5-HT1A and alpha-2 adrenergic receptors mediate the hyperglycemic and hypoinsulinemic effects of 8-hydroxy-2-(di-n-propylamino)tetralin in the conscious rat

The ability of the 5-hydroxytryptamine (5-HT)1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to affect plasma glucose levels and insulin release was assessed in rats bearing chronic jugular catheters. The i.v. administration of 8-OH-DPAT (150 micrograms/kg) rapidly promoted a transient hyperglycemia. Despite high glucose levels, insulinemia remained constant. Dose-response curves revealed that maximal hyperglycemia was associated with hypoinsulinemia. Increased glycemia, which was also found to be induced by other 5-HT direct and indirect agonists, lasted longer in food-deprived rats. Evidence for a strong inhibitory effect of 8-OH-DPAT on insulin release was reported in rats submitted to i.v. glucose tolerance tests. Pretreatments with the dopaminergic blocker haloperidol, the alpha-1 adrenoceptor antagonist prazosin or the 5-HT2 blocker ketanserin were ineffective. In contrast, the alpha-2 adrenoceptor antagonist idazoxan and the unspecific 5-HT antagonist methiotepin prevented the hyperglycemic and the hypoinsulinemic effects of 8-OH-DPAT. Blockade of these changes by (-)-propranolol (a 5-HT1 blocker), but not by (+)-propranolol, indicated that 5-HT1 and alpha-2 adrenergic receptors mediated 8-OH-DPAT-induced hyperglycemia. Reserpine pretreatment did not prevent the effects of 8-OH-DPAT. Central injection of 8-OH-DPAT induced hyperglycemia, the amplitude of which was equivalent to that measured after i.v. administration. Selective degeneration of serotonergic nerve cells by 5,7-dihydroxytryptamine did not prevent 8-OH-DPAT-induced alterations, thus rendering a key role for presynaptic mechanisms unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of the 5-hydroxytryptamine(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin on spontaneous activity, cocaine-induced hyperactivity and behavioral sensitization: a microanalysis of locomotor activity

The influence of the 5-hydroxytryptamine(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT) on locomotor hyperactivity induced by the acute and chronic administration of cocaine was assessed. Horizontal activity was measured in the periphery and center of an open field test enclosure equipped with photobeams; vertical activity was also recorded. Peripheral hyperactivity induced by an acute administration of cocaine (10 or 20 mg/kg) was significantly enhanced by 0.2 mg/kg DPAT. In contrast, central and vertical activities were reduced in a dose-related manner by DPAT (0.1 and 0.2 mg/kg); DPAT also suppressed central (0.2 mg/kg) and vertical (0.1 and 0.2 mg/kg) activities when administered alone. Similar observations were made on day 1 of chronic treatment with DPAT (0, 0.1, or 0.2 mg/kg) injected 15 min before an injection of cocaine (0, 10, or 15 mg/kg) administered twice daily for 7 days. By day 7 of repeated DPAT treatment, sensitization of DPAT-evoked peripheral activity developed, which contrasted with tolerance to the central and vertical hypoactivity evoked by DPAT. Sensitization developed to the repeated treatment with 15 mg/kg cocaine but not 10 mg/kg cocaine. Interestingly, enhancements of all activity measures were observed between days 1 and 7 in rats cotreated with DPAT plus either dose of cocaine. This sensitization to DPAT plus cocaine was expressed on challenge with DPAT and cocaine but not with cocaine alone. The present study implies that the stimulation of 5-hydroxytryptamine(1A) receptors is capable of modulating the hyperactivity evoked by cocaine, possibly via modulation of the mesoaccumbens dopamine circuit thought to mediate the behavioral effects of cocaine.     

5-hydroxytryptamine (HT)1A receptors and the tail-flick response. II. High efficacy 5-HT1A agonists attenuate morphine-induced antinociception in mice in a competitive-like manner.

