Psychedelics: Threshold of a Therapeutic Revolution

This Special Issue of Neuropharmacology on psychedelics provides a timely and comprehensive update on progress following the previous Neuropharmacology Special Issue "Psychedelics: New Doors, Altered Perceptions". Remarkable advances have been made in basic and clinical research on psychedelics in the five years since 2018. It is partly based on the seminar series focused on psilocybin organized by the National Institutes of Health (NIH), USA from April to June 2021, the "NIH Psilocybin Research Speaker Series". Participants were world leading experts, including scientists, medical practitioners, clinical psychologists and oncologists, and attendees from additional disciplines of patient advocacy, law, government science policy and regulatory policy. To provide a global perspective, their contributions are complemented with reviews by some of the world's most eminent scientists in the field. The US Food and Drug Administration (FDA) has granted two breakthrough therapy designations for psilocybin in treatment resistant depression (TRD) in 2018 and major depressive disorder (MDD) in 2019, as well as for MDMA for the treatment of post-traumatic stress disorder (PTSD) in 2017. Clinical trials are in progress to assess the therapeutic value of psilocybin in MDD and TRD, and in other indications such as cancer-related anxiety and depression, anorexia, PTSD, substance use disorders and various types of chronic pain. The contributors' insights should assist basic and applied science for transition of psychedelics from bench to potential mainstream therapies. The implications are global, because FDA approval of these new medicines will increase international interest and efforts.

Experimental strategies to discover and develop the next generation of psychedelics and entactogens as medicines

Research on classical psychedelics (psilocybin, LSD and DMT) and entactogen, MDMA, has produced a renaissance in the search for more effective drugs to treat psychiatric, neurological and various peripheral disorders. Psychedelics and entactogens act though interaction with 5-HT_2A and other serotonergic receptors and/or monoamine reuptake transporters. 5-HT, which serves as a neurotransmitter and hormone, is ubiquitously distributed in the brain and peripheral organs, tissues and cells where it has vasoconstrictor, pro-inflammatory and pro-nociceptive actions. Serotonergic psychedelics and entactogens have known safety and toxicity risks. For these drugs, the risks been extensively researched and empirically assessed through human experience. However, novel drug-candidates require thorough non-clinical testing not only to predict clinical efficacy, but also to address the risks they pose during clinical development and later after approval as prescription medicines. We have defined the challenges researchers will encounter when developing novel serotonergic psychedelics and entactogens. We describe screening techniques to predict clinical efficacy and address the safety/toxicity risks emerging from our knowledge of the existing drugs: 1) An early-stage, non-clinical screening cascade to pharmacologically characterise novel drug-candidates. 2) Models to detect hallucinogenic activity. 3) Models to differentiate hallucinogens from entactogens. 4) Non-clinical preclinical lead optimisation technology (PLOT) screening to select drug-candidates. 5) Modified animal models to evaluate the abuse and dependence risks of novel psychedelics in Safety Pharmacology testing. Our intention has been to design non-clinical screening strategies that will reset the balance between benefits and harms to deliver more effective and safer novel psychedelics for clinical use. This article is part of the Special Issue on 'National Institutes of Health Psilocybin Research Speaker Series'.

A review of late-stage CNS drug candidates for the treatment of obesity

Obesity is an important causative factor in morbidity, disability and premature death. Increasing levels of obesity will impose enormous health, financial and social burdens on worldwide society unless effective interventions are implemented. For many obese individuals, diet and behavioural modification need to be supplemented by pharmacotherapy. Preclinical research has revealed a greater understanding of the complex nature of the hypothalamic regulation of food intake and has generated a wide range of new molecular targets for the development of drug candidates for obesity treatment. As shown by the clinical results that have been obtained with this next generation of therapies, some approaches, for example, fixed-dose drug combinations, have already demonstrated an ability to deliver levels of efficacy that are not achievable with the current antiobesity drug therapies. The regulatory and marketing landscape for development, registration and commercialisation of novel centrally acting drugs for treatment of obesity and related metabolic disorders has changed substantially in recent years. Now a much greater emphasis is placed on tolerability and safety, as well as efficacy. In this review we briefly describe the therapeutic approaches to tackle obesity that are in late-stage clinical development. We then discuss drugs in late-stage development for the treatment of obesity and also future directions.

The neuropharmacology of ADHD drugs in vivo: insights on efficacy and safety

Results from in vivo techniques, especially intracerebral microdialysis in freely-moving rats, have provided insights into potential mechanisms responsible for the efficacy and safety of catecholaminergic drugs for ADHD treatment. The drugs reviewed come from distinct pharmacological classes: psychostimulant releasing agents, eg d-amphetamine; psychostimulant reuptake inhibitors, eg dl-threo-methylphenidate (dl-MPH), and non-stimulant reuptake inhibitors, eg atomoxetine. Psychostimulants, which currently deliver the best efficacy in treating ADHD, exhibit the following characteristics on extraneuronal catecholamine concentrations in rodent brain in vivo: 1) They enhance the efflux and function of both noradrenaline and dopamine in the central nervous system. 2) The increase of dopamine efflux that they produce is not limited to cortical regions. 3) They have a rapid onset of action with no ceiling on drug effect. d-Amphetamine has a mechanism independent of neuronal firing rate, displacing intraneuronal stores of catecholamines, delaying their reuptake and inhibiting catabolism by monoamine oxidase. dl-MPH has an enigmatic, extraneuronal action that is neuronal firing rate-dependent and reuptake transporter-mediated, yet paradoxically, almost as powerful as that of d-amphetamine. In safety terms, these powerful catecholaminergic effects also make the psychostimulants liable for abuse. Since efficacy and safety derive from the same pharmacological mechanisms, it has not yet been possible to separate these two components. However, the development of once-daily psychostimulant formulations and a prodrug, lisdexamfetamine, has improved patient compliance and markedly reduced scope for their diversion/abuse. This review will discuss the in vivo pharmacological profiles of approved catecholaminergic drugs for treatment of ADHD and implications for their clinical efficacy and abuse liability.

The effects of electroconvulsive therapy and antidepressant drugs on monoamine receptors in rodent brain--similarities and differences

Repeated administration to rodents of electroconvulsive shock (ECS) produces changes in brain monoamine biochemistry and function, several of which are also seen after repeated administration of antidepressant drugs. Both repeated ECS and antidepressant drug administration decrease cortical beta-adrenoceptor density and attenuate the alpha 2-adrenoceptor-mediated sedation response to clonidine injection. Neither procedure alters phenylephrine-induced locomotor activity in mice, a measure of alpha 1-adrenoceptor function. Most antidepressant drugs decrease type 2 5-hydroxytryptamine (5-HT2) receptor density in frontal cortex and 5-HT2 receptor-mediated head-twitch behaviour in mice. In contrast, repeated ECS increases both 5-HT2 receptor density and the head-twitch response, making it difficult to propose any simple hypothesis linking changes in this receptor with antidepressant activity. The putative agonist for the 5-HT1A receptor 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) produces a hypothermic response in mice, apparently by acting as an agonist at presynaptic 5-HT1 receptors. Repeated administration of antidepressant drugs and lithium markedly attenuates this hypothermic response. Repeated ECS also attenuates this response, the attenuation lasting for at least 20 days after the last ECS. Repeated ECS, but not antidepressant drug administration, markedly enhances dopamine-mediated behaviour. While the similarities in action between ECS and antidepressant drugs may help explain the therapeutic action of electroconvulsive treatment, the differences may provide clues to the efficacy of this treatment in drug-resistant depressive illness.

Mechanism of the thermogenic effect of Metabolite 2 (BTS 54 505), a major pharmacologically active metabolite of the novel anti-obesity drug, sibutramine

OBJECTIVE

To investigate the pharmacological mechanisms underlying the induction of thermogenesis by Metabolite 2 (M2; BTS 54 505), a major pharmacologically active metabolite of the anti-obesity drug, sibutramine.

DESIGN

Adult female Wistar rats were treated with M2 or vehicle, with or without various monoamine receptor antagonists, prazosin, RS79948, metergoline, propranolol and (+)butaclamol.

MEASUREMENTS

Colonic temperature and food intake at room temperature (21+/-1 degrees C), thermoregulatory behavioural response, operant responding for exogenous heat at -8 degrees C and oxygen consumption at thermoneutrality (29 degrees C).

RESULTS

M2 (10 mg/kg, p.o.) significantly increased colonic temperature during the 4.5 h period following drug administration. This effect was abolished by the non-selective 5-HT receptor antagonist, metergoline (1 mg/kg, p.o.), and alpha(1)-adrenoceptor antagonist, prazosin (1 mg/kg, p.o.), measured at 1.5-2.5 h post-M2 administration, and was partially antagonized by each antagonist at 3.5-4.5 h. The non-selective beta-adrenoceptor antagonist, propranolol (1 mg/kg, p.o.), had no effect on the M2-induced increase in colonic temperature, whereas at 20 mg/kg (p.o.), propranolol partially inhibited the effect of M2 on colonic temperature. By contrast, the selective alpha(2)-adrenoceptor antagonist, RS79948 (1 mg/kg, p.o.), and the D2/D1 receptor antagonist, (+)butaclamol (200 micro g/kg, p.o.), did not alter the effect of M2 on colonic temperature. In the thermoregulatory study, M2 (10 mg/kg, i.p.)-treated rats required significantly less radiant heat at -8 degrees C to maintain body temperature, and this effect was not affected by the D2/D1 receptor antagonist (+)butaclamol (100 micro g/kg(-1), i.p.). The hypophagia induced by M2 (10 mg/kg) measured up to 24 h was partially antagonized by the alpha(1)-adrenoceptor antagonist, prazosin, whereas metergoline, RS79948, propranolol and (+)butaclamol had no effect on M2-induced hypophagia.

CONCLUSION

It is concluded that 5-HT, alpha(1)- and beta(3)-adrenoceptors are involved in the induction of thermogenesis by M2, whereas the hypophagic effect is mainly mediated via alpha(1)-adrenoceptors. These findings are consistent with M2 increasing 5-HT and noradrenaline tone via potent reuptake inhibition which subsequently results in increased efferent sympathetic activity to brown adipose tissue (BAT).

