Classically conditioned responses following repeated insulin and glucose administration in humans

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Cephalic phase insulin release: A review of its mechanistic basis and variability in humans

Cephalic phase insulin release (CPIR) is a transient pulse of insulin that occurs within minutes of stimulation from foods or food-related stimuli. Despite decades of research on CPIR in humans, the body of literature surrounding this phenomenon is controversial due in part to contradictory findings . This has slowed progress towards understanding the sensory and neural basis of CPIR, as well as its overall relevance to health. This review examines up-to-date knowledge in CPIR research and identifies sources of CPIR variability in humans in an effort to guide future research. The review starts by defining CPIR and discussing its presumed functional roles in glucose homeostasis and feeding behavior. Next, the types of stimuli that have been reported to elicit CPIR, as well as the sensory and neural mechanisms underlying the response in rodents and humans are discussed, and areas where knowledge is limited are identified. Finally, factors that may contribute to the observed variability of CPIR in humans are examined, including experimental design, test procedure, and individual characteristics. Overall, oral stimulation appears to be important for eliciting CPIR, especially when combined with other sensory modalities (vision, olfaction, somatosensation). While differences in experimental design and testing procedure likely explain some of the observed inter- and intra-study variability, individual differences also appear to play an important role. Understanding sources of these individual differences in CPIR will be key for establishing its health relevance.

Effect of hepatic sympathetic nerve removal on energy metabolism in an animal model of cognitive impairment and its relationship to Glut2 expression

The aims of this study were to investigate the effect of hepatic sympathetic nerve removal on glucose and lipid metabolism in rats with cognitive impairment and to evaluate the relationship between these effects and liver Glut2 expression. Hippocampal injection of Aβ_1–42 was used to induce cognitive impairment. Impaired rats were divided into experimental, sham, and control groups. The experimental group was injected with 6-hydroxydopamine to remove the sympathetic nerve. At 4 weeks post injection, body weight, food and water intake, blood sugar, and blood lipids were measured, and periodic acid-Schiff (PAS) staining was used to assess the liver glycogen content. Liver Glut2 mRNA and protein were also detected. The experimental group showed reduced body weight, food intake, and blood glucose levels and elevated insulin levels compared with the control group. PAS staining showed higher glycogen contents in the experimental group than in controls. The expression levels of Glut2 mRNA and protein in the experimental group were significantly lower than in the controls. Metabolism was significantly impacted in rats with cognitive impairment following removal of the hepatic sympathetic nerve. Disruption to Glut2 liver expression via sympathetic nerve disruption represents a possible underlying mechanism.

Endocrine Cephalic Phase Responses to Food Cues: A Systematic Review

Cephalic phase responses (CPRs) are conditioned anticipatory physiological responses to food cues. They occur before nutrient absorption and are hypothesized to be important for satiation and glucose homeostasis. Cephalic phase insulin responses (CPIRs) and pancreatic polypeptide responses (CPPPRs) are found consistently in animals, but human literature is inconclusive. We performed a systematic review of human studies to determine the magnitude and onset time of these CPRs. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were used to develop a search strategy. The terms included in the search strategy were cephalic or hormone response or endocrine response combined with insulin and pancreatic polypeptide (PP). The following databases were searched: Scopus (Elsevier), Science Direct, PubMed, Google Scholar, and The Cochrane Library. Initially, 582 original research articles were found, 50 were included for analysis. An insulin increase (≥1μIU/mL) was observed in 41% of the treatments (total n = 119). In 22% of all treatments the increase was significant from baseline. The median (IQR) insulin increase was 2.5 (1.6-4.5) μIU/mL, 30% above baseline at 5± 3 min  after food cue onset (based on study treatments that induced ≥1 μIU/mL insulin increase). A PP increase (>10 pg/mL) was found in 48% of the treatments (total n = 42). In 21% of the treatments, the increase was significant from baseline. The median (IQR) PP increase was 99 (26-156) pg/mL, 68% above baseline at 9± 4 min  after food cue onset (based on study treatments that induced ≥1 μIU/mL insulin increase). In conclusion, CPIRs are small compared with spontaneous fluctuations. Although CPPPRs are of a larger magnitude, both show substantial variation in magnitude and onset time. We found little evidence for CPIR or CPPPR affecting functional outcomes, that is, satiation and glucose homeostasis. Therefore, CPRs do not seem to be biologically meaningful in daily life.

