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.     

Cardiovascular response to 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) in the rat: site of action and pharmacological analysis

The cardiovascular response to 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a selective putative 5-HT1A receptor agonist, has been investigated in the rat. Comparisons were made with clonidine, a centrally acting hypotensive agent with negligible affinity for 5-HT receptors. In conscious, spontaneously hypertensive (SH) rats, 8-OH-DPAT caused dose-related and sustained falls in blood pressure and heart rate that were unaffected by depletion of brain 5-HT by p-chlorophenylalanine. 8-OH-DPAT caused hypotension and bradycardia in anesthetized normotensive rats. In pithed rats, 8-OH-DPAT neither lowered blood pressure nor affected the cardiovascular response to spinal sympathetic stimulation or to phenylephrine. The response to 8-OH-DPAT was blocked selectively by intracisternal injection of 8-methoxy-2-(N-2-chloroethyl-N-n propyl) amino tetralin (8-MeO-CIEPAT), a putative irreversible 5-HT1A receptor antagonist, and was abolished in animals whose central monoamine transmitter stores were depleted selectively by combined treatment with DL-alpha-monofluoromethyl-dopa and dopamine. The cardiovascular response to 8-OH-DPAT was inhibited selectively by metergoline, methiothepin, and 8-MeO-CIEPAT; it was nonselectively inhibited by (+/-)-pindolol, (+/-)-cyanopindolol, buspirone, yohimbine, idazoxan, and WY 26392; and was unaffected by prazosin and cis-flupenthixol. These results establish that the cardiovascular response to 8-OH-DPAT in the rat is centrally mediated and point to the putative 5-HT1A receptor as the key site involved. An indirect link involving a catecholaminergic mechanism is suggested by the fact that alpha 2-adrenoceptor antagonists are also inhibitory despite 8-OH-DPAT having no direct agonist effects at alpha 2-adrenoceptors per se.     

Involvement of dopamine autoreceptors in the hypoactivity induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in mice.

The effect of 8-OH-DPAT, a 5-HT1A receptor agonist, on the locomotor activity was analyzed in Albino Swiss mice. The studied drug (0.5-5 mg/kg) inhibited the spontaneous locomotor activity in mice. The hypoactivity induced by 8-OH-DPAT (1.5 mg/kg) was abolished by the dopamine (D1 and D2) receptor antagonist-haloperidol (0.00125 and 0.0025 mg/kg, but not in higher doses) and by the D2 antagonist with affinity for 5-HT1A and 5-HT2 receptors-spiperone (0.0025 and 0.005 mg/kg, but not in higher doses). The effect of 8-OH-DPAT was slightly reduced by the alpha 2-adrenoceptor antagonists: idazoxan (4 mg/kg), yohimbine (2 and 4 mg/kg) and rauwolscine (4 mg/kg). On the other hand, the non-selective 5-HT antagonist metergoline (0.5-4 mg/kg), the 5-HT1A antagonist NAN-190 (0.5-2 mg/kg), the beta-adrenoceptor blockers with high affinity for 5-HT1A and 5-HT1B receptors: pindolol and SDZ 21009 (2-8 mg/kg) and the agonist/antagonist of 5-HT1A receptors ipsapirone (2.5 and 5 mg/kg) did not affect the 8-OH-DPAT-induced hypoactivity. The obtained results suggest that the reduction of the spontaneous locomotor activity induced by 8-OH-DPAT results from a stimulation of dopamine autoreceptors, but not 5-HT receptors. Involvement of an alpha 2-adrenergic mechanism cannot be excluded.     