Determination of body composition in conscious adult female Wistar utilising total body electrical conductivity

Total body electrical conductivity (TOBEC) is a noninvasive method for estimating fat free mass (FFM) in live animals. In this study, we have evaluated the use of the Em-Scan SA-3000, which is claimed by the manufacturers to perform better than earlier analysers. Previous studies in rats using these earlier versions of the TOBEC analyser have always used anaesthesia to minimise movement artefacts. As repeated anaesthesia also has the potential to induce artefacts by disrupting food intake, for example, we have also attempted to determine if this TOBEC analyser can be used to predict body composition in conscious adult weight-stable female Wistar rats. A simplified cafeteria diet was used to produce large variations in body composition (40-350 g fat/carcass) and a full chemical body composition analysis was performed to generate a TOBEC calibration equation. The TOBEC parameter was more strongly correlated to FFM (r(2)=.785) than it was to body weight (r(2)=.669) or other body composition parameters. Using the TOBEC calibration equation to predict fat mass on these data, there was an excellent correlation with the value obtained by chemical analyses (r(2)=.952, slope=0.958). To determine if the TOBEC calibration equation derived from this calibration study would then be useful for the routine estimation of body composition an additional, validation study was performed. This validation study was performed 6 months later, used an independent group of obese female Wistar rats and was undertaken by different TOBEC operators. This validation study, again, showed a good correlation between the TOBEC- and chemical-derived fat mass (r(2)=.918, slope=1.003) indicating stability of the calibration equation with time and independence from operator. We therefore conclude that it is possible to meaningfully estimate body fat changes in conscious rats using this TOBEC analysis system.

Additive effects on rat brain 5HT release of combining phentermine with dexfenfluramine

OBJECTIVE AND DESIGN

This study examined the effects of the anti-obesity agents, phentermine and dexfenfluramine given alone or in combination, on in vitro and in vivo 5HT release from rat brain tissue.

RESULTS

In vitro, phentermine was without effect on basal [3H]5HT efflux from hypothalamic slices whereas dexfenfluramine (10 microM) evoked a 131% increase in [3H]5HT release. In combination, the two drugs did not alter [3H]5HT release beyond that caused by dexfenfluramine alone. At pharmacologically equivalent doses, phentermine (5.7 mg/kg, i.p.) caused a rapid, modest elevation, and dexfenfluramine (3 mg/kg, i.p.) a larger but equally rapid elevation of extracellular 5HT in the microdialysates from the rat anterior hypothalamus. In combination, the increase in extracellular 5HT evoked by these drugs was not significantly greater than the sum of their individual effects.

CONCLUSIONS

This study provides evidence that phentermine's actions are not restricted to catecholamine systems and indicates that combining phentermine with dexfenfluramine results in an additive increase in neuronal 5HT release.

Monoamine oxidase inhibition is unlikely to be relevant to the risks associated with phentermine and fenfluramine: a comparison with their abilities to evoke monoamine release

OBJECTIVE AND DESIGN

It has been proposed that the anti-obesity agent, phentermine, may act in part via inhibition of monoamine oxidase (MAO). The ability of phentermine to inhibit both MAO(A) and MAO(B) in vitro has been examined along with that of the fenfluramine isomers, a range of selective serotonin reuptake inhibitors and sibutramine and its active metabolites.

RESULTS

In rat brain, harmaline and lazabemide showed potent and selective inhibition of MAO(A) and MAO(B), their respective target enzymes, with IC(50) values of 2.3 and 18 nM. In contrast, all other drugs examined were only weak inhibitors of MAO(A) and MAO(B) with IC(50) values for each enzyme in the moderate to high micromolar range. For MAO(A), the IC(50) for phentermine was estimated to be 143 microM, that for S(+)-fenfluramine, 265 microM and that for sertraline, 31 microM. For MAO(B), example IC(50)s were as follows: phentermine (285 microM), S(+)-fenfluramine (800 microM) and paroxetine (16 microM). Sibutramine was unable to inhibit either enzyme, even at its limit of solubility.

CONCLUSION

We therefore suggest that MAO inhibition is unlikely to play a role in the pharmacodynamic properties of any of the tested drugs, including phentermine. Instead, the lack of potency of these drugs as MAO inhibitors is contrasted with their powerful ability either to inhibit the uptake of one or more monoamines (fluoxetine, paroxetine, sertraline, sibutramine's active metabolites) or to evoke the release of one or more monoamines (S(+)-fenfluramine, S(+)-norfenfluramine, phentermine). These differences in mode of action may be linked to the adverse cardiovascular events experienced with some of the releasing agents.

BTS 72664-- a novel CNS drug with potential anticonvulsant, neuroprotective, and antimigraine properties

BTS 72664, (R)-7-[1-(4-chlorophenoxy)]ethyl]-1,2,4-triazolo(1,5-alpha)pyrimidine, was identified as a drug development candidate from a research program designed to discover novel, broad-spectrum, non-sedative anticonvulsant drugs. BTS 72664 antagonized bicuculline (BIC)- and maximal electroshock (MES)-induced convulsions with ED(50) values of 1.9 and 47.5 mg/kg p.o., respectively. In rodents, it has a wide spectrum of activity preventing seizures induced by picrotoxin, pentylenetetrazol, i.c.v. 4-aminopyridine or NMDA, and audiogenic seizures in DBA-2 mice and GEPR-9 rats. BTS 72664 was also effective in preventing convulsions in amygdala-kindled rats The lack of sedative potential was predicted on the basis of wide separation between ED(50) in anticonvulsant models and TD(50) for motor impairment in mice in rotating rod and inverted horizontal grid tests. BTS 72664 is likely to produce its anticonvulsant effect by enhancing chloride currents through picrotoxin-sensitive chloride channels, and by weak inhibition of Na(+) and NMDA channels. It does not act, however, at the benzodiazepine binding site. In addition to its potential use in the treatment of epilepsy BTS 72664 may be useful in the treatment of stroke. At 50 mg/kg p.o. x 4, given to rats at 12 hourly intervals, starting at 15 min after permanent occlusion of middle cerebral artery (MCA), it reduced cerebral infarct size by 31% (measured at 2 days after insult) and accelerated recovery in a functional behavioral model. BTS 72664 prevented increases in extraneuronal concentrations of glutamate, glycine and serine brain levels induced by a cortical insult to rats (cf. cortical spreading depression). It may, therefore, have also antimigraine activity.

Comparison of the effects of sibutramine and other weight-modifying drugs on extracellular dopamine in the nucleus accumbens of freely moving rats

The acute effects of systemic administration of the anti-obesity agent sibutramine on extracellular dopamine (DA) in the nucleus accumbens of freely moving rats were studied using in vivo microdialysis and compared with the actions of phentermine and d-amphetamine at doses 1x and 3x their respective 2 h ED(50) values to reduce food intake in rats. At the lower dose, sibutramine did not elevate extracellular DA concentrations; however, at the higher dose (6.0 mg kg(-1), i.p.) it caused a modest and prolonged increase in extraneuronal DA. A maximal rise was observed at 60 min post-sibutramine treatment (+231% compared to controls) with DA levels remaining elevated for up to 160 min post treatment. In contrast, phentermine and d-amphetamine significantly enhanced DA efflux at both the lower and higher doses. These elevations of DA levels were significantly greater than that seen with the corresponding dose of sibutramine over 0-80 min post treatment. Maximal rises in DA levels resulting from the higher dose of each drug were +733% (phentermine, 3.9 mg kg(-1), i.p.) and +603% (d-amphetamine, 1.5 mg kg(-1), i.p.) compared to controls 40 min post treatment. The highest doses of phentermine and d-amphetamine increased rat locomotor activity up to 100 min and 160 min post treatment, respectively, whereas the equivalent sibutramine dose had no effect. These findings therefore suggest that dopaminergic reward mechanisms are not involved in the reduction of food intake by sibutramine. Furthermore, they are consistent with the view that sibutramine lacks abuse potential.

Sibutramine does not decrease the number of 5-HT re-uptake sites in rat brain and, like fluoxetine, protects against the deficits produced by dexfenfluramine

The effect of sibutramine and dexfenfluramine on 5-HT re-uptake sites, labelled with [(3)H]paroxetine, have been determined in various rat brain regions. In addition, the ability of fluoxetine and sibutramine to protect against the changes in [(3)H]paroxetine binding produced by dexfenfluramine was examined. Sibutramine (9 mg/kg, p.o.) and dexfenfluramine (1, 3 and 10 mg/kg, p.o.) were administered twice daily (before 09.00 h and after 16.00 h) for four days, followed by a 14 day drug-free period. In the protection studies, fluoxetine (10 mg/kg, i.p.) and sibutramine (9 mg/kg, p.o.) were given 1 h prior to dexfenfluramine (10 mg/kg, p.o.) using the same dosing regimen as described above. Sibutramine (9 mg/kg, p.o.; three times its ED(50) to inhibit food intake at 2 h) had no significant effect on the number or affinity of 5-HT re-uptake sites the brain regions studied. In contrast, dexfenfluramine at an equivalent dose (3 mg/kg, p.o.) significantly decreased the number of 5-HT re-uptake sites in frontal cortex (by 35%), hippocampus (by 47%) and hypothalamus (by 27%). This effect was dose-dependent with marked decreases (by 58-84%) in the number of sites following 10 mg/kg, p.o. These effects were not associated with changes in binding affinity. Fluoxetine (10 mg/kg, i.p.) completely blocked the effect of dexfenfluramine (10 mg/kg, p.o.) without having any significant effect alone. Sibutramine (9 mg/kg, p.o.) also blocked the effects of dexfenfluramine, although the reversal was only partial in frontal cortex, hippocampus and hypothalamus. Thus sibutramine, unlike dexfenfluramine, does not alter brain 5-HT re-uptake sites. Furthermore, sibutramine and fluoxetine protect against the deficits in 5-HT re-uptake sites produced by dexfenfluramine. These data provide further evidence that sibutramine is a 5-HT re-uptake inhibitor and it does not have neurotoxic potential.

A comparison of the acute effects of zotepine and other antipsychotics on rat cortical dopamine release, in vivo

The acute effects of systemic administration of the antipsychotic drug, zotepine, on extracellular dopamine (DA) in the frontal cortex of freely-moving rats were studied using in vivo microdialysis and compared with the actions of clozapine, olanzapine and haloperidol. Treatment with zotepine (1.0 mg/kg, i.p.) resulted in a prolonged elevation of cortical DA levels for up to 180 min post-drug. A maximal rise of +333% was observed at 120 min post-zotepine treatment. Clozapine (10.0 mg/kg, i.p.) also evoked a rise in extracellular DA which was similar in duration (200 min) to that resulting from treatment with zotepine. A maximal rise of +223% was observed at 100 min post-clozapine treatment. Olanzapine (1.0 mg/kg, i.p.) resulted in an immediate increase in DA levels which was maximal 40 min post-treatment (+280%) with levels returning to pre-injection values by 100 min after dosing. In contrast, haloperidol (0.1 mg/kg, i.p.) had no measurable influence on cortical DA levels. Local perfusion with the NA uptake inhibitor, nisoxetine (10 microM), resulted in an increase in cortical DA levels which was maximal at 100 min post-onset of perfusion (+257% above baseline). Administration of zotepine (1.0 mg/kg, i.p.) during nisoxetine perfusion elevated DA levels to a maximum of +301% above baseline, 60 min post-zotepine. These results show that acute administration of each of three drugs with an atypical antipsychotic profile causes an elevation of cortical DA in freely-moving rats at doses relevant to those derived from animal models which predict antipsychotic activity. As a dysfunction in cortical DA is thought to be involved in both the negative symptoms of schizophrenia and cognitive deficits in schizophrenic patients, it is possible that zotepine's ability to elevate cortical DA levels may underlie its effectiveness in successfully treating these components of schizophrenia. Furthermore, the ability of zotepine to elevate cortical DA is more likely to derive from its inhibition of the NA transporter rather than DA receptor blockade in this region.

