Pavlovian conditioning of drug-induced changes in body temperature

Cellular signaling pathways as plastic, proto-cognitive systems: Implications for biomedicine

Many aspects of health and disease are modeled using the abstraction of a "pathway"-a set of protein or other subcellular activities with specified functional linkages between them. This metaphor is a paradigmatic case of a deterministic, mechanistic framework that focuses biomedical intervention strategies on altering the members of this network or the up-/down-regulation links between them-rewiring the molecular hardware. However, protein pathways and transcriptional networks exhibit interesting and unexpected capabilities such as trainability (memory) and information processing in a context-sensitive manner. Specifically, they may be amenable to manipulation via their history of stimuli (equivalent to experiences in behavioral science). If true, this would enable a new class of biomedical interventions that target aspects of the dynamic physiological "software" implemented by pathways and gene-regulatory networks. Here, we briefly review clinical and laboratory data that show how high-level cognitive inputs and mechanistic pathway modulation interact to determine outcomes in vivo. Further, we propose an expanded view of pathways from the perspective of basal cognition and argue that a broader understanding of pathways and how they process contextual information across scales will catalyze progress in many areas of physiology and neurobiology. We argue that this fuller understanding of the functionality and tractability of pathways must go beyond a focus on the mechanistic details of protein and drug structure to encompass their physiological history as well as their embedding within higher levels of organization in the organism, with numerous implications for data science addressing health and disease. Exploiting tools and concepts from behavioral and cognitive sciences to explore a proto-cognitive metaphor for the pathways underlying health and disease is more than a philosophical stance on biochemical processes; at stake is a new roadmap for overcoming the limitations of today's pharmacological strategies and for inferring future therapeutic interventions for a wide range of disease states.

How does homeostasis happen? Integrative physiological, systems biological, and evolutionary perspectives

Homeostasis is a founding principle of integrative physiology. In current systems biology, however, homeostasis seems almost invisible. Is homeostasis a key goal driving body processes, or is it an emergent mechanistic fact? In this perspective piece, I propose that the integrative physiological and systems biological viewpoints about homeostasis reflect different epistemologies, different philosophies of knowledge. Integrative physiology is concept driven. It attempts to explain biological phenomena by continuous formation of theories that experimentation or observation can test. In integrative physiology, "function" refers to goals or purposes. Systems biology is data driven. It explains biological phenomena in terms of "omics"-i.e., genomics, gene expression, epigenomics, proteomics, and metabolomics-it depicts the data in computer models of complex cascades or networks, and it makes predictions from the models. In systems biology, "function" refers more to mechanisms than to goals. The integrative physiologist emphasizes homeostasis of internal variables such as Pco₂ and blood pressure. The systems biologist views these emphases as teleological and unparsimonious in that the "regulated variable" (e.g., arterial Pco₂ and blood pressure) and the "regulator" (e.g., the "carbistat" and "barostat") are unobservable constructs. The integrative physiologist views systems biological explanations as not really explanations but descriptions that cannot account for phenomena we humans believe exist, although they cannot be observed directly, such as feelings and, ultimately, the conscious mind. This essay reviews the history of the two epistemologies, emphasizing autonomic neuroscience. I predict rapprochement of integrative physiology with systems biology. The resolution will avoid teleological purposiveness, transcend pure mechanism, and incorporate adaptiveness in evolution, i.e., "Darwinian medicine."

Hangover Links Nuclear RNA Signaling to cAMP Regulation via the Phosphodiesterase 4d Ortholog dunce

The hangover gene defines a cellular stress pathway that is required for rapid ethanol tolerance in Drosophila melanogaster. To understand how cellular stress changes neuronal function, we analyzed Hangover function on a cellular and neuronal level. We provide evidence that Hangover acts as a nuclear RNA binding protein and we identified the phosphodiesterase 4d ortholog dunce as a target RNA. We generated a transcript-specific dunce mutant that is impaired not only in ethanol tolerance but also in the cellular stress response. At the neuronal level, Dunce and Hangover are required in the same neuron pair to regulate experience-dependent motor output. Within these neurons, two cyclic AMP (cAMP)-dependent mechanisms balance the degree of tolerance. The balance is achieved by feedback regulation of Hangover and dunce transcript levels. This study provides insight into how nuclear Hangover/RNA signaling is linked to the cytoplasmic regulation of cAMP levels and results in neuronal adaptation and behavioral changes.

