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Circadian rhythms organize behavior and physiological processes to be appropriate to the predictable cycle of daily events. These rhythms are entrained by stimuli that provide time of day cues (zeitgebers), such as light, which regulates the sleep-wake cycle and associated rhythms. But other events, including meals, social cues, and bouts of locomotor activity, can act as zeitgebers. Recent evidence shows that most organs and tissues contain cells that are capable of some degree of independent circadian cycling, suggesting the circadian system is more broadly and diffusely distributed. Within laboratory studies of behavior, circadian rhythms tend to be treated as a complication to be minimized, but they offer a useful model of predictable shifts in behavioral tendencies. In the present review, we summarize the evidence that formed the basis for a hypothesis that drugs of abuse can entrain circadian rhythms and describe the outcome of a series of experiments designed to test that hypothesis. We propose that such drug-entrained rhythms may contribute to demonstrated daily variations in drug metabolism, tolerance, and sensitivity to drug reward. Of particular importance, these rhythms may be evoked by a single episode of drug taking, strengthen with repeated episodes, and reemerge after long periods of abstinence, thereby contributing to drug abuse, addiction, and relapse.

The hypothalamic-pituitary-adrenal (HPA) axis self-regulates through a glucocorticoid negative feedback mechanism that is stereotypically slow and long lasting. Rapid (seconds to minutes) glucocorticoid feedback, however, inhibits stress-induced adrenocorticotropic hormone (ACTH) secretion too quickly to result from classic transcriptional effects of the occupied glucocorticoid receptor. Cannabinoids may act as rapid intermediary messengers between glucocorticoids and HPA activation via retroactive inhibition of afferent glutamate stimulation of the corticotropin-releasing factor neurons in the paraventricular nucleus. We demonstrated fast feedback effects of GR stimulation and blockade and observed the effect of cannabinoid receptor (CB1) antagonist AM251 on HPA axis reactivity in vivo. Rats were injected intraperitoneally with varying doses of the specific GR agonist RU28362, the GR antagonist RU486, or AM251 2 min before restraint. Blood was collected at predetermined times and corticosterone and ACTH concentrations were measured. RU28362 blunted stress-induced ACTH secretion while RU486 and AM251 significantly increased stress-induced ACTH release 15 min after restraint onset. Next, we injected AM251 58 min before RU28362, 2 min before restraint, to determine if inhibition of ACTH by RU28362 was contingent on CB1 activation. Unexpectedly, CB1 blockade failed to prevent glucocorticoid negative feedback and instead enhanced it. These studies not only establish an in vivo fast feedback model but show that rapid glucococorticoid negative feedback is similarly altered by GR and CB1 blockade. Although the hormonal consequences of acute AM251 treatment were strikingly similar to those of RU486 treatment, we are unable to draw conclusions about the serial nature of the interaction between GR activation and CB release from these results.

Corticosterone and insulin play complex roles in the amount and composition of calories ingested, and the utilization and deposition of this energy. Understanding the interplay of these two hormones is complicated because increasing concentrations of corticosterone dose-dependently increase circulating insulin levels. We addressed individual contributions of each hormone by controlling, at steady-state levels, corticosterone (by adrenalectomy and exogenous replacement) and insulin (by streptozotocin-induced destruction of pancreatic β-cells and exogenous replacement) across a spectrum of concentrations in rats, creating 8 hormonal combinations. For 5 days after surgery, all rats received chow. At day 5, they were subdivided into those that continued to receive chow and those that had a choice between chow, lard, and 32% sucrose for a further 5 days. During the choice/chow period, total calories ingested were stimulated by corticosterone and choice diet, and subject to a corticosterone-insulin interaction. Sucrose, but not lard, intake was stimulated by insulin. Body weight was increased by insulin, decreased by high corticosterone, and unaffected by diet. White adipose tissue depot weights were stimulated by insulin, corticosterone, and diet. Plasma triglycerides, free fatty acids, total ketone bodies, glucose, and glycerol were all significantly increased by corticosterone and the choice diet but inhibited by insulin. In contrast, plasma leptin was only increased by insulin and diet, plasma glucagon and liver glycogen was only affected by insulin and liver triglycerides, and arcuate nucleus proopiomelanocortin mRNA was only influenced by diet. Collectively, these data show that corticosterone and insulin determine the intake, form, and compartmentalization of energy both independently and interactively.

