Dr Neil Hardingham
Synaptic transmission and plasticity in the cerebral cortex
The cerebral cortex is a six layered structure involved in the processing of many different sensory inputs, including visual, auditory and somatosensory. It is also involved in the combination of sensory inputs in association cortex, and in primates it is the highly developed cortex which carries out their unique higher functions; memory, thought, language and consciousness. The cortex has a columnar structure in that all the neurons in a particular column of cortex process the same information. This columnar nature of the cortex is typified in the barrel cortex where visible striations in layer IV (the input layer) refer to the cortical representations of individual whiskers.
A biocytin filled layer II/III neuron with the layer IV barrels beneath it visualized by a Nissl stain
Cartoon of the experimental set-up
The effect of sensory deprivation on synaptic plasticity (Long Term Potentiation (LTP) and
Long Term Depression (LTD)) in the barrel cortex. LTP is increased by sensory deprivation (top)
and LTD is occluded by deprivation (bottom)
Synaptic connections between neurons are critical to the passage of excitation in the cortex. Cortical neurons commonly signal to one another via multiple chemical synapses and synaptic properties at these sometimes weak and unreliable synapses can be modified by synaptic plasticity. I am interested in the properties of synaptic transmission at central synapses, especially in the cortex. Synaptic connections between neurons are critical to the spread of excitation in the cortex. Cortical neurons commonly signal to one another via multiple chemical synapses and synaptic properties at these sometimes weak and unreliable synapses can be modified by synaptic plasticity. An individual barrel column may contain over 10,000 neurons and each of these may receive inputs from thousands of other cells in that cortical column. Therefore a barrel column may contain over a million synaptic connections. We are now able to resolve down to the single synapse level both functionally and structurally. Knowledge of individual connection properties can give important information on the cortical circuitry. Individual cortical connections have been shown to have quite different properties to one another (Hardingham et al., 2010). The cortex operates despite these large differences in synaptic efficacy and each synaptic connection is the way it is because of its’ physiological history which has shaped its properties to the current state.
Connections have also been shown to be developmentally regulated e.g. the response of a synaptic connection to a burst of stimulations (the multi-pulse response) is developmentally regulated: immature synapses show short term depression while more mature synapses show less short-term depression and signal bursts of stimulation more faithfully.
Synaptic plasticity describes the way in which synaptic connections alter in their properties and may underlie how memories are formed in the brain. Physiologically important connections are developed via long term potentiation (LTP) and other connections are depressed via long term depression (LTD). Synapses may change in their properties by either pre- or post-synaptic alterations at existing synapses or by the creation of new synapses. I have studied the role of GluR1 in postsynaptic potentiation and nitric oxide (NO) in presynaptic potentiation (Hardingham and Fox, 2006) and also on the role of the initial connection properties in determining synaptic plasticity (Hardingham et al., 2007). Mechanisms of synaptic plasticity are often developmentally regulated and can also be pathway specific. For example, the conversion of silent, NMDA receptor only containing synapses to functional AMPA receptor containing synapses has been shown to be present in developing cortex but not in adult cortex. Therefore, when describing properties of synaptic transmission and mechanisms of synaptic plasticity, it is important not to generalise between different synaptic connections or between different stages of development, as this can lead to conflicting results. The history of activity in the barrel cortex has been shown to affect both properties of synaptic transmission and synaptic plasticity (Hardingham et al., 2008). I am also interested in neuronal morphology and correlating neuronal structure with physiological function (Hardingham et al., 2010). The location of the synaptic inputs on the dendritic tree can be important in determining their somatic efficacy, while the post-synaptic efficacy of each synaptic input can be investigated by neuronal modelling (Hardingham et al., 2010).
Quantal analysis of synaptic connections
Quantal analysis is a way in which properties in synaptic connections can be quantified in more detail. It enables each connection to be quantified in terms of the presynaptic strength (release probability, Pr), the number of synapses the connection is mediated by (N), and the postsynaptic efficacy of each synapse (Quantal amplitude, Q) (Hardingham et al., 2010). These three values are called the quantal parameters for a synaptic connection. Synaptic properties and quantal parameters can be quite heterogeneous for individual connections (Hardingham et al., 2010), and they may be modified via processes of synaptic plasticity. Quantal analysis also enables identification of the locus of changes in synaptic efficacy (eg LTP or LTD) (Hardingham and Fox, 2006; Hardingham et al., 2007).
Mean amplitude = N * Pr *Q
Quantal analysis: single release site connection (N=1)
Quantal analysis: multiple release site connection (N=3)
Increasingly, neurological disorders are being linked to structural and functional deficits in the brain. The DISC1 gene has been linked to various mental disorders, including schizophrenia. I am currently working on a project comparing development of the barrel cortex in wild type and DISC1-cc mice. It has been shown that expression of the mutant DISC-1 protein postnatally produces a deficit in plasticity in the adults, and I am looking for electrophysiological and structural deficits in the developing cortex of the DISC-1 mouse as the circuitry is wired up.
Bioctyin filled layer IV and layer V neurons
Biocytin Filled layer 2/3 neuron with background Nissl stain
A pair of biocytin filled, synaptically connected layer 2/3 neurons
A synaptically connected pre-synaptic layer 2/3 cell and postsynaptic layer 5 cell (both biocytin filled)
Left: GFP labelled neurons in cortex (c-fos tagged), viewed using a 2-photon microscope.
Centre: Visualising synapses in cortex using confocal and multi-photon microscopy. A collapsed z stack of a layer 2/3 neuron injected with Alexa 594.
Right: Dendrites of a layer 2/3 neuron injected with Alexa 594 showing spines in more detail.
Dr Emma Blain (School of Biosciences, Cardiff University)
Dr Riccardo Brambilla (School of Biosciences, Cardiff University)
Dr Guy Major (School of Biosciences, Cardiff University)
Prof Julian Jack (Wolfson Institute for Biomedical Research, UCL)
Dr Jenny Read (Institute of Neuroscience, Newcastle University)
Dr Neil Bannister (Department of Anatomy, Bristol University)
Dr Andy Trevelyan (Institute of Neuroscience, Newcastle University)