Acute ketamine challenge increases resting state prefrontal-hippocampal connectivity in both humans and rats

This placebo-controlled, cross-species, translational comparison study (n=24: humans; n=18: male rats) examines the acute effects of ketamine (rats: 10 mg/400g; humans: 35 mg/70kg) on resting-state functional connectivity and found a robust increase in the coupling between the hippocampus and the prefrontal cortex in both species. The authors believe this to reflect increased levels of excitatory neurotransmitters, such as glutamate, acetylcholine, and histamine and the disinhibition GABAergic interneurons via ketamine.

Abstract

“Rationale: Aberrant prefrontal-hippocampal (PFC-HC) connectivity is disrupted in several psychiatric and at-risk conditions. Advances in rodent functional imaging have opened the possibility that this phenotype could serve as a translational imaging marker for psychiatric research. Recent evidence from functional magnetic resonance imaging (fMRI) studies has indicated an increase in PFC-HC coupling during working-memory tasks in both schizophrenic patients and at-risk populations, in contrast to a decrease in resting-state PFC-HC connectivity. Acute ketamine challenge is widely used in both humans and rats as a pharmacological model to study the mechanisms of N-methyl-D-aspartate (NMDA) receptor hypofunction in the context of psychiatric disorders.

Objectives: We aimed to establish whether acute ketamine challenge has consistent effects in rats and humans by investigating resting-state fMRI PFC-HC connectivity and thus to corroborate its potential utility as a translational probe.

Methods: Twenty-four healthy human subjects (12 females, mean age 25 years) received intravenous doses of either saline (placebo) or ketamine (0.5 mg/kg body weight). Eighteen Sprague-Dawley male rats received either saline or ketamine (25 mg/kg). Resting-state fMRI measurements took place after injections, and the data were analyzed for PFC-HC functional connectivity.

Results: In both species, ketamine induced a robust increase in PFC-HC coupling, in contrast to findings in chronic schizophrenia.

Conclusions: This translational comparison demonstrates a cross-species consistency in pharmacological effect and elucidates ketamine-induced alterations in PFC-HC coupling, a phenotype often disrupted in pathological conditions, which may give clue to understanding of psychiatric disorders and their onset, and help in the development of new treatments.“

Authors: Oliver Grimm, Natalia Gass, Wolfgang Weber-Fahr, Alexander Sartorius, Esther Schenker, Michael Spedding, Celine Risterucci, Janina Isabel Schweiger, Andreas Böhringer, Zhenxiang Zang, Heike Tost, Adam James Schwarz & Andreas Meyer-Lindenberg 

Summary

Abstract

Rat functional imaging studies have shown that the prefrontal-hippocampal (PFC-HC) connection is disrupted in several psychiatric and at-risk conditions. This connectivity may serve as a translational imaging marker for psychiatric research.

Twenty-four healthy human subjects and 18 Sprague-Dawley male rats received intravenous doses of either saline (placebo) or ketamine (0.5 mg/kg body weight). Resting-state fMRI was used to analyze PFC-HC functional connectivity.

Introduction

The dorsolateral prefrontal cortex (DLPFC) is one of the major anatomical hubs implicated in the pathophysiology of schizophrenia. Resting-state functional magnetic resonance imaging (rs-fMRI) suggests that the DLPFC-hippocampus circuit may be modulated in a consistent way by the same pharmacological manipulation in humans and rats.

Ketamine is a dissociative anesthetic that acts as an N-methyl-D-aspartate (NMDA) receptor antagonist. It is also used to investigate mechanisms underlying psychosis.

Ketamine has been applied in humans and animals to model translationally relevant behavioral effects, but there is a lack of data on how acute ketamine treatment affects the functional coupling between prefrontal and subcortical regions.

In our earlier study, we hypothesized that ketamine challenge may be used to induce a hyperglutamatergic state similar to that of the early (prodromal) stage of schizophrenia. The present study investigated whether an acute ketamine challenge has analogous effects in rats and humans on PFC-HC connectivity measures.

Subjects

We performed a subject- and observer-blind, placebo-controlled, randomized, three-period cross-over study with 24 participants. The subjects received either ketamine (0.5 mg/kg body weight) or scopolamine (4 g body weight).

Ketamine application—humans

On each day of the experiment, subjects received an infusion of ketamine via a certified intravenous pump. MRI scanning took place after drug administration.

Ketamine application—animals

The experimental design comprised two groups of N =9 rats each. S-ketamine was injected subcutaneously at a dose of 25 mg/kg dissolved in saline.

Data acquisition—animal fMRI

Experiments were conducted at a 9.4-T MRI scanner using a linear whole-body volume transmitter coil and anatomically shaped four-channel receive-only coil array.

Rats were anesthetized under 4 % isoflurane in a mixture of N2 (70 %)/O2 (30 %), and then a continuous infusion of medetomidine solution started at 0.14 mg/kg/h rate.

A respiration pad and pulse oximeter were attached to the hindpaw to record breathing and cardiac signals.

A FieldMap and a rs-fMRI measurement were acquired with an echo-planar imaging (EPI) sequence. The slices covered the brain from the cerebellum to the posterior olfactory bulb.

Data processing and analysis—human fMRI

Data were processed using statistical parametric mapping (SPM8) and the complementary CONN toolbox v13. Images were realigned, slice-time corrected, spatially normalized, resampled to 3-mm isotropic voxels, smoothed with an 8-mm full-width at half maximum (FWHM) Gaussian kernel, and noise corrected using an aCompCor-strategy.

