The 5-HT1A agonist 8-OH-DPAT increases the number of spike-wave discharges in a genetic rat model of absence epilepsy

The effects of the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) on the epileptiform activity has been investigated in adult WAG/RIJ rats. Either intraperitoneal (0.1–0.5 mg/kg) or intracerebroventricular (2–20 μg/rat) administration of 8-OH-DPAT caused marked, dose-dependent increases in the number and mean cumulative duration of spike-wave discharges. These effects were attenuated by NAN-190, a 5-HT_1A receptor antagonist. These data indicate that serotonergic system regulates the epileptiform activity in this genetic model of human absence epilepsy.     

Inhibitory effects of tandospirone, a 5-HT1A agonist, on medial vestibular nucleus neurons responding to lateral roll tilt stimulation in rats

An electrophysiological study was performed using chloral hydrate-anesthetized rats to determine whether tandospirone, a 5-HT_1A agonist, affects neuronal activities of the medial vestibular nucleus (MVN), since serotonergic innervation and 5-HT_1A receptors are present in this nucleus. Tandospirone applied microiontophoretically at a current of 20–60 nA caused an inhibition of tilt-induced firing of α-type neurons, which showed increased and decreased firing with lateral tilt ipsilateral and contralateral to the recording site, respectively, along with that of β-type neurons which exhibited the reverse responses to ipsilateral and contralateral tilt stimulation. The inhibition was antagonized during simultaneous, iontophoretic application of WAY-100635 (20–60 nA), a 5-HT_1A receptor antagonist, although WAY-100635 alone rarely affected spontaneous or tilt-induced firing in either type of neurons. These results suggest that tandospirone acts on a 5-HT_1A receptor to inhibit transmission of otolith information to α- and β-type MVN neurons.     

Effects of 8-OH-DPAT, a 5-HT1A receptor agonist, and DOI, a 5-HT2A/2C agonist, on monosynaptic transmission in spinalized rats

We investigated the effect of the 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) and the 5-HT_2A/2C receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on monosynaptic transmission in spinalized rats. 8-OH-DPAT significantly inhibited the excitation of α-motoneurons evoked by monosynaptic transmission without a direct effect on α-motoneuron excitation. DOI potentiated the excitation of α-motoneurons by both direct stimulation and monosynaptic transmission. These results indicate that activation of 5-HT_1A receptors inhibits monosynaptic transmission, whereas activation of 5-HT_2A/2C receptors enhances it.     

5-HT1A agonists induce hippocampal theta activity in freely moving cats: role of presynaptic 5-HT1A receptors

Electrical activity in the dorsal hippocampus was recorded in freely moving cats in response to intravenous administration of 5-HT_1A agonist and antagonist drugs. Administration of low doses of the selective 5-HT_1A agonists 8-OH-DPAT (5–20 μg/kg) and ipsapirone (20–100 μg/kg) produced rhythmic slow activity (theta) in the hippocampal EEG within 30 s. Similar effects were observed with BMY 7378 (20 and 100 μg/kg), which acts as an agonist at presynaptic (somatodendritic) 5-HT_1A receptors and as an antagonist at postsynaptic 5-HT_1A receptors. Power spectral analyses showed that all three compounds produced a dose-dependent increase in the EEG power occurring in the theta frequency band (3.5–8.0 Hz) as a proportion of total power from 0.25 to 30.0 Hz (relative theta power). The increase in relative theta power produced by 8-OH-DPAT (20 μg/kg) was greatly attenuated by spiperone (1 mg/kg), a highly effective 5-HT_1A autoreceptor antagonist. Administration of spiperone alone had no significant effect on relative theta power. These results are discussed in relationship to the effects of these drugs on serotonergic neuronal activity. Our results suggest that preferential activation of presynaptic 5-HT_1A receptors, and subsequent inhibition of serotonin neurotransmission, facilitates the appearance of hippocampal theta activity in awake cats.     

