ACTA NEUROBIOL. EXP. 1989, 49: 153-169
DIFFERENTIAL EFFECT OF THE DAMAGE TO THE LATERAL
HYPOTHALAMIC AREA ON HIPPOCAMPAL THETA RHYTHM
DURING WAKING AND PARADOXICAL SLEEP
E JURKOWLANIEC, W. TROJNIAR, T. OZOROWSKA and J. TOKARSKI
Department of Animal Physiology, University of Gda6sk
24 Kladki St., 80-822 Gdafisk, Poland
Key words: hippocampal theta rhythm, lateral hypothalamus, lesion, waking, paradoxical sleep
Abstract. Hippocampal theta rhythm was analyzed in rats subjected
to bilateral, electrolytic lesions of the lateral hypothalamic (LH) region
at different levels of its rostro-caudal axis. It was found that damage
to the LH disturbed the hippocampal theta activity both during waking
and paradoxical sleep. The main effect consisted in the lowering of the
theta frequency. Typically, a decrease of frequency was accompanied
by an increase of amplitude during waking, and an amplitude fall during
paradoxical sleep. Extensive lesions increased the amount of rhythmic
slow activity during waking and induced long trains of immobilityrelated theta. The general picture of impairments of the hippocampal
theta rhythm in particular subjects depended on the size of the lesion
and, to some extent, also on its localization within the LH. It is concluded
that LH region contains systems of fibers which transmit impulses from
the brain stem reticular formation to the prosencephalic structures generating the rhythmicity of theta.
INTRODUCTION
It is well established that the rhythmic slow activity (theta rhythm)
of the dorsal hippocampus is generated in the system involving the
medial septum and diagonal band of Broca, enthorinal cortex and the
hippocampus itself (for review see 3). At least two independent generators of theta activity were found in different cellular components of the
dorsal hippocampus and the dentate gyrus (2, 9, 30). It is also well known
that the hippocampal rhythmic activity depends on the inputs from the
reticular nuclei of the lower brain stem (1, 15, 22, 27-29). The exact
nature of these influences is still to be cleared up, however, as the theta
activity survives well pretrigeminal (7, 17, 35) and cerveau isole (8, 16,
24, 35) transections, although afterwards it shows a number of abnormalities.
On the basis of the electrophysiological and lesion experiments (1, 15,
22, 27-29) it was found that ascending fibers essential for the hippocampal theta follow the dorsal and medial longitudinal fasciculi, central
tegmental tract and the medial forebrain bundle. The latter pathway
contains also fibers involved in low voltage fast activity.
The damage to the medial forebrain bundle at the level of the
lateral hypothalamus evokes disturbances in the hippocampal theta
rhythm (4, 11, 12, 23). In rats they consist in the decrease in the amplitude and frequency of motor-related theta and the appearance of long
trains of theta rhythm during immobility, the phenomenon which is
very rarely observed in normal rats.
In our previous investigations (23) in which we studied quantitative
sleep-waking relations and cortical EEG activity in lateral hypothalamic
rats, we observed that LH animals suffered also from disturbances in the
hippocampal rhythmic slow activity. In the present paper we analyzed
these disturbances.
In rats theta rhythm in the dorsal hippocampus accompanies voluntary behaviors (25, 26, 30, 32, 33) and paradoxical phase of sleep (20, 21,
25, 34). As we did not find data concerning the influence of LH damage
on paradoxical sleep theta, we have analyzed it in the present paper
separately from the changes of theta activity dGring waking. Moreover,
we have tried to relate the observed abnormalities in waking and sleep
theta to the localization of the damage within the LH.
MATERIAL AND METHOD
The experiment was carried out on 18 male albino rats of the Wistar
strain, weighing 250-300 g on the day of surgery. The animals were kept
in individual home cages with food and water ad lib., in an artificially
maintained 12:12 h lightldark cycle.
