5HT Turnover Research Articles

The persistence of stress-induced physical inactivity in rats: an investigation of central monoamine neurotransmitters and skeletal muscle oxidative stress.

Sedentary lifestyles have reached epidemic proportions world-wide. A growing body of literature suggests that exposures to adverse experiences (e.g., psychological traumas) are a significant risk factor for the development of physically inactive lifestyles. However, the biological mechanisms linking prior stress exposure and persistent deficits in physical activity engagement remains poorly understood. The purpose of this study was twofold. First, to identify acute stress intensity thresholds that elicit long-term wheel running deficits in rats. To that end, young adult male rats were exposed to a single episode of 0, 50, or 100 uncontrollable tail shocks and then given free access to running wheels for 9 weeks. Second, to identify stress-induced changes to central monoamine neurotransmitters and peripheral muscle physiology that may be maladaptive to exercise output. For this study, rats were either exposed to a single episode of uncontrollable tail shocks (stress) or left undisturbed in home cages (unstressed). Eight days later, monoamine-related neurochemicals were quantified by ultra-high performance liquid chromatography (UHPLC) across brain reward, motor, and emotion structures immediately following a bout of graded treadmill exercise controlled for duration and intensity. Additionally, protein markers of oxidative stress, inflammation, and metabolic activity were assessed in the gastrocnemius muscle by Western blot. For experiment 1, stress exposure caused a shock number-dependent two to fourfold decrease in wheel running distance across the entire duration of the study. For experiment 2, stress exposure curbed an exercise-induced increase of dopamine (DA) turnover measures in the prefrontal cortex and hippocampus, and augmented serotonin (5HT) turnover in the hypothalamus and remaining cortical area. However, stress exposure also caused several monoaminergic changes independent of exercise that could underlie impaired motivation for physical activity, including a mild dopamine deficiency in the striatal area. Finally, stress potently increased HSP70 and lowered SOD2 protein concentrations in the gastrocnemius muscle, which may indicate prolonged oxidative stress. These data support some of the possible central and peripheral mechanisms by which exposure to adverse experiences may chronically impair physical activity engagement.

Decreased brain serotonin turnover rate following administration of Sharbat-e-Ahmed Shah produces antidepressant and anxiolytic effect in rats.

Sharbat-e-Ahmed Shah (SAS) has usually been used in Traditional Unani Medicine (TUM) for depression and insomnia but still not evaluated for its anti-depressant and Neuropharmacological activity. In the present study, a Human dose of SAS (0.6ml/kg/d) was administered orally to the rats for 15 consecutive days. Antidepressant and anxiolytic were screened scientifically in rats by using Forced swim test and light and dark box test. At the end of study high-performance liquid chromatographic (HPLC) method with electrochemical (EC) detector was used for the measurement of blood and brain tryptophan and brain serotonin levels. The present reported results are according to what is known in TUM, where is prescribed as an antidepressant agent. After the administration, SAS (at a human dose for 15days) reduced the immobility time in rats analogous to Imipramine (positive control) indicating the antidepressant effect of SAS. In the present study, Diazepam or SAS (0.6ml/kg/day) treated rats stayed in the illuminated side of the light-dark box, as compare to control rats (Veh, 134.62±4.430s; SAS 0.6ml/kg, 192.2±8.11 s; DZP 1.0mg/kg, 205.21.20±10.26 s, p<0.05). It was also observed that SAS increased the availability of tryptophan in blood and brain and hence increases 5-hydroxytryptamine (Serotonin: 5HT) in the brain. At the end, it was concluded that SAS contains some active principles which increase the availability of neurochemical (tryptophan and 5HT) and decrease the 5HT turnover rate thus causes antidepressant and anxiolytic effects in experimental animals.

Metabolomics of Neurotransmitters and Related Metabolites in Post-Mortem Tissue from the Dorsal and Ventral Striatum of Alcoholic Human Brain.