Modulation of sibutramine-induced increases in extracellular noradrenaline concentration in rat frontal cortex and hypothalamus by alpha2-adrenoceptors

1. The effects of sibutramine (0.25 - 10 mg kg-1 i.p.) on extracellular noradrenaline concentration in the frontal cortex and hypothalamus of freely-moving rats were investigated using microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of sibutramine in these brain areas was also determined. 2. Sibutramine induced an increase in extracellular noradrenaline concentration, the magnitude of which paralleled dose, in both brain areas. In the cortex, this increase was gradual and sustained, whereas in the hypothalamus it was more rapid and of shorter duration. 3. In both the cortex and hypothalamus, pretreatment of rats with the alpha2-adrenoceptor antagonist RX821002 (3 mg kg-1 i.p.) potentiated increases in the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i. p.), by 7 and 10 fold respectively. RX821002 also reduced the latency of sibutramine to reach its maximum effect in the cortex, but not in the hypothalamus. 4. Infusion of RX821002 (1 microM) via the probe increased the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i.p.) in both brain areas. In the hypothalamus, the effects of RX821002 on the accumulation of noradrenaline induced by sibutramine were 2 fold greater than those in the cortex. 5. These findings support evidence that sibutramine inhibits the reuptake of noradrenaline in vivo, but that the accumulation of extracellular noradrenaline is limited by noradrenergic activation of presynaptic alpha2-adrenoceptors. Furthermore, the data suggest that terminal alpha2-adrenoceptors in the hypothalamus exert a greater inhibitory effect over the control of extracellular noradrenaline accumulation than do those in the cortex.

The effects of the peptide-coupling agent, EEDQ, on 5-HT2A receptor binding and function in rat frontal cortex

This ex vivo study in rat frontal cortex determined the influence of 5-HT receptor agonists and antagonists on EEDQ-induced depletion of 5-HT2A binding sites and reduction in their functional coupling to phospholipid hydrolysis. Twenty-four hours after EEDQ (6 mg/kg) administration a marked reduction (66%) of cortical 5-HT2A binding sites with no change in binding affinity was observed. The 5HT2A antagonists ritanserin (1 mg/kg), ketanserin (1 and 5 mg/kg), metergoline (3 mg/kg) or the 5HT2A agonist, DOI (3 and 10 mg/kg) also significantly reduced (by 15-44%) these binding sites 24 h after injection. Thirty minute pretreatment with ritanserin, ketanserin, metergoline or DOI (at the doses above) afforded 49-65% protection against the loss of 5-HT2A binding sites induced by EEDQ (6 mg/kg). DOI (10 mg/kg) pretreatment (-24 h) decreased by 26% the accumulation of [3H]inositol phosphates (IPs) evoked by 5-HT (100 microM), but did not affect that produced by DOI (100 microM). Ketanserin (5 mg/kg, -24 h) decreased 5-HT- and DOI-induced IP formation by 65% and 53%, respectively. The EEDQ (6 mg/kg, -24 h)-evoked reductions (-50%) of 5-HT- and DOI-induced IP formation were not altered by DOI (10 mg/kg) or ketanserin (5 mg/kg) given 30 min before EEDQ. G-protein-stimulated IP accumulation was unaffected by EEDQ (6 mg/kg). Overall, EEDQ reduces 5-HT2A binding sites and function in rat frontal cortex, whereas its effects on binding were attenuated by various 5-HT receptor antagonists and agonists, its effects on function was unaltered by these drugs.

Comparison of changes in the extracellular concentration of noradrenaline in rat frontal cortex induced by sibutramine or d-amphetamine: modulation by alpha2-adrenoceptors

1. The effects of sibutramine (0.25 - 10 mg kg-1, i.p.) on extracellular noradrenaline concentration in the frontal cortex of halothane-anaesthetized rats were compared with those of d-amphetamine (1 - 3 mg kg-1, i.p.) using in vivo microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of these drugs on extracellular noradrenaline concentration were also investigated by pretreating rats with the selective alpha2-adrenoceptor antagonist, RX821002. 2. Sibutramine induced a gradual and sustained increase in extracellular noradrenaline concentration. The dose-response relationship was described by a bell-shaped curve with a maximum effect at 0.5 mg kg-1. In contrast, d-amphetamine induced a rapid increase in extracellular noradrenaline concentration, the magnitude of which paralleled drug dose. 3. Pretreatment with the alpha2-adrenoceptor antagonist, RX821002 (dose 3 mg kg-1, i.p.) increased by 5 fold the accumulation of extracellular noradrenaline caused by sibutramine (10 mg kg-1) and reduced the latency of sibutramine to reach its maximum effect from 144 - 56 min. 4. RX821002-pretreatment increased by only 2.5 fold the increase in extracellular noradrenaline concentration caused by d-amphetamine alone (10 mg kg-1) and had no effect on the latency to reach maximum. 5. These findings support evidence that sibutramine acts as a noradrenaline uptake inhibitor in vivo and that the effects of this drug are blunted by indirect activation of presynaptic alpha2-adreno-ceptors. In contrast, the rapid increase in extracellular noradrenaline concentration induced by d-amphetamine is consistent with this being mainly due to an increase in Ca2+-independent release of noradrenaline.

A comparison of the effects on central 5-HT function of sibutramine hydrochloride and other weight-modifying agents

1. Effects on 5-HT function of sibutramine and its active metabolites, BTS 54 354 and BTS 54 505, were compared with fluoxetine, (+)-fenfluramine and (+)-amphetamine. 2. In vitro sibutramine weakly inhibited [3H]-5-HT uptake into brain synaptosomes. BTS 54 354, BTS 54 505 and fluoxetine were powerful [3H]-5-HT uptake inhibitors, whereas (+)-fenfluramine and (+)-amphetamine were very much weaker. Conversely, whilst sibutramine, its metabolites and fluoxetine did not release [3H]-5-HT from brain slices at < or = 10(-5)M, (+)-fenfluramine and (+)-amphetamine concentration-dependently increased [3H]-5-HT release. 3. Sibutramine and fluoxetine had no effect on 5-hydroxytryptophan (5-HTP) accumulation in either frontal cortex or hypothalamus at doses < 10 mg kg(-1). In contrast, (+)-amphetamine ( > or = 3 mg kg(-1)) reduced 5-HTP in hypothalamus, whilst (+)-fenfluramine (> or =1 mg kg(-1)) decreased 5-HTP in both regions. 4. Sibutramine (10 mg kg(-1) i.p.) and fluoxetine (10 mg kg(-1) i.p.) produced slow, prolonged increases of extracellular 5-HT in the anterior hypothalamus. In contrast, (+)-fenfluramine (3 mg kg(-1) i.p.) and (+)-amphetamine (4 mg kg(-1) i.p.) induced rapid, short-lasting increases in extracellular 5-HT. 5. Only (+)-fenfluramine (10 mg kg(-1)) altered 5-HT2A receptors in rat frontal cortex when given for 14 days, producing a 61% reduction in receptor number and a 18% decrease in radioligand affinity. 6. These results show that sibutramine powerfully enhances central 5-HT function via its secondary and primary amine metabolites; this effect, like that of fluoxetine, is almost certainly mediated through 5-HT uptake inhibition. By contrast, (+)-fenfluramine enhances 5-HT function predominantly by increasing 5-HT release. (+)-Amphetamine, though weaker than (+)-fenfluramine, also enhances 5-HT function by release.

Sibutramine: a novel anti-obesity drug. A review of the pharmacological evidence to differentiate it from d-amphetamine and d-fenfluramine

Sibutramine (BTS 54 524; N-[1-[1-(4-chlorophenyl)cyclobutyl]-3-methylbutyl]-N,N-dimethylamine hydrochloride monohydrate) is a novel 5-HT (serotonin) and noradrenaline reuptake inhibitor (SNRI) anti-obesity drug. Sibutramine reduces the food intake of rodents and this effect is partially or completely reversed by pretreating with 5-HT or noradrenaline antagonists, indicating that both neurotransmitters are involved in sibutramine's hypophagic effect. In addition, fluoxetine and nisoxetine, which are selective reuptake inhibitors of 5-HT and noradrenaline, respectively, have no effect on food intake when given alone, but they profoundly inhibit food intake when given in combination (equivalent to the actions of the SNRI, sibutramine), demonstrating a synergistic interaction of those two monoamines in the control of ingestive behaviour. Sibutramine reduces food intake by enhancing the physiological response of post-ingestive satiety. This reduction of food intake is a CNS-mediated effect because it is induced by intracerebroventricular injection of sibutramine's potently active secondary and primary amine metabolites (BTS 54 354 and BTS 54 505). Sibutramine increases energy expenditure (thermogenesis) in rats. Once again, whilst fluoxetine and nisoxetine have no thermogenic effect when given alone, the combination of these two selective monoamine reuptake inhibitors profoundly enhances thermogenesis, demonstrating a synergistic interaction of 5-HT and noradrenaline neurotransmission in the regulation of energy expenditure. Sibutramine-induced thermogenesis is abolished by administration of a high non-selective dose of atenolol or ICI 118,551 which blocks beta3-adrenoceptors in addition to beta1- and beta2-adrenoceptors, but not by a low dose of atenolol or ICI 118,551 which blocks beta1- and beta2-adrenoceptors, respectively. Glucose utilization studies demonstrate that sibutramine-induced thermogenesis is mediated via selective sympathetic activation of brown adipose tissue, and it is a centrally mediated effect because it is prevented by pretreating the animals with the ganglionic blocker, chlorisondamine. The SNRI mode of action of sibutramine is clearly differentiated from those of the two major classes of anti-obesity drugs, viz, the 5-HT releasing agents, for example, fenfluramine and dexfenfluramine, and the noradrenaline + dopamine-releasing agents, for example, dexamphetamine. In the case of the 5-HT-releasing agents, this mechanism has been linked in animal studies to profound and prolonged depletion and dysfunction of CNS 5-HT neurons. With noradrenaline + dopamine-releasing agents, it is the enhancement of central dopaminergic function which is believed to be responsible for their stimulant, rewarding and reinforcing properties and it is their releasing mechanism which makes them such powerful psychostimulant drugs of abuse. By utilizing noradrenaline and 5-HT for its anti-obesity effects, sibutramine is differentiated from other weight-reducing drugs which act through either 5-HT alone or noradrenaline + dopamine. In addition, sibutramine is further differentiated because it enhances monoamine function by reuptake inhibition, rather than by monoamine release.

