Dysfunction of the glutamatergic photoreceptor synapse in the P301S mouse model of tauopathy.

Tauopathies, including Alzheimer's disease, are characterized by retinal ganglion cell loss associated with amyloid and phosphorylated tau deposits. We investigated the functional impact of these histopathological alterations in the murine P301S model of tauopathy. Visual impairments were demonstrated by a decrease in visual acuity already detectable at 6 months, the onset of disease. Visual signals to the cortex and retina were delayed at 6 and 9 months, respectively. Surprisingly, the retinal output signal was delayed at the light onset and advanced at the light offset. This antagonistic effect, due to a dysfunction of the cone photoreceptor synapse, was associated with changes in the expression of the vesicular glutamate transporter and a microglial reaction. This dysfunction of retinal glutamatergic synapses suggests a novel interpretation for visual deficits in tauopathies and it highlights the potential value of the retina for the diagnostic assessment and the evaluation of therapies in Alzheimer's disease and other tauopathies.

Stereospecific blockade of marble-burying behaviour in mice by selective, non-peptidergic neurokinin1 (NK1) receptor antagonists.

By analogy with the selective serotonin reuptake inhibitor, fluvoxamine, and the tricyclic agent, clomipramine, the novel, selective, non-peptidergic NK(1) receptor antagonist, GR205,171, dose-dependently and completely blocked marble-burying behaviour in mice: Inhibitory Dose(50)s (ID(50)s), 4.5, 4.8 and 7.6 mg/kg, respectively. In contrast to GR205,171, its isomer, GR226,206, which displays substantially lower affinity for NK(1) receptors, was inactive (> 40.0 mg/kg). By analogy with GR205,171, a further, selective NK(1) antagonist, RP67,580, abolished marble-burying behaviour with an ID(50) of 11.9 mg/kg. At doses significantly reducing marble-burying behaviour, GR205,171 and RP67,580 little influenced motor behaviour. In conclusion, like fluvoxamine and clomipramine, selective, non-peptidergic NK(1) receptor antagonists block marble-burying in mice. Although the biological bases of this behaviour remain unclear, these observations underpin the contention that NK(1) receptors may be implicated in affective disorders.

S18616, a highly potent, spiroimidazoline agonist at alpha(2)-adrenoceptors: I. Receptor profile, antinociceptive and hypothermic actions in comparison with dexmedetomidine and clonidine.

S18616 ((S)-spiro[(1-oxa-2-amino-3-azacyclopent-2-ene)-4, 2'-(8'-chloro-1',2',3',4'-tetrahydronaphthalene)]) displayed high affinity at native rat alpha(2)-adrenoceptors (AR)s (pK(i), 9.8), native human (h)alpha(2A)-ARs (9.6), and cloned halpha(2A)- (9.5), halpha(2B)- (9.2), and halpha(2C)- (9.0) ARs. It showed 40-fold lower affinity for halpha(1A)-ARs (8.4) and >/=100-fold lower affinity for rat alpha(1)-ARs (7.1), halpha(1B)-ARs (7.7), halpha(1D)-ARs (7.6), imidazoline(1) (7.4), and imidazoline(2) (7.4) sites and >100-fold lower affinity for all other (>50) sites. At halpha(2A)-ARs, in guanosine-5'-O-(3-[(35)S]thio)triphosphate binding studies, S18616 was a potent (partial) agonist: log effective concentration (pEC(50)), 9.3/maximal effect, 51%. This observation was corroborated employing a halpha(2A)-Gi1alpha fusion protein/GTPase assay (9.0/40%) in which the actions of S18616 were blocked by pertussis toxin. Employing guanosine-5'-O-(3-[(35)S]thio)triphosphate binding assays, S18616 was also a partial agonist at halpha(2C)-ARs (8.2/63%) but a full agonist (8.4/124%) at halpha(2B)-ARs. At halpha(2A)-, halpha(2B)-, and halpha(2C)-ARs, the selective alpha(2)-AR antagonist, atipamezole, abolished the actions of S18616: pK(b) values of 9.1, 9. 1, and 9.4, respectively. As determined by depletion of membrane-bound [(3)H]phosphatidyl inositols, S18616 behaved as a (less potent) agonist (7.8/79%) at halpha(1A)-ARs, an action abolished by prazosin (pK(b), 8.9). Reflecting alpha(2)-AR agonist properties, S18616 potently (>/=1 microg/kg, s.c.) and dose dependently elicited hypothermia and antinociception (nine diverse models) in rodents. These actions were dose dependently inhibited by chemically diverse alpha(2)- versus alpha(1)-AR antagonists, atipamezole, idazoxan, RX821,002, and BRL44418 (a preferential alpha(2A)-AR ligand). In contrast, the actions of S18616 were unaffected by the alpha(1)-AR antagonists, ARC239 and prazosin (which preferentially block alpha(2B/2C)- versus alpha(2A)-ARs). Although the affinity of dexmedetomidine at alpha(2)-ARs was lower than S18616; it displayed a similar receptor and functional profile. Clonidine displayed lower efficacy than S18616, was substantially less potent, and had marked affinity for imidazoline(1) sites and alpha(1)-ARs. In conclusion, S18616 is a novel, selective, and highly potent agonist at alpha(2)-ARs.

