Pentylenetetrazol: Effect on carbaryl-induced changes of serotonin metabolism in pons-medulla of rat brain

Oral administration of carbaryl to adult male albino rats produced a dose dependent increase in the steady state level of 5-hydroxytryptamine (5-HT) at 1.00 h in pons-medulla (PM). 5-Hydroxyindole acetic acid (5-HIAA) concentration was significantly elevated only in response to a higher dose of this pesticide under similar conditions. A time course study with carbaryl and pentylenetetrazol (PTZ) showed a characteristic elevation of the steady state level of 5-HT in PM, but the 5-HIAA level was significantly elevated at 0.5 h only after carbaryl treatment. No significant change of the 5-HIAA level was evident after administration of PTZ alone or in combination with carbaryl. Tryptophan concentration was significantly elevated in PM at 0.5 h after carbaryl treatment and at 1.0 h after carbaryl + PTZ treatment. No significant change of tryptophan concentration was evident after the administration of PTZ alone under similar conditions. Measurement of (1) pargyline induced (a) accumulation of 5-HT and (b) depletion of 5-HIAA levels, and (2) probenecid-induced accumulation of 5-HIAA level in presence and absence of carbaryl and revealed that carbaryl accelerated the synthesis as well as the breakdown of 5-HT, whereas PTZ alone or in combination with carbaryl accelerated the synthesis of 5-HT without affecting its catabolism. The potency of this pesticide in elevating the pargyline-induced accumulation of 5-HT is in the order of carbaryl + PTZ greater than PTZ congruent to carbaryl. These results suggest that the carbaryl-induced increase in the synthesis of 5-HT is potentiated, and the turnover is reduced, in PM when PTZ is administered to the carbaryl-intoxicated rats.     

Carbaryl-induced elevation of corticosterone level and cholinergic mechanism

Administration of a single dose (200 mg/kg, p.o.) of carbaryl to rats produced a significant rise in adrenal and plasma corticosterone levels and an increase of tyrosine alpha-ketoglutarate transaminase activity in the liver cytosol. Synaptosomal acetylcholinesterase activity of the hypothalamic and the striatal regions of rat brain was decreased by carbaryl treatment under similar conditions. Pretreatment (0.5 h) with atropine sulphate (10 mg/kg, i.p.) failed to counteract the carbaryl-induced elevation of adrenal and plasma corticosterone levels and hence the liver tyrosine alpha-ketoglutarate transaminase activity. Present results suggest that the carbaryl-induced rise in the corticosterone level in the adrenal gland and plasma is not due to a cholinergic mechanism.     

Possible involvement of 5-HT and 5-HT2 receptors in acceleration of gastrointestinal transit by escin Ib in mice

We have reported previously that escin Ib accelerated gastrointestinal transit (GIT) in mice, and that its effect may be mediated by the release of endogenous prostaglandins (PGs) and nitric oxide (NO). In this study, the possible involvement of 5-HT and 5-HT receptors in the GIT acceleration of escin Ib was investigated in mice. The acceleration of GIT by escin Ib (25 or 50 mg/kg, p.o.) was attenuated by pretreatment with ritanserin (0.5-5 mg/kg, s.c., a 5-HT(2A/2C/2B) receptor antagonist), but not with MDL 72222 (1 and 5 mg/kg, s.c.) and metoclopramide (10 mg/kg, s.c.) (5-HT3 receptor antagonists) or tropisetron (1 and 10 mg/kg, s.c., a 5-HT(3/4) receptor antagonist). Furthermore, pretreatment with ketanserin (0.05-5 mg/kg, s.c.), haloperidol (1-5 mg/kg, s.c.) and spiperone (0.5-5 mg/kg, s.c.) (5-HT2A receptor antagonists), as well as a bolus of dl-p-chlorophenylalanine methyl ester (PCPA, 1000 mg/kg, p.o., 1, 6 or 24 h before administration of the sample) (an inhibitor of 5-HT synthesizing enzyme tryptophan hydroxylase) and reserpine (5 mg/kg, p.o.) (a 5-HT depletor), but not 6-hydroxydopamine (80 mg/kg, i.p., a dopamine depletor) or repeated PCPA (300 mg/kg x2, p.o., 72 and 48 h before administration of the sample), also attenuated the effects of escin Ib. It is postulated that escin Ib accelerates GIT, at least in part, by stimulating the synthesis of 5-HT to act through 5-HT2, possibly 5-HT2A receptors, which in turn causes the release of NO and PGs.     

