Opioid modulation of attention-related responses: delta-receptors modulate habituation and conditioned bradycardia

Endogenous opioids modulate attention-related heart rate responses evoked by novel stimuli and conditioned signals in ways that differ from their better-known effects on motivation and memory functions. We investigated the role of delta-opioids in modulating bradycardiac orienting and Pavlovian conditioned responses in rabbits, following i.v. treatment with the highly selective delta-receptor antagonist naltrindole (NTI; 0.037-0.370 mg/kg). When administered immediately before testing, NTI induced modest but detectable effects: the lowest dose increased cardiac discrimination near the end of the first training session, whereas the higher doses of NTI impaired discrimination, compared to saline-treated controls. NTI treatment immediately before testing also appeared to promote habituation of bradycardiac orienting responses elicited by novel tones, but NTI did not alter unconditioned heart rate responses following tone-shock pairs or extinction of conditioned responses. In contrast, the low dose of NTI administered 20 min, rather than immediately before testing, facilitated conditioned bradycardia during extinction, as well as during training. These results provide evidence that endogenous delta-opioid modulators normally delay the disappearance of bradycardiac orienting responses during habituation, inhibit or promote the development of bradycardiac conditioned responses during Pavlovian training depending on dose, and promote the disappearance of conditioned responses during extinction. These findings suggest that endogenous delta-opioid activity, probably involving both peripheral and central systems, coincides with, and may reflect, uncertainty about stimulus significance.