Age-related changes in brain catecholamine responses to a single footshock

Effects of age on feeding response: Focus on the rostral C1 neuron and its glucoregulatory proteins

BACKGROUND

Older people are likely to develop anorexia of aging. Rostral C1 (rC1) catecholaminergic neurons in rostral ventrolateral medulla (RVLM) are recently discovered its role in food intake control. It is well established that these neurons regulate cardiovascular function.

OBJECTIVE

This study aims to determine the effect of age on the function of rostral C1 (rC1) neurons in mediating feeding response.

METHOD

Male Sprague Dawley rats at 3-months (n = 22) and 24-months (n = 22) old were used and further divided into two subgroups; 1) treatment group with 2-deoxy-d-glucose (2DG) and 2) vehicle group. Feeding hormones such as cholecystokinin (CCK), ghrelin and leptin were analysed using enzyme-linked immunosorbent assay (ELISA). Rat brain was carefully dissected to obtain the brainstem RVLM region. Further analysis was carried out to determine the level of proteins and genes in RVLM that were associated with feeding pathway. Protein expression of tyrosine hydroxylase (TH), phosphorylated TH at Serine40 (pSer40TH), AMP-activated protein kinase (AMPK), phosphorylated AMPK (phospho AMPK) and neuropeptide Y Y5 receptor (NPY5R) were determined by western blot. Expression of TH, AMPK and NPY genes were determined by real-time PCR.

RESULTS

This study showed that blood glucose level was elevated in young and old rats following 2DG administration. Plasma CCK-8 concentration was higher in the aged rats at basal and increased with 2DG administration in young rats, but the leptin and ghrelin showed no changes. Old rats showed higher TH and lower AMPK mRNA levels. Glucoprivation decreased AMPK mRNA level in young rats and decreased TH mRNA in old rats. Aged rC1 neurons showed higher NPY5R protein level. Following glucoprivation, rC1 neurons produced distinct molecular changes across age in which, in young rats, AMPK phosphorylation level was increased and in old rats, TH phosphorylation level was increased.

CONCLUSION

These findings suggest that glucose-counterregulatory responses by rC1 neurons at least, contribute to the ability of young and old rats in coping glucoprivation. Age-induced molecular changes within rC1 neurons may attenuate the glucoprivic responses. This situation may explain the impairment of feeding response in the elderly.

Epinephrine and glucose modulate training-related CREB phosphorylation in old rats: relationships to age-related memory impairments

Epinephrine enhances memory in young adult rats, in part, by increasing blood glucose levels needed to modulate memory. In old rats, epinephrine is deficient at raising blood glucose levels and thus is only moderately effective at enhancing memory. In contrast, systemic glucose injections improve memory in old rats, with resulting memory performance equal to that of young rats. The diminished response of glucose to training in old rats may blunt downstream neurochemical and molecular mechanisms needed to upregulate memory processes. In the first experiment, young adult and old rats were trained on an inhibitory avoidance task with immediate post-training injections of aCSF or glucose into the dorsal hippocampus. Old rats had significant memory impairments compared to young rats 7 days after training. Intrahippocampal injections of glucose reversed age-related deficits, improving memory scores in old rats to values seen in young rats. A second experiment examined age-related changes in activation of the transcription factor CREB, which is widely implicated in memory formation and may act downstream of hormonal and metabolic signals. Activation was assessed in response to training with systemic injections of epinephrine and glucose at doses known to enhance memory. Young adult and old rats were trained on inhibitory avoidance with immediate post-training systemic injections of saline, epinephrine, or glucose. After training, old rats had significant impairments in CREB phosphorylation in area CA1 and the dentate gyrus region of the hippocampus, and in the basolateral and lateral amygdala. Epinephrine and glucose attenuated age-related deficits in CREB phosphorylation, but were more effective in the amygdala and hippocampus, respectively. Together, these results support the view that age-related changes in blood glucose responses to epinephrine contribute to memory impairments, which may be related to alterations in regional patterns of CREB phosphorylation.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Reduced cortical noradrenergic neurotransmission is associated with increased neophobia and impaired spatial memory in aged rats

