Physiopharmacological interactions between stress hormones and central serotonergic systems

Seasonality in suicides: the influence of suicide method, gender and age on suicide distribution in Italy

In order to ascertain if and how age, gender and choice of lethal means influence the seasonal distribution of suicide in Italy, data concerning all suicides registered in Italy from 1984 to 1995 have been analyzed, taking these variables into account. In the age group 14-65 years and over a total of 31771 male suicides (mean yearly rate, 12.6 per 100000) and 11984 female suicides (mean yearly rate, 4.4 per 100000) have been identified in Italy during the study period. Suicides in the younger age ranges, both among males and females, show a less marked asymmetrical seasonal distribution than those in the older age groups. Only suicides committed by violent methods (ICD 953-958) show clear evidence of seasonality, with a peak in spring and a low in late autumn. Suicides committed by non-violent methods (950-952) follow no seasonal trend in either sex. Spectral analysis reveals a circannual rhythm for violent suicides (ICD 953-958) in both genders. For male non-violent suicides (ICD 950-952), a period with a frequency of 0.0833 (12 months) has been identified, but with a polarity opposite to that of male violent suicides. For female non-violent suicides, no period of frequency of 0.0833 could be identified, but, as for female violent suicides, a period with frequency close to 0.2500 (4 months) has been found. Changes in climate, then, correlate with the monthly distribution of violent and non-violent suicides in opposite ways: male violent suicides show a significant positive relationship with indicators of temperature and exposure to the sun, and a significant negative relationship with indicators of humidity and rainfall. Female suicides show less significant relationships with climate indicators. Work aimed at suicide prevention should therefore take into account the complex influence of seasonal climate both on human biological rhythms (particularly on 5-HT related functions and their actions on mood and impulsivity) and on sociorelational habits.

Effects of anxiolytics, diazepam and tandospirone, on immobilization stress-induced hyperglycemia in mice

The benzodiazepine anxiolytic diazepam did not affect immobilization-elicited hyperglycemia, although a high dose increased blood glucose levels in normal mice. The serotonergic anxiolytic tandospirone reduced immobilization-induced hyperglycemia dose-dependently. Hyperglycemia elicited by immobilization stress was completely prevented by adrenalectomy but not by pretreatment with the corticosterone synthesis inhibitor dexamethasone. These results suggest that the effects of two anxiolytics, diazepam and tandospirone, on immobilization stress-induced hyperglycemia are quite different, although both drugs reduce anxiety. Furthermore, our results indicate that immobilization stress-elevated hyperglycemia is closely related to adrenaline release from the adrenal gland and that tandospirone may inhibit stress-induced hyperglycemia by modifying this mechanism.

Counteraction of spaceflight-induced changes in the rat central serotonergic system by adrenalectomy and corticosteroid replacement

The effects of a 17-day spaceflight duration on serotonergic measures in various parts of rat brain have been studied (flight-SHAM group). The contribution of the activation of the hypothalamo-pituitary-adrenal axis (HPA) related to the response of the central serotonin system was evaluated in adrenalectomized with chronic corticosterone replacement rats (flight-ADX+CORT group). These two groups of rats were compared to their respective ground-based controls. Physiological parameters (body, adrenal and thymus weights) and corticosterone levels were measured. In flight-SHAM group as compared to controls, adrenal hypertrophy and elevation in plasma corticosterone levels (174%) were observed, without change in thymus mass. In most brain areas studied, significant decreases in TRP, 5-HTP and 5-HIAA were found associated with lower levels of 5-HT in cortex, thalamus and striatum. Conversely, there were elevations in TRP, 5-HTP levels in striatum and increases in 5-HIAA/5-HT ratios, an index of 5-HT turnover, in cortex, striatum and olfactory bulb while the hypothalamus was the sole region where a fall was observed. In ADX rats with chronic corticosterone replacement these effects were not observed in the majority of brain areas. It is concluded that a 17-day spaceflight exerted an inhibitory effect on serotonin metabolism, probably by activation of the HPA axis. The results could not distinguish between the effects of microgravity and the stress associated with landing.

Vulnerability to stressor-induced disturbances in self-stimulation from the dorsal and ventral A10 area: differential effects of intraventricular D-Ala2-Met5-enkephalinamide, D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin, and D-Pen2, D-Pen5-enkephalin administration

D-Ala2-Met5-enkephalinamide (DALA) (1.0 microg/microl) was administered intraventricularly to mice responding for electrical stimulation from the dorsal or ventral aspects of the VTA immediately prior to footshock (Experiment 1). Predictably, footshock reduced self-stimulation from the dorsal but not the ventral VTA immediately, 24, and 168 h following the stressor. Intraventricular DALA administration effected a partial attenuation of stressor-induced self-stimulation reductions from the dorsal VTA immediately and 24 h poststressor. Deficits appeared among DALA-Shocked mice responding for brain stimulation from the ventral VTA during comparable test intervals. The long-term depressant influence of footshock on self-stimulation from the dorsal VTA was abolished among DALA-treated mice and DALA-associated reductions in self-stimulation from the ventral A10 region among stressed mice were not evident 1 week later. Administration of D-Ala2, N-Me-Phe4, Gly-Ol5-enkephalin (DAGO) (0.01 microg/microl) or D-Pen2, D-Pen5-enkephalin (DPDPE) (1.0 microg/microl) intraventricularly prior to footshock effected an immediate and a delayed antagonism, respectively, of the stressor on self-stimulation from the dorsal VTA, which persisted for 1 week. Prophylactic administration of 0.001 microg/microl DAGO or 0.01 microg/microl DPDPE prior to the stressor failed to influence self-stimulation from the ventral VTA (Experiment 2). Administration of 0.01 microg/microl DAGO or 1.0 microg/microl DPDPE among mice responding for brain stimulation from the dorsal VTA following footshock produced a weak therapeutic effect immediately poststressor, but effected protracted amelioration of footshock-induced reductions of self-stimulation from the dorsal VTA (Experiment 3). Taken together, mu, delta, and mu-delta activation influenced self-stimulation differentially from the dorsal and ventral VTA according to the temporal order of opioid peptide challenge relative to stressor imposition. These data are discussed with respect to stressors, motivational alterations, and the putative modulating influence of endogenous enkephalin activity in subareas of the VTA.

Alterations in serotonergic responsiveness during cocaine withdrawal in rats: similarities to major depression in humans

BACKGROUND

Withdrawal from long-term cocaine use is accompanied by symptoms resembling major depression. Because acute cocaine affects serotonin (5-HT) neurons, and 5-HT dysfunction is implicated in the pathophysiology of depression, we evaluated the effects to 5-HT agonists in rats withdrawn from repeated injections of cocaine (15 mg/kg i.p., b.i.d., 7 days) or saline.

METHODS

In the first study, prolactin (PRL) responses elicited by the 5-HT-releasing agent fenfluramine, the 5-HT1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), and the 5-HT2A/2C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) were examined as indices of postsynaptic 5-HT receptor function. In a second study, specific responses induced by 8-OH-DPAT, namely inhibition of brain 5-HT synthesis and stimulation of feeding, were examined as correlates of 5-HT1A autoreceptor function.

RESULTS

Prior treatment with cocaine did not modify fenfluramine-evoked PRL release; however, the PRL secretory response to 8-OH-DPAT was blunted and the PRL response to DOI was potentiated after chronic cocaine treatment. Cocaine exposure did not alter the inhibitory effect of 8-OH-DPAT on 5-HT synthesis. 8-OH-DPAT-induced feeding was influenced by prior cocaine, but this effect was secondary to pronounced baseline hyperphagia in the cocaine-treated group.