Behavioral Conditioning, the Placebo Effect, and Emergency Department Pain Management

Animals and humans can be readily conditioned to associate a novel stimulus (often a unique taste) by pairing it with the effects of a drug or other agent. When later presented with the stimulus alone, their body's systems respond as if the drug or agent were given. The earliest clinical applications demonstrated both conditioned suppression and enhancement of immune processes. Unique benign stimuli, paired with chemotherapy, come to elicit T-cell suppression when administered alone. The beneficial immune responses to an antigen can be conditioned in the same manner. Further study of what came to be called "psychoneuroimmunology" led to the understanding that the familiar placebo effect, previously attributed to suggestion and expectation, is at least equally dependent on the same sorts of behavioral conditioning. The demonstrated ability to manipulate the immune system by a conditioned taste stimulus is, by definition, a placebo: a therapeutic effect caused by an inactive agent. The purpose of this analysis was to stimulate research in, and the application of, placebo-response conditioning to emergency medicine. Clinical and experimental studies confirm the usefulness of conditioned placebos in analgesia and in placebo-controlled dose reduction. Such conditioning paradigms demonstrate "one-trial learning," making them potentially useful in pain and addiction management within a single emergency department encounter.

Mapping sign-tracking and goal-tracking onto human behaviors

As evidenced through classic Pavlovian learning mechanisms, environmental cues can become incentivized and influence behavior. These stimulus-outcome associations are relevant in everyday life but may be particularly important for the development of impulse control disorders including addiction. Rodent studies have elucidated specific learning profiles termed 'sign-tracking' and 'goal-tracking' which map onto individual differences in impulsivity and other behaviors associated with impulse control disorders' etiology, course, and relapse. Whereas goal-trackers are biased toward the outcome, sign-trackers fixate on features that are associated with but not necessary for achieving an outcome; a pattern of behavior that often leads to escalation of reward-seeking that can be maladaptive. The vast majority of the sign- and goal-tracking research has been conducted using rodent models and very few have bridged this concept into the domain of human behavior. In this review, we discuss the attributes of sign- and goal-tracking profiles, how these are manifested neurobiologically, and how these distinct learning styles could be an important tool for clinical interventions in human addiction.

Placebos in the era of open-label trials: An update for clinicians

Placebos have been used extensively by vast numbers of physicians, in a majority of clinical trials. Placebo effects involve behavioural, psychological and genetic factors and have been subject to ethical controversies stemming from the use of deception in treating patients. The patient-physician encounter, endogenous pharmacological pathways, personality traits and genetic diversity have all been reported to be key players in placebo responses. In the last decade, a new methodological paradigm of placebo research has emerged, using open-label placebos to investigate their effects which showed promising results for various common medical conditions. In this review, we will summarize the current body of evidence on placebos in clinical practice, with a view to open-label placebo trials in particular. It is our view that future larger-scale randomized blinded open placebo trials will benefit physicians and improve patient outcomes.

The cephalic phase insulin response to nutritive and low-calorie sweeteners in solid and beverage form

The purpose of the study was to examine the role of the cephalic phase insulin response (CPIR) following exposure to nutritive and low-calorie sweeteners in solid and beverage form in overweight and obese adults. In addition, the role of learning on the CPIR to nutritive and low-calorie sweetener exposure was tested. Sixty-four overweight and obese adults (age: 18-50years, BMI: 24-37kg/m2, body fat percentage>25% for men and >32% for women) were sham-fed (at 2-minute intervals for 14min) a randomly assigned test load comprised of a nutritive (sucrose) or low-calorie sweetener (sucralose) in beverage or solid form in phase 1 of the study. A 2-3ml blood sample was collected before and 2, 6, 10, 14, 61, 91 and 121min after oral exposure for serum insulin and glucose analysis. During phase 2, participants underwent a 2-week training period to facilitate associative learning between the sensory properties of test loads and their post-ingestive effects. In phase 3, participants were retested for their cephalic phase responses as in phase 1. Participants were classified as responders if they demonstrated a positive insulin response (rise of serum insulin above baseline i.e. Δ insulin) 2min post-stimulus in phase 1. Among responders exposed to the same sweetener in Phases 1 and 3, the proportion of participants that displayed a rise of insulin with oral exposure to sucralose was significantly greater when the stimulus was in the solid form compared to the beverage form. Sucralose and sucrose exposure elicited similarly significant increases in serum insulin 2min after exposure and significant decreases after 2min in responders in both food forms. The solid food form elicited greater CPIR over 2, 6 and 10min than the beverage form. There was no effect of learning on insulin responses after training. The results indicate the presence of a significant CPIR in a subset of individuals with overweight or obesity after oral exposure to sucralose, especially when present in solid food form. Future studies must confirm the reliability of this response.

