Viral Vectors for Optogenetics of Hypothalamic Neuropeptides - Viral Vector Approaches in Neurobiology and Brain Diseases - page 313

elicit neuropeptide release, one should always keep this possibility
in mind. A possible solution in vitro experiments could imply dis-
secting somas from axons and/or operating by two opsins allowing
the inhibition of cell bodies and simultaneous activation of distant
axons. While ChR2 and HaloR do not work properly together in
the same neuron due to the overlap of wave lengths, other light-
activated pumps, such as Arch may do the job.
Intra-axonal stocks could easily be depleted. In addition, recep-
tors of various neuropeptides are often subject to desensitization.
While not of the same nature and very poorly documented, these two
aspects must be considered when using optogenetics of neuropep-
tides by carefully characterizing the in vitro response with repeated
stimulations. In addition, using a minimal stimulation protocol that
still allows the secretion of the peptide can be an elegant solution to a
potential fast depletion of neuropeptide or desensitization of the
receptor.
Hypothalamic neuropeptides are often subject to hormonal
regulation [ 69 ]. Thus, it is important to carefully address the ques-
tion of gender effects for the study.
One should consider the importance of hormonal status if
females are chosen for the study. Indeed, hormonal regulation dur-
ing the different cycles, have been shown to act on the expression
of different neuropeptide receptor [ 70 ]. In addition, lactation can
induce drastic changes in hypothalamic cellular organization [ 71 ,
72 ] and, thus, lead to potentially critical modifi cations in neuro-
peptidergic production and modulatory action.

4 Perspectives

Although the optogenetic approach is based on the discovery of
bacterial light-sensitive channels [ 73 , 74 ] and tremendously
explored through the revolutionizing work of Boyden, Deisseroth,
Svoboda and their colleagues [ 6 , 75 ], implementation of this tech-
nique for study of neuropeptide functions has been limited up until
now to only a few reports (as demonstrated and cited in this chap-
ter). There are several important questions in neuroendocrinology
which can be answered with the help of optogenetics.
Important unresolved questions regarding neuropeptide
actions in the brain are how neuropeptides are released and how
they reach the target brain region. Although release of neuropep-
tides from every cell compartment (i.e., soma, dendrites, and
axons) has been proposed [ 76 , 77 ], the somatodendritic release
has been documented only for hypothalamic magnocellular neu-
rons that express either OT or VP. Optogenetics allows direct stim-
ulation of somatodendritic release of OT and VP and subsequent
analysis of dynamics of diffusion of these nonapeptides in hypotha-
lamic and extrahypothalamic regions. To this purpose, one needs