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Journal of Vestibular Research, Vol. 6, No. 4, pp. 243-253, 1996
Copyright © 1996 Elsevier Science Inc.
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Contribution
EFFECT OF HEMILABYRINTHECTOMY ON MONOAMINE METABOLISM
IN THE MEDIAL VESTIBULAR NUCLEUS, LOCUS COERULEUS, AND
OTHER BRAINSTEM NUCLEI OF ALBINO AND PIGMENTED RATS
H. Cransac,* L. Peyrin,* F. Farhat,t J. M. Cottet-Emard,* J. M. Pequignot,* and t .. Rebert
*Laboratoire de Physiologie, Faculte de Medecine Grange-Blanche, 8 avenue
Rockefeller, 69373 Lyon Cedex 08, France;
tLaboratoire de Neurosciences et Environnement, Faculte des Sciences de Rouen,
76821 Mont Saint Aignan Cedex, France.
0 Abstract- We compared in albino and pigmented rats the early effect of unilateral labyrinthectomy (UL) on the concentrations of monoamines (norepinephrine, dopamine, serotonin) and
their respective metabolites-3-methoxy, 4-hydroxyphenylglycol (MHPG), 3,4-dihydroxy-phenylacetic acid (DOPAC), and 5-hydroxyindoleac~tic acid
(5HIAA)-in medial vestibular nuclei (MVN), locus coeruleus, raphe dorsalis, and cochlear nuclei.
The study was conducted 6 hours after UL in both
strains, differing by the functional optokinetic responses of their central vestibular neurons and the
time-course of their vestibular compensation. The
results show that the monoaminergic changes are
different in the two rat strains. In the MVN of albino rats, there was a bilateral increase of MHPG
and an ipsilateral increase of dopamine suggesting
activation of norepinephrine synthesis and metabolism, whereas no such changes were observed in
the MVN of pigmented rats. On the other hand,
the simultaneous increase of norepinephrine and
DOPAC observed in the contralatera[ locus coeruleus of albino rats suggested enhancec
nephrine synthesis, whereas in pigmented rats the
decreased norepinephrine content found in the ipsilateral locus coeruleus might reflect an earlier
strong stimulation of NE release. These biochemical results confirm the relevant role of' locus coeruleus noradrenergic innervation in vestibular
compensation and also point out the involvement
of norepinephrine of the MVN in the early stages
of this process. The different strain-related noradrenergic responses observed on the 6th hour suggest that the involvement of central norepinephrine,
RECEIVED
particularly from locus coeruleus innervation,
may be more crucial and more sustained in the albino than in pigmented rats. No serotonin and
5HIAA changes were found in either brainstem
nuclei of albino rats. In contrast, the increase of
the ratio 5HIAA/5HT observed in raphe dorsalis,
in ipsilateral locus coeruleus, and in both sides of
MVN of pigmented rats suggested that UL induced an extended and enhanced utilization of
5HT in this strain.
0 Keywords -labyrinthectomy; monoamines;
medial vestibular nucleus; locus coeruleus; albino
rats; pigmented rats; compensation.
Introduction
After unilateral labyrinthectomy
vestibular compensation is correlated with the recovery
in ipsilateral vestibular nuclei
of neuraJ
are mentioned
the
time-course of recovery from one species to another (cat and rat for example) (] ); such differences could also affect different strains of rats.
The neural mechanisms underlying this tendency to normal resting activity are unknown.
Neural inputs from the remaining labyrinth acting through brainstem commissures (2) seem to
