5-HT2 and 5-HT3 receptors in the lateral parabrachial nucleus mediate opposite effects on sodium intake

The present study investigated the role of several 5-HT receptor subtypes in the lateral parabrachial nucleus (LPBN) in the control of sodium appetite (i.e. NaCl consumption). Male Holtzman rats had cannulas implanted bilaterally into the LPBN for the injection of 5-HT receptor agonists and antagonists in conjunction with either acute fluid depletion or 24-h sodium depletion. Following these treatments, access to 0.3 M NaCl was provided and the intakes of saline and water were measured for the next 2 h. Bilateral injections of the 5-HT2A receptor antagonist, ketanserin or the 5-HT2C receptor antagonist, mianserin into the LPBN increased 0.3 M NaCl intake without affecting water intake induced by acute fluid-depletion. Bilateral injections of the 5-HT2B receptor agonist, BW723C86 hydrochloride, had no effect on 0.3 M NaCl or water intake under these conditions. Treatment of the LPBN with the 5-HT2B/2C receptor agonist, 2-(2-methyl-4-clorophenoxy) propanoic acid (mCPP) caused dose-related reductions in 0.3 M NaCl intake after 24 h sodium depletion. The effects of mCPP were prevented by pretreating the LPBN with the 5-HT2B/2C receptor antagonist, SDZSER082. Activation of 5-HT3 receptors by the receptor agonist, 1-phenylbiguanicle (PBG) caused dose-related increases in 0.3 M NaCl intake. Pretreatment of the LPBN with the 5-HT3 receptor antagonist, 1-methyl-N-[8-methyl-8-azabicyclo (3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584) abolished the effects of PBG, but LY-278,584 had no effects on sodium or water intake when injected by itself. PBG injected into the LPBN did not alter intake of palatable 0.06 M sucrose in fluid replete rats. The results suggest that activation of the 5-HT2A and 5-HT2C receptor subtypes inhibits sodium ingestion. In contrast, activation of the 5-HT3 receptor subtype increases sodium ingestion. Therefore, multiple serotonergic receptor subtypes in the LPBN are implicated in the control of sodium intake, sometimes by mediating opposite effects of 5-HT. The results provide new information concerning the control of sodium intake by LPBN mechanisms. (C) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.

Activation of serotonergic 5-HT1A receptors in the lateral parabrachial nucleus increases NaCl intake

Previous studies using non-specific serotonergic agonists and antagonists have shown the importance of serotonergic inhibitory mechanisms in the lateral parabrachial nucleus (LPBN) for controlling sodium and water intake. In the present study, we investigated whether the serotonergic 5-HTIA receptor subtype in the LPBN participates in this control. Male Holtzman rats had cannulas implanted bilaterally into the LPBN. Bilateral injections of the 5-HTIA receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.1, 1.25, and 2.5 mu g/ 0.2 mu l), into the LPBN enhanced 0.3 M NaCl and water intake of rats injected subcutaneously with the diuretic furosemide (10 mg/kg bw) and a low dose of the angiotensin-converting enzyme inhibitor, captopril (5 mg/kg bw). The increase in NaCl intake produced by 8-OH-DPAT injections was reduced in dose-related manner by pre-treating the LPBN with the selective 5-HTIA serotonergic antagonist, WAY-100635 (WAY, I and 2 mu g/0.2 mu l). In contrast, WAY did not affect water intake produced by 8-OH-DPAT. WAY-100635 injected alone into the LPBN had no effect on NaCl ingestion. Injections of 8-OH-DAPT (0.1 mu g/0.2 mu l) into the LPBN also increased 0.3 M NaCl intake induced by 24-h sodium depletion (furosemide, 20 mg/kg bw plus 24 h of sodium-free diet). Serotonin (5-HT, 20 mu g/0.2 mu l) injected alone or combined with 8-OH-DPAT into the LPBN reduced 24-h sodium depletion-induced 0.3 M NaCl intake. Therefore, the activation of serotonergic 5-HTIA receptors in the LPBN increases stimulated hypertonic NaCl and water intake, while 5-HT injections into the LPBN reduce NaCl intake and prevent the effects of serotonergic 5-HTIA receptor activation. (c) 2005 Elsevier B.V. All rights reserved.

