Regional changes in monoamine metabolism in the aging constant estrous rat

Studies were undertaken to determine levels of monoamines and their metabolites in brain regions in young (3–4 months) normally cycling and old (25–26 month) constant estrous female rats. Dopamine (DA) concentrations were reduced in old rats in the median eminence (ME), medial basal hypothalamus (MBH), preoptic area-anterior hypothalamus (POA-AH) and the striatum. Similarly, concentrations of dihydroxyphenylacetic acid (DOPAC), the major acid metabolite of DA, were reduced significantly in all 4 regions. In the ME, a strong positive correlation was observed between DA and DOPAC concentrations in both young and old rats. Concentrations of norepinephrine (NE) were reduced in old rats in the MBH and POA-AH but not in the ME or striatum. Concentrations of serotonin (5HT) and its major metabolite, 5-hydroxyindoleacetic acid (5HIAA) were generally unchanged with age in all of the regions examined. These studies indicate the age-related regional alterations in DA and 5HT metabolism can be monitored by methods which quantitate monoamines and their metabolites.     

Reinstatement of LH surges by serotonin neuroleptics in aging, constant estrous rats

Aged rats in constant estrous (CE) were treated with drugs which effect monoamine metabolism in an attempt to restore the positive-feedback response to estrogen. Prior to treatment, the rats were ovariectomized so as to eliminate indirect effects of drug: ovary interaction which might alter the steroid environment. They then received silastic capsules containing estradiol 17β, SC, immediately (IME, immediate implant group) or 4 weeks after surgery (DEL, delayed implant group), to provide a constant, stable source of estrogen, mimicking the CE condition. Blood samples were taken 3 days after estrogen capsules were implanted to determine if the exogenous steroid induced LH surges in IME or DEL rats. LH surges occurred in 80% of DEL rats, but no IME rats suggesting that a 30-day hiatus from ovarian steroids restored the positive-feedback response in previously acyclic rats. Progesterone (0.5 mg) administered at 1100 hr increased the size of the surge in DEL rats but did not change serum LH levels in IME rats. After 3 days of estrogen exposure, hypothalamic content of serotonin was significantly lower (p <0.05) in DEL rats vs. IME rats, while norepinephrine was not different in either group. Differential effects of estrogen on norepinephrine were not apparent in IME or DEL rats. The changes in serotonin metabolism following estrogen treatment in DEL rats occurred with the onset of LH surges, suggesting a functional correlation. On the other hand, serotonin levels did not change in response to estrogen in IME rats, and LH surges were absent as well. Monoamine neuroleptics were administered to IME rats in an attempt to reestablish positive feedback. I-Dihydroxyphenylalanine (l-DOPA; 200 mg/kg at 1100 hr) stimulated LH surges in 31% of the rats; however, its effectiveness was diminished (8% responding with LH surges) by pretreatment with p-chlorophenylalanine (pCPA; 250 mg/kg at 1600 hr, 19 hours before l-DOPA). LH surges were reinstated in the majority (77%) of IME rats following combined treatment with pCPA + 5-hydroxytryptophan (5-HTP; 50 mg/kg at 1100 hr, 43 hours after pCPA). Since this combination of pCPA + 5-HTP enhances sensitivity to serotonin signals, the drugs may have restored the facilitatory effect of serotonin on LH secretion which is lost in aging constant estrous rats. In conclusion, loss of the positive-feedback response in aging female rats may follow changes in monoamine metabolism resulting from life-long exposure to estrogen. Although metabolic changes occur in all hypothalamic monoamines, this study suggests that a major deficit with regard to this feedback loss residues with serotonin.     

The effect of hypoxia on monoamine levels in discrete regions of aged rat brain

Aged rats were exposed to 10% oxygen for 2, 13, and 36 hr. Norepinephrine levels in cerebral cortex, hypothalamus, hippocampus, midbrain, cerebellum, ponsmedulla and dopamine levels in striatum were determined after each exposure. While there was no significant change in monoamine levels in brain regions after 2 hr, norepinephrine concentration in hypothalamus and midbrain decreased significantly after 13 hr of hypoxia. After 36 hr in a hypoxic environment, levels of the monoamines in brain regions were similar to the controls. This would suggest NE metabolism is most vulnerable to hypoxia in two regions of the aged brain. The precise mechanism of these changes is unknown, but they suggest both a vulnerability and an adaptive recovery of central adrenergic metabolism by the aged brain under hypoxia.     

