Viral Vectors for Optogenetics of Hypothalamic Neuropeptides - Viral Vector Approaches in Neurobiology and Brain Diseases - page 314

to combine a guide cannulae for the optic fi ber with a microdialysis
probe, which then allows monitoring of neuropeptide release after
the optic stimulation. This technique can be applied either in anes-
thetized or in freely moving animals. However, an obvious limita-
tion of application of optogenetics for studying dendritic
neuropeptide release is the potential simultaneous release of neu-
ropeptides from axons, which we recently characterized (upon
axonal light stimulation) in the central amygdala [ 8 ]. To separate
somatodendritic and axonal neuropeptide release, different stimu-
lation parameters could be applied (for instance, lower frequency
of stimulation, such as 5 or 10 Hz applied for dendritic release
instead of high frequency stimulation (30 Hz) applied for axonal
release in the case of OT). Additionally, light-sensitive channels can
be tagged with proteins that preferentially target them to dendrites
(such as postsynaptic density protein PSD95) or axons (ankyrin-
G), as presently achieved in dissociated neuronal cell culture [ 78 ].
In our previous work and in this chapter, we demonstrated
effects of endogenously released OT from axons in an extrahypo-
thalamic region—the central amygdala [ 8 ]. This fi nding opens
possibilities to study effects of OT and other peptides distantly
from the hypothalamus and to combine optic stimulation with
microdialysis while at the same time monitoring behavior.
Furthermore, optogenetics can be combined with neuroimaging
techniques, such as functional magnetic resonance, as reported for
virally mediated activation of cortical and thalamic glutamatergic
neurons [ 25 ]. This combination is applicable to neuropeptidergic
axons to monitor temporal and spatial dynamics of changes in
BOLD (blood oxygenation level dependent) signal in the illumi-
nated region and brain areas connected to this particular region.
The unavoidable limitation of this setup for animals is the require-
ment of anesthesia or restraint of the head in the scanner. This may
be circumvented by combining optogenetics either with EEG
techniques or multiple unit recordings. Indeed, it is now becoming
possible to combine such in vivo electrophysiological recordings
with optical stimulation through the development of so-called
optrodes, i.e., electrodes that include optical fi bers [ 79 ].
Neuropeptides are often coexpressed with conventional neu-
rotransmitters such as L -glutamate or GABA. Magnocellular hypo-
thalamic neurons, for instance, produce L -glutamate, whereas
neuroendocrine neurons of the arcuate nucleus produce GABA.
Such colocalization has been reported for many years, but the sig-
nifi cance of this corelease still remains to be studied. Although
both types of release require Ca 2+ -dependent exocytosis, intracel-
lular mechanisms underlying the packaging of neuropeptides into
dense core vesicles and their transport in axons and dendrites [ 80 ]
are thought to be entirely different from vesicular traffi cking and
release of conventional transmitters (Ludwig and Leng [ 77 ]). Also
for the release from OT fi bers within the central amygdala that we