Determination of Diffusion Kinetics of Ketamine in Brain Tissue: Implications for in vitro Mechanistic Studies of Drug Actions

Abstract

Ketamine has been in use for over 50 years as a general anesthetic, acting primarily through blockade of N-methyl-D-aspartate receptors in the brain. Recent studies have demonstrated that ketamine also acts as a potent and rapid-acting antidepressant when administered at sub-anesthetic doses. However, the precise mechanism behind this effect remains unclear. We examined the diffusion properties of ketamine in brain tissue to determine their effects in in vitro studies related to the antidepressant action of ketamine. Brain slices from adult mice were exposed to artificial cerebrospinal fluid (aCSF) containing ∼17 μM ketamine HCl for varying amounts of time. The amount of ketamine within each slice was then measured by tandem high-performance liquid chromatography – mass spectrometry to characterize the diffusion of ketamine into brain tissue over time. We successfully modeled the diffusion of ketamine into brain tissue using a mono-exponential function with a time constant of τ = 6.59 min. This curve was then compared to a one-dimensional model of diffusion yielding a diffusion coefficient of approximately 0.12 cm2⋅s–1 for ketamine diffusing into brain tissue. The brain:aCSF partition coefficient for ketamine was determined to be approximately 2.76. Our results suggest that the diffusion properties of ketamine have a significant effect on drug concentrations achieved within brain tissue during in vitro experiments. This information is vital to determine the ketamine concentration necessary for in vitro slice preparation to accurately reflect in vivo doses responsible for its antidepressant actions.

Effects of ketamine and propofol on muscarinic plateau potentials in rat neocortical pyramidal cells

“…This strongly suggests that our wash-in applications did not reveal the full effect of the applied drug concentrations, presumably because of delayed equilibration between the medium and the slice. This interpretation is strongly supported by studies of diffusion of ketamine and propofol (Gredell et al 2004; Geiger et al 2021). Therefore, our discussion and conclusions are primarily based on our 2 h pre-incubation results.…”

“…These slices were incubated for at least two hours before performing recordings, to allow the drugs to equilibrate in the tissue. It was previously shown that 2 hours incubation is sufficient to reach concentrations close to equilibrium for propofol near the surface of the slices, at depths down to ~50 µm, where all our somatic whole-cell recordings were obtained, and 2 hours far exceeded the time taken to reach equilibrium for ketamine (Gredell et al 2004;Geiger et al 2021). Control cells in these experiments were from slices incubated for the same duration in standard aCSF, to compensate for any potential time-dependent effects of the incubation per se.…”

“…Since ketamine is lipophilic, diffusion from the bath into the slice may be slower than occurs through delivery from capillaries in vivo . Thus, Geiger et al (2021) found that in brain slices, ketamine can take up to 40 minutes to fully equilibrate in the tissue. Therefore we decided to use an extended pre-incubation to achieve equilibrium.…”

Changes in white matter microstructure following serial ketamine infusions in treatment resistant depression

“…Both studies suggest that greater FA in the cingulum (Sydnor et al, 2020 ; Vasavada et al, 2016 ), superior longitudinal fasciculus (Sydnor et al, 2020 ) or forceps major (Vasavada et al, 2016 ) present a biomarker for improved clinical outcomes following single dose ketamine. However, clinical (Sydnor et al, 2020 ) and preclinical (Geiger et al, 2021 ) studies have so far either only examined or reported changes in diffusion properties during and immediately following ketamine administration. To the best of our knowledge, this study is the first to investigate changes in WM microstructure following serial ketamine treatment, and to utilize NODDI in this context.…”