Retinal input to the suprachiasmatic nucleus before and after puberty in Djungarian hamsters

Constructing the suprachiasmatic nucleus: a watchmaker's perspective on the central clockworks

The circadian system constrains an organism's palette of behaviors to portions of the solar day appropriate to its ecological niche. The central light-entrained clock in the suprachiasmatic nucleus (SCN) of the mammalian circadian system has evolved a complex network of interdependent signaling mechanisms linking multiple distinct oscillators to serve this crucial function. However, studies of the mechanisms controlling SCN development have greatly lagged behind our understanding of its physiological functions. We review advances in the understanding of adult SCN function, what has been described about SCN development to date, and the potential of both current and future studies of SCN development to yield important insights into master clock function, dysfunction, and evolution.

Development of retinal projections and response to photic input in the suprachiasmatic nucleus of New Zealand White Rabbits

In rabbit pups, nursing by the mother is the prevailing entraining signal for their circadian rhythms during at least the first two weeks of life. Therefore, they are considered a natural model of food anticipatory activity. However, the photic entrainment of the circadian system in rabbit pups during this developmental stage is not well understood. The present study examined the retinal projections to the suprachiasmatic nucleus (SCN) and the functional responses of the SCN to light exposure. Using the anterograde tracer cholera toxin-B, we examined the retinal projections to the SCN at postnatal days (PD) 1, 9, 19 and in adult animals. The results revealed that the retinal projections were present at PD1 with a bilateral symmetry, and with a contralateral tendency at PD19 and adults. We also explored the response of the SCN to a light pulse by assessing the induction of FOS protein, a marker of neuronal activation, at PD1, 12, 19 and in adults. Light-induced FOS was observed during day and night at PD1, but mainly during night at PD12, 19 and adults. We conclude that in the SCN there is a "gating" mechanism to FOS induction by light that develops several days after birth, as in other mammals, and in the rabbit is already present at PD12. Moreover, in contrast to other altricial mammals, the circadian visual system, although not essential for entraining the rhythm during first two weeks of life, is present and functional in rabbit pups from birth.

Photoperiodic control of oestrous cycles in Syrian hamsters: mediation by the mediobasal hypothalamus

To assess whether the mediobasal hypothalamus (MBH) is necessary for photoperiodic control of oestrous cycles and prolactin secretion, we tested intact female Syrian hamsters (controls) and those that had sustained unilateral or bilateral lesions of the MBH. All hamsters displayed 4-day oestrous cycles postoperatively in the long-day photoperiod (14 h light/day); control females and those with unilateral MBH damage ceased to undergo oestrous cycles approximately 8 weeks after transfer to a short-day photocycle (10 h light/day), whereas 12 of 15 females with bilateral MBH lesions continued to generate 4-day oestrous cycles throughout 22 weeks in short days. Serum prolactin concentrations were either undetectable or low in all hamsters 8 or 14 weeks after the transfer to short-day lengths, but increased above long-day baseline values by week 22. We conclude that melatonin-binding sites in the MBH mediate suppression of oestrous cycles but not prolactin secretion by short-day lengths; recovery of prolactin secretion in females during prolonged exposure to short-day lengths reflects development of refractoriness to melatonin in a substrate distinct from the MBH. These findings suggest that separate neural pathways mediate photoperiodic control of gonadotropin and prolactin secretion in female hamsters.

Environmental light-darkness conditions induce changes in brain and peripheral pyroglutamyl-peptidase I activity

To evaluate the influence of light and darkness on brain pyroglutamyl-peptidase I (pGluPI) activity, four experimental groups of rats were compared at the same time-point (10.00 h). Two groups were designed with a standard 12-12 h light-dark cycle: In group A, the lights were on from 7.00 h to 19.00 h, and the experiment was done under light conditions; in group B, the lights were on from 19.00 h to 7.00 h, and the experiment was done under darkness conditions. Two additional groups were designed with nonstandard light-dark conditions: In group C, the animals were subjected to constant light, and the experiment was done under light conditions. In group D, animals were subjected to constant darkness, and the experiment was done under darkness conditions. Light (vs darkness) and standard (vs nonstandard) conditions produced significant changes on pGluPI activity in specific structures; the data suggested that endogenous substrates of pGluPI such as thyrotropin-releasing hormone and gonadotropin-releasing hormone, might be modified in parallel. There was left predominance in the retina under light conditions on a standard schedule (group A). The regional pattern of distribution of activity was similar in groups on a standard schedule (A vs B) and in groups tested under constant light-dark conditions (C vs D). However, this pattern differed between groups subjected to standard vs constant light-dark conditions (A and B vs C and D). These results support an influence of environmental light and darkness on pGluPI activity, which may reflect concomitant changes in its susceptible substrates and consequently in their functions.