This study examined the influence of s.c. administration of 5-hydroxytryptamine (HT)1A agonists upon the antinociceptive action of s.c. injected morphine in tail-flick tests to noxious heat and pressure. The selective 5-HT1A agonist, (+-)-8-hydroxy-diprolaminotetralin HBr (8-OH-DPAT), dose-dependently antagonized morphine-induced antinociception (MIA) without affecting the latency to respond when applied alone. In the presence of increasing doses of 8-OH-DPAT (0.16-0.63 mg/kg), the morphine dose-response curve was shifted progressively in parallel to the right and the maximal effect of morphine was not altered; Schild analysis yielded a slope of close to -1.0. 8-OH-DPAT both prevented and reversed the action of morphine. The action of 8-OH-DPAT was reversible (at 24 hr). In distinction, 8-OH-DPAT neither blocked morphine-induced Straub tail nor precipitated withdrawal in morphine-dependent animals; thus, it lacked opioid-antagonist properties. The antagonism of MIA by 8-OH-DPAT was mimicked by additional drugs acting as high efficacy 5-HT1A agonists: lisuride, 5-methoxy-N,N-dimethyltryptamine hydrogen oxalate, RU 24969 [methoxy-3-(1,2,3.6-tetrahydropyridin-4-yl)-1H-indole] and d-lysergic acid diethylamide. In contrast, the 5-HT1B/1C agonist, TFMPP m-trifluromethylphenylpiperazine HCl, and the 5-HT1C/2 agonist, DOI (+-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCI, were ineffective. The putative selective 5-HT1A antagonists, BMY 7378 [(8-[-[4-(2-,ethoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol[4]- decane-7,9-dione-2-HCL] and spiperone, did not reduce MIA. Indeed, BMY 7378 blocked the ability of 8-OH-DPAT to antagonize MIA. Under the present conditions, agonists and antagonists at adrenergic and dopaminergic receptors did not attenuate MIA. These data show that, over a certain range of doses, the systemic administration of 8-OH-DPAT and other high efficacy 5-HT1A agonists functionally antagonizes the antinociceptive action of systemically applied morphine in a competitive-like manner. It is suggested that 5-HT1A receptors play an important role in the modulation of opioidergic antinociceptive mechanisms.     

5-hydroxytryptamine (HT)1A receptors and the tail-flick response. III. Structurally diverse 5-HT1A partial agonists attenuate mu- but not kappa-opioid antinociception in mice and rats.

This study examined the effects of structurally diverse 5-hydroxytryptamine (HT)1A partial agonists upon opioid-induced antinociception against noxious heat and pressure stimuli in rats and mice. The pyrimidinylpiperazines, buspirone, ipsapirone and gepirone, the halogenated phenylpiperazine, LY 165, 163 [1-(2-(4-aminophenyl)ethyl-4-(3-trifluoromethylphenyl)-piperazine], the heterobicylic arylpiperazine, (+/-)-flexinoxan, and the benzodiaxane, MDL 728328-[(4-(1,4-benzodioxon-2-ylmethylamino)butyl-8-azasp iro-(4,5)-decane-7,9-dione], exerted little or no effect upon basal latencies. In both mice and rats, each dose-dependently attenuated the antinociceptive action of the mu-opioid, morphine, against heat and pressure. In their presence, the morphine dose-response curve was shifted in parallel to the right with no loss of maximal effect. In mice, Schild analysis of the action of ipsapirone and gepirone yielded slopes of close to -1. In contrast to the partial agonists, the buspirone metabolite, 1-pyrimidinylpiperazine, which lacks 5-HT1A affinity, and the putative 5-HT1A antagonists, methiothepin, spiperone, BMY 7378 [(8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspirol [4]-decane-7,9-dione) 2HCl] and alprenolol, did not reduce the action of morphine. In rats, the antagonistic effect of buspirone, gepirone and ipsapirone could be blocked by BMY 7378. The 5-HT1A partial agonists also antagonized the antinociception-induced by the mu-opioid, sufentanil, but were virtually inactive against the selective kappa-opioid agonists, U69,593 (5 alpha,7 alpha,8 beta-(+)-N-methyl-N-[7-(l-pyrrolidinyl)-1-oxaspirol-(4,5)-dec-8-yl ] benzene-acetamide) and U50,488H (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide methane sulfonate hydrate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reconsideration of 5-hydroxytryptamine (5-HT)(7) receptor distribution using [(3)H]5-carboxamidotryptamine and [(3)H]8-hydroxy-2-(di-n-propylamino)tetraline: analysis in brain of 5-HT(1A) knockout and 5-HT(1A/1B) double-knockout mice

