5-HT1A agonists and dopamine: the effects of 8-OH-DPAT and buspirone on brain-stimulation reward

Direct serotonin release in humans shapes aversive learning and inhibition

The role of serotonin in human behaviour is informed by approaches which allow in vivo modification of synaptic serotonin. However, characterising the effects of increased serotonin signalling in human models of behaviour is challenging given the limitations of available experimental probes, notably selective serotonin reuptake inhibitors. Here we use a now-accessible approach to directly increase synaptic serotonin in humans (a selective serotonin releasing agent) and examine its influence on domains of behaviour historically considered core functions of serotonin. Computational techniques, including reinforcement learning and drift diffusion modelling, explain participant behaviour at baseline and after week-long intervention. Reinforcement learning models reveal that increasing synaptic serotonin reduces sensitivity for outcomes in aversive contexts. Furthermore, increasing synaptic serotonin enhances behavioural inhibition, and shifts bias towards impulse control during exposure to aversive emotional probes. These effects are seen in the context of overall improvements in memory for neutral verbal information. Our findings highlight the direct effects of increasing synaptic serotonin on human behaviour, underlining its role in guiding decision-making within aversive and more neutral contexts, and offering implications for longstanding theories of central serotonin function.

NLX-112, a highly selective 5-HT1A receptor biased agonist, does not exhibit misuse potential in male rats or macaques

NLX-112 (a.k.a. F13640 or befiradol) exhibits nanomolar affinity, exceptional selectivity and biased agonism at serotonin 5-HT_1A receptors. NLX-112 displays robust analgesic activity in a number of rodent models of pain, and is currently developed as a treatment for l-DOPA-induced dyskinesia (LID) in Parkinson's disease (PD) patients. Noteworthy, PD patients can suffer from comorbid chronic pain, thus necessitating the use of analgesic drugs, such as opioids, which have potential for misuse. Additionally, dopamine agonists used to treat PD can produce cocaine-like effects in preclinical assays of misuse potential. The present study investigated whether NLX-112 possesses misuse potential of its own using two behavioural assays routinely used for this purpose: intracranial self-stimulation (ICSS) in rats, and cocaine discrimination in macaque monkeys. In rats, low doses of NLX-112 (0.03 and 0.1 mg/kg p.o.) did not alter ICSS frequency-rate curves, while higher doses (0.3 and 1.0 mg/kg) shifted the curve to the right and flattened it, i.e., reduced ICSS. As expected, cocaine (10 mg/kg i.p.) shifted the curve to the left, i.e., facilitated ICSS, but NLX-112 (0.03 and 0.1 mg/kg p.o.) did not further enhance cocaine-induced facilitation of ICSS. In monkeys trained to discriminate cocaine (0.4 mg/kg i.m.) from saline, NLX-112 (0.01-0.1 mg/kg p.o.) did not substitute for cocaine. Taken together, these results suggest that NLX-112, at doses displaying anti-dyskinetic activity in rat, marmoset and macaque models of LID, is free from misuse potential. From a translational perspective, this is a desirable property for a compound destined to be used in PD patients, who can suffer from comorbid chronic pain necessitating the use of potentially misused analgesic drugs.

Pindolol Rescues Anxiety-Like Behavior and Neurogenic Maladaptations of Long-Term Binge Alcohol Intake in Mice

Long-term binge alcohol consumption alters the signaling of numerous neurotransmitters in the brain including noradrenaline (NE) and serotonin (5-HT). Alterations in the signaling of these neuronal pathways result in dysfunctional emotional states like anxiety and depression which are typically seen during alcohol withdrawal. Interestingly, studies have demonstrated that the development of alcohol-induced negative affective states is linked to disrupted neurogenesis in the dentate gyrus (DG) region of the hippocampus in alcohol-dependent animals. We have previously shown that modulation of NE and 5-HT activity by pharmacological targeting of β-adrenoreceptors (β-ARs) and 5-HT1A/1B receptors with pindolol reduces consumption in long-term alcohol-consuming mice. Since these receptors are also involved in emotional homeostasis and hippocampal neurogenesis, we investigated the effects of pindolol administration on emotional and neurogenic deficits in mice consuming long-term alcohol (18 weeks). We report that acute administration of pindolol (32 mg/kg) reduces anxiety-like behavior in mice at 24 h withdrawal in the marble-burying test (MBT) and the elevated plus-maze (EPM). We also show that chronic (2 weeks) pindolol treatment (32 mg/kg/day) attenuates alcohol-induced impairments in the density of immature neurons (DCX⁺) but not newborn cells (BrdU⁺) in the hippocampal DG. Pindolol treatment also restores the normal proportion of newborn proliferating cells (BrdU⁺/Ki67⁺/DCX⁻), newborn proliferating immature neurons (BrdU⁺/Ki67⁺/DCX⁺) and newborn non-proliferating immature neurons (BrdU⁺/Ki67⁻/DCX⁺) following long-term alcohol intake. These results suggest that pindolol, through its unique pharmacology may rescue some but not all deficits of long-term alcohol abuse on the brain, adding further value to its properties as a strong pharmaceutical option for alcohol use disorders (AUDs).

