Potentiation of DOM-induced stimulus control by non-competitive NMDA antagonists: a link between the glutamatergic and serotonergic hypotheses of schizophrenia

The present investigation examined the interaction between 2,5-dimethoxy-4-methylamphetamine [DOM] and non-competitive NMDA antagonists in rats trained with DOM [0.6 mg/kg; 75 min pretreatment time] as a discriminative stimulus. Pretreatment with phencyclidine [PCP] at a dose of 3 mg/kg shifted the DOM dose-response relationship to the left. When a fixed dose of DOM [0.1 mg/kg] which by itself yielded 32% DOM-appropriate responding was combined with a range of doses of PCP, dizocilpine, and ketamine, DOM-appropriate percentages increased to maxima of 73%, 84%, and 79%, respectively. When given alone, PCP, dizocilpine, and ketamine were followed by maxima of 36%, 15%, and 13%, respectively. It is concluded that the effects of DOM as a discriminative stimulus are potentiated by pretreatment with non-competitive antagonists of glutamate receptors of the NMDA subtype. These data suggest that the application of the technique of drug-induced stimulus control may prove useful in the reconciliation and integration of current hypotheses as to the etiology of psychotic disorders.     

Fluoxetine, a selective inhibitor of serotonin uptake, potentiates morphine analgesia without altering its discriminative stimulus properties or affinity for opioid receptors

The analgesic effect of morphine in the rat tail jerk assay was enhanced by the serotonin uptake inhibitor, fluoxetine. Tail jerk latency was not affected by fluoxetine alone. Morphine's affinity for opioid receptors labeled in vitro with 3H-naloxone or 3H-D-Ala2-D-Leu5-enkephalin was not altered by fluoxetine, which has no affinity for these sites at concentrations as high as 1000 nM. In rats trained to discriminate morphine from saline, fluoxetine at doses of 5 or 10 mg/kg were recognized as saline. Increasing the fluoxetine dose to 20 mg/kg did not result in generalization to either saline or morphine. The dose response curve for morphine generalization was not significantly altered by fluoxetine doses of 5 or 10 mg/kg. Those rats treated with the combination of morphine and 20 mg/kg of fluoxetine did not exhibit saline or morphine appropriate responding. Fluoxetine potentiates the analgesic properties of morphine without enhancing its affinity for opioid receptors or its discriminative stimulus properties.     

Role of 5-hydroxytryptamine1B receptors and 5-hydroxytryptamine uptake inhibition in the cocaine-evoked discriminative stimulus effects in rats.

Mesolimbic dopamine pathways play a critical role in the behavioural effects of cocaine in rodents. Nonetheless, research has also demonstrated involvement of 5-hydroxytryptamine (5-HT; serotonin) transmission in these effects. The present study investigated the ability of selective 5-HT1B receptor ligands and a 5-HT reuptake inhibitor to substitute for or to alter (enhance or antagonise) the discriminative stimulus effects of cocaine. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, i.p.) from saline (i.p.) in a two-choice, water-reinforced fixed ratio (FR) 20 drug discrimination paradigm. In substitution tests, the selective 5-HT1B receptor agonist 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-propoxypyrrolo[3,2-b]pyridine (CP 94253; 2.5-5 mg/kg, i.p.) and the 5-HT reuptake inhibitor fluoxetine (5-10 mg/kg, i.p.) elicited ca. 40 and 0% drug-lever responding, respectively. In combination experiments, CP 94253 (2.5-5 mg/kg) given with submaximal doses of cocaine (0.3-2.5 mg/kg) produced a leftward shift in the cocaine dose-response curve; pretreatment with CP 94253 (5 mg/kg) prior to a dose of cocaine (2.5 mg/kg) which elicited lower than 40% drug-lever responding, caused full substitution. Fluoxetine (5 and 10 mg/kg) given in combination with a submaximal dose of cocaine (2.5 mg/kg) produced a 100% drug-lever responding. Pretreatment with the 5-HT1B receptor antagonists N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-1,1'-biphenyl-4 carboxamide (GR 127935; 0.5-5 mg/kg, s.c.) and 3-(3-dimethylamino)-propyl)-4-hydroxy-N-[4-(4-pyridinyl)-phenyl]benzamide (GR 55562; 1 mg/kg, s.c.) failed to modulate the dose-effect curve for cocaine (0.6-5 mg/kg). On the other hand, GR 127935 (5 mg/kg) and GR 55562 (1 mg/kg) significantly attenuated the enhancement of cocaine discrimination evoked by a combination of CP 94253 (5 mg/kg) or fluoxetine (5 mg/kg) and cocaine (2.5 mg/kg). These results indicate that 5-HT1B receptors are not directly involved in the cocaine-induced discriminative stimuli in rats. On the other hand, they indicate that pharmacological stimulation of 5-HT receptors--that also seem to be a target for fluoxetine-mediated increase in 5-HT neurotransmission--can enhance the overall effects of cocaine.     

