Involvement of 5-HT2 receptors in the wet-dog shake behaviour induced by 5-hydroxytryptophan in the rat

C-LTMRs mediate wet dog shakes via the spinoparabrachial pathway

Mammals perform rapid oscillations of their body- "wet dog shakes" -to remove water and irritants from their back hairy skin. The somatosensory mechanisms underlying this stereotypical behavior are unknown. We report that Piezo2-dependent mechanosensation mediates wet dog shakes evoked by water or oil droplets applied to hairy skin of mice. Unmyelinated low-threshold mechanoreceptors (C-LTMRs) were strongly activated by oil droplets and their optogenetic activation elicited wet dog shakes. Ablation of C-LTMRs attenuated this behavior. Moreover, C-LTMRs synaptically couple to spinoparabrachial (SPB) neurons, and optogenetically inhibiting SPB neuron synapses and excitatory neurons in the parabrachial nucleus impaired both oil droplet- and C-LTMR-evoked wet dog shakes. Thus, a C-LTMR- spinoparabrachial pathway mediates wet dog shakes for rapid and effective removal of foreign particles from back hairy skin.

5-Hydroxytryptophan (5-HTP): Natural Occurrence, Analysis, Biosynthesis, Biotechnology, Physiology and Toxicology

L-5-hydroxytryptophan (5-HTP) is both a drug and a natural component of some dietary supplements. 5-HTP is produced from tryptophan by tryptophan hydroxylase (TPH), which is present in two isoforms (TPH1 and TPH2). Decarboxylation of 5-HTP yields serotonin (5-hydroxytryptamine, 5-HT) that is further transformed to melatonin (N-acetyl-5-methoxytryptamine). 5-HTP plays a major role both in neurologic and metabolic diseases and its synthesis from tryptophan represents the limiting step in serotonin and melatonin biosynthesis. In this review, after an look at the main natural sources of 5-HTP, the chemical analysis and synthesis, biosynthesis and microbial production of 5-HTP by molecular engineering will be described. The physiological effects of 5-HTP are discussed in both animal studies and human clinical trials. The physiological role of 5-HTP in the treatment of depression, anxiety, panic, sleep disorders, obesity, myoclonus and serotonin syndrome are also discussed. 5-HTP toxicity and the occurrence of toxic impurities present in tryptophan and 5-HTP preparations are also discussed.

Evaluating the abuse potential of psychedelic drugs as part of the safety pharmacology assessment for medical use in humans

Psychedelics comprise drugs come from various pharmacological classes including 5-HT_2A agonists, indirect 5-HT agonists, e.g., MDMA, NMDA antagonists and κ-opioid receptor agonists. There is resurgence in developing psychedelics to treat psychiatric disorders with high unmet clinical need. Many, but not all, psychedelics are schedule 1 controlled drugs (CDs), i.e., no approved medical use. For existing psychedelics in development, regulatory approval will require a move from schedule 1 to a CD schedule for drugs with medical use, i.e., schedules 2-5. Although abuse of the psychedelics is well documented, a systematic preclinical and clinical evaluation of the risks they pose in a medical-use setting does not exist. We describe the non-clinical tests required for a regulatory evaluation of abuse/dependence risks, i.e., drug-discrimination, intravenous self-administration and physical dependence liability. A synopsis of the existing data for the various types of psychedelics is provided and we describe our findings with psychedelic drugs in these models. FDA recently issued its guidance on abuse/dependence evaluation of drug-candidates (CDER/FDA, 2017). We critically review the guidance, discuss the impact this document will have on non-clinical abuse/dependence testing, and offer advice on how non-clinical abuse/dependence experiments can be designed to meet not only the expectations of FDA, but also other regulatory agencies. Finally, we offer views on how these non-clinical tests can be refined to provide more meaningful information to aid the assessment of the risks posed by CNS drug-candidates for abuse and physical dependence. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Changes in the sexual behavior and testosterone levels of male rats in response to daily interactions with estrus females

Male rat sexual behavior has been intensively studied over the past 100 years, but few studies have examined how sexual behavior changes over the course of several days of interactions. In this experiment, adult male rats in the experimental group (n=12) were given daily access to estrus females for 30 min per day for 15 consecutive days while control males (n=11) did not interact with females. Ovariectomized females were induced into estrus with hormonal injections, and males interacted with a different female each day. The amount of sexual activity (mounts, intromissions, and ejaculations) was found to cycle with a period of approximately 4 days in most male rats. Additionally, blood was collected every other day following sexual interactions to assess serum testosterone levels. Testosterone was found to peak on the first day of interaction and then fell back to near the level of control rats that did not interact with females. Following the initial peak, testosterone concentrations fluctuated less in males exposed to females than in controls. Sexual activity was not found to predict testosterone concentration. We conclude that when male rats have daily sexual interactions, sexual behavior tends to show cyclic changes and testosterone is significantly elevated only on the first day of interactions.

Animal models of the serotonin syndrome: a systematic review

The serotonin syndrome (SS) is a potentially life-threatening disorder in humans which is induced by ingestion of an overdose or by combination of two or more serotonin (5-HT)-enhancing drugs. In animals, acute administration of direct and indirect 5-HT agonists also leads to a set of behavioral and autonomic responses. In the current review, we provide an overview of the existing versions of the animal model of the SS. With a focus on studies in rats and mice, we analyze the frequency of behavioral and autonomic responses following administration of 5-HT-enhancing drugs and direct 5-HT agonists administered alone or in combination, and we briefly discuss the receptor mediation of these responses. Considering species differences, we identify a distinct set of behavioral and autonomic responses that are consistently observed following administration of direct and indirect 5-HT agonists. Finally, we discuss the importance of a standardized assessment of SS responses in rodents and the utility of animal models of the SS in translational studies, and provide suggestions for future research.

