Physiology & Behavior, Vol. 15, pp. 323--331. Pergamon Press and Brain Research Publ., 1975. Printed in the U.S.A.

Effects of Lateral Hypothalamic Lesions on Consummatory Behavior in Developing R a t s

I

LOY D. LYTLE 2

Massachusetts Institute o f Technology, Cambridge, MA 02139 AND BYRON A. CAMPBELL

Princeton University, Princeton, N J 08540

(Received 24 June 1974)

LYTLE, L. D. AND B. A. CAMPBELL. Effects of lateral hypothalamic lesions on consummatory behavior in developing rats. PHYSIOL. BEHAV. 15(3) 323-331, 1975. - Bilateral lesions aimed at the lateral hypothalami of preweanling rats result in decreased weight gain and eventual death within 5 days after the operation. Ten-day-old rats with unilateral damage to the lateral hypothalamus show retarded growth, food and water intake regulatory deficits, and altered sensitivity to quinine adulteration of the water for up to 60 days of age. These results suggest that at least one neural mechanism that regulates food and water intake matures early in the postnatal life of the rat. Lateral hypothalamus

Consummatory behavior

Development

Feeding and drinking

Brain lesions

METHOD

MANY studies have explored the critical role played by the hypothalamus in the regulation of food and water intake in the adult organism. However, the neural systems controlling feeding and drinking in the adult may not be the same as those mediating these behaviors in the infant. Weanling rats eat solid food before they drink [1], do not respond to dehydration by increasing water intake [ 1 ], and drink only in the presence of food [4,20]. The developing rat does not decrease food intake in response to amphetamine [ 13 ], nor does it increase food intake following injections of insulin [4, 13, 20] or 2-deoxyglucocose [9]. The food and water regulatory deficits observed in weanling rats are strikingly similar to those of adult rats recovering from the effects of damage to the lateral hypothalamus [3, 4, 20, 21]. These parallels have been described previously by Teitelbaum and his co-workers [4,20] and suggest that essential components of the neural mechanisms regulating consummatory behavior may not be functional in the preweanling rat.

Animals One hundred four male and female albino rats (Camm Breeders, N. J. parent stock) bred and reared in our vivarium were used. Each litter was reduced to 6 - 8 pups within 3 days after birth and was otherwise left undisturbed until weaning. All animals were weaned at 21 days of age with the exception of one group (the 20-day-old group), which was weaned at 16 days of age. The animals were either 10, 20, 30, or 50 days old at the time of the operation.

Procedure Ten-day-old animals were left with the mother before and after the operation. Twenty-day-old animals were weaned at 16 days of age on cornflakes (Kellogg's Special K) and water and then fed the normal colony diet (Purina Lab Chow pellets) for the 2 days preceding the operation. All other animals were weaned on the lab chow diet at age 21 days. Surgery was performed under sodium pentobarbital anesthesia (Nembutal Sodium, 50 mg/cc) as required

EXPERIMENT 1 In the first experiment we examined the extent to which lesions of the lateral hypothalamus affect body weight, and consummatory behavior in rats of various ages.

1This paper represents a more complete report of data presented at the Eastern Psychological Association Meeting held in New York, New York in April, 1971. 2Send reprint requests to: Loy D. Lytle, Massachusetts Institute of Technology, Building 37, Room 311, Cambridge, MA 02139. 323

324

LYTLE AND CAMPBELL

(approximately 40 mg/kg). The animal was fixed in a Kopf stereotaxic instrument (Kopf model No. 900 for animals 20 days and older; special Kopf adapted head holder for the youngest group), and bilateral anodal electrolytic lesions (1.2 mA for 15 sec, passed through Teflon-coated 0.009 in. dia. platinum-iridium wire) were made with a constant current power supply. All animals were randomly assigned to 1 of 3 groups: a bilaterally lesioned lateral hypothalamic group, a group in which lesions were made in the same anterior-posterior, medial-lateral plane as the lateral hypothalamic lesioned group but only half way down the dorsal-ventral axis, and a group that was anesthetized and placed in the stereotaxic instrument but was otherwise undisturbed. Coordinates for the lateral hypothalamic lesioned groups varied as a function of age and were derived from pilot experiments: 50 days old: 5.7 mm anterior to intra-aural line (A), 2.0 mm lateral to the midsagittal suture (L), and 7.8 mm ventral to dura (V); 30 days old: A 5.0, L 1.8, V 7.6; 20 days old: A 4.8, L 1.6, V 7.4; 10 days old: 5.7 mm anterior to lambda, L 1.4, V 3.7. Lesions in the 10-day-old group were performed at a 20 ° angle to the skull; all other lesions were accomplished with the skull in the horizontal position. Each group consisted of at least 10 animals at each age (5 males, 5 females), with the exception of the 50-day-old group. This group included 6 animals with bilateral lesions of the lateral hypothalamus, 6 with control lesions, and 2 unanesthetized normals. After recovery from the anesthesia, 10-day-olds were returned to the litter cage with the mother. All animals were weighed daily and the 10-day-old animals were examined visually twice a day for the presence of milk in the stomach. The older animals were housed individually in colony cages with ad lib access to food and water. Body weight, food intake (to the nearest g), and water consumption (to the nearest cc) were measured daily except for the 10-day-olds whose body weights only were determined. Most of the aphagic and adipsic lateral hypothalamic lesioned animals died and their brains were rapidly removed and soaked in Formalin for 2 - 3 weeks prior to sectioning. In other cases, when death was obviously imminent (i.e., when there was no evidence of food or water consumption for at least 5 days, and when body weight had decreased to 6 0 - 7 0 percent of preoperative weight), the aphagic and adipsic lateral hypothalamic lesioned animals were given an overdose of Nembutal and perfused with saline followed by 10 percent Formalin. The appropriate control lesioned littermates were also sacrificed at this time. Brains were removed and serial coronal slices were made at 50 kt on a freezing microtome. Each section through the lesions was stained with cresyl violet and examined microscopically. RESULTS AND DISCUSSION The lateral hypothalamic lesions were localized at the level of the ventromedial nuclei (Figs. 1 and 2). Destruction was confined mostly to the area immediately lateral to the fornices (with occasional unilateral or bilateral damage to these tracts) and extended into the medial aspects of the internal capsule. The lesions extended in some cases as far posteriorly as the mammillary bodies and as far anteriorly as the paraventricular nuclei. Dorsally, destruction included damage to the zona incerta and subthalamus; ventrally, damage occasionally included the ventromedial aspects of the optic tracts. The lesions for the 10-day-old group were localized somewhat more posteriorly than in the other

