Synaptic changes in the hypothalamic arcuate nucleus of old male rats

Age-related changes in cerebellar and hypothalamic function accompany non-microglial immune gene expression, altered synapse organization, and excitatory amino acid neurotransmission deficits

We describe age-related molecular and neuronal changes that disrupt mobility or energy balance based on brain region and genetic background. Compared to young mice, aged C57BL/6 mice exhibit marked locomotor (but not energy balance) impairments. In contrast, aged BALB mice exhibit marked energy balance (but not locomotor) impairments. Age-related changes in cerebellar or hypothalamic gene expression accompany these phenotypes. Aging evokes upregulation of immune pattern recognition receptors and cell adhesion molecules. However, these changes do not localize to microglia, the major CNS immunocyte. Consistent with a neuronal role, there is a marked age-related increase in excitatory synapses over the cerebellum and hypothalamus. Functional imaging of these regions is consistent with age-related synaptic impairments. These studies suggest that aging reactivates a developmental program employed during embryogenesis where immune molecules guide synapse formation and pruning. Renewed activity in this program may disrupt excitatory neurotransmission, causing significant behavioral deficits.

Serum testosterone and estradiol in sudden infant death

OBJECTIVE

To test the hypothesis that among infants who die unexpectedly, testosterone and/or estradiol levels are elevated in those diagnosed with SIDS versus those with known causes of death (controls).

STUDY DESIGN

Postmortem blood was collected and coded from infant autopsies, and serum was prepared and frozen until assayed for total testosterone and estradiol by fluoroimmunoassay. Subject information was then collected from the medical examiner's report.

RESULTS

Testosterone, but not estradiol, was significantly higher in 127 SIDS cases versus 42 controls for both males (4.8 +/- 0.4 vs 2.2 +/- 0.4 nmol, respectively; P < .005) and females (2.4 +/- 0.2 vs 1.6 +/- 0.2 nmol, respectively; P < 0.03).

CONCLUSIONS

Higher testosterone levels in infant victims of unexpected, unexplained death may indicate a role for testosterone or related steroids in SIDS. Further research is needed to understand the potential utility of testosterone as an indicator of SIDS risk.

Androgen stimulates neuronal plasticity in the perineal motoneurons of aged male rats

Ten aged male rats (24 months of age) were castrated and implanted subcutaneously with Silastic capsules containing testosterone (T)(5 males) or nothing (5 males). Five sham-castrated males (25 months of age) served as controls. Four weeks after castration, cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 150 CT-HRP-labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the control and experimental SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in castrated, aged males treated with T was significantly greater than that in control or castrated animals. Size and number of synaptic contacts per unit length of somatic membranes in castrated, aged males treated with T were also significantly greater than those in control or castrated animals. Plasma levels of T in castrated, aged males treated with T were significantly greater than that in controls. These results suggest that the SNB motoneurons of aged male rats retain a considerable synaptic plasticity in response to androgen, and that androgen may be, at least in part, involved in the process of aging of the SNB system in male rats.

Arcuate nucleus of the hypothalamus: effects of age and sex

The arcuate nucleus of the hypothalamus (ARN) is involved in a variety of functions known to be sexually dimorphic and altered by aging. Although the effects of sex and age on the synaptic organization and neurochemistry of the ARN have been extensively analyzed, data regarding sex-related differences and age-induced effects on the total number of neurons and volume of the ARN in adult and aged male and female rats are controversial. To address this issue, we have quantitatively analyzed the ARN of male and female Wistar rats aged 6 and 24 months. The optical fractionator, the optical rotator, and the Principle of Cavalieri were used as the estimators of the total number of neurons, mean nuclear volume of ARN neurons, and volume of the ARN, respectively. In addition, a Golgi study was carried out to analyze the dendritic trees of its neurons. We found that in young adult rats, the volume of the ARN is 0.9 mm3 in males and 0.7 mm3 in females, whereas the total number of neurons is 100 x 10(3) in males and 86 x 10(3) in females. ARN neurons of males and females have identical mean nuclear volumes, which we estimated to be 300 microm3. No significant effects of age were found in these parameters, both in males and in females. In adult rats, no sex-related differences were detected in the number of dendritic segments and in the total dendritic length, but the dendritic branching density and the spine density were greater in females than in males. In aged rats there was a significant reduction in the number of dendritic segments, in the total dendritic length, and in the branching and spine densities that, although evident in both sexes, was more marked in females. Our results show that the total number of neurons and the volume of the ARN are sexually dimorphic in adult and aged rats and that neither of these parameters is altered by aging. Conversely, aging induces regressive changes in the dendritic arborizations of ARN neurons of males and females and abolishes the sexual dimorphic pattern of their organization.