We report on changes in neurotransmitter metabolome and protein expression in the striatum of humans exposed to heavy long-term consumption of alcohol. Extracts from post mortem striatal tissue (dorsal striatum; DS comprising caudate nucleus; CN and putamen; P and ventral striatum; VS constituted by nucleus accumbens; NAc) were analysed by high performance liquid chromatography coupled with tandem mass spectrometry. Proteomics was studied in CN by two-dimensional gel electrophoresis followed by mass-spectrometry. Proteomics identified 25 unique molecules expressed differently by the alcohol-affected tissue. Two were dopamine-related proteins and one a GABA-synthesizing enzyme GAD65. Two proteins that are related to apoptosis and/or neuronal loss (BiD and amyloid-β A4 precursor protein-binding family B member 3) were increased. There were no differences in the levels of dopamine (DA), 3,4-dihydrophenylacetic acid (DOPAC), serotonin (5HT), homovanillic acid (HVA), 5-hydroxyindoleacetic acid (HIAA), histamine, L-glutamate (Glu), γ-aminobutyric acid (GABA), tyrosine (Tyr) and tryptophan (Tryp) between the DS (CN and P) and VS (NAc) in control brains. Choline (Ch) and acetylcholine (Ach) were higher and norepinephrine (NE) lower, in the VS. Alcoholic striata had lower levels of neurotransmitters except for Glu (30 % higher in the alcoholic ventral striatum). Ratios of DOPAC/DA and HIAA/5HT were higher in alcoholic striatum indicating an increase in the DA and 5HT turnover. Glutathione was significantly reduced in all three regions of alcohol-affected striatum. We conclude that neurotransmitter systems in both the DS (CN and P) and the VS (NAc) were significantly influenced by long-term heavy alcohol intake associated with alcoholism.

5-HT1B and other related serotonergic proteins are altered in APPswe mutation

Serotonergic dysfunction is implicated in Alzheimer’s disease (AD). In addition, reductions in brain of both monoamine synthesis and release have been reported. Serotonin 1B receptors (5-HT1B), along with serotonin transporter (SERT) are among the regulators of extracellular 5-HT levels. We investigated the effect of the familial AD APP (Amyloid precursor protein) K670N/M671L double mutation, APP Swedish mutation (APPswe), on the expression of 5-HT1B, SERT, MAOA, p11 and 5-HT and its metabolite 5-HIAA in SH-SY5Y human neuroblastoma cell line stably transfected with APPswe mutation. In addition, hippocampal expressions of 5-HT1B and SERT were assessed in wild type and transgenic mice expressing APPswe mutation (Tg2576) at different age groups. We found a reduction of 5-HT1B as well as SERT in both APPswe in vitro and ex vivo. P11 and 5HT were also reduced, whereas 5HT turnover and MAOA were increased. Our results indicate that APPswe induced decreased 5-HT1B expression and 5-HT release, as well as increased MAOA activity and 5-HT breakdown. Further studies to explore the detailed mechanism behind reduced 5-HT1B and SERT in AD and their clinical implications are needed.

Differences in the structure of drinking, cart expression and dopamine turnover between polydipsic and non polydipsic rats in the quinpirole model of psychotic polydipsia

Dopaminergic D2/D3 agonist quinpirole (QNP) elicits nonregulatory drinking in rats, a model of psychotic polydipsia. Why only a fraction of QNP-treated rats responds to the treatment becoming polydipsic is still unclear. To unveil possible factors contributing to such variability, we analyzed drinking microstructure in saline and QNP-treated rats, the hypothalamic expression of the cocaine and amphetamine regulated transcript (CART), and the monoaminergic turnover in selected brain areas. Rats were daily treated with saline or QNP 0.5 mg/kg, and their 5-h water intake was measured for five consecutive days. The number of bouts and episodes of licking, and their duration, were also measured. Brain CART expression was measured by in situ hybridization and monoamines turnover by HPLC analysis of tissue extracts. Based on the amount of water ingested during the 5-h session, QNP-treated rats were post hoc grouped in polydipsic (PD) and in nonpolydipsic (NPD) rats, and the results compared accordingly. The number of drinking bouts and episodes increased in PD rats, while NPD rats behaved as the controls. CART expression decreased in the arcuate nucleus of the hypothalamus of the PD rats. In contrast, both PD and NPD rats showed a reduction of DA turnover in both ventral tegmental area (VTA) and nucleus accumbens (NAcc). No difference was detected in the turnover of 5HT and NA. Microstructure analysis confirms that QNP acts on the appetitive component of drinking behavior, making it compulsive. CART expression reduction in response to dopaminergic hyperstimulation might sustain excessive drinking in PD rats.

