Spontaneous activity of mediobasal hypothalamic neurons following deafferentations and lesions

Brain Research Bulletin,

Vol. 5, pp. 759-763. Printed in the U.S.A.

Spontaneous Activity of Mediobasal Hypothalamic Neurons Following Deafferentations and Lesions’ DALIA

La~orato~

PAPIR-KRICHELI

AND SHAUL

FELDMAN2

of ~europhysiology, ~epa~~rnen~ of Neurology, ~adassah University hospital Hebrew university-~adassah Medical School, ~epasalem, Israel Received

and

6 June 1980

PAPIR-KRICHELI, D. AND S. FELDMAN. Spontaneous activity of ~ediobosoi ~ypot~ula~~c neurons following deaf fe~e~turjo~~ and lesions. BRAIN RES. BULL. 5(6)X%-763, 1980.-In view of the role of ext~hy~~~~i~ influences on the hypothalamic regulatory activity, the effects of anterior, anterotated or posterolateral hypothalamic deafferentations and bilateral medial forebrain bundle (MFB) lesions on the spontaneous single cell activity of neurons in the mediobasal hypothalamus (MBH) were studied in rats under urethane anesthesia. While the detierentations and particularly the anterior cuts reduced the average rates, MFB lesions increased the rate of firing of MBH neurons, and revealed cyclic activity. These data indicate that normalIy anterior afferents have mainly a facilitatory, while the MFB projections to the MBH have an inhibitory effect on the spontaneous activity. The possible relation of these alterations to neuroendocrine changes are discussed. Hypothalamus

Single cell activity

Deafferentation

.,ediobasal h~othal~us (MBH) serves as an integratTh ing centre for the control of the secretion of trophic hormones by the anterior pituitary gland [14]. Previous studies from this laboratory have demonstrated that photic, acoustic and sciatic nerve stimulation as well as that of limbic structures, evoke potentials in wide areas of the MBH of the rat 12,181. Subsequent experiments have shown that also a high percentage of the MBH units responds to these modalities [8, 9, 13, 161. As evident from neuroendocrine experiments these sensory stimuli produce an adrenocortical discharge and partial hypothalamic deafferentation and MFB lesions block these responses [3, 4, 5, 6, 71. Furthermore, rats with complete hypoth~amic de~erentation do not ovuiate, the diurnal rhythm in ACTH secretion is absent and TSH and growth hormone secretion are abnormal [ll]. This would indicate that the neural structures located outside the MBH exert stimulatory as well as inhibitory effects, which modify its neuroendocrine control mechanism. As changes in electrical activity may possibly reflect modifications in endocrine and other hy~th~amic autonomic functions, we have studied the effects of partial hypothalamic deafferentations and lesions of the medial forebrain bundle, which is a major afferent pathway to the hypothalamus, on the spontaneous activity of MBH units.

Autocorrelation

METHOD

The experiments were performed on male albino rats of the Hebrew University strain weighing approximately 250 g. This study includes eight intact rats and four groups of eight animals each on which the following operations were performed: (a) anterior hypothalamic deafferentation (AHD) behind the optic chiasm; (b) anterolateral hypothal~ic deafferentation (ALHD) which extended 1.4 mm laterally and 1.2 mm in the caudal direction; (c) bilateral DC lesions in the medial forebrain bundle (MFB); (d) posterolateral hypothalamic detierentation (PLHD) in the premammilary region which extended 1.2 mm anteriorly Ill] (Fig. 1). One week after the production of the lesions or detierentations, the animals were exposed to ether stress. Only those rats which showed an adequate adrenocortical response, i.e. a normal plasma corticosterone elevation following ether stress, as an indication of the capacity of the hypothalamus to respond to a stressful stimulus [4] were used in the electrophysiological study, about one week later. The rats were anesthetized with urethane (150 mg/lOO g) and placed in a stereotaxic inst~ment. Following craniotomy the dura was removed and the brain covered with agar. A glass microelectrode with a tip of l-2 @rn filled with 2M NaCl, with 5% Alcian-Blue dissolved in 0.5 M Na-acetate, was introduced stereotaxically into the mediobasal hypo-

‘This investigation was supported by United States-Israel Bination~ Science Foundation, Grant No. 1554178. *Send reprint requests to: Shaul Feldman, M.D., Department of Neurology, Hadassah University Hospital, Jerusalem, Israel.

