Uptake of serotonin by adult rat corpus callosum is partially reduced by common antidepressants

The corpus callosum (CC) is the main white matter tract involved in interhemispheric brain communication. We establish that uptake of [3H]5-hydroxytryptamine (5-HT) in CC is partially inhibited by some antidepressants. Slices of the adult rat CC had a high-affinity uptake of 5-HT. About 80% of this uptake was Na+ dependent, with a Michaelis-Menten constant, Km, of 420 +/- 80 nM and a rate of 5-HT uptake, Vmax, of 9.5 +/- 0.8 pmol/mg protein/min. The 5-HT uptake was reduced approximately 60% at pH 5 compared with that at pH 7. Fluoxetine (Prozac) inhibited only 43% of 5-HT uptake in a concentration-dependent manner, with an affinity constant, Ki, of 44.7 +/- 10.0 nM. We also studied the effects of other monoamine uptake inhibitors, all at 10 microM, and found that zimelidine, imipramine, and clomipramine inhibited 5-HT uptake in the CC by approximately 30-40%. The fluoxetine-insensitive 5-HT uptake was not altered by high concentrations of dopamine plus norepinephrine. The present data show that Na(+)-dependent 5-HT uptake occurs in the CC and optic nerve and that this uptake is partially sensitive to antidepressants and probably mediated by the serotonin transporter, which may be relevant during depression.     

Effects of repeated treatment with fluoxetine and citalopram, 5-HT uptake inhibitors, on 5-HT1A and 5-HT2 receptors in the rat brain.

Repeated treatment with fluoxetine and citalopram, which are potent 5-HT reuptake inhibitors, resulted in different regulation of 5-HT1A and 5-HT2 receptors in the rat brain. Their effects were compared with those of other antidepressants: imipramine, mianserin and levoprotiline. The density of 5-HT1A receptors, labelled with [3H]8-OH-DPAT, in the rat hippocampus was enhanced after citalopram, imipramine, mianserin and levoprotiline, but not altered after fluoxetine administration. [3H]Ketanserin binding sites, which label 5-HT2 receptors, were increased after fluoxetine and levoprotiline, but decreased after citalopram, imipramine and mianserin in the rat cerebral cortex. Acute administration of fluoxetine, but not citalopram, resulted in a decreased density of 5-HT1A receptors. 5-HT2 receptors were not changed by acute administration of either fluoxetine or citalopram. The obtained results indicate that besides 5-HT reuptake inhibiting properties of both compounds, there may exist an additional mechanism(s) of their action, which leads to different regulation of 5-HT1A and 5-HT2 receptors.     

In vivo biogenic amine efflux in medial prefiontal cortex with imipramine,fluoxetine, and fluvoxamine

In vivo brain microdialysis was used to determine the effects of the standard tricyclic antidepressant imipramine and the two selective serotonin reuptake inhibitors (SSRIs) antidepressants, fluoxetine and fluvoxamine, on extracellular levels of norepinephrine (NE), dopamine (DA), and serotonin (5-HT) in rat medial prefrontal cortex. When given intraperitoneally (IP), imipramine increased NE in the microdialysis perfusate, and elevated DA and 5-HT to a lesser extent. Similar dose-dependent increases in DA and 5-HT were detected after IP fluoxetine, although NE was less affected. In contrast, IP fluvoxamine produced no change in basal NE nor DA, although a large increase in 5-HT occurred at an intermediate dose. When administered directly into cortex, all three antidepressants increased 5-HT by the same amount in a dose-dependent fashion. Intracortical imipramine and fluoxetine increased NE, and fluoxetine and fluvoxamine both increased DA, with fluoxetine doing so at a lower concentration. These data suggest that the SSRIs are not entirely selective for serotonin in vivo. © 1994 Wiley-Liss, Inc.     