Precipitated withdrawal counters the adverse effects of subchronic cannabinoid administration on male rat sexual behavior

In the present study, sexual behavior of male rats was assessed following prolonged treatment with the CB(1) receptor agonist, HU-210 (0.1mg/mg/day for 10 days) under conditions of drug maintenance, spontaneous withdrawal and precipitated withdrawal (induced via administration of the CB(1) receptor antagonist AM251; 1mg/kg). Following subchronic cannabinoid treatment, sexual activity in male rats was impaired under both the drug maintenance and spontaneous withdrawal conditions, as revealed by a reduction in frequency of both intromissions and ejaculations. Notably, the induction of precipitated drug withdrawal reversed the negative effects of subchronic HU-210 treatment on sexual activity as seen by a reversal of the suppression of ejaculations. These data illustrate that, contrary to expectations, the impairments in male sexual activity following protracted cannabinoid administration are not due to drug withdrawal, per se, but are likely mediated by neuroadaptive changes provoked by repeated drug exposure.

Decrease in the kainate-induced wet dog shake behavior in genetically epilepsy-prone rats: possible involvement of an impaired synaptic transmission to the 5-HT(2A) receptor

Genetically epilepsy-prone rats (GEPR-9s) were derived from Sprague-Dawley rats (SD). The number of kainate-induced wet dog shake behavior (WDS) responses was found to decrease significantly in GEPR-9s compared to SD. WDS responses were potentiated by 5-hydroxytryptophan or 2,5-dimethoxy-4-iodoamphetamine and antagonized by ritanserin. The antagonizing effect of ritanserin on WDS latency was more evident in GEPR-9s than in SD, and hippocampal expression of activity-regulated cytoskeleton-associated protein paralleled the severity of WDS. The results suggest that downstream serotonergic synaptic activation is less pronounced in GEPR-9s than in SD and that the serotonergic agent may directly activate postsynaptic 5-HT2A receptors in both strains.

Effects of clozapine on ethanol withdrawal syndrome in rats

AIMS

Co-morbid substance use in schizophrenic patients is common, and an important factor affects the outcome of disease. On the other hand, drug dependence is a predictive factor for psychosis. Alcohol is one of the most frequently abused psychoactive substances and may contribute psychotic symptoms in several conditions, such as withdrawal syndrome. The present study was designed to investigate the effects of clozapine on ethanol withdrawal syndrome (EWS) in rats.

METHODS

Adult male Wistar rats were used in the study. Ethanol (7.2%, v/v) was given to rats by a liquid diet for 14 days. An isocaloric liquid diet without containing ethanol was also given to control rats. Clozapine (2.5, 5 and 10 mg/kg) and its vehicle (0.1% acetic acid) were injected to rats subcutaneously at the 1.5th and 5.5th hours of ethanol withdrawal. At 2nd, 4th and 6th hours of ethanol withdrawal, rats were observed for 5 min and withdrawal signs that included locomotor hyperactivity, agitation, tremor, tail stiffness, stereotyped behaviour and wet dog shakes were recorded or rated. Following the observations at 6th hour, subjects were tested for audiogenic seizures.

RESULTS

Clozapine significantly and dose-dependently inhibited the EWS-induced locomotor hyperactivity, wet dog shake, stereotyped behaviour, tremor and tail stiffness. However, it did not produce any significant effect on agitation and audiogenic seizures. Doses of clozapine used in the present study did not produce any significant change on locomotor activities of naïve rats.

CONCLUSIONS

Our results suggest that clozapine had some significant beneficial effects on EWS in rats. Thus, this drug may be helpful for controlling some withdrawal signs in ethanol-dependent patients.

The loss of methyl-CpG binding protein 1 leads to autism-like behavioral deficits

Methyl-CpG binding proteins (MBDs) are central components of DNA methylation-mediated epigenetic gene regulation. Alterations of epigenetic pathways are known to be associated with several neurodevelopmental disorders, particularly autism. Our previous studies showed that the loss of Mbd1 led to reduced hippocampal neurogenesis and impaired learning in mice. However, whether MBD1 regulates the autism-related cognitive functions remains unknown. Here we show that Mbd1 mutant (Mbd1(-/-)) mice exhibit several core deficits frequently associated with autism, including reduced social interaction, learning deficits, anxiety, defective sensory motor gating, depression and abnormal brain serotonin activity. Furthermore, we find that Mbd1 can directly regulate the expression of Htr2c, one of the serotonin receptors, by binding to its promoter, and the loss of Mbd1 led to elevated expression of Htr2c. Our results, therefore, demonstrate the importance of epigenetic regulation in mammalian brain development and cognitive functions. Understanding how the loss of Mbd1 could lead to autism-like behavioral phenotypes would reveal much-needed information about the molecular pathogenesis of autism.