CONCLUSIONS

These data indicate that withdrawal from chronic cocaine renders specific subpopulations of postsynaptic 5-HT1A receptors subsensitive and 5-HT2A/2C receptors supersensitive. No evidence for cocaine-induced changes in 5-HT1A autoreceptor responsiveness was found. A survey of the literature reveals similarities in the profile of 5-HT dysfunction between rats withdrawn from cocaine and humans diagnosed with depression. We propose that withdrawal from chronic cocaine in rats may serve as a useful animal model of depressive disorders.

The spectrum of behaviors influenced by serotonin

The diverse array of behavioral effects of serotonin form the basis for understanding its potential role as an etiological marker in psychiatric disorders and for the successful pharmacologic intervention of drugs regulating serotonin neurotransmission in behavior. General theories of the behavioral functions of serotonin have implicated serotonin as a general inhibitor of behavioral responding and in modulating motor behavior. The ability of serotonin to regulate behavioral satiety and macronutrient selection provides the basis for pharmacologic treatment of obesity and eating disorders. The role of serotonin in behavioral suppression may be important in social behavior involving aggression and anxiety. The role of serotonin in neuroendocrine regulation provides a basis for understanding serotonin dysregulation in depression. Animal behavior tests are being used to better understand the neural substrates underlying the behavioral effects of antidepressant drugs and to address important issues in clinical treatment. The integration of information between basic and clinical studies provides the basis for future development of more sophisticated pharmacologic treatments of psychiatric disorders.

Effect of 5-hydroxytryptamine and pineal metabolites on the secretion of neurohypophysial hormones

It has been shown that 5-hydroxytryptamine and melatonin, an indoleamine for which 5-hydroxytryptamine is a precursor, influence the release of vasopressin and oxytocin from the rat hypothalamus both in vivo and in vitro. The oral administration of melatonin has been shown to decrease oxytocin release and modulate the nocturnal vasopressin release in humans. 5-hydroxytryptamine and its metabolites, 5-hydroxytryptophol, 5-methoxytryptamine and 5-methoxytryptophol, are detected within the pineal, and there is evidence that 5-methoxytryptamine and 5-methoxytryptophol may have some physiological role. The aim of this study was to evaluate the effects of 5-hydroxytryptamine, 5-hydroxytryptophol, 5-methoxytryptamine and 5-methoxytryptophol on neurohypophysial hormone release from the rat hypothalamus in vitro. It was found that 5-hydroxytryptamine and 5-hydroxytryptophol increased neurohypophysial hormone release, 5-methoxytryptamine decreased the release of vasopressin and oxytocin and 5-methoxytryptophol was found to have no effect, thus providing further evidence for a role of indole compounds in the control of neurohypophysial hormone secretion.

The adrenergic stress response in fish: control of catecholamine storage and release

In fish, the catecholamine hormones adrenaline and noradrenaline are released into the circulation, from chromaffin cells, during numerous 'stressful' situations. The physiological and biochemical actions of these hormones (the efferent adrenergic response) have been the focus of numerous investigations over the past several decades. However, until recently, few studies have examined aspects involved in controlling/modulating catecholamine storage and release in fish. This review provides a detailed account of the afferent limb of the adrenergic response in fish, from the biosynthesis of catecholamines to the exocytotic release of these hormones from the chromaffin cells. The emphasis is on three particular topics: (1) catecholamine biosynthesis and storage within the chromaffin cells including the different types of chromaffin cells and their varying arrangement amongst species; (2) situations eliciting the secretion of catecholamines (e.g. hypoxia, hypercapnia, chasing); (3) cholinergic and non-cholinergic (i.e. serotonin, adrenocorticotropic hormone, angiotensin, adenosine) control of catecholamine secretion. As such, this review will demonstrate that the control of catecholamine storage and release in fish chromaffin cells is a complex processes involving regulation via numerous hormones, neurotransmitters and second messenger systems.

Enhanced morphine-induced behavioural effects and dopamine release in the nucleus accumbens in a transgenic mouse model of impaired glucocorticoid (type II) receptor function: influence of long-term treatment with the antidepressant moclobemide

In vivo microdialysis experiments were conducted in transgenic mice with impaired glucocorticoid receptor function resulting from expression of antisense directed against glucocorticoid receptor messenger RNA. Basal corticosterone and serotonin levels in the nucleus accumbens of untreated transgenic mice were enhanced compared to control mice (B6C3F1). Following a systemic morphine injection (15 mg/kg) mesolimbic dopamine and serotonin release was markedly increased in transgenic mice compared to control mice and in parallel enhanced behavioural stimulation was observed in these animals. After pretreatment with the antidepressant moclobemide over a time period of eight weeks (15 mg/kg/day) elevated basal levels of both corticosterone and serotonin were normalized in transgenic mice. Furthermore, morphine-induced dopamine and serotonin release as well as behavioral stimulation were suppressed in transgenic mice and similar to that in control mice. The results indicate that impaired glucocorticoid receptor function influences the basal release of serotonin in the nucleus accumbens. This alteration has no effect on basal but on morphine-stimulated release of dopamine in the mesolimbic system. An enhanced sensitivity to the effects of morphine is apparently related to elevated brain corticosterone and serotonin levels and can be normalized by long-term antidepressant treatment.

Escapable and inescapable stress differentially and selectively alter extracellular levels of 5-HT in the ventral hippocampus and dorsal periaqueductal gray of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the ventral hippocampus and dorsal periaqueductal gray (dPAG) were measured by in vivo microdialysis. Inescapable, but not escapable shock increased extracellular 5-HT in the ventral hippocampus relative to restrained controls. Basal levels of 5-HT were elevated 24 h after inescapable shock, and previously inescapably shocked subjects exhibited an exaggerated 5-HT response to 2 brief footshocks. In contrast, escapable, but not inescapable shock, increased extracellular 5-HT in the dPAG, increased basal 5-HT in the dPAG 24 h later, and led to an enhanced 5-HT response to subsequent brief footshock.

Serotonin transporter messenger RNA in the developing rat brain: early expression in serotonergic neurons and transient expression in non-serotonergic neurons

Serotonin has been shown to affect the development of the mammalian nervous system. The serotonin transporter is a major factor in regulating extracellular serotonin levels. Using in situ hybridization histochemistry the rat serotonin transporter messenger RNA was localized during embryogenesis, the first four weeks postnatally and adulthood. Three general classes of serotonin transporter messenger RNA expression patterns were observed: (i) early detection with continued expression through adult age, (ii) transient expression colocalized with vesicular monoamine transporter 2 messenger RNA but with no detectable tryptophan hydroxylase immunoreactivity, and (iii) transient expression in the apparent absence of both vesicular monoamine transporter 2 messenger RNA and tryptophan hydroxylase immunoreactivity. For example, hybridization for serotonin transporter messenger RNA was strong in serotonin cell body-containing areas beginning early in gestation, and remained intense through adulthood. Immunoreactivity for tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis, was completely overlapping with the presence of serotonin transporter messenger RNA in raphe nuclei postnatally. Sensory relay systems including the ventrobasal nucleus (somatosensory), lateral and medial geniculate nuclei (visual and auditory, respectively) as well as trigeminal, cochlear and solitary nuclei were representative of the second class of observations. In general, the limbic system expressed serotonin transporter messenger RNA in the third pattern with various limbic structures differing in the timing of expression. Septum, olfactory areas and the developing hippocampus contained serotonin transporter messenger RNA early in the developing brain. Other regions such as cingulate and frontopolar cortex exhibited hybridization peri- and postnatally, respectively. Several hypothalamic nuclei and pituitary transiently expressed serotonin transporter messenger RNA either postnatally or perinatally, respectively. If the observed patterns correlate with functional protein expression, distinct classes of serotonin transporter messenger RNA expression may reflect different functional roles for the serotonin transporter and serotonin, itself. Since the serotonin transporter is a target for a number of addictive substances including cocaine and amphetamine derivatives as well as antidepressants, transient expression of the serotonin transporter might suggest a window of vulnerability of associated cells to fetal drug exposure. Re-uptake, storage and re-release from non-serotonergic neurons might serve as a feedback mechanism from target neurons to serotonergic neurons. Alternatively, the transient expression of serotonin transporter messenger RNA may reflect critical periods important for tight regulation of extracellular serotonin in several brain regions, and may indicate previously unappreciated roles for serotonin as a developmental cue.

