The 5-HT1A receptor and behavioral stimulation in the rat: effects of 8-OHDPAT on spontaneous and cocaine-induced behavior

The 5-HT1A receptor agonist 8-OH-DPAT modulates motor/exploratory activity, recognition memory and dopamine transporter binding in the dorsal and ventral striatum

In the present studies, we assessed the effect of the 5-HT1A receptor (R) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on motor and exploratory behaviors, object and place recognition and dopamine transporter (DAT) and serotonin transporter (SERT) binding in the rat brain. In Experiment I, motor/exploratory behaviors were assessed in an open field after injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle for 30 min without previous habituation to the open field. In Experiment II, rats underwent a 5-min exploration trial in an open field with two identical objects. After injection of either 8-OH-DPAT (0.1 and 3 mg/kg) or vehicle, rats underwent a 5-min test trial with one of the objects replaced by a novel one and the other object transferred to a novel place. Subsequently, N-o-fluoropropyl-2b-carbomethoxy-3b-(4-[123I]iodophenyl)-nortropane ([123I]FP-CIT; 11 ± 4 MBq) was injected into the tail vein. Regional radioactivity accumulations were determined post mortem with a well counter. In both experiments, 8-OH-DPAT dose-dependently increased ambulation and exploratory head-shoulder motility, whereas rearing was dose-dependently decreased. In the test rial of Experiment II, there were no effects of 8-OH-DPAT on overall activity, sitting and grooming. 8-OH-DPAT dose-dependently impaired recognition of object and place. 8-OH-DPAT (3 mg/kg) increased DAT binding in the dorsal striatum relative to both vehicle and 0.1 mg/kg 8-OH-DPAT. Furthermore, in the ventral striatum, DAT binding was decreased after 3 mg/kg 8-OH-DPAT relative to vehicle. Findings indicate that motor/exploratory behaviors, memory for object and place and regional dopamine function may be modulated by the 5-HT1AR. Since, after 8-OH-DPAT, rats exhibited more horizontal and less (exploratory) vertical motor activity, while overall activity was not different between groups, it may be inferred, that the observed impairment of object recognition was not related to a decrease of motor activity as such, but to a decrease of intrinsic motivation, attention and/or awareness, which are relevant accessories of learning. Furthermore, the present findings on 8-OH-DPAT action indicate associations not only between motor/exploratory behavior and the recognition of object and place but also between the respective parameters and the levels of available DA in dorsal and ventral striatum.

The role of serotonin in modulating common waxbill behaviour

Abstract

Serotonin or 5-hydroxytryptamine (5-HT) is a monoaminergic neurotransmitter that is known to influence behaviour in various animal species. Its actions, however, are complex and not well-understood yet. Here, we tested whether and how two 5-HT receptor agonists and a 5-HT receptor antagonist influence behaviour in common waxbills (Estrilda astrild), focusing on aggression, movement and feeding. We applied acute administration of either 8-OH-DPAT (a 5-HT1A receptor agonist), fluoxetine (a selective serotonin reuptake inhibitor; SSRI) or WAY 100,635 (a 5-HT1A receptor antagonist), and then quantified behaviour in the context of competition for food. Waxbills treated with the SSRI fluoxetine showed an overall decrease of aggressive behaviour, activity and feeding, while we found no significant effects of treatment with the other serotonergic enhancer (8-OH-DPAT) or with the antagonist WAY 100,635. Since both 8-OH-DPAT and WAY 100,635 act mainly on 5-HT1A receptor pathways, while fluoxetine more generally affects 5-HT pathways, our results suggest that receptors other than 5-HT1A are important for serotonergic modulation of waxbill behaviour.

Significance statement

The serotonergic system is of interest for current behavioural research due to its influence on a range of behaviours, including aggression, affiliative behaviour, feeding and locomotion in various species. There are, however, numerous discrepancies regarding the behavioural effects of serotonin across studies. We used acute pharmacological manipulations of the serotonergic system in common waxbills, using two serotonin enhancers (8-OH-DPAT and fluoxetine) and a serotonin blocker (WAY 100,635). Behavioural effects of these pharmacological manipulations on aggressiveness, movement and feeding, during tests of competition over food, indicated an anxiogenic-like effect of fluoxetine, but not of 8-OH-DPAT and WAY 100,635. This suggests a distinct role for different serotonergic pathways on waxbill behaviour.

On the role of serotonin 5-HT1A receptor in autistic-like behavior: сross talk of 5-HT and BDNF systems

Autism spectrum disorders (ASDs) are some of the most common neurodevelopmental disorders; however, the mechanisms underlying ASDs are still poorly understood. Serotonin (5-HT) and brain-derived neurotrophic factor (BDNF) are known as key players in brain and behavioral plasticity and interact with each other. 5-HT_1A receptor is a principal regulator of the brain 5-HT system, which modulates normal and pathological behavior. Here we investigated effects of adeno-associated-virus-based 5-HT_1A receptor overexpression in the hippocampus of BTBR mice (which are a model of autism) on various types of behavior and on the expression of 5-HT₇ receptor, proBDNF, mature BDNF, and BDNF receptors (TrkB and p75^NTR). The 5-HT_1A receptor overexpression in BTBR mice reduced stereotyped behavior in the marble-burying test and extended the time spent in the center in the open field test. Meanwhile, this overexpression failed to affect social behavior in the three-chambered test, immobility time in the tail suspension test, locomotor activity in the open field test, and associative learning within the "operant wall" paradigm. The 5-HT_1A receptor overexpression in the hippocampus raised hippocampal 5-HT₇ receptor mRNA and protein levels. Additionally, the 5-HT_1A receptor overexpression lowered both mRNA and protein levels of TrkB receptor but failed to affect proBDNF, mature BDNF, and p75^NTR receptor expression in the hippocampus of BTBR mice. Thus, obtained results suggest the involvement of the 5-HT and BDNF systems' interaction mediated by 5-HT_1A and TrkB receptors in the mechanisms underlying autistic-like behavior in BTBR mice.

Serotonin and Consciousness-A Reappraisal

The serotonergic system of the brain is a major modulator of behaviour. Here we describe a re-appraisal of its function for consciousness based on anatomical, functional and pharmacological data. For a better understanding, the current model of consciousness is expanded. Two parallel streams of conscious flow are distinguished. A flow of conscious content and an affective consciousness flow. While conscious content flow has its functional equivalent in the activity of higher cortico-cortical and cortico-thalamic networks, affective conscious flow originates in segregated deeper brain structures for single emotions. It is hypothesized that single emotional networks converge on serotonergic and other modulatory transmitter neurons in the brainstem where a bound percept of an affective conscious flow is formed. This is then dispersed to cortical and thalamic networks, where it is time locked with conscious content flow at the level of these networks. Serotonin acts in concert with other modulatory systems of the brain stem with some possible specialization on single emotions. Together, these systems signal a bound percept of affective conscious flow. Dysfunctions in the serotonergic system may not only give rise to behavioural and somatic symptoms, but also essentially affect the coupling of conscious affective flow with conscious content flow, leading to the affect-stained subjective side of mental disorders like anxiety, depression, or schizophrenia. The present model is an attempt to integrate the growing insights into serotonergic system function. However, it is acknowledged, that several key claims are still at a heuristic level that need further empirical support.