Discriminative stimulus properties of the dopamine D3 receptor agonists, PD128,907 and 7-OH-DPAT: a comparative characterization with novel ligands at D3 versus D2 receptors.

Rats were trained to recognize a discriminative stimulus (DS) elicited by the preferential dopamine D3 receptor agonists, PD128,907 (0.16 mg/kg, i.p.) and 7-OH-DPAT (0.16 mg/kg, i.p.). PD128,907 and 7-OH-DPAT showed "full" (> or = 80%) and mutual generalization. Chemically-diverse, preferential D3 versus D2 agonists, quinelorane, CGS15855A, pramipexole, ropinirole and piribedil, generalized to PD128,907 (and 7-OH-DPAT) in this order of potency, which correlated more strongly with affinity/activity at cloned human (h)D3 (r=0.68/0.81, n=7) than hD2 (0.27/0.64, n=7) receptors. Further, generalization potency strongly correlated with potency for suppression of response rates (0.86), induction of hypothermia (0.92), reduction of striatal dopamine turnover (0.92) and diminution of immobility in a forced-swim procedure (0.97). Nafadotride, UH232 and AJ76, which show a mild preference for D3 versus D2 sites, blocked the PD128,907 DS, and the modestly-selective D3 antagonist, U99194A, was partially effective. Both nafadotride and U99194A blocked the 7-OH-DPAT DS. However, antagonist potency (n=4) versus PD128,907 correlated better with affinity at D2 (0.89) versus D3 (0.27) sites. Further, whereas the preferential D2 versus D3 antagonist, L741,626, antagonized the PD128,907 DS, the selective D3 antagonists, S11566, S14297 (its eutomer) and GR218,231 were ineffective against PD128907 and 7-OH-DPAT DS. S11566 and GR218,231 likewise did not generalize to PD128,907. In conclusion, under the present conditions, D2 receptors are principally implicated in the DS properties of PD128,907 and 7-OH-DPAT.

5-HT2C receptors are involved in the discriminative stimulus effects of citalopram in rats.

Rats were trained on a fixed ratio 10, food-reinforced schedule to recognize a discriminative stimulus (DS) elicited by the selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram (2.5 mg/kg, IP). The preferential, high efficacy agonist at 5-HT2C receptors, Ro60-0175, dose-dependently generalized to citalopram with an ED50 of 0.3 mg/kg, IP. Further, the selective 5-HT2C receptor antagonist, SB242,084, dose-dependently (ED50=0.1 mg/kg, IP) blocked the citalopram DS. These data suggest that 5-HT2C receptors are involved in the DS properties of the SSRI, citalopram, in rats. They do not, however, exclude a potential role of other 5-HT receptor types.

