Passive avoidance behavior in lean and obese rats with ventromedial hypothalamic lesions

Physiology & Behamor, Vol 20,

pp 5 7 - 6 5

Pergamon Press and Bram Research Publ, 1978 Pnnted m the U S A

Passive Avoidance Behavior in Lean and Obese Rats with Ventromedial Hypothalamic Lesions BRUCE M KING, CLARK D C A R R I N G T O N AND SEBASTIAN P GROSSMAN 2

Department o f Behavtoral Sciences, Commlttee on Btopsychology, University o f Chwago, 1L 60637 (Received 9 September 1977)

KING, B M, C D CARRINGTON AND S P GROSSMAN Passtve avoidance behavtor m lean and obese rats with ventromedtal hypothalamw leswns PHYSIOL BEHAV 20(1) 57-65, 1978 - Lean and obese rats with ventromedml hypothalamle lesaons performed rehably worse than control ammals m the acqmsmon of a step-down passwe avoidance task However, obese rats performed s~mficantly better than lean VMH anmaals, which consistently leaped off the platform on the second and succeedmg trmls While there were no stgnfficant differences between groups m the acqulslt~on of a step-throngh passive avoidance task, lean and obese rats with VMH lemons took rehably longer than control annnals to reach criterion when an identical step-through response had previously been remforced (pumshment-extmctaon of a one-way conchtaoned avoidance response) Both lean and obese VMH-damaged rats made more pumshed approach responses to water than control ammals fonowmg water-depnvataon to 88% of body we~,ht, but only lean VMH rats made a slgmficantly greater number of p u n ~ e d approach responses to hqmd food than unoperated animals following food-depnvataon to 88% of body weight The number of pumshed consummatory responses appeared to be mfluenced by baselme retake Among the ammals tested m more than one paradigm, there was a slgntfieant posllave correlalaon between the number of punudaed consummatory responses and the number of shocks received dunng pumshment-extanctlon of the one-way CAR, but no relataonslup was observed between the performances m either of these and the step-down avoidance paradigm The Imparted passive avoidance behavaor by rats with VMH lesions Is attributed to both an mablhty to inhibit a prewously remforced response and a change m response tendencies to averslve st~muh Ventromedtal hypothalamus

Passiveavoidance behavior

Obesity

leston-lnduced increase in basehne water and/or food retake Although cholmerglc blockade of the VMH m rats has been reported to increase a pumshed operant response for food reinforcement wxthout affecting ad hb food intake [18], others have found no effect or a further decrease m responding following dtrect apphcatlon of atropine to the VMH [22] We recently reported that the effects of VMH lemons on actwe avoidance behavior depended on the amount of lesion-reduced weight gain at the time of testing [ 12,16] Rats held to preoperative body weight performed as well or better than control ammals in a variety of actwe avoidance paradigms, but obese VMH rats were impaired in the acqmsRlon of all but a simple one-way conditioned avoidance task [ 12] Additionally, while rats with VMH lesions emitted more pumshed mtertnal interval (ITI) crossings than control ammals m a two-way condlhoned avoidance paradigm, it was observed that lean VMH rats made significantly more crossings than obese rats, suggestmg that lean and obese rats with VMH lesions may differ m passive as well as actwe avoidance behavior The present series of experiments exarmned the behawor of both lean and obese rats with VMH lesions m a variety of

NUMEROUS studies have reported that damage to the ventromedlal hypothalamus reduces the suppressant effects of pumshment on behavior [4, 5, 7, 10, 12, 16, 26, 28] Many hypotheses have been advanced to explain this effect of VMH lemons, including deficient fear [3,8], a deficit m response inhibition [28,32], and exclusive rehance on external stlmuh for the control of behavior [23] With only one exception [5], however, studies reporting an increase m punished behavior by VMH-damaged animals have employed only two types of passive avoidance paradigms, that m which an approach response to water is punished [7, 10, 26, 28], and shuttle box avoidance tasks m which mtertnal interval crossings are pumshed [4, 12, 16] While increased responding m these two paradigms may indeed reflect a defxclt in passive avoidance behavior, it should be noted that VMH lesions often increase the water/food ratlo [ 13, 15, 26], and results of recent experiments suggest that the impaired passwe avoidance behavior observed m the water-shock paradigm may be at least partially due to an increase m drive level 0 e , thtrst) underlying the approach response [26,28] Such findings suggest a need for examining the behavior of VMH-ablated animals in paradigms wluch ehmmate the possible confounding effects of a

t The authors thank George F Albeld for lus asslstance wlth the mstrumentatlon 2Support for thls research was provlded by USPHS Grant MH-26934 to SebasUan P Grossman Requests for reprints should be addressed to S P Grossman, Department of Behavloral Sclences, Unlverslty of Chlcago, 5848 South Umverslty Avenue, Chlcago, IL 60637 57

58

KING, CARRINGTON AND GROSSMAN

passive avoidance paradigms, including one m which approach responses to food or water were pumshed, and several others whtch did not revolve consummatory responses G E N E R A L METHOD

Antmals One-hundred female Long-Evans hooded rats (Slmonsen Laboratories, Gilroy, CA) were used The ammals were 1 0 0 - 1 3 0 days old and weighed 2 7 3 - 3 8 5 g at the begmnmg of the experiment All ammals were individually caged m a temperature controlled colony ( 2 1 - 2 4 ° C ) with a 1 2 h r hght/dark cycle throughout the course of the experiment

