hypothalmic mechanisms of ageing and of specific ... - Science Direct

Exp. Geronr. Vol. 14, pp. 161-174. © Pergamon Press Ltd. 1979. Printed in Great Britain.

HYPOTHALMIC SPECIFIC THRESHOLD

0531-5565/79/0801-0161502.00]0

MECHANISMS AGE

OF AGEING

PATHOLOGY--I. STIMULI

IN THE

OF

SENSITIVITY

OF HYPOTHALAMO-PITUITA~RY

TO HOMEOSTATIC

AND

COMPLEX

REPRODUCTIVE

SYSTEM V. M . DILMAN AND V. N . ANISIMOV Laboratory of Endocrinology, Prof. N. N. Petrov Research Institute of Oncology, U.S.S.R. Ministry of Public Health, Leningrad, U.S.S.R. (Received 2 August 1978)

A DISTURBED homeostasis is one of the most important manifestations of ageing. It is, therefore, easy to understand the reasons for the current search for the factors responsible for the development of homeostatic insufficiency with advancing age (see Comfort, 1964, 1969). In 1958, one of the authors of this paper published the results of his investigations entitled "On the Age-Associated Elevation of Activity of Certain Hypothalamic Centres" (Dilman, 1958). Although hypothalamic function was there evaluated on the basis of indirect evidence, some of the conclusions are worth considering now. First, the concept of a hypothalamic mechanism of ageing was advanced in this paper. Second, it was claimed there that the advance of age involves an elevation of activity of some hypothalamic centres, whereas previously the mechanism of ageing was usually believed to be associated with a decline in the activity of a particular system. Third, the paper drew the attention of the reader to those features which the hypothalamic mechanism of ageing and formation of specific age diseases have in common. This last point calls for some explanation even now. Conventionally, normal physiological ageing is strictly distinguished from ageassociated pathology. Such a distinction, however, is not quite obvious in some cases. For example, the climacteric is a typical manifestation of normal ageing and a pathological process (considering the homeostatic disturbances it involves), at the same time. V. Baranov and V. Dilman claimed in 1949 that climacteric neurosis is caused by the primary disturbances of hypothalamic function rather than estrogen deficit, as it was generally believed at the time. The lack of a strict distinction between age-related and pathologic phenomena has proved to be a common feature of all those pathological processes, the development of which depends on elevation of hypothalamic activity to a considerable degree. Therefore, some later publications (see Dilman, 1958, 1968) discuss a special group of age pathology diseases, characterized by elevation of hypothalamic activity. Subsequently, the term "elevation of hypothalamic activity" was replaced by "elevation of the hypothalamic threshold sensitivity to regulatory homeostatic stimuli" (Dilman, 1968, 1971, 1974). Stressor reaction may be used as an illustration of the differences between the elevation of hypothalamic activity and that of hypothalamic sensitivity threshold. The stress-induced elevation of hypothalamo-pituitary activity is of key importance for the realization of this reaction. However, it is easy to see that the hormono-metabolic shifts inherent in stress, e.g. the rise in blood-cortisol, glucose and free fatty acid levels, would result in the inhibition of the activity of the hypothalamo-pituitary complex by negative feedback, thus eliminating a defence mechanism, unless the sensitivity threshold of this 161

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V. M. D I L M A N A N D V. N. ANISIMOV