play the major role, at least in the recovery of
dynamic symptoms (1,3). However, the role of
other sensory inputs, that is, vision (4) and pro-
6 June 1995; AccEPTED 10 November 1995.
243
244
prioceptive syste1ns (5), or of other CNS areas
has been suggested in the initiation of vestibular
compensation. In this line, the locus coeruleus
(LC), the main brainstem noradrenergic nucleus,
which is under the control of macular labyrinth
and neck receptors (6), has been particularly
studied. LC projects largely to cerebrocortical
and subcortical structures, to cerebellum, brainstem, and spinal cord (7) and provides the majority of the noradrenergic innervation of the
renergic
or ~""'-~'-'""''U""
tion of norepinephrine
it has been shown
that the noradrenergic neural input from the locus coeruleus could exert a short-term modulatory influence on the vestibulo-spinal ( 11) and
the vestibula-ocular (12) reflexes and also intervene in long-term plastic changes, which could
be the basis of the vestibular adaptation and
compensation (6, 11, 13). Local injections of ~
noradrenergic substances in cerebellum affect
adaptation of vestibulo-spinal reflex in cats (13)
and vestibula-ocular reflex in rabbits (14). Although it has been postulated that besides cerebellum other brain regions receiving a noradrenergic innervation may be involved in these
processes (13), no such data are available in the
literature.
In addition to noradrenergic endings, the
MVN contains a substantial serotonergic innervation originating from the dorsal raphe nucleus
(9,15). A possible role of brain serotonin (5HT)
in adaptation of the vestibula-ocular reflex has
been suggested from the results obtained after
depletion of 5HT in rabbit brain (16), however,
no specific regional studies are available in either species.
Recently, using high performance liquid
chromatography
we found great amounts
of NE, SHT, and their metabolites-3-methoxy,4hydroxyphenylglycol (1\!IHPG) and 5-hydroxyindoleacetic acid (5HIAA)-and smaller amounts
of dopamine (DA) and its metabolites-3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HV A)-in the MVN of intact albino (Sprague-Dawley) and pigmented rats
(DA-HAN). Furthermore, we observed that at
basal state NE turnover - assessed from the ratio
MHPG/NE and/or by the decrease in the NE
content after treatment with the tyrosine hy-
droxylase inhibitor, a-methylparatyrosine, was
faster in MVN than in LC in both strains (47,48).
The direct neurochemical effects of UL on
monoamine metabolism in vestibular nuclei and
related areas have not yet been studied. In the
present study, we report the early effect of UL
on the concentrations of monoamines (NE, DA,
5HT) and their metabolites (1\!IHPG, DOPAC,
and 5HIAA) in MVN,
and dorsal raphe
and, as a comparison, in the cochlear nuclei
'vbose
innervation (noradrenerand
originates also from LC
and dorsal raphe (7 ,9). According to literature
data on DOP AC significance in brain areas predominantly innervated by noradrenergic neurons, DOPAC levels in the MVN and LC were
interpreted as a ret1ection of NE synthesis
(17,18). On the other hand, the ratio 5HIAA/
5HT was considered an index of 5HT utilization
(release and metabolism) ( 19). Two strains of
rats, albino Sprague Dawley and pigmented
DA-HAN rats, differing by the functional optokinetic responses of their central vestibular neurons were used (20). Since anesthesia can impair monoamine release and uptake (21), the
study, was conducted in the 6th hour after surgery. At this time, the monoaminergic effects of
anesthesia were reduced (22), the rats were
awake but exhibited the motor and ocular symptoms usually observed in the first hours after
UL (postural asymmetry, circling behavior, spontaneous nystagmus).
Methods
All animal experiments were carried out
according to the recommendations of the Declaration of Helsinki and the Animal Welfare
Guidelines of the Society for Neuroscience,
February 1992.
Surgery
Sixteen adult male albino (Sprague-Dawley;
240-260 g) and 16 adult male pigmented (DAHAN; 200-220 g) rats were used in this study.
The rats were maintained under a normal darklight cycle, with free access to food and drink-
Brainstem Monoamines After Hemilabyrinthectomy
245
ing. The lesioned groups consisted of 8 rats of
each strain. The rats were anesthetized with
ether, and the left labyrinth was surgically destroyed by a ventral approach through the tympanic bulla. After surgery, xylocaine (1%) was
infiltrated locally several times to minimize discomfort. The complete lesion was confirmed by
the occunence of body twist and oculomotor
unbalance as previously described (23). Shamoperated rats (8 of each strain) were submitted
to the same procedure (anesthesia and opening
of the tympanic bulla) except for the vestibular
lesion.
Bioche1nical Detern1inations
Eye Movement Recordings-Behavioral
Dete rn1inations
Detennination of monoanzines and their metabolites. 40 11-L of the supernatant were di-
Immediately before UL, 6 albino rats were
prepared with a head device for later recording
eye movements using a magnetic search coil as
previously described (24). Exactly 6 hours after
UL, the spontaneous eye movements were recorded with an electromagnetic device in a
head-fixed situation in darkness as well as in
light.
