The mammalian central nervous system relies on precise synaptic connections to function correctly.
The development of precise neuronal circuitry is regulated by axon guidance molecules as well as by specific pattern of activity between the pre and the post synaptic elements.
In this thesis I focused on activity dependent mechanisms, and we analyzed the role of spontaneous afferent activity in the topographic organization of the olfactory bulb. To address this point we analyzed the intrabulbar connections between isofunctional glomeruli in a line of mice genetically modified to have very little spontaneous afferent activity due to the overexpression of the inward rectifying potassium channel Kir2.1 in the olfactory sensory neurons (Yu et al., 2004).
Since previous studies were limited to adults (Bellusio et al., 2002; Lodovichi et al., 2003), we first defined the development of the intrabulbar projections between isofunctional glomeruli at early stages of development in control mice. Targeting focal tracer injections to the glomerular layer, we found that the intrabulbar projection is present as early as P7 and it is targeted between homologous glomeruli. However, at this early stage of development, the projection is not confined exclusively to the homologous glomerulus but larger. We found that the connections undergo a refinement process between P15 and P30, when they reach the mature organization of a point to point projection. We then analyzed the formation and the specific targeting of the intrabulbar link in animals with reduced spontaneous activity, i.e. Kir 2.1 mice. We found that the connections are preserved in these mice, but are not exclusively confined to the homologous glomeruli. The link remains larger than in control mice at all the ages tested, from postnatal day 30 to 70, due to the lack of developmental refinement.
We then assess the effects of the unrefined connectivity of the bulbar circuits on olfactory behaviour using a classical behavioral test designed to assess the ability to discriminate between two different odorants. We found that Kir 2.1 mice were hampered in discriminating odorants that elicit similar spatial patterns of activated glomeruli (functional maps), such enantiomers, while retaining the ability to discriminate odorants that activate very distinct spatial pattern of glomeruli .
Spontaneous activity is thought to play a prominent role in circuit formation at very early stages of development. Once sensory systems become responsive to sensory stimuli, evoked activity contributes to the stabilization and further refinement of neuronal connections. It has been clearly demonstrated that sensory experience often modulates the development of neuronal circuitry within a defined period of time (critical period) in which the brain is particularly plastic.
Whether spontaneous activity can modulate synaptic connections in adult life, remains unknown.
We addressed this topic in the olfactory system and we studied whether manipulation of afferent spontaneous activity in adulthood could affect the already established and refined synaptic contacts, i.e. the intrabulbar connections. Taking advantage of the inducible nature of the Kir 2.1 construct, we allow the expression of the Kir 2.1 channels only in adulthood (P30-P60), for 30 days. We found that the expression of the Kir 2.1 channel in adults was able to induce a regression of the intrabulbar link to an unrefined and enlarged status