Surgery and Htstology Bilateral ventromedlal hypothalamlc lestons were produced under sodium pentobarbltal (Nembutal) anesthesia (50 mg/kg) by passing a 2 0 mA anodal current between the 0 5 mm unmsulated hp of a teflon insulated stainless steel electrode (No 1 insect pro) and a rectal cathode for 20 sec With the upper mc:sor bar pos:honed 5 mm above the mteraural hne, the electrodes were stereotaxlcally pomtloned 0 8 mm posterior to bregma, 0 7 mm lateral to the mldsaglttal suture, and 10 0 mm below the surface of the skull Upon complet:on of the experiment, ammals with VMH lesions were anesthetized and mtracard:ally perfused w:th lSotomc sahne followed by a 10% formol sahne solution Htstologlcal analyms was performed by hght microscopic examination of cresyl violet stained 80 u coronal sechons, cut on a freezing mtcrotome The atlas of PeUegrmo and Cushman [20] was used m estimating the extent of the lesions Histological examination revealed that all of the operated ammals suffered extenswe bilateral damage to the ventromedml hypothalamus Half of the animals had addmonal unilateral or bilateral damage to the dorsome&al hypothalamus, but there was no difference m the extent of the damage between the lean and obese groups m any of the experiments There were also no apparent d:fferences m the avoidance behavior of rats with les~ons restricted to the ventromedlal hypothalamus and rats with addlhonal damage to the dorsomedml hypothalamus The lesions were bordered laterally by the formx and rarely extended beyond the rostral or caudal borders of the VMN Two representatwe lemons are shown m Fig 1 EXPERIMENT 1 The first experiment examined the effects of ventrome&al hypothalamlc lesions on the acqu:sltlon of a step-down passive avoidance task METHOD

Antmals Fourteen unoperated control rats and 27 rats with VMH lesions were used Ammals with lesions were used only ff they displayed a minimum weight gain of 12 g per day for the first three days followang leslonmg

Apparatus The ammals were tested m a Plexlglas chamber (46 × 46 × 62 cm high) w:th a grid floor conmstmg of 0 4 cm dm

FIG 1 Two representatwe examples of lesions of the ventromedlal hypothalamus stamless steel rods mounted 1 8 cm apart A wooden platform (12 7 x 12 7 x 8 9 cm high), which served as the safe area, was firmly attached against the middle of one of the Plexlglas walls (thus the ammals could step down from three sides of the platform) Shock (0 85 mA) was dehvered through the grid floor by a Lehigh Valley Electromcs constant current shocker and scrambler (Model 1311 )

Pro cedure The ammals were dxv:ded into three groups an unoperated control group (n = 14), a lean lesioned group maintained at preoperatwe body weight by restricting the daily diet (the animals were deprwed after first determining if they met the minimum criterion for weight gain) (n = 10), and an obese lesioned group allowed ad hb feeding throughout the experiment (n = 17) The lean VMH-damaged ammals lost all of their lesion-reduced weight gain within two days of complete food deprwatlon and were rece:vmg quannt~es of food equal to thetr preoperative retake for several days prior to the start of testing 10 days postoperattvely Obese lesioned ammals began testing after 35 days of ad hb feeding Our previous experience has md:cated that while rats w:th VMH lesions are hyperreactwe to body handhng, they are considerably less reactwe when handled by the proximal

VMH LESIONS AND PASSIVE AVOIDANCE BEHAVIOR end of the tail, and tlus method of handhng was used to transport all animals to and from the Plextglas test chamber On the first day of testing, each rat was placed on the wooden platform with tts body parallel to the back Plextglas wall The antmals recetved a 5 see shock (0 85 mA) when they stepped o f f the platform and all four feet made contact with the grid floor The rat was then removed from the test chamber and returned to its home cage Thts procedure was repeated once a day until the ammal remained on the platform for 5 mln without steppmg down, or for a maxtmum of 4 days Two measurements were used to evaluate performance, the difference between Days 1 and 2 m the latency to step down, and the number of trials to criterion (5 m m ) For purposes of data analysts, rats that dtd not reach criterion were asstgned a score of 5 trials