complex to homeostatic suppression were raised simultaneously. A similar situation develops during normal ageing, and contributes to the maintenance of homeostatic disturbances. The concept of age-related elevation of hypothalamic activity was originally associated with the mechanism of ageing and age pathology formation only. Therefore, it was natural to inquire into the causes of the hypothalamic functional shift underlying this pathology. At the same time, the formation of the concept of age-associated elevation of hypothalamic threshold of sensitivity to homeostatic inhibition made it easier to show the similarity between this phenomenon and the age-related decline in hypothalamic sensitivity to inhibition by sex hormones, which, according to the hypothesis of Donovan and Van der Werf ten Bosch (1959), plays a key role in the developmental program involved in the ageconnected switching-on of reproductive function. It became evident that instead of seeking for specific factors of ageing, it is necessary to consider the possible role of hypothalamic threshold elevation both in the mechanism of ageing or age pathology formation and in the mechanism of body development. A general conclusion of such a study would be that elevation of hypothalamic threshold to inhibition in the system of the three major superhomeostats--energy, reproductive and adaptive-constitutes the only possible mode of implementation of the neuroendocrine program of body development (Dilman, 1972, 1974). Indeed, it is easy to show that the development and growth of the body would not proceed, unless the capacity of reproductive, adaptive and energy homeostats were increased, yielding homocorrhesis. Yet the capacity of these homeostatic systems controlled chiefly by negative feedback would not increase, unless the sensitivity threshold of the hypothalamus to feedback control were elevated. This interrelationship is vividly illustrated by the mechanism of the age-associated switching-on of reproductive function. For example, if the age-related build-up of the capacity of the reproductive homeostat were due to the primary increase in the activity of the peripheral part of the homeostatic system and, more precisely, primary intensification of sex gland activity, the hypothalamo-pituitary complex would become persistently inhibited, thus preventing the cyclic functioning of reproductive homeostat. Conversely, the age-associated elevation of hypothalamic threshold to feedback control leads to a situation which contributes to further increase in the capacity of the system as a whole. In other words, the phenomenon of hypothalamic threshold elevation assures the self-development of the homeostatic system. Cybernetically speaking, it may be said that elevation of hypothalamic threshold results in the formation of the central type of homeostatic failure (Dilman, 1974),* which is responsible for the age-connected "drift of regulation or homeostasis", characteristic of ageing, according to Comfort (1964). At the same time, the build-up of homeostatic system capacity should inevitably lead to an ever-growing deviation from the requirements imposed by the "law of constancy of internal environment". Since any persistent departure from the optimal parameters of internal environment is a disease, the process of physiological ageing is a disease or a confluence of diseases of homeostasis. Therefore, elevation of hypothalamic threshold, as an element of the genetic program of development, should convert this program, on completion of growth, to the mechanism of ageing and age pathology formation. On the basis of some of its features, it was designated as the eleva*The principle of self-developmentof homeostatic systemsduring the ontogenesisof higher organisms, postulated here, is fundamentallydifferentfrom the behaviour of classical cybernetic systems, since the operation of the latter is oriented toward the maintenanceof a parameter of the homeostaticsystemfunctioning at constant level.

HYPOTHALAMIC MECHANISMS OF AGEING AND OF SPECIFIC AGE P A T H O L O G Y ~ I

163

tional mechanism of development, ageing and age pathology formation (Dilman, 1971). However, some later results necessitated the development of a more complex model. It is likely that ageing involves a decline in the threshold of sensitivity of the hypothalamo-pituitary complex to thyroid hormones, rather than an increase in the hypothalamus-pituitary-thyroid system (Part IV). It may be supposed that these peculiarities are at the background of the high activity of the thyroid gland during body growth. Apart from this, according to Aschheim (1976), the sensitivity threshold of the hypothalamo-pituitary complex, which governs the secretion of prolactin, shows a decrease during ageing, too. In this context, relevant changes taking place in the energy homeostat appear to be more complex than they were thought to be before (Part III). Therefore, it might seem reasonable to introduce into our model such a factor as the age-associated changes of the hypothalamic "set-point" of sensitivity to regulatory stimuli, if we stick to the basic principle of the agerelated "drift of homeostasis", instead of elevation of hypothalamic threshold--the key mechanism of this drift. However, even at present it seems wise to keep the term "elevation mechanism". First, elevation of hypothalamic threshold of sensitivity to homeostatic stimuli constitutes an age-dependent factor which dominates over reproductive, adaptive and energy homeostats (Parts I-III). Second, this term includes both the effect of this hypothalamic factor and the overall build-up of the functioning capacity of the main homeostatic systems during development and ageing. The metabolic shift toward the predominant utilization of free fatty acids for energy supply (instead of glucose) is the most common characteristic of the age-associated "drift of metabolism" (see Part V). Therefore, the rates of ageing and age pathology formation may be slowed down by administration of inhibitors of the activity of the main homeostatic systems, rather than stimulators (see Dilman, 1971; Comfort, 1969, 1974). Third, the term "elevation mechanism" implies that the same process is initially responsible for meeting the requirements of the growing organism and then, as it becomes more and more intensified, for the development of the pattern of ageing. For instance, it is likely that the age-connected switching-on and switching-off of reproductive function comes about due to the unceasing elevation of hypothalamic threshold (see below). As to the mechanism responsible for the age-associated changes in hypothalamic sensitivity threshold to homeostatic stimuli, numerous investigations, subsequent to the hypothesis of Stoll (1972) on the role of catecholamines in bringing about this phenomenon, showed that such factors as the levels of biogenic amines, affinity of hormone receptors and metabolic processes in the body contribute to the drift of hypothalamic sensitivity to feedback control. This statement is discussed later, using the data on the age-associated changes in reproductive homeostat as an illustration. As it was mentioned above, Donovan and Van der Werf ten Bosch (1959) postulated on the basis of the classical experiments of Hohlweg (see Hohlweg and Dtrn, 1931) that the decrease in sex centre sensitivity to inhibition by sex hormones constitutes a fundamental change which paves the way to sexual maturity. This phenomenon was confirmed by the results of a number of investigations carried out both on female and male rats (Ramirez and McCann, 1963; Smith and Davidson, 1968; Eldridge et al., 1974). But these authors studied the changes of hypothalamic sensitivity to sex hormones during a period directly associated with sexual maturity. Therefore, a problem remained to be solved as to whether hypothalamic sensitivity continues to change in the mature animal, i.e. after reproductive function is switched on. To provide the answer to the problem, we used estrogen-induced

