Neural mechanisms for sodium appetite: hypothalamus positive-hypothalamofugal pathways negative

Physiology andBeluavlor. Vol. 6, pp. 381-389. Pergamon Preaa, 1971. Printed in Great Britain

Neural Mechanisms for Sodium Appetite: Hypothalamus Positive - - Hypothalamofugal Pathways Negative' GEORGE

WOLF

Department of Anatomy, Mount Sinai School o f Medicine, New York, N.Y., U.S.A. (Received 15 M a y 1970) WOLF, G. Neural mechanismsfor sodium appetite: hypothalamuspositive--hypothalamofugalpathways negative. PrIYSlOL. BEHAV. 6 (4) 381--389, 1971.--Previous studies showed that lesions of the lateral or ventromedial hypothalamus impair sodium appetite but that lesions of the anterior or posterior medial forebrain bundle or periventricular system which carry hypothalamofugal fibers have no observable effect. In the present study other known or potential hypothalamofugal pathways were disjoined with no observable effect upon sodium appetite. Lateral hypothalamic lesions completely abolished sodium appetite under the same experimental conditions. It was concluded that no single neural pathway is necessary for transmission of hypothalamic natrorectic functions. In addition to data on sodium appetite, observations on disturbances of feeding behavior following certain subthalamic lesions are described, and motor and motivational functions of the subthalamus and hypothalamus are discussed. Sodium appetite Thirst Electrolytic lesions Hypothalamic feeding area Hypothalamic efferent pathways Extrapyramidal pathways

SoDItrM appetite is, in many ways, an ideal model primary drive system [12, 34, 43] and the study of its neural substrates has recently been receiving increased attention (e.g. [2, 6, 14] and below). Several studies have shown that lesions of the lateral [47] or the ventromedial [27, 29] hypothalamus at the level of the tuber cinerium result in lasting impairments in sodium appetite. We have been attempting to determine the efferent pathways for this hypothalamic function. Lesionsof the medial forebrain bundle [40] or of the periventricular system [41] which carry efferent fibers from the critical tuberal hypothalamic regions did not produce observable impairments of sodium appetite. Furthermore, hypophysectomy did not seriously impair sodium appetite [16] so that the possibility that the hypothalamic information is conveyed to other brain structures via a circuitous route through the pituitary and the blood stream seems unlikely. In the present experiments other possible hypothalamofugal pathways were studied. EXPOSer

Prerubral feeding syndrome

these fibers turn caudally to course to the midbrain. Fibers going to the lateral hypothalamus from the midbrain reticular formation via presumably the same subthalamic trajectory as the hypothalamofugal fibers of Szentagothai et aL have been observed in both degeneration [42, 45] and Golgi [23] studies. The first experiment tests the hypothesis that the efferent pathway necessary for transmission of hypothalamic natrorectic functions passes from the lateral hypothalamus dorsally to the subthalamus to join the ansa lenticularis and enter the midbrain tegmentum where motivational, sensory, and motor information converge [42]. Lesions were placed either in the zona incerta directly above the lateral hypothalamic feeding area or more caudally in the prerubral area to interrupt the proposed hypothalamofugal pathway at two different levels. The sodium intake of rats bearing these lesions was compared to the sodium intake of an operated control group and a group bearing lateral hypothalamic lesions.

Method

1

Adult male Sprague-Dawley rats weighing about 350 g were used. The rats were given 0.5 M NaCI solution and distilled water ad lib in graduated containers with metal spouts which were placed in fixed positions about 3 cm apart on the fronts of the individual cages. The food was Bordens Magnolia condensed milk which was given in a beaker also attached to the front of the cage. Fluid and food intake and body weight were measured daily.

