Mammalian dentin universally shows circadian increments. However, little is known about the mechanism of this phenomenon. The purpose of the present study was to investigate the role of the suprachiasmatic nucleus (SCN) in the generation of circadian rhythm in dentin increment. Rats underwent lesion of the SCN by electrodes and were maintained under constant light to examine whether the circadian increment free runs. The rats were injected with nitrilotriacetato lead to chronologically label the growing dentin. Two weeks after the operation, maxillary incisors and the locations of lesions in the brain were examined histologically. A harmonic (Fourier) analysis was performed to examine the densitometric pattern of the dentin increments to determine their periodicity. In rats with a completely lesioned SCN, ultradian increments, but no circadian increments, were observed in the dentin. Alternatively, in rats with an intact or only partially lesioned SCN, circadian increments persisted or were only temporarily disturbed. These results suggest that the SCN plays an important role in the generation of the circadian dentin increment in rats.

ABSTRACT-Excessive glucocorticoids induce osteoporosis. However, there is some controversy regarding the mechanism of action, and even the endpoint result. The present study was carried out to obtain further insight into the action of glucocorticoids on bone formation and resorption in rats. Growing rats were injected subcutaneously with methylprednisolone (mPSL) at doses of 0, 2.5, 5, 10 or 20 mg /kg per day for 4 weeks. Bone mineral density (BMD), enchondral and periosteal bone formation, collagen synthetic activities of osteoblasts, numbers of osteoblasts and osteoclasts, and serum markers to assess bone turnover were determined. Administration of mPSL dose-dependently increased the BMD in the tibial metaphysis, while it dose-dependently decreased the BMD in the diaphysis. Both enchondral and periosteal bone formation were decreased in a dose-dependent fashion. The incorporation and secretion of 3 H-proline by osteoblasts were both decreased in trabecular and cortical bones. The number of osteoclasts, together with the number of osteoblasts, in the tibial metaphysis was drastically decreased. Serum alkaline phosphatase and osteocalcin were decreased at higher doses. These results support the recent notion that glucocorticoids inhibit both bone formation and resorption. In addition, BMD as an endpoint result might differ from site to site in bone due to a different balance between bone formation and resorption.Keywords: Methylprednisolone, Osteoblast, Osteoclast, Bone formation, Bone resorption It has been documented that prolonged exposure to glucocorticoids (GCs) at supraphysiological doses induces osteoporosis associated with an increased risk of bone fracture (1 -3). The mechanism of action has been proposed to be decreased osteoblast differentiation and matrix-synthesis (4 -7), increased bone resorption (8 -12) resulting from hyperparathyroidism due to decreased intestinal calcium absorption (13 -19), and decreased secretion of sexsteroids including estrogen (20, 21).However, the results of in vivo studies in man and animals, and even endpoint results on bone mass and bone mineral density (BMD), are inconsistent; high dose and /or long-term GC therapy causes rapid bone resorption and results in a decrease in BMD (22,23), whereas a recent animal study demonstrated that, in contrast to the findings in man, GCs treatment increased bone mass in rats, which might be related to a relatively greater suppression of bone resorption than of bone formation (24 -26). Thus, comparisons of the results of previous studies are complicated by differences in steroid formulation, the dose and duration of administration, and the age and strain of the animals. These discrepancies might be resolved by obtaining basic data on the dose-dependent effects of GCs on bone formation and resorption.The purpose of the present study was to obtain further insight into the effects of methylprednisolone (mPSL), one of the most widely used corticosteroids which has almost no mineralcorticoid effects, on bone metabolism in rats. Generally, w...

Circadian rhythmicity is an essential feature of bone metabolism. The present study was undertaken to (Aoshima et al., 1998) determine the changes in bone resorption and formation in rats over 24h, (Black et al., 1999) evaluate the effect of the consecutive administration of etidronate on circadian rhythms of serum bone markers, and (Blumsohn et al., 1994) determine whether the effect of etidronate on bone metabolism is circadian time-dependent. One hundred twenty male Wistar rats, which had been adapted to a 12/12h light/dark cycle, were injected subcutaneously once daily with either 0.5 mgP/kg etidronate or 0.9% NaCl (control group) at 0090, 1300, 1700, 2100, 0100, or 0500h for 10d. Serum was collected and tibiae were dissected 24h after the last injection. Serum pyridinoline (Pyd), tartrate-resistant acid phosphatase (TRAP), osteocalcin (OC), alkaline phosphatase (ALP), calcium (Ca), phosphorus (Pi), calcitonin (CT), and parathyroid hormone (PTH) were determined. Bone mineral density (BMD) in the proximal tibia, and the rate of formation of longitudinal trabecular bone over the past 48h were also determined using a chronological labeling method with NTA-Pb. The results showed characteristic circadian rhythms in serum bone markers in rats, with peaks in both bone resorption and bone formation during the animals' rest span. The administration of etidronate at the different times of the day decreased the level of bone-resorption markers (Pyd and TRAP) without affecting the circadian patterns of markers of bone formation (OC and ALP). However, the magnitude of the decrease due to etidronate was not uniform throughout the day, and was greatest during the daytime. Etidronate increased the BMD in the tibial metaphysis in all of the time-treatment groups, but the magnitude of the increase did not vary with the time of etidronate administration. The present data provide a physiological basis for future studies on bone metabolism and may be important in the design of future experiments and in the interpretation of experimental data.

The above results suggest that pregnancy and subsequent lactation could be risk factors for alveolar bone loss, especially under conditions of Ca insufficiency or deficiency and that Ca insufficiency or deficiency in the mother caused decreases in the BMD of alveolar bone in the pups.

Activin A, a member of the TGF-b superfamily, is abundant in bone matrix, but little is known about its physiological role in bone metabolism. The present study was undertaken to determine whether topical activin A can increase the bone mass of isografted bone. The tibiae were bilaterally dissected from a donor C3H/HeJ mouse and transplanted subcutaneously in the dorsal region of a recipient mouse. One isografted tibia was topically infused for either 1, 2, 3, or 4 weeks with activin A, using an osmotic minipump at a dose of 0.02, 0.2, or 2 ng/hr. The other tibia was infused with 0.9% NaCl (control). The following results were obtained: (1) Topical activin A (2 ng/hr) stimulated periosteal bone formation after 2 or 3 weeks. The bone area in a standardized transverse section averaged 1.3 fold that in the control. (2) Numerous cuboidal or conical osteoblasts appeared on the surface of newly formed bone after the infusion of activin A for 2 or 3 weeks. Autoradiographic studies using 3H-proline revealed that the surface area of newly formed bone labelled with autoradiographic silver grains was greater in activin A-treated bone than in the control, suggesting an increased synthesis and secretion of collagen by osteoblasts. (3) Topical activin A increased the number of osteoclasts after 2 to 4 weeks. Furthermore, enhanced or increased bone resorption was observed in the projected anterior site of activin A-treated bone after 4 weeks. These results suggest that topical activin A increases the bone mass of isografted bone by increasing bone turnover.