164

V. M . D I L M A N A N D V. N . ANI'SIMOV

suppression of compensatory ovarian hypertrophy caused by hemicastration in rats. Application of this method provides an assessment of the condition of the hypothalamopituitary complex which controls predominantly the secretion of FSH (Benson et al., 1969; Mfiller et al., 1972; Butcher, 1977). Figure 1 demonstrates that, as age advances, an

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ever-increasing dose of estrogens is required to suppress the compensatory hypertrophy of ovaries. It should be particularly emphasized that this relationship persists after sexual maturity as well. However, since the age-connected gain of body weight may influence the distribution of exogenous estrogens in the animal body, producing a misleading impression of hypothalamic threshold elevation, we conducted experiments involving the injection of estradiol-1713 into the 3rd ventricle of the brain of hemicastrated rats. Figure 2 shows that, even under such experimental conditions, the inhibitory dose of estradiol is higher in 14-16 month-old rats than at the age of 3-4 months. Thus, it may be inferred that the sensitivity threshold of the hypothalamus to inhibition by estrogens grows during the greater part of the animal's lifetime. It was suggested on the basis of indirect evidence that the rise in FSH excretion should start prior to the onset of the climacteric due to the primary elevation of hypothalamic activity and, therefore, a decrease in the inhibition by estrogens (Dilman, 1958). In fact, Albert 0956) showed that the excretion of total gonadotrophins in females with normal menstrual cycles nearly doubles in the age group of 30-39 yr, as compared with 20-29 yr, i.e. in a period when there is as yet no substantial age-associated decrease in estrogen secretion. Subsequently, similar results were reported by different authors at different times (Dilman, 1960; Dilman, 1968; Loraine and Bell, 197 l). The age-associated decline in the sensitivity of hypothalamic neurons to estrogens (Babichev, 1973) seems to be related to the age-connected elevation of hypothalamic threshold of sensitivity. Some results of Gosden and Bancroft (1976) are also in agreement with the conclusion that the hypothalamic threshold of sensitivity to inhibition by estrogens rises during the greater part of the lifespan. A close examination of the experimental data of Shaar et al. (1975, Table I) also shows that the drop in LH level in ovariectomized young rats was much greater than in old females at day 13 after administration of estradiol. It should be emphasized that these data point to the age-related elevation of hypothalamic