Morgane [24] has suggested that pallidofugal fibers with alimentary functions pass through the far lateral hypothalamus. There is reason to believe that fibers arising in the far lateral hypothalamus may join the pallidofugal fibers and terminate with them in the midbrain. Szentagothai et al. [35] have described fibers which leave the lateral hypothalamus dorsally to enter the zona incerta and have suggested that

xWork done during tenure of an Established Investigatorship of the American Heart Association and supported by Grant No. 411 from the Nutrition Foundation. I acknowledge with gratitude the contributions to comparative phenomenology of my dear teacher Dr. E. E. Krieckhaus. The following papers not included in the References provide data relevant to various ideas presented here. Runnels and Thompson, Brain Res., 1969, 13: 328; Sorenson and Ellison, Expl Neurol., 1970, 29: 162; Grossman and Grossman, J. comp. physiol. Psychol., 1971, 74: 148. 381

382 After a few days when fluid intake was stable (NaC1 intake less than 2 ml and water intake more than 5 ml per day) the rats received bilateral lesions of various diencephalic structures. The control group was comprised of 12 rats with assymetric lesions or lesions of areas previously found to be ineffective [42]. Rats of the experimental groups had bilateral lesions of the lateral hypothalamic feeding area (n = 6), the zona incerta generally above the feeding area (n = 8) or the prerubral area (n : 13). Since the pallidofugal systems and the prerubral fields were originally defined in primate brains, and there is some difference in their topography in the rat it is important to define certain of the terms used here. The prerubral area refers to the caudal subthalamic region traversed by the ansa lenticularis (i.e. rostral pallidal outflow). Thus, the prerubral area in the rat as defined here is directly rostral to the red nucleus and ventral and ventromedial to the medial lenmiscus above the mammillary bodies. Additional information on pallidofugal systems and caudal subthalamic anatomy in the rat is given by Knook [18]. Coordinates with bregma and lambda at the same horizontal plane and current intensities for the various lesion areas were generally as follows. For the lateral hypothalamic feeding area: 2.7 mm behind bregrna, 1.5-2.0 mm lateral, and 8.0 mm below the top of the longitudinal sinus; 0.8 mA 10 sec. For the zona incerta" 2.0-3.0 m m behind bregma, 1.5-2.0 mm lateral, and 6.5-7.0 mm below sinus; 1.0 mA 10 sec. For the prerubral area: 4.5 mm behind bregrna, 1.6 mm lateral and 7.5 mm below sinus; 1.0 mA 10 sec. In all but two cases the lesions were made by passing anodal d.c. through stainless steel insect pins insulated except for 0.5 m m at the tip. The cathode was attached either to the stereotaxic instrument or to a needle inserted under the skin of the back. In rats 456 and 462 the lesions were made by slowly infusing 1 ~1 of 1.0 % formalin into the zona incerta through a 32 ga hypodermic needle held in the electrode carrier. The original intent was to make lesions selective for cell bodies by injecting an agent sufficiently toxic to destroy local perikarya but not myelinated fibers of passage which might be more resistant. As can be seen in Fig. 1B the formalin solution was much too strong and actually caused a cavitation. However, the shapes of the lesions were interesting--tending to conform to the boundaries of the wedge shaped zona incerta. In all rats numbered above 800 the ear bars were inserted into notches drilled in the sides of the skull next to the parieto-interparietal suture (lambda) to avoid damage to the eorda typani nerve [32]. A few days after postoperative fluid intake reached the preoperative criterion the rats were injected subcutaneously with 2.5 ml of 1.5 % formalin to induce sodium depletion [34]. The measure of sodium appetite was the change in sodium intake from the day preceding to the day following formalin injection. Rats which did not eat or drink after lesioning were tube fed until recovery. Baseline daily intakes prior to formalin injection were generally within the following ranges: saline 0-1 ml, water 10-20 ml, and milk 20-30 ml. The rats were sacrificed on the day following formalin injection (generally 3-5 days after the recovery day shown in Table 1) and brains were prepared for histologic analysis by staining 100 ~ thick frozen sections with a modified Kluver method. In selected cases sections were also stained with cresyl violet to better reveal gliotic boundaries and distortions of nuclear groups and with the Weil method to better delineate diffuse fiber pathways. The difference between the present "acute" testing procedure

WOLF TABLE 1 RECOVERYTIMEAND CHANGESIN SODIUMAND WATERINTAKE AFTER SODIUMDEPLETIONFOR INDIVIDUAL RATSOF VARIOUS GROUPS Group Control

