dl-N-methyl-3-(o-methoxyphenoxy)-3-phenylpropylamine hydrochloride, Lilly 94939, a potent inhibitor for uptake of norepinephrine into rat brain synaptosomes and heart

Profiling the chemical properties of Foeniculum vulgare Mill. and its flavonoids through comprehensive LC-MS/MS to evaluate their anti-motion sickness effect

Foeniculum vulgare Mill. is a medicinal and food homologous plant, and it has various biological activities. Yet, no research has explored its anti-motion sickness effects. Chemical properties of fennel extracts (FvE) and flavonoids (Fvf) were analyzed based on UPLC-QTRAP-MS to elucidate its potential anti-motion sickness components in the present study. The mice models of motion sickness were stimulated by biaxial rotational acceleration. Behavioral experiments such as motion sickness index and open field test and the measurement of neurotransmitters were used to evaluate the efficacy of compounds on motion sickness. Results showed that FvE contains terpenes, alkaloids, flavonoids, etc. Eight flavonoids including quercetin-3β-D-glucoside, rutin, hyperoside, quercetin, miquelianin, trifolin, isorhamnetin and kaempferol were identified in the purified Fvf. FvE and Fvf significantly reduced the motion sickness index of mice by 53.2% and 48.9%, respectively. Fvf also significantly alleviated the anxious behavior of mice after rotational stimulation. Among the eight flavonoids, isorhamnetin had the highest oral bioavailability and moderate drug-likeness index and thus speculated to be the bioactive compound in fennel for its anti-motion sickness effect. It reduced the release of 5-HT and Ach to alleviate the motion sickness response and improve the work completing ability of mice and nervous system dysfunction after rotational stimulation. This study provided in-depth understanding of the anti-motion sickness bioactive chemical properties of fennel and its flavonoids, which will contribute to the new development and utilization of fennel.

Biochemical analysis reveals the systematic response of motion sickness mice to ginger (Zingiber officinale) extract's amelioration effect

ETHNOPHARMACOLOGICAL RELEVANCE

As a common medicinal and edible plant, Zingiber officinale Roscoe (ginger) is often used for the prevention of motion sickness. However, the mechanism of its anti-motion sickness remains to be elucidated.

AIM OF THE STUDY

To explore novel treatment for motion sickness with less side effects, anti-motion sickness effect of ginger (Zingiber officinale) extract (GE) and the possible molecular mechanisms were investigated.

MATERIALS AND METHODS

The anti-motion sickness effect of ginger was evaluated through mice animal experimental models. Components of ginger that might contribute to the anti-motion sickness effect were analyzed by LC-MS/MS. Subsequently, biochemical analysis integrated with serum metabolomic profiling were performed to reveal the systematic response of motion sickness mice to ginger extract's amelioration effect.

RESULTS

Exhaustive swimming time of mice in the GE group reached 8.9 min, which was 52.2% longer than that in the model group. Motion sickness index scores and time taken traversing balance beam of mice in the GE group were decreased by 53.2% and 38.5%, respectively. LC-MS/MS analysis suggested that various active ingredients in GE, such as gingerol, ginger oil and terpenoids, might contribute to its appealing anti-motion sickness activity. Biochemical analysis revealed that GE can relieve motion sickness through reducing histamine and acetylcholine release in vestibular system, regulating fatty acid oxidation, sugar metabolism and bile acid metabolism in mice.

CONCLUSION

Gavage of mice with GE can effectively relieve the symptoms of autonomic nervous system dysfunction, improve the balance and coordination ability and ameliorate the ability to complete complex work after rotation stimulation. GE has attractive potential for development and utilization as novel anti-motion sickness food or drugs.

Illuminating the norepinephrine transporter: fluorescent probes based on nisoxetine and talopram

The utilization of fluorescent ligands to study the monoamine transporters (MATs) has increased our knowledge of their function and distribution in live cell systems. In this study, we extend SAR for nisoxetine and talopram as parent compounds, to identify high affinity rhodamine-labeled fluorescent probes for the norepinephrine transporter (NET). Nisoxetine-based fluorescent probe 6 demonstrated high binding affinity (K _i = 43 nM) for NET and an overall selectivity compared to the other transporters for dopamine (DAT; K _i = 1540 nM) and serotonin (SERT; K _i = 785 nM) in competitive radioligand binding assays. Using confocal microscopy, compound 6 was shown to stain both NET and SERT, but not DAT, at low nanomolar concentrations, in transporter-expressing cells.

18F-labeled norepinephrine transporter tracer [18F]NS12137: radiosynthesis and preclinical evaluation

INTRODUCTION

Several psychiatric and neurodegenerative diseases are associated with malfunction of brain norepinephrine transporter (NET). However, current clinical evaluations of NET function are limited by the lack of sufficiently sensitive methods of detection. To this end, we have synthesized exo-3-[(6-[¹⁸F]fluoro-2-pyridyl)oxy]-8-azabicyclo[3.2.1]-octane ([¹⁸F]NS12137) as a radiotracer for positron emission tomography (PET) and have demonstrated that it is highly specific for in vivo detection of NET-rich regions of rat brain tissue.

METHODS

We applied two methods of electrophilic, aromatic radiofluorination of the precursor molecule, exo-3-[(6-trimethylstannyl-2-pyridyl)oxy]-8-azabicyclo-[3.2.1]octane-8-carboxylate: (1) direct labeling with [¹⁸F]F₂, and (2) labeling with [¹⁸F]Selectfluor, a derivative of [¹⁸F]F₂, using post-target produced [¹⁸F]F₂. The time-dependent distribution of [¹⁸F]NS12137 in brain tissue of healthy, adult Sprague-Dawley rats was determined by ex vivo autoradiography. The specificity of [¹⁸F]NS12137 binding was demonstrated on the basis of competitive binding by nisoxetine, a known NET antagonist of high specificity.

RESULTS

[¹⁸F]NS12137 was successfully synthesized with radiochemical yields of 3.9% ± 0.3% when labeled with [¹⁸F]F₂ and 10.2% ± 2.7% when labeled with [¹⁸F]Selectfluor. The molar activity of radiotracer was 8.8 ± 0.7 GBq/μmol with [¹⁸F]F₂ labeling and 6.9 ± 0.4 GBq/μmol with [¹⁸F]Selectfluor labeling at the end of synthesis of [¹⁸F]NS12137. Uptake of [¹⁸F]NS12137 in NET-rich areas in rat brain was demonstrated with the locus coeruleus (LCoe) having the highest regional uptake. Prior treatment of rats with nisoxetine showed no detectable [¹⁸F]NS12137 in the LCoe. Analyses of whole brain samples for radiometabolites showed only the parent compound [¹⁸F]NS12137. Uptake of ¹⁸F-radioactivity in bone increased with time.

