Age-related changes in calbindin and calretinin immunoreactivity in the central auditory system of the rat

Age-related changes in the levels of major intracellular calcium buffers are known to occur in different parts of the mammalian brain, including the central auditory pathway. In the present study, we evaluate with immunohistochemistry and the western blot technique the effect that aging has on the calbindin- and calretinin-expressing system of neurons in the higher structures of the central auditory pathway, in the inferior colliculus (IC), medial geniculate body (MGB) and auditory cortex (AC) of two rat strains, the slowly aging Long-Evans and the fast aging Fischer 344. Interestingly, the age-related changes demonstrated a similar character regardless of the rat strain. In the IC of young animals, the majority of calbindin and calretinin immuno-reactive (CB and CR-ir) cells were found in the dorsal and external cortices and only sparse positive cells were present in the central nucleus of the IC. With aging, the number of CB-ir and CR-ir neurons decreased significantly in both the dorsal and external cortices. Furthermore, these declines were accompanied by an age-related reduction in the mean volumes of CB- and CR-ir neuronal somas. In the MGB of young rats, CB-ir neurons were present in abundant numbers in both the dorsal and ventral subdivisions, while CR-ir neurons were practically absent in this structure. With aging, the number and mean volume of CB-ir cells in the ventral subdivision of the MGB were significantly decreased. In comparison with the IC and MGB, age-related numerical and volumetric declines of both CB-ir and CR-ir neurons in the AC were less pronounced. Western blot protein analysis revealed a pronounced age-related decline in the levels of calbindin in both strains and in all examined brain regions. In contrast, the decline in calretinin levels with aging was less prominent, with a significant decline only in the IC of both strains. The observed age-related changes in the calbindin- and calretinin-expressing systems may contribute significantly to the deterioration of hearing function known as central presbycusis.     

Age-related changes in GAD levels in the central auditory system of the rat

Changes in the levels of gamma-aminobutyric acid (GABA) are known to occur in different parts of the brain during aging. In our study we attempted to define the effect that aging has on glutamate decarboxylase (GAD), the key enzyme in the synthesis of GABA, in the central parts of the auditory system. Age-related changes in GAD65 and GAD67 levels were investigated using immunohistochemistry and Western blotting in the inferior colliculus (IC), the auditory cortex (AC) and the visual cortex in Long-Evans rats. The results show that aging is associated with a decrease in the numbers of GAD65- and 67-immunoreactive neurons and the optical density of their somas in both the IC and AC. Western blot analysis revealed a pronounced age-related decline in the levels of GAD65 and 67 proteins in both the IC and AC. For comparison, in the visual cortex the decrease in both proteins was less pronounced than in the IC and AC. A similar pattern of age-related changes was found in Fischer 344 rats, a strain that manifests a rapid loss of hearing function with aging. The observed age-related decline in the levels of GAD65 and 67 may contribute significantly to the deterioration of hearing function that accompanies aging in mammals, including man.     

Collagen changes in the cochlea of aged Fischer 344 rats

Hearing function in the Fischer 344 (F344) albino inbred strain of rats deteriorates with aging faster than in other strains, in spite of the small hair cell loss in old F344 animals [Popelar, J., Groh, D., Pelanova, J., Canlon, B., Syka, J., 2005. Age-related changes in cochlear and brainstem auditory function. Neurobiol. Aging, in press.]. This study was aimed at elucidating the structural changes in the inner ear of this rat strain during aging. Cochlear histopathology was examined in 20-24-month-old F344 rats and compared with that of young F344 rats (4 months) and of old rats of the Long-Evans (LE) strain. Hematoxylin/eosin staining in aged F344 rats showed degenerative changes in the organ of Corti, consisting of a damaged layer of marginal cells, reduced vascularization of the stria vascularis and a distorted tectorial membrane detached from the organ of Corti. Age-related changes in collagen distribution were observed with Masson's trichrome staining in the spiral ligament of old F344 rats. The results of immunohistochemical staining for type II collagen revealed a marked decrease in collagen fibers in the area connecting the spiral ligament and stria vascularis and a decrease in area IV fibrocytes in old F344 but not in LE rats. These findings may contribute to an explanation of the substantial hearing loss found in old F344 rats.     

