Early expression of odorant receptors distorts the olfactory circuitry.

The odor response properties of a mammalian olfactory sensory neuron (OSN) are determined by the tightly regulated expression of a single member of a very large family of odorant receptors (ORs). The OR also plays an important role in focusing the central projections of all OSNs expressing that particular receptor to a pair of stereotypic locations (glomeruli) in each olfactory bulb (OB), thus creating a spatial map of odor responses in the brain. Here we show that when initiated late in neural development, transgenic expression of one OR in almost all OSNs has little influence on the architecture of the OB in mice. In contrast, early OR-transgene expression (mediated by the Ggamma8-promoter) in 50-70% of OSNs grossly distorts the morphology of glomeruli and alters the projection patterns of many residual OSNs not expressing the transgene. Interestingly, this disruption of targeting persists in adult animals despite the downregulation of Ggamma8 and transgenic OR expression that occurs as olfactory neurogenesis declines. Indeed, functional imaging studies reveal a dramatic decrease in the complexity of responses to odorants in adult Ggamma8-transgenic OR mice. Thus, we show that initiation of transgenic OR expression early in the development of OSNs, rather than just the extent of transgene expression, determines its effectiveness at modifying OB anatomy and function. Together, these data imply that OR-expression timing needs to be very tightly controlled to achieve the precise wiring and function of the mammalian olfactory system.

Prominent roles for odorant receptor coding sequences in allelic exclusion.

Mammalian odorant receptors (ORs) are crucial for establishing the functional organization of the olfactory system, but the mechanisms controlling their expression remain largely unexplained. Here, we utilized a transgenic approach to explore OR gene regulation. We determined that although olfactory sensory neurons (OSNs) are capable of supporting expression of multiple functional ORs, several levels of control ensure that each neuron normally expresses only a single odorant receptor. Surprisingly, this regulation extends beyond endogenous ORs even preventing expression of transgenes consisting of OR-coding sequences driven by synthetic promoters. Thus, part of the intrinsic feedback system must rely on elements present in the OR-coding sequence. Notably, by expressing the same transgenic ORs precociously in immature neurons, we have overcome this suppression and established a generic method to express any OR in approximately 90% of OSNs. These results provide important insights into the hierarchy of OR gene expression and the vital role of the OR-coding sequence in this regulation.

Activity-dependent plasticity in the olfactory intrabulbar map.

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.

Effects of sleep disruption on rat dentate granule cell LTP in vivo.

Sleep frequently fragmented or disrupted for prolonged periods can result in mood changes and impaired mental ability and performance. Sleep deprivation is defined as depriving a person or organism of sleep for various periods of fixed durations. Sleep disruption (SD) occurs when a person is awakened at any time when they would normally be sleeping; sometimes on a schedule but usually unexpectedly. It seems as if any disruption of an entrained sleep pattern can induce learning and memory impairment; and mood changes including irritability and aggression. Because memory is impaired under these conditions several studies have been conducted recently to examine changes in long term potentiation (LTP) in hippocampal brain slices following various periods of sleep deprivation in rats. Results of the present study show clearly that LTP is also decreased following SD but to a greater extent than that observed following sleep deprivation. The purpose of the present study was to measure dentate granule cell LTP in anesthetized rats following 1-, 2-, or 3-day schedules of SD using a modified flower pot procedure. Results showed that a single disruption of 3 h reduced LTP from a normal 38.7-7.6%; that endured for at least 14 h; and 9 h reduced it completely. Easy to handle animals become irritable, hyperactive, and aggressive following SD. Results are discussed in terms of stress related effects of SD and changes in synaptic plasticity.