Lateral hypothalaruns

Rat No. Recovery Day 1 1 2 1 1 1 1 2 2 1 1 2

4.5 3.0 3.5 2.0 4.0 0.0 3.0 3.5 4.5 9.0 3.0 8.0

10 31 11 7 8 6 8 6 13 27 l1 21

Median

1.0

3.5

10.5

553 562 611 623 625 831

456 462 547 555 557 590 591 609 Median

Prerubral area

Change in H20 (ml)

511 525 580 581 585 586 603 613 616 651 682 683

Median Zona incerta

Change in Na (mEq)

688 689 737 739 749 814 816 818 822 824 827 828 829 Median

2 1 1 4 4 54 3.0 (N.S.)* 1 1 1 5 ! 1 2 1 1.0 (N.S.) 3 11 3 3 5 16 2 12 18 14 7 18 14 11.0 (0.01)

0.0 0.0 0.5 2.5 0.0 0.0 0.0 (0.01) 2.0 6.0 5.5 5.5 3.5 2.5 4.5 -- 1.0 5.0 (N.S.) 4.5 0.0 3.0 0.0 6.0 -0.3

1 8 2 8 --7 --1 1.5 (0.01) -4 14 11 27 10 6 11 -- 1 11.0(N.S.) 5 - 2 32 10 8 --3

7.5 4.3 4.5 2.5 5.0 3.7 1.0

21 10 23 15 19 18 8

3.7 (N.S.)

10.0(N.S.)

*p's in comparison to controls by Mann-Whitney Test. and the "chronic" procedure used in other experiments may be noted (cf. [29, 41]). One purpose of the present sign is to test and sacrifice the animals before too much distortion of the lesions occurs [44]. Thus the rats are adapted to the dietary conditions prior to lesioning and are tested as soon as possible after recovery. Results and Discussion Figure 2 shows projection drawings of sections through the lesions. The lateral hypothalamic lesions were centered in the

FIG. I. Photomicrographs of selected sections from each of the experimental groups of both experiments. (A) Rat 831. The section corresponds to the center projection drawing and shows contracted lateral hypothalamic lesions. (B) Rat 456. The section is intermediate between the center and caudal projection drawings. At this level the lesions are almost totally confined to the boundaries of the zona incerta. (C) Rat 689. The section corresponds to the center projection drawing and shows prerubral lesions which extend ventrally to encapsulate the cerebral peduncle. (D) Rat 828. The section is intermediate between the rostral and center projection drawings and shows more dorsally centered prerubral lesions which do not reach the cerebral peduncle. Note that this section which is rostral to the center of the lesions shows the electrode tracts and demonstrates the point made in the discussion that electrolytic lesions may be drawn towards a caudal cathode. (E) Rat 870. The section corresponds to the center projection drawing and shows lesions which appeared to completely disjoin the hypothalamic input to the inferior thalamic peduncle and the ventral amygdaloid pathway. (F) Rat 820. The section is adjacent to the center projection drawing and was stained with cresyl violet without counterstain to more clearly demonstrate the morphology of a severely contracted prerubral lesion.

(facing page 382)

HYPOTHALAMIC NATRORECTIC PATHWAYS

625

D

l

383

625

831

591

FIG. 2. Lesions of Experiment 1. Projection drawings of Kluver stained sections near the centers of lesions in rats of the control group and near rostral tips, centers, and caudal tips of lesions in rats of each of the 3 experimental groups. A, Control Group. B, Lateral Hypothalamus. C, Zona Incerta. D, Prerubral Area.

lateral hypothalamic area between the ventromedial nucleus and the internal capsule. In each case the lesions encroached on the zona incerta dorsally and spared some midlateral and ventrolateral hypothalamic tissue at the critical tuberal level. The incertal lesions were centered in the zona incerta primarily at the rostrocaudal level of the ventromedial nucleus, but in some cases (e.g. No. 557) the lesions extended as far rostrally as the optic chiasm. While the zona incerta was almost totally destroyed by the chemically induced lesions (No. 456 and No. 462) either lateral or ventral portiOns were spared by the electrolytic lesions. The electrolytic lesions extended dorsally into the ventromedial thalamus to damage primarily the ventromedial thalamic nucleus and the nucleus gelatinosus. The prerubral lesions were centered in the caudal subthalamus at the level of the mammillary body and usually

