Altered serotonergic neurotransmission as a main player in the pathophysiology of Alzheimer's disease : structural and ultrastructural studies in a triple transgenic mouse model of the disease

Alzheimer’s disease (AD) is a neurodegenerative disease that deteriorates cognitive functions and associated brain regions such as the hippocampus, being the primary cause of dementia. Serotonin (5-HT) is widely present in the hippocampus, being an important neurotransmitter involved in learning and memory. Although recent evidence suggests alterations in 5-HT neurotransmission in AD, it is not clear how hippocampal 5-HT innervation is modified. Here, we studied hippocampal 5-HT innervation by analysing: (i) the expression, density and distribution of 5-HT transporter (SERT)-immunoreactive fibres; (ii) the specific morphological characteristics of serotonergic fibres and their relation to amyloid plaques; and (iii) the total number of 5-HT neurons within the raphe nuclei in triple transgenic mouse model of AD. We used quantitative light microscopy immunohistochemistry comparing transgenic and nontransgenic animals of different ages (3, 6, 9, 12 and 18 months). The transgenic animals showed a significant increase in SERT fibres in the hippocampus in a subfield-, strataand age-specific manner. The increase in SERT fibres was specific to the CA1 stratum lacunosum-moleculare. An increase in SERT fibres in transgenic animals was observed at 3 months (by 61%) and at 18 months (by 74%). No changes, however, were found in the total number of raphe 5-HT neurons at any age. Our results indicate that triple transgenic mice display changes in the expression of SERT and increased SERT fibres sprouting, which may account for imbalanced serotonergic neurotransmission associated with (or linked to) AD cognitive impairment. Introduction Alzheimer’s disease (AD) is an age-related, irreversible and progressive neurodegenerative disease impairing cognition and memory (Braak et al., 1999; Garcia-Alloza et al., 2005). AD neuropathological hallmarks are plaques (amyloid beta; Ab), neurofibrillary tangles (NFT), neuronal death, synaptic loss and synaptic plasticity impairment (Braak et al., 1999; Scheff et al., 2006). Classically, the loss of cholinergic neurons and a decrease in choline acetyltransferase activity were linked with AD (Birks et al., 2008). However, drugs enhancing cholinergic function have modest efficacy, suggesting the involvement of other neurotransmitters (Birks et al., 2008). Serotonin (5-HT) plays a critical role in cognition and memory (Schmitt et al., 2006; Evers et al., 2007). The dorsal (DR) and medial (MR) raphe nuclei, which contain the majority of 5-HT neurons, widely project throughout the CNS including the hippocampus (Vertes et al., 1999), showing different axon morphology (Bjarkam et al., 2005; Keuker et al., 2005; Hensler, 2006). DR axons are fine fibres (FF) with small and regularly spaced varicosities, whilst MR axons are either non-varicose (SA; stem axons) or thick with large irregular varicosities (beaded fibres – BF; Bjarkam et al., 2005; Keuker et al., 2005; Hensler, 2006). Increasing evidence emphasizes the involvement of 5-HT in AD (Tohgi et al., 1995; Meltzer et al., 1998; Lai et al., 2002; Mowla et al., 2007; Truchot et al., 2007). Patients with AD have a 5-HT deficit, and treatments with selective 5-HT re-uptake inhibitors or specific receptor ligands enhance cognitive functions (Porter et al., 2000; Mowla et al., 2007; Mossello et al., 2008; Terry et al., 2008). Patients with AD have altered levels of 5-HT, its metabolite and receptors within the cortex and hippocampus (Meltzer et al., 1998; Lai et al., 2002; Garcia-Alloza et al., 2005; Kepe et al., 2006; Lorke et al., 2006; Bowen et al., 2008; Hasselbalch et al., 2008), and a decrease in 5-HT neurons and projections (Chen et al., 2000; Thomas et al., 2006), which are correlated with cognitive and behavioural alterations (Lai et al., 2002; Garcia-Alloza et al., 2005). However, these studies are prone to misinterpretation due to contradictory results. Sprouting of 5-HT fibres and increased activity is observed Correspondence: Dr J. J. Rodrı́guez, IKERBASQUE, Basque Foundation for Science, as above. E-mail: j.rodriguez-arellano@ikerbasque.org Received 14 January 2010, revised 15 March 2010, accepted 12 April 2010 European Journal of Neuroscience, Vol. 32, pp. 71–79, 2010 doi:10.1111/j.1460-9568.2010. 07274.x a The Authors (2010). Journal Compilation a Federation of European Neuroscience Societies and Blackwell Publishing Ltd European Journal of Neuroscience