Disruption of dorsal hippocampal – prefrontal interactions using chemogenetic inactivation impairs spatial learning
Introduction
Animals need to form, maintain and retrieve memories of their experiences in novel environments for survival. This capacity to utilize internal representations of the external environment to guide behavior depends upon functional networks distributed across multiple brain regions. The hippocampus (HPC) and medial prefrontal cortex (PFC), anatomically and functionally connected brain regions, both play key roles in our ability to learn, form and use memories to guide behavior (Eichenbaum, 2017, Preston and Eichenbaum, 2013, Shin and Jadhav, 2016). These regions have complementary and overlapping roles in memory processes, with the hippocampus critical for encoding, storage and retrieval of new memories (Day et al., 2003, Eichenbaum and Cohen, 2001, Moser and Moser, 1998, Riedel et al., 1999); and PFC playing an integral role in long-term memory storage and retrieval, as well as executive functions such as working memory and decision making (Euston et al., 2012, Frankland et al., 2004, Jung et al., 2008, Miller and Cohen, 2001, Takehara-Nishiuchi and McNaughton, 2008, Tse et al., 2007).
Inactivation studies in rodents have established the role of both regions in spatial memory formation and retrieval. It is known that bilateral inactivation of either HPC or PFC impairs the ability of rats to perform spatial tasks that require working memory (Churchwell et al., 2010, Floresco et al., 1997, Riedel et al., 1999). In addition, functional interactions between these regions have been shown to be involved in these tasks (Churchwell et al., 2010, Floresco et al., 1997). Anatomically, the HPC and PFC are strongly connected via multiple direct and indirect projections in the rodent brain (Cenquizca and Swanson, 2007, Delatour and Witter, 2002, Shin and Jadhav, 2016, Vertes, 2004, Vertes et al., 2007). Here, PFC is used to denote the prelimbic (PrL) and infra-limbic (IL) regions of the medial prefrontal cortex. A prominent monosynaptic, ipsilateral and unidirectional projection arises from the ventral and intermediate CA1 and subicular regions of the hippocampus (vHPC, iHPC respectively), and terminates across various regions of PFC (Cenquizca and Swanson, 2007, Swanson, 1981). Previous functional disconnection studies have reported impairments in the ability of rodents to perform spatial tasks upon disruption of vHPC – PFC and iHPC – PFC interactions mediated by these projections (Churchwell et al., 2010, Floresco et al., 1997, Wang and Cai, 2006, Wang and Cai, 2008). These studies used a contralateral inactivation approach where the vHPC/iHPC and PFC regions are inactivated in different hemispheres, thereby disrupting interactions mediated by the ipsilateral projections. Using this method, iHPC – PFC interactions have been shown to play a role in for encoding and retrieval in a spatial maze task (Churchwell et al., 2010). In addition, vHPC – PFC interactions are important in spatial working memory performance in a delayed T-maze alternation task (Wang & Cai, 2006) and delayed radial-arm maze performance task (Floresco et al., 1997), as well as in spatial navigation learning in a Morris water maze task (Wang & Cai, 2008). Other indirect anatomical connections between these regions also play a role in spatial working memory; for example, disrupting indirect projections from PFC to HPC via the nucleus reuniens also leads to memory impairments and disruption of hippocampal representations (Hallock et al., 2013, Ito et al., 2015, Layfield et al., 2015, Viena et al., 2018).
Physiologically, multiple network patterns have been shown to mediate the coordination of hippocampal – prefrontal activity, which could sub serve the functional interactions indicated by the inactivation studies. Interestingly, these physiological interactions are seen both with respect to dorsal HPC (dHPC) as well as vHPC. Network patterns that mediate these interactions include phase-locking and coherence during theta oscillations (6–12 Hz) (Benchenane et al., 2010, Gordon, 2011, Hyman et al., 2005, Jones and Wilson, 2005, Siapas et al., 2005), coordinated reactivation during sharp wave ripples (150–250 Hz) (Jadhav et al., 2016, Peyrache et al., 2009, Tang and Jadhav, 2018, Tang et al., 2017), and cross-regional theta-gamma coupling (Spellman et al., 2015, Tamura et al., 2017), all of which have been implicated in spatial working memory. Numerous studies have thus established that theta oscillations and SWRs mediate dHPC – PFC interactions during learning and performance of spatial memory tasks. Despite the abundance of electrophysiological studies suggesting the importance of dHPC – PFC interactions in spatial memory, to our knowledge, whether disruption of interactions between these two regions leads to spatial learning impairments has yet to be tested.