GABA-evoked depolarisations in the rat cortical wedge: involvement of GABAA receptors and HCO3- ions

The effect of gamma-aminobutyric acid (GABA) was investigated on cortical wedges prepared from male Sprague-Dawley rats. GABA evoked concentration-dependent depolarisations (EC50: 0.8 mM), which were attenuated by up to 60% when bicarbonate-buffered aCSF was replaced with HEPES-buffered aCSF. Responses to 1 mM GABA were attenuated by (-)-bicuculline and picrotoxin and were potentiated by chlordiazepoxide and pentobarbitone. Ionotropic glutamate receptor antagonists had no effect. We conclude that GABA-evoked depolarisations are mediated via GABAA receptors, arising in part from HCO3- efflux from cells.

Elevation of extracellular cortical noradrenaline may contribute to the antidepressant activity of zotepine: an in vivo microdialysis study in freely moving rats

The antipsychotic, zotepine, as well as possessing affinity for dopamine D1- and D2-1ike receptors, has high affinity for the noradrenaline (NA) transporter and inhibits [3H]NA uptake by rat frontal cortex synaptosomes, in vitro. The present studies investigated the effects of zotepine on extracellular NA in the frontal cortex of freely moving rats using in vivo microdialysis. Removal of calcium from the perfusate reduced extracellular NA by 70.5% and prevented the 50 mM KCl-stimulated increase in NA levels. Zotepine (0.5-1.5 mg kg(-1) i.p.), evoked biphasic, dose-dependent rises in extracellular NA with maximal increases observed at 60 min (+ 171.0%) and 240 min (+ 211.5%) post-treatment. The increases in NA levels were sustained for up to 100 min post-dosing. Clozapine (10.0 mg kg(-1) i.p.), resulted in a smaller, transient increase in NA levels (+ 72.0%) which lasted for 20 min post-treatment. Neither ziprasidone (3.0 mg kg(-1) i.p.) nor olanzapine (1.0 mg kg(-1) i.p.) influenced extracellular NA. Systemic treatment with the antidepressant desipramine (0.3 mg kg(-1) i.p.) resulted in a prolonged elevation of NA levels over 240 min (maximal increase of + 354.3%), whilst local infusion of nisoxetine (1-100 microM) through the dialysis probe increased NA levels in a concentration-dependent manner (up to 587.8% of control values). These data suggest that the inhibition of NA uptake by zotepine and its subsequent prolonged elevation of extracellular cortical NA may underlie the reported antidepressant properties of zotepine in schizophrenic patients.

The anxiogenic agents, yohimbine and FG 7142, disrupt the noradrenergic response to novelty

Whether or not abnormal noradrenergic transmission can be a causal factor in anxiety is controversial. The present experiments examined this question by comparing the effects of two anxiogenic agents on noradrenaline efflux in the frontal cortex of freely moving rats. A single anxiogenic dose of either yohimbine (2.5 or 5 mg/kg) or FG 7142 (10 or 20 mg/kg) was administered to rats by i.p. injection. Yohimbine increased spontaneous efflux of noradrenaline, but FG 7142 had no effect. However, subsequent exposure of rats to a novel environment increased noradrenaline efflux in vehicle-, but not drug-treated rats. Calculation of the net change in noradrenaline efflux caused by transfer to the novel environment showed that this was reduced by yohimbine, whereas FG 7142 increased it. These two compounds also had different effects on locomotor activity in the novel environment. The results suggest that anxiety is unlikely to be invariably associated with increased noradrenergic transmission, in the frontal cortex at least. However, it remains possible that any disruption of the noradrenergic response to stress could be an underlying feature of anxiety.

In vivo criteria to differentiate monoamine reuptake inhibitors from releasing agents: sibutramine is a reuptake inhibitor

Because monoamine reuptake inhibitors and releasing agents both increase extracellular neurotransmitter levels, establishing in vivo experimental criteria for their classification has been difficult. Using microdialysis in the hypothalamus of unanesthetized rats, we provide evidence that serotonin- (5-HT) selective and nonselective reuptake inhibitors can be distinguished from the 5-HT-releasing agent fenfluramine by four criteria: 1) Systemic fenfluramine produces a much greater increase in 5-HT than the reuptake inhibitors. 2) The 5-HT somatodendritic autoreceptor agonist, (+/-)-8-hydroxy-(dipropylamino)tetralin (8-OH-DPAT), attenuates the increase in 5-HT produced by reuptake inhibitors, but not by fenfluramine. 3) The large increase in 5-HT produced by infusion of reuptake inhibitors into the hypothalamus is attenuated by their systemic administration. However, systemic injection of fenfluramine during its local infusion does not attenuate this increase. 4) Reuptake inhibitor pretreatment attenuates fenfluramine-induced increases in 5-HT. According to these criteria, the in vivo effects of the novel antiobesity drug sibutramine are consistent with its characterization as a 5-HT reuptake inhibitor and not a 5-HT releaser. Thus, sibutramine produced increases in hypothalamic 5-HT similar in magnitude to the effects of the known reuptake inhibitors, and the increase was attenuated by 8-OH-DPAT. Also, sibutramine attenuated fenfluramine-induced 5-HT release. Systemic administration of sibutramine failed to attenuate the increase in 5-HT produced by its local infusion, suggesting that this criterion is not applicable to compounds with low affinity for the 5-HT transporter.

Weak blockade of AMPA receptor-mediated depolarisations in the rat cortical wedge by phenytoin but not lamotrigine or carbamazepine

The effects of the anticonvulsants, lamotrigine, phenytoin and carbamazepine, were investigated on NMDA and non-NMDA receptor agonist-evoked responses and against spontaneous epileptiform discharges, in the in vitro rat cortical wedge. Lamotrigine weakly attenuated responses to (RS)-alpha-amino-3-hydroxy-5-methoxy-4-isoxazole propionic acid (AMPA) and quisqualate (IC50 values >> 100 microM), but was without effect on responses to NMDA. Phenytoin weakly, but concentration-dependently, attenuated responses to AMPA and quisqualate, but much less potently attenuated responses to NMDA (IC50 values 163, 248 and >> 300 microM, respectively). Carbamazepine (3-100 microM) significantly attenuated responses to NMDA and at 100 microM attenuated responses to AMPA and quisqualate. These effects were not concentration dependent, with the IC50 values >> 100 microM. Lamotrigine and phenytoin weakly, but concentration-dependently, reduced the frequency (IC50 values 254 and > 300 microM, respectively) and amplitude (IC50 values 141 and > 300 microM, respectively) of spontaneous epileptiform discharges, whereas carbamazepine had no effect. The results show that the anticonvulsant effects of these antiepileptics are unlikely to involve antagonism of ionotropic glutamate receptors, although blockade of non-NMDA responses may play a role in the anticonvulsant profile of phenytoin. Furthermore, the data show that clinically effective anticonvulsants do not necessarily attenuate spontaneous epileptiform discharges in the rat cortical wedge.

Investigation of the mechanisms underlying the hypophagic effects of the 5-HT and noradrenaline reuptake inhibitor, sibutramine, in the rat

1. Sibutramine is a novel 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor (serotonin-noradrenaline reuptake inhibitor, SNRI) which is currently being developed as a treatment for obesity. Sibutramine has been shown to decrease food intake in the rat. In this study we have used a variety of monoamine receptor antagonists to examine the pharmacological mechanisms underlying sibutramine-induced hypophagia. 2. Individually-housed male Sprague-Dawley rats were maintained on reversed phase lighting with free access to food and water. Drugs were administered at 09 h 00 min and food intake was monitored over the following 8 h dark period. 3. Sibutramine (10 mg kg-1, p.o.) produced a significant decrease in food intake during the 8 h following drug administration. This hypophagic response was fully antagonized by the alpha 1-adrenoceptor antagonist, prazosin (0.3 and 1 mg kg-1, i.p.), and partially antagonized by the beta 1-adrenoceptor antagonist, metoprolol (3 and 10 mg kg-1, i.p.) and the 5-HT receptor antagonists, metergoline (non-selective; 0.3 mg kg-1, i.p.); ritanserin (5-HT2A/2C; 0.1 and 0.5 mg kg-1, i.p.) and SB200646 (5-HT2B/2C; 20 and 40 mg kg-1, p.o.). 4. By contrast, the alpha 2-adrenoceptor antagonist, RX821002 (0.3 and 1 mg kg-1, i.p.) and the beta 2-adrenoceptor antagonist, ICI 118,551 (3 and 10 mg kg-1, i.p.) did not reduce the decrease in food intake induced by sibutramine. 5. These results demonstrate that beta 1-adrenoceptors, 5-HT2A/2C-receptors and particularly alpha 1-adrenoceptors, are involved in the effects of sibutramine on food intake and are consistent with the hypothesis that sibutramine-induced hypophagia is related to its ability to inhibit the reuptake of both noradrenaline and 5-HT, with the subsequent activation of a variety of noradrenaline and 5-HT receptor systems.

Comparison of the effects of sibutramine and other monoamine reuptake inhibitors on food intake in the rat

1. The effects of the potent 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor (serotonin-noradrenaline reuptake inhibitor, SNRI), sibutramine, on the cumulative food intake of freely-feeding male Sprague-Dawley rats during an 8 h dark period were investigated and compared to those of the selective 5-HT reuptake inhibitor (selective serotonin reuptake inhibitor, SSRI), fluoxetine; the selective noradrenaline reuptake inhibitor, nisoxetine; the 5-HT and noradrenaline reuptake inhibitors, venlafaxine and duloxetine; and the 5-HT releaser and 5-HT reuptake inhibitor, (+)-fenfluramine. 2. Sibutramine (3 and 10 mg kg-1, p.o.) and (+)-fenfluramine (1 and 3 mg kg-1, p.o.) produced a significant, dose-dependent decrease in food intake over the 8 h dark period. These responses became apparent within the first 2 h following drug administration. 3. Fluoxetine (3, 10 and 30 mg kg-1, p.o.), and nisoxetine (3, 10 and 30 mg kg-1, p.o.) had no significant effect on food intake during the 8 h dark period. However, a combination of fluoxetine and nisoxetine (30 mg kg-1, p.o., of each) significantly decreased food intake 2 and 8 h after drug administration. 4. Venlafaxine (100 and 300 mg kg-1, p.o.) and duloxetine (30 mg kg-1, p.o.) also significantly decreased food intake in the 2 and 8 h following drug administration. 5. The results of this study demonstrate that inhibition of 5-HT and noradrenaline reuptake by sibutramine, venlafaxine, duloxetine, or by a combination of fluoxetine and nisoxetine, markedly reduces food intake in freely-feeding rats and suggest that this may be a novel approach for the treatment of obesity.

