Modulation of 5-HT receptor subtype-mediated behaviours by corticosterone

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI): From an Obscure to Pivotal Member of the DOX Family of Serotonergic Psychedelic Agents - A Review

1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI, or DOX where X = -I) was first synthesized in 1973 in a structure-activity study to explore the effect of various aryl substituents on the then newly identified, and subsequently controlled, hallucinogenic agent 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM, or DOX where X = -CH3). Over time, DOI was found to be a serotonin (5-HT) receptor agonist using various peripheral 5-HT receptor tissue assays and later, following the identification of multiple families of central 5-HT receptors, an agonist at 5-HT2 serotonin receptors in rat and, then, human brain. Today, classical hallucinogens, currently referred to as serotonergic psychedelic agents, are receiving considerable attention for their potential therapeutic application in various neuropsychiatric disorders including treatment-resistant depression. Here, we review, for the first time, the historical and current developments that led to DOI becoming a unique, perhaps a landmark, agent in 5-HT2 receptor research.

Pimavanserin reverses multiple measures of anxiety in a rodent model of post-traumatic stress disorder

Pimavanserin is a highly selective 5-HT_2A inverse agonist in current medical use. Prior studies suggest that 5-HT_2A serotonin receptors may play a role in anxiety and emotional memory. Therefore, pimavanserin was tested in a rat model of PTSD to determine whether it might ameliorate PTSD-like symptoms. The lifetime prevalence of PTSD is estimated to be 125% higher in women than men. Consequently, in an effort to create a robust model of PTSD that was more representative of human PTSD prevalence, 20-week old female rats of the emotionally hyperreactive Lewis strain were used for these studies. The rats were single-housed and exposed twice to restraint stress coupled with predator odor or to a sham-stressed condition. Twenty days after the second stress or sham-stress exposure, rats were injected with saline alone or with 0.3 or 1.0 mg/kg pimavanserin, doses that were confirmed to substantially block 5-HT_2A receptor activity in this study without causing any non-specific behavioral or adverse effects. One hour later, rats were tested for anxiety through acoustic startle response, the elevated plus-maze and three parameters of open field behavior. Five days later, blood was sampled for plasma corticosterone. The stressed/saline-injected rats had higher anxiety scores and corticosterone levels than sham-stressed/saline-injected rats. Pimavanserin significantly and generally dose-dependently reversed these persistent stress effects, but had no significant effect on the behavioral measures in normal, non-stressed rats. These results, consistent with a role for the 5-HT_2A receptor, suggest that pimavanserin might have potential to reduce some consequences of traumatic stress.

Glucocorticoid receptor dysregulation underlies 5-HT2AR-dependent synaptic and behavioral deficits in a mouse neurodevelopmental disorder model

Prenatal environmental insults increase the risk of neurodevelopmental psychiatric conditions in the offspring. Structural modifications of dendritic spines are central to brain development and plasticity. Using maternal immune activation (MIA) as a rodent model of prenatal environmental insult, previous results have reported dendritic structural deficits in the frontal cortex. However, very little is known about the molecular mechanism underlying MIA-induced synaptic structural alterations in the offspring. Using prenatal (E12.5) injection with poly-(I:C) as a mouse MIA model, we show here that upregulation of the serotonin 5-HT_2A receptor (5-HT_2AR) is at least in part responsible for some of the effects of prenatal insults on frontal cortex dendritic spine structure and sensorimotor gating processes. Mechanistically, we report that this upregulation of frontal cortex 5-HT_2AR expression is associated with MIA-induced reduction of nuclear translocation of the glucocorticoid receptor (GR) and, consequently, a decrease in the enrichment of GR at the 5-HT_2AR promoter. The translational significance of these preclinical findings is supported by data in postmortem human brain samples suggesting dysregulation of GR translocation in frontal cortex of schizophrenia subjects. We also found that repeated corticosterone administration augmented frontal cortex 5-HT_2AR expression and reduced GR binding to the 5-HT_2AR promoter. However, virally (AAV)-mediated augmentation of GR function reduced frontal cortex 5-HT_2AR expression and improved sensorimotor gating processes via 5-HT_2AR. Together, these data support a negative regulatory relationship between GR signaling and 5-HT_2AR expression in the mouse frontal cortex that may carry implications for the pathophysiology underlying 5-HT_2AR dysregulation in neurodevelopmental psychiatric disorders.

Contingency Training Alters Neurobiological Components of Emotional Resilience in Male and Female Rats

Prior research with a rat model of behavioral therapy [i.e., effort-based reward (EBR) contingency training] suggests that strengthened associations between physical effort and desired outcomes enhance neurobiological indices of resilience. In the current study, male and female Long-Evans rats were exposed to either six weeks of EBR training or noncontingent training prior to 10 days of exposure to chronic unpredictable stress (CUS). Subsequently, all animals were exposed to a problem-solving task and then trained in a spatial learning/foraging task, the Dry Land Maze (DLM). Following habituation training and test trials, rats were assessed in a probe trial that generated a prediction error (cognitive uncertainty). Results indicated that, during CUS exposure, contingency-training enhanced dehydroepiandrosterone/corticosterone ratios (consistent with healthier stress responses), especially in male rats. Additionally, contingency training increased exploratory behaviors in the probe trial as well as differentially influenced on-task problem-solving performance in males and females. Following the probe trial, brains were exposed to histological analyses to determine the effects of sex and contingency training on various neurobiological markers. Contingency training decreased BDNF-immunoreactivity (ir) in the hippocampus CA1 and lateral habenula, implicating differential neuroplasticity responses in the training groups. Further, coordinated fos-ir activation in areas associated with emotional resilience (i.e., motivation-regulation) was observed in contingent-trained animals. In sum, the current findings confirm that behavioral training is associated with neurobiological markers of emotional resilience; however, further assessments are necessary to more accurately determine the therapeutic potential for the EBR contingency training model.

