Tuesday, January 25, 2005

ClockNews #16: Circadian Rhythm in Visual Sensitivity

NYU biologists find new function for pacemaker neurons

A study by New York University researchers reveals a
new function for the nerve cells that regulate circadian rhythms of behavior in
fruit flies.

The nerve cells, called pacemaker neurons, contain a molecular
clock that controls a 24-hour circadian rhythm in activity similar to the
rhythms in sleep/wake cycles found in humans and many other organisms. It was
previously known that pacemaker neurons receive visual signals to reset their
molecular clocks, but scientists did not have any evidence that they transmitted
information to their target cells, as most other neurons do.

The current study shows that pacemaker neurons do in fact transmit signals and are required
for a rapid behavior, according to the paper, published in the January 20th
issue of Neuron. The study was conducted by Esteban O. Mazzoni, a graduate
student in NYU's Biology Department, Biology Professor Claude Desplan, and
Assistant Biology Professor Justin Blau. The finding suggests it may be possible
to identify genes that can be used to treat problems such as sleep disorders and
jet lag.

The researchers examined the role that pacemaker neurons play in
helping Drosophila larvae avoid light. Drosophila is a species of fruit fly
commonly used in biological research. Fruit fly larvae foraging for food avoid
light, presumably to keep away from predators. Unlike adult Drosophila, the
larvae only have one structure for gathering visual cues, called Bolwig's Organ.
This organ senses the amount of light in the environment and transmits that
information to the pacemaker neurons to reset their molecular clocks.

In the experiments described by Mazzoni, Desplan, and Blau, fly larvae were placed in
the center of a Petri dish with one side dark and the other illuminated. Normal
larvae exhibited the natural behavior and clustered on the dark side. However,
when the larvae had their pacemaker neurons disabled, they were as blind as
larvae that had their light-sensing organs removed and distributed themselves
evenly between the light and dark halves of the Petri dish.

Further experiments showed that, in addition to transmitting the light information, the
pacemaker neurons also modulate the sensitivity of larvae to light, generating a
circadian rhythm in visual sensitivity. The experiments revealed that fruit fly
larvae are most sensitive to light at dawn and least sensitive toward dusk.
The study demonstrates that pacemaker neurons are doing much more than
scientists had suspected. They not only relay visual signals to target cells,
but are also act as filters, using their molecular clocks to adjust the
intensity of the transmitted signal depending on the time of day.

Almost all of the genes that make up Drosophila's molecular clock have counterparts with
similar functions in mammals. Because of this similarity, it may be possible to
identify genes in fruit flies that can be used to treat problems in people, such
as sleep disorders and jet lag.

Hmmmm, this is one messily written press release. I have not seen the paper yet, but from what I can glean from the release, the pacemakers control a circadian rhythm in photosensitivity. When the pacemaker is deleted, the rhythm is abolished. This may or may not be sufficient to explain the data, i.e., the larvae are not blind, but just do not "care" any more about light intensity. I will update once I read the actual paper.

Categories:Clock News



Blogger Coeruleus said...

LOL! That really was a terribly written press release.

From having only read the abstract, intro and discussion (me bad), I think what's novel here is the following. The circadian clock in drosophila is located in the "lateral neurons" (LN). These neurons can both directly sense light and receive light inputs from either the eyelet (in the adult fly) or "Bolwig's Organ" (BO) in the larval stage. Thus far, the dominant mental picture is that circadian pacemaker cells are responsible mainly for transmitting temporal information that is relevant for long-term--24 hour--behaviors, and therefore light inputs to the clock are only relevant for long-term circadian behaviors. In this case, ablating the larval "eye"--Bolwig's Organ--allowed the larvae to continue to seek out the dark (fast behavior), while ablating both the BO and the LN light sensors did not, so lateral neurons can sense light and transmit relevant short-term information. Additionally, the researchers found that there was a circadian rhythm in the response. Thus circadian pacemaker neurons are an integral part of both a slow (circadian) process and a fast neuronal network that allows for larval movement.

In a way, my graduate thesis was looking at a similar concept. Basically, how do neurons that communicate with one another on the order of milliseconds when they use their synapses form information that convey 24-hour temporal information

So, borrowing from busy busy busy, a shorter coeruleus: "pacemaker neurons are cool because they are, well, neurons and do the things that other neurons do."

11:39 PM  

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