The animals were implanted, under Nembutal anesthesia, with electrodes for lesion bilaterally in the region of the lateral hypothalamus,
and with EEG recording electrodes. A bipolar, concentric electrode was
placed in the CAI pyramidal cell layer of the left dorsal hippocampus;
a cortical screw electrode was positioned over the right occipital cortex
and a reference electrode - on the os frontale. A silver wire electrode
was sutured into the neck muscles for recording the EMG activity.
The localization of lesion electrodes varied in different animals along
the rostro-caudal axis of LH and stereotaxic coordinates were as follows:
1.2-2.8 mm posterior to the bregma, 1.5-1.8 mm lateral to the midline
and 7.7-9.0 below the surface of the skull. The hippocampal recording
electrode was implanted 2.5-2.8 mm posterior to the bregma, 2.5 rnrn
lateral to the midline, and 2.5-3.5 mm below the skull surface. The
neocortical recording electrode was screwed 5 mm posterior to the
bregma, 3 mm lateral to the midline at a depth of 1 rnrn below the
skull surface. The construction and implantation of electrodes was described in detail elsewhere (23).
EEG recording began after 10 days of recovery period, during which
the rats were adapted to the experimental conditions. The recording was
carried out in glass cages measuring 260 X 260 X 400 mm, placed in an
illuminated, sound attenuating chamber. The experiment lasted 1 hour
daily and was performed always at the same time (11:OO-12:OO a.m.).
As we found earlier, this part of the daily cycle allows to record all
types of the rat's EEG activity (waking, slow wave sleep and paradoxical
sleep patterns) as they transit spontaneously from one to another. The
hippocampal theta rhythm was recorded bipolarly from the concentric
electrode between the inside and the outside pole, and for comparison
also monopolarly between each pole of the concentric electrode and the
reference electrode, using 16 channels Medicor polygraph (passband 0.5350.0 Hz). The animals were continuously observed by the experimenter
through a camera connected to a monitoring system and their behavior
(waking, rearing, probable sleep etc.) was noted concomitant with EEG
records.
Normal EEG pattern was recorded for 3 days. Then the rats were
subjected, under light ether anesthesia, to electrolytic lesions made by
passing 1.2-2.0 mA anodal current for 15-20 s through previously implanted electrodes.
Starting from the day following the brain damage, the EEG pattern
was recorded for 7-8 consecutive days and then on the l l t h , 14th, 18th,
and in some animals periodically up to the 36th postlesion day. Regardless whether the EEG recording was performed or not on the particular
day, up to the end of the experiment the animals were put in the recording chamber every day at the time of the experimental session.
All rats were observed for behavioral disturbances (somnolence, ingestive impairments, body weight loss) resulting from the brain damage.
The rats which were aphagic and adipsic or extremely hypophagic and
hypodipsic for more than 2 days were artificially fed and watered by
means of a gastric tube. In the rats with anterior LH lesions the rectal
temperature was measured at the end of each recording session.
After the completion of the experiment the rats were treated with
an overdose of ether anesthesia, the brains were removed from the
skull and placed in 10°/o Formalin. After fixation, brain sections 30 pm
thick were cut using a frozen tissue technique. The sections were stained with cresyl violet for cell bodies.
All records were visually inspected for the presence of hippocampal
theta rhythm during waking and paradoxical sleep, and their amounts
in experimental hour were assessed. Three 10 s samples of waking and
sleep theta were taken randomly from each prelesion and postlesion
daily record. Peak-to-peak frequency of theta rhythm was determined
by counting the number of theta waves in 10 s samples. Amplitude was
measured with the use of a transparent plastic ruler (millimeter scale).
Amplitudes and the frequency of theta waves in each postlesion day
were compared with a preoperative baseline. Depending on distribution,
the data were statistically analyzed with the use of the Student's t-test
or nonparametric Mann-Whitney U test.