Effects of olanzapine on ethanol withdrawal syndrome in rats

The present study was designed to investigate the effects of olanzapine, a serotonin-dopamine antagonistic atypical antipsychotic agent, on ethanol withdrawal syndrome in rats. Adult male Wistar rats were subjects. Ethanol (7.2%, v/v) was given to rats by a liquid diet for 21 days. Control rats were pair fed with an isocaloric liquid diet containing sucrose as a caloric substitute to ethanol. After 2nd, 4th and 6th h of ethanol withdrawal, rats were observed for 5 min, afterwards withdrawal signs that included locomotor hyperactivity, agitation, stereotyped behavior, tremor, wet dog shakes, abnormal posture and abnormal gait were recorded or rated. Olanzapine (0.5, 1 and 2 mg/kg) and saline were injected to the rats intraperitoneally 30 min before ethanol withdrawal assessment. A second series of injections was also given 30 min before the 6th-h-observation, and subjects were then tested for audiogenic seizures. Olanzapine (2 mg/kg) produced significant inhibitory effects on stereotyped behaviors and wet dog shakes at the 6th h of ethanol withdrawal. Contrary, the same dose caused some increases in the intensity of posture and gait impairments at the 2nd h of ethanol withdrawal. In addition, that dose was found to be ineffective on agitation, tremor, tail stiffness and audiogenic seizures. Our results suggest that acute olanzapine treatment has beneficial effects on stereotyped behavior and wet dog shakes, but it also has some adverse effects on posture and gait during ethanol withdrawal in rats. Overall, olanzapine does not seem to be an adequate and suitable drug in controlling of ethanol withdrawal syndrome.

Functional selectivity of hallucinogenic phenethylamine and phenylisopropylamine derivatives at human 5-hydroxytryptamine (5-HT)2A and 5-HT2C receptors

2,5-Dimethoxy-4-substituted phenylisopropylamines and phenethylamines are 5-hydroxytryptamine (serotonin) (5-HT)(2A/2C) agonists. The former are partial to full agonists, whereas the latter are partial to weak agonists. However, most data come from studies analyzing phospholipase C (PLC)-mediated responses, although additional effectors [e.g., phospholipase A(2) (PLA(2))] are associated with these receptors. We compared two homologous series of phenylisopropylamines and phenethylamines measuring both PLA(2) and PLC responses in Chinese hamster ovary-K1 cells expressing human 5-HT(2A) or 5-HT(2C) receptors. In addition, we assayed both groups of compounds as head shake inducers in rats. At the 5-HT(2C) receptor, most compounds were partial agonists for both pathways. Relative efficacy of some phenylisopropylamines was higher for both responses compared with their phenethylamine counterparts, whereas for others, no differences were found. At the 5-HT(2A) receptor, most compounds behaved as partial agonists, but unlike findings at 5-HT(2C) receptors, all phenylisopropylamines were more efficacious than their phenethylamine counterparts. 2,5-Dimethoxyphenylisopropylamine activated only the PLC pathway at both receptor subtypes, 2,5-dimethoxyphenethylamine was selective for PLC at the 5-HT(2C) receptor, and 2,5-dimethoxy-4-nitrophenethylamine was PLA(2)-specific at the 5-HT(2A) receptor. For both receptors, the rank order of efficacy of compounds differed depending upon which response was measured. The phenylisopropylamines were strong head shake inducers, whereas their phenethylamine congeners were not, in agreement with in vitro results and the involvement of 5-HT(2A) receptors in the head shake response. Our results support the concept of functional selectivity and indicate that subtle changes in ligand structure can result in significant differences in the cellular signaling profile.

Nalfurafine, the kappa opioid agonist, inhibits icilin-induced wet-dog shakes in rats and antagonizes glutamate release in the dorsal striatum

Icilin, a cooling compound, produces vigorous wet-dog shakes in rats. We have reported previously that icilin-induced wet-dog shakes are blocked by the kappa opioid receptor agonists, nalfurafine and U50,488H, and that icilin evokes a dose- and time-dependent increase in glutamate within the dorsal striatum. Since activation of kappa opioid receptors inhibits glutamate release intrastriatally, we targeted glutamate release within the dorsal striatum using nalfurafine and examined the role of the dorsal striatum in icilin-induced wet-dog shakes, more specifically, the effect that icilin-evoked intrastriatal glutamate release has on the overt stimulant behavior. We report that nalfurafine (0.04mg/kg) inhibits icilin (0.50mg/kg)-induced wet-dog shakes and that this inhibition is reversed by intrastriatal perfusion of the kappa opioid receptor antagonist, norbinaltorphimine (100nM). Furthermore,we antagonized icilin-evoked glutamate release with nalfurafine (0.04mg/kg), and reversed inhibition of glutamate release with intrastriatal norbinaltorphimine (100nM). These findings support a central component in the behavioral response to icilin and suggest that activation of kappa opioid receptors antagonizes icilin-induced wet-dog shakes in rats by inhibiting glutamate release within the dorsal striatum.

Effect of the 5-HT receptor agonist DOI on female rat sexual behavior

INTRODUCTION

Female rats display a repertoire of behaviors during a sexual encounter with a male, including sexually receptive (the lordosis response) and proceptive (darts and hops, and ear wigglings) behaviors.

AIM

We investigated the effects of subcutaneous injection of the 5-HT(2A/2C) receptor agonist (2,5-dimethoxy-4-idophenyl)-2-aminopropane hydrochloride (DOI) on sexual behaviors of ovariectomized female rat hormonally supplemented with estradiol benzoate (10 microg) and progesterone (250 microg).

METHODS

Both female and male sexual behaviors were observed for 10 minutes (pretest). Then females were injected with the treatment and after a 10-minute delay replaced with the same male for a 30-minute mating test (posttreatment period).

RESULTS

DOI (0.5 and 1 mg/kg) significantly increased the number of darts and hops/mounts. In contrast, no significant differences in ear wigglings/mounts were observed. In addition, DOI failed to modify sexual receptivity.

CONCLUSION

These data suggest that 5-HT(2A/2C) receptors are important in the regulation of female proceptivity.