In the present study, young (5-month-old (mo)) and aged (24 mo) adult male Fischer-344 (F344) rats were assigned to experimental groups based upon their performance of a reference memory task in the Morris water maze and reactivity to a novel palatable taste in a gustatory neophobia task. Levels of norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) were assayed via high performance liquid chromatography (HPLC) in brain regions associated with the locus coeruleus (LC)-hippocampus-cortex system and A1/A2-hypothalamic system. Binding of ligands specific for alpha-1, alpha-2, beta-1, and beta-2 receptors was assessed in hippocampus and cortex with receptor autoradiography. Impaired acquisition and retention of the water maze task and gustatory neophobia in aged rats was primarily associated with decreased NE activity in cingulate cortex (CC) as indicated by a significant reduction in the MHPG/NE ratio coupled with increased NE content. No significant changes in adrenergic receptor binding were detected in any region sampled. The results suggest that an aging-related reduction in cortical NE neurotransmission is associated with the expression of increased neophobia and deficits in spatial learning and memory performance occurring with advanced age in rats.

Lead and conditioned fear to contextual and discrete cues

Male Fischer 344 rats received either tap water or water containing 250 ppm lead for 90 days prior to training in either Pavlovian fear conditioning or consummatory contrast, an aversive reward reduction paradigm. In Experiment 1, lead-exposed and -unexposed rats were trained in operant chambers over a 6-min session. After 3 min elapsed, three tone-shock pairings were presented over the remainder of the session. Rats then received 7 days of extinction training in an identical procedure with only tones presented, no shocks. Lead-exposed rats exhibited greater behavioral suppression to both the contextual and auditory cues that predicted shock. In Experiment 2, rats were placed in operant chambers daily and allowed to consume either a 15% or a 5% fructose solution for 7 days. On Day 8, the rats consuming the 15% fructose solution were shifted to the 5% solution for 3 days. Lead-exposed rats did not differ from their controls in either initial consumption of either solution or in the suppression of their consumption after shifting to the 5% solution. Taken together, these findings suggest that lead impairs the extinction of fear conditioning and this finding is not due to a nonspecific increase in aversive emotionality.

Vasopressin prolongs behavioral and cardiac responses to mild stress in young but not in aged rats

In young male Wistar rats sudden silence superimposed on low intensity background noise evokes a relative decrease in heart rate. This bradycardia is accompanied by immobility behavior. In the present study, involving young (3 month), late-adult (14 month), aged (20 month), and senescent (25 month) rats the magnitude of the stress-induced bradycardia shows an age-related reduction while the behavioral immobility response remained unchanged during the process of aging. Arginine-8-vasopressin (AVP, 6 micrograms/kg SC) administered 60 min prior to the experiment led to a prolonged behavioral and cardiac stress response in young and late-adult rats, but not in aged and senescent animals. The peripheral and central mechanisms possibly involved in the failure of systemically applied AVP to improve bradycardiac stress responses in aged rats are discussed.

Effect of low amphetamine doses on cardiac responses to emotional stress in aged rats

In young Wistar rats conditioned emotional stress can be characterized by a learned bradycardiac response to an inescapable footshock. In aged rats this bradycardiac response is attenuated and accompanied by suppressed behavioral arousal in response to novelty. In the present study, cardiac responses to emotional stress and behavioral reactivity to a novel experience in an open field were tested in aged and young rats under the influence of a low dose of d-amphetamine (AMPH, 0.5 mg/kg IP). AMPH administration in 27-month-old rats reinstated the bradycardiac response to emotional stress, while it failed to influence the resting heart rate in the home cage. Age-associated differences in open-field ambulation, present in drug-free conditions, were antagonized by low doses of AMPH (0.25-1.0 mg/kg). It is concluded that enhanced arousal by aminergic stimulation with AMPH in the aged rat invoked cardiac and behavioral response patterns resembling those at younger ages.

