An antagonistic 5-HT receptor system in the auricles of the systemic heart complex of Sepia officinalis L. (Cephalopoda) shows 5-HT1 and 5-HT4 subtype properties

In pharmacological bioassays on isolated ring-shaped auricle preparations of Sepia officinalis, the action of the specific 5-hydroxytryptamine (5-HT) agonists 8-OH-DPAT (5-HT1a), CP-93129 (5-HT1b), TFMPP (5-HT1b) and RS-67333 (5-HT4) on these autonomously contractile compartments was studied. 8-OH-DPAT and CP-93129 induced mainly positive effects on frequency and tone on the isotonically suspended auricles. The positive effect of 8-OH-DPAT on frequency was blocked by the specific 5-HT1a antagonist NAN-190. The 5-HT1b agonist TFMPP caused similar effects on tone and a positive impact on the auricular amplitude. The highly specific 5-HT4 agonist RS-67333 induced an effect opposite to the action of 5-HT1 agonists inducing mainly negative effects on frequency, amplitude and tone, causing a diastolic standstill at a concentration of 10(-6) M. These negative effects were blocked by the adenylyl cyclase inhibitor SQ-22,536 in the absence of a diastolic standstill. The opposing action of 5-HT1 and 5-HT4 agonists on auricular contractile activity suggests that an antagonistic 5-HT-receptor system exists within the auricular myocardial cells of S. officinalis, probably consisting of 5-HT1- and 5-HT4-like subtypes. The results also suggest that adenylyl cyclase acts as the intracellular target enzyme of both signal transduction mechanisms.     

Mixed agonist/antagonist properties of NAN-190 at 5-HT1A receptors: behavioural and in vivo brain microdialysis studies

1-(2-Methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine, NAN-190, is a novel compound with putative 5-HT1A antagonist properties. In the present study, the effects of NAN-190 were examined with regard to functional pre- and post-synaptic 5-HT1A receptor-mediated events, using in vivo brain microdialysis and behavioural techniques. Our findings provide evidence that NAN-190 acts as a mixed agonist/antagonist at central 5-HT1A receptors. Thus, NAN-190 blocked (+)8-OH-DPAT-induced behaviour in reserpinized rats, indicating antagonist properties at postsynaptic 5-HT1A receptors. However, the compound was also able to decrease the release of 5-HT in vivo, tentatively due to an agonist action at somatodendritic 5-HT1A autoreceptors. These data extend previous information on the pharmacological profile of NAN-190 and further emphasizes the difference between pre- and postsynaptic 5-HT1A receptors in brain.     

Antagonists of cholinergic and serotonergic responses of Aplysia buccal muscle

Cholinergic and serotonergic receptors of Aplysia californica buccal muscles were characterized pharmacologically by determining compounds that effectively inhibited contractile responses to acetylcholine (ACh) and modulatory effects of serotonin (5-HT), respectively. pA50 for ACh to elicit contraction averaged 4.7 ± 0.1 (mean ± SE, equivalent to 2 × 10⁻⁵ M). Both hexamethonium bromide and atropine inhibited ACh-elicited contractions, but neither inhibited the response completely, nor were the two together able to antagonize the response completely. Curare caused inhibition only at low ACh doses, and muscarinic antagonists pirenzapine and 4-diphenylacetoxy-N-methylpiperidine methiodide caused partial inhibition. The most effective blocker of ACh-elicited contractions was the nicotinic antagonist mecamylamine. 10⁻⁴M mecamylamine completely blocked the cholinergic response. ACh contractions were inhibited 90% within 10 min and took >40 min to recover from mecamylamine. Specificity was indicated by the lack of effect of mecamylamine on potassium-elicited contraction. NAN-190 blocked the potentiating effect of 5-HT without having inhibitory or potentiating effects by itself on ACh-elicited contractions. NAN-190 blocked the potentiating effect of 8-OH-DPAT. Cholinergic receptors on Aplysia buccal muscles are most effectively inhibited by mecamylamine and may have mixed nicotinic/muscarinic character. Serotonergic receptors have pharmacological similarities to vertebrate 5-HT_1A receptors and may be closely related to the gastropod 5-HTlym receptor.     

Peripheral 5-carboxamidotryptamine induces hindlimb scratching by stimulating 5-HT1A receptors in rats.