The characterization and anatomical distribution of 5-hydroxytryptamine (5-HT)(7) receptor binding sites in brain tissue has been hampered by the lack of a specific radioligand. In the present autoradiographic study, we took advantage of 5-HT(1A) knockout and 5-HT(1A/1B) double-knockout mice to revisit the pharmacological characterization and anatomical localization of 5-HT(7) binding sites in mouse brain using [(3)H]5-carboxamidotryptamine (5-CT) and [(3)H]8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT). The distribution pattern of [(3)H]5-CT binding sites (2 nM) in the brain of mice lacking the 5-HT(1A/1B) receptor was scarce and confined to the septum, globus pallidus, thalamus, hypothalamus, amygdala, cortex, and substantia nigra. The low densities of [(3)H]5-CT binding sites detected in septum, thalamus, hypothalamus, amygdala, and cortex were displaced by 10 microM of the selective 5-HT(7) receptor antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl) ethyl)pyrrolidine-1-sulfonyl) phenol (SB-269970). The SB-269970-insensitive [(3)H]5-CT binding sites detected in globus pallidus and substantia nigra of 5-HT(1A/1B) knockout mice were displaced by N-[3-(2-dimethylamino)ethoxy-4-methoxy-phenyl]-2'-methyl-4'- (5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide hydrochloride (SB-216641) (1 microM), demonstrating the 5-HT(1D) nature of these binding sites. In contrast to the low densities of [(3)H]5-CT binding sites, high-to-moderate densities of [(3)H]8-OH-DPAT binding sites (10 nM) were found throughout the brain of 5-HT(1A) and 5-HT(1A/1B) knockout mice (olfactory system, septum, thalamus, hypothalamus, amygdala, CA3 field of the hippocampus, cortical mantle, and central gray). These [(3)H]8-OH-DPAT binding sites were displaced by 10 microM SB-269970, risperidone, and methiothepin but not by pindolol, N-tert-butyl-3-[4-(2-methoxyphenyl)piperazin-1-yl]-2-phenylpropanamide (WAY- 100135), or citalopram. We conclude that despite its high affinity for the 5-HT(7) receptor in tissue homogenates, [(3)H]5-CT is not a good tracer for measuring 5-HT(7) receptor binding sites autoradiographically. Also, the lower affinity ligand [(3)H]8-OH-DPAT is a much better tracer for autoradiographic studies at the 5-HT(7) receptor binding sites.     

Temporal regulation of agonist efficacy at 5-hydroxytryptamine (5-HT)1A and 5-HT 1B receptors