The analgesic effect of crotoxin on neuropathic pain is mediated by central muscarinic receptors and 5-lipoxygenase-derived mediators

Crotoxin (CTX). a neurotoxin isolated from the venom of the South American rattlesnake Crotalus durissus terrificus. induces analgesia. In this study, we evaluated the antinociceptive effect of CTX in a model of neuropathic pain induced by rat sciatic nerve transection. Hyperalgesia was detected 2 h after nerve transection and persisted for 64 days. Immersion of proximal and distal nerve stumps in CTX solution (0.01 mM for 10 s), immediately after nerve transection, blocked hyperalgesia. The antinociceptive effect of CTX was long-lasting, since it was detected 2 h after treatment and persisted for 64 days. CTX also delayed, but did not block, neurectomy-induced neuroma formation. The effect of CTX was blocked by zileuton (100 mg/kg, p.o.) and atropine (10 mg/kg. i.p.), and reduced by yohimbine (2 mg/kg, i.p.) and methysergide (5 mg/kg, i.p.). on the other hand. indomethacin (4 mg/kg, i.v.). naloxone (1 mg/kg, i.p.). and N-methyl atropine (30 mg/kg, i.p.) did not interfere with the effect of CTX. These results indicate that CTX induces a long-lasting antinociceptive effect in neuropathic pain, which is mediated by activation of central muscarinic receptors and partially, by activation of alpha-adrenoceptors and 5-HT receptors. Eicosanoids derived from the lipoxygenase pathway modulate the action of crotoxin. (C) 2008 Elsevier B.V. All rights reserved.

Pimavanserin, a selective serotonin (5-HT)2A-inverse agonist, enhances the efficacy and safety of risperidone, 2 mg/day, but does not enhance efficacy of haloperidol, 2 mg/day: Comparison with reference dose risperidone, 6 mg/day

Most atypical antipsychotic drugs (APDs), e. g. risperidone (RIS), produce more extensive blockade of brain serotonin (5-HT)(2A) than dopamine (DA) D-2 receptors. This distinguishes them from typical APDs, e.g. haloperidol (HAL). Our objective was to test the hypothesis that augmentation of low doses of RIS or HAL (2 mg/day) with pimavanserin (PIM), a selective 5-HT2A inverse agonist, to enhance 5-HT2A receptor blockade, can achieve efficacy comparable to RIS, 6 mg/day, but with lesser side effects. In a multi-center, randomized, double-blind, 6 week trial, 423 patients with chronic schizophrenia experiencing a recent exacerbation of psychotic symptoms were randomized to RIS2mg + placebo (RIS2PBO), RIS2mg + PIM20mg (RIS2PIM), RIS6mg + PBO (RIS6PBO), HAL2mg + PBO (HAL2PBO), or HAL2mg + PIM20mg (HAL2PIM). Improvement in psychopathology was measured by the PANSS and CGI-S. The reduction in PANSS Total Score with RIS2PIM at endpoint was significantly greater than RIS2PBO: -23.0 vs. -16.3 (p = 0.007), and not significantly different from the RIS6PBO group: -23.2 points. The percentage of patients with >= 20% improvement at day 15 in the RIS2PIM group was 62.3%, significantly greater than the RIS6PBO (42.1%; p = 0.01) and the RIS2PBO groups (37.7%; p = 0.002). Weight gain and hyperprolactinemia were greater in the RIS6PBO group than the RIS2PIM group but there was no difference in extrapyramidal side effects (EPS). HAL2PBO and HAL2PIM were not significantly different from each other in efficacy but HAL2PIM had less EPS at end point. Both HAL groups and RIS6PBO showed equal improvement in psychopathology at endpoint, indicating HAL 2 mg/day is effective to treat an acute exacerbation in chronic schizophrenia patients. In conclusion, a sub-effective RIS dose combined with PIM to enhance 5-HT2A receptor blockade provided faster onset of action, and at endpoint, equal efficacy and better safety, compared to standard dose RIS. These results support the conclusion that 5-HT2A receptor blockade is a key component of the action of some atypical APDs and can reduce EPS due to a typical APD. (C) 2012 Elsevier B.V. All rights reserved.

