Modular organization of the silkmoth antennal lobe macroglomerular complex revealed by voltage-sensitive dye imaging
Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan
Author for correspondence (e-mail:
kanzaki{at}biol.tsukuba.ac.jp)
Accepted 10 November 2003
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Summary |
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Key words: insect, Bombyx mori, optical recording, postsynaptic response, GABA, topology, brain
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Introduction |
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To date, little has been reported about the processing of the information
of the topology of the antenna. In the American cockroach Periplaneta
americana, the projection neurons can be assigned to two subgroups on the
basis of their receptive fields from the antenna. An identified neuron with
local receptive fields has profusely branching dendrites in the lateral region
of the macroglomerulus (MG). Another identified neuron with global receptive
fields has dendrites uniformly distributed throughout the MG. However, it is
not clear whether there is spatial partitioning of the pheromone response in
the macroglomerular projection neurons according to which region of the
antenna is stimulated (Hosl,
1990). In M. sexta, the arborizations of the
macroglomerular complex projection neurons (MGC-PNs) with different receptive
fields are similar in terms of extent, indicating there is no spatial
partitioning of the pheromone response in the MGC according to which region of
the antenna is stimulated (Heinbockel and
Hildebrand, 1998
). To investigate whether there is a spatial
partitioning in the MGC, it is necessary to compare the postsynaptic
activities in the MGC sub-regions. We succeeded in separating postsynaptic
activities from presynaptic activities in the AL pharmacologically by using
optical recording techniques (Okada et
al., 1996
; Ai and Inouchi,
1996
; Ai et al.,
1998
). Moreover, it has become possible to separate the
postsynaptic activities from the presynaptic activities in the MGC sub-regions
using this technique. In the present study, the postsynaptic activities were
compared between the MGC sub-regions.
In B. mori, the antennal nerve (AN) bifurcates into the medial
nerve (MN) and the lateral nerve (LN;
Koontz and Schneider, 1987).
In the present study, these two distinct pathways from the antennae to the MGC
were characterized by optical recording in order to understand the functional
synaptic organization of topological information through the sensory fibers of
the AN to the MGC.
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Materials and methods |
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Histology
To investigate the source of the MN and the LN in the antenna, the sensory
fibers of either the MN or the LN were retrogradely stained with 0.1 mol
l1 cobalt-lysine for 24 h at 4°C. Pharate adults before
melanization were used because the stained fibers were hard to differentiate
from the outside of the cuticle of the antenna in melanized adults. The
antenna was rinsed in saline, developed in hydrogen sulfide gas and fixed in
4% paraformaldehyde for 24 h at 4°C. The brain was subsequently
intensified with silver (Bacon and Altman,
1977), dehydrated and cleared by methylsalicylate. To clarify
projection areas of sensory axons through the MN and LN, the brains were
dissected from the head and then the cut end of either the MN or the LN was
immersed in 1% tetramethylrhodaminedextran solution (Molecular Probes,
Eugene, OR, USA). The bundles were exposed to this solution for 24
h at 4°C. The brains were prefixed in 4% paraformaldehyde in 0.2 mol
l1 phosphate buffer (pH 7.4) for 2 min and immersed in 0.1%
Lucifer yellow solution (Sigma, St Louis, MO, USA) for 4 h at room temperature
(Kanzaki et al., 2003
). The
brains were then fixed in 4% paraformaldehyde for 1 h at room temperature and
dehydrated with an ethanol series and cleared in methylsalicylate. Each
stained brain was imaged frontally using a laser-scanning microscope (LSM-510;
Carl Zeiss, Jena, Germany) with a plan apochromat x10, x20 or
x40 objective. Serial optical slices were acquired at 4.0 µm
intervals for the x10 objective, 1.5 µm intervals for the x20
objective and 1.1 µm intervals for the x40 objective.