Complex role of 5-HT in the regulation of TSH secretion in the male rat

The role of 5-hydroxytryptamine (5-HT) in the regulations of TSH secretion was studied in male rats using both peripheral and central administration of the drugs. Basal TSH levels were not modified by moderate doses of 5-HT (subcutaneously) or its precursors or antagonists (intraperitoneally) given 1 h before decapitation. The cold-stimulated TSH secretion was decreased by L-tryptophan (L-TRP, 400 mg/kg i.p.), quipazine (10 mg/kg i.p.) and 5-HT (1 or 5 mg/kg s.c. or i.v.) as well as by p-chlorophenylalanine (pCPA, 20 or more mg/kg i.p.) when the drugs were given 1 h before sampling. pCPA (100-400 mg/kg i.p.) was active 24-48 h after the injection but repetitive administration did not affect TSH levels. 5-HT (5 mg/kg s.c.) was effective also in pinealectomized animals. L-TRP and 5-hydroxytryptophan potentiated the TRH-stimulated TSH secretion when given 1 h before killing. 5-HT (10 microgram/rat) infused into the third ventricle enhanced the cold-stimulated TSH secretion when given 30-45 min before sampling. When injected into the medial basal hypothalamus, 50-HT (1-10 microgram/rat) had no effect on basal or stimulated TSH levels. The results suggest: (1) 5-HT does not play any role in the regulation of basal TSH secretion; (2) in the cold-stimulated TSH secretion 5-HT has a stimulatory action evidently inside the blood-brain barrier and also an inhibitory effect obviously outside this barrier.     

Concurrent Determination of Brain Dopamine and 5-Hydroxytryptamine Turnovers in Individual Freely Moving Rats Using Repeated Sampling of Cerebrospinal Fluid

5-Hydroxytryptamine (5-HT) turnover and dopamine (DA) turnover values were obtained in individual conscious rats by measuring the rates of accumulation of 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) in cisternal CSF samples taken from each rat at 0, 30, and 60 min after probenecid (200 mg/kg i.p.) administration. In a separate experiment, 5-HT and DA turnover values were determined in CSF, striatum, and rest of brain of groups of rats killed 0, 30, or 60 min after probenecid. Whole brain turnover values were calculated from striatal and rest of brain values. Mean turnover values using CSF were comparable with both procedures. DA turnover values were greater when based on total (i.e., free + conjugated) DA metabolites than when based on free metabolites. After partial inhibition of monoamine synthesis with the decarboxylase inhibitor DL-alpha- monofluoromethyl -DOPA ( MFMD , 100 mg/kg p.o.) DA and 5-HT turnover values were comparably reduced in whole brain, rest of brain, and CSF but more markedly reduced in the striatum. Mean DA and 5-HT turnover values obtained using CSF were similar with probenecid doses over the range 150-250 mg/kg i.p. but were variable when repeatedly determined in the same rats after administration of 200 mg/kg probenecid. Results in general show that the CSF procedure may be used to determine concurrently both 5-HT and DA turnover (when estimated from the sum of total but not free metabolites) and that it provides a good index of whole brain turnover of these transmitters in the conscious individual rat.     

Enhancement in extracellular serotonin levels by 5-hydroxytryptophan loading after administration of WAY 100635 and fluoxetine

It has been demonstrated that synthesis of serotonin (5-HT) is dependent on the availability of precursor, as well as the activity of 5-HT neurons. In the present series of experiments, we examined the effects of precursor (5-HTP) loading on extracellular hypothalamic 5-HT after administration of fluoxetine alone or in combination with WAY 100635, a selective 5-HT1A antagonist. In the first experiment, fluoxetine alone (10 mg/kg i.p.) caused 5-HT levels to significantly increase to 150% of basal levels. Subsequent administration of 5-HTP at 10, 20, and 40 mg/kg i.p. caused 5-HT levels to further increase to a maximum value of 254%, 405%, and 618%, respectively. In the second experiment, either vehicle or WAY 100635 (1 mg/kg/hour s.c.) was infused, then fluoxetine (10 mg/kg i.p.) and 5-HTP (10 mg/kg i.p.) were administered. By itself, WAY 100635 led to a slight but significant increase in hypothalamic 5-HT levels one hour after the start of administration (130% of basal levels). In the WAY 100635-treated group, fluoxetine caused an increase to 240% of basal levels after one hour, which rose to 290% of basal levels after two hours. Subsequent administration of 5-HTP further increased 5-HT levels to 580% of basal levels after one hour. In the vehicle-treated group, fluoxetine caused an increase of 160% of basal levels which was stable over two hours, and subsequent administration of 5-HTP led to a slight increase in 5-HT levels of 220% after one hour. These results suggest that combining blockade of 5-HT1A autoreceptors with 5-HT uptake inhibition results in a synergistic increase in synthesis and release of 5-HT when precursor is administered.     

