In many sensory systems, lateral connections between the neurons are thought to be inhibitory, fine-tuning sensory responses the way a sculptor might shape a piece of wood. According to a new study, however, many lateral connections in the olfactory system are not inhibitory at all, but excitatory, suggesting the brain’s olfactory processing may be more complex than previously believed.
The key to these findings, published by HMS neurobiologists in the April 5 Neuron, was a combination of genetic and physiological techniques currently possible only in Drosophila. Yet the basic organization of the fruit fly olfactory system is very similar to that of noninsect species, such as rodents and humans.
Olfaction begins when odor molecules bind to receptor proteins on the surface of olfactory receptor neurons. Humans have about 350 types of these neurons, but fruit flies have only 50. All the receptor neurons of the same type relay information to the same small compartment, the glomerulus, in the fly’s antennal lobe, a region of the brain analogous to the vertebrate olfactory bulb.
A single odor can activate multiple receptor neuron types, so multiple glomeruli are co-activated in a combinatorial code. The implications of this compartmentalized anatomy have been unclear. “People didn’t really understand to what degree glomeruli represent isolated information channels versus an interconnected network,” said Rachel Wilson, HMS assistant professor of neurobiology.
Wilson, graduate student Shawn Olsen, and research fellow Vikas Bhandawat used microdissections and genetic lesions to investigate the cross-talk between channels. Their strategy was to remove direct receptor neuron input to specific glomeruli and then to record odor-evoked activity from neurons postsynaptic to the affected glomerulus. Any odor-evoked input would have to come from lateral connections. “We thought maybe we would see nothing, which would indicate the channels are acting in isolation,” Wilson said. “Or, maybe we would find that the interactions are purely inhibitory, which would be what the dominant theory suggests.”
They saw neither of the above. “We found predominately excitatory connections,” Wilson said.
The researchers proceeded to establish rules of the connectivity patterns among the channels and found that every channel was interconnected, but that different connections had different strengths. Moreover, the connectivity patterns were stereotyped. “From fly to fly to fly, it was the same,” Wilson said. “We think the connectivity pattern is probably encoded in the genome.”
The results indicate that the olfactory combinatorial code in receptor neurons is altered by synaptic interactions between different processing channels in the brain. The specific functional meaning of these excitatory connections is unclear. Wilson speculates that they might simply not currently know the underlying logic of this surprising arrangement. “Or, maybe it’s a gain-control mechanism, where weak stimuli can be boosted,” Wilson said.
One clue may come from watching how flies behave in response to odors and how this behavior changes when lateral excitatory connections are genetically disrupted. “We are trying to find out how this is useful to the fly,” said Bhandawat, who is currently developing techniques to study how odor stimuli alter the flight path of flying drosophila.
Fly behavior provided a critical clue in another recent study by Wilson’s research team. In the April issue of Nature Neuroscience, she and research fellow Michelle Schlief published the first report combining behavioral measures with in vivo physiological recordings from single neurons in the Drosophila brain.
The researchers sought to understand how the brain responds to inputs from special types of olfactory receptor neurons. Unlike most receptor neurons, which are somewhat broadly tuned, these specialized neurons are very narrowly tuned to just one odor. Wilson and Schlief found that these pheromone-sensitive specialist receptors project to highly specialized circuits in the brain. Such a narrowly tuned pathway may serve to tightly link pheromone perception with innate behavioral responses.
To understand the importance of narrowly tuned receptor neurons, Schlief first used a behavioral assay to ask whether eliminating the genes for these receptors affects the innate behavioral response to pheromones. She used two flasks, one containing odorant and the other odorless, connected by a skywalk-like tube. Flies were placed in the middle of the tube and allowed to choose which flask to enter.
The insects appeared to weigh their scent options. “They would go back and forth a bit,” Schlief said. “Seventy-five percent of the flies would make a choice within three minutes.” She found that flies had an innate attraction to certain odors, and she focused on two of them—the pheromone cis-vaccenyl acetate and the plant odor geranyl acetate.
Strikingly, Schlief found that behavioral attraction to each of these odors was abolished by genetically ablating a type of receptor neuron that is narrowly tuned to that odor. This result implied that in each case, a single “specialized” receptor neuron type is required to trigger an innate behavioral response. “On the face of it, this looks like it’s a labeled line,” said Schlief. “But it’s more complicated than that.”
Schlief next asked how the brain responds to signals from these critical specialized receptor cells. She found that neurons postsynaptic to the pheromone-selective cells were also extremely selective for that pheromone. In contrast, she found that receptor neurons specialized to detect geranyl acetate actually projected to broadly tuned higher order neurons. This suggests that the plant odor is encoded by a “general” olfactory processing system whereby neurons participate in encoding multiple odors, and unlike the pheromone-sensitive cells, are not dedicated to just a single odor.
These results demonstrate that the olfactory system handles pheromones differently from other odors. Pheromones, said Wilson, are explicit signals that have evolved to trigger a stereotyped behavioral response, like mating.
Most odors, by contrast, are encoded in a more complex combinatorial fashion and require a flexible behavioral response because their significance depends on context. “It’s like driving,” Wilson explained. “A stop sign always means you should stop your car. But you also have to keep your eyes open for more complex signals, like seeing another car running through the stop sign. Reacting to this probably requires a more complicated neural circuit.”
