Research report
Long-term effects of BIBN-99, a selective muscarinic M2 receptor antagonist, on improving spatial memory performance in aged cognitively impaired rats

https://doi.org/10.1016/S0166-4328(03)00116-5Get rights and content

Abstract

Aged Long–Evans rats were screened for spatial memory deficits using the Morris water maze task. Rats found to have impaired performance on the task (aged-impaired, AI) were then treated with a selective muscarinic M2 receptor antagonist, 5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1-oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (BIBN-99; 0.5 mg/kg, s.c.), for 3 successive days while receiving additional water maze training. BIBN-99 significantly improved performance in the task during the 3 days of drug treatment. Treatment was then ceased for the remainder of the study and rats were tested again in the water maze on days 10, 17, and 24. Compared to vehicle-treated rats, enhanced performance was observed in the AI rats that had previously been treated with BIBN-99. These results indicate that BIBN-99 enhances spatial learning in AI animals and that enhanced (or long-term) memory persists in the absence of the drug.

In a second experiment, a 2-month delay was imposed in between the original water maze screening and the drug treatment regime. Again, BIBN-99 significantly improved performance in AI rats. This latter study suggests that reference memory does not decay, even in an AI animal that had displayed poor learning following original water maze screening. Together, these studies help provide further insight into possible mechanism(s) of reference memory and its potential clinical usefulness.

Introduction

Alzheimer’s Disease (AD) is the most widely recognized age-related disease that results in cognitive impairment. A prominent neuropathological feature of AD is neuronal loss in the basal forebrain, resulting in reduced cholinergic input to the hippocampus and neocortex (for recent reviews, see [1], [2], [14], [37]). Several animal models have been adapted to study the cholinergic decline that characterizes AD [4], [10], [13]. Our laboratory has demonstrated that alterations in the cholinergic system occur simply as a function of age in male Long–Evans rats, and that these alterations correlate with cognitive ability [1], [28], [32]. Using the Morris water maze task to evaluate spatial learning and memory, 24–25-month-old rats can be identified as either aged cognitively impaired (AI) or aged cognitively unimpaired (AU) relative to 6-month-old (young adult) animals (for details, see [17], [36]). One of the major findings is that degeneration of spatial learning and memory is not an inevitable consequence of aging. Indeed, the preservation of cognitive function in aged rats, like that in aged humans, shows a great deal of individual variability. Behavioral screening of more than 800 animals over the last decade has revealed spatial memory impairments in 25–30% of aged (24-month-old) Long–Evans rats while 36–40% were found to be unimpaired [16], [17], [32], [35], [36], [46]. Other laboratories have also reported individual differences amongst populations of aged rats, with the percentages of impaired animals varying as a function of both the strain as well as the sex of the animals [12], [23], [38], [42], [51], [52].

While some muscarinic agonists, such as oxotremorine and carbamylcholine (carbachol), decrease endogenous or radiolabeled ACh release [2], [19], [33], non-selective antagonists (atropine and scopolamine) and purported muscarinic M2 antagonists (AFDX 116, AFDX 384, BIBN-99, and SCH 72788) stimulate the release of ACh from cortical, hippocampal, and striatal areas in the rat brain [18], [19], [31], [32], [41]. Lesions of the basal forebrain or destruction of cholinergic neurons result in decreased numbers of muscarinic M2 binding sites in cortex and hippocampus, suggesting that a proportion of these receptors may be located on pre-synaptic cholinergic terminals [6], [20], [25]. This observation is consistent with the finding that the muscarinic M2 gene and its receptor protein are expressed in the basal forebrain in a distribution pattern that is similar to that of cholinergic neurons [20], [30], [34], [47]. Accordingly, we and others have hypothesized that the selective blockade of pre-synaptically located M2 autoreceptors could increase ACh release into the synaptic cleft and thereby enhance cognitive performance [8], [10], [30], [31].

Although many studies have documented a decline in spatial memory performance in the rat with increased age, there have only been a handful of studies testing the ability of rats to retain a learned task over a long period of time. Two separate studies [7], [26] describe reference memory (i.e. the location of the platform) being retained for at least 14 weeks between testing and re-testing in the Morris water maze. In another study, Van Groen et al. [44] demonstrated that rats that were trained to find the platform in the Morris water maze at 12 months of age displayed significantly enhanced performance compared to age-matched controls (no pre-training), when re-tested at 24 months of age. Somehow, the pre-training experience in the repeated acquisition paradigm assisted the rat in remembering for long periods of time. What mechanism may underlie this long-term memory? Could long-term memory be induced pharmacologically? In the present studies, we examine a selected population of aged rats that display poor reference memory performance, the AI rat, and study long-term memory effects of therapeutic drug treatment. We administered the muscarinic M2 receptor antagonist, 5,11-dihydro-8-chloro-11-[[4-[3-[(2,2-dimethyl-1-oxopentyl)ethylamino]propyl]-1-piperidinyl]acetyl]-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one (BIBN-99), a compound known to enhance reference memory performance in the AI rat to an optimal level [32]. In this experiment, however, we ceased drug application after 3 days and examined how long the enhanced memory performance could endure. Next, we delayed the onset of the BIBN-99 treatment for 8 weeks and assessed water maze performance. If the AI animal’s memory for the platform location was already compromised, as witnessed following initial screening, then allowing a long interval of time to pass before pharmacological intervention was to begin might allow for memory decay to occur and potentially decrease the likelihood that reference memory could still be enhanced in these animals.