Mutual independence of 5-HT(2) and α1 noradrenergic receptors in mediating deficits in sensorimotor gating

RATIONALE

Prepulse inhibition (PPI), a preattentional information-filtering mechanism, is disrupted by serotonin (5-HT) or norepinephrine (NE) agonists to model deficits seen in schizophrenia, but whether this effect occurs through interactions between these systems is not known.

OBJECTIVES

These studies investigated whether PPI/activity changes induced by agonists of one system were dependent on neurotransmission within the other.

METHODS

Male Sprague-Dawley rats received the 5-HT(2) receptor agonist DOI (1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane) (0, 0.3 mg/kg), with or without antagonists for α1 (prazosin:0, 0.3, or 1 mg/kg) or β (timolol:0, 3, or 10 mg/kg) receptors or their combination (0 or 0.3 mg/kg prazosin + 3 mg/kg timolol), or the 5-HT(2) antagonist ritanserin (0, 2 mg/kg). Separately, the α1-adrenergic receptor agonist cirazoline (0, 0.68 mg/kg) was given with and without ritanserin (0, 0.5, or 2 mg/kg) or the NE antagonists (0 or 0.3 mg/kg prazosin + 3 mg/kg timolol). Finally, combinations of subthreshold doses of DOI (0, 0.01, 0.025 mg/kg) and cirazoline (0, 0.1, 0.25 mg/kg) were tested for their ability to disrupt PPI, and concomitant administration of all three antagonists (0 vs. 0.3 mg/kg prazosin + 3 mg/kg timolol + 2 mg/kg ritanserin) was assessed for its ability to modify PPI. Locomotion was assessed in an additional set of experiments.

RESULTS

Doses/combinations of prazosin and timolol that reversed cirazoline-induced effects did not alter DOI-induced effects, and ritanserin did not affect cirazoline at doses that blocked DOI-mediated effects. Concomitant antagonism of α1 + β + 5-HT(2) receptors did not modify PPI, nor did combinations of subthreshold doses of cirazoline and DOI.

CONCLUSIONS

5-HT(2) receptors and α1 and β NE receptors may act through independent mechanisms to modulate sensorimotor gating and locomotor activity.

TRH and TRH-like peptide expression in rat following episodic or continuous corticosterone

Sustained abnormalities of glucocorticoid levels have been associated with neuropsychiatric illnesses such as major depression, posttraumatic stress disorder (PTSD), panic disorder, and obsessive compulsive disorder. The pathophysiological effects of glucocorticoids may depend not only on the amount of glucocorticoid exposure but also on its temporal pattern, since it is well established that hormone receptors are down-regulated by continuously elevated cognate hormones. We have previously reported that TRH (pGlu-His-Pro-NH2) and TRH-like peptides (pGlu-X-Pro-NH2) have endogenous antidepressant-like properties and mediate or modulate the acute effects of a single i.p. injection of high dose corticosterone (CORT) in rats. For these reasons, two accepted methods for inducing chronic hyperglucocorticoidemia have been compared for their effects on brain and peripheral tissue levels of TRH and TRH-like peptides in male, 250 g, Sprague-Dawley rats: (1) the dosing effect of CORT hemisuccinate in drinking water, and (2) s.c. slow-release pellets. Overall, there were 93% more significant changes in TRH and TRH-like peptide levels in brain and 111% more in peripheral tissues of those rats ingesting various doses of CORT in drinking water compared to those with 1-3 s.c. pellets. We conclude that providing rats with CORT in drinking water is a convenient model for the pathophysiological effects of hyperglucocorticoidemia in rodents.

Neuroendocrinological mechanisms of actions of antidepressant drugs

Noradrenaline or serotonin (5-HT) reuptake-inhibiting antidepressants such as reboxetine or citalopram acutely stimulate cortisol and adrenocorticotrophic hormone (ACTH) secretion in healthy volunteers, whereas mirtazapine acutely inhibits the ACTH and cortisol release, probably due to its antagonism at central 5-HT(2) and/or H(1) receptors. These differential effects of antidepressants on cortisol and ACTH secretion in healthy subjects after single administration are also reflected by their different time course in the down-regulation of hypothalamic-pituitary-adrenocortical (HPA) axis hyperactivity in depressed patients as assessed by serial dexamethasone (DEX)/corticotrophin-releasing hormone (CRH) tests: Reuptake-inhibiting antidepressants such as reboxetine gradually normalise HPA axis hyperactivity in depressed patients during several weeks of treatment via up-regulation of mineralocorticoid and glucocorticoid receptor function and by step-by-step restoration of the disturbed feedback control. By contrast, mirtazapine markedly reduces HPA axis activity in depressed patients within 1 week, but there is a partial re-enhancement of HPA hormone secretion after several weeks of therapy. In all studies performed to date, the short-term effects of daily treatment with antidepressants on the DEX/CRH test results are comparable in responders and nonresponders. Moreover, a reduction in HPA axis activity is not necessarily followed by a favourable clinical response and some depressed patients keep on showing nonsuppression in the DEX/CRH test despite clinical improvement. Therefore, the importance of HPA axis dysregulation for the short-term efficacy of antidepressants continues to be a matter of debate. However, there are convincing data suggesting that persisting nonsuppression in the DEX/CRH test despite clinical remission predicts an enhanced risk for relapse of depressive symptomatology with respect to the medium- and long-term outcome.