Five others albino rats were operated, and
their behavior was followed until the 30th hour
after the lesion. Eye movement recordings and behavioral determinations were perforn1ed earlier
in the same conditions for pigmented rats (25).
Sampling of Brain Nuclei
Six h after surgery, the rats were kil1ed
cervical dislocation without anesthesia and then
decapitated. The brain was rapidly dissected out
and frozen on dry ice. MVN.
RD, and cochlear nuclei (anterior ventral [A VCN] and posterior ventral and dorsal part (PCN) were dissected with micropunches (900 11-m diameter)
from serial coronal sections (320 11-m) using the
atlas of Paxinos and Watson (26). Histological
controls of the punch sites were performed in
preliminary experiments. The punches were
stored in perchloric acid 0.4 M (100 11-L) at
- 80°C until assay (that is, less than 2 months).
Monoamines (NE, DA, 5HT) and their respective 1netabolites MHPG (free + sulfate),
DOP AC, and 5HIAA were assayed by HPLC
with electrochemical detection. The samples
were homogenized by ultrasonic disruption (10 s,
20 kHz) over ice. After removal of perchloric
acid excess by means of 6.5 M KOH/4.2 M
formate, the homogenates were centrifuged
(8800 g, 13 min) at +4°C and shared into two
portions: one portion was used to assay simultaneously NE, DA, 5HT, DOPAC, and 5HIAA
and the other one for total MHPG (free + conjugated).
rectly injected through an automatic injector
into a high petformance liquid chromatograph
coupled to an electrochemical detector to quantify NE, DA, SHT, DOPAC, and 5HIAA. The
mobile phase was a mixture of 27.5 mM citric
acid, 50 mM sodium acetate, 525 mM acetic
acid, 1 mM disodic EDTA, 1.9 mM sodium
heptane sulphonate, and 7% methanol. The final
pH was 3.44. The flow rate was 0.7 mL/rnin.
Retention times were as follows: NE, 6.1 min;
DOPAC, 10.3 min; DA, 11.9 min; 5HIAA, 16.7
min; and 5HT, 28.2 min. The selectivity of the
peaks was assessed by the comparison of hydrodynamic voltamograms obtained at different
potentials for the components of the brain sample and the corresponding standards.
Determination of total MHPG. The remammg
homogenate was diluted with 20 IJ.L water and
55
of the super~~r)(\
dorf tube containing 5.5 !JuL of a sulfuric-formic
mixture. The tube \Vas tightly closed and heated
to 95°C for 7 min to hydrolyze MHPG sulfate.
After cooling, 40 11-L of the mixture were injected into the HPLC column. The mobile phase
was a mixture of 30 mM citric acid, 50 mM sodium acetate, 1 mM disodic EDTA, and 4%
methanol. At a flow rate of 0.7 mL/n1in, theretention time of MHPG was 12.8 min.
Detection liTnits. The detection limits, calculated by doubling the noise ratios and expressed
H. Cransac et al
246
Table 1. Monoamine and Metabolite Levels (ng/mg proteins} in the MVN and LC of Sham-operated Rats.
Means :± SEM from 8 Rats
MVN
Intact side
Albino
NE
MHPG
DA
DO PAC
5HT
5HIAA
3.54
1.81
0.82
0.29
7.67
4.72
:±
:±
:±
:±
3.89
1.65
1.95
0.53
7.55
3.71
:c= 0.70
MHPG
DA
DO PAC
5HT
5HIAA
0.50
0.23
0.25
0.02
:!:: 0.40
:± 0.36
:±: 0.20
:± 0.22
:± 0.05
:± 1.07
0.42
LC
Lesioned
Intact side
:±
:±:
:±
:±:
:±
0.42t
0.17
0.03t
0.05
0.46t
0.43t
11.35 :± 1.59
4.50 0.23
1.04 0.22
1.63 :±: 0.25
7.85 :±: 0.50
6.96
0.41
9.56
4.48
0.64
1.27
8.32
8.12
± 0.23
:± 0.06
:± 1.24t
:± 0.35t
17.01:::1.72
4. i9::!:: 0.37
1.74 ± 0.23
2.22 :± 0.29
7.85 :± 0.77
6.28 :± 0.25
14.29
3.96
1.49
1.94
8.86
7.52
2.96
2.02
0.38
0.29
9.45
5.54
=
3.55 =0.39
1.-+5 =0.18
1.40
0.46
9.35
4.50
=
Lesioned
:±
:±:
:±
:±
:±:
::::
1.58
0.33
0.14t
0.30
0.44t
0.55t
1.44
:± 0.34
:c= 0.24
:± 0.25
:± 1.15
:± 0.58
tLesioned side compared with intact side, p < 0.05 using Wilcoxon test for paired values.