Age-related alterations in cardiac response to emotional stress: relations to behavioral reactivity in the rat

Age and behavioral characteristics are considered as risk factors for disturbances of the cardiac rhythm. Emotional stress may be a disseminating factor. Therefore, cardiac responsiveness and behavioral reactivity and their relation as a function of age have been studied in the rat. Young (3-month-old) and young adult (5-month-old) rats display a relative deceleratory cardiac response with bradyarrhythmias in the initial phase of response to emotional stress evoked by stimuli associated with a previous painful experience. The behavioral response is immobility. The immobility response is also displayed by aged (21-month-old) and senescent (33-month-old) rats but the initial bradycardiac heart response to emotional stress is absent, and the incidence of repetitive extrasystoles is increasing with age. An inverse correlation between behavioral reactivity to novel stimuli and the bradycardiac heart responsiveness is observed in young and young adult individual animals. The behavioral reactivity of the aged and senescent rats is diminished, but the correlation with cardiac reactivity remains preserved. It is suggested that the behaviorally coupled inhibitory influences on the heart are diminishing during aging either due to impairments in the descending cholinergic (vagal) system or secondarily, due to a decrease in the central "drive" of this system.

Individual differences in aging: behavioral and neurobiological correlates

The goal of this experiment was to determine the correlations among different behavioral and neurobiological measures in aged rats. Aged Sprague-Dawley rats were given a battery of cognitive and sensorimotor tests, followed by electrophysiological assessment of sleep and biochemical measurements of various neurotransmitter systems. The behavioral tests included the following: Activity level in an open field; short-term and long-term memory of a spatial environment as assessed by habituation: spatial navigation, discrimination reversal, and cue learning in the Morris water pool; spatial memory in a T-maze motivated by escape from water; spatial memory and reversal on the Barnes circular platform task; passive avoidance; motor skills. Sleep was assessed by electrographic cortical records. The following neurotransmitter markers were examined: Choline acetyltransferase; the density of nicotinic, benzodiazepine and glutamine receptors in the cortex and caudate nucleus; endogenous levels of norepinephrine, dopamine, and serotonin in the cortex and hippocampus. The duration of bouts of paradoxical sleep was strongly correlated with several cognitive measures and selected serotonergic markers. This finding suggests that changes in sleep patterns and brain biochemistry contribute directly to deficits in learning and memory, or that the same neurobiological defect contributes to age-related impairments in sleep and in learning and memory.

Transplantation of norepinephrine neurons into aged rats improves performance of a learned task

A reproducible behavioral correlate of aging in rodents is deficient performance of inhibitory avoidance memory tasks. Impaired performance has been attributed, in part, to age-related changes in brain norepinephrine (NE) system function. To determine whether supplementation of brain NE can ameliorate avoidance deficits in aged animals, we transplanted noradrenergic locus coeruleus neurons from fetal rat donors into the third cerebral ventricle of 24-month-old male F344 rats. Aged rats that received NE-containing grafts exhibited significant improvement of inhibitory avoidance retention performance compared to both unoperated aged animals and aged animals that received grafts of cerebellar tissue. Improved behavioral performance was prevented by pretreatment of NE graft recipients with the beta-adrenergic receptor blocking agent, propranolol, and was mimicked by chronic intraventricular infusion of NE. Taken together, our findings support the view that age-related declines in brain NE content contribute to age-related deficits in inhibitory avoidance performance, and that NE replacement therapy can improve performance of this task in aged rats.

Differential response to immobilization stress of striatal dopaminergic and hippocampal noradrenergic systems in aged rats

The effect of restraint stress on synthesis of central norepinephrine (NE) and dopamine (DA) was studied in adult and old rats. The rate of in vivo synthesis of the two catecholamines was determined in hippocampus (a prevalently noradrenergic area) and in striatum (a prevalently dopaminergic area) by measuring the accumulation of DOPA for 60 min after decarboxylase inhibition. NE synthesis was stimulated by stress in the first 30 min, after which the accumulation of DOPA declined. The stimulation was much greater in old rats. In striatum, endogenous DOPA concentration was significantly lower in old rats. Stress significantly enhanced DOPA accumulation in the first 30 min in both age groups but after this interval accumulation continued linearly only in young rats. These results indicate that in aged rats the response to stress of some noradrenergic and dopaminergic systems may be altered in opposite directions.

