Prolonged Continuous Light Research Articles

Light-induced hyphal branching of germinated AM fungal spores

Hyphal branches of the primary germ tubes and secondary hyphae of Gigaspora gigantea, Gigaspora rosea, and Glomus intraradices were induced by exposure to light. The photo- induced branching of G. rosea was increased if the germinated spores were first grown in the presence of 10 μM quercetin before exposure to light. Further analyses with G. gigantea showed that at low intensity light (13.4 μE s-1m-2), maximum branching was achieved after a 6 h exposure and at high intensity light (10,800 μE s-1m-2), maximum branching was reached after an 8 min exposure. Multiple exposures to alternating low light followed by a dark incubation period indicated that the photo-effect was not additive. Photo-induced branching did not need a subsequent dark period for the growth of hyphal branches because branching occurred during prolonged continuous light. The light-induced branching appeared to have ecological relevance. Corn seedlings (Zea maize L.) grown in AM fungal inocula exposed to light had a higher percentage of their root system colonized by G. gigantea than those in inocula that remained in the dark.

Pharmacologic restoration of suppressed temperature rhythms in rats by melatonin, melatonin receptor agonist, S20242, or 8-OH-DPAT.

Endogenous circadian rhythms in body temperature and locomotor activity rhythms are suppressed in Sprague-Dawley rats exposed to prolonged continuous light, possibly as a result of a profound alteration of the melatonin secretion rhythm. The ability to restore circadian system function with either exogenous melatonin, or melatonin receptor agonist S20242 (N-[2-(7-methoxy napth-1-yl)ethyl] propionamide), or 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), was investigated under these conditions. Seven rats received a daily 6-h intravenous infusion of melatonin (0.01 mg kg(-1)) for 10 days, which generates a nearly physiological circadian rhythm of urinary 6-sulfatoxy-melatonin, the main urinary metabolite of melatonin. Nevertheless, there was no effect on body temperature or locomotor activity rhythms. Then, 49 rats received daily subcutaneous melatonin (0.01, 1 or 5 mg kg(-1) day(-1)), S20242 (1 or 5 mg kg(-1) day(-1)) or 8-OH-DPAT (5 mg kg(-1) day(-1)) for 30 days. The circadian rhythm in body temperature was restored by subcutaneous melatonin or by S20242 as a function of the dose or by 8-OH-DPAT. The effect started within the first 10 days of treatment and persisted for I to 3 weeks following the end of treatment in 8 of 10 rats receiving melatonin, in 9 of 11 rats treated with S20242 and in 1 of 4 rats treated with 8-OH-DPAT. Activity was less susceptible to entrainment than temperature with these drugs, since circadian rhythmicity was restored in only 2 of 6 rats treated with melatonin and in 1 of 4 rats treated with 8-OH-DPAT. These data demonstrate a specific action of subcutaneous melatonin, S20242 or 8-OH-DPAT on temperature rather than on activity rhythms. This differential effect on two major outputs of the suprachiasmatic nucleus further supports the existence of two independent oscillators in this hypothalamic circadian clock, which may be considered as separate pharmacological targets in the circadian system.

A photosensitive circadian oscillator in an insect endocrine gland: photic induction of rhythmic steroidogenesis in vitro

The paired prothoracic glands of the insect Rhodnius prolixus each comprise a group of about 200 structurally identical cells. The synthesis (and release) of steroid moulting hormones (ecdysteroids) by these glands is under circadian control in vivo. We monitored ecdysteroid synthesis by single glands during long-term incubations in vitro. Synthesis is rhythmic in vitro and persists in continuous darkness. Glands which are arrhythmic (from prolonged continuous light) respond to transfer to darkness in vitro with the initiation of a free-running circadian rhythm of ecdysteroid synthesis. Therefore, the glands possess a light-sensitive circadian oscillator. These properties are conventionally associated with nervous tissue of animals. It is suggested that rhythmicity is synchronized within the gland by the known structural and electrical coupling between its component cells. The glands share properties with known pacemakers such as the avian pineal. However, the glands in vivo receive input from both light cues and the cerebral neuropeptide, prothoracicotropic hormone. Rhythmic release of this neuropeptide is controlled by a second oscillator located in the brain. We conclude that the pacemaker in the endocrine system of R. prolixus comprises at least three oscillators, one in each prothoracic gland and one in the brain, which are coupled hormonally. We conclude that the prothoracic gland is an important component of the circadian system controlling development in R. prolixus and that peripheral endocrine glands may play a more active role in the generation of animal circadian organization than has been thought.

Persistence of circadian oscillation while locomotor activity and plasma melatonin levels became aperiodic under prolonged continuous light in the rat

In order to examine the mechanism for a loss of circadian rhythms in several functions under prolonged continuous light (LL), rats were blinded following LL over 5 months, and the mode of reappearance of circadian rhythms were analyzed in locomotor activity and plasma melatonin levels. Locomotor activity and plasma melatonin levels in individual rats became aperiodic after the exposure to LL. On the day of blinding, plasma melatonin levels showed circadian rhythms having a peak coincided with the activity time of locomotor rhythm which was restored after blinding. The time of melatonin peak was not related to the time of blinding (onset of darkness) nor to the initial time of blood sampling. Circadian rhythm in plasma melatonin levels reappeared faster than those in locomotor activity. The findings suggest that aperiodism developed in these functions under prolonged LL is not due to disruption of the circadian oscillation but to uncoupling of overt functions from the circadian pacemaker.