Prenatal dexamethasone, as used in preterm labor, worsens the impact of postnatal chlorpyrifos exposure on serotonergic pathways

This study explores how glucocorticoids sensitize the developing brain to the organophosphate pesticide, chlorpyrifos. Pregnant rats received a standard therapeutic dose (0.2mg/kg) of dexamethasone on gestational days 17-19; pups were given subtoxic doses of chlorpyrifos on postnatal days 1-4 (1mg/kg, <10% cholinesterase inhibition). We evaluated serotonin (5HT) synaptic function from postnatal day 30 to day 150, assessing the expression of 5HT receptors and the 5HT transporter, along with 5HT turnover (index of presynaptic impulse activity) in brain regions encompassing all the 5HT projections and cell bodies. These parameters are known targets for neurodevelopmental effects of dexamethasone and chlorpyrifos individually. In males, chlorpyrifos evoked overall elevations in the expression of 5HT synaptic proteins, with a progressive increase from adolescence to adulthood; this effect was attenuated by prenatal dexamethasone treatment. The chlorpyrifos-induced upregulation was preceded by deficits in 5HT turnover, indicating that the receptor upregulation was an adaptive response to deficient presynaptic activity. Turnover deficiencies were magnified by dexamethasone pretreatment, worsening the functional impairment caused by chlorpyrifos. In females, chlorpyrifos-induced receptor changes reflected relative sparing of adverse effects compared to males. Nevertheless, prenatal dexamethasone still worsened the 5HT turnover deficits and reduced 5HT receptor expression in females, demonstrating the same adverse interaction. Glucocorticoids are used in 10% of U.S. pregnancies, and are also elevated in maternal stress; accordingly, our results indicate that this group represents a large subpopulation that may have heightened vulnerability to developmental neurotoxicants such as the organophosphates.

Neonatal parathion exposure disrupts serotonin and dopamine synaptic function in rat brain regions: Modulation by a high-fat diet in adulthood

The consequences of exposure to developmental neurotoxicants are influenced by environmental factors. In the present study, we examined the role of dietary fat intake. We administered parathion to neonatal rats and then evaluated whether a high-fat diet begun in adulthood could modulate the persistent effects on 5HT and DA systems. Neonatal rats received parathion on postnatal days 1–4 at 0.1 or 0.2 mg/kg/day, straddling the cholinesterase inhibition threshold. In adulthood, half the animals in each exposure group were given a high-fat diet for 8 weeks. We assessed 5HT and DA concentrations and turnover in brain regions containing their respective cell bodies and projections. In addition, we monitored 5HT 1A and 5HT 2 receptor binding and the concentration of 5HT presynaptic transporters. Neonatal parathion exposure evoked widespread increases in neurotransmitter turnover, indicative of presynaptic hyperactivity, further augmented by 5HT receptor upregulation. In control rats, consumption of a high-fat diet recapitulated many of the changes seen with neonatal parathion exposure; the effects represented convergent mechanisms, since the high-fat diet often obtunded further increases caused by parathion. Neonatal parathion exposure causes lasting hyperactivity of 5HT and DA systems accompanied by 5HT receptor upregulation, consistent with “miswiring” of neuronal projections. A high-fat diet obtunds the effect of parathion, in part by eliciting similar changes itself. Thus, dietary factors may produce similar synaptic changes as do developmental neurotoxicants, potentially contributing to the increasing incidence of neurodevelopmental disorders.