3-D reconstruction of the AL
In order to acquire 3-D reconstruction images of the AL, brains were
immersed in 0.1% LY solution for 4 h at room temperature, fixed in 4%
paraformaldehyde for 1 h at room temperature, dehydrated with an ethanol
series and cleared in methylsalicylate. The brains were viewed with a
laser-scanning confocal microscope. Serial optical sections through the AL
were acquired at 1.5 µm intervals for the x20 objective. These images
were read into image-processing software (Amira; TGS, Berlin, Germany) to make
a 3-D image of the AL.
Optical recording
Optical recording methods employed in this study were essentially identical
to those described by Ai et al.
(1998). After cooling (4°C,
30 min) to achieve anesthesia, the animals were initially fixed on an
experimental chamber. The head capsule was opened above the brain and most of
the muscles in the head were removed. The brain was desheathed using fine
forceps, removed from the head capsule and stained with a voltage-sensitive
dye (RH414; Molecular Probes; 23 mg ml1) dissolved in
physiological saline at room temperature for 510 min. Subsequently,
excess dye was washed off with physiological saline. In optical recording
experiments, the brains were fixed anterior side down on the inverted
microscope (Axiovert S100; Carl Zeiss). The brains were mounted on a coverslip
(24 mmx32 mm) and held down gently with a strip of coverslip on the
posterior side of the brain to prevent movement during perfusion. The aperture
between the two coverslips was held at 400 µm by inserting coverslips. The
contralateral antennal nerve was also fixed by insertion into a slit of the
glass tips (200 µm in width) glued on the coverslips. The recording plane
was always focused 50 µm from the surface of the AL because most of the
afferent fibers spread around the surface of the AL and enter the glomeruli at
this depth. The camera unit of the optical imaging system (Fuji HR Deltaron
1700; Fujifilm Microdevice Co., Tokyo, Japan) is a MOS-type image sensor and
has a resolution of 128x128 photopixels. Each photopixel corresponds to
a tissue area of 7.16 µmx7.16 µm or 3.58 µmx3.58 µm,
depending on whether a 10x (NA, 0.45) or 20x (NA, 0.75) objective
is used. The whole array corresponded to an area of 916 µmx916 µm
(10x) or 458 µmx458 µm (20x). The excitation filter
used in the optical system passed light lengths of 535±25 nm. Adichroic
mirror was placed in the light path of a metal halide lamp that was also the
emission path as an emission filter (>615nm; KMH-250, BMH-250; Kiyohara
Optical Laboratory, Tokyo, Japan).
Data analysis
In our optical recording experiments, a background fluorescence image
(F) was acquired, then fluorescence images were acquired every 0.6 ms
before, during and after the electrical stimulation (Fx).
All these raw data were processed through the following programs. First, each
fluorescence change per frame during data acquisition was calculated by
subtracting F from each Fx (F).
These data were processed by a `
F/F' program to
calculate the fractional change of fluorescence divided by the background
fluorescence (
F/F), then processed by a `bleaching'
program to compensate for photobleaching. Finally, the data were processed by
a `filter' program to separate neural signals from noise. In this study, the
signals were cut off above 246 Hz(Fc).
Stimulation
To apply electrical stimulus pulses, both the MN and LN were cut, and the
proximal cut stump was held in a glass capillary suction electrode. To prevent
leakage of the stimulus pulses to the other nerve, both nerves were isolated
by a Vaseline wall at the proximal region of each nerve and were then
differentially stimulated. Eight responses to electrical stimulation of the MN
or the LN (intensity, 50 µA; duration, 0.5 ms; 0.2 Hz) were averaged
in each recording session. In each preliminary experiment, the intensity of
the electrical stimulation of the MN and the LN was fixed to the value that
induced the maximal response.
Pharmacological experiments
In our study, we sought to compare the postsynaptic responses blocked under
Ca2+-free conditions with the GABAergic responses blocked by a GABA
antagonist (bicuculline) for each AL preparation. First, the optical response
evoked by electrical stimulation to the MN or the LN was acquired under normal
Ringer saline as a control (control for Ca2+-free experiments).