destroyed most of the region occupied by the ansa lenticularis ([18], p. 48). In most cases the lesions extended dorsally into the thalamus to destroy portions of the ventral posterior nucleus, the posterior nucleus, and the parafascicular nucleus as well as the medial part of the medial lemniscus and other smaller structures in the caudal ventromedial thalamus. In rats 689, 814, 824 and 829 the lesions were more ventral and included Forel's H-2 field, the subthalamic nucleus, and the medial forebrain bundle to variable extents as can be seen in the drawings and photomicrograph. Relevant behavioral data are shown in Table 1. Recovery of preoperative fluid intake levels (saline less than 2 ml and water more than 5 ml) was delayed significantly only in the prerubral group. The delay was due to total absence of drinking rather than to excessive drinking of the saline

384 solution. The adipsia appeared to be due to a motor impairment. Unlike rats manifesting the lateral hypothalamic syndrome these rats, although totally aphagic and adipsic postoperatively, did not disregard the food and water in their cages but made repeated efforts to eat and drink. In the initial postoperative period (one day to a week or more) they scattered and spilled wet mashed food (given in shallow dishes), walked in it, and then attempted to lick it off their paws. Also they would gnaw frantically at the rims of the food containers but failed to ingest any of the food inside---in some cases the animals were at first able to ingest food from a beaker only when it was filled to the very top. This period of total aphagia and adipsia was followed by a fairly consistent pattern of recovery. The rats first ate soft, wet, highly oderous and tasty food such as mashed chili con came in milk given in wide shallow dishes. Then they began to ingest dry powdered chow and water given in separate 50 ml beakers, and finally after a relatively long period they took water from a standard metal drinking spout. Note, however, that there was great variability in the duration of the feeding impairments and that this variability was not closely related to lesion size or placement within the group (cf. Table 1 and Fig. 2). The rats with lateral hypothalamic lesions recovered feeding and drinking rapidly (with the exception of No. 831). Presumably the rapid recovery of most of the rats is attributable to the sparing of some lateral hypothalamic tissue and minimal encroachment of the lesions into adjacent motor areas. The kinds of abnormal feeding behavior typical of the prerubral rats and suggestive of a motor impairment were not noted in any of the rats in this group. It may be noted in Fig. 2 that the lesions of rat 831 seem paradoxically smaller than those of the other rats of this group. This is an artifact of the longer postoperative survival time of this rat which allowed greater contraction of the lesions. The lesions of the zona incerta caused no observable impairment in feeding and drinking thus differentiating the function of this area from that of the lateral hypothalamus ventrally and the prerubral area caudally in this regard. Only the lateral hypothalamic group showed consistent evidence of a deficit in sodium appetite. Rats in this group also did not generally increase their water intake after formalin in accordance with previous findings. Neither the incertal nor the prerubral group showed consistent evidence of a deficit in sodium appetite or hypovolemic thirst. However, the four rats with the most ventrally placed lesions in the prerubral group tended to have the lowest sodium appetite scores.

The results refute the hypothesis that a hypothalamofugal pathway which joins the ansa lenticularis is necessary for transmission of the natrorectic functions of the hypothalanms. While lesions within the lateral hypothalamic feeding area completely abolished sodium appetite large bilaterally symmetrical lesions in the zona incerta above the feeding area or in the prerubral area behind it had no effect upon sodium appetite under the same experimental conditions. It is important to note that the same differential effect was manifested in water intake and thus the conclusions from the results hold for hypovolemic thirst as well as for sodium appetite. The prerubral lesions often destroyed the medial portions of the ventral posterior nuclei of the thalamus and thus included the thalamic taste relay. In a previous experiment [42] it was found that thalamic lesions which included the taste relay and (albeit to a lesser extent) tegrnental lesions just behind the taste relay blocked sodium appetite. Those

WOLF lesions were centered only about 1.5 mm away from the present prerubral lesions. Although there appears to be considerable overlap in the areas destroyed in the two experiments it must be kept in mind that the rats of the previous experiment were not sacrificed until about 3 months after lesioning when the lesions were severely contracted. On the basis of other recent findings [46] we have concluded that the deficit in the previous experiment was not due to destruction of the taste relay but to other structures such as the intralaminar or dorsomedial nuclei which were also involved in those lesions. Nevertheless, the earlier findings with thalamic and tegmental lesions look more and more hairy and the problem requires reinvestigation. EXPERIMENT

