PCPA Pretreatment Research Articles

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine’s actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Azathioprine and mesalamine can impair pancreatic ductal bicarbonate secretion in mice

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine’s actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

S-Ketamine Mediates Its Acute and Sustained Antidepressant-Like Activity through a 5-HT1B Receptor Dependent Mechanism in a Genetic Rat Model of Depression.

Rationale: The mechanisms responsible for the unique antidepressant properties of ketamine have only been partly resolved. Recent preclinical reports implicate the neurotransmitter serotonin [5-hydroxytryptamine (5-HT)] in the antidepressant-like response of ketamine, and modulation of 5-HT1B receptors has been hypothesized to attain an important role.Objectives: To evaluate the role of endogenous stimulation of 5-HT1B heteroreceptors in the antidepressant-like activity of S-ketamine.Method: Flinders sensitive line (FSL) rats, a genetic model of depression, were depleted of endogenous 5-HT by 4-chloro-DL-phenylalanine methyl ester HCl administration (pCPA; 86 mg/kg/day for 3 days). In pCPA-pretreated and control FSL rats, the acute and sustained effects of a single dose of S-ketamine (15 mg/kg) and the selective 5-HT1B receptor agonist CP94253 (1–6 mg/kg) alone and in combination with S-ketamine were studied in the forced swim test (FST), a commonly used assay that detects antidepressant activity.Results: pCPA pretreatment decreased cortical 5-HT levels to ∼6% but did not affect the baseline behavioral phenotype of FSL rats. S-ketamine demonstrated acute and sustained antidepressant-like activity, both of which were abolished by 5-HT depletion. Combining S-ketamine with a sub-effective dose of CP94253 (1 mg/kg) rescued S-ketamine’s acute and sustained antidepressant-like effects, when CP94253 was administered 2 h prior to the FST. Co-administration of S-ketamine and CP94253 did not affect the plasma level of either compound, suggesting that the observed behavioral interaction could not be ascribed to a kinetic drug-drug interaction.Conclusion: 5-HT1B receptor activation during testing appears to be critical for S-ketamine’s antidepressant-like potentials in this model.

Involvement of the monoamine system in antidepressant-like properties of 4-(1-phenyl-1h-pyrazol-4-ylmethyl)-piperazine-1-carboxylic acid ethyl ester

Piperazinic derivatives have therapeutic potential by acting as analgesic, antidepressant-like, anticonvulsant and antipsychotic in preclinical studies. In order to develop new drugs to treat mental disorders, we designed and synthesized the 4-(1-phenyl-1H-pyrazol-4-ylmethyl)-piperazine-1-carboxylic acid ethyl ester (PPMP), a new piperazine derivative with putative activities on central nervous system that seems to involve serotonergic system. In order to investigate the antidepressant-like activity of PPMP, mice were treated acutely and tested in the forced swimming test (FST) and tail suspension test. Pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA, 100 mg/kg, i.p., 4 days), and the non-selective blocker of catecholamine synthesis α-methyl para-tyrosine (AMPT, 100 mg/kg, i.p.) were used to assay the involvement of serotonergic and catecholaminergic systems. “Ex vivo” monoamine oxidase (MAO) enzymatic assay and quantification of hippocampal level of brain derived neurotrophic factor (BDNF) were carried out. PPMP reduced the immobility time in both tests. PCPA or AMPT (100 mg/kg, i.p.) pretreatment blocked the effects of PPMP, thereby suggesting the involvement of serotonergic and catecholaminergic systems in the antidepressant-like effect of PPMP. PPMP did not inhibit the activity of MAO. Moreover, after 14 days of treatment, PPMP 15 mg/kg/day induced antidepressant-like effect and increased hippocampal level of BDNF. None of the treatments in this study altered the locomotor activity in the open field test. In conclusion, PPMP demonstrates antidepressant-like effect that involve both serotonergic and catecholaminergic systems without inhibition of MAO activity. PPMP administration increased the hippocampal levels of BDNF.