Reversible inactivation of rostral nucleus raphe pallidus attenuates acute autonomic responses but not their habituation to repeated audiogenic stress in rats

The medullary nucleus raphe pallidus (RPa) mediates several autonomic responses evoked by acute stress exposure, including tachycardia and hyperthermia. The present study assessed whether the RPa contributes to the decline/habituation of these responses observed during repeated audiogenic stress. Adult male rats were implanted with cannulae aimed at the RPa, and abdominal E-mitters that wirelessly acquire heart rate and core body temperature. After surgical recovery, animals were injected with muscimol or vehicle (aCSF) in the RPa region, followed by 30 min of 95-dBA loud noise or no noise control exposures on 3 consecutive days at 24-h intervals. Forty-eight hours after the third exposure, animals were exposed to an additional, but injection-free, loud noise or no noise test to assess habituation of hyperthermia and tachycardia. Three days later, rats were restrained for 30-min to evaluate their ability to display normal acute autonomic responses following the repeated muscimol injection regimen. The results indicated that the inhibition of cellular activity induced by the GABAA-receptor agonist muscimol centered in the RPa region reliably attenuated acute audiogenic stress-evoked tachycardia and hyperthermia, compared with vehicle-injected rats. Animals in the stress groups exhibited similar attenuated tachycardia and hyperthermia during the injection-free fourth audiogenic stress exposure, and displayed similar and robust increases in these responses to the subsequent restraint test. These results suggest that cellular activity in neurons of the RPa region is necessary for the expression of acute audiogenic stress-induced tachycardia and hyperthermia, but may not be necessary for the acquisition of habituated tachycardic responses to repeated stress.

Persistence of a hyperthermic sign-reversal during nitrous oxide inhalation despite cue-exposure treatment with and without a drug-onset cue

We asked whether chronic tolerance and the hyperthermic sign-reversal induced by repeated 60% N2O exposures could be extinguished using a cue-exposure paradigm. Rats received 18 N2O administrations in a total calorimetry system that simultaneously measures core temperature (Tc), metabolic heat production (HP), and body heat loss (HL). Each exposure entailed a 2-h baseline period followed by a 1.5-h N2O exposure. The 18 drug exposures induced a robust intra-administration hyperthermia in which the initial hypothermic effect of N2O inverted to a significant hyperthermic sign-reversal during N2O inhalation due primarily to an acquired robust increase in HP. The rats were then randomized to one of three extinction procedures (n=8/procedure) over a 20-d interval: 1) a N2O-abstinent home-cage group (HC) that received only the usual animal care; 2) a cue-exposure group (CEXP) in which the animals were placed in the calorimeter 8 times but received no N2O; and 3) a drug-onset-cue group (DOC) in which animals received a brief N2O exposure in the calorimeter that mimicked the first 3 min of an actual 60% N2O trial. Following the extinction sessions, all rats received a 60% N2O test trial and Tc, HP and HL were assessed. The hyperthermic sign-reversal remained fully intact during the test trial, with no significant differences observed among groups in any post-baseline change in any thermal outcome. These data suggest that cue exposure may not be an efficacious strategy to reduce sign-reversals that develop with chronic drug use.

Assessment of heat production, heat loss, and core temperature during nitrous oxide exposure: a new paradigm for studying drug effects and opponent responses

Studies using core temperature (T(c)) have contributed greatly to theoretical explanations of drug tolerance and its relationship to key features of addiction, including dependence, withdrawal, and relapse. Many theoretical accounts of tolerance propose that a given drug-induced psychobiological disturbance elicits opponent responses that contribute to tolerance development. This proposal and its theoretical extensions (e.g., conditioning as a mechanism of chronic tolerance) have been inferred from dependent variables, such as T(c), which represent the summation of multiple underlying determinants. Direct measurements of determinants could increase the understanding of opponent processes in tolerance, dependence, and withdrawal. The proximal determinants of T(c) are metabolic heat production (HP) and heat loss (HL). We developed a novel system for simultaneously quantifying HP (indirect calorimetry), HL (direct gradient layer calorimetry), and T(c) (telemetry) during steady-state administrations of nitrous oxide (N(2)O), an inhalant with abuse potential that has been previously used to study acute and chronic tolerance development to its hypothermia-inducing property. Rats were administered 60% N(2)O (n = 18) or placebo gas (n = 16) for 5 h after a 2-h placebo baseline exposure. On average, N(2)O rapidly but transiently lowered HP and increased HL, each by approximately 16% (P < 0.001). On average, rats reestablished and maintained thermal equilibrium (HP = HL) at a hypothermic T(c) (-1.6 degrees C). However, some rats entered positive heat balance (HP > HL) after becoming hypothermic such that acute tolerance developed, i.e., T(c) rose despite continued drug administration. This work is the first to directly quantify the thermal determinants of T(c) during administration of a drug of abuse and establishes a new paradigm for studying opponent processes involved in acute and chronic hypothermic tolerance development.

