eNOS Controls Behavior
The researchers sought to determine under which conditions eNOS was active in the NGC neurons. More specifically, they wanted to assess if eNOS was activated in NGC neurons in response to changing environments. To do this, the researchers subjected mice to various environments, and monitored the activation of eNOS positive neurons by measuring nitrotyrosine levels, a protein modification that occurs due to NO production.
The researchers exposed the mice to three environments: a clean cage containing novel scents, a shock chamber that delivered 15 mild and unpredictable foot shocks for the duration of one hour, and three repeated sessions of forced swim with periods of recovery under a heat lamp. For each of the three exposures, the brains of the animals were collected and nitrotyrosine was measured. When mice were in their home cages, eNOS remained less active. However, upon switching to a new environment, the levels of eNOS increased. Out of the three conditions, the novel scents and forced swim conditions produced the largest amount of nitrotyrosine. Although the foot shock condition did not have the same effect—a result that Dr. Stern finds a “bit perplexing.” He also finds it interesting that different environments may result in varied levels of eNOS production.
To test the role of increasing levels of NO in the modulation of GA, behavioral assays were coupled with the microinfusion of a NOS production blocker N(G)-Nitro-L-arginine methyl ester (L-NAME) into the NGC. The animals that had their NO production blocked exhibited a strikingly different behavior upon returning to their home cage, after exposure to environmental perturbation (in this case, a cage with novel scents), then those who had received the control (saline) injection. The L-NAME–injected mice remained highly active, lacking the ability to recover after a highly stimulating event. This suggests a link between eNOS production and certain mental disorders that occur due to a dysregulation of arousal.
“We were not initially expecting that inhibiting eNOS would alter this particular behavior,” says Dr. Stern. However, he cites that “there have been associations between certain psychiatric disorders and the eNOS locus in GWAS studies that have looked at different patient populations with bipolar disorders, suicidality and ADHD.” Dr. Stern adds that their current study “sheds light onto the link between eNOS mutations and these disorders.”
Unfortunately, Not a Great Drug Target
When asked about the potential for the development of therapeutics from this work, the first author of the paper, Inna Tabansky, Ph.D., cautions that eNOS-based drugs are probably not a possibility. She cites that because eNOS is a main player in the circulatory system with multiple roles, inhibiting it may lead to many unwanted side effects.
Dr. Tabansky says that their goal is to “increase their understanding of how the eNOS expressing neurons interact with other neurons in the brain region to modulate behavior.” Researchers are trying to analyze that relationship by removing eNOS from these neurons. They also want to uncover the molecular profiles of other neurons in the region, as Dr. Tabansky suspects that other neurons may express eNOS as well, something that she is interested in researching further.