Coactivation of purinergic (P 2Y) receptors reduces agonist efficacy at serotonin 1B (5-HT 1B), but not 5-HT 1A receptors. Herein, we report that pretreatment for 5 min with the P 2Y receptor agonist ATP reduced agonist responsiveness at the 5-HT 1A, but not at the 5-HT 1B, receptor. The effect of ATP pretreatment on the 5-HT 1A receptor response rapidly reversed within a 10 min time frame between P 2Y receptor and 5-HT 1A receptor activation. ATP pretreatment effects on 5-HT 1A agonist responsiveness were blocked by the protein kinase inhibitors staurosporine and bisindolylmaleimide, suggesting that the ATP-mediated temporal regulation involves activation of protein kinase C (PKC). Moreover, the temporal effect of ATP was blocked by incubation with 1% ethanol, suggesting that consequences of phospholipase D (PLD) activation play a role. ATP pretreatment blocked the inhibitory effect produced by 5-HT 2C receptor activation on the 5-HT 1A, but not the 5-HT 1B, receptor response, suggesting that the 5-HT 1A receptor itself was the target for PLD/PKC action. Finally, ethanol did not block the reduction in responsiveness of the 5-HT 1A receptor system produced by activation of PKC with phorbol ester treatment, suggesting that PKC activation lies downstream of PLD. Taken together, these data suggest that activation of P 2Y receptors can reduce responsiveness of the 5-HT 1A receptor system via a PLD/PKC-dependent mechanism that is highly dependent upon the temporal pattern of receptor activation. Moreover, this work underscores the importance of time as a variable in receptor signaling cross talk and serves to further illustrate differences between the 5-HT 1A and 5-HT 1B receptor systems.     

Differential cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), flesinoxan, 5-methyl-urapidil and MDL 75,608A in conscious spontaneously hypertensive rats

Summary— The effects of intravenous (iv) administration of four agonists at central 5-HT_1A receptors were investigated and compared. Acute iv injection of 0.1 mg/kg of 8-OH-DPAT induced a decrease in blood pressure both in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). The maximal hypotensive effect was observed 15 and 10 min after injection, respectively, but the effect was greater and longer-lasting in the SHR. 8-OH-DPAT significantly decreased heart rate in WKY and, to a lesser extent, in SHR. The iv injection of 1 mg/kg of flesinoxan caused a similar fall in blood pressure and heart rate in SHR and WKY. The iv administration of 1 mg/kg of 5-methyl-urapidil or MDL 75,608A caused a fall in blood pressure which was significantly more pronounced in SHR than in WKY. 5-methyl-urapidil induced a significant tachycardia in WKY, but had little effect on heart rate in SHR. MDL 75,608A caused a short-lasting tachycardia in SHR and WKY. In conscious SHR, the intracerebroventricular (icv) injection of 10 μg of 8-OH-DPAT or 100 μg of either flesinoxan or MDL 75,608A caused a decrease in blood pressure and heart rate. The icv injection of 100 μg of 5-methyl-urapidil caused only a decrease in blood pressure. Chronic pre-treatment with these compounds, by daily iv injection, did not significantly influence the hypotensive or bradycardic effects in an acute experiment. The involvement of α₁-adrenoceptors in the effects of these compounds was studied by administering phenylephrine (1 μg/iv) at 5- min intervals before and after the iv injection of the experimental compounds. The injection of phenylephrine reproducibly increased blood pressure by 35–40 mm Hg after saline pre-treatment, and these responses were not affected by the iv injection of 0.1 mg/kg of either 8-OH-DPAT or 1 mg/kg of flesinoxan. In contrast, the phenylephrine-induced pressor responses were markedly diminished at 5 min after treatment with 1 mg/kg of either 5-methyl-urapidil or MDL 75,608A, but slowly recovered thereafter. These results show that the 5-HT_1A recptor agonists 8-OH-DPAT, flesinoxan, 5-methyl-urapidil and MDL 75,608A show antihypertensive properties in conscious SHR after iv or icv injection. However, the mechanism of action of the compounds differs: 8-OH-DPAT and flesinoxan may act predominantly as 5-HT_1A receptor agonists, where as 5-methyl-urapidil and MDL 75,608A also seem to have an effect on peripheral α₁-adrenoceptors.     