Elevation of brain 5-HT activity, POMC expression, and plasma cortisol in socially subordinate rainbow trout

Agonistic behavior, brain concentrations of serotonin (5-hydroxytryptamine, 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA, the main 5-HT metabolite), plasma cortisol levels, and the pituitary expression of pro-opiomelanocortin (POMC) A and B mRNA were determined in socially dominant and subordinate rainbow trout after 1 or 7 days of social interaction. Telencephalic and brain stem 5-HIAA/5-HT ratios, plasma cortisol levels, and pituitary POMC mRNA concentrations were elevated in fish being subordinate for 1 day. Furthermore, neither telencephalic 5-HIAA/5-HT ratios nor pituitary POMC A or POMC B mRNA expression showed any decline after 7 days of social interaction. By contrast, plasma cortisol concentrations of subordinate fish declined after 7 days but were still significantly higher than in dominant fish. Furthermore, in subordinate fish, hypothalamic 5-HIAA/5-HT ratios and plasma cortisol levels were highly correlated, suggesting an important role of hypothalamic 5-HT in the regulation of the teleost hypothalamic-pituitary-interrenal (HPI) axis. The number of aggressive acts received and plasma cortisol levels were highly correlated in 1-day subordinates, a relationship not seen in fish subjected to 1 wk of subordination. Thus the chronic stress experienced by subordinates in established dominance hierarchies appears to be more closely related to the threat imposed by the presence of the dominant fish than to actual aggressive encounters. The sustained elevation of pituitary POMC mRNA expression, an effect mainly related to an increase of melanotropic POMC expression, in subordinates could be a mechanism serving to maintain HPI axis excitability and promote acclimation in these individuals.

Effects of housing conditions and 5-HT2A activation on male rat sexual behavior

Adult male rats were housed individually or in groups for a period of 39 days. In Experiment 1, the effects of housing conditions on sexual behavior and concurrent spontaneous "wet dog shaking" (WDS) were investigated. Individual housing significantly impaired male sexual behavior and resulted in a trend toward increased WDS. In Experiment 2, the effects of housing conditions were examined following administration of the serotonergic type 2A (5-HT2A) agonist DOI. Individual housing significantly increased DOI-induced WDS. The implications of these findings are discussed in the context of stress-induced corticosterone secretion and the possible regulatory effect on 5-HT2A receptors.

Feeding behaviour, brain serotonergic activity levels, and energy reserves of Arctic char (Salvelinus alpinus) within a dominance hierarchy

The relationships between feeding activity, brain serotonergic activity level, and energy reserves in eight groups of Arctic char (Salvelinus alpinus) were evaluated using self-feeders in combination with passive integrated transponder (PIT) tags. Serotonin concentrations were measured in the brain stem, telencephalon, and hypothalamus. Energy reserves were measured, as fat levels, in muscle and mesenteric tissue. The self-feeding activity level was found to correlate positively (P < 0.05) with the growth rate and negatively (P < 0.05) with the serotonergic activity levels in the brain stem and telencephalon. No significant relationship was found, however, between the number of trigger activations and the muscle or mesenteric tissue lipid level. One or, in some cases, two fish in each group clearly dominated the activation of the trigger, and showed the highest growth rates and lowest stress levels, measured as brain serotonergic activity. Subordinate fish (one to three individuals) hardly ever activated the trigger, but showed relatively high growth rates and brain serotonergic activity levels that did not differ significantly from those of dominant fish. The remaining individuals showed very little feeding activity and low growth rates. In addition, brain serotonergic activity levels were significantly higher than in fish in the former two categories, indicating that the subordinate fish suffered from stress and had a low social rank.

Topical application of 5-HT antibodies reduces edema and cell changes following trauma of the rat spinal cord

Involvement of serotonin in the early microvascular reactions and cell changes following trauma of the spinal cord was examined using topical application of serotonin antibodies to the traumatised cord in a rat model. A focal trauma of the spinal cord was produced by making an incision into the right dorsal horn of the T10-11 segments (about 2 mm deep and 5 mm long); the animals were allowed to survive 5 h after injury. Monoclonal 5-HT antibodies (1:20) were applied (25 microliters in 10 sec) to the traumatised spinal cord 2 min after injury. There was a significant reduction in the breakdown of the blood-spinal cord barrier, edema formation, and cell changes in the traumatised rats which received 5-HT antiserum compared to the injured rats given saline. These results show that 5-HT is an important mediator involved in the early pathophysiological responses of spinal cord injury.

Differential effects of social stress on central serotonergic activity and emotional reactivity in Lewis and spontaneously hypertensive rats

Social stress by repeated defeat has been shown to be endowed with neuroendocrine and behavioural effects that render this stress model useful to identify adaptive mechanisms. Among these mechanisms, those related to central serotonergic systems (e.g., hippocampal 5-HT1A receptors, cortical 5-HT2A receptors) have been particularly underlined. Nonetheless, how (i) the neuroendocrine and behavioural effects of social stress are affected by the genetic status of the animal, and (ii) this status affects the relationships between central serotonergic systems and adaptive processes has not been studied so far. The present study has thus analysed the effects of repeated defeat (once a day for seven days) by Long-Evans resident rats upon the psychoneuroendocrine profile of Lewis rats and spontaneously hypertensive rats previously characterized for their contrasting social and anxiety-related behaviours. Repeated defeat decreased in a time-dependent manner, body weight growth and food intake in both strains, these decreases being, however, more severe and longer lasting in Lewis rats. This strain-dependent difference could not be accounted for by differences in physical contacts with the resident rats as the number of attacks and their latency throughout the stress period were similar between spontaneously hypertensive and Lewis rats. When exposed to an elevated plus-maze test of anxiety, the unstressed Lewis rats entered less the open arms than their spontaneously hypertensive counterparts, thus confirming that Lewis rats are more anxious than spontaneously hypertensive rats. This difference was amplified by stress as the latter increased anxiety-related behaviours in Lewis rats only. These strain- and stress-related differences were associated with differences in locomotor activity, this being increased in unstressed Lewis compared with spontaneously hypertensive rats; moreover, stress triggered hypolocomotion in the former but not the latter strain. Lastly, in the forced swimming test. Lewis rats spent more time immobile than spontaneously hypertensive rats with stress increasing immobility in a strain-independent manner. Beside the aforementioned metabolic changes, the activity of the hypothalamo-pituitary-adrenal axis was slightly stimulated in a strain-independent manner by the stressor, as assessed by increased corticosterone levels and adrenal weights, and decreased thymus weights. In Lewis, but not in spontaneously hypertensive rats, midbrain serotonin metabolism was increased by stress, a difference associated with an increased Bmax value of cortical [3H]ketanserin binding at 5-HT2A receptors. On the other hand, the Bmax value of hippocampal [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding at 5-HT1A receptors was decreased by stress, this reduction being amplified in spontaneously hypertensive compared with Lewis rats. This study shows that the psychoneuroendocrine responses to social stress may have a genetic origin, and that the use of socially stressed Lewis and spontaneously hypertensive rats may provide an important paradigm to study adaptive processes. However, whether the aforementioned strain-dependent differences in central serotonergic systems (partly or totally) underlie the distinct profiles of emotivity measured in spontaneously hypertensive and Lewis rats, is discussed in the context of the relationships between serotonergic systems and behavioural responses to novel environments.