Involvement of 5-HT7 receptors in serotonergic effects on spike afterpotentials in presumed jaw-closing motoneurons of rats

Intracellular recordings were obtained from rat presumed jaw-closing motoneurons in slice preparations to investigate the involvement of the serotonin₇ (5-HT₇) receptors in serotonergic inhibition of the postspike medium-duration afterhyperpolarization (mAHP) and enhancement of the afterdepolarization (ADP). 5-HT-induced suppression of the mAHP and enhancement of the ADP were mimicked by application of the 5-HT_1A/7 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and antagonized by the 5-HT_2/6/7 receptor antagonist clozapine, whereas the 5-HT₂ receptor agonist α-methyl-5-hydroxytryptamine (α-methyl-5-HT) did not affect the mAHP and ADP. 8-OH-DPAT-induced attenuation of the mAHP and enhancement of the ADP were also antagonized by clozapine and another 5-HT_2/6/7 receptor antagonist ritanserin, whereas the 5-HT_1A receptor antagonist pindolol failed to block the 8-OH-DPAT-induced effects on the mAHP and ADP. 8-OH-DPAT-induced suppression of the mAHP and enhancement of the ADP were also antagonized by a protein kinase A (PKA) inhibitor H89, whereas 8-OH-DPAT could inhibit the mAHP and enhance the ADP in the presence of a protein kinase C (PKC) inhibitor chelerythrine. The 8-OH-DPAT-induced suppression of the mAHP was enhanced under raised [Ca²⁺]_o and this enhancement was reduced by chelerythrine. It is suggested that the 5-HT₇ receptors are involved in 5-HT-induced attenuation of the mAHP and enhancement of the ADP through activation of PKA, and the attenuation of mAHP through the 5-HT₇ receptors is enhanced under raised [Ca²⁺]_o by PKC activation.     

Effect of alcohols on G-protein coupling of serotonin(1A) receptors from bovine hippocampus

The serotonin(1A) (5-hydroxytryptamine [5-HT](1A)) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins. Serotonergic signalling has been shown to play an important role in alcohol intake, preference and dependence. G-protein coupling of the 5-HT(1A) receptor serves as an important determinant for serotonergic signalling. We have studied the effect of alcohols on G-protein coupling of bovine hippocampal 5-HT(1A) receptors in native membranes. This was done by monitoring the modulation of ligand (agonist and antagonist) binding in presence of alcohols by guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S), a non-hydrolyzable analogue of GTP. Our results show that alcohols inhibit the specific binding of the agonist 8-hydroxy-2-(di-N-propylamino)tetralin (except in case of ethanol) and the antagonist 4-(2'-methoxy)-phenyl-1-[2'-(N-2"-pyridinyl)-p-fluorobenzamido]eth yl- piperazine to 5-HT(1A) receptors in a concentration-dependent manner. Further, we show here that the action of alcohols on the bovine hippocampal 5-HT(1A) receptors could be modulated by guanine nucleotides and that the mode of action of ethanol on the 5-HT(1A) receptor may be quite different than that of other alcohols. The effect of GTP-gamma-S on the agonist and the antagonist binding is found to be markedly different. Our results could be significant in the overall context of the role of G-protein coupling in serotonergic neurotransmission and its role in alcohol tolerance and dependence.     

Serotonin 2A receptor agonist binding in the human brain with [11C]Cimbi-36

[¹¹C]Cimbi-36 was recently developed as a selective serotonin 2A (5-HT_2A) receptor agonist radioligand for positron emission tomography (PET) brain imaging. Such an agonist PET radioligand may provide a novel, and more functional, measure of the serotonergic system and agonist binding is more likely than antagonist binding to reflect 5-HT levels in vivo. Here, we show data from a first-in-human clinical trial with [¹¹C]Cimbi-36. In 29 healthy volunteers, we found high brain uptake and distribution according to 5-HT_2A receptors with [¹¹C]Cimbi-36 PET. The two-tissue compartment model using arterial input measurements provided the most optimal quantification of cerebral [¹¹C]Cimbi-36 binding. Reference tissue modeling was feasible as it induced a negative but predictable bias in [¹¹C]Cimbi-36 PET outcome measures. In five subjects, pretreatment with the 5-HT_2A receptor antagonist ketanserin before a second PET scan significantly decreased [¹¹C]Cimbi-36 binding in all cortical regions with no effects in cerebellum. These results confirm that [¹¹C]Cimbi-36 binding is selective for 5-HT_2A receptors in the cerebral cortex and that cerebellum is an appropriate reference tissue for quantification of 5-HT_2A receptors in the human brain. Thus, we here describe [¹¹C]Cimbi-36 as the first agonist PET radioligand to successfully image and quantify 5-HT_2A receptors in the human brain.     

