Onset of odorant receptor gene expression during olfactory sensory neuron regeneration

Expression of the NMDA receptor subunit GluN3A (NR3A) in the olfactory system and its regulatory role on olfaction in the adult mouse

Glutamate is an excitatory neurotransmitter in the olfactory system and its N-methyl-D-aspartate-(NMDA) receptor subunits [GluN1 (NR1), GluN2A (NR2A), and GluN2B (NR2B)] are expressed at synapses in the olfactory bulb and olfactory epithelium. Thus, glutamatergic neurons and NMDA receptors play key roles in olfaction. GluN3A (NR3A) is a unique inhibitory subunit in the NMDA receptor complex; however, the expression and functional role of GluN3A in the olfactory bulb and epithelium remain unclear. The present study examined the expression patterns of GluN3A in the olfactory bulb and epithelium and explored its functional role in the olfactory system. Immunohistochemical and Western blot analyses revealed that GluN3A is abundantly expressed in different cellular layers of the olfactory bulb and epithelium of the adult wild type (WT) mice. In littermate GluN3A knockout (GluN3A(-/-); KO) mice, the expression of olfactory marker protein normally found in mature olfactory sensory neurons was significantly reduced in the olfactory bulb and epithelium. A butyl alcohol stimulus increased immediate-early gene c-Fos expression in the olfactory system of WT mice, while this response was absent in GluN3A KO mice. The level of phosphorylated Ca(2+)/calmodulin-dependent kinase II was significantly lower in GluN3A KO mice compared to WT mice. In buried food finding test, GluN3A mice took significantly longer time to find food compared to WT mice. Consistently, impaired odor distinguishing ability was seen in GluN3A KO mice. These findings suggest that GluN3A, expressed in the adult olfactory system, plays a significant regulatory role in olfactory development and functional activity.

Odorant receptor gene expression changes during the parr-smolt transformation in Atlantic salmon

The ability of salmon to home accurately to their natal stream to spawn has long intrigued biologists and has important consequences for the maintenance of population structure in these species. It is known that olfaction is crucial to homing, and that the transition from the freshwater to the marine environment (the parr-smolt transformation; PST) is a period of increased olfactory sensitivity and learning, resulting in a permanent memory of natal site odours that is retained, at least in part, in peripheral sensory neurones. These odours are then used as cues by sexually maturing fish on their homeward migration. We used quantitative polymerase chain reaction techniques to demonstrate transient increases in expression of odorant receptor transcripts (of up to fifty-fold over pre-PST levels) coincident with PST. Both olfactory (SORB) and vomeronasal receptors (SVRA and SVRC) are involved, which suggests that the fish learn both environmental odours and semiochemicals (pheromones). Receptor expression varies between families and changes over time indicating both genetic differences in odour stimuli and multiple periods of olfactory sensitivity. We suggest that changes in OR gene expression may have a role in homing behaviour and thus the maintenance of population structure in Atlantic salmon.

Odorant receptor expression as a function of neuronal maturity in the adult rodent olfactory system

Odorant receptors (ORs) are expressed in a spatially restricted manner in the mammalian olfactory epithelium (OE), and this patterning probably contributes to innervation specificity within the olfactory bulb (OB). Furthermore, glomerular targeting appears to be contingent on receptor choice. Central to the mechanism by which ORs influence axonal specificity is the timing of OR expression during the life cycle of the olfactory sensory neurons (OSNs). Data indicate that OSNs express ORs in the absence of the OB but do not address whether OR expression is an early event in OSN differentiation. Accordingly, we evaluated whether ORs are expressed in mature [olfactory marker protein (OMP(+))] and/or immature [growth-associated protein of 43 kDa m.w. (GAP-43(+))] OSNs by assessing the expression of the P2 OR subtype via immunostaining for beta-gal and concurrent OMP or GAP-43 expression in P2-IRES-tauLacZ mice. Nearly 90% of P2(+) OSNs expressed OMP, whereas approximately 10% expressed GAP-43. One month after unilateral bulb ablation, the number of P2(+) OSNs decreased on the lesioned side; however, the percent of P2(+)/GAP-43(+) OSNs dramatically increased. We also determined that onset of P2 OR expression is slightly delayed when evaluated in the context of neuronal differentiation. Additionally, we defined the expression of OR(+) OSNs in the OE of rats via in situ hybridization with a panel of eight ORs followed by OMP immunostaining. All eight ORs were found in neurons situated throughout the height of the OE, including those OSNs deep to OMP staining, thus demonstrating definitively that ORs are expressed prior to the maturational state defined by OMP expression.

The olfactory bulb as an independent developmental domain

The olfactory system is a good model to study the mechanisms underlying guidance of growing axons to their appropriate targets. The formation of the olfactory bulb involves differentiation of several populations of cells and the initiation of the central projections, all under the temporal and spatial patterns of gene expression. Moreover, the nature of interactions between the olfactory epithelium, olfactory bulb and olfactory cortex at early developmental stages is currently of great interest. To explore these questions more fully, the present review aims to correlate recent data from different developmental studies, to gain insight into the mechanisms involved in the specification and development of the olfactory system. From our studies in the pax6 mutant mice (Sey(Neu)/Sey(Neu)), it was concluded that the initial establishment of the olfactory bulb central projections is able to proceed independently of the olfactory sensory axons from the olfactory epithelium. The challenge that now remains is to consider the validity of the olfactory bulb as an independent development domain. In the course of evaluating these ideas, we will review the orchestra of molecular cues involved in the formation of the projection from the OB to the olfactory cortex.

Evidence for gradients of gene expression correlating with zonal topography of the olfactory sensory map

Signals regulating diversification of olfactory sensory neurons to express odorant receptors and other genes necessary for correct assembly of the olfactory sensory map persist in the olfactory epithelium of adult mouse. We have screened for genes with an expression pattern correlating with the topography odorant receptor-expression zones. The Msx1 homeobox gene and a semaphorin receptor (Neuropilin-2) showed graded expression patterns in the olfactory epithelium. The gradients of Msx1 and Neuropilin-2 expression in basal cells and neurons, respectively, correlated with expression of a retinoic acid-synthesizing enzyme (RALDH2) in lamina propria. A BMP-type I receptor (Alk6) showed a reverse gradient of expression in the supporting cells of the epithelium. Considering known functions of identified genes in cell specification and axon guidance this suggests that zonal division of the olfactory sensory map is maintained, during continuous neurogenesis, as a consequence of topographic counter gradients of positional information.