An investigation into the role of proteinase-activated receptor 2 on neuronal excitability and synaptic transmission in the hippocampus

Proteinase-activated receptor 2 (PAR-2) belongs to a novel family of G-protein coupled receptors that are unique in their activation mechanism by which a proteolytic cleavage at N-terminus by a proteinase reveals a ‘tethered ligand’ to activate the receptor. Albeit at a low level, PAR-2 is extensively expressed in normal and pathological brains, including the hippocampus. Qualitative studies into the expression of PAR-2 in several disease conditions, including ischaemia, HIV-associated dementia, Parkinson’s disease, Alzheimer’s disease, as well as multiple sclerosis, have suggested that PAR-2 plays either degenerative or protective role depending on in which cell type an increase in PAR-2 expression is observed. However, its potential roles in modulating neuronal excitability, synaptic transmission as well as network activities remain to be determined. Utilising the whole-cell patch clamp recording technique, I demonstrate, for the first time, that the activation of PAR-2 leads to a depolarisation of cultured hippocampal neurones following application of SLIGRL (100microM), a selective PAR-2 activating peptide (5.52 ± 1.48mV, n=16, P<0.05) and paradoxically a reduction of spontaneous action potential (AP) frequency (29.63 ± 5.03% of control, n=13, P<0.05). Pharmacological manipulation reveals that the PAR-2-mediated depolarisation is most likely dependent on astrocytic glutamate release, which takes effect on ionotropic glutamate receptors. In addition, an overt depression of synaptic transmission among the cultured neurones upon PAR-2 activation is more likely to cause the reduction of spontaneous APs. In further experiments, I show, for the first time, that the activation of PAR-2 induces a long term depression (LTD) of glutamatergic synaptic transmission at the Schaffer collateral-to-CA1 synapse in acute hippocampal slices following SLIGRL (100microM) application (80.75 ± 2.54% of control at 30 minute, n=12, P<0.05). Additionally, this novel form of LTD is independent of metabotropic glutamate receptors but mediated by NR2B subunit-containing N-methyl-D-aspartic acid (NMDA) receptors. It is also suggested from these experiments that glial-neuronal signalling is contributing to this novel form of LTD. In the final set of experiments, by monitoring field potentials in the stratum pyramidale of the CA3 area in acute hippocampal slices, I demonstrate that PAR-2 activation depresses the frequency of epileptiform activities induced by the application of 4-AP/0 Mg2+, an in vitro model of epilepsy (1.53 ± 0.21Hz to 1.18 ± 0.17Hz, n=13, P 0.05, 100microM SLIGRL). In summary, in this thesis, I demonstrate that PAR-2 modulates neuronal excitability and depresses excitatory synaptic transmission in the hippocampus. These data indicate that PAR-2 may play a regulatory role in neuronal signalling at single cell level by controlling neuronal intrinsic properties, as well as at synaptic level by tuning excitatory synaptic strength, which ultimately affects global excitability in the neural circuits as a whole. Therefore, this investigation suggests a novel physiological/pathophysiological role for PAR-2 in the brain. These data may reveal valuable clues for the development of drugs targeting a novel and potentially promising candidate.