Quantitative analysis of the cervical sympathetic trunk in young adult and aged rats

Age-related sympathetic ganglionic neuropathology: human pathology and animal models

Systematic studies of the autonomic nervous system of human subjects and development of well-defined animal models have begun to substantially improve our understanding of the pathogenesis of autonomic dysfunction in aging and may eventually provide strategies for intervention. Neuropathological studies of the sympathetic ganglia of aged human subjects and rodent models have demonstrated that neuroaxonal dystrophy involving intraganglionic terminal axons and synapses is a robust, unequivocal and consistent neuropathological finding in the aged sympathetic nervous system of man and animals. Quantitative studies have demonstrated that markedly swollen argyrophilic dystrophic axon terminals develop in the prevertebral superior mesenteric (SMG) and coeliac, but to a much lesser degree in the superior cervical ganglia (SCG) as a function of age, sex (males more than females) and diabetes. Dystrophic axons were immunoreactive for neuropeptide Y, tyrosine hydroxylase, dopamine-beta-hydroxylase, trkA and p75NTR, an immunophenotype consistent with their origin from postganglionic sympathetic neurons, and contained large numbers of highly phosphorylated neurofilaments or tubulovesicular elements. The sympathetic ganglia of aged rodents also showed the hallmark changes of neuroaxonal dystrophy as a function of age and location (many more in the SMG than in the SCG). Plasticity-related synaptic remodeling could represent a highly vulnerable target of the aging process. The fidelity of animal models to the neuropathology of aged humans suggests that similar pathogenetic mechanisms may be involved in both and that therapeutic advances in animal studies may have human application.

Plasticity of aged perivascular axons following exogenous NGF: analysis of catecholamines

The present study investigated the atrophy of aged perivascular sympathetic axons and the response of these cerebrovascular neurons to the neurotrophin nerve growth factor (NGF). Using high performance liquid chromatography coupled with electrochemical detection (HPLC-ECD) to quantify catecholamines and immunohistochemical methods to quantify the density of TH immunoreactive fibers, we found a significant decrease in norepinephrine (NE) and TH in aged sympathetic axons. However, following in vivo administration of exogenous neurotrophin, aged neurons exhibited a robust response to NGF that was similar to the young adult, suggesting little decline in the capability of aged neurons to utilize exogenous neurotrophin. These results suggest that the age-related atrophy of aged sympathetic axons may result primarily from reduced availability of target-derived neurotrophin rather than from intrinsic alterations of neuronal function.

Regulation of neurotrophin signaling in aging sensory and motoneurons: dissipation of target support?

A hallmark of senescence is sensorimotor impairment, involving locomotion and postural control as well as fine-tuned movements. Sensory and motoneurons are not lost to any significant degree with advancing age, but do show characteristic changes in gene-expression pattern, morphology, and connectivity. This review covers recent experimental findings corroborating that alterations in trophic signaling may induce several of the phenotypic changes seen in primary sensory and motoneurons during aging. Furthermore, the data suggests that target failure, and/or breakdown of neuron-target interaction, is a critical event in the aging process of sensory and motoneurons.

Vacuolar neuritic dystrophy in aged mouse superior cervical sympathetic ganglia is strain-specific

We have developed a model of autonomic nervous system aging using the mouse superior cervical sympathetic ganglion (SCG) which is characterized by the reproducible development of distinctive, markedly-enlarged neuritic swellings (vacuolar neuritic dystrophy, VND). These structures contained an admixture of lucent vacuoles and subcellular organelles, and involved both presynaptic and postsynaptic ganglionic elements. Quantitation of the frequency of VND was accomplished at the light microscopic level and validated by ultrastructural examination. VND lesions were 30-100-fold more frequent in the aged mouse paravertebral SCG than in the prevertebral celiac/superior mesenteric (C/SMG) sympathetic ganglia. Although VND was identified in all ages of mice examined, the number of lesions increased significantly with age. The frequency of VND was a function of the strain of mouse examined with a 40-fold difference in VND frequency between C57BL6 mice, the least involved strain, and the DBA/2J strain, which was most affected and began to develop significant numbers of lesions at an early age. As in our human studies of aging in the sympathetic nervous system, there was a prominent gender effect with males developing twofold greater numbers of VND lesions than females. Mice maintained on a significant calorie restricted diet for 30 months developed 70% fewer lesions than ad libitum-fed, age and sex matched controls. The aging mouse SCG, therefore, represents a robust animal model with reproducible, quantifiable and unambiguous neuropathology. Insights into pathogenetic mechanisms gained in the subsequent analysis of this relatively simple peripheral sympathetic nervous system model may contribute to the understanding of some of the most complex and significant problems involving higher brain function.