Age-related changes in the acoustic startle reflex in Fischer 344 and Long Evans rats

The behavioral consequences of age-related changes in the auditory system were studied in Fischer 344 (F344) rats as a model of fast aging and in Long Evans (LE) rats as a model of normal aging. Hearing thresholds, the strength of the acoustic startle responses (ASRs) to noise and tonal stimuli, and the efficiency of the prepulse inhibition (PPI) of ASR were assessed in young-adult, middle-aged, and aged rats of both strains. Compared with LE rats, F344 rats showed larger age-related hearing threshold shifts, and the amplitudes of their startle responses were mostly lower. Both rat strains demonstrated a significant decrease of startle reactivity during aging. For tonal stimuli, this decrease occurred at an earlier age in the F344 rats: middle-aged F344 animals expressed similar startle reactivity as aged F344 animals, whereas middle-aged LE animals had similar startle reactivity as young-adult LE animals. For noise stimuli, on the other hand, a similar progression of age-related ASR changes was found in both strains. No significant relationship between the hearing thresholds and the ASR amplitudes was found within any age group. Auditory PPI was less efficient in F344 rats than in LE rats. An age-related reduction of the PPI of ASR was observed in rats of both strains; however, a significant reduction of PPI occurred only in aged rats. The results indicate that the ASR may serve as an indicator of central presbycusis.     

GABAergic feedforward projections from the inferior colliculus to the medial geniculate body.

A novel and robust projection from gamma-aminobutyric acid-containing (GABAergic) inferior colliculus neurons to the media] geniculate body (MGB) was discovered in the cat using axoplasmic transport methods combined with immunocytochemistry. This input travels with the classical inferior colliculus projection to the MGB, and it is a direct ascending GABAergic pathway to the sensory thalamus that may be inhibitory. This bilateral projection constitutes 10-30% of the neurons in the auditory tectothalamic system. Studies by others have shown that comparable input to the corresponding thalamic visual or somesthetic nuclei is absent. This suggests that monosynaptic inhibition or disinhibition is a prominent feature in the MGB and that differences in neural circuitry distinguish it from its thalamic visual and somesthetic counterparts.     

Aging cochleas in the F344 rat: morphological and functional changes

The Fischer 344 rat strain has been frequently used as an animal model of rapid aging. The present study was aimed at evaluating the incidence of apoptotic cells in the inner ear of 20-24-month-old F344 rats and to correlate it with cochlear function using otoacoustic emissions. Staining with cresyl violet and the enzymatic labeling (terminal deoxynucleotidyl transferase, TdT) of fragmented DNA revealed large numbers of apoptotic cells in the marginal and basal layers of the stria vascularis and in adjacent cells of the spiral ligament. The amplitudes of distortion products otoacoustic emissions (DPOAEs), which reflect functional state of the outer hair cells, were significantly reduced or totally absent in these animals. In contrast to old F344 rats, no marked DPOAE amplitude reduction and smaller numbers of apoptotic cells were found in young 4-month-old F344 rats or in aged 24-28-month-old Long Evans rats. The accumulation of apoptotic cells, mainly in the basal layer of the stria vascularis and in adjacent cells of the spiral ligament, leads to a detachment of the stria vascularis from the spiral ligament and results in the impairment of outer hair cell function. This specific type of strial deterioration suggests that aged F344 rats can serve as an animal model of strial presbycusis.     

Neurohypophyseal aging: differential changes in oxytocin and vasopressin release, studied in Fischer 344 and Sprague-Dawley rats

We had previously shown that the hypothalamo-neurohypophyseal vasopressin secreting system is suppressed in aged rats. In the present study, using aged (26 months) male Fischer 344 (F344) rats, we showed that in contrast to vasopressin, oxytocin plasma concentration and hypothalamic content were unaltered in comparison with young (2-3 months) rats; however, based on data from our past and current studies, the neurohypophyseal concentrations of both hormones were found to be decreased in aged rats. We also compared the effect of aging on the oxytocin and vasopressin in secretory functions. Superfusion technique was employed to examine oxytocin and vasopressin release from isolated neural lobes of young (2-3 months) and old (26 months) male F344 and young (2-3 months), middle-aged (12 months) and old (30 months) Sprague-Dawley (SD) rats. Aging affected basal release of oxytocin and vasopressin in a differential manner. Expressed per gland, basal release of oxytocin increased in aged rats of both strains; whereas vasopressin release decreased in SD, and did not change in F344, old rats. The vasopressin responses to electrical stimulation, 56 mM K+ and initial traumatic release were decreased in aged rats; whereas oxytocin responses were either unaltered or decreased much less. All age-related changes were more pronounced in SD than in F344 rats. Thus, while the aging process is associated with a significant impairment in the vasopressin secretory function, the oxytocin secretory function is much less affected by that process. Significant strain differences were observed in the effects of aging on oxytocin and vasopressin release.     