The pathophysiology of serotonin toxicity in animals and humans: implications for diagnosis and treatment

Serotonin toxicity (or serotonin syndrome) has become an increasingly common and important clinical problem in medicine over the last 15 years with the introduction of many new antidepressants that can cause increased levels of serotonin (5-HT) in the central nervous system (CNS). Severe and life-threatening cases are almost exclusively a result of combinations of antidepressants (usually monoamine oxidase inhibitors and selective serotonin reuptake inhibitors). Unfortunately, the term serotonin syndrome has a number of quite different meanings, and many people writing on this subject have failed to differentiate them. This has led to false conclusions regarding the 5-HT receptor subtypes responsible for the life-threatening effects in animal and human toxicity, and suggestions of ineffective treatment strategies. This review primarily addresses the serotonin receptor subtypes that underlie the clinical manifestations of excess CNS serotonin in humans and animals, and their implications for diagnosis and treatment. More specific diagnostic criteria for serotonin toxicity are required to identify situations when specific antidotes are likely to be useful. However, the mainstay of treatment of severe cases is good supportive care and early intubation and paralysis in life-threatening serotonin toxicity.

Long-term kindling of the basolateral amygdala impairs copulatory behavior in male rats

This experiment examined the consequences of long-term kindling of the basolateral amygdala on male sexual behavior and the frequency of both spontaneous wet dog shakes (WDS) and those induced by the 5-HT2A receptor agonist DOI. Results demonstrated that following 60 stimulations of the left basolateral amygdala over a 4-week period, male Long-Evans rats exhibited decrements in every aspect of sexual behavior. Specifically, latencies to mount, intromit and ejaculate were all prolonged following long-term kindling, and ejaculation frequencies were significantly reduced. Furthermore, spontaneous peri-copulatory WDS were increased in kindled rats, suggesting a possible role of the 5-HT2A receptor. However, countering this suggestion, there were no differences between sham and kindled rats on WDS induced by the 5-HT2A receptor agonist DOI. These results suggest that kindled rats may exhibit elevated levels of endogenous serotonin during exposure to a female rat, which would attenuate copulatory behavior, while concurrently increasing WDS expression.

Functional role of the endocannabinoid system and AMPA/kainate receptors in 5-HT2A receptor-mediated wet dog shakes

These experiments sought to determine the influence of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and the endocannabinoid system in the functional expression of the serotonin (5-HT) type 2A receptor-mediated wet dog shake response. Male Long-Evans rats were pretreated with either 1 mg/kg i.p. of the 5-HT(2A/2C) receptor antagonist ketanserin; 1, 10 or 30 mg/kg i.p. of the AMPA/kainate antagonist 6,7-dinitroquinnoxaline-2,3-dione (DNQX); 1, 5 or 10 mg/kg i.p. of the endocannabinoid uptake inhibitor AM404; or 1, 5 or 10 mg/kg i.p. of the cannabinoid CB(1) receptor antagonist AM 251 prior to injection of the 5-HT(2A/2C) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI, 1 mg/kg i.p.). Results demonstrated that 10 mg/kg of AM404 significantly reduced the expression of DOI-induced wet dog shakes, but lower doses were ineffective. Administration of AM251 did not induce wet dog shakes behavior when administered alone, but significantly potentiated DOI-induced wet dog shaking behavior at a dose of 10 mg/kg. Pretreatment with DNQX significantly reduced the expression of DOI-induced wet dog shakes at all doses tested. These data suggest that AMPA/kainate receptors play a role in the mediation of 5-HT(2A) receptor activity, whereas the endocannabinoid system may act as a regulatory buffer system during periods of elevated activity, but not under basal conditions.

Is antagonism of alpha3beta4 nicotinic receptors a strategy to reduce morphine dependence?

18-Methoxycoronaridine, a synthetic iboga alkaloid congener, has been previously shown to attenuate several signs of morphine withdrawal in rats. The recently discovered action of 18-methoxycoronaridine to block alpha3beta4 nicotinic receptors may be responsible for this effect. To test this hypothesis the effects of non-selective alpha3beta4 receptor antagonists, dextromethorphan, mecamylamine, bupropion, and their combinations, were assessed on of acute naltrexone-precipitated (1 mg/kg i.p.) morphine withdrawal in rats. Dextromethorphan (5-40 mg/kg, s.c.), mecamylamine (0.25-4 mg/kg, i.p.) and bupropion (10-30 mg/kg, i.p.) alone produced variable effects on signs of withdrawal. However, two low-dose combinations, i.e., dextromethorphan (5 mg/kg, s.c.) and mecamylamine (0.25 mg/kg, i.p.), mecamylamine (0.25 mg/kg, i.p.) and bupropion (10 mg/kg, i.p.) as well as the three-drug combination significantly attenuated diarrhea and weight loss; none of the agents administered alone had these effects. The results of the present study provide evidence that alpha3beta4 nicotinic receptors are involved in the expression of at least two signs of opioid withdrawal.

Influence of nociceptin(1-17) fragments and its tyrosine-substituted derivative on morphine-withdrawal signs in rats

Our previous studies demonstrated that endogenous ligand of nociceptin (NOP) receptor, nociceptin(1-17) (also known as orphanin FQ), inhibits morphine-withdrawal syndrome measured as wet dog shakes in rats [Life Sci. 66 (2000) PL119]. This peptide is metabolized in the spinal cord, both in vitro and in vivo, to shorter fragments, including nociceptin(1-11) and nociceptin(1-6). These fragments, formed after cleavage by endogenous peptidase, are behaviorally active and modulate nociception in a bi-phasic process [Peptides 20 (1999) 239]. As these peptides induced transient naloxone-reversible analgesia in behavioral tests [Peptides 20 (1999) 239], in the present study we tested the influence of nociceptin(1-11) (10 and 20 microg) and nociceptin(1-6) (10, 20 and 40 microg) on the morphine-withdrawal syndrome in rats. Furthermore, the modified fragment of nociceptin(1-6) with an opioid-message domain achieved by replacement of Phe1 with Tyr was tested. Morphine-withdrawal syndrome was precipitated by the i.p. injection of naloxone hydrochloride (2 mg/kg), 72 h after implantation of morphine pellets. The wet-dog shakes were chosen for statistical analyses of the abstinence signs. The results show that nociceptin(1-11) and (1-6) attenuate this morphine-withdrawal symptom. The replacement of Phe1 with Tyr in nociceptin(1-6) fragment did not potentiate the influence of nociceptin(1-6) on wet dog shakes precipitated by naloxone in morphine-dependent rats.