Hypothalamic-pituitary-adrenal axis function and platelet serotonin concentrations in depressed patients

Plasma cortisol and platelet serotonin (5-hydroxytryptamine, 5-HT) concentrations were determined in 39 male psychotic and 39 male non-psychotic depressed inpatients, and in 69 male healthy control subjects. Psychotic or non-psychotic depressed patients had higher predexamethasone plasma cortisol levels than found in the control group. After the dexamethasone suppression test (DST), psychotic and non-psychotic depressed patients were subdivided into suppressors and non-suppressors. Psychotic and non-psychotic patients had significantly different platelet 5-HT concentrations among themselves and compared with the control group. However, there was no significant correlation between plasma cortisol levels and platelet 5-HT concentrations. Dexamethasone administration did not affect platelet 5-HT concentrations within subtypes of depressed patients. Abnormal cortisol suppression after the DST occurred more frequently in psychotic than in non-psychotic patients. Platelet 5-HT and plasma cortisol concentrations were decreased in patients with pronounced suicidal behaviour. Our results suggest that plasma cortisol and platelet 5-HT concentrations might serve as independent biological markers for different subtypes of depression.

Differential effects of restraint stress on hippocampal 5-HT metabolism and extracellular levels of 5-HT in streptozotocin-diabetic rats

Streptozotocin (STZ)-elicited diabetes reduces central serotonin (5-hydroxytryptamine, 5-HT) synthesis/metabolism, but whether this reduction leads to decreased release of 5-HT has only scarcely been investigated. We have thus analysed the impact of STZ diabetes on hippocampal extracellular 5-HT levels both under basal conditions and during restraint stress, a procedure known to stimulate hippocampal 5-HT synthesis/metabolism and release. The pretreatment with STZ (3 weeks beforehand) and the 1 h restraint session respectively decreased and increased hippocampal 5-HT metabolism, as assessed by tissue analysis of 5-HT and 5-hydroxyindoleacetic acid. On the other hand, hippocampal microdialysis revealed no difference in basal levels of extracellular 5-HT levels in (conscious) vehicle- and STZ-pretreated rats, but a differential effect of restraint. Thus, extracellular 5-HT levels increased throughout restraint (maximal increase: 194%) in vehicle-, but not in STZ-pretreated rats. In the latter rat group, plasma corticosterone levels were, however, increased, thus indicating a significant aversiveness to stress. Lastly, because anxiety-related behaviours may be affected by hippocampal serotonergic systems, resting and restrained vehicle- and STZ-pretreated rats were compared (immediately after stress) in an elevated plus-maze of anxiety. Pretreatment with STZ reduced the percent number of open arm entries and the number of closed arm entries, indicating increased anxiety and reduced locomotor activity, respectively. Restraint tended to increase anxiety-related behaviours in all rats, but this trend never reached significance. Our results confirm that gross analyses of 5-HT metabolism do not yield information on 5-HT release, and suggest that the prevalence of diabetes among patients suffering affective disorders could be related to the lack of hippocampal serotonergic response to aversive stimuli.

Autoradiographic characterization of [3H]-5-HT-moduline binding sites in rodent brain and their relationship to 5-HT1B receptors

5-HT-moduline is an endogenous tetrapeptide [Leu-Ser-Ala-Leu (LSAL)] that was first isolated from bovine brain tissue. To understand the physiological role of this tetrapeptide, we studied the localization of 5-HT-moduline binding sites in rat and mouse brains. Quantitative data obtained with a gaseous detector of beta-particles (beta-imager) indicated that [3H]-5-HT-moduline bound specifically to rat brain sections with high affinity (Kd = 0.77 nM and Bmax = 0. 26 dpm/mm2). Using film autoradiography in parallel, we found that 5-HT-moduline binding sites were expressed in a variety of rat and mouse brain structures. In 5-HT1B receptor knock-out mice, the specific binding of [3H]-5-HT-moduline was not different from background labeling, indicating that 5-HT-moduline targets are exclusively located on the 5-HT1B receptors. Although the distribution of 5-HT-moduline binding sites was similar to that of 5-HT1B receptors, they did not overlap totally. Differences in distribution patterns were found in regions containing either high levels of 5-HT1B receptors such as globus pallidus and subiculum that were poorly labeled or in other regions such as dentate gyrus of hippocampus and cortex where the relative density of 5-HT-moduline binding sites was higher than that of 5-HT1B receptors. In conclusion, our data, based on autoradiographic localization, indicate that 5-HT-moduline targets are located on 5-HT1B receptors present both on 5-HT afferents and postsynaptic neurons. By interacting specifically with 5-HT1B receptors, this tetrapeptide may play a pivotal role in pathological states such as stress that involves the dysfunction of 5-HT neurotransmission.

Tissue-specific glucocorticoid regulation of tryptophan hydroxylase mRNA levels

A potential long-term target of glucocorticoid modulation of serotonin (5-HT) production is tryptophan hydroxylase (TPH) gene expression. However, studies on TPH gene expression have been hampered by the extremely low levels of TPH mRNA in the brain, and there have been contradictory reports on the effects of glucocorticoids on 5-HT levels. To overcome these obstacles, we have developed a sensitive competitive RT-PCR assay to directly measure TPH mRNA levels from the rat brain. We observed a tissue-specific modulation of TPH mRNA levels in the melatonin producing pineal gland and the serotonin producing raphe nuclei of the brain. Following chronic treatment of adrenalectomized rats with the synthetic glucocorticoid dexamethasone for 1 week, there was a 16-fold increase in TPH mRNA in the pineal gland that was contrasted by a decrease in TPH mRNA to 16% of the control levels in the brain. To address the mechanism of dexamethasone repression of TPH mRNA levels, we then tested a serotonergic neuronal-like cell line derived from rat thyroid C cells. Dexamethasone caused a rapid decrease in TPH mRNA levels to approximately 20% of control values in CA77 C cells. This was measured by both competitive RT-PCR and a standard hybridization assay, which confirmed the validity of the RT-PCR assay. Furthermore, the reduction of TPH mRNA levels was associated with a decrease in 5-HT levels in the CA77 C cells. Hence, glucocorticoids may alter serotonin and melatonin biosynthetic capacity by cell-specific modulation of the TPH gene.

Activity of tryptophan hydroxylase and content of indolamines in discrete brain regions after a long-term hypoxic exposure in the rat

The influence of long-term hypoxia (10% O2, 14 days) on in vivo activity of tryptophan hydroxylase and on 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentration in discrete brain regions of rats was assessed. The activity of tryptophan hydroxylase was determined through 5-hydroxytryptophan accumulation (5-HTPacc) following the administration of NSD 1015. The 5-HTPacc was significantly decreased in the dorsal and median raphe (56 and 42%, respectively) and in the striatum (62%). Both 5-HTPacc and the ratio of the concentrations of 5-HIAA to 5-HT were decreased in the nucleus raphe magnus (46 and 27%, respectively), the dorsomedian medulla oblongata (52 and 51%), the locus coeruleus (62 and 40%) and the anterior hypothalamic nucleus (30 and 50%). In contrast, 5-HTPacc was increased in the ventrolateral medulla oblongata (55%) and the preoptic area (83%), but the 5-HIAA/5-HT ratio was lower in these two regions. Finally, 5-HIAA/5-HT ratio was also decreased in the periventricular nucleus and in the frontal cortex. Since various patterns of variations in 5-HTPacc and in 5-HIAA/5-HT ratio were observed, the factors affecting serotonin metabolism in hypoxic rats can be different among brain regions. These results show that, in the rat, long-term hypoxia induces changes in in vivo activity of tryptophan hydroxylase and in 5-HT and 5-HIAA content of some brain structures; some of these biochemical changes may be linked to adaptative mechanisms.