Synaptic changes in the perineal motoneurons of aged male rats

Cholera toxin-horseradish peroxidase (CT-HRP) was injected into the bulbocavernosus muscles of young (2 months of age) and old (19-20 months of age) male rats, and animals were killed 2 days later. The spinal cords containing the spinal nucleus of the bulbocavernosus (SNB) were dissected, processed with a modified tetramethylbenzidine method for visualization of retrogradely transported CT-HRP, and examined ultrastructurally. Neuronal structures apposing the membranes of 120 CT-HRP-labeled SNB motoneurons were analyzed by measuring the percentage of somatic membranes covered by synaptic contacts, synaptoid contacts, and neuron-neuron contacts. Most of the neuronal structures in the young and old SNB motoneurons consisted of synaptic contacts. The mean percentage of somatic membranes covered by synapses in old rats was significantly smaller than that in young ones. Size and number of synaptic contacts per unit length of somatic membranes in old animals were also significantly reduced. Plasma levels of testosterone in old males were significantly smaller than those in young ones. These age-related changes in synaptic inputs to SNB motoneurons and plasma levels of androgen seem to correlate with aging of the SNB system.

Age and sex do not affect the volume, cell numbers, or cell size of the suprachiasmatic nucleus of the rat: an unbiased stereological study

The circadian rhythms displayed by numerous biological functions are known to be sex specific and affected by aging. It has not been settled yet whether the sex- and age-related characteristics of circadian rhythms derive from changes in the anatomy of the suprachiasmatic nucleus. To shed light on these issues, we applied unbiased stereological techniques to estimate the volume of the suprachiasmatic nucleus as well as the total number of its cells and the mean volume of their somata and nuclei in progressively older groups of male and female Wistar rats (aged 1, 6, 12, 18, 24, and 30 months). The volume of the nucleus was estimated with the Cavalieri principle on serial sections. The total numbers of neurons and astrocytes were estimated by applying the optical fractionator, and the mean somatic and nuclear volumes of cells were estimated by using isotropic, uniform random sections and the nucleator method. On average, the volume of the suprachiasmatic nucleus was 0.044 mm3, and the total number of neurons and astrocytes was 17,400. Cells of the dorsomedial and ventrolateral components of the nucleus, which are morphologically different, have identical mean perikaryal and nuclear volumes, which we estimated to be 750 microns3 and 400 microns3, respectively. We further demonstrated that, at all ages analysed, the volume of the suprachiasmatic nucleus, the total cell number, and the mean somatic and nuclear volumes of its cells are affected neither by the age nor by the sex of the animal, regardless of the presence of sex- and age-related variations in circadian rhythms. However, the possibility that females may display changes in the volume of the suprachiasmatic nucleus at older ages cannot be ruled out. No effect of aging was observed in the total number of neurons or in the total number of astrocytes.

A possible role of glutamate in the aging process

Glutamate (Glu) is considered here for its possible role as a naturally occurring mammalian 'age inducing' substance. The existence of 'Glu elicited headaches', may serve as an indication that Glu could negatively affect the human adult CNS. The prevalence of Glu induced headaches was found to be 28.8% in a study population of 201 subjects. Circumstantial similarities between brain aging and Glu toxicity are presented in the paper. Finally, it is mentioned that Vitamin E is partially effective in blocking Glu induced headaches.