PAPIR-KRICHELI

AND FELDMAN

FIG. 1. Sites of deafferentations and lesions of the hypothalamus. Anterior hypothalamic deafferentation (AHD), anterolateral hypothalamic deafferentation (ALHD), bilateral medial forebrain bundle lesions (MFB) and posterolateral hypothalamic deafferentation (PLHD).

thalamus at the following coordinates: A 5.4, L 0.2-0.3, H -3 - -4, according to the atlas of de Groot. The microelectrode was driven by a hydraulic microdrive in micrometer steps and controlled outside the faradic cage. The indifferent electrode was connected to the neck muscles. All recordings, which started about one hour after completion of surgery, were made using a cathode follower (WPI) and a Grass HIP 511 preamplifier. The responses were monitored on a CRT and recorded on a 7 channel FM Ampex tape recorder and the data were transferred, via a peak detector, to a PDP 11145 diskette for statistical analyses. All analyses were carried out off-line. At the end of each experiment a current was passed through the recording electrode, in order to mark the site with the dye. The position of each electrode and of the brain lesions was determined histologically. These were found to be located in the area of the arcuate nucleus. Experiments with inadequate brain lesions were discarded. The average rate of firing and patterns of activity, examined by autocorrelations were determined for each cell. Also averages of the above individual analyses of activity, for all the cells in the same experimental groups, were computed. The rates of firing were compared using the unpaired r-test. The autocorrelations were analyzed with significance lines at 99.5% level of confidence. Frequency histograms were compared using the Kolmogoroff-Smimoff test.

AHD

ALHD I I h

fl

r)

MFB

PLHD

RESULTS

The average spontaneous rate of firing of the deafferentated rats was found to be lower than the average rate of firing in the intact animals. While the rate in intact rats (n=36) was 3.36 spikes/set, in the AHD rats (n=32) it was 1.70 spikes/set, in the ALHD group (n=38) it was 1.57 spikes/ set and in the PLHD rats (n=37) the rate of firing was 2.82

spikeslsec. On the other hand in rats with bilateral MFB lesions (n=26), the rate was increased to 3.70 spikes/set. All these values were significantly different from the control rate at the level of p
7

l-l

Control

4

n tmilm 0

1

2

3

FREQUENCY

L

5

6 >6

(splkes/sec)

FIG. 2. Frequency histograms of spontaneous activity in intact rats and in rats with anterior hypothalamic deafferentation (AHD), anterolateral hypothalamic deafferentation (ALHD), bilateral medial forebrain bundle (MFB) lesions and posterolateral hypothalamic deafferentation (PLHD).

761

SPONTANEOUS ACTIVITY OF MEDIOBASAL HYPOTHALAMIC NEURONS

ALHO

AHD

PLHD

FIG. 3. Examples of single cell recordings from the mediobasa1 hypothalamus in the different experimental groups. Time bar-l

sec.

is no clear pattern in intact rats. In the anterior deaf&rented rats there is a peak at frequencies between 0.5-l spikeslsec

and a tendency of grouping at higher frequencies. The antero-lateral deafferented group showed a p~domin~ce at the lower frequencies with a peak at the range of l-1.5 spikeslsec. The MFB lesioned rats showed a peak at frequencies between 1.5 and 2 spikesisec. The rap&$&firing cells in the MFB group discharged in the range of 14-19 spikestsec in comparison to 8-10 spikesisec, which was found in the intact rats and in the deafferented animals. In the posterolateral deafferented rats there was no clear pattern of frequency distribution in the histogram. All tbhefrequency dist~butions in the four experimental groups were found to be significantJy different from the control animals at 95% level of confidence. Figure 3 shows examples of unit firing in the different experimental groups. In rats with AHD and ALHD there is a slowing in relation to controls, while in the MFB lesioned rats there is a regular firing. From the pattern of fuing, as shown by the use of autocorrelations, it is apparent that each experimental group has a different pattern (Fig. 4). In the intact rats, the autocomelation shows burst spike activity, without any evidence for periodicity, which would indicate that there is a higher probability for the occurrence of another spike at short time intervals, after the appearance of a given spike [17]. In the AHD group there is a very similar autoco~elation, but with a

steeper initial slope, which would indicate a smaller number of spikes per burst. The ALHD rats show a similar steep autocorrelation, with a second peak at 250-750 msec. The MFB lesioned rats show no burst activity, but a clear cyclic aut~a~elation, which indicates that the spikes appear periodically. The PLHD rats show burst activity with a superimposed cyclic activity of IYsec. DtSCUSSION The present data indicate that partial hypothalamic deafferentatMns ha\re reduced signi&& the spontaneous av-