8-OH-DPAT-induced mydriasis in mice: a pharmacological characterisation

8-Hydroxy(di-n-propylamino)tetralin (8-OH-DPAT; 0.1-50 mg/kg i.p.) evoked a dose-dependent mydriatic response in conscious mice (ED50 = 5.8 mg/kg i.p.) which was maximal after 10 min. 8-OH-DPAT (2 mg/kg i.p.)-induced mydriasis was attenuated by the alpha 2-adrenoceptor antagonists, idazoxan (1 and 3 mg/kg i.p.) and yohimbine (1 and 3 mg/kg i.p.), by the 5-HT1 receptor antagonists, pindolol (10 mg/kg i.p.) and quipazine (2 mg/kg i.p.), and by the selective 5-HT1A receptor antagonist, (-)-N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenyl propionamide ((-)-WAY 100135; 1-10 mg/kg s.c.). These data argue that both central alpha 2-adrenoceptors and 5-HT1A receptors are involved in the mediation of mydriasis induced by 8-OH-DPAT. The synaptic location of these receptors was determined using either N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4; 100 mg/kg i.p.) or 5,7-dihydroxytryptamine (5,7-DHT; 75 micrograms i.c.v.)+p-chlorophenylalanine (PCPA; 200 mg/kg i.p.); these lesioning procedures respectively produced highly significant losses of whole brain noradrenaline (72% depletion) and 5-HT (78% depletion). The former abolished 8-OH-DPAT (5 mg/kg i.p. (ED50)) mydriasis, whereas the latter was without effect. 8-OH-DPAT (0.5-5 mg/kg i.p.) also dose-dependently increased the noradrenaline metabolite, 3-methoxy-4-hydroxy-phenylglycol (MHPG), in mouse whole brain minus cerebellum. Taken together these results show that 8-OH-DPAT initially stimulates 5-HT1A receptors, and it is likely that this is followed by release of noradrenaline onto postsynaptic alpha 2-adrenoceptors, the latter effect being responsible for the mydriatic response.

Zotepine: preclinical tests predict antipsychotic efficacy and an atypical profile

Zotepine, an atypical antipsychotic structurally similar to clozapine, is in Phase III clinical trials in the United States and Europe for the treatment and management of acute and chronic schizophrenia. Zotepine's pharmacological profile has been compared with those of the atypical antipsychotic clozapine, and the typical neuroleptics haloperidol and chlorpromazine in preclinical tests that predict antipsychotic efficacy and extrapyramidal symptoms (EPS). Because zotepine causes potent, long-lasting inhibition of dopaminergic behavioral responses in animals, it may have an efficacious prolonged antipsychotic action in humans. In contrast, it induces little catalepsy, indicating that it should cause minimal motor side effects, such as EPS. Like clozapine but unlike the typical neuroleptics, zotepine's affinities for cloned human D1 and D2 receptors are very similar. Since a stimulation imbalance favoring D1 over D2 receptors has been suggested to induce dyskinesias, zotepine's reduced EPS profile in humans may derive, in part, from balanced inhibition of these receptors.

[3H]nisoxetine--a radioligand for noradrenaline reuptake sites: correlation with inhibition of [3H]noradrenaline uptake and effect of DSP-4 lesioning and antidepressant treatments

Nisoxetine is a potent and selective inhibitor of noradrenaline uptake into noradrenergic neurones. [3H]Nisoxetine binding to rat frontal cortical membranes was of high affinity. The binding data of both competition and saturation studies fitted a single site binding model. [3H]Nisoxetine binding was potently inhibited by the selective noradrenaline uptake inhibitors desipramine and protriptyline. In addition, a very good correlation was obtained between the ability of 25 monoamine reuptake inhibitors and related compounds both to inhibit [3H]nisoxetine binding and to inhibit [3H]noradrenaline uptake in rat frontal cortex. DSP-4 (10-100 mg/kg, i.p.) dose-dependently depleted cortical noradrenaline concentrations (51-100%), with no significant effects on 5-HT and dopamine. These depletions, which were used as a marker of loss of noradrenergic nerve terminals, were associated with a dose-dependent decrease in the number of [3H]nisoxetine binding sites (20-97%) with no change in binding affinity. Furthermore, a good correlation was obtained between cortical noradrenaline concentrations and the number of [3H]nisoxetine binding sites. These data support the view that [3H]nisoxetine binds to a single population of homogeneous sites associated with the noradrenaline transporter complex. Using this ligand, the effects of repeated administration of both antidepressant drugs with a range of pharmacological actions and of electroconvulsive shock on noradrenaline reuptake sites were examined. The number and affinity of [3H]nisoxetine binding sites were unaltered by all treatments. It is unlikely, therefore, that antidepressant therapy would produce adaptive changes in noradrenaline uptake sites.

Dopamine D2 receptors: a potential pharmacological target for nomifensine and tranylcypromine but not other antidepressant treatments

Treatment for 1 or 14 days by IP injection with the antidepressants, amitriptyline (10 mg/kg), bupropion (30 mg/kg), desipramine (10 mg/kg), GBR 12909 (10 mg/kg), sibutramine HCl (3 mg/kg), mianserin (5 mg/kg), and zimeldine (10 mg/kg), did not affect the number or affinity of dopamine D2 receptors determined by [3H]raclopride binding to rat striatal membranes. Similarly, neither did a single, nor repeated (five times over 10 days), electroconvulsive shock, given under halothane anaesthesia, have any effect on [3H]raclopride binding parameters. By contrast, the noradrenaline and dopamine reuptake inhibitor, nomifensine (5 mg/kg), and the monoamine oxidase inhibitor, tranylcypromine (5 mg/kg), decreased the number of dopamine D2 receptors by 12% and 11%, respectively, when given for 14 days. Administration of the D2 receptor antagonist, haloperidol (1 mg/kg), for 14 days increased the number of [3H]raclopride binding sites by 17%. Thus, the data demonstrate that although nomifensine and tranylcypromine decrease D2 receptor number after 14 days administration, this adaptive change is not observed with other antidepressant treatments. However, the findings do not preclude a contribution of altered dopamine D2 receptor function to the efficacy of those drugs with potent effects on dopaminergic neuronal function.

Receptor binding and functional evidence suggest that postsynaptic alpha 2-adrenoceptors in rat brain are of the alpha 2D subtype

This study has determined the subtype(s) of postsynaptic alpha 2-adrenoceptors in rat brain. This question has been addressed by using two separate approaches, i.e. ligand displacement of [3H]2-(2-methoxy)-1,4-benzodioxan-2-yl)-2-imidazoline ([3H]RX 821002) from membranes prepared from rat cortex after noradrenergic denervation and, secondly, by antagonism of clonidine-induced mydriasis. After rats had been lesioned using N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4; 100 mg/kg i.p., 30 min after zimeldine 10 mg/kg i.p.), noradrenaline was undetectable in the cortex 3 days later. Displacement of [3H]RX 821002 with a range of agonists and antagonists which distinguish between the known alpha 2-adrenoceptor subtypes (alpha 2A-2D) yielded pKi values which correlated very well with reported values for the alpha 2D-adrenoceptor (r = 0.929; P < 0.001), but not the alpha 2A (r = 0.450; P = 0.192), alpha 2B (r = 0.280, P = 0.434) or alpha 2C (r = 0.283; P = 0.460) subtypes. Similarly, the potencies of various alpha 2-adrenoceptor antagonists to inhibit clonidine (0.03 mg/kg i.p.)-induced mydriasis in conscious rats correlated strongly with their pKi values for alpha 2D-adrenoceptors (r = 0.899; P = 0.015) but not alpha 2A-(r = 0.369; P = 0.472), alpha 2B-(r = -0.224; P = 0.670) or alpha 2C-adrenoceptors (r = 0.253; P = 0.584). These data are, therefore, consistent and argue strongly that postsynaptic alpha 2-adrenoceptors in the rat cortex and Edinger-Westphal nucleus are of the alpha 2D subtype.

Long-lasting effects of an acute stress on the neurochemistry and function of 5-hydroxytryptaminergic neurones in the mouse brain

The present experiments investigated the effects of a novel stress challenge (6-min swim test) on behaviour (immobility) and the neurochemistry of cortical 5-hydroxytryptaminergic neurones in CD1 mice. The influence of previous experience of stress (once-daily saline injection) or administration of the noradrenaline and 5-hydroxytryptamine (5-HT) uptake blocker, sibutramine hydrochloride, on any changes was also evaluated. 5-HT2A receptor binding was unchanged 24 h after the last injection of either saline or sibutramine alone but immobility in the swim test was reduced to the same extent by these pretreatments. Seven days, but not 3 h, after the swim test, the density of 5-HT2A receptors and the frequency of 5-HT2A receptor-mediated head-twitches were increased significantly. These increases were prevented by saline injection, but sibutramine prevented the increase in head-twitches only. Sibutramine, but not saline, reduced 5-HT synthesis and 5-HT2A receptor-mediated head-twitches 3 h after the swim and increased synthesis at 7 days. The results indicate that a brief stress can have long-term effects on central 5-hydroxytryptaminergic neurones. Previous experience of stress or sibutramine has marked, but dissimilar, effects on these changes. These findings might be relevant to long-lasting CNS disorders provoked, or aggravated, by stress.

D1 receptor binding in rat striatum: modification by various D1 and D2 antagonists, but not by sibutramine hydrochloride, antidepressants or treatments which enhance central dopaminergic function

[3H]SCH 23390 is a selective high affinity ligand for D1 receptors in vitro. Using this ligand persistent blockade of D1 receptors by SCH 23390 and cis-flupenthixol was shown to significantly increase the number of D1 receptor binding sites in rat striatum. In contrast, repeated administration of the D2-selective antagonist, clebopride, resulted in a small, but significant, reduction in number. No differences in binding affinity were observed and a single dose of these compounds was without effect. The D2-selective antagonist, haloperidol, the non-selective D1/D2 receptor antagonist, chlorpromazine, the dopamine reuptake inhibitors, bupropion, GBR 12909 and nomifensine, and the dopamine releasing agent, d-amphetamine, had no effect on D1 receptors. The antidepressant treatments, desipramine, zimeldine, amitriptyline, tranylcypromine, mianserin and ECS and the monoamine reuptake inhibitor, sibutramine, similarly did not alter striatal D1 sites. Thus, of the treatments investigated only chronic receptor blockade by high affinity antagonists altered D1 receptor binding in rat striatum.