We sought to examine insulin-sensitive food intake behavior and neuroendocrine and metabolic variables of rats that had undergone a duodenal-jejunal bypass (DJB).A DJB that circumvents the duodenum and proximal jejunum while leaving the stomach unperturbed rapidly improves insulin sensitivity in type 2 diabetic rats. This segment of proximal small intestine is innervated by the gastroduodenal branch of the vagus nerve, the transection of which influences food intake choices in streptozotocin-diabetic rats.Rats were first placed on a choice of chow and lard for 7 days and additionally provided with an enriched liquid diet for another 7 days before surgery and were allowed only the liquid diet for 7 days after either a sham or DJB operation.After surgery, DJB-operated rats initially consumed less than the sham-operated counterparts. When the rats were subsequently provided with the choice of chow and lard for 7 days, there were no differences in intake between the DJB and sham-operated groups. Similarly, the majority of metabolic and neuroendocrine variables measured were unchanged. However, DJB-operated rats exhibited greater mesenteric white adipose tissue weight, fecal output, arcuate nucleus neuropeptide Y mRNA expression, plasma corticosterone, and glucagon levels together with reduced plasma leptin concentrations.DJB surgery does not produce significant differences in food intake choices after a period of recovery; however, there are enduring metabolic and neuroendocrine changes, which are collectively important to understanding the beneficial outcomes of the operation.

Two groups of rats, one bearing bilateral excitotoxic lesions of the medial prefrontal cortex (mPFC) and one sham-lesioned group, were run in a successive negative contrast paradigm. Both groups had telemeters implanted to monitor core temperature and activity. After ad libitum baseline and food restriction to 85% body weights, rats received a sucrose solution once daily for 5 min and 30 s at 10:30 h. They received their preshift 32% sucrose solution for 14 days followed by a sucrose concentration reduction (downshift) to 4% sucrose for 12 days. Rats were then upshifted to 32% for six additional days before being downshifted to 4% for the next 6 days. There were no differences in intake of the 32% sucrose during the preshift. All rats showed profound suppression of intake upon the shift to 4% sucrose. On the first day of the unexpected 4% sucrose, lesioned rats showed an enhanced psychogenic fever compared with Shams, whereas on the second day of 4% sucrose they showed an impaired ability to blunt that fever compared with Shams. In addition, lesioned rats showed greater rates of recovery and asymptotic drinking of the subsequent 4% sucrose solution than Shams, indicating impairments in the encoding or retrieval of the shift. In addition, lesioned rats showed enhanced entrainment to the 32% sucrose meals, normal damping of anticipation, and enhanced spontaneous recovery of anticipatory thermal responses to the calorically impoverished 4% solutions. These failures to inhibit responding point to a failure in interference learning in rats bearing lesions of the mPFC.

In addition to the inhibitory role of central insulin on food intake, insulin also acts to promote lard intake. We investigated the neural pathways involved in this facet of insulin action. Insulin or saline was infused into either the superior mesenteric or right external jugular veins of strep-tozotocin-diabetic rodents with elevated steady-state circulating corticosterone concentrations. After postsurgical recovery, rats were offered the choice of chow or lard to eat. Irrespective of the site of venous infusion, insulin increased lard and decreased chow intake. After 4 days, lard was removed for 8 h. On return for 1 h, only insulin infused into the superior mesenteric vein resulted in lard intake. This facilitated distinction between the effects of circulating insulin concentrations (similar in the two insulin-infused groups) and lard ingestion on the patterns of c-Fos + cells in the brain, termed insulin- and lard-associated patterns, respectively. Insulin-associated changes in c-Fos + cell numbers were evident in the arcuate nucleus, bed nucleus of the stria terminalis and substantia nigra pars compacta, concomitant with elevated leptin levels and reduced chow intake. Lard-associated changes in c-Fos + cell numbers were observed in the nucleus of the tractus solitarius, lateral parabrachial nucleus, central nucleus of the amygdala, ventral tegmental area, nucleus accumbens shell and the prefrontal cortex, and were associated with lower levels of triglycerides and free fatty acids. The anterior paraventricular thalamic nucleus exhibited both patterns. These data collectively fit into a framework for food intake and reward and provide targets for pharmacological manipulation to influence the choice of food intake.