CONCLUSIONS

The two electrophilic ¹⁸F-labeling methods proved to be suitable for synthesis of [¹⁸F]NS12137 with the [¹⁸F]Selectfluor method providing an approximate three-fold higher yield than the [¹⁸F]F₂ method. As an electrostatically neutral radiotracer [¹⁸F]NS12137 crosses the blood-brain barrier and enabled specific labeling of NET-rich regions of rat brain tissue with the highest concentration in the LCoe.

Monoamine reuptake inhibitors in Parkinson's disease

The motor manifestations of Parkinson's disease (PD) are secondary to a dopamine deficiency in the striatum. However, the degenerative process in PD is not limited to the dopaminergic system and also affects serotonergic and noradrenergic neurons. Because they can increase monoamine levels throughout the brain, monoamine reuptake inhibitors (MAUIs) represent potential therapeutic agents in PD. However, they are seldom used in clinical practice other than as antidepressants and wake-promoting agents. This review article summarises all of the available literature on use of 50 MAUIs in PD. The compounds are divided according to their relative potency for each of the monoamine transporters. Despite wide discrepancy in the methodology of the studies reviewed, the following conclusions can be drawn: (1) selective serotonin transporter (SERT), selective noradrenaline transporter (NET), and dual SERT/NET inhibitors are effective against PD depression; (2) selective dopamine transporter (DAT) and dual DAT/NET inhibitors exert an anti-Parkinsonian effect when administered as monotherapy but do not enhance the anti-Parkinsonian actions of L-3,4-dihydroxyphenylalanine (L-DOPA); (3) dual DAT/SERT inhibitors might enhance the anti-Parkinsonian actions of L-DOPA without worsening dyskinesia; (4) triple DAT/NET/SERT inhibitors might exert an anti-Parkinsonian action as monotherapy and might enhance the anti-Parkinsonian effects of L-DOPA, though at the expense of worsening dyskinesia.

Case history: the discovery of fluoxetine hydrochloride (Prozac)

In the early 1970s, evidence of the role of serotonin (5-hydroxytryptamine or 5-HT) in depression began to emerge and the hypothesis that enhancing 5-HT neurotransmission would be a viable mechanism to mediate antidepressant response was put forward. On the basis of this hypothesis, efforts to develop agents that inhibit the uptake of 5-HT from the synaptic cleft were initiated. These studies led to the discovery and development of the selective serotonin-reuptake inhibitor fluoxetine hydrochloride (Prozac; Eli Lilly), which was approved for the treatment of depression by the US FDA in 1987. Here, we summarize this research and discuss the many challenges that we encountered during the development of fluoxetine hydrochloride, which has now been widely acknowledged as a breakthrough drug for depression.

[11C]-DASB, a tool for in vivo measurement of SSRI-induced occupancy of the serotonin transporter: PET characterization and evaluation in cats

The in vivo pharmacological profile of [(11)C]-DASB, a new radioligand developed for in vivo imaging of the serotonin transporter (SERT), was evaluated in the cat brain using positron emission tomography (PET). The in vivo distribution of [(11)C]-DASB binding was consistent with the known distribution of SERT sites in the cat brain in vitro with high uptakes of radioactivity in the midbrain and thalamus, intermediate levels in striatum, and modest to low levels of radioactivity in the neocortex and cerebellum, respectively. [(11)C]-DASB binding potential (BP) values ranged from about 0.2 in the neocortex to 2.2 in the midbrain. Radioligand binding in all brain regions except cerebellum was markedly reduced following pretreatment with fluoxetine and citalopram, but was unaffected by pretreatment with GBR12909, maprotiline, and haloperidol, indicating specificity of [(11)C]-DASB binding to the SERT. Two cats were each examined using PET and [(11)C]-DASB in a longitudinal fashion (from 30 min and up to 24 days) following a single i.v. dose of: 1) fluoxetine, and 2) citalopram at different dosages. Both drugs induced similar degrees of SERT occupancy at 30 min postinjection (approximately 90%). A comparison of citalopram and fluoxetine pharmacokinetics in the same animal and at the same dosage (1 mg/kg) showed that citalopram SERT occupancy and plasma half-lives were 9 times and 14 times shorter, respectively, than those of fluoxetine and norfluoxetine. In addition, studies performed after injection of the monoamine oxidase inhibitor tranylcypromine suggested that high levels of synaptic serotonin may compete with [(11)C]-DASB for binding on the SERT. These studies indicate that [(11)C]-DASB is a suitable PET radioligand for measuring drug occupancy of the SERT in vivo and has potential for monitoring in vivo changes in serotonin levels.

Dopamine, but not norepinephrine or serotonin, reuptake inhibition reverses motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated primates

Monoamine reuptake inhibitors that do not discriminate between the transporters for dopamine (DA), norepinephrine (NE), or 5-hydroxytryptamine (5-HT, serotonin) can reverse locomotor deficits and motor disability in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmosets. DA reuptake inhibition is presumed to be primarily responsible, but the role played by inhibition of NE and 5-HT reuptake is unknown. We now evaluate the efficacy of a range of monoamine reuptake inhibitors either alone or in combination in MPTP-treated common marmosets to determine the actions required for effective antiparkinsonian activity. Monoamine reuptake inhibitors not discriminating between the DA, NE, and 5-HT transporters [1-[1-(3,4-dichlororphenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio)ethanone monocitrate (BTS 74 398) and nomifensine] reversed locomotor deficits and motor disability in MPTP-treated marmosets but bupropion was without effect. The selective DA reuptake inhibitor 1-(2-(bis-(4-fluorophenyl)-methoxy)ethyl)-4-(3-phenylpropyl) piperazine) dihydrochloride (GBR 12909) also reversed these motor deficits. The relative efficacy of the compounds (BTS 74 398 > GBR 12909 > nomifensine >> bupropion) paralleled their potency in inhibiting DA uptake in vitro and in vivo. In contrast, the selective NE reuptake inhibitor nisoxetine and the 5-HT reuptake inhibitor sertraline administered alone failed to improve motor function and tended to worsen the deficits. Coadministration of nisoxetine attenuated the improvement in motor deficits produced by GBR 12909. Coadministration of sertraline also abolished the reversal of motor deficits produced by GBR 12909. Coadministration of both sertraline and nisoxetine similarly abolished the improvement of motor deficits produced by GBR 12909. Molecules possessing potent DA reuptake inhibitory activity may be useful in the treatment of the motor symptoms of Parkinson's disease. In contrast, there seems to be no role for NE or 5-HT reuptake inhibitors, and they may impair antiparkinsonian activity mediated through dopaminergic mechanisms.