59

AI 151 -r-

"d ~

|||.m

7 tlJ "~ --J

|

2

CONT

RESULTS AND DISCUSSION The control, and lean and obese rats with VMH lemons weighed an average of 314 7, 321 l and 639 5 g (wetght gain = 295 7 g), respectively, on the first day of testing All of the lean VMH ammals dtsplayed m m t m u m wetght gams of 15 g per day for the ftrst three clays following lestonmg (before being placed on a restricted dtet) Because of a great variabiltty in respondmg, analyses of variance of the latenctes to step down on Days 1 and 2 revealed no statlsttcally mgmficant differences between groups on etther day (see Ftg 2A) An analysts of vanance of the difference m the latenctes to step down between Days 1 and 2 ( D a y 2 - D a y 1), however, was htghly stgmficant (F(2,38) = 5 50, p < 0 01) The control ammals remamed on the platform stgmficantly longer on the second day than on the first ( p < 0 01), while the obese VMH rats showed a nonstgmficant moderate Improvement The lean VMH rats, on the other hand, remamed on the platform an average of 4 sec less the second day than on the first day In fact, only one of the l 0 lean VMH rats dtsplayed an mcrease in latency on the second day (reactung criterion), with seven showing a decrease and two remammg on the platform for only 2 sees as they had on Day 1 Indtvtdual comparisons revealed that controls had stgmficantly greater Day 2 - D a y 1 difference scores than either the lean 09<0 005) or obese (/7<0 05) rats with VMH lesions, while a comparison of the latter two groups revealed no stgmficant difference An analysts of variance of the number of trials to criterion revealed stgmficant differences between groups (F(2,38) = 12 42, p < 0 001), with a mean of 2 71, 4 70 and 3 53 trials for the control, lean and obese rats, respecttvely (see Ftg 2B) Individual comparisons revealed that control rats reached criterion m stgnlficantly fewer trials than etther the lean (/9<0 001) or obese (39<0 05) rats with VMH lemons, while the obese rats reached criterion m fewer trials than the lean rats ( p < 0 005) Only one of the l0 lean VMH rats reached criterion within 4 days, while I l of 17 obese and all 14 control rats reached criterion It should be noted that due to thetr stze, some of the obese VMH rats were qmte awkward and two obese rats that reached criterion on Day 3 actually fell off the platform on Day 2 These results suggest that lean and obese rats with VMH lemons differ m the acqutsttton of passtve as well as active avotdance behavior [ 12] While unoperated rats performed rehably better than either of the operated groups, the obese VMH rats reached criterion m stgnlficantly fewer trials than

T

5| -

|

"." LEAN

I I

I I

2 OBESE

B

--F

n,. MJ

n.-

-F

t,.j

o

3-

i-

m

<

1

I--

I CONT

LEAN

OBESE

FIG 2 Mean latency to step down by Days 1 and 2 (A) and number of trials to criterion (B) by control, and lean and obese rats with VMH lemons m a step-down passive avmdance paradigm the lean VMH antmals We recently reported a mmtlar pattern of results for the number of puntshed mtertnal interval crossings m a shuttle box avoidance paradtgrn [12,16] The difference between groups was parttcularly evadent m the topography of the step-down response of those rats that recewed shock on Days 2 - 4 While most of the control and obese VMH rats that recmved shock on Days 2 - 4 stepped o f f the platform (one foot at a time), as all ammals had done on the first day, 8 of the 9 lean lemoned rats that failed to reach c n t e n o n leaped off the platform (1 e , jumped htgh into the atr as if attempting to escape) on at least two of the last three trials Such behavtor was observed m only 1 of the 9 control and 2 of the 14 obese VMH rats that recewed shock on Days 2, 3 or 4 Many researchers have noted that rats with VMH lemons dtsplay strong escape tendenctes prior to the onset of and following shock [14, 19, 31], whtch m m contrast to the normal freezing response by rats Much of the step-down passtve avoidance deficit by lean VMH rats appears to be the result of such inappropriate escape behaviors (I e , leaping) We recently reported that the mcreased number of pumshed lfltertrial interval crossmgs by rats with VMH lemons m the shuttle box avoidance paradtgm appeared to be related to a general increase m mmilar exaggerated escape behawors [12] The increase m escape responses prior to

60

KING, CARRINGTON AND GROSSMAN

and m response to shock probably also accounts for the deficit m pasmve avoidance recently reported for rats with VMH lemons m a parachgm reqmrmg the cessation of locomotion [ 5 ] EXPERIMENT 2 Several experiments have found that animals with VMH lemons make more pumshed lntertrml interval crossings than controls m a shuttle box avoidance paradigm [4, 12, 16] In the second experiment, a mmilar behavior 0 e , crossing between two compartments) was pumshed under two different conditions, one m which the response had not previously been reinforced (step-through passive avoidance), and one m wluch it had (pumshment-extmctlon of a one-way conditioned avoidance response) METHOD

Ammals In Part 1, 8 unoperated control rats and 16 rats with VMH lemons were used Animals with lemons were used only if they displayed a minimum weight gain of 12 g per day for the fLrst three days following lemonmg In Part 2, 10 of the control rats and 20 of the ammals with VMH lesions from Experiment 1 wore used

Apparatus The rats were tested m a Plexlglas box (53 × 22 × 32 cm lugh) with a grid floor consisting of 0 4 em dia stainless steel rods mounted 1 0 cm apart The box was diwded mto two equal compartments by a partition with an open doorway measuring 10 2 x 12 7 cm tugh In Part 2, an opaque Plexiglas gutllotme shdmg door was used to prevent crossings between the two compartments The start compartment was illuminated by a 7 W l~.ht bulb recessed m an enclosure mounted m the center of the ceiling Shock (0 85 mA) was dehvered through the grid floor by the same Lelugh Valley Electronics constant current shocker and scrambler used m Experiment 1