Next, the optical responses were acquired after perfusing with
Ca2+-free saline for 510 min. Then, the optical response was
acquired after washing with normal Ringer saline for >10 min (control for
bicuculline experiments). Next, the optical response was acquired after
perfusing with bicuculline solution for 510 min. Finally, the optical
response was acquired after washing with normal Ringer saline for >10 min.
In our optical recording protocol, the intensity of the signal often slightly
decreased with time; however, when the optical response was compared between
the postsynaptic response and the GABAergic response, the optical responses
were normalized with the peak amplitude of the signal. Ca2+-free
saline was adjusted to maintain osmotic pressure by adding 8 mmol
l1 MgCl2. (+)-Bicuculline (Sigma) was dissolved
in physiological saline within a range of
107104 mol l1.
Solutions were applied by polyethylene tubes connected to a syringe.
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Results |
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Projections of axons from the two antennal nerves to the antennal lobe
The AL contains 58±1 ordinary glomeruli (Gs; diameter 3050
µm) and the MGC (150200 µm), which is composed of three
compartments: the cumulus, the toroid and the horseshoe
(Fig. 2;
Ai et al., 1998;
Kanzaki et al., 2003
). By
forward-filling with tetramethylrhodaminedextran solution from the
proximal cut end of the MN (Fig.
3A) or the LN (Fig.
4A), the projection patterns of the axons from the two ANs to the
AL were revealed. Both the axons of the MN and those of the LN separately
projected towards the AL (Figs
3A,
4A). When the axons of the MN
or the LN were filled from the cut end of the AN, a similar pattern of
staining was revealed (N=6). The axons of the stained sensory neurons
terminated with varicosities (data not shown). The stained axons were
0.51 µm in diameter, and their varicosities swelled to 2 µm. It
was found that there is a topological bias along the mediallateral axis
in the MGC but not in the ordinary glomeruli (data not shown). The density of
the sensory fibers in the MGC is always denser than those in the ordinary
glomeruli (Figs 3B,
4B). Sensory fibers in the MN
are biased towards the medial MGC; the medial toroid and the medial cumulus
were stained more strongly than the lateral toroid and lateral cumulus
(Fig. 3B). In the anterior MGC,
the bias of the sensory fibers in the MGC was clear
(Fig. 3D,E); however, in the
posterior MGC, the sensory fibers projected homologously in both the toroid
and the cumulus (Fig. 3F). On
the other hand, sensory fibers in the LN are biased towards the lateral MGC
(N=6); the lateral toroid and the lateral cumulus were stained more
strongly than the medial toroid and medial cumulus, respectively
(Fig. 4B). In depth of the AL,
the bias of the sensory fibers in the MGC was clear
(Fig. 4DF). Thick
sensory fibers, probably not olfactory fibers, run through the posterior AL to
the antennal mechanosensory and motor center (AMMC), otherwise known as the
posterior antennal center (PAC; Figs
3C,
4C).
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Optical recording
In this study, we attempted to clarify the spatial partitioning in the MGC
related to the topography of the antenna by using optical recording with a
voltage-sensitive dye. We used electrical stimulation of the AN and analyzed
the optical signals in the MGC, which has been clearly demonstrated to be the
sex pheromone processing center in the AL
(Kanzaki et al., 2003).
Similar spatio-temporal response patterns in the MGC were evoked by electrical
stimulation of the MN or the LN in repetitive sessions. In
Fig. 5, the pattern was
initially depolarization of the AN (3.67.2 ms after the onset of the
stimulation) and, subsequently, a depolarization of the MGC (4.89.6
ms). At 7.2 ms after stimulation of the MN, the depolarization was distributed
throughout the MGC, and the area strongly (>0.4% of the background
fluorescence) responding to stimulation of the MN was restricted to the medial
half of the MGC (Fig. 5B, upper
panels). On the other hand, the area strongly (>0.4% of the background
fluorescence) responding to stimulation of the LN was restricted to the
lateral half of the MGC (Fig.