Serotonin as a regulator of hypothalamic-pituitary-interrenal activity in teleost fish

Evidence for the presence of a serotonin1A (5-HT1A) receptor subtype in the salmonid fish brain has recently been presented. In the present study the potent 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) was tested for its effect on plasma cortisol concentrations in rainbow trout (Oncorhynchus mykiss). Blood was sampled and 8-OH-DPAT administered through a catheter in the dorsal aorta. Thirty minutes after the injection of 40 microg of 8-OH-DPAT/kg, plasma cortisol levels had increased from 12 to 149 ng/ml, whereupon they fell, reaching baseline levels after 4 h. The effect of 1-40 microg 8-OH-DPAT/kg on plasma cortisol concentrations was dose-dependent. The results lends further support to the hypothesis that the brain serotonergic system plays a key role in integrating autonomic, behavioral and neuroendocrine stress-responses in fish as well as mammals, suggesting that not only the structural and biochemical organization, but also the function of the serotonergic system has been conserved during vertebrate evolution.

The role of paraventricular nucleus of hypothalamus in stress-ulcer formation in rats

The rat stress model of restraint and cold water immersion was used to investigate the effect of stimulating the paraventricular nucleus (PVN) of hypothalamus on the development of stress-induced gastric ulceration. The results were (1) electric stimulation of the PVN increased the stress ulceration, while electrolytic lesion of the PVN decreased it; (2) intracerebroventricular injection (i.c.v.) of acetylcholine (Ach) enhanced the effect of PVN stimulation on stress ulcers, and the M-receptor was involved; (3) i.c.v. norepinephrine (NE) attenuated the effect of PVN stimulation on stress ulcers in a dose-dependent manner, and the beta-receptor was involved; (4) i.c.v. 5-hydroxytryptamine (5-HT) enhanced the effect of PVN stimulation on stress ulcers; (5) electrolytic lesions of dorsal raphe nucleus (DR) attenuated the effect of PVN stimulation on stress ulcers, while electrolytic lesions of the locus ceruleus (LC) aggravated the effect; (6) thyroidectomy, adrenalectomy, ovariectomy, vagotomy and sympathectomy all attenuated the effect of PVN stimulation on stress ulcers; (7) electric stimulation of the PVN produced no effect on gastric juice volume, acidity, total acid output, pepsin activity or the gastric barrier mucus; but greatly reduced gastric mucosal blood flow. These results indicate that the PVN is an important brain site regulating the development of stress-induced gastric ulcers, that the classical neurotransmitters Ach, NE and 5-HT are involved, and that in the periphery, both the parasympathetic and sympathetic nervous systems and the three endocrine glands (thyroid, adrenal and gonad) take part in the effect.

5-HT2A receptor-mediated regulation of brain-derived neurotrophic factor mRNA in the hippocampus and the neocortex

The influence of 5-HT receptor agonists on the expression of BDNF in brain was determined. Administration of a hallucinogenic 5-HT2A /2C receptor agonist, but not a 5-HT1A receptor agonist, resulted in a significant but differential regulation of BDNF mRNA levels in hippocampus and neocortex. In the hippocampus, the 5-HT2A /2C receptor agonist significantly decreased BDNF mRNA expression in the dentate gyrus granule cell layer but did not influence expression of the neurotrophin in the CA subfields. In parietal cortex and other neocortical areas, but not piriform cortex, the 5-HT2A /2C receptor agonist dramatically increased the expression of BDNF mRNA. The effect of the 5-HT2A /2C receptor agonist on BDNF mRNA in both the hippocampus and the neocortex was blocked by pretreatment with a selective 5-HT2A, but not 5-HT2C, receptor antagonist. The expression of BDNF mRNA in the hippocampus is reported to be decreased by stress, raising the possibility that the 5-HT2A receptor mediates this effect. Pretreatment with ketanserin, a 5-HT2A /2C receptor antagonist, significantly blocked the stress-induced downregulation of BDNF mRNA in hippocampus, in support of this hypothesis. The results of this study raise the possibility that regulation of BDNF expression by hallucinogenic 5-HT2A receptor agonists leads to adaptations of synaptic strength in the hippocampus and the neocortex that may mediate some of the acute and long-term behavioral effects of these agents.

Effect of repeated experience of victory and defeat in daily agonistic confrontations on brain tryptophan hydroxylase activity

The rate-limiting enzyme of serotonin biosynthesis, tryptophan hydroxylase (TPH), was studied in brain areas of male mice with repeated experience of victory (winners) or defeat (losers) gained in 10 daily agonistic confrontations. A reduction of TPH activity in the midbrain and an increase in the hypothalamus was demonstrated for winners compared with controls. In contrast, repeated defeat in social confrontations was associated with higher TPH activity in the striatum and hypothalamus in losers compared with controls. Agonistic interactions did not affect TPH activity in the amygdala, nucleus accumbens or hippocampus in either winners or losers. The sensory contact technique used in this work for generating winners and losers may be productive in the analysis of TPH gene regulation.

Effects of food deprivation on midbrain 5-HT1A autoreceptors in Lewis and SHR rats

Food deprivation stimulates the activity of the hypothalamo-pituitary-adrenal axis and brain serotonin (5-hydroxytryptamine, 5-HT) synthesis. Because midbrain somato-dendritic 5-HT1A autoreceptors may obey homologous and heterologous (e.g. by glucocorticoids) down-regulation, we have analyzed whether 24 hr of fasting affects midbrain 5-HT1A receptor binding and sensitivity in Lewis and SHR rats (i.e. strains that differ in behavioral/neuroendocrine responses to stressors). Fasting affected neither [3H]8-hydroxy-2-(di-N-propylamino)tetralin ([3H]8-OH-DPAT) binding at 5-HT1A autoreceptors nor 8-OH-DPAT-induced inhibition of midbrain 5-HT synthesis (an index of 5-HT1A autoreceptor sensitivity). Because fasting increased 5-HT precursor (tryptophan) levels to similar extents in the midbrains of saline- and 8-OH-DPAT-treated rats, we conclude that food deprivation does not affect 5-HT1A autoreceptors. In turn, our results suggest that the differential effects of 5-HT1A receptor agonists on food intake, in fed and fasted rats may be independent from 5-HT1A autoreceptors.

Impact of adrenalectomy on 5-HT6 and 5-HT7 receptor gene expression in the rat hippocampus

Both glucocorticoid excess and decreased serotonergic (5-HT) transmission may cause depression. The recently cloned 5-HT6 and 5-HT7 receptors have high affinity for antidepressants. Here, we show that pharmacological adrenalectomy increases 5-HT6 and 5-HT7 receptor mRNA expression in specific hippocampal subfields, effects partly reversed by corticosterone replacement. Increased 5-HT6 and 5-HT7 receptor expression may provide a basis, in part, for the therapeutic actions of adrenal steroid synthesis inhibitors in resistant depression.

A critical review of 5-HT brain microdialysis and behavior

Serotonin (5-HT) has been implicated in many central nervous system-mediated functions including sleep, arousal, feeding, motor activity and the stress response. In order to help establish the precise role of 5-HT in physiology and behavior, in vivo microdialysis studies have sought to identify the conditions under which the release of 5-HT is altered. Extracellular 5-HT levels have been monitored in more than fifteen regions of the brain during a variety of spontaneous behaviors, and in response to several physiological, environmental, and behavioral manipulations. The vast majority of these studies found increases (30-100%) in 5-HT release in almost all brain regions studied. Since electrophysiological studies have shown that behavioral arousal is the primary determinant of brain serotonergic neuronal activity, we suggest that the increase in 5-HT release seen during a wide variety of experimental conditions is largely due to one factor, namely an increase in behavioral arousal/motor activity associated with the manipulation.

