Olfactory Nerve Cells Expressing Same Receptor Display A Varied Set Of Reactions

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Olfactory Nerve Cells Expressing Same Receptor Display A Varied Set

Of Reactions

http://www.medicalnewstoday.com/medicalnews.php?newsid=37100

(Philadelphia, PA) - In a mouse model, University of Pennsylvania

School of Medicine researchers discovered that olfactory sensory

neurons expressing the same receptor responded to a specific odor

with an array of speeds and sensitivities, a phenomenon previously

not detected in the mammalian sense of smell. The group published

their findings this week in the online edition of the Proceedings of

the National Academy of Sciences.

" We assumed that the sensory neurons that express the same receptor

would respond to a specific odor in the same way, " says senior author

Minghong Ma, PhD, Assistant Professor of Neuroscience at Penn. " But

in real biology, these olfactory neurons keep regenerating, and even

though they all express the same receptor, they're probably at

different states of maturation, displaying different qualities. By

knowing that olfactory neurons can respond differently, we're adding

another layer to understanding how the olfactory system receives

outside information. "

Ma's group measured 53 different olfactory neurons that express the

MOR23 odor receptor. As a group, the neurons reacted differently from

one another in their response to lyral, an artificial odor used in

fragrances and flavoring. After subjecting all cells to a short pulse

(200-300 milliseconds) of lyral, the researchers measured the cells'

sensitivity to the odor. Some cells responded to very low

concentrations of lyral; others, to higher concentrations. Regarding

the cells' reaction time, some neurons finished firing within 500

milliseconds, but for others, the response time was up to five

seconds.

Detection of odor molecules depends on about 1000 different odor

receptors in the rodent nose. Different sets of receptors respond to

different sets of odors. To date, no one has been able to record

electrical impulses from a specific subtype of olfactory sensory

neuron expressing a known receptor. This is important, says Ma,

because prior to this paper, when researchers would work with

olfactory cells, there was no way to know what odor receptor that

cell expressed. " It could literally be one out of 1000, " she says.

All the sensory neurons expressing the same receptor merge to a

common region called a glomerulus, a region in the brain's olfactory

bulb. In one bulb there are about 2000 glomeruli. (The brain has two

olfactory bulbs.) There are thousands of sensory neurons dedicated to

expressing the same receptor, and in the case of MOR23 they all merge

to two glomeruli.

The researchers used genetically engineered mice that express MOR23

together with green fluorescent protein (GFP), which was generated by

colleagues from Rockefeller University. The GFP allows the

investigators to visualize the MOR23 cells separate from other

neurons. They also recorded their measurements using cells still

intact within the lining of the nose, which allows the researchers to

study these cells in their natural biochemical environment.

The researchers made their measurements from the endings of olfactory

neuron dendrites. A single dendrite extends from the cell body of the

olfactory neuron into the nasal cavity. The dendrite has a swelling

at the end called the knob, where about 10 to 15 hair-like extensions

called cilia contain the odor receptors.

Ma and colleagues are now working out the implications of their

findings. She says this study points to a more finely tuned response

in the brain to odors than previously thought. " Olfactory neurons may

be able to respond to an even wider range of odor concentrations than

we realized, " she says. The heterogeneity in odor sensitivity and the

wide response range in single cells provides new insights into why

mammals, including humans, perceive odors with unchanged quality over

a broad concentration range.