5B, lower panels). In our preparations, responses in the Gs were
also observed but did not have sufficient amplitude for analysis (<0.1% of
the background fluorescence). Fig.
5C shows the time courses of the optical signals recorded in the
areas that had a response greater than 0.3%
(
F/F) at 7.2 ms from the stimuli, evoked by
stimulation of the MN or the LN. The response, evoked in the MGC, had a peak
at 7.2 ms after the stimulus onset. This time delay varied with each
preparation because it depended upon the distance from the stimulation site to
the recording region. The negative deflection, as shown in
Fig. 5C, often appeared when
the electrode for the electrical stimulation was placed close to the AL (data
not shown). In the following pharmacological experiments, the electrodes were
placed at a distance from the AL so that the optical signal didn't include the
negative deflection.
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Pharmacological analysis
Effects of Ca2+-free saline
As clearly observed in the time course of the optical signals in the MGC
elicited by stimulation of the MN or the LN, the responses had another slow
component (arrowheads in Fig.
5C) after the first peak of depolarization. When the MN or the LN
was stimulated, the slow component had a peak (5.0 ms) after the first
peak (N=8). To investigate the source of the slow component,
pharmacological experiments were applied to the AL with optical recording. In
M. sexta, the axons of antennal olfactory receptor neurons make
chemical synapses with dendrites of AL neurons
(Tolbert and Hildebrand,
1981
). Since chemical synaptic transmission is blocked under
Ca2+-free conditions, electrical stimulation of the AN successfully
excites only primary neurons. Therefore, by subtracting the optical responses
under Ca2+-free saline conditions from those under normal saline
conditions we can separate possible postsynaptic activities
(Ai et al., 1998
). When the MN
was electrically stimulated, the optical responses were measured in the whole
MGC; in three regions of the toroid (medial, central and lateral toroid) and
two regions of the cumulus (medial and lateral cumulus)
(Fig. 6A). These five regions
have the characteristic features of the 3-D structure of the MGC as follows:
(1) medial toroid (mT) is the densely stained region observed by
forward-filling of the tetramethylrhodaminedextran solution from the
MN, through the anterior to posterior plane of the MGC
(Fig. 3DF); (2) lateral
toroid (lT) is the densely stained region observed by forward-filling of the
tetramethylrhodaminedextran solution from the LN, through the anterior
to posterior plane of the MGC (Fig.
4DF); (3) central toroid (cT) is a region where there is a
core structure composed of axons of AL interneurons (Figs
3E,
4E) and, on both the anterior
(Figs 3D and
4D) and the posterior planes
(Figs 3F,
4F), there are some projecting
terminals from both the MN and the LN; (4) medial cumulus (mC) is the medial
half of the cumulus (Figs
3DF,
4DF); (5) lateral
cumulus (lC) is the lateral half of the cumulus (Figs
3DF,
4DF). The AL in the real
image of optical recording was superimposed on the typical 3-D confocal image
of the AL (Fig. 2B, anterior)
by fitting the outline of the AL, and then the boundary between the toroid and
cumulus were determined in the optical recording images. The toroid was then
divided equally into three sections: mT, cT and lT. When the MN was
stimulated, the peak amplitudes of the optical responses in each mT, cT and mC
were always larger than that in each lT and lC. In each MGC sub-region, the
slow component of responses was blocked under Ca2+-free conditions
(arrowheads in Fig. 6A). These
results indicate that the slow component was caused by postsynaptic activities
in the MGC. In the toroid, the peak amplitude of the slow component of the
optical signals in both the mT and cT evoked by electrical stimulation of the
MN was significantly larger than that of the lT (mT>lT, P<0.05;
cT>lT, P<0.05; one-way repeated-measures ANOVA;
Fig. 7A). In the cumulus, the
peak amplitude of the slow component in the optical signals in the mC evoked
by electrical stimulation of the MN was also significantly larger than that of
the lC (mC>lC, P<0.05; one-way repeated-measures ANOVA;
Fig. 7A).