Light and electron microscopic analysis of two divisions of the suprachiasmatic nucleus in the young and aged rat

The suprachiasmatic nucleus (SCN) is a principal controller of mammalian circadian rhythms. However, in spite of documented disturbance of biological rhythms in old animals, few significant age-related changes have been observed in this nucleus. This study examined age-related differences in SCN volume, neuronal number, density, and ultrastructural features in the entire rat SCN and in its two divisions, the denser ventromedial (compacta) and less dense dorsolateral (dissipata). Light and electron microscopic morphometric techniques were utilized in weanlings (21-28 days), young adults (3-6 mo), and aged (30-36 mo) animals. The total SCN volume, as well as volumes of the compacta region, were significantly greater in young adult and aged rats than in weanlings. Thus, as the rat ages the SCN increases in total size. However, the dissipata region appears to decrease in volume while the compacta increases. Even though the total number of SCN neurons was quite constant in the three age groups, the number of neurons in the dissipata region was decreased significantly in the young adult and aged groups as compared to the weanling. Neurons in the compacta region were usually spindled-shaped with two dendritic processes, while oval to spheroidal cells with 3-4 processes predominated in the dissipata. Nuclei of SCN cells were often invaginated. In weanlings, more SCN neuronal nuclei had invaginated nuclei in the dissipata region (66%) compared to the compacta (37%). In the two older age groups of rats, a higher percentage of invaginated neuronal nuclei were found in both regions. However, more were still found in the dissipata (90%) compared to the compacta (72%), even though the number of these cells in the compacta doubled. Thus, there was a large increase in the number of invaginated nuclei, as well as the number of invaginations, in the young adult rats compared to the weanling group, and this increase persisted in aged rats. SCN neurons usually had nuclei surrounded by a thin perimeter of cytoplasm containing sparse mitochondria and granular endoplasmic reticulum, multiple Golgi regions, and a moderate number of free ribosomes. In weanlings, mitochondria contained dense cristae and the granular endoplasmic reticulum was relatively prominent. Degenerative ultrastructural changes which included mitochondrial enlargement/vacuolation, Golgi vacuolation, lysosome, and lipofuscin development occurred in less than 10% of young adult SCN cells, and were more frequently found in the dissipata. In aged, rats 30% of the neurons showed degenerative changes in the dissipata compared with 18% in the compacta. Degenerative changes appeared highly correlated with the degree of membrane folding.(ABSTRACT TRUNCATED AT 400 WORDS)

Chronic estradiol administration did not cause loss of hypothalamic LHRH or TIDA neurons in young or middle-aged C57BL/6J mice

Age-related decline in estrous cycle frequency and impaired pre-ovulatory gonadotropin surges at mid-life are modelled in young C57BL/6J mice by chronic (3 months) oral administration of estradiol (E2). However, the cellular events that induce damage to the neuroendocrine center that regulate gonadotropins with age or following E2 treatment are unclear. To address this issue, possible neuron loss was examined in relation to the loss of estrous cyclicity in E2-treated mice, in particular neurons of the hypothalamic luteinizing hormone releasing hormone (LHRH) and/or tuberoinfundibular dopaminergic (TIDA) systems. By immunocytochemical methods, there was no change in the number of LHRH or TIDA neurons in mice that have become acyclic due to age or E2 treatment. We conclude that the onset of acyclicity at middle-age or following chronic E2 treatment is not associated with loss of LHRH or TIDA neurons and that other neuroendocrine changes must be considered for the cause of acyclicity, particularly those involved in the synaptic regulation of LHRH secretion.

Loss of sexual dimorphism in rat arcuate nucleus neuronal membranes with reproductive aging

Arcuate neurons of the rat hypothalamus have a sexual dimorphic membrane phenotype: quantitative analysis of freeze-fracture replicas has revealed that a population of intramembrane protein particles (IMP) of small size (less than 10 nm) is enriched in the plasma membrane of perikarya and dendritic shafts of cycling females compared to males, whereas a population of large IMPs (greater than 10 nm) is enriched in the membrane of dendritic shafts of males. This different membrane organization is associated with a sex dimorphic synaptic connectivity. To determine whether sex differences in neuronal membrane are affected by reproductive senescence, IMPs were assessed in freeze-fracture replicas of arcuate neuronal plasma membranes of male and female Sprague-Dawley rats aged 3, 15, and 18 months. Three-month-old cycling females were studied on the morning of estrus. Senescent females were in constant estrus (15 months old) or in constant diestrus (18 months old). Young females had more IMPs with diameters under 10 nm in the inner and outer leaflets of the plasma membrane of the perikarya and dendritic shafts compared to males of the same age. In addition, young males showed an increased number of large (greater than 10 nm) IMPs in the outer membrane leaflet of dendritic shafts. No sex differences were detected in the membrane of dendritic spines. In senescent females the number of small IMPs was decreased in the perikarya and dendritic shafts compared to young females while the number of large particles was increased in the outer leaflet of the membrane of dendritic shafts, reaching values similar to those observed in males. IMP counts were not modified with aging in males and in dendritic spines of females. These results indicate that reproductive aging in female rats is associated with a remodeling of neuronal plasma membranes in arcuate neurons.

