Activity-Dependent Plasticity in the Olfactory Intrabulbar Map

Abstract

In mammals, each olfactory bulb contains two mirror-symmetric glomerular maps. Isofunctional glomeruli within each bulb are specifically linked through a set of reciprocal intrabulbar projections (IBPs) to form an intrabulbar map. We injected neural tracers into the glomerular layer on one side of the bulb and examined the resulting projection on the opposite side. In adult mice, the size of the projection tuft is directly proportional to the size of the injected region. Using this ratio as a measure of IBP maturity, we find an immature 5:1 projection to injection ratio at 1 week of age that gradually refines to a mature 1:1 by 7 weeks. Moreover, whereas the glomerular map is able to form despite the elimination of odorant-induced activity, the intrabulbar map shows clear activity dependence for its precise formation. Here we show through experiments with both naris-occluded and anosmic mice that odorant-induced activity is not required to establish IBPs but is crucial for projection refinement. In contrast, increased glomerular activation through exposure to distinct odorants during map development can accelerate the refinement of projections associated with the activated glomeruli. These findings illustrate a clear role for odorant-induced activity in shaping the internal circuitry of the bulb. Interestingly, activity deprivation can alter the organization of both the developing and the mature map to the same degree, demonstrating that intrabulbar map plasticity is maintained into adulthood with no discernible critical period.

Postnatal reduction of BDNF regulates the developmental remodeling of taste bud innervation

“…As with many other systems (Erzurumlu and Kind, 2001; Espinosa and Stryker, 2012; Marks et al, 2006; Walsh and Lichtman, 2003), postnatal remodeling is likely required for the formation of a mature taste bud with appropriate connections between taste receptor cells and gustatory neurons (Kinnamon et al, 2005; Mistretta et al, 1988; Nagai et al, 1988; Yuan and Yankner, 2000). BDNF regulates this remodeling in other systems (Cabelli et al, 1997; Gonzalez et al, 1999; Kaneko et al, 2008), and in the gustatory system is expressed in postnatal and adult taste buds (Huang and Krimm, 2010).…”

Construction of functional neuronal circuitry in the olfactory bulb

“…It has been shown that coarse topography of intrabulbar projection is formed in an activity-independent manner; however, refinement to establish the precise connectivity requires sensory-evoked activity [73]. Unlike other sensory systems, there is no critical period for this activity-dependent refinement process, i.e., plasticity persists throughout life.…”

Continuous Neural Plasticity in the Olfactory Intrabulbar Circuitry

“…Enhancing odorant-induced activity with odorant conditioning during development has been shown to accelerate the refinement of both glomerular (Kerr and Belluscio, 2006) and intrabulbar circuitry (Marks et al, 2006). Long-term naris closure causes altered glomerular refinement if it begins soon after birth (Nakatani et al, 2003; Zou et al, 2004), but not when initiated later in life.…”

“…These projections connect isofunctional odor columns through synapses in the internal plexiform layer (IPL) to produce an intrabulbar map. Developmental studies show that initially intrabulbar projections broadly target the IPL on the opposite side of the bulb and refine postnatally to their adult precision by 7 weeks of age in an activity-dependent manner (Marks et al, 2006). In this study, we sought to determine the capacity of intrabulbar map to recover its precision after disruption.…”