Cold and immobilization stress-induced changes in pain responsiveness and brain Met-enkephalin-like immunoreactivity in the rat.
Pain responsiveness and Met-enkephalin-like immunoreactivity (MLI) were studied in the rat after cold (25 degrees C) and immobilization stress for different lengths of time (30 min, 90 min and 180 min). The 30-min stress-induced analgesia (as measured by the tail-flick method), which was partially antagonized by pretreatment with naloxone (5 mg/kg, SC), and the magnitude of this analgesia was less than that of the 90-min stress or 180-min stress. The 30-min stress resulted in a significant decrease in MLI in the mesolimbic area, striatum, hypothalamus and thalamus. After the 90-min stress, MLI was found to be significantly decreased in the prefrontal cortex, amygdaloid nuclei and piriform cortex, thalamus and hippocampus, while the 180-min stress failed to induce a significant change in any brain area tested. As such a change in MLI content was thought to be related to an increase in the activity of the endogenous Met-enkephalin neuronal system, the activation of this system by the stress, especially in the hypothalamus and thalamus, seemed to be associated with analgesia.
D000698:Analgesia; D000818:Animals; D001921:Brain / Q000378:metabolism*; D003080:Cold Temperature; D004744:Enkephalin, Methionine / Q000378:metabolism*; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D010146:Pain / Q000503:physiopathology*; D051381:Rats; D011919:Rats, Inbred Strains; D012149:Restraint, Physical; D013312:Stress, Physiological / Q000503:physiopathology*; D013315:Stress, Psychological / Q000503:physiopathology; D014018:Tissue Distribution
D009270:Naloxone; D004744:Enkephalin, Methionine