EXPERIMENTAL
NEUROLOGY
57, 102-111 (1977)
Morphologic
JEsI;‘sMACEIADO-SALAS, I~cpurtvzcnfs
of CaBfor&,
Changes in the Hypothalamus of the Old Mouse MADCE E. SCHEIBEL,
AND ARNOLD B. SCHEIBEL
r
of drhadolny and Los Angeles, National
Psychiatry and Brain Research Institute, University California 90024, and Department of Scientific Research, Medical Center IMSS, Mdxico, D.F. Received
February
28, 1977
The hypothalami of old mice were studied with Golgi methods. Our observations showed progressive disruption of hypothalamic architecture, paralleled by deterioration and loss of dendritic surface. Even though these changes were found in all hypothalamic regions studied, they were not evenly distributed. These patterns of distribution plus their possible functional importance are significant in relationship to neuroendocrinological changes in the aged. Emphasis is placed on the aging hypothalamus as an integrator of information originating in similarly aging limbic structures.
INTRODUCTION Aging and senescencemarkedly impair the responsesto environmental challenges which otherwise would be easily coped with by younger subjects. The neuroendocrine system, and especially the hypothalamus, has a primary role in the elaboration of adaptive behavior. Due to its unique anatomophysiologic position, the hypothalamus has been assumed to play a crucial role in the initiation of the aging process (11-13, 15, 16, 20). Clinical and experimental observations have shown that some of the neuroendocrinological responsesof the aged do not differ from those of the young controls (18, 24, 26, 43). I n some casesin which those changes are marked, they are considered as normal physiological adjustments to old age (23). However, when progesterone-4-Cl4 was administered to old subjects, 1 This study was supported by U.S. Public Health Service Grant NS 11468. Dr. Machado-Salas was supported in part by National Institutes of Health International Fellowship (lFOS+W02179-01/02), and by the Department of Scientific Research, National Medical Center, IMSS, Mexico, D.F. The authors are indebted to Dr. Roy L. Walford (Department of Pathology, University of California, Los Angeles) for providing the animals for this study and to Ms. Arlene Koithan and Mr. Abe Green for their technical assistance. 102 All rights of reproduction
in any form reserved.
ISSN 0014 4886
OLD
MOUSE
HYPOTHALAMUS
103
they metabolized it slowly (37). Other studies made in aged volunteers by Pincus (34) showed that the aging status is paralleled by a decrement in the daily output of urinary steroids. Further evidence supporting impairment of the brain-endocrine system has been provided by Hess and Riegle (24)) Bowman and Wolf (7)) Rodeck et al. (36), and Hall (21)) among others. In spite of the profound variability shown by some neuroendocrinological systems in the aged, there is well-established agreement concerning the decline of gonadal function in old placental mammalia (3, 19, 44). Elegant studies by Clemens and collaborators (10) and by Peng and Huang (33) argue for the aging hypothalamus as the responsible structure in the deterioration of gonadal function. Several light-microscope studies have described some aspects of old hypothalamic structures. Note the thorough descriptions of Buttlar-Brentano (9) and Azcoaga (5) on the magnocellular nuclei, and Azcoaga (4) on the aged glial elements of hypothalamus. Furthermore, Rodeck et al. (36) and Frolkis et al. (15) have provided experimental evidence concerned with the altered neurosecretory activities of old rat hypothalamus, and more recently, Hasan and collaborators have shown the ultrastructural features of aged guinea pig hypothalamus (22). Because recent experiences in our laboratory have shown the power of the Golgi technique in the study of the neuropil of the aging human brain (4032) and of old mouse spinal cord and lower brain stem (30), we decided to review the neuronal and neuropilic changes of some of the neurosecretory nuclei of the hypothalamus, attempting to correlate those changes with the neuroendocrine disturbances of the aged. MATERIAL
AND
METHOD
The silver impregnation technique for the nervous system, originally described by Camilo Golgi 100 years ago, and its multiple variants still remain the most suitable methods for the study of the surface of the nerve cell body and its complex arborization. To accomplish the present study, we used 180 mice of either gender (C57B1/6 J) , which were reared under standard conditions. Their ages were 1 to 34 months. The group of young controls included mice from 1 to 9 months of age, and more than 80 mice were included in the old group (26 to 34 months of age). A comple:e account of experimental procedures has been published elsewhere (30). Nissl and periodic acid Schiff (PAS) stains were also utilized as complementary techniques. RESULTS The aging process of the nervous system is not restricted to the lower stem and spinal cord. It is also noticeable throughout the neuro-