Rapid modulation of TRH and TRH-like peptide levels in rat brain and peripheral tissues by corticosterone

Disturbance of glucocorticoid signaling has been implicated in several neuropsychiatric disorders including unipolar and bipolar depression and anxiety induced by maternal deprivation. Antidepressants have been shown to be neuroprotective and able to reverse damage to glia and neurons. Thyrotropin-releasing hormone (TRH) is an endogenous antidepressant that reduces the expression of glycogen synthase kinase-3beta (GSK-3beta), an enzyme that hyperphosphorylates tau and is implicated in bipolar depression, diabetes and Alzheimer's disease. In order to understand the potential role of TRH and TRH-like peptides both as mediators of the depressogenic effects of glucocorticoids and as potential therapeutics for neuropsychiatric disease, 300 g male Sprague-Dawley rats were injected i.p. with 4 mg corticosterone/0.5 ml 50% DMSO+50% ethanol and sacrificed 0, 2, 4 and 8h later. Levels of TRH and TRH-like peptides were measured in various brain regions involved in mood regulation and pancreas and reproductive tissues that mediate the metabolic and reproductive impairments associated with high glucocorticoid levels. Significant increases, ranging from 2- to 12-fold, in TRH or TRH-like peptide levels were observed in almost all brain regions studied at 4h after corticosterone injection. In cerebellum, TRH and TRH-like peptides increased 4-14-fold by 8h. TRH-like peptide levels fell 86-98% at 4h after treatment in testis. TRH, derived only from Leydig cells, was not affected. TRH and TRH-like peptides increased 2-4-fold at 8h in pancreas. TRH and TRH-like peptide concentrations in prostate were not affected by corticosterone up to 8h after injection. The 4h needed to detect a highly significant change in the TRH and TRH-like peptide levels in brain and peripheral tissues is consistent with the mediation of most corticosterone-effects via alterations in gene transcription.

A serotonin receptor antagonist, but not melatonin, modulates hormonal responses to capture stress in two populations of garter snakes (Thamnophis sirtalis parietalis and Thamnophis sirtalis concinnus)

Hormonal and behavioral responses to a stressor depend on many factors, including the influence of other hormones. We examined the role of melatonin in modulating hormonal responses to capture stress in two populations of male garter snakes, Thamnophis sirtalis. Studies of red-sided (T. sirtalis parietalis) and red-spotted (T. sirtalis concinnus) garter snakes were conducted in the field with free-living snakes. Populations of red-sided garter snakes in south-central Manitoba, Canada undergo a period of winter dormancy for approximately 8 months each year followed by an attenuated mating season (4-5 weeks) in early spring. In contrast, the mid-latitude red-spotted garter snake in western Oregon, USA has an extended breeding season and can be active during 10-12 months of the year given appropriate environmental conditions. We chose to study these two populations of garter snakes to investigate possible variation in melatonin function among snakes with different suites of environmental adaptations. To better address these questions, we also examined the effects of 5-hydroxytryptophan (a precursor of melatonin synthesis) and ketanserin (a serotonergic type 2A receptor antagonist) on hormonal responses to capture stress. We observed a trend of increased corticosterone and decreased androgen concentrations in northern-latitude red-sided garter snakes (T. sirtalis parietalis) subjected to 4 h of capture stress during the spring. However, these differences were not statistically significant. During the fall, red-sided garter snakes showed no change in corticosterone or androgen concentrations in response to the capture stress treatments. We speculate that northern-latitude red-sided garter snakes suppress hormonal responses to capture stress during preparation for winter dormancy. Treatment with melatonin, 5-hydroxytryptophan, or ketanserin did not significantly influence corticosterone or androgen concentrations of northern-latitude red-sided garter snakes during the spring or fall. Mid-latitude red-spotted garter snakes (T. sirtalis concinnus) from Oregon showed a statistically significant increase in corticosterone concentrations in response to 4 h of capture stress; treatment with melatonin, 5-hydroxytryptophan, or ketanserin prior to capture stress had no significant influence on plasma corticosterone concentrations. Androgen concentrations of mid-latitude red-spotted garter snakes in response to capture stress were significantly lower than those of non-stressed control snakes. Neither melatonin nor 5-hydroxytryptophan influenced the change in androgen concentrations during capture stress. However, androgen concentrations of snakes treated with ketanserin prior to 4 h of capture stress did not differ significantly from those of non-stressed control snakes. These studies suggest that melatonin does not modulate hormonal responses to capture stress in this ectothermic model. Our results also suggest that a serotonin-regulated system may play a role in modulating the activity of the hypothalamic-pituitary-gonadal axis during physiological stress responses.