Role of 5-HT1A Receptor in the Anxiolytic-Relaxant Effects of Bergamot Essential Oil in Rodent

The essential oil obtained by the fresh fruit of Citrus bergamia Risso et Poiteau is used worldwide in aromatherapy to reduce pain, facilitate sleep induction, and/or minimize the effects of stress-induced anxiety. Preclinical pharmacological data demonstrate that bergamot essential oil (BEO) modulates specific neurotransmissions and shows an anxiolytic-relaxant effect not superimposable to that of the benzodiazepine diazepam, suggesting that neurotransmissions, other than GABAergic, could be involved. Several studies on essential oils indicate a role for serotonergic (5-HT) neurotransmission in anxiety. Interestingly, among serotonergic receptors, the 5-HT1A subtype seems to play a key role in the control of anxiety. Here, we report that modulation of the 5-HT1A receptor by selective agonist ((±)8-OH-DPAT) or antagonist (WAY-100635) may influence some of the anxiolytic-relaxant effects of BEO in Open Field and Elevated Plus Maze tests.

Effects of combined escitalopram and aripiprazole in rats: role of the 5-HT1a receptor

RATIONALE

Pre-clinical and clinical studies have suggested that the antidepressant efficacy of escitalopram (ESC) can be augmented by co-administration of aripiprazole (ARI).

OBJECTIVE

To establish if the effects of ESC + ARI can be altered by modulating the 5-HT_1a receptor.

METHODS

Sprague-Dawley male rats received ESC + ARI (10 and 2 mg/kg/day, respectively, via osmotic or by cumulative injections), as well as the 5-HT_1a antagonist WAY-100635 (WAY; 0.01-1 mg/kg) and the 5-HT_1a agonist 8-OH-DPAT (DPAT; 0.3-1 mg/kg) prior to testing in locomotion chambers and in the forced swim test (FST). Expression of the 5-HT_1a receptor mRNA in the dorsal raphe nucleus, hippocampus, septum, and entorhinal cortex was also assessed.

RESULTS

WAY generally synergized, while DPAT antagonized, the effect of ESC + ARI on motor activity. All groups showed significantly lower 5-HT_1a mRNA in the dorsal raphe nucleus. In the hippocampus, ESC + ARI and WAY + ESC + ARI groups displayed equivalent elevations of 5-HT_1a mRNA, but this was not observed in groups that received DPAT + ESC + ARI. Finally, the addition of ARI to ESC augmented the effect that ESC alone had on reducing immobility in the FST. Importantly, WAY antagonized this effect, while DPAT had no consequences.

CONCLUSIONS

Taken together, these results in rats indicate that the 5-HT_1a receptor is involved in the behavioral and brain region-specific mRNA effects of ESC + ARI.

A dose-response study of separate and combined effects of the serotonin agonist 8-OH-DPAT and the dopamine agonist quinpirole on locomotor sensitization, cross-sensitization, and conditioned activity

Chronic treatment with the dopamine D2/D3 agonist, quinpirole, or the serotonin 1A agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), induces behavioral sensitization. It is not known whether both drugs produce sensitization through a shared mechanism. Here, we examine whether quinpirole and 8-OH-DPAT show cross-sensitization and impact sensitization, as would be expected from shared mechanisms. Male rats (N=208) were assigned randomly to 16 groups formed by crossing four doses of quinpirole (0, 0.03125, 0.0625, or 0.125 mg/kg) with four doses of 8-OH-DPAT (0, 0.03125, 0.625, or 0.125 mg/kg). After a course of 10 drug treatments administered twice per week in locomotor activity chambers, all groups were challenged on separate tests with quinpirole (0.1 mg/kg), 8-OH-DPAT (0.1 mg/kg), or saline, and locomotor activity was evaluated. Challenge tests with quinpirole and 8-OHDPAT showed no cross-sensitization between the drugs. Chronic quinpirole (0.125 mg/kg) administration induced a sensitized quinpirole response that was attenuated dose-dependently by chronic 8-OH-DPAT cotreatment. Cotreatment with quinpirole (0.0625 mg/kg) and 8-OH-DPAT (all doses) induced quinpirole sensitization. Chronic 8-OH-DPAT (0.125 mg/kg) induced a sensitized 8-OHDPAT response that was prevented by chronic cotreatment with the lowest but not the highest dose of quinpirole. Cotreatment with 8-OHDPAT (0.0625) and quinpirole (0.125 mg/kg) induced sensitization to 8-OH-DPAT. The saline challenge test showed elevated locomotor activity in chronic quinpirole (0.125 mg/kg) and 8-OHDPAT (0.0625, 0.125 mg/kg) alone groups, and in seven of nine cotreated groups. The absence of cross-sensitization suggests separate mechanisms of sensitization to quinpirole and 8-OH-DPAT. Cotreatment effects suggest that induction of sensitization can be modulated by serotonin 1A and D2/D3 activity.

Efficacy of Low-Dose Buspirone for Restricted and Repetitive Behavior in Young Children with Autism Spectrum Disorder: A Randomized Trial

OBJECTIVES

To determine safety and efficacy of the 5HT1A serotonin partial agonist buspirone on core autism and associated features in children with autism spectrum disorder (ASD).

STUDY DESIGN

Children 2-6 years of age with ASD (N = 166) were randomized to receive placebo or 2.5 or 5.0 mg of buspirone twice daily. The primary objective was to evaluate the effects of 24 weeks of buspirone on the Autism Diagnostic Observation Schedule (ADOS) Composite Total Score. Secondary objectives included evaluating the effects of buspirone on social competence, repetitive behaviors, language, sensory dysfunction, and anxiety and to assess side effects. Positron emission tomography measures of tryptophan metabolism and blood serotonin concentrations were assessed as predictors of buspirone efficacy.

RESULTS

There was no difference in the ADOS Composite Total Score between baseline and 24 weeks among the 3 treatment groups (P = .400); however, the ADOS Restricted and Repetitive Behavior score showed a time-by-treatment effect (P = .006); the 2.5-mg buspirone group showed significant improvement (P = .003), whereas placebo and 5.0-mg buspirone groups showed no change. Children in the 2.5-mg buspirone group were more likely to improve if they had fewer foci of increased brain tryptophan metabolism on positron emission tomography (P = .018) or if they showed normal levels of blood serotonin (P = .044). Adverse events did not differ significantly among treatment groups.

CONCLUSIONS

Treatment with 2.5 mg of buspirone in young children with ASD might be a useful adjunct therapy to target restrictive and repetitive behaviors in conjunction with behavioral interventions.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT00873509.

Whole tobacco smoke extracts to model tobacco dependence in animals

Smoking tobacco is highly addictive and a leading preventable cause of death. The main addictive constituent is nicotine; consequently it has been administered to laboratory animals to model tobacco dependence. Despite extensive use, this model might not best reflect the powerful nature of tobacco dependence because nicotine is a weak reinforcer, the pharmacology of smoke is complex and non-pharmacological factors have a critical role. These limitations have led researchers to expose animals to smoke via the inhalative route, or to administer aqueous smoke extracts to produce more representative models. The aim was to review the findings from molecular/behavioural studies comparing the effects of nicotine to tobacco/smoke extracts to determine whether the extracts produce a distinct model. Indeed, nicotine and tobacco extracts yielded differential effects, supporting the initiative to use extracts as a complement to nicotine. Of the behavioural tests, intravenous self-administration experiments most clearly revealed behavioural differences between nicotine and extracts. Thus, future applications for use of this behavioural model were proposed that could offer new insights into tobacco dependence.