I)raill
104
MACHADO-SALAS,
SCHEIBEL
AND
SCHEIBEL
secretory structures of old mice. In our Golgi preparations, significant changes are found in the architecture of the dendrite matrix made familiar by the studies of Krieg (27), Leontovich (28)) Millhouse (31) , and Ram& y Cajal (35). Furthermore, the usually smooth and regular somatic silhouette (oval, spheroidal, triangular, or polyhedrical) shown by young cells has been replaced by irregularities on the surface and accentuated constrictions and convexities of the somatodendritic profiles. At the beginning of this deteriorative process, it is possible to observe lumpiness of the nerve cell body, and early nodulation and shortening of its dendrites (Fig. la). A sagittal view of a suprachiasmatic nerve cell (Fig. lb) is used for illustrating a further step in the process of somatic distortion, enhanced swelling of the dendritic stalk, and decrease in its arborization. In several nuclei, we frequently found marked swelling of the nerve cell body and thin, irregular, and shortened dendrites bearing no spines (Fig. 1~). Figure Id depicts two adjacent neurons of the nuclei paraventricularis, showing somatodendritic changes ; one of them displays an advanced stage of cell body deformity (bell-shaped) and few shrunken dendrites. We should mention that the magnocellular nuclei were not a very important source of pathologic cells, although we were able to detect neuronal alterations at all stages of the deteriorative process. In all neurosecretory nuclei studied, we observed some advanced changes which we consider typical of neuronal senescence. In those instances (Fig. le, f), the already swollen cells are now shrunken, and, simultaneously, the impoverishment of dendritic arborizations reaches its maximum. The few remaining dendrite stumps are beaded and have lost their spines. Undoubtedly the presence of these features of advanced senility in the nerve cells reflects varying degrees of functional incompetence and eventual cell death. We are impressed with the high incidence of neuropathology in the anterior hypothalamus and especially in the preoptic area. Figure 2a exemplifies early lumpiness of the somata and moderate dendritic loss. Normally, the dendritic stalks of the preoptic area lie perpendicular to the main afferent fibers, which are clearly illustrated in this picture. Some of the very old mice show a considerable number of multibeaded axons in the anterior hypothalamus (Fig. 2b). These dilations may be related to a possible impairment in the axonal flow; however, their anatomopathological significance is not yet clear. Under other conditions (Fig. 2c), dendrites show a dilated initial segment, followed by a severe constriction (arrow, same figure) and another dilation. Recently (30), we proposed that this image may precede the final amputation of dendritic stalks. This picture is observed rather frequently in a number of nuclear centers, and in all cases it is accompanied by the
OLD
MOUSE
HYPOTHALAMUS
10.5
FIG. 1. Photomicrographs of Golgi-impregnated neurons from various hypothalamic nuclei of old mice. a-Neuron of n. suprachiasmaticus showing moderate somatodrndritic distortion; 34-month-old mouse. b-Sagittal view of a neuron of n. suprachiasmaticus. Notice the accentuated deformity of its soma as well as its short and thick dendrites ; 33-month-old mouse. c-Neuron of anterior hypothalamus displaying a swollen soma and a few thin and shrunken dendrites; 30-month-old mouse. d-Two neurons of n. paraventricularis showing marked alterations in cell body contour; 30month-old mouse. e-Two neurons of n. supraopticus which exemplify moderate and very advanced cases of senile changes; 30-month-old mouse. f-Neuron of preoptic area (medialis) showing swollen somata with irregular surface. The remaining dendritic stump appears severely distorted ; 33-month-old mouse. hlagnifications : b, e, x300; a, c, d, f, i:ooo.
106
MACHADO-SAT.AS,
SCHEIBEL
AND
SCHEIBEL
FIG. 2. Photomicrographs of Golgi-impregnated neurons from anterior hy@halamic region. a-Neuron of preoptic area showing developing lumpiness of the soma and main dendritic stalks. Observe the spatial relation between dendrites and axons which is peculiar to this region; 34-month-old mouse. LMultibeaded axons are present in the anterior hypothalamus of very old mice; 33-month-old mouse. c-A neuronal group of preoptic region (medialis) showing moderate aging changes. However, notice how one cf the neurons displays a marked strangulation (arrow) at the initial portion of one of its dendrites. In the insert (c‘), at higher magnification, note the “negative image” of an unstained glial cell in the depth of the constriction (arrow) ; 2%month-old mouse. d-Neuron of preo,ptic area with severe dilation and deformity of the cell body and almost complete lack of dendrites; 34-month-old mouse. Magnifications: a, b, c, x290; c’ (insert), d, x580.
