Dynorphin A

Dynorphin A binds kappa-opioid receptors (KOR) with very high potency, as well as mu-opioid (MOR) and delta-opioid (DOR) receptors at higher concentrations. KOR is a Gi-coupled GPCR; activation inhibits adenylyl cyclase, activates GIRK channels (hyperpolarizing neurons), and recruits beta-arrestin for receptor internalization and biased signaling. KOR activation in the limbic system and prefrontal cortex produces the dysphoric and aversive effects characteristic of kappa agonism.

The dynorphin/KOR system is activated by repeated stress exposure, chronic pain, and drug withdrawal. KOR activation in the nucleus accumbens and mesolimbic dopamine pathway reduces dopamine release and tone, producing anhedonia and dysphoria. Dynorphin is the neurochemical substrate of stress-induced negative affect, and KOR antagonists represent a novel antidepressant strategy for stress-related mood disorders.

Dynorphin released during acute stress produces stress-induced analgesia (SIA) through spinal KOR activation, an evolutionarily conserved mechanism for suppressing pain during acute threat. However, repeated stress leads to sustained dynorphin elevation and KOR activation in limbic circuits, producing anhedonia and behavioral despair in animal models of depression.

Paradoxically, while acute dynorphin mediates analgesia, excess spinal dynorphin after cord injury may contribute to secondary injury and chronic pain through non-opioid receptor mechanisms (NMDA receptor activation at high concentrations). Dynorphin's excitotoxic potential at spinal injury sites has prompted research into KOR antagonists as neuroprotective agents.

The dynorphin/KOR system as a target for depression and stress disorders has advanced to clinical trials with KOR antagonists (aticaprant, CERC-501). Phase II trials showed antidepressant effects in major depressive disorder and anhedonia, directly validating the role of dynorphin-mediated KOR activation in depression pathophysiology.