Mediation of mydriasis in conscious rats by central postsynaptic alpha 2-adrenoceptors

The alpha 2-adrenoceptor agonist, clonidine (0.001-1 mg/kg, IP), dose-dependently induced mydriasis in conscious rats (ED50 0.088 mg/kg). This response was maximal when measured 10 min after clonidine injection and was of about 30-min duration. The noradrenaline releasing agent, methamphetamine (0.75 mg/kg, IP), also increased pupil diameter. Clonidine (0.03 mg/kg, IP)-induced mydriasis was inhibited in a dose-related fashion by the alpha 2-adrenoceptor antagonists, idazoxan (0.03-3 mg/kg, IP) and yohimbine (0.03-3 mg/kg, IP), but was unaltered by the alpha 1- or beta-adrenergic antagonists, prazosin (1 and 3 mg/kg, IP) or pindolol (1 and 3 mg/kg, IP). Methamphetamine (0.75 mg/kg, IP)-induced mydriasis was similarly inhibited by idazoxan (1 mg/kg, IP) and yohimbine (1 mg/kg, IP). These data argued strongly that central alpha 2-adrenoceptors are involved in the mediation of mydriasis. The synaptic location of these receptors was determined using DSP-4 (50 mg/kg x 2, IP) to lesion noradrenergic neurones: this produced a 64% depletion of noradrenaline in the midbrain (containing the Edinger-Westphal nucleus responsible for mydriasis) and reduced the mydriatic effect of methamphetamine (0.75 mg/kg, IP) to a similar extent (72%), whereas clonidine mydriasis remained unaltered. Therefore, these results show that the mydriasis responses induced by either clonidine or methamphetamine are mediated by central postsynaptic alpha 2-adrenoceptors.

Evidence for altered alpha 2-adrenoceptor function following isolation-rearing in the rat

This study investigated central alpha 2-adrenoceptor function in differentially reared rats. Rats reared from weaning were either housed singly or in groups of five. Measurements of spontaneous ambulatory activity at 4 weeks postweaning showed that isolates were more hyperactive on exposure to a novel environment than grouped rats. alpha 2-Adrenoceptors were investigated using alpha 2-adrenoceptor agonist-induced behaviours, [3H]-idazoxan binding and measurement of forskolin-stimulated cyclic AMP accumulation. Clonidine (0.001-1.0 mg/kg IP) induced mydriasis in both groups with no difference observed in the response between the isolation and group-reared animals. Clonidine (0.01-0.5 mg/kg IP) induced hypoactivity in both groups, with the effect significantly greater in the isolation-reared rats. Idazoxan markedly attenuated both responses, confirming their induction by alpha 2-adrenoceptor stimulation. Clonidine-induced hypoactivity and mydriasis are mediated by pre- and postsynaptic alpha 2-adrenoceptors, respectively; therefore the results suggest rats reared in isolation have enhanced presynaptic but unchanged postsynaptic alpha 2-adrenoceptor function. Saturation binding experiments using [3H]-idazoxan were undertaken to determine alpha 2-adrenoceptor number (Bmax) and affinity (Kd) in membranes prepared from the frontal cortex and hippocampus. Analysis of binding data revealed an increase in receptor number in the hippocampus of isolates. Cyclic AMP accumulation was measured in hippocampal slices from differentially reared rats. Isolation-rearing did not affect cyclic AMP accumulation in response to stimulation by forskolin (30 microM). However, the selective alpha 2-adrenoceptor agonist, UK14304, produced a significantly greater inhibition of cyclic AMP accumulation in slices from isolated rats, confirming changes in alpha 2-adrenoceptor function following isolation rearing.

Common profile of D1 receptor antagonists and atypical antipsychotic drugs revealed by analysis of dopamine turnover

1. Selective antagonists of dopamine D1 and D2 receptors enhanced 3-methoxytyramine (3-MT) accumulation in the striata and accumbens of tranylcypromine pretreated rats. Selective D1 and D2 agonists produced opposite effects. The smaller changes produced by the D1 agonists and antagonists were probably mediated by neuronal feedback, whereas the larger effects produced by the D2 ligands predominantly reflected pharmacological actions at prejunctional dopaminergic autoreceptors. 2. Atypical antipsychotics evoked small increases in 3-MT similar to the effects of the selective D1 inhibitors, whereas the mixed D1/D2 antagonists mimicked the selective D2 inhibitors by inducing much larger elevations in 3-MT. 3. gamma-Butyrolactone, an inhibitor of dopaminergic neuronal firing, dose-dependently decreased 3-MT accumulation in both the striata and accumbens. 4. gamma-Butyrolactone pretreatment abolished the small increases in 3-MT induced by the selective D1 antagonists and the atypical antipsychotics and also the large increases produced by the mixed D1/D2 antagonists. By contrast, gamma-butyrolactone only partially reversed the marked elevation of 3-MT evoked by the selective D2 antagonists. 5. The above data suggest that in vivo the atypical antipsychotics behave predominantly as selective D1 antagonists.

Evidence that RU 24969-induced locomotor activity in C57/B1/6 mice is specifically mediated by the 5-HT1B receptor

1. The behavioural effects of the 5-HT1B receptor agonists, RU 24969 and CGS 12066B, have been investigated in C57/B1/6 mice. 2. RU 24969 (1-30 mg kg-1) produced intense and prolonged hyperlocomotion and other behavioural changes. 3. CGS 12066B caused similar effects, but they were much less pronounced, inconsistent and transient irrespective of whether this drug was given i.p. (1-15 mg kg-1) or i.c.v. (0.2-40 micrograms). However, CGS 12066B (7.5 and 15 mg kg-1) caused a dose-related inhibition of RU 24969 (7.5 mg kg-1)-induced hyperlocomotion indicating that the former is a 5-HT1B partial agonist. 4. RU 24969 (7.5 mg kg-1 i.p.)-induced hyperlocomotion was inhibited by the (-)-, but not (+)-isomers of pindolol (4 mg kg-1) and propranolol (20 mg kg-1) but not by metoprolol (10 mg kg-1) or ICI 118,551 (5 mg kg-1), consistent with an involvement of 5-HT1A or 5-HT1B receptors. 5. The response was not altered by the selective 5-HT1A receptor antagonist, WAY 100135 (5 mg kg-1, s.c.), the 5-HT2A/5-HT2C receptor antagonist, ritanserin (0.1 mg kg-1), the selective 5-HT3 receptor antagonist, ondansetron (1 mg kg-1) or the non-selective 5-HT receptor antagonists methysergide (3 mg kg-1) and metergoline (3 mg kg-1). 6. Although spiroxatrine (0.1 mg kg-1) and ketanserin (1 mg kg-1) inhibited RU 24969-induced hyperlocomotion, these effects were probably due to antagonism of dopamine D2 receptors and alpha 1-adrenoceptors respectively. 7. Taken together, these results indicate that RU 24969-induced hyperlocomotion results specifically from activation of central 5-HTIB receptors.8. Lesioning of 5-HT neurones with 5,7-dihydroxytryptamine (75 microg, i.c.v.) or depletion with pchlorophenylalanine(200 mg kg-1, i.p. for 14 days) had no effect on RU 24969-induced hyperlocomotiondemonstrating that the 5-HTIB receptors involved are postsynaptic and that they do not show super sensitivity.9. The involvement of other monoamine neurotransmitter systems in RU 24969-induced hyperlocomotionwas also examined. The response was inhibited by the al-adrenoceptor antagonist, prazosin(1 mg kg-1), the dopamine DI receptor antagonist, SCH 23390 (0.05 mg kg-1) and the dopamine D2 receptor antagonist, BRL 34778 (0.03 mg kg-1), but not by the M2-adrenoceptor antagonist, idazoxan(1 mg kg-1). Lesioning noradrenergic neurones with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine(100 mg kg-1) markedly attenuated this behaviour. These results show that the hyperlocomotion is expressed via noradrenergic and dopaminergic neurones acting on alpha 1-adrenoceptors, DI and D2 receptors.10. RU 24969 decreased brain concentrations of 5-hydroxyindoleacetic acid whilst simultaneously increasing 5-HT, consistent with the reduction of 5-HT neuronal activity by activation of 5-HTlA and 5-HTIB autoreceptors. RU 24969 increased brain 3-methoxy-4-hydroxyphenylglycol, but not noradrenaline, concentrations which supports the involvement of noradrenergic neurones in the expression of hyperlocomotion. RU 24969 did not alter dopamine, dihydroxyphenylacetic acid or homovanillic acid concentrations in the nucleus accumbens suggesting that the dopaminergic neurones terminating there are not directly involved.

Quantification of presynaptic alpha 2-adrenoceptors in rat brain after short-term DSP-4 lesioning

The relative numbers of pre- and postsynaptic alpha 2-adrenoceptors were determined in various rat brain regions after short-term DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine) lesioning. For these studies, rats pretreated with zimeldine (10 mg/kg i.p.) were injected with DSP-4 (100 mg/kg i.p.) and were killed either 3 or 15 days later. At the 3 day time-point, DSP-4 treatment produced marked reductions in the noradrenaline content of the cortex (93%), hippocampus (89%), hypothalamus (83%) and cerebellum (92%) with no change in the levels of dopamine or 5-HT. This treatment also decreased the number of alpha 2-adrenoceptors labelled with [3H]idazoxan in the cortex (20%), hippocampus (18%), cerebellum (24%) and hypothalamus (39%). Fifteen days after DSP-4 lesioning, the marked reductions of noradrenaline were sustained in the cortex, hippocampus and cerebellum, but there was a considerable reversal of the effect of DSP-4 in the hypothalamus. At this time-point, the decrease in alpha 2-adrenoceptors was attenuated in cortex (4%) and cerebellum (0%) and their number was increased in hippocampus (8%) and hypothalamus (7%). Together, the data argue that presynaptic alpha 2-adrenoceptors comprise approximately 20% of the total alpha 2-adrenoceptor population in the cortex, hippocampus and cerebellum, but about 40% of it in the hypothalamus. Furthermore, they also demonstrate that although the number of presynaptic alpha 2-adrenoceptors in rat brain can be determined by the reduction of radioligand-receptor binding shortly after DSP-4 lesioning, this effect is rapidly masked by receptor proliferation in response to noradrenergic denervation.

[3H]paroxetine binding in rat frontal cortex strongly correlates with [3H]5-HT uptake: effect of administration of various antidepressant treatments

Paroxetine is a selective and potent inhibitor of 5-hydroxytryptamine (5-HT) uptake into serotonergic neurones. [3H]Paroxetine binding to rat frontal cortex was of high affinity with a high percentage of specific binding. The binding data of both competition and saturation studies fitted a single site binding model. [3H]Paroxetine binding was potently inhibited by the selective 5-HT uptake inhibitors. In addition, a very good correlation was demonstrated between the ability of twenty-three compounds to inhibit [3H]paroxetine binding to rat frontal cortical membranes and [3H]5-HT uptake into rat frontal cortical synaptosomes. These data support the view that [3H]paroxetine binds to a single site which corresponds to the 5-HT uptake site. Using this ligand, the effects of repeated administration of antidepressant drugs with a wide range of pharmacological actions and electroconvulsive shock on 5-HT reuptake sites were examined. [3H]Paroxetine binding parameters (Kd and Bmax) were unaltered by all treatments. It would, therefore, appear that antidepressant therapy does not produce adaptive changes in 5-HT uptake sites.