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When the LN was electrically stimulated, optical responses were evoked in the lT and lC (Fig. 8A). In the lT and the lC, the slow components were isolated under Ca2+-free conditions (arrowheads on red lines in Fig. 8A). In the other MGC sub-regions, the slow component was hardly observed (red lines in Fig. 8A).
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With both MN stimulation and LN stimulation, the slow components gradually increased from 1.5±0.5 ms after the first peak, then rapidly decreased after 3.7±0.7 ms and disappeared at 24.4±2.5 ms (mean ± S.E.M., N=7). After washing with normal saline (5 min), the time course of the falling phase recovered to that under normal saline.
Effects of bicuculline
We have demonstrated that many GABA-like immunoreactive neurons exist in
the AL of male B. mori (Yokokawa
and Kanzaki, 1997). In order to separate possible GABAergic
secondary responses, optical recordings were made under GABA blocker
conditions. When the MN was stimulated, bicuculline had no effect on the first
peak of the optical response but increased the slow component of the optical
response (arrowheads in all MGC sub-regions in
Fig. 6B). When the LN was
stimulated, bicuculline had no effect on the rising phase of the optical
response but increased the intensity of the falling phase of the optical
response (arrowheads in MGC sub-regions except mT in
Fig. 8B). The differences in
the time courses of the response amplitudes were caused by blocking the
secondary GABAergic inhibitory potentials of AL interneurons. Therefore, the
possible time courses of the GABAergic inhibitory potentials were calculated
by subtracting the responses under bicuculline conditions from those under
normal conditions (Ai et al.,
1998
). The GABAergic inhibitory potentials gradually increased
from 0.2±0.2 ms and reached a peak at 2.8±0.5 ms, then gradually
decreased (mean ± S.E.M., N=7). The delay time of
the GABAergic inhibitory potentials from the first peak (0.2±0.2 ms)
was shorter than the delay time of the postsynaptic activities blocked by
Ca2+-free saline (1.5±0.5 ms) (P<0.05; paired
t-test, N=7; Fig.
9). After washing with normal saline, the time course of the
falling phase often recovered to that under normal saline. The amplitudes of
the GABAergic inhibitory responses in the MGC sub-regions were not
significantly different from each other (all MGC sub-regions in
Fig. 6B and in
Fig. 8B, P>0.05,
one-way repeated-measures ANOVA). These results show that the GABAergic
inhibitory responses were equally distributed throughout the MGC sub-regions
where an optical response was remarkably evoked.
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Excitatory postsynaptic activities
Since polyclonal antibodies raised against GABA have been shown to label
all of the AL glomeruli and many somata of AL interneurons in B. mori
(Yokokawa and Kanzaki, 1997),
M. sexta (Hoskins et al.,
1986
) and P. americana
(Distler, 1990
), it has been
suggested that GABA is a major transmitter of most inhibitory interneurons in
the insect AL. In the present study, excitatory postsynaptic responses were
calculated by subtracting GABAergic inhibitory potentials from the
postsynaptic activities. Before subtracting, each GABAergic inhibitory
potential was calculated using the following equation:
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When the MN was stimulated, the maximum amplitude of the excitatory postsynaptic response of the optical signals in both the mT and cT was significantly larger than that of the lT, and the maximum amplitude of the excitatory postsynaptic response of the optical signals in the mC was also significantly larger than that of the lC (mT>lT, cT>lT, mC>lC, P<0.05; oneway repeated-measures ANOVA; Fig. 7A). When the LN was stimulated, the maximum amplitude of the excitatory postsynaptic response of the optical signal in the lT was significantly larger than that of the cT, and the maximum amplitude of the excitatory postsynaptic response of the optical signals in the lC was also significantly larger than that of the mC (lT>cT and lC>mC, P<0.05; one-way repeated-measures ANOVA; Fig. 7B). These results suggest that the differences in the peak amplitudes of the postsynaptic responses, blocked under Ca2+-free saline conditions (red lines in Figs 6A, 8A), reflect a nonhomologous distribution of excitatory postsynaptic activity (blue lines in Figs 6A, 8A) not of inhibitory postsynaptic activity (orange lines in Figs 6B, 8B).