No involvement of 5-HT(7) or 5-HT(1D) receptors in the (R)-8-OH-DPAT-induced depression of the monosynaptic reflex in spinalized rats

(R)-8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) depressed the monosynaptic reflex. This effect was not antagonized by 5-HT(1A) receptor antagonists. We examined whether 5-HT(1D) and 5-HT(7) receptors are involved in (R)-8-OH-DPAT-induced inhibition of the monosynaptic reflex in spinalized rats. Pretreatment with methiothepin and mesulergine, but not clozapine, inhibited (R)-8-OH-DPAT-induced monosynaptic reflex depression. Pretreatment with 2a-(4-phenyl-1,2,3,6-tetrahydropyridal)butyl)-2a,3,4,5-tetrahydrobenzo[c,d]indol-2(1H)-one (DR4004) and (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrolidine (SB-269970), new selective 5-HT(7) receptors antagonists, and N-[methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide (GR127935), a selective 5-HT(1D) receptor antagonist, had no effect on (R)-8-OH-DPAT-induced depression. These results suggested that 5-HT(7) and 5-HT(1D) receptors are not involved in (R)-8-OH-DPAT-induced monosynaptic reflex depression.     

The behavioural, but not the hypothermic or corticosterone, response to 8-hydroxy-2-(DI-n-propylamino)-tetralin, is antagonized by NAN-190 in the rat

The 5-HT1A receptor antagonistic properties of 1-(2-methoxyphenyl)-4-[4-(2-phthalimmido)butyl] piperazine (NAN-190) were studied in rats: its effect on the 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)-induced behavioural syndrome (flat body posture and reciprocal forepaw treading), hypothermia and secretion of corticosterone, i.e. responses mediated by 5-HT1A receptors, were examined. The drug NAN-190 (1-8 mg/kg) antagonized dose-dependently behavioural effects of 8-OH-DPAT (in both non-reserpinized and reserpine-pretreated animals); however, when administered in doses of 0.5-4 mg/kg, it did not affect the hypothermic or the hormonal response to 8-OH-DPAT. However, NAN-190 (1-8 mg/kg) given alone, produced hypothermia and increased the concentration of corticosterone in serum. The latter effects of NAN-190 were not reduced by (-)pindolol or spiperone. Moreover, the NAN-190-induced secretion of corticosterone was not affected by ketanserin, prazosin or yohimbine. The above results indicate that NAN-190 acts as a 5-HT1A receptor antagonist, only in the model of the 8-OH-DPAT-induced behavioural syndrome. The lack of effect of NAN-190 on the hypothermic or corticosterone response to 8-OH-DPAT most probably results from its own action which mimics the effects of 8-OH-DPAT. The mechanisms responsible for the NAN-190-induced hypothermia and secretion of corticosterone are still unknown, though stimulation of 5-HT1A receptors (either effect), 5-HT2 receptors and alpha 1- and alpha 2-adrenoceptors (corticosterone response) seems to be excluded.     

The behavioural effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in mice

The effects of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on the behaviour of mice were studied. 8-OH-DPAT given i.v. in doses greater than 1 mg/kg induced the distinct 5-HT syndrome, including head weaving, hindlimb abduction, forepaw treading and tremor. The 8-OH-DPAT-induced behaviour was not affected by the 5-HT depleter, p-chlorophenylalanine. Reserpine, which depletes monoamines, significantly decreased the head weaving elicited by 8-OH-DPAT, although it did not reduce the other components of the behavioural syndrome. The non-specific 5-HT receptor antagonist, metergoline, attenuated the 8-OH-DPAT-induced behaviour, while the 5-HT2 receptor antagonist, ketanserin, was without effect. In addition, the 5-HT1A receptor antagonist, spiperone, inhibited the 5-HT syndrome elicited by 8-OH-DPAT, while the dopamine receptor antagonist, haloperidol, affected only the head weaving. These results suggest that 8-OH-DPAT-induced behaviour in mice is mediated by the postsynaptic 5-HT1A receptor.     

Characterization of 8-OH-DPAT-induced hypothermia in mice as a 5-HT1A autoreceptor response and its evaluation as a model to selectively identify antidepressants.

1. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) dose-dependently induced hypothermia in mice. 2. The 5-HT1A receptor partial agonists, buspirone, gepirone and ipsapirone, also dose-dependently induced hypothermia. 3. The 8-OH-DPAT temperature response was antagonized by the 5-HT1 receptor antagonists quipazine (2 mg kg-1, i.p.), (+/-)-propranolol (10 mg kg-1, i.p.). (+/-)-pindolol (5 mg kg-1, i.p.), spiroxatrine (0.5 mg kg-1, i.p.) and metitepine (0.05 mg kg-1, i.p.), but not by 5-HT2 (ketanserin) or 5-HT3 (MDL 72222, GR 38032F) receptor antagonists. 4. The response was also antagonized by the dopamine D2 receptor antagonists, haloperidol and BRL 34778. No other catecholamine or muscarinic receptors were involved in mediating the response. 5. Destruction of 5-hydroxytryptamine (5-HT)-containing neurones with the neurotoxin, 5,7-dihydroxytryptamine (75 micrograms, i.c.v.), abolished the response to 8-OH-DPAT indicating that the 5-HT1A receptors involved were located on 5-HT neurones. 6. Chronic antidepressant treatment down-regulated this 8-OH-DPAT response. In addition, chronic administration of anxiolytics and neuroleptics was also effective in this respect. Down-regulation was also observed following repeated administration of 8-OH-DPAT (0.5 mg kg-1, s.c.), (+/-)-pindolol (10 mg kg-1, i.p.) and ketanserin (0.5 mg kg-1, i.p.). 7. In conclusion, these data confirm that 8-OH-DPAT-induced hypothermia is mediated by 5-HT1A autoreceptors. They also indicate that the response involves D2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulation of corticosterone and beta-endorphin secretion in the rat by selective 5-HT receptor subtype activation

Changes in plasma concentrations of corticosterone and beta-endorphin (beta-END) were determined in male rats after treatment with the selective 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) or the non-selective 5-HT agonist 6-chloro-2-(1-piperazinyl)pyrazine (MK-212). The administration of either 8-OH-DPAT or MK-212 increased plasma concentrations of both corticosterone and beta-END in a dose-related manner. The corticosterone and beta-END responses to 8-OH-DPAT were antagonized by spiperone and (-)-pindolol, both of which have been shown to have high affinity for the 5-HT1A binding site. In contrast, antagonist which are selective for the 5-HT2 receptor or non-selective 5-HT antagonists were without effect on the hormone responses to 8-OH-DPAT. The MK-212-induced increase in plasma concentrations of corticosterone and beta-END were not affected by treatment with the 5-HT1A antagonists spiperone and (-)-pindolol. However, the corticosterone and beta-END responses to MK-212 were attenuated by the selective 5-HT2 antagonists ketanserin, ritanserin and altanserin, as well as by the non-selective 5-HT antagonist metergoline. It is concluded that stimulation of either 5-HT1A or 5-HT2 receptors results in an activation of the hypothalamic-pituitary-adrenal axis in the rat.     

Corticosterone selectively attenuates 8-OH-DPAT-mediated hypothermia in mice

The 5-HT1A agonist 8-OH-DPAT produces a hypothermia in mice mediated by somatodendritic 5-HT1A receptors, that is attenuated by antidepressants and corticosterone. The present study investigated if the effect of corticosterone is specific to the serotonergic system or a non-specific effect on thermoregulation. Administration of corticosterone for 3 d had no effect on dopaminergic (apomorphine) or adrenergic (clonidine) hypothermic challenges. However in addition to 8-OH-DPAT, nicotine-induced hypothermia was attenuated by corticosterone. Administration of the selective nicotinic antagonist mecamylamine had no effect on 8-OH-DPAT-induced hypothermia, although nicotine-induced hypothermia was attenuated by the selective 5-HT1A antagonist WAY-100635. This demonstrates a serotonergic-nicotinic interaction in the generation of hypothermia in mice and is consistent with corticosterone selectively attenuating somatodendritic 5-HT1A receptor function.     

