Potentiation of the Fluoxetine-Induced Increase in Dialysate Levels of Serotonin (5-HT) in the Frontal Cortex of Freely Moving Rats by Combined Blockade of 5-HT1A and 5-HT1B Receptors with WAY 100,635 and GR 127,935

Abstract: In this study, we examined the influence of blockade of serotonin (5-HT)_1A and/or 5-HT_1B autoreceptors on the fluoxetine-induced increase in dialysate levels of 5-HT as compared with dopamine (DA) and noradrenaline (NAD) in single samples of the frontal cortex (FCx) of freely moving rats. Fluoxetine (10.0 mg/kg, s.c.) elicited a twofold increase in dialysate levels of 5-HT relative to baseline values. The selective 5-HT_1A antagonist WAY 100,635 (0.16 mg/kg, s.c.) did not influence 5-HT release alone but doubled the influence of fluoxetine on basal levels. Similarly, the selective 5-HT_1B/1D antagonist GR 127,935 (2.5 mg/kg, s.c.) did not alter basal 5-HT levels alone and doubled the fluoxetine-induced increase in 5-HT levels. Combined administration of WAY 100,635 and GR 127,935 elicited an (at least) additive rise in the fluoxetine-induced increase in 5-HT levels to eightfold basal values, without modifying resting 5-HT levels. These changes were selective for 5-HT inasmuch as the parallel (twofold) increase in DA and NAD levels provoked by fluoxetine was not potentiated. The present data demonstrate that combined blockade of 5-HT_1A and 5-HT_1B autoreceptors markedly and selectively potentiates the fluoxetine-induced increase in dialysate levels of 5-HT versus DA and NAD in the FCx of freely moving rats. These observations suggest that 5-HT_1A/1B antagonism may represent a novel strategy for the improvement in the therapeutic profile of 5-HT reuptake inhibitor antidepressant agents and that 5-HT may be primarily involved in such interactions.     

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.     

Differential Regulation of Somatodendritic Serotonin 5-HT1A Receptors by 2-Week Treatments with the Selective Agonists Alnespirone (S-20499) and 8-Hydroxy-2-(Di-n-Propylamino)tetralin

Abstract : Single treatment with the serotonin (5-hydroxytryptamine) 5-HT_1A receptor agonists 8-hydroxy-2-(di- n -propylamino)tetralin (8-OH-DPAT) and alnespirone (S-20499) reduces the extracellular 5-HT concentration (5-HT_ext) in the rat midbrain and forebrain. Given the therapeutic potential of selective 5-HT_1A agonists in the treatment of affective disorders, we have examined the changes in 5-HT_1A receptors induced by 2-week minipump administration of alnespirone (0.3 and 3 mg/kg/day) and 8-OH-DPAT (0.1 and 0.3 mg/kg/day). The treatment with alnespirone did not modify baseline 5-HT_ext but significantly attenuated the ability of 0.3 mg/kg s.c. alnespirone to reduce 5-HT_ext in the dorsal raphe nucleus (DRN) and frontal cortex. In contrast, the ability of 8-OH-DPAT (0.025 and 0.1 mg/kg s.c.) to reduce 5-HT_ext in both areas was unchanged by 8-OH-DPAT pretreatment. Autoradiographic analysis revealed a significant reduction of [³H]8-OH-DPAT and [³H]WAY-100635 {³H-labeled N -[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N -(2-pyridyl)cyclohexanecarboxamide · 3HCl} binding to somatodendritic 5-HT_1A receptors (but not to postsynaptic 5-HT_1A receptors) of rats pretreated with alnespirone but not with 8-OH-DPAT. In situ hybridization analysis revealed no change of the density of the mRNA encoding the 5-HT_1A receptors in the DRN after either treatment. These data indicate that continuous treatment for 2 weeks with alnespirone, but not with 8-OH-DPAT, causes a functional desensitization of somatodendritic 5-HT_1A receptors controlling 5-HT release in the DRN and frontal cortex.     