Synaptology of luteinizing hormone-releasing hormone neurons in the preoptic area of the male rat: effects of gonadectomy

Ultrastructural analysis of the synaptic input to luteinizing hormone-releasing hormone neurons has previously shown that in male rats these cells acquire an increased density of innervation with increasing age [Witkin J. W. (1987) Neuroscience 22, 1003-1013]. To determine if this aging phenomenon might be due to changes in the steroid environment, we examined luteinizing hormone-releasing hormone neurons, using methods identical to the earlier study, in sham operated and 1 day and 4 week castrated male rats. The density of synaptic input to luteinizing hormone-releasing hormone neurons did not vary among the three experimental groups. Furthermore, there were no differences in the relative numbers of various morphological categories of synaptic boutons (characterized by vesicle type: clear vs dense and round vs pleomorphic) among the groups. Long term castration did result in a slight decrease in immunocytochemically detectable luteinizing hormone-releasing hormone neuron numbers. These results suggest that gonadal steroid deprivation does not alter the total density or morphological characteristics of synaptic input to the luteinizing hormone-releasing hormone neuron in the male rat.

Age-related loss of synaptic terminals in the rat medial nucleus of the trapezoid body

The effect of aging on axosomatic synaptic terminals in the rat medial nucleus of the trapezoid body was studied using quantitative electron microscopy. In young adult rats (3 months of age), the mean percentage of the surface area of principal cells covered by synaptic terminals is 61.7% (S.E.M. = 4.1) while in aged animals (27-33 months of age) the per cent coverage is 43.7% (S.E.M. = 3.3). Likewise, between 3 and 27-33 months of age, the average number of synaptic terminals present along a 100 micron length of principal cell surface decreases significantly (P less than 0.001) from 28.3 (S.E.M. = 1.3) to 18.9 (S.E.M. = 1.3). Only terminals derived from calyces of Held are lost in the aged animals, displaying a 37% reduction between 3 and 27-33 months of age. The length of apposition by synaptic terminals in the medial nucleus of the trapezoid body does not change significantly with aging. We conclude that because of a significant loss of calycine synaptic endings, the structure of calyces of Held becomes less complex with advancing age in rats. This would presumably result in an age-related partial deafferentation of principal cells, causing significant alterations in the processing of auditory information in the medial nucleus of the trapezoid body.

Neuron numbers and dendritic extent in normal aging and Alzheimer's disease

Factors which limit the interpretation of studies of aging brain include: secular trends, species and strain differences, effects of tissue processing, and bias which may be introduced at many levels of an experimental design. With these limitations considered, evidence is reviewed regarding neuron numbers and dendritic extent in normally aging rodent, monkey and human brain and in Alzheimer's disease. It is concluded that neuron loss and change in dendritic extent in normal aging are regionally specific, and that corresponding brain regions do not always change in similar ways in rodents and primates. It is suggested that such differences may, in part, be due to inconsistent definitions of 'aged' among species. In Alzheimer's disease there is excess neuron loss and dendritic regression in some, but not all, brain regions. Measures of the morphological substrates of brain function show appreciable overlap between AD and control groups. It is hypothesized that the static, post-mortem status of brain morphology may not adequately reflect the functional capabilities of the dynamic morphology of the living brain.

