The generation of mature myelinating and non-myelinating Schwann cells from immature Schwann cells is regulated by signals from the axon. In the absence of axonal signals, for example during nerve injury, Schwann cells will return to an immature-like phenotype. However, Schwann cells retain the ability to re-differentiate after injury, demonstrating remarkable plasticity throughout adult life. Although these cellular transformations are normally beneficial, during diseases of the peripheral nervous system, they are often disrupted producing negative consequences. In this thesis, I have demonstrated a synergistic relationship between two signals, cyclic adenosine monophosphate (cyclic AMP) and p-neuregulin-1 (NRG1) in inducing myelin differentiation in cultured mouse Schwann cells. Furthermore, I found that the cyclic-AMP response element binding protein (CREB) family of transcription factors are required for this differentiation. Second, I have investigated the role of the transcription factor c-Jun in controlling Schwann cell responses after nerve injury. Using in vivo and in vitro approaches, I have found that c-Jun is an axonal-regulated injury factor that controls Schwann cell demyelination and dedifferentiation. Furthermore, Schwann cell derived c-Jun is crucial for adequate nerve regeneration and repair after injury. Finally, I investigated the functional properties of a mutant form of the my el in-related transcription factor Egr2 (Krox-20), which leads to severe congenital hypomyelinating neuropathy in humans. I found that mutant Egr2 is not transcriptionally inactive but retains residual wild-type Egr2 functions. More importantly, mutant Egr2 also demonstrates aberrant effects in Schwann cells, enhancing DNA synthesis, both in the presence and absence of the putative axonal mitogen, p-neuregulin 1. This is in stark contrast to wild-type Egr2, which causes withdrawal from the cell cycle.

The role of temperature sensations elicited from the skin, include object identification, thermoregulation and the conscious perception of pain. Humans can differentiate at least three distinct cold sensations: innocuous cooling, cold pain and pain evoked by freezing. However, patients with painful neuropathies often suffer from cold allodynia, where normally non-painful cool stimuli begin to induce pain or cold hyperalgesia, a heightened sensitivity to a painful cold stimulus. It is therefore vital to understand the mechanisms by which cold is signalled under normal conditions and to investigate which changes occur under pathological conditions. The thesis will describe sets of electrophysiological recordings carried out from rodent primary afferent neurons using the in vitro skin nerve preparation in an attempt to reveal mechanisms underlying thermal sensitivity. The discovery of thermally sensitive transient receptor potential (TRP) ion channels has given insights into the molecular mechanisms of thermal transduction. These include the heat sensitive TRPV1 ion channel and cold sensitive TRPM8 and TRPA1 ion channels. However, the role of the TRPA1 receptor in cold transduction remains controversial. Cold sensitivity of primary afferents in adult rat was studied. Selective TRP channel agonists capsaicin, menthol, and mustard oil were then applied onto the receptive field of primary afferents to determine the expression pattern of thermosensitive TRP channels. The majority of cold sensitive A and C fibre nociceptors as well as thermoreceptors were sensitive to menthol, indicating that TRPM8 is the transducer of cold on these afferents. The poor correlation of TRPA1 expression and cold sensitivity in nociceptive A and C fibres indicates that TRPA1 is unlikely to play a significant role is detecting noxious cold. TRPV2 is another heat activated ion channel. The sensory phenotype of TRPV2 knock-out mice was studied and compared against TRPV2 wild-type mice in both hairy and glabrous skin. Mice lacking TRPV2 had normal heat sensitive nociceptors and afferents retained mechanical sensitivity. The involvement of potassium (K+) channels in mediating and/or modulating thermosensation has been suggested. Based on these previous findings, the effects of the broad spectrum potassium channel blockers 4-aminopyridine (4- AP) and Tetraethylammonium (TEA) were studied on primary afferents neurons. Application of 4-AP or TEA directly on the receptive fields induced a novel cold sensitivity in a proportion of low threshold mechanoreceptors and increased the cold responses in a proportion of cold sensitive A and C fibre nociceptors. Interestingly 4-AP or TEA had no effect on the cold responses of innocuous cold thermoreceptors. Drug induced cold sensitivity was investigated using the chemotherapeutic agent oxaliplatin, which induces a sensory neuropathy in patients. Following infusion of the drug, patients experience abnormal skin sensations (paresthesias), which are triggered or aggravated by exposures to cold. The receptive properties of afferents were investigated before and after oxaliplatin application to provide an insight into the mechanism by which this abnormal cold sensitivity develops. This study shows for the first time, that oxaliplatin applied directly on the receptive fields induces a novel cold sensitivity in half of previously cold insensitive Aβ mechanoreceptors. Just over a third of Aδ nociceptors also displayed a novel or increased sensitivity to cold after oxaliplatin application. In contrast, receptive properties of C fibres remained unchanged. Overall, the results of the thesis provide evidence that TRPM8 is involved in the transduction of cold stimuli and that potassium and sodium conductances are involved in modulating the final response to a cold stimulus.

