µ, σ and β in synaptic plasticity: A study of synapse organization and plasticity based on variance in synaptic responses
We constantly need to adapt to and learn from our environment in order for us to function properly. These adaptations are dependent on changes in the brain. The adult human brain contains over 100 billion neurons, and each neuron is interconnected with other neurons by thousands of contact points called synapses. The strength of these synapses can change, and it is this phenomenon, called synaptic plasticity, that underlies learning, memory and adaptive behavior. However, for many cases in which synaptic plasticity occurs, the underlying cellular mechanism is still unclear and new tools to study the synaptic changes underlying synaptic plasticity are still required.
The strength of synaptic transmission onto a neuron largely depends on three parameters: the number of functional neurotransmitter release sites (N), the probability of presynaptic vesicle release (Pr), and the quantal size (Q), which is the size of the postsynaptic response to the release of a single vesicle of neurotransmitter. Q therefore depends on the density, conductance and open-channel probability of postsynaptic receptors. A change in synaptic strength is caused by a modulation of one or more of the three parameters N, Pr and Q. However, current methods cannot always distinguish which of these parameters is altered. Therefore, in Chapter 2, we developed the variance-to-mean ratio (VMR) as a tool to study the mechanisms underlying synaptic plasticity and validated its use at the synapses that are most commonly studied in synaptic plasticity research: synapses onto hippocampal CA1 pyramidal neurons that receive Schaffer collateral input from CA3 neurons (Sc-CA1 synapses). The VMR is a statistical tool based on the quantal model of synaptic transmission, and can be used in combination with the conventional quantal measure of the inverse square of the coefficient of variation (1/CV2). While the VMR is dependent on Q and inversely related to Pr, but is independent of N, the 1/CV2 depends on N and Pr, but not on Q. We found that calculating both 1/CV2 and VMR values of evoked synaptic currents before and after an alteration in synaptic strength allows for a fast and reliable prediction of whether the synaptic plasticity is caused by a change in N, Pr, and/or Q.
In the rest of the thesis, I used this variance analysis in combination with other experimental tools in an effort to better understand several synaptic plasticity phenomena. To understand the possibilities for, and effects of, synaptic plasticity it is useful to first gain more insight into how the receptors involved in plasticity are regulated and organized at the synapse under basal conditions. Two of the key players for mediating plasticity are the major types of ionotropic glutamate receptors: the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and the N-methyl-D-aspartate receptors (NMDARs). Both these types of glutamate receptors can be divided into different subtypes, which have different properties. In Chapter 3 I investigated the regulation and spatial organization of the different AMPAR and NMDAR subtypes at Sc-CA1 synapses. The different properties of the different receptor subtypes are determined by their subunit composition. In adult excitatory neurons, the two most commonly expressed AMPAR subtypes are composed of GluA1/GluA2 heterodimers or GluA2/GluA3 heterodimers. Common NMDAR subtypes are composed of GluN1/GluN2A heterodimers or GluN1/GluN2B heterodimers. We showed that under basal conditions, GluA1-homomers and GluA3-homomers are largely excluded from synapses due to basal levels of intracellular calcium. We also found results suggesting that GluA1- and GluA3-containing AMPARs can be colocalized at the same Sc-CA1 synapses. However, we demonstrated that GluN2A- and GluN2B-containing NMDARs are segregated at Sc-CA1 synapses. Because of the differences between GluN2A- and GluN2B-containing NMDARs, the segregation of NMDAR subtypes at Sc-CA1 synapses suggests that synaptic plasticity rules might vary between different glutamate release sites.
Synaptic AMPAR and NMDAR currents are weakened by oligomers of the amyloid-β (Aβ) peptide, which accumulates in the brain in Alzheimer’s disease. This aberrant synaptic plasticity is thought to be crucially involved in the cognitive decline seen in the early stage of Alzheimer’s disease. In Chapter 4, I used our extended variance analysis to tease out the different components contributing to Aβ-induced synaptic depression (= weakening) at Sc-CA1 synapses. We found that Aβ-induced synaptic depression consists of the loss of functional synapses and a reduced AMPAR strength in the remaining synapses. Furthermore, we found that the loss of functional synapses requires both the removal of GluA3-containing AMPARs and a GluN2A-dependent process. This GluN2A-dependent process might involve an increase in the number or conductance of GluN2A-containing NMDARs. How all these different factors contributing to Aβ-mediated synaptic depression are linked to each other exactly remains to be further elucidated in future studies. Understanding the Aβ-driven signaling cascade in detail may offer insight into which interventions could be promising in therapies for Alzheimer’s disease.
In Chapter 5, we moved to the primary visual cortex (V1) to study the contribution of GluA3-containing AMPARs to synaptic transmission and β-adrenergic activation-induced plasticity in that brain region. β-adrenergic receptors are activated by the neurotransmitter noradrenaline, the release of which is increased during wakefulness compared to sleep, and increases further during a state of arousal. In the hippocampus, β-adrenergic activation increases the open-channel probability and single-channel conductance of GluA3-containing AMPARs, leading to synaptic potentiation (= strengthening). Since the GluA3 subunit is particularly prevalent in the cortex, synaptic transmission through GluA3-containing AMPARs as well as GluA3-plasticity could play a prominent role in the functioning of V1. We found that indeed, unlike in the hippocampus, GluA3-containing AMPARs contribute to basal synaptic transmission onto layer 2/3 pyramidal neurons in V1 and that β-adrenergic activation causes GluA3-dependent postsynaptic potentiation at synapses onto these neurons, as well as presynaptic plasticity and GluA1-dependent postsynaptic potentiation. Our experiments reveal that GluA3 is a prominent player in synaptic communication in V1.
This thesis demonstrates that the VMR is a useful tool to extract more information from electrophysiological recordings. We furthermore uncovered several unknown facts about the organization of synapses, synaptic transmission and plasticity, emphasizing once more that fundamental research into plasticity mechanisms remains critical to get to know all the ways our brain can adapt to the environment and form memories, and to understand how these processes go wrong in cognitive disorders.