Dual serotonin and noradrenaline uptake inhibitor class of antidepressants potential for greater efficacy or just hype?

Preclinical and clinical studies support the rationale that development of single molecules, which would promote serotonergic and noradrenergic neurotransmission by inhibiting simultaneously the uptake of both monoamines, would potentially result in improved antidepressant drugs. Currently, the dual inhibitors of serotonin and noradrenaline uptake are venlafaxine, milnacipran and duloxetine. Based on the preclinical studies, the three drugs do show properties of inhibiting uptake of both monoamines in vitro and in vivo in the following order of decreasing potency: duloxetine, venlafaxine and milnacipran, and all exhibit low affinity at neuronal receptors of neurotransmitters, suggesting low side-effect potential. In double-blind, controlled studies, venlafaxine and milnacipran were repeatedly shown to be as efficacious as tricyclic antidepressant drugs in treating major depressive disorder, while one double-blind, placebo-controlled trial showed the antidepressant efficacy of duloxetine. Specifically designed comparative trials of dual uptake inhibitors against the other agents are needed to establish whether the dual uptake inhibitors show improvement in efficacy, rate of responders, antidepressive effects and/or remission.

[3H]Nisoxetine binding sites in the cat brain: an autoradiographic study

The binding of [3H]nisoxetine, a selective inhibitor of the high-affinity noradrenaline uptake sites, was studied on frontal frozen sections of the cat brain. The highest densities in autoradiographic signal were observed in the nucleus locus coeruleus and its ascending pathways, in the area postrema and in the dorsal part of the inferior olive, the pontine nuclei, the raphe nuclei, the colliculi, the periventricular and lateral areas of the hypothalamus, the suprachiasmatic nucleus, the nucleus accumbens and the olfactory bulb. A moderately high concentration of binding sites was observed in the hippocampal formation, especially in the molecular layer of Ammon's horn, in the superficial layers of the entorhinal cortex and in the indusium griseum. Binding sites were visualized in all the subdivisions of the neocortex. The highest density of binding was generally detected in the outer edge of the superficial layer I. In some cortical areas, especially in the visual cortex, labeling with a prevalent laminar distribution in the superficial layers I-III and in the deep layers V-VI was clearly observed. Moderate to low densities of binding sites were seen in most other areas of the brain except in the white matter, the caudate nucleus and putamen, which were devoid of labeling. Overall these findings indicate a good correlation between the distribution of [3H]nisoxetine binding sites and the noradrenergic systems. Furthermore, data suggest that in several areas, high-affinity noradrenaline reuptake mechanisms could play an important role in local interactions between the noradrenergic system and the other monoaminergic systems.

Prozac (fluoxetine, Lilly 110140), the first selective serotonin uptake inhibitor and an antidepressant drug: twenty years since its first publication

In this review, we describe the evolutionary process involved in the discovery of the selective 5-HT uptake inhibitor, fluoxetine, and summarize some of the large body of scientific research performed on fluoxetine in the 20 years since the first publication. The historical background of the proposed involvement of 5-HT in psychiatric disorders and the activity of tricyclic antidepressants in depression is reviewed. The effects of fluoxetine in various in vitro assays and in animal studies including receptor down-regulation, neurochemical and behavioral models are summarized. In addition, the clinical effectiveness of fluoxetine in depression and obsessive compulsive disorders and its potential use in other disorders are examined.

Novel halogenated analogs of tomoxetine that are potent and selective inhibitors of norepinephrine uptake in brain

Halogenated analogs of the potent norepinephrine (NE) uptake inhibitor, tomoxetine, were synthesized and their affinities for the serotonin (5HT) and NE uptake sites evaluated. One of the most potent was the 2-iodo substituted analog (289306) that inhibited [3H]tomoxetine binding to rat cerebral cortex with a Ki of 0.37 nM. The compound also inhibited the uptake of [3H]NE into rat hypothalamic synaptosomes with a Ki of 3.5 nM. This analog was significantly less potent at the 5HT uptake site, as exhibited by a Ki of 25 nM in the inhibition of [3H]paroxetine binding and a Ki of 121 nM in [3H]5HT uptake. The resolved (R) enantiomer (303926) was 10 times more potent as a [3H]NE uptake inhibitor and 29 times more potent as an inhibitor of [3H]tomoxetine binding than the (S) enantiomer (303884). Administration of 289306 to rats prior to an i.c.v. injection of 6-hydroxydopamine prevented the depletion of hypothalamic NE and Epi with ED50 values of 0.28 and 0.47 mg/kg, respectively. Thus, 289306 was a potent inhibitor of NE uptake in vitro and in vivo. In addition, these compounds provide structures for potential ligands for the study of NE uptake sites by autoradiography, PET or SPECT imaging.

Facilitatory and inhibitory effects of selective norepinephrine reuptake inhibitors on hypogastric nerve-evoked urethral contractions in the cat: a prominent role of urethral beta-adrenergic receptors

Tricyclic antidepressants (TCA), such as imipramine, are well-accepted for the treatment of urinary incontinence and enuresis, but their mechanism of action remains undefined due to their multiple pharmacological actions. To explore only the contribution imparted by sympathomimetic effects on the urethra by norepinephrine (NE) reuptake inhibition, two selective NE reuptake inhibitors (nisoxetine and tomoxetine) that possess no antimuscarinic or serotonergic properties were examined for their effects on sympathetic hypogastric nerve (HgN) evoked urethral contractions in chloralose anesthetized cats. Under control conditions, HgN stimulation produced a biphasic response composed of a consistent initial contraction that was prazosin- (alpha adrenergic antagonist) sensitive, followed by a more variable relaxation that was propranolol- (beta adrenergic antagonist) sensitive. Unexpectedly, nisoxetine (0.03 to 1.0 mg./kg. intravenously, n = 6) and tomoxetine (0.3 to 3 mg./kg. intravenously, n = 3) produced decreases (about 50% to 60% of control) in HgN-evoked contractions. These inhibitory effects of the reuptake inhibitors were reversed by propranolol. In cats that were pretreated with propranolol, nisoxetine produced a significant increase in HgN-evoked contractions. In conclusion, these results indicate that inhibition of NE reuptake into the sympathetic nerve terminal produces a relative increase in the activation of beta adrenergic receptors compared with alpha adrenergic receptors in the urethra. This increased beta receptor stimulation might be due to a greater diffusion of NE away from the neuro-effector junction to extrajunctional sites following blockade of junctional reuptake. These findings should highlight the importance of urethral beta adrenergic receptors, which is not well-recognized in the literature.