Aging alters mechanical and contractile properties of the Fisher 344/Nnia X Norway/Binia rat aorta

Vascular mechanical and contractile properties were compared in adult (6 months old) and very-aged (36 months old) Fischer 344/NNiaHSd X Brown Norway/BiNia (F344/NXBN) rats. Our previous work has indicated that aging is associated with aortic medial thickening. This morphological alteration was accompanied by a leftward shift in the aortic stress/strain curve indicating increased vessel stiffness in very-aged animals. Disruption of the endothelium as well as pretreatment of tissues with the nitric oxide (NO) donor sodium nitroprusside eliminated differences, suggesting a link between deficient endothelial NO release and reduced compliance in very-aged aortae. In addition, the Rho kinase inhibitor Y-27632 increased vessel compliance in both adult and very-aged tissues suggesting that the Rho cascade contributed to the stress/strain relationship. Maximal force developed in response to high potassium (K⁺) was reduced by ∼70% in intact and endothelium-denuded aortae from very-aged rats. In contrast to contractile force development, calcium-dependent stress relaxation was increased in very-aged aorta. Finally, gel electrophoresis indicated a significantly higher tissue content of myosin heavy chain and a higher ratio of SM1/SM2 isoforms with aging. The results suggest multiple molecular changes with aging, which may be expected to alter vascular tissue function.     

Serotonergic modulation of ethanol-induced electrophysiological depression in young and aged rats

BACKGROUND: Ethanol (EtOH)-induced electrophysiological depressions in cerebellar Purkinje neurons have been shown to be potentiated by exogenously applied serotonin (5HT). In this study, we determined whether this modulatory action can be activated by endogenous release from presynaptic serotonergic terminals, and whether such a response is altered by age or rat strain. METHODS: Extracellular 5HT levels in cerebellar cortex were measured in real time by in vivo chronoamperometry, by using Nafion-coated carbon fiber electrodes, in anesthetized young (3-5 months old) or aged (18-24 months old) Sprague Dawley and Fischer 344 rats. Some animals were prelesioned with 5,7 dihydroxytryptamine (5,7 DHT). Single unit electrophysiological activity was recorded from cerebellar Purkinje neurons. Serotonin or its presynaptic antagonist methiothepin was applied directly to cerebellar neurons through multibarrel pipettes. RESULTS: Local application of methiothepin dose-dependently induced 5HT overflow in young Sprague Dawley and Fisher 344 rats. Methiothepin-induced 5HT release was decreased significantly in aged or 5,7 DHT-lesioned rats. Local application of methiothepin or 5HT potentiated EtOH-induced electrophysiological depression of Purkinje neurons in young animals of both strains. Methiothepin-potentiated, EtOH-elicited neuronal inhibition was reduced greatly in aged or 5,7 DHT-lesioned rats. Serotonin-facilitated EtOH responses were reduced in the aged Sprague Dawley rats. CONCLUSIONS: EtOH-induced electrophysiological responses in cerebellum can be facilitated by endogenous 5HT release by using a 5HT autoreceptor antagonist. Such actions are attenuated in aged rats perhaps through a presynaptic serotonergic mechanism.     

Genetic diversity contributes to abnormalities in pain behaviors between young and old rats

Aging has profound yet unpredictable effects on pain perception and incidence of anxiety disorders. However, the mechanisms underlying age-related pathologies are confounded by contradictory observations in rodent models. Therefore, the goal of our study was to test the hypothesis that genetic variability contributes to age-related pain behaviors and susceptibility to anxiety. To address this hypothesis, we examined pain and anxiety-like behavior in young or old Brown Norway (BN), Fisher 344, and BN/F344 (F1), three rat strains used in studies to evaluate the effect of aging. Mechanosensitive thresholds were assessed using the Von Frey assay, and visceral pain sensitivity was measured via the visceromotor response to colorectal distension. Anxiety-like behavior and exploration was quantified in the elevated plus maze. In the BN strain, old rats exhibited increased mechanosensitive thresholds compared to young rats; however, age did not affect visceral sensitivity in this strain. In F344-BN rats, the number of abdominal contractions induced by the highest colonic distension pressure was significantly lower in old rats. However, following colonic sensitization, a difference was no longer apparent. In the F-344 strain, visceral hypersensitivity following afferent sensitization was evident in young rats at all distension pressures but was not observed in older animals at 20 mmHg. Aging significantly reduced maze exploration across all strains. Our data demonstrate that age- and strain-related alterations exist in pain behavior and highlight the effects of aging on exploratory behavior. These findings suggest that strain differences contribute to the controversial data on the effects of aging on pain perception.     