Preferential Potentiation of the Effects of Serotonin Uptake Inhibitors by 5-HT1A Receptor Antagonists in the Dorsal Raphe Pathway: Role of Somatodendritic Autoreceptors

Abstract: 5-HT_1A autoreceptor antagonists enhance the effects of antidepressants by preventing a negative feedback of serotonin (5-HT) at somatodendritic level. The maximal elevations of extracellular concentration of 5-HT (5-HT_ext) induced by the 5-HT uptake inhibitor paroxetine in forebrain were potentiated by the 5-HT_1A antagonist WAY-100635 (1 mg/kg s.c.) in a regionally dependent manner (striatum > frontal cortex > dorsal hippocampus). Paroxetine (3 mg/kg s.c.) decreased forebrain 5-HT_ext during local blockade of uptake. This reduction was greater in striatum and frontal cortex than in dorsal hippocampus and was counteracted by the local and systemic administration of WAY-100635. The perfusion of 50 µmol/L citalopram in the dorsal or median raphe nucleus reduced 5-HT_ext in frontal cortex or dorsal hippocampus to 40 and 65% of baseline, respectively. The reduction of cortical 5-HT_ext induced by perfusion of citalopram in midbrain raphe was fully reversed by WAY-100635 (1 mg/kg s.c.). Together, these data suggest that dorsal raphe neurons projecting to striatum and frontal cortex are more sensitive to self-inhibition mediated by 5-HT_1A autoreceptors than median raphe neurons projecting to the hippocampus. Therefore, potentiation by 5-HT_1A antagonists occurs preferentially in forebrain areas innervated by serotonergic neurons of the dorsal raphe nucleus.     

Monoamine Interactions Measured by Microdialysis in the Ventral Tegmental Area of Rats Treated Systemically with (±)-8-Hydroxy-2-(Di-n-Propylamino)tetralin

Abstract: The effect of (±)-8-hydroxy-2-(di- n -propylamino)tetralin (8-OH-DPAT), a selective serotonin 5-HT_1A agonist, on levels of extracellular norepinephrine (NE), dopamine (DA), and 5-HT (measured simultaneously) was investigated by microdialysis in the ventral tegmental area (VTA) of freely moving rats, and their behavioral activity was monitored. At 50 µg/kg s.c., 8-OH-DPAT reduced 5-HT levels but enhanced NE and DA levels in VTA dialysate. These effects were not altered by pretreatment with systemic idazoxan (5 mg/kg i.p.), a selective α₂ antagonist, or local sulpiride (10 µ M ), a selective D₂/D₃ antagonist. At 500 µg/kg s.c., 8-OH-DPAT further enhanced or more persistently reduced dialysate NE or 5-HT content but had little effect on dialysate DA content. Its DA level-increasing effect could be seen dramatically with local infusion of cocaine (30 µ M ) and, to a lesser extent, sulpiride (10 µ M ). Depletion of endogenous 5-HT with p -chlorophenylalanine attenuated both the 5-HT level-reducing and DA level-enhancing effects of 8-OH-DPAT without affecting its maximal NE effect and the locomotor-stimulatory effect. Partial depletion of endogenous NE with N -(2-chloroethyl)- N -ethyl-2-bromobenzylamine failed to change the monoamine response but diminished the locomotion induced by 8-OH-DPAT. These results suggested that (a) the low dose of 8-OH-DPAT may act at presynaptic 5-HT_1A receptors to modulate 5-HT and DA release, while acting at postsynaptic 5-HT_1A receptors to modulate NE release; (b) the high dose of 8-OH-DPAT may activate D₂ receptors to offset its DA level-increasing effect; and (c) the locomotor-stimulatory effect of 8-OH-DPAT might be mediated primarily by postsynaptic 5-HT_1A receptors and the NE system.     

Neuronal Release of Serotonin in the Cerebellum of Behaving Rats: An In Vivo Microdialysis Study

Abstract: Release of endogenous serotonin [5-hydroxy-tryptamine (5-HT)] in the cerebellum of awake rats was characterized using in vivo microdialysis. 5-HT output was increased (∼70%) by local application of KCl (100 m M ) and was reduced (∼60%) by both tetrodotoxin (0.5 µ M ) and omission of Ca²⁺ from the perfusion fluid. 5-HT release was decreased (∼70%) by the selective 5-HT_1A agonist 8-hydroxy-2-(di- n -propylamino)tetralin (0.25 mg/kg, s.c.), and this effect was rapidly reversed by a selective 5-HT_1A antagonist, N -[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N -(2-pyridinyl)cyclohexane-carboxamide trihydrochloride (WAY-100635; 0.1 mg/kg, i.p.). These results indicate that a large portion of the measurable 5-HT output in the cerebellum is of neuronal origin, is dependent on impulse flow, and is sensitive to 5-HT_1A autoreceptor activation. Further studies examined the relationship between 5-HT levels and general activity of the animals across the light-dark transition and during behavioral manipulations. Both 5-HT levels and behavioral activity were significantly elevated during the dark period, with changes in 5-HT efflux closely paralleling changes in activity. Similar increases (∼40%) in 5-HT output were observed during both feeding and feeding in the presence of a stressor (tail pinch). These findings suggest that behavioral state is an important factor determining neuronal 5-HT release in cerebellum under physiological conditions.     