Uninjured aged sympathetic neurons sprout in response to exogenous NGF in vivo

The extent to which the loss of plasticity by aged neurons is due to changes in the neuronal environment or to a loss of growth potential of the neurons has not been determined. In previous studies we observed that young adult cerebrovascular axons undergo a sprouting response following a 2-week intracerebroventricular infusion of nerve growth factor (15 microg; NGF). The present study used electron microscopy to examine the innervation of the intradural segment of the internal carotid artery of the aged rat and to determine whether aged sympathetic perivascular axons would respond to in vivo infusion of NGF. Young adult and aged Fischer 344 female rats received a 2-week intracranial infusion of NGF (15 microg) or vehicle (VEH) and were perfused for electron microscopy. Although there was no change in the total number of perivascular axons associated in aged VEH when compared with young adult VEH, a significant reduction was observed in aged VEH when total axons and sympathetic axons were expressed per microm2 vascular wall, reflecting an age-related increase in blood vessel size. Following NGF infusion, aged sympathetic axons were significantly increased by 192% compared with aged VEH cases. These results suggest that there is a proportional reduction in sympathetic cerebrovascular neurons with aging but that they exhibit robust sprouting in response to an exogenous neurotrophin.

Neurons and microvessels of the nodose (vagal sensory) ganglion in young adult and aged rats: morphometric and enzyme histochemical studies

The neuron cell bodies and microvessels in sections of the nodose (vagal sensory) ganglion (NG) of Wistar rats of 4- and 24-months of age have been examined morphometrically and by quantitative enzyme histochemistry. The range of neuronal somata areas was similar at the two ages and distributed unimodally, ranging approximately from 200-1500 microns 2 with the largest somata occurring in the older age group. The range of microvessel diameters was also comparable but the largest microvessels were seen in the older animals. The histological arrangement of the ganglion permitted analyses to be made of 'neuronal' and 'axonal' areas independently. The number of microvessels per unit area was less in regions of the ganglion occupied by axons at both ages. Random transects indicated that the percentage area occupied by neuron somata decreases and that of axons increases with age. Overall, however, the results suggest that the histological organization, the size of vagal sensory neurons, the ganglionic microvessels, and the relationship between them, does not change greatly in Wistar rats of up to 2 years of age. Ultrastructural features of the aged sensory neurons included the presence of secondary lysosomes, disrupted rough endoplasmic reticulum, swollen Golgi cisternae, and the presence of much filamentous material in the perikaryon similar to that seen in chromatolytic neurons. However, analysis of electron micrographs did not reveal significant changes in the numbers of mitochondria or Golgi bodies. There was an overall increased thickness in the microvascular wall in the older animals with the endothelium and pericyte covering being significantly increased, but the thickness of the basal lamina was unchanged. The activities of neuronal NADH tetrazolium reductase, succinate dehydrogenase and cytochrome oxidase were all increased with age. The results suggest that vagal sensory neurons are not greatly affected by age in the rat.

Preservation of the cervical vagus nerve in aged rats: morphometric and enzyme histochemical evidence