The common hepatic branch of the vagus nerve is a two-way highway of communication between the brain and the liver, duodenum, stomach and pancreas that regulates many aspects of food intake and metabolism. In this study, we utilized the afferent-specific neurotoxin capsaicin to examine if common hepatic vagal sensory afferents regulate lard intake. Rats implanted with a corticosterone pellet were made diabetic using streptozotocin (STZ) and a subset received steady-state exogenous insulin replacement into the superior mesenteric vein. These were compared with non-diabetic counterparts. Each group was then subdivided into those whose common hepatic branch of the vagus was treated with vehicle or capsaicin. Five days after surgery, the rats were offered the choice of chow and lard to consume for a further 5 days. The STZ-diabetic rats ate significantly less lard than the non-diabetic rats. Capsaicin treatment restored lard intake to that of the insulin-replaced, STZ-diabetic rats, but modified neither chow nor total caloric intake. This increased lard intake led to selective fat deposition into the mesenteric white adipose tissue depot, as opposed to an increase in all visceral fat pad depots evident after insulin replacement-induced lard intake. Capsaicin treatment also increased the levels of circulating glucose and triglycerides and negated the actions of insulin on these and free fatty acids and ketone bodies. Collectively, these data suggest that afferent signalling through the common hepatic branch of the vagus inhibits lard, but not chow, intake, directs fat deposition and regulates plasma metabolite levels.

Glucocorticoids act primarily in a feed-forward fashion on brain to activate CNS pathways that implement wanting appropriate to physiological needs. Thus, depending on the available conditions, elevated glucocorticoids may augment the behavioural want to run, fight or feed. Although glucocorticoids stimulate intake of chow, fat and sucrose, insulin appears to sculpt calorie-associated desires toward foods high in fat, acting through hepatic branch afferents of the vagus nerve. Both conditions of reduced food allowance and chronic stress excite glucocorticoid-augmented central neural networks that may lead toward ultimate abdominal obesity.

Although high insulin concentrations reduce food intake, low insulin concentrations promote lard intake over chow, possibly via an insulin-derived, liver-mediated signal. To investigate the role of the hepatic vagus in voluntary lard intake, streptozotocin-diabetic rats with insulin or vehicle replaced into either the superior mesenteric or jugular veins received a hepatic branch vagotomy (HV) or a sham operation. All rats received a pellet of corticosterone that clamped the circulating steroid at moderately high concentrations to enhance lard intake. After 5 d of recovery, rats were offered the choice of lard and chow for 5 d. In streptozotocin-diabetic rats, HV, like insulin replacement, restored lard intake to nondiabetic levels. Consequently, this reduced chow intake without affecting total caloric intake, and insulin site-specifically increased white adipose tissue weight. HV also ablated the effects of insulin on reducing circulating glucose levels and attenuated the streptozotocin-induced weight loss in most groups. Collectively, these data suggest that the hepatic vagus normally inhibits lard intake and can influence glucose homeostasis and the pattern of white adipose tissue deposition. These actions may be modulated by insulin acting both centrally and peripherally.

In mammals, glucocorticoid actions appear to have evolved to maintain and enhance energy stores to be used for life-saving gluconeogenesis. They act on the brain to stimulate search behaviors, palatable feeding and emotionally relevant memories, and they act on the body to mobilize stored peripheral energy and direct it to central depots that serve the substrate needs of the liver. Our work in rats shows that searching and intake of palatable foods (sucrose, saccharin and lard) are stimulated by corticosterone in a dose-related fashion. Adrenalectomized rats gain weight poorly, have low fat content, increased sympathetic neural and hypothalamo-pituitary-adrenal outflow, and altered behaviors. Replacement with corticosterone reverses these effects. Surprisingly, when such rats are provided with 30% sucrose to drink, in addition to saline, all of the usual effects of adrenalectomy are corrected without corticosterone. We hypothesize that there is a metabolic feedback system that decreases stress-responsiveness. Although we have not yet identified the signal associated with sucrose drinking, the weight of mesenteric fat correlates inversely with hypothalamic corticotropin-releasing factor (CRF). When rats eat lard and sucrose ad libitum, fat stores increase and CRF, ACTH and corticosterone responses are reduced. During stress, chow intake decreases but intake of lard and sucrose does not. Our current working model suggests that palatability signals and neural signals from fat stores act on brain to reduce activity in the central stress response system. Correlative results from a clinical study support the powerful role of small changes in glucocorticoids in type 2 diabetes.

