Altered synaptic gain in amygdala circuits after Grik4 over-expression

The amygdala is involved in emotional behaviors and its relation with some psychiatric illnesses is the subject of active research. Kainate receptors are ionotropic glutamate receptors, which carry out excitatory synaptic transmission and the modulation of both inhibitory and excitatory transmission between neurons. Alteration in the copy number of genes coding for Kainate receptor subunits have been linked to neuropsychiatric syndromes such as Autism and depression. Kainate receptors are heteromeric arrangements of low (GluK1-3) and high (GluK4-5) affinity subunits. To start delineating the role played by GluK4 kainate receptor subunits in human brain disease, the lab has generated a transgenic mouse over-expressing grik4 in the forebrain under the control of the CaMKII promoter, which drives expression of exogenous grik4 only in the forebrain (C57BL/6J-Tg(Camk2a-grik4); nicknamed to GluK4over. These mice displayed anhedonia, enhanced anxiety and depressive states, as well as altered social interaction, common endophenotypes associated to autism spectrum disorders. To start looking for electrophysiological correlates of this behavior in amygdala, the objective of this study was to elucidate the effect of exogenous grik4 overexpression in the forebrain on the pre- and postsynaptic excitatory activity in the characterized cells of Basolateral (interneurons and pyramidal cells) and central amygdala (late firing and regular firing GABAergic interneurons) through the patch clamp technique. First,we looked at the expression pattern of the exogenous GluK4 kainate receptor subunit in the basolateral amygdala (BLA) in the wild type and GluK4Over mice. Grik4 was expressed in all subnuclei of the Amygdala and the transgenic protein, as assessed by myc-immunoreactivity, was also present at principal cells within the BLA. Overexpression of Grik4 led to enhanced spontaneous and external capsule evoked glutamate-mediated excitatory activity in the BLA and a decreased short term facilitation in EC BLA synapses, suggesting more presynaptic presence of high affinity GluK4-containing KARs leading to a high amount of glutamate release from the presynaptic vesicles in the EC–BLA synapses. There was also increased AMPA/NMDA ratio of amplitudes of evoked EPSCs in Grik4 overexpressing mice in BLA pyramidal cells, indicating postsynaptic insertion of AMPARs and removal of NMDARs in EC-BLA synapses. We further noticed a consequently increased excitatory input to the regular firing cells of CeLA and, on the other hand, no change in the excitatory inputs to the late firing cells of the CeLA. Although, functional net inhibition of the late firing cells was found to be increased as evidenced by increased frequency of AMPARs mediated sIPSCs and a decreased ratio of frequency of sEPSCs/sIPSCs. This feedforward inhibition could lead to decreased functional inhibitory output to Centro medial amygdala (CeMA) effector neurons that in turn inhibit with a higher inhibitory control the neurons belonging to, e.g. Antero-dorsal bed nucleus of stria terminalis (major output pathway of amygdala). This increased inhibition has been shown earlier to promote anxiety behaviour [1]. Such a significant alteration in the excitatory-inhibitory synaptic transmission balance within intra-amygdala circuits may justify to a large extent the behavioral abnormalities observed in our GluK4Over model of ASD.