Evidence of an enhanced central 5HT response in irritable bowel syndrome and in the rat maternal separation model

Efforts to define either a biomarker for irritable bowel syndrome (IBS) or a valid animal model have proven disappointing. The aims of this study were to determine if buspirone stimulates prolactin release through the 5-hydroxytryptamine (5HT)1a receptor and whether this response is altered in patients with IBS and in the rat maternal separation model. Buspirone (30 mg) was used to stimulate prolactin release in 40 patients with IBS and in 40 healthy controls. In study 1, 10 IBS patients and 10 controls underwent pretreatment with pindolol (5HT1a antagonist) or placebo followed by buspirone. In study 2, 30 patients with IBS and 30 healthy controls had prolactin release stimulated by buspirone. Maternally separated and nonseparated rats were also treated with buspirone and prolactin monitored. Serotonin metabolites were measured together with the expression of the 5HT1a and serotonin transporter (SERT) gene. Pindolol produced a dose-dependent decrease in the buspirone prolactin response. Patients with IBS and maternally separated rats showed an exaggerated release of prolactin in response to buspirone. In the animal model, an increased turnover of 5HT was found in the brainstem together with a trend toward increased activity of the SERT gene. In conclusion altered central serotonin responses are found in both IBS and in an animal model.

Developmental exposure to terbutaline and chlorpyrifos, separately or sequentially, elicits presynaptic serotonergic hyperactivity in juvenile and adolescent rats

Developmental exposure to unrelated neurotoxicants can nevertheless converge on common final targets so as to exacerbate damage or functional deficits. We examined the effects of developmental exposure to terbutaline, a β 2-adrenergic receptor agonist used to arrest preterm labor, and chlorpyrifos, a widely used organophosphate pesticide, on serotonin (5HT) systems. Treatments were chosen to parallel periods typical of human developmental exposures, terbutaline (10 mg/kg) on postnatal days (PN) 2–5 and chlorpyrifos (5 mg/kg) on PN11–14, with assessments conducted in juvenile and adolescent stages (PN21, PN30 and PN45), comparing each agent alone as well as sequential administration of both. By itself, terbutaline produced persistent 5HT presynaptic hyperactivity as evidenced by increased 5HT turnover in brain regions containing 5HT terminal zones; this effect was similar to that seen in earlier studies with chlorpyrifos administration during the same early postnatal period. Later administration of chlorpyrifos (PN11–14) produced a transient increase in 5HT turnover during the juvenile stage, and the sequential exposure paradigm, terbutaline followed by chlorpyrifos, showed a corresponding increase in effect over either agent alone. In combination with our earlier work on 5HT receptors, these results indicate that terbutaline is a developmental neurotoxicant that targets the 5HT system, findings that lend a mechanistic underpinning to clinical indications of elevated childhood psychiatric disorders in the offspring of women treated with β-agonist tocolytics. Equally importantly, the interaction between terbutaline and chlorpyrifos suggests that tocolytic therapy may alter the subsequent susceptibility to common environmental toxicants.

Prenatal chlorpyrifos exposure elicits presynaptic serotonergic and dopaminergic hyperactivity at adolescence: Critical periods for regional and sex-selective effects

Fetal or neonatal chlorpyrifos (CPF) exposure affects serotonin (5HT) synaptic function and related behaviors in adulthood. We examined the critical period and dose threshold for effects on 5HT and assessed their emergence in adolescence. Pregnant rats were given CPF (1 or 5 mg/kg/day) from gestational days (GD)17–20 or GD9–12 and 5HT levels and turnover were evaluated on postnatal day 30; the lower dose lies below the threshold for inhibition of fetal brain cholinesterase. GD17–20 exposure increased 5HT turnover in brain regions containing either 5HT projections or cell bodies, with a preferential effect in males. Shifting the exposure to GD9–12 also augmented 5HT activity but only in the cerebral cortex and without sex preference. Similar, but lesser effects were seen for dopamine turnover in the same regions. These results indicate that, in a critical developmental period, apparently subtoxic exposures to CPF produce long-term activation of 5HT systems.