Treatment of rats with 5-carboxamidotryptamine (5-CT) or 5-methoxy-tryptamine (5-MeOT) induces a hindlimb scratch response. These compounds have high affinity for 5-HT1A and 5-HT1D receptors. The selective 5-HT1A receptor agonist N,N-dipropyl-5-CT (DP-5-CT) also induced hindlimb scratching while the selective 5-HT1D receptor agonist, sumatriptan, did not. 5-CT-induced hindlimb scratching was inhibited dose-dependently by several 5-HT1A antagonists (BMY 7378, NAN-190, MDL 73005EF and pindobind-5-HT1A) as well as the non-selective 5-HT antagonist, methiothepin. Pretreatment of rats with the serotonin (5-HT) synthesis inhibitor, p-chlorophenylalanine (PCPA) or the 5-HT depleting agent, reserpine, markedly attenuated 5-CT-induced hindlimb scratching. These data suggest that hindlimb scratching induced by 5-HT agonists may not be centrally mediated but rather may be mediated by a neuronal 5-HT1A receptor localized outside the blood-brain barrier.     

Involvement of some 5-HT receptors in methamphetamine-induced locomotor activity in mice.

Effects of some selective 5-HT antagonists on methamphetamine-induced locomotor activity were investigated in male mice in order to study whether this effect of methamphetamine is selectively or at least partially, induced through stimulation of a specific serotonin receptor subtype. Methamphetamine (1.5 mg/kg, IP) produced a significant increase in locomotor activity. Methamphetamine-induced hyperactivity by the above mentioned dose was significantly antagonized by NAN-190 ( 5-HT(1A) antagonist) at a dose of 4 mg/kg, IP, methiothepin (5-HT(1B/1D) antagonist) at a dose of 0.1mg/kg, IP or mianserin ( 5-HT(2C) antagonist) at a dose of 8 mg/kg, IP. On the other hand, methysergide ( 5-HT(2A/2B) antagonist) at a dose of 1mg/kg, IP or ondansetron ( 5-HT(3) antagonist) at a dose of 0.5mg/kg, IP potentiated the methamphetamine-induced hyperactivity. None of the above mentioned doses of 5-HT antagonists altered the spontaneous activity of mice when administered alone. The results of the present study indicate a possible role for serotonergic mechanisms, in addition to the catecholaminergic systems, in the locomotor stimulant activity of methamphetamine in mice. This role is possibly mediated through direct stimulation of some 5-HT receptor subtypes. Stimulation by methamphetamine of 5-HT(1A), 5-HT(1B/1D) and/or 5-HT(2C) receptor subtypes may result in hyperactivity, whereas stimulation by methamphetamine of 5-HT(2A/2B) and/or 5-HT(3) receptor subtypes may result in decreased activity.     

Effects of 5-HT1A receptor agonist and antagonist on anxiety in intact and ovariectomized female rats

The purpose of this work was to study the role of 5-HT1A receptors on the level of anxiety in adult intact and ovariectomized (OVX) female rats. The influence of chronic administration of 5-HT1A receptor agonist 8-OH-DPAT (0.05 mg/kg, s.c.) and 5-HT1A receptor antagonist NAN-190 (0.1 mg/kg, i.p.) given for 14 days alone or in combination with 17beta-estradiol (0.5 microg i.m./rat/day) was studied on behavior in the elevated plus maze. In intact females administration of NAN-190 resulted in significant increase in the number of enterings and the time spent on the open arms in every phase of the estrous cycle, however, 8-OH-DPAT failed to modify these parameters. In OVX females 8-OH-DPAT alone or in combination with 17beta-estradiol significantly increased the number of enterings and time spent on the open arms. On the contrary, NAN-190 alone or in combination with 17beta-estradiol in OVX females failed to evoke behavioral changes in the elevated plus maze. Thus, the 5-HT1A receptor antagonist NAN-190 induced anxiolytic effect in intact female rats, while 5-HT1A receptor agonist 8-OH-DPAT produced an anxiolytic profile on OVX rats. Results of this work specify the involvement of 5-HT1A receptors in behavioral mechanisms of anxiety in OVX female rats.     