Decreased Incentive Motivation Following Knockout or Acute Blockade of the Serotonin Transporter: Role of the 5-HT2C Receptor

Acute pharmacological elevation of serotonin (5-hydroxytryptamine; 5-HT) activity decreases operant responding for primary reinforcers, suggesting that 5-HT reduces incentive motivation. The mechanism by which 5-HT alters incentive motivation is unknown, but parallel evidence that 5-HT2C receptor agonists also reduce responding for primary reinforcers implicates this receptor as a potential candidate. These experiments examined whether chronic and acute disruptions of serotonin transporter (SERT) activity altered incentive motivation, and whether the 5-HT2C receptor mediated the effects of elevated 5-HT on behavior. To assess incentive motivation, we measured responding for three different reinforcers: a primary reinforcer (saccharin), a conditioned reinforcer (CRf), and an unconditioned sensory reinforcer (USRf). In the chronic condition, responding was compared between SERT knockout (SERT-KO) mice and their wild-type littermates. In the acute condition, responding was examined in wild-type mice following treatment with 10 or 20 mg/kg citalopram, or its vehicle. The ability of the selective 5-HT2C antagonist SB 242084 to prevent the effects of SERT-KO and citalopram on responding was subsequently examined. Both SERT-KO and citalopram reduced responding for saccharin, a CRf, and a USRf. Treatment with SB 242084 enhanced responding for a CRf and a USRf in SERT-KO mice and blocked the effects of citalopram on CRf and USRf responding. However, SB 242084 was unable to prevent the effects of SERT-KO or citalopram on responding for saccharin. These results support a powerful inhibitory function for 5-HT in the control of incentive motivation, and indicate that the 5-HT2C receptor mediates these effects of 5-HT in a reinforcer-dependent manner.

Preclinical Abuse Potential Assessment of Flibanserin: Effects on Intracranial Self-Stimulation in Female and Male Rats

INTRODUCTION

Flibanserin is a serotonin receptor subtype 1A agonist and 2A antagonist that has been approved by the Food and Drug Administration for treating female sexual interest and arousal disorder. Little is known about the abuse potential of flibanserin.

AIM

To examine abuse-related effects of flibanserin in rats using an intracranial self-stimulation (ICSS) procedure previously used to evaluate the abuse potential of other drugs.

METHODS

Adult female and male Sprague-Dawley rats with electrodes implanted in the medial forebrain bundle were trained to press a lever for electrical brain stimulation under a "frequency-rate" ICSS procedure. In this procedure, increasing frequencies of brain stimulation maintain increasing rates of responding. Drugs of abuse typically increase (or "facilitate") ICSS rates and produce leftward and upward shifts in ICSS frequency-rate curves, whereas drugs that lack abuse potential typically do not alter or only decrease ICSS rates. Initial studies determined the potency and time course of effects on ICSS produced by acute flibanserin administration (1.0, 3.2 and 10.0 mg/kg). Subsequent studies determined the effects of flibanserin (3.2-18 mg/kg) before and after a regimen of repeated flibanserin administration (5.6 mg/kg/d for 5 days). Effects of the abused stimulant amphetamine (1.0 mg/kg) were examined as a positive control.

MAIN OUTCOME MEASURES

Flibanserin effects on ICSS frequency-rate curves in female and male rats were examined and compared with the effects of amphetamine.

RESULTS

Baseline ICSS frequency-rate curves were similar in female and male rats. Acute and repeated administrations of flibanserin produced only decreases in ICSS rates, and rate-decreasing effects of the highest flibanserin dose (10 mg/kg) were greater in female than in male rats. In contrast to flibanserin, amphetamine produced an abuse-related increase in ICSS rates that did not differ between female and male rats.

CONCLUSION

These results suggest that flibanserin has low abuse potential. In addition, this study suggests that female rats might be more sensitive than male rats to the rate-decreasing effects of high flibanserin doses.

Uncoupling the dopamine D1-D2 receptor complex: a novel target for antidepressant treatment

Depression, or major depressive disorder (MDD), is a serious mental illness that causes substantial worldwide disability. Current antidepressant medications mostly target the serotonin and norepinephrine neurotransmitter systems. These drugs are ineffective in many patients, and there are limited options for treatment-resistant depression. The dopamine neurotransmitter system has recently been identified as another modulator of mood and depressive symptoms, and a recently discovered interaction between the dopamine D1 and D2 receptor may be a novel antidepressant target.

5-HT receptors and reward-related behaviour: a review

The brain's serotonin (5-HT) system is key in the regulation of reward-related behaviours, from eating and drinking to sexual activity. The complexity of studying this system is due, in part, to the fact that 5-HT acts at many receptor subtypes throughout the brain. The recent development of drugs with greater selectivity for individual receptor subtypes has allowed for rapid advancements in our understanding of this system. Use of these drugs in combination with animal models entailing selective reward measures (i.e. intracranial self-stimulation, drug self-administration, conditioned place preference) have resulted in a greater understanding of the pharmacology of reward-related processing and behaviour (particularly regarding drugs of abuse). The putative roles of each 5-HT receptor subtype in the pharmacology of reward are outlined and discussed here. It is concluded that the actions of 5-HT in reward are receptor subtype-dependent (and thus should not be generalized) and that all studied subtypes appear to have a unique profile which is determined by content (e.g. receptor function, localization - both throughout the brain and within the synapse) and context (e.g. type of behavioural paradigm, type of drug). Given evidence of altered reward-related processing and serotonergic function in numerous neuropsychiatric disorders, such as depression, schizophrenia, and addiction, a clearer understanding of the role of 5-HT receptor subtypes in this context may lead to improved drug development and therapeutic approaches.

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.

Differential effects of 5-HT2C receptor ligands on place conditioning and locomotor activity in rats

Effects of the 5-hydroxytryptamine (5-HT)(1A/1B/2C) receptor agonist N-[3-(trifluoromethyl)phenyl] piperazine (TFMPP, 0-3.0 mg/kg s.c.) and the 5-HT2C receptor agonist 8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one (WAY 161503, 0-3.0 mg/kg s.c.) in place conditioning were measured in male Sprague-Dawley rats. Effects of TFMPP, alone and with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-cyclohexanecarboxamine (WAY 100635), the 5-HT(1B) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1,1'-biphenyl-4-carboxamide (GR 127935) or the 5-HT2C receptor antagonist 6-chloro-5-methyl-1-[[2-(2-methylpyrid-3-yloxy)pyrid-5-yl]carbamoyl]indoline (SB 242084) and of WAY 161503 alone and with SB 242084 on locomotor activity were also assessed. Neither TFMPP nor WAY 161503 induced place conditioning. WAY 161503 (1.0 and 3.0 mg/kg s.c.) decreased locomotor activity; SB 242084 (1.0 mg/kg i.p.) blocked this effect. Reduced locomotor activity following TFMPP was blocked by SB 242084 but not WAY 100635 (0.1 mg/kg s.c.) or GR 127935 (3.0 mg/kg s.c.). Behaviourally relevant levels of 5-HT2C receptor stimulation may not exert reinforcing effects, although other studies indicate that such manipulations alter reinforcing effects of drugs of abuse.