Conditioning Immune and Endocrine Parameters in Humans: A Systematic Review

BACKGROUND

Conditioned pharmacological effects may provide relevant clinical opportunities to improve treatment for patients with a variety of conditions. The aim of this systematic review was to create an overview of studies in this field of research and to investigate whether specific characteristics of the study design make for successful conditioning.

METHODS

The protocol of this review was registered in Prospero (PROSPERO 2015: CRD42015024148). A systematic literature search was conducted in the databases PubMed, Embase, and PsychInfo. Studies were included if they were placebo-controlled trials in humans in which the effects of a pharmacological agent on immune or endocrine outcomes (e.g., interleukin-2 and cortisol) were conditioned, using a specific conditioned stimulus. The risk of bias of each study was assessed using the Cochrane risk-of-bias tool.

RESULTS

The final selection included 16 studies. Overall, those studies indicate that conditioning of immunosuppression, conditioning of allergic responses, and conditioning of insulin and glycemic responses is possible. Regarding immunostimulants, antiallergic effects, and cortisol conditioning, the preliminary results are promising, but additional studies are needed.

CONCLUSIONS

This systematic review shows classical conditioning of immune and endocrine responses for various pharmaceutical substances. The studies reviewed here indicate that the number of acquisition and evocation sessions, and characteristics of the unconditioned and conditioned stimuli, are important determinants of the effectiveness of pharmacological conditioning on immune and endocrine parameters. In the future, conditioned pharmacological effects may be used clinically as adjunct therapy in various patient populations.

Pancreatic regulation of glucose homeostasis

In order to ensure normal body function, the human body is dependent on a tight control of its blood glucose levels. This is accomplished by a highly sophisticated network of various hormones and neuropeptides released mainly from the brain, pancreas, liver, intestine as well as adipose and muscle tissue. Within this network, the pancreas represents a key player by secreting the blood sugar-lowering hormone insulin and its opponent glucagon. However, disturbances in the interplay of the hormones and peptides involved may lead to metabolic disorders such as type 2 diabetes mellitus (T2DM) whose prevalence, comorbidities and medical costs take on a dramatic scale. Therefore, it is of utmost importance to uncover and understand the mechanisms underlying the various interactions to improve existing anti-diabetic therapies and drugs on the one hand and to develop new therapeutic approaches on the other. This review summarizes the interplay of the pancreas with various other organs and tissues that maintain glucose homeostasis. Furthermore, anti-diabetic drugs and their impact on signaling pathways underlying the network will be discussed.

Teaching neurons to respond to placebos

KEY POINTS

We analysed the placebo response at the single-neuron level in the thalamus of Parkinson patients to see the differences between first-time administration of placebo and administration after pharmacological pre-conditioning. When the placebo was given for the first time, it induced neither clinical improvement, as assessed through muscle rigidity reduction at the wrist, nor neuronal changes in thalamic neurons. However, if placebo was given after two, three or four prior administrations of an anti-Parkinson drug, apomorphine, it produced both clinical and neuronal responses. Both the magnitude and the duration of these placebo responses depended on the number of prior exposures to apomorphine, according to the rule: the greater the number of previous apomorphine administrations, the larger the magnitude and the longer the duration of the clinical and neuronal placebo responses. These findings show that learning plays a crucial role in the placebo response and suggest that placebo non-responders can be turned into placebo responders, with important clinical implications.