Flies, genes, and learning

Flies can learn. For the past 25 years, researchers have isolated mutants, engineered mutants with transgenes, and tested likely suspect mutants from other screens for learning ability. There have been notable surprises-conventional second messenger systems co-opted for intricate associative learning tasks, two entirely separate forms of long-term memory, a cell-adhesion molecule that is necessary for short-term memory. The most recent surprise is the mechanistic kinship revealed between learning and addictive drug response behaviors in flies. The flow of new insight is likely to quicken with the completion of the fly genome and the arrival of more selective methods of gene expression.

Mechanisms and mediators of psychological stress-induced rise in core temperature

OBJECTIVE

Despite numerous case reports on "psychogenic fever," it remains uncertain how psychological stress raises core temperature and whether the rise in core temperature is a real fever or a hyperthermia. This article reviews studies on the psychological stress-induced rise in core temperature (PSRCT) in animals with the aim to facilitate studies on the mechanisms of so-called psychogenic fever in humans.

METHODS

To address this question, we reviewed the mechanisms and mediators of the PSRCT and classic conditioning of the fever response in animals.

RESULTS

The PSRCT is not due to the increased locomotor activity during stress, and the magnitude of the PSRCT is the same in warm and cold environments, indicating that it is a centrally regulated rise in temperature due to an elevated thermoregulatory "set point." The PSRCT caused by conventional psychological stress models, such as open-field stress, is attenuated by cyclooxygenase inhibitors, which block prostaglandin synthesis. On the other hand, the PSRCT elicited by an "anticipatory anxiety stress" is not inhibited by cyclooxygenase inhibitors but by benzodiazepines and serotonin Type 1A receptor agonists. The febrile response can be conditioned to neutral stimuli after paired presentation with unconditioned stimuli such as injection of lipopolysaccharide, a typical pyrogen.

CONCLUSIONS

Most findings indicate that the PSRCT is a fever, a rise in the thermoregulatory set point. The PSRCT may occur through prostaglandin E2-dependent mechanisms and prostaglandin E2-independent, 5-HT-mediated mechanisms. The febrile response can be conditioned. Thus, these mechanisms might be involved in psychogenic fever in humans.

Functional ethanol tolerance in Drosophila

In humans, repeated alcohol consumption leads to the development of tolerance, manifested as a reduced physiological and behavioral response to a particular dose of alcohol. Here we show that adult Drosophila develop tolerance to the sedating and motor-impairing effects of ethanol with kinetics of acquisition and dissipation that mimic those seen in mammals. Importantly, this tolerance is not caused by changes in ethanol absorption or metabolism. Rather, the development of tolerance requires the functional and structural integrity of specific central brain regions. Mutants unable to synthesize the catecholamine octopamine are also impaired in their ability to develop tolerance. Taken together, these data show that Drosophila is a suitable model system in which to study the molecular and neuroanatomical bases of ethanol tolerance.

