Antidepressant and memory affecting influence of estrogen and venlafaxine in ovariectomized rats

The experiments presented in this paper aimed to investigate whether estrogen level changes in oviariectomized rats (OVX) may lead to depression and memory disorders, and whether the effects of such changes may be reversible following administration of a new antidepressant, venlafaxine (CAS 9930-78-4, VEN, Efectin). The Porsolt forced swimming test and Morris water maze test were carried out on female Wistar rats after ovariectomy and in sham-ovariectomized rats. VEN 20 mg/kg was administered orally 30 min before the tests for the period of 14 days. Estradiol (17beta-estradiol benzoate, CAS 50-28-2, E2) administration (5 microg E2/0.2 ml sesame oil s.c.) was started 24 h after ovariectomy and was continued for 14 days--each dose was administered 180 min before the test. In the immobility test, which reflects antidepressant drug activity, it was found that VEN shortened immobility time (IT) after the 1st, 7th and 14th administration (days 1, 7, 14, respectively) in ovariectomized rats, whereas in the control group (sham-ovariectomized rats) VEN exerted antidepressant action only after single administration (day 1) and after 7 days of administration. E2 significantly reduced immobility behaviour both after single and chronic treatment in ovariectomized rats. After joint administration of VEN and E2 potentiation of the antidepressant activity of VEN could be observed in both groups except for concurrent administration of VEN and E2 after 14 days in sham-ovariectomized rats. VEN improved the spatial memory in the Morris water maze test, whereas E2 did not affect the memory of the tested animals. Joint administration of VEN and E2 maintained the memory improving effect induced by VEN. The regulatory role of the steroid hormone and the new antidepressant drug (VEN) in antidepressant activity and memory function could be related to the interactions between noradrenergic and serotoninergic systems.     

Quantitative Untersuchungen über die blutzuckersteigernde Wirkung von synthetischem Oxytocin und Vasopressin sowie über das Auftreten dieses Hormoneffektes unter physiologischen Bedingungen

Zusammenfassung Die blutzuckersteigernde Wirkung von synthetischen Oxytocin und Vasopressin wurde bei verschiedenen Tierarten quantitativ untersucht. Bei Hunden ließen sich linear verlaufende Dosis-Wirkungskurven bestimmen. Meßbare Blutzuckererhöhungen traten nach der i.v. Injektion von 1,25 mE/kg Oxytocin und von 20 mE/kg Vasopressin auf. In Dosen von 1,28 E/kg steigern beide Hormone den Blutzuckergehalt um ca. 100%. Auch mehrfach wiederholte Einzelinjektionen bleiben wirksam. In Dosen von 15 mE/kg i.v. wirkt Vasopressin antagonistisch gegen Oxytocin. In der Wirkung der Hypophysenhinterlappenhormone auf den Blutzucker bestehen große Species-Unterschiede. Diese Differenzen wurden an den gebräuchlichsten Laboratoriumstieren untersucht. Auch bei Fröschen wirken Oxytocin und Vasopressin blutzuckersteigernd.Da bekannt ist, daß Oxytocin die Milchejektion hervorruft und während des Stillens sezerniert wird, wurden Blutzuckerbestimmungen an lactierenden Hunden während des Stillaktes ausgeführt. Regelmäßig stieg die Blutglucosekonzentration um 16–20% an, was einer Wirkung von 3 mE/kg Oxytocin entspricht. Es konnte ausgeschlossen werden, daß Vasopressin oder Adrenalin für diesen Effekt verantwortlich sind. Diese Ergebnisse geben erstmalig einen Hinweis dafür, daß Oxytocin beim Hund eine physiologische Rolle bei der Regulation des Kohlenhydratstoffwechsels spielt.