Fluoxetine Increases Extracellular Dopamine in the Prefrontal Cortex by a Mechanism Not Dependent on Serotonin

Fluoxetine at 10 and 25 mg/kg increased (167 and 205%, respectively) the extracellular dopamine concentration in the prefrontal cortex, whereas 25 (but not 10) mg/kg citalopram raised (216%) dialysate dopamine. No compound modified dialysate dopamine in the nucleus accumbens. The effect of 25 mg/kg of both compounds on cortical extracellular dopamine was not significantly affected by 300 mg/kg p-chlorophenylalanine (PCPA) (fluoxetine, saline, 235%; PCPA, 230%; citalopram, saline, 179%; PCPA, 181%). PCPA depleted tissue and dialysate serotonin by approximately 90 and 50%, respectively, and prevented the effect of fluoxetine and citalopram on dialysate serotonin (fluoxetine, saline, 246%; PCPA, 110%; citalopram, saline, 155%; PCPA, 96%). Citalopram significantly raised extracellular serotonin from 0.1 to 100 microM (251-520%), whereas only 10 and 100 microM increased dialysate dopamine (143-231%). Fluoxetine similarly increased extracellular serotonin (98-336%) and dopamine (117-318%). PCPA significantly reduced basal serotonin and the effects of 100 microM fluoxetine (saline, 272%; PCPA, 203%) and citalopram (saline, 345%; PCPA, 258%) on dialysate serotonin but did not modify their effect on dopamine (fluoxetine, saline, 220%; PCPA, 202%; citalopram, saline, 191%; PCPA, 211%). The results clearly show that the effects of fluoxetine and of high concentrations of citalopram on extracellular dopamine do not depend on their effects on serotonin.     

Discriminative stimulus properties of the serotonergic agent 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)

Using a two-lever drug discrimination procedure, six rats were trained to discriminate 0.5 mg/kg of racemic 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) from saline. Once trained, the animals demonstrated a dose-related decrease in discriminative performance upon administration of lower doses of DOI (ED50 = 0.16 mg/kg). DOI-stimulus generalization occurred with the putative 5-HT2 agonist DOM (ED50 = 0.49 mg/kg), but not with the 5-HT1A agonist 8-OH DPAT, or the 5-HT1B agonist TFMPP. Furthermore, the DOI stimulus could be antagonized by pretreatment of the animals with the 5-HT2 antagonist ketanserin. The present results, coupled with the prior demonstration that DOI possesses a significant affinity and selectivity for 5-HT2 binding sites, suggest that the discriminative stimulus effects of DOI may be 5-HT2-mediated.     

Comparative behavioural profile of newer antianxiety drugs on different mazes

Anxiety is associated with diverse range of psychiatric conditions. In the present study, antianxiety effect of fluoxetine, citalopram (SSRI's), gabapentin (antiepileptic drugs), venlafaxine (SNRI), clozapine and resperidone (atypical antipsychotics) and a herbal preparation ashwagandha on elevated zero maze and elevated plus maze paradigms was examined. Anti-anxiety potentials of these drugs were compared with diazepam. The drugs tested i.e. fluoxetine (10 mg/kg), citalopram (10 mg/kg), clozapine (0.25, 0.5, 1 mg/kg), resperidone.(0.5, 1 mg/kg), venlafaxine (4, 8, 16 mg/kg), citalopram (10 mg/kg), fluoxetine (10 mg/kg), gabapentin (10, 20 mg/kg) and ashwagandha (100, 200 mg/kg) significantly increased the number of open arm entries and time spent in open arm. These drugs also decreased the latency to enter in open arm as compared to control in both the paradigms. Present study confirms the antianxiety activity of different newer classes of drugs and found some of them comparable to diazepam in both the elevated zero maze and elevated plus maze paradigm.     