groups but the extent of damage was quite similar to that noted for the older animals. The damage following the control lesions was confined for the most part to the anterior-ventral aspects of the hippocampus and the dorsal portion of the lateral thalamic nuclei anterior to the lateral habenular nuclei and posterior to the ventral hippocampal commissure. Occasional damage extended medially to the stria medullaris (especially in the 10-day-old group) and laterally to the hippocampal fimbria. The 20-, 30-, and 50-day-old lateral hypothalamic lesioned groups showed the typical aphgic-adipsic syndrome first described in detail by Teitelbaum and Epstein [21]. None of the animals in these groups ate any food or drank any water during the course of the experiment, although no attempt was made to offer the animals anything but ad lib access to the standard laboratory pellet diet and water in their home cages. The 20-day-old animals weighed less than normal suckling rats left with the dam (approximately 80 percent of colony controls) as a result of the early weaning. Animals of this age with lateral hypothalamic lesions rapidly lost weight following the operation whereas animals in the control groups gained weight and continued to eat and drink during the postoperative phase of the experiment. The 10-day-old lateral hypothalamic lesioned animals died at a significantly higher percent weight, F(3,32) = 12.3, p ,~ + 2 0 tic tu 0 O. o t-- - 2 0 z w ¢J-40 fie bJ o_

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FIG. 3. Median percent weight change (top panels), food intake (middle panels), and water intake (bottom panels) for animals with lateral hypothalamic lesions, control lesions, or anesthetized controls (see text). Animals were operated on at 10, 20, 30, or 50 days of age; no food or water intake was determined for the 10-day-old groups. Each point is the median for at least 8 animals per group at each age. The medians are based only on the surviving animals at each postoperative day. Lente Iletin insulin (Eli Lilly and Co.) was administered and weight and food and water intake were again measured over the same time periods. At the completion of the tests the surviving animals were sacrificed and perfused, and histology was performed as described previously. RESULTS

Lesions aimed at the lateral hypothalamus were generally smaller in size and more asymmetrical when compared with those lesions that produced aphagia and adipsia in the 10-day-old lateral hypothalamic group described in Experiment I. However, at least unilateral damage to the lateral hypothalamus was observed for all animals in these groups. The other lesion was often displaced medially and included, on occasion, damage to the fornix, dorsomedial hypothalamic nuclei, dorsal premammillary nuclei, nucleus subparafasciculus, mammillothalamic tract, fasciculus retroflexus at the level of the supramammillary decussations, the anterior hypothalamic nuclei, and the paraventricular nuclei.

The extent of the damage following the control lesions was similar to that observed in Experiment 1. The two control groups (control lesion and anesthetized normals) did not differ significantly from one another on any of the recovery test measures and so were combined into one group for purposes of analysis.

Impaired Weight Regulation. Animals with unilateral damage to the lateral hypothalamus lost more weight than the controls on the first postoperative day but gained weight thereafter; however, the body weights of these animals remained significantly below the weights of the control group throughout the testing period (Table 1). This finding suggests that longterm weight regulation is permanently impaired, and is similar to the reduced weight gain in adult rats with either unilateral or bilateral lesions of the lateral hypothalamus [ 17 ]. Concomitant with the reduced body weights, animals incurring damage to the lateral hypothalamus at age 10 days tended to consume less food and drink less water throughout the course of the experiment (Table 1),

LYTLE AND CAMPBELL

328 TABLE 1

MEAN FOOD AND WATER INTAKE AND WEIGHT GAIN FOLLOWING DAMAGE TO THE LATERAL HYPOTHALAMUS INCURRED AT 10 DAYS OF AGE* Age (in Days) Group

22

30

43

49

55

67

Body Weight (g) Control

52 ± 2*

96 ± 2

107-+ 3

130±4

185 ± 5

221±

Lesion

45 ± 2 t

85 ± 4 t

98 -+ 5

112 ± 55

148 ± 11:~

178 -+ 155

7

Control

5 +- 1

18 ± 2

21 ± 1

23 ± 1

24 ± 1

24 _+ 1

Lesion

4 ± 0.3

16 ± 1

18 ± 2

21 ± I

23 ± 1

21 +_ 11-

Control

7 +- 1

24 ± 1

29 ± 1

23 ± 1

24 +_ 1

33 ± 2

Lesion

6± 1

19 +_ 15

24 ± 2 t

21 ± 1

23 + 1

30±

Food Intake (g)

Water Intake (ml) 4

Values are the body weights after the first day of weaning (22 days of age) or represent the 24 hr ad lib food and water intake for the day immediately preceding the amphetamine anorexia test (beginning at 30 days of age), the hypertonic saline test (at 43 days of age), the prandial drinking test (at 49 days of age), the quinine aversion test (beginning at age 55 days), or the insulin food intake test (beginning at age 67 days). See text for descriptions of these various tests. *All values are the means ± S.E. tp