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Summary The blood glucose increasing effect of synthetic oxytocin and vasopressin was investigated quantitatively in various animals. In dogs linear dose response curves were obtained. First measurable enhancements occur after i.v. injection of 1.25 mU/kg oxytocin and of 20 mU/kg vasopressin. In doses of 1.28 U/kg i.v. both hormones raise blood glucose concentrations by approx. 100%. Frequently repeated injections remain effective. Vasopressin antagonises oxytocin in the dose range of 15 mU/kg i.v. There are great species differences in the action of the hormones of the posterior pituitary lobe on blood glucose elevation. These differences were investigated in common laboratory animals. In the frog too, oxytocin and vasopressin raise blood sugar level.Since it is known that oxytocin evokes milk ejection and is secreted during suckling, blood glucose determinations were done in nursing dogs. In all cases blood glucose concentrations increased by 16 to 20%, corresponding to 3 mU/kg of oxytocin. It could be ruled out, that vasopressin or epinephrine are responsible for this effect. These results therefore are considered to give strong evidence that in dogs oxytocin plays a physiological role in regulating carbohydrate metabolism.

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Mit Unterstützung der Deutschen Forschungsgemeinschaft.     

The effect of venlafaxine on behaviour, body weight and striatal monoamine levels on sleep-deprived female rats

Partial sleep deprivation is clinically associated with fatigue, depressive symptoms and reduced memory. Previously, it has been demonstrated that venlafaxine, an atypical antidepressant, increases the levels of noradrenaline and serotonin in rat hippocampus. The aim of this study was to evaluate the effects of venlafaxine on depression, anxiety, locomotor activity and memory in a model of REM sleep (REMs) deprivation in rats. We have also studied the influence of venlafaxine on monoamine levels in the striatum. Six groups of animals (N=20 each) were treated with saline or venlafaxine (1 or 10 mg/kg) during 10 days, submitted or not to REMs deprivation and studied with the forced swimming test of Porsolt (STP), plus-maze, passive avoidance and open-field tests right after sleep deprivation. Animals were also studied for passive avoidance 24 h later (rebound period). Brain samples for monoamine measurements were collected either immediately after REMs deprivation or after 24 h. Both REMs deprivation and venlafaxine showed an antidepressant effect. An anxiolytic effect was also observed after REMs deprivation. Previous treatment with venlafaxine blocked the antidepressant and anxiolytic effects of REMs deprivation. REMs deprivation alone and treatment with venlafaxine 10 mg/kg increased locomotor activity, and this effect was inhibited by venlafaxine in REMs deprived rats. Both venlafaxine treatment and REMs deprivation induced weight loss. Venlafaxine treatment, but not REMs deprivation, induced an increase in striatal dopamine (DA) levels. The combination of REMs deprivation and venlafaxine treatment was associated with an increase in serotonin turnover 24 h after rebound sleep. In this study, venlafaxine treatment hindered most behavioral effects of REMs deprivation and was associated with an interference on dopamine and serotonin systems in the striatum.     

Untersuchungen über die Wirkungsweise der Hypophysenhinterlappenhormone auf den Kohlenhydrat- und Fettstoffwechsel

Zusammenfassung Die Wirkungsweise von synthetischem Oxytocin und Vasopressin auf den Kohlenhydrat- und Fettstoffwechsel wurde in Versuchen am intakten Tier und an Organschnitten untersucht. Die Hinterlappenhormone steigerten den Blutzuckergehalt auch an adrenalektomierten Katzen und Ratten. Da die Unterschiede gegenüber normalen Tieren nicht signifikant waren, ließ sich ausschließen, daß eine Adrenalinsekretion an der Blutzuckererhöhung wesentlich beteiligt ist. An isolierten Rattenleberschnitten hatten Oxytocin und Vasopressin eine glykogenolytische Wirkung, die bereits bei Konzentrationen um 10^–8 einsetzte. An isolierten Rattenzwerchfellstreifen, die 20 min lang in glucosehaltiger Krebs-Henseleit-Lösung inkubiert wurden, hemmten Oxytocin und Vasopressin meßbar nur den Glykogenaufbau, während Adrenalin auch im Muskel deutlich glykogenolytisch wirkte. In Versuchen an Hunden, die 0,2–20 mE/kg/min Oxytocin bzw. 0,2–40 mE/kg/min Vasopressin 60–90 min lang i.v. infundiert erhielten, zeigten sich folgende Stoffwechseleffekte: Der Blutzuckeranstieg war stets nur vorübergehend, die Ausgangswerte wurden nach 30–60 min trotz fortgesetzter Infusion wieder erreicht. Der Leberglykogengehalt nahm deutlich ab, das Muskelglykogen zeigte dagegen keine wesentlichen Änderungen. Regelmäßig und während der ganzen Infusionsdauer wurde die Konzentration der freien Fettsäuren im Plasma bis um etwa 50% herabgesetzt. Die minimal wirksamen Dosen betrugen beim Oxytocin 0,2 mE/kg/min bzw. beim Vasopressin 1 mE/kg/min. Dagegen hatte Oxytocin in Infusionsversuchen an Menschen selbst in Dosen bis zu 3 mE/kg/min für 60 min keinen deutlichen Einfluß auf die Konzentrationen von Glucose, Milchsäure, Brenztraubensäure und freien Fettsäuren im Blut.