Reserpinization severs the cooperative but not the oppositional interaction between D1 and D2 receptors

Dopamine D1 and D2 receptors in rat brain interact synergistically to produce stereotypy and antagonistically to induce atypical jerking. This study showed that repeated reserpine treatment (5 daily injections of 1 mg/kg, s.c.), which depleted dopamine stores by 98%, rendered D1 and D2 receptors independent with respect to the production of stereotypy, i.e. either D1- or D2-selective agonists given alone induced stereotypy. In contrast, the atypical jerking induced by the D2 agonist, quinpirole, was still inhibited by the D1 agonist, SKF 82958, indicating that reserpine treatment did not render these two receptors independent with respect to initiation of this behavior.

Mediation of the antidepressant-like effect of 8-OH-DPAT in mice by postsynaptic 5-HT1A receptors

1. The 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) has been evaluated in a mouse model for detecting potential antidepressants (Porsolt test). The effects of various receptor antagonists, lesions of brain monoaminergic neurones and chronic drug treatments on this 8-OH-DPAT-induced response have also been determined. 2. 8-OH-DPAT (0.3-10.0 mg kg-1, s.c.) dose-dependently increased the mobility of mice in the Porsolt test. Other selective 5-HT1A receptor ligands (0.3-30 mg kg-1, s.c.) either mimicked the 8-OH-DPAT response (ipsapirone, at 10 and 30 mg kg-1, s.c.) or were inactive (buspirone and gepirone). However, each of these compounds (< or = 100 mg kg-1, p.o.) inhibited the response to 8-OH-DPAT (3 mg kg-1, s.c.) when given concurrently. 3. The putative 5-HT1A antagonists, spiroxatrine (1-30 mg kg-1, p.o.), (+/-)-pindolol (30 mg kg-1, p.o.) and methiothepin (3-10 mg kg-1, p.o.), each attenuated the 8-OH-DPAT (3 mg kg-1, s.c.)-induced increase in mobility. 4. The dopamine D1 receptor antagonist, SCH 23390 (3-10 mg kg-1, p.o.), weakly reversed the 8-OH-DPAT response. Antagonists at 5-HTlc/5-HT2 receptors (ketanserin; 0.1-3.0 mg kg-1, p.o.),5-HT3 receptors (ondansetron; 0.03-10mg kg-1, p.o.), at-adrenoceptors (prazosin; 1-3mgkg-1, p.o.),alpha2 -adrenoceptors (idazoxan; 3-30mg kg-1, p.o.), alpha 1-adrenoceptors (metoprolol; 1-30mgkg-1, p.o.),beta 2-adrenoceptors (ICI 118,551; 1-30 mg kg-1, p.o.), dopamine D2 receptors (sulpiride; 10-300mg kg-',p.o.) and opiate receptors (naloxone; 3-100 mg kg-', p.o.) had no effect on the 8-OH-DPAT response.5. Selective destruction of 5-HT neurones with 5,7-dihydroxytryptamine or inhibition of 5-HT synthesis with p-chlorophenylalanine did not change the 8-OH-DPAT response in the Porsolt test. This response was also unaltered by pretreatment with the noradrenergic neurotoxin, DSP-4.6. Administration of 8-OH-DPAT (3 mg kg-1, s.c.) twice-daily for 10 days attenuated the hypothermia,but not the increased mobility, induced by 8-OH-DPAT (3 mg kg-1, s.c.). Similarly, repeated administration of amitriptyline (3-30 mg kg-1), desipramine (3-30 mg kg-1) or dothiepin (10-100 mg kg-1) also attenuated the former, but not the latter, response.7. We conclude that 8-OH-DPAT produces an antidepressant-like effect in the Porsolt test which is mediated via postsynaptic 5-HT1A receptors.

Characterization of 8-OH-DPAT-induced hypothermia in mice as a 5-HT1A autoreceptor response and its evaluation as a model to selectively identify antidepressants

1. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) dose-dependently induced hypothermia in mice. 2. The 5-HT1A receptor partial agonists, buspirone, gepirone and ipsapirone, also dose-dependently induced hypothermia. 3. The 8-OH-DPAT temperature response was antagonized by the 5-HT1 receptor antagonists quipazine (2 mg kg-1, i.p.), (+/-)-propranolol (10 mg kg-1, i.p.). (+/-)-pindolol (5 mg kg-1, i.p.), spiroxatrine (0.5 mg kg-1, i.p.) and metitepine (0.05 mg kg-1, i.p.), but not by 5-HT2 (ketanserin) or 5-HT3 (MDL 72222, GR 38032F) receptor antagonists. 4. The response was also antagonized by the dopamine D2 receptor antagonists, haloperidol and BRL 34778. No other catecholamine or muscarinic receptors were involved in mediating the response. 5. Destruction of 5-hydroxytryptamine (5-HT)-containing neurones with the neurotoxin, 5,7-dihydroxytryptamine (75 micrograms, i.c.v.), abolished the response to 8-OH-DPAT indicating that the 5-HT1A receptors involved were located on 5-HT neurones. 6. Chronic antidepressant treatment down-regulated this 8-OH-DPAT response. In addition, chronic administration of anxiolytics and neuroleptics was also effective in this respect. Down-regulation was also observed following repeated administration of 8-OH-DPAT (0.5 mg kg-1, s.c.), (+/-)-pindolol (10 mg kg-1, i.p.) and ketanserin (0.5 mg kg-1, i.p.). 7. In conclusion, these data confirm that 8-OH-DPAT-induced hypothermia is mediated by 5-HT1A autoreceptors. They also indicate that the response involves D2 receptors.The present study also shows that a wide range of antidepressant drugs down-regulate this response although this property is not restricted to antidepressant treatments. Therefore, care should be exercised when interpreting data from this paradigm.

Opposing roles for 5-HT1B and 5-HT3 receptors in the control of 5-HT release in rat hippocampus in vivo

1. Intracerebral microdialysis was used to determine whether 5-hydroxytryptamine (5-HT) release in the ventral hippocampus of rats anaesthetized with chloral hydrate was modulated by 5-HT3 receptors. 2. It was confirmed that 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), a selective 5-HT1B receptor agonist, decreased 5-HT release in a dose- and concentration-related manner when administered i.p. (1 and 5 mg kg-1) or via the dialysis probe (0.1 and 1 microM) respectively. The effect of RU 24969 infusion (1 microM) was attenuated by concurrent infusion of metitepine (10 microM) into the hippocampus. 3. When infused into the hippocampus for 15 min, the selective 5-HT3 receptor agonist, 2-methyl-5-hydroxytryptamine (2-methyl-5-HT; 0.1- 10 microM) increased dialysate 5-HT levels in a concentration-related manner; an effect which was abolished by concurrent infusion of 3-tropanyl-3,5-dichlorobenzoate (1 microM, MDL 72222), a selective 5-HT3 antagonist. 4. MDL 72222 had no effects on hippocampal 5-HT release when administered via the dialysis probe (1 or 10 microM). 5. The data show that 5-HT3 and 5-HT1B receptors have opposing roles in the control of 5-HT release in the hippocampus, with 5-HT3 receptors facilitating and 5-HT1B receptors inhibiting 5-HT efflux, respectively. They also indicate that the facilitatory 5-HT3 receptors are not tonically activated.

A comparison of various antidepressant drugs demonstrates rapid desensitisation of alpha 2-adrenoceptors exclusively by sibutramine hydrochloride

The functional status of presynaptic and postsynaptic alpha 2-adrenoceptors in murine brain was respectively monitored using the hypoactivity (sedation) and mydriasis (pupil dilatation) responses to clonidine (0.1 mg/kg IP). Both responses were attenuated 24 h after 3 days of injection of sibutramine hydrochloride (3 mg/kg IP). To ascertain whether this property was exclusive to sibutramine, the following antidepressant drugs were also tested for their ability to down-regulate alpha 2-adrenoceptors rapidly: amitriptyline, doxepin, nomifensine, desipramine, amoxapine, fluoxetine, zimeldine, tranylcypromine and mianserin. When given for 3 or 5 days at the low dose of 3 mg/kg IP, none of the other antidepressants reduced clonidine-induced hypoactivity or mydriasis. Furthermore, increasing the dose of amitriptyline, doxepin, nomifensine, desipramine, amoxapine and tranylcypromine to 10 mg/kg IP did not enable these antidepressants to attenuate the alpha 2-adrenoceptor-mediated responses after 3 days of treatment. An electroconvulsive shock (ECS; 200 V, 2 s) given once daily attenuated clonidine-induced mydriasis, but not hypoactivity, when administered for 3 days and both responses when administered for 5 days. In conclusion, this comparative study using antidepressant treatments with differing pharmacological modes of action demonstrated that sibutramine was the only drug which rapidly down-regulated pre- and postsynaptic alpha 2-adrenoceptors. ECS down-regulated postsynaptic alpha 2-adrenoceptors when given for 3 days, but required 5 days to desensitise both alpha 2-adrenoceptor populations.

A comparison of the effects of sibutramine hydrochloride, bupropion and methamphetamine on dopaminergic function: evidence that dopamine is not a pharmacological target for sibutramine

Sibutramine hydrochloride, a novel monoamine reuptake inhibitor antidepressant, has been studied to determine whether it alters dopaminergic function in the brain. Its effects have been compared with bupropion, a dopamine reuptake inhibitor, and methamphetamine, a dopamine reuptake inhibitor and releasing agent. Sibutramine (0.1-3 mg/kg PO) and methamphetamine (0.3-30 mg/kg PO) both prevented reserpine (0.75 mg/kg IV) ptosis in rats with ED50 values of 0.6 mg/kg and 4.2 mg/kg, respectively. Bupropion (10-100 mg/kg PO) was ineffective against reserpine ptosis. The efflux of [3H]-dopamine from preloaded rat striatal slices was not altered by 10(-7)-10(-5) M concentrations of sibutramine, BTS 54,354, BTS 54,505 (secondary and primary amine metabolites, respectively) or bupropion. In contrast, methamphetamine (10(-8)-10(-4) M) caused a significant concentration-dependent increase in [3H]-dopamine release. Sibutramine (3 mg/kg IP or 6 mg/kg PO) and bupropion (10 mg/kg IP or 30 mg/kg PO) did not alter 3-methoxytyramine (3-MT) levels in rat striatum. Striatal 3-MT concentrations were, however, dose-dependently increased by methamphetamine (0.3-10 mg/kg IP or 0.42-4.2 mg/kg PO). Sibutramine (6 mg/kg PO) did not induce circling in rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopaminergic neuronal tract. Bupropion (10-100 mg/kg PO) did not induce circling at the lowest dose, but caused increasing ipsilateral rotation at higher doses. Methamphetamine (0.42 or 4.2 mg/kg PO) induced ipsilateral circling with marked effects at the higher dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of the role of noradrenergic and 5-hydroxytryptaminergic neurones in postsynaptic alpha 2-adrenoceptor desensitization by desipramine and ECS