This thesis focuses on the putative roles of the Robo2 and Robo3 receptors in regulating the development of cortical interneurons and Cajal-Retzius cells in the embryonic mouse forebrain. A detailed analysis of the expression patterns of the Robo3 receptor is elucidated for the first time. Further comparison of all three Robos with interneuron markers confirms that different populations of cortical interneurons express these receptors during development. The putative roles of the Robo2 and Robo3 receptors in specifying the total number and distribution of cortical interneurons during development is investigated in vivo, using transgenic mice deficient in these receptors. This analysis shows that removal of the Robo2 or Robo3 receptors alone does not result in significant changes in the total numbers or positioning of interneurons within the cortex, suggesting that these receptors are not involved in the ventral-dorsal tangential migration of interneurons from their origins within the ganglionic eminences to the cortex. However, both Robo2 and Robo3 receptors significantly regulate the morphology of migrating interneurons during development. Preliminary analysis in triple Robo mutant mice points to a complex interplay between these receptors, and highlights the importance of understanding the functional relationship between these. In addition, a population of pioneering Cajal- Retzius cells express Robo receptors during preplate stages of development. Analysis in single Robo mutant animals suggests that Robo2 has a role in determining the total numbers of (reelin immunopositive) Cajal-Retzius cells within the hippocampal cortex.

The cerebellum is vitally important for motor learning and this behaviour is heavily reliant on the plasticity of excitatory and inhibitory synaptic transmission. Over the past two decades, numerous forms of synaptic plasticity within the cerebellum have been described, particularly affecting the principle output inhibitory neuron, the Purkinje cell (PC). In this study, I have focused on the mechanism underlying the phenomenon of rebound potentiation (RP), which is a long-lasting enhancement of inhibitory transmission at interneuron-PC synapses. RP is triggered by strong climbing fibre induced depolarization of postsynaptic PCs. Subsequent Ca^{2+} influx via voltage-dependent calcium channels, and Ca^{2+} release from intracellular stores, synergise to activate Ca^{2+} dependent kinase pathways resulting in a persistent enhancement of synaptic γ aminobutryic acid type-A receptor (GABA_AR) mediated current on PCs. The types of RP that are induced can be classified as early and late RP. The induction of RP critically depends on both Ca^{2+}/Calmodulin-dependent protein kinase II (CaMKII) and protein tyrosine kinase (PTK) activation, as selective inhibitors (CaMKIINtide and genistein) blocked the RP of miniature inhibitory postsynaptic currents (mIPSCs). These kinases were found to work through inter-dependent pathways with PTK acting dowmtream of CaMKII. Furthermore, CaMKII activity is required for both the induction and the maintenance of RP. In PCs, the majority of GABA_ARs are comprised of α1β2γ2 subunits. The α1 subunit was essential for mediating the phasic inhibition observed in PCs; whereas, the β2 subunit-containing receptors underlied the large amplitude, fast rise time mIPSCs and were also critically important for the induction of RP. Thirdly, tyrosine phosphorylation of the γ2 subunit was found to determine the direction of plasticity, converting RP to a new phenomenon of rebound depression. Finally, by using inhibitors of SNARE-based exocytosis pathways, we determined that GABA_A receptor insertion is the underlying mechanism of RP. In conclusion, RP is a phosphorylation-dependent subunit-specific plasticity of GABA_ARs, which is manifest by postsynaptic GABA_A receptor insertion. As such, it may be an important contributory factor to motor learning in the cerebellum.

The development of the nervous system is a complex task that involves precise connections between billions of neurons. This is achieved, at least in part, by the overproduction of neurons and the survival of a select few that compete for limited survival and growth promoting factors, such as neurotrophic factors. The neurotrophin brain-derived neurotrophin factor (BDNF) has been shown to play an important role in proliferation and differentiation of cortical neuronal precursors (Bartkowska et al 2007). Moreover, BDNF induces binding of the transcription factor CREB to gene promoters in a nitric oxide (NO)-dependent manner (Riccio et al 2006). NO positively regulates a large number of transcription factors and genes in the nervous system (Hemish et al 2003; Dhakshinamoorthy et al 2007). I have demonstrated that NO achieves this broad level of gene regulation by influencing chromatin remodelling. My data also show that NO accumulates within the nucleus of cortical neurons upon BDNF stimulation, thereby inducing Snitrosylation of a wide array of nuclear proteins. S-nitrosylation of histone deacetylase 2 (HDAC2) decreases its affinity for chromatin, leading to increased histone acetylation levels. This NO-dependent regulation of HDAC2 promotes changes in endogenous gene expression and affects the dendritic length of cortical neurons.