Study of mechanisms of calcitonin analgesia in mice: Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT_1A agonist (±)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT_1A antagonist (WAY 100, 135). As the stimulation of 5-HT_1A autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT_2A–2C agonist (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT_2A antagonist ketanserin failed to prevent this effect. The 5-HT₃ agonist (±)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT₃ antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT₃ receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT₃ receptors.     

Effects of dorsal rhizotomy and selective lesion of serotonergic and noradrenergic systems on 5-HT1a , 5-HT1b and 5-HT3 receptors in the rat spinal cord

Summary Autoradiographic studies were performed in combination with dorsal rhizotomy or selective lesion of descending serotonergic or noradrenergic systems in an attempt to identify the neuronal cell types endowed with the serotonin 5-HT_1a , 5-HT_1b and 5-HT₃ receptors in the rat spinal cord. Unilateral sectioning of seven dorsal roots (C4–T2) at the cervical level produced a marked decrease (–75%, 10 days after the surgery) in the binding of [¹²⁵I]iodozacopride to 5-HT₃ receptors in the superficial layers of the ipsilateral dorsal horn, further confirming the preferential location of these receptors on primary afferent fibres. In addition, a significant decrease (20%) in the binding of [³H]8-OH-DPAT to 5-HT_1A receptors and of [¹²⁵I]GTI to 5-HT_1B receptors was also observed in the same spinal area in rhizotomized rats, suggesting that a small proportion of these receptors are also located on primary afferent fibres. The labelling of 5-HT_1B receptors was significantly decreased (–12%) in the dorsal horn at the cervical (but not the lumbar) level, and that of 5-HT₃ receptors was unchanged in the whole spinal cord in rats whose descending serotonergic projections had been destroyed by 5,7-dihydroxytryptamine. Conversely, the labelling of 5-HT_1A receptors was significantly increased in the cervical (+13%) and lumbar (+42%) dorsal horn in 5,7-dihydroxytryptamine-lesioned rats. Similarly, [³H]8-OH-DPAT binding to 5-HT_1A receptors significantly increased (+26%) in the lumbar (but not the cervical) dorsal horn in rats whose noradrenergic systems had been lesioned by DSP-4. The labelling of 5-HT_1B receptors was also increased (+31% at the cervical level; +17% at the lumbar level), whereas that of 5-HT₃ receptors remained unchanged in these animals. These data indicate that complex adaptive changes in the expression of 5-HT_1A and 5-HT_1B receptors occurred in the rat spinal cord following the lesion of descending monoaminergic systems.Abbreviations CP 93129 3-(1,2,5,6-tetrahydropyrid-4-yl) pyrrolo [3,2-b]pyrid-5-one - 5,7-DHT 5,7-dihydroxytryptamine - DSP-4 N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine - GTI serotonin-O-carboxymethylglycyl-iodotyrosinamide - HEPES 4-(2-hydroxyethyl)-l-piperazine ethane sulfonic acid - 5-HT 5-hydroxytryptamine - i.c. intracisternally - NA noradrenaline - 8-OH-DPAT 8-hydroxy-2-(di-n-propylamino)tetralin     

Cataleptic effect of neurosteroid 3α-hydroxy-5α-pregnan-20-one in mice: modulation by serotonergic agents

