Multioscillatory Circadian Organization in a Vertebrate,Iguana iguana

Abstract

The lizard Iguana iguana when kept in constant ambient temperature displays endogenously generated circadian rhythms of body temperature and locomotor activity. Although surgical removal of the parietal eye has only slight effects on overt circadian rhythmicity, subsequent pinealectomy completely abolishes the rhythm of body temperature. However, the rhythm of locomotor activity is only slightly affected by parietalectomy plus pinealectomy. Our results demonstrate that the pineal complex is centrally involved in the generation and control of the circadian rhythm of body temperature but is only marginally involved in locomotor rhythmicity. Plasma melatonin levels are not significantly reduced by parietalectomy, whereas pinealectomy dramatically lowers the level and completely eliminates the circadian rhythm of melatonin in the circulation. Isolated parietal eye, pineal, and retina all synthesize melatonin with robust circadian rhythmicity when maintained for Ն4 d in culture, although in the intact animal all or almost all of the circulating melatonin comes from the pineal. The circadian system of I. iguana is composed of multiple circadian oscillators that reside in different tissues and have specific and different roles.

Circadian Photoreception in Vertebrates

“…The multiple extraretinal photoreceptors of nonmammalian vertebrates are often contained within tissues that are themselves circadian oscillators, e.g., the pineal gland of lamprey, birds, and lizards and the parietal eye of lizards (Zimmerman and Menaker 1979;Underwood 1990;Morita et al 1992;Tosini and Menaker 1998). In zebra fish, peripheral oscillators in many organs and tissues are directly entrainable by light (Whitmore et al 2000), a situation similar to that of Drosophila melanogaster, in which clock tissues all over the body use the flavin-based photoreceptor crypotochrome (Plautz et al 1997;Emery et al 2000).…”

Isolation and characterization of melanopsin and pinopsin expression within photoreceptive sites of reptiles

“…The pineal gland and parietal eye of the lizard are of particular interest with regard to the circadian timing system because they contain: (1) self-sustained oscillators, (2) a photic-input pathway to the oscillators and (3) an overt output represented by the rhythmic production of melatonin (Tosini and Menaker 1998;Bertolucci et al 2003). Detailed studies in chicken pineal suggested the presence of at least two distinct photopigments mediating the acute light suppression of melatonin synthesis or the phase shift of the melatonin rhythm (Zatz 1996).…”

Interactions between endocrine and circadian systems

“…Owing to the widespread expression of melatonin receptors, melatonin has been reported to modulate several physiological systems such as immune function (Srinivasan et al 2011), metabolism (Nduhirabandi et al 2012), and higher brain functions (Srinivasan et al 2012). In birds and reptiles, the pineal-melatonin system is an essential part of the circadian clockwork (Gaston & Menaker 1968, Tosini & Menaker 1998. In contrast, no overt circadian disruption is observed in pinealectomized mammals (Quay 1970(Quay , 1972, but melatonin may play an important regulatory role in distributing the time signal of the SCN (see below).…”