Possible effects of nitric oxide synthases on odor-induced behavioral changes in mice

The eNOS isoform exhibits increased expression and activation in the main olfactory bulb of nNOS knock-out mice

The main olfactory bulb (MOB) is a neural structure that processes olfactory information. Among the neurotransmitters present in the MOB, nitric oxide (NO) is particularly relevant as it performs a wide variety of functions. In this structure, NO is produced mainly by neuronal nitric oxide synthase (nNOS) but also by inducible nitric oxide synthase (iNOS) and endothelial nitric oxide synthase (eNOS). The MOB is considered a region with great plasticity and the different NOS also show great plasticity. Therefore, it could be considered that this plasticity could compensate for various dysfunctional and pathological alterations. We examined the possible plasticity of iNOS and eNOS in the MOB in the absence of nNOS. For this, wild-type and nNOS knock-out (nNOS-KO) mice were used. We assessed whether the absence of nNOS expression could affect the olfactory capacity of mice, followed by the analysis of the expression and distribution of the NOS isoforms using qPCR and immunofluorescence. NO production in MOB was examined using both the Griess and histochemical NADPH-diaphorase reactions. The results indicate nNOS-KO mice have reduced olfactory capacity. We observed that in the nNOS-KO animal, there is an increase both in the expression of eNOS and NADPH-diaphorase, but no apparent change in the level of NO generated in the MOB. It can be concluded that the level of eNOS in the MOB of nNOS-KO is related to the maintenance of normal levels of NO. Therefore, our findings suggest that nNOS could be essential for the proper functioning of the olfactory system.

The temporal expression profile of a Nos3-related natural antisense RNA in the brain suggests a possible role in neurogenesis

Experimental work over the past several years has revealed an unexpected abundance of long natural antisense transcripts (NATs) in eukaryotic species. In light of the proposed role of such RNA molecules in the regulation of gene expression in the brain, attention is now focused on specific examples of neuronal NATs. Of particular interest are NATs that are complementary to mRNAs encoding nitric oxide synthase (NOS), the enzyme responsible for production of the important gaseous neurotransmitter nitric oxide (NO). Here we study the temporal expression profile of murine Nos3as NAT in the brain. Notably, Nos3as NAT is known to act as a negative regulator of Nos3 gene expression. The results of our quantitative analysis reveal differential expression of Nos3as NAT during embryonic and post-embryonic stages of development of the brain. Also, they show that the low levels of Nos3as NAT coincides with active neurogenesis. In addition we report on an inverse correlation between the relative expression level of Nos3as NAT and the level of Nos3 protein. Thus our data raise the hypothesis that the Nos3as NAT regulates neurogenesis through suppression of Nos3 gene activity. This idea is further supported by experiments conducted on the olfactory bulbs and cultured neuroblastoma cells.

Endothelial Nitric Oxide Synthase Is Present in Dendritic Spines of Neurons in Primary Cultures

Nitric oxide exerts important regulatory functions in various brain processes. Its synthesis in neurons has been most commonly ascribed to the neuronal nitric oxide synthase (nNOS) isoform. However, the endothelial isoform (eNOS), which is significantly associated with caveolae in different cell types, has been implicated in synaptic plasticity and is enriched in the dendrites of CA1 hippocampal neurons. Using high resolution microscopy and co-distribution analysis of eNOS with synaptic and raft proteins, we now show for the first time in primary cortical and hippocampal neuronal cultures, virtually devoid of endothelial cells, that eNOS is present in neurons and is localized in dendritic spines. Moreover, eNOS is present in a postsynaptic density-enriched biochemical fraction isolated from these neuronal cultures. In addition, qPCR analysis reveals that both the nNOS as well as the eNOS transcripts are present in neuronal cultures. Moreover, eNOS inhibition in cortical cells has a negative impact on cell survival after excitotoxic stimulation with N-methyl-D-aspartate (NMDA). Consistent with previous results that indicated nitric oxide production in response to the neurotrophin BDNF, we could detect eNOS in immunoprecipitates of the BDNF receptor TrkB while nNOS could not be detected. Taken together, our results show that eNOS is located at excitatory synapses where it could represent a source for NO production and thus, the contribution of eNOS-derived nitric oxide to the regulation of neuronal survival and function deserves further investigations.

The neuropeptide F/nitric oxide pathway is essential for shaping locomotor plasticity underlying locust phase transition

Behavioral plasticity is widespread in swarming animals, but little is known about its underlying neural and molecular mechanisms. Here, we report that a neuropeptide F (NPF)/nitric oxide (NO) pathway plays a critical role in the locomotor plasticity of swarming migratory locusts. The transcripts encoding two related neuropeptides, NPF1a and NPF2, show reduced levels during crowding, and the transcript levels of NPF1a and NPF2 receptors significantly increase during locust isolation. Both NPF1a and NPF2 have suppressive effects on phase-related locomotor activity. A key downstream mediator for both NPFs is nitric oxide synthase (NOS), which regulates phase-related locomotor activity by controlling NO synthesis in the locust brain. Mechanistically, NPF1a and NPF2 modify NOS activity by separately suppressing its phosphorylation and by lowering its transcript level, effects that are mediated by their respective receptors. Our results uncover a hierarchical neurochemical mechanism underlying behavioral plasticity in the swarming locust and provide insights into the NPF/NO axis.

DOI:http://dx.doi.org/10.7554/eLife.22526.001

Migratory locusts are widespread throughout the Eastern Hemisphere, especially in Asia, Australia and Africa. Although usually solitary insects, locusts can also form swarms made up of millions of individuals, which can devastate crops. Swarming can be studied on a smaller scale in the laboratory by forcing locusts to crowd together. This causes the locusts to enter a so-called gregarious state in which they are more active and sociable, which in turn promotes swarming. Isolating individual locusts has the opposite effect, causing the insects to enter a solitary state in which they are less active.

Chemicals in the locust brain called neuropeptides control phase transitions between solitary and gregarious behavior. Neuropeptides bind to specific proteins called receptors in the outer membranes of neurons and initiate unique signaling cascades inside cells. However, exactly how neuropeptides regulate the changes in locust behavior that affect swarming was not clear.

Hou et al. now reveal the role that two related neuropeptides, NPF1a and NPF2, play in this process. Crowding causes the levels of NPF1a and NPF2 in the locust brain to decrease, whereas isolating individual locusts causes the levels of two NPF receptors to increase. Both neuropeptides reduce levels of a molecule called nitric oxide in the brain. NPF1a does this by reducing the activity of the enzyme that produces nitric oxide, whereas NPF2 reduces the production of this enzyme. The reduction in nitric oxide in turn makes the locusts less active.

Similar NPF neuropeptides had previously been shown to affect activity levels in other invertebrates, such as roundworms and fruit flies. This, combined with the results now presented by Hou et al., suggests that the NPF/nitric oxide pathway may regulate activity in insects in general. Future work should investigate this possibility, as well as whether the NPF/nitric oxide pathway controls changes in other insect behaviors such as feeding and mating.

DOI:http://dx.doi.org/10.7554/eLife.22526.002