| Further work in our lab has shown
that the formation of dendritic branches in ON-type ganglion cells
is retarded by dark rearing (Liu et al. 2007). These findings reveal
that at least some of the synaptic organization of the inner retina
is responsive to visual inputs.
Future work using transgenic mice
having cell-specific markers will be used to characterize dendritic
morphogenesis in more detail.
In addition variable dark rearing protocols will establish whether
a "critical period" exists over which synaptic pathway
maturation is most sensitive to light deprivation.
Neurotrophins and the Development of Synaptic Pathways in Neonatal
Retina.
We, and others, have demonstrated recently that BDNF (brain-derived
neurotrophic factor) activation of its cognate receptor, TrkB,
plays a critical role in visual experience-dependent laminar refinement
of ganglion cell dendrites during neonatal maturation (Liu et al.
2007). BDNF levels are reduced in the retinas of dark reared animals.
Refinement of dendrites is accelerated in BDNF overexpressing mice
while it is retarded in mice that have reduced or conditionally-deleted
TrkB receptors. The effects of dark rearing on laminar refinement
are over-ridden in BDNF overexpressing mice.
Future work will focus on regulation of synaptic pathway development
by other neurotrophins. In addition, we have available now a transgenic
mouse line in which we can temporally interrupt BDNF-TrkB signaling.
This mouse line will give us a unique tool to explore in much more
detail how BDNF-TrkB signaling regulates synaptic development.
One particular focus will be on how dark rearing, BDNF-TrkB signaling
and GABAergic circuits interact during maturation of the retina.
BDNF-TrkB signaling may also regulate the development of the dopaminergic
system in the retina. Dopamine is a neuromodulator that governs
the adaptational state in the retina. In brighter background conditions
dopamine release is enhanced, which in turn causes retinal circuits
to become less sensitive to rod-driven inputs and more sensitive
to cone-driven inputs. The maturation of the dopamine system could
have consequences on the ability of the retina to "adapt" its
operating range to different background illumination. Initial work
will extend our preliminary results (Grishanin et al. 2008) to
characterize how BDNF-TrkB regulates the numbers and morphology
of dopamine neurons in retina.
Glutamate Transporters and Receptors in the Retinal Circuits
Our lab has a long-standing interest in identifying and characterizing
the synaptic molecules that serve to conduct the visual signals
from photoreceptors to ganglion cells. With Robert Edwards' lab
we have localized the vesicular glutamate transporters in specific
cell types-VGLUT1 in photoreceptors and bipolar cells, VGLUT2 in
ganglion cells and VGLUT3 in a class of amacrine cell. Using VGLUT1
knockout mice we have demonstrated that synaptic signaling from
rods and cones is eliminated, as would be predicted from the immunolocalization
studies (Johnson et al., 2007). In earlier work we showed that
glutamatergic inputs to ganglion cells simultaneously activate
both AMPA and NMDA receptors.
Future work will focus on the lateral mobility of NMDA receptors
near the active zone of the synapses in ganglion cells. Physical
movement of NMDA receptors could provide a mechanism of adjusting
the gain of bipolar to ganglion cell synapses. In addition, using
conditional NMDA receptor knockout mice we will be investigating
the role of NMDA receptors in the development and maturation of
synaptic pathways in the inner retina.
Melanopsin Expressing Ganglion Cells and Visual Behavior Before
the Onset of Rod and Cone Function.
We have recently found that mouse and rat pups as young as postnatal
day 6 exhibit negative phototaxis whereby they turn away from light.
Rods and cones don't begin signaling to postsynaptic cells until
postnatal day 11. Because melanopsin is expressed as early as the
day of birth and these melanopsin-expressing ganglion cells can
be excited by light, we hypothesized that this class of retinal
neuron mediated the phototactic behavior. In support of this idea,
melanopsin knockout mice don't show negative phototaxis. These
findings underscore the existence of a secondary photosensitive
system in the retina that could regulate not only instinctual survival
behavior but also sleep-wake cycles and circadian rhythms. This
secondary photosensitive system could operate in premature infants
and in degenerative diseases in which rod and cone function is
compromised.
Future work will further characterize and quantify negative phototaxis
in rodents. In addition we will be studying the behavior of the
melanopsin ganglion cells by recording their responses directly
using a multi-electrode array system. Of particular interest is
whether the melanopsin ganglion cells are coupled electrotonically
together, what the receptive structure of these cells is, and are
there drugs that can block the light responses of melanopsin ganglion
cells? |
