influence of ventral mesencephalic lesions on various spontaneous and conditioned behaviors in the...

I )hyxh~h~ , l ' t/ml B c h a v i , , r . Vol. 4. pp. 567 573. Pergamon Press. 1969. Printed in Great Britain

Influence of Ventral Mesencephalic Lesions on Various Spontaneous and Conditioned

Behaviors in the Rat'

M I C H E L L E M O A L , B E R N A R D C A R D O A N D L O U I S S T I N U S

Laboratoire de Psychophysiologie, Facultd des Sciences, Avenue des Facultds - 33 - Talence, France

(Rece ived 15 F e b r u a r y 1969)

LE MOAL, M., B. CAROO AND L. STINUS. Influence o f ventral mesencephalic lesions on various spontaneous and conditioned behaviors in the rat. PHYSIOL. B~]AV. 4 (4) 567-573, 1969.--In the first set of experiments, we used groups of rats with either bilateral or unilateral ventral mesencephalic lesions and a group of controls in the following tests: diurnal exploratory activity, nocturnal activity, Skinner box and avoidance conditioning. Both groups of lesioned rats were found to be hyperactive in both types of activity tests, but those with unilateral lesions showed hyperactivity to a lesser extent. Neither of the experimental groups showed any evidence of extinction in the Skinner box. Rats with bilateral lesions learn a conditioned avoidance response more easily than the control animals but they also tend to show inadaptive behavior during the course of the testing procedure. In a second experiment, stimulating and recording electrodes were implanted in the lateral hypothalamus in certain of the animals of the three previous groups, having lesions as well. Such lesions were found to significantly decrease the threshold of arousal to central hypothalamic stimulation. From the results of our lesioning and testing procedures, a syndrome has been defined. However, the complex structure of the region that was destroyed does not permit a clear interpretatim) of the results.

Ventral mesencephalic lesions Activity levels Conditioning Threshold of arou~l

VARXOVS investigations on self-stimulation have shown that the response rates are very high when the stimulating electrode is p l a c ~ in the area of the ventral tegmentum and above the interpeduncular nucleus [6, 10, 27, 28, 32, 37]. These rates are often even higher than those obtained from the lateral hypothalamus, of which the median forebrain bundle (M.F.B.) represents the classical anatomical locus of the "reward system".

Recently, the anatomical studies of the M.F.B. that were done using the method of degeneration [16, 29, 38] have been enriched by newer histochemical studies [1, 2, 3, 9, 10, 12, 17] which have shown the existence of catecholamine containing cells in the ventral tegrnental region whose axons constitute a part of the M.F.B.

With these results in mind, we undertook the study of the effects of lesioning the mesencephalic region. This area is bounded ventrally by the interpeduncular nucleus, dorsally by the red nucleus, and laterally by the substantia nigra and the medial lemniscus.

Chronologically, our experiments were done in two stages. The effects of iesioning on various spontaneous and conditioned behaviors were first studied (Experiment 1). In the second stage, the effects of these same lesions on the level of excitability of the hypothalamus were studied (Experiment II).

EXPERIMENT I

I. M E T H O D S

Animals

Forty-nine male rats of the Sprague-Dawley strain were used. Their average weight was 290 g at the time of the operation.

Stereotaxic Coordinates and Extent o f the Lesions The definitive coordinates, which had been determined in

the course of preliminary experiments, were the following: 4 mm posterior to bregma, 0,4 nun lateral from the midline, 8,5 mm below the calvarium. These lesions were done under ether anesthesia using a high frequency current. The electrodes were made from stainless steel wire having a dia. of 0,3 ram. The animals were divided into three groups: group I or control group: 14 rats in whom electrodes were inserted bilaterally, but no coagulation was done; group II : 16 rats with unilateraJ lesions, there were 8 rats with lateral lesions (4 rats with lesions on the right, 4 with lesions on the left) and 8 rats with medial lesions; group III : 19 rats all with bilateral lesions.

Behavior Studied After allowing 15 days for post-operative recovery, three

types of behavior were studied successively:

2We wish to thank Mrs. McNeil who was kind enough to translate this article into English.

