A critical period for activity-dependent synaptic development during olfactory bulb adult neurogenesis

Abstract

New neurons integrate in large numbers into the mature olfactory bulb circuit throughout life. The factors controlling the synaptic development of adult-born neurons and their connectivity remain essentially unknown. We examined the role of activity-dependent mechanisms in the synaptic development of adult-born neurons by genetic labeling of synapses while manipulating sensory input or cell-intrinsic excitability. Sensory deprivation induced marked changes in the density of input and output synapses during the period when new neurons develop most of their synapses. In contrast, when sensory deprivation started after synaptic formation was complete, input synapses increased in one domain without detectable changes in the other dendritic domains. We then investigated the effects of genetically raising the intrinsic excitability of new neurons on their synaptic development by delivering a voltage-gated sodium channel that triggers long depolarizations. Surprisingly, genetically increasing excitability did not affect synaptic development but rescued the changes in glutamatergic input synapses caused by sensory deprivation. These experiments show that, during adult neurogenesis in the olfactory bulb, synaptic plasticity is primarily restricted to an early period during the maturation of new neurons when they are still forming synapses. The addition of cells endowed with such an initial short-lived flexibility and long-term stability may enable the processing of information by the olfactory bulb to be both versatile and reliable in the face of changing behavioral demands.

Figure 1.

Sensory deprivation changes glutamatergic input synapse development in adult-born GCs. A, Progenitor cells were infected with retroviruses in the SVZ in combination with unilateral naris occlusion either at the same day or after synaptic development was complete (starting at 42 dpi). Genetically labeled GCs were examined at different days after infection. B, GCs have three main dendritic domains: a basal and a proximal domain that receive glutamatergic input synapses and a distal dendritic domain that contains recurrent input and output synapses. C, At 28 dpi, PSDG⁺ clusters were examined in adult-born GCs in sensory-deprived (starting on the day of retroviral labeling) or contralateral control olfactory bulbs. To attribute PSDG⁺ clusters (green) to a particular GC, dendritic morphology was visualized by red dye-labeled GFP immunofluorescence against the diffuse PSD-95:GFP present in the cytosol that was otherwise undetectable. The three main dendritic domains were analyzed separately (from top): distal, proximal, or basal domain. Scale bar, 10 μm.

Figure 2.

Sensory deprivation changes glutamatergic synapses in specific dendritic domains during a critical period. A, Scatter plot and mean density of PSDG⁺ clusters (clusters per micrometer) of adult-born GCs in sensory-deprived (starting on the day of retroviral labeling) and contralateral control (red and black circles, respectively) olfactory bulbs at different days after infection. Statistical significance is only indicated if p < 0.05 (t test). The bottom graph shows the ratio of the mean cluster density of sensory-deprived over control GCs in a specific domain at a given day after infection. B, When sensory deprivation started after synaptic development was complete, the mean density of PSDG⁺ clusters (clusters per micrometer) only changed in the proximal domain (unilateral naris occlusion at 42 dpi and examined at 63 dpi).

Figure 3.

Sensory deprivation during synaptic development reduces output synapse density. A, At 28 dpi, synaptophysin:GFP⁺ clusters were examined in the distal domain of adult-born GCs from sensory-deprived (starting on the day of retroviral labeling) and contralateral control olfactory bulbs. Scale bars, 10 μm. B, Scatter plot and mean density of SypG⁺ clusters (clusters per micrometer) of adult-born GCs from sensory-deprived (starting on the day of retroviral labeling) and contralateral control (red and black circles, respectively) olfactory bulbs at 28 dpi (t test). C, When sensory deprivation started after synaptic development was complete, there were no changes in the mean density of SypG⁺ clusters (clusters per micrometer) (unilateral naris occlusion at 42 dpi and examined at 63 dpi).

Figure 4.

Genetically increased excitability does not change glutamatergic input synapse development. A, Whole-cell recordings were obtained from acute slices containing GCs that either expressed mCherry as control or NaChBac:GFP. Bottom left, At 28 dpi, a short current injection (4 nA, 5 ms) evoked a sustained depolarization in NaChBac expressing GCs but not in controls of adult-born GCs. Right, The current–voltage relationship revealed a voltage-dependent inward current that was only observed in NaChBac-expressing GCs (10 mV steps, V_h = −70 mV, 16 dpi). B, At 28 dpi, PSDG⁺ clusters were examined in adult-born GCs expressing either the synaptic marker alone or with NaChBac. Scale bar, 10 μm.

Figure 5.

Genetically increased excitability does not change glutamatergic input synapse development and blocks the synaptic changes induced by sensory deprivation. A, Scatter plot and mean density of PSDG⁺ clusters (clusters per micrometer) in a dendritic domain of new control or NaChBac-expressing GCs (black and red circles, respectively) born in the adult and examined at different dpi. Statistical significance is only indicated if p < 0.05 (t test). The bottom graph shows the ratio of the mean cluster density of NaChBac-expressing over control GCs in a specific dendritic domain at a given day after infection. B, Same as A, but instead, adult-born GCs coexpressed PSD-95:GFP and NaChBac in both the sensory-deprived (starting on the day of retroviral labeling) and contralateral control olfactory bulb. Statistical significance is only indicated if p < 0.05 (t test).

Figure 6.

Genetically increased excitability does not change output synapse development. A, At 28 dpi, SypG⁺ clusters were examined in the distal domain of adult-born GCs expressing either the synaptic marker alone or with NaChBac. Scale bar, 10 μm. B, Scatter plot and mean density of SypG⁺ clusters (clusters per micrometer) of new control or NaChBac-expressing GCs (black and red circles, respectively) born in the adult at 28 dpi. No significant differences were detected (t test).

Figure 7.

Sensory deprivation and intrinsic excitability differently control synaptic development in the dendritic domains of adult-born neurons. Reduced sensory input during synaptic development changed synaptic densities in all dendritic domains. When sensory deprivation started after the completion of synaptic development, the only detectable changes were increases in the density of glutamatergic input synapses in the proximal domain. Genetically increasing the excitability of new neurons did not affect the synaptic development of cells that matured in a normal sensory stimulation but rescued the synaptic changes triggered by sensory deprivation.