Differential effects of intra-midbrain raphe and systemic 8-OH-DPAT on VTA self-stimulation thresholds in rats

RATIONALE

Intra-median raphe nucleus (MRN) administration of the 5-HT(1A) receptor agonist 8-OH-DPAT decreases lateral hypothalamic self-stimulation thresholds and is reported to have biphasic effects following systemic administration. These experiments attempted to extend the previous findings to mesolimbic pathway self-stimulation at ventral tegmental area (VTA) electrodes.

OBJECTIVES

This study was conducted to provide comparative data for systemic and intra-dorsal raphe nucleus (DRN) and intra-MRN effects of 8-OH-DPAT on VTA self-stimulation.

METHODS

Male Sprague-Dawley rats with VTA electrodes were trained to respond for electrical stimulation. Systemic and intra-midbrain raphe 8-OH-DPAT effects on rate-frequency thresholds were measured. Systemic administration of WAY 100635 was used to confirm 5-HT(1A) receptor mediation of 8-OH-DPAT effects.

RESULTS

8-OH-DPAT (0.003-0.3 mg kg(-1) SC) increased rate-frequency thresholds and decreased maximal response rates. WAY 100635 alone (0.0125-0.1 mg kg(-1) SC) did not alter these measures. Intra-DRN and intra-MRN 8-OH-DPAT (5.0 microg) decreased rate-frequency thresholds without altering maximal response rates. Intra-DRN 8-OH-DPAT (0.1-5.0 microg) induced a slight decrease and intra-MRN 8-OH-DPAT a slight increase in locomotor activity. WAY 100635 (0.1 mg kg(-1)) blocked effects of 8-OH-DPAT on VTA self-stimulation.

CONCLUSION

These results confirm threshold-decreasing effects of intra-MRN 8-OH-DPAT and extend this to the DRN and to VTA thresholds. Monophasic dose dependent increases in VTA thresholds following systemic 8-OH-DPAT are not equivalent to reports for hypothalamic self-stimulation. Differences between studies may be attributable to stimulation site and/or differences in threshold measurement procedures. Effects of WAY 100635 in this study indicate 5-HT(1A) receptor mediation of these 8-OH-DPAT effects.

Serotonin and drug reward: focus on 5-HT2C receptors

Pharmacological manipulation of the 5-hydroxytryptamine (5-HT; serotonin) system has long been associated with a regulation of feeding behaviour, however, the initial part of this article reviews evidence that central 5-HT systems similarly modulate reward-related behaviours, particularly drug reward. The second part of this article considers what we believe to be strong emerging pharmacological and genetic evidence that many of these effects are mediated through 5-HT(2C) receptor signalling mechanisms. Finally, we consider the potential for selective 5-HT(2C) agonists as therapies for substance abuse disorders and the medical implications for different 5-HT(2C) receptor isoforms generated by RNA editing.

Effects of 8-OH-DPAT and WAY-100635 on performance on a time-constrained progressive-ratio schedule

RATIONALE

Performance on progressive-ratio schedules has been proposed as a means of assessing the effects of drugs on motivation. We have adopted a mathematical model proposed by Killeen to analyse the effects of drugs acting at 5-HT(1A) receptors on progressive-ratio performance. According to this model, the relationship between response rate and ratio size is described by a bitonic (inverted-U) function. One parameter of the function, a, expresses the motivational or "activating" effect of the reinforcer (duration of activation of responding produced by the reinforcer), whereas another parameter, delta, expresses the minimum time needed to execute a response and is regarded as an index of "motor capacity".

OBJECTIVE

To examine the effect of the selective 5-HT(1A) receptor agonist 8-OH-DPAT [8-hydroxy-2-(di- n-propylamino)tetralin] and the antagonist WAY-100635 [ N-[2-(4-[2-methoxyphenyl]-1-piperazinyl)ethyl]- N-2-pyridinylcyclo-hexanecarboxamide] on progressive-ratio schedule performance.

METHODS

Sixteen rats responded for a food-pellet reinforcer on a time-constrained progressive-ratio schedule (55-min sessions). In phase 1, they received single doses (s.c.) of 8-OH-DPAT (25, 50, 100, 200 microg kg(-1), four treatments at each dose) or the vehicle (0.9% saline solution). In phase 2, they received WAY-100635 (30, 100, 300 microg kg(-1)) according to the same regimen. In phase 3, they received 8-OH-DPAT (100 microg kg(-1)) alone or in combination with WAY-100635 (30 microg kg(-1)). 8-OH-DPAT dose dependently increased the value of a, significant increases being seen with the 50, 100 and 200 microg kg(-1) doses. The highest dose also increased delta. WAY-100635 did not significantly alter either a or delta. WAY-100635 significantly attenuated the effect of 8-OH-DPAT on both a and delta.

CONCLUSIONS

The results suggest that 8-OH-DPAT enhanced the activating effect of the reinforcer (the highest dose may also have induced motor debilitation). The finding that the effect of 8-OH-DPAT on a was attenuated by WAY-100635 implicates 5-HT(1A) receptors in this effect. The results are consistent with previous reports that 8-OH-DPAT facilitates feeding and food-reinforced operant responding in rats and suggest that these effects may be brought about by an increase in food motivation.