1. Experiments were conducted to determine the respective roles which noradrenergic and 5-hydroxytryptaminergic neurones play in the down-regulation of postsynaptic alpha 2-adrenoceptors by desipramine and electroconvulsive shock (ECS). The functional status of these receptors was monitored by use of clonidine-induced mydriasis in conscious mice. 2. Mydriasis to clonidine (0.1 mg kg-1, i.p.) was markedly attenuated by administration of either desipramine (10 mg kg-1, i.p.) for 14 days or ECS (200 V, 2s) given five times over ten days confirming our previous observations. 3. The neurotoxin, DSP-4 (100 mg kg-1, i.p. X 2), reduced brain noradrenaline levels by 64% and abolished the mydriasis induced by the noradrenaline releasing agent and reuptake inhibitor, methamphetamine, without significantly altering the response to clonidine, confirming our earlier results. This lesion prevented the attenuation of clonidine mydriasis by repeated administration of desipramine, but not ECS. 4. Lesioning of central 5-hydroxytryptaminergic neurones with 5,7-dihydroxytryptamine (75 micrograms, i.c.v.) had no influence on the reduction in clonidine mydriasis produced by repeated administration of either desipramine or ECS. 5. Since noradrenergic neurones are essential for the desensitization of postsynaptic alpha 2-adrenoceptors by desipramine, it indicates that this effect is probably the result of increased synaptic noradrenaline levels. This mechanism is not responsible for the change induced by ECS because this adaptation is independent of an intact noradrenergic input. 5-HT-containing neurones do not play a permissive role in the down-regulation of postsynaptic alpha 2-adrenoceptors by either antidepressant treatment.

Effects of antidepressant drugs and electroconvulsive shock on pre- and postsynaptic alpha 2-adrenoceptor function in the brain: rapid down-regulation by sibutramine hydrochloride

Clonidine (0.1 mg/kg IP)-induced hypoactivity and mydriasis responses were respectively used as functional indices of pre- and postsynaptic alpha 2-adrenoceptors in mouse brain. A single injection of various antidepressant drugs had no effect on either response when measured 24 h later. However, 14 days' treatment with sibutramine HCl (3 mg/kg IP), dothiepin (50 mg/kg IP), amitriptyline (10 mg/kg IP), desipramine (10 mg/kg IP) or tranylcypromine (10 mg/kg IP) markedly attenuated both clonidine-induced hypoactivity and mydriasis. Repeated administration of zimeldine (10 mg/kg IP), mianserin (10 mg/kg IP) or clenbuterol (5 mg/kg IP) had no effect on either response. Subchronic treatment with sibutramine HCl (3 mg/kg IP; 3 days) also attenuated pre- and postsynaptic alpha 2-adrenoceptor function. Five ECS (200 V, 2 s) spread over 10 days, but not a single shock, reduced the hypoactivity and mydriasis responses to clonidine. Together, the results indicate that pre- and postsynaptic alpha 2-adrenoceptor function is attenuated by repeated treatment with those antidepressants which acutely increase synaptic levels of noradrenaline. These adrenergic receptor populations are also desensitized by ECS, although this effect is probably mediated via a different mechanism. Finally, the rapid down-regulation observed with sibutramine HCl is not confined to beta-adrenoceptors alone, because pre- and postsynaptic alpha 2-adrenoceptor function is also attenuated by 3 days of treatment with this novel antidepressant drug.

A new and highly sensitive method for measuring 3-methoxytyramine using HPLC with electrochemical detection. Studies with drugs which alter dopamine metabolism in the brain

3-Methoxytyramine (3-MT) is a minor metabolite of dopamine which is suggested to reflect the turnover and utilization of dopamine. A novel, isocratic HPLC method has been developed which can be used to analyse 3-MT in homogenates of rat brain without the need for additional purification procedures. Furthermore, the coulometric electrochemical detection system is sensitive enough to measure 3 pg of 3-MT (equivalent to 0.6 ng/g tissue wet weight). 3-Methoxytyramine was measured in the striatum and n. accumbens after decapitation and rapid freezing, using 3-methoxy-4-hydroxybenzylamine as the internal standard. The effects of dopaminergic and other drugs on this metabolite were examined using this method. alpha-Methyl-p-tyrosine (200 mg/kg i.v.) produced parallel linear decreases in dopamine and 3-MT in naive rats, but not those pretreated with tranylcypromine (5 mg/kg i.p.). Methamphetamine (0.3-10 mg/kg i.p.) and amphetamine (0.3-10 mg/kg i.p.) both dose-dependently increased 3-MT in naive and tranylcypromine-pretreated rats. In naive animals, 3-MT was not altered by intraperitoneal injection of the dopamine reuptake inhibitors, bupropion (10 mg/kg) and nomifensine (10 mg/kg) or by sibutramine HCl (3 mg/kg), amitriptyline (10 mg/kg), desipramine (10 mg/kg) and zimeldine (10 mg/kg). 3-Methoxytyramine was decreased by apomorphine (5 mg/kg i.p.) and also by large doses of the selective D2 antagonist, BRL 34778 (5 mg/kg i.p.) or L-DOPA (50 mg/kg i.p.). The selective D1 antagonist, SCH 23390 (0.1 or 5 mg/kg i.p.) was without effect. In tranylcypromine-pretreated rats, 3-MT was dose-dependently reduced and increased by apomorphine (0.01-5 mg/kg i.p.) and BRL 34778 (0.1-5 mg/kg i.p.), respectively. The drug SCH 23390 (0.1-5 mg/kg i.p.) produced much smaller increases in 3-MT which were probably mediated through the striatonigral pathway. Overall, the data suggest that measurement of 3-MT, after inhibition of monoamine oxidase, is a useful index of the release and utilization of dopamine. However, after substantial and prolonged depletion of dopamine, levels of 3-MT in naive animals are a better index. Also, the formation of 3-MT in naive rats rats provides a sensitive method for distinguishing between dopamine releasing agents and reuptake inhibitors.

The effects of acute and chronic lithium treatment on pilocarpine-stimulated phosphoinositide hydrolysis in mouse brain in vivo

1. Measurements were made of the in vivo formation of inositol phosphates in the brains of C57/B1/601a mice treated acutely or chronically with lithium chloride (LiCl). 2. A single injection of LiCl (10 mEquiv kg-1, s.c.) 18 h before death increased the accumulation of [3H]-inositol phosphates ([3H]-Ins P's) in the brains of mice injected i.c.v. with [3H]-myo-inositol 24 h previously. 3. Pilocarpine (200 mg kg-1, i.p.) injected 15 min before death further enhanced the formation of [3H]-Ins P's in the brains of LiCl-treated, but not saline-treated, mice. The enhancement due to pilocarpine was abolished by injection of atropine sulphate (10 mg kg-1, i.p.) 10 min earlier. 4. Chronic (14 days) LiCl feeding produced an accumulation of [3H]-Ins P's significantly less than that due to a single injection of LiCl, but the response to pilocarpine was markedly greater in mice chronically fed with LiCl when compared with mice acutely injected with LiCl. 5. Mass measurements of endogenous inositol 1,4,5 triphosphate revealed increases due to pilocarpine and chronic LiCl feeding alone. A combination of the two treatments produced levels greater than either alone. 6. These results demonstrate that LiCl treatment enhances both basal and pilocarpine-stimulated inositol phospholipid hydrolysis in vivo and this might be relevant to its therapeutic effects.

An investigation of the role of 5-hydroxytryptamine in the attenuation of presynaptic alpha 2-adrenoceptor-mediated responses by antidepressant treatments

Changes in the function of presynaptic alpha 2-adrenoceptors in the brain were assessed by rating the hypoactivity (sedation) response of mice to clonidine (0.1 mg/kg). A single injection of 5,7-dihydroxytryptamine (5,7-DHT, 75 micrograms ICV) or administration of p-chlorophenylalanine (PCPA; 200 mg/kg) daily for 11-15 days produced 62-77% reductions in brain 5-HT concentrations and marked supersensitivity of 5-HT2 receptor function, as indicated by the enhancement of the head-twitch response to 5-methoxy-N,N-dimethyltryptamine (2 mg/kg). Clonidine-induced hypoactivity was moderately enhanced after 5,7-DHT lesioning, but not after repeated PCPA injection. In addition, 5,7-DHT lesioning prevented the adaptive attenuation of this alpha 2-adrenoceptor-mediated response produced by daily injection of desipramine (10 mg/kg) for 14 days, but had no effect on the reduction caused by five electroconvulsive shocks (ECS, 200 V, 2 s) spread over 10 days. In contrast, repeated PCPA treatment did not prevent the reduction of clonidine-induced hypoactivity produced by repeated desipramine or ECS administration. Together, these results indicate that 5-HT (or possibly a cotransmitter contained within 5-hydroxytryptamine neurones) influences presynaptic alpha 2-adrenoceptor function. Furthermore, an intact 5-HT neuronal input is a prerequisite for the attenuation of clonidine-induced hypoactivity by desipramine, but not ECS. The probable explanation for a contrasting requirement for a functional 5-HT input is that desipramine and ECS induce this common adaptive response by different pharmacological mechanisms.

Biochemical and in vivo voltammetric evidence for differences in striatal dopamine levels in inbred strains of mice

High performance liquid chromatography with electrochemical detection and differential pulse voltammetry were used to provide a direct measurement of tissue content of dopamine and its metabolites and extracellular dopamine levels, respectively, in the striata of BALB/c and CBA inbred strains of mice. We found that levels of striatal dopamine and its metabolite, dihydroxyphenylacetic acid, were significantly higher in the CBA strain than in the BALB/c strain, whereas levels of homovanillic acid were not significantly different between the strains. Levels of the dopamine metabolite 3-methoxytyramine, on the other hand, were higher in the BALB/c mice. Dopamine turnover rates were significantly higher in the CBA strain when the homovanillic acid/dopamine ratio was used as an index of dopamine activity. Voltammetric recording showed that the local infusion of K+ in pargyline-treated mice resulted in the immediate appearance of a peak at +85 mV, which has been shown to correspond to extracellular dopamine in the rat. The mean height of this peak detected in vivo following K+ stimulation corresponds to in vitro dopamine concentrations of 25 +/- 8 microM for BALB/c mice and 7 +/- 2 microM for CBA mice. K(+)-stimulated dopamine release in the BALB/c mice could be evoked every 10-15 min with similar magnitude. In contrast, very little dopamine release in CBA mice could be evoked after the first stimulation. Since striatal dopamine levels are higher in CBA mice, these data suggest that (a) BALB/c strain may have more dopamine in the readily releasable pool, whereas the CBA mice have a larger storage pool of dopamine, and/or (b) that dopamine uptake in the CBA mice is much more avid than in BALB/c.