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.

Central 5-HT receptor hypersensitivity in migraine without aura

Serotonin has long been implicated as a key neurotransmitter in migraine. There is a dearth of research specifically examining 5-HT1A receptor sensitivity in migraine despite the importance of this receptor in regulating central serotonergic tone. In this study we examined the hypothesis that migraine without aura is associated with hypersensitivity of central 5-HT1A receptors, using a 5-HT1A neuroendocrine challenge drug and comparing serum prolactin responses between a test group with migraine and a matched group of healthy controls. Twelve female subjects fulfilling International Headache Society (IHS) criteria for migraine without aura were evaluated. Following an overnight fast, subjects presented for testing at 9am. An intravenous canula was inserted and serum prolactin was assessed at baseline and every 30 min for 3 h following a single dose of 30 mg oral buspirone, a 5-HT1A-receptor agonist. Subjects were assessed during the first 5 days of the menstrual cycle. No subjects were taking psychotropic medication or migraine prophylactic treatment. Patients with current or previous psychiatric disorder, daily headache or analgesic overuse were excluded. 16 healthy female volunteers matched for age and menstrual status were also evaluated and served as controls. There was no difference in baseline prolactin between groups. There was a significant rise in prolactin following buspirone in both groups. Subjects with migraine had a significantly increased prolactin response to buspirone (delta max) compared to controls (P < 0.001). This study supports the hypothesis that migraine without aura is associated with a relative hypersensitivity of central 5-HT1A receptors. This is of relevance given the role of the 5-HT1A receptor in controlling raphe 5-HT tone and in the possible association between migraine and anxiety and depression.

Nitric oxide synthase inhibition suppresses wet dog shakes and augments convulsions in rats

Electrical stimulation of limbic structures, pharmacological interventions, and getting wet induces wet dog shakes (WDS) in rats. WDS are often associated with the occurrence of seizures. In this study, we evaluated the effects of reduced NO production on physiologically (wetting)- or pharmacologically (kainic acid; KA)-induced WDS and KA-triggered seizures. Following wetting, naive and saline-treated rats displayed more WDS than rats treated with NO synthase (NOS) inhibitor, N omega-nitro-L-arginine (L-NA). In another experiment, WDS and seizures were monitored after KA treatment alone or in combination with L-NA. Again, NOS inhibition reduced the number of KA-triggered WDS but augmented the number and severity of seizures. Our results suggest that not only do physiologically- and kainate-induced WDS share a common mechanism that includes NO, but that there is also an antagonism between WDS and convulsions.

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.

Effect of 5-HT1A and 5-HT2A/2C receptor modulation on neuroleptic-induced vacuous chewing movements

Tardive dyskinesia is a serious motor side effect of chronic neuroleptic therapy. Chronic treatment or rats with neuroleptics leads to the development of abnormal oral movements called vacuous chewing movements. Vacuous chewing movements in rats are widely accepted as an animal model of tardive dyskinesia. Atypical antipsychotics such as clozapine and rispiridone are associated with a lower incidence of extrapyramidal side effects and tardive dyskinesia. The present study was aimed to explore the role of 5-HT1A, 5-HT2A/2C receptors in the expression of neuroleptic-induced orofacial dyskinesia. In the present study rats were chronically (for 21 days) treated with haloperidol (1.5 mg/kg, i.p.) to elicit vacuous chewing movements. The neuroleptic-induced vacuous chewing movements, viz., vertical jaw movements, tongue protrusions and bursts of jaw tremors, were counted during a 5-min observation period. Acute treatment with 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, dose-dependently (0.05, 0.1 and 0.2 mg/kg, i.p.) reduced the haloperidol-induced vacuous chewing movements and headshakes. Both acute and chronic administration of seganserin, ketanserin and ritanserin, 5-HT2A/2C receptor antagonists, also reduced haloperidol-induced vacuous chewing movements in a dose-dependent (0.05, 0.1 and 0.2 mg/kg, i.p.) manner. In acute studies a higher dose of ritanserin (1 mg/kg) but not ketanserin (1 mg/kg) increased vacuous chewing movements, whereas a higher dose of seganserin (1 mg/kg) did not have any effect on vacuous chewing movements. All the drugs reduced haloperidol-induced headshakes in a dose-dependent fashion. These findings indicate that the serotonergic system, and particularly 5-HT1A and 5-HT2A/2C receptors, may be involved in haloperidol-induced orofacial dyskinesia, and that 5-HT receptors may provide novel targets for the development of drugs that can be used to reverse or prevent the extrapyramidal side effects associated with long-term antipsychotic treatment.

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.