Cytokine mediation of experimental heart failure-induced anhedonia

Immune system dysfunction is hypothesized to influence several disease states, including cardiovascular disease and psychological depression. The comorbidity of depression and coronary artery disease may be influenced by immune system-brain interactions involving proinflammatory cytokines. The present studies evaluated an index of depression in a rodent model of heart failure by measuring responses to rewarding electrical brain stimulation, which provides an experimental procedure to operationally define anhedonia in rats. Heart failure led to a rightward shift in the current-response relationship in the brain stimulation paradigm, indicative of reduced rewarding properties of the brain stimulation (i.e., anhedonia). Acute treatment with a tumor necrosis factor antagonist, etanercept, reduced circulating tumor necrosis factor- levels in rats with heart failure and restored responding for electrical brain stimulation. The current findings have implications for the study of pathophysiological mechanisms underlying the association of cardiovascular disease and depression.

Naltrexone and relapse prevention treatment for cocaine-dependent patients

A double-blind, placebo-controlled clinical trial examining the joint action of naltrexone (NTX) in combination with relapse prevention (RP) therapy for the treatment of cocaine dependence was conducted. Eighty-five participants who achieved initial abstinence during the intake evaluation and detoxification phase of the study were randomized into 1 of 4 combined NTX (0 vs. 50 mg) by therapy (RP vs. Drug Counseling) experimental conditions for the 12-week outpatient treatment phase of the study. A random effects regression model to test for group differences on percentage of cocaine-positive urines indicated a significant time by medication by therapy interaction, suggesting less cocaine use over time among subjects receiving RP-50 mg than those in the other conditions. No differences were found for retention or time until first cocaine-positive urine. Naltrexone was well tolerated by participants, with acceptable rates of medication compliance observed. Treatment integrity measures confirmed successful manipulation of the psychotherapy. These results are consistent with the notion that substance use in dependent patients can be reduced with a combination of coping skills training and pharmacologic treatments.

Lateral hypothalamic serotonin inhibits nucleus accumbens dopamine: implications for sexual satiety

Dopamine (DA) is released in several brain areas, including the nucleus accumbens (NAcc), before and during copulation in male rats. DA agonists administered into this area facilitate, and DA antagonists inhibit, numerous motivated behaviors, including male sexual behavior. Serotonin (5-HT) is generally inhibitory to male sexual behavior. We reported previously that 5-HT is released in the anterior lateral hypothalamic area (LHA(A)) and that a selective serotonin reuptake inhibitor microinjected into that area delayed and slowed copulation. Our present results, using high temporal resolution microdialysis, (1) confirm previous electrochemical evidence that extracellular levels of DA increase in the NAcc during copulation and decrease during the postejaculatory interval (PEI) and (2) reveal that LHA(A) 5-HT can inhibit both basal and female-elicited DA release in the NAcc. These findings suggest that the neural circuit promoting sexual quiescence during the PEI includes serotonergic input to the LHA(A), which in turn inhibits DA release in the NAcc. These findings may also provide insights concerning the inhibitory control of other motivated behaviors activated by the NAcc and may have relevance for understanding the sexual side effects common to antidepressant medications.

Neurotransmitter system interactions revealed by drug-induced changes in motivated behavior

The present article reviews studies conducted either in collaboration with Jac Herberg, or in parallel with those studies that used consummatory behavior and responding for intracranial self-stimulation (ICSS) to investigate interactions between neurotransmitter systems. The studies reviewed include investigations of the role of dopamine in 8-OH-DPAT-induced feeding; the role of 5-HT3 receptors in the stimulant and depressant effects of nicotine on responding for ICSS; the interaction of D2 and 5-HT2 antagonists in sucrose consumption, and the differential contributions of alpha2-adrenoceptor and 5-HT2 antagonism to the rapid recovery of ICSS responding from depression produced by atypical neuroleptics. Further studies of the role of alpha2-adrenoceptor antagonism in the pattern of response decrements produced by neuroleptics on schedule-controlled responding for food confirm that the behavioral effects of monoamine interactions vary, depending on the specific receptor subtypes targeted and the behavioral paradigm employed. Consequently, the clinical relevance of findings will crucially depend on the choice of appropriate behavioral measures.

An investigation into the effects of 5-HT agonists and receptor antagonists on ethanol self-administration in the rat

Pharmacological manipulation leading to altered 5-HT function has been widely demonstrated to reduce ethanol intake in free choice tests. The aim of the present study was to examine the effects of a range of compounds known to influence 5-HT neurotransmission, including selective 5-HT receptor agonists and antagonists, on ethanol ingestion and maintained behaviour in an operant self-administration paradigm. Female Sprague-Dawley rats were trained to respond for 8% ethanol (v/v) in a 60-min test by a previously described technique. The number of responses and ethanol reinforcers (dipper deliveries), ethanol consumption (g/kg of body weight), and locomotor activity (LMA) were measured following administration of 5-HT agonists (5-HT, d-fenfluramine, fluoxetine, buspirone, TFMPP, and DOI) and antagonists (metergoline, ritanserin, and ondansetron) 30 min prior to testing. d-Fenfluramine, fluoxetine, buspirone, TFMPP, and DOI all produced a reduction in ethanol ingestion and maintained behaviour at doses that failed to reduce LMA. Conversely, metergoline and ritanserin only reduced ethanol self-administration at doses that concomitantly reduced LMA. 5-HT and ondansetron were without effect on any measure. These results demonstrate that, under the present experimental conditions, activation of central 5-HT1A, 5-HT1B, and 5-HT2 receptors reduced ethanol intake and reinforced behaviour in an operant paradigm.

