Thermal and Nutritional Modulation of Sleep in Infant Rats

Spontaneous activity, including myoclonic twitching, is a prominent feature of mammalian and avian behavior early in ontogeny (Corner, 1977). Twitches in infants resemble the movements exhibited by adult mammals during REM sleep and are readily distinguished from the high-amplitude coordinated movements that characterize awake behavior. Although the developmental relations between infant and adult sleep continue to be debated (Blumberg, Karlsson, Seelke, & Mohns, in press; Frank & Heller, 2003), it is clear that myoclonic twitching is a sleep-related behavior (Blumberg & Lucas, 1996; Gramsbergen, Schwartze, & Prechtl, 1970; Jouvet-Mounier, Astic, & Lacote, 1970). In previous studies in which myoclonic twitching was used as the sole indicator of infant sleep, it was previously found that twitching is tightly linked to thermoregulatory competence during thermal challenges (Blumberg & Lucas, 1996; Sokoloff & Blumberg, 1998). For example, rat pups exhibited similar rates of twitching at thermoneutrality and during exposure to moderate air temperatures (defined as the range of air temperatures that stimulate endogenous heat production without exceeding thermogenic capacity), but that extreme air temperatures (defined as the range of air temperatures that exceed a pup’s thermogenic capacity) suppressed twitching. Recent work has indicated that other measures of sleep can be used to supplement the information provided by myoclonic twitching. In particular, soon after birth in rats, a strong association already exists between myoclonic twitching and nuchal muscle atonia (Karlsson, Kreider, & Blumberg, 2004). Combining these behavioral and EMG measures, a typical sleep-wake cycle in a week-old rat evolves as follows: High-amplitude movements of the limbs and elevation of the head occurs during periods of high nuchal tone, after which muscle tone rapidly decreases and atonia begins; initially, atonia is accompanied by a brief period of behavioral quiescence, which has been designated as quiet sleep (QS) (Jouvet-Mounier et al., 1970); then, twitching of the limbs and tail begins while background levels of atonia are maintained, a state that has been designated as active sleep (AS) (Jouvet-Mounier et al., 1970); finally, there is an abrupt transition to high-amplitude waking movements and high muscle tone, thus completing the cycle. The effect of air temperature on this sleep-wake cycle – as defined using both behavioral and EMG criteria – has not been investigated. In addition to temperature, food deprivation is also considered an important modulator of sleep. For example, Lorenz and colleagues found that active sleep in 11–13-day-old (P11–13) rats was suppressed after 9–12 h of separation from the dam, and that infusion of milk into the gut restored high levels of AS (Lorenz et al., 1998). In adults, food deprivation also suppresses sleep, but only after several days of fasting (Borbely, 1977; Danguir & Nicolaidis, 1979; Dewasmes, Duchamp, & Minaire, 1989; Jacobs & McGinty, 1971). It is not known, however, how infant sleep changes over the course of food deprivation; nor is it known how the effects of food deprivation on sleep interact with cold exposure. In this experiment, myoclonic twitching and nuchal EMG were measured in P8 rats during exposure to three air temperature conditions: 35°C (thermoneutral), 28°C (moderate thermal challenge), and 20°C (extreme thermal challenge). Pups in the 35°C and 28°C conditions were tested across an 8-h period to assess the effects of food deprivation on sleep. Based on previous work, we hypothesized that moderate thermal challenge would have negligible effects on the expression of sleep and wakefulness and that the effects of extreme thermal challenge would be pronounced. Furthermore, we hypothesized that 8 h of food deprivation would result in increased wakefulness, especially during moderate thermal challenge because of the increased use of energy to support thermogenesis.