Intracerebroventricular injection of orexin-A stimulates monoamine metabolism but not HPA axis in neonatal chicks

We investigated the effects of intracerebroventricular (ICV) injection of orexin-A on plasma corticosterone (CORT) concentration and brain monoamine metabolism to clarify the mechanism by which ICV orexin-A induced arousal in chicks. In Experiment 1, plasma CORT concentrations were measured as an indicator of hypothalamic-pituitary-adrenal (HPA) axis activity. There was no significant difference in CORT concentration between the control and orexin-A administered groups. In Experiment 2, the concentrations of monoamines (norepinephrine, dopamine and serotonin), their metabolites, and their metabolic turnover rates in the telencephalon, mesencephalon, and diencephalon were investigated. All metabolic turnover rates studied were increased at all brain sites after ICV orexin-A injection. In conclusion, the HPA axis does not appear to be involved in arousal-inducing mechanisms of orexin-A in neonatal chicks; however, several monoaminergic systems do.     

Serotonin concentrations in normal aging human brains: Relation to serotonin receptors

Concentrations of serotonin (5-HT) and its deaminated metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in 7 regions of normal human brains and, in some of the regions, were compared to the number of serotonin receptors (S1 and S2). Neither 5-HT nor 5-HIAA concentrations correlated significantly with increasing age (from 17–100 years) in any of the regions investigated. Positive correlations between 5-HT and 5-HIAA were found in all regions studied, significantly (p<0.05) so in 5 of the areas. When comparing 5-HT transmitter and metabolite concentrations to the number of S1 and S2 receptors, no significant correlations were found either within any brain area of between different brain regions. These data confirm that 5-HT transmitter concentrations are not altered by increasing age, support the ideas that S1 and S2 receptors are not presynaptic and also that 5-HT transmitter concentrations and receptor densities are separately controlled.     

Age-related changes in the pattern of luteinizing hormone release induced in female rats by male rat urine

The release of luteinizing hormone (LH) in young (3–4 months) and aged (24–25 months) ovariectomized female Sprague-Dawley rats, s.c. implanted with a 17β-estradiol benzoate silastic capsule, was studied in the presence and absence of stimulation by exposure to male rat urine. After taking an initial blood sample at 12:00 (reference sample), either urine, collected from young adult male rats, or distilled water was poured into the female's cage. Blood samples were then collected hourly up to 18:00 via a catheterized jugular cannula. The concentration of LH in the plasma was measured by RIA. The basal plasma LH level in young control rats was found to increase significantly at 16:00 compared with the 12:00 reference sample while no statistically significant change in plasma LH concentration occurred in old controls over the same period. Male rat urine caused a significantly earlier (at 15:00) and prolonged (from 15:00 to 18:00) elevation of plasma LH in young rats compared with young controls. In contrast, exposure of old female rats to male rat urine resulted in no marked change in plasma LH levels. These results suggest that both basal LH release and the response to pheromonal stimulation by male rat urine may be modified with increasing age in female rats.     

Urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA) in the rat after immobilization stress

An increase of 100% in urinary 5-hydroxyindoleacetic acid, as a measure of release and catabolism of serotonin, is associated with the stress of 18 hr of restraint. Seventy percent of the rats developed gastric ulcers. Increased urinary 5-hydroxyindoleacetic acid levels were not observed in rats deprived of food for 42 hr.     

航天特定环境下抑郁症模型大鼠血清、海马及延髓中单胺神经递质含量变化

目的 研究航天特定环境下抑郁症模型大鼠血清、海马及延髓中单胺神经递质的变化.方法 选择旷场实验行为学评分相近的30只大鼠,按完全随机法分为对照组和模型组,每组15只.模型组采用经典抑郁症模型,结合航天特定环境,建立大鼠抑郁症模型,对照组不做任何处理.观察航天特定环境下抑郁症模型大鼠行为学变化,测定血清、海马、延髓组织中5-羟色胺(5-HT)、多巴胺(DA)及去甲肾上腺素(NE)含量的变化.结果 与对照组比较,造模后模型组大鼠旷场实验中的穿行格数和直立次数减少,理毛次数、中央格停留时间增加(均P＜0.05);模型组大鼠血清中5-HT、DA及NE含量均高于对照组[(334.23±110.45)μg/L比(248.76±51.83)μg/L,(109.19±26.86)μg/L比(88.42±12.63)μg/L,(102.46±39.31)μg/L比(74.88±12.04)μg/L,均P＜0.05)],而海马组织中NE含量低于对照组[(9.99±8.02)μg/L比(25.41±5.68)μg/L,P＜0.05].结论 航天抑郁症的产生可能与血清中5-HT、DA降低无关,而与海马组织中的NE降低有关.     