Acute and chronic fluoxetine treatment decreases the sensitivity of rats to rewarding brain stimulation

The effects of fluoxetine on rewarding brain stimulation were determined in eight Wistar rats using a rate-independent discrete-trial threshold measure. Rats were implanted with bipolar, stainless steel electrodes either into the ventral tegmental area (VTA) or medial forebrain bundle (MFB). Acute administration of fluoxetine significantly raised the reward threshold (decreased sensitivity) at doses of 2.5, 5.0, 10.0, and 20.0 mg/kg, i.p., without altering latency of response. There were no significant differences between VTA and MFB groups. To determine the effects of chronic treatment, daily injections of 5.0 mg/kg fluoxetine were administered to rats for 21 days. Chronic treatment of fluoxetine continued to significantly elevate reward thresholds with no evidence of tolerance. The results of these experiments suggest that fluoxetine does not possess abuse potential and that serotonin produces an inhibitory effect on the mesolimbic dopaminergic reward system. Furthermore, these results suggest that the antidepressant effects of fluoxetine are not the direct result of excitation of brain reward systems, at least in the same manner as abused substances, for example, cocaine.

Serotonergic modulation of 3,4-methylenedioxymethamphetamine (MDMA)-elicited reduction of response rate but not rewarding threshold in accumbal self-stimulation

In a fixed interval 5-s rate-frequency function paradigm with rats, 3,4-methylenedioxymethamphetamine (MDMA; 0.5, 2 and 4 mg/kg) dose-dependently decreased response rate for nucleus accumbens self-stimulation while both D-amphetamine (0.3 and 1 mg/kg) and cocaine (5 and 15 mg /kg) increased response rates. The highest doses of MDMA caused a cessation of responding in many of the rats tested, but in those rats that continued to respond a significant reduction in frequency threshold for self-stimulation was seen. Cocaine and amphetamine dose-dependently reduced frequency threshold in all rats tested. The non-specific serotonin antagonist, methysergide (5 mg/kg), reversed the inhibitory effects of MDMA on response rates and caused all rats to respond following MDMA (4 mg/kg). Methysergide did not affect MDMA's threshold-lowering properties and when administered alone methysergide had not effect on self-stimulation. These results suggest serotonergic involvement in the performance but not reinforcement-modulating effect of MDMA in the self-stimulation paradigm.

Multiple serotonin receptors: opportunities for new treatments for obesity?

Recent progress in the molecular pharmacology of 5-HT receptors and the development of selective ligands for various 5-HT receptor subtypes has advanced our understanding of the role of 5-HT mechanisms in the control of food intake and bodyweight. The most intensively investigated 5-HT receptor subtypes have been the 5-HT1A receptor, the 5-HT1B receptor and the 5-HT2C receptor. The overall pattern of results to date suggests that selective 5-HT2C agonists may be novel anorectic drugs and prove useful in the treatment of obesity. However, a number of issues remain unresolved, particularly regarding potential side-effects, as the 5-HT2C receptor agonist mCPP has been reported to induce anxiety and nausea in humans, actions that would clearly limit its therapeutic utility. In addition, the possible role of recently cloned 5-HT receptor subtypes such as 5-ht5, 5-ht6 and 5-ht7, remains unexplored and the development of selective ligands for these sites has the potential to lead to new treatments for obesity.

Median raphe injections of 8-OH-DPAT lower frequency thresholds for lateral hypothalamic self-stimulation

The selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reduces the activity of brain 5-HT neurons via somatodendritic autoreceptors located in the midbrain raphe nuclei. This action of 8-OH-DPAT results in reduced 5-HT synthesis and release of 5-HT in terminal regions. Previous studies have shown that injecting 8-OH-DPAT into the raphe nuclei stimulates feeding, sexual behaviour, and locomotor activity, and serves as an unconditioned stimulus for inducing a conditioned place preference. This behavioural profile suggests that raphe injections of 8-OH-DPAT facilitate reward-related behaviour. The present study tested this hypothesis by investigating the effects of median raphe injections of 8-OH-DPAT on sensitivity to lateral hypothalamic (LH) self-stimulation. Frequencies required to sustain half-maximal rates of responding were determined following injection of saline or various doses of 8-OH-DPAT (0.2-5 micrograms) into the median raphe. In four rats with accurate injection sites 8-OH-DPAT dose-dependently lowered frequency thresholds by up to 40%. In the remaining rats injection sites were located outside the median raphe, and 8-OH-DPAT either slightly raised or failed to lower frequency thresholds. These results show that 8-OH-DPAT injected into the median raphe facilitates brain stimulation reward, and suggest that acute reductions in 5-HT neurotransmission may enhance sensitivity to rewarding stimuli. The possible interactions between 5-HT neurons and efferent systems utilizing dopamine and acetylcholine as neurotransmitters in mediating this effect are discussed.

The effects of repeated treatment with 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) on the lever press responding of the rat under FI and DRL schedules of food reinforcement

In general, the effects of 8-OH-DPAT on the body temperature of rats or in inducing the 5-HT syndrome show rapid tolerance. However, in contrast, the 8-OH-DPAT-induced increase in the activity of rats in a two-way active avoidance task only occurs after repeated administration, i.e. there is sensitisation. The present study was conducted to examine whether this developing hyperactivity may also be expressed as increased rates of lever press responding, and if so, under which conditions it occurs. Rats were trained to press levers under fixed interval 60-s (FI 60) or differential reinforcement of low rates 20-s or 72-s (DRL20, DRL72) schedules of food reinforcement. Groups of trained rats were then treated daily 5 min before testing with doses of 0.01, 0.1 and 1.0 mg/kg 8-OH-DPAT SC for 10-21 days. In all three procedures, in the first couple of days of drug treatment, 8-OH-DPAT generally suppressed lever pressing in a dose-dependent manner. Thereafter, tolerance to this effect was seen to a greater (DRL20, DRL72) or lesser (FI60) extent. Some evidence for stimulation of low rates of lever press responding was seen after 10 days treatment under FI60, but not in DRL20 or DRL72 during short 30 to 60 min long daytime tests although in the latter case, the rats responded to the stimulating effects of 0.8 mg/kg SC amphetamine administered once at the end of the experiment. However, when rats were allowed to respond under DRL72 testing for 12 h during the night, after 10 days treatment a clear stimulation of lever pressing was observed. This stimulation was not specific to lever pressing, however, since a stimulation of entries into the food tray and licking were also seen. From these results, it may be concluded that the stimulating effect of 8-OH-DPAT after repeated administration may be expressed as increased rates of lever pressing, but not under all conditions in which psychomotor stimulation by amphetamine is seen. The potential for 8-OH-DPAT and related compounds to stimulate motor responding in this way should be taken into account when interpreting the effects of these drugs in animal models of psychiatric disorders.