Mediation of the discriminative stimulus properties of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) by the putative 5-HT1A receptor

Male Sprague-Dawley rats were trained to discriminate the putative 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) from saline in a 2-lever operant drug discrimination paradigm. The 8-OH-DPAT cue was found to be highly selective; neither the 5-HT receptor agonists, quipazine, LSD, MK 212 and RU 24969, the 5-HT releasing agent, p-chloroamphetamine, nor the alpha 2-adrenoceptor agonist, clonidine, were able to substitute for 8-OH-DPAT in tests of generalization. In contrast, both buspirone and TVX Q 7821, which like 8-OH-DPAT have high affinity and selectivity for the 5-HT1A recognition site, generalized to the 8-OH-DPAT cue in a dose-dependent manner. The discriminative stimulus properties of 8-OH-DPAT were not antagonized by the 5-HT2 receptor antagonist, ketanserin, or the selective beta 1- and beta 2-adrenoceptor antagonists, betaxolol and ICI 118.551, indicating that neither 5-HT2 receptors, nor beta-adrenoceptors play a significant role in the behaviour. However, the 8-OH-DPAT cue was antagonized stereoselectively by pindolol and alprenolol, which have relatively high affinity and stereoselectivity for 5-HT1, but not 5-HT2, recognition sites. Similarly, the capacity of TVX Q 7821 to generalize to the 8-OH-DPAT cue could be blocked by pindolol. In view of the fact that 8-OH-DPAT has negligible affinity for the 5-HT1B site, the above results are consistent with its discriminative stimulus properties being mediated by the putative 5-HT1A receptor. Moreover, agonist activity at central 5-HT1A receptors may be an important mechanism contributing to the anxiolytic properties of buspirone and TVX Q 7821.     

Modulation of 8-OH-DPAT-induced hypothermia by imipramine in rats

The effects of imipramine on 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), the 5-hydroxytryptamine (5-HT)(1A)-receptor full agonist, -induced hypothermia was examined in rats. Single administration of imipramine (30 mg/kg, i.p.) attenuated 8-OH-DPAT-induced hypothermia. This effect of imipramine was blocked by the 5-HT(2A)-receptor antagonist ketanserin. 8-OH-DPAT-induced hypothermia was not altered 24 h after repeated administration of imipramine (1 - 10 mg/kg per day) for 14 days. However, 8-OH-DPAT-induced hypothermia was significantly enhanced in repeated imipramine (10 mg/kg per day)-treated rats that received 8-OH-DPAT plus imipramine 24 h after the final imipramine administration for 14 days. The 5-HT(2A)-receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((+/-)-DOI) attenuated the 8-OH-DPAT-induced hypothermia in drug naive rats. The inhibitory effect of (+/-)-DOI (0.3 mg/kg, s.c.) on 8-OH-DPAT-induced hypothermia was attenuated by repeated administration of imipramine (10 mg/kg per day) for 14 days. These findings suggest that enhancement of the 5-HT(1A) receptors by repeated administration of imipramine may be due to reduction of the inhibitory effects from the 5-HT(2A) receptors to the 5-HT(1A) receptors.     

Counteraction of the rapid tolerance to 8-hydroxy-2-(di-n-propylamino)tetralin-induced hypothermia in rats by activation of the GABAA receptor-chloride channel complex