Evidence for a spinal serotonergic control of the peripheral inflammation in the rat

We investigated the effect of serotonergic agonists and antagonists injected intrathecally by direct punction of the spinal cord at the lumbar level (between L5-L6) on peripheral inflammatory edema. Edema was induced by carrageenan injected subcutaneously in one hindpaw 30 min after spinal treatments. Serotonin (0.1, 1, 10 pmol) caused a graded-inhibition of the inflammatory paw edema. The corticosteroid inhibitor aminoglutethimide (100 mg/kg, p.o. 1.5 h before spinal treatment) did not modify this effect. The 5-HT1A agonist buspirone and the 5-HT1B/1D agonist sumatriptan (0.1, 1.0 and 10 nmol) also inhibited paw edema. The 5-HT1,2 antagonist methysergide (10 and 100 pmol) enhanced edema, but higher doses ( 4 and 8 nmol) diminished edema. NAN-190 (5-HT1 antagonist; 1 and 10 nmol) increased paw edema, while ritanserin (5-HT2 antagonist; 1 nmol) inhibited paw edema. Ondansetron (5-HT3 antagonist; up to 10 nmol) did not affect edema, but metoclopramide (5-HT3 antagonist / 5-HT4 agonist; 5, 10 and 30 pmol) inhibited edema. These data suggest that a tonic release of serotonin in the spinal cord may occurs during ongoing peripheral inflammation, modulating the neurogenic component of edema either by an inhibitory action on 5-HT1 receptors or by a stimulatory action on 5-HT2 receptors. A disfunction in such mechanism may be involved in the pathophysiology of certain types of headaches or migraine, which seem to depend on neurogenic vasodilation, and may also help to explain the therapeuthic effectiveness of some serotonergic agents in these conditions.     

5-HT(7)-like receptors mediate serotonergic modulation of photo-responsiveness of the medulla bilateral neurons in the cricket,Gryllus bimaculatus

Serotonin (5-HT) suppresses the photo-responsiveness of medulla bilateral neurons (MBNs) that are involved in the coupling mechanism of the bilaterally paired optic lobe circadian pacemakers in the cricket, Gryllus bimaculatus. We found that forskolin, a highly specific activator of adenylate cyclase, mimicked the effects of serotonin on the MBNs. This fact suggests the involvement of cyclic 3', 5'-adenosine monophosphate (cAMP) in mediating the action of serotonin. We therefore tested the effects of various 5-HT receptor agonists and antagonists that are coupled to adenylate cyclase to specify the receptor involved. Application of 8-OH-DPAT that has affinity for both 5-HT(1A) and 5-HT(7) receptors suppressed the photo-responsiveness, like forskolin. The inhibitory effect of 8-OH-DPAT was effectively blocked by clozapine, a high affinity 5-HT(7) receptor antagonists with a very low affinity for 5-HT(2). Ketanserin, a selective 5-HT(2) antagonist, and NAN-190, a 5-HT(1A) antagonist, did not block it. These results suggest that serotonergic suppression of the photo-responsiveness of the MBNs is mediated by 5-HT(7)-like receptor subtypes.     

Involvement of 5-HT1A receptors in passive avoidance learning in intact and ovariectomized female rats

The influence of chronic administration of 5-HT1A receptor agonist 8-OH-DPAT (0.05 mg/kg, s.c.) and 5-HT1A receptor antagonist NAN-190 (0.1 mg/kg, i.p.) injected for 14 days alone or in combination with 17beta-estradiol (0.5 microg i.m./rat/day) was studied on passive avoidance performance (PAR) and on behavior in the open field test in adult intact and ovariectomized (OVX) female rats. Administration of 5-HT1A receptor antagonist NAN-190 alone significantly improved PAR (p<0.05) in intact females with proestrus and estrus and in OVX females. Administration of 5-HT1A receptor agonist 8-OH-DPAT alone or in combination with 17beta-estradiol significantly (p<0.05) improved PAR in OVX rats and failed to normalize PAR in intact rats with proestrus and estrus. Results of the work specify the involvement of 5-HT1A receptors in the mechanisms of passive avoidance learning in OVX female rats.     

