[3H]spiroxatrine labels a serotonin1A-like site in the rat hippocampus

Thermodynamics of peptide and non-peptide interactions with the human 5HT1a receptor

The human serotonin 1a receptor (H5HT1aR) is a highly studied member of the 7 transmembrane G protein-coupled receptors. This model receptor, negatively coupled to adenylyl cyclase via Gi, is linked to physiological processes such as cognition and mood regulation and to associated disorders like anxiety and depression. Gibb's free energies, enthalpies, and entropies were calculated for the agonist [(3)H]8-OH-DPAT in the presence of synthetic peptides derived from sequences of intracellular loops 2 and 3 of the H5HT1aR. For comparative purposes, the thermodynamic parameters were also determined in the presence of a limited number of ligand-binding site substances (the partial agonist dipropyltryptamine [DPT], and the full agonist [(3)H]8-OH-DPAT alone). All of these thermodynamic measurements were based on binding data accumulated over a range of temperatures (0-35 degrees C). Representative examples of binding constant experiments and van't Hoff plots are shown to establish the thermodynamic variables. Although differences exist between the peptides themselves and the non-peptide agonists, in all situations the binding events are highly entropy driven. Differences between this information and published data for rat 5HT1aR are discussed, as are relationships to other receptor systems. Overall, the conclusions should be useful in further defining a comprehensive model of 5HT1aR, and for future development of binding-site and non-binding-site directed agents for the receptor.

Regulation of G proteins by human 5-HT1a receptor TM3/i2 and TM5/i3 loop peptides

A bioactive synthetic 11 amino acid peptide probe (P11) was constructed according to the published sequence of the human 5HT1a receptor. The probe was used to enhance understanding of cytoplasmic loop 2/G protein coupling and activation. Additionally, two peptides (P8, P9) from the cytoplasmic loop 3 region were synthesized and studied. These probes were tested in a model system of human 5HT1a receptor stably expressed in Chinese Hamster Ovary cells. In agonist inhibition studies, P11 was active in all three receptor preparations tested: whole cells, membrane bound, and solubilized. In analyses of the membrane bound receptor system, P11 demonstrated uncompetitive inhibition characteristics. When forskolin-stimulated cAMP levels were measured, P11 was inactive in this negatively coupled system. Utilizing a [35S]gamma-S-GTP incorporation assay, P11 was unable to stimulate G protein incorporation of GTP. While P8 and P9 were also broadly active as non-competitive agonist inhibitors, their characteristics differed in the signal transduction system. P8 and P9 did not significantly change forskolin-stimulated cAMP levels. However, P8 increased [35S]gamma-S-GTP incorporation, while P9 decreased incorporation. Thus, P11, a synthetic peptide from the TM3/i2 region of the receptor, provides suggestive evidence that this receptor region is involved in G protein coupling but not activation. On the other hand, P8 and P9 activities suggest that the TM5/i3 region is involved in both coupling to and regulation of G protein activity. The current evidence from these cytoplasmic loop regions is discussed in the overall context of an emerging model for human 5HT1a receptor-G protein interactions.

Agonistic Properties of Cannabidiol at 5-HT1a Receptors

Cannabidiol (CBD) is a major, biologically active, but psycho-inactive component of cannabis. In this cell culture-based report, CBD is shown to displace the agonist, [3H]8-OH-DPAT from the cloned human 5-HT1a receptor in a concentration-dependent manner. In contrast, the major psychoactive component of cannabis, tetrahydrocannabinol (THC) does not displace agonist from the receptor in the same micromolar concentration range. In signal transduction studies, CBD acts as an agonist at the human 5-HT1a receptor as demonstrated in two related approaches. First, CBD increases [35S]GTPgammaS binding in this G protein coupled receptor system, as does the known agonist serotonin. Second, in this GPCR system, that is negatively coupled to cAMP production, both CBD and 5-HT decrease cAMP concentration at similar apparent levels of receptor occupancy, based upon displacement data. Preliminary comparative data is also presented from the cloned rat 5-HT2a receptor suggesting that CBD is active, but less so, relative to the human 5-HT1a receptor, in binding analyses. Overall, these studies demonstrate that CBD is a modest affinity agonist at the human 5-HT1a receptor. Additional work is required to compare CBD's potential at other serotonin receptors and in other species. Finally, the results indicate that cannabidiol may have interesting and useful potential beyond the realm of cannabinoid receptors.