ABSTRACT

Placebos have been found to affect the patient's brain in several conditions, such as pain and motor disorders. For example, in Parkinson's disease, a placebo treatment induces a release of dopamine in the striatum and changes the activity of neurons in both thalamic and subthalamic nuclei. The present study shows that placebo administration for the first time induces neither clinical nor neuronal improvement in Parkinson patients who undergo implantation of electrodes for deep brain stimulation. However, this lack of placebo responsiveness can be turned into substantial placebo responses following previous exposure to repeated administrations of the anti-Parkinson agent apomorphine. As the number of apomorphine administrations increased from one to four, both the clinical response and the neuronal activity in the ventral anterior and anterior ventrolateral thalamus increased. In fact, after four apomorphine exposures, placebo administration induced clinical responses that were as large as those to apomorphine, along with long-lasting neuronal changes. These clinical placebo responses following four apomorphine administrations were again elicited after a re-exposure to a placebo 24 h after surgery, but not after 48 h. These data indicate that learning plays a crucial role in placebo responsiveness and suggest that placebo non-responders can be turned into responders, with important implications in the clinical setting.

The Pervasive Problem With Placebos in Psychology: Why Active Control Groups Are Not Sufficient to Rule Out Placebo Effects

To draw causal conclusions about the efficacy of a psychological intervention, researchers must compare the treatment condition with a control group that accounts for improvements caused by factors other than the treatment. Using an active control helps to control for the possibility that improvement by the experimental group resulted from a placebo effect. Although active control groups are superior to "no-contact" controls, only when the active control group has the same expectation of improvement as the experimental group can we attribute differential improvements to the potency of the treatment. Despite the need to match expectations between treatment and control groups, almost no psychological interventions do so. This failure to control for expectations is not a minor omission-it is a fundamental design flaw that potentially undermines any causal inference. We illustrate these principles with a detailed example from the video-game-training literature showing how the use of an active control group does not eliminate expectation differences. The problem permeates other interventions as well, including those targeting mental health, cognition, and educational achievement. Fortunately, measuring expectations and adopting alternative experimental designs makes it possible to control for placebo effects, thereby increasing confidence in the causal efficacy of psychological interventions.

Placebo and nocebo effects: a complex interplay between psychological factors and neurochemical networks

Placebo and nocebo effects have recently emerged as an interesting model to understand some of the intricate underpinnings of the mind-body interaction. A variety of psychological mechanisms, such as expectation, conditioning, anxiety modulation, and reward, have been identified, and a number of neurochemical networks have been characterized across different conditions, such as pain and motor disorders. What has emerged from the recent insights into the neurobiology of placebo and nocebo effects is that the psychosocial context around the patient and the therapy, which represents the ritual of the therapeutic act, may change the biochemistry and the neuronal circuitry of the patient's brain. Furthermore, the mechanisms activated by placebos and nocebos have been found to be the same as those activated by drugs, which suggests a cognitive/affective interference with drug action. Overall, these findings highlight the important role of therapeutic rituals in the overall therapeutic outcome, including hypnosis, which may have profound implications both in routine medical practice and in the clinical trials setting.

Placebo and the new physiology of the doctor-patient relationship

Modern medicine has progressed in parallel with the advancement of biochemistry, anatomy, and physiology. By using the tools of modern medicine, the physician today can treat and prevent a number of diseases through pharmacology, genetics, and physical interventions. Besides this materia medica, the patient's mind, cognitions, and emotions play a central part as well in any therapeutic outcome, as investigated by disciplines such as psychoneuroendocrinoimmunology. This review describes recent findings that give scientific evidence to the old tenet that patients must be both cured and cared for. In fact, we are today in a good position to investigate complex psychological factors, like placebo effects and the doctor-patient relationship, by using a physiological and neuroscientific approach. These intricate psychological factors can be approached through biochemistry, anatomy, and physiology, thus eliminating the old dichotomy between biology and psychology. This is both a biomedical and a philosophical enterprise that is changing the way we approach and interpret medicine and human biology. In the first case, curing the disease only is not sufficient, and care of the patient is of tantamount importance. In the second case, the philosophical debate about the mind-body interaction can find some important answers in the study of placebo effects. Therefore, maybe paradoxically, the placebo effect and the doctor-patient relationship can be approached by using the same biochemical, cellular and physiological tools of the materia medica, which represents an epochal transition from general concepts such as suggestibility and power of mind to a true physiology of the doctor-patient interaction.