“negative
Advanced
of a glial cell, which lies in the depths of the constriction, senile changes in neurons of this area are depicted in Fig. 2~1.
image”
OLD
MOUSE
IIYPOTIIALAMUS
107
Even though the “corpora amylacea” are not specific to the aging process, they are frequently seen in old human brains. We had the opportunity to observe them in the case of extremely old mice (34 months old), and they were situated in the hypothalamic regions close to the pial surface of the diencephalic floor and spreading laterally toward the amygdalar nuclei. Some complementary data have been obtained from the study of Nissland PAS-stained material, which are briefly presented : (i) The supraoptic neurons tend to show greater basophilia of the cytoplasm than the periventricular ones, a fact that is possibly related to differences in cellular metabolic rate. (ii) Important concentrations of lipofuscin are found in the neurons of the supraoptic nuclei, preoptic nuclei, and anterior hypothalamic nuclei. The intracytoplasmic concentration in paraventricular neurons is rather sparse. (iii) The epentlymal cells lining the III ventricle showed a positive PAS reaction clue to accumulation of glycogen and/or lipofuscin. It is well known that one of the properties of the Golgi techniques is to impregnate blood vessels. Taking advantage of this fact, we compared the vascularity of the hypothalamus at different ages and received the impression of disruption of the vascular architecture, as well as a decrease in the number of branches, especially in the magnocellular nuclei. More detailed observations on the organization of the microcirculation will be discussed elsewhere. DISCUSSION The vital role played by neuroendocrine mechanisms in the maintenance of homeostasis becomes evident when the individual ages. In this regard, the aging hypothalamus has been the subject of several anatomical and physiological studies, including analyses of its secretory and chemical functions (2, 46, 9, 14-16, 36). Some of those studies have shown remarkable alterations suggesting, for this diencephalic structure, a prime place in the initiation of aging (1 l-13, 15, 16, 20). Review of the literature does not yield any description of the architectural state of the aging hypothalamic nuclei ; however, our observations on Golgi-impregnated material from old mice aim to throw some light on this issue. They indicate that we are dealing with a progressive deteriorative process which, in early stages, manifests itself as localized irregularities of the somatodendritic silhouette and a decrease in the spine-like processes of many neurons. Later, the gross architectural appearance of those nuclei becomes distorted by progressive loss of dendritic domains and deformities of the nerve cell bodies. In the final stage, extremely swollen or distorted somata bear a few amputated dendritic stumps prior to complete cell disappearance. This spectrum of change has been found in all neurosecretory nuclei studied. Nevertheless, they are more frequently observed in rostra1 hypo-
108
MACIIADO-SALAS,
SCHEIBEL
AND
SCHEIBEL
thalamic regions where neuropil impoverishment seems to be greater. These structural changes, unequally distributed throughout the neurosecretory nuclei of the aging hypothalamus, are in general agreement with the experimental findings of Babichev and Frolkis and collaborators (6, 15, 16), and clinical observations on the dissociation of the neuroendocrine alterations in aged individuals (18, 19, 21). It has been this “spotty” pattern of physiologic and anatomic changes which has originated the concept of “hypothalamic disregulation” and its central position in the aging process (11-13, 15, 16, 20). It seems probable that an impoverished hypothalamic neuropil, progressively deprived of its dendritic surfaces, becomes increasingly unable to elaborate adequate responses even to minimal environmental challenges. Note also that the hypothalamus is the recipient of massive afferent tracts originating in various limbic structures ( 1, 29, 31, 32)) some of which also show important age-related changes as already indicated in previous studies from this laboratory (30,41,42). The importance of modulation of ovarian function by limbic structures is well established, and its significance has recently been stressed by Sawyer (35, 39). On the other hand, two hypothalamic centers have been identified (17) as the source of “tonic” and “phasic” influences upon the function of the female reproductive system. These structures are : (i) the arcuate-ventromedial nuclei, which maintain adequate levels of hypophysotropins, and consequently of gonadotropins, throughout the sexual cycle, and (ii) the preoptic-anterior hypothalamus nucleus, which is related to the surge of Iuteinizing hormone