Neonatal infection modulates behavioral flexibility and hippocampal activation on a Morris Water Maze task

Abstract

Neonatal infection has enduring effects on the brain, both at the cellular and behavioral levels. We determined the effects of peripheral infection with Escherichia coli at postnatal day (P) 4 in rats on a water maze task in adulthood, and assessed neuronal activation in the dentate gyrus (DG) following the memory test. Rats were trained and tested on one of 3 distinct water maze task paradigms: 1) minimal training (18 trials/3days), 2) extended training (50 trials/10days) or 3) reversal training (extended training followed by 30 trials/3days with a new platform location). Following a 48h memory test, brains were harvested to assess neuronal activation using activity-regulated cytoskeleton-associated (Arc) protein in the DG. Following minimal training, rats treated neonatally with E. coli had improved performance and paradoxically reduced Arc expression during the memory test compared to control rats treated with PBS early in life. However, neonatally-infected rats did not differ from control rats in behavior or neuronal activation during the memory test following extended training. Furthermore, rats treated neonatally with E. coli were significantly impaired during the 48h memory test for a reversal platform location, unlike controls. Specifically, whereas neonatally-infected rats were able to acquire the new location at the same rate as controls, they spent significantly less time in the target quadrant for the reversal platform during a memory test. However, neonatally-infected and control rats had similar levels of Arc expression following the 48h memory test for reversal. Together, these data indicate that neonatal infection may improve the rate of acquisition on hippocampal-dependent tasks while impairing flexibility on the same tasks; in addition, network activation in the DG during learning may be predictive of future cognitive flexibility on a hippocampal-dependent task.

Keywords: Arc; Immediate early gene; Reversal learning; Spatial learning.

Figure 1. Neonatally-infected rats have better memory and decreased neuronal activation in the DG following minimal training

A) Both groups of adult rats (PBS- vs. E. coli-treated on P4) acquired the platform location on the water maze task equally well with minimal training (F_(1,76)=91.3, p<0.001). B) Neonatally-infected rats had significantly better performance on the memory probe at 48HR (t₍₃₀₎= −1.90, p=0.067). C) Arc protein in the DG of neonatally-infected rats was significantly lower at the 48HR probe than in PBS-treated control rats (t₍₂₉₎=2.47, p=0.020). There were no differences in Arc protein expression in the CA1 (not shown). D) After a single trial (SWIM), both treatment groups had equal Arc expression in the DG (t₍₂₅₎=−0.32, p=0.75) and greater average expression compared to trained rats.

Figure 2. All rats exhibit similar acquisition, memory and neuronal activation following extensive training

A) Both neonatally-infected rats and controls acquired the platform location at the same rat during extensive training (5 days of training with 10 trials per day) (F_(1,56)=31.0, p<0.001). B) When tested 48HR after their last training trial, both groups performed equally on the probe test (t₍₁₄₎= 1.08, p=0.298). C) When the brains of the trained rats were assessed, both groups had similar numbers of Arc-positive cells in the DG (t₍₁₄₎=−0.781, p=0.448). D) The “yoked” rats that matched the trained rats in latency in the pool had similar Arc-positive cell counts in the DG, regardless of neonatal treatment (t₍₁₄₎=−0.44, p=0.67).

Figure 3. Neonatally-infected rats are significantly impaired on a reversal task, despite acquisition of both the original and reversal platform locations comparable to controls

A) Both neonatal treatment groups acquired the platform at the same rate during the 5 days of extensive training (F_(4,159)=142.4, p<0.001) and the 3 days of reversal training (F_(2,95)=140.0, p<0.001). B) Controls and neonatally-infected rats performed similarly on the 48HR (t₍₁₃₎= 0.78, p=0.451) and 7 Day (t₍₁₃₎=0.33, p=0.745) memory probe tests for the original platform. After reversal training, however, neonatally-infected rats had an impaired memory for the reversal platform location (t₍₂₈₎=2.98, p=0.006).

Figure 4. Neuronal activation does not differ between treatment groups at the 48HR reversal probe

In spite of a behavioral difference in which neonatally-infected rats performed worse on the probe test, both groups had equal numbers of Arc-positive cells in the DG during that probe (t₍₁₄₎=−0.259, p=0.8).

Figure 5. Representative images from immunohistochemical staining

Each image shows the dentate gyrus in one hemisphere at 10X magnification. The scale bars indicate 100 μm and the arrows indicate representative Arc+ cells. (A) DG from a minimally trained control rat after the 48HR probe. (B) DG from a minimally trained neonatally-infected rat after the 48HR probe. (C) DG from an extensively trained control rat after the 48HR probe. (D) DG from an extensively trained neonatally-infected rat after the 48HR probe.