Regulation of extracellular concentrations of 5-hydroxytryptamine (5-HT) in mouse striatum by 5-HT(1A) and 5-HT(1B) receptors

The ability of selective serotonin (5-HT) receptor agonists to reduce the extracellular concentration of 5-HT was examined in the striatum of awake, unrestrained mice by in vivo microdialysis. Systemic administration of either 8-OH-PIPAT (R-(+)-trans-8-hydroxy-2-[N-n-propyl-N-(3'-iodo-2'-propenyl)] aminotetralin), a novel 5-HT(1A) receptor agonist, or CP 94,253, a selective 5-HT(1B) receptor agonist, resulted in significant dose-related reductions of striatal 5-HT. The effect of 8-OH-PIPAT (1.0 mg/kg) was blocked by pretreatment with WAY 100635 (0.1 mg/kg), a selective 5-HT(1A) receptor antagonist, but it was not blocked by pretreatment with GR 127935 (0.056 mg/kg), a selective 5-HT(1B/1D) receptor antagonist. The effect of CP 94,253 (1.0 mg/kg) was blocked by pretreatment with GR 127935 (0.056 mg/kg) but was not blocked by pretreatment with WAY 100635 (0.1 mg/kg). Neither WAY 100635 nor GR 127935 altered extracellular 5-HT levels at the doses that were able to completely block the effects of either 8-OH-PIPAT or CP 94,253. The present findings suggest that, on systemic administration, both 8-OH-PIPAT and CP 94,253 are potent and selective agonists at the somatodendritic 5-HT(1A) autoreceptor and terminal 5-HT(1B/1D) autoreceptor, respectively, and are each able to cause decreases in extracellular levels of 5-HT in the mouse striatum by activating a distinct set of receptors.     

Pindolol suppresses serotonergic neuronal activity and does not block the inhibition of serotonergic neurons produced by 8-hydroxy-2-(di-n-propylamino)tetralin in awake cats.

Clinical studies have shown that pindolol can enhance the effects of antidepressant drugs, presumably by acting as an antagonist at somatodendritic 5-hydroxytryptamine (5-HT)(1A) autoreceptors, which regulate the firing rate of central serotonergic neurons. The current study characterized the action of pindolol on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus of freely moving cats. (+/-)-Pindolol produced a dose-dependent inhibition of neuronal activity after i.v. (ED(50) = 0.25 mg/kg) and s.c. (ED(50) = 1.23 mg/kg) administration. The active enantiomer (-)-pindolol (1 mg/kg i.v.) also suppressed neuronal activity (maximal decrease, 88%). Upon p.o. administration, (+/-)-pindolol (10 mg/kg) produced a marked, long-acting suppression of neuronal activity similar to that observed after s.c. administration. In all cases, the reduction in firing rate produced by pindolol was completely reversed by low doses of N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)cycloh exanecarboxamide (WAY-100635) (0.1 mg/kg i.v. or 0.2 mg/kg s.c.), a selective 5-HT(1A) antagonist. Systemic administration of (-)-tertatolol (1-5 mg/kg i.v.), another beta-adrenoceptor blocker/putative 5-HT(1A) antagonist, had no significant effect on neuronal activity. The ability of i.v. (+/-)-pindolol (0.1-1 mg/kg) to reverse the suppression of serotonergic neuronal activity produced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10 microg/kg i.v.), a selective 5-HT(1A) agonist, also was examined. (+/-)-Pindolol had no appreciable effect on the action of 8-OH-DPAT. In contrast, the 5-HT(1A) antagonist drugs WAY-100635 (0.1 mg/kg i.v. ), 4-fluoro-N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl benzamide (0.1 mg/kg, i.v.), N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylprop anamid e [(S)-WAY-100135] (0.5 mg/kg i.v.), and (-)-tertatolol (1-5 mg/kg i. v.) reversed the effect of 8-OH-DPAT to varying degrees. Overall, these results indicate that pindolol acts as an agonist rather than an antagonist at 5-HT(1A) autoreceptors in awake animals.     

Central 5-hydroxytryptamine (5-HT) receptors in blood pressure regulation.