Binding properties of dipropyltryptamine at the human 5-HT1a receptor

Dipropyltryptamine (DPT) is a synthetic indolealkylamine first characterized in the 1960s. Largely forgotten since the discovery of multiple serotonin receptor subtypes, some of the properties of DPT at the cloned human 5-HT1a receptor are described here. When [3H]8-OH-DPAT is bound to the receptor, DPT inhibits the interaction with an IC50 of 0.1 micromol/l. This interaction is shown to be competitive when double-reciprocal plots of the DPT/agonist interaction are analyzed. DPT's effects in the signal transduction system are complex. While DPT alone (0.1-1,000 micromol/l) activates Gi when both cAMP and gamma-S-GTP incorporation are measured, in the presence of 5-HT (0.1-10 micromol/l), DPT blocks the agonist effect. In combination, the findings suggest that DPT is a moderate affinity partial agonist at the human 5-HT1a receptor. These results provide evidence that DPT has potential as a versatile experimental tool at 5-HT1a receptors.

8-[3H]-hydroxy-2-(di-n-propylamino)tetralin binding sites in blood lymphocytes of rats and the modulation by mitogens and immobilization

Serotonin 5-HT(1A) receptors were characterized in rat resting lymphocytes obtained by cardiac puncture with the use of the ligand [3H]8-hydroxy-2-(di-n-propylamino)tetralin. Selectivity of the specific binding was demonstrated by inhibition experiments with various serotonergic and nonserotonergic drugs. The rank order of potency for inhibition was WAY-100478>pindobind>NAN-190>buspirone>imipramine>serotonin. While pimozide, desipramine, nomifensine, haloperidol and sulpiride did not inhibit the binding. Kinetic parameters calculated from saturation experiments indicated one site of interaction, with an equilibrium dissociation constant of 2.50 nM and maximum binding capacity of 487.21 nmol/10(6) cells. Complete dissociation was obtained with serotonin as the displacement agent, and equilibrium dissociation constant calculated by association and dissociation experiments was 2.03 nM. Thus, serotonin 5-HT(1A) receptors are present in resting lymphocytes. The in vivo administration of the mitogens lipopolysacharide (0.1 mg/kg, 18 h) or concanavalin A (0.2 mg/kg, 18 h) increased the number of sites. The elevation produced by the latter was of higher magnitude than that of lipopolysacharide, and two sites of the binding were determined by isotopic dilution. Immobilization stress (1 h daily for 7 days) also resulted in a significant increase of binding capacity, but was smaller than that produced by the mitogens. The affinity of binding was not affect by the treatments. The results indicate that serotonin 5-HT(1A) receptors are modulated by unspecific and specific immune system activation, as well as by a potent stress condition, which might result in relevant functional modifications in the response of rat lymphocytes.

Characterization of 5-HT(₁A,B) and 5-HT(₂A,C) serotonin receptor binding

This unit describes assays for measuring the binding of radioligands to two major types of receptors for 5-hydroxytryptamine (5-HT or serotonin), 5-HT₁ and 5-HT₂ receptors, in homogenates of brain tissue or cloned into cells in culture. The specific receptor subtypes covered are 5-HT(₁A), 5-HT(₁B), 5-HT(₂A), and 5-HT(₂C). In addition, methodology for using quantitative autoradiography to measure radioligand binding to serotonin receptors in brain slices is described. Protocols are provided for characterization of both saturation and competition binding assays, and instructions for data analysis of these assays is also described. In addition, methodology is provided for the quantification (image analysis) of radioligand binding in brain tissue sections to determine receptor density, preparation of rat brain sections for quantitative autoradiography, and thionin staining of thaw-mounted tissue sections to define certain brain regions.

Structural variants of a human 5-HT1a receptor intracellular loop 3 peptide

To better understand cytoplasmic loop 3/G protein coupling, variations in a bioactive synthetic peptide probe (P1) were constructed according to the published sequences of the human 5HT1a receptor. These probes were tested in a model system of human 5HT1a receptor stably expressed in Chinese hamster ovary cells. In agonist inhibition studies, peptides with amino acid substitutions of residues 6-9 from the amino terminus of loop 3 were less active than P1. Truncated peptide P4, conserving the residue 6-9 region, was also less active than P1. Truncates P5 and P6, deleting the residue 6-9 region, were inactive. When cAMP levels were measured, both substituted peptides were more active than P1 in this negatively coupled system. In contrast, the truncated peptides were without activity in the cAMP assays. Thus, P1 and its derivatives (P2-P6) constitute a small group of peptides with differential uncoupling (agonist inhibition) and signal transduction (cAMP) activities in this G-protein-linked system. It is proposed that these peptides will be useful in future studies detailing the molecular determinants at the receptor/G protein interface.