Nitrous oxide-induced hypothermia in the rat: acute and chronic tolerance

Although inhalation of nitrous oxide (N2O) causes hypothermia in rats, there is a paucity of information as to whether tolerance develops to this effect. The purpose of this study was to determine whether tolerance to N2O hypothermia develops within a single administration as well as over repeated administrations. Temperature was measured telemetrically by implanting intraperitoneal thermal sensors/transmitters in male Long-Evans rats. Experimental rats received an initial 2-h exposure to 60% N2O and became hypothermic relative to controls breathing placebo gas. Only a few rats demonstrated evidence of acute tolerance over the 120 min. Over the next 10 days, the experimental rats received five additional 30-min exposures to 60% N2O and five 30-min exposures to placebo while the control rats received only placebo gas exposures. Chronic tolerance developed to N2O hypothermia over these repeated administrations. A test for Pavlovian drug conditioning found no evidence that conditioned temperature effects contributed to chronic tolerance development. In a second experiment, naive rats were given a 380-min exposure to 60% N2O and a 380-min exposure to placebo gas in a counterbalanced order. Acute tolerance did develop to N2O hypothermia, with the recovery of temperature beginning after a mean of 141 min of gas administration. Hence, both acute and chronic tolerance develop to N2O's hypothermic effects in rats.

The effects of preexposure to the drug on state dependent retention

The present experiment examined whether previous experience with a drug would decrease the potential of the drug to produce state dependent retention (SDR) for a passive avoidance response in rats. In the first experiment, a single injection of sodium pentobarbital (20 mg/kg) given on six consecutive days before the training day slightly reduced, but did not block, pentobarbital-induced SDR. In Experiment Two, four preexposure injections of 5 IU/kg insulin reduced the magnitude of memory loss produced by administration of the hormone prior to training. As with pentobarbital, however, preexposure to insulin did not completely block the amnestic effect of the hormone. A subsequent experiment demonstrated that the decrease in the strength of insulin-induced SDR in insulin preexposed rats was not the result of enhanced acquisition. Collectively, these data indicate that noncontingent preexposure to an amnestic treatment may decrease the magnitude of memory loss that would normally result from the administration of that treatment during training.

Cellular and learned tolerance to pentobarbital ataxia

Pentobarbital was administered to 4 groups of rats: 1) intermittently before testing on the rotarod (RR) (experienced, EXP), 2) chronically (CHR) before testing on the RR (EXP), 3) intermittently (INT) after being tested on the RR (NONEXP), and 4) chronically (CHR) after being tested on the RR (NONEXP). On postchronic testing, Group 1 (INT/EXP) failed to show tolerance to the RR decrement, related to prechronic scores, while Group 3 (INT/NONEXP) actually showed an enhanced RR decrement. Group 2 (CHR/EXP) and Group 4 (CHR/NONEXP) both exhibited prominent tolerance to RR impairment at the postchronic test, with a nonsignificant trend for greater tolerance in Group 2. The lack of an expressed behavioral tolerance in INT/EXP rats and the enhanced RR decrement in INT/NONEXP subjects at the postchronic test was attributed to repeated use of a towel wrap restraint during the chronic treatment period. When the prechronic tests for INT/EXP animals were separated into the first 3 and last 3 days, pentobarbital impairment of RR during days 4-6 was significantly less than during days 1-3. This tolerance in INT/EXP rats was lost at the postchronic testing, while INT/NONEXP subjects had by then developed an enhanced RR impairment to pentobarbital. Following postchronic testing, chronic pentobarbital (CHR/EXP and CHR/NONEXP groups) and chronic vehicle (INT/EXP and INT/NONEXP groups) were discontinued for 9 days (withdrawal), after which an intermediate dose of the drug was tested on RR performance. Next, 9 days of extinction training involved vehicle injection daily before testing RR performance, after which the intermediate drug dose was again tested. INT/EXP and INT/NONEXP groups showed no change in RR impairment at the postwithdrawal and postextinction tests. However, in CHR/EXP rats pentobarbital tolerance was partly lost at the postwithdrawal test, with a significantly greater loss at the postextinction test.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective cross-tolerance to 5-HT1A and 5-HT2 receptor-mediated temperature and corticosterone responses