Long-term cortical plasticity evoked by electric stimulation and acetylcholine applied to the auditory cortex

Auditory fear conditioning with tone bursts followed by electric leg stimulation activates neurons not only in the auditory and somatosensory systems but also in many other regions of the brain and elicits shifts in the best frequencies (BFs) of collicular and cortical neurons, i.e., reorganization of the frequency (co-chleotopic) maps in the inferior colliculus and auditory cortex (AC). What are the neural elements minimally necessary for evoking long-term cortical BF shifts? We found that: (i) both electric stimulation and acetylcholine applied to the AC evoke the long-term cortical BF shift as does the conditioning; (ii) both electric stimulation of the AC and acetylcholine applied to the inferior colliculus increase the short-term collicular BF shift evoked by the cortical electric stimulation but do not change it into long-term; and (iii) as this short-term collicular BF shift is blocked by atropine, the development of the long-term cortical BF shift becomes slow and small. Therefore, the most essential neural elements for evoking the long-term cortical BF shift are the AC, corticofugal feedback and the cholinergic nucleus. Our current data support the Gao-Suga model, which hypothesizes that the small short-term cortical BF shifts are evoked by tonal stimuli without the association of conditioned and unconditioned stimuli in the multisensory thalamic nuclei and that these BF shifts are augmented and changed into the large long-term BF shifts by cholinergic neurons.     

Age-related alterations in noradrenergic neurotransmission in Sprague-Dawley and Fischer 344 rat strains

This study compares the electrophysiological effects of locally applied norepinephrine and phencyclidine between 3 month old, 18.20 month old, and 24–26 month old Sprague-Dawley and Fischer 344 rats. Micro-pressure injection through multi-bareled micropipettes was used to apply drugs and record depressant effects on the discharge rate of single cerebellar Purkinje neurons. Neurons from the 3 month old animals were significantly more sensitive to locally applied norepinephrine and phencyclidine than cerebellar Purkinje cells recorded from the older age groups. These results were observed in both rat strains tested. The results suggest that decreased post-synaptic sensitivity to norepinephrine is a concomitant of aging in these animals.     

Age-associated alterations of cardiac structure and function in the female F344xBN rat heart

The Fischer 344/NNiaHSD × Brown Norway/BiNia F1 (F344xBN) rat model exhibits an increased life span and fewer age-associated pathologies compared to commonly used Fischer 344 (F344). How aging may affect cardiac structure and function in these animals, has to our knowledge, not been investigated. Echocardiography was performed on female F344xBN rats at 6, 26, and 30 months of age using a Phillips 5500 Echocardiography system. Before sacrifice, electrocardiograms were measured in the female F344xBN in order to determine heart rhythm interval changes. Aging was associated with an increase in heart to body weight ratio, cardiomyocyte cross-sectional area, posterior wall thickening, and left ventricle chamber dilatation. Aging was associated with slight evidence of diastolic dysfunction. Alterations in heart rhythm intervals were associated with alterations in the spatial distribution of connexin 43. The incidence of arrhythmias was not different with age; however, valvular dysfunction was increased. These data suggest that aging in the female F344xBN rat heart is associated with changes in cardiac structure as well as function. Further investigation regarding other parameters of cardiac biochemistry and function is needed to better understand the normal compensated cardiovascular aging process in the female F344xBN.

Electronic supplementary material

The online version of this article (doi:10.1007/s11357-014-9684-6) contains supplementary material, which is available to authorized users.     