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Discussion |
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Analysis of the source of the optical response
We succeeded in separating excitatory and inhibitory postsynaptic
activities from presynaptic activities in the MGC and demonstrated that MGC
sub-regions code not only odor quality but also topological information from
the antenna. A model of two opposing parallel pathways in the MGC of M.
sexta was proposed by Christensen et al.
(1998) based on their
pharmacological, anatomical and electrophysiological data. These pathways
comprise two feedforward pathways and one feedback pathway to the MGC-PNs. One
of these feedforward pathways (pathway 1) is a route passing through a
GABAergic local interneuron (GABA-LN), resulting in an inhibitory response
(I1) in an MGC-PN. Bicuculline blocks this inhibitory input
(I1). The other feedforward pathway (pathway 2) is a polysynaptic
route passing through local interneurons (LNs), resulting in an excitatory
response (E) in an MGC-PN. Bicuculline does not block this excitatory pathway.
The feedback pathway to MGC-PNs (pathway 3) comprises a feedback neuron (FN)
and a GABA-LN, resulting in hyperpolarization (I2) in the PN.
Bicuculline does not block I2. Recently, in B. mori,
several types of MGC-PNs were morphologically and physiologically identified
(Kanzaki et al., 2003
).
Kanzaki et al. (2003
) showed
that there is a difference between the MGC-PN pheromone response patterns of
B. mori and M. sexta. The difference is that, in M.
sexta MGC-PNs, pheromone stimulation evokes a brief hyperpolarization
(I1) followed by a depolarization with bursting potentials, but in
B. mori MGC-PNs, I1 was not observed
(Kanzaki et al., 2003
).
Bicuculline blocks I1 in M. sexta MGC-PNs
(Waldrop et al., 1987
). In the
present study, the postsynaptic activities in the MGC were enhanced under
bicuculline conditions. This result suggests that GABAergic inhibitory input
also exists in Bombyx MGC-PNs, as in Manduca MGC-PNs. We do
not know the reason why I1 cannot be observed in B. mori
MGCPNs electrophysiologically. With this one exception, there are many
similarities between B. mori and M. sexta, suggesting that
there is a similar neural circuit in both MGCs. Assuming that this is the
case, we can suppose the following hypothesis about the source of the optical
responses.
Pathway 1
In the present study, the inhibitory postsynaptic responses in the MGC were
separated under bicuculline conditions (Figs
6B,
8B). Most of the local
interneurons in the AL are GABAergic in M. sexta
(Waldrop et al., 1987;
Christensen et al., 1998
) and
in P. americana (Boeckh and
Tolbert, 1993
; Distler,
1989
,
1990
). In these studies, most
of these LNs have wide field arborizations in the AL. In B. mori,
wide field LNs were also identified in the AL (Y. Seki, unpublished
observations). In the present study, we demonstrated that GABAergic inhibitory
responses were uniformly distributed in the whole MGC, where the excitatory
postsynaptic activity is generated. It is likely that most of the signals
blocked by bicuculline originated from inhibitory postsynaptic potentials of
MGC-PNs induced by similar wide field LNs.
Pathways 2 and 3
The inhibitory postsynaptic response always preceded the postsynaptic
responses separated under Ca2+-free conditions
(Fig. 9). Therefore, we
speculate that the excitatory components of the postsynaptic responses
separated under Ca2+-free conditions are compound potentials
originating from pathways 2 and 3.