Serotonergic mechanism of the relieving effect of bee venom acupuncture on oxaliplatin-induced neuropathic cold allodynia in rats.

BackgroundOxaliplatin, an important chemotherapy drug for advanced colorectal cancer, often induces peripheral neuropathy, especially cold allodynia. Our previous study showed that bee venom acupuncture (BVA), which has been traditionally used in Korea to treat various pain symptoms, potently relieves oxaliplatin-induced cold allodynia in rats. However, the mechanism for this anti-allodynic effect of BVA remains poorly understood. We investigated whether and how the central serotonergic system, a well-known pathway for acupuncture analgesia, mediates the relieving effect of BVA on cold allodynia in oxaliplatin-injected rats.MethodsThe behavioral signs of cold allodynia in Sprague–Dawley (SD) rats were induced by a single injection of oxaliplatin (6 mg/kg, i.p.). Before and after BVA treatment, the cold allodynia signs were evaluated by immersing the rat’s tail into cold water (4°C) and measuring the withdrawal latency. For BVA treatment, a diluted BV (0.25 mg/kg) was subcutaneously administered into Yaoyangguan (GV3) acupoint, which is located between the spinous processes of the fourth and the fifth lumbar vertebra. Serotonin was depleted by a daily injection of DL-p-chlorophenylalanine (PCPA, 150 mg/kg, i.p.) for 3 days. The amount of serotonin in the spinal cord was measured by ELISA. Serotonergic receptor antagonists were administered intraperitoneally or intrathecally before BVA treatment.ResultsThe serotonin levels in the spinal cord were significantly increased by BVA treatment and such increase was significantly reduced by PCPA. This PCPA pretreatment abolished the relieving effect of BVA on oxaliplatin-induced cold allodynia. Either of methysergide (mixed 5-HT1/5-HT2 receptor antagonist, 1 mg/kg, i.p.) or MDL-72222 (5-HT3 receptor antagonist, 1 mg/kg, i.p) blocked the anti-allodynic effect of BVA. Further, an intrathecal injection of MDL-72222 (12 μg) completely blocked the BVA-induced anti-allodynic action, whereas NAN-190 (5-HT1A receptor antagonist, 15 μg, i.t.) or ketanserin (5-HT2A receptor antagonist, 30 μg, i.t.) did not.ConclusionsThese results suggest that BVA treatment alleviates oxaliplatin-induced acute cold allodynia in rats via activation of the serotonergic system, especially spinal 5-HT3 receptors. Thus, our findings may provide a clinically useful evidence for the application of BVA as an alternative therapeutic option for the management of peripheral neuropathy, a dose-limiting side effect that occurs after an administration of oxaliplatin.

5-Hydroxyindolacetic Acid (5-HIAA), a Main Metabolite of Serotonin, is Responsible for Complete Freund's Adjuvant-Induced Thermal Hyperalgesia in Mice

BackgroundThe role of serotonin (5-hydroxytrptamine, 5-HT) in the modulation of pain has been widely studied. Previous work led to the hypothesis that 5-hydroxyindolacetic acid (5-HIAA), a main metabolite of serotonin, might by itself influence pain thresholds.ResultsIn the present study, we investigated the role of 5-HIAA in inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) into the hind paw of mice. Wild-type mice were compared to mice deficient of the 5-HT transporter (5-HTT-/- mice) using behavioral tests for hyperalgesia and high-performance liquid chromatography (HPLC) to determine tissue levels of 5-HIAA. Wild-type mice reproducibly developed thermal hyperalgesia and paw edema for 5 days after CFA injection. 5-HTT-/- mice treated with CFA had reduced thermal hyperalgesia on day 1 after CFA injection and normal responses to heat thereafter. The 5-HIAA levels in spinal cord and sciatic nerve as measured with HPLC were lower in 5-HTT-/- mice than in wild-type mice after CFA injection. Pretreatment of wild-type mice with intraperitoneal injection of para-chlorophenylalanine (p-CPA), a serotonin synthesis inhibitor, resulted in depletion of the 5-HIAA content in spinal cord and sciatic nerve and decrease in thermal hyperalgesia in CFA injected mice. The application of exogenous 5-HIAA resulted in potentiation of thermal hyperalgesia induced by CFA in 5-HTT-/- mice and in wild-type mice pretreated with p-CPA, but not in wild-type mice without p-CPA pretreatment. Further, methysergide, a broad-spectrum serotonin receptor antagonist, had no effect on 5-HIAA-induced potentiation of thermal hyperalgesia in CFA-treated wild-type mice.ConclusionTaken together, the present results suggest that 5-HIAA plays an important role in modulating peripheral thermal hyperalgesia in CFA induced inflammation, probably via a non-serotonin receptor mechanism.