Rabbit cerebral cortex 5HT1a receptors

Selective 5HT1a agonist binding to membranes from rabbit cerebral cortex was concentration-dependent and saturable; the Kd was 1.1 nM and Bmax of 480 fmols/mg protein. Scatchard as well as Hill plots were linear; the Hill coefficient was 0.96, suggesting a single, non-interacting binding site. Agonist binding was inhibited in a concentration-dependent fashion by gamma S GTP, a result consistent with the coupling of this binding site to the G protein signal transduction system. In competition experiments involving agonist and a series of agents with known affinities and specificities at 5HT1a receptors, a rank order relationship was found consistent with this binding site being a 5HT1a binding site. Direct comparisons of agonist and antagonist binding at rat cerebral cortex 5HT1a receptors and cloned human 5HT1a receptors also suggested that the rabbit binding site belongs to the 5HT1a class. The only rank order anomalies were with methiothepin in rabbit cerebral cortex, where a comparatively high Ki was observed and with buspirone in cloned human 5HT1a receptor, where a low Ki was determined; these anomalies bear further study in light of the comparative pharmacology of 5HT1a receptors. Finally, the natural product parthenolide was tested for affinity in the rabbit, rat, and human systems, where it uniformly was unable to displace agonist, suggesting that the 5HT1a receptor is not a target for this compound. Overall, these results suggest that a functional 5HT1a receptor exists in rabbit cerebral cortex.

Electrophysiological effects of serotonin in the solitary tract nucleus of the rat

The nucleus tractus solitarius (nTS) is an important site for the integration of visceral information and its modification by afferent neural systems. One such afferent system arises from the raphe nuclei. This study investigated the electrophysiological effects of the primary transmitter of the raphe nuclei, serotonin (5-HT), on neurones in the nTS of the rat. Extracellular single unit recordings were made of the spontaneous activity of nTS neurones in isolated, superfused brainstem slices during bath-application of 5-HT (50, 100, 250, 500, 1000 nM). Twenty-seven of 46 neurones studied (approximately 59%) showed concentration-dependent decreases of firing rate, with a calculated EC50 of 261 nM. An additional 3 neurones displayed excitatory responses, while the remaining 16 were unaffected. The broad-spectrum 5-HT1 antagonist methysergide (200 nM) was highly effective in producing blockade of 5-HT-evoked depressions of neuronal activity (4 of 4), whereas the 5-HT1A receptor-selective antagonist spiroxatrine (5 nM) and the 5-HT2-selective antagonist mianserin (200 nM) were considerably less effective (1 of 5 and 2 of 5, respectively). Seven additional neurones were examined during exposure to the 5-HT1-selective agonist 5-carboxamidotryptamine (5-CT) or the 5-HT2-selective agonist DOI. 5-CT depressed the activity of three of the four neurones tested, the remaining neurone being unresponsive at concentrations up to 50 nM. DOI at concentrations up to 100 nM failed to affect the activity of two of the three neurones tested, and depressed the activity of the third at a concentration of 50 nM.(ABSTRACT TRUNCATED AT 250 WORDS)

8-[3H]hydroxy-2-(di-n-propylamino) tetralin binding sites in goldfish retina

The binding sites of 8-[3H]hydroxy-2-(di-n-propylamino)tetralin ([3H]DPAT) were characterized in the retina of goldfish in order to evaluate the selectivity of the ligand for serotonin1A (5HT1A) receptors. Specificity of the binding was performed in the presence of serotonergic and dopaminergic agonists and antagonists. Buspirone, spiroxatrine and 5-methoxy-N,N-dimethyltryptamine were potent inhibitors, followed by propranolol, citalopram, imipramine and desipramine. Serotonin was not a potent inhibitor, and its interaction with the binding sites of [3H]DPAT was complex. Nomifensine displayed an important inhibition, however, other dopamine uptake blockers, such as bupropion and GBR-12909, were less potent. Haloperidol was also a good inhibitor, but the D1 receptor agonist, SKF-38393, the D2 receptor antagonist, sulpiride, and dopamine did not inhibit the binding. GppNHp inhibited the binding in the micromolar range. The analysis of saturation experiments by isotopic dilution, using buspirone to determine nonspecific binding, revealed two sites. The number of binding sites defined by buspirone were higher than the ones defined by nomifensine. The specific binding, using buspirone for definition, was reduced by the intraocular injection of 6-hydroxydopamine. This investigation demonstrates that [3H]DPAT labels 5HT1A receptors in goldfish retina, but also interacts with a non-5HT receptor site. These receptors seem to be localized in dopaminergic neurons.