Previous findings (N. Pecoraro, J. Chou-Green, & M. F. Dallman, 2003; N. Pecoraro & M. F. Dallman, 2005) indicated that unexpected reductions in sucrose concentration in once daily meals result in a febrile response on the 1st, but not the 2nd day of a concentration shift. This study shows that this day-specific fever is blocked by adrenalectomy accompanied by constant low corticosterone replacement. Rats implanted with telemetry probes were adrenalectomized and given low-corticosterone pellets or were sham operated. Food-restricted rats were given 2 rounds of sucrose concentration downshifts, as follows: 32% sucrose (14 days), 4% sucrose (6 days), 32% sucrose (4 days), and 4% sucrose (4 days). Intact rats showed more pronounced anticipation of the sucrose than did rats having low, clamped corticosterone. Only intact rats showed a 4-hr, postshift temperature burst on the 1st, but not the 2nd day of the shift to 4% sucrose, during both rounds of shifting. Increased activity accompanied the fever. These data confirm previous findings, show them to be dependent on high corticosterone, and appear to be related to a host of day-specific alterations in other motor outflows following unexpected downward shifts in palatable sucrose concentrations.

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

To test whether glucocorticoids amplify incentive motivation, three groups of rats were adrenalectomized and replaced subcutaneously with pellets of corticosterone (B), containing 0, 30, or 80% B and cholesterol. A fourth group of sham adrenalectomized rats received cholesterol pellets. Animals were placed on a four-arm maze baited with 32% sucrose for 5-min daily sessions. After 7 days of minimal drinking on the maze at free-feeding weights, their body weights were gradually reduced to 90% of their free-feeding weights for the next 12 days (pre-shift phase). The sucrose concentration was reduced to 4% for the next 2 days (post-shift phase). B dose-dependently increased 32% sucrose intake, insulin secretion, and relative fat depots. Intake was reduced similarly in all groups following the shift to 4%, resulting in a relative B-dependent intake suppression following the shift to 4%. Videoscoring of locomotor activity indicated that search behaviors were not different between groups prior to the shift to 4%, whereas the increase in search behavior following the shift to 4% sucrose was entirely B-dependent. These data were predicted by a new model of chronic stress positing two regulatory axes on brain by glucocorticoids: one indirect axis by which glucocorticoids remodel energy stores to provide metabolic feedback and a direct axis whereby glucocorticoids act directly on brain to remodel appetitive structure.

This study tested whether pre-training pairings of ingestion of a 32% sucrose solution and injection(s) of corticosterone (B) would enhance later ingestion in the absence of B, and whether these effects would carry over into later contrast-like effects when animals were subsequently shifted to 4% sucrose. Frequency-dependence of these pairings was also examined. Three groups of male Sprague-Dawley rats were adrenalectomized (ADX). A fourth group was sham ADX. Each ADX group received three presentations of sucrose and B (666 microg/kg, s.c.). One received unpaired presentations (separated by days), one received two unpaired presentations and one paired (i.e., simultaneous) presentation, and one received three paired presentations. Shams received three sucrose presentations paired with saline. Single, but not multiple pairings of B with ingestion of a 32% sucrose solution enhanced later sucrose ingestion, a memorial-like effect that carried over into later, opposite contrast-like effects upon presentation of a less-preferred 4% sucrose solution. These effects could not be easily ascribed to differences in training, other than the pairing regimen itself, nor to motivational differences at the time of testing, and were presumed to be memorial. The pairing and frequency-dependence of these appetitive phenomena are analogous to what is frequently observed during acute or chronic exposure to aversive situations and/or neuromodulatory stress hormones, in terms of their bidirectional effects on memory. Through effects on memory, stress hormones may modulate reward and reward relativity.