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166

V. M. DILMAN AND V. N. ANI,qLMOV

(Kelch et aL, 1973). However, in reproductive and menopausal women, much larger doses of ethinyl-estradiol were used (see Yen and Tsai, 1972), which might have led to a failure to detect age-associated changes. Moreover, two factors should be taken into consideration. First, it is necessary to compare sensitivity to estrogens in those women of different ages who have retained menstrual cycles, because otherwise a long-term deficit of estrogens in the postmenopausal period may cause secondary changes in the sensitivity of the hypothalamo-pituitary complex (see similar experimental data reported by Damassa and Davidson, 1973). Second, a single administration of estrogens, which is often resorted to, is not desirable, because even when administered at days 3-6 of the menstrual cycle, estrogens may start to exert their effect by positive feedback (Yen and Tsai, 1972). There is thus substantial evidence in support of our earlier hypothesis (Dilman, 1968) that both age-associated switching-on and switching-off of reproductive function are brought about by one and the same phenomenon---elevation of hypothalamic threshold of sensitivity to homeostatic stimuli. This conclusion is corroborated by the data on the mechanism of hypothalamic threshold elevation proper. Stoll (1972) suggested that elevation of hypothalamic threshold to homeostatic inhibition by estrogen by negative feedback is associated with the age-dependent decrease in dopamine level in the hypothalamus. This hypothesis is supported by our findings (Anisimov, 1975; Dilman and Anisimov, 1975). Figure 3 indicates that different adrenolytic preparations, varying in their mechanism of action, raise the threshold of hypothalamo-pituitary complex sensitivity to estrogens. Conversely, L-dopa raises the sensitivity of this complex to inhibition by estrogens, i.e. it lowers the hypothalamic threshold of sensitivity. At the same time, it is known that advancing age is accompanied by a decrease in the hypothalamic level of catecholamines and, particularly, the concentration and metabolism of dopamine, whereas the latter, characteristics of the serotoninergic system, are susceptible to changes to a lesser degree (Finch, 1973, 1976; Robinson, 1975; Segal et al., 1975; Anisimov et al., 1977). We obtained similar results when catecholamine, serotonin and histamine levels were assayed at the same time (Table 1). TABLE 1. LEVELS OF BIOGENIC AMINES IN THE HYPOTHALAMUS OF MALE RATS OF DIFFERENT AGES (A~sL~tovet al., 1977)

Age Number (months) of rats Noradrenaline 1 32 1142 4- 70 2-3 72 1466 4- 63 6 32 1393 4- 152 13-15 28 1256 4- 73* 36 12 1216 4- 60t

Dopamine 319 ± 58* 486 4- 34 390 4- 65 261 4- 54* 543 4- 134

Serotonin (ng/g wet tissue) 721 + 79 810 4- 97 625 4- 94 778 4- 71 671 4- 143

5-HIAA 737 + 64 814 4- 121 751 4- 72 894 4- 198 662 4- 79

Histamine 608 -1- 56t 1067 4- 111 802 4- 94 853 4- 259 758 4- 167

*Difference from parameter value for 2-3 month old rats is statistically significant, p < 0"05. "['p < 0.01. Note: Rise in biogenic amine levels between 1 and 2-3 months of life is probably associated with the process of the hypothalamus development.

Accordingly, the administration of L-dopa to old animals restored the ability of estrogens to inhibit compensatory ovarian hypertrophy (Fig. 4). Therefore, the above results point to a positive correlation between the decrease in biogenic amine levels and elevation of hypothalamic threshold to inhibition by estrogens. Some indirect evidence also suggests that the age-associated changes taking place in the cyclic centre are caused by similar shifts in biogenic amine levels. For example, administra-

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FIG. 5. The effect of cholinergic and serotoninergic agents on compensatory ovarian hypertrophy and inhibition by diethylstilbestrol in young rats (Krylova et al., 1978; Anisimov, 1978). Experimental procedure and symbols as in Fig. 3. lighting ( K l e d z i k a n d Meites, 1974; T a k a h a s h i et al., 1973). M o r e o v e r , we f o u n d that the a d m i n i s t r a t i o n o f histamine in the 3rd ventricle o f the brain, or H l - a g o n i s t IEM-813 parenterally, r e m o v e d the i n h i b i t o r y effect o f estrogens on c o m p e n s a t o r y o v a r i a n hypert r o p h y , while a n t i h i s t a m i n e drugs failed to affect the estrogen ability to inhibit c o m p e n s a t o r y ovarian hypertrophy. The decreased b i n d i n g o f a h o r m o n e by the receptors o f the target tissue m a y be one of the factors which d e t e r m i n e h y p o t h a l a m i c sensitivity to h o m e o s t a t i c stimuli. Peng and Peng (1973) r e p o r t e d a decreased u p t a k e o f 3H-labelled estradiol by the a n t e r i o r hypothalamus, pituitary a n d uterus in o l d female rats with s p o n t a n e o u s c o n s t a n t estrus. However, T a b l e 2 shows t h a t the u p t a k e o f aH-estradiol-1713 by the a n t e r i o r a n d m e d i o b a s a l h y p o t h a l a m u s decreases with a d v a n c i n g age even before the onset o f age-associated disturbances in o v a r i a n cyclicity a n d this decline becomes a p p a r e n t at an earlier stage t h a n similar changes in the p i t u i t a r y or uterus. The a d m i n i s t r a t i o n o f L - d o p a , on the o t h e r h a n d , TABLE2.