Schistosoma mansoni: effects of serotonin and serotonin receptor antagonists on motility and length of primary sporocysts in vitro

The effects of serotonin (5-hydroxytryptamine; 5-HT) on in vitro transformed primary sporocysts of Schistosoma mansoni were investigated. Serotonin treatment significantly increased parasite motility (percentage of motile sporocysts) and length at concentrations as low as 1 microM. These effects were mimicked by the 5-HT agonist tryptamine, albeit with 10- to 100-fold less potency. The effects of 10 microM 5-HT on sporocyst motility were observed within 15 min posttreatment and on parasite length by 6 h posttreatment, and both effects were stable for up to 48 h. Receptor antagonists with varying affinities for defined vertebrate neurotransmitter receptor subtypes were examined for their effects on parasite behavior in the absence and presence of 10 microM 5-HT. In the absence of 5-HT, only methiothepin significantly inhibited normal parasite growth after 48 h of incubation. In the presence of 10 microM 5-HT, the serotonin receptor antagonists mianserin, ketanserin (both at 100 microM), and methiothepin (at 10 microM) significantly inhibited 5-HT-induced lengthening of primary sporocysts, while 3-tropanyl-indole-3-carboxylate and chlorpromazine had no significant effect. The effects of these same drugs on parasite motility were also examined. In the absence of 5-HT, 10 microM chlorpromazine increased parasite motility, while the other antagonists had no effect. When sporocysts were treated with 10 microM 5-HT for 2 h in the continued presence of antagonist, 100 microM mianserin, ketanserin, 3-tropanyl-indole-3-carboxylate, and 10 microM methiothepin inhibited 5-HT induced increases in parasite motility, while 10 microM chlorpromazine had no effect. These results show that primary sporocysts of S. mansoni exhibit behavioral responses to serotonin much like adult stages of this parasite. Furthermore, these responses appear to be mediated via receptors with pharmacological similarities to those previously described in adult worms.     

Serotonergic mechanisms mediating spawning and oocyte maturation in the zebra mussel,Dreissena polymorpha

Summary

The zebra mussel, Dreissena polymorpha, is a freshwater biofouling bivalve unintentionally introduced in the 1980s into North America from Europe. Oocyte maturation (germinal vesicle breakdown, GVBD) and spawning of the zebra mussel can be triggered with serotonin (5-hydroxytryptamine, 5-HT). In pharmacological experiments to characterize the receptor mediating spawning, the serotonin receptor agonists 8-OH-DPAT, TFMPP, and 1-(1-naphthyl)piperazine were effective at stimulating spawning; whereas, 2-methylserotonin and alpha-methylserotonin had no effect. In experiments with antagonists of serotonin receptors ketanserin and propranolol had no effect; mianserin, NAN-190, and cyproheptadine had partial inhibitory effects; and methiothepin was a very effective antagonist. Metergoline had mixed agonist/antagonist properties. Ergotamine was the most effective activator of spawning in females. Compared to serotonergic receptors in other organisms, the receptors that activate spawning in zebra mussels resemble 5HTlym, 5HTdro2 and human 5HT1Dβ, which are receptors that may act both by inhibiting adenylyl cyclase and by activating phospholipase C. In zebra mussels, 5-HT and 8-OH-DPAT activate GVBD in gonad fragments, a process also initiated by manual dissection of gonad fragments. GVBD can be inhibited by pre-treatment of ovaries with forskolin and theophylline, suggesting an inhibitory role for cyclic AMP. The Ca²⁺ ionophore A23187 can trigger GVBD and polar body formation. Thus, oocyte maturation in zebra mussels may be initiated via serotonergic receptors simultaneously inhibiting adenylyl cyclase and activating Ca²⁺ mechanisms.     

Contribution of the peripheral 5-HT 2A receptor to mechanical hyperalgesia in a rat model of neuropathic pain

We investigated the effect of 5-HT receptor antagonists on mechanical hyperalgesia observed in a neuropathic pain rat model prepared by chronic constriction injury of the sciatic nerve. NAN-190, a 5-HT 1A receptor antagonist, (-)-pindolol, a 5-HT 1A/1B receptor antagonist, and tropisetron, a 5-HT(3/4) receptor antagonist, did not affect the pain threshold in the hyperalgesic hind limb to the same extent as in the normal hind limb. However, sarpogrelate and ketanserin, 5-HT 2A receptor antagonists, significantly elevated the pain threshold in the hyperalgesic hind limb, but not in the normal hind limb. In spite of its high affinity for the 5-HT 2A receptor, methysergide only slightly elevated the pain threshold in the hyperalgesic hind limb. Pre-treatment with methysergide significantly antagonized the inhibitory effect of sarpogrelate on hyperalgesia. Furthermore, the 5-HT 2A receptor specific binding activity of 3H-ketanserin determined for the hyperalgesic hind limb did not differ from that of the normal hind limb. From these results, we propose that the 5-HT 2A receptor in the hyperalgesic hind paw function as an agonist-independent active receptor following constriction of the sciatic nerve, and that sarpogrelate and ketanserin act as inverse agonists of this receptor and suppress its activation. Methysergide may act as a neutral antagonist that blocks the effect of inverse agonists on the 5-HT 2A receptor.     