Persistence of circadian oscillation while locomotor activity and plasma melatonin levels became aperiodic under prolonged continuous light in the rat

In order to examine the mechanism for a loss of circadian rhythms in several functions under prolonged continuous light (LL), rats were blinded following LL over 5 months, and the mode of reappearance of circadian rhythms were analyzed in locomotor activity and plasma melatonin levels. Locomotor activity and plasma melatonin levels in individual rats became aperiodic after the exposure to LL. On the day of blinding, plasma melatonin levels showed circadian rhythms having a peak coincided with the activity time of locomotor rhythm which was restored after blinding. The time of melatonin peak was not related to the time of blinding (onset of darkness) nor to the initial time of blood sampling. Circadian rhythm in plasma melatonin levels reappeared faster than those in locomotor activity. The findings suggest that aperiodism developed in these functions under prolonged LL is not due to disruption of the circadian oscillation but to uncoupling of overt functions from the circadian pacemaker.

Pharmacokinetically guided melatonin scheduling in rats with circadian system suppression

To obtain a pharmacologic effect of melatonin in rats kept under prolonged continuous light exposure, conditions known to produce functional suppression of the circadian system, mimicking of the physiologic 24-h pattern of melatonin secretion, a hormonal signal of darkness exposure may be needed. The delivery scheme for melatonin was established in rats in the present studies. First, the plasma pharmacokinetics of [ 3H]melatonin were determined in rats kept under continuous light and in rats synchronized by exposure to alternating 12 h light and 12 h darkness (LD 12:12) in the early light span. The pharmacokinetics of total radioactivity were similar in both groups. Further quantitation of melatonin by thin-layer chromatography revealed differences dependent on light conditions. The mean plasma clearance and steady-state distribution volume were ∼ twice as low with continuous light as with LD 12:12. Plasma protein binding of melatonin was ∼ 33%, irrespective of group or sampling time. These pharmacokinetic parameters were used to devise a 24-h periodic delivery schedule consisting of a 6-h constant infusion of exogenous melatonin, followed by an 18-h melatonin-free interval. In a second study, the melatonin 24-h pattern was estimated from the measurement of 2-h fractions of urinary 6-sulfatoxymelatonin excretion for 4 days. 6 unrestrained rats kept under continuous light received melatonin for 2 days from 22:00 to 04:00 h through an indwelling jugular catheter, connected to a reservoir from a programmable pump. Only the administration of low doses (0.01 mg/kg/day) resulted in both a circadian pattern for 6-sulfatoxymelatonin excretion and normal physiological values during the infusion-free intervals. The resynchronizing efficacy of this schedule should be tested in rats with functional suppression of the circadian system.

Aperiodism induced by prolonged continuous light: Different mechanisms for locomotor and melatonin rhythms

Aperiodism induced by prolonged continuous light: Different mechanisms for locomotor and melatonin rhythms

Effects of prior exposure to prolonged continuous light on the pattern of melatonin secretion in sheep held under continuous darkness.

To gain insight into the nature of the neural centre regulating melatonin production by the pineal gland we have measured hourly changes in plasma melatonin in 16 ewes held for 42-56 h under continuous darkness following a 28-day period of exposure to continuous light. Plasma melatonin was undetectable while animals were in constant light but increased to normal night time levels (120-2,200 pmol/litre) within 10 min of the onset of darkness in 14 out of the 16 ewes regardless of whether this occurred at 1,200 h (group 1) or 2,400 h (group 2). In four ewes melatonin secretion was maintained for approximately 36 h, indicating that the pineal gland remained responsive to a static stimulatory signal presumably emanating from the supra chiasmatic nucleus. In all ewes, melatonin secretion ceased at approximately 12 and or 36 h after the onset of darkness and did not require a separate zeitgeber. Although the increase and the decline of plasma melatonin levels were synchronized within groups 1 and 2, there were sufficient differences in the pattern of melatonin secretion between the groups to suggest that the regulatory mechanism had an intrinsic component. A hypothesis involving a diurnal change in sensitivity to circulating melatonin is advanced to explain the observed differences in the synchrony of melatonin secretion and the prolonged episodes of melatonin secretion.

Endogenous ultradian rhythms in rats exposed to prolonged continuous light.

Circadian rhythms of locomotor activity, body temperature, and plasma corticosterone were determined simultaneously in individual rats that were exposed to 200 lx continuous light for over 3 mo. Free-running circadian rhythms of locomotor activity persisted for about 2 mo under continuous light and then the rhythms gradually decomposed. After 3 mo of exposure, circadian rhythms disappeared and activity bursts of 1- to 2-h duration manifested themselves several times during a 24-h period. Body temperature also exhibited several bursts of fluctuation and these bursts were closely correlated in their temporal sequence with those of locomotor activity. A least-squares spectrum analysis revealed that the burst had regular 4- to 6-h periods. Plasma corticosterone, determined by serial sampling at 2-h intervals from individual rats, also exhibited several secretion episodes in a day. These episodic secretions synchronized with bursts of locomotor activity. These results suggest that the ultradian component, manifested under prolonged continuous light, is a fundamental unit of the circadian rhythm and an oscillator for the ultradian rhythm is common to the three functions examined.

The relationship of the hyaluronidase and antihyaluronidase activity of the retina to the intensity of a light stimulus

The retina possesses hyaluronidasic and antihyaluronidasic activity which depends on the strength of the light stimulus. In prolonged continuous light stimulation the retinal hyaluronidasic activity is diminished. The dynamics of decrease of the hyaluronidasic activity depends on its initial value: the higher the initial activity, the quicker its decrease.