Rats were used in a successive negative contrast procedure to determine which brain structures were activated by sucrose concentration downshifts, and on what day this occurred. Subjects were given preshift solutions for 12 days before being shifted to their postshift concentrations. Groups included 2 unshifted controls (32%-32% and 4%-4%) and 1 shifted group (32%-4%). Half of each group was killed 1 hr after the first exposure to the shifted solution (Shift 1), and half after the second exposure (Shift 2). Brains were processed for c-Fos-like immunoreactivity (FLI). Two major patterns emerged. Terminal drinking of 32% sucrose activated minimal brainstem regions involved in palatable taste, visceral feedback, and fluid homeostasis. In contrast, shifted subjects showed extensive cortical activation with selective activation in cerebral nuclei and brainstem. Robust FLI on Shift 1 was absent on Shift 2, consistent with evidence for rapid (1-trial) changes in all major motor outputs mediated by expectancy.

We suggested a new model of the effects of glucocorticoids (GCs) exerted during chronic stress, in which GCs directly stimulate activities in the brain while indirectly inhibiting activity in the hypothalamo-pituitary-adrenal (HPA) axis through their metabolic shifts in energy stores in the periphery. This study is an initial test of our model. In a 2 x 2 design, we provided ad lib access to calorically dense lard and sucrose (comfort food) + chow or chow alone, and repeatedly restrained half of the rats in each group for 5 d (3 h/d). We measured caloric intake, body weight, caloric efficiency, ACTH, corticosterone (B), and testosterone during the period of restraint and leptin, insulin, and fat depot weights, as well as hypothalamic corticotropin-releasing factor mRNA at the end of the period. We hypothesized that chronically restrained rats would exhibit a relative increase in comfort food ingestion and that these rats would have reduced HPA responses to repeated restraint. Although total caloric intake was reduced in both groups of restrained rats, compared with controls, the proportion of comfort food ingested increased in the restrained rats compared with their nonrestrained controls. Moreover, caloric efficiency was rescued in the stressed, comfort food group. Furthermore, ACTH and B responses to the repeated restraint bouts were reduced in the rats with access to comfort food. Corticotropin-releasing factor mRNA was reduced in control rats eating comfort food compared with those eating chow, but there were no differences between the stressed groups. The results of this experiment tend to support our model of chronic effects of stress and GCs, showing a stressor-induced preference for comfort food, and a comfort-food reduction in activity of the HPA axis.

Glucocorticoids have a major effect on food intake that is underappreciated, although the effects of glucocorticoids on metabolism and abdominal obesity are quite well understood. Physiologically appropriate concentrations of naturally secreted corticosteroids (cortisol in humans, corticosterone in rats) have major stimulatory effects on caloric intake and, in the presence of insulin, preference. We first address the close relationship between glucocorticoids and energy balance under both normal and abnormal conditions. Because excess caloric intake is stored in different fat depots, we also address the systemic effects of glucocorticoids on redistribution of stored energy preponderantly into intraabdominal fat depots. We provide strong evidence that glucocorticoids modify feeding and then discuss the role of insulin on the choice of ingested calories, as well as suggesting some central neural pathways that may be involved in these actions of glucocorticoids and insulin. Finally, we discuss the evolutionary utility of these actions of the stress hormones, and how dysregulatory effects of chronically elevated glucocorticoids may occur in our modern, rich societies.

_ Negative incentive contrast effects (NCEs) have typically been attributed to frustration or the decremental generalization of learned associations. The purpose of these experiments was to clarify the relation of NCEs to the repertoires of functional search behaviors evoked by incentive downshifts. Rats shifted from 32% to 4% sucrose-solution decreased consummatory responses but increased nose-down locomotion, orientation, location entries, and sampling of alternatives relative to unshifted controls. These changes in behavior were terminated or failed to occur under incentive upshifts. Furthermore, reward downshifts did not produce avoidance of the location of the shifted incentive. Increased search occurred whether or not alternative reward locations were available. Together the evidence suggests that NCEs are related to evoked search modes supporting a repertoire of functional behaviors related to finding food. Negative contrast effects (NCEs) are abrupt changes in behavior that usually follow a discriminable reduction in the accustomed size or quality of reward. These changes have been of particular interest because they are abrupt, exaggerated, and have been assumed to have an emotional basis. The abruptness of the behavioral changes suggests little dependency on habit formation. The behavioral changes are considered exaggerated because they are larger than would be expected if the animals simply adjusted to the new level of reward (i.e., adopted the behavior of unshifted animals never experiencing the larger reward). For example, compared with unshifted rats always receiving small rewards, rats shifted from large to small rewards show lower intake of foods (Flaherty, 1982) and slower approach speeds in runways (Crespi, 1942). Changes in behavior also appear to be emotion based in that rats may show escape or withdrawal from the reward location (Daly, 1974a, 1974b) and avoidance in the form of increased choice errors on a maze (Elliott, 1928). The dominant explanation for NCE has focused on emotional constructs such as disappointment or frustration (Amsel, 1951; Crespi, 1942; Spence, 1956; Tinkelpaugh, 1928; see Flaherty, 1982 and 1996 for comprehensive reviews). Many experiments support the frustration explanation of reward-shift effects in runway situations (Amsel