Central and peripheral serotonergic influences on viscerovisceral inhibitory reflex during duodenal distension in sheep

The effects of duodenal distension on forestomach and abomasal motility were investigated in conscious sheep chronically fitted with intraparietal electrodes, a duodenal cannula, and an intracerebroventricular cannula. Duodenal distensions with a balloon inflated with 40 ml (DD40) of water reduced the frequency of forestomach and abomasal contractions by 45 and 32%, respectively, while distension with 80 ml (DD80) induced a total inhibition. Methysergide, a mixed 5HT1-5HT2 antagonist administered intravenously (200 micrograms/kg) or intracerebroventricularly (20 micrograms/kg) suppressed the DD40-induced inhibition and reduced that induced by DD80. Spiroxatrine, a selective 5HT1A antagonist, intravenously (100 micrograms/kg) or intracerebroventricularly (10 micrograms/kg), suppressed the DD40 and DD80-induced inhibition, which was also attenuated by the 5HT2 antagonist ritanserin given intravenously (200 micrograms/kg) or intracerebroventricularly (20 micrograms/kg). Granisetron, a 5HT3 antagonist, injected intravenously (150 micrograms/kg), abolished the effects of DD40 and DD80 while it had no antagonistic action on DD40 and DD80 when given intracerebroventricularly (15 micrograms/kg). It is concluded that in sheep, duodenal distension inhibits forestomach and abomasal motility through 5HT1A and 5HT2 receptors at the level of the central nervous system and 5HT3 receptors located peripherally.

Serotonin is a directly-acting hyperalgesic agent in the rat

In this study, we have investigated serotonin hyperalgesia employing the mechanical paw withdrawal nociceptive threshold test in the rat. Intradermally injected serotonin was found to produce a dose-dependent hyperalgesia that was not attenuated by procedures which eliminate the known indirect mechanisms of hyperalgesia such as sympathectomy, polymorphonuclear leukocyte depletion or cyclooxygenase inhibition. In addition, the latency to onset of serotonin hyperalgesia is extremely short, with maximal hyperalgesia observed in less than 1 min, a similar temporal onset to direct-acting hyperalgesic agents such as prostaglandin E2. The results suggest, therefore, that the hyperalgesic effects of serotonin in our animal model are exerted by direct action on primary afferent neurons. Only the intradermal injection of selective serotonin (5-hydroxytryptamine; 5-HT) agonists for the 1A receptor subset (5-HT1A), (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthaline hydrobromide and N,N-dipropyl-5-carboxamido-tryptamine maleate, produced dose-dependent hyperalgesia. No hyperalgesia was seen after 5-HT1B, CGS-12066B maleate and m-trifluoromethylphenyl-piperazine hydrochloride; 5-HT2+IC, alpha methyl 5HT and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl; or 5-HT3, 2-methyl-5-hydroxytryptamine maleate and phenylbiguanide, agonists. Similarly, only the 5-HT1A antagonists, spiroxatrine and spiperone, attenuated the hyperalgesia induced by intradermally injected serotonin. 5-HT2+IC antagonists, mesulergine and ketanserin, and 5-HT3 antagonists, quipazine and 3-tropanyl-indole-3-carboxylate, did not significantly attenuate 5-HT hyperalgesia. We conclude that serotonin produces hyperalgesia by a direct action on the primary afferent neuron via the 5-HT1A subset of serotonin receptors.

α1-adrenoceptor blocking properties of spiroxatrine in rat aorta

This preliminary study has analyzed the potential ability of the 5-HT1A ligand spiroxatrine to interact with vascular alpha 1-adrenoceptors. Norepinephrine and the selective alpha 1-adrenoceptor agonist, methoxamine, elicited concentration-dependent contractions of rat aortic rings. In contrast, (+/-)-spiroxatrine (from 10(-8) to 3.1X10(-7) M) was devoid of any effect on vascular tone per se, but shifted the concentration-response curves of norepinephrine and methoxamine to the right in a concentration-dependent manner with pA2 values of 8.48 +/- 0.22 and 8.93 +/- 0.33, respectively. Endothelium removal did not significantly affect the above pA2 values of (+/-)-spiroxatrine. These data, taken in concert, support the contention that (+/-)-spiroxatrine displays alpha 1-adrenoceptor blocking properties in rat aortic rings.

