Spatial and visual discrimination reversals in adult and geriatric rats exposed during gestation to methylmercury and n − 3 polyunsaturated fatty acids

Abstract

Fish contain essential long chain polyunsaturated fatty acids (PUFAs), particularly docosahexaenoic acid (DHA), an omega-3 (or n − 3) PUFA, but are also the main source of exposure to methylmercury (MeHg), a potent developmental neurotoxicant. Since n − 3 PUFAs support neural development and function, benefits deriving from a diet rich in n − 3s have been hypothesized to protect against deleterious effects of gestational MeHg exposure. To determine whether protection occurs at the behavioral level, female Long-Evans rats were exposed, in utero, to 0, 0.5, or 5 ppm of Hg as MeHg via drinking water, approximating exposures of 0, 40, and 400 μg Hg/kg/day and producing 0, 0.29, and 5.50 ppm of total Hg in the brains of siblings at birth. They also received pre- and postnatal exposure to one of two diets, both based on the AIN-93 semipurified formulation. A “fish-oil” diet was high in, and a “coconut-oil” diet was devoid of, DHA. Diets were approximately equal in α-linolenic acid and n − 6 PUFAs. As adults, the rats were first assessed with a spatial discrimination reversal (SDR) procedure and later with a visual (nonspatial) discrimination reversal (VDR) procedure. MeHg increased the number of errors to criterion for both SDR and VDR during the first reversal, but effects were smaller or non-existent on the original discrimination and on later reversals. No such MeHg-related deficits were seen when the rats were retested on SDR after 2 years of age. These results are consistent with previous reports and hypotheses that gestational MeHg exposure produces perseverative responding. No interactions between diet and MeHg were found, suggesting that n − 3 PUFAs do not guard against these behavioral effects. Brain Hg concentrations did not differ between the diets, either. In geriatric rats, failures to respond were less common and response latencies were shorter for rats fed the fish-oil diet, suggesting that exposure to a diet rich in n − 3s may lessen the impact of age-related declines in response initiation.

Introduction

Developmental exposure to methylmercury (MeHg) at doses that result in low micromolar concentrations of mercury in whole brain (Newland and Reile, 1999, Newland et al., 2006b) produce long-lasting behavioral effects that are manifested in adulthood and become exacerbated in aging (Kinjo et al., 1993, Newland and Rasmussen, 2000, Newland et al., 2004, Rice, 1996). Included among these effects are impaired sensory function in nonhuman primates (Rice and Gilbert, 1982, Rice and Gilbert, 1990, Rice, 1996), retarded behavior in transition in rodents and nonhuman primates (Newland et al., 1994, Newland et al., 2004, Paletz et al., 2006), deficits in complex, high-rate operant behavior (Newland and Rasmussen, 2000), disrupted performance on timing in fixed-interval schedules of reinforcement (Rice, 1992), delayed object permanence (Burbacher et al., 1988, Gunderson et al., 1988b), and impaired facial recognition (Gunderson et al., 1988a). Performance on tasks that tap memory function, however, are relatively spared after developmental MeHg exposure (Elsner et al., 1988, Gilbert et al., 1993, Goldey et al., 1994, Newland and Paletz, 2000). This behavioral toxicity is consistent with damage to the cerebral cortex, a region that is especially susceptible to low-level developmental MeHg exposure. Even at very low, submicromolar, concentrations, MeHg disrupts cell signaling, calcium currents, and induces dysmorphology in in vitro models of cortical development (Barone et al., 1998, Parran et al., 2003, Shafer et al., 2002).

Consumption of certain types of fish, notably long-lived predators, is the primary route of MeHg exposure. Marine fish also contain docosahexaenoic acid (DHA), an essential omega-3 polyunsaturated fatty acid (n − 3 PUFA) that supports brain development and visual and motor function (Egeland and Middaugh, 1997, Reisbick and Neuringer, 1997). Dietary deficiencies in n − 3 PUFAs reduce brain weight (Wainwright et al., 1992), impair vision (Okaniwa et al., 1996, Reisbick et al., 1997, Yamamoto et al., 1987, Yamamoto et al., 1988), and produce motor deficits (Greiner et al., 1999). There is mixed evidence about whether n − 3 PUFAs affect such cognitive functions as learning, behavior change, or memory (Becker and Kyle, 2001, Wainwright, 1997, Wainwright et al., 1999). Nevertheless, it is reasonable to hypothesize that benefits derived from consumption of n − 3 PUFAs may protect against or otherwise counter some of MeHg's effects (Davidson et al., 1998, Egeland and Middaugh, 1997, Grandjean et al., 1997, Mahaffey, 1998).

In studies involving a choice between two sources of reinforcement, the acquisition of choice but not its steady-state expression was especially sensitive to gestational MeHg. In a common choice procedure, an animal is presented with a left and right lever to press, and the reinforcement rate on one lever is scheduled to occur independently of that on the other lever. Under these conditions, animals allocate responses to each lever in accordance with the relative ratio of the two reinforcement rates (Davison and McCarthy, 1988). When the ratio changes, allocation of behavior adjusts to the new conditions, but gestational MeHg exposure retards this adjustment (Newland et al., 1994, Newland et al., 2004). In these studies, MeHg exposure may have strengthened the impact of reinforcers delivered prior to the change of conditions to such an extent that the reinforced behavior was resistant to change. This outcome is consistent with accounts of response perseveration in that allocation of behavior under the previous arrangements would persist despite the change in the structure of the environment. Response perseveration has also been observed in a study of the acquisition of fixed-ratio responding in gestationally exposed rats (Paletz et al., 2006).

The present study was designed to determine the effects of gestational MeHg and gestational plus lifetime exposure to n − 3 PUFAs using discrimination reversal procedures, which permit an examination of perseverative responding on a lever that no longer produces any reinforcers. The first phase comprised a spatial discrimination reversal task in which up to seven reversals were evaluated. In the second phase, the same animals performed a more difficult visual discrimination reversal. Finally, the rats were reevaluated on the spatial task, 9 months after their behavior was initially assessed and when most were over two years of age. Female Long-Evans rats, used as breeders, began their diets 5 weeks, and MeHg exposure 2 weeks, before breeding commenced. MeHg exposure of the offspring ended at weaning. Female offspring were used in the present study so as to facilitate comparisons with chronic exposure to the female breeders, which were studied in other experiments (Day et al., 2005).