The neurosteroid 3α-hydroxy-5α-pregnan-20-one (3α,5α-THP) induced catalepsy in mice is modified by dopaminergic, adenosinergic and GABAergic agents. In light of serotonergic agents being implicated in antipsychotic-induced catalepsy and their ability to increase brain neurosteroid content, the present study was undertaken to investigate the effect of various 5-HT agents on catalepsy induced by 3α,5α-THP in mice. Pretreatment with selective serotonin reuptake inhibitor, fluoxetine (5 mg/kg, i.p.), 5-HT releaser, fenfluramine (10 mg/kg, i.p.), 5-HT_1A receptor agonist, 8-OH-DPAT (0.3 mg/kg, s.c.), 5-HT_1B/1C receptor agonist, TFMPP (3 mg/kg, i.p.), 5-HT_2A/1C receptor agonist, DOI (1.5 mg/kg, s.c.) and 5-HT₃ agonist, 2-methylserotonin (5 mg/kg, i.p.) potentiated the catalepsy induced by exogenous administration of 3α,5α-THP. Furthermore, FGIN 1–27, an MDR agonist that increases endogenous content of 3α,5α-THP although per se failed to exhibit any cataleptic effect but enhanced the cataleptic response in combination with these serotonergic agents. The potentiating action of 5-HT_1A, 5-HT_2A/1C or 5-HT₃ receptor agonist on 3α,5α-THP induced catalepsy was not blocked by prior administration of sub-effective dose (1 mg/kg, s.c.) of their respective receptor antagonists pindolol, ritanserin or ondansetron or by pretreatment with serotonergic neurotoxin 5,7-DHT (100 μg/mouse, i.c.v.). However this effect of different serotonergic agents was antagonized by the GABA_A receptor antagonist, bicuculline (1 mg/kg, i.p.) or the 3α-hydroxysteroid oxidoreductase enzyme inhibitor, indomethacin (5 mg/kg, i.p.). The 5-HT agents enhance neurosteroid-induced catalepsy by increasing GABAergic tone, likely as a consequence of increased brain content of 3α,5α-THP.     

Melatonin reversal of DOI-induced hypophagia in rats; possible mechanism by suppressing 5-HT2A receptor-mediated activation of HPA axis

Serotonin type 2A (5-HT_2A) receptor-mediated neurotransmitter is known to activate hypothalamic–pituitary–adrenal (HPA) axis, regulate sleep–awake cycle, induce anorexia and hyperthermia. Interaction between melatonin and 5-HT_2A receptors in the regulation of the sleep–awake cycle and head-twitch response in rat have been reported. Previous studies have shown that melatonin has suppressant effect on HPA axis activation, decreases core body temperature and induces hyperphagia in animals. However, melatonin interaction with 5-HT_2A receptors in mediation of these actions is not yet reported. We have studied the acute effect of melatonin and its antagonist, luzindole on centrally administered (±)-1-(2,5-dimethoxy-4-iodophenyl) 2-amino propane (DOI; a 5-HT_2A/2C agonist)-induced activation of HPA axis, hypophagia and hyperthermia in 24-h food-deprived rats. Like ritanserin [(1 mg/kg, i.p.) 5-HT_2A/2C antagonist], peripherally administered melatonin (1.5 and 3 mg/kg, i.p.) did not affect the food intake, rectal temperature or basal adrenal ascorbic acid level. However, pretreatment of rats with it significantly reversed DOI (10 μg, intraventricular)-induced anorexia and activation of HPA axis. But the hyperthermia induced by DOI was not sensitive to reversal by melatonin. Mel₁ receptor subtype antagonist luzindole (5 μg, intraventricular) did not modulate the DOI effect but antagonized the melatonin (3 mg/kg, i.p.) reversal of 5-HT_2A agonist response. The present data suggest that melatonin reversal of DOI-induced hypophagia could be due to suppression of 5-HT_2A mediated activation of HPA axis.     

