Strain dependent effects of the enkephalin analogue FK 33-824 on locomotor activity in mice.

Administration of the enkephalin analogue FK 33-824 was followed in the DBA/2 (DBA) strain by dose related depressant effects which, after the injection of 40 mg/kg, were still present 6 hrs after treatment; on the contrary, in the C57BL/6 (C57) strain, activity levels were enhanced by treatment, except for the dose of 40 mg/kg, which induced a short lasting behavioral stimulation followed by activity depression and catatonia and later on again by behavioral stimulation. All the effects of FK were naloxone reversible. Cross tolerance between FK and morphine was moreover observed both as concerns the excitatory effects (C57 strain) and the depressant effects (DBA strain) evident following their acute administration. The results are discussed in terms of differences in type, number and/or distribution of the receptors responsible, in the two strains, for the behavioral responses to opiate administration.