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Summary The mode of action of synthetic oxytocin and vasopressin on carbohydrate and fat metabolism was investigated in experiments on intact animals and on organ slices. The hormones of the posterior pituitary raised blood sugar concentration also in adrenalectomized cats and rats. Since differences to normal animals proved to be insignificant epinephrine secretion could be ruled out as essential for the increase of blood sugar. In isolated rat liver slices oxytocin and vasopressin had a glycogenolytic action, beginning already in concentrations of approx. 10^–8. In isolated rat diaphragm slices, incubated for 20 min in glucose containing Krebs-Henseleit-solution, oxytocin and vasopressin only inhibited glycogen formation while epinephrine had a provable glycogenolytic action in muscles. In experiments on dogs which received continuous infusions of 0.2 to 20 mU/kg/min oxytocin respectively 0,2–40 mU/kg/min vasopressin for 60 to 90 min, the following effects on metabolism were observed: the increase of blood sugar proved to be only transient. In spite of continuous infusions initial values were reached within 30 to 60 min. The glycogen content of the liver decreased markedly while muscle glycogen did not show essential changes. Regularly and during the whole infusion time the concentration of free fatty acids in plasma was lowered down to 50%. Minimal effective dosages amounted to 0.2 mU/kg/min for oxytocin and 1 mU/kg/min for vasopressin. In infusion experiments on man, however, oxytocin in doses up to 3 mU/kg/min for 1 hour, had no clear effect on blood concentrations of glucose, lactic acid, pyruvic acid and free fatty acids.

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Effect of acutely and chronically administered venlafaxine on the anticonvulsant action of classical antiepileptic drugs in the mouse maximal electroshock model

The influence of acute and chronic treatments with intraperitoneal venlafaxine, a selective serotonin/norepinephrine reuptake inhibitor, on the anticonvulsant activity of selected antiepileptic drugs was studied in the maximal electroshock test in mice. Venlafaxine (12.5 and 25 mg/kg), given either acutely or chronically, significantly increased the electroconvulsive threshold. Moreover, both acute and chronic venlafaxine, applied at the highest subprotective dose of 6.25 mg/kg, enhanced the anticonvulsant effect of valproate, without affecting the protective action of carbamazepine, phenobarbital and phenytoin. The antidepressant did not affect brain concentration of valproate, indicating that the interaction between the two drugs seems pharmacodynamic in nature. Despite the lack of effect on the antielectroshock action of the remaining antiepileptics, acute venlafaxine increased the brain concentration of phenobarbital, while chronic venlafaxine reduced the brain level of phenytoin. In terms of adverse effects, acute/chronic venlafaxine and antiepileptic drugs alone, as well as their combinations, did not produce significant motor or long-term memory deficits in mice. Summing up, it seems that venlafaxine may be considered as a safe drug for the clinical use in patients with epilepsy and depressive disorders.     