The effects of compounds that open the GABAA receptor-chloride channel complex on the rapidly developed tolerance to the 8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide(8-OH-DPAT)-induced hypothermia in rats were examined. The test compound was injected 15 min. before 1 mg/kg subcutaneous 8-OH-DPAT or saline and 24 hr later a challenge dose of 0.1 mg/kg subcutaneous 8-OH-DPAT was given. The rectal temperature was measured before the challenge dose and 30, 60, 90 and 120 min. thereafter. The hypothermic effect was calculated as the area under the curve. It was found that all the GABAergic compounds examined significantly counteracted the 8-OH-DPAT-induced tolerance to the hypothermic response: muscimol at 3 mg/kg subcutaneous, diazepam at 1 - 3 mg/kg subcutaneous, pentobarbitone sodium at 20 mg/kg subcutaneous, and chlormethiazole at 40 mg/kg subcutaneous. Combined treatment of the rats with the GABAA receptor antagonist, bicucculine, or the GABAA receptor-chloride channel blocker, picrotoxin and diazepam, pentoparbitone sodium or chlormethiazole significantly antagonised this counteraction of the 8-OH-DPAT-induced tolerance. Depletion of 5-HT by pretreatment of the rats with the tryptophan hydroxylase inhibitor p-chlorophenylalanine did not counteract the 8-OH-DPAT-induced tolerance to the hypothermic response. Pretreatment of the rats with dexamethazone did not change the development of the tolerance to 8-OH-DPAT-induced hypothermic effect which seems to exclude the involvement of the hypothalamo-pituitary-adrenocortical axis in the tolerance development. It is concluded that the results support the hypothesis that GABA neurones beyond the 5-HT neurones are involved in the mechanism causing tolerance to the 5-HT1A receptor-mediated hypothermia in rats.     

Agonist action at 5-HT1C receptors facilitates 5-HT1A receptor-mediated spontaneous tail-flicks in the rat

In rats lightly restrained in plastic cylinders, subcutaneous administration of the selective, high efficacy 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), induced spontaneous tail-flicks, that is, tail-flicks in the absence of extraneous stimulation. The putative 5-HT1B receptor agonist, CGS 12066B, the mixed 5-HT1B/1C receptor agonists, 1-((3-(trifluoromethyl)phenyl]piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP), the 5-HT1C/2 receptor agonist, [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1B/1C/2 receptor agonist, quipazine, did not, in contrast, elicit tail-flicks when applied alone. However, TFMPP, mCPP, DOI and quipazine, but not CGS 12066B, each potentiated the action of 8-OH-DPAT. Further, in the presence of TFMPP, mCPP and DOI, the dose-response curve for the induction of tail-flicks by 8-OH-DPAT was both steeper and shifted to the left. Tail-flicks induced by another high efficacy 5-HT1A receptor agonist, lisuride, were also enhanced by TFMPP, mCPP and DOI. The 5-HT1A receptor partial agonists, buspirone and (+/-)-flesinoxan, evoked tail-flicks only in the presence of TFMPP, mCPP or DOI. The mixed 5-HT1C/2 receptor antagonists, ritanserin and ICI 169,369, did not modify the action of 8-OH-DPAT alone but abolished the potentiation of 8-OH-DPAT-induced tail-flicks by DOI and TFMPP. Further, the selective 5-HT1A receptor antagonist, BMY 7378, blocked tail-flicks induced by both 8-OH-DPAT alone and 8-OH-DPAT plus DOI or TFMPP. A common property of those drugs potentiating 8-OH-DPAT-induced tail-flicks is an agonist action at 5-HT1C receptors and the data indicate that it is this mechanism which underlies the facilitation of tail-flicks.     

The 5-HT1A receptor full agonist, 8-OH-DPAT inhibits ACTH-induced 5-HT2A receptor hyperfunction in rats: involvement of 5-HT1A receptors in the DOI-induced wet-dog shakes in ACTH-treated rats

We examined the influence of 8-hydroxy-2-di-n-propylamino tetralin (8-OH-DPAT), a serotonin 1A (5-HT1A) receptor full agonist, on the wet-dog shake response induced by the (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2A receptor agonist, in adrenocorticotropic hormone (ACTH)-treated rats. Chronic ACTH (100 microg/rat, s.c.) treatment for 14 d increased the wet-dog shake response induced DOI. The 8-OH-DPAT inhibited the wet-dog shake response induced by DOI in rats with ACTH for 14 d. On the other hand, the 8-OH-DPAT-induced hypothermia and flat body posture were inhibited when ACTH was administered for 14 d. These findings suggest that chronic treatment with ACTH decreased the sensitivity of the 5-HT1A receptor system; however, the inhibitory effects from the 5-HT1A receptors to the 5-HT2A receptor system is not inhibited in ACTH-treated rats.     

