A Role for α4β2 δGABA A Receptors during the Critical Pubertal Period on Post-Pubertal Spine Pruning and Behavioral Flexibility

Abstract

Neuroplastic modifications, like synaptic pruning, underlie the transition from childhood to adulthood through refinements of neuronal circuits that are poorly understood although signaling cascades triggered by NMDA receptors (NMDARs) are implicated. Activation of these receptors is impaired at puberty due to the emergence of perisynaptic α4βδ GABA-A receptors (GABAARs) on the spines of CA1 pyramidal cells of the hippocampus, which generate a shunting inhibition and hamper NMDAR activation. Therefore, I tested the hypothesis that α4βδ GABAARs play a role in synaptic pruning during adolescence.

To this end, I treated female mice during the pubertal period (puberty, 35-44 PND, identified by vaginal opening) with GABA drugs which target α4βδ GABAARs or used α4-/- mice and compared spine density during post-puberty (post-pub, 8 wks of age) with vehicle-treated wild-type (WT) mice by examining Golgi impregnated pyramidal neurons from the CA1 hippocampus using the Neurolucida system. Spine density of CA1 hippocampal pyramidal cells was decreased by 46% in post-pub hippocampus from pubertal WT conditions (P<0.05) but was not decreased in α4-/- mice, implicating α4βδ GABAARs. Selective enhancement of α4βδ GABAAR function with gaboxadol (GBX, .1mg/kg) during puberty resulted in decreased spine density, while administration of the neurosteroid THP (3α-OH-5β-pregnan-20-one, 10 mg/kg), which decreases α4βδ expression and function during puberty, increased spine density in post-pub hippocampus consistent with my hypothesis.

I also assessed behavioral flexibility, which measures novelty detection and re-acquisition of a behavior, using the active place avoidance (APA) task and the multiple placement object recognition task (MPORT) where the target area/object is moved after initial learning occurred. Performance on these tasks was compromised with all treatments and α4 knock-out, compared to WT, suggesting that the naturally occurring spine density in WT post-pub hippocampus may be optimal for these tasks. These findings suggest that α4βδ GABAARs play a role in adolescent synaptic pruning, and that optimal spine density is necessary for behavioral flexibility.