Role of α4-containing GABAA receptors in limiting synaptic plasticity and spatial learning of female mice during the pubertal period

Abstract

Expression of α4βδ GABA_A receptors (GABARs) increases at the onset of puberty on dendritic spines of CA1 hippocampal pyramidal cells. These receptors reduce activation of NMDA receptors (NMDARs), impair induction of long-term potentiation (LTP) and reduce hippocampal-dependent spatial learning. These effects are not seen in the δ-/- mouse, implicating α4βδ GABARs. Here we show that knock-out of α4 also restores synaptic plasticity and spatial learning in female mice at the onset of puberty (verified by vaginal opening). To this end, field excitatory post-synaptic potentials (fEPSPs) were recorded from the stratum radiatum of CA1 hippocampus in the slice from +/+ and α4-/- pubertal mice (PND 35-44). Induction of LTP, in response to stimulation of the Schaffer collaterals with theta burst stimulation (TBS), was unsuccessful in the +/+ hippocampus, but reinstated by α4 knock-out (~65% potentiation) but not by blockade of α5-GABARs with L-655,708 (50nM). In order to compare spatial learning in the two groups of mice, animals were trained in an active place avoidance task where the latency to first enter a shock zone is a measure of learning. α4-/- mice had significantly longer latencies by the third learning trial, suggesting better spatial learning, compared to +/+ animals, who did not reach the criterion for learning (120s latency). These findings suggest that knock-out of the GABAR α4 subunit restores synaptic plasticity and spatial learning at puberty and is consistent with the concept that the dendritic α4βδ GABARs which emerge at puberty selectively impair CNS plasticity. This article is part of a Special Issue entitled SI: Adolescent plasticity.

Keywords: Alpha-4; Alpha-5; Delta; GABA-A receptor; Hippocampus; Long-term potentiation; Puberty; Spatial learning.

Figure 1. Pubertal impairments in LTP induction are not observed in α4 −/− hippocampus

LTP was induced by theta burst stimulation (TBS) of the Schaffer collaterals to the CA1 hippocampus (arrow). LTP was not successfully induced in pubertal +/+ hippocampus (left), but was robustly induced in the α4 −/− at puberty (middle). Bath application of 50 nM L-655,708 to block α5-GABARs (right) did not reinstate LTP induction at puberty. (Dashed line, average potentiation at 2 h post-TBS, α4 −/−) Inset, Representative fEPSPs before and after TBS (arrow). Scale, 0.5 mV, 50 ms; n=10–12 slices/group; *P<0.05 vs. pre-TBS.

Figure 2. Pubertal impairments in spatial learning are not observed in α4 −/− mice

A, Spatial learning was assessed by the latency to enter the shock zone (hatched sector) on a rotating arena in an active avoidance task (longer latency=improved learning) across 3 10-min learning trials. B, Representative traces of mouse trajectory during the final training trial for +/+ (left) and α4 −/− (right) pubertal mice, reflecting a greater number of shocks (open circles, 13) for the +/+ compared to the α4 −/− (1). C, Latency to first entry for training trials #1–3. Latencies were significantly longer in pubertal α4 −/− mice compared to wild-type (WT), signifying improved learning by learning trial #2. *P<0.05 vs. +/+. D, A greater percentage of α4 −/− mice reached learning criterion (120 s latency) compared to +/+. E, #shocks/entry was unaltered across groups suggesting that the shock was equally aversive for all WT and α4 −/− mice. F, #entries for the acclimation trial (no shock), a measure of locomotor activity. n=14 mice/group.