in terms of pmoles of injected amounts, were less
than 0.03 for all compounds, and intra-assay coefficients were 0.2% for all of these (n = 15).
The Mann-Whitney U test was used to compare the values between sham and operated animals, and the Wilcoxon test for paired values
was used to compare the values between the left
and right sides of MVN, LC, and cochlear nuclei. Difference was considered significant
when p < 0.05.
strains: the albino rats were more hyperactive
and had more pronounced postural imbalance
than the pigmented rats; they had a more pronounced spontaneous nystagmus that was not
influenced by light. The slow phase velocity
(SPV) of the spontaneous nystagmus was directed toward the lesioned side. As reported
earlier (25), in pigmented rats the SPV quantified 6 hours after the lesion was weaker in light
(SPV: 19.3°/s ± 10.9) (mean ± SD) than in
darkness (SPV: 30.3°/s ± 21.8); in contrast, in
albino rats, the spontaneous nystagmus was not
modulated by the light (SPV: 68°/s ± 24 in the
light; SPV: 70°/s ::±: 28 in darkness).
On the 30th hour after the lesion, the spontaneous nystagmus was no longer observed in the
pigmented rats (25), whereas the albino rats
still exhibited an intermittent spontaneous nystagmus.
Results
Effects of UL on i\1/onoamine Content and
Metabolism in JY!VN and LC
Determination ofproteins. The centrifuged pellets were resuspended, and the protein content
was determined with use of a Coomassie Blue
Protein assay reagent (Biorad Laboratories, Munich, Germany).
Statistical Analysis
Postural and Motor Deficits in the Two
Rat Strains
On the 6th hour after surgery, all of the rats
exhibited the motor, postural, and behavioral
deficits currently seen after hemilabyrinthectomy. However, some postural and oculomotor
differences were observed between the two
The values in the sham-operated animals are
shown in Table 1, and the UL-induced effects
are represented in Figures l and 2.
The comparison of the values obtained in
both sides of sham-operated animals showed
that anesthesia and opening the left tympanic
bulla without left labyrinth lesion decreased NE
and DA concentrations in the ipsilateral MVN
Brainstem Monoamines After Hemilabyrinthectomy
200
"8
";;;
[5
0..
0
~
*
NE
180
247
200
160
160
140
140
120
120
~
100
100
4-<
80
80
c
60
60
u
40
40
20
20
0
ll)
'""
ll)
0..
MVN
200
"0
180
~ll)
160
ll)
LC
MVN
140
..r::: 100
100
80
80
c
60
60
~
40
40
20
20
ll)
~
Intact
LC
MHPG
160
120
0
Lesioned
180
* *
120
~
1m
0
200
MHPG
0.. 140
0
VJ
4-<
NE
180
0
MVN
LC
MVN
350
350
300
B
c<:j
300
'""
&
250
250
200
200
"0
LC
DA
0
~
..r:::
VJ
150
150
c
100
100
~
50
50
0
0
200
200
180
180
4-<
0
ll)
~
MVN
"0
B
c<:j
160
160
&140
0
140
120
120
c<:j
100
..r:::
VJ
100
8
4-<
0
c
ll)
~
G)
0..
