Ketamine administration disturbs behavioural and distributed neural correlates of fear conditioning in the rat

doi:10.1016/j.pnpbp.2006.02.019

Progress in Neuro-Psychopharmacology and Biological Psychiatry

Volume 30, Issue 7, 30 September 2006, Pages 1209-1218

https://doi.org/10.1016/j.pnpbp.2006.02.019 Get rights and content

Abstract

The neurotransmitter glutamate and its associated receptors perform an important role in the brain circuitry underlying normal fear processing. The glutamate NMDA receptor, in particular, is necessary for the acquisition and recollection of conditioned-fear responses. Here the authors examine how acute blockage of the NMDA receptor with sub-anaesthetic doses of ketamine affects behavioural assays of fear-conditioned stress (e.g. freezing) and cFos expression in a network of brain areas that have previously been implicated in fear processing. Fear-conditioned rats displayed significantly more freezing behaviour than non-conditioned controls. In fear-conditioned rats that also received ketamine, this conditioning effect was largely neutralised. Fear conditioning also led to increased cFos expression in various areas central to fear processing, including the basolateral nucleus of the amygdala, the paraventricular nucleus of the hypothalamus and the anterior cingulate. Ketamine abolished such increases in cFos expression in most brain areas investigated. The present study therefore demonstrates that systemic ketamine administration in rats interferes with fear conditioning on a behavioural level and in a network of brain regions associated with fear and anxiety. The combination of ketamine and fear conditioning may therefore provide a useful model of abnormal fear processing, as observed in certain psychiatric conditions.

Introduction

Classical fear conditioning is a technique generally used to explore fear circuits in the brain. It involves learning an association between a neutral conditioned stimulus and an aversive unconditioned stimulus (Walker and Davis, 2002). The neural pathways underlying fear conditioning, and more generally, fear processing, have been thoroughly investigated in the rat, with the primary focus being the amygdala (Davis et al., 1994, Maren and Fanselow, 1996, LeDoux, 1998, LeDoux, 2000, Maren, 2001).

Previous studies have shown that NMDA receptors in the amygdala are essential for long-term potentiation (LTP), a process that underlies fear learning (Li et al., 1995, Lee et al., 2001). Amygdalar NMDA receptors are necessary for the convergence and association of the unconditioned and conditioned stimuli (Walker and Davis, 2002). Kim and McGaugh (1992) for example, examined how injecting various NMDA antagonists (AP5, MK-801, CPP) into the rat amygdala altered conditioned-fear behaviour in an inhibitory avoidance task. While acquisition of conditioned responses remained intact, deficits in inhibitory avoidance were noted 48 h later. It has been hypothesized that temporally precise fear responses to specific threats are mediated by the amygdala, whereas sustained anxiety responses that persist beyond the immediate threat are mediated, at least in part, by structures such as the anterior cingulate, nucleus accumbens, orbitofrontal cortex and insula (Davidson and Irwin, 1999, Cardinal et al., 2002, Walker and Davis, 2002). In one study, for example, the pairing of anterior-cingulate stimulation with an auditory tone produced conditioned-fear responses in the rat (Tang et al., 2005). These responses were then blocked by the infusion of an NMDA-receptor antagonist into the amygdala. Such studies indicate a functional relationship between the anterior cingulate and the amygdala during fear processing.

To further investigate the functional basis of normal and altered fear processing, we induced a state of fear-conditioned stress (Suzuki et al., 2002) – a state dependent on the environmental context in which classical conditioning occurs – in the rat. We then examined behavioural (e.g. freezing) and neural (cFos expression) assays of fear conditioning following the (systemic) administration of ketamine, an NMDA antagonist, and in control animals. Based on available knowledge of the processing regions likely to be involved in fear and stress responses (LeDoux, 1998, Davidson and Irwin, 1999, Cardinal et al., 2002, Walker and Davis, 2002), we identified several candidate brain regions for investigation, including the amygdala and anterior cingulate. We hypothesized that ketamine would abolish neural and behavioural responses associated with fear conditioning, thereby inducing a state of abnormal fear processing which may prove useful in developing animal models of certain psychiatric conditions, such as schizophrenia.

Section snippets

Animals

All animals were cared for in accordance with the principles laid down by the European Communities Council Directive (1986) for the Protection of Vertebrate Animals used for Experimental or Other Scientific Purposes (86/EEC). Sprague–Dawley rats (n = 24) weighing between 225–250 g were obtained from the central animal facility (Groningen, The Netherlands) and were housed individually in a temperature (± 23 °C) and humidity controlled (40% to 60%) environment. Food and water were delivered ad

Behaviour

The results of the ANOVA on frequency, percentage total duration of the half-hour session and the mean time that the animal displayed certain behaviour are analysed and are stated here. Results of the post hoc test for individual comparisons are represented in Fig. 2.

Behavioural and neural correlates of stress

The main aim of this study was to examine how the influence of systemic ketamine administration manifests itself in neural and behavioural assays of fear-conditioned stress in rats. Fear conditioning was successful in eliciting stress, as witnessed by the increased (decreased) freezing (grooming) behaviour (Fig. 2) up to 30 min after the last conditioned tone. Consistent with our behavioural observations, the locus coeruleus, the dorsal raphe and the paraventricular nucleus, all areas involved

Conclusion

The authors conclude that the administration of the glutamate antagonist ketamine blocks the expression of fear-conditioned stress in rats at multiple neural sites and at the behavioural level. We further suggest that the combination of fear conditioning and ketamine may provide an effective model in linking the breakdown of fear processing to hypoglutamatergic states. In particular, our work could have implications for disorders in which fear processing is abnormal, such as schizophrenia (Tsai

Acknowledgements

We would like to thank Miranda van de Zeyden and Janine Doorduin for the behavioural analyses, as well as Joseph Haas and Michael Johnson (Eli Lilly, Indianapolis) for the statistical analyses. The help of Tony Vladusich in the preparation of this manuscript is deeply appreciated.

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Ketamine administration disturbs behavioural and distributed neural correlates of fear conditioning in the rat

Abstract

Introduction

Section snippets

Animals

Behaviour

Behavioural and neural correlates of stress

Conclusion

Acknowledgements

Neurosci Biobehav Rev

Trends Cogn Sci

Trends Neurosci

Pain

Brain Res Bull

Brain Res

Curr Opin Neurobiol

Biol Psychiatry

Behav Brain Res

Neuron

Prog Neuropsychopharmacol Biol Psychiatry

Physiol Behav

Schizophr Res

Curr Opin Pharmacol

Eur J Pharmacol

NeuroImage

Physiol Behav

Pharmacol Biochem Behav

Intra-amygdala infusion of the N-methyl-d-aspartate receptor antagonist AP5 blocks acquisition but not expression of fear-potentiated startle to an auditory conditioned stimulus

Behav Neurosci

Serotonin, stress and corticoids

J Psychopharmacol

The NMDA receptor glycine modulatory site: a therapeutic target for improving cognition and reducing negative symptoms in schizophrenia

Psychopharmacology (Berl)

Maternal separation in rats leads to anxiety-like behavior and a blunted ACTH response and altered neurotransmitter levels in response to a subsequent stressor

Metab Brain Dis