Serotonin-1A receptor function in the dorsal raphe nucleus following chronic administration of the selective serotonin reuptake inhibitor sertraline

Serotonin-1A (5-HT_1A) receptors in the dorsal raphe nucleus (DRN) function as somatodendritic autoreceptors, and therefore play a critical role in controlling serotonergic cell firing and serotonergic neurotransmission. We hypothesized that a decrease in the capacity of 5-HT_1A receptors to activate G proteins was a general mechanism by which 5-HT_1A receptors in the DRN are desensitized following chronic administration of selective serotonin reuptake inhibitors (SSRIs). Using in vivo microdialysis, we found that the ability of the 5-HT_1A receptor agonist 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT) (0.025 mg/kg, s.c.) to decrease extracellular 5-HT levels in striatum was attenuated following chronic treatment of rats with the SSRIs sertraline or fluoxetine. This apparent desensitization of somatodendritic 5-HT_1A autoreceptor function was not accompanied by a decrease in 5-HT_1A receptor sites in the coupled, high-affinity agonist state as measured by the binding of [3H]8-OH-DPAT. In marked contrast to what was observed following chronic administration of fluoxetine, 5-HT_1A receptor-stimulated [³⁵S]GTPγS binding in the DRN was not altered following chronic sertraline treatment. Thus, desensitization of 5-HT_1A somatodendritic autoreceptor function following chronic sertraline administration appears not to be due to a decrease in the capacity 5-HT_1A receptors to activate G proteins in the DRN. Our findings suggest that the SSRIs may not be a homogeneous class of antidepressant drug with regard to the mechanism by which the function of somatodendritic 5-HT_1A autoreceptors is regulated.     

Flesinoxan Dose-Dependently Reduces Extracellular 5-Hydroxytryptamine (5-HT) in Rat Median Raphe and Dorsal Hippocampus Through Activation of 5-HT1A Receptors

Abstract: The effects of systemic administration of the serotonin (5-hydroxytryptamine) 5-HT_1A receptor agonists flesinoxan and 8-hydroxy-2-(di- n -propylamino)tetralin on extracellular 5-HT were measured using microdialysis probes in both median raphe nucleus and dorsal hippocampus. Both 5-HT_1A agonists dose-dependently decreased dialysate 5-HT levels from both brain regions. The effects of flesinoxan in the median raphe (0.3 mg/kg) and dorsal hippocampus (1.0 mg/kg) could be blocked by the 5-HT_1A receptor antagonist N -[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]- N -(2-pyridyl)cyclohexane carboxamide trihydrochloride (WAY 100,635) at a dose of 0.05 mg/kg s.c. The antagonist itself had no effect at this dosage. Local perfusion of flesinoxan for 30 min through the dialysis probe into the median raphe region at concentrations of 20, 100, and 1,000 n M resulted in a significant decrease in dialysate 5-HT content from both regions. The effect of 100 n M flesinoxan could be blocked by coperfusion of 1,000 n M WAY 100,635. The data indicate that flesinoxan is a potent 5-HT_1A receptor agonist and also support the notion that somatodendritic 5-HT_1A autoreceptors regulate both terminal and somatodendritic 5-HT release.     

Combined Administration of a 5-Hydroxytryptamine (5-HT)1D Antagonist and a 5-HT Reuptake Inhibitor Synergistically Increases 5-HT Release in Guinea Pig Hypothalamus In Vivo

Abstract: In vivo microdialysis in guinea pig hypothalamus was used to study the effect of serotonin [5-hydroxytryptamine (5-HT)] subtype 1D autoreceptor blockade on the increase in extracellular 5-HT levels produced by a selective 5-HT reuptake inhibitor (SSRI). Administration of the selective 5-HT_1D antagonist GR127935 at 0.3 mg/kg had no effect, but 5 mg/kg significantly increased extracellular levels of 5-HT and 5-hydroxyindoleacetic acid to 135% of basal values. Moreover, at these doses GR127935 significantly attenuated the decrease in extracellular 5-HT levels following local perfusion with the selective 5-HT_1D agonist CP-135,807. The SSRI sertraline at 2 mg/kg increased 5-HT levels to 130% of basal levels. The combination of this low dose of sertraline with either dose of GR127935 resulted in a pronounced, long-lasting increase in 5-HT levels to 230% of basal values. These results indicate that the effects of an SSRI on terminal 5-HT are significantly enhanced by coadministration of a 5-HT_1D antagonist and confirm that in addition to somatodendritic 5-HT_1A autoreceptors, terminal 5-HT_1D autoreceptors mitigate the effect of SSRIs on terminal 5-HT. As such, antagonists of the 5-HT_1D autoreceptor could be useful as rapidly acting antidepressants and may shorten the onset of antidepressant action when combined with SSRIs.     