Beyond specificity: effects of serotonin and serotonergic treatments on psychobiological dysfunction

Serotonin is a ubiquitous neurotransmitter with widespread projections that provide for the involvement of serotonin in the regulation of many biological and psychological functions. A variety of serotonin receptor subtypes exist that mediate overlapping psychobiological functions and that are targets for a new generation of medications. Although these new generation medications appear to possess great biochemical specificity, their actions extend to many psychiatric disorders, reflecting the many interactions of serotonergic subsystems.

Dose-dependent effects of buspirone on behavior and cerebral glucose metabolism in rats

In this study we compared the effects of the anxiolytic buspirone on behavior and regional cerebral metabolic rates for glucose (rCMRglc) with those of the reference serotonin (5-HT)1A agonist 8-hydroxy-2(di-N-propylamino)tetralin (DPAT). Behavioral effects were assessed by scoring the 5-HT syndrome. rCMRglc was measured in 56 brain regions by using the quantitative autoradiographic [14C]2-deoxyglucose technique, at 10 min after i.p. injection of DPAT (1 mg/kg) or buspirone (0.4, 4 and 40 mg/kg) in awake male Fischer-344 rats. Whereas DPAT produced an intense 5-HT syndrome, buspirone had no behavioral effect. A low dose (0.4 mg/kg) of buspirone reduced rCMRglc in 18 brain areas (32%), more markedly in limbic areas and raphe nuclei. These were the only rCMRglc effects buspirone had in common with the potent 5-HT1A agonist DPAT and suggest that low dose buspirone activates preferentially 5-HT1A receptors. Hence, this receptor subtype may mediate buspirone functional effects on the limbic system and, given the role of these brain areas in mood control, possibly buspirone therapeutic actions. High doses (4 and 40 mg/kg) of buspirone produced widespread rCMRglc decreases in 46 (82%) and 44 (79%) of the areas studied and increased rCMRglc in one brain area, the lateral habenula, that was not affected by DPAT or a low dose of buspirone. The topographic distribution and direction of rCMRglc changes by high doses of buspirone differ from those produced by the 5-HT1A agonist DPAT. Instead these changes resemble the rCMRglc effects of dopaminergic D2 antagonists like haloperidol and are consistent with some pharmacological and binding properties of buspirone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of d-fenfluramine and metergoline on responding for conditioned reward and the response potentiating effect of nucleus accumbens d-amphetamine

These studies investigated the effects of the 5-hydroxytryptamine (5-HT) releaser, and re-uptake inhibitor, d-fenfluramine, and the non-selective 5-HT receptor antagonist metergoline, on responding for conditioned reward (CR), and on the potentiation of responding for CR following amphetamine injected into the nucleus accumbens. Water deprived rats were trained to associate a compound stimulus with water delivery during a conditioning phase. During a test phase, water was not delivered but the compound stimulus was delivered according to a random ratio 2 schedule following a response on one of two levers; responding on the other lever was not reinforced. Overall, rats responded at a higher rate on the lever delivering the CR. d-Amphetamine (1, 3 and 10 micrograms) injected into the nucleus accumbens dose-dependently enhanced responding on the CR lever. Treatment with d-fenfluramine (0.5 and 1 mg/kg) reduced responding for the CR, and abolished the potentiating effect of d-amphetamine. Responding on the inactive lever was also reduced by 1 mg/kg but not 0.5 mg/kg d-fenfluramine. The reduction of d-amphetamine's effect on responding for CR was prevented by prior treatment with the 5-HT receptor antagonist metergoline (1 mg/kg). Control experiments showed that changes in thirst and motor performance, as well as deficits in learning ability, cannot account for the effects of d-fenfluramine in this paradigm. In a separate experiment, 1 mg/kg metergoline failed to enhance responding for CR, and to augment the response potentiating effect of a low dose (2 micrograms) of d-amphetamine injected into the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)

Low doses of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH DPAT) increase ethanol intake

Previous work has reported that the 5-hydroxytryptamine (5-HT)1A agonist, 8-hydroxy 2-(di-n-propylamino)tetralin (8-OH DPAT), reduces ethanol intake by rats. However, as 8-OH DPAT reduces 5-HT neurotransmission, these findings are inconsistent with the proposed inhibitory role of central 5-HT neurons on ethanol intake. We examined the effect of 8-OH DPAT on ethanol, water and food intake in rats maintained on a limited access schedule using a lower dose range (6-250 micrograms/kg) and by assessing concomitant changes in behaviour. Low doses of 8-OH DPAT enhanced ethanol intake even when food and water were offered as alternatives. Suppression in ethanol intake was observed at higher doses where elements of the 5-HT syndrome were apparent. Similar observations were made in both fluid and non-fluid deprived water drinking rats, suggesting the latter effect is non-selective. Therefore 8-OH DPAT may both increase or decrease ethanol consumption in the rat depending on the dose used.