Buspirone inhibits corticotropin-releasing factor and stress-induced cecal motor response in rats by acting through 5-HT1A receptors

The effect of buspirone on corticotropin-releasing factor (CRF) and stress-stimulated cecal motility and its antagonism by 5-HT1A (spiroxatrine) and sigma (BMY 14802) antagonists were evaluated by electromyography in rats equipped with chronically implanted electrodes on the cecum and a small catheter into the right lateral ventricle of the brain. Exposure to mental stress, consisting of a fear-conditioned response, increased during 30 min the frequency of cecal spike bursts significantly (P less than 0.01). The frequency of cecal spike bursts was also increased following intracerebroventricular injection of CRF (500 ng/kg). Buspirone (1 mg/kg s.c.) abolished the stimulatory effects of mental stress and CRF on cecal motility. Whereas spiroxatrine (0.5 mg/kg s.c.) blocked the effect of buspirone on the colonic hypermotility induced by i.c.v. injection of CRF, BMY 14802 at a similar dose (0.5 mg/kg s.c.) was unable to block the action of buspirone. It is concluded that s.c. administration of buspirone suppresses the stress-induced cecal motor response through 5-HT1A receptors, probably by inhibiting the central or peripheral pathways involved in CRF mediation of these effects.

Structural determinants of 5-HT1A versus 5-HT1D receptor binding site selectivity

A structure-activity analysis was used to identify selective 5-HT1A versus 5-HT1D receptor agents. An analysis of published data identified 13 drugs which display nanomolar affinity for the 5-HT1A receptor and that have been analyzed at 5-HT1D receptor binding sites. Four agents display greater than or equal to 100-fold selectivity for the 5-HT1A receptor. Two structural features were identified which hypothetically result in selectivity for 5-HT1A versus 5-HT1D binding sites. The linkage of an indole ring to a basic nitrogen atom via the 4 position on the indole ring or the absence of an indole ring are two features which lower the affinity for the 5-HT1D receptor, but do not necessarily lower the affinity for the 5-HT1A receptor. A series of 7 agents (5 indoles, 2 quinolines) was identified which met these hypothetical selectivity criteria. These compounds were then analyzed in radioligand binding studies. These 7 agents display affinities of 1.3-170 nM for the 5-HT1A receptor binding site, and 1,800-13,000 nM for the 5-HT1D receptor binding site. All 7 agents display greater than or equal to 47-fold selectivity for the 5-HT1A versus 5-HT1D site and 4 of the agents are greater than 100-fold selective. Compound No. 1 (N,N'-bis[3-(4-indolyloxy)-2-hydroxypropyl]-(Z)-1,8-diamino-p-meth ane) and compound No. 2 (N8-[3-(4-indolyloxy)-2-hydroxypropyl]-N1-(propioloyl)-(Z)-1 ,8-diamino-p-methane) are the most selective agents yet described for 5-HT1A versus 5-HT1D receptor binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of 5-hydroxytryptamine1A receptor agents using a composite pharmacophore analysis and chemical database screening

A composite pharmacophore analysis and computer-assisted chemical database screening were used to identify a previously unrecognized class of 5-hydroxy-tryptamine1A (5-HT1A) receptor active agents. An analysis of published data led to the identification of 20 different chemical structures which share nanomolar affinity for the 5-HT1A receptor. From a composite pharmacophore analysis of all 20 potent agents, we hypothesized that compounds containing a novel (in terms of 5-HT1A receptor analysis) 3 ring structure might be active at the 5-HT1A receptor. To test this hypothesis, the Chemical Abstracts database, which contains over 10 million compounds, was screened electronically for compounds that contain this core structure. A series of 319 agents was identified which contain this core structure. A total of 6 compounds was then obtained commercially and evaluated in radioligand binding studies. A single agent (Compound 69/183) conformed most closely to the composite 5-HT1A pharmacophore and displayed an affinity of 20 nmol/l for the 5-HT1A receptor binding site. Two other agents displayed affinities of 170 and 500 nmol/l, respectively, for the 5-HT1A receptor site. The 3 agents which differed most significantly from the composite 5-HT1A pharmacophore displayed affinities of 1,200- greater than 10,000 nmol/l for the 5-HT1A receptor binding site. These data suggest that a composite pharmacophore analysis and computer-assisted chemical database screening can be an effective technique for the identification of previously unrecognized receptor active agents.