LC
DOPAC
80
80
60
60
40
40
20
20
0
MVN
LC
MVN
LC
Figure 1. Effect of 6 hours unilateral labyrinthectomv or: the medial vestibula~- (MVN: anC:: locus coeruieus (LC)
content in noradrenaline (NE), MHPG, dopamine (DA), and DOPAC in albino (left) and pigmented (right; rats. Results are expressed in percentage of the mean data calculated for the same structures on the same side in the
sham operated animals. Lesioned side in grey, intact side in white, error bars are::±:: SEM. *Sham compared with
Ul, p < 0.05 using Mann-Whitney U test.
and DA concentrations in the ipsilateral LC of
albino rats, but did not induce asymmetric alterations of these amines in the pigmented sham
group (Table 1). On the other hand, in the shamoperated groups, 5HT and 5HIAA levels were
higher in the ipsilateral MVN of both strains
and higher in the ipsilateral LC of albino rats
than in the corresponding contralateral nuclei
(Table 1).
The effect of UL was analyzed by comparing
the UL group and the corresponding shamgroup side by side. Six hours after UL, differen-
H. Cransac et al
248
2:l
~
ll)
0..
0
~
...c:
C/J
'-+-<
0
5
C_)
:u
c..
180
180
II
Lesioned
160
160
D
Intact
140
140
120
120
100
100
80
80
60
60
40
40
20
20
MVN
:200
'">::.:)
ll)
'§
g_
0
8
C'(!
~
4-<
0
c:
ll)
8ll)
c..
SHT
200
SHT
200
'">::.:)
LC
180
160
11i0
140
140
120
120
100
100
80
80
60
60
40
40
20
20
200
LC
SHIAA/SHT
200
'">::.:)
180
180
C'(!
.....
<!.)
160
160
140
140
120
~
120
100
100
4-<
80
80
c:
60
60
8
40
40
20
20
...::::
C/J
0
ll)
&
MVN
LC
0
MVN
0..
0
LC
200
3HJIAA
180
0
s
MVN
SHIAA/SHT
* *
*
0
0
MVN
LC
MVN
LC
Figure 2. Effect of 6 hours unilateral labyrinthectomy on the medial vestibular (MVN) and locus coeruleus (LC)
content in serotonin (SHT), its main metabolite SHIAA, and the ratio SHIAAISHT in albino (left) and pigmented
(right) rats. Results are expressed in percentage of the mean data calculated for the same structures on the
same side in the sham operated animals. Lesioned side in grey, intact side in white, error bars are ± SEM.
*Sham compared with UL, p < 0.05 using Mann-Whitney U test.
tial neurochemical changes were observed according to the rat strain and lesioned (left) or
contralateral (right) side.
In the UL albino rats, the greatest catecholamine changes were seen in the contralateral
LC, which exhibited a large increase of NE and
DOPAC (Figure l), indicating enhanced NE
synthesis (17, 18). On the other hand, in the
MVN of this strain there was a bilateral increase
of MHPG and an increase of DA, which was
significant only on the lesioned side (Figure 1).
These IYIHPG and DA increases were not different between the two sides of MVN.
In the UL pigmented rats, the only noradrenergic change was a decrease of the NE content
in the ipsilateral LC, which could be interpreted
as the consequence of an earlier enhanced NE
release (27) (Figure 1).
No significant changes of serotonergic indices were seen in either nuclei of albino rats; in
contrast, in pigmented rats, in spite of insignificant changes of 5HT and 5HIAA levels in the
Brainstem Monoamines After Hemilabyrinthectomy
Table 2. Effect of Unilateral Labyrinthectomy (UL) on
the 5HT and 5HIAA Levels (ng/mg proteins) and on the
Ratio 5HIAA/5HT in the Raphe Dorsalis (RD)
5HT
Sham
UL
5HIAA
Sham
UL
5HIAA/5HT
Sham
UL
Albino
Pigmented
21.97:!: 0.90
23.96:!: 1.67
23.61 :!: 2.86
18.01 :!: 0.36
20.70:!: 1.35
21.21 :!: 1.42
22.22:!: 1.82
22.07:!: 1.98
0.87:!: 0.05
0.83:!: 0.06
0.91 :!: 0.05
i.12±0.03*
Means :!: SEM from 8 rats.