Age related changes in the control of prolactin secretion in the female rat

The purpose of this investigation was to evaluate the effects of advancing age on the control of pituitary prolactin secretion. The effects of dopaminergic inhibition and estrogen stimulation of pituitary prolactin secretion were tested both in vivo and in vitro. Estrogen stimulated prolactin secretion in both old and young animals, and elevated estrogen levels in old rats may be partially responsible for elevated prolactin levels. Oral L-DOPA administration induced cycles in old rats but had no effect on prolactin levels in either old or young rats. Injections of L-DOPA lowered prolactin in young but not in old rats, while apomorphine reduced prolactin levels in both groups. The pituitaries of young rats secrete more prolactin in vitro than old pituitaries, further supporting a decrease in hypothalamic DA turnover as a cause for elevated prolactin levels in old rats.     

Alterations in mouse aggressive behavior and brain monoamine dynamics as a function of age.

Male mice of the BALB, ICR and C57B1/6J strains were tested for conspecific aggressive behavior at either 6 or 30 weeks of age. Only the BALB animals were aggressive at 6 weeks of age while both BALB and ICR mice fought at 30 weeks of age. C57B1/6J mice did not show aggressive behavior at either of the time intervals studied. Changes in housing conditions altered the degree of aggressive behavior of the ICR but not the C57B1/6J mice. Whole-brain levels, rate constants (k) and utilization rates (K) of norepinephrine (NE) and dopamine (DA) in these mice were fluorometrically determined. C57B1/6J mice had significantly higher NE levels while BALB mice had higher DA levels, compared to the other strains at both time periods. The NE rate constants and utilization rates for C57B1/6J mice were lower at 6 weeks and higher at 30 weeks than either BALB or ICR mice as was the dopamine rate constant at the 6 week period. There was a tendency for BALB mice to have consistently higher DA utilization rates. These findings are discussed in relation to the catecholamine hypothesis of affective disorders.     

Sexually dimorphic age-related changes in the distribution of immunoreactive luteinizing hormone releasing hormone in the platyfish

In this study on platyfish we demonstrate that the distribution and intensity of immunoreactive luteinizing hormone releasing hormone (ir-LHRH) in the brain and pituitary gland fluctuate at specific ages between 8 and 30 months. These changes are not the same in males and females, and they vary for each LHRH-containing region in the brain. In males, ir-LHRH intensity is greatest in the nucleus olfactoretinalis (NOR), nucleus preopticus periventricularis (NPP) and nucleus lateralis tuberis (NLT) at 18 months of age. In females, by comparison, the NOR displays the same degree of immunoreactivity at all ages examined, as does the NLT, where ir-LHRH perikarya are absent at all ages and immunoreactivity is restricted to a few fibers. In the NPP, females show fluctuations similar to those seen in males, however, the intensity of ir-LHRH is always less, and the number of ir-LHRH perikarya appear to be fewer, than in males. Thus, the age-related changes in LHRH systems in platyfish follow different patterns in males and females. Our study clearly indicates that immunocytochemical data must be evaluated with caution and that only after the age and sex of the animal and the maturational state of the gonads have been carefully scrutinized, can meaningful interpretations be made.     

Effects of midbrain lesions on female sexual behavior in the rat

Female sexual behavior was observed in ovariectomized female rats treated with varying doses of estrogen and progesterone, following electrolytic midbrain lesions. Severe deficits in receptivity were produced by lesions aimed at the dorsal and ventral norepinephrine pathways (DV lesions) and by lesions intended to destroy meso-limbic (A-10) dopamine-producing cells. Significant but less profound impairments were seen following lesions aimed at the serotonin-producing raphe nucleus or the nigro-striatal dopamine cells. Systemically administered d-amphetamine was found to increase receptivity in severely impaired A-10 and DV animals. The observed deficits may result from damage to midbrain structures involved in the sensory-motor or hormonal mediation of the lordosis reflex, or from disruption of a neurochemical system involved in control of lordosis.     

