By Rex Graham
There’s more going on in a bird’s brain than meets the eye.
Circumstantial evidence hinted decades ago that non-mammal vertebrates could detect light directly with their brains, but most scientists were skeptical. A new study shows it’s no longer as far-fetched as it sounds.
Birds eye within bird brain
Amazing the skeptics, Japanese scientists have actually documented how light sensing actually occurs inside the vertebrate brain. In a study published online July 7, 2014, in Current Biology, the researchers showed that light-sensing occurs in specialized light-sensing neurons in the brain of the Japanese Quail.
“Their findings are astonishing,” Juraj Sevc, a professor at the Institute of Biology and Ecology at P.J. Safarik University, Kosice, Slovak Republic, said in an email. “This groundbreaking news shall influence the view of these enigmatic cells even in the central nervous system of mammals and human, too.”
Several neurobiologists echo Sevc’s reaction, saying the new study amounts to elegant proof of the “eye within the brain” theory for non-mammalian vertebrates. The team that made the discovery was led by Takashi Yoshimura, a professor at the Institute of Transformative Bio-Molecules of Nagoya University.
The eye in the brain idea had been based on a handful of classic studies. For example, fish that have been trained to rise to the water’s surface for food at a light signal will continue to respond that way after they’ve been blinded. Also, ducks that are blinded grow gonads in response to longer exposure to light, preparing for spring mating like ducks with perfect vision.
Yoshimura’s team showed that in the quail, cells called cerebrospinal fluid (CSF)-contacting neurons in the hypothalamus contain the photo-active pigment Opsin-5 and respond to light without input from eyes or other sensory inputs. The neurons are tucked within the paraventricular organ of the quail mediobasal hypothalamus, a small part of the brain that links the nervous system to the hormone-secreting endocrine system.
The quail’s light-sensing neurons are involved in detecting the arrival of the longer days of spring and are thus in regulating breeding activities in the birds. The findings are expected to lead to greater productivity of Japanese quail at domestic quail farms. The findings also may spark greater interest in the evolution and development of non-visual photo-receptors.
Crucial 30 minutes of daylight
Successful breeding of any species of wild bird hinges on adapting to seasonal changes. Birds are well known to use the changing length of each day as part of an internal calendar. For migratory birds, knowing when to take flight on a long trip is just as important.
“Quail recognize 11 hours, 30 minutes of light as a short winter day, but they recognize 12 hours of light as a long summer day,” Yoshimura said in an email. “Each bird species may have a different critical photoperiod to trigger reproductive activity. Animals living in the far north are thought to have a longer critical photoperiod than animals at the equator.”
Proving bird’s eye in brain
To satisfy a legion of skeptics that the brain of a bird can see light, Yoshimura’s team needed more than circumstantial evidence. They needed proof.
“The gold standard for the demonstration of intrinsic photosensitivity is neurophysiological recording of light responses from individual cells isolated from other possible photoreceptor inputs,” wrote Michael Menaker, a biology professor at the University of Virginia, in an editorial in Current Biology accompanying Yoshimura’s paper. “This can be technically demanding, especially if the putative photoreceptors are buried in the deep brain. The authors have solved these technical problems elegantly.”
Evolution of non-visual light perception
Animal species that can initiate breeding activities at the most opportune times would have an evolutionary advantage over those that can’t.
The underlying behaviors and anatomical changes associated with breeding in birds are controlled by hormones. These powerful chemicals often at act in concert. While activation of the process is known to be initiated by longer exposure to light, the exact trigger was anybody’s guess.
Scientists worldwide also are seeking to understand circadian rhythms in humans and other vertebrates. A small structure in the vertebrate brain called the pineal gland releases melatonin, a hormone that modulates sleep patterns in circadian rhythms. Melatonin is secreted at night, when little or no light comes from eyes.
In the quail experiments completed by Yoshimura’s team, the pineal glands, which respond to light in non-mammalian primates, were removed so that the effects of circadian rhythms could be eliminated. The team also chemically neutralized neurotransmitters to eliminate their effects on the neurons being studied.
The steps allowed Yoshimura to pinpoint the effect only of light on the cerebrospinal fluid (CSF)-contacting neurons.
Light’s biochemical domino effect
The new findings reveal in exquisite detail the first step in a biochemical domino effect, sort of a cascade of events that leads to the reproductive response of a Japanese Quail. Here are the sequential scientific details of those dominos:
- Domino #1: Light-detection by the photo-active pigment Opsin-5 in the cerebrospinal fluid (CSF)-contacting neurons. Yoshimura’s team confirmed this step.
- Domino #2: Opsin5-positive cerebrospinal fluid (CSF)-contacting neurons transmit light information to the pituitary gland, causing it to release thyroid-stimulating hormone. This effect was demonstrated by Yoshimura’s group and published in 2010.
- Domino #3: Thyroid-stimulating hormone, the “spring calling hormone,” stimulates long-day-induced gonadotropin secretion by local activation of thyroid hormone within the hypothalamus. This was reported by Yoshimura’s group in 2008.
- Domino #4: Gonadotropin secretion triggers the spring breeding response.
While the order of the steps in the biochemical domino effect is known, the multitude of possible influencing factors have not yet been fully studied.
Mammals don’t have the non-vision light-sensing cells as quail. The only non-visual photo-receptors in mammals are certain clusters of nerve cells in the retina, the inner surface of the eye. However, there are other light-sensing structures scattered at various place in the non-mammalian vertebrate brain. These structures pre-date the evolutionary origin of eyes, but undoubtedly gave rise to the advanced eye. These ancestral light-sensing structures continue to play important roles in regulating reproductive and other behaviors.
“The current paper, although it does not provide final answers to the many questions raised by this complexity, is an elegant beginning to their in-depth analysis,” Menaker said in his editorial. “Working out the details of the photo-receptive response to long days is likely to be complicated. It will be technically difficult to determine the relative roles of other photo-reception structures, of which there are several.”