Does central serotonergic function correlate inversely with aggression? A study using D-fenfluramine in healthy subjects

Much research has investigated possible links between serotonergic function and aggression in violent or personality disordered populations, but few studies have looked at healthy subjects. In this study we administered 30 mg of the specific 5-HT releasing agent D-fenfluramine to 35 healthy subjects, along with questionnaire measures of hostility and aggression. Prolactin and cortisol responses were used as indices of central 5-HT function. In males, there were significant inverse correlations between 5-HT mediated cortisol responses and both the Buss-Durkee Hostility Inventory total score and the aggression factor. There were no such relations in female subjects or using prolactin responses. There was also an inverse relation between basal cortisol levels and both prolactin and cortisol responses, but no relation between basal cortisol levels and aggressive measures. These results provide some support for the existence of an inverse relationship between central serotonin function and aggression/hostility in healthy males, similar to that seen in previous studies using violent or highly aggressive populations.

Release of serotonin in the rat locus coeruleus: effects of cardiovascular, stressful and noxious stimuli

To investigate the function of serotonergic neurons within the locus coeruleus, this brain nucleus of conscious, freely moving rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the extracellular concentration of serotonin was determined in the superfusate. Serotonin release was increased by depolarization with veratridine (5 microM) or 80 mM K+, while superfusion with tetrodotoxin (1 microM) or systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin substantially diminished the release rate of serotonin in the locus coeruleus. The pressor response to intravenous infusion of noradrenaline (4 micrograms/kg/min) was associated with a pronounced increase in the release rate of serotonin. Superfusion of the locus coeruleus with tetrodotoxin (1 microM) abolished the increase in serotonin release evoked by the pressor response. A fall of blood pressure produced by intravenous administration of nitroprusside (150 micrograms/kg/min) or chlorisondamine (3 mg/kg) diminished the release rate of serotonin. Immobilization, noise (95 dB) or tail pinch increased the release of serotonin in the locus coeruleus and slightly elevated blood pressure. Chlorisondamine abolished the rise in blood pressure elicited by tail pinch without influencing the increased serotonin release. Tail pinch-induced serotonin release was abolished by superfusion with tetrodotoxin. The findings demonstrate that neuronal serotonin release in the locus coeruleus responds to cardiovascular and sensory stimuli, suggesting a function of serotonergic neurons in central blood pressure regulation, as well as in the modulation of locus coeruleus activity by stress and noxious stimuli.

Stress-induced alterations of somatodendritic 5-HT1A autoreceptor sensitivity in the rat dorsal raphe nucleus--in vitro electrophysiological evidence

Somatodendritic 5-HT1A autoreceptors play a key role in the control of the electrical and metabolic activity of serotoninergic neurons in the dorsal raphe nucleus. These neurons also possess intracellular glucocorticoid receptors which may be involved in the well established modulation of serotonin (5-hydroxytryptamine, 5-HT) metabolism by corticosterone in stressed animals. The possible mediation by somatodendritic 5-HT1A autoreceptors of such corticosterone-dependent changes in serotoninergic neuron activity was investigated using an in vitro electrophysiological approach. 5-HT1A autoreceptor-mediated inhibition of the firing of serotoninergic neurons was examined in brain stem slices from rats whose serum corticosterone concentrations had been markedly increased (+100-200%) by two different stressful conditions. Immobilization for 30 or 90 min (restraint stress) did not modify the concentration-dependent inhibition of the firing of serotoninergic neurons by the 5-HT1A receptor agonist ipsapirone. In contrast, placing the rats in novel uncontrolled environmental conditions for 16 h significantly reduced the cell response to ipsapirone, indicating a decreased sensitivity of somatodendritic 5-HT1A autoreceptors. Such a change was not observed in adrenalectomized rats subjected to the same stressful conditions. These data show that some forms of stress can reduce the 5-HT1A autoreceptor-dependent inhibitory control of the electrophysiological activity of serotoninergic neurons in the dorsal raphe nucleus. Both the nature and duration of stress seem to be critical factors for triggering the (corticosterone-dependent) mechanism(s) responsible for the functional desensitization of 5-HT1A autoreceptors in stressed rats.

Role of central neural mechanisms in the regulation of hepatic glucose metabolism

Central monoamine neurotransmitters affect blood glucose homeostasis. Activation of central cholinergic, noradrenergic histaminergic, and serotonergic neurons rapidly increase hepatic glucose output via the sympathetic nervous system. Acute hyperglycemia is mediated by three distinct pathways: the action of epinephrine on the liver, the action of glucagon on the liver, and the direct innervation of the liver. The relative contribution of these factors to hyperglycemia can be altered by diet and the kinds of neurotransmitters evoked in the central nervous system, but the magnitude of epinephrine secretion is closely related to the magnitude of hyperglycemia. On the other hand, neuropharmacological stimulation of central cholinergic muscarinic receptors, histaminergic H1 receptors, and serotonergic 5-HT2 receptors increases hypothalamic noradrenergic neuronal activity, which is associated with hyperglycemia. In contrast, central GABAA receptors play an inhibitory role in the regulation of hepatic glucose metabolism. Thus, central monoaminergic neurons could be linked together, and play a homeostatic role in the regulation of hepatic glucose metabolism.

Activity wheel running reduces escape latency and alters brain monoamine levels after footshock

We examined the effects of chronic activity wheel running on brain monoamines and latency to escape foot shock after prior exposure to uncontrollable, inescapable foot shock. Individually housed young (approximately 50 day) female Sprague-Dawley rats were randomly assigned to standard cages (sedentary) or cages with activity wheels. After 9-12 weeks, animals were matched in pairs on body mass. Activity wheel animals were also matched on running distance. An animal from each matched pair was randomly assigned to controllable or uncontrollable inescapable foot shock followed the next day by a foot shock escape test in a shuttle box. Brain concentrations of norepinephrine (NE), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), 5-hydroxytryptamine (5-HT), and 5-hydroxyindole acetic acid (5-HIAA) were assayed in the locus coeruleus (LC), dorsal raphe (DR), central amygdala (AC), hippocampus (CA1), arcuate nucleus, paraventricular nucleus (PVN), and midbrain central gray. After prior exposure to uncontrollable foot shock, escape latency was reduced by 34% for wheel runners compared with sedentary controls. The shortened escape latency for wheel runners was associated with 61% higher NE concentrations in LC and 44% higher NE concentrations in DR compared with sedentary controls. Sedentary controls, compared with wheel runners, had 31% higher 5-HIAA concentrations in CA1 and 30% higher 5-HIAA concentrations in AC after uncontrollable foot shock and had 28% higher 5-HT and 33% higher 5-HIAA concentrations in AC averaged across both foot shock conditions. There were no group differences in monoamines in the central gray or in plasma prolactin or ACTH concentrations, despite 52% higher DA concentrations in the arcuate nucleus after uncontrollable foot shock and 50% higher DOPAC/DA and 17% higher 5-HIAA/5-HT concentrations in the PVN averaged across both foot shock conditions for sedentary compared with activity wheel animals. The present results extend understanding of the escape-deficit by indicating an attenuating role for circadian physical activity. The altered monoamine levels suggest brain regions for more direct probes of neural activity after wheel running and foot shock.

