Compensatory responses to age-related decline in odor quality acuity: Cholinergic neuromodulation and olfactory enrichment

Abstract

The perceptual differentiation of odors can be measured behaviorally using generalization gradients. The steepness of these gradients defines a form of olfactory acuity for odor quality that depends on neural circuitry within the olfactory bulb and is regulated by cholinergic activity therein as well as by associative learning. Using this system as a reduced model for age-related cognitive decline, we show that aged mice, while maintaining almost the same baseline behavioral performance as younger mice, are insensitive to the effects of acutely elevated acetylcholine, which sharpens generalization gradients in young adult mice. Moreover, older mice exhibit evidence of chronically elevated acetylcholine levels in the olfactory bulb, suggesting that their insensitivity to further elevated levels of acetylcholine may arise because the maximum capacity of the system to respond to acetylcholine has already been reached. We propose a model in which an underlying, age-related, progressive deficit is mitigated by a compensatory cholinergic feedback loop that acts to retard the behavioral effects of what would otherwise be a substantial age-related decline in olfactory plasticity.

We also treated mice with 10-day regimens of olfactory environmental enrichment and/or repeated systemic injections of the acetylcholinesterase inhibitor physostigmine. Each treatment alone sharpened odor quality acuity, but administering both treatments together had no greater effect than either alone. Age was not a significant main effect in this study, suggesting that some capacity for acetylcholine-dependent plasticity is still present in aged mice despite their sharply reduced ability to respond to acute increases in acetylcholine levels.

These results suggest a dynamical framework for understanding age-related decline in neural circuit processing in which the direct effects of aging can be mitigated, at least temporarily, by systemic compensatory responses. In particular, a decline in cholinergic efficacy can precede any breakdown in cholinergic production, which may help explain the limited effectiveness of cholinergic replacement therapies in combating cognitive decline.

Introduction

The olfactory bulb (OB) is a cortical structure that directly receives afferent input from primary olfactory sensory neurons. It is the final common path for olfactory afferent signals before they diverge to multiple cortical and subcortical targets, and it receives copious projections from several regions of the brain that regulate its processing of olfactory sensory information. The OB performs several computational tasks common to early sensory processing across modalities; in particular, it is thought to play a major role in odor stimulus decorrelation, a process that defines the early delineation and categorization of odors based upon stimulus features and learned contingencies (Cleland et al., 2007, Cleland and Sethupathy, 2006). This process is measured behaviorally using generalization gradients (Shepard, 1987), in which experience with one odor is generalized to a range of novel but perceptually similar odors (Cleland et al., 2002, Cleland and Narla, 2003). This gradient is steepened by increased learning (Cleland et al., 2009) and by cholinergic neuromodulation within the OB (Chaudhury et al., 2009, Mandairon et al., 2006a). Owing to the relatively well-described circuitry of the OB, the cellular and circuit mechanisms of this cortical network can be related to behavioral performance via computational modeling (Cleland et al., 2007, Mandairon et al., 2006a). As a reduced model system for integrated cortical function, the OB enables study of learning mechanisms and the convergence of bottom–up and top–down factors on sensory representations across levels of organization, from cellular physiology to functional networks to behavior.

Normal aging results in correspondingly increased rates of self-reported chronic olfactory dysfunction in humans (Hoffman et al., 1998). In particular, olfactory identification deficits, in which odors become more difficult to differentiate from one another, increase with age and are predictive of broader cognitive and learning impairments (Wilson et al., 2007) including clinical dementias (Serby et al., 1991). In rodents, aging induces deficits in olfactory perception (Nakayasu et al., 2000), including odor discrimination (Enwere et al., 2004, Prediger et al., 2005, Prediger et al., 2006) and impairments in olfactory learning and memory (Guan and Dluzen, 1994, Prediger et al., 2005, Rosenzweig and Bennett, 1996, Schoenbaum et al., 2002, Terranova et al., 1994). Notably, two mouse models of human dementias also exhibit odor habituation deficits and/or a broadening of olfactory generalization gradients, both characteristic of impaired olfactory learning (Bath et al., 2008, Guerin et al., 2009).

The olfactory system receives cholinergic input from the basal forebrain, specifically the horizontal limb of the diagonal band of Broca (Shipley and Ennis, 1996, Wenk et al., 1977). Cholinergic activity in the olfactory system influences a range of behavioral tasks reflecting learning, memory, and the regulation of perceptual differentiation, including habituation (Hunter and Murray, 1989, Mandairon et al., 2006a), short-term memory (Ravel et al., 1994, Roman et al., 1993), perceptual learning (Fletcher and Wilson, 2002), proactive interference (De Rosa and Hasselmo, 2000, De Rosa et al., 2001) and behavioral generalization (Linster and Cleland, 2002, Linster et al., 2001). Cholinergic dysfunction is widely implicated in degenerative dementias, most prominently Alzheimer's dementia but including normal age-related cognitive decline. However, the nature of this dysfunction is unclear, as cholinergic replacement therapies have yielded somewhat disappointing results.

Environmental enrichment alters physiological responses, enhances learning and neural plasticity, and influences cholinergic neuromodulatory systems (Del Arco et al., 2007a, Del Arco et al., 2007b, Rosenzweig and Bennett, 1996, van Praag et al., 2000). In the olfactory analogue of environmental enrichment (see Section 2.7), odor response patterns in the OB can be modified by odor exposure in both juvenile (Woo et al., 2007) and adult rodents (Buonviso and Chaput, 2000, Spors and Grinvald, 2002). At a behavioral level, olfactory enrichment can improve short-term odor memory (Rochefort et al., 2002) and improve olfactory discrimination capacities (Escanilla et al., 2008, Mandairon et al., 2006b, Mandairon et al., 2006c). Moreover, olfactory enrichment selectively affects the perception of odorants which activate at least partially overlapping regions of the OB, and manipulation of the OB network suffices to produce long-lasting perceptual changes (Mandairon et al., 2006b). While general environmental enrichment can delay or mitigate cognitive decline (Berardi et al., 2007, O’Callaghan et al., 2009), it is not yet known whether this olfactory analogue of enrichment will have a similar effect upon olfactory learning.

We sought to test whether aged wildtype mice would exhibit a broadening of associative olfactory generalization gradients (an olfactory correlate of reduced learning, hence a model for age-related cognitive decline) in comparison to young adult mice, and whether such deficits could be mitigated by acute cholinergic potentiation via acetylcholinesterase inhibition. We then assessed the capacities of two 10-day treatment regimens, olfactory enrichment and/or daily administration of an acetylcholinesterase inhibitor, to persistently sharpen generalization gradients (i.e., at least 24 h after the final treatment) in both aged and young animals. Finally, we measured the levels of acetylcholinesterase present in the OB to assess the chronic level of acetylcholine release in that structure. We found that age sharply reduces the capacity of mice to respond to acetylcholine. Chronic treatments of enrichment or cholinergic potentiation, however, were still somewhat effective, indicating that the aged system retained some capacity for plasticity on a slower timescale. Aged mice exhibited substantially higher levels of acetylcholinesterase in the OB than did younger mice, suggesting that they maintain chronically elevated rates of acetylcholine release therein. Notably, the effect of age alone on generalization gradients in these studies was weak; specifically, it was not significant in the two behavioral experiments described above, though it was shown to be significant in an third, larger study. We suggest that aged mice are able to retain near-normal behavioral capacities in response to progressive underlying degeneration by employing a compensatory strategy of chronically elevating the level of acetylcholine released into the OB.