Modafinil enhances the increase of extracellular serotonin levels induced by the antidepressant drugs fluoxetine and imipramine: a dual probe microdialysis study in awake rat

In view of a postulated role of the vigilance-promoting drug modafinil in depression, the interaction of modafinil and two classical antidepressant drugs, fluoxetine and imipramine, were studied in 5-HT levels in the dorsal raphe-cortical system using dual-probe microdialysis. Fluoxetine (1-10 mg/kg) dose-dependently increased dorsal raphe-cortical 5-HT levels. Modafinil at a very low dose (3 mg/kg), by itself ineffective, enhanced the fluoxetine (5 mg/kg)-induced increases of 5-HT levels in both brain areas. A synergistic interaction was observed in the prefrontal cortex with fluoxetine (1 mg/kg) in terms of 5-HT release, but not in the dorsal raphe. Imipramine (1.3 mg/kg) increased 5-HT levels in the dorsal raphe, but not in the prefrontal cortex, while the higher doses (10.9-21.8 mg/kg) caused substantial increases in both brain areas. Modafinil (3 mg/kg), injected before imipramine (1.3 mg/kg), which by itself was ineffective on cortical 5-HT levels, increased cortical 5-HT levels. On other hand, modafinil failed to affect the high-dose imipramine (10.9 mg/kg)-induced increase of 5-HT levels in the prefrontal cortex and the imipramine (1.3; 10.9 mg/kg)-induced increase of 5-HT levels in the dorsal raphe nucleus. These results demonstrate that modafinil in low doses enhances the acute effects of fluoxetine and imipramine on 5-HT levels in the dorsal raphe nucleus (fluoxetine only) and especially in the prefrontal cortex of the awake rat. These findings suggest a therapeutic potential of low doses of modafinil in the treatment of depression when combined with low doses of classical antidepressants, especially by increasing 5-HT transmission in cortical regions.     

Interactions of a non-selective monoamine oxidase inhibitor, phenelzine, with inhibitors of 5-hydroxytryptamine, dopamine or noradrenaline re-uptake

Interactions of combined antidepressants which occur in man were reproduced in rats pretreated with phenelzine, features elicited including myoclonic phenomena, an augmented lower limb flexor reflex, muscle fasiculation and fatalities, particularly with combinations incorporating 5-hydroxytryptamine (5-HT) re-uptake inhibitors. Combinations of antidepressants included phenelzine with 5-HT re-uptake inhibitors (paroxetine, fluoxetine, clomipramine); with "mixed" re-uptake inhibitors affecting 5-HT and noradrenaline (imipramine, amitriptyline); with noradrenaline re-uptake inhibitors (desipramine, maprotiline, nisoxetine) and with dopamine re-uptake inhibitors (benztropine, nomifensine). Myoclonic phenomena such as forelimb flexor-extensor movements, head and body twitches, occurred in phenelzine pretreated rats after paroxetine, fluoxetine, clomipramine, imipramine, amitriptyline and desipramine. Wet dog shakes, the most intense phenomenon, were obtained only after paroxetine, fluoxetine, clomipramine and imipramine. Myoclonic features were prevented when pretreatment included p-chlorophenylalanine but were unaffected when this incorporated alpha-methyl-p-tyyrosine; there were attenuated by methysergide, cyproheptadine, clozapine or pimozide. The myoclonic phenomena were reproduced by combination of 5-hydroxytryptophan but not L-3,4-dihydroxyphenylalanine with clomipramine. Electrocortical changes observed included 2-4 Hz, 5-8 Hz, large amplitude potentials unrelated to the myoclonic incidents and unaffected by sensory stimulation. Following phenelzine, brain monoamine oxidase (MAO) A inhibition was 99% and that of MAO B, 88%; 5-HT concentration was significantly elevated in the cortex and hypothalamus, as was hypothalamic noradrenaline. Peak and basal tensions of a lower-limb flexor reflex were elevated in phenelzine pretreated spinal rats by fluoxetine, paroxetine, clomipramine and imipramine, effects attenuated by cyproheptadine. Forelimb flexor-extensor movements and body twitches were elicited by fluoxetine and paroxetine in phenelzine pretreated spinal rats in the presence of electrical stimulation of the central stump of a divided posterior tibial nerve. Pressor responses were observed in phenelzine pretreated spinal rats given 5-HT re-uptake inhibitors, "mixed" re-uptake inhibitors and those affecting noradrenaline re-uptake; ECG anomalies occurred in such rats given clomipramine.     