Effects of acute physical exercise on central serotonergic systems

This paper reviews data concerning the effects of acute physical exercise (treadmill running) in trained rats. Works from the 1980's have established that acute running increases brain serotonin (5-hydroxytryptamine: 5-HT) synthesis in two ways. Lipolysis-elicited release of free fatty acids in the blood compartment displaces the binding of the essential amino acid tryptophan to albumin, thereby increasing the concentration of the so-called "free tryptophan" portion, and because exercise increases the ratio of circulating free tryptophan to the sum of the concentrations of the amino acids that compete with tryptophan for uptake at the blood-brain barrier level, tryptophan enters markedly in the brain compartment. However, this marked increase in central tryptophan levels increases only to a low extent brain 5-HT synthesis, as assessed by the analysis of 5-hydroxyindoleacetic acid levels, thereby suggesting that exercise promotes feedback regulatory mechanisms. Indirect indices of 5-HT functions open the possibility that acute exercise-induced increases in 5-HT biosynthesis are associated with (or lead to) increases in 5-HT release. Lastly, the hypothesis that training and/or acute exercise triggers changes in 5-HT receptors has been examined in several studies; actually, both positive and negative results have been reached. Taken together, all these data support the need for future studies on the functional effects of exercise on 5-HT, including those related to the hypothesis that the positive mood effects of exercise rely (partly or totally) on central serotonergic systems.

Thyroid parafollicular cells. An accessible model for the study of serotonergic neurons

Serotonergic neurons play key roles in modulating a wide variety of behavioral and homeostatic processes. However, there is a paucity of good model systems to study these neurons at a molecular level. In this review we will present evidence that cell lines derived from an unexpected source, thyroid parafollicular cells (PF) (also called C cells), fit the criteria for use as models for the study of serotonergic neurons. A strength of PF cell lines over other cell lines is that the parental PF cells have serotonergic properties and a neuronal potential that is consistent with their neural crest origin. Furthermore, PF cells and PF cell lines are capable of expressing the fundamental properties of serotonergic neurons, including: (1) serotonin (5-HT) biosynthesis by tryptophan hydroxylase (TPH), (2) vesicular 5-HT storage and regulated release, (3) expression of a 5-HT autoreceptor, and (4) expression of the 5-HT transporter. In this review, we will focus primarily on the serotonergic and neuronal properties of the rat CA77 PF cell line and the parental rat PF cells. The applicability of CA77 cells for molecular analyses will be described. First, their use for studies on the glucocorticoid regulation of the TPH gene will be discussed. Second, control of the calcitonin/calcitonin gene-related peptide (CT/CGRP) gene will be discussed, with particular emphasis on the application of serotonergic drugs in treating migraine headaches. These examples highlight the versatility of thyroid PF cell lines as a system for studying the control of both serotonin biosynthesis and physiological actions.

Effects of tryptophan and/or acute running on extracellular 5-HT and 5-HIAA levels in the hippocampus of food-deprived rats

The present microdialysis study has examined whether exercise-elicited increases in brain tryptophan availability (and in turn 5-HT synthesis) alter 5-HT release in the hippocampus of food-deprived rats. To this end, we compared the respective effects of acute exercise, administration of tryptophan, and the combination of both treatments, upon extracellular 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels. All rats were trained to run on a treadmill before implantation of the microdialysis probe and 24 h of food deprivation. Acute exercise (12 m/min for 1 h) increased in a time-dependent manner extracellular 5-HT levels (maximal increase: 47%), these levels returning to their baseline levels within the first hour of the recovery period. Besides, exercise-induced increases in extracellular 5-HIAA levels did not reach significance. Acute administration of a tryptophan dose (50 mg/kg i.p.) that increased extracellular 5-HIAA (but not 5-HT) levels in fed rats, increased within 60 min extracellular 5-HT levels (maximal increase: 55%) in food-deprived rats. Whereas 5-HT levels returned toward their baseline levels within the 160 min that followed tryptophan administration, extracellular 5-HIAA levels rose throughout the experiment (maximal increase: 75%). Lastly, treatment with tryptophan (60 min beforehand) before acute exercise led to marked increases in extracellular 5-HT and 5-HIAA levels (maximal increases: 100% and 83%, respectively) throughout the 240 min that followed tryptophan administration. This study indicates that exercise stimulates 5-HT release in the hippocampus of fasted rats, and that a pretreatment with tryptophan (at a dose increasing extracellular 5-HT levels) amplifies exercise-induced 5-HT release.

A microdialysis examination of serotonin release in the rat forebrain induced by behavioral/environmental manipulations

Serotonin (5-HT) has been implicated in a variety of behavioral and physiological processes mediated by the central nervous system. However, the exact nature of 5-HT release under naturalistic or physiologic conditions remains unclear. The present study investigated this issue by employing in vivo microdialysis to examine 5-HT release in the hippocampus, corpus striatum, amygdala, and prefrontal cortex of the rat during manipulations that induced varying behavioral and physiological responses. In each rat, two sites were examined simultaneously during two of the following manipulations: tail pinch, tail pinch with food present, feeding, cat exposure, floating, and swimming. Results indicate that 5-HT levels increased 20-65% above baseline levels in response to each manipulation in all forebrain areas examined. The only significant difference found across manipulations was that tail pinch with food present produced a greater increase in 5-HT than cat exposure, which may be attributable to differences in the degree of general activation induced by these manipulations rather than to specific physiological or behavioral aspects. Furthermore, there was a strong positive correlation between 5-HT release and time spent in alert waking. Finally, there was an overall significantly smaller increase in 5-HT release in the corpus striatum compared to the other three sites. These data suggest that the release of 5-HT in the forebrain is closely related to behavioral state, not to specific behaviors, and that the magnitude of 5-HT release may reflect the degree of activation. In addition, this study suggests there can be some degree of differential pattern of release of 5-HT in the forebrain.

The 5-HT1A receptor is not involved in emotional stress-induced rises in stress hormones

To determine whether emotional stress-induced rises in stress hormone levels are mediated by activation of 5-HT1A receptors, we studied the effects of the selective 5-HT1A receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY-100635) on plasma ACTH, corticosterone, prolactin, and glucose levels in the conditioned ultrasonic vocalisation (USV) model in adult rats. The effects of WAY-100635 on USVs were also investigated in this paradigm. WAY-100635 (0.3, 1, and 3 mg/kg SC) had no clear effects on basal plasma ACTH, corticosterone, and glucose levels, but the 3 mg/kg dose significantly increased the plasma prolactin levels. The increases in plasma ACTH, corticosterone, and prolactin levels induced by the USV procedure were not affected by WAY-100635. This indicates that the 5-HT1A receptor does not play a major role in the distress-induced activation of the hypothalamic-pituitary-adrenal axis and prolactin secretion. The USVs were significantly enhanced by low doses of WAY-100635 (0.03 and 0.3 mg/kg SC), whereas higher doses (1.0 and 3.0 mg/kg SC) had no effect. These findings suggest that blockade of 5-HT1A receptors during stress may enhance the behavioural stress-response.

Diet and Parkinson's disease. II: A possible role for the past intake of specific nutrients. Results from a self-administered food-frequency questionnaire in a case-control study

In a case-control study, we compared the past dietary habits of 342 Parkinson's disease (PD) patients recruited from nine German clinics with those of 342 controls from the same neighborhood or region. Data were gathered with a structured interview and a self-administered food-frequency questionnaire. Nutrient intakes were calculated from the reported food intakes through linkage with the German Federal Food Code and analyzed using multivariate conditional logistic regression to control for total energy intake, educational status, and cigarette smoking. At the macronutrient level, patients reported higher carbohydrate intake than controls after adjustment for total energy intake, smoking, and educational status (OR = 2.74, 95% confidence interval [CI]: 1.30-6.07, for the highest versus lowest quartile, p trend = 0.02). This was reflected in higher monosaccharide and disaccharide intakes at the nutrient level. There was no difference between patients and controls in protein and fat intake after adjustment for energy intake. We found an inverse association between the intakes of beta-carotene (OR = 0.67, 95% CI: 0.37-1.19, p trend = 0.06) and ascorbic acid (OR = 0.60, 95% CI: 0.33-1.09, p trend = 0.04) by patients, although only the trend for ascorbic acid intake reached statistical significance. There was no difference between groups for alpha-tocopherol intake after adjustment for energy intake. We also found that patients reported a significantly lower intake of niacin than controls (OR = 0.15, 95% CI: 0.07-0.33, p trend < 0.00005). Our results suggest that if antioxidants play a protective role in this disease, the amounts provided by diet alone are insufficient. Although the interpretation of the inverse association between niacin intake and PD is complicated by the high niacin content in coffee and alcoholic beverages, which were also inversely associated with PD in this study, the strength of this association and its biologic plausibility warrant further investigation.

