Physiology and Plasticity of Sensory Synapses in the Piriform Cortex

Abstract

In this thesis, I utilise in vitro slice models to investigate the structure, physiology and plasticity of synapses delivering sensory (afferent) information from the olfactory bulb (OB) to the primary excitatory neurons of the PCx, semilunar (SL) and superficial pyramidal (SP) cells.

Given the relationship between postsynaptic morphology and synaptic transmission, I first used enhanced-resolution confocal microscopy to investigate the dendritic structure and distribution of spines on the apical arbours of the two target cell types. SP cells displayed more complex dendritic trees and a higher spine density than SL cells. Interestingly, although arranged at a lower density, spines on the distal apical dendrites of SL cells (where afferent input is received) were significantly larger than those on SP cells. As a result, synaptic opportunity (defined as total spine head surface area per unit dendritic length) was similar for both cell types. In view of the substantial differences in spine structure and distribution, I next combined minimal electrical stimulation with whole-cell somatic recordings to investigate the properties of afferent synaptic transmission. These experiments showed that SL cells receive significantly larger unitary synaptic inputs from the OB than do SP cells. Variance-mean analysis and strontium-evoked asynchronous EPSCs revealed that these powerful inputs to SL cells are driven by both a higher neurotransmitter release probability (Pr) and higher quantal amplitude (Q). As a result, SL cells can be lifted to action potential threshold with fewer afferent inputs than SP cells.

Next, I asked whether the disparity in afferent synaptic structure and transmission to SL and SP cells extended to their ability to express synaptic plasticity. Using patterned extracellular stimulation, N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) and depression (LTD) could be observed using field recordings. When isolating unitary inputs, however, LTP and LTD could only be induced at afferent-SL synapses, with no reliable plasticity inducible at afferent-SP inputs. Interestingly, afferent-SL LTD was expressed both pre- and postsynaptically, with variable changes in Pr (presynaptic) and Q (postsynaptic) observed. On the other hand, afferent-SL LTP was predominantly presynaptic and appeared to depend on the prior Pr of the stimulated input, such that low Pr synapses could be potentiated while high Pr synapses could not.

In a final set of exploratory experiments, I investigated the excitability of SL and SP cell dendrites. Dendritic NMDA spikes can evoke synaptic potentiation at afferent inputs, so it was important to understand whether the plasticity I observed depended on this mechanism. I made whole-cell recordings from both somas and dendrites of SL and SP cells while systematically increasing the strength of afferent extracellular stimulation. Although putative dendritic electrogenesis was observed at high stimulation strengths, synaptic responses scaled linearly through the minimal stimulation range, indicating that the unitary stimulation strengths used in my plasticity experiments did not evoke active dendritic events.

Overall, the experiments presented in this thesis reveal distinct differences in how SL and SP cells receive and process monosynaptic sensory information from the OB. From dendritic spine structure through to synaptic transmission and plasticity, SL and SP cells are markedly distinct. Given that these cells are the dominant glutamatergic cell types of the PCx, and receive the majority of monosynaptic afferent input from the OB, understanding how they process olfactory input will be critical for our ultimate understanding of how information is encoded and stored within this trilaminar sensory cortex. Show more