Mediation of the antidepressant-like effect of 8-OH-DPAT in mice by postsynaptic 5-HT1A receptors.

1. The 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) has been evaluated in a mouse model for detecting potential antidepressants (Porsolt test). The effects of various receptor antagonists, lesions of brain monoaminergic neurones and chronic drug treatments on this 8-OH-DPAT-induced response have also been determined. 2. 8-OH-DPAT (0.3-10.0 mg kg-1, s.c.) dose-dependently increased the mobility of mice in the Porsolt test. Other selective 5-HT1A receptor ligands (0.3-30 mg kg-1, s.c.) either mimicked the 8-OH-DPAT response (ipsapirone, at 10 and 30 mg kg-1, s.c.) or were inactive (buspirone and gepirone). However, each of these compounds (< or = 100 mg kg-1, p.o.) inhibited the response to 8-OH-DPAT (3 mg kg-1, s.c.) when given concurrently. 3. The putative 5-HT1A antagonists, spiroxatrine (1-30 mg kg-1, p.o.), (+/-)-pindolol (30 mg kg-1, p.o.) and methiothepin (3-10 mg kg-1, p.o.), each attenuated the 8-OH-DPAT (3 mg kg-1, s.c.)-induced increase in mobility. 4. The dopamine D1 receptor antagonist, SCH 23390 (3-10 mg kg-1, p.o.), weakly reversed the 8-OH-DPAT response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction between 5-HT(1A) and 5-HT(1B) receptors: effects of 8-OH-DPAT-induced hypothermia in 5-HT(1B) receptor knockout mice

To test for adaptive compensatory changes that may have occurred in the functional activity of somatodendritic 5-HT(1A) receptors during the development of constitutive "knockout" mice lacking the 5-HT(1B) receptor subtype (5-HT(1B) -/- KO), we assayed for decrease in body temperature induced by an acute subcutaneous injection of the 5-HT(1A) receptor agonist, 8-hydroxy 2(di-n-propyl(amino)tetralin (8-OH-DPAT), either alone or in the presence of a selective 5-HT(1A) receptor antagonist, N-[4-(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide (WAY 100635). We compared dose-response curves, time course study, calculated ED(50) values (potency), maximal response to 8-OH-DPAT (efficacy) as well as measurements of the dose-dependent blockade of this response by WAY 100635 between wild-type controls and mutant mice. We found a higher efficacy of 8-OH-DPAT-induced hypothermia in 5-HT(1B) -/- KO compared to wild-type mice suggesting that an adaptive thermoregulatory process involving the functional activity of somatodendritic 5-HT(1A) receptors is altered in mutant mice lacking 5-HT(1B) receptors.     

Rapid desensitization of 5-HT(1A) receptors in Fawn-Hooded rats after chronic fluoxetine treatment.