We extracted the first eigenvariate from the right and left DLPFC, Fisher-Z-transformed the consecutive correlation map, and used flexible factorial models to calculate group statistics. We modeled the ketamine versus placebo comparison alone and treated the other factor levels as nuisance variables.

Data processing and analysis—animal fMRI

Data were processed using SPM8, corrected for magnetic field inhomogeneities, filtered out respiratory and cardiac signals, and spatially normalized to a rat brain template with co-registered anatomical atlas positioned in the Paxinos stereotactic coordinate system.

We extracted time courses from the right and left prelimbic cortex, smoothed them by 0.8 mm, and calculated correlation coefficients. We then used Fisher Z-scores to test connectivity between the right and left hippocampi.

Pharmacokinetics—humans

Ketamine and norketamine levels were measured in blood samples 10 and 80 min after the end of the infusion. The levels were correlated with DLPFC-HC connectivity.

Pharmacokinetics—animals

After the ketamine/vehicle injection, blood samples were collected and analyzed to determine exposure to ketamine and levels of its major metabolite norketamine.

Human pharmacological rs-fMRI

Ketamine significantly increased the correlation between the left and right DLPFC and the left hippocampus, but not between the left DLPFC and either the left or right hippocampus.

Ketamine and norketamine levels were high in the first blood sample, but not in the second blood sample. There were no statistically significant correlations between plasma levels and connectivity between the DLPFC and hippocampus.

Animal pharmacological rs-fMRI

Ketamine significantly elevated the correlation between the left PRL and the right and left hippocampus, as well as between the right PRL and the left hippocampus. There was no significant effect between the right PRL and the right hippocampus.

Discussion

Both humans and rats showed enhanced PFC-HC coupling in response to ketamine in their rs-fMRI analyses. This suggests that functional imaging could be used as a translational biomarker.

The observed increase in PFC-HC coupling might reflect the neuromodulatory effect of ketamine on this network. Ketamine increases the levels of excitatory neurotransmitters and disinhibits GABAergic interneurons.

Ketamine increases glucose utilization and blood flow in the hippocampus and cerebral cortex, which might explain the findings. Additionally, ketamine increases gamma band oscillations, which correlate with BOLD connectivity in humans.

Hippocampal and prefrontal pathophysiology

Ketamine increases connectivity in the dorsal hippocampus, which is in accordance with the higher expression of NMDA receptors in the dorsal hippocampus along a ventral-dorsal axis in both rats and humans. This could partly explain the cognitive impairment observed after acute administration of ketamine.

Postmortem and neuroimaging studies reveal disrupted hippocampal function in schizophrenic patients, which might underlie abnormal recruitment of hippocampal circuits during cognitive tasks. Ketamine administration in humans may have more relevance to cognitive deficits associated with disorders such as schizophrenia, rather than plain psychosis.

The posterior region of the rat hippocampal formation correlates more strongly with the mPFC than the anterior dorsal hippocampus, and a homologous relationship exists in human. The DLPFC is a region of the human cortex that exhibits aberrant functional connectivity with the hippocampus in disease. Rat mPFC is the closest homolog of human DLPFC in terms of anatomical connections, electrophysiological and computational properties, and plays a similar role in spatial working memory as the human DLPFC.

Hemispheric lateralization of the effects

We found that the left hippocampus responded more strongly to ketamine than the right hippocampus. This could be due to differences in NMDA/AMPA receptor density, subunit composition profile, or expression of glutamate transporters.

The left hippocampus seems to be more affected in schizophrenic patients, compared to the right. Additionally, the disease phenotype and at-risk effects are left-lateralized with regard to the connectivity pattern.

In rats, both left and right hippocampi showed increased coupling with the PrL, which could be due to a reduced laterality in terms of connections, NMDA/AMPA receptor profile and/or glutamate transporters.

Ketamine effects in depressive patients were asymmetries in response patterns, with a decreased post-ketamine metabolism in the right DLPFC and increased in the left parahippocampal region.

Differences between human and rat experiments

The rat PFC-HC circuit has robust functional connectivity patterns that closely reflect known anatomical connectivity, and the rat brain still preserves global network properties of the awake brain.

In this study, the human subjects had lower plasma levels of ketamine than the rats, reflecting the lower dose administered. However, the effect of ketamine on PrL-hippocampal connectivity in both species was comparable at different, species-specific, dose and exposure levels.

Humans received racemic ketamine (mixture of S(+) and R() enantiomers), while rats received S-ketamine, the S(+)-enantiomer of ketamine, which is discussed as the main compound responsible for the NMDA-antagonistic effect. However, a direct analogy between pharmacokinetics and pharmacodynamics in both species was not possible.

Comparison with schizophrenia

The directionality of the ketamine effect is opposite to the observed reduced PFC-HC coupling during resting-state in chronic schizophrenia. The ketamine effect resembles a hyperglutamatergic state and cognitive disruption as observed in prodromal stage of schizophrenia.

While it is attractive to discuss the PFC-HC connectivity as a marker for schizophrenia, the exact changes in mind states induced by ketamine anesthesia are different from those seen in schizophrenia.

Future questions

Several questions remain open for future studies, such as the reason for the asymmetry of ketamine response and the crossing steps in the transition from prodromal to chronic schizophrenia.

Ketamine effects may be observed in other regions and networks of the brain, but current studies do not elucidate the links or circuits in which the connectivity is most strongly increased.

Conclusion

Our data suggest that the systems-level alterations induced by ketamine treatment are comparable across species, and that this finding may help in the development of new treatments.