The role of serotonin in drug use and addiction

The use of psychoactive drugs is a wide spread behaviour in human societies. The systematic use of a drug requires the establishment of different drug use-associated behaviours which need to be learned and controlled. However, controlled drug use may develop into compulsive drug use and addiction, a major psychiatric disorder with severe consequences for the individual and society. Here we review the role of the serotonergic (5-HT) system in the establishment of drug use-associated behaviours on the one hand and the transition and maintenance of addiction on the other hand for the drugs: cocaine, amphetamine, methamphetamine, MDMA (ecstasy), morphine/heroin, cannabis, alcohol, and nicotine. Results show a crucial, but distinct involvement of the 5-HT system in both processes with considerable overlap between psychostimulant and opioidergic drugs and alcohol. A new functional model suggests specific adaptations in the 5-HT system, which coincide with the establishment of controlled drug use-associated behaviours. These serotonergic adaptations render the nervous system susceptible to the transition to compulsive drug use behaviours and often overlap with genetic risk factors for addiction. Altogether we suggest a new trajectory by which serotonergic neuroadaptations induced by first drug exposure pave the way for the establishment of addiction.

5-HT1A receptor activation reduces fear-related behavior following social defeat in Syrian hamsters

Social defeat leads to selective avoidance of familiar opponents as well as general avoidance of novel, non-threatening intruders. Avoidance of familiar opponents represents a fear-related memory whereas generalized social avoidance indicates anxiety-like behavior. We have previously shown that serotonin signaling alters responses to social defeat in Syrian hamsters, although it is unclear whether serotonin modulates defeat-induced fear, anxiety, or both. In this study we focus on 5-HT1A receptors, in part, because their activation had been linked to the acquisition of conditioned fear. We hypothesized that pharmacological activation of 5-HT1A receptors prior to social defeat would reduce avoidance of familiar opponents and impair Arc expression in the basolateral amygdala (BLA), but not alter anxiety-like behavior. We administered 8-OH-DPAT, a 5-HT1A receptor agonist, prior to 3, 5-minute social defeats and 24h later exposed hamsters to a social interaction test to measure the conditioned defeat response immediately followed by either a Y-maze test or an open field test. In a separate experiment, we administered 8-OH-DPAT prior to 3, 5-minute social defeats and later removed the brains for Arc immunohistochemistry. Social defeat increased the number of Arc immunopositive cells in the central amygdala (CeA), prelimbic cortex (PL), and BLA, and 8-OH-DPAT treatment reduced Arc immunoreactivity in the PL. These results suggest that 5-HT1A receptor activation impairs the fear memory associated with social defeat, but does not alter defeat-induced anxiety. Overall, 5-HT1A receptor activation may impair Arc expression in select brain regions such as the PL and thereby disrupt the development of a fear memory essential for the conditioned defeat response.

Chronic administration of 5-HT1A receptor agonist relieves depression and depression-induced hypoalgesia

Previous studies have shown that depressed patients as well as animal models of depression exhibit decreased sensitivity to evoked pain stimuli, and serotonin is indicated to be involved in depression-induced hypoalgesia. The purpose of this study was to investigate the potential role of 5-HT1A receptor in the depression-induced hypoalgesia. Acute or chronic administration of 5-HT1A receptor agonist, 8-OH-DPAT, was performed in olfactory bulbectomy (OB) and sham-operated rats. The depression-like behavior and pain thresholds were measured using open-field test and radiant heat thermal pain test, respectively. We found that acute administration of 8-OH-DPAT increased locomotor activity and pain thresholds in the sham rats but had no effect on the OB rats. In contrast, chronic administration of 8-OH-DPAT reduced locomotor activity and pain thresholds and restored them to normal level. Increased pain thresholds were also observed in the sham rats after the chronic administration. These results demonstrated that chronic administration of 8-OH-DPAT reversed the depression-induced decrease in pain sensitivity in rats, suggesting that 5-HT1A receptor may play a role in the depression-associated hypoalgesia.

The role of serotonin in memory: interactions with neurotransmitters and downstream signaling

Serotonin, or 5-hydroxytryptamine (5-HT), is found to be involved in many physiological or pathophysiological processes including cognitive function. Seven distinct receptors (5-HT1-7), each with several subpopulations, have been identified for serotonin, which are different in terms of localization and downstream signaling. Because of the development of selective agonists and antagonists for these receptors as well as transgenic animal models of cognitive disorders, our understanding of the role of serotonergic transmission in learning and memory has improved in recent years. A large body of evidence indicates the interplay between serotonergic transmission and other neurotransmitters including acetylcholine, dopamine, γ-aminobutyric acid (GABA) and glutamate, in the neurobiological control of learning and memory. In addition, there has been an alteration in the density of serotonergic receptors in aging and Alzheimer's disease, and serotonin modulators are found to alter the process of amyloidogenesis and exert cognitive-enhancing properties. Here, we discuss the serotonin-induced modulation of various systems involved in mnesic function including cholinergic, dopaminergic, GABAergic, glutamatergic transmissions as well as amyloidogenesis and intracellular pathways.

Extending therapeutic use of psychostimulants: focus on serotonin-1A receptor

INTRODUCTION

Despite a number of medicinally important pharmacological effects, the therapeutic use of psychostimulants is limited because of abuse potential and psychosis following long term use. Development of pharmacological agents for improving and extending therapeutic use of psychostimulants in narcolepsy, attention deficit hyperactivity disorder, Parkinson's disease, obesity and as cognitive enhancer is an important research imperative. In this regard, one potential target system is the 5-hydroxytryptamine (5-HT; serotonin) neurotransmitter system. The focus of the present article is to evaluate a potential role of 5-HT-1A receptor in the alleviation of abuse potential and psychosis-induced by prescription psychostimulants amphetamines and apomorphine.

METHOD

Synaptic contacts between dopamine systems and 5-HT-1A receptors are traced. Studies on serotonin-1A influences on the modulation of dopamine neurotransmission and psychostimulant-induced behavioral sensitization are accumulated.

RESULTS

Inhibition of amphetamine and apomorphine-induced behavioral sensitization by co administration of 5-HT-1A agonists cannot be explained in terms of direct activation of 5-HT-1A receptors, because activation of pre- as well as postsynaptic 5-HT-1A receptors tends to increase dopamine neurotransmission.

CONCLUSION

Long term use of amphetamine and apomorphine produces adaptive changes in 5-HT-1A receptor mediated functions, which are prevented by the co-use of 5-HT-1A agonists. In view of extending medicinal use of psychostimulants, it is important to evaluate the effects of co-use of 5-HT-1A agonists on potential therapeutic profile of amphetamine and apomorphine in preclinical research. It is also important to evaluate the functional significance of 5-HT-1A receptors on psychostimulant-induced behaviors in other addiction models such as drug self-administration and reinstatement of drug seeking behavior.

