Osaka: Noradrenaline is a neuromodulator secreted in the brain depending on behavioral context and physiological states of animal, influencing a wide range of physiological functions by modulating brain activity. It may be best known as a hormone to regulate heart rate and blood flow, and many drugs, such as well-known beta blockers, target its effects. It also modulates the visual system. "Noradrenaline administration modulates the primary visual cortex (V1)", says Osaka University Associate Professor Satoshi Shimegi. "It changes the spatial sensitivity of this region."
Understanding how noradrenaline functions in the brain is expected to
give new insights on how the brain processes spatial information, which
has important implications on patient care and machine learning.
Noradrenaline exerts its effects by binding to adrenergic receptors.
Beta blockers target b-adrenergic receptors, but there are also exist
a-adrenergic receptors, for which there exists alpha blockers. The
binding of noradrenaline to its receptors leads to a reduction of
spontaneous neural activity in V1, but studies on different animals have
been inconclusive about which receptors are primarily responsible for
In a new set of experiments, the Shimegi lab observed the effects of
noradrenaline on the vision of free-moving rats. "We wanted to observe
behaving animals, because this is a better representation of nature,"
explained Ryo Mizuyama, first author of the study, which can be read in PLOS ONE.
The rats were subjected to one of two different inhibitors of
a-adrenergic receptors or one inhibitor of b-adrenergic receptors. Of
the three, only the b-adrenergic receptor inhibitor, propranolol
hydrochloride, had an effect on vision performance.
"Contrast sensitivity was suppressed", said Mizuyama, who further
added that, "contrast sensitivity defines one's ability to distinguish
objects at different light and dark contrasts."
Interestingly, however, the effect on contrast sensitivity was found only for a specific range of spatial frequencies.
"The contrast sensitivity at optimal spatial frequencies was
suppressed by propranolol hydrochloride," observed Shimegi. "This result
suggests that only neurons sensitive to specific spatial frequencies
are affected by the inhibitor. Therefore, noradrenaline could improve
vision by targeting a very small subgroup of neurons or neural
Noradrenaline (NA) is released from the locus coeruleus in the
brainstem to almost the whole brain depending on the physiological
state or behavioral context. NA modulates various brain functions
including vision, but many questions about the functional role of its
effects and mechanisms remain unclear. To explore these matters, we
focused on three questions, 1) whether NA improves detectability of a
behavior-relevant visual stimulus, 2) which receptor subtypes
contribute to the NA effects, and 3) whether the NA effects are
specific for visual features such as spatial frequency (SF). We
measured contrast sensitivity in rats by a two-alternative forced
choice visual detection task and tested the effects of NA receptor
blockers in three SF conditions. Propranolol, a β-adrenergic receptor
inhibitor, significantly decreased contrast sensitivity, but neither
prazosin nor idazoxan, α1- and α2-adrenergic
receptor inhibitors, respectively, had an effect. This β blocker effect
was observed only at optimal SF. These results indicate that endogenous
NA enhances visual detectability depending on stimulus spatial
properties via mainly β-adrenergic receptors.
Figure 1. Endogenously-released noradrenaline improves visual
detectability via β adrenergic receptor, and thereby animal is able to
discern the difficult-to-find object (grating stimulus in the present
To learn more about this research, please view the full research
report entitled “Noradrenaline Improves Behavioral Contrast Sensitivity
via the β-Adrenergic Receptor” at of the PLOS ONE website.