Scheduled daily injections of methamphetamine (MA) produced locomotor activity that preceded and followed the usual time of injection in rats housed under conditions of constant, moderately dim light and temporally distributed feeding. A circadian basis for pre-injection time activity was supported by its anticipatory timing in the apparent absence of reliable preceding external cues and by its persistence on a test day on which the rats remained undisturbed. Post-injection time locomotor activity also persisted on the test day, occurring from 24 to 29 h after the final MA injection. These results indicate that MA injections engage circadian processes underlying locomotor activity, and they raise the possibility that intake of drugs of abuse by humans may facilitate drug taking or relapse at times of day related to previous drug use.

Previous studies have shown reduced hypothalamo-pituitary-adrenal responses to both acute and chronic restraint stressors in rats allowed to ingest highly palatable foods (32% sucrose ± lard) prior to restraint. In this study we tested the effects of prior access (7 d) to chow-only, sucrose/chow, lard/chow, or sucrose/lard/chow diets on central corticotropin-releasing factor (CRF) expression in rats studied in two experiments, 15 and 240 min after onset of restraint. Fat depot, particularly intraabdominal fat, weights were increased by prior access to palatable food, and circulating leptin concentrations were elevated in all groups. Metabolite concentrations were appropriate for values obtained after stressors. For unknown reasons, the 15-min experiment did not replicate previous results. In the 240-min experiment, ACTH and corticosterone responses were inhibited, as previously, and CRF mRNA in the hypothalamus and oval nucleus of the bed nuclei of the stria terminalis were reduced by palatable foods, suggesting strongly that both neuroendocrine and autonomic outflows are decreased by increased caloric deposition and palatable food. In the central nucleus of the amygdala, CRF was increased in the sucrose-drinking group and decreased in the sucrose/lard group, suggesting that the consequence of ingestion of sucrose uses different neural networks from the ingestion of lard. The results suggest strongly that ingestion of highly palatable foods reduces activity in the central stress response network, perhaps reducing the feeling of stressors.

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.

Circadian rhythms prepare organisms for predictable events during the Earth's 24-h day. These rhythms are entrained by a variety of stimuli. Light is the most ubiquitous and best known zeitgeber, but a number of others have been identified, including food, social cues, locomotor activity, and, most recently drugs of abuse. Given the diversity of zeitgebers, it is probably not surprising that genes capable of clock functions are located throughout almost all organs and tissues. Recent evidence suggests that drugs of abuse can directly entrain some circadian rhythms. We have report here that entrainment by drugs of abuse is independent of the suprachiasmatic nucleus and the light/dark cycle, is not dependent on direct locomotor stimulation, and is shared by a variety of classes of drugs of abuse. We suggest that drug-entrained rhythms reflect variations in underlying neurophysiological states. This could be the basis for known daily variations in drug metabolism, tolerance, and sensitivity to drug reward. These rhythms could also take the form of daily periods of increased motivation to seek and take drugs, and thus contribute to abuse, addiction and relapse.