Neural Network Detects the Effects of p-CPA Pre-treatment on Brain Electrophysiology in a Rat Model of Focal Brain Injury

To examine the performance of Artificial Neural Network (ANN) in evaluation of the effects of pretreatment of para-Chlorophenylalanine (p-CPA), a serotonin blocker, in experimental brain injury. Continuous 4 h digital electroencephalogram (EEG) recordings from male Charles Foster rats and its power spectrum analysis by using fast Fourier transform (FFT) were performed in two experimental (i) drug untreated injury group; (ii) p-CPA pretreated injury group as well as a control group. The EEG power spectrum data were tested by ANN containing 60 nodes in input layer, weighted from the digital values of power spectrum from 0 to 30 Hz, 18 nodes in hidden layer and an output node. The effects of injury and of the drug pretreatment were confirmed with the help of calculation of edematous swelling in the brain. The changes in EEG spectral patterns were compared with the ANN and the accuracy was determined in terms of percent (%). Overall performance of the network was found the best in control group (97.9%) in comparison to p-CPA untreated injury group (96.3%) and p-CPA pretreated injury group (71.9%). The decrease in accuracy in p-CPA pretreated injury group of subjects have occurred due to increase in misclassified patterns due to faster recovery in brain cortical potentials. EEG spectrum analysis with ANN was found successful in identifying the changes due to brain swelling as well as the effect of pretreatment of p-CPA in focal brain injury condition. Thus, the training and testing of ANN with EEG power spectra can be used as an effective diagnostic tool for early prediction and monitoring of brain injury as well as the effects of drugs in this condition.

P-CPA Pretreatment Reverses the Changes in Sleep and Behavior Following Acute Immobilization Stress Rats

The effects of p-CPA (para-chlorophenylalanine) pretreatment was studied on the sleep-wake parameters and patterns of behavioral activities in an animal model of acute immobilization stress. For the experiments, young male Charles Foster rats were divided into three groups, subjected to (i) acute immobilization stress for four hours on specially designed wooden boards, (ii) a similar model of acute immobilization stress after pretreatment of p-CPA (injected through i.p. route), and (iii) control rats (p-CPA untreated and unstressed). Three channels of electrographic signals, i.e., EEG (electroencephalogram), EOG (electrooculogram), and EMG (electromyogram) were recorded continuously for four hours for all three groups of rats to analyze the changes in sleep-wake stages. The assessment of behavior was performed just after the stress on separate groups of rats in Open-Field (OF) and Elevated Plus-Maze (EPM) apparatuses. The significant changes in total sleep time (P < 0.05), total time for rapid eye movement sleep (P < 0.01), and total time in wakefulness (P < 0.01) following acute immobilization stress were found reversed in the p-CPA (a serotonin inhibitor) pretreated group of rats. Simultaneously, the results of the present work also revealed that the changes in grooming behavior (P < 0.05) in OF and the total time spent on the center of EPM (P < 0.05) were observed altered in p-CPA pretreated group of rats.