Effects of melatonin on the behavioral and hormonal responses of red-sided garter snakes (Thamnophis sirtalis parietalis) to exogenous corticosterone

We investigated possible interactions between melatonin and corticosterone in modulating the reproductive behavior of male red-sided garter snakes (Thamnophis sirtalis parietalis) following spring emergence. We also examined whether melatonin's modulatory actions could be explained by its potential properties as a serotonin receptor antagonist. Exogenous corticosterone significantly reduced courtship behavior of male snakes in a dose-dependent manner. Melatonin also significantly reduced courtship behavior of male garter snakes. Pretreatment with melatonin before administering corticosterone treatments further suppressed courtship behavior of red-sided garter snakes. These results indicate additive inhibitory effects of melatonin and corticosterone in modulating reproductive behavior. Snakes receiving ketanserin, a serotonergic type 2A receptor antagonist, followed by corticosterone also showed reduced courtship behavior; this serotonin receptor antagonist followed by treatment with vehicle did not significantly influence courtship behavior of male snakes. Neither melatonin nor corticosterone treatments significantly influenced testosterone + 5-alpha-dihydrotestosterone concentrations of male garter snakes, supporting a direct effect of melatonin and corticosterone on courtship behavior that is independent of any effect on androgen concentrations. We propose that a serotonin system is involved in the modulation of male courtship behavior by melatonin and corticosterone. In addition, our data support the hypothesis that melatonin may function as a serotonin receptor antagonist. Further research is necessary to discern whether the actions of melatonin and corticosterone are converging on the same pathway or if their effects on different pathways are having additive inhibitory effects on courtship behavior.

Effect of lesioning the suprachiasmatic nuclei on behavioral despair in rats

The suprachiasmatic nucleus (SCN) is involved in regulating many biological rhythms. Several lines of research implicate the SCN in affective behavior. The SCN is directly involved in regulating the daily rhythms of the hypothalamo-pituitary-adrenal (HPA) axis hormones involved in stress. Bilateral lesions of the SCN disrupt both the rhythms and the basal levels of the HPA axis hormones involved in coping with stress. Moreover, stress can affect the biological rhythms regulated by the SCN, and disruption of biological rhythms in turn can cause stress. The present study assessed the effect of bilateral destruction of the SCN on behavioral despair, an animal model of depression sensitive to antidepressant treatment. The results indicate that bilateral destruction of the SCN results in reduced immobility in the second forced swimming test (FST) compared to sham controls and animals with incomplete lesions. These results indicate that bilateral destruction of the SCN has a protective effect in the induction of behavioral despair which may arise out of disruption of the secretion of the HPA axis hormones and/or of the neural connections between the SCN and the limbic structures that modulate the response to swim stress.

Different roles of 5-HT2A and 5-HT2C receptors in regulation of female rat paced mating behaviour

Serotonin (5-HT) receptor 2A (5-HT2A) and 2C (5-HT2C) agonists have been reported to facilitate female rat lordosis behaviour. This study investigated the acute effects of the 5-HT2A receptor agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and the 5-HT2C receptor agonist MK-212 (2-chloro-6-(1-piperazinyl) pyrazine) on paced mating behaviour in a population of sexually receptive female rats in order to explore the role of 5-HT2A and 5-HT2C receptors in the mediation of female rat sexual motivation. DOI (0.5 and 1.0 mg/kg) increased female rat sexual motivation during the first of two consecutive copulatory series seen as a tendency towards a decrease in return latencies following ejaculation and decreased inter-intromission intervals. MK-212 generally increased approach latencies. 0.5 mg/kg MK-212 increased post-ejaculatory exit latency, while 1.0 and 2.0 mg/kg MK-212 increased both post-ejaculatory exit latencies and post-ejaculatory return latencies. The possibility that an increased level of anxiety confounded the effects of MK-212 on sexual motivation behaviour is discussed. The results may support the hypothesis that both 5-HT2A and 5-HT2C receptors are important in regulation of female sexual behaviour.

Long-lasting effects of chronic stress on DOI-induced hyperthermia in male rats

RATIONALE

Exposure to chronic stress can affect the serotoninergic (5-HT) system and behavioral measures associated with 5-HT. Repeated stress increases 5-HT receptor subtype 2 (5-HT2) mediated behaviors in rodents, such as wet dog shakes and head twitch.

OBJECTIVES

The current study investigated whether exposure to chronic unpredictable stress would augment 5-HT(2A/C) receptor-mediated hyperthermia. Furthermore, the persistence of these hyperthermic effects was investigated by testing rats up to 60 days after the stress procedure terminated.

METHODS

For 2 or 10 days, rats were either not stressed (controls) or exposed to chronic unpredictable stress, i.e. two stressors per day of the following: cage rotation, cold exposure, swim, restraint, light cycle manipulations, single housing, and food and water deprivation. After the termination of stress (day 3 or 11), the 5-HT(2A/C) receptor agonist DOI (1.5 mg/kg) or saline, was injected and the rectal temperature of the rats was monitored. In a separate experiment, the 5-HT2 receptor antagonist, LY-53,587, was injected 30 min prior to the injection of DOI or saline. Finally, DOI was injected into rats 8, 30 or 60 days after the 10-day stress procedure ended.