Procedure Part 1 (step-through pasmve avoidance) The ammals were divided rote three groups an unoperated control group (n --- 8), a lean lesioned group mamtamed at preoperative body weight by rest•ctmg the daily diet (the ammals were deprived after f~rst determining if they met the minimum criterion for weight gain) (n = 8), and an obese lemoned group allowed ad hb feeding throughout the experiment (n = 8) The lean VMH-damaged ammals lost all of their lemon-mduc~d weight gain within two days of complete food deprivation and were receiving quantities of food equal to thetr preoperative mtake for several days prior to the start of testing 10 days postoperatively Obese lemoned ammals began testing after 30 days of ad hb feeding On the first day of testing, each rat was placed m the illuminated start compartment facmg away from the center partition When the animals crossed rote the adjoinmg dark compartment (entxre body through the open doorway), they received footshock (0 85 m A ) u n t i l they escaped back rote the start compartment The tat was then removed from the test chamber and returned to its home cage Tail handling was used to transport the rats to and from the test chamber (see procedure section, Expert-

ment 1) Tlus procedure was repeated once a day until the animal remained m the start compartment for 5 mm, or for a maximum of 3 days Two measurements were used to evaluate performance, the difference between Days 1 and 2 m the latency to step-through, and the number of trials to c n t e n o n (5 mm) For purposes of data analyms, rats that did not reach c n t e n o n were asmgned a score of 4 trials Part 2 (pumshment-extmctlon of a one-way CAR) Ten rats from each of the three groups m Experiment 1 were used The ammals were tested starting 6 - 7 days after the last step-down passwe avoidance trial (begmmng 1 7 - 2 0 and 4 2 - 4 5 days postoperatively for the lean and obese VMH rats, respectively) On the day of testing, the rats were placed rote the darkened start compartment facing toward the center parhtlon Ten sec later, the shdmg door was raised mmultaneously with the illumination of the overhead hght (CS) and the animal recewed continuous footshock (US) if it did not avoid by moving rote the adjoining dark compartment within 6 see The C S - U S combination remained on until the rat had remained m the darkened safe compartment for 15 see, at which time the shdmg door was lowered and the ammal was removed to its home cage A erossmg durmg the 6 see CS interval was scored as an avoidance response even if the ammal later returned to the Illuminated start chamber and recewed shock Ten sec later the rat was removed from its home cage and the procedure was repeated until the animal made rune avoidance responses in I 0 consecutwe trials Training was terminated if the ammal did not reach criterion within 60 trials Pumshment-extmctlon of the one-way avoidance response began immediately after each rat had reached c n t e n o n On the next trml, the rats were placed rote the darkened start compartment as before and 10 see later the shding door was raised simultaneously with the illumination of the overhead light wtuch prewously had served as the CS When the ammal crossed rote the adjoining dark compartment it received continuous footshock until escaping back rote the start compartment, after which the shdmg door was lowered and the ammal returned to its home cage On succeeding trmls, the ammals could avoid receiving shock by remaining m the start compartment for 20 sec after the onset of the overhead hght and ralsmg of the door, after which the door was lowered and the ammal returned to its home cage. Testing was continued until the ammals made rune pasmve avoidance responses m 10 consecutwe tnals Tail handling was used to transport the rats to and from the test chamber throughout the expenment

RESULTS AND DISCUSSION

Part 1 The control, and lean and obese rats with VMH lemons weighed an average of 298 4, 297 6 and 562 3 g (weight gain = 249 2 g), respectively, on the first day of testing Separate analyms of variance of the latencles to stepthrough on Days 1 and 2 revealed no stat~stlcally significant differences between groups on either day (see Fig 3A) Separate analyses of variance also revealed that there were no mgnlficant differences between groups m the latency difference scores (Day 2 - D a y l ) or m the number of trials to criterion (see Fig 3B) All animals except one control and one lean VMH rat reached criterion by the third day It should be noted that a prchmmary study indicated that unhke albino rats, which displayed an aversion to hght,

VMH LESIONS AND PASSIVE AVOIDANCE BEHAVIOR

300 -

61

A

A 20-

200

15 -Ik-

10-

5B

0

'CON~"

LEAN

OBESE

CONT.

LEAN

OBESE

B

03 6 v

o

4-

03

CONT

LEAN

OBESE

FIG 3 Mean latency to step through on Days I and 2 (A) and number of trials to criterion (B) by control, and lean and obese rats wlth VMH lesmns m a step-through passlve avmdance parad~m

hooded rats showed no preference between the dark and illuminated compartment

Part 2 The control, and lean and obese rats with VMH lesmns weighed an average of 319 5, 322 2 and 648 1 g (weight gain = 318 5 g), respectively, on the day of testing The results for the acqmsltion of the one-way active avoidance response are reported elsewhere [12] In brief, there were no significant differences between groups m either the number of tnals (a mean of 29 4, 28 8 and 34 7 trials for the control, lean and obese rats, respectively) or shocks recewed to criterion Both the control and lean VMH rats made more punished return crossings to the start compartment than the obese VMH rats, but this was statistically significant only for the control group One control and one obese VMH rat faded to reach criterion v~thm 60 tnals and were not tested for pumshment-extmction of the one-way avoidance response The results for the remaining 28 animals are shown in Fig 4 An analysis of vanance of the n u m b e r of trials to criterion revealed slgmflcant differences between groups, F(2,25) = 4 0 1 , p < 0 0 5 , (a mean of 11 7, 1 8 2 and 17 1 trials for the control, lean and obese rats, respectively, see