5-HT has contrasting effects in the frontal cortex, but not the hypothalamus, on changes in noradrenaline efflux induced by the monoamine releasing-agent, d-amphetamine, and the reuptake inhibitor, BTS 54 354

There is extensive evidence for functional interactions between central noradrenergic and serotonergic neurones. Here, dual-probe microdialysis was used in freely-moving rats to compare the effects of 5-HT on noradrenergic transmission in the rat frontal cortex and hypothalamus. We studied the effects of the 5-HT synthesis inhibitor, para-chlorophenylalanine ( pCPA; which depleted 5-HT stores in both the frontal cortex and the hypothalamus), on spontaneous efflux of noradrenaline and on the noradrenergic responses to d-amphetamine, and the monoamine reuptake inhibitor, BTS 54 354. pCPA pretreatment alone did not affect spontaneous noradrenaline efflux in either brain region, whether or not α 2-autoreceptors were inactivated by administration of the α 2-antagonist, atipamezole (1 mg/kg i.p). However, in the frontal cortex, pCPA pretreatment augmented the amplitude of, and prolonged, the noradrenergic response to local infusion of d-amphetamine (10 μM). In contrast, pCPA abolished the increase in cortical noradrenaline efflux induced by local infusion of BTS 54 354 (50 μM). In the hypothalamus, pCPA did not affect the amplitude of the response to either of these agents but did prolong the effects of d-amphetamine on noradrenaline efflux. These findings suggest that serotonergic transmission has complex effects on the noradrenergic response to drugs that increase noradrenergic transmission in the frontal cortex, but has less influence in the hypothalamus.

Effects of fenfluramine and antidepressants on protein kinase C activity in rat cortical synaptoneurosomes.

Fenfluramine releases serotonin (5-HT) via the 5-HT transporter (SERT). Previous work has shown that amphetamine increases particulate protein kinase C (PKC) activity in striatal synaptoneurosomes. The increased PKC activity is linked to the outward transport of dopamine, and when release is diminished, the inward transport of amphetamine inhibits PKC instead. Since there is homology among monoamine transporters, this study was undertaken to determine if D-fenfluramine has similar effects on PKC. The role of 5-HT receptors and endogenous 5-HT were also examined. Naive rats and rats pretreated with p-chlorophenylalanine (PCPA), a 5-HT synthesis inhibitor, were sacrificed. Cortical synaptoneurosomes were prepared and incubated with fenfluramine. PKC activity was determined by thiophosphorylation of endogenous substrates. It was found that 5-HT, D/L-fenfluramine, and D-fenfluramine increased PKC activity in a time- and dose-dependent manner. The 5-HT-mediated increase in PKC activity was attenuated by pretreatment with the 5-HT(2) antagonist ketanserin, but not with the SERT inhibitor fluoxetine. The D-fenfluramine-induced increase in PKC activity was completely prevented, however, by pretreatment with SERT inhibitors and partially with ketanserin. It was also attenuated by pretreatment with PCPA, resulting in a dose-dependent inhibition of PKC instead. Thus, when 5-HT release was diminished the uptake of D-fenfluramine inhibited PKC. Similar effects have been observed with amphetamine. Unlike D-fenfluramine, the D/L-fenfluramine-induced increase in PKC activity was partially resistant to PCPA pretreatment but was attenuated with bupropion, a dopamine transporter (DAT) inhibitor. SERT inhibitors (sertraline, paroxetine, citalopram, and fluoxetine) also increased PKC activity. Nefazodone and bupropion increased PKC activity, but mirtazapine was relatively inactive. The SERT inhibitor-induced increase in PKC was unaffected by pretreatment with PCPA but was inhibited by calcium. Similar effects on PKC activity have been observed with DAT inhibitors. These results, showing that D-fenfluramine altered PKC activity similar to D-amphetamine, suggest that the topographic homology between DAT and SERT may extend to their effects on PKC activity.

Serotonin depletion enhances the intracerebroventricularly administered MK-801-induced plasma interleukin-6 levels in mice.