Inhibition by venlafaxine of the increase in norepinephrine output in rat prefrontal cortex elicited by acute stress or by the anxiogenic drug FG 7142

Venlafaxine is an antidepressant drug that inhibits the reuptake of serotonin and norepinephrine with different efficacies. The effects of repeated administration of this drug on the increase in the extracellular concentration of norepinephrine in the prefrontal cortex, induced by stress or by the anxiogenic drug FG 7142, were studied in freely moving rats. Exposure to foot-shock stress induced a marked increase (+120%) in the extracellular norepinephrine concentration in the prefrontal cortex of control rats. Long-term administration of venlafaxine (10 mg/kg i.p., once a day for 21 days) reduced the effect of stress on norepinephrine output by 75%. This effect of venlafaxine persisted for at least 5 days after discontinuation of drug treatment. Acute administration of FG 7142 (20 mg/kg i.p.), a benzodiazepine receptor inverse agonist, increased norepinephrine output (+90%) in control rats. Chronic treatment with venlafaxine prevented the effect of FG 7142. In contrast, the acute administration of this antidepressant had no effect on the stress- or FG 7142-induced increase in norepinephrine output. These plastic changes in the sensitivity of norepinephrine neurones to foot-shock stress and to an anxiogenic drug may reveal an important neuronal mechanism for the physiological regulation of emotional state. Furthermore, this mechanism might be relevant to the anxiolytic and antidepressant effects of venlafaxine.     

Dose-dependent noradrenergic and serotonergic properties of venlafaxine in animal models indicative of antidepressant activity

The present study was undertaken to investigate thoroughly the preclinical psychopharmacological profile of venlafaxine, testing a wide range of doses in animal models indicative of antidepressant-like effects. Venlafaxine was found to be active in mouse forced swimming test (at 8, 16, 32 and 64 mg/kg) and to increase spontaneous locomotor activity (at 16, 32 and 64 mg/kg). Venlafaxine antagonised apomorphine-induced (16 mg/kg) hypothermia (at 2, 4, 8, 16, 32 and 64 mg/kg). Pretreatment with PCPA significantly attenuated the anti-immobility effects of venlafaxine (8 and 16 mg/kg; P < 0.01) in the mouse forced swimming test. Venlafaxine at a dose of 32 mg/kg remained active, despite PCPA pretreatment. DSP-4 significantly attenuated the anti-immobility effects of venlafaxine (16 mg/kg; P < 0.05), whereas venlafaxine at 32 mg/kg remained active, despite DSP-4 pretreatment. Venlafaxine was active in the forced swimming test when administered at sub-effective doses in combination with (±) pindolol (venlafaxine: 1 and 2 mg/kg), RU 24969 (venlafaxine: 1, 2 and 4 mg/kg), 8-OH-DPAT (venlafaxine: 4 mg/kg), clonidine (venlafaxine: 1, 2 and 4 mg/kg), lithium (venlafaxine: 1, 2, and 4 mg/kg) and quinine (venlafaxine: 1 and 2 mg/kg). Prior administration with NAN-190 antagonised the anti-immobility effects of venlafaxine (8, 16 and 32 mg/kg). Interaction studies did not induce changes in locomotor activity. The results of the present study indicated that, at low doses, venlafaxine inhibited serotonin reuptake, while at higher doses it inhibited both serotonin and noradrenaline reuptake. Received: 19 May 1997/Final version: 1 December 1997     

Antidepressant-like effects of serotonin type-3 antagonist, ondansetron: an investigation in behaviour-based rodent models

The present behavioural investigation evaluates the antidepressant potential of ondansetron (OND), a widely used (in management of cancer chemotherapy-induced nausea and emesis) 5-HT3 receptor antagonist. Separate groups of mice received acute or chronic treatment of OND (0.005-1000 microg/kg), and were subjected to spontaneous locomotor activity test or antidepressant assays, namely, the forced swim and tail suspension tests. Interaction studies with fluoxetine, venlafaxine, desipramine and 8-hydroxy-2-(di-n-propylamino) tetralin were conducted in the forced swim test. The effect of OND (0.01-1000 microg/kg) in combination with paroxetine (10 mg/kg, for 14 days) on the behaviour of male bulbectomized or sham-operated rats was also assessed. The postbulbectomy behavioural analysis included exploration in the open field and elevated plus maze. OND exhibited a biphasic dose-response profile, with antidepressant-like effects peaking at 0.1 microg/kg, in the forced swim and tail suspension tests. None of the tested doses influenced spontaneous locomotor activity. Chronic OND pretreatment augmented the antidepressant effects of fluoxetine and venlafaxine but did not influence the effects of desipramine or 8-hydroxy-2-(di-n-propylamino) tetralin. Chronic OND (10 microg/kg) reversed hyperactivity in the open field, and decreased the percentage entry and time spent in open arms in the elevated plus maze. Summing up, it is observed that OND exhibits antidepressant-like effects, possibly mediated through postsynaptic 5-HT3 receptors.     