Specific PET-ligands for selected 5-HT and GABA A-receptor subtypes = PET-Liganden mit Spezifität für definierte 5-HT und GABA A-Rezeptor-Subtypen

Für diese Arbeit wurden sechs neue Benzodiazepinderivate, TC07, TC08, TC09, TC10, TC11 und TC12, hergestellt. Diese wurden mittels Radioligandenbindungsassay sowohl auf ihre Bindungseigenschaften für Membranen des Cerebellum, des Hippo-campus und des Cortex der Ratte hin untersucht, als auch für Membranen von HEK293 Zellen, die transient rekombinante GABAA Rezeptoren exprimierten. Zusätz-lich wurden kompetitive in situ Rezeptorautoradiographien an Rattenhirnschnitten mit den Liganden [3H]Ro15-4513 und [3H]R015-1788 durchgeführt. Zusammen ergaben sich aus diesen Experimenten deutliche Hinweise auf eine Selektivität der Verbindun-gen TC07, TC11 und TC12 für a5-Untereinheiten enthaltende GABAA Rezeptoren mit a5-Affinitäten im niedrigen nanomolaren Bereich. In vivo Bindungsexperimente in Ratten, mit [3H]Ro15-1788 als Tracer und TC07 als Kompetitor, ergaben, dass TC07 mehr [3H]Ro15-1788 im Vorderhirn als im Cerebellum verdrängt. Bezog man die regionale Verteilung der a5-Untereinheit des GABAA Rezep-tors im Rattenhirn mit ein – sehr wenige a5-Untereinheiten im Cerebellum, etwa 20 % der GABAA Rezeptor-Untereinheiten im Hippocampus – untermauerten diese Ergeb-nisse die Vermutung, TC07 könne a5-selektiv sein. Diese Daten bestätigten darü-berhinaus, dass TC07 die Blut-Hirn-Schranke passieren kann. Für elektrophysiologische Messungen mit TC07 und TC12 wurden die oben erwähnten transient transfizierten HEK293 Zellen verwendet, welche die GABAA Rezeptor Unte-reinheitenkombination a5b3g2 exprimierten. Das Dosis-Antwort Verhalten ergab keinen signifikanten Effekt für TC12. Die Daten von TC07 dagegen lassen auf einen schwach negativ modulatorischen Effekt schließen, was, zumindest theoretisch, die Möglichkeit eröffnet, TC07 auch als sogenannten cognitive enhancer einzusetzen. Der errechnete Ki-Wert lag in derselben Größenordnung wie der Ki-Wert, der anhand der Bindungsas-saydaten errechnet wurde. Insgesamt rechtfertigen die bisherigen Ergebnisse die radiochemische Markierung mit 18F von drei der sechs getesteten Verbindungen in der Reihenfolge TC07, TC12 und TC11. Des Weiteren wurde [18F]MHMZ, ein potentiell 5-HT2A selektiver Ligand und PET-Tracer einschließlich Vorläufer und Referenzverbindungen, mit hohen Ausbeuten syn-thetisiert (Herth, Debus et al. 2008). Autoradiographieexperimente mit Rattenhirn-schnitten zeigten hervorragende in situ Bindungseigenschaften der neuen Verbindung. Die Daten wiesen eine hohe Selektivität für 5-HT2A Rezeptoren in Verbindung mit einer niedrigen unspezifischen Bindung auf. [18F]MHMZ erfährt in vivo eine schnelle Metabo-lisierung, wobei ein polarer aktiver Metabolit entsteht, welcher vermutlich nicht die Blut-Hirn-Schranke passieren kann. Transversale, sagittale und coronale Kleintier-PET-Bilder des Rattenhirns zeigten eine hohe Anreicherung im frontalen Cortex und im Striatum, während im Cerebellum so gut wie keine Anreicherung festzustellen war. Diese Verteilung deckt sich mit der bekann-ten Verteilung der 5-HT2A Rezeptoren. Die in vivo Anreicherung scheint sich ebenfalls gut mit der Verteilung der in den Autoradiographieexperimenten gemessenen Bindung zu decken. Nach Berechnungen mit dem 4-Parameter Referenzgewebe Modell beträgt das Bindungspotential (BP) für den frontalen Cortex 1,45. Das Cortex zu Cerebellum Verhältnis wurde auf 2,7 nach 30 Minuten Messzeit bestimmt, was bemerkenswert nah an den von Lundkvist et al. für [11C]MDL 100907 publizierten Daten liegt. Abgesehen von der etwas niedrigeren Affinität waren die gemessenen in vitro, in situ und in vivo Daten denen von [3H]MDL 100907 und [11C]MDL 100907 sehr ähnlich, so dass wir ein [18F]Analogon in der Hand haben, das die bessere Selektivität von MDL 100907 verglichen mit Altanserin mit der längeren Halbwertszeit und den besse-ren Eigenschaften für die klinische Routine von 18F verglichen mit 11C verbindet. Die Ergebnisse von [18F]MHMZ rechtfertigenden weitere Experimente, um diesen Liganden für die klinische Routine am Menschen nutzbar zu machen.