Acute and Chronic Treatments with Citalopram Lower Somatostatin Levels in Rat Brain Striatum Through Different Mechanisms

Abstract: The suggestion that somatostatin is involved in the pathophysiology of obsessive-compulsive disorder and the evidence that selective serotonin reuptake inhibitors show significant antiobsessional effect prompted us to examine the effect of citalopram, a selective and potent serotonin reuptake inhibitor, on the somatostatinergic system in different brain regions of the rat. A single intraperitoneal injection of 10 mg/kg citalopram significantly reduced somatostatin levels in the striatum and nucleus accumbens after 4 but not 1, 8, or 24 h. No changes were found in hippocampus. In addition, we found that the K⁺-evoked overflow of somatostatin-like immunoreactivity from striatal slices was significantly increased 1 h after a single injection of citalopram and was still higher, although not significantly, 4 h after the drug injection. Levels of preprosomatostatin mRNA were unchanged in striatum and accumbens 1 and 4 h after a single drug administration. In rats treated with citalopram (10 mg/kg i.p.) twice daily for 14 days, the levels of somatostatin and its mRNA were significantly decreased in the striatum but not in other brain regions 24 h after the last dose. No change was found in the basal or K⁺-evoked overflow of somatostatin-like immunoreactivity at 1, 4, and 24 h after the last drug injection. These results suggest that acute and chronic treatment with citalopram reduces somatostatin levels in striatum by different mechanisms. Whereas a single dose of the drug reduces somatostatin levels by increasing the release of the peptide, repeated drug treatment reduces the biosynthesis of somatostatin.     

Discriminative stimulus properties of the serotonin agonist 1-(3-trifluoromethylphenyl)piperazine (TFMPP)

Using a standard two-lever drug discrimination procedure, twelve rats were trained to discriminate 1.0 mg/kg of the serotonin (5-HT) agonist TFMPP from saline. Once trained, the animals displayed a dose-related decrease in discriminative performance upon administration of lower doses of TFMPP. Tests of stimulus generalization were performed using the purported 5-HT agonist RU-24, 969 and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM). While TFMPP produced stimulus effects similar to those of RU-24,969, these effects seem to be dissimilar to those of DOM. The results of the present study suggest that the discriminative stimulus effects of TFMPP may involve a 5-HT1-related mechanism.     

Effect of subchronic lithium treatment on citalopram-induced increases in extracellular concentrations of serotonin in the medial prefrontal cortex

We investigated the effect of citalopram [a selective serotonin (5-HT) reuptake inhibitor; SSRI] and MKC-242 (a selective 5-HT1A agonist), following treatment with subchronic lithium (p.o., 1 week) on extracellular 5-HT concentrations in the medial prefrontal cortex (mPFC). Acute treatment with citalopram (3 and 30 mg/kg) led to significant increases in extracellular 5-HT concentrations. The subchronic lithium group showed significantly higher basal levels of extracellular 5-HT than normal diet controls. Acute citalopram (3 and 30 mg/kg) treatment together with subchronic lithium treatment showed significant increases in the extracellular 5-HT concentrations, compared with citalopram treatment alone. Acute MKC-242 (1 mg/kg) treatment showed significant decreases in extracellular 5-HT concentrations, in both the normal diet and lithium diet groups to the same extent. The addition of lithium did not change the effect of the 5-HT1A agonist on extracellular 5-HT concentrations. This study suggests that lithium augmentation of the antidepressant effect of SSRI is mediated by the additional increases in extracellular 5-HT concentrations following the co-administrations of lithium and SSRI.     