The role of the dorsal raphé nucleus in reward-seeking behavior

Pharmacological experiments have shown that the modulation of brain serotonin levels has a strong impact on value-based decision making. Anatomical and physiological evidence also revealed that the dorsal raphé nucleus (DRN), a major source of serotonin, and the dopamine system receive common inputs from brain regions associated with appetitive and aversive information processing. The serotonin and dopamine systems also have reciprocal functional influences on each other. However, the specific mechanism by which serotonin affects value-based decision making is not clear. To understand the information carried by the DRN for reward-seeking behavior, we measured single neuron activity in the primate DRN during the performance of saccade tasks to obtain different amounts of a reward. We found that DRN neuronal activity was characterized by tonic modulation that was altered by the expected and received reward value. Consistent reward-dependent modulation across different task periods suggested that DRN activity kept track of the reward value throughout a trial. The DRN was also characterized by modulation of its activity in the opposite direction by different neuronal subgroups, one firing strongly for the prediction and receipt of large rewards, with the other firing strongly for small rewards. Conversely, putative dopamine neurons showed positive phasic responses to reward-indicating cues and the receipt of an unexpected reward amount, which supports the reward prediction error signal hypothesis of dopamine. I suggest that the tonic reward monitoring signal of the DRN, possibly together with its interaction with the dopamine system, reports a continuous level of motivation throughout the performance of a task. Such a signal may provide "reward context" information to the targets of DRN projections, where it may be integrated further with incoming motivationally salient information.

Effects of 5-HT1A receptor stimulation on D1 receptor agonist-induced striatonigral activity and dyskinesia in hemiparkinsonian rats

Accumulating evidence supports the value of 5-HT1A receptor (5-HT1AR) agonists for dyskinesias that arise with long-term L-DOPA therapy in Parkinson's disease (PD). Yet, how 5-HT1AR stimulation directly influences the dyskinetogenic D1 receptor (D1R)-expressing striatonigral pathway remains largely unknown. To directly examine this, one cohort of hemiparkinsonian rats received systemic injections of Vehicle + Vehicle, Vehicle + the D1R agonist SKF81297 (0.8 mg/kg), or the 5-HT1AR agonist ±8-OH-DPAT (1.0 mg/kg) + SKF81297. Rats were examined for changes in abnormal involuntary movements (AIMs), rotations, striatal preprodynorphin (PPD), and glutamic acid decarboxylase (GAD; 65 and 67) mRNA via RT-PCR. In the second experiment, hemiparkinsonian rats received intrastriatal pretreatments of Vehicle (aCSF), ±8-OH-DPAT (7.5 mM), or ±8-OH-DPAT + the 5-HT1AR antagonist WAY100635 (4.6 mM), followed by systemic Vehicle or SKF81297 after which AIMs, rotations, and extracellular striatal glutamate and nigral GABA efflux were measured by in vivo microdialysis. Results revealed D1R agonist-induced AIMs were reduced by systemic and intrastriatal 5-HT1AR stimulation while rotations were enhanced. Although ±8-OH-DPAT did not modify D1R agonist-induced increases in striatal PPD mRNA, the D1R/5-HT1AR agonist combination enhanced GAD65 and GAD67 mRNA. When applied locally, ±8-OH-DPAT alone diminished striatal glutamate levels while the agonist combination increased nigral GABA efflux. Thus, presynaptic 5-HT1AR stimulation may attenuate striatal glutamate levels, resulting in diminished D1R-mediated dyskinetic behaviors, but maintain or enhance striatal postsynaptic factors ultimately increasing nigral GABA levels and rotational activity. The current findings offer a novel mechanistic explanation for previous results concerning 5-HT1AR agonists for the treatment of dyskinesia.

Effects of repeated exposure to MDMA on 5HT1a autoreceptor function: behavioral and neurochemical responses to 8-OHDPAT

A consistent effect of repeated exposure to 3,4 methylenedioxymethamphetamine (MDMA) is a decrease in the tissue levels of serotonin (5-HT). A variety of behavioural and neurochemical tests were conducted to determine whether the tissue deficits were accompanied by an increased sensitivity of the 5-HT1a autoreceptor. Tests were conducted 2 weeks following MDMA exposure (four injections of 10.0 mg/kg, IP, administered at 2-h intervals in a single day). The response to the 5-HT1a agonist, 8-OHDPAT (0.003-0.5 mg/kg, SC), was assessed using lower lip retraction (LLR), hypoactivity, and 5-hydroxytryptophan (5-HTP) accumulation following decarboxylase inhibition. The 8-OHDPAT produced a dose-dependent increase in LLR and hypoactivity, but these effects were comparable for MDMA and saline pretreated groups. MDMA decreased tissue levels of 5-HT and the accumulation of 5-HTP, but these effects were not reflected in the changes in autoreceptor sensitivity. The data suggest that the decrease in tissue levels of 5-HT produced by MDMA is accompanied by a decrease in tryptophan hydroxylase activity but cannot be explained by supersensitivity of the 5-HT1a autoreceptor.

Effect of chronic l-DOPA treatment on 5-HT(1A) receptors in parkinsonian monkey brain

After chronic use of l-3,4-dihydroxyphenylalanine (l-DOPA), most Parkinson's disease (PD) patients suffer from its side effects, especially motor complications called l-DOPA-induced dyskinesia (LID). 5-HT(1A) agonists were tested to treat LID but many were reported to worsen parkinsonism. In this study, we evaluated changes in concentration of serotonin and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) and of 5-HT(1A) receptors in control monkeys, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkeys, dyskinetic MPTP monkeys treated chronically with l-DOPA, low dyskinetic MPTP monkeys treated with l-DOPA and drugs of various pharmacological activities: Ro 61-8048 (an inhibitor of kynurenine hydroxylase) or docosahexaenoic acid (DHA) and dyskinetic MPTP monkeys treated with l-DOPA+naltrexone (an opioid receptor antagonist). Striatal serotonin concentrations were reduced in MPTP monkeys compared to controls. Higher striatal 5-HIAA/serotonin concentration ratios in l-DOPA-treated monkeys compared to untreated monkeys suggest an intense activity of serotonin axon terminals but this value was similar in dyskinetic and nondyskinetic animals treated with or without adjunct treatment with l-DOPA. As measured by autoradiography with [(3)H]8-hydroxy-2-(di-n-propyl) aminotetralin (8-OH-DPAT), a decrease of 5-HT(1A) receptor specific binding was observed in the posterior/dorsal region of the anterior cingulate gyrus and posterior/ventral area of the superior frontal gyrus of MPTP monkeys compared to controls. An increase of 5-HT(1A) receptor specific binding was observed in the hippocampus of MPTP monkeys treated with l-DOPA regardless to their adjunct treatment. Cortical 5-HT(1A) receptor specific binding was increased in the l-DOPA-treated MPTP monkeys alone or with DHA or naltrexone and this increase was prevented in low dyskinetic MPTP monkeys treated with l-DOPA and Ro 61-8048. These results highlight the importance of 5-HT(1A) receptor alterations in treatment of PD with l-DOPA.