Kinetics and autoradiography of high affinity uptake of serotonin by primary astrocyte cultures

Primary astrocyte cultures prepared from the cerebral cortices of neonatal rats showed significant accumulation of serotonin (5-hydroxytryptamine; [3H]-5-HT). At concentrations in the range of 0.01 to 0.7 microM [3H]-5-HT, this uptake was 50 to 85% Na+ dependent and gave a Km of 0.40 +/- 0.11 microM [3H]-5-HT and a Vmax of 6.42 +/- 0.85 (+/- SEM) pmol of [3H]-5-HT/mg of protein/4 min for the Na+-dependent component. In the absence of Na+ the uptake was nonsaturable. Omission of the monoamine oxidase inhibitor pargyline markedly reduced the Na+-dependent component of [3H]-5-HT uptake but had a negligible effect on the Na+-independent component. This suggest significant oxidative deamination of serotonin after it has been taken up by the high affinity system, followed by release of its metabolite. We estimated that this system enabled the cells to concentrate [3H]-5-HT up to 44-fold at an external [3H]-5-HT concentration of 10(-7) M. Inhibition of [3H]-5-HT uptake by a number of clinically effective antidepressants was also consistent with a specific high affinity uptake mechanism for 5-HT, the order of effectiveness of inhibition being chlorimipramine greater than fluoxetine greater than imipramine = amitriptyline greater than desmethylimipramine greater than iprindole greater than mianserin. Uptake of [3H]-5-HT was dependent on the presence of Cl- as well as Na+ in the medium, and the effect of omission of both ions was nonadditive. Varying the concentration of K+ in the media from 1 to 50 mM had a limited effect on [3H]-5-HT uptake.(ABSTRACT TRUNCATED AT 250 WORDS)     

St. John's wort extract Ze 117 (Hypericum perforatum) inhibits norepinephrine and serotonin uptake into rat brain slices and reduces 3-adrenoceptor numbers on cultured rat brain cells

Despite almost forty years of widespread use, the mode of action of antidepressant drugs is still largely unknown. There is agreement that these drugs interact with central neurotransmission. Common findings are acute inhibitory actions on reuptake mechanisms for norepinephrine (NE) and for serotonin (5-HT) at presynaptic axons and chronic adaptive effects on neurotransmitter receptors on postsynaptic membranes. In particular, beta-adrenoceptor downregulation has been observed after chronic treatment with most antidepressants in vivo and in cell culture systems. We studied the effectiveness of Ze 117 (St. John's wort) extract (Hypericum perforatum) on NE- and 5-HT-uptake into rat brain slices. Potency and efficacy of the Ze 117 extract were compared with those of tricyclic (TCA) and selective serotonin reuptake inhibitor (SSRI)-type antidepressants. A dose-dependent inhibition was seen on NE and 5-HT uptake into brain slices. The Ze 117 extract was more selective for the uptake of NE than for that of 5-HT. The maximal extent of uptake inhibition by Ze 117 extract was comparable to that of imipramine (IMI), desipramine (DMI) or fluvoxamine for 5-HT, but lower for NE transport, than that of the synthetic antidepressants. Chronic exposure (8 days) of confluent C6-cell cultures to Ze 117 extract resulted in a dose-dependent beta-adrenoceptor downregulation equal to that induced by DMI, a potent TCA. None of these effects could be achieved with either hypericin or hyperforin alone in a relevant dose range. Our results indicate that the St. John's wort extract Ze 117 contains active, but as yet unknown antidepressant principles with effects comparable to those of TCAs.     

Antidepressants are functional antagonists at the serotonin type 3 (5-HT3) receptor