Discriminative stimulus properties of the novel serotonin (5-HT)2C receptor agonist, RO 60-0175: a pharmacological analysis.

Employing a Fixed-Ratio 10, food-reinforced protocol, rats were trained to recognize the discriminative stimulus (DS) properties of the novel, potent, 5-HT2C agonist, Ro 60-0175 (2.5 mg/kg, i.p.). This schedule generated appropriate drug versus vehicle responding after 50 + 5 training sessions and Ro 60-0175 elicited full (100%) drug selection with an effective dose50 (ED50) of 0.6 mg/kg, i.p.. The 5-HT2C receptor agonists, mCPP and MK 212, fully generalized to Ro 60-0175 with ED50s of 0.8 and 0.4 mg/kg, s.c., respectively, whereas the preferential 5-HT2B agonist, BW 723C86 ( > 10.0 mg/kg, s.c.) and the 5-HT2A agonist, DOI ( > 2.5 mg/kg, s.c.), were ineffective. The 5-HT2A/2B/2C receptor antagonist, mianserin, dose-dependently blocked the DS properties of Ro 60-0175 with an ED50 of 0.7 mg/kg, s.c. This action was mimicked by the novel, 5-HT2B/2C antagonist, SB 206,553 (ED50 = 0.3 mg/kg, s.c.), whereas the selective 5-HT2A antagonist, MDL 100,907 ( > 0.63 mg/kg, s.c.), was ineffective. Further, the selective 5-HT2C antagonist, SB 242,084, dose-dependently and fully blocked drug selection (ED50 = 0.2 mg/kg, i.p.), whereas the selective 5-HT2B antagonist, SB 204,741, was not active ( > 0.63 mg/kg, i.p.). In conclusion, these data demonstrate that Ro 60-0175 generates a robust DS and suggest that activation of 5-HT2C receptors is the principal mechanism underlying its DS properties.

Citalopram elicits a discriminative stimulus in rats at a dose selectively increasing extracellular levels of serotonin vs. dopamine and noradrenaline.

Citalopram (2.5 mg/kg, i.p.) increased (+145-+180%) extracellular levels of serotonin (5-hydroxytryptamine, 5-HT) in the frontal cortex, nucleus accumbens and striatum of freely-moving rats, whereas dopamine and noradrenaline were unaffected. At this dose, employing a two-lever, food-reinforced, drug discrimination procedure, citalopram generated reliable recognition and fully (> 80%) generalized to itself with an Effective Dose50 (ED50) of 0.1 mg/kg, s.c. Two further selective 5-HT reuptake inhibitors, sertraline and paroxetine, fully generalized with ED50s of 0.01 and 0.04 mg/kg, s.c., respectively. In contrast, the anxiolytic, diazepam (0.63), and the antipsychotic, clozapine (2.5), did not (< or = 20%) generalize. In conclusion, the selective 5-HT reuptake inhibitor, citalopram, elicits a pharmacologically-specific discriminative stimulus in rats at a dose selectively elevating extracellular concentrations of 5-HT.

8-OH-DPAT-induced spontaneous tail-flicks in the rat are facilitated by the selective serotonin (5-HT)2C agonist, RO 60-0175: blockade of its actions by the novel 5-HT2C receptor antagonist SB 206,553.

The 5-HT1A receptor agonist, 8-OH-DPAT ((+/-)-8-dihydroxy-2-(di-n-propylamino) tetralin), (0.63 mg/kg, s.c.) elicited spontaneous tail-flicks (STFs) in rats. This response was potentiated by the selective 5-HT2C receptor agonist, RO 60-0175 ((S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine) fumarate) (0.16 mg/kg, s.c.), the action of which was abolished by the novel 5-HT2C antagonist, SB 206,553 (5 methyl-1-(3-pyridil-carbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3 -f]indole) (0.16 mg/kg, s.c.). These data show that 5-HT1A receptor-mediated STFs in rats are facilitated by activation of 5-HT2C receptors supporting the existence of functional interactions between these sites.