Expression and function of 5-HT7 receptors in smooth muscle preparations from equine duodenum, ileum, and pelvic flexure

In horses, gastrointestinal (GI) disorders occur frequently and cause a considerable demand for efficient medication. 5-Hydroxytryptamine receptors (5-HT) have been reported to be involved in GI tract motility and thus, are potential targets for treating functional bowel disorders. Our studies extend current knowledge on the 5-HT(7) receptor in equine duodenum, ileum and pelvic flexure by studying its expression throughout the intestine and its role in modulating contractility in vitro by immunofluorescence and organ bath experiments, respectively. 5-HT(7) immunoreactivity was demonstrated in both smooth muscle layers, particularly in the circular one, and within the myenteric plexus. Interstitial cells of Cajal (ICC), identified by c-Kit labeling, show a staining pattern similar to that of 5-HT(7) immunoreactivity. The selective 5-HT(7) receptor antagonist SB-269970 increased the amplitude of contractions in spontaneous contracting specimens of the ileum and in electrical field-stimulated specimens of the pelvic flexure concentration-dependently. Our in vitro experiments suggest an involvement of the 5-HT(7) receptor subtype in contractility of equine intestine. While the 5-HT(7) receptor has been established to be constitutively active and inhibits smooth muscle contractility, our experiments demonstrate an increase in contractility by the 5-HT(7) receptor ligand SB-269970, suggesting it exerting inverse agonist properties.

Synthesis and Characterization of Photoaffinity Probes that Target the 5-HT3 Receptor

Abstract: The 5-HT3 receptor is one of several ion channels responsible for the transmission of nerve impulses in the peripheral and central nervous systems. Until now, it has been difficult to characterize transmembrane receptors with classical structural biology approaches like X-ray crystallography. The use of photoaffinity probes is an alternative approach to identify regions in the protein where small molecules bind. To this end, we present two photoaffinity probes based on granisetron, a well known antagonist of the 5-HT3 receptor. These new probes show nanomolar binding affinity for the orthosteric binding site. In addition, we investigated their reactivity using irradiation experiments.

The muscarinic antagonists scopolamine and atropine are competitive antagonists at 5-HT3 receptors