Both intravenous and central administration of ketanserin, a 5-HT2 receptor antagonist, decrease blood pressure and sympathetic nerve activity, suggesting a central origin of its effects. However, ketanserin also possesses alpha 1-adrenoceptor blocking properties. Selective 5-HT2 receptor antagonists devoid of alpha 1-adrenoceptor blocking properties, e.g. LY 53857 and cinanserin, fail to reduce blood pressure and sympathetic nerve activity. In addition, 5-HT2 receptor agonists increase blood pressure and sympathetic nerve discharge. Therefore, it seems improbable that blockade of central 5-HT2 receptors alone could lead to a reduction in blood pressure. In contrast, the selective 5-HT1A receptor agonists 8-OH-DPAT and flesinoxan decrease blood pressure and heart rate by a centrally-mediated decrease in sympathetic tone and an increase in vagal tone. The sympatho-inhibitory effects of 5-HT1A receptor agonists result from the stimulation of postsynaptic 5-HT1A receptors within the ventrolateral pressor area. These results suggest that selective 5-HT1A receptor agonists acting in the central nervous system could be developed for the treatment of hypertension. Indeed, drugs such as flesinoxan and urapidil are effective in this setting.     

The 5-hydroxytryptamine1A receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin, increases cardiac output and renal perfusion in rats subjected to hypovolemic shock

The 5-hydroxytryptamine(1A) receptor agonist, (+)-8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), raises blood pressure (BP) and venous tone in rats subjected to hemorrhagic shock. Here, BP, ascending aortic blood flow [i.e., estimate of cardiac output (CO)] and venous blood gases were measured to determine the hemodynamic effects of 8-OH-DPAT (30 nmol/kg i.v., n = 10), saline (n = 10), or an equipressor infusion of epinephrine (n = 10) in unanesthetized rats subjected to hemorrhagic shock (25 min of hypotensive hemorrhage, approximately 50 mm Hg). Renal and iliac blood flow were measured in separate groups of similarly hemorrhaged rats given the same dose of 8-OH-DPAT (n = 7) or saline (n = 6). Compared with saline treatment, 8-OH-DPAT produced a sustained rise in BP (+32 +/- 4 versus +9 +/- 2 mm Hg, 15 min after injection, P < 0.01) and CO (+27 +/- 5 versus +4 +/- 6 ml/min/kg, P < 0.01) but did not affect total peripheral resistance (TPR). Infusion of epinephrine reduced CO (-12 +/- 6 ml/min/kg, P < 0.01) and dramatically increased TPR [+0.37 +/- 0.11 versus +0.05 +/- 0.05 log (mm Hg/ml/min/kg), P < 0.01]. 8-OH-DPAT increased renal conductance (+7 +/- 1 versus +4 +/- 1 microl/min/mm Hg, P < 0.01) but did not significantly affect iliac conductance. 8-OH-DPAT attenuated further development of acidosis compared with either saline or epinephrine (-5.6 +/- 1.6 versus -13.0 +/- 2.0 versus -11.3 +/- 2.6 mmol/liter base excess 45 min after start of hemorrhage, both P < 0.01 versus 8-OH-DPAT). These data demonstrate that 8-OH-DPAT improves hemodynamics during circulatory shock, in part, through renal vasodilation and mobilizing of blood stores.     

5-hydroxytryptamine3 (5-HT3) receptors mediate spinal 5-HT antinociception: an antisense approach.