Exogenous Tryptophan Increases Synthesis, Storage, and Intraneuronal Metabolism of 5-Hydroxytryptamine in the Rat Hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.     

Effects of Milnacipran and Pindolol on Extracellular Noradrenaline and Serotonin Levels in Guinea Pig Hypothalamus

Abstract: Milnacipran, a dual noradrenaline (NA) and serotonin (5-hydroxytryptamine, 5-HT) uptake inhibitor, increased extracellular levels of NA and 5-HT in hypothalamus of freely moving guinea pigs as measured by microdialysis. The basal levels of both monoamines, which were tetrodotoxin sensitive, were increased in a dose-dependent manner and to a similar extent after the intraperitoneal administration of milnacipran (10 and 40 mg/kg i.p.). Levels of the NA metabolite 4-hydroxy-3-methoxyphenylglycol (MHPG) were decreased by milnacipran at 10 and 40 mg/kg i.p., whereas those of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) showed no effect. Subcutaneous injection of 5-HT_1A and β-adrenergic receptor antagonist (−)-pindolol alone, at 10 mg/kg, had no effect on the extracellular levels of NA or 5-HT. The concomitant administration of (−)-pindolol (10 mg/kg s.c.) with milnacipran (10 mg/kg i.p.) increased severalfold the effect of milnacipran on the extracellular levels of NA and 5-HT. These results indicate that milnacipran, by blocking the uptake of NA and 5-HT, increases virtually equipotently the extracellular levels of NA and 5-HT, confirming previous in vitro studies. In addition, the antagonism of 5-HT_1A autoreceptors by (−)-pindolol potentiates the action of milnacipran on both NA and 5-HT systems, without modifying the ratio of these activities.     

Effect of chronic D-fenfluramine administration on rat hypothalamic serotonin levels and release

D-fenfluramine, an anorectic agent in rats and man, was administered daily at doses 1.25, 2.5, 5 or 10 mg/kg/day for 10 days, and sacrificed 6 days later. Hypothalamic serotonin (5-HT) levels were unchanged in rats receiving 1.25-5 mg/kg/day of d-fenfluramine but reduced by 22% (p less than 0.01) at the highest drug dose (10 mg/kg/day); hypothalamic 5-hydroxyindole acetic acid (5-HIAA) levels were reduced by 15% (p less than 0.05) or 28% (p less than 0.01) in rats receiving 5 or 10 mg/kg/day of the drug, respectively. Hypothalamic slices prepared from rats which were previously treated with any of the drug doses spontaneously released endogenous 5-HT at rates that did not differ from those of vehicle-treated rats. 5-HT released with electrical field-stimulation was unaffected by prior d-fenfluramine treatment at doses of 1.25-5 mg/kg/day, and was reduced by 20% (p less than 0.05) from slices prepared from rats which received 10 mg/kg/day. 5-HIAA efflux was also attenuated by the highest drug dose. These data indicate that chronic administration to rats of customary anorectic doses of d-fenfluramine (i.e. 0.06-1.25 mg/kg) fail to cause long-lasting reductions in brain 5-HT release.     