Scopolamine is a high affinity muscarinic antagonist that is used for the prevention of post-operative nausea and vomiting. 5-HT3 receptor antagonists are used for the same purpose and are structurally related to scopolamine. To examine whether 5-HT3 receptors are affected by scopolamine we examined the effects of this drug on the electrophysiological and ligand binding properties of 5-HT3A receptors expressed in Xenopus oocytes and HEK293 cells, respectively. 5-HT3 receptor-responses were reversibly inhibited by scopolamine with an IC50 of 2.09 μM. Competitive antagonism was shown by Schild plot (pA2 = 5.02) and by competition with the 5-HT3 receptor antagonists [3H]granisetron (Ki = 6.76 µM) and G-FL (Ki = 4.90 µM). The related molecule, atropine, similarly inhibited 5-HT evoked responses in oocytes with an IC50 of 1.74 µM, and competed with G-FL with a Ki of 7.94 µM. The reverse experiment revealed that granisetron also competitively bound to muscarinic receptors (Ki = 6.5 µM). In behavioural studies scopolamine is used to block muscarinic receptors and induce a cognitive deficit, and centrally administered concentrations can exceed the IC50 values found here. It is therefore possible that 5-HT3 receptors are also inhibited. Studies that utilise higher concentrations of scopolamine should be mindful of these potential off-target effects.

5-HT2A and 5-HT2C receptors as hypothalamic targets of developmental programming in male rats

Funding The research reported in this publication was supported by the Biotechnology and Biological Sciences Research Council (E007821/1 to M.S.M-G, R.L.C and E00797X/1; BB/K001418 /1 to L.K.H), the British Heart Foundation (FS/09/029/27902 to S.E.O.), the UK Medical Research Council Metabolic Diseases Unit (MC_UU_12012/4 to S.E.O and MC_UU_12012/1 to G.S.H.Y), the Wellcome Trust (WT081713 and WT098012 to L.K.H), the European Union (FP7-HEALTH-266408 Full4Health to G.S.H.Y) and the Helmholtz Alliance ICEMED to G.S.H.Y.

Brain lipids that induce sleep are novel modulators of 5-hydroxytrypamine receptors.

Amide derivatives of fatty acids were recently isolated from cerebrospinal fluid of sleep-deprived animals and found to induce sleep in rats. To determine which brain receptors might be sensitive to these novel neuromodulators, we tested them on a range of receptors expressed in Xenopus oocytes. cis-9,10-Octadecenamide (ODA) markedly potentiated the action of 5-hydroxytryptamine (5-HT) on 5-HT2A and 5-HT2C receptors, but this action was not shared by related compounds such as oleic acid and trans-9,10-octacenamide. ODA was active at concentrations as low as 1 nM. The saturated analog, octadecanamide, inhibited rather than potentiated 5-HT2C responses. ODA had either no effect or only weak effects on other receptors, including muscarinic cholinergic, metabotropic glutamate, GABA(A), N-methyl-D-asparate, or alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionic acid receptors. Modulation of 5-HT2 receptors by ODA and related lipids may represent a novel mechanism for regulation of receptors that activate G proteins and thereby play a role in alertness, sleep, and mood as well as disturbances of these states.

Functional serotonin 5-HT4 receptors in porcine and human ventricular myocardium with increased 5-HT4 mRNA in heart failure

Serotonin (5-hydroxytryptamine, 5-HT) increases contractile force and elicits arrhythmias through 5-HT4 receptors in porcine and human atrium, but its ventricular effects are unknown. We now report functional 5-HT4 receptors in porcine and human ventricle. 5-HT4 mRNA levels were determined in porcine and human ventricles and contractility studied in ventricular trabeculae. Cyclic AMP-dependent protein kinase (PKA) activity was measured in porcine ventricle. Porcine and human ventricles expressed 5-HT4 receptor mRNA. Ventricular 5-HT4(b) mRNA was increased by four times in 20 failing human hearts compared with five donor hearts. 5-HT increased contractile force maximally by 16% (EC50=890 nM) and PKA activity by 20% of the effects of (-)-isoproterenol (200 muM) in ventricular trabeculae from new-born piglets in the presence of the phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine. In ventricular trabeculae from adult pigs (3-isobutyl-1-methylxanthine present) 5-HT increased force by 32% (EC50=60 nM) and PKA activity by 39% of (-)-iso-proterenol. In right and left ventricular trabeculae from failing hearts, exposed to modified Krebs solution, 5-HT produced variable increases in contractile force in right ventricular trabeculae from 4 out of 6 hearts and in left ventricular trabeculae from 3 out of 3 hearts- range 1-39% of (-)-isoproterenol, average 8%. In 11 left ventricular trabeculae from the failing hearts of four beta-blocker-treated patients, pre-exposed to a relaxant solution with 0.5 mM Ca2+ and 1.2 mM Mg2+ followed by a switch to 2.5 mM Ca2+ and 1 mM Mg2+, 5-HT (1-100 muM, 3-isobutyl-1-melhylxanthine present) consistently increased contractile force and hastened relaxation by 46% and 25% of (-)-isoproterenol respectively. 5-HT caused arrhythmias in three trabeculae from 3 out of I I patients. In the absence of phosphodiesterase inhibitor, 5-HT increased force in two trabeculae, but not in another six trabeculae from 4 patients. All 5-HT responses were blocked by 5-HT4 receptor antagonists. We conclude that phosphodiesterase inhibition uncovers functional ventricular 5-HT4 receptors, coupled to a PKA pathway, through which 5-HT enhances contractility, hastens relaxation and can potentially cause arrhythmias.