Synaptic structural changes during development and aging

Although a great deal is known about the development of synaptic number, comparatively little is known about the effects of development, and particularly aging, on the structure of the synapse. The present study examined synaptic structure in the molecular layer of the motor-sensory neocortex during early development (postnatal days (P) 1, 3, 5, 7, 10, 15, 20, 30), adulthood (P60, 90), and old age (28 months). Tissue was stained with osmium tetroxide (osmium) or ethanol phosphotungstic acid and the following synaptic characteristics were quantified: (1) presynaptic element length, area, thickness, maximal projection height and smoothness, and number and size of vesicles adjacent to the presynaptic element; (2) postsynaptic element length, area, and thickness; and (3) cleft width. There is an early developmental increase in synaptic element length, followed by an increase in thickness into adulthood. During development the height and width of the presynaptic dense projections increase, after which they remain stable. While the number of adjacent synaptic vesicles increases throughout the lifespan, there is a parallel decrease in their size. During the period of rapid synaptogenesis in this brain region there are no decreases in any of the synaptic structural parameters examined, indicating that newly generated synapses are either formed the same size as the existing mature synapses, or are extremely plastic and grow very rapidly. Unlike age-associated changes in synaptic number, no changes were found in synaptic structure during aging.

Aging changes in synaptology of luteinizing hormone-releasing hormone neurons in male rat preoptic area

This study was undertaken to examine some aspects of the anatomical substrate for reproductive senescence. Immunocytochemically identified luteinizing hormone-releasing hormone neurons and their processes in the male rat brain preoptic area were compared in young adult (2-4 months), middle-aged (12-14 months) and old (20-23 months) animals. At the light microscopic level there were no age-dependent differences in total numbers or sizes of LHRH neurons nor in their distribution in the brain. Examination of these neurons at the electron microscopic level did reveal significant differences in certain organelles and in the degree and kind of synaptic input. Random sections of middle-aged luteinizing hormone-releasing hormone neurons more frequently passed through the nucleolus and the incidence of nematosomes was higher than in luteinizing hormone-releasing hormone neurons from the young and old animals. Quantitative measures of synaptic input to luteinizing hormone-releasing hormone soma and dendrites as well as to unidentified neurons in the same thin section were made. These are reported as percent of membrane that showed synaptic structure. Dendrites of both luteinizing hormone-releasing hormone and nonidentified neurons were more densely innervated than perikarya. The density of synaptic input to luteinizing hormone-releasing hormone neurons was significantly greater than that to nonidentified neurons in young and middle-aged animals, but was equal to that of nonidentified neurons by old age. Age-related changes were noted in synaptic organization with the most significant change being an increased input to luteinizing hormone-releasing hormone perikarya. Indeed, synaptic input to luteinizing hormone-releasing hormone perikaryal membrane was increased three-fold by middle age and ten-fold by old age. Density of synaptic input to luteinizing hormone-releasing hormone dendritic membrane did not change with age. There were no aging changes in percentage of membrane with synaptic structure in nonidentified elements. Synapses were also classified on the basis of their synaptic vesicle content. There were proportionately more synaptic boutons containing round clear than pleomorphic vesicles in the young sample. The proportion of synapses with pleomorphic vesicles increased with age onto both luteinizing hormone-releasing hormone perikarya and their dendrites. The proportion of boutons containing some electron dense-core vesicles along with clear vesicles decreased with age onto both luteinizing hormone-releasing hormone and nonidentified neurons and their processes.

Neocortical synaptogenesis, aging, and behavior: lifespan development in the motor-sensory system of the rat

Little evidence presently exists on the development and aging of synaptic contacts and their relationship to behavior, particularly in nonvisual brain areas. To investigate this interrelationship, rats at a series of developmental ages [postnatal day 1 (P1) to P90] were initially examined on a battery of motor tasks. The battery, ranging from simple reflexive tests to tests of complex locomotor capacities, consisted of tactile-induced forelimb placing, chin-induced placing, body righting, climbing an inclined plane, traversing a narrow beam, and keeping up with a revolving wheel. Following completion of the behavioral testing, the animals, together with an additional group of aged (28- to 29-month-old) rats, were killed and their motor-sensory cortex was removed, stained with osmium tetroxide or ethanol phosphotungstic acid (EPTA), and examined under electron microscopy for density of synaptic contacts. Simple motor abilities such as tactile-induced placing was present by the end of the first postnatal week, with locomotor performance reaching a mature level by the end of the third postnatal week, and intermediate task abilities maturing within this range. Paralleling the development of complex locomotor skills was a sharp increase in synaptic density in the molecular layer of the motor-sensory cortex, commencing in the second postnatal week and peaking at P30. After P30 there was a sharp decline in synaptic density as well as a decline in performance on some motor tasks, although these two functions seemed to be occurring independently. There was a continued, but less dramatic synaptic loss evident in the aged rats.