567

568 LE MOAL, CARDO AND STINUS

Spontaneous activio,. The apparatus consisted of a circular corridor 12cm wide and 1,7 meter long. Four beams of infrared light cross the corridor, 3 cm above the ground. These are placed across the two diameters of the corridor and are perpendicular to each other. Each beam makes contact with a photo-electric cell. When an animal intercepts the beam, he connects a circuit which sends an impulse to a counter. This counter marks the number of impulses received in intervals of 10 rain. Two measures of activity were taken: one, during the day for a i-hr period; the other, during the night and extending over 12 hr. In this last case, the animals had food ( l0 g) and water ad libitum.

Alimentary conditioning. We used a Skinner box such as the one previously described [7, 24]. Three situations were studied in turn: 1st session: acquisition; 2nd session: retention; 3rd session: extinction. In the last case the distributor operated but no longer delivered pellets. Each session lasted 90 rain. In this presentation we will describe only the results obtained in extinction.

Avoidance conditioning. The paradigm used has been described previously [7]. The floor of the conditioning cage consisted of an electrifyable grid separated into two equal compartments by a barrier. The rat rnczived an electric shock from the bars Of the grill (unconditioned stimulus, U.C.S.). This was always preceded by a tone (condi t ion~ stimulus C.S.) with a duration of 5 see. A conditioned response (C.R.), was considered to occur if tim animal jumped over the barrier during the tone. An unconditioned r~ponse (U.C.R.) occurred if the animal jumped the barrier only after the electric shock had been turned on. The apparatus we used al- lowed us to establish a permanent current in the unoccupied compartment during the intervals between trials. Thus, any anticipatory responses of the animal, made before the onset

1000

800

600

400

200

O GROUP fit Nffi16 o/ , , ," L GRouP _ ~ , , / H 71" N=16

~ / 0 " - - - 0 GROUP T Nffi 8

o 1'o 2'o 3'o ,'0 FIG. 1. Cumulative exploratory activity of the three groups of animals (mean of 2 sessions).

abscissa: time in rain. ordinate: mean number of passages across the photoelectric ceil.

of the C.S., were penalized. This penalty was not al~'ays utilized at the beginning of the conditioning procedure. Each daily session consisted of 20 trials. The experimenter noted the number of C.R. 's and the number of anticipatory responses. Only groups 1 and II were used in this test. The groups had been mixed and the rats given new identities. This was done to avoid any possibility of recognition on the part of the experimenter.

I I . R E S U L T S

I. Daytime activity The three groups were tested on this measure on two

occasions. Figure 1 shows the mean activity, recorded cumulatively over a 60 rain period. Note that the control group had the lowest activity levels (601 5=42); the group of rats with bilateral lesions showed the highest activity (989 ± 58), and the unilaterally lesioned group showed an activity level intermediate between that of the other two groups (767+42). The differences between the three groups are statistically significant: F=7,99 df2/37 0.01>p>0.001.

2. Nocturnal Activity Figure 2 summarizes the results. The three groups were

again divided in the same manner as in the former experiment: group II[ : 3876+540, group I[: 2653~290, group [: 2161-t-224; (F=5,18 df 2/37 p=0.01).

4 0 0 0

3 0 0 0

2000

4000

.0-"" .0- - - / ..,7 / H _: . . ,o

~ f f ~ r ~-o-"0 C.,ROUP I N - ' 1 3

i " '1 I 1 i " r "

FIG. 2. Cumulative nocturnal activity of the three groups of animals.

abscissa: time in hr. o r d i n a t e : m e a n n u m b e r o f p a s s a l ~ a c r o s s the p h o t o - e l e c t r i c cel l .

3. Extinction of Alimentary Conditioning Figure 3 represents the cumulative record of the number

of responses given by the animals in the different groups during the extinction session (90 rain.). Once again, the three groups are seen to be divided in the same manner as in the previous experiments. The bilaterally lesioned rats (HI) gave more rmpouses than those with only one lesion (II) and those in turn show higher scores than the control rats: group I I I : 429-4-52, group II : 322+43, group I : 2195=33; (F=38,3 df2/35 p<0.001).

