Control of food delivery in food-deprived rats mediates analgesia

Chronic food restriction enhances memory in mice--analysis with matched drive levels

We compared the effects of chronic and acute food deprivation on learning and memory using a dry-type water maze, active avoidance and passive avoidance in C57BL/6L mice. The drive level of the animals--under acute and chronic food deprivation--was matched by a progressive ratio schedule. Both deprivations led to a high degree of activity in the animals; however, the animals on an acute dietary restriction did not exhibit a significantly better performance than those on ad libitum feeding, while those on a chronic food deprivation exhibited memory enhancement. These effects were subtle and were found at a later stage of learning. These findings suggest that chronic food restriction induces memory consolidation or resistance to memory reduction in addition to increased activity.

Blunted prolactin response to fentanyl in depression. Normalizing effect of partial sleep deprivation

There is some evidence that sleep deprivation (SD) might exert its antidepressant properties by involving endogenous opioid mechanisms. The authors investigated the effects of mu-receptor agonist administration on prolactin release in depressed patients before and after partial SD. Medication-free female depressed inpatients (N=18) were participating in two fentanyl challenge tests after partial SD and undisturbed sleep, 3 days apart in random order. Healthy volunteer women (N=10) were enrolled after full night sleep as comparison subjects. Five of them had placebo trials. Participants were given an intravenous injection of 0.1 mg/70 kg fentanyl at 9:00 AM. The prolactin secretory response to the opiate agonist was investigated for 1 h with serial blood sampling. After a night of undisturbed sleep, fentanyl administration prompted increases in plasma prolactin concentrations with blunted responses found in the depressed group. Following partial SD, the stimulated prolactin secretion of depressed patients increased significantly and was comparable to the response of comparison subjects. These findings suggest that SD acts via an opioid/dopamine-related mechanism. An alternative explanation, based on serotonin involvement is addressed in the discussion.

The effects of uncontrollable, unpredictable aversive and appetitive events: similar effects warrant similar, but not identical, explanations?

Exposure to uncontrollable, unpredictable appetitive events produces a variety of cognitive debilitations and vegetative changes, as does exposure to uncontrollable, unpredictable aversive events. Similarities include impaired escape from aversive events, impaired discrimination, finicky consumption, analgesia, and body weight loss. However, in stark contrast, uncontrollable aversive stress causes reduced motor activity where as similar appetitive treatment does not; aversively induced debilitation is causally related to energy regulation, whereas the appetitively induced effects are not. Parallel mechanisms are suggested to explain these effects in terms of a revised anxiety account of the aversive effects, and a frustration account of the appetitive effects. Finally, factors likely to limit important research to resolve the many remaining issues are identified: negative presentation of animal research, political decision making, and ignorance and fear in committees which review the ethics of research.

The role of hypophyseal and adrenal mechanisms in the hypoalgesic response to non-contingent food delivery in the rat

Exposure to non-contingent food delivery has been shown to elicit an increase in nociceptive thresholds in rats. This hypoalgesia has previously been shown to be reversible by the opiate antagonist naloxone. Given that most opioid forms of SIA are found to rely on an intact pituitary-adrenal axis, this research examined the possibility that the hypoalgesic response to non-contingent food delivery is also hormonally mediated. Hypophysectomy (Expt. 1) but not bilateral adrenalectomy (Expt. 2) was found to completely attenuate the hypoalgesic response to non-contingent food delivery. Preliminary data from ongoing research is presented that points to the possible involvement of the hypophyseal peptides beta-endorphin and adrenocorticotropin hormone (ACTH).

Interactions between vasopressin and food restriction on stress-induced analgesia

To investigate the role of the hormone vasopressin (VP) in mediating the response of an organism to food restriction stress-induced analgesia, tail flick latencies and scored qualitative behavioral responses were recorded in VP-containing (LE) rats and VP-deficient (DI) rats. These variables were measured under nonstressed (ad lib) and stressed (food restriction) conditions. In the ad lib condition, DI and LE rats had a similar tail flick latency and scored qualitative behavioral response to the stimulus eliciting the tail flick. During the food restriction condition, however, LE animals developed significant food restriction stress-induced analgesia, as measured by tail flick latency. On the other hand, DI animals did not develop significant analgesia. In addition, DI animals exhibited a significantly greater scored qualitative behavioral response to the stimulus eliciting the tail flick than LE animals. These results demonstrate that VP plays an important role in the regulation of food restriction stress-induced analgesia, as well as the scored qualitative behavioral response elicited by the tail flick stimulus.

The impact of inter-pellet interval and polydipsia on hypoalgesia elicited by non-contingent food delivery

Exposure to non-contingent food delivery has been shown to elicit an increase in nociceptive thresholds in rats. The conditions which elicit analgesia are similar to those that elicit schedule-induced polydipsia. In both instances animals are food-deprived and receive food on an intermittent schedule. Interpellet interval has been found to be an important predictor of schedule-induced polydipsia. Experiment 1 therefore investigated the effect of varying the interval between pellet deliveries on tail flick latencies in rats. The relationship between nociceptive threshold and inter-pellet interval was found to be bitonic in nature given that animals submitted to fixed time schedules of 30 or 60 s, but not 15 or 120 s, exhibited significant increases in tail flick latencies. Experiment 2 examined the effect of providing animals with water during exposure to non-contingent food delivery. Under these conditions animals exhibited polydipsic behaviour, the development of which attenuated the hypoalgesic response to non-contingent food delivery.

The economic context of drug and non-drug reinforcers affects acquisition and maintenance of drug-reinforced behavior and withdrawal effects

The focus of this review is to examine the effect of non-drug alternative reinforcers on drug-reinforced behavior. An increasing number of animal laboratory as well as human clinical studies have demonstrated the effectiveness of non-drug reinforcers in reducing steady-state levels of drug self-administration. One goal of this review was to determine what behavioral economic conditions are optimal for reducing drug-reinforced behavior. Variables such as price of the drug and non-drug reinforcer have been manipulated by changing fixed-ratio (FR) value of these commodities. Income has been changed by limiting the amount of access to the commodities or by changing session length. Substitution was evaluated by determining whether decreased demand for a drug (due to increased price) was related to increased demand for a non-drug reinforcer. A second goal of this review was to investigate transition states in the drug addiction process with respect to the role of alternative non-drug reinforcers. Animal models of acquisition and withdrawal were examined to identify behavioral economic conditions under which acquisition may be prevented or withdrawal effects (and potential for relapse) may be alleviated.