Interactions between the central nervous system and pancreatic islet secretions: a historical perspective

The endocrine pancreas is richly innervated with sympathetic and parasympathetic projections from the brain. In the mid-20th century, it was established that α-adrenergic activation inhibits, whereas cholinergic stimulation promotes, insulin secretion; this demonstrated the importance of the sympathetic and parasympathetic systems in pancreatic endocrine function. It was later established that insulin injected peripherally could act within the brain, leading to the discovery of insulin and insulin receptors within the brain and the receptor-mediated transport of insulin into the central nervous system from endothelial cells. The insulin receptor within the central nervous system is widely distributed, reflecting insulin's diverse range of actions, including acting as an adiposity signal to reduce food intake and increase energy expenditure, regulation of systemic glucose responses, altering sympathetic activity, and involvement in cognitive function. As observed with central insulin administration, the pancreatic hormones glucagon, somatostatin, pancreatic polypeptide, and amylin can each also reduce food intake. Pancreatic and also gut hormones are released cephalically, in what is an important mechanism to prepare the body for a meal and prevent excessive postprandial hyperglycemia.

Olfaction under metabolic influences

Recently published work and emerging research efforts have suggested that the olfactory system is intimately linked with the endocrine systems that regulate or modify energy balance. Although much attention has been focused on the parallels between taste transduction and neuroendocrine controls of digestion due to the novel discovery of taste receptors and molecular components shared by the tongue and gut, the equivalent body of knowledge that has accumulated for the olfactory system, has largely been overlooked. During regular cycles of food intake or disorders of endocrine function, olfaction is modulated in response to changing levels of various molecules, such as ghrelin, orexins, neuropeptide Y, insulin, leptin, and cholecystokinin. In view of the worldwide health concern regarding the rising incidence of diabetes, obesity, and related metabolic disorders, we present a comprehensive review that addresses the current knowledge of hormonal modulation of olfactory perception and how disruption of hormonal signaling in the olfactory system can affect energy homeostasis.

Placebo mechanisms across different conditions: from the clinical setting to physical performance

Although the great increase in interest in the placebo phenomenon was spurred by the clinical implications of its use, the progressive elucidation of the neurobiological and pharmacological mechanisms underlying the placebo effect also helps cast new light on the relationship between mind (and brain) and body, a topic of foremost philosophical importance but also a major medical issue in light of the complex interactions between the brain on the one hand and body functions on the other. While the concept of placebo can be a general one, with a broad definition generally applicable to many different contexts, the description of the cerebral processes called into action in specific situations can vary widely. In this paper, examples will be given where physiological or pathological conditions are altered following the administration of an inert substance or verbal instructions tailored to induce expectation of a change, and explanations will be offered with details on neurotransmitter changes and neural pathways activated. As an instance of how placebo effects can extend beyond the clinical setting, data in the physical performance domain and implications for sport competitions will also be presented and discussed.

Unconditioned and conditioned effects of intravenous insulin and glucose on heart rate variability in healthy men

We examined whether an injection of intravenous insulin and intravenous glucose would affect frequency-domain measures of heart rate variability (HRV), i.e., the high-frequency (HF-) band and the ratio of the low frequency (LF-) to the HF-band in healthy humans. Using a classical conditioning protocol, we also assessed whether the measures of HRV are subject to classical conditioning. Thirty healthy men were divided into three groups, given a conditioned stimulus (CS) and an intravenous injection of either insulin (0.05IU/kg) in Group 1, glucose (15%, 0.5g/kg) in Group 2, or placebo (physiological saline [0.9%]) in Group 3 during the 4-day acquisition phase. All subjects were given an olfactory CS (rosewood-peppermint smell) and placebo injection on day 5 (test). Due to their high inter-individual variability, HF and LF/HF-ratio were analysed by intragroup comparisons, using a pre-injection baseline interval (min -15 to -5), and three functional post-injection intervals: a) the interval to the maximum insulin level, i. e. insulin peak (min 0-5) in Groups 1 and 2, b) the interval to the maximum of insulin-induced hypoglycaemia (min 20-25) in Group 1, and c) the end of the session (min 70-75). On days 1 to 4, we found significant increases of the HF-band from baseline to interval min 0-5 in Group 1, and an even more pronounced increase in the glucose-treated Group 2. At the test (Day 5), both experimental groups responded with an HF-increase in the interval of the former insulin peak, and also at the other measurement intervals, reflecting some general increase of vagal activity remaining as a conditioned response. On days 1 to 4, the HF-band was positively correlated with the change of peripheral insulin levels in Group 1, reaching statistical significance on days 3 and 4. This pattern only emerged in tendency on Day 4 in Group 2. In conclusion, insulin triggers an increase in parasympathetic tone at maximum hyperinsulinaemia, and our data support the notion that this response pattern can become classically conditioned.