Fluoxetine Partly Exerts its Actions Through GABA: A Neurochemical Evidence

Fluoxetine, as a serotonin re-uptake inhibitor augments serotonin concentration within the synapse by inhibiting the serotonin transporter. The contribution of amino acids has also been shown in depression. We hypothesized that fluoxetine exerts its actions at least in part by intervening brain signaling operated by amino acid transmitters. Therefore the aim of this study is to supply neurochemical evidence that fluoxetine produces changes in amino acids in cerebrospinal fluid of rats. Sprague-Dawley rats were anesthetized and concentric microdialysis probes were implanted stereotaxically into the right lateral ventricle. Intraperitoneal fluoxetine (2.5 or 5 mg/kg) or physiological saline was administered and the probes were perfused with artificial cerebrospinal fluid at a rate of 1 μl/min. In the chronic fluoxetine group, the rats were treated daily with oral fluoxetine solution or inert syrup for 3 weeks. The microdialysis probes were placed on the 21st day and perfused the next day. Fluoxetine was ineffective in changing the cerebrospinal fluid GABA levels at the dose of 2.5 mg/kg but produced a significant increase in the perfusates following injection of 5 mg/kg of fluoxetine (P < 0.05). Oral fluoxetine administration (5 mg/kg) for 21 days also elevated the CSF GABA levels by approximately 2-fold (P < 0.05). l-glutamic acid levels were not affected in all groups. These neurochemical findings show that fluoxetine, a selective serotonin re-uptake inhibitor affects brain GABA levels indirectly, and our results suggest that acute or chronic effects may be involved in beneficial and/or adverse effects of the drug.     

Effects of citalopram and fluoxetine on the corticocerebral blood flow in conscious rabbits

Depression, which is associated with an increased incidence of vascular events, frequently occurs following stroke. Selective serotonin reuptake inhibitory drugs (SSRIs) as antidepressants, are well tolerated, and also seem to be effective in post-stroke depression. The aim of this study was to investigate the effects of the SSRIs citalopram and fluoxetine, on the corticocerebral blood flow (cCBF) in rabbits with unilateral carotid occlusion induced cerebral ischemia. The cCBF was measured by the hydrogen clearance technique. After determination of the mean baseline cCBF, the effects of individual doses (0.1, 0.3 and I mg/kg) of citalopram or fluoxetine on the cCBF were investigated. Following the induction of an impaired cCBF, the changes in cCBF after drug treatments in this condition were likewise measured. The mean arterial blood pressure (MABP) and the heart rate (HR) from the electrocardiogram (ECG) were also determined. Neither citalopram nor fluoxetine influenced the cCBF in the control group. Fluoxetine improved the cCBF only very slightly in the ischemic animals. In contrast, all the doses of citalopram exerted pronounced and dose-dependent cCBF-increasing effects in the animals with unilateral carotid occlusion (maximal mean ACBF: 10, 16 and 27 ml/min/100 g tissue). The HR was decreased in both groups. Only citalopram treatment led to a slight MABP-decreasing effect. Besides enhancement of the serotonergic transmission in the brain, the cCBF-increasing effect of citalopram under ischemic conditions may be of benefit in post-stroke and vascular depression.     

Benzodiazepine receptor mediated discriminative cues: Effects of GABA-ergic drugs and inverse agonists

Rats were exposed to a two-layer drug discrimination procedure using the benzodiazepine (BZ) receptor inverse agonists N′-methyl-β-carboline-3-carboxamide (FG 7142) or methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM). FG 7142 (30 mg/kg) failed to acquire discriminative stimulus control, although it did suppress responding. The same group of animals was trained successfully to discriminate diazepam (DZP, 2.5 mg/kg) from vehicle. The DZP cue was potentiated by the GABA agonist 4,5,6,7-tetrahydro-isoxazolo [5, 4-c] pyridin-3-ol (THIP, 1–3 mg/kg); THIP alone produced vehicle-appropriate responding. In addition, clonazepam (0.2 mg/kg) and chlordiazepoxide (5 mg/kg) substituted for DZP (with potencies of 7.5 and 0.25 times that of DZP, respectively). In antagonism tests, FG 7142 (5–17.5 mg/kg), methyl-β-carboline-3-carboxylate (β-CCM, 2.5 mg/kg), nicotine (0.3 mg/kg), harmaline (5 mg/kg) and naltrexone (10 mg/kg) did not effect, bicuculine (2 mg/kg) and DMCM (1 mg/kg) partially blocked, and the BZ receptor antagonist Ro 15–1788 (40 mg/kg) completely blocked the discriminative stimulus effects of DZP. In animals trained to discriminate DMCM (0.2 mg/kg) from vehicle, 95% substitution occured with bicuculline (2 mg/kg); DZP (1–5 mg/kg) completely antagonized DMCM. These results indicate that the DZP cue is mediated by GABA-coupled BZ receptors and that GABA may modulate the efficacy of a BZ at its receptor site. However, since inverse BZ receptor agonists (FG 7142, DMCM and β-CCM) were, at best, only marginally effective in antagonizing DZP, the DZP cue may be mediated by a distinct subclass of BZ receptors.     

