Glutamatergic regulation prevents hippocampal-dependent age-related cognitive decline through dendritic spine clustering

The dementia of Alzheimer’s disease (AD) results primarily from degeneration of neurons that furnish glutamatergic corticocortical connections that subserve cognition. Although neuron death is minimal in the absence of AD, age-related cognitive decline does occur in animals as well as humans, and it decreases quality of life for elderly people. Age-related cognitive decline has been linked to synapse loss and/or alterations of synaptic proteins that impair function in regions such as the hippocampus and prefrontal cortex. These synaptic alterations are likely reversible, such that maintenance of synaptic health in the face of aging is a critically important therapeutic goal. Here, we show that riluzole can protect against some of the synaptic alterations in hippocampus that are linked to age-related memory loss in rats. Riluzole increases glutamate uptake through glial transporters and is thought to decrease glutamate spillover to extrasynaptic NMDA receptors while increasing synaptic glutamatergic activity. Treated aged rats were protected against age-related cognitive decline displayed in nontreated aged animals. Memory performance correlated with density of thin spines on apical dendrites in CA1, although not with mushroom spines. Furthermore, riluzole-treated rats had an increase in clustering of thin spines that correlated with memory performance and was specific to the apical, but not the basilar, dendrites of CA1. Clustering of synaptic inputs is thought to allow nonlinear summation of synaptic strength. These findings further elucidate neuroplastic changes in glutamatergic circuits with aging and advance therapeutic development to prevent and treat age-related cognitive decline.Cognitive decline often occurs with aging in rodents (1), nonhuman primates (2), and humans (3). Memory loss (4) and executive impairment (5) are of the most functional importance, mediated primarily by the hippocampus and related areas of the medial temporal lobe and the prefrontal cortex (PFC), respectively. The neural circuits vulnerable to aging are composed of glutamatergic pyramidal neurons that furnish corticocortical connections between the association cortices as well as the excitatory hippocampal connections (2, 6). Dendritic spine changes, which appear to be the primary site of structural plasticity in the adult brain (7), occur in the pyramidal neurons of the PFC (5) and in the hippocampus (8, 9) with aging and correlate with behavioral decline. Spines form the postsynaptic component of most excitatory synapses in the cerebral cortex and are capable of rapid formation, expansion, contraction, and elimination (10, 11).Synaptic glutamatergic activity is neuroprotective and critical for long-term potentiation (LTP) and memory formation, whereas extrasynaptic NMDA receptor activity promotes long-term depression and excitotoxicity (12, 13). There is some evidence that astrocytic glutamate transporters decrease with aging (14, 15), and consequently reduce glutamate uptake (14, 16, 17). Reduced glutamate uptake can lead to glutamate spillover to the extrasynaptic space with electrophysiological repercussions (14). The potential use of glutamate modulators as a therapeutic target to regulate the synaptic age-related glutamatergic dysregulation in those vulnerable neural circuits remains to be further investigated.Riluzole is a glutamate modulator that decreases glutamate release (18) and facilitates astrocytic glutamate uptake (19–21). These actions have been suggested to increase glutamate-glutamine cycling, enhancing synaptic glutamatergic activity while preventing excessive glutamate overflow to the extrasynaptic space in rodents (21, 22) and humans (23). Riluzole has also been shown to increase oxidative metabolism with mitochondrial enhancing properties (24) and to increase BDNF expression (25). We hypothesized that improved regulation of the glutamatergic synapse with the glutamate modulator riluzole would promote synaptic NMDA receptor activation while preventing extrasynaptic NMDA activity, thereby protecting against age-related cognitive decline, through induction of neuroplastic changes in the hippocampus and PFC. An important neuroplastic mechanism is clustering of dendritic spines because it significantly empowers neural circuits with nonlinear summation of synaptic inputs (26, 27) and is dependent on neuronal activity (28, 29). For this study, we focused on pyramidal neurons within CA1 and pyramidal neurons in layer 3 of the prelimbic region of medial PFC, an area where we have demonstrated age-related spine loss in middle-aged animals previously (30).