How placebos change the patient's brain

Although placebos have long been considered a nuisance in clinical research, today they represent an active and productive field of research and, because of the involvement of many mechanisms, the study of the placebo effect can actually be viewed as a melting pot of concepts and ideas for neuroscience. Indeed, there exists not a single but many placebo effects, with different mechanisms and in different systems, medical conditions, and therapeutic interventions. For example, brain mechanisms of expectation, anxiety, and reward are all involved, as well as a variety of learning phenomena, such as Pavlovian conditioning, cognitive, and social learning. There is also some experimental evidence of different genetic variants in placebo responsiveness. The most productive models to better understand the neurobiology of the placebo effect are pain and Parkinson's disease. In these medical conditions, the neural networks that are involved have been identified: that is, the opioidergic-cholecystokinergic-dopaminergic modulatory network in pain and part of the basal ganglia circuitry in Parkinson's disease. Important clinical implications emerge from these recent advances in placebo research. First, as the placebo effect is basically a psychosocial context effect, these data indicate that different social stimuli, such as words and rituals of the therapeutic act, may change the chemistry and circuitry of the patient's brain. Second, the mechanisms that are activated by placebos are the same as those activated by drugs, which suggests a cognitive/affective interference with drug action. Third, if prefrontal functioning is impaired, placebo responses are reduced or totally lacking, as occurs in dementia of the Alzheimer's type.

Neurobiological mechanisms of placebo responses

Expectations, positive or negative, are modulating factors influencing behavior. They are also thought to underlie placebo effects, potentially impacting perceptions and biological processes. We used sustained pain as a model to determine the neural mechanisms underlying placebo-induced analgesia and affective changes in healthy humans. Subjects were informed that they could receive either an active agent or an inactive compound, similar to routine clinical trials. Using PET and the mu-opioid selective radiotracer [(11)C]carfentanil we demonstrate placebo-induced activation of opioid neurotransmission in a number of brain regions. These include the rostral anterior cingulate, orbitofrontal and dorsolateral prefrontal cortex, anterior and posterior insula, nucleus accumbens, amygdala, thalamus, hypothalamus, and periaqueductal grey. Some of these regions overlap with those involved in pain and affective regulation but also motivated behavior. The activation of endogenous opioid neurotransmission was further associated with reductions in pain report and negative affective state. Additional studies with the radiotracer [(11)C]raclopride, studies labeling dopamine D2/3 receptors, also demonstrate the activation of nucleus accumbens dopamine during placebo administration under expectation of analgesia. Both dopamine and opioid neurotransmission were related to expectations of analgesia and deviations from those initial expectations. When the activity of the nucleus accumbens was probed with fMRI using a monetary reward expectation paradigm, its activation was correlated with both dopamine, opioid responses to placebo in this region and the formation of placebo analgesia. These data confirm that specific neural circuits and neurotransmitter systems respond to the expectation of benefit during placebo administration, inducing measurable physiological changes.

Classical conditioning of endocrine effects

PURPOSE OF REVIEW

Classical conditioning is a form of associative learning, based on the pioneering work of I. P. Pavlov: due to its association with an unconditioned stimulus that induces an unconditioned response, an initially neutral stimulus will become a conditioned stimulus evoking a conditioned response in the absence of the unconditioned stimulus. One important area for the application of conditioning principles is the regulation of physiological systems in general, and endocrine responses and its concomitant changes specifically. Conditioned endocrine responses were predominantly addressed in animal studies so far, mainly examining conditioned insulin production (and blood-glucose change) and corticosterone release.