20 FIG 4 Mean number of trmls (A) and shocks (B) to cntenon by control, and lean and obese rats w3th VMH lesions durmg pumshment-extmcUon of a one-way conditioned avoidance response Fig 4A), with individual compansons indicating that the control animals reached cnterion m significantly fewer trials than either the lean (p<0 025) or obese (/7<0 05) rats with VMH lesions There was no slgnlflcant difference between the lean and obese VMH rats An analysls of variance of the n u m b e r of shocks received to c n t e n o n was also significant, F(2,25) = 3 79, p < 0 05, see Fig 4B, wlth mdiwdual comparisons indlcatmg that the lean, but not obese, rats with VMH lesions received significantly more shocks than the control animals (p<0 025) Further analysls revealed a slgnlflcant positive correlation between the number of trials to crlterlon m the actlve and passwe avoidance paradigms for both the lean (r = 89) and obese (r = 86) VMH rats, but not the control animals ( r = - 16), wluch dlsplayed httle Varlabfltty in the acqulsitton of the passive avoidance response (range I 1 - 1 3 trials) Interestmgly, there was no relatlonslup ( r = - 3 1 ) between the performances m the step-down avoidance paradigm (trials

62

KING, CARRINGTON AND GROSSMAN

to criterion or Day 2 - D a y 1 difference scores) and the present experiment The results of these two experiments indicate that the animal's reinforcement history is an important factor m the passive avoidance behavior of rats with VMH lesions VMH-lesmned rats displayed no evidence of a deficit in passive avoidance when they were punished the first time they ever crossed between two compartments (Part 1), but a strong deficit when the same response m the same apparatus had prewously been reinforced (Part 2) These results are congruent with the findings that rats wath VMH lesions make more punished antertnal interval crossings than control animals m a two-way c o n d m o n e d avoidance paradigm where crossings are often reinforced [8, 12, 16], but make only as many or fewer pumshed return crossings to the start compartment than unoperated animals in a one-way active avoidance paradigm where such responses are not reinforced Unlike the step-down and shuttle box avoidance paradigms [12], there was httle evidence of a difference in passive avoidance behavior between lean and obese rats with VMH lesions in either paradigm Although lean, but not obese, VMH rats recewed slgmficantly more shocks to criterion than controls in Part 2, the results suggest that prior reinforcement of the punished response is of greater mfluence on the passive avoidance behavior of obese rats with VMH lesmns than is the previously demonstrated attenuating effects of obesity EXPERIMENT 3 Several experiments have found that rats with VMH lesmns make more pumshed approach responses to water than control animals [7, 10, 26, 28], but there IS some exqdence that the deficit is related to a lesmn-mduced increase in the water/food ratio [26,28], suggesting that the results may have been at least partmlly due to increased motlvatmn Numerous studies have reported that while rats with VMH lesmns are hyperreactwe to adulteration of their water or food supply, the flmcklness is greatest in obese rats [6, 11, 17, 29] Obese rats with VMH lesmns have also been found to be less wllhng than control or lean VMH rats to work for food [ 13,21], suggesting to some that obese VMH rats are less motivated for food than rats of normal body weight Experiment 3 compared the effects of punishmg approach responses to water or food m control, and lean and obese rats with VMH lesions METHOD

Animals Sixty-one of the rats from Experiments 1 and 2 were used for the water-shock paradigm Thlrty-fwe additional ammals were used for the food-shock paradigm Animals with lesmns were used only ff they displayed a minimum weight gain of 12 g per day for the first three days following lesiomng

Apparatus The ammals were tested in a Plexiglas chamber (24 x 24 x 30 cm high) with a stainless-steel mesh floor A cahbrated bottle was mounted on the outside of the front wall with the tube protruding into, but not through (thus preventing accidental body contacts), a 2 0-cm dla hole located 2 5 cm above the floor Constant current DC shock was

produced by a high voltage transformer (600 VAC) connected m series with a large variable resistance and a full wave bridge The output resistance was adjusted to produce 0 4 mA of current whenever the rat completed the circuit between the mesh floor and the tube Each circuit closure was recorded by an automatic counting device

Procedure The animals were dwlded into six groups two unoperated control groups (n = 22 and 12 for the water- and food-shock paradigms, respectwely), two lean lesioned groups maintained at preoperatwe body weight by restrictmg the dally diet (the ammals were deprived after first determining ff they met the minimum criterion for weight gain (n water-shock = 18, n food-shock = 12), and two obese lesioned groups allowed ad hb feeding throughout the experiment (n water-shock = 21, n food-shock = 11) Animals in the water-shock paradlgm were tested starting 10 days after the completion of Experiment 2, while the lean and obese VMH rats in the food-shock paradigm were tested beginning 20 and 35 days following leslonlng, respectively In the food-shock paradigm, animals were fed a hquld diet of sweetened condensed milk Animals in previous water-shock passive avoidance experiments were motwated by an hours-of-deprwatlon criterion [7, 10, 26, 28], but Collier and his colleagues [1,2] have demonstrated that the hunger and thirst motivated behavior of normal animals varies much more consistently wath percentage body weight loss In the present experiment, ammals were either food-deprwed (water ad lib) or water-deprived (food ad hb) to 88% of their body weight Animals in the food-shock paradigm were fed the hquld diet for the first time throughout the 24 hr period immediately preceding the onset of deprwatmn All the rats readily consumed the previously novel diet In order to adapt the animals to the test chamber and location of the drinking tube, all the rats were allowed ad lib water or hquld food (water-shock and food-shock rats, respectively) for 1 hr in the test chamber after the first 24 hr of water or food deprwatmn After having reached 88% of their body weight, the ammals were placed in the test chamber and the latency to begin consuming was recorded The shock source was turned on approximately 15 sec later (while each rat was in the process of hckmg) and testing was terminated 10 mm later RESULTS AND DISCUSSION The control, and lean and obese rats with VMH lesions m the water-shock paradigm weighed an average of 323 0, 314 5 and 647 6 g (weight gain = 336 3 g), respectively, just prior to water deprivation The control, lean and obese rats in the food-shock paradigm weighed an average of 349 6, 340 4 and 580 5 g (weight gain = 264 1 g), respectively, just prior to food deprwatlon There were no slgmficant differences between any of the groups in the latency to begin consuming The control, and lean and obese VMH rats in the water-shock paradigm began drinking in an average of 1 60, 2 00 and 2 17 mm, respectively, while the control, lean and obese food-shock animals began feeding after 1 69, 1 46 and 1 5 0 m m , respectively The mean number of shocks received by the animals in each of the six groups is presented m Fig 5 A two-way analysis of variance revealed a highly slgmficant Conditions effect (F(1,90) = 6 44, p < 0 025), indicating