Episodic memory is the memory for our personal past experiences. Although numerous functional magnetic resonance imaging (fMRI) studies investigating its neural basis have revealed a consistent and distributed network of associated brain regions, surprisingly little is known about the contributions individual brain areas make to the recollective experience. In this thesis I address this fundamental issue by employing a range of different experimental techniques including neuropsychological testing, virtual reality environments, whole brain and high spatial resolution fMRI, and multivariate pattern analysis. Episodic memory recall is widely agreed to be a reconstructive process, one that is known to be critically reliant on the hippocampus. I therefore hypothesised that the same neural machinery responsible for reconstruction might also support ‘constructive’ cognitive functions such as imagination. To test this proposal, patients with focal damage to the hippocampus bilaterally were asked to imagine new experiences and were found to be impaired relative to matched control participants. Moreover, driving this deficit was a lack of spatial coherence in their imagined experiences, pointing to a role for the hippocampus in binding together the disparate elements of a scene. A subsequent fMRI study involving healthy participants compared the recall of real memories with the construction of imaginary memories. This revealed a fronto-temporo-parietal network in common to both tasks that included the hippocampus, ventromedial prefrontal, retrosplenial and parietal cortices. Based on these results I advanced the notion that this network might support the process of ‘scene construction’, defined as the generation and maintenance of a complex and coherent spatial context. Furthermore, I argued that this scene construction network might underpin other important cognitive functions besides episodic memory and imagination, such as navigation and thinking about the future. It is has been proposed that spatial context may act as the scaffold around which episodic memories are built. Given the hippocampus appears to play a critical role in imagination by supporting the creation of a rich coherent spatial scene, I sought to explore the nature of this hippocampal spatial code in a novel way. By combining high spatial resolution fMRI with multivariate pattern analysis techniques it proved possible to accurately determine where a subject was located in a virtual reality environment based solely on the pattern of activity across hippocampal voxels. For this to have been possible, the hippocampal population code must be large and non-uniform. I then extended these techniques to the domain of episodic memory by showing that individual memories could be accurately decoded from the pattern of activity across hippocampal voxels, thus identifying individual memory traces. I consider these findings together with other recent advances in the episodic memory field, and present a new perspective on the role of the hippocampus in episodic recollection. I discuss how this new (and preliminary) framework compares with current prevailing theories of hippocampal function, and suggest how it might account for some previously contradictory data.

The work carried out in this doctoral thesis investigated the representation of statistical sound properties in human auditory cortex. It addressed four key aspects in auditory neuroscience: the representation of different analysis time windows in auditory cortex; mechanisms for the analysis and segregation of auditory objects; information-theoretic constraints on pitch sequence processing; and the analysis of local and global pitch patterns. The majority of the studies employed a parametric design in which the statistical properties of a single acoustic parameter were altered along a continuum, while keeping other sound properties fixed. The thesis is divided into four parts. Part I (Chapter 1) examines principles of anatomical and functional organisation that constrain the problems addressed. Part II (Chapter 2) introduces approaches to digital stimulus design, principles of functional magnetic resonance imaging (fMRI), and the analysis of fMRI data. Part III (Chapters 3-6) reports five experimental studies. Study 1 controlled the spectrotemporal correlation in complex acoustic spectra and showed that activity in auditory association cortex increases as a function of spectrotemporal correlation. Study 2 demonstrated a functional hierarchy of the representation of auditory object boundaries and object salience. Studies 3 and 4 investigated cortical mechanisms for encoding entropy in pitch sequences and showed that the planum temporale acts as a computational hub, requiring more computational resources for sequences with high entropy than for those with high redundancy. Study 5 provided evidence for a hierarchical organisation of local and global pitch pattern processing in neurologically normal participants. Finally, Part IV (Chapter 7) concludes with a general discussion of the results and future perspectives.

