What have we learned recently using non-mammalian model organisms?
Over the past few days I have seen a variety of posters under the Sensory Systems theme (theme D). One thing I am struck by is the usefulness of comparing a variety of species in hopes of finding common underlying features of the brain and nervous system that will help us understand general principles of sensory development and processing. In this blog post, I’m focusing on the so-called “nontraditional” model systems – to me, this means nonmammalian systems. Perhaps my personal bias is showing (my research is in tadpoles) but I think it is very important to understand how the senses work and how sensory information is processed in a variety of species.
On Saturday afternoon, my wanderings were well rewarded – I found Brendon Billings presenting his recent work using fMRI to locate sensory processing areas of the Nile crocodile! I was impressed. Not only did they find discrete, well defined areas that process visual and auditory information, but they put actual crocodiles into a 7T MRI scanner! The logistics of the experiment are heroic to me, someone who has been too close to alligators in Florida. Yet, the reptile brain is understudied and this research provides new insight into the locations of sensory processing, and also located a multimodal area that processes visual and auditory information.
On Sunday morning, I saw a great poster investigating how archerfish locate and aim at prey. Archerfish are really cool because they shoot waterjets at the surface of the water to stun their prey, insects. Their highly developed visual system allows them to precisely select targets. In their research, Volotsky and colleagues found that most cells exhibit global inhibition when distracting stimuli outside the receptive field are presented. An interesting extension of this result is that mutual suppression of activity requires a neural network that is connected in a particular way. Cells of the optic tectum are often highly interconnected in other species. Here, I see an accumulation of evidence across species for a certain type of neural network for fast and accurate sensory processing.
In my adventures in the poster hall, I also came across sensory processing being studied in crickets, lampreys and goldfish. In crickets, Someya and colleagues shows that multisensory enhancement is represented in the burst firing of the AN2 neuron found in the cercal ganglion. Specifically, the burst represents the coincidence of the stimulus onset. If two inputs arrive near each other, a burst is more likely. In the lamprey optic tectum, Suzuki and colleagues showed that GABA-ergic neurons control selectivity for the side of motor output activated. Here, inhibition generates specificity for avoidance versus orienting. In goldfish, Opalka and colleagues investigated changes in startle responses with telecephalon lesions. They showed that a medial telecephalon lesion decreases the mean startle angle while a lateral telecephalon lesion increases mean startle angle. Furthermore, a dopamine antagonist decreases mean startle angle while a dopamine agonist rescues the medial telecephalon lesion.
These projects vary in scope and intention, but they all help us to understand basic features of sensory processing. The common thread I see through all of these investigations is somewhat basic: a neural network that processes sensory information in a particular way that allows fast and accurate responses to stimuli. By examining neural responses and behavior in animals with a specialized system for a particular predator or prey behavior, we can build a general framework to define principles of neural network design.
Also, it’s fun to hear about studies that are not being done in mammals!
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