a temperature cycle and constant dim illumination, a pineal melatonin rhythm is observed which peaks during the cool phase of the temperature cycle, but the amplitude of the rhythm is depressed relative to that observed under LD (Fig. 2). Illumination up to 2 h in duration does not suppress the nocturnal melatonin peak in theAnolis pineal (Fig. 4). No melatonin rhythm was observed in the eyes ofAnolis under either 24 h LD cycles and a constant temperature (Fig. 1), or under simultaneous light and temperature cycles (Fig. 3). Ocular melatonin content was, in all cases, either very low or non-detectable.

Pineal melatonin rhythms in the lizard Anolis carolinensis: I. Response to light and temperature cycles.

Both light and temperature can influence the pineals synthesis of the indoleamine melatonin. An investigation of the effects of light and temperature cycles on the pineal melatonin rhythm (PMR) showed the following: (1) Both daily light cycles and daily temperature cycles could entrain the PMR; melatonin levels peaked during the dark phase of a light-dark cycle or the cool phase of a temperature cycle. (2) The PMR could be entrained by a temperature cycle as low as 2°C in amplitude in lizards held in constant light or constant darkness. (3) The length of the photoperiod or thermoperiod affected the phase, amplitude, or duration of the PMR. (4) When presented together, the effects of light and temperature cycles on the PMR depended on the phase relationship between the light and temperature cycles, as well as on the strength of the entraining stimuli, such as the amplitude of the temperature cycle. (5) Exposure to a constant cold temperature (10°C) eliminated the PMR, yet a rhythm could still be expressed under a 24-hr temperature cycle (32°C/10°C), and the rhythm peaked during the 10°C phase of the cycle. (6) A 6-hr dark pulse presented during the day did not elicit a premature rise in melatonin levels. These studies show how environmental stimuli can control the pineal rhythm of melatonin synthesis and secretion. Previous studies have supported a model in which the lizards pineal acts as a circadian pacemaker within a multioscillator circadian system, and have implicated melatonin as a hormone by which the pineal may communicate with the rest of the system. The lizard pineal, therefore, may act as a photo- and thermoendocrine transducer translating light and temperature information into an internal cue in the form of the PMR. The PMR, in turn, may control the phase and period of circadian clocks located elsewhere, insuring that the right internal events occur at the right time of day.

Circadian organization in the lizardSceloporus occidentalis: The effects of pinealectomy, blinding, and melatonin

SummaryPinealectomy of the iguanid lizardSceloporus occidentalis freerunning in either continuous illumination or continuous darkness typically causes changes in the period of the activity rhythm as well as changes in the amount of daily activity (α). Blinding also alters the period of the freerunning activity rhythm. Continuous long term administration of melatonin via subcutaneous capsules causes a significant lengthening of the period of the activity rhythm (as well as a decrease in α) of pinealectomized and/or blinded lizards showing that melatonin exerts its action at extrapineal and extraocular sites. However, the amount of lengthening induced by melatonin is significantly greater in pinealectomized lizards than in intact lizards. The results indicate that the pineal (and possibly the eyes) act as coupling devices or as the loci of circadian pacemakers within a multioscillator system. Melatonin may function as a chemical messenger between the pineal (or eyes) and the rest of the circadian system.

Entrainment of the circadian activity rhythm of a lizard to melatonin injections

The circadian activity rhythms of lizards (Sceloporus occidentalis) can be entrained (synchronized) to a period of 24 hr by melatonin injections given every other day at the same time of day, but not by saline injections. The activity onsets of the entrained lizards exhibited two preferred phase-relationships (approximately 165 degrees and approximately 30 degrees) with the time of melatonin injections with the 30 degree phase only rarely observed. These results suggest that endogenous rhythms of melatonin secretion (i.e., from the pineal organ) may be involved in synchronizing circadian oscillations within the lizards multioscillator circadian system.

Circadian Rhythms in Lizards: Phase Response Curve for Melatonin

Single biweekly injections of melatonin were administered to lizards (Sceloporus occidentalis) free‐running (exhibiting their endogenous circadian activity rhythm) in constant dim illumination. The injections caused phase shifts in the activity rhythm whose magnitude and direction were a function of the time of the melatonin injections, relative to activity onsets. Plotting the direction and amount of phase shift versus the time (phase) at which the injection was given generates a phase‐response curve (PRC). The PRC shows that injections administered between midsubjective day and early subjective night (6–15 hr after activity onset) elicit phase advances in the activity rhythm, whereas injections given at other phases of the activity cycle induce phase delays. The existence of a PRC for melatonin suggests that the daily endogenous rhythm of melatonin (i. e., of pineal origin) may be involved in phasing, or entraining, the circadian system of lizards. The shape of the PRC also allows predictions as to the effects of continuous exogenous melatonin administration on the period of free‐running activity rhythms as well as on the mechanism of entrainment of activity rhythms to daily melatonin injections.

The circadian rhythm of thermoregulation in Japanese quail

Japanese quail exhibit a robust circadian rhythm in body temperature. This rhythm is readily entrainable by 24 h light-dark (LD) cycles and persists under constant conditions. Because both the pineal organ and the eyes have been implicated as major components of the circadian system of birds, the role of these organs in generating the rhythm of body temperature was investigated. Pinealectomy, when performed alone, had little effect on the body temperature rhythm of quail either under LD or under constant darkness (DD). Most birds subjected to optic nerve section alone remained rhythmic in DD although the “robustness” of the rhythm was decreased, and 25% became arrhythmic. Birds subjected to both pinealectomy and optic nerve section behaved similarly to birds subjected to optic nerve section alone. However, complete eye removal, when performed alone or in combination with pinealectomy, caused all birds to become arrhythmic in DD. The data support the hypothesis that the eyes are the loci of circadian pacemakers in quail that act, via both neural and hormonal outputs, to preserve the integrity of (self-sustaining or damped) circadian oscillators located elsewhere.

Melatonin Rhythms in Quail: Regulation by Photoperiod and Circadian Pacemakers

The profile of melatonin in the eyes, pineal, and blood of Japanese quail was assessed in birds held under LD 16:8 and LD 6: 18 photoperiods. Melatonin levels in all three tissues showed a robust daily rhythm with higher levels occurring at night. The amplitude of the rhythm was depressed and its duration lengthened on LD 6: 18 relative to LD 16:8. The blood melatonin rhythm precisely reflected the rhythms shown by the pineal and eyes, supporting the idea that the blood rhythm is a result of melatonin secretion by both the eyes and pineal.