Biomedical Engineering Reference
In-Depth Information
These experiments were however extremely difficult to per-
form and raised several problems: (i) The effect of drugs was
unpredictable, sometimes rapidly reversible due to drug washout
by blood flow, (ii) High concentrations of NBQX decreased
the signal-to-noise ratio of fluorescent signals and occasionally
decreased or blocked basal blood flow. We repeated these superfu-
sion experiments in the mouse in which the olfactory nerve layer is
thin as opposed to rat's, favoring the drug access to the glomeru-
lar layer. We used G-CaMP2 transgenic mice
(61)
which express
G-CaMP2 fluorescence in mitral cells and thus strongly improve
our capacity to detect responding glomeruli. In G-CaMP2 mice,
superfusion of glutamate receptor antagonists strongly reduced or
even fully blocked both postsynaptic Ca
2
+
and vascular responses.
These results allowed us to conclude that activation of postsynap-
tic glutamate receptor plays a major role in triggering the vascular
response. It is thus possible that intrinsic signals observed in the
presence of glutamate antagonists
(11)
reflect changes in light
scattering involving either olfactory nerve terminals or astrocytes.
8. Concluding
Remarks
Can we eliminate a possible involvement of glial cells in glomeru-
lar vascular responses? Our data show that direct activation of glial
cells by the glutamate release from olfactory nerve terminals is not
sufficient to trigger vascular responses. However, blocking postsy-
naptic neuronal responses also blocked dendritic release. Is it thus
conceivable that dendritic release from mitral cells or from some
juxtaglomerular cell type activates glial cells and a downstream
cascade leading to vasodilation. The question of a “disynaptic”
activation of glial cells remains open. Still, the regulation of neu-
rovascular coupling seems different in the olfactory bulb and in
the neocortex.
(62)
In the course of our investigations, our data questionned the
use of local field potential (LFP) responses as reliable markers of
neuronal activity to study neurovascular coupling
(60)
.Weini-
tially characterized which component of the odor-evoked LFP
response was generated locally, within glomerular boundaries. We
placed extracellular fluorescent recording pipettes in the center of
glomeruli under TPLSM “visual” control and found that odor
evoked various types of LFP responses with several components.
However, one of them was exclusively glomerulus-specific: It con-
sisted of “rapid” negativities that were locked to the 2 Hz respi-
ration frequency, disappeared at the glomerular border and were
sensitive to intraglomerular NBQX or TTX. As suggested by Rall
and Shepherd decades ago
(63)
, we demonstrated that LFP neg-
ativities were most likely generated by mitral/tufted cell EPSPs.
