Abstract
The synaptic plasticity processes of LTP and LTD is vital for memory formation and overall neural health, as unconstrained plasticity compromises learning and, in the extreme, maybe injurious. Thus, mechanisms must be in place to prevent pathologically excessive LTP and LTD. Such regulation comes partly through "metaplasticity'', whereby neural activity at one-point influences subsequent plasticity. Recent evidence suggests that such heterosynaptic regulation of plasticity thresholds exists in the hippocampus, a brain region involved in spatial and episodic memory formation. It was demonstrated that a strong ''priming'' activity delivered to synapses in the CA1 of the hippocampus inhibited subsequent LTP and facilitated subsequent LTD at a neighbouring pathway quiescent during priming, both heterosynaptically and even heterodendritically
The principal aim of this thesis was to expand upon the previous findings of heterosynaptic and heterodendritic metaplasticity in CA1, and in particular, to determine the spatial spread and the mode of long-distance communication, which allows synaptic innervation to alter plasticity thresholds in hippocampal subregions that are located hundreds of microns away. To this end, extracellular and intracellular electrophysiological experiments were conducted in acute hippocampal slices taken from rats or mice. Electrical priming stimulation was delivered to synapses in stratum oriens (SO) prior to testing LTP induction in a second pathway in either SO, stratum radiatum (SR), stratum-lacnuosum molecular (SLM) of CA1 or the middle molecular layer (MML) of the dentate gyrus (DG). The metaplasticity effect was highly pathway-specific in its spatial spread, as LTP was inhibited in synapses of SR but not in SO or SLM of CA1. Notably, the LTP was inhibited at the synapses in the MML, and this was indeed surprising as the DG is located far away from the SO of CA1 and is physically separated from CA1 by the hippocampal fissure. Further, both high-frequency stimulation and theta-burst stimulation pattern of priming delivered to SO inhibited subsequent LTP in SR and MML. The robustness of the metaplasticity effect was further supported as it was independent of sex and was seen in both rats and mice.
The possible roles for astrocyte-mediated bidirectional communication in the trans-regional metaplasticity in the DG were tested, considering the lack of direct neuronal innervations between CA1 and DG subregions in the hippocampus. The whole-cell patch-clamp dialysis of a single MML astrocyte with the calcium chelator EGTA (''calcium clamp") completely restored locally induced LTP from the inhibition generated by SO electrical priming stimulation. Moreover, the trans-regional metaplasticity effect was shown to occur via astrocytic calcium release from IP3 receptor-coupled channels in the endoplasmic reticulum. Notably, the effect was absent in mice lacking the IP3 receptor 2 which is expressed only in astrocytes.
To investigate the underlying mechanism by which the trans-regional metaplasticity effect is mediated, pharmacological methods were utilised to target the common transmitters involved in intercellular signalling communication. The trans-regional metaplasticity was independent of glutamatergic receptor activation, namely, AMPARs, KARs, NMDARs, mGluRs, or the muscarinic AChRs or the purinergic P2Y1R activation at the time of delivering the priming stimulus in SO, and the neuronal trigger required for the activation of astrocytes was not ascertained.
The calcium elevations in astrocytes modulate the neuronal synaptic activity via gliotransmission, ultimately leading to LTP inhibition. Astrocytes release modulatory transmitters such as glutamate and cytokines such as tumour necrosis factor-alpha (TNFα). Pharmacological priming via the exogenous application of TNFα protein for 10 min inhibited MML LTP recorded 30 min later, similar to the electrical priming effect. Further, the TNFα- mediated trans-regional metaplasticity effect required activation of the TNFR1. As TNFα is considered to trigger glutamate release from astrocytes, which acts predominantly on neuronal GluN2B-containing NMDARs, the contribution of these receptors in mediating the trans-regional metaplasticity was also assessed. Electrical priming in the presence of the GluN2B selective antagonist ifenprodil significantly prevented LTP inhibition in MML. The result presented in this thesis supports a model of neuron-astrocyte-neuron involved in the long-distance communication between dendritic layers for mediating the metaplasticity effect.
Taken together, these data demonstrate a novel hippocampus network-wide yet pathway-specific metaplastic regulation of synaptic plasticity that is mediated by intercellular communication between neurons and astrocytes. This network-level regulation can perhaps serve as a neuroprotective mechanism and play an important role in hippocampal information processing.