Regional differences in brain norepinephrine and dopamine uptake kinetics in inbred rat strains with hypertension and/or hyperactivity

High-affinity uptake of norepinephrine (NE) and dopamine (DA) were determined in synaptosomes of brain regions from four genetically related inbred rat strains, all derived from the Wistar-Kyoto rat: SHR, WKY, WKHA and WKHT strains. SHRs express hypertension and hyperactivity, WKHAs express hyperactivity alone, WKHTs express hypertension alone, and WKYs are neither hypertensive nor hyperactive. Significant increases in NE uptake, primarily in Vmax, in cerebral cortical areas and the cerebellum, were associated with the hypertensive trait. Significant increases in DA uptake Vmax in the frontal cortex were associated with the inheritance of hyperactivity among these strains. A limited study in SHRs indicated that DA uptake in the frontal cortex increased with age, and that males did not differ from females. No changes in DA uptake in the neostriatum were found with respect to either strain, or age or sex. These findings revealed changes in brain catecholamine neuronal function that are of relevance to both hypertension and hyperactivity. This was made possible by the availability of WKHA and WKHT, in addition to WKYs, as appropriate controls for the SHR.

4-Iodotomoxetine: a novel ligand for serotonin uptake sites

The tomoxetine analog, R-4-iodotomoxetine, binds in vitro to a single site of rat cortical membranes with high affinity (Kd = 0.03 +/- 0.01 nM, n = 4) and can be blocked by a selective serotonin reuptake site inhibitor, paroxetine. The [125I]R-4-iodotomoxetine binding at equilibrium is saturable and is temperature- and Na(+)-dependent. The number of specific [125I]R-4-iodotomoxetine binding sites (Bmax = 356 +/- 20 fmol/mg protein) is similar to that of [3H]citalopram (329 +/- 30 fmol/mg protein), a known serotonin uptake inhibitor. The binding of [125I]R-4-iodotomoxetine is selectively inhibited by several serotonin uptake blockers, and a good correlation is demonstrated between the potency of various drugs to inhibit in vitro binding of [125I]R-4-iodotomoxetine and [3H]citalopram. In addition, lesions performed with the neurotoxin p-chloroamphetamine, which destroys monoamine neurons, including serotonergic neuronal system, result in a 90% reduction of [125I]R-4-iodotomoxetine binding when compared to sham controls. These results indicate that the binding sites labeled by [125I]R-4-iodotomoxetine are associated with the neuronal serotonin uptake sites. However, the in vivo and ex vivo results do not show regional localization corresponding to the distribution of serotonin uptake sites. The nonspecific uptake may be related to this compound's high lipophilicity (octanol-buffer partition coefficient = 1100 - 1400 at pH 7). Although the in vivo properties of [125I]R-4-iodotomoxetine make it an unlikely candidate for mapping serotonin uptake sites with SPECT, the high affinity and selectivity should make it a useful tool for in vitro studies of the serotonin uptake sites.

Characterization of norepinephrine accumulation by a crude synaptosomal-mitochondrial fraction isolated from rat heart

Norepinephrine (NE) uptake into a heart synaptosomal-mitochondrial fraction was assessed under conditions where neuronal uptake (type 1) was linear with respect to both time and protein concentration. The NE accumulation process was sensitive to incubation temperature, sodium ion concentration and medium osmolality. Furthermore, NE uptake was attenuated by the neuronal uptake inhibitor desmethylimipramine (DMI) in a concentration dependent manner; the IC50 value was approximately 10 nM and maximum inhibition was obtained at 100 nM. In contrast, the extraneuronal uptake inhibitor, metanephrine did not significantly attenuate NE uptake. Kinetic analysis demonstrated that the DMI sensitive NE accumulation is saturable with a KM of approximately 400 nM and that NE uptake occurs via a single uptake process. This demonstration of neuronal type NE uptake by a synaptosomal-mitochondrial fraction constitutes a successful demonstration of the preparation of a rat heart subcellular fraction containing functional synaptosomes.

Fluoxetine, a selective inhibitor of serotonin uptake

In summary, fluoxetine is a highly selective serotonin uptake inhibitor in vitro and in vivo. The conformation of fluoxetine, which resembles that of sertraline and other serotonin uptake inhibitors, appears to be a key feature that enables its high affinity and selective interaction with the serotonin transporter. The para-trifluoromethyl substituent, however, is also a pivotal structural element. The molecular pharmacology of fluoxetine has been well-defined, and its in vivo pharmacological effects appear to be mediated almost exclusively by serotonin uptake inhibition. Its selectivity for the serotonin transporter, lack of affinity for neurotransmitter receptors, and retention of selectivity following metabolism to norfluoxetine make fluoxetine a useful tool to explore pharmacologically induced increases in serotonin neurotransmission. Fluoxetine has found a variety of therapeutic application. Its use in treating depression has been most extensively studied, but controlled clinical studies also suggest the drug may have a role in treating obesity and bulimia. Moreover, a variety of other psychiatric disorders may be treatable with this drug. Regardless of the outcome of these clinical trials, it is apparent that fluoxetine has found a useful niche in therapy, and can be used as a probe to determine the role of serotonin in modulating human pathophysiologies.

Characterization of binding of [3H]GBR 12935 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine) to membranes and to solubilized membrane extracts from terminal field regions of mesolimbic, mesocortical and nigrostriatal dopamine pathways

The binding characteristics of [3H]GBR 12935 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine), a selective dopmaine uptake inhibitor, were examined in intact membrane preparations and solubilized extracts of terminal field regions of dopamine pathways in the brain of the rats. There were many similarities in the properties of binding sites for [3H]GBR 12935 in the striatum, nucleus accumbens and olfactory tubercle. The binding of [3H]GBR 12935 was saturable and the affinity constants were not significantly different between regions of the brain. The binding of [3H]GBR 12935 was inhibited by amfonelic acid, GBR 12909, mazindol, methylphenidate and cocaine, with comparable affinities in each region of the brain and with the same order of potency in both preparations. Furthermore, the rank order of potencies for inhibiting the binding of [3H]GBR 12935 was the same as for inhibiting the uptake of [3H]dopamine in these regions of the brain. There did appear to be some degree of heterogeneity of binding sites for [3H]GBR 12935 in each of these regions of the brain, as both amfonelic acid and mazindol were best fitted by two-site models. Whether this apparent heterogeneity was due to the existence of two distinct binding sites or to two components of a single site is unclear. It did not, however, appear to be due to binding to uptake sites for norepinephrine or serotonin, as neither nisoxetine nor fluoxetine, selective inhibitors of the uptake of norepinephrine and serotonin, respectively, inhibited the binding of [3H]GBR 12935, at concentrations which inhibit the uptake of norepinephrine or serotonin.

