Ketamine selectively enhances AMPA neurotransmission onto a subgroup of identified serotoninergic neurons of the rat dorsal raphe
Although the fast antidepressant effect of ketamine is now well established clinically, neither its mechanism(s) nor its main site(s) of action is clearly defined. Because enhanced serotoninergic (5-HT) transmission is an important part of the antidepressant effect of various drug classes, we asked whether ketamine and one of its metabolites (hydroxynorketamine [HNK]), both used in their racemic form, may modulate the excitatory drive onto these neurons. Using whole-cell recordings from pharmacologically identified 5-HT and non-5-HT neurons in juvenile rat dorsal raphe slices, we found that both ketamine and HNK (10 {micro}M) increase excitatory AMPA neurotransmission onto a subset (50%) of 5-HT neurons, whereas other 5-HT cells were unaffected. Both compounds increased the amplitude as well as the frequency of spontaneous excitatory post-synaptic currents (sEPSCs) mediated by AMPA receptors. The effect of ketamine was more robust than the one of HNK, since it significantly enhanced the charge transfer through AMPA channels, whereas HNK did not. The increase in the excitatory drive induced by ketamine was dependent on NMDA receptor blockade. In the presence of tetrodotoxin, the effect of ketamine was markedly reduced. Non-5-HT neurons, on the other hand, were unaffected by the drugs. We conclude that ketamine and HNK increase the excitatory drive onto a subset of 5-HT neurons by promoting glutamate release and possibly also through a postsynaptic action. The effect of ketamine is dependent on NMDA receptor modulation and appears to involve a network effect. These findings improve our understanding of the fast-acting antidepressant effect of ketamine. SIGNIFICANCE STATEMENTThe mechanisms of ketamines antidepressant effect are currently controversial. We asked whether the drug would produce changes is the strength of synaptic inputs onto serotoninergic neurons of the dorsal raphe. We found that this is indeed the case in about half of these neurons. The action of ketamine was mimicked to some extent by its well-known metabolite hydroxynorketamine, was dependent on NMDA receptor activation and probably involved a local network effect. It remains to be determined if the differential susceptibility of serotoninergic neurons to the drug correlates with any differential inputs and/or outputs.