RESULTS
Prelesion hippocampal theta activity. Before the lesion, the rhythmic
slow activity (theta rhythm) in the hippocampal records appeared during
waking mainly in association with exploratory behaviors (waking around
the cage, sniffing, rearing) and with head movements. In normal rats we
have not observed theta activity during immobile waking. Our observations concerning the relation of theta rhythm to behavior were compatible with those described previously (25, 26, 30, 32, 33). The frequency
of waking theta varied from 6.0 to 7.6 Hz. This may reflect differences
in the intensity of particular movements (20, 25, 33). The amplitude of
theta waves showed a marked interindividual variability (average values:
140-440 pV), which was probably due to slight variations in the localization of electrodes' tips within the dorsal hippacampus (18). However,
the amplitude was relatively stable intraindividually.
Similarly to the previous reports (19, 21, 25, 34), the theta rhythm
was also recorded in the paradoxical phase of sleep. Its amplitude varied
from 200 to 780 pV in different subjects and its frequency was 7.08.0 Hz.
Theta rhythm in the hippocampus was accompanied in the neocortex
either by desynchronization or by rhythmic synchronous activity re-
sembling the hippocampal theta, especially during paradoxical sleep. As
was suggested, such cortical activity is probably volume-conducted from
the hippocampus (6).
Before the lesion, the mean duration of waking theta in the hippocampus was l l O / o of the total recording time. Paradoxical sleep theta
took 13O/o.
Abnormalities in theta rhythm after LH damage. Damage to the
lateral hypothalamus resulted in a number of abnormalities in the hippocampal rhythmic slow activity. The intensity and duration of these disturbances corresponded with the size of the lesion.
The main change concerned the frequency of theta waves. It was
depressed during waking in 8 rats and during paradoxical sleep in 12
subjects. Typically, a decrease of frequency was accompanied by an increase of amplitude (n = 6 ) in waking and by an amplitude fall (n = 6)
in paradoxical sleep. Unfortunately, in 3 rats an assessment of amplitude
of paradoxical sleep theta was impossible because of either extreme
shortening (e.g. only one episode in the entire recording session) or
even complete cessation of this phase of sleep in the early postlesion
period, or because of unsatisfactory quality of EEG records.
Disturbances in theta rhythm during waking and paradoxical sleep in particular rats subjected to
brain damage. Explanations: 'It" increase in comparison with the baseline; "4" decrease in comparison with the baseline; "=" lack of change; "-" not analyzed
Waking theta
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Out of 18 investigated rats, simultaneous abnormalities in the waking
and sleep theta of a typical course (with only minor deviations) occurred
in 5 subjects, mainly those with the most extensive lesions of the LH
region. In two animals there were impairments in the waking theta
only, and further 4 were disturbed only in paradoxical sleep. In 4 other
rats the changes of theta rhythm were atypical, for instance a decrease
of amplitude of the waking theta without change of frequency or a decrease of frequency of the paradoxical sleep theta accompanied by an
increase of its amplitude. Three rats did not show any changes of hippocampal activity after LH damage. Table I summarizes the results
obtained in particular subjects.
Figures 1-3 demonstrate the examples of the main three types of
disturb'ances in the hippocampal theta activity. Each figure consists of
diagrams of the amplitude and frequency of the theta rhythm during
waking and paradoxical sleep and of segments of polygraph records
taken before the damage and in the early postlesion period. In particular
figures some postlesion days are omitted, usually due to unsatisfactory
quality of EEG records. Figures 4-6 illustrate anatomical verification of
the lesion placements in rats shown in Figs. 1-3.
Figure 1 illustrates an example (rat no. 32) of severe disturbances in
both waking and paradoxical sleep theta, lasting up to the end of the
experiment. Regardless of the state of vigilance, LH lesions affected
mainly the frequency of theta rhythm. Its depression (by 0.5-1.7 Hz
during waking and by 0.6-1.8 Hz during paradoxical sleep) was highly
significant and persisted at least up to the 18th postlesion day. Changes
in amplitude were not as consistent, particularly during sleep. In this
animal the increase of amplitude of the waking theta ranged from 40 to
150 pV in particular postlesion days, and the amplitude fall was 110120 pV during paradoxical sleep. This constituted 13-45O/o and 21-24"o
of the prelesion baseline respectively. In other animals from this group
the changes were similar, although the relative intensity and duration
of disturbances might differ in particular subjects.
