Effects of Photoperiod, Melatonin, and the Pineal Gland on Compensatory Gonadal Hypertrophy during Postnatal Development in the Marsh Rice Rat (Oryzomypalustris)

Bone Structural, Biomechanical and Histomorphometric Characteristics of the Hindlimb Skeleton in the Marsh Rice Rat (Oryzomys palustris)

INTRODUCTION

The rice rat (Oryzomys palustris) is a non-conventional laboratory rodent species used to model some human bone disorders. However, no studies have been conducted to characterize the postcranial skeleton. Therefore, we aimed to investigate age- and gender-related features of the hindlimb skeleton of this species.

METHODS

We used femurs and tibiae from 94 rats of both genders aged 4-28 wks. Bone mineral content (BMC), volumetric bone mineral density (vBMD), and biomechanical properties were determined in femurs. In addition, bone histomorphometry of tibiae was conducted to assess bone cell activities and bone turnover over time.

RESULTS

Bone length, total metaphysis BMC and vBMD, mid-diaphyseal BMC and vBMD, cortical thickness, and cortical area progressively augmented with age. Whereas the increase in these parameters plateaued at age 16-22 wks in female rats, they continued to rise to age 28 wks in male rats. Furthermore, bone strength parameters increased with age, with few differences between genders. We also observed a rapid decrease in longitudinal growth between ages 4-16 wks. Whereas young rats had a greater bone formation rate and bone turnover, older rice rats had greater bone volume and trabecular thickness, with no differences between genders.

CONCLUSIONS

1) Sexual dimorphism in the rice rat becomes grossly evident at age 16 wks; 2) the age-related increases in bone mass, structural cortical parameters, and in some biomechanical property parameters plateau at an older age in male than in female rats; and 3) bone growth and remodeling significantly decreased with age irrespective of the gender.

A melatonin-independent seasonal timer induces neuroendocrine refractoriness to short day lengths

The duration of nocturnal pineal melatonin secretion transduces effects of day length (DL) on the neuroendocrine axis of photoperiodic rodents. Long DLs support reproduction, and short DLs induce testicular regression, followed several months later by spontaneous recrudescence; gonadal regrowth is thought to reflect development of tissue refractoriness to melatonin. In most photoperiodic species, pinealectomy does not diminish reproductive competence in long DLs. Turkish hamsters (Mesocricetus brandti) deviate from this norm: elimination of melatonin secretion in long-day males by pinealectomy or constant light treatment induces testicular regression and subsequently recrudescence; the time course of these gonadal transitions is similar to that observed in males transferred from long to short DLs. In the present study, long-day Turkish hamsters that underwent testicular regression and recrudescence in constant light subsequently were completely unresponsive to the antigonadal effects of short DLs. Other hamsters that manifested testicular regression and recrudescence in short DLs were unresponsive to the antigonadal effects of pinealectomy or constant light. Long-term suppression of melatonin secretion induces a physiological state in Turkish hamsters similar or identical to the neuroendocrine refractoriness produced by short-day melatonin signals (i.e., neural refractoriness to melatonin develops in the absence of circulating melatonin secretion). A melatonin-independent interval timer, which would remain operative in the absence of melatonin during hibernation, may determine the onset of testicular recrudescence in the spring. In this respect, Turkish hamsters differ from most other photoperiodic rodents.

Photoperiodic regulation of compensatory testicular hypertrophy in hamsters

In mammals, removal of one testis results in compensatory testicular hypertrophy (CTH) of the remaining gonad. Although CTH is ubiquitous among juveniles of many species, laboratory rats, laboratory mice, and humans unilaterally castrated in adulthood fail to display CTH. We documented CTH in pre- and postpubertally hemi-castrated Syrian and Siberian hamsters and tested whether day length affects CTH in juvenile and adult Siberian hamsters. Robust CTH was evident in long-day hemi-castrates of both species and was preceded by increased serum FSH concentrations in juvenile Siberian hamsters. In sharp contrast, CTH was undetectable in short-day hemi-castrated Siberian hamsters for several months and only made its appearance with the development of neuroendocrine refractoriness to short day lengths; serum FSH concentrations of juveniles also did not increase above sham-castrate values until the onset of refractoriness. Long-day hemi-castrated Siberian hamsters with hypertrophied testes underwent complete gonadal regression after transfer to short days, albeit at a reduced rate for the first 3 weeks of treatment. Blood testosterone concentrations of adult hamsters did not differ between long-day hemicastrates and sham-castrates 9-12 weeks after surgery. We conclude that CTH is suppressed by short day lengths in Siberian hamsters at all ages and stages of reproductive development; in short day lengths, but not long day lengths, the remaining testis produces sufficient negative feedback inhibition to restrain FSH hypersecretion and prevent CTH.