RESULTS

Rats exposed to 10 days, but not 2 days, of unpredictable stress exhibited higher rectal temperatures following DOI than non-stressed rats. The DOI-induced hyperthermia was attenuated by LY-53,587. The augmentation of DOI-induced hyperthermia in stressed rats persisted when examined 8, 30 and 60 days following the stress procedure.

CONCLUSIONS

The enhancement of 5-HT receptor function by chronic stress persists even after the environmental stressor is removed. This lasting increase in 5-HT receptor function may have implications for clinical disorders associated with stress, such as depression or post-traumatic stress disorder.

Conditioned taste aversion: modulation by 5-HT receptor activity and corticosterone

Two experiments were designed to elucidate the involvement of the hypothalamic-pituitary-adrenal axis and the 5-hydroxytryptamine (5-HT) system in the acquisition of lithium chloride-conditioned taste aversion. In Experiment 1, rats were administered either vehicle or 50 mg/kg nefazodone daily for 4 weeks. Rats were treated with 22 mg/kg of lithium chloride in order to produce conditioned taste aversion to a sucrose solution. Three days later, nefazodone completely blocked the lithium chloride-conditioned taste aversion. In Experiment 2, the effects of chronic corticosterone administration on lithium chloride-conditioned taste aversion were investigated. Twenty male rats received either corticosterone at a dose of (50 mg/kg) or vehicle injections over a period of 14 consecutive days. Lithium chloride-conditioned taste aversion was potentiated in rats treated with corticosterone. Additionally, corticosterone-treated animals required more trials to reach extinction. These results suggest the involvement of both the 5-HT system and the hypothalamic-pituitary-adrenal axis in lithium chloride-conditioned taste aversion.

Endocrinological effects of mirtazapine in healthy volunteers

OBJECTIVE

Unlike other antidepressants, mirtazapine does not inhibit the reuptake of norepinephrine or serotonin (5-HT) but acts as an antagonist at presynaptic alpha2-receptors and at postsynaptic 5-HT2, 5-HT3 and histamine H1-receptors. In the present investigation, the influence of acute oral administration of 15-mg mirtazapine on the cortisol (COR), adrenocorticotropin (ACTH), growth hormone (GH) and prolactin (PRL) secretion was examined in 12 healthy male subjects, compared to placebo.

METHODS

After insertion of an intravenous catheter, both the mean arterial blood pressure (MAP) and the heart rate were recorded and blood samples were drawn 1 h prior to the administration of mirtazapine or placebo (7:00 a.m.), at time of administration (8:00 a.m.) and during 5 h thereafter in periods of 30 min. Concentrations of COR, ACTH, GH and PRL were measured in each blood sample by double antibody radioimmunoassay and chemiluminescence immunoassay methods. The area under the curve (AUC; 0-300 min after mirtazapine or placebo administration) was used as parameter for the COR, ACTH, GH and PRL response. Furthermore, the urinary free cortisol excretion (UFC) was determined beginning at 8:00 a.m. (time of administration of placebo or mirtazapine) up to 8:00 a.m. the day after.

RESULTS

Two-sided t-tests for paired samples revealed significantly lower COR AUC, ACTH AUC, UFC and PRL AUC values after 15-mg mirtazapine compared to placebo, whereas no significant differences were found with respect to GH AUC, MAP and heart rate.

CONCLUSIONS

Since the acute inhibition of COR secretion in the healthy volunteers was paralleled by a simultaneous decrease of ACTH release, central mechanisms (e.g., inhibition of hypothalamic corticotropin releasing hormone (CRH) output) are suggested to be responsible for the inhibitory effects of mirtazapine on COR secretion. Our results are of particular interest in the light of the hypercortisolism observed in depressed patients and new pharmacological approaches such as CRH1 receptor antagonists.

Alteration in 5-hydroxytryptamine agonist-induced behaviour following a corticosterone implant in adult rats

Hypercortisolism and altered serotonergic function may account for the pathological symptoms seen in depression. This study examines the impact of 4 days continuous corticosterone treatment on 5-HT agonist-induced behaviour to delineate changes in 5-HT receptor function in the adult rat. The flat body posture, reciprocal forepaw treading, elevated corticosterone, hyperglycaemia, hypothermia and reduced hippocampal 5-HT induced by the 5-HT(1A) agonist 8-OHDPAT (0.3 mg/kg ip) were all significantly attenuated by the corticosterone implant. The elevation in plasma corticosterone and back muscle contractions evoked by the 5-HT(2A) agonist DOI (1 mg/kg ip) were attenuated, whilst wet-dog shakes were enhanced by corticosterone treatment. 5-HT(2B) agonist-induced behaviour and the hypolocomotion and hypophagia induced by the 5-HT(2C) agonist m-CPP (2.5 mg/kg ip) were unaltered but the mCPP-induced elevation in corticosterone was abolished by corticosterone treatment. Hypothalamic 5-HT receptors mediating corticosterone- and 5-HT(1A) receptors, whether on serotonergic nerve terminals or postsynaptic neurones, were downregulated by corticosterone treatment. In contrast, 5-HT(2A) receptors may be up- or downregulated dependent on whether they are on supraspinal or spinal neurones, respectively. A comparison of the brain region-dependent alteration in serotonergic function produced by hypercorticosterone in the rat with that seen in depression is discussed.