The Fischer 344/NNiaHSd X Brown Norway/BiNia is a Better Model of Sarcopenia than the Fischer 344/NNiaHSd: a Comparative Analysis of Muscle Mass and Contractile Properties in Aging Male Rat Models

Sarcopenia, characterized by profound muscle atrophy and the loss of contractile function, contributes significantly to the development of frailty and functional impairment in older age. Although present in aging humans, rat models have failed to clearly demonstrate a similar degree of this age-associated loss of muscle mass and function. This investigation compared two models of rats raised specifically for aging studies, the Fischer 344/NNiaHSd (F344/N) and the Fischer 344/NNiaHSd X Brown Norway/BiNia (F344/NXBN), and sought to determine which model provides the most accurate representation of human sarcopenia. We found that aging had no effect on F344/N muscle mass or contractile function in the extensor digitorum longus (EDL) and soleus (SOL). Conversely, in the F344/NXBN model, aging was found to decrease EDL and SOL mass and contractile function. These changes were sufficient to satisfy the proposed criteria for the diagnosis of human sarcopenia based upon muscle mass and contractile function. Results indicate that the F344/NXBN provides a better model of the alterations seen in aging human muscle than the F344/N rat model.     

Sugar-induced hypertension in Fischer 344 and F1-hybrid rats (F344/BN) at different ages

Epidemiological evidence suggests that some nutrients, like sodium and potassium, participate in age-related hypertension. A role for refined carbohydrates (CHO), principally sugar, in blood pressure regulation is not generally recognized. This may be unfortunate, since modern lifestyle is associated with large amounts of dietary refined CHO. We examined the effect of a high sugar diet on systolic blood pressure (SBP) in Fischer 344 rats (F344) and the F1-hybrid of this strain (F344/BN). These genotypes have been used to develop experimental models for studies on different aspects of aging. Age-dependent hypertension has not been reported in either strain. In fact, insensitivity to salt-induced hypertension has been found in F344 rats. Upon arrival at our laboratory, the mean difference in SBP between the youngest (4 months) and oldest (18 months) F344 and F1-hybrid rats was 10 mm Hg, the highest mean SBP was 123 mm Hg. These values remained relatively constant over the next month when both strains consumed a low sugar diet. Differently, a steady increase in SBP occurred in both strains when rats of all ages were fed a diet high in sucrose content, mean SBP increasing to over 170 mm Hg at termination of study. Older rats proved more sensitive initially to sugar-induced SBP elevations. Associated with rising SBP was evidence of Na retention. We conclude that a diet containing excess sugar can create a gradual elevation of SBP into a hypertensive range with aging of F344 and F1-hybrid rats. This contrasts with previous findings.     

Experience-dependent plasticity in the auditory cortex and the inferior colliculus of bats: role of the corticofugal system

In the big brown bat, Eptesicus fuscus, the response properties of neurons and the cochleotopic (frequency) maps in the auditory cortex (AC) and inferior colliculus can be changed by auditory conditioning, weak focal electric stimulation of the AC, or repetitive delivery of weak, short tone bursts. The corticofugal system plays an important role in information processing and plasticity in the auditory system. Our present findings are as follows. In the AC, best frequency (BF) shifts, i.e., reorganization of a frequency map, slowly develop and reach a plateau approximately 180 min after conditioning with tone bursts and electric-leg stimulation. The plateau lasts more than 26 h. In the inferior colliculus, on the other hand, BF shifts rapidly develop and become the largest at the end of a 30-min-long conditioning session. The shifted BFs return (i. e., recover) to normal in approximately 180 min. The collicular BF shifts are not a consequence of the cortical BF shifts. Instead, they lead the cortical BF shifts. The collicular BF shifts evoked by conditioning are very similar to the collicular and cortical BF shifts evoked by cortical electrical stimulation. Therefore, our working hypothesis is that, during conditioning, the corticofugal system evokes subcortical BF shifts, which in turn boost cortical BF shifts. The cortical BF shifts otherwise would be very small. However, whether the cortical BF shifts are consequently boosted depends on nonauditory systems, including nonauditory sensory cortices, amygdala, basal forebrain, etc., which determine the behavioral relevance of acoustic stimuli.     

Advanced glycation end-product accumulation and associated protein modification in type II skeletal muscle with aging

One mechanism that may influence the quality of skeletal muscle proteins, and explain the age-related decline in contractility, is protein damage. Advanced glycation end-products (AGE) in vivo are useful biomarkers of damage. In this study, comparison of extensor digitorum longus (EDL) muscles from young (8 months), old (33 months), and very old (36 months) Fischer 344 Brown Norway F1 (F344BNF1) hybrid rats shows that muscles from the very old rats have a significantly higher percentage of myofibers that immunolabel intracellularly for AGE-antibody 6D12 compared to the younger age group. The AGE-modified proteins, determined in the semimembranosus muscles from young (9 months) and old (27 months) F344 rats, identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry include creatine kinase, carbonic anhydrase III, beta-enolase, actin, and voltage-dependent anion-selective channel 1. Moreover, there is a significant increase in AGE modification of beta-enolase with age. These results identify a common subset of proteins that contain AGE and suggest that metabolic proteins are targets for glycation with aging.     