VENTRAL MESENCEPHAL1C LESIONS 569

400

300

2 0 0

I 0 0 _

- / o'

y p., O " /

p "

• .o..o. ~ .o..z~ -°" " ° " ° " " °

o..o" P'" o.--o GROUP 111" N - I 6

e-.--e GROUP "ll" N='I0 o---o GROUP 1" N-12

I ; I 0 30 60 90

FIG. 3. Extinction of alimentary conditioning: cumulative record of r ~ p o n ~ s .

absaiBa: time in rain. ordinaie: mean number of responses.

I 00_

- - • # " ~

SO_ l

d .o..,.<' ",. ,,

, 0 o - - 0 GROUP T N=13 -

O

I I i I I I I I I I I I I i I

0 I 2 3 4 5 6 7 8 9 I01~ 12 '13'14 15

FIG. 4. Avoidance conditioning to a tone: mean number of the conditioned responses of groups I and IH.

ab~i~a : sex~ of 20 trials. ordinate: perccntnge of correct responsa.

4. Avoidance Conditioning In the realm of ganeral observation it should b¢ noted that

rats with bilateral lesions showed behavior that was quite different from that of the controls. This consisted notably of a more thorough exploration of the cage, of more rapid and accurate escape reactions, of the appearance, precociously, of anticipatory responses, despite the penalty that was imposed for this, and of generally a more rapid recuperation after being subjected to different stresses. This general hyperactivity had two opposite consequences: on the one hand, acquisition of an avoidance response by these rats was more rapid and efficient, whereas on the other hand, they all persisted in making quite inadaptive, anticipatory responses.

Figure 4 shows graphically the first of these consequences. It also proves that the rats in group III were conditioned much more rapidly than those in group I. A comparison of tbe mean number of C.R. 's made during each of the 15 sessions of 20 trials, allows us to differentiate quite clearly the performance of each of the groups: the mean number of C.R. 's: group I l I : 178,8+12,5; group I: 122,5+15,6; t=3 ,10; 0.01 >p>0.001 .

Figure 5 gives the mean number of anticipatory responses given during each session of 20 trials. This allows us to say that the bilaterally lesioned rats always gave a grmter number of anticipatory responses than the controls. The mean values calculated for tbe 15 sessions of 20 trials each are, for group I : 12,9+3,7 and for group I I l : 40,S~7,7. The difference between tbese two values is clearly significant: t = 2 , 9 ; p=0.01.

5 _ 0 0 GROUP ]]Z N=lS / (~ / \ / \

3_

'- / ,...o X i", / \ I 'l I ~ # ,- ? i \ / ',._,.<" < . - - < > . . i ° - , o . - D " ~ 5 o " -v

. . . . . . . . . ; ' A I 2 3 4 s 6 7 o 9 o 11 s ",4

FIG. 5. Avoidan~ conditioning to a tone: mean number of anticipatiom for groups I and HI.

a l ~ - i m : ~ of 20 tria~. ordinate: mean number of anticipatory responses.

I IL C O N C L U S I O N

Tbe results obtained from the three preceding tests clearly show disturbances in behavior in the rats with lesions in the ventral tegrnental area which are quite consistent. These disturbances consist of two complementary aspects:

(1) on one hand these animals are definitely hyperactive. This is true both for the activity levels measured during the daily l -hr sessions and for the nocturnal activity; (2) on the other hand, this very hyperactivity seems to make it impossible for the animal to adapt its behavior to the experi- n~-ntal situation: extinction in the Skinner box proved to be

570 LE MOAL, CARDO AND STINUS

very difficult; inadaptive responses that appear during the course of avoidance conditioning procedures do not dis- appear.

In a first estimate of results therefore, we can say that lesions in the ventral tegmental area bring about a general hyperactivity which is also associated with a defective adjustment of the animal's behavior to the imposed experimental conditions.

In order to clarify the type of hyperactivity involved here, we decided to do further experiments to compare the threshold of arousal to central stimulation in control animals with that in animals having ventral tegmental lesions.

EXPERIMENT II

Stimulating and recording electrodes were implanted in certain of the animals from the three previous groups.