Serotonergic agents modulate antidepressant-like effect of the neurosteroid 3α-hydroxy-5α-pregnan-20-one in mice

The present study demonstrated the antidepressant-like effect of neurosteroid 3α-hydroxy-5α-pregnan-20-one (3α, 5α THP) in mouse forced swim test of depression and its modulation by different serotonergic agents. Pretreatment with the selective serotonin reuptake inhibitor, fluoxetine (5 mg/kg, i.p.), the 5-HT releaser, fenfluramine (10 mg/kg, i.p.), the 5-HT_1A receptor agonist, 8-OH-DPAT (0.1 mg/kg, s.c.), the 5-HT_1B/1C receptor agonist, TFMPP (4 mg/kg, s.c.) and the 5-HT_2A/1C receptor agonist, DOI (2 mg/kg, s.c.) potentiated the antidepressant-like effect of 3α, 5α THP. At these doses the serotonergic agents per se did not modify the duration of immobility. However, fluoxetine (20 mg/kg, i.p.), fenfluramine (20 mg/kg, i.p.) or imipramine (5 or 20 mg/kg, i.p.) not only reduced immobility but also enhanced the antidepressant-like effect of 3α, 5α THP. Such a potentiating effect of the 5-HT_1A or the 5-HT_2A/1C receptor agonist was not antagonized by the sub-effective dose (0.1 mg/kg, s.c.) of their respective antagonists p-MPPI or ketanserin. Pretreatment with p-CPA (300×3 mg/kg, i.p.), a depleter of 5-HT neuronal store failed to block the influence of fluoxetine and fenfluramine on antidepressant-like effect of 3α, 5α THP. The accelerated effect of 3α, 5α THP in presence of serotonergic agents was antagonized by the GABA_A receptor antagonist, bicuculline (1 mg/kg, i.p.) or the 3α-hydroxysteroid oxidoreductase enzyme inhibitor, indomethacin (5 mg/kg, i.p.). These findings for the first time demonstrate that serotonergic agents potentiate the antidepressant-like action of 3α, 5α THP, by enhancing the GABAergic tone as a likely consequence of increased brain content of this neurosteroid.     

Effect of estradiol benzoate microinjection into the median raphe nucleus on contextual conditioning

Estrogen deficiency has been associated with stress, anxiety and depression. Estrogen receptors have been identified in the median raphe nucleus (MRN). This structure is the main source of serotonergic projections to the hippocampus, a forebrain area implicated in the regulation of defensive responses and in the resistance to chronic stress. There is reported evidence indicating that estrogen modulates 5-HT_1A receptor function. In the MRN, somatodendritic 5-HT_1A receptors control the activity of serotonergic neurones by negative feedback. The present study has evaluated the effect of intra-MRN injection of estradiol benzoate (EB, 600 or 1200 ng/0.2 μl) on the performance of ovariectomized rats submitted to contextual conditioning. Additionally, the same treatment was given after intra-MRN injection of Way 100635 (100 ng/0.2 μl), a 5-HT_1A receptor antagonist. Both doses of EB decreased freezing and increased rearing, indicating an anxiolytic effect. Pretreatment with Way 100635 antagonized the anxiolytic effect of estradiol. On the basis of these results, it may be suggested that estrogens modulate anxiety by acting on 5-HT_1A receptors localized in the MRN.     

Quantitative autoradiography of 5-HT1D and 5-HT1E binding sites labelled by [H]5-HT, in frontal cortex and the hippocampal region of the human brain

