Circadian Timing

Most investigators of interval timing and circadian rhythms regard timing in short-interval and circadian ranges as mediated by separate mechanisms (e.g., Aschoff, 1984; Church, 1984; Gibbon et al., 1997a; Hinton and Meck, 1997), with interactions that may occur at the behavioral level (Gibbon et al., 1984, 1997a; Lustig and Meck, 2001; Meck, 1991; Pizzo and Crystal, in press; Terman et al., 1984) or in the range of hours (Aschoff, 1985a, 1985b; Aschoff and Daan, 1997). However, the observation that the scalar property applies to the anticipation of daily meals, as it does to short-interval timing, raises the prospect that the mechanisms of short-interval and circadian timing may be more related than previously supposed. Moreover, if the location of a local maximum in short-interval sensitivity identifies the period of an oscillator, then a local maximum in sensitivity to approximately 24 h is predicted. Therefore, a series of experiments investigating meal anticipation was undertaken

Time Prior to Meal (hr) Time/Interval

FIGURE 3.2 Anticipatory responses superimpose when plotted as a function of time as a proportion of the interval between light offset and the meal (right panel). Superposition is not observed when anticipation is plotted as a function of time prior to the meal (left panel). Meals started 3 or 7 h after light offset for independent groups of rats (n = 3 per group). Variability in timing (i.e., the width of the response rate function indicated by dashed lines) was higher for 7- than for 3-h groups (left panel: t(4) = 3.75, P < .05). Variability on a proportional timescale (i.e., width/interval) was the same for 3- and 7-h groups (right panel: t(4) < 1). This demonstrates that the rats timed with respect to light offset. The same data are plotted in left and right panels as a function of different x-axes. The width of the functions was measured at 70% of the maximum rate. The same conclusions were reached when the width was measured at 25, 50, and 75% of the maximum rate. Left panel: The meal started at 0 h. Right panel: End of the meal corresponds to 1 on the x-axis. (From Crystal, J.D., J. Exp. Psychol. Anim. Behav. Process, 27, 68-78, 2001a. Copyright © 2001 by the American Psychological Association. Reprinted with permission.)

to test the hypothesis that the well-established feeding-entrainable circadian oscillator (for review, see Mistlberger, 1994) is characterized by a local maximum in sensitivity to time (Crystal, 2001a).

Sensitivity to time was examined for intervals in the circadian range by measuring anticipation of restricted feeding in constant darkness. Anticipation of restricted meals was examined for intermeal intervals near the circadian range (22 to 26 h) and outside this range (14 and 34 h). The rats inspected the food trough before meals started (Crystal, 2001a). Response rate increased later into the interval for intermeal intervals near the circadian range than for intervals outside this range, as shown in Figure 3.3. The spread of the response distributions was characterized by two percentages of the interval: low variability for intervals near the circadian range and high variability for intervals outside this range, as shown in Figure 3.4 (Crystal, 2001a). The data plotted in Figure 3.4 document a local maximum in

Time/Interval

FIGURE 3.3 Response rate increased later into the interval for intermeal intervals near the circadian range (unfilled symbols) relative to intervals outside this range (filled symbols); dashed lines indicate width of response rate functions. Anticipatory responses increase immediately prior to the meal for all intermeal intervals except 34 h. Each 45-mg food pellet was contingent on a photobeam break after a variable interval during 3-h meals. Intermeal intervals were tested in separate groups of rats (n = 3 to 5 per group). The end of the meal corresponds to 1 on the x-axis. Testing was conducted in constant darkness. (From Crystal, J.D., J. Exp. Psychol. Anim. Behav. Process., 27, 68-78, 2001a. Copyright © 2001 by the American Psychological Association. Reprinted with permission.)

Time/Interval

FIGURE 3.3 Response rate increased later into the interval for intermeal intervals near the circadian range (unfilled symbols) relative to intervals outside this range (filled symbols); dashed lines indicate width of response rate functions. Anticipatory responses increase immediately prior to the meal for all intermeal intervals except 34 h. Each 45-mg food pellet was contingent on a photobeam break after a variable interval during 3-h meals. Intermeal intervals were tested in separate groups of rats (n = 3 to 5 per group). The end of the meal corresponds to 1 on the x-axis. Testing was conducted in constant darkness. (From Crystal, J.D., J. Exp. Psychol. Anim. Behav. Process., 27, 68-78, 2001a. Copyright © 2001 by the American Psychological Association. Reprinted with permission.)

sensitivity to time near 24 h (Crystal, 2001a). These data are consistent with the hypothesis that a function of an oscillator is improved sensitivity to time.

It is generally accepted that food-anticipatory activity to a daily meal develops only when the interval between successive meals is near 24 h. The limited range of food entrainment has been estimated to be between 22 and 31 h (Aschoff et al., 1983; Boulos et al., 1980; Mistlberger and Marchant, 1995; Stephan, 1981; Stephan et al., 1979a, 1979b; White and Timberlake, 1999) and between 22.17 and 26.67 h (Madrid et al., 1998). It is noteworthy that the animals in the study described above anticipated the 14-h intermeal interval. For additional evidence of timing the 14-h interval, see Figure 3.5, which plots anticipation functions from individual rats (right panel) and activity records (left and center panels). Although it is generally accepted that animals cannot anticipate intermeal intervals outside a limited range near 24 h, this conclusion is based on a relatively limited data set. When these data are reex-amined, there is evidence for timing long intermeal intervals that are substantially

0 0

Post a comment