A kappa-selective opioidergic pathway is involved in the reversal of a behavioural effect of adrenalectomy.

We have shown previously that adrenalectomized rats are immobile for only approximately 30% of a 5 min retest period, 24 h after an initial 15 min swimming exposure, compared with approximately 70% immobility for intact animals. The administration of ketocyclazocine, dynorphin-(1-17) or [Met5]enkephalin[Arg6,Phe7] immediately after initial exposure reversed the effect of adrenalectomy, whereas equimolar doses of morphine sulphate, [D-Ala2,D-Leu5]enkephalin and dynorphin-(1-8) were inactive. MR2266, a kappa-selective partial agonist/predominant antagonist, did not reverse immobility but antagonized the reversing effect of ketocyclazocine. In conjunction with our previous studies, we interpret these data to show that one of the opioidergic pathways for the incorporation of information post-stress is kappa-selective. This kappa selectivity in turn suggests a possible physiological role for prodynorphin-derived peptides in memory.