Antidepressants are commonly supposed to enhance serotonergic and/or noradrenergic neurotransmission by inhibition of neurotransmitter reuptake through binding to the respective neurotransmitter transporters or through inhibition of the monoamine oxidase. Using the concentration-clamp technique and measurements of intracellular Ca2+, we demonstrate that different classes of antidepressants act as functional antagonists at the human 5-HT3A receptor stably expressed in HEK 293 cells and at endogenous 5-HT3 receptors of rat hippocampal neurons and N1E-115 neuroblastoma cells. The tricyclic antidepressants desipramine, imipramine, and trimipramine, the serotonin reuptake inhibitor fluoxetine, the norepinephrine reuptake inhibitor reboxetine, and the noradrenergic and specific serotonergic antidepressant mirtazapine effectively reduced the serotonin-induced Na(+)- and Ca(2)(+)-currents in a dose-dependent fashion. This effect was voltage-independent and, with the exception of mirtazapine, noncompetitive. Desipramine, imipramine, trimipramine, and fluoxetine also accelerated receptor desensitization. Moclobemide and carbamazepine had no effect on the serotonin-induced cation current. By analyzing analogues of desipramine and carbamazepine, we found that a basic propylamine side chain increases the antagonistic potency of tricyclic compounds, whereas it is abolished by an uncharged carboxamide group. The antagonistic effects of antidepressants at the 5-HT3 receptor did not correlate with their effects on membrane fluidity. In conclusion, structurally different types of antidepressants modulate the function of this ligand-gated ion channel. This may represent a yet unrecognized pharmacological principle of antidepressants.     

Changes in [3H]5-HT uptake and [3H]imipramine binding in platelets after chlorimipramine in healthy volunteers. Comparison with maprotiline and amineptine

In the platelets of normal healthy volunteers (n = 8) taking chlorimipramine (50 mg/day) for 1 week, the saturable uptake of [3H]5-hydroxytryptamine (5-HT) was fully inhibited at the end of the week, but returned to control values after 2 weeks washout. The Bmax of [3H]imipramine binding was decreased by 63% at the end of the treatment and remained significantly decreased below control values after 1 week washout, whereas the Kd values were increased at the end of the treatment, but had returned to baseline values after 1 week washout. The time course of recovery following the administration of chlorimipramine showed some variation between subjects, but it was necessary to wait up to 4 weeks of washout before the Bmax of [3H]imipramine returned to baseline levels. In contrast, neither 1-week treatment with maprotiline (50 mg/day) nor with amineptine (100 mg/day) changed the parameters of [3H]5-HT uptake or [3H]imipramine binding in platelets from healthy volunteers. These results support the following conclusions. (1) [3H]Imipramine binding in platelets can be down-regulated by relatively low, subtherapeutic doses of chlorimipramine. (2) It is possible to dissociate [3H]imipramine binding parameters from [3H]5-HT uptake because the time course of recovery was clearly different, indicating that [3H]imipramine labels a site linked with, but different from, the 5-HT recognition site in the transporter complex. (3) A washout of antidepressants of 4 weeks may be needed when studying the parameters of [3H]imipramine binding in platelets from depressed patients if the previous medication involved chlorimipramine. For antidepressants like maprotiline or amineptine, that act through mechanisms other than inhibition of 5-HT uptake, the time of washout appears to be less critical, although it is not possible to rule out the existence of some secondary modifications influencing the 5-HT transporter complex.     

Serotonin uptake in cerebral cortex cultures: imipramine-like inhibition by N-isopropyl-p-iodoamphetamine

In cultured rat neocortex, uptake of [3H]serotonin (5-HT) and the SPECT radiopharmaceutical N-isopropyl-p-[123I]iodoamphetamine (IMP) was demonstrated after 4 and 14 days in vitro. Both imipramine and cold IMP inhibited [3H]5-HT uptake. Uptake of [123I]IMP was inhibited by imipramine but not by cold 5-HT. The similarity in the behaviors of IMP and imipramine indicates that uptake of IMP might be related to a serotonergic uptake system in a way that is similar to that in which imipramine is related to such a system.     