Pro- and antinociceptive actions of serotonin (5-HT)1A agonists and antagonists in rodents: relationship to algesiometric paradigm.

In mice injected with formalin into the hindpaw, the 5-HT1A receptor agonists, 8-OH-DPAT and flesinoxan, equipotently inhibited the early phase (EP) and late phase (LP) of licking. At higher doses, they provoked ataxia and inhibited the writhing elicited by intra-abdominal acetic acid. The antagonists, (-)-alprenolol, (-)-tertatolol, WAY-100,135 and S 15931 were more potent against the LP than the EP. They also inhibited writhing, and only at very high doses did they elicit ataxia. In rats, 8-OH-DPAT and flesinoxan increased the current required to elicit vocalisation upon electrical stimulation of the tail. The action of 8-OH-DPAT was blocked by WAY-100,135, which, like other antagonists, was inactive alone. Interestingly, a low dose of 8-OH-DPAT partially inhibited the antinociceptive action of the mu-opioid agonist, morphine, the action of which was dose-dependently facilitated by (-)-alprenolol and S 15931. Administered s.c., 8-OH-DPAT elicited spontaneous tail-flicks (STFs) in rats: these were abolished by WAY-100,135, (-)-tertatolol, (-)-alprenolol and S 15931. STFs were also eliminated by s.c. or i.t. administration of the alpha 2-adrenergic receptor agonist, clonidine, the GABAA agonist, muscimol or the GABAB agonist, baclofen. The mu-opioid, morphine, blocked STFs only at high doses and the kappa-opioid agonists, U 50,488 and U 69,593, even at supra-ataxic doses, were inactive. Antagonists at neurokinin (NK)1 (RP 67580), NK2 (SR 48,968) and bradykinin (BK)2 (Hoe 140) receptors, as well as aspirin, did not block STFs, though indomethacin was effective. Antagonists at the glycine B site coupled to the NMDA receptor, L 687,414, L 701,324 and (+)-HA966, blocked STFs. Furthermore, (+)-HA 966 and the competitive NMDA receptor antagonist, CPP, were active upon i.t. administration. STFs were also blocked by s.c. or i.t. administration of the AMPA antagonists, YM 900 and NBQX. In conclusion, the influence of 5-HT1A ligands upon nociception is dependent upon the algesiometric paradigm. Intriguingly, modulation of 5-HT1A receptor-mediated STFs reveals parallels to neuropathic pain.

Functional correlates of dopamine D3 receptor activation in the rat in vivo and their modulation by the selective antagonist, (+)-S 14297: 1. Activation of postsynaptic D3 receptors mediates hypothermia, whereas blockade of D2 receptors elicits prolactin secretion and catalepsy.