at the time of ovulation. The preceding data were obtained in normal mature animals in which anatomical circuitry and neuropil in the hypothalamus, as well as in the hindbrain and forebrain limbic systems, are presumably intact ; thus the brain-gonadal system may be considered to behave with chronometric cyclicity, allowing reproductive functions during the mature stage of the development of the individual. Afterward, a progressive decline of the function of the brain-gonadal axis becomes evident. Initially, this deterioration is subtle, due undoubtedly to redundancy of limbic connections ; later, when the behavioral manifestations of sexual activity have ceased and the sexual organs have undergone atrophy, the aging process in the hypothalamus becomes maximally evident ; the neuropil is scarce, afferents are quantitatively and qualitatively impoverished, the nerve cell bodies are physically distorted, and there is significant loss of dendritic surface. Undoubtedly, all those changes contribute importantly to aging phenomena in the brain-gonadal system. As already mentioned, there seems to be a degree of dissociation in agerelated neuroendocrine changes. Brain-gonadal functions decline in old individuals without exception, whereas other brain-endocrine relationships may show little if any change. This functional dissociation bears some resemblance to the uneven distribution of morphologic changes in the aging
hypothalamus which would provide a structural basis for the deterioration of gonadal function in old age, with relative sparing of other endocrine activities. Recently, it was demonstrated (45) that the only source of luteinizing hormone-releasing hormone is the hypothalamus, whereas the sources of thyrotropin-releasing hormone are spread throughout the nervous system (8, 25), thereby providing redundant sources for the latter and a higher probability of continuing thyroid function into later life. We have described the progressive deteriorative process found in the neuropil of the aging hypothalamus and attempted to interpret its significance in relationship to neuroendocrine alterations, with due regard to the broad framework and complexity of the mechanisms involved. We have also stressed the dissociative characteristics of the morphologic changes and emphasized the position of the old hypothalamus as an integrator of signals originating in similarly aging limbic structures. REFERENCES A. DAHLSTROI~, K. FUXE, I
NEUROLOGY
57, 102-111 (1977)
Morphologic
JEsI;‘sMACEIADO-SALAS, I~cpurtvzcnfs
of CaBfor&,
Changes in the Hypothalamus of the Old Mouse MADCE E. SCHEIBEL,
AND ARNOLD B. SCHEIBEL
r
of drhadolny and Los Angeles, National
Psychiatry and Brain Research Institute, University California 90024, and Department of Scientific Research, Medical Center IMSS, Mdxico, D.F. Received
February
28, 1977
The hypothalami of old mice were studied with Golgi methods. Our observations showed progressive disruption of hypothalamic architecture, paralleled by deterioration and loss of dendritic surface. Even though these changes were found in all hypothalamic regions studied, they were not evenly distributed. These patterns of distribution plus their possible functional importance are significant in relationship to neuroendocrinological changes in the aged. Emphasis is placed on the aging hypothalamus as an integrator of information originating in similarly aging limbic structures.
INTRODUCTION Aging and senescencemarkedly impair the responsesto environmental challenges which otherwise would be easily coped with by younger subjects. The neuroendocrine system, and especially the hypothalamus, has a primary role in the elaboration of adaptive behavior. Due to its unique anatomophysiologic position, the hypothalamus has been assumed to play a crucial role in the initiation of the aging process (11-13, 15, 16, 20). Clinical and experimental observations have shown that some of the neuroendocrinological responsesof the aged do not differ from those of the young controls (18, 24, 26, 43). I n some casesin which those changes are marked, they are considered as normal physiological adjustments to old age (23). However, when progesterone-4-Cl4 was administered to old subjects, 1 This study was supported by U.S. Public Health Service Grant NS 11468. Dr. Machado-Salas was supported in part by National Institutes of Health International Fellowship (lFOS+W02179-01/02), and by the Department of Scientific Research, National Medical Center, IMSS, Mexico, D.F. The authors are indebted to Dr. Roy L. Walford (Department of Pathology, University of California, Los Angeles) for providing the animals for this study and to Ms. Arlene Koithan and Mr. Abe Green for their technical assistance. 102 All rights of reproduction
in any form reserved.