Serotonin uptake and serotonin uptake inhibition

Serotonin uptake carriers occur on serotonin neurons, on glial cells and on blood platelets. The uptake carrier on serotonin neurons inactivates serotonin that has been released into the synaptic cleft by transporting it back into the nerve terminal. The serotonin uptake carrier is the means by which blood platelets acquire serotonin, since they do not synthesize it. The function of the serotonin uptake carrier on glial cells is poorly understood. Selective inhibitors of serotonin uptake enhance neurotransmission via serotonergic neurons and have been useful pharmacologic tools for studying physiologic roles of serotonin neurons. Some serotonin uptake inhibitors are finding therapeutic uses in mental depression and other psychiatric disorders and in treating obesity and bulimia; other therapeutic applications continue to be evaluated.

Comparison of the effects of cocaine and other inhibitors of dopamine uptake in rat striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex

It is thought that inhibition of dopamine reuptake into neurons may play a major role in the mechanisms by which cocaine produces its reinforcing effects. The striatum, while rich in dopamine terminals, is not implicated in drug reinforcement, whereas the mesolimbic dopamine pathway appears to play a primary role. It is therefore possible that the properties and drug sensitivities of the dopamine uptake systems in the nigrostriatal, mesolimbic, and mesocortical tracts differ. The effects of cocaine, GBR 12909, amfonelic acid, and methylphenidate on dopamine uptake in the striatum, nucleus accumbens, olfactory tubercle, and medial prefrontal cortex were examined. Over 80% of the dopamine uptake in each of the 4 regions was sodium-dependent and exhibited Km values of approximately 100 nM. Cocaine, GBR 12909, amfonelic acid, and methylphenidate each biphasically inhibited uptake in the striatum, nucleus accumbens and olfactory tubercle with GBR 12909 and amfonelic acid being approximately 50-fold more potent than cocaine or methylphenidate. In the medial prefrontal cortex, cocaine and GBR 12909 could inhibit only about 40% of the [3H]dopamine uptake. There are similarities in the properties and drug sensitivities of the dopamine uptake systems in brain areas which are implicated in drug reinforcement and those which are not.

Supersensitivity to noradrenaline in preparations of rat cauda epididymis after vasectomy

Preparations of cauda epididymides were taken from rats unilaterally vasectomised by medial transection of the vas deferens. Responses of preparations from the operated side to field stimulation (10 pulses, 70 V, 1 ms, 0.1-20 Hz) declined in parallel with decreases in catecholamine fluorescence. There were leftward shifts in the mean cumulative concentration-response curves to noradrenaline on vasectomised cauda epididymides of 5-, 10-, and 12-fold at days 2, 7 and 28, respectively compared to unoperated cauda epididymides following vasectomy. Nisoxetine (0.1 mumol/l) enhanced the potency of noradrenaline upon unoperated but not upon vasectomised segments. There were leftward shifts of 7- and 8-fold in the mean cumulative concentration-response curves to acetylcholine in vasectomised cauda epididymides at days 2 and 7, respectively, following surgery. Responses to methoxamine and carbachol were unaffected by vasectomy. Thus neurotransmission to the smooth muscle of the rat cauda epididymis resembles that to the vas deferens, in that vasectomy results in the development of prejunctional supersensitivity to noradrenaline and acetylcholine.

Extracellular ATP stimulates norepinephrine uptake in PC12 cells

This study examined the effects of extracellular ATP on norepinephrine (NE) uptake, using PC12 cells as a model of noradrenergic neurons. Previous experiments with synaptosomes led to the hypothesis that extracellular ATP can regulate NE uptake via an ecto-protein kinase. In the present study, we examined the high-affinity uptake of NE (referred to as uptake 1) in PC12 cells in the presence of varying concentrations of extracellular ATP. In the presence of Ca2+, low concentrations of ATP (0.1 microM) increased uptake 1 by approximately 36%. This increase could be mimicked by adenosine-5'-O-(3-thiotriphosphate) tetralithium salt (ATP gamma S), an analogue of ATP which can be utilized by protein kinases, and not by 5'-adenylylimidodiphosphate tetralithium salt, a nonhydrolyzable analogue of ATP, GTP, ADP, and adenosine also had no effect on uptake 1. Preincubation of the cells with NE and ATP gamma S, followed by washing and assaying NE uptake 30 min later, resulted in a persistent increase in uptake 1. Similar pretreatment with ATP did not show this increase; however, simultaneous pretreatment with ATP and ATP gamma S blocked the activation produced by ATP gamma S alone. Kinetic analysis showed that ATP gamma S pretreatment produces an increase in the Vmax of uptake 1 without altering the apparent Km for NE. These results support the hypothesis that extracellular ATP can regulate NE uptake via an ecto-protein kinase.

Effect of monoamine uptake inhibitors on norepinephrine-stimulated phosphatidylinositol hydrolysis in rat cortex

The effects of the monoamine uptake inhibitors cocaine, nisoxetine, and desipramine (DMI) on norepinephrine (NE) stimulated phosphatidylinositol (PI) hydrolysis were investigated. Rat cortical tissue slices were labeled with [3H]inositol. Slices were then stimulated, in vitro, with NE in LiCl containing buffer in the presence and absence of monoamine uptake inhibitors. Cocaine and nisoxetine, but not DMI, potentiated NE-stimulated PI hydrolysis with a significant decrease in the EC50. Nisoxetine appeared to be more potent than cocaine with respect to the potentiation of NE-stimulated PI hydrolysis. The potentiating effect of cocaine was biphasic and dependent upon the concentrations of cocaine and NE. The NE concentration-effect curve was shifted to the right 100-fold in the presence of 0.1 microM prazosin. Cocaine at 10 microM did not potentiate NE-stimulated PI hydrolysis in the presence of 0.1 microM prazosin. Cocaine at 10 microM did not affect significantly the binding of [3H]prazosin or the NE-[3H]prazosin competition binding to cortical membranes. The results suggest that NE-uptake inhibition by cocaine and nisoxetine is the mechanism for the enhancement of NE-stimulated PI hydrolysis.