I. METHODS

Implantation After anesthetizing the animals with thiopental (80 mg/kg)

a trepanation of the skull was performed and a miniature carrier was implanted under stereotaxic control. The carrier contains: One stimulating electrode consisting of two palla- dium-silver wires which are twisted and enamelled. Each wire has a diameter of 0,17 nun and is bare at the tip for 0,3 to 0,5 mm. The coordinates chosen for implantation were the following: 0,6 mm posterior to bregma, 1,7 mm lateral from the midiine, 8,7 mm below the calvarium (lateral hypothalamus). Recording electrodes made of silver. These are placed on the dura in occipital and in frontal positions.

Eight control animals (group I) and 14 animals with bilateral lesions (group III) received only one stimulating electrode, either on the right or on the left side. In contrast to this, eight of the rats in group II, those with unilateral lesions, received two stimulating electrodes. These were symmetrically placed. The animals surviving the operations were: 8 in group I, 3 in group II and 12 in group III.

Research on the Threshold of Arousal Fifteen days after the operation the animals were placed

into a soundproof enclosure in which the floor was sensitive to changes in pressure. Thus, the slightest movements of the animals were picked up and recorded. Electrocortical activity (E.E.G.) was continuously recorded using the usual techniques. As soon as an animal was in a state of sleep characterized by slow waves for at least 1 rain, the stimulation (sinusoidai current of 250 c/s, duration 4 sec) was turned on. I t was delivered first with progressively increasing and then with decreasing intensities. The value of the threshold was always taken to be that which gave the required response in 50 per cent of the cases [5]. Two types of responses were used as criteria: Sb a desynchronization of the E.E.G. appearing during stimulation and lasting for at least 4 sec after it is turned off. Sll, a desynchronization plus a motor reaction that is as small as possible.

H. RESULTS

1. Study Comparing the Thresholds Obtained in the Rats of Group I and Group III

Table 1 and Figure 6A summarize the results. It should be noted that for every level of threshold studied (SI and SH), the differences observed were clearly significant: (for SI: F=12,7df=l/18p<O.Ol; for SH: F=12df=l /18 p<0.01)

TABLE 1 THRESHOLDS OF EXCITABILITY IN THE LATERAL HYPOTHALAMUS.

COMPARISON OF GROUPS [ AND l I I .

Mean threshold Group Reaction N (~A)

I SI 8 17+2,7 SH 8 20 4- 3,2

III St 12 9,3 4-0,7 SH 12 12+0,8

30.

20_

10.

Sl

C~I ~K

B t +

c i c i

51I

FIG. 6. (A) Mean values of the E.E.G. (S0 and behavioral (SII) thresholds of group I ((3. I) and group l l I (G.III).

ordinate: intensity in ~amps. group I: N = 8 group III: N=12

(B) Mean values of E.E.G. (SI) and behavioral (SII) thresholds of 3 rats in group II.

c: intensity of stimulation utilizing the electrode ¢ontralateral to the lesion; i: intensity using the ipsilateral electrode.

Study of the Thresholds of Rats in Group II One cannot draw definite conclusions from this experiment

since only three animals survived. However, it is still in- teresting since it does permit a comparison of the thresholds of arousal between the two sides of the same animal. I t

TABLE 2 Co~0PAa~SON Ot~ "mE Ttm~.st~t.Ds OF ExcrrAan.n'Y IN ~E

LKrI~,AL HYPOT~,LXMUS, DEPENDING ON V4HETHER THE STIMUI.&TI~D SIDE WAS CONTRALATERAL OR IPSn~TEgAL TO

T ~ V~ST~L MES~qC~HAUC L~ION (N=3).

Side Mean threshold stimulated Reaction (~.A)

Side contra- $1 27_ 2,8 lateral to the lesion. SR 34 ~ 6,7 Side ipsi- S[ 13,64-2,9 lateral to the lesion SII 16,6:i:3,5

VENTRAL MESENCEPHALIC LESIONS 571

should be kept in mind that there is a lesion in the ventral tegmentum behind only one of the stimulating electrodes. It should also be mentioned that during the determination of the effective thresholds of each electrode, the experimenter was ignorant of whether the electrode being used was contralateral or ipsilateral to the lesion. Table 2 and Figure 6B show that the thresholds S! and Si! are systematically lower when the electrode is ipsilateral to the lesion than when it is contralateral; (for Si: F = 2 1 df 1/4 p=0 .Ol ; for Sll: F=7 ,25 df 1/4 p=0.05).