Proanthocyanidins alleviate pentylenetetrazole-induced epileptic seizures in mice via the antioxidant activity

The role of oxidative stress in the initiation and progress of epilepsy is well established. Proanthocyanidins (PACs), a naturally occurring polyphenolic compound, have been reported to possess a broad spectrum of pharmacological and therapeutic properties against oxidative stress. However, the protective effects of proanthocyanidins against epilepsy have not been clarified. In the present study, we used the pentylenetetrazole (PTZ)-induced epilepsy mouse model to explore whether proanthocyanidins could help to reduce oxidative stress and protect against epilepsy. Mice were allocated into four groups (n?=?14 per each group): control, PTZ (60 mg/kg, intraperitoneally), PACs?+?PTZ (200 mg/kg, p.o.) and sodium valproate (VPA)?+?PTZ (200 mg/kg, p.o.). PTZ injection caused oxidative stress in the hippocampal tissue as represented by the elevated lipid peroxidation and NO synthesis and increased expression of iNOS. Furthermore, depleted levels of anti-oxidants, GSH, GR, GPx, SOD, and CAT also indicate that oxidative stress was induced in mice exposed to PTZ. Additionally, a state of neuroinflammation was recorded following the developed seizures. Moreover, neuronal apoptosis was recorded following the development of epileptic convulsions as confirmed by the elevated Bax and caspase-3 and the decreased Bcl2 protein. Moreover, AChE activity, DA, NE, 5-HT, brain-derived neurotrophic factor levels, and gene expression of Nrf2 have decreased in the hippocampal tissue of PTZ exposed mice. However, pre-treatment of mice with PACs protected against the generation of oxidative stress, apoptosis, and neuroinflammation in the PTZ exposed mice brain as the biomarkers for all these conditions was bought to control levels. In addition, the gene expression of Nrf2 was significantly upregulated following PACs treatment. These results suggest that PACs can ameliorate oxidative stress, neuroinflammation, and neuronal apoptosis by activating the Nrf2 signaling pathway in PTZ induced seizures in mice.

     

Enhancement of Brain Dopamine Metabolism by Tyrosine During Immobilisation: An In Vivo Study Using Repeated Cerebrospinal Fluid Sampling in Conscious Rats

Central dopamine (DA) and 5-hydroxytryptamine (5-HT) metabolism was monitored in conscious, freely moving rats by determination of levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) in CSF samples withdrawn repeatedly from the cisterna magna and treated with acid to hydrolyse DOPAC and HVA conjugates. The effect of tyrosine on DA metabolism was investigated. Time courses of metabolite concentrations in individual rats in a quiet room showed that tyrosine (20, 50, or 200 mg/kg i.p.) was without significant effect; brain changes were essentially in agreement. However, the increases of CSF DOPAC and HVA levels that occurred on immobilisation for 2 h were further enhanced by tyrosine (200 mg/kg). The associated increases of 5-HIAA level were unaffected. The corresponding increases of DA metabolite concentrations in the brains of immobilised rats given tyrosine were less marked than the CSF changes and only reached significance for "rest of brain" DOPAC. The CSF studies revealed large interindividual variation in the magnitude and duration of the effects of immobilisation on transmitter amine metabolism. These results may help toward the elucidation of possible relationships between the neurochemical and behavioural effects of stress.     

Dose-dependent dual effect of corticosterone on cerebral 5-HT metabolism

The action of 1.0 and 10.0 mg/kg (i.p.) of corticosterone on serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents and on serotonin turnover, measured by an MAO-inhibitor method, was studied at 30 and 120 min after administration. A 1.0 mg/kg dose of corticosterone increased the serotonin content and turnover in the hypothalamus and mesencephalon 30 min after administration; however, it was ineffective on dorsal hippocampus and frontal and parietal cortex. 5-HIAA content did not change significantly in any of the brain areas studied. A 10.0 mg/kg dose of corticosterone decreased the serotonin content and turnover in the hypothalamus and mesencephalon; it was ineffective in other brain areas investigated. 5-HIAA content significantly decreased in the hypothalamus while it increased in the mesencephalon and dorsal hippocampus. In the parietal and frontal cortex, 5-HIAA content did not change following administration of 10.0 mg/kg of corticosterone. At 120 min after corticosterone administration, neither 5-HT content and turnover nor 5-HIAA content showed any change in the brain areas investigated. The results suggest that corticosteroids might change the activity of the brain serotoninergic system in a dose- and time-dependent manner, and in this way the serotoninergic system might play an important role in mediation of the corticosteroid effect exerted on brain function.     