To investigate the effect of serotonin depletion on the intracerebroventricularly (i.c.v.) administered MK-801-induced plasma interleukin-6 (IL-6) levels, we pretreated mice with parachlorophenylalanine (PCPA, 300 mg/kg, i.p.) 3 d before an i.c.v. injection of MK-801 (1 microg). The i.c.v. MK-801-induced rise of plasma IL-6 level was markedly enhanced in the PCPA-pretreated mice. However, serotonin depletion by PCPA pretreatment did not affect the i.c.v. MK-801-induced increase in plasma corticosterone level. These results suggest that serotonergic system is involved in the suppressive regulation of MK-801-induced plasma IL-6 level.

Role of Monoamine Oxidase Inhibition and Monoamine Depletion in Fenfluramine‐Induced Neurotoxicity and Serotonin Release

Abstract: The role of both monoamine synthesis and monoamine oxidase inhibition in mediating the fenfluramine‐induced damage to serotonin neurones was examined; as pretreatment agents, both alpha‐methyl‐para‐tyrosine (AMPT) and para‐chlorophenylalanine (PCPA) were used to deplete dopamine and serotonin, respectively, while clorgyline and deprenyl were used to inhibit monoamine oxidase types A and B. While both AMPT and deprenyl did not alter fenfluramine‐induced serotonin or 5‐hydroxyindoleacetic acid (5‐HIAA) depletion in any area, PCPA did partially reduce the serotonin depletion in the hippocampus and hypothalamus. Although pretreatment with clorgyline did not significantly alter fenfluramine‐induced serotonin depletion, it did produce a 65% mortality rate in animals treated with both drugs. Both PCPA and clorgyline significantly increased the depletion of striatal 5‐HIAA concentration consequent to fenfluramine; however, these drugs also produced a long‐term depletion of striatal 5‐HIAA when administered alone, therefore, the changes seen after the coadministration with fenfluramine may be viewed as additive. Finally, acute PCPA pretreatment attenuated the rapid rise in 3,4‐dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (homovanillic acid) induced by fenfluramine, and acute clorgyline reversed the drop in serotonin and rise in 5‐HIAA induced by fenfluramine. These results indicate that the rapid increase in dopamine activity induced by fenfluramine is partially dependent on serotonin concentration and release and that the mechanism of fenfluramine‐induced toxicity is unlike that of the other substituted amphetamines.

Role of monoamine oxidase inhibition and monoamine depletion in fenfluramine-induced neurotoxicity and serotonin release.

The role of both monoamine synthesis and monoamine oxidase inhibition in mediating the fenfluramine-induced damage to serotonin neurones was examined; as pretreatment agents, both alpha-methyl-para-tyrosine (AMPT) and parachlorophenylalanine (PCPA) were used to deplete dopamine and serotonin, respectively, while clorgyline and deprenyl were used to inhibit monoamine oxidase types A and B. While both AMPT and deprenyl did not alter fenfluramine induced serotonin or 5-hydroxyindoleacetic acid (5-HIAA) depletion in any area, PCPA did partially reduce the serotonin depletion in the hippocampus and hypothalamus. Although pretreatment with clorgyline did not significantly alter fenfluramine-induced serotonin depletion, it did produce a 65% mortality rate in animals treated with both drugs. Both PCPA and clorgyline significantly increased the depletion of striatal 5-HIAA concentration consequent to fenfluramine; however, these drugs also produced a long-term depletion of striatal 5-HIAA when administered alone, therefore, the changes seen after the coadministration with fenfluramine may be viewed as additive. Finally, acute PCPA pretreatment attenuated the rapid rise in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (homovanillic acid) induced by fenfluramine, and acute clorgyline reversed the drop in serotonin and rise in 5-HIAA induced by fenfluramine. These results indicate that the rapid increase in dopamine activity induced by fenfluramine is partially dependent on serotonin concentration and release and that the mechanism of fenfluramine-induced toxicity is unlike that of the other substituted amphetamines.

