The observed olfactory learning performances of trisomic and disomic mice in the olfactometer () constitutes the first demonstration that olfactory-based associative learning is negatively affected in Ts65Dn mice. The poor performance in the associative learning task, combined with documented deficits in spatial cognition, strongly support a widespread effect of this trisomy on adaptive behaviors. This wide array of cognitive and sensory deficits validate the Ts65Dn mouse as a valid model of Down Syndrome. The deficit in olfactory-based associative learning is likely due to both deficits in learning and odor perception. These deficits are well-documented in human: mild to moderate mental retardation is evident in verbal and mental performance3
and olfactory deficits33,34,35,36,37,38
The presence of a third copy and presumed overexpression of genes from Chr 21 in humans with DS, and Chr 16 in Ts65Dn mouse39
, result in a complex perturbation of multiple processes involved in neurological development and function10
. Many genes expressing olfactory membrane receptors40
are located on Chr 21 in humans41
and Chr 16-17 in mice39
, but the neurological effects of the presumed overexpression of these olfactory genes is not known. On one hand, olfactory receptor proteins are expressed from a single allele, and an overrepresentation of the olfactory receptor genes is unlikely to lead to an overexpression of olfactory receptor proteins42,43
. However it is unknown if this overrepresentation of the olfactory receptor genes could generate a down-regulation effect on the expression of certain olfactory receptor proteins that could decrease the sensitivity of the olfactory system to certain odorants. Conversely, changes in expression of other genes such as ApoE epsilon 4 could mediate an olfactory associative learning deficit36
. Previous work has provided evidence of olfactory impairment in patients with DS34,35,36,38
and a role for cholinergic neurodegeneration in this aspect of DS pathology is supported by the observation that olfactory loss is also a common facet of Alzheimer's disease44,45
. It is difficult to dissociate the olfactory effects that could be caused by the overrepresentation of olfactory receptor genes from those caused by the neural degeneration associated with the early onset of Alzheimer's disease34,35
. This is a limitation of our approach, since the olfactometer cannot separate learning and olfactory impairment. Thus, we do not know how changes in protein expression induced by DS affect olfactory learning and further studies will be required to investigate these questions.
Another neurological consequence associated to the overexpression of genes in Ts65Dn mice is the diminished number of cholinergic neurons on the basal forebrain19,46
, which likely is one of the main causes of their learning deficits, and could underlie olfactory deficits. The cholinergic neurons on the basal forebrain innervate the cerebral cortex in a relatively widespread manner47,48,49
, and the acetylcholine release from these basal forebrain cholinergic neurons modulates the cortical neurons and consequently the cerebral cortex50
. It is known that the manifestations of dementia in DS have been associated with a frontal lobe dysfunction10
, which could be associated with the reduced number of cholinergic neurons19
. In addition, decreased number of cholinergic neurons in the horizontal diagonal band of Broca in the basal forebrain would result in decreased projection of axons to the olfactory bulb thereby affecting olfactory processing32,51,52,53
If the deficit in neural activity is due to decreased Ach neuron expression, it should be compensated by using pharmacological activators of cholinergic neurons54,55
, like galantamine that boosts Ach responses in the neural terminals and nicotinic Ach Receptors56,57
. In this way, the galantamine activation of cholinergic neuron in the basal forebrain that project to cortical neurons and olfactory areas could conceivably activate the cholinergic neurons and possibly rescue some of the neurological and behavioral deficits associated with DS. This possibility is consistent with previous work that has shown that perinatal dietary supplementation with choline acts to significantly improve cognition and emotion regulation in the Ts65Dn mouse model58
In the present study we observed that the galantamine treatment enabled trisomic mice to reach a performance level comparable with that of disomic mice (). A significant improvement in the percent of correct choices occurred in disomic and trisomic mice under chronic treatment with galantamine. Galantamine treatment improved the performance of trisomic mice in both tasks, while it only improved the performance of disomic mice on the most difficult of them (task B). An explanation for the lack of effect of galantamine on the performance of disomic mice on odor pair A comes from the fact that they already achieved high scores on this task leaving little scope for improvement. Thus, galantamine an acetylcholinesterase inhibitor and an agonist of nicotinic receptors elicits improved performance in an olfactory learning task in trisomic Ts65Dn mice. Interestingly, in the past no improvement in behavioral performance in the Morris Water Maze was found in these mice after treatment with the acetylcholine esterase inhibitor donepezil59
. In the future it makes sense to test galantamine and donepezil in parallel experiments in olfactory learning and MWM. Finally, in spite of the fact that we cannot dissociate totally olfactory deficits from learning deficits at a behavioral level, the positive action of galantamine in our mice taken together with evidence from literature showing that non-olfactory learning deficits in Ts65Dn are linked to a diminished number of cholinergic neurons in the basal forebrain, reinforce the hypothesis that the lower scores on the olfactory discrimination task is more likely an impairment in the acquisition of the learning rule in consequence of the impaired basal forebrain-neocortex circuitry than due to a malfunctioning of the olfactory system. Future studies will have to determine whether these are olfactory or sensory deficits.
Galantamine has been used clinically to stabilize cognition in patients with Alzheimer's disease60
. Since DS has been strongly linked with the early development of Alzheimer's disease10
, and autopsy of patients with Alzheimer's disease revels lesions in the cholinergic neurons of the basal forebrain61
, the results from the present work indicate that galantamine has significant therapeutic potential for alleviating learning deficits in humans with DS.
However, additional studies of galantamine treatment in Ts65Dn mice and controls are needed to dissociate their transient and permanent effects prior to translation to a clinical trial.