RECENT FINDINGS

There are very few studies on classical conditioning of endocrine responses in the annual period of this review. The advancement, however, is that some are conducted with humans. Recently, as a new avenue, hormones and neurotransmitters have been examined as mediators of basic conditioning processes. Moreover, there is an increasing interest in gender-specific conditioning responses which are influenced by gonadal hormones.

SUMMARY

Research on classical conditioning demonstrates that endocrine systems are adaptable to environmental signals. Likewise, the endocrine status of an organism (at least with regard to glucocorticoids and gonadal hormones) was shown to modify classically conditioned responses. Partly due to the high expenditure of conducting conditioning experiments, the quantity of studies is limited, but there is a need to extend this research to humans. In sum, the application of conditioning paradigms constitutes an important research tool for behavioral medicine as well as psychiatry to examine brain-behavior relationships.

Insulin and the insulin receptor in experimental models of learning and memory

Insulin is best known for its action on peripheral insulin target tissues such as the adipocyte, muscle and liver to regulate glucose homeostasis. In the central nervous system (CNS), insulin and the insulin receptor are found in specific brain regions where they show evidence of participation in a variety of region-specific functions through mechanisms that are different from its direct glucose regulation in the periphery. While the insulin/insulin receptor associated with the hypothalamus plays important roles in regulation of the body energy homeostasis, the hippocampus- and cerebral cortex-distributed insulin/insulin receptor has been shown to be involved in brain cognitive functions. Emerging evidence has suggested that insulin signaling plays a role in synaptic plasticity by modulating activities of excitatory and inhibitory receptors such as glutamate and GABA receptors, and by triggering signal transduction cascades leading to alteration of gene expression that is required for long-term memory consolidation. Furthermore, deterioration of insulin receptor signaling appears to be associated with aging-related brain degeneration such as the Alzheimer's dementia and cognitive impairment in aged subjects suffering type 2 diabetes mellitus.

Septal infusions of glucose or pyruvate, but not fructose, produce avoidance deficits when co-infused with the GABA agonist muscimol

Although glucose typically enhances memory or reverses memory deficits, glucose can also produce memory deficits when co-infused with the gamma-aminobutyric acid (GABA) agonist muscimol into the medial septum (Parent & Gold, 1997; Parent, Laurey, Wilkniss, & Gold, 1997). To date the mechanisms underlying the memory-impairing interaction between GABA and glucose remain unknown. Here we investigate whether this effect is the result of hyperosmolar conditions or may involve glucose metabolism. Male Sprague-Dawley rats were given one-trial inhibitory avoidance training after receiving septal infusions of vehicle (phosphate-buffered saline, 0.5 microl), the GABA(A) agonist muscimol (3 nmol), glucose (16.5, 33, or 66 nmol), fructose (33 nmol), pyruvate (33 nmol), or a solution containing muscimol combined with glucose, fructose, or pyruvate. Retention performance was tested 48 h later. Infusions of glucose, pyruvate, fructose, or muscimol alone did not affect retention performance. However, co-infusions of all doses of glucose (16.5, 33, or 66 nmol) or the glycolytic end product pyruvate with muscimol impaired retention performance. Co-infusions of fructose with muscimol did not affect retention performance. These results suggest that the memory-impairing interaction between glucose and muscimol does not result from hyperosmolar conditions, because equiosmolar concentrations of fructose do not mimic the effects of glucose and the memory deficits do not vary as a function of glucose concentration. The finding that pyruvate mimicked the effects of glucose and impaired memory when combined with muscimol suggests that glucose metabolism may be involved in the memory-impairing interaction between glucose and GABA(A) receptors in the medial septum.

The house economist and the eating paradox

An important observation of the experiments of George Collier is that animals normally prefer to maintain their body weight by eating a large number of small meals each day. However, as the effort to obtain access to food increases, the animals adapt by changing to a schedule of eating a small number of large meals each day. A strong implication of this is that there is a hidden cost to eating large meals, and this is the basis of the eating paradox that states that although food is a necessary commodity, the act of ingesting it poses certain metabolic problems for animals. Experiments on cephalic insulin secretion, conditioned insulin secretion and meal feeding are discussed to make the point that the economy demonstrated by rats in Collier's paradigm is dictated in part by predictions of the eating paradox.