Effects of aging on lens transmittance and retinal input to the suprachiasmatic nucleus in golden hamsters Yan Zhang c , George C. Brainard d , Phyllis C. Zee a,b , Lawrence H. Pinto a,b , Joseph S. Takahashi a,b , Fred W. Turek a,b, * a Center for Circadian Biology and Medicine, Department of Neurobiology and Physiology, Northwestern University, Evanston, IL 60208, USA b Department of Neurology, Northwestern University Medical School, Chicago, IL 60611, USA c Department of Neurology, Cleveland Clinic Foundation, Cleveland, OH 44195, USA d Department of Neurology, Jefferson Medical College, Philadelphia, PA 19107, USA Received 19 June 1998; accepted 27 October 1998 Abstract Old animals are less sensitive by almost an order of magnitude to the phase-shifting effects of a low intensity light pulse on the locomotor activity rhythm and the associated induction of immediate early genes in the circadian clock. The transmittance of energy from 200 to 700 nm through the excised lens of the eyes of young and old golden hamsters was measured to determine if an age-related difference exists in the transmittance of light. There is only a small decrease (8–50%) in transmittance, with the magnitude being dependent upon wavelength. No significant differences were detected between young and old animals in the retinal innervation of the suprachiasmatic nucleus (SCN). These results support the hypothesis that the observed decrease in sensitivity to light in the aged circadian system occurs within the SCN itself and/or retino-hypothalamic tract photoreceptors.  1998 Published by Elsevier Science Ireland Ltd. All rights reserved Keywords: Lens; Spectral transmittance; Retinohypothalamic tract; Suprachiasmatic nucleus; Aging; Circadian rhythm It is well established that the suprachiasmatic nucleus (SCN) of hypothalamus is the predominant pacemaker gov- erning circadian rhythms of mammals [6,14]. The endogen- ous circadian clock in the SCN is normally synchronized to the 24 h changes in the light-dark (LD) cycle. Entrainment by light involves at least two neural pathways from the eyes to the SCN: the direct retino-hypothalamic tract (RHT) [4] and the indirect retino-geniculo-suprachiasmatic tract (GHT) [5]. Changes in several basic parameters of circadian rhythms in mammals are associated with aging. These changes include alterations of circadian period length, amplitude and/or phase of overt rhythms regulated by the clock [1,10] In addition, the response of the circadian clock to both photic and non-photic stimuli is altered in advanced age [15]. Recently, it has been shown that there is a decreased photic induction of immediate-early gene (IEG) mRNAs such as c-fos and nerve growth factor one A (NGFI-A) in the SCN of aged rats [12]. Furthermore, it has been demon- strated that, with aging, the thresholds for both the phase- shifting effects of light on the circadian rhythm of locomo- tor activity and the photic induction of Fos in the hamster SCN, are shifted towards higher light intensities [17]. Old hamsters were approximately 20 times less sensitive to the resetting effects of low intensity light pulse on the locomo- tor activity rhythm than young hamsters [17]. The mean number of Fos-positive cells induced by a bright pulse of light was reduced by about 43% in old hamsters [17]. Alterations of a number of possible processes in the trans- duction pathway of light to the clock could contribute to the observed decrease in sensitivity to light. For example, an age-related change in ocular transmission through the cor- Neuroscience Letters 258 (1998) 167–170 0304-3940/98/$ - see front matter  1998 Published by Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(98)00887-8 * Corresponding author. Department of Neurobiology and Physiol- ogy, Northwestern University, 2153 North Campus Drive, Evanston, IL 60203, USA. Tel.: +1 847 4912865; fax: +1 847 4674065; e-mail: fturek@nwu.edu