ISSN 0014 4886
OLD
MOUSE
HYPOTHALAMUS
103
they metabolized it slowly (37). Other studies made in aged volunteers by Pincus (34) showed that the aging status is paralleled by a decrement in the daily output of urinary steroids. Further evidence supporting impairment of the brain-endocrine system has been provided by Hess and Riegle (24)) Bowman and Wolf (7)) Rodeck et al. (36), and Hall (21)) among others. In spite of the profound variability shown by some neuroendocrinological systems in the aged, there is well-established agreement concerning the decline of gonadal function in old placental mammalia (3, 19, 44). Elegant studies by Clemens and collaborators (10) and by Peng and Huang (33) argue for the aging hypothalamus as the responsible structure in the deterioration of gonadal function. Several light-microscope studies have described some aspects of old hypothalamic structures. Note the thorough descriptions of Buttlar-Brentano (9) and Azcoaga (5) on the magnocellular nuclei, and Azcoaga (4) on the aged glial elements of hypothalamus. Furthermore, Rodeck et al. (36) and Frolkis et al. (15) have provided experimental evidence concerned with the altered neurosecretory activities of old rat hypothalamus, and more recently, Hasan and collaborators have shown the ultrastructural features of aged guinea pig hypothalamus (22). Because recent experiences in our laboratory have shown the power of the Golgi technique in the study of the neuropil of the aging human brain (4032) and of old mouse spinal cord and lower brain stem (30), we decided to review the neuronal and neuropilic changes of some of the neurosecretory nuclei of the hypothalamus, attempting to correlate those changes with the neuroendocrine disturbances of the aged. MATERIAL
AND
METHOD
The silver impregnation technique for the nervous system, originally described by Camilo Golgi 100 years ago, and its multiple variants still remain the most suitable methods for the study of the surface of the nerve cell body and its complex arborization. To accomplish the present study, we used 180 mice of either gender (C57B1/6 J) , which were reared under standard conditions. Their ages were 1 to 34 months. The group of young controls included mice from 1 to 9 months of age, and more than 80 mice were included in the old group (26 to 34 months of age). A comple:e account of experimental procedures has been published elsewhere (30). Nissl and periodic acid Schiff (PAS) stains were also utilized as complementary techniques. RESULTS The aging process of the nervous system is not restricted to the lower stem and spinal cord. It is also noticeable throughout the neuro-
I)raill
104
MACHADO-SALAS,
SCHEIBEL
AND
SCHEIBEL
secretory structures of old mice. In our Golgi preparations, significant changes are found in the architecture of the dendrite matrix made familiar by the studies of Krieg (27), Leontovich (28)) Millhouse (31) , and Ram& y Cajal (35). Furthermore, the usually smooth and regular somatic silhouette (oval, spheroidal, triangular, or polyhedrical) shown by young cells has been replaced by irregularities on the surface and accentuated constrictions and convexities of the somatodendritic profiles. At the beginning of this deteriorative process, it is possible to observe lumpiness of the nerve cell body, and early nodulation and shortening of its dendrites (Fig. la). A sagittal view of a suprachiasmatic nerve cell (Fig. lb) is used for illustrating a further step in the process of somatic distortion, enhanced swelling of the dendritic stalk, and decrease in its arborization. In several nuclei, we frequently found marked swelling of the nerve cell body and thin, irregular, and shortened dendrites bearing no spines (Fig. 1~). Figure Id depicts two adjacent neurons of the nuclei paraventricularis, showing somatodendritic changes ; one of them displays an advanced stage of cell body deformity (bell-shaped) and few shrunken dendrites. We should mention that the magnocellular nuclei were not a very important source of pathologic cells, although we were able to detect neuronal alterations at all stages of the deteriorative process. In all neurosecretory nuclei studied, we observed some advanced changes which we consider typical of neuronal senescence. In those instances (Fig. le, f), the already swollen cells are now shrunken, and, simultaneously, the impoverishment of dendritic arborizations reaches its maximum. The few remaining dendrite stumps are beaded and have lost their spines. Undoubtedly the presence of these features of advanced senility in the nerve cells reflects varying degrees of functional incompetence and eventual cell death. We are impressed with the high incidence of neuropathology in the anterior hypothalamus and especially in the preoptic area. Figure 2a exemplifies early lumpiness of the somata and moderate dendritic loss. Normally, the dendritic stalks of the preoptic area lie perpendicular to the main afferent fibers, which are clearly illustrated in this picture. Some of the very old mice show a considerable number of multibeaded axons in the anterior hypothalamus (Fig. 2b). These dilations may be related to a possible impairment in the axonal flow; however, their anatomopathological significance is not yet clear. Under other conditions (Fig. 2c), dendrites show a dilated initial segment, followed by a severe constriction (arrow, same figure) and another dilation. Recently (30), we proposed that this image may precede the final amputation of dendritic stalks. This picture is observed rather frequently in a number of nuclear centers, and in all cases it is accompanied by the