Melatonin protects against the effects of chronic stress on sexual behaviour in male rats

The effects of chronic mild stress (CMS) on both sexual behaviour and wet dog shakes (WDS), a serotonergic type 2A (5-HT2A) receptor-mediated behaviour, were explored in the male rat. In addition, the possible attenuation of these effects by chronic treatment with melatonin, a putative 5-HT2A antagonist, was examined. The CMS procedure resulted in a significant increase in WDS and an overall decrease in all aspects of sexual behaviour. Concurrent melatonin administration attenuated the CMS-induced effects on sexual behaviour, but not the effects on either spontaneous WDS or WDS in response to the 5-HT2A agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, suggesting a mechanism of action other than exclusive 5-HT2A antagonism. These results are the first to demonstrate that melatonin significantly protects against the detrimental effects of a chronic stressor on sexual behaviour.

Ketanserin attenuates the behavioural effects of corticosterone: implications for 5-HT(2A) receptor regulation

The effects of chronic corticosterone treatment on sexual behaviour and wet-dog shakes were investigated in both female and male rats. The serotonergic type 2A (5-HT(2A)) receptor antagonist ketanserin was administered to test the hypothesis that the behavioural effects of corticosterone were mediated by increased 5-HT(2A) receptor activity. Rats were randomly assigned to one of four chronic treatment groups: control, ketanserin alone, corticosterone alone, or ketanserin and corticosterone. Ketanserin attenuated the corticosterone-induced changes in both sexual behaviour and wet-dog shakes. Ketanserin alone had no effect on these behaviours. Results suggest that increased 5-HT(2A) receptor activity mediates the effects of corticosterone on sexual behaviour and wet-dog shakes.

Paradoxical effects of chronic corticosterone on forced swim behaviours in aged male and female rats

The effects of chronically administered corticosterone on forced swim test and open field test behaviours were explored in aged male and female rats. Though corticosterone has typically been associated with depressive behaviours, recent data have suggested a putative antidepressive effect of corticosterone. The current study used the forced swim test as a model of antidepressant efficacy in order to explore this. Aged male and female rats received either corticosterone (20 mg/kg) or the vehicle for 10 days before testing in the forced swim test, then for an additional 3 days before testing in the open field test. On day 11, each animal was individually tested on the duration of swimming, immobile, and struggling behaviours, and on day 14, for the display of rearing and line crossing behaviours. Results revealed that corticosterone significantly increased swimming and decreased immobility behaviour in females, but failed to do so in males. Additionally, there was a main effect of corticosterone on struggling behaviour such that it decreased it in males. There were no effects of corticosterone or sex on open field test behaviours, suggesting that the present findings are not accounted for by a general effect of corticosterone on motor behaviour. Overall, the data suggest that chronically administered corticosterone possesses effects that are sex-specific, and that it may exert mildly antidepressive effects in females, but the opposite effects in males. These data are consistent with emerging evidence that corticosterone may play a paradoxical antidepressive effect.

Enhanced corticosterone release after a modified forced swim test in Fawn hooded rats is independent of rearing experience

Recent findings have demonstrated that Fawn hooded (FH/Har) rats exhibit enhanced plasma corticosterone (CORT) responses compared to Wistar rats after exposure to an open field, whereas this effect was not influenced by early social experience. In contrast, it was found that behavior in a modified version of the Porsolt Forced Swim Test (Porsolt FST) was affected by both strain and social experience. An important part of this study included modifications of the Porsolt FST that allowed separation of multiple behavioral endpoints. The present experiment was conducted to determine if FH/Har rats also exhibit enhanced CORT responses after exposure to the modified forced swim test, and whether CORT levels might predict the behavioral response in this context. After the initial exposure in the modified forced swim test FH/Har rats had higher CORT levels than Wistar rats, but this difference was not affected by isolation rearing. However, CORT levels were not correlated with the main behavioral measures assayed in this test. Nonetheless, the data confirm that FH/Har rats have altered HPA axis responses to stressors.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.

Serotonin, stress and corticoids

There is evidence for stressor- and brain region-specific selectivity in serotonergic transmission responses to aversive stimuli. The aim of the present review is to provide an overview of the effects of different acute and repeated/chronic stressors on serotonin (5-HT) release and reuptake, extracellular 5-HT levels, and 5-HT pre- and postsynaptic receptors in areas tightly linked to the control of fear and anxiety, namely the dorsal and median raphe nuclei, the frontal cortex, the amygdala and the hippocampus. In addition, our knowledge of the impacts of corticoids on serotonergic systems in these brain areas is also briefly provided to examine whether the hypothalamo-pituitary-adrenal axis may play a role in stress-induced alterations in 5-HT neurotransmission. Taken together, the data presented reinforce the hypothesis that stress affects such a transmission, partly through the actions of corticoids. However, we are still left with unanswered, albeit crucial questions. First, the question of the specificity of the serotonergic responses to stress, with regard to the site of action and the nature of the stressor still remains open due to the heterogeneity of the results obtained so far. This could indicate that environmental factors, other than the stressor itself, may have enduring consequences on 5-HT sensitivity to stress. Second, the question regarding the role of stress-elicited changes in 5-HT transmission within coping processes finds in most cases no clearcut answer. In keeping with human symptomatology, the need to consider the environment (including the early one) and the genetic status when assessing the effects of stress on 5-HT neurotransmission is underlined. Such a consideration could help to answer the questions raised.

