INTRACEREBRAL METHIONINE-ENKEPHALIN, SERUM CORTISOL, AND SERUM BETA-ENDORPHIN DURING ACUTE EXPOSURE OF SHEEP OR PHYSICAL OR ISOLATION STRESS

In vivo microdialysis was used to estimate extracellular concentrations of methionine-enkephalin in 19 brain sites for 5 h on each of three consecutive days (trials) in six conscious ewes. Following control procedures on d 1, ewes were completely isolated from other sheep for 60 min on d 2 (psychological stress). Physical stress was imposed on d 3 and consisted of continuous pinching of the skin for 60 min during the middle of the 5-h experimental period. Imposition of both physical and psychological stress rapidly increased serum concentrations of cortisol, and the induced increase persisted for at least 30 min after termination of the stress. Psychological stress of isolation initially increased cortisol to a greater extent than the physical stress of skin pinch, but this difference disappeared after 30 min of stress exposure. Psychological stress also transiently increased serum concentrations of beta-endorphin/beta-lipotropin, whereas physical stress did not. Average concentrations of methionine-enkephalin in dialysate ranged between 1.52 and 1.85 ng/mL when the intracerebral probes were placed into the caudate nucleus, the preoptic area of the hypothalamus, or the thalamus. The concentration of methionine-enkephalin was consistently less than 1.0 ng/mL when probes were placed into major fiber tracts of the brain (corpus callosum, internal capsule). Potassium-induced depolarization around the probe tip located in the caudate nucleus increased dialysate concentrations of methionine-enkephalin by 2.7-fold. Imposition of physical or psychological stress did not consistently increase or decrease dialysate concentrations of methionine-enkephalin in any of the brain sites studied. In summary, stress was reliably induced in sheep, and extracellular concentrations of methionine-enkephalin in local brain areas were estimated by microdialysis. However, neither psychological nor physical stress affected concentrations of methionine-enkephalin within the brain.