Regional Variations of 5HT Concentrations in Rora sg (staggerer) Mutants

Ataxic Rora(sg) (staggerer) mouse mutants, containing a deletion of the Rora gene which encodes a retinoid-like nuclear receptor, were compared to non-ataxic controls for concentrations of 5-hydroxytryptamine (HT), its main metabolite (5-hydroxy-indole acetic acid, 5HIAA), and its precursor (tryptophan) in cerebellum, brainstem, and forebrain. In Rora(sg) cerebellum, 5HT concentrations increased relative to controls, while tryptophan concentrations decreased. 5HIAA concentrations increased in mutant cerebellum and brainstem, but the 5HIAA/5HT ratio declined only in cerebellum. These results indicate that 5HT turnover decreased in cerebellum of an ataxic mutant, perhaps indicative of presynaptic accumulation and compromised neurotransmission and susceptible to be modified by 5HT pharmacotherapy.

Serotonin turnover in different duration of sleep recovery in discrete regions of young rat brain after 24 h REM sleep deprivation

Sleep plays an important role in restorative function and serotonin (5-hydroxytryptamine: 5HT) equally plays important roles in sleep. Though various studies have revealed the roles of 5HT in sleep/wake cycle, the mechanism involved is yet unclear. In the present study we investigated alteration of the 5HT turnover in various regions of the young rat brains after 24 hours (h) REM sleep (sREM) deprivation to elucidate the roles of 5HT in sleep restoration function in the these regions. The 5HT turnover was evaluated by the ratio of 5-hydroxyindole acetic acid against 5HT. The sREM deprivation was performed by the inverted flowerpot technique. The 5HT turnover showed significant alteration in the all regions of the brain examined after 24 h sREM deprivation, particular depending on the brain region. These results revealed that sREM modulates the 5HT turnover in the brain with region specificity and this may be one of the restorative functions of sleep indicating that sREM is regionally generated.

Effects of Perinatal Exposure to Bisphenol A on Brain Neurotransmitters in Female Rat Offspring

Pregnant Sprague-Dawley (CD IGS) rats were orally administered doses of bisphenol A (BPA) at 4, 40, and 400 mg/kg, from gestation days 6 to postnatal day 20. Neurotransmitters such as dopamine (DA) and serotonin (5HT) were extracted from the brains of dams and female offspring, and measured using liquid chromatography. BPA at 400 mg/kg was toxic and dosed rats died. At 3 wk after birth, brain levels of 3,4-dihydroxyphenylacetic acid (DOPAC, a DA metabolite), homovanillic acid (HVA, a DA metabolite), 5HT, 5-hydroxyindoleacetic acid (5HIAA, a 5HT metabolite) in female offspring were increased and the HVA/DA ratio was high in some brain areas of BPA-treated groups as compared with controls. At the age of 6 wk, levels of choline (Ch) in BPA-treated groups at 4 and 40 mg/kg were higher than control in all of eight brain areas. No changes were observed in acetylcholine (ACh) contents. In 9-wk-old offspring, changes in monoamines and metabolites were scattered and not great. At 3 wk after delivery, levels of 5HIAA in some brain areas of dams treated with BPA were higher than in control dams. Dose dependent increases in HVA and the HVA/DA ratio of the occipital cortex, and in the HVA/DA ratio of the frontal cortex were observed. The turnover of DA and 5HT was accelerated in 3-wk-old offspring and dams. BPA possesses very weak estrogenic activity. Changes in cerebral neurotransmitters observed in offspring and dams in this study may have been related to the estrogenic activity of BPA. However, further investigation is needed to examine the contribution of hormonal activity to such neurotransmitter changes.