UPTAKE OF aH-ESTRAD1OL-17 ~ BY TISSUES OF HYPOTHALAMUS, CEREBRAL CORTEX, ADENOHYPOPHYSIS AND UTERUS OF RATS OF VARYING AGES*

Age of animals (month) 2 6 10 16 24

No. of animals 4 4 2 10 3

Mediobasal Anterior hyhypopothalamus thalamus 103± 15 119i22 109±27 110±32 80±4 9 2 ± 10 61±7t 43±6t 60±147 54±8f

Specific radioactivity (dpm/mg) Dorsal hypoCerebral Adenohythalamus cortex pophysis 7 7 ± 10 70±3 261!46 6 9 ± 10 57±4 251 ± 2 6 66±7 70~20 254±23 7 3 ± 11 50±8 263±29 59±5 66:k4 161±16?

Uterus 503±85 537± 141 347± 18 332±33 261 ± 1 2 2 t

*Rats with regular estral cyclicity (2-16 months) and spontaneous constant estrus (24 months) were ovariectomized 1 week before experiment. On the first day of experiment, rats were killed 1 hr after an intraperitoneal administration of 0.2 ~tg/100 g body weight of 8H-6"7 estradiol-1713 (specific radioactivity-10.2 mCi/mM). The hypothalamus was dissected, according to Maurer and Wolley (1974). The radioactivity of tissues was measured in a liquid scintillation counter, Mark II (Nuclear Chicago, U.S.A.). tDiffearence in the indices for 6 month old rats is statistically significant, p ,< 0.05.

HYPOTHALAMIC MECHANISMS OF AGEING AND OF SPECIFIC AGE P A T H O L O G Y - - 1

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raises the hypothalamic uptake of ZH-estradiol, producing no marked effect on its binding in the pituitary or uterus (Table 3), while reserpine exerts a contrary effect in young animals (Fig 6). All this points to a certain causality between the hypothalamic levels of biogenic amines, the binding ability of the hypothalamic receptors of estrogens and elevation of hypothalamic threshold of sensitivity. Apart from the levels of biogenic amines and functional state of hormone receptors in the hypothalamus, some metabolic factors seem to affect the age dynamics of hypothalamic threshold, too. For example, Kennedy and Mitra (1963) demonstrated that the age-associated switching-on of reproductive function in rats is correlated with the body weight of the animal and that this function may switch on at an earlier stage if the animal is fed a fat-rich diet. This finding is corroborated by the statistical data of Frisch (1973) on a relationship between body weight and age of menarche. Since the switching-on of reproductive function is associated with elevation of hypothalamic threshold, it may be inferred that this phenomenon in the reproductive system is regulated by processes which take place in the energy system. TABLE 3. T H E EFFECT OF TREATMENT W I T H L - D O P A AND DILANTIN ON UPTAKE OF aH-ESTRADIOL-17~ BY TISSUES OF OLD RATS

Group Control L-dopa Dilantin Phenformin

Specificradioactivity (dpm/mg) Anterior Mediobasal Dorsal No. of hypohypohypoCerebral Adenohyanimals thalamus thalamus thalamus cortex pophysis 15 72 ± 10 74 i 8 43 ± 5 35 ± 2 170 ± 16 9 133± 18" 251 ±57" 101 ±20* 72 ± 12" 168 ~ 26 6 64±5 85 i 18 40±10 32 ± 5 157~30 8 73± 12 91 ± 13 58± 13 51 ± 10 241 ±40

Uterus 237 ± 21 292 ± 28 234± 18 315±48

L-dopa was injected intraperitoneally 100 mg/kg, 30 min before 3H-estradiol; Dilantin was administered per os, 30 mg/kg, 1 h before aH-estradiol; phenforrnin--per os, 5 mg/rat/day × 1 week.