Deletion of the 5-HT(3A)-receptor subunit blunts the induction of cocaine sensitization

Serotonin (5-HT) receptors are classified into seven groups (5-HT_1–7), comprising at least 14 structurally and pharmacologically distinct receptor subtypes. Pharmacological antagonism of ionotropic 5-HT₃ receptors has been shown to modulate both behavioral and neurochemical aspects of the induction of sensitization to cocaine. It is not known, however, if specific molecular subunits of the 5-HT₃ receptor influence the development of cocaine sensitization. To address this question, we studied the effects of acute and chronic intermittent cocaine administration in mice with a targeted deletion of the gene for the 5-HT_3A-receptor subunit (5-HT_3A−/−). 5-HT_3A (−/−) mice showed blunted induction of cocaine-induced locomotor sensitization as compared with wild-type littermate controls. 5-HT_3A (−/−) mice did not differ from wild-type littermate controls on measures of basal motor activity or response to acute cocaine treatment. Enhanced locomotor response to saline injection following cocaine sensitization was observed equally in 5-HT_3A (−/−) and wild-type mice suggesting similar conditioned effects associated with chronic cocaine treatment. These data show a role for the 5-HT_3A-receptor subunit in the induction of behavioral sensitization to cocaine and suggest that the 5-HT_3A molecular subunit modulates neurobehavioral adaptations to cocaine, which may underlie aspects of addiction.     

CB1 knockout mice display impaired functionality of 5-HT1A and 5-HT2A/C receptors

Interaction between brain endocannabinoid (EC) and serotonin (5-HT) systems was investigated by examining 5-HT-dependent behavioral and biochemical responses in CB₁ receptor knockout mice. CB₁ knockout animals exhibited a significant reduction in the induction of head twitches and paw tremor by the 5-HT_2A/C receptor selective agonist (±) DOI, as well as a reduced hypothermic response following administration of the 5-HT_1A receptor agonist (±)-8-OH-DPAT. Additionally, exposure to the tail suspension test induced enhanced despair responses in CB₁ knockout mice. However, the tricyclic antidepressant imipramine and the 5-HT selective reuptake inhibitor fluoxetine induced similar decreases in the time of immobility in the tail suspension test in CB₁ receptor knockout and wild-type mice. No differences were found between both genotypes with regard to 5-HT_2A receptor and 5-HT_1A receptors levels, measured by autoradiography in different brain areas. However, a significant decrease in the ability of both, the 5-HT_1A receptor agonist (±)-8-OH-DPAT and the 5-HT_2A/C receptor agonist (−)DOI, to stimulate [³⁵S]GTPγS binding was detected in the hippocampal CA₁ area and fronto-parietal cortex of CB₁ receptor knockout mice, respectively. This study provides evidence that CB₁ receptors are involved in the regulation of serotonergic responses mediated by 5-HT_2A/C and 5-HT_1A receptors, and suggests that a reduced coupling of 5-HT_1A and 5-HT_2A receptors to G proteins might be involved in these effects.     

Serotonergic Regulation of Acetylcholine Release in Rat Frontal Cortex

Abstract: The extent to which serotonin regulates the activity of cortically projecting cholinergic neurons was studied using in vivo microdialysis to monitor interstitial concentrations of acetylcholine in the frontal cortex of freely moving rats. Systemic administration of the serotonin release-inducing agent fenfluramine (3 or 10 mg/kg, i.p.) increased acetylcholine release by 110–130%. The fenfluramine-induced increase in acetylcholine release was significantly attenuated by pretreatment with the selective serotonin uptake inhibitor fluoxetine (10 mg/kg, i.p.). Pretreatment with the selective dopamine D₁ receptor antagonist SCH-23390 (0.3 mg/kg, s.c.) failed to prevent the fenfluramine-induced increase in acetylcholine release. In contrast, the serotonin 5-HT_2A receptor antagonist ketanserin (5 mg/kg, i.p.) blocked fenfluramine-induced increases in acetylcholine release. In contrast to previous studies that have concluded that serotonin has inhibitory actions on cortical acetylcholine release, the present results indicate that fenfluramine increases cortical acetylcholine release in vivo by its ability to enhance serotonin transmission and that serotonin produces these effects at least in part via actions at serotonin 5-HT_2A receptors.     