Local modulation of striatal glutamate efflux by serotonin 1A receptor stimulation in dyskinetic, hemiparkinsonian rats

Serotonin 1A receptor (5-HT(1A)R) agonists reduce both L-DOPA- and D1 receptor (D1R) agonist-mediated dyskinesia, but their anti-dyskinetic mechanism of action is not fully understood. Given that 5-HT(1A)R stimulation reduces glutamatergic neurotransmission in the dopamine-depleted striatum, 5-HT(1A)R agonists may diminish dyskinesia in part through modulation of pro-dyskinetic striatal glutamate levels. To test this, rats with unilateral medial forebrain bundle dopamine or sham lesions were primed with L-DOPA (12 mg/kg+benserazide, 15 mg/kg, sc) or the D1R agonist SKF81297 (0.8 mg/kg, sc) until abnormal involuntary movements (AIMs) stabilized. On subsequent test days, rats were treated with vehicle or the 5-HT(1A)R agonist ±8-OH-DPAT (1.0 mg/kg, sc), followed by L-DOPA or SKF81297, or intrastriatal ±8-OH-DPAT (7.5 or 15 mM), followed by L-DOPA. In some cases, the 5-HT(1A)R antagonist WAY100635 was employed to determine receptor-specific effects. In vivo microdialysis was used to collect striatal samples for analysis of extracellular glutamate levels during AIMs assessment. Systemic and striatal ±8-OH-DPAT attenuated L-DOPA-induced dyskinesia and striatal glutamate efflux while WAY100635 reversed ±8-OH-DPAT's effects. Interestingly, systemic ±8-OH-DPAT diminished D1R-mediated AIMs without affecting glutamate. These findings indicate a novel anti-dyskinetic mechanism of action for 5-HT(1A)R agonists with implications for the improved treatment of Parkinson's disease.

Inactivation of the dorsal raphé nucleus reduces the anxiogenic response of rats running an alley for intravenous cocaine

Rats traversing a straight alley once a day for delivery of a single i.v. injection of cocaine develop over trials an ambivalence about entering the goal box. This ambivalence is characterized by the increasing occurrence of "retreat behaviors" where animals leave the start box and run quickly to the goal box, but then stop at the entry point and "retreat" back toward the start box. This unique pattern of retreat behavior has been shown to reflect a form of "approach-avoidance conflict" that stems from the animals' concurrent positive (cocaine reward) and negative (cocaine-induced anxiety) associations with the goal box. Cocaine blocks reuptake of the serotonergic (5-HT) transporter and serotonin has been implicated in the modulation of anxiety. It was therefore of interest to determine whether inactivation of the serotonergic cell bodies residing in the dorsal raphé nucleus (DRN) and projecting to brain areas critical for the modulation of anxiety, would alter the anxiogenic state exhibited by rats running an alley for single daily i.v. injections of 1.0mg/kg cocaine. Reversible inactivation of the DRN was accomplished by intracranial application of a mixed solution of the GABA agonists baclofen and muscimol. While DRN inactivation had no impact on the subjects' motivation to initiate responding (i.e., latencies to leave the start box were unaffected) it reliably reduced the frequency of approach-avoidance retreat behaviors (conflict behavior). These data suggest that inactivation of the dorsal raphé reduces the conflict/anxiety otherwise present in experienced cocaine-seeking animals.

The role of the dorsal raphe nucleus in the development, expression, and treatment of L-dopa-induced dyskinesia in hemiparkinsonian rats

Convergent evidence indicates that in later stages of Parkinson's disease raphestriatal serotonin neurons compensate for the loss of nigrostriatal dopamine neurons by converting and releasing dopamine derived from exogenous administration of the pharmacotherapeutic L-3,4-dihydroxyphenyl-L-alanine (L-dopa). Because the serotonin system is not equipped with dopamine autoregulatory mechanisms, it has been postulated that raphe-mediated striatal dopamine release may fluctuate dramatically. These fluctuations may portend the development of abnormal involuntary movements called L-dopa-induced dyskinesia (LID). As such, it has been hypothesized that reducing the activity of raphestriatal neurons could dampen supraphysiological stimulation of striatal dopamine receptors thereby alleviating LID. To directly address this, the current study employed the rodent model of LID to investigate the contribution of the rostral raphe nuclei (RRN) in the development, expression and treatment of LID. In the first study, dual serotonin/dopamine selective lesions of the RRN and medial forebrain bundle, respectively, verified that the RRN are essential for the development of LID. In a direct investigation into the neuroanatomical specificity of these effects, microinfusions of +/-8-OH-DPAT into the intact dorsal raphe nucleus dose-dependently attenuated the expression of LID without affecting the antiparkinsonian efficacy of L-dopa. These current findings reveal the integral contribution of the RRN in the development and expression of LID and implicate a prominent role for dorsal raphe 5-HT1AR in the efficacious properties of 5-HT1AR agonists.

Inhibition of monoamine oxidases desensitizes 5-HT1A autoreceptors and allows nicotine to induce a neurochemical and behavioral sensitization

Although nicotine is generally considered to be the main compound responsible for addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other substances of abuse. We recently showed that a pretreatment with mixed irreversible monoamine oxidases inhibitors (MAOIs), such as tranylcypromine, triggers a locomotor response to nicotine in mice and allows maintenance of behavioral sensitization to nicotine in rats. Moreover, we showed by microdialysis in mice that behavioral sensitization induced by compounds belonging to main groups of drugs of abuse, such as amphetamine, cocaine, morphine, or alcohol, was underlain by sensitization of noradrenergic and serotonergic neurons. Here, this neurochemical sensitization was tested after nicotine, tranylcypromine, or a mixture of both compounds. Data indicate that, whereas neither repeated nicotine nor repeated tranylcypromine alone has any effect by itself, a repeated treatment with a mixture of nicotine and tranylcypromine induces both behavioral sensitization and sensitization of noradrenergic and serotonergic neurons. The development of neurochemical and behavioral sensitizations is blocked by prazosin and SR46349B [(1Z,2E)-1-(2-fluoro-phenyl)-3-(4-hydroxyphenyl)-prop-2-en-one-O-(2-dimethylamino-ethyl)-oxime hemifumarate], two antagonists of alpha1b-adrenergic and 5-HT(2A) receptors, respectively, but not by SCH23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride], a D(1) receptor antagonist. Finally, we found that pretreatments with WAY 100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-hexane carboxamide trihydrochloride], a 5-HT(1A) receptor antagonist, can also induce a behavioral and neurochemical sensitization to repeated nicotine. Complementary experiments with 8-OHDPAT (8-hydroxy-dipropylamino-tetralin), a 5-HT(1A) receptor agonist, and analysis of 5-HT(1A) receptors expression in the dorsal raphe nucleus after a tranylcypromine injection indicate that MAOIs contained in tobacco desensitize 5-HT(1A) autoreceptors to trigger the strong addictive properties of tobacco.