2

The second experiment tests two alternative hypotheses. The first hypothesis was suggested by the anatomical studies of Cowen et al. [8] indicating the existence of fibers from the hypthalamus to the amygdala via the substantia innominata and of Millhouse [23] indicating a much more massive hypothalamic input to the inferior thalamic peduncle than was apparent from previous studies. Since these were the only known hypothalamofugal pathways which had not yet been studied it was hypothesized that either or both constitute the necessary efferent pathway. In order to maximize the chances of uncovering an effect both of these pathways as well as the anterior hypothalamic medial forebrain bundle which contributes fibers to them were destroyed in each rat. The second hypthesis was suggested by the results of the preceding experiment in which the four rats with the most ventrally placed prerubral lesions tended to have subnormal sodium intakes. These lesions encroached on the medial forebrain bundle and the H-2 field (lenticular fasciculus) and it seemed possible that inclusion of one of these structures in the lesions induced the deficit in sodium appetite. In the present experiment the prerubral lesions were aimed more ventrally to include these structures as well as the ansa lenticularis. Methods

Operative, recuperative, and testing procedures were identical to those of the first experiment. There were three groups in the present experiment. The control group was comprised of 10 rats with assymetric lesions or lesions of areas previously found to be ineffective. One experimental group of 6 rats received lesions of the substantia innominata (1.2 mm behind bregma, 2.5 mm lateral, and 8.0 mm below sinus--l.0 m A for 10 sec), the inferior thalamic peduncle (1.2 mm behind bregma, 1.3 mm lateral, and 6.5 mm below sinus--l.0 m A for 10 sec) and the medial forebrain bundle at the level of the anterior hypothalamus (1.2 mm behind bregma, 1.3 mm lateral, and 8.0 mm below sinus--l.0 m A for 10 sec). These pathways shall be referred to as the rostral pathways. Since there were three lesions on each side of the brain, the lesions were made in 2-3 stages--usually one side first and the other side a few weeks later. The rats were injected with 5 mg/kg of phenoxybenzamine after the second medial forebrain bundle lesion since we found that this sympathetic blocker improves postoperative recovery from such lesions [40]. A second experimental group of 6 rats received lesions like those of the prerubral group of the first experiment but generally aimed 0.5-0.7 mm more ventrally to include the medial forebrain bundle and the H-2 field. This group shall be referred to as the caudal pathways group. Rats with lateral hypothalamic

HYPOTHALAMIC NATRORECI'IC PATHWAYS

385

lesions were not included in this experiment since it was demonstrated in the preceding experiment that these lesions produce deficits under the present experimental conditions. The control and the caudal pathway groups were usually sacrificed within one week after the recovery day shown on Table 2. The rostral pathways group was not sacrificed until about 3 months later.

TABLE 2 RECOVERYTIME AND CHANGESIN SODIUMAND WATERINTAKE AFTER SODIUMDEPLETIONFOR INDIVIDUALRATSOF VARIOUS GROUPS Group Control

Rostral pathways

Rat No. Recovery Day

Change in H~O (ml)

806 808 809 835t 868 869J885 886 889 897

1 1 1 2 4 3 10 1 2 5

5.0 6.0 5.0 8.0 4.0 7.0 8.0 14.5 4.0 11.5

7.0 10.0 8.0 13.0 7.0 5.0 20.0 4.0 19.0 12.0

Median

2

5.5

9.0

865J" 870 871 873 875 878

5 2 2 2 2 2

6.5 6.0 3.0 2.5 2.5 4.5

19.0 32.0 27.0 5.0 1.0 --16.0

2 (N.S.)*

3.8 (N.S.)

12.0 CN.S.)

6.0 2.5 2.0 6.0 3.0 5.0

18.0 18.0 10.0 12.0 6.0 22.0

4.0 (N.S.)

15.0 (N.S.)

