Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

EXPERIMENTAL

98,103-

NEUROLOGY

109 ( 1987)

Hypothalamic Afferent Connections Mediating Adrenocortical Responses That Follow Hippocampal Stimulation SHAUL FELDMAN, Department

of Neurology,

DAVID

SAPHIER, AND NISSIM CONFORTI’

Hadassah University Hospital and Hebrew Medical School, Jerusalem, Israel 91 120 Received

February

University-Hadassah

25, 1987

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fomix and lateral sep tal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic dealferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hip pocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fomix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response. 0 1987 Academic press, I~C. INTRODUCTION

Previous studies from this laboratory delineated the role of anterior and posterior hypothalamic inputs as well as of the medial forebrain bundle (MFB) in mediating adrenocortical responses that follow hippocampal stimulation (2-4). Those experiments indicated that the dorsal hippocampal impulses arrive in the hypothalamus via an anterior input and the MFB. However, the inhibitory effect of posterior hypothalamic deafferentation suggested that either the neural cues that arrive from the hippocampus are propagated via the MFB posteriorly and reenter the medial hypothalamus Abbreviations: CS-corticosterone, MFB-medial forebrain bundle, MRF-midbrain reticular formation, PVN-paraventricular nucleus of the hypothalamus. ’ This research was supported by the Lena P. Harvey Endowment Fund for Neurological Research. The technical assistance of A. Itzik and E. Reinhartz is gratefully acknowledged. 103 0014-4886/87 $3.00 Copyright @ 1987 by Academic Press, Inc. All rights of reproduction in any form reserved.

104

FELDMAN,

SAPHIER, AND CONFORTI

SCh.n

LHA

VMN Arc

N

III v

PHA

2.0 lnm I I

2.8mm 1.8rnm

I I I

FIG. 1. Schematic representation of posterior hypothalamic dea6erentations. OC-optic chiasm, SCh.n.-suprachiasmatic nucleus, LHA-lateral hypothalamus, VMH-ventromedial hypothalamus, Arc N-arcuate nucleus, III V-third ventricle, PHA-posterior hypothalamus.

from the caudal direction, or alternatively, that a tonic input to the hypothalamus from brain stem structures is necessary in order that full adrenocortical responses be elicitated by hippocampal stimulation. With the purpose of further elucidating the neural pathways involved in the dorsal and ventral hippocampal influences, on corticosterone (CS) secretion, we studied the effects of various brain lesions and hypothalamic deafferentations on this mechanism. MATERIALS

AND

METHODS

The experiments were carried out on male rats of the Hebrew University strain weighing approximately 250 g. They were housed in the animal room of our laboratory in groups of five to six per cage under artificial illumination between 0600 and 1800 h. Purina Chow and water were available ad libitum. Ambient temperature was 22 to 23°C. Posterior hypothalamic deafferentations were carried out according to the methods of Halasz and Pupp (7) with minor modifications. The dimensions of the knives used had a radius of 1.0, 1.4, and 1.9 mm, and a height 2.4 mm. In this preparation a semicircular cut is made around the caudal extreme of the hypothalamus, at the level of the mammillary nuclei (Fig. 1). The dorsal fornix was transected bilater-

