The pharmacologically active (R,S)-ketamine (ketamine) metabolite (2R,6R)-hydroxynorketamine (HNK) maintains ketamine's preclinical antidepressant profile without adverse effects. While hypotheses have been proposed to explain how ketamine and its metabolites initiate their antidepressant-relevant effects, it remains unclear how sustained therapeutic actions arise following drug elimination. To distinguish the physiological mechanisms involved in the rapid from sustained actions of HNK, we utilized extracellular electrophysiology combined with pharmacology to develop an in vitro hippocampal slice incubation model that exhibited pharmacological fidelity to the 1) rapid synaptic potentiation induced by HNK at the Schaffer collateral-CA1 (SC-CA1) synapse during bath-application to slices collected from mice, and 2) maintenance of metaplastic (priming) activity that lowered the threshold for N-methyl-D-aspartate receptor (NMDAR) activation-dependent long-term potentiation (LTP) hours after in vivo dosing. We then used this model to reveal novel druggable mechanisms engaged in HNK's temporally- sensitive antidepressant synaptic actions, finding that the induction of synaptic potentiation by HNK did not require NMDAR activity, but NMDAR activity was necessary to maintain synaptic priming. HNK required protein kinase A (PKA) activity to rapidly potentiate SC-CA1 neurotransmission to facilitate synaptic priming that persistently promoted LTP formation. HNK's rapid actions were blocked by inhibitors of adenylyl cyclase 1 (AC1), but not an AC5 inhibitor. We conclude that HNK rapidly potentiates SC-CA1 synaptic efficacy, which then stimulates priming mechanisms that persistently favor antidepressant-relevant plasticity. Targeting such priming mechanisms may be an effective antidepressant strategy, and using approaches such as our incubation model may aid in revealing novel pharmacological targets.