*Sham compared with UL, p < 0.05 using Mann-Whitney U
test
MVN and decrease of both compounds in the LC,
the ratio 5HIAA/5HT was increased in the ipsilateral LC and in both MVN sides, thus indicating enhanced 5HT utilization in this strain (19)
(Figure 2).
Effect of UL on 5HT and 5HIAA Levels
and the Ratio 5HIAA/5HT
in the Raphe Dorsalis
No serotonergic changes were induced by
the UL lesion in the raphe dorsalis of albino rats
(Table 2). In contrast, in the pigmented rats, although the changes in 5HT and 5HIAA concentrations did not reach a significant level, the increased 5HIAA/5HT ratio suggested an increased
5HT utilization (19).
Eff'ect
on
Metabolism in Cochlear Aluclei (Tal?ie
The effect of UL was also analyzed in cochlear nuclei as a comparison to assess the specificity of the UL-induced neuroche1nical responses (Table 3). No monoamine changes
were found in the sham-operated groups. Six
hours after UL, the 5HT and 5HIAA contents of
the ventral or posterior parts of cochlear nuclei
were unaltered in albino rats. In pigmented rats,
no changes of NE or MHPG were observed in
these nuclei, however, there was a significant
249
decrease of 5HT in the cochlear nuclei and an
increase of the ratio 5HIAA/5HT on the contralateral posterior ventral and dorsal cochlear
nucleus.
Discussion
This is the first neurochemical report on the
monoaminergic effects of UL in the MVN, LC,
and raphe dorsalis nucleus which contain, respectively, the cell bodies of the nm·adrenergic
and serotonergic input to MVN. To assay simultaneously the three monoamines (NE, DA,
5HT) and their main metabolites (MHPG,
DOPAC, and 5HIAA) in these small brainstem
nuclei, we developed a sensitive and specific
HPLC method. The main interest of our method
is that it allows assay of all the compounds studied (except MHPG, which needs a preliminary
hydrolysis of its sulfate conjugate) in a single
step with a very high sensitivity and without
losses through extraction steps. Indeed, the direct HPLC analysis of the deproteinized extract
avoids the losses of compounds through column
extraction steps as was previously described by
Mefford et al. (28). Because central monoamines are very sensitive to stress, we used as a
comparison group sham-operated rats, that were
submitted to the same experimental protocol
(ether anaesthesia and opening of tympanic
bulla) as the UL-groups, except for the labyrinth lesion. The comparison of the biochemical
values obtained in the present study in the
sham-operated group to those that we previously found in intact rats of both strains in basal
conditions (48) shows that the sham-operation
(that is. NE
mauces
and DA decreases and 5HlA..f\ mcreases) in the
LC (Table 1), ancl cochlear nuclei (Table
3), but not in the raphe dorsalis (Table 2). These
changes are more pronounced on the lesioned
side than on the intact side of the sham-operated
rats. Such surgery-induced monoaminergic
changes confirm the great sensi6vity of LC to
stress, as already shown by other authors
(29,30), and extend this finding to MVN and
cochlear nuclei. Furthermore, our data point out
the relevance of the sham-operated group and
the need to compare the UL group and the cor-
H. Cransac et al
250
Table 3. Effect of Unilateral Labyrinthectomy (UL) on the Monoamine (NE and 5HT) and Metabolite (MHPG and
5HIAA) Content of Cochlear Nuclei (AVCN: Anterior Ventral Part and PCN: Posterior Ventral+ Dorsal Part)
in Albino and Pigmented Rats
PCN
AVCN
Intact side
Albino
NE
Sham
UL
MHPG
Sham
LJL
SHT
Sham
UL
5HIAA
Sham
UL
5HIAA/5HT
Sham
UL
Pigmented
NE
Sham
UL
MHPG
Sham
UL
5HT
Sham
UL
5HIAA
Sham
UL
5HIAA/5HT
Sham
UL
Lesioned side
Intact side
Lesioned side
2.97 ::':: 0.48
4.05 2:: 0.67
3.08 ::':: 0.45
3.28 ::':: 0.32
0.33
2.37
3.92 ::':: 0.49
3.48 ::':: 1.06
3.91 ::':: 0.45