Potentiation of LSD-induced stimulus control by fluoxetine in the rat

The present investigation examined the interaction between fluoxetine enantiomers and LSD in rats trained with LSD as a discriminative stimulus. The dose response relationships for LSD alone and in combination with either (+)- or (−)-fluoxetine were determined. In both instances, the LSD dose response relationship was shifted to the left. However, statistical significance was observed only for the (+)- enantiomer. It is concluded that the effects of LSD as a discriminative stimulus are potentiated by the acute administration of fluoxetine. These results are consistent with anecdotal reports of interactions between LSD and acutely administered SSRIs in humans.     

Blockade of substance P (neurokinin 1) receptors enhances extracellular serotonin when combined with a selective serotonin reuptake inhibitor: an in vivo microdialysis study in mice

Abstract Substance P antagonists of the neurokinin-1 receptor type (NK1) are gaining growing interest as new antidepressant therapies. It has been postulated that these drugs exert this putative therapeutic effect without direct interactions with serotonin (5-HT) neurones. Our recent microdialysis experiment performed in NK1 receptor knockout mice suggested evidence of changes in 5-HT neuronal function (Froger et al. 2001). The aim of the present study was to evaluate the effects of coadministration of the selective 5-HT reuptake inhibitor (SSRI) paroxetine with a NK1 receptor antagonist (GR205171 or L733060), given either intraperitoneally (i.p.) or locally into the dorsal raphe nucleus, on extracellular levels of 5-HT ([5-HT]ext) in the frontal cortex and the dorsal raphe nucleus using in vivo microdialysis in awake, freely moving mice. The systemic or intraraphe administration of a NK1 receptor antagonist did not change basal cortical [5-HT]ext in mice. A single systemic dose of paroxetine (4 mg/kg; i.p.) resulted in a statistically significant increase in [5-HT]ext with a larger extent in the dorsal raphe nucleus (+ 138% over basal AUC values), than in the frontal cortex (+ 52% over basal AUC values). Co-administration of paroxetine (4 mg/kg; i.p.) with the NK1 receptor antagonists, GR205171 (30 mg/kg; i.p.) or L733060 (40 mg/kg; i.p.), potentiated the effects of paroxetine on cortical [5-HT]ext in wild-type mice, whereas GR205171 (30 mg/kg; i.p.) had no effect on paroxetine-induced increase in cortical [5-HT]ext in NK1 receptor knock-out mice. When GR205171 (300 micro mol/L) was perfused by 'reverse microdialysis' into the dorsal raphe nucleus, it potentiated the effects of paroxetine on cortical [5-HT]ext, and inhibited paroxetine-induced increase in [5-HT]ext in the dorsal raphe nucleus. Finally, in mice whose 5-HT transporters were first blocked by a local perfusion of 1 micro mol/L of citalopram into the frontal cortex, a single dose of paroxetine (4 mg/kg i.p.) decreased cortical 5-HT release, and GR205171 (30 mg/kg i.p.) reversed this effect. The present findings suggest that NK1 receptor antagonists, when combined with a SSRI, augment 5-HT release by modulating substance P/5-HT interactions in the dorsal raphe nucleus.     

Agmatine attenuates neuropathic pain in rats: possible mediation of nitric oxide and noradrenergic activity in the brainstem and cerebellum

Effect of agmatine (10-400 mg/kg) on neuropathic pain in a rat model produced by loose ligatures around the common sciatic nerve was studied. The involvement of possible alterations in nitric oxide (NO) levels [measured as its stable metabolites nitrate + nitrite] and in noradrenergic activity [measured as norepinephrine and 3-methoxy-4-hydroxyphenylethylene glycol (MHPG) levels] in this effect was also investigated biochemically in the brainstem and cerebellum. Agmatine increased the neuropathic pain threshold at 300 and 400 mg/kg. There was almost a twofold increase in nitrate + nitrite levels in the brainstem and cerebellum of the rats with neuropathic pain and agmatine decreased the high nitrate + nitrite levels only in the brainstem at 300 mg/kg and both in the brainstem and cerebellum at 400 mg/kg. Ligation of sciatic nerve resulted in almost twofold increase in norepinephrine and MHPG levels only in the brainstem of the rats. Agmatine decreased MHPG levels at 300 and 400 mg/kg, however it decreased norepinephrine levels only at the higher dose. These findings indicate that agmatine decreases neuropathic pain, an effect which may involve the reduction of NO levels and noradrenergic activity in the brain.     

