Nick P. van Goethem

Object recognition testing: methodological considerations on exploration and discrimination measures

The object recognition task (ORT) is a popular one-trial learning test for animals. In the current study, we investigated several methodological issues concerning the task. Data was pooled from 28 ORT studies, containing 731 male Wistar rats. We investigated the relationship between 3 common absolute- and relative discrimination measures, as well as their relation to exploratory activity. In this context, the effects of pre-experimental habituation, object familiarity, trial duration, retention interval and the amnesic drugs MK-801 and scopolamine were investigated. Our analyses showed that the ORT is very sensitive, capable of detecting subtle differences in memory (discrimination) and exploratory performance. As a consequence, it is susceptible to potential biases due to (injection) stress and side effects of drugs. Our data indicated that a minimum amount of exploration is required in the sample and test trial for stable significant discrimination performance. However, there was no relationship between the level of exploration in the sample trial and discrimination performance. In addition, the level of exploration in the test trial was positively related to the absolute discrimination measure, whereas this was not the case for relative discrimination measures, which correct for exploratory differences, making them more resistant to exploration biases. Animals appeared to remember object information over multiple test sessions. Therefore, when animals have encountered both objects in prior test sessions, the object preference observed in the test trial of 1h retention intervals is probably due to a relative difference in familiarity between the objects in the test trial, rather than true novelty per se. Taken together, our findings suggest to take into consideration pre-experimental exposure (familiarization) to objects, habituation to treatment procedures, and the use of relative discrimination measures when using the ORT.

Object recognition testing: rodent species, strains, housing conditions, and estrous cycle.

The object recognition task (ORT) allows assessing learning and memory processes in rodents. In this study, two areas in which knowledge about the ORT could be extended were addressed; i.e. generality to species and strains, and intervening variables including housing and estrous cycle. Regarding generality to species and strains, the ORT performance of golden hamsters was assessed. The hamsters showed sufficient exploration times, object recognition performance, and a retention-interval dependent decline similar to rats and mice. Subsequently, we tested three mouse strains which have not been described before in the ORT; i.e. OF1, NMRI, and SJL mice. OF1 and NMRI strains performed equally well, whereas the SJL strain showed low exploration times and no memory retention. Therefore, the SJL strain is unsuited for ORT experiments using a 1h retention interval and a fixed (3 min) trial duration. Furthermore, the sensitivity to a pharmacological memory deficit model (scopolamine) was tested in three rat strains. Each strain showed a dose dependent relationship, but the least effective dose of scopolamine differed among the three strains, the effect being greater in the order of Wistar, Long-Evans, Hooded Lister rats. Finally, to investigate potential intervening variables in the ORT, the effects of housing conditions and estrous cycle were investigated with rats. Single housing resulted in absolute higher performance than social housing. Furthermore, females in pro-estrus/estrus showed better performance compared to females in met-estrus/di-estrus. Taken together, object recognition appears to be a common ability of rodent species, but different strains have different memory capacities and sensitivities to scopolamine, individual housing leads to higher performance, and performance of females is dependent on the estrous cycle phase. Thus, rodent species, strain, housing, and estrous cycle should be taken into consideration in ORT studies.

Memory deficits in the transgenic rat model of Huntington's disease

Memory deficits are common in patients with Huntingtons disease (HD) and have a substantial impact on the quality of life of patients and their relatives. A good model resembling the human memory deficits is needed for research purposes. In this study we investigated the memory function of the transgenic rat model of Huntingtons disease (tgHD) in the object location (OLT) and the object recognition task (ORT). Several studies have shown that the recent developed tgHD rat model resembles the human phenotype of HD. Impairments of spatial and object recognition memory in the OLT and ORT, however, have to our knowledge not yet been reported in this transgenic model. Our findings show that in both early and late stages of the disease the tgHD rats have clear deficits for both visuospatial and visual object memory. Since HD patients are known to be impaired in both types of memory, these results confirm the validity of this tgHD rat as a model for the human HD phenotype.

Continuous infusion of the alpha 7 nicotinic acetylcholine receptor agonist EVP-6124 produces no signs of tolerance at memory-enhancing doses in rats: a pharmacokinetic and behavioral study

We investigated whether the effects of acutely administered EVP-6124, an α7 nicotinic acetylcholine receptor (α7 nAChR) agonist, on cognition were maintained after 6-day continuous minipump administration. Performance in a delay-dependent forgetting test was measured in the object recognition task after single-oral doses of 0.3 or 1 mg/kg, or at plasma steady-state concentrations (Css) of 0.6 or 2 ng/ml, which were similar to the efficacious plasma concentrations after single-oral dosing. The 0.3 mg/kg acute dose enhanced memory at a total plasma concentration of ∼0.3 ng/ml at 1-4 h after dosing. Continuous treatment produced total plasma Css values of 0.48 and 1.93 ng/ml on day 6 and enhanced memory. At EVP-6124 plasma concentrations that optimally enhance memory in the object recognition task, tolerance did not develop after 6 days of continuous treatment.

Donepezil and the alpha-7 agonist PHA 568487, but not risperidone, ameliorate spatial memory deficits in a subchronic MK-801 mouse model of cognitive impairment in schizophrenia.