Nad-299 antagonises 5-HT-stimulated and spiperone-inhibited [35S]GTPgammaS binding in cloned 5-HT1A receptors

In Chinese Hamster Ovary (CHO) cells expressing cloned human 5-hydroxytryptamine1A A (5-HT1A) receptors, (R)-3-N,N-dicyclobutylamino-8-fluoro-[6-3H]-3,4-dihydro-2H-1-benzopyan-5-carboxamide ([3H]NAD-299) exhibited high affinity (Kd = 0.16 nM) and labeled 34% more receptors than 8-hydroxy-2-([2,3-3H]di-n-propylamino)tetralin ([3H]8-OH-DPAT). NAD-299 behaved as a silent antagonist in [35S]GTPgammaS binding similar to N-tert-butyl-3-(4-(2-methoxyphenyl)-piperazin-1-yl)-2-phenylpropanamide (WAY-100635) and (S)-5-fluoro-8-hydroxy-2-(di-n-propylamino)tetralin ((S)UH-301). 5-HT and 5-carboxamidotryptamine (5-CT) stimulated [35S]GTPgammaS binding 2.5-fold while spiperone and methiothepin inhibited [35S]GTPgammaS binding 1.4-fold. Furthermore, NAD-299 antagonised both the 5-HT stimulated and the spiperone inhibited [35S]GTPgammaS binding to basal levels. The KiL/KiH ratios for spiperone (0.66), methiothepin (0.39), WAY-100635 (0.32), (S)UH-301 (0.94), NAD-299 (1.29), NAN-190 (1.23), (S)pindolol (5.85), ipsapirone (13.1), buspirone (24.6), (+/-)8-OH-DPAT (47.3), flesinoxan (55.8), 5-HT (200) and 5-CT (389) correlated highly significantly with the intrinsic activity obtained with [35S] GTPgammaS (r = 0.97).     

Effects of Methiothepin and Lysergic Acid Diethylamide on Serotonin Release In Vitro and Serotonin Synthesis In Vivo: Possible Relation to Serotonin Autoreceptor Function

Abstract: An in vitro system characterizing the presyn- aptic serotonin (5-HT) autoreceptor which controls the release of 5-HT from rat brain slices is described. Using this system, methiothepin (1–10 μ M) demonstrated 5-HT autoreceptor antagonist activity -by enhancing 5-HT release, while several recognized postsynaptic 5-HT receptor antagonists were inactive: mianserin, cinanserin, cyproheptadine, methysergide. The activity of methiothepin was highest in hypothalamic slices and lowest in striatal slices and was inhibited by the autoreceptor agonists lysergic acid diethylamide (LSD) and 5-methoxy- tryptamine (5-MT). The reversal of the methiothepin-enhanced 5-HT release from hypothalamic slices by LSD was not influenced by 0.3 μ M tetrodotoxin. The peripheral administration of LSD to rats has been shown to reduce 5-HT synthesis and release by a mechanism thought to involve, in part, an autoreceptor-mediated reduction in impulse flow of 5-HT neurons. In the present experiments, intraperitoneal injection of methiothepin antagonized the LSD-induced reduction in hypothalamic 5-HT synthesis (5-hydroxytryptophan accumulation) while exerting no influence by itself. Conversely, compounds which were not active as 5-HT autoreceptor antagonists in vitro (i.e., cyproheptadine, methysergide, cinanserin) did not influence the effect of LSD on 5-HT synthesis. Further, the reduction in 5-hydroxytryptophan (5-HTP) accumulation by LSD showed regional differences in inhibition by methiothepin (hypothalamus > cortex > striatum) which paralleled the autoreceptor antagonist activity of methiothepin in vitro. These data suggest that similar autoreceptor mechanisms control 5-HT release and synthesis in terminal 5-HT projection areas and that the reduction in 5-HT accumulation by LSD and the antagonism by methiothepin may represent a useful biochemical measure of 5-HT autoreceptor activity in vivo.     