Median Caudal pathways

Change in Na (mEq)

820~: >150 887 20 890 15 893 7 894 6 902 43 Median

18 (0.01)

*p's in comparison to controls by Mann-Whitney Test. tRats 835, 869 and 865 sporadically ingested large amounts of the 0.SM saline after reaching the recovery criterion so the concentration was increased to 1.0M. ~:Rat 820 began to drink water from a beaker 17 days after surgery but never recovered the ability to drink from a spout--the solutions for the appetite test were given in beakers 150 days after operation

Results and Discussion Figure 3 shows projection drawings of sections through the lesions. Lesions of the rostral pathways usually extended rostrocaudally from the anterior commissure to the anterior tip of the ventromedial nucleus and mediolaterally from the lateral edge of anterior hypothalamic nucleus to the medial edge of the anterior amygdaloid nucleus. It is difficult to assess the exact extent of the damage to the three pathways because of contraction of the lesions and distortion of topographical relations. Also the tracts are diffuse so that their boundaries are somewhat obscure. The caudal limits of the fibers ascending from the hypothalamus into the inferior

hypothalamic peduncle and of the fibers turning laterally into the amygdala have not been clearly established. However, on the basis of available published data and of microscopic analysis of serial sections of normal as well as of the lesioned brains I feel quite confident that at least 80 per cent bilateral transection of each pathway was induced in each case. The most caudal fibers entering the substantia innominata and the inferior thalamic peduncle were probably spared in rats 865, 871 and 878, but both pathways appeared completely disjoined in the other 3 rats. The caudal pathway lesions were similar in extent to those induced in the four rats with the most ventral prerubral lesions of the first experiment but were centered somewhat more caudally. Again it is not possible to precisely assess the percentage of damage to the pathways for the reasons discussed above. The ansa lenticularis was probably completely disjoined in rats 820 (severely contracted lesions), 890 and 902. The medial part of the ansa may have been spared unilaterally in the remaining 3 rats. The ventromedial portion of the medial forebrain bundle and the lateral portion of the H-2 field appeared spared at least unilaterally in all six rats. However, damage to these pathways was no less extensive in these rats than in the four rats of the preceding experiment. Table 2 gives the relevant behavioral data. Lesions of the rostral pathways produced no postoperative feeding impairments while lesions of the caudal pathways produced transient or prolonged deficits apparently due to motor impairments as described in the first experiment. Neither the rostral nor the caudal lesions produced impairments in sodium appetite or hypovolemic thirst. As can be seen in Table 2, the increases in sodium and water intake were well within the normal range in both groups. Thus both hypotheses of this experiment are clearly refuted by the results. Neither the inferior thalamic peduncle nor the hypothalamoamygdaloid pathway of Cowan et aL [8] appear to be necessary for the transmission of hypothalamic natrorectic functions since these these pathways were completely disjoined in three rats and severely damaged in another three with no resulting impairment on the sodium appetite test. The caudal pathway lesions also had no effect upon sodium intake. The abnormal behavior of the rats with the ventral prerubral lesions in the first experiment can be attributed to chance factors or possibly to damage to the caudal part of the feeding area. The most rostral sections through the lesions (Fig. 2) are at the level of the ventromedial nucleus and it may be that the actual amount of damage was greater than it appears because the lesions are contracted. GENERAL DISCUSSION

Lesions of either the lateral or the ventromedial hypothalamus at the level of the tuber cinerium completely block the normal increase in sodium intake which is elicited by body sodium deficiency or elevated mineralocorticoid levels in the rat. Rats with lateral hypothalamic lesions manifest no indication of sodium appetite even under the most favorable environmental and strongest motivational conditions. In one study [47] recovered laterals were adrenalectomized and given isotonic saline (which they prefer to plain water) ad lib. They drank the saline when they were fed dry food (presumably to wet their mouths [17]) but they did not drink any saline when they were given liquid food even though they became so severely sodium depleted that circulatory collapse and death ensued.

386

WOLF

889

~

897

~'~/

FIG. 3. Lesions of Experiment 2. Projection drawings of Kluver stained sections near centers of lesions in rats of the control group and near rostral tips, centers, and caudal tips of lesions in rats of the two experimental groups. A, Control Group. B, Rostral Pathways. C, Caudal Pathways.