Cocaine conditioning: reversal by autoreceptor dose levels of 8-OHDPAT

In order to investigate the contribution of serotonergic effects of cocaine to Pavlovian conditioning of cocaine locomotor stimulant effects, two experiments were conducted in which groups of rats (N=10) received cocaine treatments (10 mg/kg) paired or unpaired to placement in an open-field environment. Initially, a cocaine conditioned locomotion stimulant effect was established. Next, additional Coc-P and Coc-UP pairings were carried out in conjunction with pretreatment injections of the 5-HT1A agonist, 8-OHDPAT (0.01, 0.025 and 0.05 mg/kg) or saline. In experiment 1, the Coc-P group which received the saline pretreatment again exhibited conditioning but in the 8-OHDPAT pretreatment Coc-P group conditioning was eliminated. In the second experiment, the protocol of the first experiment was repeated but expanded in the post-conditioning phase to include an 8-OHDPAT plus the 5-HT1A antagonist pretreatment Coc-P group. As in the first experiment, the 8-OHDPAT pretreatment Coc-P group did not exhibit a cocaine conditioned locomotion stimulant effect; whereas, the saline pretreatment Coc-P and the 8-OHDPAT plus WAY-100635 pretreatment Coc-P groups did exhibit the cocaine conditioned locomotion stimulant effect. These findings are consistent with an important role for serotonin in the maintenance of cocaine Pavlovian conditioned effects.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

Adaptations in pre- and postsynaptic 5-HT1A receptor function and cocaine supersensitivity in serotonin transporter knockout rats

RATIONALE

While individual differences in vulnerability to psychostimulants have been largely attributed to dopaminergic neurotransmission, the role of serotonin is not fully understood.

OBJECTIVES

To study the rewarding and motivational properties of cocaine in the serotonin transporter knockout (SERT-/-) rat and the involvement of compensatory changes in 5-HT1A receptor function are the objectives of the study.

MATERIALS AND METHODS

The SERT-/- rat was tested for cocaine-induced locomotor activity, cocaine-induced conditioned place preference, and intravenous cocaine self-administration. In addition, the function and expression of 5-HT1A receptors was assessed using telemetry and autoradiography, respectively, and the effect of 5-HT1A receptor ligands on cocaine's psychomotor effects were studied.

RESULTS

Cocaine-induced hyperactivity and conditioned place preference, as well as intravenous cocaine self-administration were enhanced in SERT-/- rats. Furthermore, SERT-/- rats displayed a reduced hypothermic response to the 5-HT1A receptor agonist 8-OHDPAT. S-15535, a selective somatodendritic 5-HT1A receptor agonist, reduced stress-induced hyperthermia (SIH) in wild-type controls (SERT+/+), while it increased SIH in SERT-/- rats. As 5-HT1A receptor binding was reduced in selective brain regions, these thermal responses may be indicative for desensitized 5-HT1A receptors. We further found that both 8-OHDPAT and S-15535 pretreatment increased low-dose cocaine-induced locomotor activity in SERT-/- rats, but not SERT+/+ rats. At a high cocaine dose, only SERT+/+ animals responded to 8-OHDPAT and S-15535.

CONCLUSION

These data indicate that SERT-/- -associated 5-HT1A receptor adaptations facilitate low-dose cocaine effects and attenuate high-dose cocaine effects in cocaine supersensitive animals. The role of postsynaptic and somatodendritic 5-HT1A receptors is discussed.

Behavior selectively elicited by novel stimuli: modulation by the 5-HT1A agonist 8-OHDPAT and antagonist WAY-100635

The serotonergic system has a broad influence on behavior, but its specific contribution to novel object exploration remains to be examined. Toward this end, we assessed the impact of the 5-HT1A agonist, 8-OHDPAT (0.01-0.05 mg/kg) and the 5-HT1A antagonist, WAY-100635 (0.01-0.05 mg/kg) on novel object exploration in a familiar open-field environment. 8-OHDPAT produced a dose-related inhibition of responding to the novel object, whereas, WAY-100635 treatment induced a dose-related increase in the investigatory response to the novel object. Combined, the effects of WAY and 8-OHDPAT treatments were statistically indistinguishable from saline. In terms of locomotor activity, only the highest dose of 8-OHDPAT (0.05 mg/kg) altered locomotor activity and the effect was inhibitory. These findings provide evidence for an involvement of the serotonergic system in the response to novel stimuli and indicate that this effect can be dissociated from effects on overall activity including locomotor, rearing and grooming behaviors.

Involvement of the serotonin system in L-dopa-induced dyskinesias

The ability of L-dopa to relieve the motor impairments in Parkinson's disease patients declines over time, and side-effects, such as dyskinesias, appear--limiting the use of the drug in the advanced stage of the disease. Serotonergic neurons are able to convert L-dopa to dopamine and to store this neurotransmitter in synaptic vesicles. This peculiarity might be very important in the advanced disease, when most of the dopaminergic neurons have degenerated. Indeed, an increasing body of evidence points to dopamine released as a false neurotransmitter from the serotonin terminals as the main pre-synaptic determinant of L-dopa-induced dyskinesias in animal models of Parkinson's disease. These findings make the serotonin system an intriguing target for anti-dyskinetic therapies.

Maternal fluoxetine treatment decreases behavioral response to dopaminergic drugs in female pups

Since serotonin (5-HT) acts as neurotrophic factor, the use of fluoxetine (FLX) by mothers during pregnancy and/or lactation could disrupt brain development of the progeny. To unveil if maternal FLX exposure could compromise the functional integrity of monoaminergic and GABA-ergic neurotransmission, the behavioral responses of male and female mouse pups to diethylpropion, apomorphine, 8-OH-DPAT and diazepam were evaluated. Swiss dams were gavaged daily with FLX (7.5 mg/kg) or tap water during pregnancy day zero to weaning (postnatal day 21). Pups were evaluated on postnatal day 40. The behavioral response to diethylpropion was assessed in the open-field and drug-induced stereotyped behavior; to apomorphine in the drug-induced stereotyped behavior; to diazepam, in the elevated plus maze test and to 8-OH-DPAT in the open-field and forced swimming tests. Exposure to FLX did not influence any drug-induced behavioral response in males. Conversely, in females, FLX exposure significantly prevented diethylpropion-induced hyperactivity in the open-field and reduced stereotyped behavior induced by diethylpropion and apomorphine. In conclusion, the results showed that maternal exposure to FLX induced in female pups long-lasting decreased dopaminergic-mediated behaviors, suggesting altered development of the dopaminergic system. If this alteration also occurs in humans, female children of women who use FLX during pregnancy and lactation may express dopaminergic behavioral alterations and/or altered responsiveness to psychotropic medications later in life.

Mescaline effects on rat behavior and its time profile in serum and brain tissue after a single subcutaneous dose

RATIONALE

Mescaline is a nonselective serotonin receptor agonist. It has relatively delayed onset of action and prolonged duration. Mescaline attenuates various behavioral parameters in rats; however, no information is available about its pharmacokinetics in rats and its relation to the behavioral changes produced by the drug.

OBJECTIVES

The present study evaluates the spontaneous locomotor activity and sensorimotor gating in relation to mescaline concentrations in the serum and the brain of rats

MATERIALS AND METHODS

Behavioral changes induced by mescaline [10, 20, and 100 mg/kg subcutaneously (s.c.)] were evaluated in an open-field test and testing of the prepulse inhibition of acoustic startle reaction (PPI) 15 and 60 min after drug administration. The time disposition of mescaline 20 mg/kg s.c. in rat serum and brain homogenates was analyzed by gas chromatography-mass spectrometry.