Using [125I]-iodosulpride as a radioligand, the novel naphthofurane, (+/-)-S 11566 [(+/-)-[7-(N,N-dipropylamino)-5,6,7,8-tetra-hydro- naphtho(2,3b)dihydro,2,3-furane]) showed a marked preference for human, recombinant D3 as compared with D2 receptors stably transfected into Chinese hamster ovary cells (Kis = 24/529 nM). This activity resided in its (+)-eutomer, (+)-S 14297 (13/297 nM) as compared with its (-)-distomer, (-)-S 17777 (406/3544 nM). In contrast, (+)-AJ 76 manifested only a mild 2-fold preference for D3 sites (70/154 nM), whereas haloperidol and six additional antagonists showed a mild (2-7-fold) preference for D2 sites. As concerns agonists, (+)-7-OH-DPAT, (+/-)-CGS 15855A, quinelorane, (-)-quinpirole and N-0434 displayed a preference (6-40-fold) for D3 receptors, whereas piribedil showed a slight, 2-fold, preference for D2 sites (243/126 nM). (+)-S 14297 showed low (> 1.0 microM) affinity at rat D1 and D2 sites and at cloned, human D4 and D5 receptors and only low affinity (145 to > 10,000 nM) at all other sites examined. In vivo, administered s.c., (+)-7-OH-DPAT, CGS 15855A, quinelorane, (-)-quinpirole and N-0434 potently evoked hypothermia. Across all (8) agonists tested, potency correlated significantly with affinity at D3 sites (r = .84, P < .001) but not D2 sites (r = .50, P > .05). (+)-S 14297 (0.16-1.25 mg/kg, s.c.) blocked the induction of hypothermia by (+)-7-OH-DPAT, CGS 15855A and (-)-quinpirole, but not by the alpha 2-adrenergic agonist, clonidine, without influencing core temperature alone. In contrast, (-)-S 17777 (10.0 mg/kg, s.c.) was only partially active. Across all (9) antagonists, potency for inhibition of (+)-7-OH-DPAT-induced hypothermia correlated more strongly with affinity at D3 (r = .96, P < .001) than D2 (r = .75, P < .02) sites. Whereas haloperidol and the other antagonists provoked prolactin secretion and elicited catalepsy, (+)-S 14297 and (+/-)-S 11566 at doses of up to 10.0 and 40.0 mg/kg, s.c., respectively, were not significantly effective (P > .05). Across all antagonists, potency for eliciting prolactin secretion and catalepsy correlated better with affinity at D2 (r = .95 and .96) than D3 (r = .76 and .91) sites. In conclusion, these data demonstrate that the novel naphtofurane, (+)-S 14297, is a selective ligand (antagonist) at dopamine D3 receptors and suggest that their activation mediates hypothermia in the rat.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of potent and selective antagonists at postsynaptic 5-HT1A receptors in a series of N4-substituted arylpiperazines.

Benzocycloalkyl and benzocycloalkenyl moities linked, directly or via an alkyl chain, to oxygen-bearing heteroarylpiperazines were synthesized, in an attempt to obtain potent and selective antagonists at postsynaptic 5-HT1A receptors. From the numerous arylpiperazines described in the literature, 1-(2,3-dihydro-1,4-benzodioxin-5-yl)piperazine (3a) was chosen as a model of an arylpiperazine in view of its selectivity for 5-HT1A receptors versus alpha 1-, alpha 2-, and beta-adrenergic receptors, as well as dopamine D1 and D2 receptors. Two other closely-related arylpiperazines, 1-(1,5-benzodioxepin-6-yl)piperazine (3b) and 1-(benzofuran-7-yl)piperazine (3c), were also examined in this study. All compounds showed high affinity at 5-HT1A sites (8.10 < or = pKis < or = 9.35), and the majority behaved as antagonists in vivo in blocking the hypothermia induced by the 5-HT1A agonist 8-OH-DPAT in the absence of a marked effect alone at equivalent doses. An in vivo evaluation of dopamine D2 receptor antagonist properties revealed that the majority of compounds was devoid of activity at this site, in marked contrast to BMY 7378 which displayed virtually no selectivity for 5-HT1A versus dopamine D2 receptors. Moreover, six compounds of the present series, 8, 10, 11, 14, 25, and 37, showed > 10-fold selectivity in vitro for 5-HT1A versus alpha 1-adrenergic receptors. Compound 14 displayed an optimal compromise between potency (pKi = 8.75), marked antagonist activity, and selectivity toward alpha 1-adrenergic (81-fold) and dopamine D2 (195-fold) receptors. These characteristics clearly distinguish 14 from previously-reported ligands such as the postsynaptic 5-HT1A antagonist BMY 7378 and the weak partial agonist NAN 190 which, in contrast to the compounds of this series, belong to the well-exemplified class of imido derivatives of (o-methoxyphenyl)piperazines. The availability of 14 (S 15535) should facilitate the further elucidation of the functional role and potential therapeutic significance of 5-HT1A receptors.

Antinociceptive profiles of non-peptidergic neurokinin1 and neurokinin2 receptor antagonists: a comparison to other classes of antinociceptive agent.

