Abstracts

Rationale: Fragile X is the leading inherited cause of mental retardation. It is a developmental disorder that results from a 'loss of function mutation' in the FMR1 gene, which encodes an mRNA binding protein (fragile X mental retardation protein, FMRP). Approximately 40% of Fragile X patients are also diagnosed with autism spectrum disorder, an even higher proportion in different forms of anxiety, and 20% with epilepsy. Disorders such as these indirectly implicate dysfunctional inhibitory neurotransmission and/or increased glutamatergic transmission. Direct evidence implicating inhibitory neurotransmission is limited to a few studies that point to decreased subunit expression of the GABA-A receptor system and decreases in inhibitory neuron number, mainly in cortex. However, little is known about inhibitory synaptic transmission in Fmr1-/- mutant mice, the animal model of FXS, especially in the amygdala. Methods: Using a combined neuroanatomical and electrophysiological approach, we examined inhibitory circuitry in the basolateral complex (BLC) of the amygdala in FMR1 -/- mutant mice. Immunocytochemical localization of inhibitory neuron markers (parvalbumin, NPY, calretinin) and GABA-A receptor subunits (alpha1, alpha2) were used to determine relative abundance of presynaptic inhibitory neurons and postsynaptic receptors in the BLC of the amygdala. Whole cell patch clamp recordings of excitatory neurons in the amygdala of FMR1 -/- mutant mice were obtained to gather data on excitability of the major excitatory neuron in the BLC. In other experiments whole cell voltage clamp recordings of inhibitory postsynaptic currents were obtained in order to evaluate fast (GABA-A) and slow (GABA-B) inhibitory neurotransmission in excitatory neurons. Results: Our results reveal increased excitability of excitatory neurons in the BLC of the amygdala in FMR1 -/- mutant mice. Additionally we find specific alterations of inhibitory neuron markers; however, these changes were not accompanied by changes in interneuronal subtype cell numbers. We also find that inhibitory postsynaptic events were significantly reduced in frequency and amplitude, yet decay kinetics were slightly enhanced. Inhibitory dysfunction was also affected at the circuit level as the GABA-B component of the feedforward inhibitory response in excitatory BLC neurons was greatly reduced. Conclusions: Taken together, these data show the presence of profound and specific alterations in inhibitory and excitatory synaptic transmission in the amygdala in FMR1 -/- mutant mice. These data also show that the high risk for anxiety in Fragile X patients may involve a decrease in inhibitory neurotransmission in the amygdala. Further, the increase in synaptic decay kinetics may also reveal an attempt at achieving homeostasis in an increased excitatory environment. Funding: NIH (NIDA020140 to JC, NS050719 to MH); FRAXA Research Foundation.