[I]5-iodo-6-nitroquipazine: a potent and selective ligand for the 5-hyfroxytryptamine uptake complex. I. In vitro studies

In search of a potent and selective radioiodinated ligand for the 5-hydroxytryptamine (serotonin or 5-HT) uptake complex, we synthesized and evaluated the in vitro properties of [¹²⁵I]5-iodo-6-nitroquipazine. The binding properties and pharmacological profile of this radioligand were studied in rat brain homogenates, and it was found to display high affinity and selectivity for the serotonin uptake complex. Scatchard analysis of the binding data indicated a single population of sites with aK_d of23 ± 6pM and aB_max of 430 ± 50 fmol/mg protein (mean ±S.E.M., n - 7). Inhibitors of serotonin uptake were the most efficient competitors for [¹²⁵I]5-iodo-6-nitroquipazine binding withK_i values similar in rank order and magnitude to those obtained in studies of other established serotonin uptake blockers. Inhibitors of dopamine and norepinephrine uptake as well as a wide variety of postsynaptic receptor agents were relatively ineefective in inhibiting [¹²⁵I]5-iodo-6-nitroquipazine binding to rat brain membranes. Serotonin was the only monoaminergic neurotransmitter capable of effectively competing for [¹²⁵I]5-iodo-6-nitroquipazine binding sites and gave aK_i value of 2.8 ± 0.6 μM. Lesions of the serotonergic system withp-chloroamphetamine resulted in a dramatic loss 90%of [¹²⁵I]5-iodo-6-nitroquipazine binding to rat cortical membranes. Non-radiolabeled 5-iodo-6-nitroquipazine potently inhibited the binding of [³H]paroxetine to serotonin reuptake sites in rat cortical membranes with aK_i of 0.17 ± 0.06nM. These results demonstrate that 5-iodo-6-nitroquipazine is a potent and selective ligand for the serotonin uptake complex, and the [¹²⁵I]-labeled radioligand has an advantage of detecting low levels of binding sites.     

Effects of benzylpiperazine derivatives on the neurotoxicity of 3,4-methylenedioxymethamphetamine in rat brain

The neurotoxicity of 3,4-methylenedioxymethamphetamine (MDMA) in rat brain was attenuated significantly by coadministration of several benzylpiperazines (p-nitrobenzylpiperazine, p-chlorobenzylpiperazine and 1-piperonylpiperazine), which were weak inhibitors for [³H]6-nitroquipazine binding to the 5-hydroxytryptamine (5-HT) transporter in rat brain. These results suggest that these benzylpiperazines may inhibit the MDMA-induced neurotoxicity by a novel neuropharmacological effect other than 5-HT uptake inhibition.     

Effect of subchronic antidepressants administration on serotonin-stimulated phosphoinositide hydrolysis in para-chlorophenylalanine-treated rat hippocampal slices.

1. This study examines the effect of subchronic parachlorophenylalanine (PCPA) treatment upon serotonin (5-HT)-stimulated inositol monophosphate (IP-1) accumulation in rat hippocampal slices and also the effect of antidepressants upon this 5-HT response in the hippocampus from rats treated with or without concurrent administration of PCPA. 2. For high dose PCPA treatment, animals were injected intraperitoneally with 300 mg/kg daily for the first 5 days and then 100 mg/kg for 5 days, while for low dose PCPA treatment animals were injected for 10 days at a dose of 100 mg/kg. Imipramine or iprindole (15 mg/kg i.p.) was given once daily for 10 consecutive days. 3. 10-Day treatment with high dose of PCPA resulted in a significant increase in 5-HT-stimulated IP-1 accumulation, whereas low dose of PCPA had no significant effect upon the 5-HT response as compared to vehicle. 5-HT-stimulated IP-1 accumulation in rat hippocampus was not affected by subchronic treatment with imipramine or iprindole. The enhancement of the 5-HT response induced by high dose of PCPA was not attenuated by repeated antidepressants treatment.     

Serotonin uptake in cerebral cortex cultures: Imipramine-like inhibition by N-isopropyl-p-iodoamphetamine

In cultured rat neocortex, uptake of [³H]serotonin (5-HT) and the SPECT radiopharmaceutical N-isopropyl-p-[¹²³I]iodoamphetamine (IMP) was demonstrated after 4 and 14 days in vitro. Both imipramine and cold IMP inhibited [³H]5-HT uptake. Uptake of [¹²³I]IMP was inhibited by imipramine but not by cold 5-HT. The similarity in the behaviors of IMP and imipramine indicates that uptake of IMP might be related to a serotonergic uptake system in a way that is similar to that in which imipramine is related to such a system.     

Serotonin via presynaptic 5-HT1 receptors attenuates synaptic transmission to immature rat motoneurons in vitro.