Modulation of certain 5-HT-related behaviours by serotonergic and noradrenergic systems

The modulation of 5-hydroxytryptamine (5-HT)-related head-twitchbehaviour by antimigraine drugs and migraine triggers was examined inmice. The antimigraine drugs examined produced either inhibition or noeffect on 5-HT-related head-twitching. On the basis of these resultsit is suggested that 5-HT-related head-twitching is unlikely to beuseful in the preclinical screening and discovery of systemically-activeantimigraine agents. The migraine triggers examined, tyramineand beta-PEA initially produced a repeatable complex time-relatedeffect on 5-HT-related head-twitching, with both inhibition andpotentiation of this behaviour being observed, however, when furtherexamination of the effect of the migraine triggers on 5-HT-relatedhead-twitching was attempted some time later the effects seeninitially were no longer produced. The effect of (±)-1-<2, 5-dimethoxy-4-iodophenyl)-2-aminopropane,((±)DOl), on on-going behaviour of mice and rats was examined. Shakingbehaviour was observed in both species. In mice, excessive scratchingbehaviour was also present. (±)DOl-induced scratching and shakingbehaviour were found to be differentially modulated by noradrenergicand serotonergic agents, however, the fact that both behaviours wereblocked by ritanserin (5-HT2/5-HT1c receptor antagonist) and inhibitedby FLA-63 (a dopamine-beta-oxidase inhibitor which depletesnoradrenaline), suggests the pathways mediating these behaviours mustbe convergent in some manner, and that both behaviours require intact5-HT receptors, probably 5-HT2 receptors, for their production. Ingeneral, the behavioural profile of (±)DOI was as expected for anagent which exhibits high affinity binding to 5-HT2/5-HT1c receptors.Little sign of the 5-HTl-related '5-HT syndrome' was seen in eithermice or rats. The effect of a variety of noradrenergic agents on head-twitchinginduced by a variety of shake-inducing agents was examined. A patternof modulatory effect was seen whereby the modulatory effect of thenoradrenergic agents on 5-hydroxytryptophan <5-HTP) (and in some cases, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)) was found to be the opposite of that observed with quipazine and (±)DOI. The relationship between these effects, and their implications for understanding the pharmacology of centrally acting drugs is discussed.

5-Hydroxytryptamine induced excitation and inhibition in the subthalamic nucleus:action at 5-HT2C, 5-HT4 and 5-HT1A receptors