2.24
2.09
2.02 ::':: 0.32
2.27
c).45
1.61:2::0.16
2.25
0.39
1.62 ::':: 0.26
2.74 ::':: 0.59
0.38
u.i4
=
=
5.17 ::':: 0.83
5.71 ::':: 1.30
5.17
5.40
0.75
1.13
3.96 ::+:: 0.24
4.59 ::+:: 0.66
4.48 ::':: 0.64
4.05 ::':: 0.64
2.31
2.00
0.48
0.26
2.18 ::':: 0.28
1.85 ::':: 0.23
2.26 ::+:: 0.16
2.36 ::':: 0.24
2.44 ::':: 0.35
1.89 ::+:: 0.23
0.43 ::':: 0.05
0.43 ::+:: 0.10
0.41 ::+:: 0.05
0.43 ::':: 0.09
0.53 ::':: 0.04
0.57 ::':: 0.11
0.51 ::':: 0.05
0.53 ::+:: 0.10
3.27 ::':: 0.26
2.68 ::+:: 0.25
3.28 ::':: 0.46
2.94 ::':: 0.31
2.25::'::0.18
1.88 ::':: 0.18
2.99 ::':: 0.38
2.31 ::+:: 0.10
2.00 ::':: 0.45
1.54 ::':: 0.09
1.61 ::':: 0.31
1.37 ::':: 0.17
1.80 ::':: 0.19
1.87 ::':: 0.24
1.12::'::0.16
1.32::'::0.10
8.72 ::':: 2.39
3.81 ::+:: 0.48*
6.67 ::':: 1.53
3.74 ::+:: 0.25
7.13::+::1.31
3.56 ::+:: 0.16*
6.19 ::':: 1.00
3.43 ::+:: 0.19*
1.99 ::+:: 0.36
1.50 ::':: 0.27
1.51 ::+:: 0.30
1.34 ::+:: 0.19
2.10 ::+:: 0.29
1.88 ::+:: 0.28
2.11 ::':: 0.20
1.63 ::+:: 0.22
0.28 ::+:: 0.05
0.36 ::+:: 0.03
0.24 ::+:: 0.04
0.33 ::+:: 0.04
0.31 ::+:: 0.03
0.48 ::+:: 0.05*
0.35 ::+:: 0.03
0.43 ::+:: 0.04
::+::
::+::
::+::
Results are expressed as the mean ± SEM in ng/mg proteins.
*Sham compared with UL, p < 0.05 using Mann-Whitney Utest.
responding sham-group side by side to assess
the proper effect of UL. The main findings are
that UL induces short-term monoaminergic
changes mainly in the LC and the MVN. Furthermore, the comparison with the cochlear nuclei and raphe dorsalis shows that the ULinduced noradrenergic changes are restricted to
MVN and LC whereas the serotonergic effects
seem to be more extended. This finding confirms biochemically the relevant role of LC
noradrenergic innervation in vestibular compensation (6) and points out the involvement of
NE of the MVN in the early stages of this process.
It has been shown that in contrast to other
mammals, the rat shows a very fast compensation after UL: spontaneous nystagmus and postural imbalance disappear within few hours (5).
In fact, our present results show that 6 h after
UL the noradrenergic changes are different in
the two strains of rats. Alternative explanations
for these interstrain biochemical differences
could be the differential time-course of compensation between the two strains and some differences in the neural pathways involved in the
compensation. Indeed, as far as vestibular ret1exes are concerned, some relevant features of
albino rats are the anomalies of the visual system and a defective functional optokinetic response of central vestibular neurons (31). The
role of vision in vestibular compensation has
been well recognized in several mammals, including rat (4,5). Despite the failure of efficient
visual pathways and the deficient optokinetic
reflex, electrophysiological recording of vestibular neurons in hemilabyrinthectomized albino
rats has shown the occurrence of re-equilibra-
Brainstem Monoamines After Hemilabyrinthectomy
tion of the gain of vestibular neurons between
the lesioned side and the intact side, which is
necessary for compensation of UL behavioral
effects. However, the time-course of vestibular
compensation was shorter in pigmented rats
than in albino rats (5,32). This is confirmed by
early observations showing that 30 h after the
UL, spontaneous nystagmus has disappeared in
the pigmented rats (25) and by present observations, showing that it was still present intermittently at the same time in albino rats. Indeed. it
has been shown that the restoration of the optokinetic responses occurs in rats when the spontaneous nystagmus has vanished (25). Accordingly, one can assume that, to compensate for
vestibular loss, each particular species or strain
utilizes the remaining vestibular system and
other central pathways with different kinetics.