Intracellular recordings were made from motoneurons in transverse spinal cord slices from immature (12-20 day) rats and the effects of 5-HT on dorsal root evoked excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials were assessed. With or without causing a membrane polarization, 5-HT (1-300 microM) depressed synaptic responses; the IC50 was 6 microM. The inhibitory effect was potentiated by the uptake inhibitor fluoxetine. The 5-HT1A/1B agonists 5-CT and 8-OH-DPAT and the 5-HT1B/1C agonist TFMPP reduced the synaptic responses as well, with an IC50 of 0.26, 2.2 and 0.28 microM, respectively. The synaptic depressant effect was not antagonized by methysergide (0.1-1 microM), ketanserin (1-5 microM) and MDL 72222 (1-10 microM). Methysergide alone diminished the synaptic responses in some of the motoneurons. Spiperone (1-10 microM) partially and fully antagonized the depressant effect of 5-HT and 8-OH-DPAT, but was ineffective against 5-CT and TFMPP. The 5-HT-induced synaptic depression was not accompanied by a concomitant reduction of glutamate-induced depolarizations; the latter were enhanced after repeated exposure to 5-HT in some motoneurons. Finally, 5-HT reduced the afterhyperpolarization following a single spike or a train of spikes. The results indicate that 5-HT inhibits synaptic responses in motoneurons via presynaptic 5-HT1 receptors, the activation of which reduces the liberation of excitatory and inhibitory transmitters from respective nerve endings.     

High affinity [3H]paroxetine binding to serotonin uptake sites in human brain tissue

[3H]Paroxetine binding to human brain tissue was characterized. Competition studies in the putamen and frontal cortex revealed single-site binding models for binding sensitive to 5-hydroxytryptamine (5-HT) (Ki 1-3 microM) and citalopram (Ki 0.6 nM), which displaced the same amount of binding. However, desipramine, norzimeldine and fluoxetine displaced additional binding (10-20%) and these competitors fitted two-site binding models with high affinity components in the nanomolar range and low affinity components in the micromolar range. The high affinity components approximated the 5-HT- and citalopram-sensitive binding fraction. Most of the [3H]paroxetine binding sites were protease-sensitive, but the low-affinity (microM) sites appeared to be protease-resistant. Based on these findings, only the [3H]paroxetine binding representing the fraction sensitive to 30 microM 5-HT (or e.g. 0.3 microM norzimeldine), was regarded as specific binding. This binding fraction was saturable with an apparent binding affinity (Kd) of 0.03-0.05 nM throughout the brain. The highest binding densities were obtained in the hypothalamus and substantia nigra (Bmax 500 fmol/mg protein). The basal ganglia reached intermediate densities (Bmax 200 fmol/mg protein), whereas cortical areas had low Bmax values (less than 100 fmol/mg protein). The lowest B max value was noted in cerebellar cortex (30 fmol/mg protein). The [3H]paroxetine binding was competitively inhibited by low concentrations of 5-HT, imipramine and norzimeldine, suggesting that the substrate recognition site for 5-HT uptake was labeled. Compounds active at dopaminergic, noradrenergic, histaminergic, 5-HT1, 5-HT2 and cholinergic muscarinic sites did not affect the binding at 100 microM concentrations. It is concluded that [3H]paroxetine is a marker for the 5-HT uptake site in the human brain, provided that an adequate pharmacological definition of specific binding is performed.     