Cognitive impairment associated with schizophrenia (CIAS) is an important etiological feature of this disorder with implications for symptom severity and quality of life. Acute N-methyl-d-aspartate receptor (NMDAR) blockade using MK-801, a non-competitive antagonist to NMDARs, is assumed to produce temporary cognitive impairments in mice similar to those seen in schizophrenia patients. Less is known, however, about the effects of subchronic MK-801 administration on cognition. In the current study, twenty-eight male C57/BL6 mice received a daily dose of MK-801 (0.1mg/kg, i.p.) for seven days. Spatial memory was assessed using an object location task prior to MK-801 administration as well as at multiple time points after the treatment. Subchronic treatment with MK-801 caused lasting memory deficits, which were ameliorated by acute doses of an acetylcholinesterase inhibitor (donepezil) and an alpha-7 nicotinic agonist (PHA 568487), but were unaffected by acute administration of the atypical antipsychotic risperidone. Subchronic administration of MK-801 may lend this pharmaceutical model increased face validity, while its resemblance to prodromal schizophrenia makes it suitable for screening new CIAS treatments.

New insights into selective PDE4D inhibitors: 3-(Cyclopentyloxy)-4-methoxybenzaldehyde O-(2-(2,6-dimethylmorpholino)-2-oxoethyl) oxime (GEBR-7b) structural development and promising activities to restore memory impairment

Phosphodiesterase type 4D (PDE4D) has been indicated as a promising target for treating neurodegenerative pathologies such as Alzheimers Disease (AD). By preventing cAMP hydrolysis, PDE4 inhibitors (PDE4Is) increase the cAMP response element-binding protein (CREB) phosphorylation, synaptic plasticity and long-term memory formation. Pharmacological and behavioral studies on our hit GEBR-7b demonstrated that selective PDE4DIs could improve memory without causing emesis and sedation. The hit development led to new molecule series, herein reported, characterized by a catechol structure bonded to five member heterocycles. Molecular modeling studies highlighted the pivotal role of a polar alkyl chain in conferring selective enzyme interaction. Compound 8a showed PDE4D3 selective inhibition and was able to increase intracellular cAMP levels in neuronal cells, as well as in the hippocampus of freely moving rats. Furthermore, 8a was able to readily cross the blood-brain barrier and enhanced memory performance in mice without causing any emetic-like behavior. These data support the view that PDE4D is an adequate molecular target to restore memory deficits in different neuropathologies, including AD, and also indicate compound 8a as a promising candidate for further preclinical development.

Translational issues with the development of cognition enhancing drugs.

Franco and Cedazo-Minguez, in their review, raised the issue that animal models have a poor predictability with respect to drug development in the field of Alzheimer’s disease (AD) (1). Although this critical paper touches upon highly relevant issues in the development of new drugs, various other factors can be suggested, which also complicate the translation from animal studies to human studies. Here, we briefly discuss these issues relating to the development of cognition enhancers.

Physiological and pathological processes of synaptic plasticity and memory in drug discovery : Do not forget the dose-response curve

Abstract The response of a biological system to an endogenous or exogenous molecule depends upon the dose. For this reason, performing dose‐response curves is crucial to understand physiological and pathophysiological phenomena, and to predict the effect of a drug. Most of the studies in pharmacological research have been performed according to the classical threshold model, focusing on higher doses able to ensure a biological effect. However, recent evidences pointed out the need to investigate the effect of low doses. Indeed, several molecules behave in a hormetic fashion, i.e. low‐doses stimulate whereas high‐doses inhibit a biological response. This is particularly interesting in CNS, where several physiological molecules involved in neuronal transmission during learning and memory have shown a biphasic effect that might represent the link between physiology and pathology. In this review we will focus on cholinergic, glutamatergic and nitrinergic transmission, because of their central role in learning and memory and their impairment in neurodegenerative disorders such as Alzheimer’s disease. Pre‐clinical studies performed on healthy adult animals and aged animals, as well as transgenic animal models of AD, have suggested a biphasic DR for acetylcholine, glutamate and nitric oxide. This stresses the relevance to perform DR curves when studying the mechanisms underlying synaptic plasticity and memory, the pharmacological profile of cognitive‐enhancing drugs acting on these systems, and the possibility to combine low/ineffective doses of drugs that might have additive/synergistic effects, reducing the unwanted side effects associated to the high doses.

Assessing spatial pattern separation in rodents using the object pattern separation task

Pattern separation is the process of transforming highly similar sensory inputs into distinct, dissimilar representations. It takes place in the hippocampus and is thought to be used in episodic memory. Impaired pattern separation performance has been recognized as a predictor for the development of cognitive impairments such as dementia in humans and as being present in patients with schizophrenia and post-traumatic stress disorder (PTSD). In this protocol, we describe how to implement a simple and robust object pattern separation (OPS) task in mice and rats that we have previously established and validated. This two-trial memory task uses specific object locations so differences in performance can be calibrated with the extent of object movement. Changes in performance are indicative of spatial pattern separation. In contrast to other pattern separation tasks, the OPS task allows detection of spatial pattern separation performance bidirectionally. Furthermore, the OPS task is cheaper and easier to use and interpret than other tasks that use more than two objects or that are touch-screen based. The entire protocol, from vivarium acclimatization to training of the animals, takes ~35–41 d. After successful training, the animals can be tested repeatedly, and three OPS experiments (n = 20–24 per experimental day) can be performed per week. A standard level of expertise in behavioral studies in rodents is sufficient to successfully integrate this paradigm into an existing rodent test battery.In this behavioral task, two objects are placed at specific locations at different times, and the response of mice and rats to these positions is assessed. The results indicate the rodents’ ability to recognize changes in spatial pattern separation.

Publisher Correction: Assessing spatial pattern separation in rodents using the object pattern separation task

In the HTML version of this paper originally published online, text in Table 6 was misaligned in a way that made it difficult to determine which entries in the “Problem,” “Possible reason,” and “Solution” columns corresponded to one another. Additional but less severe alignment problems were also present in the PDF and print articles. These errors have been corrected in the HTML and PDF versions of the paper.