Role of serotonin (5-HT) in the antidepressant-like properties of neuropeptide Y (NPY) in the mouse forced swim test

Neuropeptide Y (NPY) is thought to be implicated in depressive disorders. The mouse forced swim test (FST) is an animal model widely used as a predictor of the efficacy of antidepressant drugs. The present study was undertaken to explore the possible contribution of endogenous serotonin (5-HT) systems in the behavioral effects elicited by NPY in this model. The selective serotonin re-uptake inhibitor (SSRI), fluoxetine, was also tested for comparison. 5-HT was depleted prior to testing by the administration of the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA; 300 mg/kg, i.p., each day for 3 days; control mice received saline-vehicle over the same period). On the fourth day, mice received NPY (3 nmol, I.C.V.), fluoxetine (16 mg/kg, i.p.) or saline injections before testing in the FST. Both NPY and fluoxetine significantly reduced immobility time in saline-treated control animals. Pre-treatment with PCPA significantly blocked the effects of fluoxetine in the FST, confirming the role of endogenous 5-HT. Similarly, pre-treatment with PCPA also significantly attenuated the anti-immobility effects of NPY, thus suggesting a role for 5-HT in the effects of NPY in the FST. Quantitative receptor autoradiography revealed increases in specific [125I][Leu31, Pro34]PYY sites that were sensitive to BIBP3226 (Y1-like sites) in various brain regions. Specific [125I]GR231118 and [125I]PYY(3-36) binding levels were not changed following PCPA treatment, suggesting that depletion of endogenous 5-HT resulted in an apparent increase in the level of Y1 sites in their high-affinity state. Taken together, these results suggest a role for 5-HT-related systems in the antidepressant-like properties of NPY.     

Inhibition of gut- and lung-derived serotonin attenuates pulmonary hypertension in mice

Decreasing the bioavailability of serotonin (5-HT) by inhibiting its biosynthesis may represent a useful adjunctive treatment of pulmonary hypertension (PH). We assessed this hypothesis using LP533401, which inhibits the rate-limiting enzyme tryptophan hydroxylase 1 (Tph1) expressed in the gut and lung, without inhibiting Tph2 expressed in neurons. Mice treated repeatedly with LP533401 (30-250 mg/kg per day) exhibited marked 5-HT content reductions in the gut, lungs, and blood, but not in the brain. After a single LP533401 dose (250 mg/kg), lung and gut 5-HT contents decreased by 50%, whereas blood 5-HT levels remained unchanged, suggesting gut and lung 5-HT synthesis. Treatment with the 5-HT transporter (5-HTT) inhibitor citalopram decreased 5-HT contents in the blood and lungs but not in the gut. In transgenic SM22-5-HTT+ mice, which overexpress 5-HTT in smooth muscle cells and spontaneously develop PH, 250 mg/kg per day LP533401 or 10 mg/kg per day citalopram for 21 days markedly reduced lung and blood 5-HT levels, right ventricular (RV) systolic pressure, RV hypertrophy, distal pulmonary artery muscularization, and vascular Ki67-positive cells (P < 0.001). Combined treatment with both drugs was more effective in improving PH-related hemodynamic parameters than either drug alone. LP533401 or citalopram treatment partially prevented PH development in wild-type mice exposed to chronic hypoxia. Lung and blood 5-HT levels were lower in hypoxic than in normoxic mice and decreased further after LP533401 or citalopram treatment. These results provide proof of concept that inhibiting Tph1 may represent a new therapeutic strategy for human PH.     