Critical Prenatal and Postnatal Periods for Persistent Effects of Dexamethasone on Serotonergic and Dopaminergic Systems

Glucocorticoid administration to preterm infants is associated with neurodevelopmental disorders. We treated developing rats with dexamethasone (Dex) at 0.05, 0.2, or 0.8 mg/kg, doses below or spanning the range in clinical use, testing the effects of administration during three different stages: gestational days 17-19, postnatal days 1-3 or postnatal days 7-9. In adulthood, we assessed the impact on synaptic biomarkers for serotonin (5-hydroxytryptamine (5HT)) systems. Across all three regimens, Dex administration evoked upregulation of cerebrocortical 5HT1A and 5HT2 receptors and the presynaptic 5HT transporter, greatest for 5HT1A receptors. The effects were fully evident even at the lowest dose. In contrast, 5HT levels in the cerebral cortex and hippocampus showed disparate patterns of temporal sensitivity, with no change after gestational treatment, an increase with the early postnatal regimen, and a decrease with the later postnatal exposure. None of the changes in 5HT concentrations were offset by adaptive changes in the fractional 5HT turnover rate. Furthermore, the critical period of sensitivity seen for 5HT levels differed from that of dopamine even within the same brain region. These findings suggest that developmental exposure to Dex during the critical neurodevelopmental period corresponding to its use in preterm infants, elicits selective changes in 5HT and dopaminergic synaptic function over and above its effects on general aspects of neural cell development, below the threshold for somatic growth impairment, and even at doses below those used clinically. Accordingly, adverse neurobehavioral consequences may be inescapable in glucocorticoid therapy of preterm infants.

Alterations in Central Nervous System Serotonergic and Dopaminergic Synaptic Activity in Adulthood after Prenatal or Neonatal Chlorpyrifos Exposure

Exposure to chlorpyrifos (CPF) alters neuronal development of serotonin (5HT) and dopamine systems, and we recently found long-term alterations in behaviors related to 5HT function. To characterize the synaptic mechanisms underlying these effects, we exposed developing rats to CPF regimens below the threshold for systemic toxicity, in three treatment windows: gestational days (GD) 17–20, postnatal days (PN) 1–4, or PN11–14. In early adulthood (PN60), we assessed basal neurotransmitter content and synaptic activity (turnover) in brain regions containing the major 5HT and dopamine projections. CPF exposure on GD17–20 or PN1–4 evoked long-term increases in 5HT turnover across multiple regions; the effects were not secondary to changes in neurotransmitter content, which was unaffected or even decreased. When the treatment window was shifted to PN11–14, there were no long-term effects. Dopamine turnover also showed significant increases after CPF exposure on GD17–20, but only when the dose was raised above the threshold for overt toxicity; however, hippocampal dopamine content was profoundly subnormal after exposures below or above the acute, toxic threshold, suggesting outright neurotoxicity. These results indicate that, in a critical developmental period, apparently nontoxic exposures to CPF produce lasting activation of 5HT systems in association with 5HT-associated behavioral anomalies.

Exacerbation of harmaline-induced tremor by imipramine

Imipramine is a well-established tricyclic antidepressant which was first approved for the treatment of depression in the late fifties. Antidepressant effect of imipramine is attributed to inhibition of serotonin (5HT) and noradrenaline (NA) reuptake in brain. These monoamines have been implicated in a variety of neurological disorders including tremor. In the present investigation attempt was made to study the effect of imipramine on harmaline-induced tremor in rats. Male Sprague Dawley rats weighing 115±2.5 g were given harmaline (10 mg/kg, i.p.) alone or along with imipramine (30 min before harmaline) in doses of 60 and 90 mg/kg respectively. The latency of onset, intensity and duration of tremor and EMG were recorded. To substantiate the role of 5HT in aetiopathology of tremor the above experiment was repeated in the rats pretreated with P-chlorophenylalanine (PCPA), a potent 5HT depleter. The levels of 5HT and 5-hydroxyindole acetic acid (5HIAA) in the brain stem were measured using high performance liquid chromatography. Imipramine dose-dependently exacerbated the duration, intensity and amplitude of EMG following harmaline-induced tremor. Imipramine treatment further decreased harmaline-induced 5HT turnover in the brain stem. However, this was statistically insignificant. Depletion of 5HT produced a significant reduction in the intensity and duration of harmaline-induced tremor. In conclusion, this study suggests that imipramine exacerbates harmaline-induced tremor. Clinical use of imipramine for the treatment of depression in patients who also suffer from tremors may require a close monitoring.