Extracellular single-unit recordings in mouse brain slices were used to determine the effect of exogenously applied 5-HT on STN neurones. Recordings were made from 74 STN cells which fired action potentials at a regular rate of 7.19 ± 0.5 Hz. In 61 cells (82%), 5-HT application increased STN neurone firing rate (10 μM, 180 ± 16.8%, n = 35) with an estimated EC 50 of 5.4 μM. The non-specific 5-HT2 receptor agonist α-methyl 5-HT (1-10 μM) mimicked 5-HT induced excitations (15 cells). These excitations were significantly reduced by pre-perfusion with the specific 5-HT2C receptor antagonist RS102221 (500 nM, 9 cells) and the 5HT4 antagonist GR113808 (500 nM, 7 cells). In 6 cells (8%) 5-HT induced biphasic responses where excitation was followed by inhibition, while in 7 cells (9%) inhibition of firing rate was observed alone. Inhibitory responses were reduced by the 5-HT1A antagonist WAY100135 (1 μM, 4 cells). No inhibitory responses were observed following α-methyl 5-HT applications. Both the excitations and inhibitions were unaffected by picrotoxin (50 μM, n = 5) and CNQX (10 μM, n = 5) indicative of direct postsynaptic effects. Thus, in STN neurones, 5-HT elicits two distinct effects, at times on the same neurone, the first being an excitation which is mediated by 5-HT 2C and 5-HT4 receptors and the second an inhibition which is mediated by 5-HT1A receptors. © 2005 Elsevier Ltd. All rights reserved.

Regulation of conditioned and unconditioned fear in rats by 5-HT1A receptors in the dorsal periaqueductal gray

Studies on the involvement of 5-HT1-mediated mechanisms in the dorsal periaqueductal gray (dPAG) of animals with past stressful experiences have not been conducted so far. We investigated the role of 5-HT1 receptors in the dPAG of rats previously submitted to contextual fear conditioning. Defensive behaviors induced by activation of the dPAG were assessed by measuring the lowest electric current applied to this structure (threshold) able to produce freezing and escape responses during testing sessions of contextual fear conditioning, in which animals were placed in a context previously paired to footshocks. The 5-HT1A function of the dPAG was evaluated by local injections of 8-OH-DPAT (4 and 8 nmol/0.2 mu L) and WAY-100635 (10 nmol/0.2 mu L), selective agonist and antagonist of 5-HT1A receptors, respectively. In accordance with previous studies, 8-OH-DPAT increased aversive thresholds (antiaversive effects) but injections of WAY 100635 into the dPAG did not produce significant effects on the aversive thresholds in naive rats. However, the aversive thresholds of animals exhibiting contextual fear remained unchanged with both treatments. Moreover, 8-OH-DPAT and WAY 100635 did not change the dPAG post-stimulation freezing. The present results suggest that the stressful experience of being fear conditioned has an effect on the role of the 5-HT1A receptors in mediating unconditioned fear. Also, the reduction in the regulation of the defensive behaviors by 5-HT1A-mediated mechanisms in the dPAG of these animals may underlie the stress precipitated psychopathology associated with the neural substrates of aversion of the dPAG. (c) 2007 Elsevier Inc. All rights reserved.

Stimulation of 5-HT1A or 5-HT2A receptors in the ventrolateral periaqueductal gray causes anxiolytic-, but not panicolytic-like effect in rats

Evidences from studies using electrical or chemical stimulation of the midbrain periaqueductal gray (PAG) suggest that whereas the dorsal PAG is critical for the regulation of panic-related defensive behaviors, the ventrolateral PAG (vlPAG) modulates generalized anxiety-related responses. In the present study we evaluated whether the activation of 5-HT1A and 5-HT2A/2C receptors in the ventrolateral column of the periaqueductal gray (vlPAG) causes differential effects on an anxiety- and a panic-related defensive behavior, respectively, inhibitory avoidance and escape, in male Wistar rats submitted to the elevated T-maze. Our results showed that intra-vlPAG injection of the endogenous agonist serotonin, the 5-HT1A/7 agonist 8-OH-DPAT or 5-HT2A/2C agonist DOI impaired the acquisition of inhibitory avoidance, without interfering with escape performance. The same selective anxiolytic effect was also observed after local administration of the benzodiazepine receptor agonist midazolam. Moreover, as shown by the results of antagonism studies, 5-HT2A receptors are recruited for the anxiolysis caused by serotonin and DOI. while 5-HT1A receptors account for the effect of 8-OH-DPAT. In conclusion, our data show that the activation of 5-HT1A and 5-HT2A receptors in the vlPAG affects defensive responses related to generalized anxiety, but not panic disorder. (C) 2008 Elsevier B.V. All rights reserved.