Different forms of structural plasticity in the adult olfactory bulb

Abstract

The adult olfactory bulb (OB) continuously receives new interneurons that integrate into the functional neuronal network and that play an important role in odor information processing and olfactory behavior. Adult neuronal progenitors are derived from neural stem cells in the subventricular zone (SVZ) bordering the lateral ventricle. They migrate long distances along the rostral migratory stream (RMS) toward the OB where they differentiate into interneurons, mature, and establish synapses with tufted or mitral cells (MC), the principal neurons in the OB. The plasticity provided by both adult-born and pre-existing early-born neurons depends on the formation and pruning of new synaptic contacts that adapt the functioning of the bulbar network to changing environmental conditions. However, the formation of new synapses occurs over a long time scale (hours-days), whereas some changes in environmental conditions can occur more rapidly, requiring a much faster adjustment of neuronal networks. A new form of structural remodeling of adult-born, but not early-born, neurons was recently brought to light. This plasticity, which is based on the activity-dependent relocation of mature spines of GCs toward the dendrites of active principal cells, may allow a more rapid adjustment of the neuronal network in response to quick and persistent changes in sensory inputs. In this mini-review we discuss the different forms of structural plasticity displayed by adult-born and early-born neurons and the possibility that these different forms of structural remodeling may fulfill distinct roles in odor information processing.

Keywords: activity-dependent plasticity; adult neurogenesis; critical period; olfactory bulb; spine relocation; spine turnover; structural plasticity.

Figure 1.

Schematic representation of the mouse forebrain and of olfactory bulb neurogenesis. Schematic drawing of the adult mouse forebrain and illustration showing the OB neuronal circuitry and adult neurogenesis. Early-born and adult-born GCs are shown in different colors.

Figure 2.

Two forms of structural plasticity in the adult OB. (A-B) Spine turnover represented as the formation and elimination of spines on GC dendrites (green). MC dendrites are shown in red. (C-D) Activity-dependent spine relocation is shown by the relocation of a spine from one MC dendrite to another. The activity-dependent release of glutamate and BDNF is indicated by the yellow cloud.