METHYLHISTAMINE: EVIDENCE FOR SELECTIVE DEAMINATION BY MAO B IN THE RAT BRAIN IN VIVO

Abstract— A new method has been developed for the separation of histamine and its metabolites after intracisternal injection of [³H]histamine into the rat brain, involving solvent extraction and subsequent thin-layer chromatography. The effect of graded doses of the MAO inhibitors deprenil and pargyline, which at relatively low doses inhibit preferentially the B form (phenethylamine deaminating) of the enzyme, and clorgyline, which mainly inhibits the A form (serotonin, noradrenaline and dopamine deaminating) on the brain levels of intracisternally injected [³H]histamine and its labelled metabolites was studied and compared to MAO A and B activity as determined with the substrates serotonin and phenethylamine, respectively. In addition, the time-course of the effects of a single dose of pargyline (50mg/kg subcutaneously) was investigated. No [³H]imidazoleacetic acid could be detected in any of the control or treated animals. [³H]Histamine accounted for 9–12% of the total extracted radioactivity and this was not altered significantly by pretreatment with any of the MAO inhibitors up to high doses, at which both MAO A and B activities were completely inhibited. In the controls, 40–43% of the total extracted radioactivity was [³H]methylhistamine and 28–30% was [³H]methylimidazoleacetic acid. Deprenil and pargyline caused [³H]methylhistamine levels to increase in a dose-dependent manner up to about 150% of control levels and those of [³H]methylimida-zoleacetic acid to decrease concomitantly to about 10% of control levels. Clorgyline in doses up to 10 mg/kg subcutaneously (s.c.) had no effect on the levels of these two metabolites. The dose-response curves of the effects of deprenil and pargyline on [³H]methylimidazoleacetic acid levels were congruent with those of the MAOI effects on MAO B activity and not with those on MAO A activity. Pargyline (50 mg/kg s.c.) had a long lasting effect on the accumulation of [³H]methylhistamine and [³H]methylimidazoleacetic acid. Recovery occurred within 21 days, and the half-lives observed were 5.3 and 5.6 days, respectively. This compares well to the half-life for the recovery of MAO B activity reported earlier after the same dose of pargyline (5.5 days). These results suggest that methylhistamine is metabolized selectively by MAO B in rat brain. Moreover, the fact that clorgyline, at doses where phenethylamine deamination is already considerably inhibited, did not affect the deamination of methylhistamine, suggests that the latter is an even more selective substrate for MAO B than phenethylamine itself. Therefore, small doses of deprenil (0.3–3 mg/kg s.c.) or pargyline (1–3 mg/kg) can be used to influence histamine catabolism without interfering with catecholamine or serotonin deamination.     

Increased acetylcholinesterase activity in selected regions of rat brain after chronic (−)-Deprenyl administration

(−)-Deprenyl, 0.05, 1.0, 2.0, and 10.0 mg/kg body weight, was administered intraperitonially to Wistar rats for 30 days. The activity of acetylcholinesterase, and monoamine oxidase A and B were assayed in different brain regions. After the experimental period acetyl cholinesterase activity was found to be significantly increased in frontal cortex [P<0.001] and hippocampus [P<0.001] but not in striatum and brainstem at 0.1, 1.0, and 2.0 mg/kg dose, the maximum increase being at 0.1 mg/kg dose. Monoamine oxidase B activity was inhibited by more than 90% at 1.0, 2.0, and 10.0 mg/kg dose while 0.05 and 0.1 dose inhibited only about 55% and 70% respectively. Monoamine oxidase A activity was inhibited to more than 70% at 1.0 mg dose and to more than 90% at 2.0 and 10.0 mg/kg dose. At 0.05 and 0.1 mg/kg dose monoamine oxidase A activity was not significantly altered.     

Fluoxetine protects against monocrotaline-induced pulmonary arterial remodeling by inhibition of hypoxia-inducible factor-1α and vascular endothelial growth factor

The selective serotonin re-uptake inhibitor fluoxetine has been shown to protect against monocrotaline (MCT)-induced pulmonary hypertension in rats. To investigate the possible role of hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) in mediating this protective effect, MCT-treated rats were administered fluoxetine by gavage, at doses of 2?mg/kg body mass or 10?mg/kg once daily for 3 weeks. Changes in pulmonary hemodynamic parameters, pulmonary artery morphologies, and expressions of HIF-1α and VEGF were assessed. Fluoxetine at the 10?mg/kg dose, but not at the 2?mg/kg dose, attenuated the effects of MCT on pulmonary artery pressure, right ventricle index, and medial wall thickness. In addition, 10?mg/kg fluoxetine mitigated the MCT-induced up-regulation of HIF-1α and VEGF protein and reactive oxygen species (ROS) in the lungs. This dosage also decreased pERK1/2 levels and inhibited proliferation of pulmonary arterial smooth muscle cells in MCT-treated rats. In conclusion, fluoxetine can protect against MCT-induced pulmonary arterial remodeling, which linked to reduced ROS generation and decreased HIF-1α and VEGF protein levels via the ERK1/2 phosphorylation pathway.