Kádár et al. BMC Neurosci (2018) 19:48 https://doi.org/10.1186/s12868-018-0449-5 BMC Neuroscience RESEARCH ARTICLE Open Access Arc protein expression after unilateral intracranial self‑stimulation of the medial forebrain bundle is upregulated in specifc nuclei of memory‑related areas Elisabet Kádár2,4* , Eva Vico Varela1,3 , Laura Aldavert‑Vera1 , Gemma Huguet2 , Ignacio Morgado‑Bernal1 and Pilar Segura‑Torres1 Abstract Background: Intracranial Self-Stimulation (ICSS) of the medial forebrain bundle (MFB) is a deep brain stimulation procedure, which has a powerful enhancement efect on explicit and implicit memory. However, the downstream synaptic plasticity events of MFB-ICSS in memory related areas have not been described thoroughly. This study complements previous work studying the efect of MFB-ICSS on the expression of the activity-regulated cytoskeleton- associated (Arc) protein, which has been widely established as a synaptic plasticity marker. We provide new integrated measurements from memory related regions and take possible regional hemispheric diferences into consideration. Results: Arc protein expression levels were analyzed 4.5 h after MFB-ICSS by immunohistochemistry in the hip‑ pocampus, habenula, and memory related amygdalar and thalamic nuclei, in both the ipsilateral and contralateral hemispheres to the stimulating electrode location. MFB-ICSS was performed using the same paradigm which has previously been shown to facilitate memory. Our fndings illustrate that MFB-ICSS upregulates the expression of Arc protein in the oriens and radiatum layers of ipsilateral CA1 and contralateral CA3 hippocampal regions; the hilus bilat‑ erally, the lateral amygdala and dorsolateral thalamic areas as well as the central medial thalamic nucleus. In contrast, the central amygdala, mediodorsal and paraventricular thalamic nuclei, and the habenular complex did not show changes in Arc expression after MFB-ICSS. Conclusions: Our results expand our knowledge of which specifc memory related areas MFB-ICSS activates and, motivates the defnition of three functionally separate groups according to their Arc-related synaptic plasticity response: (1) the hippocampus and dorsolateral thalamic area, (2) the central medial thalamic area and (3) the lateral amygdala. Keywords: Arc, Medial forebrain bundle, Intracranial self-stimulation, Memory, Hippocampus, Amygdala, Thalamus, Habenula *Correspondence: [email protected] 4 Department of Biology, Sciences Faculty, University of Girona, C/Mª Aurèlia Capmany 40, Camous Montilivi, 17003 Girona, Spain Full list of author information is available at the end of the article © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/ publi​cdoma​in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Kádár et al. BMC Neurosci (2018) 19:48 Page 2 of 13 Background amygdala-dependent active avoidance tasks, in which Deep brain stimulation (DBS), an electrical current ICSS reverses the memory defcit caused by aging [13, delivered through stereotactically implanted electrodes 14] and/or brain damage [15, 16]. into specifc areas of the brain, is a promising therapeu- Consistent with the broad efect of MFB-ICSS on dif- tic option for patients with neurological and psychiatric ferent types of learning tasks, c-Fos expression results diseases. To date, DBS has been successfully applied to suggest ICSS activates multiple regions related to dif- alleviate movement disorders, such as Parkinson’s dis- ferent memory systems, such as the amygdala, the dor- ease [1]. DBS is now being considered for use as a treat- sal striatum, the hippocampus and the prefrontal cortex ment for neurodegenerative disorders associated with [10, 17–19]. Additionally, MFB-ICSS induces diferential memory impairments such as Alzheimer’s disease [2]. time course mRNA expression of some synaptic plas- However, the mechanisms of action of how DBS afects ticity-related genes including Arc in the amygdala and memory are not yet fully known. the hippocampus [18, 19]. ICSS-treated rats showing a Studies with laboratory animals are required to facilitated spatial memory have also long-lasting struc- explore potential targets and to analyze the underlying tural changes including extended dendritic arboriza- cellular and molecular brain changes of DBS. Recent tion of pyramidal CA1 neurons [6]. Tus, based on the studies indicate that bilateral DBS of the fornix/hypo- fact that Arc protein is a well-known marker of synaptic thalamic area drives neural activity in the cortico-hip- plasticity events associated with the memory consolida- pocampal memory circuit with a signifcant reversal tion process [20] and is also involved in long-term spine of the impaired cortical glucose utilization observed enlargement [21], we initially set out to determine the in patients with Alzheimer’s disease [3, 4]. Further efect of MFB-ICSS has on Arc protein levels