VMH LESIONS AND PASSIVE AVOIDANCE BEHAVIOR

10

T

B

t

°

I I

li

1

P

D v

I

(O

o" ! " O9

s-

-I-

4

63 than the obese rats (mtake of obese rats just prior to deprivation compared to mtake of lean rats just follovang lesmnmg), but both groups showed eqmvalent increases in the mean water/food ratio, this pattern of results is congruent wtth earlier suggestions that the deficit m passive avoidance in such paradigm is partially due to increased basehne retake [26,281 However, mcreased food and water retakes do not entirely account for the passwe avoidance deficit displayed by VMH-damaged rats m paradigms m wluch consummatory responses are pumshed The rats with VMH lesions m the water-shock paradigm also displayed deficits in step-down passwe avoidance (Expenment 1) and punxshment-extmctmn of a one-way avotdance response (Expenment 2B) There was a stamtlcally slgmficant correlation between the number of shocks received in the water-shock paradigm and d u n n g pumshment-extmctmn of the one-way avoxdance response (r = 47, dr= 26, p < 0 01), although this relatxonshxp was considerably stronger for the control and lean VMH rats than it was for the obese VMH rats There was no relation, however, between the number of shocks recewed m the water-shock paradigm and performance In the step-down passive avoidance task (r = 17) -

GENERAL DISCUSSION

0

v, CONT

f

w

f

LEAN

w

f

OBESE

FIG 5 Mean number of pumsbed approach responses to water (w) or food (f) by control, and lean and obese rats with VMH lesmns deprived to 88% of body weight that the water-deprived rats received slgmflcantly more shocks than the food-deprived ammals Individual comparisons revealed that the obese VMH water-deprived rats made more punished approach responses than the obese food-deprived rats (P<0 025), while the difference between the water- and food-deprived lean VMH rats approached statlst~cal stgmficance (p<0 10) There was no slgmflcant difference between the two control groups The analysts of variance also revealed a htghly slgmficant Groups effect, F(2,90) = 6 82, /9<0005 Among the animals m the water-shock paradigm, mdlvtdual comparisons revealed that control animals made significantly fewer pumshed approach responses than either the lean (p<0 001) or obese 09<0 002) rats wath VMH lestons, with no difference between the latter two groups Whtle obese rats took 1 - 2 days longer to lose the necessary amount of body wetght, Colher and Lewtsky [2] have demonstrated that the level of thirst-motivated behavior is determined by the percent of weight loss, not by the rate of weight loss Rats that were clearly hyperdlpstc, whether lean or obese, generally made a large number of pumshed approach responses to water One lean VMH rat, for example, had a water/food ratm of 4 77 (controls = 1 8 0 - 2 60) and recewed 72 shocks It should be noted, however, that most of the normodlpslc VMH rats also recetved more shocks than control ammals Indwtdual compansons among the three food-depnved groups revealed that although there was no significant difference between the lean and obese rats with VMH lesmns, only the more hyperphagtc lean VMH rats made more punished approach responses to the hqmd food than the control ammals (p< 0 05) As lean VMH rats displayed higher ad lab food retakes

Several previous studies have demonstrated that rats with lesions of the ventromedial hypothalamus make more pumshed approach responses to water than control animals [7, 10, 26, 28] While two of these studies found that such results were at least partially due to a lesion-reduced increase in basehne water intake [26,28], the present senes of experiments demonstrate, as do prevtous results from our lab [ 12], that VMH-damaged rats also display imparted passive avoidance behavior m paradtgms which do not mvolve consummatory responses VMH-ablated rats were lmpatred m the acqmsltlon of a step-down passwe avoidance task (Experiment 1) and took rehably longer to reach criterion durmg punishment-extraction of a one-way condxtloned avoidance response (Expenment 2) In addition, we have previously observed that rats with VMH lestons make more pumshed mtertnal interval crossings m a shuttle box avoidance task [12,16], and Colpaert and Wlepkema [5] recently reported that VMH rats displayed tmpalred passwe avoidance behavtor m a paradigm requtrmg cessation of all horizontal movements It may be concluded, therefore, that rats with VMH lesions are lmpatred m a wide vartety of passive avoidance paradigms Several factors, however, appear to influence the pattern of results within any particular paradigm As previously reported [26,28], the number of pumshed consummatory responses was mfluenced by baseline mtake (Experiment 3) Extremely hyperdtpstc rats wath VMH lestons generally made more pumshed approach responses to water than normodlpstc VMH animals, and only the lean VMH rats made more pumshed approach responses to food than control ammals A difference between lean and obese ammals, however, was also observed m the step-down paradigm, w~th lean VMH rats performing worse than obese antmals, and only the lean rats recewed more shocks than control ammals durmg pumshment-extmctton of the oneway CAR As lean VMH rats make more pumshed ITI crossmgs than obese rats m a shuttle box avotdance paradigm [ 12], and more pumshed return crossings during acqmsttlon of a one-way avoidance task [12], it may be