The common hepatic branch of the vagus nerve negatively regulates lard intake in rats with streptozotocin (STZ)-induced, insulin-dependent diabetes. However, this branch consists of two subbranches: the hepatic branch proper, which serves the liver, and the gastroduodenal branch, which serves the distal stomach, pancreas, and duodenum. The aim of this study was to determine whether the gastroduodenal branch specifically regulates voluntary lard intake. We performed a gastroduodenal branch vagotomy (GV) on nondiabetic, STZ-diabetic, and STZ-diabetic insulin-treated groups of rats and compared them with sham-operated counterparts. All rats had high steady-state corticosterone levels to maximize lard intake. Five days after surgery, all rats were provided with the choice of chow or lard to eat for another 5 days. STZ-diabetes resulted in a reduction in lard intake that was partially rescued by either GV or insulin treatment. Patterns of white adipose tissue (WAT) deposition differed after GV- and insulin-induced lard intake, with subcutaneous WAT increasing exclusively after the former and mesenteric WAT increasing exclusively in the latter. GV also prevented the insulin-induced reduction in the STZ-elevated plasma glucagon, triglycerides, free fatty acids, and total ketone bodies but did not alter the effect of insulin-induced reduction of plasma glucose levels. These data suggest that the gastroduodenal branch of the vagus inhibits lard intake and regulates WAT deposition and plasma metabolite levels in STZ-diabetic rats.

It is becoming increasingly evident that the urocortins (Ucns) and their receptors are involved in the initiation and development of inflammation in the gastrointestinal (GI) tract. There has not been a systematic study of the basal expression of Ucns or their receptors in the GI tract. Here, we examined basal expression of Ucn 2 and its high-affinity receptor, CRF-R2 in the rat GI tract. Ucn 2 mRNA was expressed throughout the small and large intestine. Surprisingly, CRF-R2 mRNA expression was detected in only a subset of GI regions that expressed Ucn 2. Immunohistochemical study showed that both Ucn 2 immuno-reactivity (Ucn 2-IR) and CRF-R2-IR were consistently seen in the neurons of the myenteric plexus and the nerve fibers innervating the circular muscle. By and large, Ucn 2-IR was detected in all layers, including the mucosal and the submucosal layers throughout the GI regions. In contrast, CRF-R2-IR was very low or undetectable in the mucosal layers of all regions examined. The role of Ucn 2 and CRF-R2 was then examined in a rat model of chemically-induced colitis. In the early phase of colitis, Ucn 2 mRNA levels peaked, whereas, in striking contrast, CRF-R2 mRNA expression decreased approximately 2.5-fold below control levels. At the peptide level, Ucn 2-IR was specifically induced in a large population of immune cells that infiltrated the lamina propria and submucosa of the distal colon, whereas CRFR2-IR was detected in only a small fraction of infiltrated immune cells. CRF-R2-IR was dramatically reduced in the neurons of the myenteric plexus. Thus, we show, for the first time, that in the acute phase of inflammation, Ucn 2 levels are increased whereas expression levels of its only identified receptor, CRF-R2, are decreased. This suggests that Ucn 2 exerts its effects only in part via CRF-R2.

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

Corticosterone (B) increases and insulin decreases food intake. However, in streptozotocin (STZ)-diabetic rats with high B, low insulin replacement promotes lard intake. To test the role of the liver on this, rats were given STZ and infused with insulin or vehicle into either the superior mesenteric or right jugular vein. Controls were nondiabetic; all rats were treated with high B. After 5 d, all rats were offered lard, 32% sucrose, chow, and water ad libitum until d 10. Diabetes exacerbated body weight loss from high B; this was prevented by insulin into the jugular, but not superior mesenteric, vein. Without insulin, STZ groups essentially consumed only chow; controls increased caloric intake about equally from the three sources. Insulin into both sites reduced chow and increased lard intake. Although circulating insulin was increased only by jugular infusion, plasma glucose and liver glycogen were similar after insulin into both sites. Fat depot weights differed: sc fat was heavier after jugular and mesenteric fat was heavier after mesenteric insulin infusions. We conclude that there are important site-specific effects of insulin in regulating the choice of, but not total, caloric intake, body weight, and fat storage in diabetic rats with high B. Furthermore, lard intake might be regulated by an insulin-derived, liver-mediated signal because superior mesenteric insulin infusion had similar effects on lard intake to jugular infusion but did not result in elevated circulating insulin levels likely associated with liver insulin removal.

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.