This study compared the antinociceptive properties of systemic administration of selective, non-peptidergic antagonists at neurokinin (NK1 and NK2) receptors to those of other classes of antinociceptive agent. (All doses are in mg/kg.) In mice, the NK1 antagonist, CP 99,994, preferentially (inhibitory dose50 (ID50) = 4.4) inhibited the late phase (LP) as compared to the early phase (EP) (16.1) of formalin-induced licking (FIL). A high dose (17.6) elicited ataxia in the rotarod test. Acetic acid-induced writhing was reduced at intermediate doses (10.0) whereas the tail-flick (TF) response to thermal and mechanical stimuli was inhibited only at high doses (22.7 and 17.7, respectively). Modulation of stimulus intensity did not modify the influence of CP 99,994 upon the response to heat. A similar pattern of data was acquired with RP 67,580, although this NK1 antagonist more potently inhibited writhing (2.8). In contrast, RP 68,651, the inactive isomer of RP 67,580, neither reduced the LP of FIL nor modified writhing indicating that these actions of RP 67,580 were stereospecific. Three further NK1 antagonists, SR 140,333, WIN 51,708 and WIN 62,577, likewise inhibited the LP of FIL and failed to modify the TF response at non-ataxic doses. Further, SR 140,333 (0.5) and WIN 51,708 (1.4) were potent ligands in the writhing procedure. The NK2 antagonist, SR 48,966, mimicked NK1 antagonists in preferentially inhibiting the LP (7.7) as compared to the EP (26.9) of FIL. Further, only at doses higher than those evoking ataxia (20.9) did SR 48,968 modify the TF response (36.5 and 32.0 for heat and pressure, respectively). However, it differed to NK1 antagonists in being inactive in the writhing test (> 40.0). In comparison to these NK1 and NK2 antagonists, the mu-opioid agonists (morphine and fentanyl) and kappa-opioid agonists (enadoline and U 69,593) equipotently inhibited all nociceptive responses at doses not provoking ataxia. While the glycine B receptor partial agonist, (+)-HA 966, selectively blocked the LP of FIL and did not evoke ataxia, the NMDA receptor channel blocker, (+)-MK 801, elicited antinociception only at doses close to those provoking ataxia. Finally, the NSAIDs, indomethacin and ibuprofen, the BK2 antagonist, Hoe 140 and the nitric oxide synthase (NOS) inhibitors, L-NAME and 7 nitroindazole, inhibited the LP (but not the EP) of FIL and (except for L-NAME) also reduced writhing: in contrast, they did not evoke ataxia and were inactive in the TF procedures.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple alpha-2 adrenergic receptor subtypes. II. Evidence for a role of rat R alpha-2A adrenergic receptors in the control of nociception, motor behavior and hippocampal synthesis of noradrenaline.

