Behavioral tests of hippocampal function: simple paradigms complex problems

Does my mouse have Alzheimer's disease?

Small animal models that manifest many of the characteristic neuropathological and behavioral features of Alzheimer's disease (AD) have been developed and have proven of great value for studying the pathogenesis of this disorder at the molecular, cellular and behavioral levels. The great progress made in our understanding of the genetic factors that either cause or contribute to the risk of developing AD has prompted many laboratories to create transgenic (tg) mice that overexpress specific genes which cause familial forms of the disease. Several of these tg mice display neuropathological and behavioral features of AD including amyloid beta-peptide (A beta) and amyloid deposits, neuritic plaques, gliosis, synaptic alterations and signs of neurodegeneration as well as memory impairment. Despite these similarities, important differences in neuropathology and behavior between these tg mouse models and AD have also been observed, and to date no perfect animal model has emerged. Moreover, ascertaining which elements of the neuropathological and behavioral phenotype of these various strains of tg mice are relevant to that observed in AD continues to be a challenge. Here we provide a critical review of the AD-like neuropathology and behavioral phenotypes of several well-known and utilized tg mice that express human APP transgenes.

Altered performance characteristics in cognitive tasks: comparison of the albino ICR and CD1 mouse strains

With the advent of recombinant DNA technology the mouse has become a favored model organism in brain research. Numerous mouse strains are available to use as a host for carrying genetic alteration induced by targeted or random mutagenesis. Most strains differ in their genetic makeup and phenotypical characteristics. The choice of the host strain thus can be crucial for the analysis of functional effects of the induced mutation. In the present paper we analyze the behavior of two related outbred albino strains of mice, ICR and CD1, that are often used in transgenic research. Using two frequently employed learning tasks, the Morris water maze (MWM) and the context-dependent fear conditioning (CFC) as well as other behavioral tests, we demonstrate significant performance differences between the strains. ICR suffers from a severe visual impairment making this strain difficult to use in several behavioral paradigms that require good visual perception, e.g. the MWM. CD1 does not suffer from grossly impaired vision but, similarly to the ICR strain, CD1 mice exhibit decreased freezing in all phases of CFC. Although the strains are able to learn, such deficits can render them significantly impaired dependent on the performance demands of the cognitive test employed. Our findings underscore the need for careful examination of the characteristics of the host strain, the choice of which must be made in accordance with the expected functional alterations induced by the mutation.

Ontogenetic quinpirole treatments produce spatial memory deficits and enhance skilled reaching in adult rats

There is a paucity of data on neurochemical abnormalities and associated effects on cognition and motor performance in rats ontogenetically treated with quinpirole, a rodent model of dopaminergic hyperfunction. The objective of the current study was to analyze the cognitive and motor effects produced by ontogenetic administration of quinpirole, a dopamine D(2)/D(3) receptor agonist. Past research from this laboratory has shown that ontogenetic quinpirole treatment sensitizes D(2) receptors and produces a variety of characteristic stereotypic behaviors in adult rats. In the current study, rats received quinpirole HCl (1 mg/kg/day) or saline from postnatal day (PD) 1 to PD 11 and went otherwise untreated until adulthood (PD 60). In Experiment 1, cognitive performance was assessed on the standard and matching-to-place versions of the Morris water task (MWT). In Experiment 2, skilled motor performance was assessed on the Whishaw reaching task and locomotor activity was also analyzed. We found that ontogenetically quinpirole-treated rats displayed a deficit on the probe trial given at the end of training of the standard version of the MWT but that there were no significant differences from control on the matching-to-place task. Additionally, rats treated in ontogeny with quinpirole showed significant enhancement in reaching accuracy on the Whishaw reaching task as well as increased locomotor activity relative to saline controls. These findings demonstrate that ontogenetic quinpirole treatments produce cognitive deficits, enhanced skilled reaching and hyperlocomotion. The behavioral changes produced by ontogenetic quinpirole treatment are consistent with dopaminergic hyperfunction, and possible mechanisms are discussed.

Hippocampal dysfunction and behavioral deficit in the water maze in mice: an unresolved issue?

Dysfunction of the hippocampal formation manifests as impaired relational learning and memory in humans and animals. One of the most frequently applied relational learning paradigms in animals is the Morris water maze (MWM), in which the subject is required to learn complex spatial relationships of visual cues. MWM has been employed as a diagnostic tool to investigate effects of drugs and mutations. However, the validity of this test and its ability to properly detect hippocampal dysfunction have been questioned. In order to corroborate the role of hippocampus in spatial learning, we employed ibotenic acid lesioning and ablated the hippocampus bilaterally or unilaterally in mice, as ascertained by magnetic resonance imaging. We found a significant impairment in response to hippocampal disruption that was more pronounced in mice with bilateral lesion than with unilateral lesion. However, the results also indicated that even the mice with bilateral lesion could improve their performance, which confirms the notion that the MWM has an important non-hippocampal component. It is thus possible that experimental alteration of brain function does not manifest as modified performance in MWM, even when hippocampal function is modified (false-negative finding), or manifest as altered performance without varying hippocampal function (false-positive finding), possibilities that have important implications for studies using genetic and pharmacological manipulation of the brain.

Impaired water maze learning performance without altered dopaminergic function in mice heterozygous for the GDNF mutation

Exogenous glial cell line-derived neurotrophic factor (GDNF) exhibits potent survival-promoting effects on dopaminergic neurons of the nigrostriatal pathway that is implicated in Parkinson's disease and also protects neurons in forebrain ischemia of animal models. However, a role for endogenous GDNF in brain function has not been established. Although mice homozygous for a targeted deletion of the GDNF gene have been generated, these mice die within hours of birth because of deficits in kidney morphogenesis, and, thus, the effect of the absence of GDNF on brain function could not be studied. Herein, we sought to determine whether adult mice, heterozygous for a GDNF mutation on two different genetic backgrounds, demonstrate alterations in the nigrostriatal dopaminergic system or in cognitive function. While both neurochemical and behavioural measures suggested that reduction of GDNF gene expression in the mutant mice does not alter the nigrostriatal dopaminergic system, it led to a significant and selective impairment of performance in the spatial version of the Morris water maze. A standard panel of blood chemistry tests and basic pathological analyses did not reveal alterations in the mutants that could account for the observed performance deficit. These results suggest that endogenous GDNF may not be critical for the development and functioning of the nigrostriatal dopaminergic system but it plays an important role in cognitive abilities.