Figure 2 shows the example of a rat (no. 28) in which a relatively
large lesion of the relevant area caused disturbances exclusively in the
waking theta. The hippocampal activity during paradoxical sleep remained virtually unchanged. In the presented animal the frequency was
decreased by 0.6-1.5 Hz.The increase of amplitude of the waking theta
was relatively small (about 40 pV, i.e. 10°/a of the baseline). The changes
in theta activity were observed in the first week after the lesion. The
other animal from this group (no. 29) showed a 3 days' marked increase
of amplitude (100-140 pV, i.e. 24-34"/0 of the baseline) and a comparable
(0.9 Hz) fall of frequency.
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Fig. 1. The example of disturbances in theta rhythm both in waking and paradoxical sleep (rat no. 32). Top: diagrams of frequency and amplitude of theta
waves; each point denotes mean (fSD) value counted from 9 samples (3 in each
day) before LH lesion (marked as day O), and from 3 samples in each postlesion
day. The level of significant difference from the baseline: * P
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Fig. 2. The example of disturbances in theta rhythm exclusiveIy in waking (rat
no. 28). Explanations as in Fig. 1. The experiment was concluded on the 7th
postlesion day.
Figure 3 shows an example (rat no. 27) of exclusive disturbances in
paradoxical sleep theta of a typical course. The decrease of frequency in
this animal ranged from 0.7 to 0.8 Hz, and the amplitude fall was
50-80 pV (21-33O/o of the baseline). These changes, although not regular,
lasted up to the end of the experiment (usually 18 days). In one rat
from this group they were particularly long-lasting and persisted up
to the 36th postlesion day (the last day of the experiment).
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Fig. 3. The example of disturbances in theta rhythm exclusively in paradoxical
sleep (rat no 27). Explanations as in Fig. 1.
The other characteristic abnormality in hippocampal EEG observed
after LH damage is the appearance in some animals, mainly those with
extensive damage to LH, of long trains of theta rhythm during imrnobility. Occasionally there were also episodes of extremely slow (4.4-4.5 Hz)
waking theta, never observed before the lesion.
Six rats showed a marked elevation of the amount of theta activity
during waking (Table I). Without exception they belonged to a group
with the largest lesions of the relevant area. Accordingly they suffered
from the most severe behavioral depression (hypokinesia, or even somnolence) and thus the theta activity occurred mainly during states of
immobility.
As we have described elsewhere (23) damage to the LH region disturbs quantitative sleep-waking relations towards the shortening of sleep,
mainly its slow wave phase, but to some extent also paradoxical sleep.
Of the animals analyzed in the present paper - 5 showed a marked
shortening of paradoxical sleep (Table I), in 3 of them there was a complete or almost complete cessation of this stage in the early postlesion
period.
We have not found disturbances in body temperature in anterior LH
lesioned animals.
Anatomical verification of electrode and lesion placements. All hippocampal recording electrodes were situated in the CAI pyramidal cell
region of the dorsal hippocampus, approximately at the level of A 4110
of the Konig and Klippel atlas (13). However in particular subjects the
position of electrode tips differed slightly in the dorso-ventral plane.
Lesions of the LH region were spread along its rostro-caudal axis
from the preoptic area up to the anterior midbrain tegmentum. Sixteen
animals received lesions within the LH region, in the remaining two the
electrodes were misplaced and damage involved mainly the periventricular tissue at the prosencephalic level. In all animals lesions were
bilateral, in the majority of cases relatively symmetrical, although there
were variations in the precise localization of damage on both sides of
the brain. It is well established that the diencephalic and prosencephalic
projections of the brain stem reticular formation are mainly ipsilateral
(5, 10, 31). Although it is not absolutely certain whether this concerns
also fibers relevant for the hippocampal theta activity, we assumed that
the hypothalamic damage ipsilateral to the recording electrode should
be more influential than the one on the opposite side of the brain. In
cases of asymmetry in localization of bilateral lesions we related eventual
disturbances in hippocampal EEG to the localization of damage ipsilatera1 to the recording electrode.