Retinohypothalamic projections in the common marmoset (Callithrix jacchus): A study using cholera toxin subunit B

Retinal projections in vertebrates reach the primary visual, accessory optic, and circadian timing structures. The central feature of the circadian timing system is the principal circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The direct projections from the retina to the SCN are considered the entrainment pathway of the circadian timing system. In this study, unilateral intravitreal injections of cholera toxin subunit B were used to trace the retinal projections to the marmoset hypothalamus. The retinohypothalamic tract reaches the ventral suprachiasmatic nucleus bilaterally, as anticipated from previous studies. However, labeled fibers were found in several other hypothalamic regions, such as the medial and lateral preoptic areas, supraoptic nucleus, anterior and lateral hypothalamic areas, retrochiasmatic area, and subparaventricular zone. These results reveal new aspects of retinohypothalamic projection in primates and are discussed in terms of their implications for circadian as well as noncircadian control systems.

Daily melatonin treatments regulate the circadian melatonin rhythm in the adult Djungarian hamster

The present study tested the hypothesis that daily melatonin treatments influence the biological clock mechanism controlling the circadian melatonin rhythm. Adult male and female Djungarian hamsters in light:dark = 16L:8D (lights on 0300-1900 h) were administered melatonin subcutaneously (s.c.) each day (5 micrograms/0.2 ml saline) in the morning at 1000 h (AM) or late afternoon at 1700 h (PM); controls received a vehicle injection (CON). After 14 days, pineal and serum melatonin concentrations were determined at various times on the last day of treatment and the next day in constant darkness (no treatment). The rhythm in pineal gland melatonin content was similar in each of the three groups on the last day of treatment (about 6 h duration). On the next day in constant dark, the rising phase was advanced and duration extended by 2 h or more in melatonin-treated hamsters compared to that in CONs (ANOVA). In circulation, the melatonin rhythm in AM and PM groups was phase advanced (onset and peak) on both days of the study. Thus duration was extended by up to 4.5 h compared to that in saline-treated controls. Moreover, amplitude of the nighttime serum melatonin rise was elevated up to fivefold relative to that in the CON group (ANOVA and Accumulated Sums analysis). The effects of repeated melatonin treatments on amplitude and phase of the serum melatonin rhythm raise the possibility that the circadian clock that controls pineal gland production of melatonin may also regulate melatonin secretion. From this and another study, the apparent half-life of melatonin in circulation was estimated to be 7.5 min; the melatonin injection initially produced pharmacological concentrations that were followed by low serum melatonin levels within 2 h. Thus, in both melatonin treatment groups, the data suggest that two distinct periods of elevated serum melatonin were present each day. The cellular mechanism for melatonin action must take into consideration how a brief interruption in elevated melatonin in circulation (about 1 h in the PM group) is recognized as a continuous duration (short daylength), whereas a more extended baseline period is transduced as an abbreviated or long daylength (about 7 h in the AM group). These data further suggest that the biological clock mechanism that generates the circadian melatonin rhythm is responsive to the influence of daily melatonin treatments and presumably to the feedback action of endogenous melatonin on its own rhythm in the Djungarian hamster in long days.

Photoperiodic exposure and time of day modulate the expression of arginine vasopressin mRNA and vasoactive intestinal peptide mRNA in the suprachiasmatic nuclei of Siberian hamsters

In hamsters, changes in ambient photoperiod lead to alterations in the circadian rhythm of pineal melatonin secretion and subsequent changes in reproductive function. The present study examined whether photoperiod also alters 24-h rhythms in neuropeptide mRNA levels in the SCN of Siberian hamsters. In situ hybridization and quantitative autoradiography were used to assess messenger RNA levels for vasopressin (AVP) and vasoactive intestinal peptide (VIP) in the SCN of hamsters sacrificed at six times of day following exposure to long (16 h light/day) or short (10 h light/day) photoperiod for 2 weeks. Both AVP mRNA and VIP mRNA in the SCN were significantly affected by time of day and photoperiodic exposure. The 24-h profiles of AVP mRNA and VIP mRNA showed different relationships to the light: dark cycle, suggesting that these profiles are differentially regulated. In general, short photoperiod tended to suppress AVP mRNA and VIP mRNA in the SCN; this effect on AVP mRNA was significant at two times of day. These results complement and extend previous findings of 24-h h profiles in neuropeptide mRNA expression in the rat SCN by showing that these 24-h profiles are also characteristic of the Siberian hamster SCN and that they can be modulated by photoperiod.

Suprachiasmatic nucleus and photoperiodic regulation of gonadal development in the Siberian hamster, Phodopus sungorus

Photoregressed Siberian hamsters exposed at 19 days of age to constant light for 24 h manifested increased testicular development 16 days later. Hamsters that sustained lesions of the suprachiasmatic nucleus (SCN) 3 days after the light pulse had significantly heavier gonads than did sham-operated animals or hamsters pinealectomized at 19 days of age. Enhanced reproductive development after SCN ablation is not due solely to lesion-induced elimination of melatonin secretion. SCN neural activity beginning 3 days after the light pulse is not necessary for light-induced gonadal growth.