Regional changes in monoamine synthesis in the developing rat brain during hypoxia

4, 14 and 28 days old rats were exposed to a hypoxic environment of 6% O₂-94% N₂ for 30 min. Tyrosine hydroxylase and tryptophan hydroxylase acivity was studied in different brain regions (hemispheres, striatum, midbrain and brainstem) in vivo by measuring the accumulation of dihydroxyphenylalanine (Dopa) and 5-hydroxytryptophan (5-HTP) respectively, after inhibition of aromatic L-amino acid decarboxylase with NSD 1015. Tyrosine and tryptophan levels in the different brain regions were measured simultaneously. The tyrosine and tryptophan levels in the various brain parts were generally not influenced during exposure to hypoxia. Tyrosine hydroxylase activity decreased in most areas in the 4 and 14 days old rats, and all brain areas studied in the 28 days old rats. Tryptophan hydroxylase activity decreased markedly in all brain areas at all ages studied. It is concluded that the enzymes tyrosine hydroxylase as well as tryptophan hydroxylase seem to be equally affected during hypoxia in the different brain regions studied.     

Plasma and pituitary concentrations of LH,FSH, and Prolactin in aging C57BL/6 mice at various times of the estrous cycle

—Plasma and pituitary concentrations of LH, FSH, and prolactin (Prl) were measured by RIA in 2–4, 7–8, 12–13 and 16–20 month-old female C57BL/6 mice during various stages of the estrous cycle. In general, gonadotropin concentrations tended to rise with increasing age and Prl concentrations tended to decline. Pronounced differences existed, however, between the four age groups around the time of the LH surge. LH secretion declined progressively with increasing age at 21.00 hr of proestrus. Aged mice, 16–20 months old, had significantly lower plasma concentrations of LH than did other age groups. It is not known whether age-related changes in the ovary, pituitary, or hypothalamus are largely responsible for differences in the secretion of LH, FSH and Prl in aging C57BL/6 mice.     

Mechanisms of the anorexia of aging in the Brown Norway rat

Aging is associated with a loss of the ability to maintain homeostasis in response to physiologic and environmental disturbances. Age-related dysregulation of food intake and energy balance appears to be the result of impaired responsiveness of hypothalamic integrative circuitry to metabolic cues, which can lead to lack of appropriate food intake (the anorexia of aging) and thus to inappropriate weight loss in response to acute or chronic illness or other stressors. Using the Brown Norway (BN) male rat model, we have shown that old animals fail to appropriately increase food intake after the metabolic challenge of a 72 h fast, resulting in the failure to re-gain lost body weight upon refeeding. Leptin levels increase with adiposity and age, and remain elevated above levels of young animals even after a 72 h fast, suggesting that hyperleptinemia may be influencing the energy balance dysregulation. It is unclear whether this age-related response is due to a failure of the network of hypothalamic neurons to appropriately integrate hormonal and neural inputs, or due to a failure of the neurons to produce the appropriate neuropeptides. We hypothesize that sequential, age-related alterations in the expression patterns of neuropeptides that maintain melanocortinergic tone, and in the hormone mediators that inform the system of the state of energy balance, result in a diminished ability to maintain energy homeostasis with increasing age. We have undertaken a number of interventional approaches to test this hypothesis, including manipulations of the hormones ghrelin, insulin and testosterone, and direct application of neuropeptides to the central nervous system in these animals.     

Selective increase in brain dopamine metabolism during REM sleep rebound in the rat

Rats implanted with electrodes for EEG and EMG recording were placed on small or large platforms for either 24 or 96 hr and the effects on the subsequent REM sleep rebound were observed. Only the 96 hr protocol produced a selective REM sleep rebound in rats placed on small platforms. Brain MOPEG-SO₄, a final metabolite of norepinephrine, was determined fluorometrically and dopamine metabolites HVA and DOPAC, as well as the serotonin metabolite 5-HIAA, were measured by using an HPLC method with electrochemical detection. The results show that selective increases in brain HVA and DOPAC were obtained only during the REM sleep rebound in rats from small platforms. Non-specific increases were observed in MOPEG-SO₄ and 5-HIAA during the recovery period in rats from both small and large platforms which could be related to the effects of immobilization stress.     

Age-related changes in dopamine, LHRH and somatostatin in the rat hypothalamus

The distribution of dopamine, somatostatin and LHRH was examined in young and aged male rats of the Fisher 344 strain. Dopamine histofluorescence and peptide immunocytochemical staining were performed together in each animal by the use of a stimultaneous visualization technique. Comparative analysis or rats at 3, 12, 20, and 30 months of age revealed a general decrease in somatostatin and LHRH in the median eminence; dopamine fluorescence intensity also was depressed in the median eminence although dopaminergic perikarya of the arcuate nucleus of the hypothalamus appeared to increase in intensity with age. The age-related decline in median eminence LHRH may point to a central locus of reproductive senescence in the rat.     