*Differencestatistically significant,p < 0"05, L-dopa vs control. In view of all this it is noteworthy that the sensitivity of the hypothalamo-pituitary complex to inhibition by estrogens was restored in our experiments by the administration of such preparations as phenformin, succinic acid and dilantin (Fig. 4), which influence different levels of metabolism (see Dilman, 1974; Muntoni, 1974). However, unlike L-dopa, phenformin and dilantin (diphenylhydantoin) failed to increase the uptake of estradiol by the hypothalamus (Table 3), which means that metabolic factors exert their effects on hypothalamic threshold by a pathway other than biogenic amines. Finally, the pineal gland contributes to the age-associated elevation of hypothalamic threshold of sensitivity to homeostatic stimuli. Apart from their direct inhibiton of the hypothalamo-pituitary system, some pineal factors are capable of raising hypothalamic sensitivity to homeostatic stimuli (Dilman, 1970). This hypothesis is supported by our data on the elevation of sensitivity of the hypothalamo-pituitary complex to estrogens and glucocorticoids, as a result of pineal polypeptide extract administration (Ostroumova and Dilman, 1972; Anisimov et aL, 1973, 1974). Moreover, direct measurements showed that pinealectomy results in elevation of the threshold of hypothalamic sensitivity to inhibition by estrogens in young rats (Fig. 7). On the contrary, the injection of pineal polypeptide extract in old animals brought hypothalamic sensitivity to inhibition by diethylstilbestrol to normal (Fig. 8). It should be mentioned that the administration of pineal polypeptides caused the resumption of cyclicity of ovarian activity in old rats with constant

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estrus (Anisimov et al., 1973). This means that pineal polypeptides raise hypothalamic sensitivity to homeostatic stimuli both in the tonic and cyclic centres of reproductive homeostat. Elevation of hypothalamic sensitivity to homeostatic stimuli induced by pineal polypeptides throws some light on the role of external environmental factors. It was reported that constant lighting accelerates the switching-on of reproductive function in sexuallyimmature rats (Wurtman, 1970). Considering that maturity of reproductive homeostat is associated with elevation of the hypothalamic threshold of sensitivity to the action of estrogens, it may be expected that such a phenomenon can be easily established in an experiment involving the use of continuous lighting. It is clear from Fig. 7 that continuous lighting results in an elevation of the hypothalamic threshold of sensitivity to inhibition by estrogens in young animals. This gives a clue as to why continuous lighting induces constant estrus in mature rats, i.e. it leads to the switching-off of reproductive function. On the basis of the data on the influence of continuous lighting on the reproductive system, Hoffman (1973) also came to the conclusion that the hypothalamic threshold of sensitivity to positive feedback rises with ageing. Accordingly, continuous lighting, like ageing, decreases estrogen uptake by the rat's hypothalamus (Illei-Donhoffer et al., 1974). Apart from continuous lighting, some other factors raise the hypothalamic threshold of sensitivity to inhibition by estrogens. They include, for example, some chcmical carcinogens and transplantation of tumours (Fig. 9). Without going into the detail of this special problem, it is necessary to point out that normal development and ageing, continuous lighting, neonatal administration of androgens and chemical carcinogens cause similar changes in the reproductive system (see Dilman, 1974). We believe, accordingly, that elevation of hypothalamic threshold of sensitivity to inhibition plays a key role, not only in the mechanism of age-associated switching-on and switching-off of reproductive function, but also in the adaptation of reproductive function to the external and internal environment. Also, the factors which can raise hypothalamic threshold

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FIG. 9. The effect of transplantation tumours or carcinogen treatment on the inhibition of compensatory ovarian hypertrophy by diethylstilbestrol in young rats (Anisimov and Dilman, 1974; Anisimov and Ermosheknov, 1975). Transplantable tumours or homogenized muscle tissue were transplanted under the skin on the back 1 week before hemicastration. Single doses of anthracene and D M N A were administered 3 days before operation. *Similar results were obtained in the experiments with sarcoma 45 and lymphosarcoma of Pliss. **Similar results were obtained after treatment with 20-methylcholantrene, N-nitrosoethylurea, 1,2, dimethylhydrazine and DDT. Experimental procedure and symbols as in Fig. 4.