The deficit in sodium appetite observed after lateral hypothalamic lesions appears to be but one manifestation of a general motivational impairment. The data [11, 33, 36, 40] suggest that there is a loss of all drives innately elicited by specific unhomeostatic conditions. Possibly the motivating functions of interoceptive stimuli in general are mediated by the lateral hypothalamus for recovered laterals apparently do not learn to avoid "poisoned" foods [31 ], and in this type of learning the UCS is an interoceptive stimulus. On the other hand the motivating functions of exteroceptive stimuli are not mediated by the lateral area for rats with lateral hypothalamic lesions manifest, if anything, hyper-responsiveness to painful electric shock [19] and to the taste qualities of foods [36, 39, 47]. Data on conditioned responses to electric shock are, in my opinion, unclear (cf. [4, 7, 39])--possibly deficits are due to extrahypothalamic involvement. Whether or not this general view of lateral hypothalamic function proves to be correct it is obvious that the hypothalamus has some function which is critical for the elicitation of sodium intake in response to sodium need. It is equally obvious that relevant hypothalamic signals must ultimately

affect the activity of the final common motor pathways in order for appropriate ingestive behavior to occur. How are such signals transmitted out of the hypothalamus ? I began the study of this problem by testing the simple hypothesis that a given hypothalamofugal pathway was necessary for the transmission of the signals. This hypothesis was repeatedly rejected. With the exception of a potential, but highly unlikely caudolateral pathway into the cerebral peduncle all known as well as potential pathways out of the hypothalamus have been tested. Destruction of tissue either rostral, caudal, dorsal, or rostrolateral to the critical areas of the hypothalamus as well as ablation of the pituitary gland had no observable effect on sodium appetite ([16, 40, 41] and present results). An alternative hypothesis, suggested by Valenstein [37] for the self-stimulation system, is that there is no single necessary pathway but that there is redundancy or plasticity in the system. Thus there may exist alternate routes which must be destroyed together in order to block the transmission of information. On the other hand, there is reason to suspect that hypothalamic drive signals may be non-axonal. We are most

HYPOTHALAMIC NATRORECTIC PATHWAYS familiar with the kind of axonal-synaptic information transfer which is seen in certain motor control systems or in sensory systems such as vision where the information is exceedingly complex and is presumably coded in a spatiotemporal mosaic composed of the firing patterns of individual units. This kind of coding in which the activity of individual units is critical is, conceivably, not necessary for conveying hypothalamic motivational information since there are presumably only a rather limited number of innate drives which could more simply be coded by gross extracellular potential changes or by chemicals. In fact, recent work by Valenstein's group with stimulus bound behavior [38] is consonant with the idea that the lateral hypothalamus sends out only a single undifferentiated "urge" message which is given specificity elsewhere in the brain. But in any case, on the basis of both empirical and phenomenal evidence primary drives do not seem sufficiently complex to warrant an axonal mosaic coding system. Izquierdo and Merlo [15] found widespread electrotonic propagation of potentials elicited by stimulation of a generator site in the anterior hypothalamus in the rat. Potentials of about 200 uV amplitude could be recorded as far away as thecontralateral neocortex and pons and could be eliminated by placing a glass plate between stimulation and recording sites but not by transections. While it is conceivable that specific changes in the volume conducted background electrical activity might alter the excitability or firing patterns of groups of neurons there is no evidence to support the conception that such activity is of physiological significance. As an alternative to electrical coding, chemical coding is suggested by a rapidly growing literature demonstrating highly specific effects of intracerebral injections of hypothalamic perfusates, neurohumors, and related chemicals upon appetitive behavior [6, 14, 22]. It may be that hypothalamic drive signals are transmitted via chemicals secreted either into (A) the interstitial fluid to reach surrounding structures by diffusion (possibly along "extracellular channels"--[5]) or into (B) a portal system connecting the hypothalamus to specific distant brain structures. Important advances in techniques for severing fiber pathways within the brain have recently been made (e.g. [1, 10]). These techniques, which use knife-like instruments to make slender incisions, allow disjunctions across broad areas and even total neural isolation of entire brain regions. Incision techniques promise to play an important role in further investigations of the general problem of hypothalamic communication pathways and especially in tests of the specific hypotheses outlined above. Certain observations made during the course of this experiment have relevance to the problems of extrahypothalamic functions in feeding behavior [13, 20, 24, 28] and of motor involvement in the lateral hypothalamic syndrome [3, 25, 30]. Lesions of the prerubral area produced a transient postoperative aphagia and adipsia. For purposes of comparison with the lateral hypothalamic syndrome the recovery process following prerubral lesions can be divided into 4 stages as follows: (I) Total aphagia and adipsia. (2) Ingestion of oderous, tasty, soft, wet foods from a wide, shallow container. (3) Ingestion of dry chow (first powdered then pellets) and water from an open container. (4) Ingestion of water from a standard metal drinking spout. While there is some superficial similarity in the stages of