Delayed effect of the vasopressin metabolite VP4-8 on the social memory of sexually naive male rats

RATIONALE: Endogenous vasopressin is involved in the social memory of the male rat and administration of exogenous vasopressin improves social memory. These findings are mainly based on studies using sexually experienced males that were tested in the social recognition test. OBJECTIVE: The present study was aimed to establish whether the modulation of social memory by vasopressin fragments depends on the sexual experience of the male rat. For this purpose, the social discrimination test was used, since this test is more suitable than the social recognition test for measuring social memory in sexually naive males. METHODS: Male rats were tested in the social discrimination test and treated subcutaneously with the vasopressin metabolite [pGlu4,Cyt6]vasopressin-(4-8) (VP4-8). VP4-8 shares with vasopressin the effects on memory processes but lacks the peripheral effects of vasopressin. RESULTS: VP4-8 (1 microgram/kg) acutely improved the social memory of sexually experienced male rats, confirming previous reports. However, in sexually naive males VP4-8 failed to improve social memory in doses ranging from 0.1 microgram/kg to 1 microgram/kg. Instead, 1 microgram/kg VP4-8 or 6 micrograms/kg desglycinamide-vasopressin were found to exert a delayed effect in sexually naive rats. This delayed effect resulted in an improved social memory 2 days after administration. CONCLUSIONS: Vasopressin sensitisation is discussed as a possible underlying mechanism of the observed delayed effect of vasopressin fragments. It is concluded that in male rats sexual experience can influence the modulation of social memory by vasopressin.     

Venlafaxine compromises the antinociceptive actions of gabapentin in rat models of neuropathic and persistent pain

Rationale Neuropathic pain is associated with a number of disease states of diverse aetiology that can share common pathophysiological mechanisms. Antiepileptic drugs modulate ion channel function and antidepressants increase extracellular monoamine levels, and both drug classes variously attenuate signs and symptoms of neuropathic pain. Thus, coadministration of the antiepileptic gabapentin and the antidepressant venlafaxine may provide superior pain relief to administration of either drug alone.Objectives To systematically establish the pain relieving efficacies of venlafaxine and gabapentin alone and in combination.Materials and methods Gabapentin (50 and 100 mg/kg, s.c.) and venlafaxine (10, 25, 50 mg/kg, s.c.) were tested alone or in combination in the rat spared nerve injury (SNI) model of neuropathic pain and the rat formalin test of persistent pain. Diuresis was measured in a separate experiment after administration of venlafaxine.Results Hindpaw mechanical allodynia was dose-dependently reversed by gabapentin (50 and 100 mg/kg, s.c.), whereas venlafaxine was ineffective (10 and 50 mg/kg, s.c.). Both gabapentin and venlafaxine also attenuated hindpaw mechanical hyperalgesia. Surprisingly, coadministration of venlafaxine (50 mg/kg) significantly lowered the antiallodynic effect of both doses of gabapentin by up to 60% in spared-nerve-injury rats and a negative antinociceptive interaction between gabapentin and venlafaxine was also observed in the rat formalin test. We demonstrated that venlafaxine administration was associated with a dose-dependent increase in urine output over the time course of the nociceptive experiments.Conclusion Venlafaxine compromises the antiallodynic effects of coadministered gabapentin most probably as consequence-increased diuresis.     

