Major depressive disorder afflicts ~16 percent of the world population at some point in their lives. Despite a number of available monoaminergic-based antidepressants, most patients require many weeks, if not months, to respond to these treatments, and many patients never attain sustained remission of their symptoms. The non-competitive glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonist, (R,S)-ketamine (ketamine), exerts rapid and sustained antidepressant effects following a single dose in depressed patients. Here we show that the metabolism of ketamine to (2S,6S;2R,6R)-hydroxynorketamine (HNK) is essential for its antidepressant effects, and that the (2R,6R)-HNK enantiomer exerts behavioural, electroencephalographic, electrophysiological and cellular antidepressant actions in vivo. Notably, we demonstrate that these antidepressant actions are NMDAR inhibition-independent but they involve early and sustained α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor activation. We also establish that (2R,6R)-HNK lacks ketamine-related side-effects. Our results indicate a novel mechanism underlying ketamine’s unique antidepressant properties, which involves the required activity of a distinct metabolite and is independent of NMDAR inhibition. These findings have relevance for the development of next generation, rapid-acting antidepressants.

SUMMARY Dopamine (DA) neurons in the ventral tegmental area (VTA) and substantia nigra (SNc) encode reward prediction errors (RPEs) and are proposed to mediate error-driven learning. However the learning strategy engaged by DA-RPEs remains controversial. RPEs might imbue cue/actions with pure value, independently of representations of their associated outcome. Alternatively, RPEs might promote learning about the sensory features (the identity) of the rewarding outcome. Here we show that although both VTA and SNc DA neuron activation reinforces instrumental responding, only VTA DA neuron activation during consumption of expected sucrose reward restores error-driven learning and promotes formation of a new cue➔sucrose association. Critically, expression of VTA DA-dependent Pavlovian associations is abolished following sucrose devaluation, a signature of identity-based learning. These findings reveal that activation of VTA- or SNc-DA neurons engages largely dissociable learning processes with VTA-DA neurons capable of participating outcome-specific predictive learning, while the role of SNc-DA neurons appears limited to reinforcement of instrumental responses.

Currently approved antidepressant drug treatment typically takes several weeks to be effective. The noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine has shown efficacy as a rapid-acting treatment of depression, but its use is associated with significant side effects. We assessed effects following blockade of the glycine B co-agonist site of the NMDA receptor, located on the GluN1 subunit, by the selective full antagonist 7-chloro-kynurenic acid (7-Cl-KYNA), delivered by systemic administration of its brain-penetrant prodrug 4-chlorokynurenine (4-Cl-KYN) in mice. Following administration of 4-Cl-KYN, 7-Cl-KYNA was promptly recovered extracellularly in hippocampal microdialysate of freely moving animals. The behavioral responses of the animals were assessed using measures of ketamine-sensitive antidepressant efficacy (including the 24-hour forced swim test, learned helplessness test, and novelty-suppressed feeding test). In these tests, distinct from fluoxetine, and similar to ketamine, 4-Cl-KYN administration resulted in rapid, dose-dependent and persistent antidepressant-like effects following a single treatment. The antidepressant effects of 4-Cl-KYN were prevented by pretreatment with glycine or the a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX). 4-Cl-KYN administration was not associated with the rewarding and psychotomimetic effects of ketamine, and did not induce locomotor sensitization or stereotypic behaviors. Our results provide further support for antagonism of the glycine B site for the rapid treatment of treatment-resistant depression without the negative side effects seen with ketamine or other channelblocking NMDA receptor antagonists.

Flexible and efficient decision-making in complex environments can be achieved through constant interactions between the goal-directed and habitual systems. While goal-directed behavior is considered dependent upon Response-Outcome (R-O) associations, habits instead rely on Stimulus-Response (S-R) associations. However, the stimuli that support the S-R association underlying habitual responding in typical instrumental procedures are poorly defined. To resolve this issue, we designed a discrete-trials procedure, in which rats must wait for lever insertion and complete a sequence of five lever presses to obtain a reward (20% sucrose or grain-based pellets). Lever insertion thus constituted an audio-visual stimulus signaling the opportunity for reward. Using sensory-specific satiety-induced devaluation, we found that rats trained with grain-based pellets remained sensitive to outcome devaluation over the course of training with this procedure whereas rats trained with a solution of 20% sucrose rapidly developed habit, and that insensitivity to outcome devaluation in rats trained with sucrose did not result from a bias in general satiety. Importantly, although rats trained with pellets were sensitive to satiety-induced devaluation, their performance was not affected by degradation of instrumental contingency and devaluation by conditioned taste aversion (CTA), suggesting that these rats may also have developed habitual responding. To test whether the discrete-trials procedure biases subjects towards habitual responding, we compared discrete-trials to free-running instrumental responding, and found that rats trained with sucrose in a fixed-ratio 5 (FR5) procedure with continuous presentation of the lever were goal-directed. Together, these results demonstrate that discrete presentations of a stimulus predictive of reward availability promoted the formation of S-R habit in rats trained with liquid sucrose. Further research is necessary to explain inconsistencies in sensitivity to outcome devaluation when rats are trained with grain-based pellets.

Although maladaptive decision-making is a defining feature of drug abuse and addiction, we have yet to ascertain how cocaine self-administration disrupts neural signals in anterior cingulate cortex (ACC), a brain region thought to contribute to attentional control. To address this issue, rats were trained on a reward-guided decision-making task; reward value was manipulated by independently varying the size of or the delay to reward over several trial blocks. Subsequently, rats self-administered either a cocaine (experimental group) or sucrose (control) during 12 consecutive days, after which they underwent a 1-month withdrawal period. Upon completion of this period, rats performed the previously learned reward-guided decision-making task while we recorded from single neurons in ACC. We demonstrate that prior cocaine self-administration attenuates attention and attentionrelated ACC signals in an intake-dependent manner, and that changes in attention are decoupled from ACC firing. These effects likely contribute to the impaired decision-making-typified by chronic substance abuse and relapse-observed after drug use.