In this study, we attempted to identify of the subtype(s) of alpha-2 adrenergic receptor (AR) involved in the control of motor behavior, nociception and the hippocampal synthesis of noradrenaline (NA) in the rat. The high efficacy alpha-2 AR agonists, xylazine and UK 14,304 [5-bromo-6-[2-imidazolin-2-yl-amino]quinoxaline], inhibited striatal accumulation of L-dopa in rats pretreated with NSD 1015 (an inhibitor of aromatic amino acid-decarboxylase), elicited a loss of the righting reflex in rats, provoked ataxia in the rotarod test in mice and elicited antinociception in the writhing and hot-plate tests in mice. Guanfacine and guanabenz, agonists acting preferentially at rat alpha-2A (R alpha-2A)/human alpha-2A (H alpha-2A) AR, mimicked the antinociceptive and motor actions of xylazine and UK 14,304 and likewise inhibited NA synthesis. The preferential R alpha-2A/H alpha-2A AR antagonist, [2-(2H-(1-methyl-1, 3-dihydroisoindole)methyl)-4, 5-dihydro-imidazole (BRL 44408), enhanced hippocampal synthesis of NA and blocked the antinociceptive and motor effects of UK 14,304, xylazine, guanfacine and guanabenz. Similarly, fluparoxan and des-fluorofluparoxan, preferential antagonists at R alpha-2A AR as compared to H alpha-2A AR, were highly active. In contrast, the preferential alpha-2B/alpha-2C AR antagonists, ARC 239 [2-(2-(4-o-methoxyphenyl)piperazine-1-yl)-ethyl)-4,4-dimethyl-1,3- (2H,4H)-isoquinolinedione] prazosin, corynanthine, spiroxatrine and [1,2-dimethyl-2,3,9,13-betetrahydro-1H-dibenzo(c,f)- imidazo(1,5-a)azepine (BRL 41992)], as well as the preferential H alpha-2A AR antagonist, [2-(2,6-dimethoxyphenoxyethyl)- aminomethyl-1,4-benzodioxane] (WB 4101), were only weakly active. Based on the actions of a total of 16, structurally diverse alpha-2 AR antagonists, a correlation matrix was constructed. This revealed a strong correlation among the tests (median r = 0.82) and allowed for a comparison between drug potency in inhibiting these alpha-2 AR-mediated actions and affinity at various populations of alpha-2 AR subtypes (see companion paper). Correlations for potency in the two motor tests were pronounced with R alpha-2A sites (0.85), modest with H alpha-2A sites (0.60) and alpha-2B sites (0.58) and poor with alpha-2C sites (0.35). For the two antinociceptive tests, correlations were likewise pronounced with R alpha-2A sites (0.80) but less marked with H alpha-2A sites (0.73), alpha-2B sites (0.62) and alpha-2C sites (0.62).(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of hypothermia as a model of 5-hydroxytryptamine1A receptor-mediated activity in the rat: a pharmacological characterization of the actions of novel agonists and antagonists.

In this study, we examined the localization of the 5-hydroxytryptamine (5-HT)1A receptors mediating hypothermia in the rat, evaluated the pharmacological specificity of this response and examined the influence of a series of novel 5-HT1A receptor ligands upon core temperature. Administered s.c., 8-hydroxy-(2-di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), an agonist at both pre- and postsynaptic 5-HT1A receptors, elicited pronounced hypothermia. In contrast, BMY 7378, which shows low efficacy at postsynaptic 5-HT1A receptors but high efficacy at presynaptic 5-HT1A receptors, elicited only mild hypothermia. Similarly, 8-OH-DPAT was more efficacious than BMY 7378 in eliciting corticosterone secretion, a response mediated by postsynaptic 5-HT1A receptors, whereas BMY 7378 was as efficacious as 8-OH-DPAT in inhibiting striatal accumulation of 5-hydroxytryptophan, a response mediated by presynaptic 5-HT1A receptors. These data suggest, by analogy, that postsynaptic 5-HT1A receptors mediate hypothermia, an interpretation supported by the observation that destruction of central 5-HT neurons with 5,7-dihydroxytryptamine failed to reduce 8-OH-DPAT-induced hypothermia (DIH). Agonists at 5-HT1B, 5-HT1C, 5-HT2 and/or 5-HT3 receptors did not elicit hypothermia, and drugs releasing 5-HT elicited hyperthermia. In contrast, DIH was fully mimicked by the novel 5-HT1A receptors agonists, eltoprazine, WY 48,723, MDL 72832, tandospirone, S 14671, S 14506 and WY 50,324, whereas the novel partial agonist, zalospirone, was less efficacious. DIH was blocked by (-)-alprenolol, (+/-)-pindolol and the novel beta-blocker, (-)-tertatolol, which also has high affinity for 5-HT1A receptors; in distinction, betaxolol and ICI 118,551, antagonists at beta-1 and beta-2 adrenoceptors, respectively, were inactive. Spiperone, NAN-190 and BMY 7378 also inhibited DIH whereas ritanserin, SCH 39166, raclopride and prazosin, antagonists at 5-HT2 receptors, D1 and D2 dopamine receptors and alpha-1 adrenoceptors, respectively, were inactive. The novel 5-HT1A antagonists, WAY 100,135, MDL 73005 EF and (very potently) SDZ 216-525 all blocked DIH. Potency for induction of hypothermia and inhibition of DIH correlated well with affinity for 5-HT1A binding sites. In conclusion, hypothermia is a highly specific and sensitive response to activation of postsynaptic 5-HT1A receptors. Furthermore, DIH is inhibited by their selective blockade. At postsynaptic 5-HT1A receptors mediating hypothermia, eltoprazine, WY 48,723, MDL 72832 and tandospirone are agonists, zalospirone is a partial agonist and (-)-tertatolol, WAY 100,135, MDL 73005 EF and SDZ 216-525 are antagonists.