RESULTS

Mescaline produced significant inhibitory effects on locomotion in low doses and a biphasic effect with the highest dose. In the PPI test, only when tested 60 min after drug administration, all doses of mescaline disrupted PPI. Besides the experimental protocol, we have observed that approximately 50% of animals receiving 100 mg/kg died within 12 h post-injection. The serum levels of mescaline rapidly increased within 30 min and subsequently quickly decreased; however, the brain concentrations reached a maximum 1 h after administration and remained high for an additional 60 min.

CONCLUSIONS

Mescaline had a delayed onset of the main behavioral changes in rats compared to other hallucinogens. Behavioral changes correlated with the pharmacokinetics of the drug.

Serotonin neuron transplants exacerbate L-DOPA-induced dyskinesias in a rat model of Parkinson's disease

Clinical trials in patients with Parkinson's disease have shown that transplants of fetal mesencephalic dopamine neurons can form a new functional innervation of the host striatum, but the clinical benefits have been highly variable: some patients have shown substantial recovery in motor function, whereas others have shown no improvement and even a worsening in the 3,4-dihydroxyphenyl-L-alanine (L-DOPA)-induced dyskinetic side effects. Differences in the composition of the grafted cell preparation may contribute to these discrepancies. In particular, the number of serotonin neurons contained in the graft can vary greatly depending on the dissection of the fetal tissue. Importantly, serotonin neurons have the ability to store and release dopamine, formed from exogenously administered L-DOPA. Here, we have evaluated the effect of transplants containing serotonin neurons, or a mixture of dopamine and serotonin neurons, on L-DOPA-induced dyskinesias in 6-hydroxydopamine-lesioned animals. As expected, dopamine neuron-rich grafts induced functional recovery, accompanied by a 60% reduction in L-DOPA-induced dyskinesia that developed gradually over the first 10 weeks. Rats with serotonin-rich grafts with few dopamine neurons, in contrast, showed a progressive worsening of their L-DOPA-induced dyskinesias over time, and no functional improvement. The antidyskinetic effect of dopamine-rich grafts was independent of the number of serotonin neurons present. We conclude that serotonin neurons in the grafts are likely to have a detrimental effect on L-DOPA-induced dyskinesias in cases in which the grafts contain no or few dopamine neurons.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Pharmacologic mechanisms of serotonergic regulation of dopamine neurotransmission

The neurotransmitter dopamine (DA) has a long association with normal functions such as motor control, cognition, and reward, as well as a number of syndromes including drug abuse, schizophrenia, and Parkinson's disease. Studies show that serotonin (5-HT) acts through several 5-HT receptors in the brain to modulate DA neurons in all 3 major dopaminergic pathways. There are at least fourteen 5-HT receptor subtypes, many of which have been shown to play some role in mediating 5-HT/DA interactions. Several subtypes, including the 5-HT1A, 5-HT1B, 5-HT2A, 5-HT3 and 5-HT4 receptors, act to facilitate DA release, while the 5-HT2C receptor mediates an inhibitory effect of 5-HT on DA release. Most 5-HT receptor subtypes only modulate DA release when 5-HT and/or DA neurons are stimulated, but the 5-HT2C receptor, characterized by high levels of constitutive activity, inhibits tonic as well as evoked DA release. This review summarizes the anatomical evidence for the presence of each 5-HT receptor subtype in dopaminergic regions of the brain and the neuropharmacological evidence demonstrating regulation of each DA pathway. The relevance of 5-HT receptor modulation of DA systems to the development of therapeutics used to treat schizophrenia, depression, and drug abuse is discussed. Lastly, areas are highlighted in which future research would be maximally beneficial to the treatment of these disorders.

Effects of osemozotan, ritanserin and azasetron on cocaine-induced behavioral sensitization in mice

Repeated intermittent administration of psychostimulants causes behavioral sensitization in rodents. Previous studies using serotonin (5-HT) receptor ligands show that the 5-HT system is involved in cocaine-induced behavioral sensitization in rats, but the role of the 5-HT system has not been studied in mice. The present study examined the effects of the 5-HT(1A) receptor agonist osemozotan, the 5-HT(2) receptor antagonist ritanserin and the 5-HT(3) receptor antagonist azasetron on cocaine-induced behavioral sensitization in male ddY mice. Repeated administration of cocaine for 7 days enhanced cocaine-induced locomotor activity, and this sensitization was observed even after withdrawal for 7-14 days. Cocaine-induced behavioral sensitization after a 7-day withdrawal was significantly reduced with the coadministration of osemozotan, ritanserin or azasetron with cocaine repeatedly for 7 days. A single injection of osemozotan or ritanserin before cocaine challenge also reduced repeated cocaine-induced behavioral sensitization. However, none of these ligands inhibited cocaine-induced behavioral sensitization, when each drug was administered for 7 days after repeated cocaine administration. These results suggest that the central 5-HT system plays a role in the development and expression, but not maintenance, of behavioral sensitization in cocaine-treated mice.

Determining the region-specific contributions of 5-HT receptors to the psychostimulant effects of cocaine

Cocaine is a drug of abuse that has complex neurochemical and behavioural profiles. When it became evident that models that involve only dopamine do not fully explain the complex effects of cocaine on behaviour, the focus of research expanded to include the 5-hydroxytryptamine (5-HT) system in the brain. The 5-HT system comprises several subtypes of 5-HT receptors, which contribute differentially to the various behavioural effects of cocaine. In this article, we describe which subtypes regulate behaviours that are related to cocaine addiction and how they might provide new therapeutic approaches. Numerous subpopulations of each 5-HT receptor can be distinguished according to their location in the brain. We also discuss how these subpopulations relate to the effects of 5-HT-receptor stimulation at the systemic level. These insights provide a new receptor-based approach for understanding the 5-HT mechanisms that subserve the actions of cocaine and possible pharmacotherapies against cocaine addiction.

Modification of cocaine-induced behavioral and neurochemical effects by serotonin1A receptor agonist/antagonist in mice

Administration of cocaine causes a locomotor stimulant effect and increases extracellular levels of serotonin (5-HT) and dopamine (DA) in the brains of rodents. Previous studies show that 5-HT1A receptor agonist and antagonist modify the cocaine-induced behavioral and neurochemical effects in the rats. However, the role of the 5-HT system on the effects of cocaine has not been studied in the prefrontal cortex. The present study examined in ddY-strain male mice the effects of the 5-HT1A receptor agonist osemozotan and the receptor antagonist WAY100635 on cocaine-induced locomotor stimulant effect and increases in extracellular levels of 5-HT and DA in the prefrontal cortex. The cocaine-induced locomotor stimulant effect was attenuated by osemozotan and enhanced by WAY100635. The cocaine-induced increase in extracellular levels of 5-HT was attenuated by osemozotan, and enhanced by WAY100635. The cocaine-induced increase in extracellular levels of DA was enhanced by osemozotan, but not affected by WAY100635. These results suggest that the prefrontal 5-HT system plays a pivotal role in the locomotor stimulant effect of cocaine in mice.

