Abstracts

Rationale: Transcription factors (TFs) establish molecular codes that orchestrate early stages of interneuron development. Some TFs remain expressed in mature interneurons, suggesting a divergent, yet unknown, function. We previously showed that postnatal conditional ablation of one TF, the Aristaless Related homeobox gene (Arx), in parvalbumin (PV) positive interneurons resulted in learning deficits, alterations in intrinsic and synaptic properties, and electrographic seizures in some animals. These results suggest that Arx loss in all mature interneurons may have a more profound effect on network function. Thus, we studied the effect of postnatal ablation of Arx in all interneurons on the excitability of hippocampal networks.Methods: To temporally control Arx expression, we crossed a floxed Arx mouse (Arxfl/fl) with a tamoxifen-inducible CreER- mouse. Cre-mediated recombination induced by tamoxifen (Tam) injection in P35-40 male mice resulted in ablation of Arx in interneurons. Tam-induced (Arx-/y;CreER) and sham injected (Arx+/y;CreER) mice were sacrificed 10 days after the injections and processed for immunohistochemistry to assess the efficiency of Cre-mediated recombination. To determine the consequence on network activity, intracranial EEG and extracellular field recordings were performed ~21 days after the injections. Mice were monitored by video EEG continuously for 5 days. Field potentials were recorded at the hippocampal CA1 region in response to Schaffer collaterals (SC-CA1) stimulation. As an index of synaptic transmission, we examined the input-output (I/O) coupling of field excitatory postsynaptic potentials (fEPSPs). To examine the effect of Arx loss on synaptic plasticity, we recorded fEPSPs and measured the probability of release and long term plasticity (LTP).Results: Preliminary results demonstrated that Tam injections resulted in nearly complete ablation of Arx in cortical and subcortical brain regions. Background EEG showed some irregular spikes, but no seizure was observed after Tam injections. Ablation of Arx postnatally resulted in a leftward shift of the I/O curve, suggesting a reduction in synaptic transmission. Paired-pulse stimulation of SC-CA1 was not affected by ablation of Arx, suggesting no change in release probability. However, both HFS and theta burst stimuli failed to induce robust LTP in the Arx-/y; CreER mice, indicating that loss of Arx in interneurons impaired LTP.Conclusions: Our preliminary study suggests that postnatal loss of Arx may alter network function via impairments of synaptic transmission and LTP. Therefore, postnatal expression of Arx may regulate the maturation of functional properties of inhibitory interneurons.