64

KING, C A R R I N G T O N A N D G R O S S M A N

concluded that obese rats w~th VMH les~ons are generally less impaired m passxve avoidance b e h a w o r than are lean VMH rats This does n o t appear to be due to a difference in postoperatwe recovery time, for obese rats make fewer punished responses than lean VMH animals even when b o t h groups are tested after an equal duration of recovery [ 12,16] It should be n o t e d that unlike hyperphagda, o t h e r VMH lesion-induced behavioral changes generally show httle attenuation w~th time [ 24,27] Several persons have suggested that rats w~th VMH les~ons suffer f r o m a general deficit m response lntublt~on [28,32] The results of the present series of experiments suggest, however, that the mabdaty to mh~b~t responding is considerably greater in s~tuat~ons m which a response has a hxgh p r o b a b d l t y of occurrence Rats with VMH lesions displayed no ~mpalrment m the acqmslt~on of a novel step-through avoidance response ( E x p e r i m e n t 2A), but faded to mlub~t responding when the same step-through response had prewously been reinforced ( E x p e r i m e n t 2B) S~mflarly, Smgh [28] r e p o r t e d that whale VMH-ablated rats learned a position m a T-maze sooner than controls, thus displaying more rap~d mhxb~t~on o f n o n r e l n f o r c e d responses, t h e y made a s~gnlflcantly greater n u m b e r o f errors during reversal training It should be n o t e d that the response that was pumshed m the c o n s u m m a t o r y shock paradigm (1 e , h c k m g a d n n k m g tube) was a response for which the ammals had been reinforced t h r o u g h o u t their hfetxme (all animals had drinking tubes protruding into their h o m e cage) A m o n g those rats that were observed m more than one paradigm, there was a statistically s~gmflcant relationship b e t w e e n the performances m the consummatory-shock and pumshment-extmct~on tests, suggestmg that the deficits by rats with VMH les~ons in these two paradigms were at least partially due to a c o m m o n dysfunction We beheve th~s dysfunction to be an mabd~ty to mlublt a response with high habit-strength Such a dysfunction, of course, can not account for the impaired acquisition of step-down avoidance behavior (where a novel and previous u n r e m f o r c e d response was punished), and indeed, no relat~onstup was observed be-

tween the performance m th~s and any of the o t h e r paradigms The step-down paradigm was most notable for the exaggerated leaping behavior by lean VMH rats on the second and succeeding trmls, which resulted m shock This behavior never occurred during the first trial prior to the mltlal shock (all lesioned rats stepped off the platform), and o n l y one of the 10 control ammals ever emitted a similar response With this one exception, control ammals that did not remain on the p l a t f o r m on the second and succeeding trmls always stepped off the platform, and remained relatwely motionless whale on it An ammal's response to averslve st~muh is usually classified into one of three types of behawor, 1 e , ~mmob~hzatlon (often described as freezing), flight, or attack Turner et al [31] r e p o r t e d that while the typical response of n o r m a l rats prior to and m response to shock was lmmoblhzat~on, rats w~th VMH les~ons generally emitted strong escape tendencies (e g , " r e p e a t e d a t t e m p t s to j u m p or c h m b out of the test apparatus") We have observed s~mdar responses to shock by rats w~th VMH les~ons [ 12,14], and Panksepp [ 19] reported that operated rats ran bhndly off the edge o f a table followang an averslve stimulus A l t h o u g h VMH lesions generally result m decreased actlwty [ 9 , 3 0 ] , VMH-ablated rats have been reported to be h y p e r a c t w e during an m~tlal exposure in a novel open field [7, 8, 25] Unoperated rats usually d~splay a transxent ~mmobthzat~on m such s~tuatlons In brief, ~t appears that one effect of VMH lesions is to change the response to averswe st~muh f r o m ~mmobflmatton to fhght (or attack when escape ~s prevented), even when such behavior ~s mapproprlate Our previous observations led us to suggest that inappropriate escape behaviors by rats w~th VMH les~ons c o n t r i b u t e d to the greater n u m b e r of pumshed m t e r t n a l interval crossings m a shuttle box avoidance paradigm [ 12], although prior r e i n f o r c e m e n t of the crossmg response probably also contributes These inappropriate escape behaviors probably also account for m u c h of the ~mpatred p e r f o r m a n c e m b o t h the step-down passive avoidance task and the paradigm r e q u m n g cessation of horizontal m o v e m e n t s [ 5 ]