The aim of this thesis was to characterise the molecular mechanisms involved in cold transduction in peripheral neurons. Members of the transient receptor potential (TRP) family of cation channels are currently recognised as the principal transducers of thermal stimuli in sensory neurons, and two channels, TRPM8 and TRPA1, have been implicated in cold sensation. Ratiometric calcium imaging and quantitative rt-PCR studies revealed that cold sensitivity in sympathetic neurons and in a significant proportion of sensory neurons is independent of TRPM8 and TRPA1 expression, suggesting the presence of a further cold-sensitive ion channel in these cells. Previous studies had suggested that voltage-gated potassium channels may also be important in the regulation of cold transduction. In calcium imaging experiments, pharmacological blockade of potassium channels unmasked a novel cold sensitivity in previously unresponsive populations of sensory and sympathetic neurons. Moreover, the individual potassium channels underlying these effects differ between neuronal populations, as demonstrated by the differing effects of specific potassium channel blockers on sensory and sympathetic neurons. The chemotherapeutic drug oxaliplatin induces an acute cold hypersensitivity in patients, and has been suggested to act on voltage-gated sodium channels. The activation of sodium channels alone did not alter cold sensitivity in sensory neurons, however the application of oxaliplatin to dissociated cells induced a qualitative change in the cold response, with neurons displaying a unique bursting response pattern. Cold-sensitive neurons are dependent on NGF during development, and a further aim of the thesis was to investigate the influence of this growth factor on cold sensitivity in neonatal sensory neurons. NGF induced an up-regulation of cold sensitivity and functional TRP channel expression. In the case of TRPM8, this effect was mediated via the Runx1 transcription factor, which is known to be expressed in NGF-dependent neurons during development.

The cerebellum is involved in maintenance of posture and balance and coordination of voluntary movements. It has previously been shown that the inhibition of granule cells by Golgi cells, in the input layer of the cerebellar cortex, is important for normal cerebellar function. However, little is known about what determines the firing of spontaneously active Golgi cells and how intrinsic activity interacts with sensory input. In particular, the excitation of the interneuron by mossy fibres, which may mediate feed-forward inhibition of granule cells, has not been characterized. I have used immuno-histochemistry, patch-clamp recordings and imaging in acute cerebellar slices of rats to study feed-forward mossy fibre input onto Golgi cells and its downstream effects. I confirm that mossy fibres, Golgi cells and granule cells form a functional feed-forward inhibitory circuit. Anatomical analysis of the circuitry suggests that only spatially correlated inputs result in feed-forward inhibition. Activation of the pathway required synchronous activity in 4 out of the approximately 10 mossy fibres contacting a Golgi cell. These inputs can reset the timing of spontaneous Golgi cell firing with remarkably high temporal precision. I found that an interaction between fast EPSC kinetics, electronic compactness and pacemaker conductances allowed precise temporal signaling while integrating only 6 quanta across the dendritic tree of a Golgi cell. Golgi cell mean firing rate was only weakly modulated by mossy fibre input due to dominant pacemaker conductances. These results suggest that the properties of the feed-forward mossy fibre - Golgi cell - granule cell pathway are tuned to detect and signal coincident synaptic activity with high temporal precision. This provides a likely synaptic basis for precisely timed Golgi cell responses observed in vivo, which may signal the onset of sensory stimulation producing spatiotemporally correlated mossy fibre activity. These findings are discussed in the context of current models of granule cell layer processing.

The cerebellum is essential for some forms of motor learning and storage of motor memories. The simple organization of the cerebellum provides special opportunities to analyse mechanisms underlying learning and memory in a real neural network. A dominant hypothesis of cerebellar function is one based on suggestions by Marr and Albus, where Purkinje cell responses to parallel fibre inputs are modified under climbing fibre control. A candidate cellular mechanism is a long-term depression of parallel fibre-Purkinje cell synapses (pf-PC LTD) seen, in vitro, after conjunctive activation of parallel and climbing fibres. The associative and persistent nature of pf- PC LTD suggests that it may underlie behavioural forms of cerebellum-dependent learning. Activation of the metabotropic glutamate receptor type 1 (mGlu 1 receptor) is critical for the induction of pf-PC LTD. If pf-PC LTD underlies learning, then blocking this receptor should lead to impairment of cerebellum-dependent learning. The studies described here test the role of the mGlu 1 receptor in one form of cerebellum-dependent associative learning. Chapters 2 and 3 describe slice electrophysiological experiments that assess the mGlu1 antagonists CPCCOEt, YM-298198 and JNJ16259685. CPCCOEt was found to have a non-specific action, while YM-298198 and JNJ16259685 were found to be very potent and highly specific. Chapter 4 describes the effects of cerebellar infusions of JNJ16259685 on classical conditioning of the rabbit nictitating membrane response and reveals that conditioning was not impaired. The result suggests there could be dissociation between pf-PC LTD and cerebellar learning. It is known that pf-PC LTD in vitro may not be a single phenomenon since it can be induced with a range of protocols that can differ significantly from the normal physiology. Thus, there may be a form of pf-PC LTD in vivo that is relatively independent of mGlu 1 receptor function. Alternatively, behavioural learning may depend upon plasticities involving other neuronal types in the cerebellar cortex.