Ketamine enhances ketogenic diet-induced resilience to activity-based anorexia of adult mice through hippocampal synaptic plasticity.

Ketogenic diet (KGD) combined with ketamine (KET) suppresses food restriction-evoked hyperactivity in activity-based anorexia (ABA) mouse model of anorexia nervosa (AN) for more than 28 days, thereby improving weight retention and reducing anxiety, even after mice return to standard diet (SD) and are re-challenged with ABA (KGD + KET → SD, "KET + KGD group"). Suppression of these AN-like behavior is not sustained in mice fed KGD without KET (KGD → SD, "KGD-only group"). We hypothesized that the sustained efficacy of KET + KGD is associated with altered prevalence of certain synapses in the dorsal hippocampus that influence behavior. We compared the prevalence of excitatory and inhibitory synapses between KET + KGD (N = 7) versus KGD-only (N = 7) groups, using electron microscopy and GAD immunolabeling to distinguish GABAergic inhibitory synapses from non-GABAergic excitatory synapses. Pyramidal cell layer (PCL) was analyzed to assess the contribution by parvalbumin + subtype of GABAergic neurons known to reside there, while stratum lacunosum-molecular (SLM) was analyzed to assess inhibition by non-PV + GABAergic neurons known to reside there. In both PCL and SLM, the KET + KGD group exhibited greater correlation between GABAergic inhibition of PNs and weight retention. In SLM of the KET + KGD group, the areal density of excitatory synapses was less. Inhibitory synapses onto GABAergic dendrites (disinhibition which may also augment excitability) correlated negatively with hyperactivity for the KET + KGD group only. KET + KGD, but not KGD-only, may sustain ABA resilience by enhancing plasticity of both glutamatergic and GABAergic synapses. This jointly sustained plasticity underlying hippocampal excitability may be the cellular bases for KET + KGD treatment's suppression of AN relapses.