In vivo evaluation by differential pulse voltammetry of the effect of thyrotropin-releasing hormone (TRH) on dopaminergic and serotoninergic synaptic activity in the striatum and nucleus accumbens of the rat

Summary In vivo differential pulse voltammetry was used to determine the effect of thyrotropin-releasing hormone (TRH) on dopaminergic and serotoninergic synaptic activity in the striatum and nucleus accumbens of the rat. Thyrotropin releasing hormone (TRH) produces marked stimulatory effects on behaviour, which have been attributed to the release of dopamine in the nucleus accumbens. Other studies indicate a close relationship between the peptide and serotonin in the brain. We have thus used an improved differential pulse voltammetry technique to evaluate the effects of TRH on the extracellular content of the dopamine and serotonin metabolites. Dihydroxyphenylacetic acid (DOPAC) and 5-Hydroxy-indolacetic acid (5HIAA) in the nucleus accumbens and striatum of the rat in vivo. TRH rapidly increased extracellular DOPAC, reaching a maximum after 60 min in the nucleus accumbens, and after 40 min in the striatum. There was also a slower increase in extracellular 5-HIAA content in both areas, reaching a plateau after 100 min. The delayed time course of the increase in 5-HIAA suggested that the increase in 5-HIAA content might be secondary to the increase in dopamine turnover produced by TRH. These results suggest that doses of TRH which produce behavioural stimulation increase the release of both dopamine and serotonin in the nucleus accumbens and striatum.     

The effects of monoamine neurotoxins on peptides in the rat spinal cord

The coexistence of two neuronally-localised peptides, substance P and thyrotropin-releasing hormone (TRH), in descending serotoninergic nerve fibres to the spinal cord was investigated using immunocytochemical and biochemical methods. Substance P-like material in the spinal cord was shown to be identical to the undecapeptide substance P by the criteria of gel filtration, high performance liquid chromatography and behaviour in substance P specific radioimmunoassays. Immunocytochemical staining for 5-hydroxytryptamine, substance P, and TRH showed that all three substances had a similar distribution in nerve fibres and terminals in the ventral and lateral grey matter of the spinal cord. After treatment with the serotonin neurotoxin 5,7-dihydroxytryptamine, neuronal elements containing 5-hydroxytryptamine, substance P and TRH degenerated and disappeared from these parts of the spinal cord in parallel with one another.Biochemical measurements of 5-hydroxytryptamine, substance P and TRH in the spinal cord after treatment with 5,7-dihydroxytryptamine confirmed that these three substances were all depleted from the ventral horn and, in addition, showed that there was a small depletion of substance P from the dorsal horn. Two other neuropeptides, somatostatin and methionine-enkephalin were not depleted from the spinal cord by treatment with 5,7-dihydroxytryptamine nor was substance P in other parts of the brain. Substance P in the spinal cord was unaffected by 6-hydroxydopamine, a drug known to destroy catecholamine-containing neurones.These results are consistent with coexistence of substance P and TRH together with 5-hydroxytryptamine in the descending axons and terminals of bulbospinal neurones.     

Differential effects of -trytophan and buspirone on biogenic amine contents and metabolism in Lurcher mice cerebellum

The effects of serotoninergic stimulation on monoamines were studied in the heterozygous Lurcher (Lc/+) mutant mouse, a model of human cerebellar ataxia. Wild type (+/+) and Lc/+ mice were treated for 40 days with -tryptophan or buspirone, and serotonin (5-HT), dopamine (DA), noradrenaline (NA) and their main metabolites were measured in the cerebellum. In +/+ mice, only buspirone increased concentrations of 5-HT metabolites. In the hypoplastic Lc/+ cerebellum, indoleamines were higher, and increased further after both treatments. The 5-HT turnover index was increased in +/+ mice by buspirone, while in Lc/+ mutants it increased after -tryptophan but was decreased by buspirone, indicating that in the mutants nerve terminals synthesize and accumulate 5-HT, but may not utilize it efficiently. Catecholamine contents remained unchanged in +/+ mice, but in Lc/+ mutants with higher endogenous NA, -tryptophan further increased NA and 3,4-dihydroxy-phenylacetic acid (DOPAC), and buspirone augmented NA, DA and DOPAC levels.