Changes in dopamine, serotonin and their metabolites in discrete brain areas of rat offspring after in utero exposure to cocaine or related drugs

The concentrations of dopamine (DA), serotonin (5HT) and their metabolites were quantified in 5 brain areas of rats exposed to saline, cocaine (15 mg/kg b.i.d.), amitriptyline (10 mg/kg), or amfonelic acid (AFA, 1.5 mg/kg) throughout gestation. Male pups from 3 similarly treated dams were fostered to 2 surrogate dams. The process of breeding and rearing was repeated 4 times with new dams to build the groups to 4-12, since only one pup per litter was used for any one measurement. AFA was used to mimic the dopamine (DA) uptake blockade and stimulant properties of cocaine and amitriptyline was used to mimic the other pharmacological effects of cocaine. At postnatal days (PND) 30, 60, and 180, one pup per litter was removed for HPLC analysis of monoamines. A second pup received 0.3 mg/kg haloperidol, catalepsy assessed after 1 hr, and the brain used for analysis. The cataleptic response to haloperidol was unaffected by any prenatal treatment. The striatum from PND 30 cocaine rats had decreased levels of DA without a decrease in DA metabolites. At PND 60 in cocaine exposed rats, DA and DOPAC concentrations were increased, and 5HT levels were decreased in the striatum. The amitriptyline-exposed group exhibited decreased 5HT and 5-HIAA levels in the striatum. The hypothalamus of the cocaine group had lower levels of 5-HIAA, and other brain areas had a trend for lower levels of 5HT and 5-HIAA. At PND 180, DOPAC was increased in the striatum and prefrontal cortex of the cocaine group. Haloperidol-induced altered monoamine metabolism was unaffected by any prenatal treatment at any age. These data suggest that age-related changes in the DA and 5HT neurotransmission systems occur in rats exposed prenatally to cocaine. However, the ability of the dopaminergic system to respond to a challenge by a DA receptor blocker is unaltered by these in utero treatments.

Conditioned place preference induced by microinjection of 8-OH-DPAT into the dorsal or median raphe nucleus

Experiments were conducted to examine the ability of the selective 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to induce a conditioned place preference following peripheral injection, and direct microinjection into the dorsal or median raphe nuclei. An unbiased place preference paradigm was used in which control animals showed no preference for either of two compartments differing in terms of colour (white versus black), floor texture (rough versus smooth) and olfactory cues (no odour versus acetic acid odour). Drug treatments were paired with access to either of the two compartments, and saline injections were paired with access to the other compartment. Rats experiencing a low dose of 8-OH-DPAT (125 micrograms/kg) with a specific compartment demonstrated a significant preference for that compartment over one paired with saline injections. The magnitude of this effect was similar to that observed in rats treated with 1.5 mg/kg d-amphetamine. A significant place preference was found in animals receiving injections of 8-OH-DPAT in the dorsal raphe at 0.1 microgram but not 1 microgram. Animals also displayed a preference for the compartment paired with 1 microgram 8-OH-DPAT injected into the median raphe; lower doses were not effective. These results indicate that the mechanism by which 8-OH-DPAT induces a conditioned place preference involves activation of raphe somatodendritic 5-HT1A autoreceptors, leading to a reduction in 5-HT neurotransmission. This demonstration of the rewarding properties of 8-OH-DPAT, together with previous results showing increased feeding and sexual behaviour following 8-OH-DPAT treatment, strongly suggests an important role for brain 5-HT systems in reward and reinforcement processes.

Serotonergic influences on male sexual behavior of rhesus monkeys: effects of serotonin agonists

Although numerous studies in rats have demonstrated an influence of serotonin (5-HT) on male copulation, no studies have yet to demonstrate whether such a relationship exists in primate species. The present study sought to characterize 5-HT influences on male copulatory behavior of rhesus monkeys by using three different 5-HT agonists: a full 5-HT1A agonist, 8-hydroxy-2-(din-propylamino) tetralin (8-OH-DPAT); a partial 5-HT1A agonist, ipsapirone; and a 5-HT 1C/ID agonist, m-chlorophenylpiperazine (m-CPP). 8-OH-DPAT had a biphasic effect upon ejaculation latency, with low doses (5-10 micrograms/kg) producing a shortening of ejaculation latency (time from initiation of copulation to ejaculation), and the highest dose (100 micrograms/kg) producing a lengthening of ejaculation latency. Intromission frequency (number of intromissions preceding ejaculation) was affected only at 10 micrograms/kg 8-OH-DPAT with monkeys requiring fewer intromissions to ejaculation at this dose. Ipsapirone administration led to a shortening of ejaculation latency at all doses tested (50-800 micrograms/kg), and a reduction in intromission frequency at 200-800 micrograms/kg ipsapirone. Administration of the 5-HT 1C/1D agonist, m-CPP, resulted in an increase in ejaculation latency at 200-400 micrograms/kg m-CPP and mount latency at 400 micrograms/kg m-CPP, but did not affect intromission frequency. In summary, stimulation of 5-HT1A receptors lowered the ejaculatory threshold of the monkeys, while stimulation of 5-HT 1C/1D receptors interfered with copulatory behavior and raised the ejaculatory threshold.(ABSTRACT TRUNCATED AT 250 WORDS)

The discriminative stimulus properties of buspirone involve dopamine-2 receptor antagonist activity

To investigate a dopaminergic component in the discriminative stimulus properties of buspirone, rats were trained to discriminate 2.5 mg/kg buspirone from saline, using a two lever, food-rewarded, fixed ratio 10 operant procedure. To test the dopamine-2 (D2) antagonist action of buspirone, a second group of rats was trained to discriminate 0.16 mg/kg apomorphine from saline. In addition to a complete generalization to 8-OH-DPAT, the D2 antagonists haloperidol, R 79598 and sulpiride showed a partial generalization to buspirone. The benzodiazepine ligands chlordiazepoxide and bretazenil did not generalize to the buspirone cue. Buspirone (2.0 mg/kg) completely blocked the apomorphine cue in the apomorphine trained rats. Haloperidol, R 79895 and sulpiride also blocked the apomorphine cue, although at doses much smaller than the doses needed to evoke buspirone responding in the buspirone trained group. 8-OH-DPAT did not antagonize apomorphine. It was concluded that the D2 action of buspirone partially contributes to its discriminative stimulus properties. Mediation of the buspirone cue by 5-HT1a receptor activation seemed predominant. Further, buspirone can act as a full D2 antagonist in drug discrimination. A model was proposed suggesting a compound discriminative stimulus complex of buspirone with a dominant 5-HT1a component that overshadows a less pronounced D2 component.