The most extensive lesions of LH were found in rats showing impairments in both waking and paradoxical sleep theta activity. In various
animals from this group lesions were situated at different levels of the
rostro-caudal axis of LH. In two rats (no. 44 and no. 45) damage was
limited to the anterior LH region and did not extend beyond the anterior
pole of the ventro-medial nucleus (A 5910 - A 4890 according to the
Kijnig and Klippel (13) atlas). The detailed localization of lesions in these
animals are shown in another paper (Trojniar et al., in preparation)
concerning cortical EEG activity and sleep-waking relations in LH rats.
Figure 4 shows an example of lesion limited to the intermediate part
of LH (the level of the ventro-medial nucleus). Still another rat (no. 9)
A 3430)
had damage involving intermediate and posterior (A 4380
parts of LH, and in one rat (no. 10) damage was limited to the posterior
(A 3750 - A 3290) course of the medial forebrain bundle. Ipsilaterally
-
Fig. 4. Anatomical localization of lesions (shaded areas) in the rat shown in Fig. 1
4no. 32). Reconstruction of the damage was superimposed on plates taken from
the atlas by KiSnig and Klippel (13). Unilateral hippocampal electrode was implanted on the left side of the brain
to the hippocampal recording electrode the lesions dissected totally or
almost totally the medial forebrain bundle, with the exception of the
rat with the most posterior localization of the lesion (no. lo), in which
the medial forebrain bundle was dissected in about 50°/0. All lesions
were located in the close proximity of the capsula interna and/or
cerebral peduncle involving pericapsular tissue. Lesions to the capsula
interna were small or altogether absent.
On the contralateral side the lesions were similarly located as far
as the rostro-caudal level is concerned. There were however some asymmetries (Fig. 4).
In two rats showing exclusive disturbances in the waking theta,
damage to the lateral hypothalamus proper, ipsilaterally to the hippocampal electrode was small (less than 114 of its coronal section, Fig. 5)
or absent (rat no. 29). Instead, large lesions involved the capsula interna
A 3990
Fig.
in Fig. 2 (no.
and the optic tract together with the adjacent tissue. Contralaterally,
in both rats lesions involved the medial border of the capsula interna
and damaged totally (rat no. 29) or partially (Fig. 5) the lateral hypothalamic-medial forebrain bundle area.
Generally, rats showing impairments of hippocampal activity exclusively in the paradoxical sleep have small to medium-sized lesions
within the LH region, which destroyed the area of the medial forebrain
bundle only partially (Fig. 6). In some cases the medial border of the
capsula interna was also invaded. Contralaterally the damage also involved the LH region, at relatively similar rostro-caudal level, but sometimes there were some asymmetries. The same concerns the animals
with an atypical course of theta changes after the brain damage.
A 4230
A
CllO
Fig. 6. Anatomical localization of lesions in the rat shown in Fig. 3 (no. 27).
Explanations as in Fig. 4.
No disturbances in the hippocampal theta were found in rats with
either small lesions of the posterior LH region, or an extensive damage,
but located outside the LH area.
The increase of the amount of theta activity during waking after
brain damage was found in rats with the most extensive lesions of the
LH region. They belonged to the first two groups described above.
DISCUSSION
The major findings of the present experiment are as follows: (i)
damage to the lateral hypothalamic region disturbs hippocampal theta
rhythm both during waking and paradoxical sleep; (ii) the main effect
consists in the lowering of theta waves frequency, which typically is
accompanied by an increase of amplitude during waking and amplitude
fall during paradoxical sleep; (iii) extensive lesions increase the amount
of slow rhythmic activity during waking and induce long trains of
immobility-related theta; (iv) general picture of impairments of hippocampal activity in particular subjects depends on the size of the lesion
and its localization within the LH region.
The effect of lateral (4, 11, 12) and posterior (20) hypothalamic
damage on the theta rhythm during waking has already been described
earlier. Our results, in general, are compatible with those reported
previously. We also found that LH lesions greatly depress the frequency
of theta rhythm, and release immobility-related theta activity, rarely
observed in normal rats.