Acute Antidepressant-Like and Antianxiety-Like Effects of Tryptophan in Mice

The antidepressant-like, antianxiety-like and sedative effects of tryptophan (TRP), in the absence and presence of p-chlorophenylalanine (p-CPA), and melatonin were studied in mice using the forced-swimming test, open-field test and activity cage, respectively. Single-dose TRP caused an antidepressant-like effect dose dependently up to 125 mg/kg. No significant effect was observed, however, when the TRP dose was increased to 250 mg/kg, i.e. a reversal of effect occurred at high dose. With p-CPA pretreatment, the effects observed at 125 and 250 mg/kg TRP were similar to those obtained at 50 and 125 mg/kg without p-CPA pretreatment, respectively. Melatonin also caused an antidepressant-like effect in a similar manner, but appeared to be less potent than TRP. These results strongly indicate that the antidepressant-like effect of TRP was due to its conversion to 5-hydroxytryptamine (5-HT). An antianxiety-like effect was observed for TRP only at 250 mg/kg dose together with p-CPA pretreatment, while no sedative effect was observed at all. In contrast, melatonin did not produce any antianxiety-like effect, but produced sedation at 200 mg/kg dose. It may be concluded that the antianxiety-like effect of TRP is unrelated to 5-HT and melatonin formation, but associated with TRP itself or, perhaps, with other anxiolytic metabolites.

Effects of p-chlorophenylalanine on striatal acetylcholinesterase activity and on biogenic amine levels in nuclei raphe and caudate-putamen during physostigmine-induced tremor in rats

The biochemical mechanism underlying tremor is unknown. We investigated the effects of p-chlorophenylalanine ( pCPA), a serotonin (5-HT) depletor, on the neurochemical processes in nuclei raphe dorsalis (NRD) and caudatus putamen (NCP) paralleling physostigmine-induced tremor in rats. Peak of physostigmine tremor correlated with increase in 5-HT in NRD and NCP, and a decrease in striatal dopamine (DA), as assayed employing high pressure liquid chromotography-electrochemistry. Administration of pCPA caused significant decrease in DA, norepinephrine (NE) and 5-HT levels in both the nuclei, without affecting striatal NE content and acetylcholinesterase activity. pCPA pretreatment significantly inhibited physostigmine-induced tremor and blocked corresponding increase in the levels of 5-HT in NRD and NCP. These results indicate involvement of central 5-HT, but not DA or NE, in the genesis of physostigmine tremor.

Hyposerotonin-induced nitric oxide supersensitivity in the cerebral microcirculation.

To investigate the relationship between hyposerotonin and cranial microvascular responses to nitric oxide (NO). Although the mechanism underlying NO supersensitivity in migraine is still unclear, an alteration of the serotonin system is a possible explanation. Wistar rats were divided into control and hyposerotonin groups. Serotonin was depleted by intraperitoneal injection with 300 mg/kg of para-chlorophenylalanine (PCPA), a tryptophan hydroxylase inhibitor. Three days after PCPA pretreatment, the animals were prepared for assessment of their NO-induced vasomotor response using glyceryl trinitrate (GTN: 8 to 10 mg/kg, intravenously) as an NO donor. Pial circulation was visualized by the intravital fluorescein videomicroscopic technique. Images of vessels at 0, 5, 15, 30, and 60 minutes post GTN infusion were digitized and measured. At the end of monitoring, the rat brains were removed for ultrastructural study of the brain microvessels. Infusion of GTN produced dose-dependent pial arteriolar dilatation. This vasodilator effect was significantly increased in the PCPA-treated groups, especially at 30 and 60 minutes. The percentage change from baseline diameter at 30 minutes after the 8-mg/kg GTN infusion was 42.6 +/- 3.1 for the hyposerotonin group and 16.8 +/- 2.9 for the control group (P<.001). Electron microscopic study revealed that exposure to the NO donor produced considerable changes in cerebral microvessels, characterized by focal ballooning of endothelial cells, increased microvillous formation, and increased endothelial pinocytosis. These anatomical changes were significantly more prominent in the hyposerotonin group. A hyposerotoninergic condition can facilitate the NO-induced physiological and pathological responses in meningeal and cerebral microvessels and, therefore, is a possible explanation for the supersensitivity to NO observed in patients with migraine.