Influence of sildenafil on the antidepressant activity of bupropion and venlafaxine in the forced swim test in mice

Recent studies highlight the involvement of the nitrergic system in the mechanism of action of antidepressant drugs. Sildenafil, a selective PDE5 inhibitor, was shown to abolish the anti-immobility effects of bupropion, venlafaxine and s-citalopram in mice. In this study we assessed the effects of sildenafil on the activity of bupropion and venlafaxine in the forced swim test in mice. Swim trials were conducted by placing mice in glass cylinders filled with water for 6 min and the duration of the behavioral immobility during the last 4 min of the test was evaluated. Locomotor activity was evaluated with photoresistor actimeters. Brain and serum concentrations of the studied antidepressants were determined by HPLC method. Sildenafil at a dose of 20 mg/kg, but not 5 and 10 mg/kg, significantly increased the anti-immobility action of bupropion (20 mg/kg). The antidepressant activity of venlafaxine (2 mg/kg) was potentiated by joint administration with sildenafil at doses of 10 and 20 mg/kg. Since the combined treatments did not increase the locomotor activity, the antidepressant-like effects were not related to non-specific behavioral activation. Data from pharmacokinetic studies revealed that sildenafil increased bupropion and venlafaxine levels in serum without affecting their concentrations in the brain. The present study demonstrates the enhancement of anti-immobility action of bupropion and venlafaxine by sildenafil co-administration. The observed changes might have been partly due to pharmacokinetic interactions. However, mechanisms underlying the effects of sildenafil on the antidepressant activity of bupropion and venlafaxine should be carefully evaluated in further studies.     

Concomitant use of carbamazepine and olanzapine and the effect on some behavioral functions in rats

As shown in clinical studies, combinations of first generation normothymics (carbamazepine - CBZ) with atypical neuroleptics (olanzapine - OLA) lead to improvements in approximately half of patients treated for relapses of bipolar affective disease. Our previous studies have shown OLA to have an antidepressant effect when administered at a dose of 0.5 mg/kg only upon single administration; the effect did not last throughout chronic administration, whereas CBZ administered at a dose of 30 mg/kg showed an antidepressant effect only after 7 days of administration. As shown in our previous studies, both OLA and CBZ improve memory in rats but only after chronic administration. The improved antidepressant effect of many drugs, including OLA and CBZ used in combined therapy - as observed in our clinic - as well as confirmed evidence of OLA's and CBZ's positive effects on cognitive functions in humans and animals substantiated commencement of research on defining the effect of combined administration of OLA and CBZ on sedation (tested in a locomotor activity test), antidepressant effect (Porsolt test) and spatial memory (Morris test) in animals. The tests were performed on male Wistar rats. It was found that in combined administration of CBZ and OLA for 7 and 14 days, OLA would completely prevent the CBZ's sedative effect. With combined administration of CBZ and OLA, both as a single dose and after prolonged treatment for 7 days, a significant reduction in immobility time was observed. Combined administration of CBZ and OLA did not improve memory in rats that received these drugs in a single dose, whereas statistically significant differences were observed in the chronic experiments. It can be assumed that the observed effects of combined administration of CBZ and OLA may be due to the pharmacokinetic interactions, but further studies are necessary to confirm these assumptions.     

Role of the opioidergic system and nitric oxide in the analgesic effect of venlafaxine

The noradrenalin and serotonin re-uptake inhibitor venlafaxine has an analgesic effect that is independent of its antidepressant activity; however, the mechanism of this effect remains to be elucidated. This study was performed to investigate the possible roles of the opioidergic system and nitric oxide (NO) pathway in the analgesic effect of venlafaxine. Eighty Wistar rats of both sexes were allocated to 10 groups. The hot plate test was used to assess the antinociceptive/analgesic effect. The temperature of the hot plate was adjusted to 52.5+/-1 degrees C, the cut-off period was set to be 50 sec; licking of the hind paw was used as a sign of pain perception. Venlafaxine alone (25 mg/kg) showed marked analgesic activity (p<0.05). N-omega-nitro-L-arginine (L-NOARG) alone (20 mg/kg) and naloxone alone (2 mg/kg and 4 mg/kg) showed no analgesic activity (p>0.05). Coadministration of low-dose naloxone (2 mg/kg) and both doses of L-NOARG (20 and 40 mg/kg) with venlafaxine (25 mg/kg) did not modify the analgesic effect but high-dose naloxone (4 mg/kg) decreased it significantly (p<0.05). In conclusion, these results suggest that the opioidergic system but not the NO pathway has a role in the analgesic effect of venlafaxine.     

