Prenatal Protein Malnutrition Produces Resistance to Distraction Similar to Noradrenergic Deafferentation of the Prelimbic Cortex in a Sustained Attention Task

Abstract

Exposure to malnutrition early in development increases likelihood of neuropsychiatric disorders, affective processing disorders, and attentional problems later in life. Many of these impairments are hypothesized to arise from impaired development of the prefrontal cortex. The current experiments examine the impact of prenatal malnutrition on the noradrenergic and cholinergic axons in the prefrontal cortex to determine if these changes contribute to the attentional deficits seen in prenatal protein malnourished rats (6% casein vs. 25% casein). Because prenatally malnourished animals had significant decreases in noradrenergic fibers in the prelimbic cortex with spared innervation in the anterior cingulate cortex and showed no changes in acetylcholine innervation of the prefrontal cortex, we compared deficits produced by malnutrition to those produced in adult rats by noradrenergic lesions of the prelimbic cortex. All animals were able to perform the baseline sustained attention task accurately. However, with the addition of visual distractors to the sustained attention task, animals that were prenatally malnourished and those that were noradrenergically lesioned showed cognitive rigidity, i.e., were less distractible than control animals. All groups showed similar changes in behavior when exposed to withholding reinforcement, suggesting specific attentional impairments rather than global difficulties in understanding response rules, bottom-up perceptual problems, or cognitive impairments secondary to dysfunction in sensitivity to reinforcement contingencies. These data suggest that prenatal protein malnutrition leads to deficits in noradrenergic innervation of the prelimbic cortex associated with cognitive rigidity.

Keywords: cognition; distractibility; norepinephrine; prefrontal cortex; rat model of malnutrition.

FIGURE 1

(A) A schematic diagram of a coronal slice from the rat brain approximately 3.7 mm anterior to Bregma. Black squares indicate the location of the prelimbic cortex (PL) (Paxinos and Watson, 2007). (B) Photomicrographs of the PL showing noradrenergic axons stained for dopamine beta hydroxylase (DBH). The top left image is taken from well-nourished control rats (25/25) and is compared to subjects exposed to prenatal malnutrition (6/25) shown on the right. Subjects from the second study are shown in the row below with SHAM-LX rats shown on the left and NE-LX shown on the right.

FIGURE 2

(A) Rats exposed to prenatal malnutrition (6/25; white bars) had significantly fewer noradrenergic axons in PL than well-nourished control subjects (25/25; black bars). There was no difference between the number of noradrenergic axons in the ACC between control and malnourished rats. (B) Data from experiment two is shown with control subjects indicated by dark gray bars (SHAM-LX) and lesioned rats indicated by light gray bars (NE-LX). Infusions of DBH-saporin produces noradrenergic deafferentation in the PL cortex while sparing fibers in the nearby ACC. Though the extent of noradrenergic damage produced by lesioning the PL was greater than the extent of damage found to result from malnutrition (38% reduction in axons due to lesioning vs. 21.7% after malnutritions), damage following both treatments was limited to the PL. ^∗ indicates p < 0.05.

FIGURE 3

Performance on the baseline SAT task in prenatally well-nourished and malnourished rats. (A) The ordinate shows the percent correct responding to signal trials. The abscissa depicts performance at each stimulus length (25, 100, and 500 ms). When accuracy was averaged across all stimulus lengths, 6/25 (empty circles) rats showed statistically significant higher accuracy on signal trials than controls (25/25; filled circles). Means and SEMs averaged across signal lengths are provided in Table 2. (B) Blocks of 54 trials are shown on the x-axis. Each block contained 27 signal trials and 27 non-signal trials presented in a pseudorandom order so that each target duration was presented on nine trials. There was no change in performance over the course of the testing session in the SAT for either 6/25 (white bars) or 25/25 rats (black bars).

FIGURE 4

(A) 6/25 rats (white bars) were more resistant to cognitive fatigue produced by the presence of a 0.5 Hz visual distractor than 25/25 rats (black bars). Prenatally protein malnourished rats did not show a significant decline in performance under these conditions until the last block of trials. In contrast, 25/25 rats were significantly impaired in detecting signals by Block 2 and their signal detection remained impaired throughout the rest of the testing session. (B) Rats with selective noradrenergic lesions of the PL cortex (light gray bars) were more resistant to the effects of a 0.5 Hz visual distractor than controls (dark gray bars) as shown by the higher number of hits (ordinate) over the course of the testing session. ^∗ indicates p < 0.05.

FIGURE 5

(A) 25/25 rats (black bars) were more impaired than 6/25 (white bars) at discriminating visual target stimuli from the unpredictable distractor (0.25, 0.5, 1.0, 1.5, 2.0, or 3.0 s on/off) during the second block of testing. While 6/25 rats were better able than 25/25 rats to maintain signal detection in the presence of this distractor, both groups of rats were impaired when performance in this session was compared to performance in the prior day’s SAT performance as reported in the results. For comparison, see data in Figure 3B. (B) Performance in the uSAT was unchanged by noradrenergic lesions with no differences found in the % hits of sham-lesioned (dark gray bars) and noradrenergically lesioned rats (light gray bars). ^∗indicates p < 0.05.

FIGURE 6

(A) The effects of a distracting, visual stimulus that overlapped in duration with targets (500, 100, or 25 ms on/off; oSAT) is shown. All rats were impaired in signal detection during this session when performance was compared to the baseline. For comparison, see data in Figure 3B. This distractor produced similar deficits in all rats regardless of prenatal protein levels. (B) Prenatally malnourished rats (6/25, white bars) emitted a greater number of false alarms when the houselight was flashed at durations equivalent to that of target stimuli (oSAT) than when the houselight was constantly illuminated (SAT). Rats from well-nourished mothers (black bars) did not show significantly higher numbers of false alarms during the overlapping distractor session (oSAT) than during than baseline SAT with the houselight constantly illuminated (SAT). These data support the hypothesis that malnutrition contributes to impairments in response inhibition. ^∗indicates p < 0.05.