Serotonergic mediation of the effects of fluoxetine, but not desipramine, in the rat forced swimming test.

The forced swimming test (FST) is a behavioral test in rodents that predicts the clinical efficacy of many types of antidepressant treatments. Recently, a behavior sampling technique was developed that scores individual response categories, including swimming, climbing and immobility. Although all antidepressant drugs reduce immobility in the FST, at least two distinct active behavioral patterns are produced by pharmacologically selective antidepressant drugs. Serotonin-selective reuptake inhibitors increase swimming behavior, while drugs acting primarily to increase extracellular levels of norepinephrine or dopamine increase climbing behavior. Distinct patterns of active behaviors in the FST may be mediated by distinct neurotransmitters, but this has not been shown directly. The present study examined the role of serotonin in mediating active behaviors in the forced swimming test after treatment with two antidepressant drugs, the selective serotonin reuptake inhibitor, fluoxetine and the selective norepinephrine reuptake inhibitor, desipramine. Endogenous serotonin was depleted by administering para-cholorophenylalanine (PCPA, 150 mg/kg, IP.) to rats 72 h and 48 h prior to the swim test. Fluoxetine (10 mg/kg, SC) or desipramine (10 mg/kg, SC) was given three times over a 24-h period prior to the FST. Behavioral responses, including immobility, swimming and climbing, were counted during the 5-min test. Pretreatment with PCPA blocked fluoxetine-induced reduction in immobility and increase in swimming behavior during the FST. In contrast, PCPA pretreatment did not interfere with the ability of desipramine to reduce immobility and increase climbing behavior. Depletion of serotonin prevented the behavioral effects of the selective serotonin reuptake inhibitor fluoxetine in the rat FST. Furthermore, depletion of serotonin had no impact on the behavioral effects induced by the selective norepinephrine reuptake inhibitor, desipramine. The effects of antidepressant drugs on FST-induced immobility may be exerted by distinguishable contributions from different neurotransmitter systems.

(3H)-Ketanserin binding and elevated plus-maze behavior after chronic antidepressant treatment in DSP-4 and P-CPA pretreated rats: Evidence for partial involvement of 5-HT2A receptors

[3H]-Ketanserin binding in rat neocortex (frontal and cerebral cortex) after 3-week treatment with desipramine (10 mg/kg) and citalopram (5 mg/kg) in vehicle-, DSP-4- (50 mg/kg) and p-CPA- (350 mg/kg before the beginning of experiments and once per week on the first and second week plus 100 mg/kg in the last week) pretreated rats was measured. The antidepressant activity of DSP-4 and p-CPA was evaluated indirectly using the elevated plus-maze test. Acute antidepressant treatment revealed an anxiogenic effect while chronic treatment elicited an anxiolytic effect. DSP-4 and p-CPA pretreatment elicited an antiexploratory effect that was not blocked by acute antidepressant treatment. After 3 weeks of antidepressant treatment, only desipramine + DSP-4 or p-CPA treatment revealed an antiexploratory effect. Three-week antidepressant treatment downregulated [3H]-ketanserin binding in the cerebral cortex. Citalopram treatment partially reversed p-CPA-induced downregulation of [3H]-ketanserin binding in rat whole neocortex. In conclusion, our experiments suggest that the 5-HT2A receptors in monoamine-impaired rats are involved in the mediation of antidepressant-induced behavioral phenomena, but chronic antidepressant treatment downregulates [3H]-ketanserin binding independently from the state of monoaminergic neurotransmission.

Rodent data and general hypothesis: antipsychotic action exerted through 5-Ht2A receptor antagonism is dependent on increased serotonergic tone.