Venlafaxine in depressed outpatients

Venlafaxine is a structurally novel compound with a biochemical and pharmacological profile suggesting antidepressant properties. We report the results of a Phase II, double-blind, placebo-controlled clinical trial assessing the efficacy and safety of venlafaxine in a sample of 93 depressed outpatients. Venlafaxine doses of 25 mg t.i.d., 75 mg t.i.d., and 125 mg t.i.d. were compared to placebo. Patients receiving venlafaxine showed a significantly greater improvement in their mood symptoms compared to those receiving placebo. Venlafaxine was well tolerated and the most common side effect was nausea. There was some evidence to suggest that venlafaxine may have antidepressant activity within the first 2 weeks of treatment.     

Distinct Neuropsychological Mechanisms May Explain Delayed- Versus Rapid-Onset Antidepressant Efficacy

The biochemical targets for antidepressants are relatively well established, but we lack a clear understanding of how actions at these proteins translate to clinical benefits. This study used a novel rodent assay to investigate how different antidepressant drugs act to modify affective biases that have been implicated in depression. In this bowl-digging task, rats encounter two equal value learning experiences on separate days (one during an affective manipulation and the other during control conditions). This induces an affective bias that is quantified using a preference test in which both digging substrates are presented together and the individual rats’ choices recorded. The assay can be used to measure affective biases associated with learning (when the treatment is given at the time of the experience) or examine the modification of previously acquired biases (when the treatment is administered before the preference test). The rapid-onset antidepressant ketamine, but not the delayed-onset antidepressant, venlafaxine, attenuated the previously acquired FG7142-induced negative bias following systemic administration. Venlafaxine but not ketamine induced a positive bias when administered before learning. We then used local drug infusions and excitotoxic lesions to localize the effects of ketamine to the medial prefrontal cortex and venlafaxine to the amygdala. Using a modified protocol we also showed that positive and negative biases amplified further when the numbers of substrate–reinforcer associations are increased. We propose that this pattern of results could explain the delayed onset of action of venlafaxine and the rapid onset of action but lack of long-term efficacy seen with ketamine.     

Dual effect of local application of nitric oxide donors in a model of incision pain in rats

The effects of chronic administration of the mixed serotonin [5-hydroxytryptamine (5-HT)]/norepinephrine re-uptake inhibitor venlafaxine (5 mg/kg daily by osmotic minipump for 28 days) on the sensitivity of somatodendritic 5-HT(1A) autoreceptors on serotonergic neurons innervating the hypothalamus, and on 5-HT(1B) autoreceptors in both hypothalamus and hippocampus, were determined using in vivo microdialysis in freely moving rats. Venlafaxine induced a reduction in sensitivity of 5-HT(1B) autoreceptors in hypothalamus, but did not affect the sensitivity of 5-HT(1A) autoreceptors, or of 5-HT(1B) autoreceptors in hippocampus. The corticosterone and oxytocin responses to the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.05 or 0.2 mg/kg), a measure of postsynaptic 5-HT(1A) receptor activity in the hypothalamus, were reduced in animals administered 5 or 10 mg/kg venlafaxine daily by intraperitoneal injection for 21 days. This desensitization of post-synaptic 5- HT(1A) receptors in the hypothalamus may be a consequence of increased 5-HT levels induced by desensitization of the presynaptic 5-HT(1B) receptors. These results taken together with those of previous studies suggest that the hypothalamus might be an important site of drug action, and that venlafaxine has an overall mechanism similar to that of selective serotonin re-uptake inhibitors.     

Venlafaxine occupation at the noradrenaline reuptake site: in-vivo determination in healthy volunteers

Venlafaxine, a serotonin and noradrenaline reuptake inhibitor, is an effective antidepressant at doses of 75 mg p.o. daily and above. Preclinical and healthy volunteer studies have demonstrated that venlafaxine is more potent at the serotonin than at the noradrenaline reuptake site, with noradrenergic blocking effects being observed at doses >75 mg p.o. in man. We used the Multiple Organs Coincidences Counter and [11C] meta hydroxy ephedrine (MHED) to test whether significant occupation of cardiac sympathetic neurones was achieved in man in vivo after the acute administration of venlafaxine 75 mg p.o. in nine healthy volunteers. MHED is a tracer which binds at the noradrenaline reuptake site. This study demonstrates that the [11C]MHED signal is significantly reduced after the administration of venlafaxine 75 mg p.o. thus showing that noradrenaline reuptake blockade is observable at this dose. This effect is predominantly seen in volunteers who received > 1 mg/kg venlafaxine.     