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.     

Differential Effects of Ipsapirone on 5-Hydroxytryptamine Release in the Dorsal and Median Raphe Neuronal Pathways

Abstract: Serotonergic neurons of the dorsal and median raphe nuclei are morphologically dissimilar. Recent results challenge previous evidence indicating a greater inhibition of dorsal raphe neurons after 5-hydroxytryptamine_1A (5-HT_1A) autoreceptor activation. As both nuclei innervate different forebrain territories, this issue is critical to understanding the changes in brain function induced by anxiolytic and antidepressant drugs. Using microdialysis, we examined the modifications of 5-HT release induced by the selective 5-HT_1A agonist ipsapirone in both neuronal pathways. Maximal and minimal basal 5-HT values (in the presence of 1 µ M citalopram) were 45.0 ± 4.8 fmol/fraction in the median raphe nucleus and 8.4 ± 0.4 fmol/fraction in the dorsal hippocampus. Ipsapirone (0.3, 3, and 10 mg/kg s.c.) reduced dose-dependently 5-HT in the two raphe nuclei and four forebrain areas. Maximal reductions (to ∼25% of predrug values) were observed in cortex and striatum and in median raphe nucleus. The effects were more moderate in dorsal and ventral hippocampus (to 66 and 50% of baseline, respectively). These results are consistent with a higher sensitivity of dorsal raphe neurons to 5-HT_1A autoreceptor activation. Yet the differential reduction of 5-HT release in the median raphe nucleus and hippocampus suggests the presence of complex mechanisms of control of 5-HT release in these neurons.     

The Third Intracellular Loop of the 5-Hydroxytryptamine2A Receptor Determines Effector Coupling Specificity

Abstract: 5-Hydroxytryptamine (5-HT) receptors contain seven putative transmembrane domains and couple via different guanine nucleotide binding proteins to specific effector enzymes. Studies with other receptors identify the second and third intracellular loops or the C-terminus of the receptor as important for selective effector coupling. However, it is not known which regions of the 5-HT receptor determine effector coupling specificity. To address this question, we constructed a chimeric 5-HT receptor in which the third intracellular (i3) loop is derived from the 5-HT_2A receptor, which is coupled to activation of phospholipase C, and the rest of the sequence is derived from the 5-HT_1B receptor, which is coupled to inhibition of adenylyl cyclase. The chimeric receptor exhibited ligand binding properties similar to those of the 5-HT_1B receptor and distinct from those of the 5-HT_2A receptor. This suggests that the i3 loop is not critical for the unique pharmacology of the 5-HT_1B receptor. In contrast, the chimeric receptor exhibited signaling properties similar to those of the 5-HT_2A receptor and distinct from those of the 5-HT_1B receptor. This indicates that the i3 loop determines the effector coupling specificity of the 5-HT_2A receptor.     

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.     

Differential Membrane Targeting and Pharmacological Characterization of Chimeras of Rat Serotonin 5-HT1A and 5-HT1B Receptors Expressed in Epithelial LLC-PK1 Cells

Abstract: The serotonin 5-HT_1A and 5-HT_1B receptors are two structurally related but pharmacologically distinguishable 5-HT receptor types. In brain, the 5-HT_1A receptor is localized on the soma and dendrites of neurons, whereas the 5-HT_1B receptor is targeted to the axon terminals. We previously showed that these two receptors are targeted in different membrane compartments when stably expressed in the epithelial LLC-PK1 cell line. Further investigations on the mechanisms responsible for their differential targeting were done by constructing chimeras of 5-HT_1A and 5-HT_1B receptors still able to bind specifically [³H]lysergic acid diethylamide and selective agonists and antagonists. Their cellular localization examined by confocal microscopy suggests that the third intracellular domain of the 5-HT_1B receptor was responsible for its Golgi-like localization in transfected LLC-PK1 cells. In contrast, the third intracellular domain of the 5-HT_1A receptor apparently allowed the sorting of the chimeras to the plasma membrane. Further inclusion of the C-terminal domain of the 5-HT_1A receptor in their sequence led to a basolateral localization, whereas that of the 5-HT_1B receptor allowed an apical targeting, suggesting the existence of a targeting signal in this portion of the receptor(s).