5-Hydroxytryptamine receptor subtypes

Significant advances in the molecular pharmacological analysis of 5-hydroxytryptamine (5-HT) receptor subtypes occurred in the 1980's. To a significant degree, this progress resulted from 2 independent approaches: molecular biology and molecular pharmacology. This review focuses on the pharmacological data derived from radioligand binding studies. At the present time, 5-HT receptor subtypes are often categorized into at least 3 major "families" as well as a few "orphan" receptors that cannot yet be placed into the major categories. Each "family" consists of multiple receptor subtypes which share similarities in their molecular biological, pharmacological, biochemical and physiological properties. In order to provide a comparative pharmacological analysis of the 7 most extensively characterized 5-HT receptor subtypes, potency information is presented on the 30 pharmacological agents that have been, to date, studied most extensively in the published literature.

Antagonism of 8-OH-DPAT-induced behaviour in rats

Selective activation of the 5-HT1A receptor induces lower lip retraction (LLR) in rats. 8-Hydroxy-dipropylamino tetralin (8-OH-DPAT)-induced LLR could not be antagonised by the 5-HT antagonists methysergide, metergoline or mesulergine. In fact, some 5-HT antagonists induced LLR. However, 8-OH-DPAT-induced LLR could be antagonised by pindolol, spiperone, spiroxatrine and NAN-190, but not by the beta 1-adrenoceptor antagonist metoprolol, the beta 2-adrenoceptor antagonist butoxamine or the dopamine antagonist haloperidol. This antagonism was competitive as the dose-response curve of 8-OH-DPAT was shifted to the right. Pindolol, spiperone, spiroxatrine and NAN-190 all have a high affinity for the 5-HT1A receptor. This indicates that blockade of 8-OH-DPAT-induced LLR is only possible by selective blockade of 5-HT1A receptors. A possible mechanism of action is discussed. The increased defecation induced by 8-OH-DPAT could be antagonised by pindolol and NAN-190. The effect of spiroxatrine and haloperidol on the 8-OH-DPAT-induced increase in defecation was bimodal: an increase after a low and a decrease after a high dose of 8-OH-DPAT. Metoprolol and butoxamine had no effect on the 8-OH-DPAT-induced increase in defecation, thereby excluding an influence of beta-adrenoceptors.

Actions of 5-HT1 ligands on excitatory synaptic transmission in the hippocampus of alert rats

1. The effects of 5-hydroxytryptamine1 (5-HT1) ligands on excitatory synaptic transmission were examined in the stratum radiatum of the CA1 region of the dorsal hippocampus of alert, gently restrained, rats. 2. 5-HT produced a dose-dependent reduction in the amplitude of the electrically evoked population excitatory postsynaptic potential (e.p.s.p.) when injected directly into the hippocampus via a cannula (dose producing 50% maximum inhibition, ED50 = 0.46 microgram). 3. Direct intrahippocampal (i.h.) application of buspirone (ED50 = 0.29 microgram), gepirone (1 microgram), ipsapirone (1 microgram), BMY 7378 (0.1 microgram) and 5-carboxamidotryptamine (5-CT, 0.02 microgram) mimicked the inhibitory effect of 5-HT. 4. Systemic injection of buspirone (ED50 = 0.88 mg kg-1, i.p.), BMY 7378 (0.01 mg kg-1, i.p.) and RU 24969 (1 mg kg-1, s.c.) also had an inhibitory effect on the amplitude of the e.p.s.p. 5. Injection of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 2 micrograms) and spiroxatrine (1 microgram) i.h. alone had no effect on the e.p.s.p. amplitude but prevented the inhibitory effect of 5-HT. 6. Systemic injection (i.p.) of methysergide (5 mg kg-1) and spiroxatrine (1 mg kg-1) antagonized the inhibitory effect of buspirone whereas pretreatment with ketanserin (1 mg kg-1), trifluoperazine (1 mg kg-1) and idazoxan (1 mg kg-1) had no effect on the response to buspirone. 7. It is concluded that hippocampal synaptic transmission is highly sensitive to the agonist and antagonist properties of 5-HT1 ligands in the alert rat.