S 14671: a naphtylpiperazine 5-hydroxytryptamine1A agonist of exceptional potency and high efficacy possessing antagonist activity at 5-hydroxytryptamine1C/2 receptors.

The interaction at 5-hydroxytryptamine (5-HT) receptors of the novel naphtylpiperazine, S 14671 (1-[2-(2-thenoylamino)ethyl]-4[1-(7- methoxynaphtyl)]piperazine), was compared to that of the 5-HT1A ligands, 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), WY 50,324 [N-(29(4-(2-pyrimidinyl)-1-piperazinyl)ethyl)tricyclo(3.3.1.1(3,7) )- decane-1-carboxamide], (+)-flesinoxan, buspirone and BMY 7378 [(8-[2-[4-(2-methoxyphenyl)- 1-piperazinyl]ethyl]-8-azaspirol[-4-]-decane-7,9-dione 2HCl]. S 14671 showed a very high affinity for 5-HT1A sites (pKi, 9.3) as compared to the reference ligands (pKi values, 9.2, 8.7, 8.7, 7.9 and 8.7, respectively). S 14671 bound in an apparently competitive manner and, in distinction to the reference compounds, possessed a Hill Coefficient (1.4) significantly superior to 1. Although showing low affinity at 5-HT1B and 5-HT3 sites, S 14671 displayed significant affinity at both 5-HT1C and 5-HT2 sites; pKi, 7.8 in each case. Furthermore, S 14671 acted as an antagonist of 5-HT-stimulated phosphoinositide turnover in rat choroid plexus (5-HT1C) and cortex (5-HT2). In vivo, upon s.c. administration, S 14671 acted as a high efficacy agonist in models of 5-HT1A receptor-mediated activity: induction of flat-body posture, spontaneous tail-flicks, hypothermia and corticosterone secretion and inhibition of morphine-induced antinociception. In every test, S 14671 was the most potent compound: it was active at doses as low as 5 micrograms/kg s.c. Relative potency across all tests was S 14671 greater than 8-OH-DPAT greater than WY 50,324 greater than (+)-flesinoxan greater than buspirone with BMY 7378 too weak for comparison to be meaningful. The action of S 14671 in 5-HT1A tests was blocked by BMY 7378 and the 5-HT1A antagonist, (-)-alprenolol, but unaffected by the 5-HT1C/2 antagonist, ritanserin, and the 5-HT3 antagonist, ondansetron. Activation of postsynaptic 5-HT1A receptors was confirmed in 5,7-dihydroxytryptamine-lesioned rats, in which the potency of S 14671 to elicit spontaneous tail-flicks was potentiated. Activation of presynaptic receptors was demonstrated by inhibition of the electrical activity of the dorsal raphe nucleus with the following order of relative potency: S 14671 greater than 8-OH-DPAT greater than WY 50,324 greater than BMY 7378 greater than buspirone. Spiperone, which acts as a pure 5-HT1A antagonist at raphe 5-HT1A receptors, blocked the action of S 14671. In conclusion, S 14671 is a structurally novel ligand manifesting high efficacy and exceptional potency at both pre- and postsynaptic 5-HT1A receptors.(ABSTRACT TRUNCATED AT 400 WORDS)