Ontogeny of a photic response in the suprachiasmatic nucleus in the Siberian hamster (Phodopus sungorus)

The ontogeny of photic responsiveness in the suprachiasmatic nucleus of the Siberian hamster (Phodopus sungorus) was studied using the enhanced expression of the immediate early gene c-fos as a marker of neuronal activation. c-fos expression was assessed by immunocytochemical localization of its protein product. Hamsters were kept on a 16 h light:8 h dark photocycle. The adult Siberian hamster showed a marked increase in the number of c-fos-immunoreactive (c-fos-ir) cells within the suprachiasmatic nuclei (SCN) in response to a 1 h light pulse delivered 1-3 h after lights off, in comparison to controls kept in the dark. This is consistent with previous studies in the Syrian hamster and rat. The development of the photic response was examined. The first study investigated the effects of a light pulse on c-fos induction in pups at 5, 9, 12 and 24 postnatal days of age (PD). The suprachiasmatic region was identified by immunocytochemical localization of peptide-histidine-isoleucine in adjacent sections, a peptide expressed early in the development of the rodent SCN. The distribution of c-fos-ir cells was also compared with the location of retinal efferents, as determined by intraocular injection of the tract tracer cholera toxin B subunit 24 h previously. At PD 9, 12 and 24, significant increases in the number of c-fos-ir cells occurred in the light pulsed animals in comparison to age-matched control animals which were moved within the non-illuminated room to provide a 'dark' pulse.(ABSTRACT TRUNCATED AT 250 WORDS)

Anterograde neuronal tracing of retinohypothalamic projections in the hamster--possible innervation of substance P-containing neurons in the suprachiasmatic nucleus

The retinohypothalamic tract (RHT) in the Djungarian hamster Phodopus sungorus was studied using anterograde neuronal transport of cholera toxin subunit B (CTB) following unilateral intraocular injection. A major projection terminates bilaterally in the hypothalamic suprachiasmatic nuclei (SCN). In the anterior ventral SCN, a light ipsilateral predominance was evident. In the medial SCN, labelling was concentrated laterally where it was seen over the dorso-ventral extension of the nuclei, pronounced contralaterally to the site of CTB injection, which was even more characteristic in the posterior aspects of the nuclei. Labelled fibers and terminals were observed in the supraoptic nuclei, but not in lateral and paraventricular hypothalamic regions. Additional experiments utilizing double immunofluorescence of CTB and of substance P (SP) in the SCN showed that SP-containing perikarya were particularly observed in a central portion of the nucleus, where CTB-stained terminals were accumulated in the vicinity of immunoreactive cell bodies, fibers and terminals. Our data provide preliminary morphological evidence for the regulation of SCN function by retinal afferents and may explain the circadian and photoperiodic fluctuations in the amount of SP in the SCN.

Influence of acute melatonin treatment and light on the circadian melatonin rhythm in the Djungarian hamster

The present study tested the hypothesis that acute melatonin treatment affects the biological clock mechanism controlling the circadian melatonin rhythm. Adult Djungarian hamsters were raised in LD 16:8 (lights-on from 0300 to 1900 hr). Pineal and serum melatonin concentrations were assessed at specific clock times during the subsequent LD cycle (day 1) and afterwards in hamsters that remained in constant darkness (day 2), to assess the effects of treatment on the circadian melatonin rhythm. In untreated controls (n = 76), melatonin increased in the pineal and in circulation within the first hour after lights-off (day 1) or subjective night (day 2); melatonin concentrations remained elevated for up to 7 hr. A single melatonin injection (5 micrograms/0.2 ml saline, s.c.), administered in the morning (0900 hr, n = 80) or late afternoon (1600 hr, n = 74), transiently increased in circulation but not in the pineal gland. In constant darkness (day 2), the circadian melatonin rhythm in both the pineal gland and circulation was the same as that in untreated controls. No significant differences were found among the three groups in the mean concentration during the day or night; the rise, fall, peak time, peak amplitude, and duration of increased melatonin were also unchanged. The ability of melatonin to override the effect of a light pulse on the circadian melatonin rhythm was also tested. The 5-min light pulse at night (2300-2305 hr of day 1, n = 64) delayed the rise and shortened the duration of increased melatonin in both pineal gland and serum by as much as 3 hr (day 2); these light effects were more pronounced on the pineal than on the serum melatonin rhythm. Injection with melatonin prior to the light pulse (5 micrograms/0.2 ml saline at 2245 hr, n = 64) failed to alter the inhibitory effects of light on the circadian melatonin rhythm. These data suggest that the biological clock mechanism controlling the circadian melatonin rhythm in the Djungarian hamster in long days is responsive to the phase-shifting effects of light, but resistant to the influence of acute melatonin treatment.