Melatonin reverses neurochemical alterations induced by 6-OHDA in rat striatum

The present work showed that the intrastriatal injection of 6-OHDA significantly decreases DA, DOPAC and HVA levels in that rat brain structure. Although there is also a decrease in 5-HT levels no changes were observed in 5-HIAA levels as compared to controls. On the other hand, melatonin (2, 5, 10 and 25 mg/kg. i.p., daily for 7 days) treatment starting 1 h after 6-OHDA lesions, partially reverses the decreases caused by 6-OHDA lesions on these neurotransmitter levels, and contents were brought to approximately 50% of that observed in the contralateral sides of controls or of melatonin treated group. Melatonin was more efficient at the doses of 5 and 10 mg/kg, i.p., and effects were similar between the lowest and highest doses characteristic of a bell-shaped type of response. The apomorphine-induced rotational behavior (3 mg/kg, i.p.) was blocked by 60, 89, 78 and 47% after the doses of 2, 5, 10 and 25 mg/kg, i.p., respectively. Similarly, in this case the doses of 5 and 10 mg/kg were also more efficient. Melatonin (5 mg/kg) produced an upregulation of D1 receptors associated with a decrease in Kd value. While no change was observed in maximum density of D2 receptors, the Kd value was also decreased.     

Selective stimulation of serotonin2c receptors blocks the enhancement of striatal and accumbal dopamine release induced by nicotine administration

The effects of acute and repeated nicotine administration on the extracellular levels of dopamine (DA) in the corpus striatum and the nucleus accumbens were studied in conscious, freely moving rats by in vivo microdialysis. Acute intraperitoneal (i.p.) injection of nicotine (1 mg/kg) increased DA outflow both in the corpus striatum and the nucleus accumbens. Repeated daily injection of nicotine (1 mg/kg, i.p.) for 10 consecutive days caused a significant increase in basal DA outflow both in the corpus striatum and the nucleus accumbens. Acute challenge with nicotine (1 mg/kg, i.p.) in animals treated repeatedly with this drug enhanced DA extracellular levels in both brain areas. However, the effect of nicotine was potentiated in the nucleus accumbens, but not in the corpus striatum. To test the hypothesis that stimulation of 5-HT (5-hydroxytryptamine, serotonin)(2C) receptors could affect nicotine-induced DA release, the selective 5-HT(2C) receptor agonist RO 60-0175 was used. Pretreatment with RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently prevented the enhancement in DA release elicited by acute nicotine in the corpus striatum, but was devoid of any significant effect in the nucleus accumbens. RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently reduced the stimulatory effect on striatal and accumbal DA release induced by an acute challenge with nicotine (1 mg/kg, i.p.) in rats treated repeatedly with this alkaloid. However, only the effect of 3 mg/kg RO 60-0175 reached statistical significance. The inhibitory effect of RO 60-0175 on DA release induced by nicotine in the corpus striatum and the nucleus accumbens was completely prevented by SB 242084 (0.5 mg/kg, i.p.) and SB 243213 (0.5 mg/kg, i.p.), two selective antagonists of 5-HT(2C) receptors. It is concluded that selective activation of 5-HT(2C) receptors can block the stimulatory action of nicotine on central DA function, an effect that might be relevant for the reported antiaddictive properties of RO 60-0175.     

Alpha melanocyte stimulating hormone (α-MSH) does not modify pentylenetetrazol- and pilocarpine-induced seizures

Aims

Alpha-melanocyte stimulating hormone (α-MSH) is a pro-opiomelanocortin (POMC)-derived peptide involved in different neurological functions that also exerts anti-inflammatory effects, including in the central nervous system (CNS). Although inflammation has been implicated in seizures and epilepsy, no study has systematically investigated whether α-MSH modifies seizures. Therefore, in the current study we determined whether α-MSH alters pentylenetetrazol (PTZ)- and pilocarpine-induced seizures.

Main methods

Adult male Swiss mice were injected with α-MSH (1.66, 5 or 15 μg/3 μL, intracerebroventricular (i.c.v.)) or systemic (0.1, 0.3 or 1 mg/kg, intraperitoneally (i.p.)). Five to sixty minutes after the injection of the peptide, animals were injected with PTZ (60 mg/kg, i.p.) or pilocarpine (370 mg/kg, i.p.). Latency to myoclonic jerks and tonic–clonic seizures, number of seizure episodes, total time spent seizing and seizure intensity, assessed by the Racine and Meurs scales were recorded. Interleukin 1 beta (IL-1β) levels in the hippocampus were measured by a commercial enzyme-linked immunoabsorbent assay (ELISA).