Serotonergic involvement in the inhibitory effects of repeated buspirone treatment on synaptic transmission in the hippocampus

The effects of acute and long-term treatment with buspirone on synaptic transmission in the hippocampus were compared in alert rats with chronic indwelling electrodes and cannula. Buspirone produced a transient dose-dependent reduction in the amplitude of the excitatory postsynaptic potential (EPSP) when acutely injected either systemically (0.3-3.0 mg/kg i.p.) or directly into the hippocampus (0.1-1.0 microgram i.h.). Whereas acute application of 0.5 mg/kg i.p. produced a 20% reduction which reversed within 2 h, during long-term treatment with this relatively low dose there was a gradual reduction of the baseline EPSP amplitude which reached a maximum (40%) between days 7-14 and which did not reverse completely until 72 h after the last injection. Intrahippocampal injection of either buspirone or 5-hydroxytryptamine did not have any additional effect during the period of baseline reduction. The 5-HT1A receptor antagonist spiroxatrine (1 mg/kg i.p.) produced a transient reversal of the effect of chronic buspirone. It is concluded that the chronic inhibitory effect of buspirone is probably an extension of its acute action on 5-HT1A receptors in the hippocampus.

Behavioral and neurochemical effects of the serotonin (5-HT)1A receptor ligand spiroxatrine

The effects of spiroxatrine, a putative antagonist with selectivity for the serotonin (5-HT)1A receptor, were compared with compounds believed to function as agonists at the 5-HT1A receptor. Schedule-controlled responding of pigeons was maintained under a multiple 30-response fixed-ratio (FR), 3-min fixed-interval (FI) schedule or under a schedule in which responding was suppressed by electric shock ("conflict" procedure). Under the multiple schedule, spiroxatrine (0.3-1.0 mg/kg) decreased FR responding but did not affect FI responding; responding was decreased in both schedule components at 3.0 mg/kg. When administered alone, buspirone, a compound believed to produce its anxiolytic effects through 5-HT1A agonist actions, produced effects similar to those of spiroxatrine; in combination, the two drugs produced greater effects than when either was administered alone. As with 5-HT1A agonists such as buspirone and 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) in the pigeon, spiroxatrine (0.01-1.0 mg/kg) increased punished responding. Spiroxatrine and buspirone were potent inhibitors of [3H]8-OH-DPAT binding to pigeon cerebral membranes with IC50 values in the nM range. Neurochemical analyses of metabolite changes produced by spiroxatrine in pigeon cerebrospinal fluid showed buspirone-like effects, with increases in MHPG, DOPAC and HVA at doses that decreased 5-HIAA levels. Spiroxatrine dose-dependently blocked the behavioral effects of the dopamine agonist piribedil indicating that, like buspirone, it also is a potent dopamine antagonist. Spiroxatrine most likely functions as an agonist at the 5-HT1A receptor. As with buspirone, however, spiroxatrine has a prominent dopamine antagonist component.

[3H]8-OH-DPAT labels the 5-hydroxytryptamine uptake recognition site and the 5-HT1A binding site in the rat striatum

The binding of [3H]8-hydroxy-2-(di-N-propylamino)-tetralin ([ 3H]8-OH-DPAT) to rat hippocampal and striatal membranes has been compared. In the hippocampus, low concentrations of [3H]8-OH-DPAT bound to a single, high affinity site which was sensitive to inhibition by spiperone, buspirone and ergotamine but not by mianserin, quipazine or (-)-propranolol. This is consistent with a selective labeling of the 5-HT1A receptor. In the striatum, [3H]8-OH-DPAT bound to two sites with high and low affinity (KD's 1.18 and 109 nM). The high affinity component was blocked by low concentrations of buspirone, spiperone and ergotamine. The low affinity component was blocked only by high concentrations of buspirone and spiperone, and was not displaced by ergotamine at concentrations up to 1 microM. The ergotamine-resistant component of striatal [3H]8-OH-DPAT binding was blocked by low concentrations of the 5-HT uptake inhibitors fluvoxamine and paroxetine, and by relatively low concentrations of 5-HT itself. Thus [3H]8-OH-DPAT labels the 5-HT transporter in the rat striatum. Unlike [3H]imipramine binding, the binding of [3H]8-OH-DPAT to the 5-HT transporter was independent of external sodium ions. It is therefore suggested that 8-OH-DPAT acts as substrate for the 5-HT transporter and labels the 5-HT recognition site of the transporter complex.