Biodistribution, dosimetry, metabolism and monkey PET studies of [18F]GBR 13119. Imaging the dopamine uptake system in vivo

The in vivo characteristics of a new radiotracer, [18F]GBR 13119, have been examined. Full body biodistribution in rats has been determined and the expected human dosimetry calculated. Pharmacological specificity of in vivo regional brain distribution in rats was examined. Blockage of specific binding was accomplished by dopamine reuptake inhibitors but no effect was observed for pretreatment with serotonin or norepinephrine reuptake inhibitors. Preliminary examination of rat blood shows the presence of radiolabeled metabolites, which can be rapidly identified using bonded-phase (Sep-Pak) chromatography. Finally, the striatum of living primates has been imaged using PET and i.v. administration of [18F]GBR 13119. These results represent the intermediate steps in the development of [18F]GBR 13119 as a radiotracer for the study of the dopamine uptake system in man.

Synthesis and regional mouse brain distribution of [11C]nisoxetine, a norepinephrine uptake inhibitor

Nisoxetine, a selective and high affinity (IC50 = 1 nM) inhibitor of NE reuptake, has been radiolabeled in high specific activity (greater than 600 Ci/mmol) by the alkylation of the nor-methyl precursor with [11C]CH3I. Synthetic yields are good (40-60% from [11C]methyl iodide, corrected for decay, 20 min synthesis), with the product purified by HPLC. In vivo studies of regional brain distribution in CD-1 mice show uptake and retention of tracer in the cortex, striatum, hypothalamus and thalamus, with the highest levels in the hypothalamus and cortex. Specific binding in the cortex and hypothalamus can be reduced by preadministration of 7 mg/kg i.v. unlabeled nisoxetine. The possible value of [11C]nisoxetine as a PET imaging agent is discussed.

Lack of generalization of nisoxetine with amphetamine in the rat

Rats were trained to discriminate between the stimulus properties of intraperitoneally administered d-amphetamine (0.8 mg/kg) and its vehicle in a two-lever, food-motivated operant task. Once trained, doses of the norepinephrine reuptake inhibiting agent nisoxetine, ranging from 10 to 20 mg/kg, were administered to investigate if the amphetamine-trained rats would generalize to this agent. This did not, however, occur. Thus, it would seem that noradrenergic mechanisms have a negligible role in the production of the amphetamine-induced discriminative stimulus cue in the rat. Previous evidence that indicated a noradrenergic mediation of amphetamine discrimination in the mouse contrasted with the present results in rats and this discrepancy should warrant caution in comparing results of discriminative studies in these two species.

Selective interaction of tricyclic antidepressants with a subclass of rat brain cholinergic muscarinic receptors

Experiments were undertaken to determine whether the anticholinergic actions of tricyclic antidepressants are mediated by a selective interaction with a subclass of muscarinic receptors. To this end, the potencies of these antidepressants to inhibit [3H]-QNB binding to rat brain cerebral cortical membranes was compared to their potencies as antagonists of carbachol-stimulated inositol phosphate accumulation in cerebral cortical slices and carbachol-induced inhibition of GTP-stimulated adenylate cyclase in striatal membranes. Whereas amitriptyline was more potent than pirenzepine, a selective muscarinic M1 receptor antagonist, in competing for [3H]-QNB binding sites and as an antagonist of carbachol-induced inhibition of adenylate cyclase, pirenzepine was substantially more active (ten-fold) than amitriptyline in blocking carbachol-stimulated phosphatidyl inositol turnover. Atropine was more potent than all other agents in these assays, failing to display any significant degree of selectivity. The results suggest that the tricyclic antidepressants, in particular amitriptyline, appear to be selective antagonists for muscarinic receptors associated with adenylate cyclase in striatal membranes. Given the current classification of cholinergic receptors, these findings indicate that the tricyclic antidepressants may be useful for defining the properties of M2 receptors in brain.

5-Hydroxytryptamine uptake and imipramine binding sites in neurotumor NCB-20 cells

NCB-20 cells (neuroblastoma X fetal Chinese hamster brain hybrids) are equipped with a [3H]5-hydroxytryptamine [( 3H]5-HT) uptake system and [3H]imipramine recognition sites. Approximately 80% of the radioactivity taken up by cells incubated with [3H]5-HT was identified with 5-HT. [3H]5-HT uptake was temperature-dependent, partially sodium-dependent, saturable (Km = 7.3 +/- 0.6 microM; Vmax = 2.0 +/- 0.6 pmol/min/mg), and inhibited by clomipramine, imipramine, fluoxetine, and desipramine, but not by iprindole, mianserin, or opipramol. Lineweaver-Burk plots showed a competitive type of inhibition by imipramine and fluoxetine. [3H]5-HT uptake was not inhibited by nisoxetine or benztropine. [3H]Imipramine binding sites had a KD of 12 +/- 2 nM and a Bmax of 22 +/- 7 pmol/mg protein. The binding was sodium-sensitive although to a lesser extent than that found with brain membranes. Imipramine binding was displaced by tricyclic antidepressants with the following order of potency: clomipramine greater than imipramine greater than fluoxetine greater than desipramine much greater than iprindole = mianserin greater than opipramol. These results suggest that imipramine binding sites are present together with the 5-HT uptake sites in NCB-20 cells and that these sites interact functionally but are different biochemically.

Quantitative autoradiography of [3H]indalpine binding sites in the rat brain: I. Pharmacological characterization

The binding of [3H]indalpine (4-[2-(3-indolyl)]ethyl piperidine) to slide-mounted sections of rat brain has been characterized. This 5-hydroxytryptamine (5-HT) uptake blocker binds to sections with high affinity (KD approximately 1 nM). The binding is saturable, and can be displaced by the addition of clomipramine (1 microM). Other drugs inhibiting the uptake of 5-HT also have the capacity to inhibit the binding of [3H]indalpine. A significant correlation (r = 0.86) was found between the capacity of these compounds to inhibit the uptake of 5-HT and their potencies as inhibitors of [3H]indalpine binding. Binding was Na+ - and Cl- -dependent and was inhibited competitively by 5-HT. Furthermore, electrolytic lesions of the dorsal raphe or medial forebrain bundle, which cause a degeneration of 5-HT cell bodies and fibers, respectively, resulted in a 30-40% reduction in the binding of [3H]indalpine. [3H]Indalpine binds to the 5-HT uptake recognition sites in a different manner from imipramine-like compounds.