OLD
MOUSE
HYPOTHALAMUS
10.5
FIG. 1. Photomicrographs of Golgi-impregnated neurons from various hypothalamic nuclei of old mice. a-Neuron of n. suprachiasmaticus showing moderate somatodrndritic distortion; 34-month-old mouse. b-Sagittal view of a neuron of n. suprachiasmaticus. Notice the accentuated deformity of its soma as well as its short and thick dendrites ; 33-month-old mouse. c-Neuron of anterior hypothalamus displaying a swollen soma and a few thin and shrunken dendrites; 30-month-old mouse. d-Two neurons of n. paraventricularis showing marked alterations in cell body contour; 30month-old mouse. e-Two neurons of n. supraopticus which exemplify moderate and very advanced cases of senile changes; 30-month-old mouse. f-Neuron of preoptic area (medialis) showing swollen somata with irregular surface. The remaining dendritic stump appears severely distorted ; 33-month-old mouse. hlagnifications : b, e, x300; a, c, d, f, i:ooo.
106
MACHADO-SAT.AS,
SCHEIBEL
AND
SCHEIBEL
FIG. 2. Photomicrographs of Golgi-impregnated neurons from anterior hy@halamic region. a-Neuron of preoptic area showing developing lumpiness of the soma and main dendritic stalks. Observe the spatial relation between dendrites and axons which is peculiar to this region; 34-month-old mouse. LMultibeaded axons are present in the anterior hypothalamus of very old mice; 33-month-old mouse. c-A neuronal group of preoptic region (medialis) showing moderate aging changes. However, notice how one cf the neurons displays a marked strangulation (arrow) at the initial portion of one of its dendrites. In the insert (c‘), at higher magnification, note the “negative image” of an unstained glial cell in the depth of the constriction (arrow) ; 2%month-old mouse. d-Neuron of preo,ptic area with severe dilation and deformity of the cell body and almost complete lack of dendrites; 34-month-old mouse. Magnifications: a, b, c, x290; c’ (insert), d, x580.
“negative
Advanced
of a glial cell, which lies in the depths of the constriction, senile changes in neurons of this area are depicted in Fig. 2~1.
image”
OLD
MOUSE
IIYPOTIIALAMUS
107
Even though the “corpora amylacea” are not specific to the aging process, they are frequently seen in old human brains. We had the opportunity to observe them in the case of extremely old mice (34 months old), and they were situated in the hypothalamic regions close to the pial surface of the diencephalic floor and spreading laterally toward the amygdalar nuclei. Some complementary data have been obtained from the study of Nissland PAS-stained material, which are briefly presented : (i) The supraoptic neurons tend to show greater basophilia of the cytoplasm than the periventricular ones, a fact that is possibly related to differences in cellular metabolic rate. (ii) Important concentrations of lipofuscin are found in the neurons of the supraoptic nuclei, preoptic nuclei, and anterior hypothalamic nuclei. The intracytoplasmic concentration in paraventricular neurons is rather sparse. (iii) The epentlymal cells lining the III ventricle showed a positive PAS reaction clue to accumulation of glycogen and/or lipofuscin. It is well known that one of the properties of the Golgi techniques is to impregnate blood vessels. Taking advantage of this fact, we compared the vascularity of the hypothalamus at different ages and received the impression of disruption of the vascular architecture, as well as a decrease in the number of branches, especially in the magnocellular nuclei. More detailed observations on the organization of the microcirculation will be discussed elsewhere. DISCUSSION The vital role played by neuroendocrine mechanisms in the maintenance of homeostasis becomes evident when the individual ages. In this regard, the aging hypothalamus has been the subject of several anatomical and physiological studies, including analyses of its secretory and chemical functions (2, 46, 9, 14-16, 36). Some of those studies have shown remarkable alterations suggesting, for this diencephalic structure, a prime place in the initiation of aging (1 l-13, 15, 16, 20). Review of the literature does not yield any description of the architectural state of the aging hypothalamic nuclei ; however, our observations on