Developmental alcohol exposure alters light-induced phase shifts of the circadian activity rhythm in rats.

IN HUMANS, THE central nervous system is susceptible to structural damage from alcohol (EtOH) exposure during critical stages of brain development (Clarren et al., 1978; Jones and Smith, 1973; Mattson et al., 1996; Sowell et al., 1996). Detailed animal studies have revealed that developmental EtOH exposure similarly disrupts the structural organization of different brain regions. In rats, EtOH exposure during the brain growth spurt (early postnatal period), which corresponds to the equivalent period of human brain development during the third trimester of gestation (Dobbing and Sands, 1979), has been shown to retard brain growth; produce microencephaly; cause neuronal loss in the hippocampus, cerebellum, and olfactory bulb; and alter neuronal circuitry (Bonthius et al., 1992; Bonthius and West, 1990; Chen et al., 1998; Goodlett et al., 1998; Goodlett and Johnson, 1999; Kelly et al., 1988; Maier et al., 1996; West et al., 1981). Furthermore, hippocampal and cerebellar cell loss in rats exposed to EtOH during the early postnatal period have been observed in association with behavioral deficits in learning/memory and motor performance tasks (Goodlett et al., 1987, 1988; Kelly et al., 1988; Thomas et al., 1996, 1998). However, further analysis is necessary to fully determine the scope of the developmental EtOH-induced damage to the brain and whether the resulting neuroanatomical changes have long-term neurobehavioral consequences. The circadian clock and its regulation of circadian behavior have recently emerged as a significant area of interest in the analysis of specific neurobehavioral disturbances associated with developmental EtOH exposure. The internal biological clock responsible for the generation of mammalian circadian rhythms is located in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. The endogenous timekeeping function of the SCN is complemented by its role in mediating the entrainment or synchronization of mammalian circadian rhythms to light/dark cycles. Entraining light/dark signals are transduced by the retina and conveyed to the SCN via the retinohypothalamic tract (RHT), a monosynaptic projection from a subpopulation of retinal ganglion cells that terminates bilaterally within the ventrolateral subfield of each nucleus (Moore, 1983; Pickard, 1982). In rodents, complete ablation of the SCN abolishes circadian rhythmicity in various physiologic and behavioral processes (Moore, 1983; Turek, 1985), whereas destruction of the RHT eliminates entrainment of the activity rhythm to light/dark cycles without affecting other visual functions (Johnson et al., 1988). Initial evidence for EtOH-induced disturbances of circadian clock function has emanated from rodent studies indicating that prenatal EtOH exposure alters the light/dark regulation of circadian rhythms (Sei et al., 2003). Consistent with these effects of developmental EtOH on the SCN clock, our preliminary findings demonstrate that postnatal EtOH treatment disrupts endogenous neurochemical rhythmicity in the SCN and alters the free-running period and light/dark entrainment of circadian rhythms (Earnest et al., 1997; Marchette et al., 2003). On the basis of the effects of developmental EtOH exposure on circadian entrainment to light/dark cycles, this study was conducted to determine whether early postnatal EtOH exposure permanently alters other aspects of the photic regulation of circadian behavior. Photoentrainment of circadian rhythms requires daily adjustment of the SCN circadian clock by an amount equal to the difference between its circadian period and the length (usually 24 hr) of the entraining light/dark cycle. Light is thought to mediate these adjustments by inducing time-dependent phase shifts of the SCN clock mechanism. Circadian rhythms freerunning in constant darkness (DD) are phase-delayed or reset to a later time in response to a brief light exposure during the early subjective night (i.e., coinciding with a previous dark phase or the animal’s active period) but are phase-advanced or displaced to an earlier time when light pulses are administered during the late subjective night. Because phase-shifting responses to light are directly correlated with circadian period (Daan and Pittendrigh, 1976, Pittendrigh and Daan, 1976), it is anticipated that light-induced phase shifts of the activity rhythm in adult rats will be altered in association with the long-term changes in free-running period caused by early postnatal EtOH exposure. Consequently, the circadian rhythm of wheel-running behavior was assessed in adult rats for evidence of developmental EtOH-related alterations in phase-shifting responses to light pulses administered at either 2 or 10 hr after the onset of activity [i.e., during the early subjective night at circadian time (CT) 14 or during the late subjective night at CT 22, respectively], because light exposure at these times induces maximal phase delays or advances of the rat activity rhythm. Because partial damage to the SCN alters circadian clock properties (e.g., period) that influence phase-shifting responses to light (Davis and Gorski, 1984; Pickard and Turek, 1982, 1985), this study also examined whether long-term changes in the photic regulation of circadian behavior are accompanied by developmental EtOH-induced insults on SCN integrity. Specifically, neuronal number and density in the SCN were analyzed for evidence of cell loss in adult rats exposed to EtOH during the early neonatal period.