Interactions of lithium and drugs that affect signal transduction on behaviour in rats

The therapeutic mechanism of the action of lithium in the treatment of bipolar affective disorder is not known, in spite of a burgeoning number of biochemical studies linking lithium to signal transduction processes. This article reviews a decade of studies examining the behavioural manifestations of manipulating inositol, cyclic adenosine monophosphate (cAMP) and G proteins in rats. Inositol, forskolin, dibutyryl cAMP and pertussis toxin all interacted with lithium when rearing behavior was measured. Lithium potentiated the increase in locomotion induced by injections of cholera toxin into the nucleus accumbens, consistent with the hypothesis that it inactivates inhibitory G proteins. More specific interactions were found between lithium and inositol following cholinergic and serotonergic stimulation. Inositol, but not forskolin, attenuated lithium-pilocarpine seizures and the enhancement of the serotonin syndrome; however, inositol had no effect on lithium-induced attenuation of wet dog shakes following an injection of 5-hydroxytryptophan. Behavioural evidence supports biochemical findings suggesting that lithium's interactions with the phoshphatidyl inositol and cyclic AMP signal transduction systems may be relevant to its therapeutic effects in bipolar disorder. Further research on more specific behaviours may elucidate the relevant pharmacological mechanisms underlying the therapeutic effect of lithium.

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.

Behavioural effects of a subconvulsive dose of kainic acid in rats

Kainic acid can induce a continuum of non-convulsive seizures characterised by epileptic automatisms and convulsive motor seizures depending on the dose. There are scarce data on the behavioural effects of low doses of kainate inducing only non-convulsive seizures. Therefore, we studied spontaneous behaviour of adult male rats using a method of positive habituation based on a detailed analysis of patterns and attention of animals to a stimulus object. Twenty-three animals were individually tested in the experimental arena on two consecutive days. Comparing the data from the first two exposures, a conspicuous habituation in all animals was observed. On experimental day 3, 12 rats received kainate (6 mg/kg intraperitoneally) and the remaining 11 animals received a physiological saline. After 1 h, animals were put into the arena with an object localised in the centre. It was found that both kainate and saline treated animals exhibited a significant increase in the total number of central area visits, and both the total and mean time spent in the vicinity of the object. However, the mean time spent was significantly shorter in kainate treated rats. Furthermore. kainate rats exhibited a significant decrease in rearing as compared with the controls. In addition, an epileptic automatism (wet dog shakes) was observed in seven out of 12 animals given kainate. The comparison of transition matrices between consecutive behavioural categories showed significant differences between the kainate and control groups. Our results demonstrate that a non-convulsive dose of kainate induced changes in the structure of spontaneous behaviour and impaired the processes related to maintenance of attention.

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.

Premature ejaculation and serotonergic antidepressants-induced delayed ejaculation: the involvement of the serotonergic system

Premature ejaculation has generally been considered a psychosexual disorder with psychogenic aetiology. Although still mainly treated by behavioural therapy, in recent years double-blind studies have indicated the beneficial effects of some of the serotonergic antidepressants (SSRIs) in delaying ejaculation. We describe here the neurophysiology and the peripheral neuroanatomy of ejaculation and provide a review of the involvement of serotonin in the central nervous system in relation to serotonergic nuclei and their projections. A hypothesis of the role of 5-HT1A and 5-HT2C receptors in premature ejaculation is postulated.

Effect of chronic amitriptyline administration on serotonergic receptors in rats with methylazoxymethanol-induced microencephaly

Methylazoxymethanol (MAM)-induced cortical hypoplasia resulted in a 20% decrease in the Bmax of 5-HT2A receptors in the frontal cortex with no change in the Bmax of 5-HT1A receptors. Chronic treatment with amitriptyline did not further decrease the Bmax of 5-HT2A receptors in the MAM-lesioned cortex, suggesting that the persistent down-regulation of cortical 5-HT2A receptors in MAM-lesioned rats was induced by serotonergic hyperinnervation.

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.

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.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

Effect of the selective 5-HT receptor agonists 8-OHDPAT and DOI on behavior and brain biogenic amines of rats

1. The behavioral responses, as well as the biogenic amines and metabolite contents in discrete brain areas were determined in male rats subcutaneously treated with a 5-HT1A (8-OHDPAT) or 5-HT2A (DOI) agonist at doses (0.5-2 mg/kg) sufficient to produce the typical effects of the stimulation of these brain receptor subtypes. 2. Besides the expected effects (i.e., forepaw treading, flat body posture and inhibition of 5-HT release and turnover), 8-OHDPAT displayed signs of increased dopaminergic transmission. 3. DOI increased dopamine turnover and provoked stereotypical behavior, in addition to head shakes and body twitches. 4. Moreover, DOI induced both forepaw treading and flat body posture, which are believed to be typical responses to the stimulation of brain 5-HT1A receptors. 5. This finding cannot be explained on the basis of actual knowledge, because the affinity of DOI for 5-HT1A receptor has been found to be very low, whereas indirect mechanisms of activation of this receptor subtype triggered by stimulation of 5-HT2A receptor are actually unknown.

Role of serotonin receptor subtypes in the development of amygdaloid kindling in rats

The present study was conducted to identify serotonin (5-HT) receptor subtypes involved in the development of amygdala (AM) kindling. We used 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A agonist, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 agonist, both of which were injected subcutaneously 15 min prior to each daily electrical stimulation to the rat AM. Treatment with 8-OH-DPAT (1 mg/kg) slightly suppressed behavioral and electrographic seizure development during the course of kindling. In contrast, DOI (1 mg/kg) strongly facilitated kindling development and reduced the number of stimulations needed to produce generalized seizures. These facilitatory effects of DOI were completely blocked by pretreatment with a 5-HT2 antagonist ketanserin. The present results suggest that the activation of 5-HT1A receptors can retard the development of AM kindling, whereas 5-HT2 receptors play a facilitatory role in this developmental seizure process.