Adrenergic and serotoninergic receptors mediate the immunological activation of corticosterone secretion in male rats

In order to elucidate the mechanism of action of immune agents on corticosterone secretion, the present study evaluated the possible involvement of some neuronal pathways (serotoninergic, noradrenergic/adrenergic) in the lipopolysaccharide (LPS)-induced corticosterone release in male rats. Serotoninergic antagonists, mianserin (5-HT2C receptor blocker) or pindolol (5HT1A receptor blocker) or noradrenergic/adrenergic antagonists, prazosin (alpha 1-adrenoceptor blocker) or propranolol (beta-adrenoceptor blocker), were intraperitoneally (i.p.) injected before (5 min) the administration of LPS. In each experiment a group of rats i.p. injected with vehicle served as controls. Animals were sacrificed by decapitation 90 min after administration of LPS and trunk blood was collected for corticosterone radioimmunoassay. Results showed that pretreatment with mianserin, but not with pindolol, significantly reduced plasma corticosterone levels following administration of LPS (p < 0.05); prazosin attenuated the plasma corticosterone response to LPS (p < 0.05), while propranolol did not induce significant change. The present study indicated that serotoninergic and noradrenergic/adrenergic pathways are involved in the immunoneuroendocrine modulation of hypothalamus-pituitary-adrenal function in rats. In particular, it is probably mediated by the activation of 5-HT2C receptors and of alpha 1-adrenoceptors, while type 1A serotonin receptors or beta-adrenoceptors do not seem to be involved in such a phenomenon.

Enhanced synaptophysin immunoreactivity in rat hippocampal culture by 5-HT 1A agonist, S100b, and corticosteroid receptor agonists

Serotonin (5-HT) has been shown to modulate brain maturation during development and adult plasticity. This effect in the whole animal may be due to activation of 5-HT1A receptors and a corresponding increases in S100b and corticosterone. Synaptophysin, an integral protein of the synaptic vesicle membrane that correlates with synaptic density and neurotransmitter release, is reduced by depletion of 5-HT in the cortex and hippocampus of the adult rat. Injections of a 5-HT1A agonist or dexamethasone can reverse the loss of synaptophysin immunoreactivity (IR). In this study we used morphometric analysis of synaptophysin-IR to study the effects of the 5-HT1A agonist, ipsapirone, and the neuronal extension factor, S100b on hippocampal neurons grown in a serum and steroid free media. Both compounds increased the synaptophysin-IR at doses previously established to be highly specific. Ipsapirone (10(-9)M) was more effective on neuronal cell bodies staining and S100b (10 ng/ml) was more effective in increasing the number of synaptophysin-IR varicosities on neuronal processes. In addition both types of corticosteroid receptor agonists, at previously established specific doses, Ru28362 (10(-8) M) and aldosterone (10(-9) M) produced smaller increases compared to control groups in both the cell body staining and the number of varicosities. The effect of these differentiating factors on the expression of synaptophysin-IR suggests multiple regulation sites for producing and maintaining pre-synaptic elements in the brain.

Effect of acute and chronic treatment with triiodothyronine on serotonin levels and serotonergic receptor subtypes in the rat brain

Hyperthyroidism is often associated with behavioral disorders, and thyroid hormones modify receptor sensitivity as well as the synthesis and/or turnover rate of many neurotransmitters. We evaluated the influence in adult rats of triiodothyronine (T3), administered s.c. (100 micrograms/kg) acutely (once only) or chronically (once a day for 3 or 7 consecutive days), on brain serotonin concentration and on the density and affinity of two brain serotonin (5-HT) receptor subtypes mainly involved in behavioral effects. After both acute and chronic T3 treatment, serotonin levels increased in the cerebral cortex but not in the hippocampus. The density and affinity of 5-HT1A receptors (using [3H]-8-OH-DPAT as ligand) were not affected, while there was a significant decrease in the number of 5-HT2 receptors in the cerebral cortex (using [3H]ketanserin as ligand). This observation might indicate that thyroid hormones enhance 5-HT concentration in certain brain areas, thus causing a down-regulation of 5-HT2 receptors. The serotonergic system could be involved in the complex brain-neurotransmitter imbalance underlying hyperthyroidism-linked behavioral changes.

Increased 5-HT2C receptor responsiveness occurs on rearing rats in social isolation

To investigate whether isolation rearing alters 5-hydroxytryptamine2C (5-HT2C) receptors, the effect of the serotonin agonist m-chlorophenylpiperazine (mCPP) was examined on elevated plus-maze behaviour, plasma corticosterone and brain 5-HT2C receptor protein levels in rats. There was no distinction between behaviour or corticosterone levels in drug-free isolates or socially housed rats exposed to the elevated plus-maze. The anxiogenic response to mCPP (decrease in open arm entry and time and an increase in stretch attend postures) on the elevated plus-maze was greater in isolation than in socially reared controls without any concomitant difference in the hypolocomotor effect of mCPP in the two groups. mCPP produced a greater elevation in plasma corticosterone in isolates than in group-housed controls. Hippocampal 5-HT2C receptor protein-like immunoreactive levels were significantly lower following mCPP than saline only in rats reared in isolation. These results indicate that increased 5-HT2C receptor responsiveness accompanies isolation-rearing and may contribute to the enhanced response to stress and the increased neophobia seen in this animal model of trait anxiety/depression. In isolation reared rats, rapid down-regulation of supersensitive 5-HT2C receptors may occur in the hippocampus following 5-HT agonist challenge.

Effects of stress on the functional properties of pre- and postsynaptic 5-HT1B receptors in the rat brain

Numerous studies have clearly shown that the turnover and release of serotonin (5-hydroxytryptamine, 5-HT) are increased under acute stressful conditions. Inasmuch as this latter process is under the control of a feedback mechanism involving the stimulation of presynaptic 5-HT1B autoreceptors, we have investigated the possible effects of acute restraint (40 min) on the functional properties of 5-HT1B receptors. The efficacy of the selective 5-HT1B receptor agonist 3-[1,2,5,6-tetrahydropyrid-4-yl]pyrrolo-[3,2-b]pyrid-5-one (CP-93,129) in inhibiting in vitro the K+-evoked release of [3H]5-HT, was significantly reduced in stressed rats as compared to naive animals. Similarly, the responsiveness of 5-HT1B receptors inhibiting the release of [3H]acetylcholine (presynaptic 5-HT1B heteroreceptors), was reduced by restraint. These effects were observed in the hippocampus, but using the inhibitory effect of CP-93,129 on forskolin-stimulated adenylyl cyclase activity as an index of 5-HT1B receptor function, it could be shown that the 5-HT1B receptors located in the substantia nigra are also desensitized by stress. The number as well as the apparent affinity constant of 5-HT1B binding sites labelled by [125I]iodocyanopindolol, as measured by quantitative autoradiography and membrane binding, were similar in naive and restraint-stressed rats suggesting that the stress-induced desensitization of 5-HT1B receptors is not due to a reduced number of 5-HT1B binding sites. As stress is thought to be a causal factor for the etiology of anxiety and depression, these results support the potential involvement of 5-HT1B receptor dysfunction in the development of these neurological disorders.