I l L C O N C L U S I O N

Both of the preceding experiments lead us to the same conclusion: the destruction of the ventral tegrnental area significantly decreases the threshold of excitability of the lateral hypothalamus to local electrical stimulation. Our second experiment also tends to indicate that the influence of the destroyed area on the lateral hypothalamus is exerted via direct pathways rather than by crossed ones.

FR / A 2'I~3)J\DTV

FIG. 8. Schematic representation of frontal sections showing the lesions of the rats in group l l I (N= 19).

H I S T O L O G I C A L CONTROL O F EXPERIMENTS I AND II

At the end of the experiments, the brains were removed and fixed in 10% neutral formalin. Frozen sections were made of 80g and stained with thionine.

Histological verification was done for the 8 rats in group I, the 15 in group II and the 19 in group III. The results of this control are summarized in Figs. 7, 8, 9 and 10.

FIG. 7. Schematic representation of frontal sections showing the lesions of the rats in group II (N=13) (Schemas are taken from Konig and Klippel, [23].)

AC: acqueductus cerebri (sylvii), DTD: decussatio tegrnenti dorsalis, DTV: decussatio tegrnenti ventralis, FLM: fasciculus longitudinalis medialis, FOR: formatio reticularis, FR: fasciculus retroflexus, IP: nucleus interpeduncularis, LC: nucleus linearis (pars caudalis), LM: lemniscus medialis, NR: nucleus ruher, pIlI: nucleus principalis n. oculomotorii, PCMA: pedunculus corporis mamiUaris, PCS: pedunculus cerehellaris superior, SNR: substantia

nigra.

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FIG. 9. Frontal section at the level of the hypothalan~ ventro- medial nucleus showing the placement of stimulating eh~odes . Group I: N = 8 Group III: N = I 2

AK: nucleus arcuatus (hypothalami), FM: nucleus paraventri- cularis (hypothalami), FMP: fasciculus medialis pmsenoephali, l iD : nucleus dorsomedialis (hypothalami), HVM: nucleus ventromedialis (hypothalami) RE: nucleus reuniens, Vm: ventriculus tertius.

Figure 7 indicates the position of the unilateral and medial lesions of group II. Figure 8 shows the extent of the bilateral lesions of group II[. The photographic reproduction in Fig. 9 shows the placement of the stimulating electrodes in the group I (control) and group II I (bilaterally lesioned rats). The Fig. 10 is composed of two parts: the first is a photo- graph indicating the position of the two stimulating electrodes in each of the three surviving rats of group II ; and the second is three drawings reproducing the unilateral lesions of each of these animals.

Note that the size of the lesions was limited in all cases and that the in to~,duncular nucleus was almost always left intact.

572 LE MEAL, CARDO AND SI'INUS

" \ t !

"~ ~ _ , , ~ AC -

I¢ 41. O

RAT Nt1~14 RAT N'~1216 RAT Nt1217

~r:, : . . ?:.~ .:, •

'" " It ,'<~-/" i; *~, '

" o N ~" - -~ '~*~ : , ;~ , :'~ v . ~.

FIG. 10. Above: Scl~matic representation of tl~ unilateral lesions of the 3 rats with double implants in the lateral hypothalamus.