(+/-)-1-(2,5-Dimethoxy-4-ethylthiophenyl)-2-aminopropane (ALEPH-2), a novel putative anxiolytic agent lacking affinity for benzodiazepine sites and serotonin-1A receptors

Serotonergic behavioral responses, effects on motor activity and core temperature, and binding properties of the novel putative anxiolytic amphetamine derivative (+/-)1-(2,5-dimethoxy-4-ethylthio-phenyl)-2-aminopropane (ALEPH-2), were examined in rodents in order to elucidate the mechanism underlying its anxiolytic-like effect. After peripheral administration in rats, ALEPH-2 induced some symptoms of the serotonergic syndrome, e.g. forepaw treading and flat body posture. Additionally, a decrease in motor activity was observed. No significant effects on the number of head shakes were observed after injection, although high inter-subject variability was noted. Higher doses of ALEPH-2, in the range exhibiting anxiolytic properties (4mg/kg), elicited significant hypothermia in mice. The affinity of the drug for 5-HT2A/2C receptors ([3H]ketanserin sites) was in the nanomolar range (Ki = 173 nM), whereas for 5-HT1A, benzodiazepine sites, and GABAA receptors, the affinity was micromolar of lower. Based on these results the mechanism of action and the anxiolytic-like properties of ALEPH-2 are discussed.

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

Malfunction of the serotonergic system and dysregulation of the hypothalamo-pituitary-adrenocortical axis have been implicated in the pathophysiology of depression. Several studies provide evidence for reciprocal influences between glucocorticoids and 5-HT receptors. The effect of repeated treatment with a high dose of corticosterone (50 mg/kg s.c. twice daily for 4 days) on 5-HT receptor subtype-mediated behaviours was studied. It was found that in rats that were repeatedly treated with corticosterone the number of 2-chloro-6-(1-piperazinyl)pyrazine HCl (MK 212)-induced, 5-HT2C receptor-mediated penile erections were reduced, whereas both MK 212 and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced 5-HT2A receptor-mediated head shakes were increased. The (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)-induced lower lip retraction mediated by presynaptic 5-HT1A receptors was unchanged, whereas the open field activity induced by 8-OH-DPAT was enhanced in corticosterone pretreated rats. These changes in 5-HT receptor subtype-mediated behaviours were not seen after a single injection with corticosterone given 24 h or 5 days before. The results suggest that 5-HT2A, 5-HT2C and postsynaptic 5-HT1A receptor-mediated behaviour can be modulated by repeated treatment with a high dose of corticosterone.

Preclinical profile of the mixed 5-HT1A/5-HT2A receptor antagonist S 21,357

This study evaluated the pharmacological and behavioral effects of S 21,357, a drug with high affinity for both 5-HT1A and 5-HT2A receptors. The drug behaved as antagonist at both 5-HT1A autoreceptors and postsynaptic 5-HT1A receptors, as it prevented the inhibitory effect of lesopitron on the electrical discharge of the dorsal raphé nucleus (DRN) 5-HT neurons and the activity of forskolin-stimulated adenylate cyclase in hippocampal homogenates. In addition, S 21,357 (4 and 128 mg/kg, PO) inhibited 5-HTP-induced head-twitch responses in mice, indicating that it possesses 5-HT2A antagonistic properties. In a test battery designed to assess defensive behaviors of Swiss-Webster mice to the presence of, or situations associated with, a natural threat stimulus (i.e., rat), S 21,357 (0.12-2 mg/kg, IP) reduced contextual defense reactions after the rat was removed, risk assessment activities when the subject was chased, and finally, defensive attack behavior. These behavioral changes are consistent with fear/anxiety reduction. Furthermore, the drug strongly reduced flight reactions in response to the approaching rat. This last finding, taken together with recent results with panic-modulating drugs, suggest that S 21,357 may have potential efficacy against panic attack. Finally, our results suggest that compounds sharing high affinities for both 5-HT1A and 5-HT2A receptors may directly or synergistically increase the range of defensive behaviors affected.

Pharmacological profile of (-)HT-90B, a novel 5-HT1A receptor agonist/5-HT2 receptor antagonist

1. HT-90B ((-)-N-([2-(8-methyl-l, 4-benzodioxane-2-ylmethyl)amino]ethyl) tricyclo[3,3,1,1(3.7)] decane-1-carboxamide) had high affinities for the 5-HT1A (Ki = 0.18 nM) and 5-HT2 (Ki = 9.2 nM) receptors. 2. HT-90B inhibited forskolin activated adenylate cyclase in rat hippocampal membranes as a 5-HT1A full agonist (IC50 = 2 nM), and the potency of the drug was higher than that of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a standard 5-HT1A agonist. 3. In the serotonin syndrome test, HT-90B behaved as a weak partial 5-HT1A agonist in reserpinized rats. 4. 5-HT2 receptor-mediated potentiation of rabbit platelet aggregation by serotonin (5-HT) was reduced by HT-90B (IC50 = 1.73 microM). 5. Head twitch response induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 agonist, was inhibited by HT-90B in mice. 6. It is concluded that HT-90B has potent 5-HT1A receptor agonist as well as 5-HT2 receptor antagonist properties in vitro and in vivo.

