Role of diurnal variation and receptor specificity in the opioidergic regulation of food intake in free-fed and food-deprived rats

Mood state effects of chocolate

BACKGROUND

Chocolate consumption has long been associated with enjoyment and pleasure. Popular claims confer on chocolate the properties of being a stimulant, relaxant, euphoriant, aphrodisiac, tonic and antidepressant. The last claim stimulated this review.

METHOD

We review chocolate's properties and the principal hypotheses addressing its claimed mood altering propensities. We distinguish between food craving and emotional eating, consider their psycho-physiological underpinnings, and examine the likely 'positioning' of any effect of chocolate to each concept.

RESULTS

Chocolate can provide its own hedonistic reward by satisfying cravings but, when consumed as a comfort eating or emotional eating strategy, is more likely to be associated with prolongation rather than cessation of a dysphoric mood.

LIMITATIONS

This review focuses primarily on clarifying the possibility that, for some people, chocolate consumption may act as an antidepressant self-medication strategy and the processes by which this may occur.

CONCLUSIONS

Any mood benefits of chocolate consumption are ephemeral.

Antinociceptive profile of salvinorin A, a structurally unique kappa opioid receptor agonist

Salvinorin A, is a structurally unique, non-nitrogenous, kappa opioid receptor (KOP) agonist. Given the role of KOPs in analgesic processes, we set out to determine whether salvinorin A has antinociceptive activity in thermal and chemo-nociceptive assays. The tail-flick assay was employed to investigate 1) salvinorin A's (0.5, 1.0, 2.0, and 4.0 mg/kg) dose-response and time-course (10, 20, and 30 min) effects in a thermal nociceptive assay, and 2) the ability for the KOP antagonist norBNI (10.0 mg/kg) to prevent salvinorin A antinociception. The hotplate assay was utilized as a second thermal nociceptive measure to test salvinorin A's dose-response effects. The acetic acid abdominal constriction assay was used to study salvinorin A's dose-response and time-course (over 30 min) effects in a chemo-nociceptive assay. Together, these studies revealed that salvinorin A produces a dose-dependent antinociception that peaked at 10 min post-injection but rapidly returned to baseline. Additionally, pretreatment with the KOP antagonist norbinaltorphimine (norBNI) reversed salvinorin A-induced antinociception. These findings demonstrate that salvinorin A produces a KOP mediated antinociceptive effect with a short duration of action.

Naloxone suppression and morphine enhancement of voluntary wheel-running activity in rats

This study examines the effects of the opioid receptor antagonist, naloxone, and the agonist, morphine, on voluntary wheel-running activity (WR) in rats. Male Sprague-Dawley rats were given 1-h access to a running wheel under non-deprived conditions. Naloxone injections (1.0, 0.5, or 0.25 mg/kg, ip), administered immediately before access to running wheels, dose-dependently suppressed WR. In another experiment, subjects were given 6-h access to running wheels under nondeprived conditions for 5 consecutive days. Morphine injections (2.0 mg/kg, sc) were found to increase WR after an initial suppression. These data demonstrate that naloxone inhibits WR, while morphine both suppresses and enhances WR depending on time and dose. These are in agreement with data on other behaviors that indicate that endogenous opioid systems play a major role in the mediation of motivational behaviors.

Food cravings, endogenous opioid peptides, and food intake: a review

Extensive research indicates a strong relationship between endogenous opioid peptides (EOPs) and food intake. In the present paper, we propose that food cravings act as an intervening variable in this opioid-ingestion link. Specifically, we argue that altered EOP activity may elicit food cravings which in turn may influence food consumption. Correlational support for this opioidergic theory of food cravings is provided by examining various clinical conditions (e.g. pregnancy, menstruation, bulimia, stress, depression) which are associated with altered EOP levels, intensified food cravings, and increased food intake.

Differential effects of intrahypothalamic administration of opioids on food intake in naive and tolerant rats

We investigated the effects of intrahypothalamic administrations of the opioid agonists morphine (MOR) and ketocyclazocine (KCZ) and antagonists naltrexone (NALTX) and Mr2266 on food intake (FI) during light and dark phases of the diurnal cycle, after acute or chronic administration in rats. Acute intralateral hypothalamic (LH) administration of MOR or KCZ (1 microgram/rat) enhanced FI during dark and light phases, respectively, whereas intraventromedial hypothalamic (VMH) injections resulted in moderate hyperphagia during dark phases by both mu and kappa agonists. The receptor specificity was evident from blockade of the responses to MOR or KCZ by the respective antagonists NALTX and Mr2266. After repeated administrations of MOR and KCZ, FI responses to the test dose of these agonists injected in LH were modulated in opposite directions. However, the adaptative changes in FI after intra-VMH injection of KCZ were similar to those seen with MOR. These results are discussed in light of a differential opioid receptor involvement and their possible functional interactions within the hypothalamus during food intake.

Effects of intrahypothalamic morphine and its interactions with oxytocin and vasopressin during food intake in rats

The effects of intra-hypothalamic microinjections of morphine (MOR) into the ventromedial (VMH) and lateral (LH) nucleus and its interactions with oxytocin (OXY) and vasopressin (AVP) were evaluated during food intake FI) in rats. Acute MOR clearly enhanced FI during the dark phase (6-24 h) of the diurnal cycle, while no appreciable changes in the light phase (0-6 h) FI was seen after intra-VMH or LH injections. Long term MOR administration into both VMH and LH attenuated the enhancements in FI seen after acute MOR during the dark phase, while FI was enhanced during the light phase. Pretreatment with OXY or AVP clearly antagonized such tolerance or reverse tolerance/sensitization development to the orexic effect of MOR. These results are discussed in the light of complex peptidergic interactions in the hypothalamus during ingestive behavior.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.