The impact of spacer structure on 5-HT7 and 5-HT1A receptor affinity in the group of long-chain arylpiperazine ligands

New cis-, trans-2-butene and 1,2-bismethylbenzene analogues of MM77 and NAN-190 (1-[4-[4-(2-methoxyphenyl)-piperazin-1-yl]-butyl]-pyrrolidine-2,5-dione and isoindole-1,3-dione, respectively) were synthesized. The differences in their in vitro affinity for serotonin 5-HT(7) and 5-HT(1A) receptors were explained using a conformational analysis. A bioactive conformation of those compounds for the 5-HT(7) receptor, different from that established for 5-HT(1A), was proposed.     

Serotonin 5-HT receptor blockade enhances Ca(2+)/calmodulin-dependent protein kinase II function and membrane expression of AMPA receptor subunits in the rat hippocampus: implications for memory formation

Stimulation of hippocampal 5-HT(1A) receptors impairs memory retention. The highly selective 5-HT(1A) antagonist, WAY-100635, prevents the cognitive deficits induced not only by 5-HT(1A) stimulation but also by cholinergic or NMDA receptor blockade. On this basis, the effects of WAY-100635 on molecular events associated with memory storage were explored. In rat hippocampus, WAY-100635 produced a rapid increase in phosphorylated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and in Ca(2+)-independent CaMKII and protein kinase A (PKA) enzyme activity. This increase was followed a few hours later by an enhanced membrane expression of AMPA receptor subunits, especially of the GluR1 subunit phosphorylated at the CaMKII site, pGluR1(Ser831). The same qualitative effects were found with the weaker 5-HT(1A) antagonist NAN-190. The effects of both antagonists were no longer apparent in rats with a previous 5-HT depletion induced by the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA), suggesting that 5-HT(1A) receptor blockade removes the tonic inhibition of 5-HT through 5-HT(1A) receptor stimulation on excitatory hippocampal neurons, with the consequent increase in PKA activity. In addition, administration of WAY-100635 potentiated the learning-specific increase in the hippocampus of phospho-CaMKII, Ca(2+)-independent CaMKII activity, as well as the phosphorylation of either the CaMKII or the PKA site on the AMPA receptor GluR1 subunit. This study suggests that blockade of hippocampal 5-HT(1A) receptors favours molecular events critically involved in memory formation, and provides an in vivo molecular basis for the proposed utility of 5-HT(1A) receptor antagonists in the treatment of cognitive disorders.     

Vasomotor effects of noradrenaline, acetylcholine, histamine, 5-hydroxytryptamine and bradykinin on snake (Trimeresurus flavoviridis) basilar arteries

We investigated the responsiveness of basilar arterial rings isolated from snakes to noradrenaline (NA), acetylcholine (ACh), histamine (His), 5-hydroxytryptamine (5-HT), mammalian bradykinin (BK) and rattlesnake BK. We also examined whether endothelial cells were involved in the responsiveness to ACh, BK, rattlesnake BK and in their resting vascular tone. NA and 5-HT induced concentration-dependent contractions. The cumulative concentration response curves of NA and 5-HT were shifted to the right in parallel by phentolamine (an alpha antagonist) and methiothepin (a 5-HT(1) and 5-HT(2) antagonist), respectively. However, ketanserin (a 5-HT(2) antagonist) had no effect on the cumulative concentration response curve of 5-HT. His, ACh, BK and rattlesnake BK had no effect on resting vascular tone; however, rattlesnake BK and sodium nitroprusside relaxed arteries precontracted by 5-HT. The rattlesnake BK-induced relaxations were almost abolished by L-nitro arginine (L-NA, a nitric oxide synthase inhibitor). L-NA and indomethacin (a cyclooxygenase inhibitor) had no effect on resting vascular tone or on precontracted arteries. These results suggest that alpha and 5-HT(1) receptor subtypes might be important in arterial contraction. Endothelial cells might play an important role in the responsiveness of snake basilar arteries to rattlesnake BK, but they might not be involved in the responsiveness to ACh, BK and in resting vascular tone.     