Anxiety, platelet serotonin (5-HT) content and functions of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) were measured in Sprague--Dawley (SD) and Fawn-Hooded (FH) rats, a strain with genetically impaired 5-HT storage and reuptake system and a putative model of depression and anxiety. In addition, the effects of 7 and 16 days treatment with the selective serotonin reuptake inhibitor (SSRI) fluoxetine on 8-OH-DPAT-induced responses were studied. FH rats showed significantly higher anxiety in the social interaction test, and much lower platelet 5-HT content compared to SD rats. The efficacy of 8-OH-DPAT (15-120 microg/kg, i.v.) to induce lower lip retraction (an effect mediated by median raphe receptors) was increased in FH rats. In most FH but only a few SD rats a special neurological syndrome, clonic movement of the masseters and in-and-out movement of the eyeballs, was induced by 8-OH-DPAT, and this behaviour like other effects of 8-OH-DPAT, was completely blocked by pretreatment with the 5-HT(1A) receptor antagonist WAY-100635. In SD rats fluoxetine (10 mg/kg/day, i.p.) caused a moderate inhibition of 8-OH-DPAT-induced hypothermia, an effect mediated most likely by hypothalamic 5-HT(1A) receptors, (-19% and -40% after 7 and 16 days of fluoxetine, 24 h after the last injection, respectively). In FH rats fluoxetine caused a rapid and complete reduction in the 8-OH-DPAT-induced hypothermia (-65% and -91% after 7 and 16 days of fluoxetine, respectively). Fluoxetine caused no change in lower lip retraction but a reduction in the masseter-eyeball syndrome in both SD and FH rats. Our data provide evidence that in FH rats, median raphe 5-HT(1A) receptors are hypersensitive, and the hypothalamic 5-HT(1A) receptor desensitization, caused by SSRI antidepressants, is faster and more complete. These data support the notion that chronic treatment with SSRIs induces a desensitization of some 5-HT(1A) receptor populations, and impaired 5-HT storage and reuptake may accelerate this process.     

Dose-dependent discriminative stimulus properties of 8-OH-DPAT

Separate groups of rats were trained to discriminate either 0.1mg/kg (low dose; L) or 2.5mg/kg (high dose; H) of 8-OH-DPAT from saline, in a standard operant task. Both cues were found to be dose, time and route dependent and generalized completely to the 5-HT(1A) agonists ipsapirone and flesinoxan. Buspirone substituted completely for 8-OH-DPAT in L and partially in H, whereas the 5-HT(1A/1B) receptor agonist eltoprazine substituted completely for 8-OH-DPAT in H but only partially in L. The 5-HT(1A/1B) receptor agonist RU24969, the 5-HT(1B/2C/1A) receptor agonist TFMPP and the 5-HT reuptake blocker fluvoxamine did not completely mimic the effect of 8-OH-DPAT in either L or H and the 5-HT(1A) mixed agonists/antagonists BMY 7378 and NAN-190 produced partial generalization in L, but no generalization in H. In antagonism tests, NAN-190 and BMY 7378 only partially blocked the 8-OH-DPAT cue in both groups. The non-selective 5-HT receptor antagonist methysergide did not completely block the 8-OH-DPAT cue in in L or H. However, in generalization studies, it completely mimicked the 8-OH-DPAT cue in L and produced partial generalization in H. The beta-adrenergic/5-HT(1A/1B) receptor antagonist pindolol completely blocked the 8-OH-DPAT cue in L and H and did not mimic the 8-OH-DPAT cue in either condition. The alpha(2)-adrenoceptor blocker yohimbine substituted fully for the 8-OH-DPAT cue in L and partially in H. Idazoxan did not substitute for the cue of 8-OH-DPAT in H, but produced nearly 80% generalization in L. The dopamine receptor antagonist pimozide neither blocked nor mimicked the cue of 8-OH-DPAT in either group. A number of other drugs (i.e. m-CPP, S(-)-propranolol, DOI, ketanserin, clonidine and apomorphine) were only tested in H. S(-)-Propranolol blocked the 8-OH-DPAT cue but the other compounds produced neither stimulus generalization nor antagonism. The present study demonstrates that the cues produced by the low and the high training dose of 8-OH-DPAT are quantitatively different and mediated by the agonistic activity of 8-OH-DPAT at 5-HT(1A) receptors. Although the results suggest that the 8-OH-DPAT cue (both L and H) is mediated via postsynaptic 5-HT(1A) receptors, the involvement of presynaptic 5-HT(1A) receptors cannot yet be ruled out.     

