Effects of the glutamic acid decarboxylase (GAD) inhibitor semicarbazide and anti-GAD autoantibodies-containing immunoglobulin G on neuronal network activity within the motor cortex

The electrical activity of the brain is the result of a complex interaction between excitation and inhibition mediated by several types of neurotransmitters. As the majority of neurons in the brain utilize either the inhibitory neurotransmitter γ-aminobutyric acid (GABA) or the excitatory neurotransmitter glutamate, the interplay of these two neurotransmitters principally controls brain excitability and, hence, imbalance between these two neurotransmitters may cause severe pathological conditions. Inhibition of glutamic acid decarboxylase (GAD), the rate-limiting enzyme of GABA synthesis, is believed to change neuronal network activity caused by impaired GABAergic inhibition. Recordings of intrinsic optical signals (IOSs) and whole-cell patch-clamp measurements of GABAA receptor-mediated miniature postsynaptic currents (GABAA Minis) and spontaneous excitatory postsynaptic currents (sEPSCs) were performed in the motor cortex in acute brain slices to unveil the effects of GAD inhibitors at the network level. The first project of this PhD thesis was to prove the IOS technique for its capability of monitoring neuronal network activity over several hours. Concurrently, new software for the analysis of IOS data was developed, which facilitates and significantly accelerates data analysis. Afterwards, changes in neuronal network activity after impairing GAD activity with the well-known GAD inhibitor semicarbazide (SMC) were observed with the IOS technique. If compared to the values of sham-drug application, a stable and reversible increase in both signal intensity and signal area was observed after 2 h of 2 mM SMC application. Consistent with these findings of IOS recordings, patch-clamp measurements of GABAA Minis revealed an SMC-induced reduction in the strength of GABAergic inhibition. The results are in line with the assumption that SMC impairs GABA synthesis by blocking GAD activity. SMC application, however, did not alter spontaneous excitatory neuronal network activity. The final aim of this study was to investigate potential effects of Anti-GAD autoantibodies-containing immunoglobulin G (IgG) derived from patients with stiff-person syndrome (SPS-IgG) on motor cortical neuronal network activity. IOS recordings do not reveal differences in neuronal network activity during SPS-IgG application and control IgG application. However, run-down of IOSs was significantly decelerated during IgG application, which possibly indicates a diminished neuronal cell death caused by an unspecific IgG effect. Compared to brain slices preincubated with IgG-free artificial cerebrospinal fluid, control IgG did not affect GABAA Mini amplitude and frequency as well as sEPSC amplitude. The sEPSC frequency, however, was significantly reduced under these conditions. This decreased excitatory transmitter release might explain the beneficial effect of immunoglobulin treatment in some forms of epilepsy. Similar to SMC, patch-clamp measurements of GABAA Minis revealed a reduction in the strength of GABAergic inhibition after preincubation with SPS-IgG. Consistent with this finding, application of SPS-IgG enhanced sEPSC frequency. This shows that IgG of SPS patients is indeed capable of altering GABAergic synaptic transmission, thus further supporting the hypothesis of an autoimmune origin of the stiff-person syndrome.