In rats the frequency of waking theta is correlated with the intensity
of movement it accompanies. Extensive lateral hypothalamic lesions are
known to produce akinesia or even somnolence (14). We observed it
also in our animals (23). The first impression is that the slowing down
of theta activity may. be secondary to the depression of locomotor
activity. It is not so, however. Similarly to other authors (4, 12) we
recorded this slower theta rhythm even during episodes of quite normal
movements.
A decrease of body temperature (32) does not seem to contribute to
the fall of theta frequency either, because we did not observe such
a change in our LH rats. Similar observations were also made by other
authors (4, 12). It seems therefore that damage to the LH region impairs
some more basic mechanism involved in the regulation of frequency of
the theta rhythm. As was found in the present experiment, this impairments concern also hippocarnpal activity during paradoxical sleep.
The effects of hypothalam5c lesions on the amplitude of theta are
more vague. Kolb and Whishaw (12) found a depression of both amplitude and frequency of the waking theta in the early postlesion period.
De Ryck and Teiteilbaum (4) and Robinson and Whishaw (20) did not
observe great changes in their lateral and posterior hypothalamic rats.
In our experiment there was a different effect of LH damage on the
amplitude of theta rhythm during waking and paradoxical sleep. Contrary to previous reports, during waking we observed rather an increase
of amplitude accompanying a depression of frequency. During paradoxic3
- Acta
Neurobiol. Exp. 4189
a1 sleep, usually both amplitude and frequency were decreased. However, it should be pointed that the change in amplitude was not as impressive as the change of frequency. Its magnitude varied in different
animals, in the majority of cases it was not particularly long-lasting,
and sometimes appeared after a period of unchanged activity in the
early postlesion days.
In particular rats investigated in this work, the lesions differed in
size, precise localization within the LH region and sometimes in symmetry. Accordingly, there were variations in disturbances of the hippocampal function. Having such a material, we tried to draw a conclusion as to the neuroanatomical basis of the observed changes in theta
activity. The main finding is that the severity of impairments depends
on the size of the lesion. Only large lesions involving the lateral hypothalamic-medial forebrain area (total or almost total dissection) ipsilaterally to the recording electrode produced impairments of the theta
activity both during waking and paradoxical sleep. It appeared that the
rostro-caudal level of the lesion was irrelevant, and the same results
were obtained with damage at the anterior, intermediate and posterior
LH level.
One may speculate that to cause exclusive disturbances of the waking
theta, a destruction of the lateral hypothalamic-medial forebrain bundle
area is not absolutely necessary. In two animals we found pronounced
impairments when large lesions, ipsilateral to the recording electrode,
were placed within the capsula interna and optic tract, omitting the
main body of LH. Of course such a conclusion is possible only with a n
assumption that the lesion ipsilateral to the hippocampal electrode is
decisive for theta disturbances. We feel that this hypothesis is worth
checking in further experiments.
Incomplete lesions of LH caused either exclusive impairments of hippocampal activity during paradoxical sleep or inconsistent changes in
both waking and paradoxical sleep theta. On the basis of this material
we feel that the theta activity in paradoxical sleep is more sensitive to
LH damage than that in waking.
Extensive lesions of LH region caused that the waking EEG activity
was dominated by the theta rhythm in hippocampal records. Similar
observations were reported after pretrigeminal (7, 17, 35) and cerveau
is016 (8, 16, 24, 35) transections in rats and cats and after medial pontine
lesions in rats (12). However, LH animals were not so deeply affected
and the rhythmic, slow activity of the hippocampus was at times intermitted by other patterns.
All these data suggest that the lateral hypothalamus and neighbouring
structures (e.g. capsula interna) contain systems of fibers which trans-
mit impulses from the brain stem reticular formation to the prosencephalic structures generating theta rhythmicity. These fibers seems to
determine the amount, frequency and possibly also amplitude of the
theta rhythm both during waking and paradoxical sleep.
This investigztion was supported by Project CPBP 04.01.6.18.
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Accepted 20 January 1989