MDL 72832: a potent and stereoselective ligand at central and peripheral 5-HT1A receptors

In receptor binding assays (+/-)MDL 72832, 8-[4-(1,4-benzodioxan-2-ylmethylamino)butyl]-8-azaspiro++ +[4,5] decane-7,9-dione, was a potent (pIC50 9.1), selective and stereospecific ligand for central 5-HT1A recognition sites. In functional tests, (+/-)MDL 72832 and its S(-) and R(+) enantiomers blocked stereoselectively the 8-OH-DPAT-induced neuronal inhibition of the transmurally stimulated guinea-pig ileum and the cardiovascular effects of 8-OH-DPAT in anaesthetized rats. In contrast, (+/-)MDL 72832 and its enantiomers were exclusively '8-OH-DPAT-like' in their ability to fully and stereoselectively generalize to the 8-OH-DPAT discriminative stimulus and, in reserpinised rats, to induce forepaw treading and flat body posture. These results characterize (+/-)MDL 72832 as a potent, stereoselective ligand with mixed agonist and antagonist properties at central and peripheral 5-HT1A receptors. The similar stereoselective requirements for the recognition site and functional effects provides compelling evidence that the 5-HT1A recognition site is indeed a functional receptor.     

Interaction of the beta adrenergic receptor antagonist bucindolol with serotonergic receptors

Bucindolol is a nonselective beta-adrenergic receptor antagonist that has additional vasodilating properties. Because some beta-adrenergic receptor antagonists such as cyanopindolol are used as 5-HT1A/5-HT1B receptor antagonists, we tested the hypothesis that bucindolol can interact with 5-HT receptors. Both in vitro and in vivo methods were used to examine the interaction of bucindolol with 5-HT receptors relevant to the cardiovascular system-the 5-HT1A, 5-HT1D, 5-HT2A, and 5-HT2B receptors-and with alpha1-adrenergic receptors. In binding studies, bucindolol displayed high affinity for the 5-HT1A receptor (Ki, 11 nM), modest affinity for the 5-HT2A receptor (Ki, 382 nM), and no measurable affinity for the 5-HT1D receptor; binding affinity for the 5-HT2B receptor was not studied. Bucindolol also displayed significant binding affinity (Ki, 69 nM) for the alpha1-adrenergic receptors. Alpha1-Adrenergic receptor antagonist activity was confirmed by the ability of bucindolol (1 mg/kg) to act as a competitive antagonist against 0.01-30 microg/kg phenylephrine-induced pressor responses in conscious rats. In conscious permanently instrumented rats, bucindolol (0.1-3.0 mg/kg, i.v.) did not cause bradycardia similar to that elicited by the 5-HT1A-receptor agonist 8-OH-DPAT (3-300 microg/kg, i.v.), nor did bucindolol (1 mg/kg) block the 8-OH-DPAT-induced bradycardia. Bucindolol (10(-9)-10(-5) M) did not cause relaxation in the PGF2alpha-contracted, endothelium-intact porcine coronary artery, nor did bucindolol (10(-5) M) block 5-HT-induced coronary artery relaxation, indicating that bucindolol does not have significant interactions at the 5-HT1D receptor. Bucindolol also displayed no agonist activity at the 5-HT2A and 5-HT2B receptor (endothelium-denuded rat thoracic aorta and rat stomach fundus, respectively), but did act as a weak 5-HT2A-receptor antagonist (-log K(B) [M] = 5.4+/-0.1) and 5-HT2B-receptor antagonist (-log K(B) [M] = 7.8+/-0.1). Thus, these data suggest that bucindolol lacks the ability to activate the 5-HT1A, 5-HT1D, 5-HT2A, and 5-HT2B receptor, but can block alpha1-adrenergic receptors and act as a weak 5-HT2A- and 5-HT2B-receptor antagonist. The relevance of these serotoninergic effects as it pertains to the mechanism of bucindolol-induced vasodilation is unknown.