¹University of Ottawa Institute of Mental Health Research at The Royal, Psychiatry, Ottawa, Canada

Background: Low-dose racemic ketamine enhances the firing rate and burst activity of rat norepinephrine neurons in the locus coeruleus, as well as the population activity of dopamine neurons in the ventral tegmental area [El Iskandrani et al, 2015]. These brainstem monoamine nuclei densely innervate the prefrontal cortex (PFC), whereas clinical studies have shown a facilitation of glutamate neurotransmission in the PFC by a single injection of ketamine [Li et al, 2016].

Aims & Objectives: Due to reciprocal interactions between PFC glutamate neurons and monoamine neurons and their key role in the neurocircuitry of depression, the present electrophysiological study was aimed at investigating whether single acute doses of racemic ketamine, its more potent enantiomer NMDA antagonist S-ketamine, R-ketamine, and the NMDA GluN2B subunit antagonist ifenprodil alter the spontaneous firing rate of medial prefrontal cortex (mPFC) glutamate pyramidal neurons and their responsiveness to AMPA and NMDA.

Method: Experiments were carried out on male Sprague Dawley rats weighing 275-300 g. They were anesthetized with chloral hydrate and mounted in a stereotaxic frame. Extracellular recordings of pyramidal neurons in the mPFC were carried out with single- and five-barreled glass micropipettes. The central barrel used for the unitary recording was filled with a 2 M NaCl solution and the side barrels contained AMPA, NMDA, and saline to balance out any net currents. mPFC pyramidal neurons were identified using previously described criteria, and their evoked firing to AMPA and NMDA applications by iontophoresis were assessed prior to and starting 30 minutes to 2 hours following administration of studied compounds (10 mg/kg, i.p.).

Results: Only S-ketamine and ifenprodil significantly enhanced the spontaneous firing rate of mPFC neurons, respectively by 63% and 50%, when compared to control rats (two-way ANOVA with repeated measures F [1, 32] = 17.5, p <0.001). Despite none of the drugs altered the responsiveness mPFC pyramidal neurons to iontophoretically applied NMDA (two-way ANOVA with repeated measures F [1, 18] = 2.7, p>0.05), an increase in responsiveness to applied AMPA was only observed following ketamine and S-ketamine (two-way ANOVA with repeated measures F [1, 26] = 15.9, p<0.001, followed by Holm-Sidak post hoc tests: t=4.5, p<0.001 and t= 3.1, p=0.005, respectively).

Discussion & Conclusions: Although both R-ketamine and ifenprodil were shown to have an antidepressant-like effect in the forced swim test [Zhang et al, 2014; Poleszak et al, 2013], they do not increase AMPA-induced response in the mPFC, unlike racemic ketamine and S-ketamine that do both. The increase in AMPA-induced response may thus constitute a common factor underlying the antidepressant effects of ketamine and S-ketamine confirmed in the clinic. Such an increase in AMPA responsiveness coincides with an enhancement in expression of GluA1 subunit of AMPA receptors in the rat PFC by racemic ketamine [Piva et al, 2021]. These results are line with clinical studies using ketamine that showed an increase in glucose metabolism, measured by ¹⁸F-FDG positron-emission tomography. Therefore, an involvement of facilitation of glutamate neurotransmission in the PFC appears to correlate with the rapid antidepressant effect of low-dose of racemic ketamine and S-ketamine in major depressive disorder [Li et al, 2016].