Key findings

Neither intracerebroventricular (1.66, 5 or 15 μg/3 μL, i.c.v.) nor systemic (0.1, 0.3 or 1 mg/kg, i.p.) administration of α-MSH altered PTZ- and pilocarpine-induced seizures. IL-1β levels in the hippocampi were not altered by α-MSH, PTZ or pilocarpine.

Significance

Although inflammation has been implicated in seizures and epilepsy and α-MSH is a potent anti-inflammatory peptide, our results do not support a role for α-MSH in seizure control.     

In Vivo Electrochemical Monitoring of Serotonin in Spinal Dorsal Horn with Nafion-Coated Multi-Carbon Fiber Electrodes

Abstract: Biosensors sensitive for in vivo monitoring of serotonin (5-HT) in the CNS by differential normal pulse voltammetry were constructed by coating treated multi-carbon fiber electrodes (mCFEs) with Nafion (N-mCFE). In vitro sensitivities of mCFE and N-mCFE were compared in solutions ranging from 5 n M to 20 µ M of uric acid (UA), 5-hydroxyindoleacetic acid (5-HIAA), and 5-HT. The mCFEs were three to seven times less sensitive for 5-HIAA or UA than for 5-HT. Nafion treatment dramatically decreased sensitivity for 5-HIAA and UA of N-mCFEs (∼10³ times), whereas it remained in the nanomolar range for 5-HT. In vivo, in the dorsal horn of the lumbar spinal cord of anesthetized rats, the monoamine oxidase inhibitor clorgyline (10 mg/kg i.p.) produced a reduction (55 ± 3% at 180 min) of peak 3 of oxidation current (characteristic of 5-hydroxyindoles) monitored with mCFEs, but with N-mCFEs (in this latter case the peak was termed 3N) peak 3N increased to 135 ± 5% at 180 min. The 5-HT release-inducer p -chloroamphetamine (PCA; 6 mg/kg i.p.) induced a slight (12 ± 3% at 150 min) decrease in peak 3 measured with mCFEs, whereas with N-mCFEs PCA induced a rapid increase of peak 3N (137 ± 6% at 90 min). The xanthine oxidase inhibitor allopurinol (10 mg/kg i.p.) produced a decrease (30 ± 3% at 180 min) in peak 3 (mCFEs), but peak 3N (N-mCFEs) was not affected (106% at 180 min). After pretreatment with allopurinol, PCA also produced an increase (135 ± 6% at 90 min) in peak 3N. These in vitro and in vivo data provide evidence for a highly preferential detection of 5-HT versus 5-HIAA and UA by N-mCFEs, which could be used to follow the extracellular 5-HT concentration within very discrete structures throughout the CNS.     

Antagonism of Serotonin 5-HT1A Receptors Potentiates the Increases in Extracellular Monoamines Induced by Duloxetine in Rat Hypothalamus

Abstract: In the current study we examined the effects of coadministration of a serotonin 5-HT_1A antagonist, (±)-1-(1 H -indol-4-yloxy)-3-(cyclohexylamino)-2-propanol maleate (LY 206130), and a dual 5-HT and norepinephrine (NE) uptake inhibitor, duloxetine, on extracellular levels of NE, 5-HT, dopamine (DA), 5-hydroxyindoleacetic acid, and 3,4-dihydroxyphenylacetic acid in rat hypothalamus microdialysates. LY 206130 (3.0 mg/kg, s.c.) alone significantly increased NE and DA levels by 60 and 34%, respectively, without affecting 5-HT levels. Duloxetine administration at 4.0 mg/kg, i.p. alone produced no significant changes in levels of 5-HT, NE, or DA. In contrast, when LY 206130 and duloxetine were coadministered at 3.0 mg/kg, s.c. and 4.0 mg/kg, i.p., respectively, 5-HT, NE, and DA levels increased to 5.7-, 4.8-, and threefold over their respective basal levels. These data demonstrate that antagonism of somatodendritic 5-HT_1A autoreceptors and concomitant inhibition of 5-HT and NE uptake with duloxetine may promote synergistic increases in levels of extracellular 5-HT, NE, and DA in hypothalamus of conscious, freely moving rats.     

