Delta-opioid stimulation by BW373U86 promotes autonomic reactivity to stressors and alters attention-related cardiac responses in rabbits.

The nonpeptide delta-opioid agonist BW373U86 (3 to 300 micrograms/kg) was tested in rabbits for effects on heart rate, cardiac orienting and Pavlovian conditioned responses to tones, and unconditioned cardiac and somatomotor responses to signaled and unsignaled shocks. BW373U86 did not alter shock-evoked somatomotor reflexes and had few effects on the development or retention of Pavlovian conditioned heart rate discrimination. However, BW373U86 appeared to modulate cardiac conditioning indirectly, by facilitating sympathetic reflexes evoked by the signaled stressor, and the dose effect was U-shaped within the dose range tested. The pronounced tachycardiac effect of BW373U86 was completely blocked, or rapidly reversed, by the selective delta-opiate antagonist naltrindole. BW373U86 was more potent in increasing signaled than unsignaled shock-evoked tachycardia, suggesting release of an endogenous substance (e.g., a delta-opioid) because of the Pavlovian conditioning contingency.