AUGMENTATION OF AGE·RELATED CHANGES OF SOMATOTROPHS OF LIFE-LONG MELATONIN DEFICIENT RATS

Many studies indicate connection between pineal gland function ami growth hormone (GH)-insulin-like growth factor (lOF-I) axis in mammals, but their results are not always synonymous (Nir, 1978; Mess, 1983), Pinealcctomy abolishes a decrease of GH concentration in the pituitary gland-of rats, as well as in plasma caused by constant darkness (Relkin, 1972). Ronnekleiv and Me Cann (1978) detected the decrease of GH secretion in rats after pinealcctomy during the day and a lack of the effect at night. Other investigators indicated intensification of circadian fluctuations of GR after pincalcctomy in rats with small increase of daily secretion (Niles et aI., 1979).
Progressive and irreversible physiological decline is a characteristic of all organisms late in life, Many studies indicating functional alterations in pars distalis cells accmpanied with aging (Mitchell et at, 1995; Velasco et aI., 1998; Velduis,
2000). Pinealcctomy is known to result in a clear decrease in the concentration of circulating melatonin (Gauer et al., 1992). Growth hormone rhythm was suppressed in the pinealectomized rats (Ostrowska et al., 2001). The dysfunctional processes that are a result of macromolecular damage constitute what is referred to as the free radical theory of aging (Sohal and Weindruch, 1996), Numerous reports documented protective actions of melatonin in various models
of oxidative stress (El-Sokkary et al., 1999; Lee et al., 2002). This is due to its  high efficacy as a free radical scavenger and indirect antioxidant (Tan et al., 2002). Additionally, melatonin stimulates the activities of enzymes that metabolize reactive species (Reiter et aI., 2000) and maintains cell membrane fluidity at an optimal level (Garcia et al., 1998). Melatonin was found to decrease with aging (Lee et aI., 2002). Reiter et al. (1999) reported that aging in the pineal-intact animals was associated with increased levels of lipid peroxidation (in the lung, kidney and skin). togethcrwith rises in an oxidatively damaged DNA (in liver, kidney and pancreas), and in the levels of protein carbonyls (in the liver). Likewise, advanced age was associated with a significant decrease in membrane fluidity of hepatic rnicrosomcs inpineal-intact rats. For all of these parameters and in a number of organs, pinealectomy caused further increase in the indices of oxidative damage. The aim of this work was to investigate the ultrastructural alterations in somatotrophcells that result from melatonin deficiency in aged (Pineal-intact) rats and to test whether further induced reduction of melatonin by pinealectomy augment these alterations, trying to detect the role of mclaronin in the aging process.