Compound excitatory postsynaptic response on pathways 13
In the present study, excitatory postsynaptic responses were calculated by
subtracting the GABAergic inhibitory responses, blocked by bicuculline, from
the postsynaptic responses, blocked by Ca2+-free saline. In the
M. sexta AL, bicuculline does not block the GABAergic I2
(Christensen et al., 1998).
There is a possibility that the excitatory postsynaptic response includes a
GABAergic hyperpolarization (I2). The GABAergic hyperpolarization
(I2) has a delay time of
50 ms after I1
(Christensen et al., 1998
). In
the present study, the postsynaptic response was separated within a range of
<25 ms from the stimulus onset and did not include I2. This
suggests that the excitatory postsynaptic response in the present study is
mostly caused by the compound excitatory postsynaptic response of all AL
interneurons (pathways 1, 2 and 3).
As shown in Figs 3 and
4, thick fibers pass through
the posterior region of the AL. These thick fibers running from the MN pass
posteriorly through the central region of the MGC
(Fig. 3B,C), and those running
from the LN pass posteriorly through the lateral region of the MGC
(Fig. 4B,C). Our optical
recording focus plane was fixed at 50 µm below the surface of the MGC.
However, there is a possibility of recording not only the signal in the MGC
but also the signal of these thick fibers because we could not recognize the
range in depth to acquire the optical recording. These thick fibers terminate
in the AMMC and not in the MGC or the posterior region of the MGC
(Koontz and Schneider, 1987).
This suggests that postsynaptic responses blocked under Ca2+-free
conditions originate purely from the MGC and not from the thick fibers.
Modular organization of the MGC
In the present study, MGC sub-regions, which have high excitatory
postsynaptic input from the MN or from the LN, were identified. Our results
raise the following question: are there specific interneurons that receive
sensory input from topologically specific sensory fibers? In a previous study,
it was found that toroid-PNs have distinct tufts in confined regions in the
toroid (Kanzaki et al., 2003).
Considering these results (Kanzaki et al.,
2003
) with restricted terminal arborizations of the olfactory
receptor neurons (Koontz and Schneider,
1987
), it was speculated that specific olfactory receptor neurons
contact specific toroid-PNs in confined regions, and such confined regions may
play roles as local circuits for olfactory coding
(Kanzaki et al., 2003
). These
results suggest that there is a possibility that the toroid-PNs receive
topological information from the MN or the LN and that these parallel pathways
send the topological information to higher olfactory centers. By contrast, the
cumulus-PNs did not have distinct tufts but had their dendritic arborizations
uniformly distributed in several cumulus sub-regions. In the cumulus there is
a possibility that some common PNs receive synaptic input from the sensory
fibers either in the MN or in the LN. Kanzaki et al.
(2003
) also demonstrated that
(1) toroid-PNs send sparse terminals to restricted areas in the calyces of the
mushroom body (MB) and dense, widely branching terminals to the inferior
lateral protocerebrum (ILPC) and that (2) cumulus-PNs send dense, widely
branching terminals to the MB and small branching terminals to the ILPC. We
have not yet determined if this topological information is retained in the
ILPC and in the MB. It is our ongoing work to investigate whether projection
neurons that have dendritic arborizations restricted to a certain sub-region
in the toroid have specific response patterns (or topological response
patterns) to the stimulation applied to the medial or lateral flagella, and
whether these PNs have projection areas in certain areas in the ILPC and in
the MB.
In the present study, we used multi-site optical recording to reveal functional differences in MGC sub-regions. Our results will contribute to gaining a better understanding of the processing of topological information from the antenna in the insect brain. We are planning to investigate the spatio-temporal patterns evoked by odor stimulation to the antenna.
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Acknowledgments |
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Footnotes |
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References |
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