Effects of mirtazapine on growth hormone, prolactin, and cortisol secretion in healthy male subjects

In the present study the effects of acute PO-administration of 15 mg mirtazapine on the growth hormone (GH), prolactin (PRL), and cortisol (COR) secretion were examined in eight physically and mentally healthy male subjects, compared to placebo. Mirtazapine is a new antidepressant agent which does not inhibit the reuptake of norepinephrine or serotonin but is an antagonist of presynaptic and, presumably, postsynaptic alpha 2-receptors as well as an antagonist of postsynaptic 5-HT2 and 5-HT3-receptors. After insertion of an i.v. catheter, blood samples were drawn 1 h prior to the administration of mirtazapine or placebo, at time of application, and during the time of 4 h after application in periods of 30 min. Plasma concentrations of GH, PRL, and COR were determined in each blood sample by double antibody RIA methods. The area under the curve (AUC) value was used as parameter for the GH, PRL, and COR response. With respect to GH and PRL secretion, mirtazapine did not show any effects in comparison with placebo. However, in all subjects, the COR concentrations were remarkably lower after mirtazapine compared to placebo, the difference being obvious in the mean value graphs 60 min after the application up to the end of the measurement period. The t-test for paired samples revealed a highly significant difference (P < 0.01) in COR-AUC-values between the mirtazapine group (mean COR-AUC: 1558.07 micrograms/100 ml x 240 min) and the placebo group (mean COR-AUC: 2698.86 micrograms/100 ml x 240 min). Further studies have to elucidate the question whether the demonstrated inhibition of COR secretion after application of 15 mg mirtazapine is caused by central or peripheral effects of this substance.

The influence of corticosterone on serotonergic stereotypy and sexual behavior in the female rat

The effects of adrenalectomy and chronic corticosterone treatment on sexual behavior in the ovariectomized female rat were investigated. The serotonergic type 2A (5-HT2A) receptor-mediated behavior 'wet dog shakes' (WDS) was measured concurrently. In Experiment 1, adrenalectomy reduced the frequency of WDS following the administration of the 5-HT(2A/2C) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) but had no effect on spontaneous WDS. In Experiment 2, chronic corticosterone treatment increased DOI-induced WDS in both adrenalectomized and sham-adrenalectomized rats. In Experiment 3, adrenalectomized and sham-adrenalectomized rats were compared on measures of spontaneous WDS and sexual behavior following the administration of estrogen alone, or estrogen in combination with progesterone. Chronic corticosterone and acute progesterone administration increased WDS and facilitated sexual receptivity and proceptivity, while adrenalectomy decreased WDS, facilitated sexual receptivity and inhibited proceptivity. These findings suggest that the behavioral effects seen following hypothalamic-pituitary-adrenal (HPA) axis disruption may, in part, be mediated by altered 5-HT2A receptor responsivity.

Nefazodone attenuates the stress-induced facilitation of wet dog shaking behaviour but not the facilitation of sexual behaviour in female rats

The effects of chronic stress both alone and in combination with the antidepressant, nefazodone, which possesses antagonistic activity at the 5-HT2A receptor, were examined on the 5-HT2A receptor-mediated behaviour, wet dog shaking and sexual behaviour. Ovariectomized female rats received either a chronic stressor or no stress for 30 days, and half of each group received concurrent nefazodone treatment (100 mg/kg/day). Following treatment with either estrogen, or estrogen combined with progesterone, sexual behaviour and wet dog shaking were recorded. Chronic stress alone was found to facilitate sexual behaviour and increase wet dog shaking, while nefazodone administration alone was without effect. Furthermore, nefazodone completely attenuated the stress-induced facilitation of wet dog shaking, but not sexual behaviour.

Corticosterone regulation of 5-HT2A receptor-mediated behaviors: attenuation by melatonin

The effects of chronic corticosterone treatment on sexual behavior and on wet-dog shakes (WDS), a serotonergic type 2A (5-HT2A) receptor-mediated behavior, were explored in the male rat. In addition, the effects of acute melatonin treatment, both alone and in combination with corticosterone, were investigated. Chronic injections of corticosterone resulted in an overall decrease in consummatory measures of sexual behavior, and an increase in WDS. Furthermore, although an acute injection of melatonin alone had no effect on any recorded behavior, it attenuated the effects of corticosterone on sexual behavior and WDS. The data suggest that in the context of 5-HT2A receptor-mediated behaviors, melatonin has possible implications as a 5-HT2A antagonist.

Blockade of the anxiolytic-like action of ipsapirone and buspirone, but not that of 8-OH-DPAT, by adrenalectomy in male rats

The effect of the 5-HT1A agonists ipsapirone (5 mg/kg), buspirone (5 mg/kg) and 8-OH-DPAT (0.5 mg/kg) on experimental anxiety was examined in sham-operated, adrenalectomized and adrenally demedullated male rats. The animal model of anxiety used was the defensive burying test. At the doses selected, all 5-HT1A compounds produced an anxiolytic-like action by reducing the burying behavior in both sham-operated and demedullated rats. However, in adrenalectomized subjects, while 8-OH-DPAT still reduced burying behavior, ipsapirone and buspirone lost their action. Data suggest that adrenocortical secretions play a role in the anxiolytic-like actions of buspirone and ipsapirone, but not in those of 8-OH-DPAT. Buspirone and ipsapirone also produced a reduction in burying behavior latency in sham-operated animals that was not observed in adrenalectomized or adrenally demedullated rats. These data suggest that adrenaline may be participating in the action of these compounds on the burying behavior latency. Present findings support possible direct relationships between the stimulation of 5-HT1A receptors and adrenal secretions.