387 recovery from lateral hypothalmic and prerubral lesions, the undei'lying impairments are probably quite different. The feeding and drinking deficits of the prerubral rats appeared to be due to an inability to execute ingestive behavior rather than to a loss of motivation. Direct observation of the lesioned rats (see results of Experiment 1) suggests that while they are able to perform the individual movements necessary to ingest food, they are unable to coordinate sensory and motor functions--i.e, the guiding and releasing functions of sensory stimuli seem inoperative. The rats appeared unable to orient and direct their snouts to the food or to emit ingestive reflexes upon appropriate exteroceptive stimulation. They were more likely to ingest food from large open containers presumably because physical contact and stimulation were maximized [21]. They probably ate soft wet palatable foods before dry lab chow not so much because of the incentive value of the palatability but because of the stronger olfactory and gustatory stimulation which helped guide behavior and elicit appropriate ingestive reflexes and because of the greater ease of mastication and swallowing of a soft wet food. The sequence of foods ingested during recovery from lesions which produce transient aphagia and adipsia obviously provides only a rather crude indication of the underlying functional impairment, but future studies attempting to relate the functional impairments following lateral hypothalamic and extra-hypothalamic lesions can also utilize other kinds of observation [3, 9, 10, 17, 30, 31, 33, 36, 39, 47]. One or more of several fiber systems passing through the prerubral area may be responsbile for the sensori-motor deficit described above. Following Morgane's notions [24] the deficit is attributable to destruction of pallidofugal fibers. The apparent sensory basis of the motor deficit points to damage to a sensory feedback system as another causative factor. The dentato-ruhro-thalamic tract, the classical lemniscal pathways, and the ascending reticular pathways to the subthalamus and thalamus, were all damaged to some extent by the lesions and may each have contributed to the observed deficits. Another ascending pathway which was probably totally destroyed by most of the prerubral lesions and which by virtue of its morphology [45] is conceivably responsible for the conduction of sensory information to the lateral hypothalamus (e.g. [26, 48D is the reticulo-hypothalamic pathway mentioned in the introduction. With regard to motor involvement in the lateral hypothalamic syndrome, I suspect that whenever lateral hypothalamic lesions produce prolonged periods of aphagia and adipsia there is involvement of extrahypothalamic tissue (e.g. subthalamus, internal capsule, entopeduncular nucleus) so that a sensori-motor impairment is superimposed upon the motivational inertia. In my experience, lesions which are restricted to the lateral hypothalamic area never cause more than a few days of aphagia (see also [13]). Extrahypothalamic involvement in the initial stages of the lateral hypothalamic syndrome has probably been overlooked because animals in which the period of aphagia and adipsia is long lasting are necessarily kept alive for a long postoperative period during which time the lesions heal and contract [44]. The original extent of the lesions may be greatly underestimated from their histologic appearance. Subthalamic damage is especially liable to escape notice. Electrolytic lesions centered in the lateral hypothalamus tend to spread dorso-laterally into the cellular region of low electrical resistance between the myelinated medial lemniscus and cerebral peduncle. The lesions tend also to be drawn caudally when the ground electrode is caudal to the head of the animal. During the healing process the

388 cerebral peduncle is drawn upward as the laterally elongated cavity collapses and glial cells disappear so that histology

WOLF reveals only a thin line of gliosis and a symmetrically narrowed subthalamic region whose atrophy may not be recognized.

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