Section snippets

Animals

Male Long–Evans rats (retired breeders) were obtained from Charles River (St. Constant, Que., Canada) at 12 months of age and were housed in our vivarium for 12–13 months before the experiments began. Young rats were obtained at 3–4 months of age and were housed for 2–3 months prior to behavioral testing. The animals were housed in groups of three (45cm×30cm×18 cm polycarbonate cages) and maintained on a 12 h/12 h light/dark schedule, with ad libitum access to food (Purina Lab Chow) and water. At

Results

Aged (24–25 months old) male Long–Evans rats were screened in the spatial version of the Morris water maze task and were classified into groups based upon their performance as described in Section 2. A significant group by days interaction was observed for both latency to platform (F(8,472)=7.045, P<0.0001) and for distance traveled (F(8,472)=4.4, P<0.0001); post hoc comparisons indicated that the AI rats were significantly impaired at finding the hidden platform on days 2, 3, 4, and 5 of

Discussion

Administration of BIBN-99 improved spatial memory in AI rats. The effect appeared very rapidly, even when drug administration was delayed for 2 months following the initial behavioral screening. Additionally, the improvement induced by BIBN-99 was enduring, outlasting the treatment period by several weeks.

In the first study, improved water maze performance was noted on the first day (day 1) following BIBN-99 treatment (Fig. 2). In fact, performance was significantly enhanced on the first trial

Conclusion

The underlying mechanism(s) for BIBN-99 induced long-term memory enhancement remains largely unknown. It may be mediated by the cholinergic system as eluded to above. M2 receptor immunoreactivity has been found to be co-localized on acetylcholinesterase-rich neurons in hippocampal, cortical and basal forebrain regions [21], [22], [39]. However, other neurotransmitter systems may also be responsible for mediating these effects as M2 receptors have been described on gamma amino butyric acid

Acknowledgements

This work was supported by research grants from the Canadian Institute for Health Research (CIHR) to R.Q. and M.J.M. Both R. Quirion’s research scholarship and W.B. Rowe’s post-doctoral fellowship were supported by Le Fonds de la Recherche en Santé du Québec. J.-P. O’Donnell was supported by a studentship award from CIHR. D. Pearson was supported by an NSERC studentship award. The authors would also like to thank the animals care staff from McGill University and the Douglas Hospital Research

References (52)

  • P.A. Lapchak et al.

    Binding sites for [3H]AF-DX 116 and effect of AF-DX 116 on endogenous acetylcholine release from rat brain slices

    Brain Res.

    (1989)
  • A.L. Markowska et al.

    Individual differences in aging: behavioral and neurobiological correlates

    Neurobiol. Aging

    (1989)
  • D.G. Mumby et al.

    Retrograde amnesia and selective damage to the hippocampal formation: memory for places and object discriminations

    Behav. Brain Res.

    (1999)
  • B.R. Pike et al.

    Post-injury administration of BIBN 99, a selective muscarinic M2 receptor antagonist, improves cognitive performance following traumatic brain injury in rats

    Brain Res.

    (1995)
  • R. Quirion et al.

    Autoradiographic distribution of putative muscarinic receptor sub-types in mammalian brain

    Prog. Brain Res.

    (1993)
  • W. Regenold et al.

    Direct visualization of brain M2 muscarinic receptors using the selective antagonist [3H]AF-DX116

    Eur. J. Pharmacol.

    (1987)
  • W.B. Rowe et al.

    Hypothalamic–pituitary–adrenal function in aged cognitively impaired and unimpaired rats: effects of antidepressant drug treatment

    Neurobiol. Aging

    (1997)
  • W.B. Rowe et al.

    Reactivity to novelty in cognitively impaired and unimpaired aged rats, and young rats

    Neuroscience

    (1998)
  • K.A. Sherman et al.

    Presynaptic cholinergic mechanisms in brain of aged rats with memory impairments

    Neurobiol. Aging

    (1981)
  • J.F. Smiley et al.

    Infracortical interstitial cells concurrently expressing M2-muscarinic receptors, acetylcholineresterase and nicotinamide adenine dinucleotide phosphate-diaphorase in the human and monkey cerebral cortex

    Neuroscience

    (1998)
  • M.J. Stillman et al.

    Effects of M2 antagonists on in vivo hippocampus acetylcholine levels

    Brain Res. Bull.

    (1996)
  • M. Vallee et al.

    Role of pregnenolone dehydroepiandrosterone and their sulfate esters on learning and memory in cognitive aging

    Brain Res. Brain Res. Rev.

    (2001)
  • T. Van Groen et al.

    Old rats remember old tricks; memories of the water maze persist for 12 months

    Behav. Brain Res.

    (2002)
  • M.G. Vannucchi et al.

    Selective muscarinic antagonists differentially affect in vivo acetylcholine release and memory performances of young and aged rats

    Neuroscience

    (1997)
  • E. Vaucher et al.

    Amyloid beta peptide levels and its effects on hippocampal acetylcholine release in aged, cognitively impaired and unimpaired rats

    J. Chem. Neuroanat.

    (2001)
  • M.T. Vilaro et al.

    Muscarinic M2 receptor mRNA expression and receptor binding in cholinergic and non-cholinergic cells in the rat brain: a correlative study using in situ hybridization histochemistry and receptor autoradiography

    Neuroscience

    (1992)
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