Intact attentional processing but abnormal responding in M₁ muscarinic receptor-deficient mice using an automated touchscreen method

Abstract

Cholinergic receptors have been implicated in schizophrenia, Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. However, to better target therapeutically the appropriate receptor subsystems, we need to understand more about the functions of those subsystems. In the current series of experiments, we assessed the functional role of M₁ receptors in cognition by testing M₁ receptor-deficient mice (M1R^−/−) on the five-choice serial reaction time test of attentional and response functions, carried out using a computer-automated touchscreen test system. In addition, we tested these mice on several tasks featuring learning, memory and perceptual challenges. An advantage of the touchscreen method is that each test in the battery is carried out in the same task setting, using the same types of stimuli, responses and feedback, thus providing a high level of control and task comparability. The surprising finding, given the predominance of the M₁ receptor in cortex, was the complete lack of effect of M₁ deletion on measures of attentional function per se. Moreover, M1R^−/− mice performed relatively normally on tests of learning, memory and perception, although they were impaired in object recognition memory with, but not without an interposed delay interval. They did, however, show clear abnormalities on a variety of response measures: M1R^−/− mice displayed fewer omissions, more premature responses, and increased perseverative responding compared to wild-types. These data suggest that M1R^−/− mice display abnormal responding in the face of relatively preserved attention, learning and perception.

Introduction

It is widely acknowledged that our attempts at developing pro-cholinergic treatments for diseases affecting cognition – schizophrenia, Alzheimer’s disease, Parkinson’s disease and Huntington’s disease, to name just a few (Bartus et al., 1982, Coyle et al., 1983, Eglen et al., 1999, Felder et al., 2000, Friedman, 2004, Wess, 2004, Youdim and Buccafusco, 2005) – have been met with limited success. One possible reason for this lack of success is our lack of understanding of the specific functional roles of the various subtypes of cholinergic receptors (Gainetdinov and Caron, 1999). To target treatments to the critical cholinergic subsystem associated with disease, we must first understand the functions of these subsystems. With respect to diseases involving changes in cognition, many authors have emphasized specifically the importance of the muscarinic (M₁–M₅) receptor subsystem (Bymaster et al., 2002, Langmead et al., 2008, Wess, 2004). In the present study we focused on the functional role of the M₁ receptor, as M₁ receptors have been repeatedly implicated in normal cognition, as well as in all of the diseases mentioned above (Dean et al., 2003, Fisher, 2008, Wess et al., 2007).

One aspect of cognitive function with which the cholinergic system has been repeatedly associated is attention (Passetti et al., 2000, Robbins, 1997, Sarter and Bruno, 2000), which can effectively be assessed in rodents using the 5-choice serial reaction time task (5-CSRTT). This task provides measures of sustained attentional function, and also measures of abnormal responding, for example premature and perseverative responses (thought to model impulsivity and compulsivity, respectively) (Carli et al., 1983, Robbins, 2002). Non-selective muscarinic receptor antagonists impair attentional function in rodents performing the 5-CSRTT when given either systemically (Humby et al., 1999, Jones and Higgins, 1995, Mirza and Stolerman, 2000, Pattij et al., 2007) or when infused directly into prefrontal cortex (Chudasama et al., 2004, Dalley et al., 2004, Muir et al., 1996). However, little is known about which specific muscarinic receptor subtypes are involved in this task. As the M₁ receptor is the predominant muscarinic receptor in cortex (Flynn et al., 1995, Levey et al., 1991), we speculated that this receptor might play an important role in attention as measured by the 5-CSRTT.

M₁ knockout (M1R^−/−) mice are an ideal model for investigating the role of these receptors in cognition, as there is a paucity of agents with which to target these receptors selectively, and the knockout mice do not show any gross behavioral or morphological abnormalities, making them well-suited to cognitive testing (Wess, 2004, Wess et al., 2007). Furthermore, several studies have indicated that disruption of one specific muscarinic receptor gene does not have major effects on the levels of the four remaining muscarinic receptors (Hamilton et al., 1997). In the present study, therefore, we trained M1R^−/− mice and wild-type (M1R^+/+) controls on a touchscreen version of the 5-CSRTT, and assessed their performance under a number of behavioral challenge conditions, as well as on several additional tests of cognition using the same touchscreen apparatus, and on a spontaneous object recognition test using 3-dimensional objects.