Schedule-controlled brain self-stimulation: has it utility for behavioral pharmacology?

We review evidence that schedule-controlled intracranial self-stimulation (ICSS) has properties in common with conventional reinforcements, such as food and water, but unlike the latter, animals will respond for ICSS for long periods of time at a near-constant rate. Schedule-controlled ICSS has proven to be more sensitive to drug-induced changes than has ICSS on a continuous reinforcement schedule, and it permits a more fine-grained analysis of the pattern of responding that results in the reinforcement. Evidence is accumulating that the schedule of ICSS itself leads to neurochemical changes in areas of the brain, such as the nucleus accumbens, in which reward processes occur. Results obtained from schedule-controlled ICSS would complement those obtained by drug self-administration studies which generally use intermittent reinforcement. A systematic examination of ICSS schedules at different brain sites would greatly facilitate our interpretation of drug effects and this would have utility for behavioral pharmacology.

Opiate antagonists inhibit feeding induced by 8-OH-DPAT: possible mediation in the nucleus accumbens

Previous work has shown that the 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) elicits a variety of behaviours, including feeding in rats. These effects are accompanied by reduced 5-HT neurotransmission resulting from activation of somatodendritic 5-HT receptors located in the midbrain raphe nuclei. Dopamine antagonists injected either peripherally or into the nucleus accumbens reverse 8-OH-DPAT-induced feeding. Thus a facilitation of dopamine activity, secondary to reduced 5-HT activity, may be involved in mediating 8-OH-DPAT-induced feeding. Opiate antagonists have been shown previously to reduce several dopamine-dependent behaviours including feeding induced by dopaminergic drugs, tail pinch and electrical brain stimulation. Therefore experiments were conducted to assess the effects of opiate antagonists on feeding induced by peripheral, and raphe injection of 8-OH-DPAT in free-feeding rats. Following SC injection naloxone (0.1-10 mg/kg) dose-dependently reduced the feeding response induced by 100 micrograms/kg 8-OH-DPAT (SC). The lowest effective dose of naloxone was 1 mg/kg. This dose of naloxone also suppressed feeding induced by 8-OH-DPAT injected into either the dorsal (1 microgram) or median (0.5 micrograms) raphe. Microinjecting 2 micrograms naloxone together with 8-OH-DPAT into either of these sites failed to prevent the increased feeding. These results indicate that the effects of naloxone are mediated at sites distal to the raphe nuclei. One possible site may be the nucleus accumbens, since methyl-naltrexone (0.3, 1 or 3 micrograms) injected into this site blocked the feeding responses to intra-raphe 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine receptor blockade in nucleus accumbens or caudate nucleus differentially affects feeding induced by 8-OH-DPAT injected into dorsal or median raphe

The 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) elicits a variety of behaviours including feeding in rats. These effects are accompanied by a reduction in 5-HT neurotransmission resulting from activation of somatodendritic 5-HT receptors located in the midbrain raphe nuclei. Previous work showing that dopamine receptor antagonists attenuate 8-OH-DPAT-induced feeding indicates that a facilitation of dopamine activity, secondary to reduced 5-HT activity, is involved in the expression of this effect. Microinjection studies were conducted to explore further the nature of this 5-HT-dopamine interaction. Injection of 8-OH-DPAT (0.125-2 micrograms) into either dorsal or median raphe induced dose-dependent increases in 1 h food intake in non-deprived rats. Pretreatment with haloperidol (0.05 and 0.1 mg/kg s.c.) attenuated the effect induced by median raphe 8-OH-DPAT (0.5 microgram) complementing previous results with dorsal raphe 8-OH-DPAT. The feeding resulting from dorsal raphe (1 microgram) or median raphe (0.5 microgram) 8-OH-DPAT was attenuated by alpha-flupenthixol (1.25 and 2.5 micrograms) injected into the nucleus accumbens. alpha-Flupenthixol in either the dorsolateral or ventrolateral aspects of the caudate nucleus attenuated also the feeding response to dorsal raphe, but not median raphe, 8-OH-DPAT. However, alpha-flupenthixol in the dorsomedial caudate failed to alter feeding resulting from dorsal raphe 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of taste and food texture on the feeding responses induced by 8-OH-DPAT and gepirone

Previously it has been shown that 5-hydroxy-tryptamine (5-HT)1A agonists such as 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and gepirone increase food intake in free-feeding rats. These experiments were conducted to examine the possible influence of taste and textural factors on the feeding responses induced by these two drugs. Separate groups of non-water-deprived rats were given access to one of a variety of different solutions of saccharin (0.02, 0.04, 0.20 and 2.0% w/v) or water for 2 h each day. Rats were then treated with different doses of 8-OH-DPAT (10, 60 or 100 micrograms/kg) or gepirone (1 or 2.5 mg/kg) in a repeated measures design. Under saline injection an inverted-U shaped concentration-response curve was obtained, with the highest level of intake occurring in rats drinking from the 0.20% saccharin solution. The highest doses of 8-OH-DPAT and gepirone suppressed drinking of saccharin, particularly over the first 30 min of the test period, leading to a flattening of the concentration response curve. At 2 h post-injection 60 micrograms/kg 8-OH-DPAT enhanced the consumption of the 0.04% saccharin solution only. In a second experiment, 8-OH-DPAT or gepirone was administered to rats eating either standard pelleted chow or the same food presented in powdered form. Both drugs stimulated feeding. However, interactions with food type were found. At 60 and 100 micrograms/kg 8-OH-DPAT increased eating of both food types equally, but with 500 micrograms/kg rats are significantly more of the pelleted food.(ABSTRACT TRUNCATED AT 250 WORDS)