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MODIFICATIONS OF ADRENO~~RTICAL RESPONSES FOLLOWING FRONTAL CORTEX SIMULATION IN

RATS WITH HYPOTHALAMIC DEAFFERENTATIONS AND MEDIAL FOREBRAIN BUNDLE LESIONS

S, FELDMAN* and N. C~NFORT~

Laboratory of ~europhysioto~, Department of Neurology, Hadassah University Hospitai and Hebrew University-Hadassah Medical School, Jerusalem, 91 120, Israel

Abstract-With the purpose of delineating the neural pathways in the rat which mediate adrenocortical responses following frontal cortex stimulation, the effects of partial hypothalamic deafferentations and medial farebrain bundle lesion were studied. In intact and sham-operated animals, cortical stimulation through permanently implanted electrodes caused a significant increase in plasma corticosterone levels. In rats with anterior hypothalamic ~erent~tion and bifateral medial forebrain bundle lesions the adrenal response to cortical stimulation was blocked completely, while in animals with posterior hypothalamic deafferentation there occurred a normal rise in plasma corticosterone.

These studies demonstrate that the frontal cortex effects on adrenocortical secretion are neurally mediated and involve an anterior hypothalamic input, more specifically the medial forebrain bundle.

A variety of peripheral and central neural stimuli can produce an increase in adrenocortical secretion in the rat. These effects are mediated by differc~t neural pathways, as evident by selective hypo- thalamic deaffereutations and extrahypothalamic brain lesions.24 Thus it was found that while anterior, and to a ksser degree posterior, hypothalamic de- afferentation inhibited the adrenocortical response following stimulation of limbic sub-cortical struc- tures, the effects of medial forebrain bundle (MFB) Iesions had a differentiaf effect on this response.5”

in view of the fact that anatomical studies have demonstrate connections between the frontal cortex and the hypothalamus in the rat,” we have studied its effect on the adrenocortical responses, as mediated by the hypothalamus, and the afierent neural pathways involved.

EXP~R~M~T~L PROCEDURES

Experiments were carried out on male rats of the Hebrew University strain weighing approximately 240 g. They were housed in the animal room of our laboratory in groups of S-6 per cage under artificial illumination between 0600 and 18tJO h. Purina chow and water were available ad l&turn. Ambient temperature was 22-23-C. H~thalami~ deaKer- entations were peformed according to the method of Halasz and Pnpp’ with minor modifications, The dimensions of the knife used were: radius, 1.4 mm; heigbt, 2.4 mm. Two types of deafferentation were carried out. (1) Anterior hypo- thalamic deafferentation: a semicircular cut is made around the rostra] hypothalamus, at the level of the posterior border of the optic chiasma. (2) Posterior hypothalamic deaffer- entation: in tltis p~paration a semicircuiar cut is made around the eaudal extreme of the hypothatmus at the level of the mammillary nu&i. Sham hypothalamic deafferentation was performed by lowering the knife into

‘To whom all correspondence should be addressed. ~~~~eu~u~~~: MFB, medial forebrain bundle.

the brain in the midline, down to the level of the roof of the hypothalamus, and then removing it without rotation. The blade of the knife is held in the sag&al plane throughout this procedure, Bilateral MFB lesions were made by passing 3.0mA of direct current for 10s through an electrode with 0.4-mm diameter, insulated except for 0.4mm at the base. Stereotaxic co-ordinates were 1.3 mm posterior to the bregma, t.3 mm lateral to the midline and 1. I mm above the base of the skulL7

Two weeks following hypothatmic deafferentations or MFB lesians the rats were implanted with a bipolar stainless-steel electrode in the frontal cortex. One week subsequent to impiantation, the animals were subjected to brain stimufation. AlI stimulations were performed between 8.00 and 11.00 h. The protocol consisted of injecting pento- barbital (40 mg/kg body weight); 15 min later stimulation was initiated and was continued for Smin @.5mA, I ms, 100-‘). Fifteen minutes fotlowing onset of stimulation, blood was collected from the jugular vein by acute venesec- tion for corticosterone determination. Control animals were subjected to identical treatment and blood samples were collected in a similar manner to that described herein, except that stimulating current was not applied (“sham stimu- lation”). Plasma corticosterone was determined by the method Glick et at8

Upon completion of an experiment, the rats were sacrificed. the brains removed and fixed in fonnalin and the deafferentations, lesions and electrode placements were verified. For ~~e~n~t~ons, the brains were first examined m8cro~opically. They were sub~qu~tly sliced in a coronal plane on a cryostat-mounted mjcrotdme and examined histoiogicaljy. This enabled us to determine whether the cuts were c&rectiy positioned, and whether they extended down to the base of the brain. For MFB-iesioned animals, frozen coronal brain sections were made and the location and size of the lesions, in both the lateral and the longitudinal planes, were examined.