The locomotor stimulation induced by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 (dizocilpine) in mice was regarded as a model of at least some aspects of schizophrenia. The serotonin synthesis inhibitor dl-p-chlorophenylalanine (PCPA) was used to evaluate the involvement of endogenous serotonin in (a) the induction of MK-801-induced hyperlocomotion in NMRI mice, and (b) the inhibition of MK-801-induced hyperlocomotion by each of five monoaminergic antagonists (M100907, clozapine, olanzapine, raclopride, SCH23390). Further, brain monoaminergic biochemistry was characterised in rats and mice after various drug treatments. PCPA pretreatment did not significantly reduce MK-801-induced hyperlocomotion in any of the experiments performed; however in a meta-analysis of six experiments, the locomotion displayed by MK-801-treated animals was diminished 17% by PCPA pretreatment. The selective 5-HT2A receptor antagonist M100907 exerted a dose-dependent inhibition of MK-801-induced hyperlocomotion. This effect was abolished in mice pretreated with PCPA, but could be restored in a dose-dependent manner by restitution of endogenous 5-HT by means of 5-hydroxytryptophan (5-HTP). On the other hand, the inhibition of MK-801-induced hyperlocomotion exerted by the selective dopamine D-2 receptor antagonist raclopride or the dopamine D-1 receptor antagonist SCH23390 was unaffected by PCPA pretreatment. The antipsychotics clozapine and olanzapine displayed a split profile. Hence, the inhibitory effect on MK-801-induced hyperlocomotion exerted by low doses of these compounds was diminished after PCPA pretreatment, while inhibition exerted by higher doses was unaffected by PCPA. These results suggest that (1) MK-801-induced hyperlocomotion is accompanied by an activation of, but is not fully dependent upon, brain serotonergic systems. (2) In the hypoglutamatergic state induced by MK-801, endogenous serotonin exerts a stimulatory effect on locomotion through an action at 5-HT2A receptors, an effect that is almost completely counterbalanced by a concomitant inhibitory impact on locomotion, mediated through stimulation of serotonin receptors other than 5-HT2A receptors. M100907, by blocking 5-HT2A receptors, unveils the inhibitory effect exerted on locomotion by these other serotonin receptors. (3) Dopamine D-2 receptor antagonistic properties of antipsychotic compounds, when they come into play, override 5-HT2A receptor antagonism. Possible implications for the treatment of schizophrenia with 5-HT2A receptor antagonists are discussed. It is hypothesized that treatment response to such agents is dependent on increased serotonergic tone.

1912 Serotonergic system involved in manifestation of analgesic action of electroacupuncture

Rats were trained to discriminate 0.3 mg/kg (IP) flesinoxan from saline in a standard two-lever operant procedure and thereafter subjected to generalization and antagonism tests with the 5-HT1A receptor agonist ipsapirone and the β-adrenergic/5-HT1 receptor antagonist pindolol. Ipsapirone (3.0 mg/kg) completely substituted for flesinoxan. Both the flesinoxan (0.3 mg/kg) and the ipsapirone cue (3.0 mg/kg) were dose-dependently blocked by (±)-pindolol. In a second group of rats, trained to discriminate 0.5 mg/kg (IP) of flesinoxan from saline, the putative 5-HT1A antagonist NAN-190 (in the dose range of 1.0 to 6.0 mg/kg) partially blocked the cue of flesinoxan. Generalization studies revealed that the flesinoxan cue could not be mimicked by NAN-190 (3.0 mg/kg). Finally, rats were pretreated with the 5-HT depletor parachlorophenylalanine (PCPA) and thereafter tested with the flesinoxan training dose (0.5 mg/kg). PCPA pretreatment did not significantly attenuate the recognition of the flesinoxan cue. The present results are in agreement with previous findings concerning the stimulus effect of flesinoxan and point to a mechanism that involves the activation of 5-HT1A receptors in the brain. Depletion of 5-HT did not significantly affect the stimulus effect of flesinoxan, suggesting that presynaptic 5-HT1A receptors do not play a crucial role in the mechanism underlying the stimulus effect of flesinoxan.