HIPPOCAMPUS

AND

ADRENAL

SECRETION

105

ally by lowering stereotaxically from the cortex at the level of the bregma, a blade of 4 mm width. The fimbria was cut bilaterally with an Lshape knife. Electrolytic bilateral lesions were made in the lateral septal nucleus (A 1.7, L 0.8, H -5) and in the mesencephalic reticular formation (MRF) (P 5.5, L 1.5, H -6.0) (12). The current used was 2.5 mA during 10 s. Ether responsiveness of the rats with the brain lesions was verified 1 week postoperatively in all operated groups by examining the plasma CS response to ether stress. The animals were removed from their cages, placed in a jar containing ether, and blood was sampled from the jugular vein within 2 min; this sample provided basal plasma CS values. A second sample was taken 15 min later for the determination of the ether stress value. Approximately 1 week after ether stress, a bipolar stainless-steel electrode was implanted either in the rats’ dorsal (P 3.0, L -3.0, H -4.5) or the ventral hippocampus (P 2.8, L 4.5, H 7.0). One week subsequent to electrode implantation the animals were subjected to brain stimulation. All stimulations were carried out between 0800 and 1100 h. The protocol consisted of injecting pentobarbital(40 mg/kg body weight); 15 min later, stimulation was initiated and was continued for 5 min (0.5 mA, 1 ms, 100/s). Fifteen minutes after onset of stimulation, blood was collected from the jugular vein by acute venesection for CS determinations. For controls, animals were subjected to identical treatment, and blood samples were collected in a similar manner, but the stimulating current was not applied (“sham stimulation”). Plasma CS was determined by the method of Glick et al. (6). Upon completion of an experiment, the rats were killed, the brains were removed and fixed in Formalin, and the deafferentations, lesions, and electrode placements were verified. The results are expressed as group means f SD (absolute CS values, pg%). Statistical comparisons between group means were made with Students’ t test. RESULTS The electrical stimulation of the dorsal hippocampus in intact rats caused a very significant (P < 0.001) increase in plasma CS, which was similar in magnitude to that produced by ether stress (Fig. 2 and Table 1). Transection of the dorsal fornix caused a complete inhibition in the adrenocortical response, as the sham and poststimulation values were not significantly different. Lesions in the lateral septal nucleus blocked very considerably the increase in plasma CS that followed dorsal hippocampal stimulation. However, this inhibition was not complete, as there was a slight difference between sham and poststimulation CS values. Also, a small (1 .O mm) posterior hypothalamic deafferentation caused a very significant inhibition in the increase of the plasma CS. A bilateral MRF lesion did not affect the adrenocortical response following dorsal hippocampal stimulation (Fig. 2).

106

FELDMAN,

SAPHIER, AND CONFORTI DF

N

0

ES *

P
ES

shs

MRF

ShS

m

ShS

PHD A

LSP

NS

P
ES NS

ShS P
ES

ShS

ES

P.402

ShS ES P
FIG. 2. Adrenocortical responses determined by plasma corticosterone (CS) concentrations (Y+ SE) to dorsal hippocampal stimulation in intact rats (N) and in animals with brain lesions. Abbreviations: ShS-sham-stimulated, ES-electrically stimulated, PHD-posterior hypothalamic deafferentation, DF-domal fomix transection, LSP-bilateral lateral septal lesions, MRF-bilateral midbrain reticular formation lesions. The numbers at the base of each column indicate the number of experimental animals. Significance values were determined by Student’s t test. The P values at the base refer to the plasma CS in each group and the asterisk refers to the Pvalue in relation to the intact rats.

Similarly, stimulation of the ventral hippocampus produced a very significant (P < 0.00 1) increase in plasma CS in intact rats. Bilateral transection of the fimbria and bilateral lesions in the lateral septal nucleus blocked completely the adrenocortical response to ventral hippocampal stimulation. The large ( 1.9 mm) and medium-size (1.4 mm) posterior hypothalamic deafferentations had a very significant, though not complete inhibitory effect on the

TABLE 1 Plasma Corticosterone Concentrations in Intact Rats and Rats with Lesions’ Treatment’

Normal PHD(I.Omm) PHD (1.4 mm) PHD(1.9 mm) DF FIM LSP MRF

Basal

Ether stress

N

9.0 Lk0.7 6.9 + 0.5 11.0&0.5 7.2 f 0.7 9.1 kO.7 8.6 f 0.6 a.7 -t 0.4 8.7 + 0.5

27.8 f 0.5 24.7 + 0.6 30.7 f 1.2 28.6 k 0.7 27.2 f 0.6 27.9 f 0.6 27.3 k 0.5 26.9 f 0.5

10 14 7 5 10 12 12 IO

a Values are pg,/IO0 ml blood. * Abbreviations: PHD-posterior hypothalamic deaIIerentation, DF-dorsal fimbria, LSP-lateral septum, MRF-mesencephalic reticular formation.

fomix, FIM-

HIPPOCAMPUS N

AND ADRENAL FIM

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LSP

PHD aa

PHD ”

300

LOz 2 g

ShS

q ES * P
2

x

x

lo-

i

OShS ES PcOQol

ShS ES NS

Ill 23 ShS

22 ES NS

ShS ES PCo!m

ShS ES Pcom2

FIG. 3. Adrenocortical response determined by plasma CS concentrations (Yk SE) to ventral hippocampal stimulation in intact rats and in animals with brain lesions. Abbreviations as in Fig. 2, FIM-fimbria transection.