Effect of isolation on behavioural models involving serotonergic 5-HT2 and 5-HT1A receptors

1. The effect of 7 days of isolation were observed in mice on behavioural models involving 5-HT2 and 5-HT1A receptors. 2. The sensitivity of 5-HT2 receptors as assessed through L-5-HTP or 5-MeODMT induced head-twitches was reduced. 3. The sensitivity of the 5-HT1A receptors implicated in the 8-OH-DPAT induced feeding was unchanged. 4. The sensitivity of the 5-HT1A receptors involved in the 8-OH-DPAT induced hypothermia was diminished. 5. On the whole, these results show that after 7 days of isolation, the responses to the stimulation of serotonergic receptors is unchanged or diminished according to both the receptor's subtype and the model used.

Study of the mechanisms involved in behavioral changes induced by flunitrazepam in morphine withdrawal

1. The attenuation of morphine withdrawal syndrome by acute benzodiazepine administration has been well documented. However, the pharmacological mechanisms implicated in this effect remain unclear. 2. In this study, the possible participation of noradrenergic, serotonergic and benzodiazepine receptors on flunitrazepam-modified morphine withdrawal syndrome was investigated in mice. Flunitrazepam was associated to the noradrenergic antagonists prazosin (1 mg/kg) and propranolol (0.5 mg/kg), the serotonergic agents ritanserine (1 mg/kg) and p-chloro phenylalanine (600 mg/kg), the benzodiazepine antagonist flumazenil (10 mg/kg), and the benzodiazepine partial inverse agonist Ro 15-4513 (5 mg/kg). 3. The decrease in jumping behavior-induced by flunitrazepam was potentiated by prazosin, while ritanserine, flumazenil and Ro 15-4513 blocked this effect. 4. Flunitrazepam-induced increase on wet dog shake frequency was partially reduced by flumazenil, and strongly antagonized by ritanserine and Ro 15-4513. 5. Noradrenergic and serotonergic systems seem to be primarily implicated in the changes induced on jumping and wet dog shakes respectively. These modifications are induced through the activation of the benzodiazepine receptors.

Interactions between 5-hydroxytryptamine receptor subtypes: is a disturbed receptor balance contributing to the symptomatology of depression in humans?

The purpose of this review is to describe the consequences of antidepressant treatment on the behaviour of rodents after activation of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes. In a summary table, the involvement of 5-HT receptors in inducing behavioural changes are described. It is emphasized that these effects are not always only exclusively linked to serotonergic functions nor that they are only initiated by central 5-HT receptors. Hereafter, the complex mutual inhibitory effects of 5-HT receptor subtype-mediated processes are discussed by interpreting effects of antagonists and describing the different effects of low and high doses of mixed 5-HT1C/5-HT2 receptor agonists. Mutual influences are seen particularly with 5-HT1A, 5-HT1C and 5-HT2, but not with 5-HT1B, 5-HT1D or 5-HT3 receptor-mediated effects. It is shown that the behavioural consequences of 5-HT1A, 5-HT1C and 5-HT2 receptor stimulation may be changed by brain lesions or chronic treatment with drugs. Among these drugs are the antidepressants. Finally, 5-HT receptor function in depressed patients is discussed, and the hypothesis is proposed that an important function of antidepressants is to restore a disturbed balance between 5-HT1A, 5-HT1C and 5-HT2 receptors in depressed patients.

Myo-inositol attenuates the enhancement of the serotonin syndrome by lithium

Lithium elicits opposite effects on two behavioural syndromes in rats: enhancement of the 5-HT1A-linked serotonin syndrome and attenuation of the 5-HT2-linked wet dog shakes. The ability of intracerebroventricular (ICV) myo-inositol or forskolin to reverse the enhancement of the serotonin syndrome by lithium was tested in rats that were fed chronic dietary lithium or control diet and injected with the serotonin agonist 5-MeODMT (5-methoxy-N, N-dimethyltryptamine). Lithium enhanced the total serotonin syndrome score and particularly flat posture and tremor. Inositol, but not forskolin, mitigated the effects of lithium. Inositol was also injected in the lateral ventricle of rats pretreated with chronic dietary lithium or regular rat chow for 3 weeks and injected with carbidopa and L-5-hydroxytryptophan (5-HTP). Lithium attenuated wet dog shakes, but inositol had no significant effect on lithium-treated or control rats. These findings suggest that the enhancement of the serotonin syndrome by lithium may be related to lithium-induced inositol depletion.

Effects of serotonin precursors on the negative feedback effects of glucocorticoids on hypothalamic-pituitary-adrenal axis function in depression

In order to investigate the relationships between brain serotonergic turnover and hypothalamic-pituitary-adrenal (HPA) axis function in unipolar depression, the authors measured intact adrenocorticotropic hormone (ACTH) and cortisol levels in baseline conditions and after combined dexamethasone (1 mg PO) and L-5-hydroxytryptophan (L-5-HTP, 200 mg PO) administration in 13 minor, 17 simple major, and 17 melancholic subjects. L-5-HTP significantly enhanced post-DST ACTH and cortisol secretion in major--but not in minor--depressed subjects. Major depressed subjects with or without melancholia exhibited significantly higher post-DST ACTH and cortisol responses to L-5-HTP than minor depressed subjects. L-5-HTP administration converted some major depressed ACTH or cortisol suppressors into nonsuppressors. L-5-HTP stimulated ACTH or cortisol secretion to the same extent in major depressed HPA-axis suppressors and nonsuppressors. It is concluded that L-5-HTP loading may augment ACTH and, consequently, cortisol escape from suppression by dexamethasone in major but not in minor depressed subjects. The findings show that serotonergic mechanisms modulate the negative feedback of glucocorticoids on central HPA-axis regulation. It is hypothesized that the higher L-5-HTP-induced post-DST HPA-axis hormone responses in major depression reflect upregulated 5-HT2 receptor-driven breakthrough secretion of pituitary ACTH from suppression by dexamethasone.