Although one limitation of our study is that we
did not sacrifice the animals at different intervals after the UL, our results support this assumption. So the involvement of central NE
may be more crucial and more sustained in the
albino than in pigmented rats. The data show
that 6 h after UL, NE metabolism (synthesis and
release) is still activated in both the MVN and
the LC of albino rats, whereas the reduced NE
content observed at this time in the LC of pigmented rats could reflect an earlier strong stimulation of NE release leading to a partial
neurotransmitter depletion. Such reduction of
monoamine content is seen when a sustained
NE release exceeds synthesis capacities (27).
The large increase of NE synthesis found in the
contralateral LC of albino rats agrees with the
previous electrophysiological observations of
Pompeiano in cats (1 1). A possible explanation
for the asymmetric NE activation in the LC and
bilateral increase of NE metabolism in MVN
(evidenced by the increase of MHPG) is that the
labyrinth input of the intact side predominantly
activates the ipsilateral LC. This, in turn activates NE projections of both MVN through
both ipsilateral and crossed pathways. Moreover, the increase of DA found in the MVN of
albino rats suggests a role for this amine, since
it has been reported that dopaminergic agonists
accelerate compensation of postural and ocular
symptoms in pigmented rats (33) and reduce the
UL-induced clinical symptoms in albino rats (34).
251
The MVN is the nucleus of the vestibular
complex exhibiting the highest density of 5HT
receptors (35,36). Microiontophoretic application of 5HT on brainstem slices modifies the firing rate of MVN neurons (37). Our study shows
that UL induces an increase of 5HT utilization
in the raphe dorsalis, in both MVN sides, and in
ipsilateral LC in pigmented but not in albino
rats. Since the raphe dorsalis is the main source
of the serotonergic innervation of MVN and LC
(9,15), it can be assumed that the 5HT changes
observed in these nuclei are the consequence of
the serotonergic activation of raphe dorsalis. In
this line, it has been reported that the electrical
activation of raphe dorsalis induced modifications (increase or decrease) of firing in neurons
of several vestibular nuclei (38). The recent
demonstration in superior colliculus and cortical visual areas of serotonergic axons originating from raphe dorsalis in pigmented LongEvans rats (39) supports the hypothesis that
raphe dorsalis and its serotonergic projections
may contribute to the oculomotor and visual adjustments occurring during vestibular compensation. The role of brain 5HT in the adaptation
of vestibula-ocular reflex has been suggested
(16), but no specific regional studies have been
conducted in UL animals.
Another noteworthy point of our study is the
differential pattern of noradrenergic and serotonergic activation observed in the two strains
6 h after UL: in albino rats, NE metabolism is
activated while 5HT is unaltered, whereas in
pigmented rats, the activation of 5HT metabolism seems to follow an earlier NE activation.
These findings can be paralleled with the modulatory effect of 5HT on the LC noradrenergic
neurons (40).
In addition to monoamines, cholinergic, glutamatergic, GABAergic, and histarninergic endings are present in vestibular nuclei (41,42).
Pharmacological manipulations using agonists
and antagonists of neurotransmitter receptors
have suggested a role for these neurotransmitters and for neuropeptides such as substance P
and ACTH in vestibular compensation (42).
However, only a few direct neurochemical studies are available in UL animals. Smith and Darlington (43) and DeWaele et al. (44) proposed
that a denervation-induced supersensitivity of
H. Cransac et al
252
NMDA receptors of the deafferented vestibular
neurons could play a role in vestibular compensation. More recently, acetylcholinesterase activity changes were found in the MVN of cats
(45) and pigmented rats (46) after UL.
Acknowledgnzents-Thanks are due to R. M. CottetEmard and A. Vouillarmet for technical assistance
and to M. Dumarest for animal care. This work was
supported by grants of DRET (Direction des Recherches Etudes et Techniques) contract N° 93 1 305.
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