REFERENCES 1

Colher, G Body weight loss as a measure of motivation m hunger and thtrst Ann N Y Acad Set U S A 157" 594-609, 1969 2 Colher, G and D Levltsky Defense of water balance in rats Behavioral and physiological responses to depletion J comp phystol Psychol 64: 59-67, 1967 3 Colpaert, F C The ventromedml hypothalamus and the control of avoidance behavior and aggression Fear hypothesis versus response-suppression theory of hmblc system function Behav Btol 15" 27-44, 1975 4 Colpaert, F C and M Callens Effects of VMH lesions on CAR learning m cats Phystol Behav 12 893-896, 1974 5 Colpaert, F C and P R Wlepkema Ventromedml hypothalamus Fear condmonmg and passive avoidance m rats Physzol Behav 16 91-95, 1976 6 Corblt, J D Hyperphagle hyperreactlvlty to adulteraUon of drml~ng water with quinine HC1 J comp physlol Psychol 60" 123-124, 1965 7 Green, P C Effects of early vs late lesions on cogmtlveaffectlvebehavlor m rats VMH Psychon Scz 7 11-12, 1967 8 Grossman, S P Aggression, avoidance, and reaction to novel envtronments m female rats with ventromedml hypothalamlc lesions J eomp physzol Psychol 78" 274-283, 1972

9

Hethermgton, A W and S W Ranson The spontaneous activity and food mtake of rats with hypothalamw lesions Am J Physxol 136" 609-617, 1942 10 Kaada, B R , E W Rasmussen and O Kvelm Impaired acqmsltlon of passive avoidance behavior by subcallosal, septal, hypothalamlc, and msular lesions m rats J comp phystol Psychol 55: 661-670, 1962 11 Kennedy, G C The hypothalamlc control of food retake m rats Proc R Soc 137. 535-549, 1950 12 King, B M, G F Alheld and S P Grossman Factors influencing acUve avoidance behavior m rats with ventromedlal hypothalamlc lesions Phystol Behav 18: 901-913, 1977 13 King, B M and M G Gaston Factors influencing the hunger and thirst moUvated behavior of hypothalanuc hyperphagw rats Physlol Behav 16 33-41, 1976 14 Kmg, B M and M G Gaston Impaired free-operant avoidance behavior following ventromedlal hypothalamlc lesions m rats Physzol 8ehav 16: 719-726, 1976 15 Kmg, B M and S P Grossman Response to glucopnvlc and hydratlonal challenges by normal and hypothalamlc hyperphaglc rats Physlol Behav 18" 463-473, 1977 16 King, B M and S P Grossman Impaired and enhanced shuttle box avoidance behavior following ventromedml hypothalamlc lesions m rats Physzol Behav 20: 51-56, 1978

VMH LESIONS AND PASSIVE AVOIDANCE BEHAVIOR 17 18

19 20 21

22

23 24

Krasne, F B Decreased tolerance of hypothalamtc hyperphaglcs to qumme m drinking water Psychon Scz 4: 313-314, 1966 Margules, D L and L Stem Chohnexglc synapses m the ventromedlal hypothalamus for the suppressmn of operant behavior by pumshment and satiety J comp physiol Psychol 67: 327-335, 1969 Panksepp, J Effects of hypothalarmc lesions on mouse-kflhng and shock-reduced fighting m rats Phystol Behav 6: 311-316, 1971 Pellegnno, L J and A J Cushman A Stereotaxw Atlas o f the Rat Brain New York Appleton-Century-Crofts, 1967 Porter, J H and J D Allen Food-motivated performance m rats with ventromedlal hypothalarmc lesions Effects of body weight, depnvataon, and preoperative training Behav Bzol 19: 238-254, 1977 Ross, J F , L J McDermott and S P Grossman Dlsmlubltory effects of mtraluppocampal or mtrahypothalanuc mjecUons of antlcholmerglc compounds m the rat Pharmac Btochem Behav 3: 631-639, 1975 Schachter, S Some extraordinary facts about obese humans and rats Am Psychol 26: 129-144, 1971 Sclafam, A Neural pathways revolved m the ventromedtal hypothalamlc lesion syndrome m the rat J comp physiol Psychol 77: 70-96, 1971

65 25 26 27 28 29 30 31 32

Sclafam, A , J D Belluzzl and S P Grossman Effects of lemons m the hypothalamus and amygdala on feeding behawor m the rat J comp physzol Psychol 72: 394-403, 1970 Sclafanl, A and S P Grossman Reactlv3ty of hyperphaglc and normal rats to quinine and electnc shock Y comp phys~ol Psychol 74: 157-166, 1971 Smgh, D Comparison of hyperemottonahty caused by lesions m the septal and ventromedlal hypothalatmc areas m the rat Psychon Sci 9: 3-4, 1969 Smgh, D Comparison of behav3oral deficits caused by lesions m septal and ventromedlal hypothalamlc areas of female rats J comp physzol Psychol 84: 370-379, 1973 Teltelbaum, P Sensory control of hypothalanuc hyperphagla J comp phystol Psychol 48: 158-163, 1955 Teltelbaum, P Random and food-dtrected activity m hyperphaglc and normal rats Y comp physiol Psychol 50: 486-490, 1957 Turner, S G, J A Sechzer and R A Llebelt Senslttvlty to electric shock after ventromedml hypothalamlc lesions Expl Neurol 19: 236-244, 1967 Wetsman, R N and L W Hamilton Two-way avoidance respondmg followmg VMH lesions Effects of varying shock intensity Phy~ol Behav 9: 243-246, 1972