In human cortex and hippocampus area, [³H]5-HT (5 nM) labels 5-HT_1A, 5-HT_1D and 5-HT_1E sites. After masking 5-HT_1A receptors by 0.1 μM 8-OH-DPAT, the binding displaced by 0.1 μM 5-CT presumably represented 5-HT_1D sites and the remaining binding 5-HT_1E sites. In frontal cortex, 5-HT_1A receptors represented the main binding in layers II and VI and a lower fraction on other layers. 5-HT_1D and 5-HT_1E sites, were more homogeneously distributed in layers II to VI (21–34% of specific [³H]5-HT binding). 5-HT_1E sites were of similar affinities (K_D close to 6–8 nM) in the cortical layers II to VI. In CA1 field of hippocampus, (pyramidal layer, stratum radiatum, molecular layer), CA2 and dentate gyrus, 5-HT_1A receptors represented the major fraction, 5-HT_1D sites a significant fraction and 5-HT_1E a minor fraction of the specific [³H]5-HT binding. In CA3–CA4 fields, 5-HT_1A receptors were less densely present, 5-HT_1D sites were predominant and 5-HT_1E sites represented a significant fraction (27%). The highest densities of 5-HT_1E sites have been measured in subiculum, where 5-HT_1A, 5-HT_1D, and 5-HT_1E binding sites were equally represented and in entorhinal cortex where 5-HT_1E sites represented the major binding in layer III. They were also present in layers II and IV (29 and 24%) and, to a lesser extent, in layers V and VI. 5-HT_1A sites were predominant in layer VI, II and V and were less abundant in other layers. 5-HT_1D were homogeneously present in layers II, III, IV and were present in low amounts in other layers. No 5-HT_1E were detected in choroid plexus, where [³H]5-HT was dramatically reduced by mesulergine (5-HT_2C receptors). No significant displacement of [³H]5-HT by mesulergine was measured in other structures.     

5-HT2 receptor regulation of acetylcholine release induced by dopaminergic stimulation in rat striatal slices

The role of 5-hydroxytryptamine (5-HT) receptor subtypes in acetylcholine (ACh) release induced by dopamine or neurokinin receptor stimulation was studied in rat striatal slices. The dopamine D₁ receptor agonist SKF 38393 potentiated in a tetrodotoxin-sensitive manner the K⁺-evoked [³H]ACh release while SCH 23390, a dopamine D₁ receptor antagonist, had no effect. [³H]ACh release was decreased by the dopamine D₂ receptor agonist LY 171555 (quinpirole) and slightly potentiated by the dopamine D₂ receptor antagonist haloperidol. The selective neurokinin NK₁ receptor agonist [Sar⁹, met(O₂)¹¹]SP also potentiated K⁺-evoked release of [³H]ACh. GR 82334, a NK₁ receptor antagonist, blocked not only the effect of [Sar⁹, met(O₂)¹¹]SP but also the release of ACh induced by the D₁ receptor agonist SKF 38393. Among the 5-HT agents studied, only the 5-HT_2A receptor antagonists ketanserin and ritanserin were able to reduce the ACh release induced by dopamine D₁ receptor stimulation. Mesulergine, a more selective 5-HT_2C antagonist, showed an intrinsic releasing effect but did not affect K⁺-evoked ACh release induced by SKF 38393. Methysergide and methiothepin, mixed 5-HT_1/2 antagonists, as well as ondansetron, a 5-HT₃ receptor antagonist, showed an intrinsic effect on ACh release, their effects being additive to that of SKF 38393. 5-HT₂ receptor agonists were ineffective. However, the 5-HT₂ agonist DOI was able to prevent the antagonism by ketanserin of the increased [³H]ACh efflux elicited by SKF 38393, suggesting a permissive role of 5-HT_2A receptors. None of the above indicated 5-HT agents was able to reduce the ACh release induced by the selective NK₁ agonist. The results suggest that 5-HT₂ receptors, probably of the 5-HT_2A subtype, modulate the release of ACh observed in slices from the rat striatum after stimulation of dopamine D₁ receptors. It seems that this serotonergic control is exerted on the interposed collaterals of substance P-containing neurons which promote ACh efflux through activation of NK₁ receptors located on cholinergic interneurons.     

Effect of chronic amitriptyline administration on serotonergic receptors in rats with methylazoxymethanol-induced microencephaly

Methylazoxymethanol (MAM)-induced cortical hypoplasia resulted in a 20% decrease in the B_max of 5-HT_2A receptors in the frontal cortex with no change in the B_max of 5-HT_1A receptors. Chronic treatment with amitriptyline did not further decrease the B_max of 5-HT_2A receptors in the MAM-lesioned cortex, suggesting that the persistent down-regulation of cortical 5-HT_2A receptors in MAM-lesioned rats was induced by serotonergic hyperinnervation.     