Below: Localisation of the bilateral el~.rod~,

DISCU~ON

(1) The disturhances produced by ventral mesencephalic lesions in the rat can be classified into four patterns:

(a) Hyperactivity. This is the clearest symptom that emerges from all the behavior that have been studied, daytime exploratory activity, nocturnal activity, Skinner box, and avoidance conditioning. Our results agr¢~ well with those of Carisson [18], who obtained quite exaggerated jumping reactions after he lesioned the mesencephalic-diencephalic junction in rats, as weU as with those of Anden et aL [31 in whose studies animals with similar lesions show a marked increase in reactivity to painful and to auditory stimuli. Certain of our results, and particularly the unusual reactivity of our rats to auditory and painful stimuli, such as was seen in the course of avvrsive conditioning, speak equally in favor of a hyper-reactivity to external stimuli. This point however merits confirmation by further study. It is also important to note that this hyperactivity is confirmed by the diminution in the threshold of arousal by electrical stimulation of the lateral hypothalamus. This result indicates a definite influence of ventral mesencephalic structures on the M.F.B. (b) Di.~iculty of behavioral adiustment. Both the decreased capacity for extinction in the Skinner box and the persistence of anticipatory responses in the avoidance situation, allow us to define a second symptom, namely, a defective adjustment to the imposed experimental situation despite the fact that the lesioned animals' capacity for learning seems to be superior to that of the controls'.

Certain investigations [i 1, 12, 15] have shown that lesions in the ventral mesencephalic region in rats markedly alter their performance in the course of conditioned avoidance tasks. These conclusions do not s ~ m to be in agreement with ours. However, it should be noted, that tbe lesions reported by these authors were much larger than ours. In

addition, the effects of lesioning were only tested in a retention situation. Lastly, the authors made use of programmed sequences in their paradigm, a condition which was certainly much more constraining for the animals. Thus it is possible, that the deficiency in adjustment of behavior shown by our eXl~'iments, could be responsible for the disturbances reported by the~ authors.

(c) Hypoemotivity. Our results in this domain are not definitively significant. However the quietness of the animals, the absence of fear reactions, the scarcity of defecation, and the negligible effect of the numerous penalties that were imposed for anticipatory responses in the course of avoidance conditioning, clearly argue in favor of a hypoemotive state.

(d) Absence o]" vicariance. None of the symptoms that appeared, and in particular, that of hyper-reactivity, regressed at all during the entire course of the experiment which lasted 190 days.

In summary, a definite syndrome, that seems due to these lesions, exists. This syndrome is defined by the following tb.re¢ permanent symptoms: hyperactivity, defect in adjustment to the experimental situation, and hypocmotivity. This syndrome bears a marked resemblance to that seen after lesions of certain limbic structures [22, 31]. (2) The interpretation of our results can only be made with caution in so far as the region that was destroyed is rather complex and consists of several nuclei, of numerous, well individualized tracts, and of diffusely distributed ascending and descending fibres as well [4].

(a) The red nucleus and the interpeduncular nucleus were left intact, although the former was sotr~times displayed downward. In contrast to this, the linear nucleus [4] (caudal part), and the nucleus of the posterior commissure, which has connections to the reticule-spinal tract, were destroyed. Also the catecholamine containing neuronal system, whose existence has been proven by recent histocbemical methods using fluorescence techniques [9, 10, 14, 17, 18, 20, 26], was destroyed. More specifically, the region A,o of Dahlstrom and Fuxe [9] was destroyed.

(b) Certain other well defined tracts were partially or completely destroyed: specifically, the fasciculus retroflexus. interp~uncular-togrncntal, rubro-spinal and tecto-rubral tracts, the decussation of the superior cerebral peduncle, and fibres of the IIIrd cranial nerve.

(c) And finally, more diffuse fibres of the mesencephalic- diencephalic junction we, re also destroyed. Both techniques of degeneration and specific electrophysiological results have clarif-~d the reciprocal rdat ionshi~ that connect the mesencephalic t e x t u r e with the hypothalamus and also with certain limbic structures, n',ost often via the M.F.B., by means of a system of ascending [13, 16, 30, 34, 38] as well as descending [16, 21, 29, 36, 38] connections.

Given the complexity of the destroyed region, it is not possible to associate the observed syndrome with the destruction of any one particular structure. We cannot be sure that the effects reported result from the destruction of local neuronal populations or from the interruption of afferent fibres in the area of the cerebral trunk which later rejoin the M.F.B.

In order to clarify this point, further experiments utilizing other techniques are necessary.

VENTRAL MESENCEPttALIC LESIONS 573

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influence of ventral mesencephalic lesions on various spontaneous and conditioned behaviors in the...

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