172

V. M. DILMANAND V. N. ANISIMOV

(neonatal administration of androgens, continuous lighting and some chemical carcinogens) accelerate the processes of maturing and ageing in the reproductive system. It appears to depend, to a considerable degree, on such factors as the age-associated decline in the hypothalamic level of catecholamines,* decreased binding of sex hormones by the receptors, decreased activity of the pineal gland and a shift in energy metabolism toward predominant utilization of free fatty acids for energy supply (Parts !11 and V). Resumption of cyclicity of the reproductive system as a result of medication provides an additional argument in support of the hypothesis on the regulatory mechanism of ageing and, particularly, the ageing of the reproductive system. SUMMARY It is shown that, on completion of sexual maturity, the level of estrogens required to suppress hemicastration-induced compensatory ovarian hypertrophy is raised in female rats, as age advances. This suggests an age-associated elevation of the threshold of sensitivity of the tonic centre of the hypothalamo-pituitary complex to inhibition by estrogens. It is shown experimentally that such factors as the age-connected decline in the hypothalamic level of catecholamines, decreased estrogen uptake by the anterior and mediobasal hypothalamus, diminished activity of the pineal gland and a shift toward predominant utilization of free fatty acids for energy supply play a key role in the mechanism of this hypothalamic phenomenon. Some external environmental factors (constant lighting, carcinogenic chemicals, transplantable tumours) serve to raise the hypothalamic threshold of sensitivity, too. Hypothalamic sensitivity to estrogen inhibition was normalized in old rats by administration of L-dopa, phenformin, dilantin, polypeptide pineal extract and succinic acid. Resumption of estral cyclicity in old rats with constant estrus due to administration of some drugs provides another proof in support of the hypothesis on the regulatory nature of ageing and, particularly, the mechanism of age-associated switching-off of reproductive function. Formation of concepts on the regulatory mechanism of ageing is briefly discussed. REFERENCES ALBERT, A. (1956) Recent Prog. Horm. Res. 12, 227. ANISIMOV, V. N. (1975) Bull. exp. Biol. Med. U.S.S.R. 80, 12, 44. ANISlMOV, V. N. (1978) Endocrinologie 71, 149. ANISIIvlOV, V. N., KHAVINSON,V. K., MOROZOV,V. G. and DILMAN, V. M. (1973) Dokl. Acad. Sci. U.S.S.R. 213, 483. ANISIMOV, V. N. and DILMAN, V. M. (1974) Vop. Onkol. 20, 5, 61. ANISIMOV, V. N., OSTROUMOVA,M. N. and DILMAN, V. M. (1974) Bull. exp. Biol. Med. U.S.S.R. 77, 4, 100. AN1SIMOV, V. N. and ERMOS~tENKOV,V. S. (1975) Vop. Onkol. 21, 3, 56. ANISlMOV, V. N., POZDEEV, V. K., DMITRIEVSKAYA,A. YA., GRACHEVA, G. M., IkIN, A. P. and D1LMAN, V. M. (1977) Sechenov Physiol. J. U.S.S,R. 68, 353. ASHttHM. P. (1976) In: Hypothalamus, Pituitary and Aging (Edited by A. V. EVI~RETT and J. A. BURGf~S. p. 376. Thomas, Springfield. BA~ICHEV, V. N. (1973) Bull. e.~7). Biol. Med. U.S.S.R. 75, 6, 3. *According to Aschheim (1976), cycling rats reveal an elevated ability to respond by pseudopregnancy to estrogen treatment with advancing age. This is a manifestation of the age-associated decline in hypotha!amic threshold of sensitivity. It is possible that tbis effect is associated both with the age-related decrease in the hypothalamic level of dopamine and the ability of estrogen to lower catecholamine level in the hypothalamus and, thus, to stimulate prolactin secretion. For example, one and the same phenomenon--ageconnected decrease in hypothalamic level of catecholamines-may produce two opposite-directed effects in the reproductive system proper and the system which controls prolaetin secretion. However, in the latter case, age-associated rise in prolactin secretion contributes to the elevation mechanism of ageing, too.

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