Interaction of zinc with antidepressants in the tail suspension test

The antidepressant-like effect of zinc has been shown in several animal models of depression. In this study, zinc chloride (ZnCl2) was given alone or in combination with different classes of antidepressants by oral route (p.o.) to mice and the behavioral response in the tail suspension test (TST), a predictive test of antidepressant action, was investigated. ZnCl2 at a dose of 10 and 30 mg/kg, p.o., reduced the immobility time in the TST, without affecting the locomotor activity in open-field test. The antidepressants fluoxetine, paroxetine, imipramine, desipramine and bupropion produced a significant reduction in the immobility time in TST at the doses of 10, 1, 1, 1 and 10 mg/kg, p.o., respectively. The combined treatment of sub-effective doses of ZnCl2 (1 mg/kg) with sub-effective doses of fluoxetine (5 mg/kg), paroxetine (0.1 mg/kg), desipramine (0.1 mg/kg), imipramine (0.1 mg/kg) or bupropion (1 mg/kg) induced a significant reduction in the immobility time in the TST when compared with the groups treated with ZnCl2 or with antidepressants alone. The treatment with sub-effective doses of ZnCl2 and antidepressants alone or in combination did not affect the locomotion in open-field test, except that desipramine alone reduced the ambulation. The results first indicate that ZnCl2 administered by p.o. route produces an antidepressant-like effect in the TST. Moreover, synergistic effects of zinc with antidepressants were shown in the TST, suggesting that an improvement in the response to the antidepressant therapy occurs when zinc is combined with different classes of antidepressants.     

The serotonin 5-HT2 receptor-phospholipase C system inhibits the induction of long-term potentiation in the rat visual cortex

The effect of serotonin 5-HT2 receptor stimulation on long-term potentiation (LTP) in the primary visual cortex was investigated by using rat brain slices in vitro. Field potentials evoked by stimulation of layer IV were recorded in layer II/III. The 5-HT2 receptor agonist 1-(2,5-dimethyl-4-iodophenyl)-2-aminopropane (DOI) did not affect baseline synaptic potentials evoked by single-pulse test stimulation, but significantly inhibited the induction of LTP in a concentration-dependent manner (0.1-10 microM). The LTP-inhibiting effect of DOI (10 microM) was blocked by the 5-HT2,7 receptor antagonist ritanserin (10 microM), but not by the 5-HT1A receptor antagonist NAN-190 (10 microM) nor by the 5-HT3,4 receptor antagonist MDL72222 (10 microM). The inhibitory effect of DOI was also blocked by the phospholipase C inhibitor U73122, but not by its inactive analogue U73343. These results suggest that visual cortex LTP is inhibited by activation of the 5-HT2 receptor-phospholipase C system. In addition, the LTP-inhibiting effect of DOI was abolished by the presence of the GABAA receptor antagonist bicuculline (10 microM), suggesting that 5-HT2 receptor-mediated inhibition of visual cortex LTP is dependent on GABAergic inhibition.     

MK-801 interaction with the 5-HT transporter: a real-time study in brain slices using fast cyclic voltammetry.

The effects of a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine ((+)-MK-801) and a competitive NMDA antagonist, (+/-)-3-2-carboxypiperazin-4-yl-propyl-1-phosphonic acid (CPP) were compared in electrically evoked 5-HT release in the brain slices incorporating the substantia nigra pars reticulata (SNr) or the dorsal raphé nucleus (DRN) using fast cyclic voltammetry (FCV). Electrical stimulation of either the SNr or the DRN with 50 pulses at frequencies greater than 10 Hz generated signals that were indistinguishable from 5-HT. In the SNr, 0.6-60 microM MK-801 concentration dependently potentiated stimulated 5-HT release. CPP 20 microM or NMDA 100 microM had no effect on 5-HT release evoked by electrical stimulation. In the SNr, 1 microM fluvoxamine or 0.6-60 microM MK-801 potentiated electrically evoked release of 5-HT. Pre-exposure to 20 microM MK-801 inhibited the enhancing effects of 1 microM fluvoxamine on electrically evoked 5-HT release in the SNr. In the DRN, the presence of 1 microM fluvoxamine or 20 microM MK-801 weakly potentiated 5-HT release. In the presence of 1 microM methiothepin (a nonselective 5-HT1-2 antagonist), 1 microM fluvoxamine or 20 microM MK-801 were equipotent in potentiating the concentration of 5-HT released in response to electrical stimulation. The T1/2 values for 5-HT release following MK-801 or fluvoxamine administration were significantly increased. Potentiation of 5-HT release by MK-801 in the SNr and the DRN and lack of effect of either CPP or NMDA on 5-HT release or uptake argues against a role for NMDA receptors in modulation of 5-HT release. Inhibition of fluvoxamine induced potentiation of 5-HT signal in the presence of MK-801 suggests that MK-801 and fluvoxamine may interact at the level of the 5-HT transporter.