Cardiovascular effects of intrathecal administration of agents active at 5-hydroxytryptamine1-receptors in the rat: modulation by substance P and a substance P antagonist

Summary The intrathecal (i.th., T 8–10) administration in conscious rats of the 5-hydroxytryptamine (5-HT)_1A agonist 8-OH-DPAT (10 nmol), and to a lesser extent the 5-HT_1B agonist CGS 12066B (6 nmol), resulted in a blood pressure reduction and a bradycardia. These responses were prevented by the i.th. pretreatment with substance P (SP) (6.5 nmol) and enhanced following pretreatment with the non-peptide SP antagonist CP-96,345 (10 nmol). The partial 5-HT_1A agonist 8-MeO-CLEPAT (10 nmol) had by itself small cardiovascular effects. Following the pretreatment with SP, 8-MeO-CLEPAT caused a pressor response and a tachycardia whereas the opposite effects were observed following pretreatment with the SP antagonist. These results support the notion of a functional interaction between serotonergic and SP mechanisms at the level of the preganglionic sympathetic nerves in the spinal cord.     

Evidence for 5-HT4 receptor involvement in the enhancement of acetylcholine release by p-chloroamphetamine in rat frontal cortex

The roles of endogenous serotonin (5-HT) and 5-HT receptor subtypes in regulation of acetylcholine (ACh) release in frontal cortex of conscious rats were examined using a microdialysis technique. Systemic administration (1 and 3 mg/kg, i.p.) of the 5-HT-releasing agent p-chloroamphetamine (PCA) elevated ACh output in a dose-dependent manner. Depletion of endogenous 5-HT by p-chlorophenylalanine significantly attenuated the facilitatory effect of PCA on ACh release. The PCA (3 mg/kg)-induced increase in ACh release was significantly inhibited by local application of the 5-HT₄ receptor antagonists RS23597 (50 μM) and GR113803 (1 μM), while the 5-HT_1A antagonist WAY-100135 (10 mg/kg, i.p.; 100 μM), 5-HT_1A/1B/β-adrenoceptor antagonists (−)-pindolol (8 mg/kg, i.p.) and (−)-propranolol (150 μM), 5-HT_2A/2C antagonist ritanserin (1 mg/kg, i.p.; 10 μM) and 5-HT₃ antagonist ondansetron (1 mg/kg, i.p.; 10 μM) failed to significantly modify the effect of PCA. These results suggest that PCA-induced enhancement of 5-HT transmission facilitates ACh release from rat frontal cortex at least in part through 5-HT₄ receptors.     

SoRI 9409, a non-peptide opioid μ receptor agonist/δ receptor antagonist, fails to stimulate [S]-GTP-γ-S binding at cloned opioid receptors

Recent work suggests that opioids which combine μ agonist and δ antagonist activity may be non-addicting antinociceptive agents. SoRI 9409 (5′-(4-Chlorophenyl)-17-(cyclopropylmethyl)-6,7-didehydro-3,14-dihydroxy-4,5α-epoxypyrido-[2′,3′:6,7]morphinan) is a naltrexone-derived non-peptide ligand which demonstrates partial μ and κ agonist activity and antagonist activity at δ receptors. Chronic administration of SoRI 9409 to mice failed to produce tolerance to its antinociceptive effect and SoRI 9409 produced less withdrawal signs than naloxone in acute and chronic morphine dependence models. To further characterize SoRI 9409 we determined its effects in the guanosine 5′-O-(3-[³⁵S]thio)-triphosphate binding assay. SoRI 9409 demonstrated no agonist activity at cloned μ, δ, or κ receptors. Other experiments demonstrated that SoRI 9409 was a potent and selective δ antagonist (K_i = 0.08 nM) which acted also as an antagonist at μ and κ receptors. Its profile of activity resembled that of naltrindole (NTI). Viewed collectively, the in vitro data reported here predict that SoRI 9409 should be a μ antagonist in vivo, which is not observed. Resolving these discrepant findings will require additional research.