Background

Numerous studies in rodents have demonstrated that memory is a process generally considered to occur during several stages: acquisition, consolidation and retrieval (Abel and Lattal, 2001). The acquisition phase is better known as learning, while consolidation is a labile phase during which the memory trace is stored. The standard model of memory consolidation posits that task engagement initially triggers intracellular signaling cascades and activation of immediate early gene (IEG) expression, within minutes to hours after learning, and ultimately elicits long-term synaptic changes primarily within the hippocampus (Dudai, 2004). Over time, hippocampal activity is theorized to promote extra-hippocampal consolidation, particularly in the neocortex, in a process termed “systems consolidation” that is largely independent of the hippocampus (Dudai, 2004). However, recent studies suggest that hippocampal and neocortical consolidation occur concurrently, and rapidly, after de novo learning (Runyan et al., 2019). Memory retrieval takes place during re-exposure to the learning context and is believed to involve reactivation of patterns of neural activity in hippocampal and neocortical networks associated with the original learning experience (Ben-Yakov et al., 2015). Surgical or chemical strategies that interfere with acquisition, consolidation and retrieval stages have shed light on the underlying processes of memory formation (Broadbent et al., 2004; Bast et al., 2005).

The Morris water maze (MWM) was originally developed by Richard Morris in order to test spatial learning and memory in rats (Morris, 1984). The MWM is a hippocampus dependent behavioral task in which rodents are placed in a circular pool filled with opaque water and learn to find (and climb up on) an escape platform hidden just below the water surface at an unchanging location within the pool. Initially, rodents placed in the pool find the escape platform by random navigation. However, over repeated training trials, mice slowly learn the escape platform location using objects or symbols placed outside the maze as cues, and progressively swim to the platform in a shorter time (decreased escape latency). This visuospatial task is a reliable test for rats and mice, and more recently in humans using an analogous virtual task (Kallai et al., 2005; Zhong et al., 2017). The popularity of the MWM task has resulted in the development of a plethora of different versions that vary in utility (D'Hooge and De Deyn, 2001). Generalized MWM protocols for mice have been described elsewhere ( Wenk, 2004; Choi et al., 2006; Vorhees and Williams, 2006; Bromley-Brits et al., 2011) and modified MWM protocols adapted for use with various strains, environments, genetics and ages have also been developed (Bromley-Brits et al., 2011; Barnhart et al., 2015; Weitzner et al., 2015; De Coninck et al., 2017).

Various pharmacological, genetic and lesion approaches have helped to define the brain regions, neural pathways and molecular processes involved in different stages of spatial memory using the MWM. Although lesion studies can be informative for discerning brain regions and neural circuits required for memory formation, the permanence of many types of lesions makes it difficult to precisely determine which stage(s) of memory the lesion affected. However, the MWM is highly amenable for testing the effects of injectable drugs on spatial memory. In many studies, memory modifying drugs are typically administered for a prolonged period of time or simply prior to the beginning of testing. The duration and timing of drug administration can have profound effects on the different stages of memory, thereby precluding proper determination of drug effects on learning or memory. Relatively few studies have attempted to systematically modify the timing of drug treatment during the MWM assay in order to tease apart the effect of the agent on learning/acquisition, consolidation and retrieval of spatial memory (Da and Takahashi, 2002; Florian and Roullet, 2004; Hou et al., 2006; Bonini et al., 2007). Herein we outline a protocol for distinguishing the influence of an intraperitoneally (IP) injected fast acting drug on different spatial learning and memory components.