Citalopram, a selective serotonin reuptake inhibitor augments harmaline-induced tremor in rats

Citalopram, a serotonin reuptake inhibitor (SSRI) is one of the widely used antidepressants. Apart from its antidepressant activity citalopram is also used for anxiety, panic disorders, obsessive-compulsive disorder and behavioral disturbances of dementia. Tremor is the second most common neurological adverse effect in patients receiving treatment with SSRIs. Use of these agents in depressed patients with essential tremor has not been studied. The present study was undertaken to investigate the effect of chronic citalopram treatment on harmaline-induced tremors in rats. Female Sprague–Dawley rats weighing 70±2 g were given citalopram in doses of 0, 10, 20 and 40 mg/kg by gavage for 2 weeks. On the 15th day, the rats were given harmaline (10 mg/kg, i.p.) 30 min after the last dose of citalopram. The latency of onset, intensity and duration of tremor and EMG were recorded. Serotonin (5HT) and 5-hydroxy indole acetic acid (5HIAA) were measured in brain stem. Citalopram dose dependently exacerbated the duration, intensity and amplitude of EMG of harmaline-induced tremor. A significant decrease in 5HT turnover (5HIAA/5HT ratio) in the brain stem was observed suggesting a possible role of serotoninergic impairment in citalopram-induced augmentation of harmaline-induced tremor. Clinical implications of these observations warrant further investigation.

Effect of Aging on 24-Hour Changes in Serotonin and Dopamine Turnover, and Somatostatin and Amino Acid Content, of the Rat Hypothalamus and Pituitary Gland

The effect of aging on neurotransmitter and peptide content in the hypothalamichypophysial unit has commonly been analyzed at single time points in the 24-h cycle. Since significant changes in circadian rhythmicity occur during aging, this study aimed to examine 24-h rhythmicity in hypothalamic and pituitary serotonin (5HT) and dopamine (DA) turnover and content, and somatostatin and amino acid content in 2 months-old and 18–20 months-old rats, killed at 6 different time intervals throughout the light-dark cycle. Aged rats showed suppressed or disrupted 24-h rhythms of 5HT and DA turnover and of somatostatin, glutamate, aspartate and taurine content (anterior hypothalamus), of 5HT and DA turnover and of somatostatin, glutamate, taurine and glycine content (medial hypothalamus) and of DA turnover and amino acid content (posterior hypothalamus). Twenty-four h variations in DA, somatostatin, aspartate, GABA and glycine content of the anterior hypophysis and in all parameters tested in the neurointermediate lobe became suppressed or disrupted in aged rats. Mean values generally decreased with age, except for DA content in the anterior pituitary lobe and aspartate content in the neurointermediate lobe. Conclusions: Examination of neurotransmitter and neuropeptide content at different times of the day is needed to analyze the effects of aging in the hypothalamic-hypophysial unit.

