Opioid receptors regulate the acquisition and extinction of second-order fear in both sequential and simultaneous conditioning protocols.

The endogenous opioid system is thought to regulate Pavlovian fear conditioning in rats by instantiating some component of the error correction signal that underlies this learning. Consistent with this view, opioid receptor antagonists enhance the acquisition of first-order conditioned fear when a novel stimulus (S1, e.g., a light) is paired with a brief but painful foot shock; and impair the extinction of this fear when an already-conditioned S1 is repeatedly presented in the absence of the shock. The present study examined whether opioid receptors also regulate the acquisition and extinction of fear in protocols where the conditioning does not involve the experience of pain. Specifically, we established second-order conditioned fear (measured in freezing) using protocols in which a second novel stimulus (S2, e.g., a tone) was sequentially or simultaneously paired with an already-conditioned S1. We then found that subcutaneous injections of the opioid receptor antagonist, naloxone (2.5 mg/kg): 1) enhanced acquisition of freezing to S2 when it was sequentially or simultaneously paired with the already-conditioned S1; and 2) impaired extinction of a sequentially or simultaneously conditioned S2 when it was repeatedly presented in the absence of the S1. These results support the proposal that opioid receptors regulate fear learning via processes that go beyond their role in conditioned analgesia and pain modulation. They are discussed with respect to a role for opioid receptors in regulating the contribution of prior learning to the calculation of prediction error, and/or inhibitory learning that occurs across the course of second-order fear conditioning and extinction.