RESULTS

As evident from Tables 1 and 2, cortical stimu- lation caused in shad-deafferented and intact rats,

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1046 S. Feldman and N. Conforti

Table 1. Plasma corticosterone levels in rats with anterior or posterior hypothalamus deafferentation following sham or electrical stimulation of the frontal cortex

Sham stimulation Electrical stimulation

Sham anterior hypothalamic deafferentation 12.7 + 0.6(13)

Sham posterior hypothalamic 22.6 k 1.2*(17)

deafferentation 13.5 + 1.0(13) Anterior hypothalamic

21.7 + 1.4* (18)

deafferentation 14.1 + 1.0(8) Posterior hypothalamic

15.1 f 1.2(14)

deafferentation 12.5 + 0.9 (6) 24.9 k 0.9* (13)

Results are presented in pg”/, corticosterone, as mean f SEM. *P < 0.001. Significance levels are according to the Student’s t-test. The numbers in brackets represent number of animals tested per group.

Table 2. Plasma corticosterone levels in intact animals and in rats with bilateral medial forebrain bundle lesions follow- ing sham stimulation or electrical stimulation of the frontal

cortex

Sham Electrical stimulation stimulation

Intact animals 13.9kO.7(14) 25.2 k 1.7* (16) Medial forebrain

bundle lesions 11.4+ 1.1(14) 13.6+ 1.4(24)

Results are presented in pgo/, corticosterone. as mean + SEM.

'P c 0.001. Significance levels are according to the Student’s r-test.

respectively, a significant (P < 0.001) rise in plasma corticosterone level, in comparison to sham stimu- lation alone.

However, in rats with anterior hypothalamic deafferentation no adrenocortical response at all occurred upon cortical stimulation, as the post- stimulation plasma corticosterone levels were not different from the basal level in the sham-stimulated animals (Table 1).

In rats with posterior hypothalamic deafferen- tation a significant 2-fold increase in plasma cortico- sterone (P < 0.001) was observed following cortical stimulation, similar to that of sham-operated or intact animals (Table 1).

No significant rise in plasma corticosterone was observed following electrical stimulation of the cortex in the rats with MFB lesions, as the post-stimulation plasma corticosterone levels were not different from the basal levels in sham-stimulated animals (Table 2).

DISCUSSION

The present study demonstrated that the adreno- cortical response following frontal cortex stimulation is mediated by anterior afferents to the hypothalmus and more specifically the MFB, as in rats with these lesions, the corticosterone was completely blocked following frontal cortex stimulation.

However, the significant finding is that anterior. and not posterior, hypothalamic deafferentation blocked the effects of cortical stimulation. These

effects are probably mediated both by indirect and direct cortical connections with the hypothalamus, which have been demonstrated in the rat. Anatomical studies, using methods of autoradiography, have indicated that the prefrontal cortex of the rat has direct projections to the hypothalamus. The axons enter the internal capsule and distribute to the lateral hypothalamic area. Some of the labelling in this region represents fibers traveling caudalwards in the MFB.’ Other studies using the retrograde horseradish peroxidase method have demonstrated labeling of pyramidal and non-pyramidal prefrontal cortical neurons following horseradish peroxidase injection into the lateral hypothalamus.” Experiments using electron microscopy have shown direct ipsilateral fiber connections between the frontal cortex and the lateral hypothalamus.”

The anatomical studies are corroborated by electrophysiological observations. Stimulation of the prefrontal cortex-evoked monosynaptic and poly- synaptic responses in the lateral hypothalamus. Be- cause lesions of the preoptic area resulted in the disappearance of evoked potentials in the lateral hypothalamus, it was suggested that they are medi- ated by this region.”

Thus, it is evident that the lateral hypothalamus plays an important role in the transmission of cortical impulses to the hypothalamus.

The pathways most probably involved are cortical afferents into the MFB which is situated in the lateral hypothalamus. These fibers after synaptic con- nections, project to the medial hypothalamus. In view of the fact that lateral hypothalamic,” and more specifically the MFB fibers,‘* have been shown to project to the paraventricular nucleus, in which are located the corticotropin-releasing factor cell~,‘~ this pathway may be involved in the mediation of the adrenocortical responses following cortical stimu- lation.

The present experiments emphasize the differential effect of hypothalamic deafferentations and MFB lesions in the mediation of adrenocortical responses following the stimulation of various neural aBerents. While in our previous studies anterior hypothalamic deafferentation completely blocked the adreno-

Cortical effects on adrenocortical respons& 1047

cortical response, following stimulation of the dorsal hippocampus, medial septum, basolateral amygdafa and the midbrain reticular formation, posterior hypo- thalamic deafferentation completely blocked only the hippocampal effect and the other modalities were affected only partially.’ On the other hand MFB lesions had no effect on plasma corticosterone responses following septal and ventral hippocampal stimulation, but inhibited very significantly the adrenocortical response to dorsal hippocampal, amygdala and midbrain reticular formation stimu- lation.6 Thus it is evident that the neural pathways mediating the cortical effects on adrenocortical regu- lation, differ from those involved in the effects of the

subcortical limbic structures previously studied, as they do riot involve a posterior input into the hypo- thalamus.

Conclusion

The present experiments demonstrate that an anterior MFB input mediates the adrenocortical responses following frontal cortex stimulation and that this input differs from that of subcortical limbic effects on the secretion of the adrenal cortex.

Acknowledgements-The technical assistance of Mrs. A. Itzik and Mrs. E. Reinhartz is gratefully acknowledged. This work was supported by the Lena P. Harvey Endow- ment Fund for Neurological Research.

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(Accepted 26 February 1985)