Enkephalin analogue effects in the amygdala central nucleus on conditioned heart rate

The experiment was conducted to assess the effects of enkephalin analogue administration into the amygdala central nucleus on the acquisition of classically conditioned heart rate responding in rabbits. Bilateral injections of either D-Ala², Met⁵-enkephalinamide (DALA) or D-Ala², D-Leu⁵-enkephalin (DADL) were administered in a 1.0 μl volume into the central nucleus immediately prior to the conditioning session. Administration of DALA significantly attenuated the acquisition of conditioned heart rate responding whereas groups which received comparable doses of DADL exhibited conditioned responding which did not differ from the vehicle-injected control group. The effect on conditioned responding produced by DALA administration was blocked by concurrent administration of the opiate antagonist naloxone. These results provide some support for the involvement of mu receptor activity within the central nucleus region of the amygdala in conditioning processes.