Rapid neuronal responses during spreading neurotoxic and neuroprotective network activity
Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system, playing critical roles in basal synaptic transmission and the molecular correlates of learning and memory, long-term potentiation and long-term depression. However, glutamate is also neurotoxic during prolonged exposure and the dysfunction of the glutamatergic system has been implicated in most neurological disorders, including stroke and epilepsy, and in certain neurodegenerative diseases, including Alzheimer’s disease. In these conditions, an increased concentration of extracellular glutamate causes an over-activation of local ionotropic glutamate receptors that trigger neuronal cell death (excitotoxicity). In this study, we have used dissociated hippocampal neurons cultured on coverslips and within novel microfluidic devices to study neuronal responses, both functional and morphological, to prolonged exposure to glutamate. We find that high glutamate concentrations evoke a rapid retraction of dendritic spines, the collapse of microtubules, the formation of dendritic beads and the inhibition of basal neurotransmitter release. These responses have been identified in many neurological disorders where excitotoxicity is reported, suggesting they may be a sign of imminent cell death. However, the development of dendritic beads and the inhibition of network activity also occurs at subtoxic concentrations of glutamate and neuronal morphological changes recover rapidly post-insult. We therefore hypothesised that beading and the inhibition of neurotransmitter release may be a protective mechanism and render neurons resistant to subsequent glutamatergic insults. However, a subtoxic stimulation is not protective against a subsequent excitotoxic insult delivered immediately afterwards. However, given that neurotransmitter release can confer protection to neurons, it is possible that protection is realised, not on the neurons exposed to the subtoxic insult, but on those neurons with which they communicate, as a ‘warning’ signal. To assess the impact of a localised insult to a wider neuronal network, hippocampal neurons were cultured in novel microfluidic devices, to environmentally isolate neuronal populations, whilst preserving synaptic contacts between them. We observe that bystander naïve neurons downstream of a localised excitotoxic insult succumb to a secondary, activity-dependent, spreading toxicity. In addition, we reveal a novel mechanism by which neuronal networks also transmit a rapid and robust (albeit transient) protection from excitotoxicity. The protective phenotype acquired by neurons during this protective process requires neuronal inhibitory activity to quench overexcitation, along with the retraction of dendritic spines and/or dendritic beading. Therefore, we highlight a dichotomous role that dendritic beading plays following a direct glutamatergic insult (large beads) and as a result of GABAergic recruitment in downstream neurons (small beads). We determine that a network neuroprotective capacity exists that limits spreading toxicity, which may be recruited from a distal site even after an excitotoxic insult has occurred. Together, we may have identified a new therapeutic opportunity to limit on-going brain damage in conditions of acute neuronal injury.
Neuroscience, Excitotoxicity, Neuronal Networks, Neuroprotection
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