Influence of aging on catecholamine levels, accumulation, and release in F-344 rats

Neuronal function in selected brain areas has been evaluated in Fischer 344 rats aged 2-4 months, 11-14 months and 21-26 months. In vitro release of 3H-norepinephrine from hypothalamus and occipital cortex and 3H-dopamine from striatum has been evaluated using potassium, amphetamine, and field-stimulation. In vitro uptake of 3H-catecholamines has been evaluated in the same tissues. Catecholamine levels were measured in six brain areas: hypothalamus, striatum, cortex, cerebellum, brainstem and midbrain. Significant age-related decreases of NE levels, uptake, and release to high frequency stimulation were seen in the hypothalamus. The decreases in 3H-NE uptake and NE levels in the hypothalamus were apparent at 12 months, whereas the decrease in 3H-NE release after high frequency stimulation was seen in the senile rats.

Synaptogenesis in the neonatal preoptic area grafted into the aged brain

The medial preoptic area (POA) of newborn female rats was transplanted into the third ventricle of young adult (2 months of age) and aged (27 months of age) female rats. The grafted POA tissues were examined ultrastructurally 2 months after transplantation. The mean numbers of axodendritic shaft and spine synapses in the POA transplants in both young adult and aged female rats were much greater than those in the medial POA of newborn female rats. These results suggest that the neuronal substrates in the medial POA grafts develop well in the brain of the aged female rats.

Estrogen stimulates neuronal plasticity in the deafferented hypothalamic arcuate nucleus in aged female rats

The hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally 3 weeks after the complete deafferentation of the medial basal hypothalamus (MBH) with the island isolation technique in ovariectomized aged female rats (720-930 days of age). The mean numbers of axodendritic and axosomatic synapses in the ARCN decreased to about one-third of those in the intact controls. However, the treatment with estradiol benzoate (2 micrograms/day) during the 3 weeks following the day of brain surgery brought about a marked increase in the numbers of these synapses. The data suggest that the ARCN neurons of aged female rats still retain plasticity to react to deafferentation under influences of estrogen.

Neuron numbers in hypothalamic nuclei of young, middle-aged and aged male rats

Morphologic analysis of nine hypothalamic areas revealed significant decreases in the number of neurons per unit area in the ventral medial and arcuate nuclei. These data suggest that altered neuron numbers in the VMW and perhaps the ARC may participate in the well documented reductions in endocrine and neuroendocrine function observed in aging rats.

Catecholamine synapses and contents in the paraventricular hypothalamic nucleus and nucleus tractus solitarius of DOCA-salt hypertensive rats

Central catecholamine (CA) neurons in the nucleus tractus solitarius (NTS) and paraventricular hypothalamic nucleus (PVN) were studied in Wistar rats that had been unilaterally nephrectomized. The experimental animals were then treated with deoxycorticosterone acetate (DOCA) and salt water. The control animals were treated with the vehicle and tap water. Blood pressure of animals 4 weeks after DOCA/salt treatment was significantly elevated when compared to control rats. Morphologically, CA terminals showed no noticeable changes in the DOCA/salt hypertensive rats. Furthermore, the density of CA terminals either in the NTS or in the PVN of the DOCA/salt hypertensive rats was not statistically different from that of normotensive controls, suggesting that salt does not cause lesions or destruction of CA terminals. However, an extensive electron-microscopic morphometric analysis indicated that there was an enhancement of CA synaptogenesis (expressed by increased synaptic frequency among all CA boutons labeled with 5-hydroxydopamine) in the PVN, but not in the NTS of DOCA/salt hypertensive rats. In addition, the high-performance liquid chromatography revealed decreased CA contents in the PVN, but not in the NTS, of DOCA/salt hypertensive animals. Since synapses are primary sites for neurotransmitter release, the above results collectively suggest that more CA synapses formed in the PVN may reflect a net CA release from CA terminals resulting in the decreased CA content in the axonal terminals. Such an increased CA release and enhanced CA synaptogenesis may consequently enhance CA function in the PVN of hypertensive rats 4 weeks after DOCA/salt treatment, and relate to the development and/or maintenance of hypertension in the DOCA/salt rats.