Analgesic response to stress is reduced in perinatally undernourished rats

Mildly negative social encounters reduce physical pain sensitivity

While previous research has demonstrated a reduction in physical pain sensitivity in response to social exclusion, the manipulations employed have arguably been far removed from typical daily experience. The purpose of this study was to investigate the effects of relatively ordinary social encounters on the perception of pain. Healthy participants rated the intensity and unpleasantness of painful stimuli before and after engaging in a structured interaction with a confederate who was instructed to either be warm and friendly or indifferent. A control group was asked to perform a similar structured activity, but alone. Consistent with predictions, participants who experienced the mildly negative social exchange reported lower pain intensity and unpleasantness after the encounter relative to baseline, whereas those exposed to the positive social exchange did not evidence any change in pain ratings. These results were not mediated by changes in mood or perceived connectedness. If mildly negative social encounters can provoke an analgesic effect, it is possible that social hypoalgesia may be considerably more commonplace than previously realized. Discussion focuses on the role of stress-induced hypoalgesia, and the implications of the results for clinical assessments of pain.

Oxidative stress induced by morphine in brain of rats fed with a protein deficient diet

The objective of the study is to determine the damage by oxidative stress induced by morphine in brain of rats fed with a protein-deficient diet. Twenty-eight malnourished male Wistar rats, 30 days old, were used in the study. The animals were divided into four groups of 7 rats per group. Group I received NaCl and the groups II; III and IV intraperitoneally received 3, 6 and 12 mg/kg of morphine sulphate, respectively, in a single dose. Animals were sacrificed and the levels of glutathione (GSH), dopamine, tryptophan and 5-hydroxyindole-3-acetic acid (5-HIAA) as well as, Na(+)/K(+) ATPase and total ATPase activity in the brain were measured. Tryptophan levels and Na(+)/K( +) ATPase activity showed non-significant changes in the experimental group. Levels of 5-HIAA decreased significantly (p = .03) in animals that received 12 mg/kg of morphine and in animals that received 3 mg/kg, levels of GSH and dopamine were found to have a significant decrease (p < .05), but a significant increase in the group that received 12 mg/kg of morphine (p < .05). Total ATPase activity increased significantly in the groups that received 3 mg/kg (p = .015) and 6 mg/kg (p = .0001) of morphine. The results show that malnutrition induces changes in cellular regulation and biochemical responses to oxidative stress caused by morphine sulphate.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Perinatal undernutrition: changes in brain opiate receptor density

The present work sought to study the binding properties of central mu-opiate receptors in whole brain and in different central areas in adult rats undernourished at perinatal age. Rats were undernourished with a hypoproteic diet containing 8% casein from day 14 of gestation until 50 days of age. The animals were thereafter fed a balanced commercial chow until 140 days of age. At this time point the experiments started. 3H-D-Ala2, N-Me-Phe4, Gly5-ol-enkephalin (3H-DAMGO) was used to selectively label the mu-receptors. The results obtained demonstrated that perinatal undernutrition induced, in the adult animal, a decreased mu-receptors density (Bmax) both in whole brain as well as in midbrain, without significant changes in affinity. In addition, no changes were found in mu-specific binding in the cortex of these undernourished animals. Taking into account that recent evidences from our laboratory have demonstrated a lower stress-induced analgesia following exposure to different stressful situations in rats undernourished in early life, the present findings seem to suggest that this lower analgesic response could be due, at least in part, to a lower density of mu-opiate receptors in the brain.

Influence of prenatal protein malnutrition on behavioral reactivity to stress in adult rats

In Experiment 1, adult prenatally protein malnourished and well-nourished male and female rats were tested in an open field after having been subjected to a 15-day regimen of varied uncontrollable and inescapable mild stress (experimental group). Their responses were compared with rats that had not been subjected to the stress regimen (control group). In the control group, females with a history of prenatal malnutrition made significantly fewer entries into the center of the arena than did well-nourished females, suggesting that baseline differences in anxiety exist between the two nutritional groups of females. In addition to open field, die experimental group of animals was also tested in a forced swim test conducted at the beginning (Day 5) and at the end of the stress regimen (Day 15). Significant differences were observed between nutritional groups on Day 15 only: prenatally malnourished males exhibited a lower latency to immobility than well-nourished males, whereas the opposite effect was found in malnourished females. In Experiment 2, separate groups of males were exposed to forced swim on two different occasions without the stress regimen between exposures. A somewhat different pattern of findings was generated. There was no significant difference in the latency to immobility between malnourished and well-nourished rats on the second forced swim. However, malnourished animals showed greater total immobility than the well-nourished controls in the second exposure to forced swim, providing further support for the interpretation that the malnourished males were less affected than well-nourished ones, or adapted more readily to the stress regimen in Experiment 1. Overall these results suggest that the relationship between prenatal malnutrition and stress depends on the level of stress (acute vs. chronic), the type of behavioral measure used to assess its effects, as well as gender.