Involvement of Spinal Serotonin Receptors in Electroacupuncture Anti-Hyperalgesia in an Inflammatory Pain Rat Model

We previously showed that electroacupuncture (EA) activates medulla-spinal serotonin-containing neurons. The present study investigated the effects of intrathecal 5,7-dihydroxytryptamine creatinine sulfate, a selective neurotoxin for serotonergic terminals, the 5-hydroxytryptamine 1A receptor (5-HT1AR) antagonist NAN-190 hydrobromide and the 5-HT2C receptor (5-HT2CR) antagonist SB-242,084 on EA anti-hyperalgesia. EA was given twice at acupoint GB30 after complete Freund’s adjuvant (CFA) injection into hind paw. CFA-induced hyperalgesia was measured by assessing hind paw withdrawal latency (PWL) to a noxious thermal stimulus 30 min post-EA. Serotonin depletion and the 5-HT1AR antagonist blocked EA anti-hyperalgesia; the 5-HT2CR antagonist did not. Immunohistochemical staining showed that spinal 5-HT1AR was expressed and that 5-HT2CR was absent in naive and CFA-injected animals 2.5 h post-CFA. These results show a correlation between EA anti-hyperalgesia and receptor expression. Collectively, the data show that EA activates supraspinal serotonin neurons to release 5-HT, which acts on spinal 5-HT1AR to inhibit hyperalgesia.     

Effects of cholecystokinin tetrapeptide (CCK(4)) and of anxiolytic drugs on GABA outflow from the cerebral cortex of freely moving rats

The effect of cholecystokinin tetrapeptide (CCK(4)) and of different anxiolytic drugs on GABA outflow from the cerebral cortex was investigated in freely moving rats, by using the epidural cup technique. CCK(4) (3-30 microg/kg, i.p.) increased GABA outflow and induced objective signs of anxiety. These neurochemical and behavioral responses were prevented by the CCK(B) antagonist GV150013 at 0.1 microg/kg (i.p.). At higher doses (up to 30 microg/kg) this compound per se reduced GABA release and caused sedation, suggesting the presence of a CCKergic positive tonic modulation on GABA interneurons. Similarly the GABA(A) receptors modulator, diazepam (2mg/kg, i.p.) and the 5-HT(1A) agonist buspirone (3mg/kg, i.p.) reduced GABA outflow and caused the expected behavioral effects (reduced muscle tone, mild 5-HT syndrome) which were prevented by the respective, selective antagonists, flumazenil (1mg/kg, i.p.) and NAN-190 (3mg/kg, i.p.). These findings support the idea that GV150013, diazepam and buspirone inhibit GABAergic cortical activity, through the respective receptors. This neurochemical effect may represent the end-effect of various anxiolytic compounds affecting the cortical circuitry.     

Serotonergic modulation of murine fundic tone

Fundic tone is maintained through a balance of excitatory and inhibitory input to fundic smooth muscle. The aim of this study was to determine the role of serotonin (5-HT) and 5-HT receptors in modulating murine fundic tone. Muscle strips were prepared from the murine fundus. Intracellular recordings were made from circular smooth muscle cells, and the effects of 5-HT on tone and excitatory and inhibitory junction potentials evoked by electrical field stimulation (EFS) were determined. 5-HT induced a concentration-dependent contraction and smooth muscle depolarization that was tetrodotoxin resistant. The 5-HT(1B/D) receptor antagonists GR-127935 and BRL-155172 significantly inhibited 5-HT-induced contractions. The 5-HT(1B/D) agonist sumatriptan contracted murine fundic muscle. The 5-HT(1A) receptor agonist buspirone relaxed fundic smooth muscle, and the relaxation was inhibited by WAY-100135 but not by N(omega)-nitro-l-arginine or tetrodotoxin. 5-HT enhanced both the excitatory and inhibitory responses to EFS. The 5-HT(3) receptor antagonist MDL-72222 partly inhibited both the excitatory and inhibitory response elicited by EFS, whereas the 5-HT(4) receptor antagonist GR-113808 partly inhibited the EFS-evoked inhibitory response. The 5-HT reuptake inhibitor fluoxetine contracted smooth muscle strips, a contraction that was partially inhibited by GR-127935 and abolished by tetrodotoxin. In conclusion, the data suggest that 5-HT modulates murine fundic contractile activity through several different receptor subtypes. Sustained release of 5-HT maintains fundic tone through postjunctional 5-HT(1B/D) receptors. 5-HT(3) receptors modulate excitatory neural input to murine fundic smooth muscle, and both 5-HT(3) and 5-HT(4) receptors modulate inhibitory neural input to murine fundic smooth muscle.