Search

Previous studies have shown that the successive negative contrast procedure, in which food-restricted rats entrained to once daily, brief presentations of 32% sucrose are unexpectedly shifted to a 4% solution, results in an adrenocortical response on the second, but not the first postshift day. We attempted to generalize that finding in our own procedure. In Experiment 1, two groups of rats were given a 32% sucrose solution once daily in their home cages for 14 days before being shifted to a 4% solution. One group was killed 10 min after the first 4% solution and one was killed after the second 4% solution. In addition, two groups receiving either 32% or 4% sucrose throughout the experiment served as unshifted controls. In contrast to previous findings, both shifted groups exhibited prominent adrenocorticotropin hormone (ACTH) and adrenocortical (B) responses on both postshift days compared to unshifted controls, which did not differ from one another. Experiment 2a employed distinctive contexts to test if the lack of generality of the delayed HPA axis response was due to suppressive effects of S(+) on the first postshift day. Rats were given once daily 32% sucrose in S(+) and equal exposure time in S(-). Half of these rats were shifted to 4% sucrose in S(+) and half were shifted in S(-). These two groups were compared to home cage controls. Half of each group was killed after their first 4% sucrose, and half after the second 4% sucrose. All rats showed ACTH and B responses comparable to shifted rats in Experiment 1. S(+) failed to suppress the HPA axis, and the stress response was higher on the first compared to the second day of the shift. Experiment 2b established that distinctive contexts predicting sucrose, S(+), or not predicting sucrose, S(-), controlled behavioral choice and contextual discrimination. Thus, there was no evidence that issues of stimulus control could explain the lack of generality of previous findings. The data indicate that thwarting sucrose expectancies is stressful, and that this stress response habituates across days.

Based on observed phenotypic differences in growth and ACTH responses to stress in Sprague Dawley rats obtained from different vendors, we ran head-to-head comparisons on rats obtained from three different vendors, Harlan, Charles River, and Simonsen, with respect to baseline phenotypic differences and a metabolic feedback hypothesis of hypothalamo-pituitary-adrenal (HPA) regulation. Charles River and Harlan rats gained weight faster than Simonsen rats, but chow intake standardized for body weight was not increased, consistent with their greater caloric efficiency. Weight gain was inversely related with mean daily temperatures, without differences in activity levels. Half of the animals given lard and 32% sucrose solutions in addition to chow increased caloric intake and core temperature, decreased caloric efficiency, and increased fat depots, leptin, and in Simonsen rats, insulin. A 5-day regimen of once-daily 2-h restraint decreased feeding and caloric efficiency. Rats from two vendors with the availability of sucrose and lard, Charles River and Simonsen, showed blunted HPA responses to restraint compared to chow controls, whereas the Harlans exhibited no adrenocorticotropin (ACTH) response and an amplified adrenocortical response on the high-energy diet compared to chow controls. Substantial phenotypic differences exist between Sprague Dawley rats from different vendors with respect to metabolism and HPA function. The metabolic feedback hypothesis was supported in two of the three vendors' rats.

Eight female Sprague-Dawley rats were housed in isolated continuous 24-h/day environments under conditions of constant dim light and a rate-limited feeding schedule. Following 2 months of free-running activity, all animals were administered methamphetamine (MA) i.p. (2 mg/kg) once every 31 h for 24 injection cycles. Average wave forms of wheel-running activity showed that animals did not anticipate the 31-h schedule of MA injections, but rather displayed circadian ensuing drug activity (CEDA) between 24 and 28 h following the injections. Post-injection meals failed to meet reliably the threshold necessary to achieve food-engendered anticipatory or ensuing activity. Cosinor analysis showed that the intensity of CEDA was strongly influenced by the relative phase of the free-running rhythm. CEDA was moderately influenced by the size of the post-injection bout of activity. Because injection times rotated daily throughout local time without repeating a time of day, CEDA resulting from a long T schedule of MA administration appeared to be based on one-trial resetting of a circadian-related mechanism by a major drug of abuse.