The repeated administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 3 mg/kg, twice daily for 14 days) significantly diminished hypothermia and corticosterone secretion induced by an acute challenge with the 5-HT1A agonist 8-OH-DPAT (0.1 mg/kg) when compared to the responses in animals treated chronically with the solvent vehicle. In contrast, the chronic administration of 5-MeODMT did not alter the magnitude of hyperthermia or corticosterone secretion induced by the acute administration of MK-212 (1.0 mg/kg). The repeated administration of the 5-HT2 agonist DOI (1.0 mg/kg, daily for 7 days) significantly reduced the increase in corticosterone, but not body temperature, produced by MK-212. Chronic treatment with DOI did not alter the hypothermia or increase in corticosterone secretion elicited by 8-OH-DPAT. These data are consistent with other evidence that these physiological effects of 8-OH-DPAT and MK-212 are mediated by 5-HT1A and 5-HT2 receptors, respectively. Thus, data presented in these studies are suggestive that the chronic administration of 5-MeODMT diminishes the responsiveness of 5-HT1A receptor-mediated changes in body temperature and corticosterone secretion without altering the responses mediated by 5-HT2 receptors. In contrast, the chronic administration of DOI selectively diminishes the magnitude of 5-HT2 receptor-mediated changes in corticosterone secretion without affecting the responsiveness of those receptors involved in thermoregulatory responses. These selective changes in receptor responsiveness following the chronic administration of these 5-HT agonists further establishes the independence of 5-HT1A and 5-HT2 receptor-mediated pharmacological effects.

Cholecystokinin-octapeptide-induced hypothermia in rats: dose-effect and structure-effect relationships, effect of ambient temperature, pharmacological interactions and tolerance

Subcutaneous injection of cholecystokinin octapeptide (CCK-8) (0.005-1.25 mg/kg) elicited dose-dependent hypothermia in rats. The threshold of the response was between 0.01 and 0.05 mg/kg and the dose-response curve levelled off at doses larger than 0.2-0.5 mg/kg. Warm and cold ambient temperatures decreased and increased the response, respectively. Pretreatment with capsaicin, morphine, naloxone, atropine, haloperidol or propranolol did not affect the response to CCK-8, whereas pretreatment with phenoxybenzamine and a large dose of proglumide, an antagonist for CCK-receptors, attenuated the hypothermia. It seems that neither capsaicin-sensitive thermal and non-thermal afferents, nor opiate mechanisms are involved in the response, but alpha-adrenoceptors might be of some importance in the hypothermia. Non-sulphated-CCK-8, the C-terminal tetrapeptide and hexapeptide, [D-Ala4]-CCK-8 and [D-Met6]-CCK-8 were ineffective. Chronic treatment with CCK-8 resulted in the development of tolerance to the thermoregulatory effect, while the hypothermic responses to apomorphine and capsaicin were not affected. It seems that the tolerance cannot be attributed to conditioned homeostatic reactions.

Learned anticipatory rise in body temperature due to handling

Hyperthermia produced by handling becomes evident at the initial daily measurement if temperature is measured at a consistent time. This hyperthermia may be a learned effect occurring in anticipation of handling. In Experiment One male Wistar rats were either unhandled or had their temperatures measured daily in the dark or the light part of the day. All animals had their temperatures measured on Day 29, in the dark. Rats usually tested in the dark were hyperthermic, 38.8 degrees C, relative to rats previously handled only in the light, 38.1 degrees C, and to naive rats, 37.9 degrees C. In Experiment Two rats were handled three times daily in either the light or the dark. On Day 9 each group was divided in two, and temperatures were measured at either the usual time or at the other time. Rats tested at their usual time were hyperthermic, relative to rats normally handled in the other part of the cycle. This suggests a conditioned hyperthermia occurs in response to stimuli predictive of handling.

Dissociation of tolerance to the hypothermic and tachycardic effects of ethanol

Tolerance to the cardioacceleratory and hypothermic effects of ethanol was studied in unanesthetized, freely-moving rats surgically implanted with EKG electrodes and biotelemetric temperature sensors. Different groups received 0.0, 1,0 or 2.0 g ethanol/kg body weight in injections given every other day for a total of nine injections. Heart rate and body temperature were recorded for 1 hr before and 2 hr after each injection. Ethanol initially induced a monophasic dose-related cardioacceleration (80 bpm) and hypothermia (1.0 degrees C) that persisted throughout the 2-hr sample period. Tolerance developed to the hypothermic, but not to the tachycardic effect of ethanol. Assuming that tolerance depends on level of impairment in specific neuronal pathways, this outcome suggests that these two effects of ethanol are not mediated through a common autonomic mechanism (e.g., vasomotor depression) and/or that tolerance to the hypothermic effect is due to alterations in pathways unique to the thermoregulatory system. Overall, the finding is consistent with those of studies showing development of tolerance to depressant, but not to excitatory drug effects.