High concentrations of oxytocin cause vasoconstriction by activating vasopressin V1A receptors in the isolated perfused rat kidney

The aim of this study was to evaluate the renal vascular effects of oxytocin in Sprague-Dawley rats and in Brattleboro heterozygous or homozygous rats, the latter being genetically deficient in vasopressin synthesis. Studies were performed in vitro, in the isolated kidney perfused in an open circuit with a Tyrode's solution. Oxytocin induced a concentration-dependent renal vasoconstriction in Sprague-Dawley rats, at rather high concentrations (EC50=170+/-39 nM, mean +/- SEM, n=6) with a maximum response amounting to 44% of that elicited by vasopressin (increase in renal vascular resistance: 11.5+/-0.9 mmHg min ml(-1) vs. 26.2+/-2.2 mmHg min ml(-1)). Oxytocin-evoked renal vasoconstriction was abolished by SR 49059, a selective vasopressin V1A receptor antagonist (10 nM), but not by d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-(NH2)9] vasotocin, an oxytocin receptor antagonist (10 nM). In the presence of SR 49059, oxytocin did not induce renal vasorelaxation. Oxytocin induced renal vasoconstriction in Brattleboro homozygotes and heterozygotes (EC50=59+/-12 nM and 262+/-110 nM; Emax=7.8+/-1.1 mmHg min ml(-1) and 6.9+/-0.4 mmHg min ml(-1), n=5 respectively) with characteristics similar as observed in Sprague-Dawley rats concerning partial agonist activity, low potency and antagonism by SR 49059. Responsiveness to vasopressin did not differ in Brattleboro homozygotes and heterozygotes (EC50 approximately 0.25 nM) and was similar as we reported in Sprague-Dawley rats. These findings indicate that high concentrations of oxytocin induce renal vasoconstriction in the rat by activating vasopressin V1A receptors. The low agonist activity makes it unlikely that oxytocin can substitute functionally for vasopressin at the renal vascular V1A receptor in Brattleboro homozygous rats which are deficient in endogenous vasopressin.     

The partial NMDA agonist D-cycloserine stimulates LH secretion in healthy volunteers

 Because of clinical interest in the effects of antidepressant drugs that exert their effects on multiple neurotransmitter systems, the behavioral effects produced by combined treatment with an SSRI (fluoxetine) with a selective norepinephrine (NE; desipramine) or dopamine (DA) reuptake inhibitor (buproprion) were examined in the forced swimming test (FST), a behavioral test in rodents that predicts the clinical activity of antidepressants. Three additional compounds with mixed activity as NE-5-HT reuptake inhibitors, milnacipran, duloxetine and venlafaxine, were also examined. Desipramine and fluoxetine both reduced immobility in the FST, but desipramine increased only climbing behavior, whereas fluoxetine increased only swimming behavior. The combination of fluoxetine with desipramine or bupropion increased both climbing and swimming behaviors at certain doses, but higher doses of desipramine when combined with fluoxetine replaced swimming behavior with climbing behavior. The mixed NE-5-HT reuptake inhibitors milnacipran and duloxetine reduced immobility and increased climbing behavior, but did not alter swimming. Venlafaxine reduced immobility and increased swimming behavior, except at the highest dose tested (80mg/kg), which increased both swimming and climbing behaviors. Thus, combining certain doses of pharmacologically selective monoamine reuptake inhibitors, or the mixed reuptake inhibitor venlafaxine, produced a pattern of mixed active behaviors in the FST (climbing and swimming) that may reflect the activity of multiple neurotransmitters, especially the combination of enhanced 5-HT and DA activity. The combination of higher doses of desipramine with fluoxetine, or compounds with mixed activity at inhibiting NE and 5-HT reuptake, demonstrated effects similar to those of desipramine alone and may reflect inhibition of the expression of serotonergic antidepressant behavioral effects by selective NE reuptake inhibitors. Received: 10 June 1997/Final version: 19 August 1997