Several lines of evidence suggest the possibility that dHPC – PFC interactions play a role in spatial learning and memory. Recently, direct connections that arise from dHPC and project unilaterally to the PFC have been reported (DeNardo et al., 2015, Hoover and Vertes, 2007, Xu and Sudhof, 2013, Ye et al., 2017), and these projections have been shown to mediate contextual fear memory (Ye et al., 2017). Indeed, it is known that dHPC place cells represent spatial information with the highest precision compared to iHPC and vHPC regions (Kjelstrup et al., 2008), and therefore encode spatial contextual information with high fidelity. Further, our previous studies have reported that SWRs in dHPC mediate coordinated reactivation of spatial information in the dHPC – PFC network (Jadhav et al., 2016, Tang et al., 2017). This coordinated dHPC – PFC reactivation is especially strong during initial learning of spatial alternation tasks (Tang and Jadhav, 2018, Tang et al., 2017), suggesting that dHPC – PFC interactions may play a role in novel task learning. In the current study, we therefore investigated whether disruption of dHPC – PFC interactions using a contralateral inactivation approach impairs spatial alternation learning.
We implemented a rapid, single-day learning paradigm in a hippocampal-dependent W-track alternation task comprising two components, a spatial reference memory component, and a spatial working memory component. Using a chemogenetic inactivation approach, we tested if dHPC – PFC interactions contribute to learning in a novel W-track maze. Virally introduced DREADDs were used for precise targeting of excitatory circuits in dHPC and PFC, with systemic Clozapine N-oxide (CNO) injections for inactivating circuits in these animals (Roth, 2016, Urban and Roth, 2015). Learning was assessed in multiple groups of animals: contralateral inactivation of dHPC and PFC, ipsilateral inactivation of these regions, systemic vehicle-injected controls, and naïve controls. We found that contralateral inactivation led to a specific impairment in learning the spatial working memory component of the task. Ipsilateral inactivation, which accounts for effects of unilateral inactivation as well as any non-specific effects of systemic CNO injections, showed no deficits in learning. In addition, we also performed experiments to confirm that bilateral PFC inactivation impairs learning in this task, as established in other spatial working memory tasks (Churchwell et al., 2010, Wang and Cai, 2006, Wang and Cai, 2008). Our results demonstrate that dHPC – PFC interactions are necessary for rapid spatial alternation learning, and provide crucial supporting evidence for the hypothesis that physiological interactions between the dHPC and PFC play an important role in spatial learning and memory.
Section snippets
Animals
Adult Long Evans rats (n = 46, 6–8 months old, 450–600 g) obtained from Charles River Laboratories were used for all experiments. All procedures were conducted in accordance with the guidelines of the US National Institutes of Health and approved by the Institutional Animal Care and Use Committee at Brandeis University. All animals used were individually housed in temperature and humidity regulated cages and kept in a facility maintained in a 12-hour light-dark cycle. Ad libitum food and water
Histology
In order to perturb the activity of dHPC and PFC during learning of the W-track task, we injected adeno-associated viral constructs of an evolved human muscarinic receptor (hM4Di DREADDs) targeting excitatory neurons in the specified regions using a CaMKIIα promoter (Fig. 2). In Experiment 1, we investigated the effects of functionally disconnecting dHPC and PFC during spatial learning by targeting these regions in contralateral hemispheres (contralateral inactivation group). Fig. 2A and B show
Discussion
Our results establish that dorsal hippocampal (dHPC) – prefrontal (PFC) interactions are required for rapid spatial alternation learning in novel environments, and specifically, learning of spatial working memory tasks. We used a chemogenetic method to inactivate dHPC and PFC regions in contralateral hemispheres during acquisition of spatial alternation learning in a novel W-track maze. This contralateral inactivation strategy leaves intact the dHPC and PFC in one hemisphere, but disrupts
Acknowledgements
We thank all members of the Jadhav lab for comments on the manuscript.
Funding sources
This work was supported by the National Institute of Health [grant number R01 MH112661]; a Sloan Research Fellowship in Neuroscience (Alfred P. Sloan Foundation), a NARSAD Young Investigator grant (Brain and Behavior Foundation), and Whitehall Foundation award to SPJ.
Conflict of interest
None.
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Cited by (0)
- 1
Present address: Cold Spring Harbor Laboratory, Cold Spring Harbor, NY 11724, USA.
- 2
These authors contributed equally to this work.
- 3
Present address: McLean Hospital, Belmont, MA 02478, USA.
- 4
Present address: Boston Children’s Hospital, Boston MA 02115, USA.