Stimulus gated cocaine sensitization: Interoceptive drug cue control of cocaine locomotor sensitization

Repeated cocaine treatments typically generate sensitization effects which are environment specific. In this study, we investigated whether drug treatments with highly selective receptor specificity can also function as contextual cues to control the expression of cocaine sensitization effects. Two experiments were conducted in which separate groups of rats (N=10) received ten paired or unpaired cocaine (10.0 mg/kg) treatments. In the experiments, autoreceptor preferring low doses of either the 5-HT1A agonist, 8-OHDPAT (8OH) (0.05 mg/kg) or the D1/D2 agonist apomorphine (APO) (0.05 mg/kg) were administered 20 min prior to cocaine administration and test environment placement (paired treatment). Under these conditions, the drug cues generated by the 8OH/APO treatments were associated with the cocaine stimulant effect in the test environment. The unpaired treatment groups received the same drug treatments but the cocaine was administered after testing, in the homecage. Consequently, for these groups, the 8OH/APO drug cues generated by the drug treatments would not become associated with the cocaine stimulant effect in the test environment. Critically, both 8OH and APO pretreatments elicited equivalent unconditioned response effects which were opposite to the cocaine unconditioned response effects; that is, behavioral inhibition vs. behavioral stimulation. Initially, the 8OH and APO pretreatments prevented the locomotor stimulant effects of cocaine; but, these inhibitory effects were reversed in the paired groups with repeated cocaine treatments, consistent with the emergence of cocaine sensitization effects. In the unpaired 8OH and APO pretreatment groups, behavioral suppression persisted throughout the treatment protocol. Subsequently, paired and unpaired groups were compared in four conditioning/sensitization tests. The conditioning tests included: a saline/saline test; and a 8OH/saline test (Experiment 1); and, a saline/saline test and a APO/saline test (Experiment 2). There were no paired/unpaired group differences in these conditioning tests. The sensitization tests included: a saline/cocaine test; and a 8OH/cocaine test (Experiment 1); and, a saline/cocaine test and a APO/cocaine test (Experiment 2). There were no paired/unpaired group differences in the saline/cocaine test for sensitization but paired/unpaired group differences were found in both the 8OH/cocaine and APO/cocaine sensitization tests. In these tests the paired but not the unpaired groups exhibited cocaine locomotor sensitization effects. Critically when, in an additional test, the pretreatments in the cocaine tests were reversed (i.e., 8OH paired group received APO and APO paired group received 8OH prior to cocaine), then there was no evidence for cocaine sensitization. Since the 8OH/APO pretreatments had equivalent inhibitory response effects, it was the stimulus properties of these drugs which controlled the expression of the cocaine locomotor sensitization effects. These findings support the critical role of associative processes in the stimulus-gating of psychostimulant drug sensitization. Importantly, this report incorporates a new methodology in which context can be specified in terms of highly specific brain receptor targets rather than in terms of global environmental situational cues.

Heightened cocaine-induced locomotor activity in adolescent compared to adult female rats

Initiation and experimentation with illicit drugs often occurs in adolescence. Evidence suggests that adolescent rats are more sensitive to some of the effects of drugs of abuse than adult rats. The present study investigated whether adolescent and adult female Sprague Dawley rats differ in cocaine-induced locomotor activity. Animals were placed in the test environment for 30 minutes, and then administered an intraperitoneal (IP) injection of either cocaine (20mg/kg) or saline (0.9%). Both adult and adolescent animals showed significant increases in locomotor activity as a result of cocaine administration compared to saline controls. Interestingly, cocaine induced significantly more locomotor activity in the adolescent females compared to the adults, demonstrating that cocaine acts differently in developing animals.

Pharmacological inhibition of DA- and 5-HT activity blocks spontaneous and cocaine-activated behavior: reversal by chronic cocaine treatment

Recently it was shown that the combined pretreatment with low autoreceptor preferring dose levels of apomorphine (0.05 mg/kg) and 8-OHDPAT (0.05 mg/kg), which decrease dopaminergic and serotonergic activity, induces a profound behavioral inhibition and also blocks the stimulant effects of cocaine. In two experiments, we report that the acute blockade of spontaneous and cocaine locomotor stimulant effects by pretreatment with 8-OHDPAT (0.05 mg/kg) plus apomorphine (0.05 mg/kg) is dose-dependently (0.0 2.5, 5.0, and 10.0 mg/kg cocaine) reversed with repeated cocaine treatments. Using a paired vs. unpaired Pavlovian conditioning protocol, we found that this reversal by cocaine (10 mg/kg) of the inhibition by the combined 8-OHDPAT plus apomorphine pretreatment occurred for the paired but not the unpaired cocaine treatment. The findings suggest that this reversal of behavioral inhibition is mediated by the transformation of the drug cues generated by 8-OHDPAT and apomorphine into cocaine-conditioned stimuli which can activate behavior.

Evidence that the 5-HT1A autoreceptor is an important pharmacological target for the modulation of cocaine behavioral stimulant effects

The psychostimulant effects of cocaine critically depend on the serotonergic (5-HT) system, of which the 5-HT1A receptor is an essential component. We recently showed divergent contributions of various pre- and postsynaptic 5-HT1A receptor populations to the behavioral effects of cocaine. Here, we further investigate the role of 5-HT1A autoreceptors in the acute and chronic stimulant effects of cocaine using 5-HT1A receptor ligands in autoreceptor preferring doses. In experiment 1, four groups of rats (N = 10) received either saline or the 5-HT1A agonist, 8-OHDPAT (0.05 mg/kg) 20 min prior to a saline or cocaine (10 mg/kg) injection on 9 consecutive days. In experiment 2, six groups (N = 10) were given either saline, the 5-HT1A antagonist, WAY 100635 (0.05 mg/kg) or 8-OHDPAT (0.05 mg/kg) plus WAY 100635 (0.05 mg/kg) 20 min before a saline or cocaine (10.0 mg/kg) treatment on 9 consecutive days. Initially, both the 8-OHDPAT and WAY 100635 pretreatments completely blocked the locomotor stimulant effects of cocaine whereas the combined 8-OHDPAT plus WAY 100635 pretreatment had no effect. In saline treated groups, neither the WAY 100635 nor the 8-OHDPAT plus WAY 100635 pretreatment influenced spontaneous activity levels, whereas the 8-OHDPAT alone severely reduced spontaneous activity. These effects persisted over the course of the 9 test sessions. A different pattern of results was obtained for the cocaine treatment groups. With repeated treatments, the WAY 100635 treatment always blocked the locomotor activation effect of cocaine, whereas the effects of 8-OHDPAT were transformed from an inhibition to an enhancement of cocaine locomotor stimulation. The combined 8-OHDPAT plus WAY 100635 pretreatment did not affect the stimulant effect of cocaine. These findings demonstrate that low dose autoreceptor preferring treatments with a 5-HT1A agonist and antagonist can strongly modify the behavioral stimulant effects of cocaine and suggest that the 5-HT1A autoreceptor may be an important pharmacological target for the development of treatments for cocaine addiction.