Neurochemical profile of Lu 19-005, a potent inhibitor of uptake of dopamine, noradrenaline, and serotonin

The neurochemical profile of a new compound, Lu 19-005 [(+/-)trans-3-(3,4-dichlorophenyl)-N-methyl-1-indanamine hydrochloride], has been investigated. Lu 19-005 is a potent inhibitor of the synaptosomal uptake of 3,4-dihydroxyphenylethylamine (dopamine, DA), noradrenaline (NA), and 5-hydroxytryptamine (5-HT, serotonin). In this respect it resembles diclofensine, whereas compounds such as GBR 13.069 and bupropion are more selective DA-uptake inhibitors. Although Lu 19-005 releases DA in in higher concentrations it must be considered as an uptake inhibitor, as the accumulation of DA is inhibited in much lower concentrations. Lu 19-005 attenuates the DA and NA depletion caused by 6-hydroxydopamine in mouse brain. These properties confirm the DA- and NA-uptake-inhibitory properties of the compound. In receptor-binding models and functional in vitro tests Lu 19-005 is devoid of dopaminergic-, serotonergic-, noradrenergic-, histaminergic-, and cholinergic-inhibiting properties. Since DA, NA, and 5-HT seem to be involved in depression, the profile of Lu 19-005--with equally potent activity on the three neuronal systems--makes it an interesting experimental tool and a potential new antidepressant agent.

Modification of alpha-adrenoceptor agonist antinociceptive activity by nisoxetine: a selective inhibitor of noradrenergic uptake

The alpha-adrenoceptor agonists, clonidine and xylazine, produced a dose-dependent antinociceptive effect in the mouse writhing assay as does morphine. Fluoxetine, a highly-specific inhibitor of serotonin uptake, enhanced the antinociceptive effect of morphine in this test but not that of clonidine or xylazine. In contrast, nisoxetine, a selective inhibitor of noradrenergic uptake, significantly potentiated the antinociceptive activity of morphine, clonidine, and xylazine. These findings strengthen the evidence for an involvement of a noradrenergic mechanism in the antinociceptive effects of alpha-adrenoceptor agonists.

Effects of norepinephrine and serotonin uptake inhibitors on the schedule-controlled behavior of the pigeon

The effects of nortriptyline, amitriptyline, desipramine, chlorimipramine, protriptyline, doxepin, nisoxetine, fluoxetine and iprindole were determined on responding by pigeons under a multiple fixed-ratio 30-response, fixed-interval 10-minute schedule of grain presentation. Those drugs which have been shown to block uptake of norepinephrine decreased fixed-interval quarter-life values. Those which are considered most selective as norepinephrine uptake inhibitors also increased overall fixed-interval responding. These increases in fixed-interval responding, both on local and overall rates, in pigeons appear to be due to the actions of these drugs to inhibit uptake of norepinephrine rather than to other actions.

Nisoxetine and amphetamine share discriminative stimulus properties in mice

The interaction of amphetamine with noradrenergic neurons could mediate a portion of the drug's discriminative stimulus properties. To test this hypothesis, mice were trained to discriminate 1.0 or 3.2 mg/kg amphetamine, 32 mg/kg of the selective norepinephrine uptake inhibitor, nisoxetine, or 32 mg/kg nisoxetine + 1.0 mg/kg amphetamine from saline. Differential drug- or saline-appropriate responding was determined using a two photocell-beam procedure with beam interruption as the operant. Reinforcement (5-sec access to evaporated milk) was presented on a fixed-ratio 20 (FR-20) schedule. Mice trained to discriminate 1.0 mg/kg amphetamine from saline generalized to nisoxetine (32 mg/kg) alone and to doses of 0.56 mg/kg amphetamine and above but not to lower doses unless pretreated with nisoxetine (20 or 32 mg/kg). Mice trained to discriminate nisoxetine (32 mg/kg) from saline generalized to 0.56, 1.0 and 3.2 mg/kg amphetamine and generalized to all amphetamine doses when pretreated with nisoxetine (32 mg/kg). Mice trained to discriminate the drug combination from saline generalized to nisoxetine (32 mg/kg) alone, and to 3.2 mg/kg amphetamine tested alone, to 0.56 mg/kg of amphetamine or above when the lower dose of nisoxetine (20 mg/kg) was used, and to all test doses of amphetamine with nisoxetine (32 mg/kg) pretreatment. Mice trained to discriminate 3.2 mg/kg amphetamine from saline generalized to no test dose of amphetamine following either saline or nisoxetine (32 mg/kg) pretreatment. Testing with several doses of pentobarbital (1.0, 3.0, 10.0 and 18.0 mg/kg) resulted in saline-appropriate responding regardless of training group.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of biogenic amines reuptake inhibition on ethanol induced hypothermia

The mechanism of ethanol induced hypothermia (EIH) was examined by the use of chemically related compounds which inhibit 5-hydroxytryptamine (5-HT) or norepinephrine (NE) reuptake. Desipramine, a tricyclic antidepressant drug (TCA) and NE reuptake inhibitor partially antagonizes EIH. However, Nisoxetine (NE reuptake inhibitor but not TCA) did not abolish EIH. Chlorimipramine, a TCA compound and 5-HT reuptake inhibitor abolishes EIH. Meanwhile, fluoxetine (5-HT reuptake inhibitor, but not TCA) potentiated ethanol induced hypothermia. It was concluded that the antagonism of EIH is probably related to the antidepressant effect of TCA compounds.

The effect of highly selective inhibitors of the uptake of noradrenaline or 5-hydroxytryptamine on TRH-induced hyperthermia in mice

Well established as supposed antidepressant drugs, desipramine (1.25-5 mg/kg), nisoxetine (0.625-2.5 mg/kg) and clomipramine (1.25-5 mg/kg) but not fluoxetine (2.5-40 mg/kg) or citalopram (2.5-40 mg/kg) dose-dependently potentiated TRH (40 mg/kg)-induced hyperthermia in mice. Alpha-adrenergic blocking agents, phenoxybenzamine (20 mg/kg) and prazosin (5 mg/kg), which when given alone lowered body temperature, did not prevent the thermogenic effect of TRH but completely abolished the potentiating effect of clomipramine and almost completely antagonized the same effect of desipramine. The potentiating effect of desipramine on TRH-induced hyperthermia was also attenuated by 4 mg/kg l-propranolol but not by the same dose of d-propranolol. l-Propranolol (4 mg/kg) did not affect the potentiating effect of clomipramine. Cyproheptadine (5 mg/kg), an antagonist of 5-hydroxytryptamine receptors (which, like the alpha-adrenoceptor antagonist, produced hypothermia in normal mice) did not prevent the effects of clomipramine or desipramine. We conclude that a noradrenergic rather than a 5-hydroxytryptaminergic mechanism is involved in the potentiating effect of antidepressant drugs on TRH-induced hyperthermia. Hence, screening tests for antidepressants, which are based on the potentiation of the TRH-induced hyperthermia will always result in false negatives for antidepressants, such as citalopram, which are highly selective inhibitors of the uptake of 5-hydroxytryptamine.