The vagus nerve trunk, sampled at a mid-cervical level, has been analysed quantitatively by light and electron microscopy principally with respect to the numbers and sizes of the myelinated and unmyelinated axon populations in Wistar rats of 4, 24 and 30 months. No significant differences in total myelinated axon numbers were seen over the age range in counts made on light microscope montages of the entire cross-section of the nerve. The overall histological organisation and appearance of the nerve trunk did not change with age but age-associated alterations in the ultrastructure of some myelinated fibres and their Schwann cells was seen. Unmyelinated axons and their associated Schwann cells rarely showed age-associated changes. The numbers of myelinated and unmyelinated axons per mm2 determined from electron micrographs were, however, slightly decreased but the ratio of myelinated to unmyelinated axons was approximately 1:4 at all ages. Measurements of myelinated fibres showed a small but significant increase in size between young and old animals. There was an increase in the thickness of the myelin sheath, a decrease in myelinated axon diameter and in the the g ratio. The diameter of unmyelinated axons decreased with age and the number of unmyelinated axons per Schwann cell unit increased. The numbers, diameters and thickness of the walls of the vagal microvessels remained unchanged. Quantitation by microdensitometry of the activity of NADH tetrazolium reductase and succinate dehydrogenase in longitudinal sections of the vagus indicated an increase in the activity of these two metabolic markers whilst an increase in the activity of cytochrome oxidase indicated that neuronal activity in the vagus was unimpaired in old age. It is concluded that the structure of the rat vagus nerve, and in particular of its predominantly unmyelinated axon population, is not significantly affected in old age.

Tyrosine hydroxylase- and nitric oxide synthase-immunoreactive nerve fibers in mitral valve of young adult and aged Fischer 344 rats

Using confocal fluorescence microscopy we studied, in whole mounts of heart mitral valves of young adult and aged Fischer 344 rats, the distribution of nerves containing the catecholamine marker tyrosine hydroxylase (TH) or the synthetic enzyme marker for nitric oxide, nitric oxide synthase (NOS). TH-IR was localized in two separate nerve plexuses which do not intermingle. The 'major' plexus arose from the annulus region, traversed the basal zone of the valve, and ramified in the intermediate zone to form a dense network of fine fibers. The 'minor' plexus was restricted to the distal zone and originated from bundles that ascended the chordae tendineae to enter the valve cusp. A concentric zone located between the major and minor plexuses was devoid of TH-IR nerve fibers. Both plexuses demonstrated (i) nerves that contained numerous varicosities along the length of each fiber, (ii) many terminal axons and (iii) different shaped terminal axon endings. With age, the density of TH-IR innervation in the mitral valve was markedly reduced; and nerve fibers of the minor plexus were limited to the chordae tendinae, without extending into the valve cusp itself. NOS-IR fibers in the mitral valve formed a loose network that extended from the annulus to more than halfway down the cusp. The varicose beads of the terminal NOS-IR axons appeared to become progressively smaller and less intensely fluorescent until they disappeared at the terminal endings, which showed no specializations. No NOS-IR fibers were observed in the distal zone of the valve leaflet or in the chordae. In the aged mitral valve, the density of NOS-IR nerves was decreased, as compared with NOS-IR innervation in the young adult valve. The existence of TH and NOS as well as other signal molecule markers in heart valve nerves and the disparate patterns of their distribution and localization provide evidence supporting the theory that heart valve nerves form a complex reflexogenic control system in the mitral heart valve. In summary, two distinct neural architectures are described for TH-IR and NOS-IR valve nerves, respectively. The former are believed to be axons dedicated to sympathetic motor functions. The NOS-IR valve nerves may have sensory and/or postganglionic parasympathetic motor functions. An implication of these findings is that different, but perhaps related, valve functions may be mediated by separate, dedicated circuits.

Reactions of rat sympathetic neurons to ethanol exposure are age-dependent

Age-differences in the sensitivity of peripheral sympathetic neurons to chronic ethanol exposure and ethanol withdrawal were studied in male Wistar rats aged 4 months, 12 months, or 24 to 25 months. The superior cervical ganglia (SCG) of the young (4 months) and the 2-year-old rats responded to a 12-day or 4-week ethanol exposure with significantly increased catecholamine turnover, while the ganglia of the middle-aged rats (12 months) showed only a minor increase in the intensity of catecholamine fluorescence and tyrosine hydroxylase immunoreactivity. Extensive neuronal vacuolation was found in the 4 months ethanol-exposed SCG, probably as a reaction of a subpopulation of neurons to increased stimulation. Ethanol-induced neuronal loss was most prominent in the SCG of the oldest age group. Contrary to the marked changes in SCG functional and morphometric parameters, the pelvic sympathetic neurons in the hypogastric ganglion showed no significant changes after ethanol exposure. The pattern of ethanol-induced morphological alterations found in the present study did not provide unambiquous support for either the "accelerated aging" or the "increased vulnerability" concept regarding ethanol-aging interactions in the nervous system.