in the dif- experiments in rats have established that forniceal DBS ferent sub-regions of the hippocampus and in the retro- (F-DBS) induces increased hippocampal expression splenial cortex. We observed that after 4.5 h of unilateral of c-Fos protein, some neurotrophic factors such as MFB-ICSS, Arc protein levels increased signifcantly in BDNF, and other synaptic plasticity markers [5], which the CA1 and DG hippocampal subfelds ipsilateral to the are prominent molecular correlates of memory con- stimulated hemisphere [22] but at this time contralateral solidation. Tese results are consistent with fndings regions were not analyzed. We also observed that MFB- from our group which reveal a memory improvement ICSS increased Arc expression in the granular retrosple- after stimulating the medial forebrain bundle (MFB), nial cortex (RSC), a hippocampus-related region involved the most important pathway of the brain reward sys- in long-lasting memory storage [23]. Tis study showed a tem, which passes through the lateral hypothalamus possible lateralization of the ICSS efects on Arc protein (LH). Activation of brain areas belonging to the reward levels since changes were only observed in the ipsilateral, system generates positive reinforcement, and actions but not the contralateral hemisphere. that are positively reinforced are more likely to be Te current study provides new integrated data of repeated than those that do not. Intracranial self-stim- the synaptic plasticity efects of the memory enhanc- ulation (ICSS) is a DBS procedure in which subjects ing MFB-ICSS treatment on additional regions related self-administer electrical stimulation to brain reward to diferent memory systems and analyzes hemisphere areas by performing an instrumental response, such as diferences. Arc protein expression was examined by pressing a lever in a Skinner box. Te use of ICSS in immunohistochemistry in both ipsilateral and contralat- animals allows us to unequivocally ensure that stimula- eral hemispheres 4.5 h after MFB-ICSS in each subject. tion activates the reward system in a functional man- Te studied areas include (1) the hippocampal subfelds, ner. In a similar way to F-DBS, ICSS of the LH has a involved in spatial memory encoding and consolida- reliable enhancing efect on hippocampus-dependent tion; (2) amygdalar nuclei involved in implicit emotional explicit memory [6–8]. However, while F-DBS afects memory, including the associative lateral amygdala (LA) fornix fbers, ICSS of the LH activates the MFB. Due and the central nucleus (Ce), responsible for the auto- to the extensive network of the MFB connects, this nomic components of emotions [24]; (3) higher order causes a widespread state of arousal and simultaneous thalamic nuclei, such as the central medial (CM) and activation of many areas, some of which are associated paraventricular (PV) nuclei related to memory through with diferent memory systems [9]. Tis could explain its role in regulation of arousal or attentional levels [25], the wide therapeutic efects that MFB-ICSS appears to and the mediodorsal (MD) and dorsolateral (DL) nuclei, have in relation to diferent memory types. In addition considered to be relays to the hippocampus, amygdala to explicit memory, MFB-ICSS also improves perfor- and/or associative cortices, as well as being involved in mance in implicit memory tasks [10–12]. Most notable learning and executive functions [26, 27]; and fnally (4) are the efects on emotional memory as measured by the medial and the lateral habenula (MHb and LHb), Kádár et al. BMC Neurosci (2018) 19:48 Page 3 of 13 involved in reward, emotional behavior and cognitive consisted of sinusoidal wave currents, of 0.3 s in duration functions [28], which could also be implicated in the cir- and 50 Hz in frequency, with an intensity ranging from cuit in which MFB-ICSS afects memory via the inactiva- 50 and 250µA. Te procedure included the reinforce- tion of structures that project to it [29, 30]. ment of successive approximations to both the lever and Our fndings show that the memory systems engaged to the required action (lever press) by the animal, while by MFB-ICSS include the hilus, the oriens and radiatum attempting to fnd the minimum current intensity that layers of CA1 and CA3 hippocampal areas, the LA, DL gave a stable response of 250 lever presses in 5 min, and and CM thalamic nuclei, in accordance with the idea that self-stimulation until 500 reinforcements were reached. separate neural systems could operate in parallel to sup- Establishment of the optimal intensity (OI) for each ani- port the efects of ICSS on memory. In addition, a dif- mal was performed in the corresponding ICSS treatment ferential hemispheric Arc dependent-synaptic plasticity session as described in Segura-Torres et al. 1988 [32]. response to the MFB-ICSS treatment was observed in some of the analyzed regions.