Repeated stress effects on nociception and on ectonucleotidase activities in spinal cord synaptosomes of female rats.

It has been reported that animals submitted to repeated restraint stress present various adaptation responses which are dependent on the sex. These adaptations include changes in nociception and adenine nucleotide hydrolysis. In this study, we report the effect of chronic administration of a gonadal steroid (17beta-estradiol) on ATP, ADP and AMP hydrolysis in spinal cord synaptosomes of adult ovariectomized (OVX) Wistar rats submitted to repeated restraint stress over 40 days. We also measured nociceptive threshold in these animals using the tail-flick test. The results show that tail-flick latencies were decreased in both stressed groups, OVX and OVX rats receiving estradiol replacement therapy, indicating reduced nociceptive threshold after exposure to repeated stress. Repeated restraint stress caused no effect on ATPase or ADPase activities. On the other hand, AMP hydrolysis in spinal cord synaptosomes from repeatedly stressed rats was decreased in OVX rats compared to non-stressed OVX ones, indicating reduced extracellular adenosine production; this effect was reversed by hormonal replacement. These observations suggest that nociceptive sensitivity to noxious stimuli is affected by repeated stress and that modulation of neurotransmission by adenine nucleotides in spinal cord may be altered by the interaction of sexual hormones and psychological factors, such as exposure to stress.

Taste modulation of nociception differently affects chronically stressed rats.

Stress responses cover a wide range of physiological changes, including alterations in the perception of and response to pain. Animals submitted to repeated stress present altered nociception and this effect is part of this process of adaptation; in addition pleasant and unpleasant experiences with tastes and odors have been shown to affect distinct behavioral aspects, such as pain perception. The aim of the present study is to verify the responses of repeatedly stressed rats (1 h of daily immobilization during 40 days) to pleasant and unpleasant tastes on nociception, when compared to control animals. An increase in the tail-flick latency (TFL) was observed 5 min after exposure to a sweet taste in the control group, whereas no effect was observed in chronically stressed animals. When submitted to an unpleasant taste (5% acetic acid), the chronically stressed group presented an increase in TFL, whereas no effect was observed in the control group. In conclusion, chronically stressed animals present different nociceptive responses to sweet and acid tastes; although control animals suitably respond to a sweet stimulus, stressed animals seem to be more apt to react to the unpleasant stimulus.