Local GABA circuit control of experience-dependent plasticity in developing visual cortex

Abstract

Sensory experience in early life shapes the mammalian brain. An impairment in the activity-dependent refinement of functional connections within developing visual cortex was identified here in a mouse model. Gene-targeted disruption of one isoform of glutamic acid decarboxylase prevented the competitive loss of responsiveness to an eye briefly deprived of vision, without affecting cooperative mechanisms of synapse modification in vitro. Selective, use-dependent enhancement of fast intracortical inhibitory transmission with benzodiazepines restored plasticity in vivo, rescuing the genetic defect. Specific networks of inhibitory interneurons intrinsic to visual cortex may detect perturbations in sensory input to drive experience-dependent plasticity during development.

Fig. 1

(A) GABA content in isolated homogenates of young (P18−19; eight WT and six GAD65 KO mice) and adult brains (>5 months old; six WT and four KO mice), normalized to young WT values by region. Open (WT) and solid (KO) bars represent means ± SEM (*P < 0.05, **P < 0.005; t test). (B) Maximal GABA output from WT and GAD65 KO binocular zone in response to brief depolarizing stimuli (100 mM KCl, arrow; three mice, each >P27; *P < 0.05, t test). Columns are individual 30-min dialysis fractions ± SEM normalized to mean before high K⁺ perfusion (basal GABA ± SD = 0.18 ± 0.02 pmol per fraction for WT, 0.22 ± 0.11 pmol per fraction for KO). (C) Expression of activity-dependent immediate-early gene zif268 in P28 visual cortex by 90 min of photostimulation (D+P) after 5 days of dark-rearing (D). Northern blots were normalized to housekeeping gene G3PDH (left panel). Sensitivity to visual stimulation in WT and KO mice is shown as (D+P)/D zif268 expression ratio (right panel, mean ± SEM, three mice each; *P < 0.01, t test).

Fig. 2

(A) Prolonged discharge of single-unit activity after visual stimulation in extracellular recordings from GAD65 KO visual cortex. Upper panel: Raster plot of spike response to four sweeps each of computer-generated light-bar stimuli moving across the visual field (5 °/s) in opposite directions (90° and 270°). Trials were randomly interleaved with periods of no stimulus presentation (Spont.). Despite crisp spike onset, neuronal response continued beyond the outer edge of the presumptive receptive field (RF), as defined by stimuli moving in the opposite direction. Lower panel: Proportions of WT and KO cells exhibiting prolonged discharge to stimuli exiting their RF (n = 670 and 1112 cells, 28 and 45 mice, respectively; P < 0.0001, t test). (B) Retinotopic organization of WT (open squares) and GAD65 KO (solid diamonds) primary visual cortex. RF-center azimuths are plotted versus electrode position relative to vertical meridian. Inset: Correlation coefficients for three WT and four KO regressions (P = 0.2, t test). (C) RF size (left panel, 82 WT and 79 KO cells; P = 0.1, t test) and proportion of cells exhibiting preference for light-bar stimuli of particular orientation (right panel, n = 670 and 1112 cells, 28 WT and 45 KO mice, respectively; P = 0.3, t test).

Fig. 3

(A) Ocular dominance scores for WT mice were normally skewed in favor of the contralateral eye (left panel, 66 cells, three mice). Four days of monocular eyelid suture (solid circle) during the critical period shifted ocular dominance toward the open, ipsilateral eye (open circle), yielding a balanced distribution (right panel, 132 cells, six mice; P < 0.0001, χ² test). Plasticity in rodents is less pronounced than in other mammals (24). (B) Ocular dominance distribution of nondeprived GAD65 KO mice is similar to WT (left panel, 99 cells, five mice; P = 0.2, χ² test). Four days of monocular vision yielded no shift in favor of the open eye (right panel, 108 cells, five mice; P = 0.4 vs. normal KO and P < 0.0001 vs. deprived WT, χ² test). As an indicator of the relative response to the two eyes, the CBI (16), shown in the upper right corner of each distribution, takes values of 0 and 1 for complete ipsilateral or contralateral eye dominance, respectively. (C) LTP and LTD in WT (open circles) and KO (solid circles) visual cortical slices after theta-burst (left panel) or low-frequency (1 Hz, right panel) stimulation, respectively [8 slices, seven mice from each group (LTP) and 7 slices, six WT and seven KO mice (LTD); P = 0.2 for both comparisons at 20 min, t test].

Fig. 4

(A) Local diazepam treatment of visual cortex restores plasticity to GAD65 KO mice in vivo. Left panel: Directed infusion restricted diazepam from regions beyond the visual cortex ipsilateral to the cannula (solid circles), as well as throughout the opposite hemisphere (open circles, mean ± SEM, three mice). Right panel: Ocular dominance shifted fully in the KO binocular zone exposed to diazepam concurrent with a brief period of monocular deprivation during the critical period (114 cells, four mice; P < 0.0001, χ² test vs. nondeprived KO; compare with Fig. 3B). (B) Neither vehicle treatment (left panel, 99 cells, four mice; P < 0.0001, χ² test vs. diazepam KO above) nor diazepam infusion into the hemisphere opposite the recording site (right panel, 77 cells, three mice; P < 0.0001, χ² test vs. diazepam KO above) restored the effect of monocular deprivation to GAD65 KO mice. (C) Monocular T TX injections for 4 days during the critical period produced significantly less plasticity in KO than in WT mice (P < 0.05, t test). Untreated and vehicle-treated monocularly deprived (MD) mice are grouped together, as are ventricle- and cortex-infused diazepam-treated animals. Shaded region indicates the range of nondeprived CBIs for both WT and KO mice. Each symbol represents one animal.