Cross-fostering does not alter the neurochemistry or behavior of spontaneously hypertensive rats

Abstract

Background: Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable developmental disorder resulting from complex gene-gene and gene-environment interactions. The most widely used animal model, the spontaneously hypertensive rat (SHR), displays the major symptoms of ADHD (deficits in attention, impulsivity and hyperactivity) and has a disturbance in the noradrenergic system when compared to control Wistar-Kyoto rats (WKY). The aim of the present study was to determine whether the ADHD-like characteristics of SHR were purely genetically determined or dependent on the gene-environment interaction provided by the SHR dam.

Methods: SHR/NCrl (Charles River, USA), WKY/NCrl (Charles River, USA) and Sprague Dawley rats (SD/Hsd, Harlan, UK) were bred at the University of Cape Town. Rat pups were cross-fostered on postnatal day 2 (PND 2). Control rats remained with their birth mothers to serve as a reference for their particular strain phenotype. Behavior in the open-field and the elevated-plus maze was assessed between PND 29 and 33. Two days later, rats were decapitated and glutamate-stimulated release of [3H]norepinephrine was determined in prefrontal cortex and hippocampal slices.

Results: There was no significant effect of "strain of dam" but there was a significant effect of "pup strain" on all parameters investigated. SHR pups travelled a greater distance in the open field, spent a longer period of time in the inner zone and entered the inner zone of the open-field more frequently than SD or WKY. SD were more active than WKY in the open-field. WKY took longer to enter the inner zone than SHR or SD. In the elevated-plus maze, SHR spent less time in the closed arms, more time in the open arms and entered the open arms more frequently than SD or WKY. There was no difference between WKY and SD behavior in the elevated-plus maze. SHR released significantly more [3H]norepinephrine in response to glutamate than SD or WKY in both hippocampus and prefrontal cortex while SD prefrontal cortex released more [3H]norepinephrine than WKY. SHR were resilient, cross-fostering did not reduce their ADHD-like behavior or change their neurochemistry. Cross-fostering of SD pups onto SHR or WKY dams increased their exploratory behavior without altering their anxiety-like behavior.

Conclusion: The ADHD-like behavior of SHR and their neurochemistry is genetically determined and not dependent on nurturing by SHR dams. The similarity between WKY and SD supports the continued use of WKY as a control for SHR and suggests that SD may be a useful additional reference strain for SHR. The fact that SD behaved similarly to WKY in the elevated-plus maze argues against the use of WKY as a model for anxiety-like disorders.

Figure 1

Effect of cross-fostering on behavior in the open-field apparatus. *SHR pups (n = 12 – 14) travelled a greater distance (two-way ANOVA, significant effect of "pup strain", F_(2,100) = 46, P < 0.0001, post-hoc Tukey's HSD test, P < 0.01), spent a longer period of time in the inner zone (two-way ANOVA, F_(2,100) = 31, P < 0.0001, post-hoc Tukey's HSD test, P < 0.005) and entered the inner zone more frequently (two-way ANOVA, F_(2,100) = 52, P < 0.0001, post-hoc Tukey's HSD test, P < 0.005) than WKY (n = 11 – 12) and SD (n = 10 – 15). SHR also entered the inner zone more rapidly than WKY (two-way ANOVA, F_(2,100) = 30, P < 0.0001, post-hoc Tukey's HSD test, P < 0.0005). ^†SD pups travelled a greater distance (P < 0.0005), spent more time in the inner zone (P < 0.0005) and entered the inner zone more frequently (P < 0.0005) than WKY pups. SD entered the inner zone more rapidly than WKY (P < 0.0005). ^§Significantly different from SD reared by SHR or WKY (two-way ANOVA, significant interaction between "strain of dam" and "pup strain", F_(4,100) > 2.8, P < 0.05, post-hoc Tukey's HSD test, P < 0.005). ‡Significantly less than SD reared by SHR, and SHR reared by WKY or SD (two-way ANOVA, significant dam*pup interaction, F_(4,100) = 4.0, P < 0.005, post-hoc Tukey's HSD test, P < 0.005).

Figure 2

Effect of cross-fostering on behavior in the elevated-plus maze. *SHR pups (n = 12 – 14) spent less time in the closed arms (two-way ANOVA, significant effect of "pup strain" F_(2,100) = 13.9, P < 0.0001, post-hoc Tukey's HSD test, P < 0.001), more time in the open arms (two-way ANOVA, F_(2,100) = 6.0, P < 0.005, post-hoc Tukey's HSD test, P < 0.05) and entered the open arms more frequently (two-way ANOVA, F_(2,100) = 22, P < 0.0001, post-hoc Tukey's HSD test, P < 0.0005) than WKY (n = 11 – 12) and SD (n = 10 – 15). ^§Significantly greater than SD reared by WKY or SD (two-way ANOVA, significant interaction between "strain of dam" and "pup strain", F_(4,100) = 3.7, P < 0.01, post-hoc Tukey's HSD test, P < 0.005).

Figure 3

Effect of cross-fostering on [³H]norepinephrine release in prefrontal cortex and hippocampus of SHR (n = 12 – 14), WKY (n = 11 – 12) and SD (n = 10 – 15). *SHR released significantly more [³H]norepinephrine in response to glutamate than SD or WKY in hippocampus (two-way ANOVA, significant effect of "pup strain" F_(2,100) = 52, P < 0.0001, post-hoc Tukey's HSD test, P < 0.0005) and prefrontal cortex (two-way ANOVA, significant effect of "pup strain" F_(2,100) = 32, P < 0.0001, post-hoc Tukey's HSD test, P < 0.001). ^†SD pups released more [³H]norepinephrine than WKY in prefrontal cortex (P < 0.0005). ^§Significantly greater than SD reared by SHR or SD dams (two-way ANOVA, significant interaction between "strain of dam" and "pup strain", F_(4,100) = 2.6, P < 0.05, post-hoc Tukey's HSD test, P < 0.0005).