Interaction between enkephalinergic and dopaminergic systems in stressful situations.

Mice exhibit a marked suppression of mobility when they are placed in the same cage in which they had previously received an electric shock. This suppression of motility is believed to be a stress-induced response. [D-Ala2,Met5]enkephalinamide (DAMEA, 40 and 80 nmol i.c.v.), a derivative resistant to [Met5]enkephalin-degrading enzyme activity, significantly attenuated the conditioned suppression of motility. The attenuation of conditioned suppression of motility induced by DAMEA was antagonized not only by naloxone (0.1 mg/kg s.c.), but also by 6-hydroxydopamine (6-OHDA, 100 micrograms/mouse, i.c.v.)-induced lesions of dopaminergic (DAergic) neurons. Pimozide (100 micrograms/kg i.p.) and a low dose of apomorphine (50 micrograms/kg s.c.) also inhibited the effect of DAMEA. In addition, DAMEA reversed the decrease in DA turnover in the striatum in the conditioned suppression group. These results suggest that the attenuation of conditioned suppression of motility induced by activation of the [Met5]enkephalinergic system may be related to the striatal DAergic system.