Effect of labetalol and YM-09538 on neuronal uptake of (3H)norepinephrine in the rat vas deferens

Labetalol and YM-09538 are combined alpha and beta receptor antagonists with demonstrated antihypertensive activity. Both compounds inhibited the uptake of (3H)norepinephrine into nerves in the rat vas deferens. However, labetalol was approximately 5 fold more potent than YM-09538 as an inhibitor of neuronal uptake. Inhibition of neuronal uptake occurred at concentrations 43 and 3800 times higher than necessary for inhibition of alpha receptors by labetalol and YM-09538, respectively. Thus YM-09538 shows a greater separation between neuronal uptake blocking properties and alpha receptor blocking properties than labetalol. This separation of activities may contribute to greater antihypertensive efficacy of YM-09538 relative to labetalol.

Effects of chronic treatment with antidepressants on aggressiveness induced by clonidine in mice

It has previously been found that a number of typical and atypical antidepressants, given chronically, intensify clonidine-induced aggressiveness in mice. Further experiments now show that chronic, but not acute, administration of nisoxetine, a selective inhibitor of noradrenaline uptake, potentiates clonidine-induced aggressiveness. Citalopram and fluvoxamine, two selective inhibitors of serotonin uptake, have no such action. Of the two isomers of flupenthixol, only the trans-form potentiates clonidine-induced aggressiveness of chronic experiments. The cis-form induces an inhibiting effect. Clonidine-induced aggressiveness is also intensified by chronic, but not by acute, administration of pizotifen, an antagonist fo serotonin and noradrenaline. The results seem to support the previous hypothesis that potentiation of clonidine-induced aggressiveness is mediated by an alpha 1-adrenergic mechanism.

Ionophore (A23187)-induced efflux of [3H]norepinephrine and endogenous norepinephrine in the rat vas deferens

The calcium ionophore, A23187, produced a concentration-dependent increase in the release of norepinephrine from nerves in the rat vas deferens. Maximum response to A23187 (10(-6) - 10(-5) M) was delayed in onset, occurring 60-80 min after initiation of continuous superfusion with A23187. In fact. after tissue exposure to A23187 (10(-5) M) for only 5 min with subsequent superfusion in A23187-free buffer, a significant but delayed increase in norepinephrine efflux occurred. The A23187-induced increase in efflux of norepinephrine was not altered when neuronal sodium conductance was blocked with tetrodotoxin (3.1 X 10(-7) M) or when Na+, K+ -stimulated ATPase was blocked with ouabain (10(-4) M). Release of norepinephrine by A23187 was calcium-dependent since A23187-induced efflux of norepinephrine was diminished (approximately 50%), although not abolished, in calcium-free buffer. Thus, one component of A23187 action was calcium independent. A23187 caused an increased efflux of both norepinephrine formed endogenously and [3H]norepinephrine taken up into neuronal stores. However, the effects of A23187, both on rate and maximum amount of release were greater for [3H]norepinephrine than for endogenous norepinephrine. The present studies demonstrate that neurotransmitter efflux can be induced by carboxylic ionophores in a calcium-dependent process, and this approach may prove useful in studies evaluating factors that modulate neurotransmitter release processes.

Use of potentiation of thyrotrophin releasing hormone (TRH)-induced hyperthermia as a test for screening antidepressants which activate alpha-adrenoceptor systems

1 The minimal dose which significantly potentiates the hyperthermia induced by thyrotrophin releasing hormone (TRH, 40 mg/kg i.p.) in mice has been established for tricyclic and other antidepressants (imipramine, amitriptyline, clomipramine, nortriptyline, maprotiline, nomifensine, viloxazine) including a specific inhibitor of noradrenaline (NA) uptake (nisoxetine). 2 The minimal effective dose in this test has been compared with the minimal dose of the same compounds antagonizing reserpine-induced hypothermia. The ratio of the two doses for each substance indicates that potentiation of TRH-induced hyperthermia is, in general, the more sensitive test. 3 A correlation seems to exist between the alpha-adrenergic effect of antidepressants and the potentiation of TRH- induced hyperthermia. Those antidepressants which do not act on alpha-adrenergic systems (butriptyline, amineptine, trazodone, danitracen, fluoxetine) are inactive in this test. 4 This property may be used to select antidepressants that activate alpha-adrenoceptor systems.

Clonidine analgesia and suppression of operant responding: dissociation of mechanism

In the rat the effects of clonidine and xylazine were compared on a measure of analgesia (tail-withdrawal from hot water) and on operant responding maintained by a fixed-ratio 20-response schedule of food presentation. Clonidine (2--8 mg/kg) produced dose-dependent increase in tail-withdrawal latency and was approximately twice as potent as xylazine (8--16 mg/kg) in this test. This analgesic effect of 8 mg/kg of clonidine was antagonized by phenoxybenzamine (10 mg/kg), and high doses of yohimbine (5--10 mg/kg), whereas the effect of 2 mg/kg of clonidine was potentiated by a nonanalgesic dose of nisoxetine (10 mg/kg). Clonidine (0.0063--0.2 mg/kg) and xylazine (0.25--8 mg/kg) produced a dose-dependent suppression of fixed-ratio responding. The potency of clonidine in this task was approximately 40 times greater than that of xylazine. The suppression of responding produced by both drugs was antagonized by low doses of yohimbine (0.5--2 kg/mg), which was maximally effective at a dose of 1 mg/kg. The data suggest that when using this type of assay for analgesia, the antinociceptive effects of clonidine which are measured are a result of stimulation of alpha-adrenergic receptors or noradrenergic neurons or by adrenergic receptors which inhibit noradrenergic neurons.

Differential effects of antidepressant treatment on brain monoaminergic receptors

Chronic (21 days) antidepressant administration to rats results in a decrease in both serotonin and beta-adrenergic, but not cholinergic muscarinic, receptor binding in selected brain regions, with the frontal cortex appearing to be somewhat more sensitive to this effect. Neither nisoxetine nor fluoxetine, potent and specific inhibitors of norepinephrine and serotonin uptake respectively, caused receptor binding changes after chronic administration, suggesting that inhibition of transmitter uptake, in itself, is insufficient to cause receptor subsensitivity. In vitro experiments indicated that antidepressants are relatively weak alpha 2-receptor blocking agents, but some are potent on the alpha 1-receptor system indicating that the norepinephrine releasing potency of some antidepressants may not be mediated by blockade of presynaptic autoreceptors.