Effects of pre-weaning exposure to a maze on stress responses in pigs at weaning and on subsequent performance in spatial and fear-related tests

Abstract

Learning, memory and regulation of the stress response are mediated by the hippocampus. Biologically relevant hippocampal-dependent tasks that develop and integrate cognitive processing of this region may not be available to piglets in some current production systems. Additionally, weaning piglets at less than two weeks of age may make them particularly vulnerable to disruptions during cognitive development. We measured the effects of a spatial maze task (MT) on the stress response of piglets at weaning (12 days of age) and subsequent fear response (at 7 weeks of age). Twenty-seven pigs from four litters were assigned to one of three treatments: maze task (MT), isolation control (IC), or control with sow (SC), then combined into same sex groups with each treatment represented. Each group was tested four times per day from 5–11 days of age. MT piglets navigated the MT in order to return to the home farrowing crate containing the sow and litter. IC piglets were isolated for the same length of time taken for the MT piglet in their group to navigate the MT. SC piglets controlled for handling and were returned to sow as MT entered the MT. Saliva was collected immediately pre- and post-MT on day 11 to measure cortisol concentrations, which were lower pre-MT ( F(1, 8) = 5.65, P = 0.04). Weaning at 12 days of age increased cortisol concentrations 2 h post-weaning ( F(4, 75) = 5.67, P < 0.001). When exposed to a modified Morris water maze (MWM), a significant interaction of sex and treatment was found with MT males and IC females faster at solving the MWM than male IC pigs ( χ 2(2) = 9.14, P = 0.01). Lower cortisol concentrations were seen pre-water maze versus post-water maze ( F(1, 19) = 27.62, P < 0.001). At 50 days of age, fear response of pigs was examined using three fear-related tests (open field test (OFT), novel object test (NOT), and human approach test (HAT), each consisting of 1 m acclimation and 4 m testing. In the HAT, MT animals had a tendency to touch the unfamiliar person more quickly ( F(2, 19) = 2.51, P = 0.10), and more times than animals in other groups ( F(2, 49) = 6.31, P = 0.008). MT may result in less fear of novel persons and ameliorate cognitive deficits in male pigs, suggesting benefits of exposing young pigs to environments requiring spatial learning.