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

Central corticotropin-releasing factor (CRF) networks are recruited by chronic stressors and elevated glucocorticoids (GCs) that initiate recruitment of central CRF activity in the amygdala. Increased central activity of the CRF network stimulates all monoaminergic cell groups, as well as premotor autonomic and other limbic structures resulting in the typical arousal, behavioral changes, autonomic, and neuroendocrine changes that accompany the chronic imposition of a stressor. By contrast, elevated GCs appear, through a variety of means to counteract the effects of central CRF, which they have initiated. Together with insulin, the GCs stimulate drive for and ingestion of "comfort foods" that may directly result in reduction of the negative effects of the chronic stressor in the nucleus Accumbens, through stimulation of the anterior, more pleasure-associated part of this cell group, thus reducing the weight of the stress-stimulated posterior, more defensive part. Furthermore, the shift in caloric intake from chow to preference for "comfort foods," together with elevated GCs and insulin, reorganize energy stores from a peripheral to a central distribution, primarily as abdominal fat. A signal associated with this fat depot appears, as with eating "comfort foods," to reduce the influence of the chronic stress network on behaviors, autonomic, and neuroendocrine outflow.

Acutely, glucocorticoids act to inhibit stress-induced corticotrophin-releasing factor (CRF) and adrenocorticotrophic hormone (ACTH) secretion through their actions in brain and anterior pituitary (canonical feedback). With chronic stress, glucocorticoid feedback inhibition of ACTH secretion changes markedly. Chronically stressed rats characteristically exhibit facilitated ACTH responses to acute, novel stressors. Moreover, in adrenalectomized rats in which corticosterone was replaced, steroid concentrations in the higher range are required for facilitation of ACTH responses to occur after chronic stress or diabetes. Infusion of corticosterone intracerebroventricularly into adrenalectomized rats increases basal ACTH, tends to increase CRF, and allows facilitation of ACTH responses to repeated restraint. Therefore, with chronic stressors, corticosterone seems to act in brain in an excitatory rather than an inhibitory fashion. We believe, under conditions of chronic stress, that there is an indirect glucocorticoid feedback that is mediated through the effects of the steroid +/- insulin on metabolism. Increased energy stores feedback on brain to inhibit hypothalamic CRF and decrease the expression of dopamine-beta-hydroxylase in the locus coeruleus. These changes would be expected to decrease the level of discomfort and anxiety induced by chronic stress. Moreover, central neural actions of glucocorticoids abet the peripheral effects of the steroids by increasing the salience and ingestion of pleasurable foods.

The effects of adrenal corticosteroids on subsequent adrenocorticotropin secretion are complex. Acutely (within hours), glucocorticoids (GCs) directly inhibit further activity in the hypothalamo–pituitary–adrenal axis, but the chronic actions (across days) of these steroids on brain are directly excitatory. Chronically high concentrations of GCs act in three ways that are functionally congruent. ( i ) GCs increase the expression of corticotropin-releasing factor (CRF) mRNA in the central nucleus of the amygdala, a critical node in the emotional brain. CRF enables recruitment of a chronic stress-response network. ( ii ) GCs increase the salience of pleasurable or compulsive activities (ingesting sucrose, fat, and drugs, or wheel-running). This motivates ingestion of “comfort food.” ( iii ) GCs act systemically to increase abdominal fat depots. This allows an increased signal of abdominal energy stores to inhibit catecholamines in the brainstem and CRF expression in hypothalamic neurons regulating adrenocorticotropin. Chronic stress, together with high GC concentrations, usually decreases body weight gain in rats; by contrast, in stressed or depressed humans chronic stress induces either increased comfort food intake and body weight gain or decreased intake and body weight loss. Comfort food ingestion that produces abdominal obesity, decreases CRF mRNA in the hypothalamus of rats. Depressed people who overeat have decreased cerebrospinal CRF, catecholamine concentrations, and hypothalamo–pituitary–adrenal activity. We propose that people eat comfort food in an attempt to reduce the activity in the chronic stress-response network with its attendant anxiety. These mechanisms, determined in rats, may explain some of the epidemic of obesity occurring in our society.

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.

Theorists of learning, regulation, and evolution explain behavior using remarkably different concepts because of pressures toward specialization, a focus on testing simple causal theories that underconceptualize the contributions of the organism and its environment, and the absence of a working model capable of surviving in a complex environment. We add suggestions for the development and testing of such a model.