Chronic stress effects on sexual behavior in male and female rats: mediation by 5-HT2A receptors

The effects of chronic psychosocial stress on sexual behavior and on the serotonergic type 2A (5-HT2A) receptor-mediated behavior "wet dog shakes" (WDS) were investigated in male and female rats. In Experiment 1, both bilaterally adrenalectomized and sham-adrenalectomized female rats were assigned to either a psychosocial stress condition or a control condition for 62 days. On the 63rd day, estrogen-primed females were compared on measures of sexual behavior and WDS. Immediately after the behavioral tests, the same rats were primed with a subthreshold level of progesterone. Three hours after the administration of progesterone, rats were again scored for sexual behavior and WDS. Psychosocial stress was found to facilitate sexual behavior and increase WDS in sham-adrenalectomized female rats providing they were primed with both estrogen and progesterone. In Experiment 2, intact male rats were assigned to either the psychosocial stress condition or the control condition for 30 days. On the 31st day, males were compared on measures of sexual behavior and WDS. No significant differences were revealed on the spontaneous expression of sexual behavior and WDS. Subsequently, males were retested following the administration of the 5-HT2A agonist, DOI. Psychosocial stress resulted in a significant decrease in male sexual behavior and a concurrent increase in WDS, following the administration of DOI. Taken together, these results suggest that chronic psychosocial stress facilitates female sexual behavior and inhibits male sexual behavior, and that the effects of stress on sexual behavior may be mediated by 5-HT2A receptor activity.

Sexual behavior and wet dog shakes in the male rat: regulation by corticosterone

The potential involvement of adrenal steroids in the regulation of 'wet dog shakes' (WDS) and sexual behavior was investigated in male rats treated or not with the serotonergic type 2A (5-HT2A) agonist DOI (5-HT2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane). In Experiment 1, the frequency of both spontaneous and DOI-induced WDS were compared in adrenalectomized and sham-adrenalectomized rats. Adrenalectomy significantly reduced the frequency of DOI-induced WDS. In Experiment 2, adrenalectomized and sham-adrenalectomized rats received either corticosterone or oil chronically and were again scored for WDS behavior. Corticosterone effectively blocked the adrenalectomy-induced reduction of WDS in the DOI treatment condition. In Experiment 3, intact male rats were chronically administered either corticosterone or oil treatment. Animals were then compared on measures of both spontaneous and DOI-induced WDS and sexual behavior. Corticosterone significantly increased WDS and inhibited sexual behavior in both the spontaneous and DOI treatment conditions. These results suggest that the adrenal steroid corticosterone is important in the regulation of WDS and sexual behavior in the male rat and that this regulation may be mediated by activity at 5-HT2A receptors.

The antidepressant, nefazodone, attenuates corticosterone-induced increases in 5-HT2A receptor-mediated behaviors in the female rat

The effects of chronic corticosterone administration on sexual behavior and on wet-dog shakes, a 5-HT2A mediated behavior, were investigated in the female rat. In addition, effects of the antidepressant nefazodone, a selective 5-HT2A receptor antagonist, both alone and in combination with corticosterone were examined. Testing was conducted in ovariectomized animals primed with estrogen and progesterone. Corticosterone was found to significantly increase sexual receptivity, sexual proceptivity and wet dog shakes. While nefazodone alone had no significant effects, it completely attenuated the corticosterone-induced increases in both sexual behavior and wet dog shakes. This suggests that corticosterone influences sexual behavior and wet dog shakes via a 5-HT2A receptor mechanism.

Inhibitory effect of serotonin on feeding behavior in goldfish: involvement of CRF

The possible action of 5-HT on feeding behavior in goldfish has been studied. Food intake was significantly reduced by intracerebroventricular (ICV) injection of serotonin (5-HT, 10 microg) at 2 h postinjection. After peripheral (intraperitoneal) administration of 5-HT (1 and 10 microg/g bw), no significant modifications in food intake were detected. Thus, it can be concluded that there is a central anoretic action of 5-HT in teleost fish. Taking in mind the inhibitory effect of corticotropin releasing factor (CRF) on feeding in teleosts and the interactions between 5-HT and CRF described in mammals, we investigated the possible involvement of CRF as mediator of the 5-HT anoretic action in goldfish. The ICV pretreatment with alpha-Helical CRF[9-41](20 microg) partially blocked the inhibitory effect of 5-HT on food consumption in goldfish. These results show that CRF mediates, at least in part, the 5-HT-induced feeding inhibition in goldfish. On the other hand, the alterations in hypothalamic indoleamines content evoked by ICV treatments would suggest that the activation of CRF neurons by 5-HT appears to inhibit hypothalamic serotoninergic transmission, supporting the intermediate role of this neuropeptide in the central anoretic effect of 5-HT in goldfish.