erage rates of firing of the neurones in the MBH, This result differs from our previous observations in rats with compIete hypothalamic deafferentation, in which a slight but significant increase in the rate of firing was found [lo]. This would indicate that the net result of the different detierentations was the interruption of mainly facilitatory ext~y~th~~i~ inputs into the MBH, thus accounting for the reduction in the rate of firing. The largest reduction in the rate of firing was produced by anterior detierentation, and this is demonstrated by the shift of the peak in the frequency histogram towards the lowest frequencies, interrupting the MFB input inio tie WW, moored slightly the peak towards higher frequencies. This would indicate that the anterior tierents to the hypotb~~us are mainly facilitatory in relation to spontaneous activity. On the other hand bilateral MFB lesions

PAPIR-KRICHELI

762

Intact

100%

0

ALHD

2500

0 lOO%-

rats

100% -

AHD

AND FELDMAN

MFB

2500

0

2500

FIG. 4. Compound autocorrelations of the spontaneous activity of all the units in intact rats, and in rats with anterior hypothalamic deafferentation (AHD), anterolateral hypothalamic deafferentation (ALHD), bilateral medial forebrain bundle lesions (MFB) and posterolateral hypothalamic deafferentation (PLHD). Abscissa-Time in msec. Ordinatt+spikes/sec/bin expressed in percentage. Maximal response is 100%. Significance lines (normal distribution) are at 99.5% level of confidence.

caused a significant increase in the rate of firing, a shift of the peak towards the higher frequencies and the appearance of rapidly firing units, indicating that the MFB has normally an inhibitory effect on the spontaneous activity of MBH neurones. This latter influence is exerted probably through synaptic connections, as no tibre connections were found between the lateral regions and the medial hypothalamic nuclei [1,19]. The shorter bursts in the AHD rats may possibly be related with the removal of anterior excitatory influences on the MBH. On the other hand the appearance of cyclic spike activity, following bilateral MFB lesions, may indicate the unmasking of a phenomenon which was suppressed by inhibitory influences mediated by the lateral hypothalamic regions. The inhibitory effect of MFB lesions on the spontaneous activity is corroborated by our experiments on the responsiveness of MBH neurones to sensory stimuli. It was found that such lesions produced a marked facilitation of responses to photic, acoustic and sciatic nerve stimulation.

It is difficult to relate specifically the described alterations in the spontaneous activity of MBH neurones, as the result of these procedures, to neuroendocrine and other changes in hypothalamic activity. However, the anterior deatferentations interrupted facilitatory afferents as well as the input from suprachiasmatic nucleus to the MBH. This nucleus has been shown to control a variety of circadian rhythms in mammals, such as drinking, sleep-wake activity, estrous cycling and adrenocortical activity [ 151. These are mediated probably by suprachiasmatic neurones, which send their axons into the MBH [12]. Also experiments involving extra-hypothalamic lesions in the hippocampus, septum, preoptic area, amygdala and brain stem indicate that these brain regions exert through the MBH an influence on the anterior pituitary secretions [3,14]. The question may arise whether the observed changes in the electrical activity in the rats with the deafferentations and lesions were the result of the interruption of neural afferents to the MBH or were the result of hormonal and nutri-

SPONTANEOUS

ACTIVITY

OF MEDIOBASAL

HYPOTHALAMIC

tional alterations produced by these lesions. As related to possible hormonal changes, experiments in our laboratory have shown changes in ACTH, TSH and FSH in rats with anterior hypothalamic deafferentation; however, in rats with MFB lesions and posterolateral hypothalamic deatferentation no changes in ACTH, TSH, FSH, LH and prolactin were found. These two latter groups showed the most marked changes in the electrical activity in the present experiments. Furthermore, there were no differences in the changes in weight of the rats in the five experimental groups between the time of the production of the various lesions and the time of the electrophysiological study. Also, the animals

NEURONS

763

had free access to food and water and did not have diabetes insipidus. In view of these considerations, we believe that the changes in unit tiring found in the different groups were the result of interruption of connections between extrahypothalamic structures and the MBH.

ACKNOWLEDGEMENTS

The technical assistance of Mrs. A. Itzik and Mrs. E. Reinhartz and the computer programming by Mr. A. Arieli are gratefully acknowledged.

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