Circadian Rhythms in Adenohypophysial Hormone Levels and Hypothalamic Monoamine Turnover in Mycobacterial-Adjuvant-Injected Rats

The effect of Freund’s adjuvant injection on 24-hour variation in circulating ACTH, prolactin, growth hormone (GH) and thyroid-stimulating hormone (TSH) levels, and of hypothalamic norepinephrine (NE) content and dopamine (DA) and serotonin (5HT) turnover was examined in adult rats. In control rats, serum ACTH and prolactin exhibited peak values at the light-dark transition while the maximum in TSH was found in the late afternoon. GH levels did not vary on a 24-hour basis. In Freund’s-adjuvant-injected rats, 24-hour variations in TSH levels became blunted while 24-hour variations in prolactin and ACTH persisted. Freund’s adjuvant treatment augmented serum ACTH and prolactin levels, and decreased GH and TSH levels. Hypothalamic NE content, and turnover of DA and 5HT varied on a 24-hour basis in rats receiving adjuvant’s vehicle. The NE content of the anterior, medial and posterior hypothalamus peaked at 04.00 h, while that of the median eminence attained its maximum at 16.00–20.00 h. Maxima in hypothalamic DA and 5HT turnover occurred at 04.00 h regardless of the region examined. In Freund’s-adjuvant-injected rats, reduced amplitude of daily variations of NE content in the median eminence and anterior and medial hypothalamus, as well as a phase advance in the 24-hour rhythm of the posterior hypothalamic NE content were seen. Mycobacterial adjuvant injection also reduced the amplitude of circadian rhythm in hypothalamic 5HT turnover, shifted the maximum in median eminence DA turnover towards light-dark transition, and decreased the amplitude of DA turnover rhythm in the anterior, medial and posterior hypothalamus. Administration of the immunosuppressant drug cyclosporine restored the augmented ACTH and prolactin levels and the depressed GH and TSH levels found in Freund’s-adjuvant-injected rats. Cyclosporine was also effective to restore 24-hour rhythmicity of serum ACTH and TSH, but not of prolactin levels. Immunosuppression restored rhythmicity of NE content and of DA and 5HT turnover in anterior, medial and posterior hypothalamic regions. Cyclosporine did not modify the effect of Freund’s adjuvant on median eminence but in was able to counteract the changes in the DA and 5HT turnover in the median eminence found after immunization. The results are in accord with a significant effect of immune-mediated inflammatory response at an early phase after Freund’s adjuvant injection on ACTH, GH, prolactin and TSH release mechanisms, which was partially sensitive to immunosuppression induced by cyclosporine.

The neuroendocrine control of clock-timed gonadotropin release in the female Syrian hamster: role of serotonin.

We hypothesized that rhythms in hypothalamic serotonergic activity were permissive to daily and estrous cycle-related rhythms of LH, FSH and prolactin (PRL). In the Syrian hamster, proestrus (PRO) is characterized by a surge of LH, FSH and PRL; diestrus (DIE) by low LH and FSH and a small surge of PRL, while in photoperiod-induced anestrous (PIA) animals there is a surge of LH and FSH and low PRL. Turnover rates of serotonin (5HT) in four brain areas were determined for the three reproductive states at 2-h intervals. Turnover in the preoptic area and arcuate nuclei did not change, indicating that 5HT projections to these regions probably do not control LH, FSH or PRL release. Serotonin turnover in the median eminence (ME) was elevated at 0600 h in PIA females, at 0600 h, 0800 h, and 1400 h on DIE and at 0600 h and 2200 h on PRO. Since the pattern of 5HT turnover in the ME is different during each of the three reproductive states, 5HT in this area is likely not crucial to the control of LH, FSH and PRL. Turnover of 5HT also did not change in the suprachiasmatic nuclei (SCN) of PRO or PIA animals. However, 5HT turnover rates in the SCN were elevated at 1200 h, 2000 h, and 2400 h on DIE. The correlation of high 5HT turnover in the SCN of DIE but not PRO and PIA animals suggested that elevated serotonergic activity in the SCN is part of the mechanism by which the gonadotropin surge is prevented on DIE. To test this, PRO and DIE hamsters were injected with 5HT receptor ligands. Administration of a 5HT agonist attenuated the PRO surge of LH and blocked the surge of PRL. In contrast, administration of two 5HT antagonists failed to elicit a surge of LH in DIE and phenobarbital-blocked PRO females, an indication that other mechanisms also contribute to inhibition of gonadotropin and PRL surges.