Centrally acting hypotensive agents with affinity for 5-HT1A binding sites inhibit forskolin-stimulated adenylate cyclase activity in calf hippocampus

1. A number of centrally acting hypotensive agents and other ligands with high affinity for 5-hydroxytryptamine1A (5-HT1A) recognition sites have been tested on forskolin-stimulated adenylate cyclase activity in calf hippocampus, a functional model for 5-HT1A-receptors. 2. Concentration-dependent inhibition of forskolin-stimulated adenylate cyclase activity was elicited by the reference 5-HT1-receptor agonists (mean EC50 value, nM): 5-HT (22), 5-carboxamidotryptamine (5-CT, 3.2), 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, 8.6), N,N-dipropyl-5-carboxamidotryptamine (DP-5-CT, 2.3), 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)-piperazine (PAPP or LY 165163, 20), 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H indole (RU 24969, 20), buspirone (65) and ipsapirone (56). Emax amounted to 18-20% inhibition for all but the latter two agonists (14%). 3. The following hypotensive agents with high affinity for 5-HT1A sites were potent agonists in this system (mean EC50 value, nM): flesinoxan (24), indorenate (99), erythro-1-(1-[2-(1,4-benzodioxan-2-yl)-2-hydroxyethyl]-4-piperidyl )- 2-benzimidazolinone (R 28935, 2.5), urapidil (390) and 5-methyl-urapidil (3.5). The first two agents were full agonists, whereas the latter three acted as partial agonists with 60-80% efficacy. 4. Metergoline and methysergide behaved as full agonists and cyanopindolol as a partial agonist with low efficacy. Spiroxatrine and 2-(2,6-dimethoxyphenoxyethyl)aminomethyl- 1,4-benzodioxane (WB 4101) which bind to 5-HT1A sites with nanomolar affinity, were agonists and inhibited potently forskolin-stimulated adenylate cyclase in calf hippocampus, showing mean EC50 values of 23 and 15 nM, respectively. Spiroxatrine and WB 4101 yielded 90% and 50% efficacy, respectively. 5. Spiperone and methiothepin (each 1 microM) caused rightward shifts of the concentration-effect curve to 8-OH-DPAT, without loss of the maximal effect, as did the partial agonist cyanopindolol (0.1 microM) and the (-)- and (+)-enantiomers of pindolol (1 microM and 0.1 mM, respectively). 6. There was an excellent correlation (r = 0.90, P = 0.0001) between the pEC50 values (ranging from 6.4 to 8.7) of the 19 agonists tested at adenylate cyclase and their pKD for 5-HT1A recognition sites. Apparent pKB values of antagonists at adenylate cyclase and their pKD values for 5-HT1A binding sites were also significantly correlated. 7. This study further indicates that the 5-HT1A recognition site and the 5-HT receptor mediating inhibition of adenylate cyclase in hippocampus are the same. The data show that a number of centrally acting hypotensive agents with high affinity for the 5-HT1,A site are potent agonists in this model, suggesting an involvement of central 5-HTIA-receptors in the control of blood pressure.

Chronic MAO A and MAO B inhibition decreases the 5-HT1A receptor-mediated inhibition of forskolin-stimulated adenylate cyclase

The effect of chronic administration of various monoamine oxidase (MAO) inhibitors on the ability of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to inhibit forskolin-stimulated adenylate cyclase activity was studied. Groups of 12 rats were given either saline, (E)-beta-fluoromethylene-m-tyrosine (MDL 72394 0.25 mg/kg p.o.), clorgyline (1 mg/kg p.o.), selegiline (1 mg/kg p.o.) or tranylcypromine (5 mg/kg p.o.) once a day for 21 days. Biochemical determinations were made 72 h after the final dose. MDL 72394 and tranylcypromine produced a nonselective inhibition of MAO but clorgyline and selegiline selectively inhibited MAO A and MAO B respectively. All treatments that inhibited MAO A also increased tissue levels of 5-HT. Chronic treatment with MDL 72394, clorgyline or tranylcypromine reduced the ability of 8-OH-DPAT to inhibit forskolin-stimulated adenylate cyclase activity. These data suggest that chronic nonselective and chronic MAO A inhibition causes a down-regulation of the 5-HT1A-mediated inhibition of forskolin-stimulated adenylate cyclase activity.