increase of CS concentrations, as the poststimulation values were little higher than those obtained in the sham-stimulated rats (Fig. 3). As demonstrated in Table 1 the adrenocortical responses to ether stress in rats with dorsal fornix, fimbria, septal, and MRF lesions as well as with various hypothalamic deafferentations, did not differ from those in intact rats. DISCUSSION The hippocampus has been demonstrated as having both facilitatory and inhibitory effects on plasma CS depending among other factors, on the prestimulus levels of adrenocortical secretion (17). In our previous studies, (3, 4), in rats chronically implanted under pentobarbital anesthesia in which the basal plasma CS was relatively low, we found that both dorsal and ventral hippocampal stimulation produced an initial rise in plasma CS concentrations. This was also found in unanesthetized rats, following dorsal or ventral hippocampal stimulation (1). Therefore, we used this poststimulatory initial increase in plasma CS as an index of hippocampal effects on adrenocortical secretion. The present experiments indicate that the dorsal fornix and fimbria mediate the adrenocortical responses that follow stimulation of the dorsal and ventral hippocampus, respectively. Furthermore, the lateral septal nucleus is a relay nucleus in the transmission of neural impulses which increase plasma CS following dorsal and ventral hippocampal stimulation. These neuroendocrine findings are in accordance with anatomic data. Morphological studies with the combined use of 3H-amino acid radiography and horseradish peroxidase histochemistry have demonstrated that the fibers from the dorsal hippocampus project through the dorsal fornix to the dorsomedial quadrant

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SAPHIER, AND CONFORTI

of the lateral septal nucleus. The fibers from the ventral hippocampus project through the fimbria to the ventrolateral quadrant of the lateral septal nucleus. Furthermore, the hippocampal projections to the septum terminate in the same region that give rise to hypothalamic projections (8,9). The paraventricular nucleus of the hypothalamus (PVN) has been found in recent years to be the major source of the corticotropin-releasing factor which reaches the median eminence and stimulates ACTH secretion ( 16). In view of this finding it is of interest that both anatomical ( 11, 15) and electrophysiological ( 13, 14) studies have demonstrated lateral septal projections to the PVN, which can certainly mediate the above adrenocortical responses. The lateral septal nucleus projects heavily to the medial septal nucleus and the remaining fibers course along the ipsilateral MFB to terminate in several hypothalamic nuclei (5, 10). On the other hand, the medial septal nucleus projects bilaterally, via a midline route, to the mediobasal hypothalamus including the PVN (5). In our previous studies we demonstrated that bilateral MFB lesions blocked the increase in plasma CS that follows dorsal hippocampal stimulation, but had very little effect after ventral hippocampal stimulation. This indicates that the effects of the dorsal hippocampus on the hypothalamic regulation of adrenocortical secretion are mediated via the lateral septum and the MFB, whereas those of the ventral hippocampus may be transmitted from the lateral to the medial septal nuclei and from there via the midline route to the PVN. In view of our findings (3), that a medium-size posterior hypothalamic deafferentation inhibited adrenocortical responses following dorsal hippocampal stimulation, we confirmed in the present study that a large or a medium-sized posterior hypothalamic deafferentation had the same effect also following ventral hippocampal stimulation. Because in our previous studies the interruption by posterior hypothalamic deafferentation of the caudal propagation of hippocampal impulses in the MFB was considered as a possible explanation of this phenomenon (4), we studied the effects of a small posterior deafferentation, which did not damage the MFB. In these rats, there was still a complete inhibition in the increase of plasma CS after dorsal hippocampal stimulation, suggesting that the hypothalamic input from a caudal structure is essential for this response. With the purpose of identifying the brain stem structure involved, we produced bilateral lesions in the MRF; however, these were without effect on the elevation of plasma corticosterone that follows dorsal hippocampal stimulation. Therefore, further studies are necessary to elucidate this problem. In conclusion, the present experiments demonstrated that the adrenocortical responses after dorsal and ventral hippocampal stimulation, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septal

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nucleus. A posterior hypothalamic input, which does not involve the MFB or the MRF, is also essential for the activation of this adrenocortical response. REFERENCES 1. CASADY, R. L., AND A. N. TAYLOR. 1976. Effect of electrical stimulation of the hippocampus upon corticosteroid levels in the freely-behaving, non stressed rat. Neuroendocrinology20: 68-78. 2. FELDMAN, S. 1985. Neural pathways mediating adrenocortical responses. Fed. Proc. 44:

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