Serotonergic Facilitation of Acetylcholine Release In Vivo from Rat Dorsal Hippocampus via Serotonin 5-HT3 Receptors

Abstract: The serotonin (5-HT) releaser d -fenfluramine and its active metabolite d -norfenfluramine, or the 5-HT-uptake inhibitor citalopram, by increasing synaptic 5-HT availability, facilitated in vivo release of acetylcholine (ACh) from dorsal hippocampi of freely moving rats as determined by the microdialysis technique. The effects of d -norfenfluramine (7.5 mg/kg i.p.) and citalopram (10 μ M , applied by reverse dialysis) were prevented by a 14-day chemical lesion of the raphe nuclei, suggesting mediation by the 5-HT system in the cholinergic action of the drugs. The increase in extracellular ACh content induced by d -norfenfluramine (5 mg/kg i.p.) was antagonized by the 5-HT₃ receptor antagonists tropisetron (0.5 mg/kg i.p.) and DAU 6215 (60 μg/kg i.p.), but not by the mixed 5-HT₁ and 5-HT₂ receptor antagonist metergoline (2 mg/kg s.c.). In accordance with an involvement of the 5-HT₃ receptor in the ACh facilitation induced by d-norfenfluramine is the finding that the selective 5-HT₃ receptor agonist 2-methyl-serotonin (250 μg i.c.v., or 10 μ M applied by reverse dialysis) raised ACh release. The effect of the intracerebroventricular drug was prevented by the 5-HT₃ antagonists DAU 6215 (60 μg/kg i.p.) and ondansetron (60 μg/kg s.c.). These antagonists by themselves did not modify the basal ACh release, indicating that 5-HT does not tonically activate the 5-HT₃ receptors involved. In conclusion, the overall regulatory control exerted by 5-HT in vivo is to facilitate hippocampal ACh release. This is mediated by 5-HT₃ receptors probably located in the dorsal hippocampi.     

5,5-Diphenylhydantoin Antagonizes Neurochemical and Behavioral Effects of p, p'-DDT but Not of Chlordecone

Abstract: Rats were given 75 mg/kg of 5,5-diphenylhydantoin (phenytoin) or vehicle 30 min prior to 75 mg/kg of 1, 1, 1-trichloro-bis( p -chlorophenyl)ethane ( p, p' -DDT) (p.o.) or chlordecone (i.p.) and tremor was measured 12 h later. Rats were then killed, and regional brain levels of biogenie amines and their acid metabolites and amino acids were determined. Pretreatment with phenytoin significantly attenuated the tremor produced by p, p' -DDT but enhanced that produced by chlordecone. p, p' -DDT had significant effects on the levels of asparate, glutamate, 5-hydroxyindoleacetic acid (5-HIAA), and 3-methoxy-4-hydroxyphenylglycol (MHPG), whereas chlordecone increased glycine, 5-HIAA, and MHPG levels. Pretreatment with phenytoin blocked p.p' -DDT-induced increases of aspartate in the brainstem and spinal cord, 5-HIAA in the hippocampus, and MHPG in the brainstem and hypothalamus. Phenytoin significantly enhanced chlordecone-induced increases of MHPG in the brainstem. These data indicate that organo-chlorine-induced increases in noradrenergic activity in the brainstem and spinal cord may be directly related to the tremorigenic effects of these chemicals.     

Regional Effects of Sodium Valproate on Extracellular Concentrations of 5-Hydroxytryptamine, Dopamine, and Their Metabolites in the Rat Brain: An In Vivo Microdialysis Study

The effects of sodium valproate (VPA; 100, 200, and 400 mg/kg, i.p.) on ventral hippocampal and anterior caudate putamen extracellular levels of dopamine (DA) and 5-hydroxytryptamine (5-HT) were examined using in vivo microdialysis. VPA induced dose-related increases in dialysate DA, 3,4-dihydroxyphenylacetic acid, and 5-HT in the ventral hippocampus. Anterior caudate putamen dialysate 5-HT was also dose dependently elevated by the drug, whereas DA levels tended to decrease with increasing VPA dose. In contrast, VPA (200, 400, and 800 mg/kg, i.p.) produced no significant elevation of DA in posterior caudate putamen dialysates, although 5-HT levels were significantly elevated at the 400- and 800-mg/kg doses. In all three regions studied, dialysate concentrations of 5-hydroxyindoleacetic acid and homovanillic acid remained at basal levels following VPA treatments. The results are discussed with regard to the possible anticonvulsant mode of action of VPA.