Delayed-Line Memory, Olfactory Neuron Fate Determination, and FlyNap

Today I Learned:
1) ...about a crazy old form of computer memory called delayed line memory. A delayed line memory device is, conceptually, a really long twisted-up wire with a transmitter and a receiver at each end. To store information in the device, you feed data to the head end. The head sends the data down the line, which takes some number of seconds to get to the other end. While that information is traveling down, the head can fire off more data down the pipe. When the data gets to the other end, it is made available for reading and is sent back to the head, which immediately retransmits. So you basically have all of your bits flowing around in a circular pipe, with a window at one end that lets you see the bits go by.

As you might imagine, programming efficient memory usage on a computer with delayed line memory was loads of fun.

2) Consider olfactory (scent-detecting) neurons. Each individually-discernable scent that a human can smell is detected by a single receptor protein, of which there are order-of-magnitude 1,000. Each olfactory neuron expresses exactly one receptor type; when that receptor fires, the neuron signals to projection neurons, which signals to interneurons, which send signals to the appropriate higher-level brain functional circuit in order to induce the experience of the scent (or, in the case of pheromones, possibly directly influence behavior).

Today I learned how each neuron picks its receptor. The thousand-odd different olfactory receptor proteins come with pretty much identical regulatory regions (promoters, enhancers, repressors, and such). These regions are uniformly repressed by methylation control via the polycomb protein, making each olfactory receptor neuron a sort of blank by default. When several of the regulatory regions for olfactory receptor proteins stochastically come together in the nucleus, they form a complex which activates one of the promoters, causing the neuron to start expressing the corresponding olfactory receptor protein.

Something I'm now wondering about -- if olfactory receptors are picked randomly for each neuron, then there must be something *downstream* of the olfactory receptor protein regulatory elements that controls how each neuron is wired through to its appropriate higher-level olfactory brain circuit. Otherwise, scent molecules would have effectively random sensations associated with them, as there would be no way to consistently wire the receptors for, say, ammonia to the sharp, painful sensory experience of smelling ammonia.

So I have this hypothesis that the receptor types chosen for a neuron somehow determines the wiring of that neuron. A consequence of this hypothesis is that receptor type must be (randomly) selected before the receptor neurons are wired up to higher-level circuitry. Does anyone know enough about olfactory development to confirm or deny this hypothesis?

3) The active (and distinctive-smelling) ingredient in FlyNap is triethylamine. Based on the structure and smell, it probably activates more or less the same scent receptors as ammonia.