The principal finding of our study is that one of the core symptoms – namely, anxiety – of the afflicted SPS patient could be reproduced in the recipient rat by passively transferring her GAD 65 antibody-containing IgG into the subarachnoid space. This points to a probable pathogenic action of IgG autoantibodies. Furthermore, application of IgG at the caudal spinal cord lead to distant IgG binding at the amygdala that is likely linked to the observed behavioral alterations.
How do our findings relate to the neuropsychiatric observations in the patient? The extension of the PET study of the SPS patient, showing a reduced 11
C-FMZ binding potential in limbic structures, supports the concept that the GABAergic system in the amygdala region is affected by the disease, and this is consistent with a significant reduction in GABA-A receptor binding observed in patients with panic disorder and other anxiety-related disorders 
. The PET findings suggest a dysfunction of the amygdala which may be interpreted as a disturbance of GABA signaling in patients with SPS, rather than a secondary effect due to repeated falls 
. These findings are in accordance with previous reports of decreased GABA levels in selected brain regions measured with MRI spectroscopy 
showing that GABAergic transmission is not uniformly affected in the brain of patients with SPS.
Based on our passive transfer experiments we like to propose that the neuropsychiatric changes may be a direct consequence of circulating IgG autoantibodies to GAD 65 although it cannot be excluded that autoantibodies with other as yet unknown specificities are instrumental in causing the behavioral abnormalities and IgG binding in recipient rats (see below).
The GABAergic involvement of the limbic cortex shown here might be due to the high demand of modulatory control in these regions, based on their finely tuned GABAergic transmission 
. Our findings are also supported by and in good agreement with reports about increased fear-related behavior and altered responses to anxiolytic drugs in GAD 65 knockout mice, resulting from a desynchronization in the amygdala-hippocampal network 
. Furthermore, mice heterozygous for the γ2 subunit of the GABA-A receptor, with reduced synaptic clustering of the GABA-A receptor, showed an exaggerated anxiety behavior 
In contrast to a recent report on the effects of GAD 65 antibody containing IgG in an acute ex-vivo preparation 
and to our own studies with SPS derived antibodies against amphiphysin 
, we could neither observe acute motor hyperexcitability or muscle stiffness, nor a chronic motor disorder after repetitive applications of the IgG. The absence of a motor phenotype was corroborated by the results of in-vivo
electrophysiological recordings including sensitive tests such as H-reflex activity and measure of presynaptic afferent depolarization 
directly by recording the dorsal rot potential amplitudes. Thus, from our present data we have no direct evidence indicating that our SPS patient's IgG also lead to a diminished GABAergic inhibition in the rat spinal cord. The discrepancy between our findings and those in the ex-vivo preparations 
is most likely resulting from the different experimental paradigms. Our results are in contrast to our earlier observations with antibodies against amphyphysin. We assume that the mechanism of disturbed vesicular endocytosis as observed by our group 
may be a more ubiquitous one when compared to the effects of GAD 65 antibodies. In contrast, GAD65 dysfunction may be the consequence of a predominant vulnerability of neuronal pathways in the amygdala-hippocampal complex, according to the findings in the animal models with genetic modification of GAD65 expression (15;26; see above).
One question that still needs to be solved is whether the anti-GAD 65 antibodies in the patient's purified IgG fraction are directly responsible for the observed effects, or whether antibodies directed against yet unidentified surface antigens of GABAergic neurons might play a role. Using western blotting, we could not detect any reactivity towards antigens other than GAD65. However, as in the example of GABA-A-receptor-associated protein (GABARAP), identified as an additional antigen in patients with anti-GAD-positive SPS 
, the possibility of additional autoantibody specificities cannot be excluded. Using patient IgG depleted of anti-GAD65 IgG in animal models and in-vitro assays would be the most direct way to address this, but attempts to produce sufficient quantities of soluble and stable recombinant human GAD65 were not successful as also found by other groups 
. Since GAD65 is a cytosolic enzyme, which is not exposed at the cell surface, anti-GAD65 antibodies would have to enter the cells and obtain access to their antigen to exert a pathogenic role. Recently we have shown that antibodies to amphiphysin, another intracellular antigen in SPS patients, can indeed be taken up by neurons and bind to their intracellular protein antigen 
. For GAD 65 antibodies this remains to be shown.
Recently a new family of paraneoplastic and idiopathic CNS disorders with neuropsychiatric symptoms, namely limbic encephalitis with psychiatric features, SPS, hyperexplexia, epilepsy and various other optional features has been identified to be linked to autoantibodies directed at CNS autoantigens including NMDA-receptors, AMPA-receptors, Caspr2 and LGI-1; some of these were formerly classified as voltage-gated potassium channelopathies 
. Many of these patients respond to immunosuppressive treatment and plasma exchange as described in GAD 65 and amphiphysin antibody-associated SPS opening up a wide field of antibody-mediated neuropsychiatric disorders.
Further studies will be necessary to confirm our findings in a larger number of patients. Another aim for follow-up studies is to identify the exact target of the SPS antibodies responsible for these observations. Taken further, our findings may imply that autoantibodies should be searched for in other anxiety related disorders, and, if detected, this may open new diagnostic and therapeutic options for patients with anxiety syndromes.