Molecular reactivity of mesocorticolimbic brain areas of high and low grooming rats after elevated plus maze exposure

doi:10.1016/j.molbrainres.2005.03.005

Molecular Brain Research

Volume 137, Issues 1–2, 13 June 2005, Pages 184-192

https://doi.org/10.1016/j.molbrainres.2005.03.005 Get rights and content

Abstract

High and low grooming rats (HG and LG), selected by extremities in stress-induced self-grooming on the elevated plus maze (EPM), display differences in stress coping style on the EPM, their motivation to self-administer cocaine, and differences in the reactivity of dopaminergic nerve terminals in mesocorticolimbic brain areas. This indicates a link between coping with a stressful/anxiogenic situation and drug intake. Here, we aimed to determine the molecular correlates of these differences by analyzing the reactivity of the mesocorticolimbic brain areas (the medial prefrontal cortex (mPFC) nucleus accumbens shell (NAS) and ventral tegmental area (VTA)) of HG and LG rats in response to EPM exposure. We report by measuring levels of immediate early gene (IEG) transcripts that EPM exposure-induced IEG expression was not significantly different between HG and LG rats. On the other hand, novel IEG expression patterns upon stress (EPM exposure) were apparent in all three areas including arc induction in the mPFC and NAS, CRH, BDNF, and Nr4a3 induction in the NAS, and serum glucocorticoid-regulated kinase (sgk) induction in the VTA. It is concluded that although the mPFC, NAS, and VTA play a role in modulating stress and grooming behavior, the neuronal reactivity in these regions measured by the IEG response is not related with behavioral extremities in stress coping style displayed on the EPM.

Introduction

A high degree of individual variation among humans exists in the vulnerability to drug abuse [9]. Therefore, it is of interest to find determinants underlying vulnerability, which may aid our understanding of potential predisposing factors of this disease, and the development of novel medications. In humans, stress or anxiety may enhance vulnerability to substance abuse [27]. The relationship between drug abuse and anxiety/stress has also been established in laboratory animals in which administration of anxiolytic compounds leads to decreased psychostimulant self-administration [8] whereas anxiogenic stimuli facilitate this process [7].

Recently, the issue to which extent individual differences in reactivity (e.g., coping style) to an anxiogenic/stressful situation could be a predisposing factor to drug abuse has been addressed. For instance, when encountered with a stressful condition, rats engage in self-grooming behavior, which is thought to be part of a “de-arousal” process that may serve to restore the homeostatic status disturbed by stressful stimuli [38]. Using the elevated plus maze (EPM), a clear relationship between grooming behavior in response to this stressful situation and drug (cocaine) taking has been observed [15]. Moreover, high and low grooming (HG and LG) animals displayed differences in the reactivity of dopaminergic and serotonergic nerve terminals towards depolarization in the medial prefrontal cortex (mPFC) and nucleus accumbens shell (NAS), and corticosterone response following EPM exposure [15], [16]. The mesocorticolimbic brain areas, such as the mPFC, NAS, and ventral tegmental area (VTA), have been implicated in the rewarding properties of drugs of abuse (e.g., opiates, psychostimulants) [43], stress responses [4], [29], [44], and coping to a stressor, such as self-grooming [14], [35].

Stimulation of neurons by neurotransmitters, e.g., dopamine (DA), results in induction of immediate early genes (IEGs), belonging to a class of transcription factors from the fos and egr family [12]. Encounters with stressful stimuli are associated with increased levels of IEGs such as fos and/or egr-related genes in mesocorticolimbic brain areas [6], [36]. Therefore, IEG induction is used as a sensitive parameter for neuronal reactivity [12]. Another class of IEGs also induced in response to stimulation are known as effector IEGs, which code for proteins that have a function that may have a more direct impact on cellular functioning than transcription factors, such as altering neuronal signaling or having a role in structural and functional synaptic modifications [10], [19]. Since HG and LG rats have shown differential corticosterone responses after EPM exposure, interesting candidates to examine stressor-induced differences in neuronal reactivity for HG and LG rats include brain derived neurotrophic factor (BDNF), corticotropin releasing hormone (CRH), and serum glucocorticoid regulated kinase (sgk), since all three IEGs can be regulated by either CRH or glucocorticoids [21], [22], [42]. Particular transcription factors, e.g., Nr4a3, can also be induced by stress-responsive molecules such as CRH [23]. Another interesting effector IEG is arc, which may play a role in synaptic modifications [20]. By measuring these aforementioned effector IEGs in conjunction with transcription factor, a more defined picture composing of both altered general reactivity and changes in neuronal signaling/structure might be obtained.

Therefore, in this study, we investigated whether upon EPM exposure differences in neuronal reactivity existed between HG and LG rats for the mPFC, NAS, and VTA. As a readout for changes in neuronal reactivity, IEG transcript levels of arc, BDNF, CRH, sgk, c-fos, egr-1, egr-2, and Nr4a3 were measured by real-time quantitative PCR (qPCR).

Section snippets

Animals

Male Wistar rats (Harlan, The Netherlands) weighing 180–200 g at arrival were housed socially (ten per cage) under controlled conditions (lights on, 07.00 h; lights off, 19.00 h) at 22 ± 2 °C. Standard food (Hope Farms, Woerden, The Netherlands) and water were available ad libitum. Animals were allowed to accustom to these housing conditions for 1 week prior to the beginning of the experiments. Rats were handled once daily during the 3 days prior to behavioral screening. All experiments were

Behavior on the EPM

For the rats to be labeled HG or LG rats, they had to self-groom more than 21 s or under 5 s, respectively (Fig. 1A). No significant effect of time (45 min vs. 7 days), group (HG vs. LG), nor time × group interaction was observed for all EPM behavioral parameters such as total distance moved (Fig. 1B), number of closed arm entries (Fig. 1C), percentage of open arm entries (Fig. 1D), except for percentage time spent on open arms, in which a group effect was observed (Fig. 1E) (F(1,20) = 4.59, P

Discussion

When encountered with a stressful situation, the brain undergoes neuronal plastic changes that may be accompanied by brief bursts in gene expression. In the present study, we have studied multiple activation markers in different mesocorticolimbic areas instead of a single marker, and have shown that upon EPM exposure, particular sets of IEGs display common and unique induction patterns between the different areas studied. Although the reactivity patterns indicate that mPFC, NAS, and VTA play a

Acknowledgments

The authors would like to thank Rob Binnekade, George Wardeh, Halfdan Raasø, Allert Jonker for their technical assistance; Rene van Elk and Michiel van den Berg for assistance with Prism and/or Photoshop 7.0; Prof. Dr. Harry Uylings for helpful comments on the dissection atlas. This project was supported by N.W.O. grant 985-10-007.

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Research reportMolecular reactivity of mesocorticolimbic brain areas of high and low grooming rats after elevated plus maze exposure

Abstract

Introduction

Section snippets

Animals

Behavior on the EPM

Discussion

Acknowledgments

Anal. Biochem.

Neuroscience

Neuroscience

Brain Res.

Pharmacol. Biochem. Behav.

Brain Res., Brain Res. Rev.

Eur. J. Pharmacol.

Neuroscience

Neuroscience

Neurobiol. Learn. Mem.

Neuron

Brain Res.

Brain Res.

Addict. Behav.

Neuropsychopharmacology

Brain Res.

Neuroscience

Neurosci. Lett.

Behav. Brain Res.

Biol. Psychiatry

Neurosci. Res.

Neuron

Research report
Molecular reactivity of mesocorticolimbic brain areas of high and low grooming rats after elevated plus maze exposure