Regional brain catecholamines and memory: effects of footshock, amygdala implantation, and stimulation

Previous findings have revealed a correlation between post-training release of whole brain norepinephrine (NE) and later retention performance. The present experiment examined changes after a training footshock in NE levels, as well as the levels of the major central NE metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), dopamine (DA), and epinephrine (EPI) in eight brain regions. Brain levels of these amines and the metabolite were assessed 10 min after training in a one-trial inhibitory (passive) avoidance task. The results indicate that NE levels decreased significantly in neocortex, neostriatum, hypothalamus, frontal pole, septum, and brainstem, but not in hippocampus or thalamus. The decreases in NE levels were generally accompanied by increases in MHPG; the MHPG/NE ratio increased significantly in all areas in which decreases in NE were observed. DA levels decreased in neostriatum and increased in neocortex and brainstem. Epinephrine levels decreased only in the brainstem sample. Thus, the effects of training on NE are widespread, probably reflecting the release of the amine in most brain regions. Such findings are consistent with the view that posttraining release of brain NE may modulate the storage of new information in many brain regions. One especially potent treatment for modulating memory storage is electrical stimulation of the amygdala. Therefore, we also examined the effects of amygdala implantation and stimulation on brain catecholamine levels to determine whether such changes might be correlated with the effects of amygdala stimulation on memory. The results indicate that electrode implantation into the amygdala results in pervasive changes in NE levels in most brain regions tested. Against this modified baseline, the results of training and electrical stimulation were region specific and very difficult to interpret. The major conclusion which can be derived from this portion of the experiment is that the amygdala damage produced by electrode implantation produces a brain which is substantially different from that of intact animals.

The effects of training, epinephrine, and glucose injections on plasma glucose levels in rats

Recent findings indicate that a post-training injection of glucose enhances memory storage, suggesting that release of glucose into plasma may mediate the effects of epinephrine and perhaps other treatments on memory. The present experiment examined the effects of handling, inhibitory (passive) avoidance training, epinephrine and glucose injections on plasma glucose levels in Sprague-Dawley rats. Handling produced a small, but significant, transient increase in plasma glucose above basal levels. Saline injections caused a similar increase in circulating glucose levels. Inhibitory avoidance training with high footshock (2.0 mA, 2.0 s) resulted in significant increases in plasma glucose levels above those of low (0.5 mA, 0.75 s) and unshocked animals suggesting that glucose release is responsive to inhibitory avoidance training. Subcutaneous injections of epinephrine (0.01-1.0 mg/kg), or glucose (10-1000 mg/kg) significantly elevated glucose levels above those of saline-injected animals in a dose-dependent manner. Memory facilitating doses of epinephrine and glucose resulted in increases in plasma glucose levels similar to those seen in rats trained with high footshock. Higher doses of epinephrine and glucose resulted in further increases in circulating glucose, to levels significantly greater than those of memory facilitating doses. These results suggest that memory modulation, both endogenous and in response to epinephrine injections, may be mediated in part by circulating glucose levels. Thus, the findings of these experiments support the view that circulating glucose levels regulate the efficacy of neural memory storage processes.

Post-training brain catecholamine levels: lack of response to water-motivated training

Rats were trained in a one-trial appetitive task using water motivation. Brain catecholamine and metabolite levels were assessed in samples collected 10 min after training. There was no evidence that brain NE levels were modified by training, although catecholamine levels increased when the animals were placed in a novel environment. These results differ from those obtained after avoidance training where the extent of a post-training decrease in brain norepinephrine predicts later retention performance.

Age-related memory deficits in rats and mice: enhancement with peripheral injections of epinephrine

Epinephrine peripherally administered to rats and mice immediately following avoidance and/or appetitive training enhances later memory retention in both young and old animals. These findings suggest a possible involvement of peripheral adrenergic systems in memory dysfunctions which accompany aging.