Prevention of cardiovascular and renal pathology of aging by the advanced glycation inhibitor aminoguanidine.

Human aging is impacted severely by cardiovascular disease and significantly but less overtly by renal dysfunction. Advanced glycation endproducts (AGEs) have been linked to tissue damage in diabetes and aging, and the AGE inhibitor aminoguanidine (AG) has been shown to inhibit renal and vascular pathology in diabetic animals. In the present study, the effects of AG on aging-related renal and vascular changes and AGE accumulation were studied in nondiabetic female Sprague-Dawley (S-D) and Fischer 344 (F344) rats treated with AG (0.1% in drinking water) for 18 mo. Significant increases in the AGE content in aged cardiac (P < 0.05), aortic (P < 0.005), and renal (P < 0.05) tissues were prevented by AG treatment (P < 0.05 for each tissue). A marked age-linked vasodilatory impairment in response to acetylcholine and nitroglycerine was prevented by AG treatment (P < 0.005), as was an age-related cardiac hypertrophy evident in both strains (P < 0.05). While creatinine clearance was unaffected by aging in these studies, the AGE/ creatinine clearance ratio declined 3-fold in old rats vs. young rats (S-D, P < 0.05; F344, P < 0.01), while it declined significantly less in AG-treated old rats (P < 0.05). In S-D but not in F344 rats, a significant (P < 0.05) age-linked 24% nephron loss was completely prevented by AG treatment, and glomerular sclerosis was markedly suppressed (P < 0.01). Age-related albuminuria and proteinuria were markedly inhibited by AG in both strains (S-D, P < 0.01; F344, P < 0.01). These data suggest that early interference with AGE accumulation by AG treatment may impart significant protection against the progressive cardiovascular and renal decline afflicting the last decades of life.     

Reduced expression of epidermal growth factor receptors in rat liver during aging

Proliferative responsiveness of hepatocytes to epidermal growth factor (EGF) declines during aging. The role of EGF receptors in mediating age-dependent changes of EGF-induced mitogenic signaling in liver remains incompletely understood. We assessed EGF receptor expression levels in whole liver specimens as well as in freshly isolated and cultured hepatocytes from young adult and senescent Fischer 344 male rats. Hepatic EGF receptor messenger RNA and protein levels, and the number of high- and low-affinity receptor binding sites, decreased with aging. Ligand-induced EGF receptor activation, determined by receptor dimerization and tyrosine phosphorylation, was reduced in old animals in parallel with the age-related decline in receptor expression. Stimulation of the extracellular signal-regulated kinase pathway by EGF was also attenuated in hepatocytes from old animals. Our results implicate decreased expression of EGF receptors as a key determinant of reduced mitogenic signaling responsive to EGF stimulation of liver during aging.     

Increased plasma concentrations, hypothalamic content, and in vitro release of arginine vasopressin in inflammatory disease-prone, hypothalamic corticotropin-releasing hormone-deficient Lewis rats.

The susceptibility of Lewis (LEW/N) rats to severe inflammatory disease has been causally associated with subnormal responsiveness of their hypothalamic CRH-secreting neurons and, consequently, their hypothalamic-pituitary-adrenal axis to several stimulatory neurotransmitters and inflammatory cytokines. In the present study we investigated in this strain the secretory dynamics of another major activator of pituitary ACTH secretion, arginine vasopressin (AVP). To accomplish this, we evaluated the circadian plasma concentrations and circadian and glucocorticoid-induced changes in hypothalamic content and in vitro release of AVP in 8- to 10-week-old female LEW/N rats and compared these measurements to those obtained in parallel from age- and sex-matched histocompatible, inflammatory disease-resistant Fischer (F344/N) rats. Plasma concentrations and hypothalamic content and in vitro release of AVP were significantly elevated in LEW/N compared to F344/N rats in both the morning and evening. These indices of higher AVP secretion in LEW/N than in F344/N rats were also present after chronic dexamethasone treatment. These findings suggest increased AVP production and release in LEW/N rats, perhaps representing an adaptive compensation for insufficient CRH and glucocorticoid secretion. The high levels of circulating AVP might contribute to the excessive inflammatory responses of these animals.