Golgi-impregnated material from old mice aim to throw some light on this issue. They indicate that we are dealing with a progressive deteriorative process which, in early stages, manifests itself as localized irregularities of the somatodendritic silhouette and a decrease in the spine-like processes of many neurons. Later, the gross architectural appearance of those nuclei becomes distorted by progressive loss of dendritic domains and deformities of the nerve cell bodies. In the final stage, extremely swollen or distorted somata bear a few amputated dendritic stumps prior to complete cell disappearance. This spectrum of change has been found in all neurosecretory nuclei studied. Nevertheless, they are more frequently observed in rostra1 hypo-
108
MACIIADO-SALAS,
SCHEIBEL
AND
SCHEIBEL
thalamic regions where neuropil impoverishment seems to be greater. These structural changes, unequally distributed throughout the neurosecretory nuclei of the aging hypothalamus, are in general agreement with the experimental findings of Babichev and Frolkis and collaborators (6, 15, 16), and clinical observations on the dissociation of the neuroendocrine alterations in aged individuals (18, 19, 21). It has been this “spotty” pattern of physiologic and anatomic changes which has originated the concept of “hypothalamic disregulation” and its central position in the aging process (11-13, 15, 16, 20). It seems probable that an impoverished hypothalamic neuropil, progressively deprived of its dendritic surfaces, becomes increasingly unable to elaborate adequate responses even to minimal environmental challenges. Note also that the hypothalamus is the recipient of massive afferent tracts originating in various limbic structures ( 1, 29, 31, 32)) some of which also show important age-related changes as already indicated in previous studies from this laboratory (30,41,42). The importance of modulation of ovarian function by limbic structures is well established, and its significance has recently been stressed by Sawyer (35, 39). On the other hand, two hypothalamic centers have been identified (17) as the source of “tonic” and “phasic” influences upon the function of the female reproductive system. These structures are : (i) the arcuate-ventromedial nuclei, which maintain adequate levels of hypophysotropins, and consequently of gonadotropins, throughout the sexual cycle, and (ii) the preoptic-anterior hypothalamus nucleus, which is related to the surge of Iuteinizing hormone at the time of ovulation. The preceding data were obtained in normal mature animals in which anatomical circuitry and neuropil in the hypothalamus, as well as in the hindbrain and forebrain limbic systems, are presumably intact ; thus the brain-gonadal system may be considered to behave with chronometric cyclicity, allowing reproductive functions during the mature stage of the development of the individual. Afterward, a progressive decline of the function of the brain-gonadal axis becomes evident. Initially, this deterioration is subtle, due undoubtedly to redundancy of limbic connections ; later, when the behavioral manifestations of sexual activity have ceased and the sexual organs have undergone atrophy, the aging process in the hypothalamus becomes maximally evident ; the neuropil is scarce, afferents are quantitatively and qualitatively impoverished, the nerve cell bodies are physically distorted, and there is significant loss of dendritic surface. Undoubtedly, all those changes contribute importantly to aging phenomena in the brain-gonadal system. As already mentioned, there seems to be a degree of dissociation in agerelated neuroendocrine changes. Brain-gonadal functions decline in old individuals without exception, whereas other brain-endocrine relationships may show little if any change. This functional dissociation bears some resemblance to the uneven distribution of morphologic changes in the aging
hypothalamus which would provide a structural basis for the deterioration of gonadal function in old age, with relative sparing of other endocrine activities. Recently, it was demonstrated (45) that the only source of luteinizing hormone-releasing hormone is the hypothalamus, whereas the sources of thyrotropin-releasing hormone are spread throughout the nervous system (8, 25), thereby providing redundant sources for the latter and a higher probability of continuing thyroid function into later life. We have described the progressive deteriorative process found in the neuropil of the aging hypothalamus and attempted to interpret its significance in relationship to neuroendocrine alterations, with due regard to the broad framework and complexity of the mechanisms involved. We have also stressed the dissociative characteristics of the morphologic changes and emphasized the position of the old hypothalamus as an integrator of signals originating in similarly aging limbic structures. REFERENCES A. DAHLSTROI~, K. FUXE, I