To examine the role of the 5-hydroxytryptamine(1B) (5-HT1B) and 5-HT3 receptor subtypes in the analgesia produced by 5-HT (serotonin) agonists, we assessed the effect of antisense oligodeoxynucleotides (AODNs) designed to "knock down" the number of these receptor subtypes on analgesia produced by intrathecal (i.t.) 5-HT, the 5-HT1B receptor agonist, 7-trifluoromethyl-4-(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline maleate (CGS-12066A), and the 5-HT3 receptor agonist, 2-methyl-5-HT. Groups of mice (n = 17-20) were injected i.t. on days 1, 3, and 5 with one of the AODNs, a mismatch oligo, or saline. On day 6, all mice were injected i.t. with 70.5 nmol of 5-HT, 44.4 nmol of CGS-12066A, or 49 nmol of 2-methyl-5-HT by lumbar puncture. Following testing, spinal cords were rapidly removed and prepared for receptor binding assays. Treatment with AODN for 5-HT1B receptors produced a 70% reduction in ligand binding to this receptor subtype. After treatment with AODN for 5-HT3 receptors, ligand binding to this receptor subtype was undetectable. In mice tested with i.t. 5-HT, tail-flick analgesia was attenuated only in mice treated with the 5-HT3 receptor AODN. Mice treated with the AODN designed to knock down 5-HT(1B) receptors or with its mismatch oligo were not significantly different from controls. In mice tested with i.t. administration of CGS-12066A, none of the oligo treatments produced a significant attenuation of analgesia. In mice tested with i.t. administration of 2-methyl-5-HT, only 5-HT3 receptor AODN attenuated analgesia. Thus, 5-HT and 2-methyl-5-HT analgesia are mediated by the 5-HT3 receptor subtype. However, spinal CGS-12066A analgesia appears not to be mediated by either the 5-HT1B or the 5-HT3 receptor subtypes.     

Autoradiographic characterization of (+-)-1-(2,5-dimethoxy-4-[125I] iodophenyl)-2-aminopropane ([125I]DOI) binding to 5-HT2 and 5-HT1c receptors in rat brain

The 5-HT2 (serotonin) receptor has traditionally been labeled with antagonist radioligands such as [3H]ketanserin and [3H]spiperone, which label both agonist high-affinity (guanyl nucleotide-sensitive) and agonist low-affinity (guanyl nucleotide-insensitive) states of this receptor. The hallucinogen 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) is an agonist which labels the high-affinity guanyl nucleotide-sensitive state of brain 5-HT2 receptors selectively. In the present study, conditions for autoradiographic visualization of (+/-)-[125I]DOI-labeled 5-HT2 receptors were optimized and binding to slide-mounted sections was characterized with respect to pharmacology, guanyl nucleotide sensitivity and anatomical distribution. In slide-mounted rat brain sections (+/-)-[125I]DOI binding was saturable, of high affinity (KD approximately 4 nM) and displayed a pharmacologic profile typical of 5-HT2 receptors. Consistent with coupling of 5-HT2 receptors in the high-affinity state to a guanyl nucleotide regulatory protein, [125I]DOI binding was inhibited by guanyl nucleotides but not by adenosine triphosphate. Patterns of autoradiographic distribution of [125I]DOI binding to 5-HT2 receptors were similar to those seen with [3H]ketanserin- and [125I]-lysergic acid diethylamide-labeled 5-HT2 receptors. However, the density of 5-HT2 receptors labeled by the agonist [125I]DOI was markedly lower (30-50%) than that labeled by the antagonist [3H]ketanserin. High densities of [125I]DOI labeling were present in olfactory bulb, anterior regions of cerebral cortex (layer IV), claustrum, caudate putamen, globus pallidus, ventral pallidum, islands of Calleja, mammillary nuclei and inferior olive. Binding in hippocampus, thalamus and hypothalamus was generally sparse. Of note, choroid plexus, a site rich in 5-HT1c receptors had a high density of [125I]DOI binding sites but [3H]ketanserin binding in this region was low. Studies in which [125I]DOI binding to 5-HT2 receptors was blocked with spiperone revealed persisting robust [125I]DOI binding in choroid plexus, which was guanyl nucleotide-sensitive and displayed a pharmacologic profile consistent with its binding to 5-HT1c receptors. These studies suggest that [125I]DOI may be useful as a radiolabel for visualizing the agonist high-affinity state of 5-HT2 receptors and for visualizing 5-HT1c receptors.     

3,4-Methylenedioxymethamphetamine- and 8-hydroxy-2-di-n-propylamino-tetralin-induced hypothermia: role and location of 5-hydroxytryptamine 1A receptors

The popular drug of abuse 3,4-methylenedioxymethamphetamine (MDMA) has complex interactions with thermoregulatory systems, resulting in either hyperthermia or hypothermia. MDMA induces hypothermia when given to animals housed at a low ambient temperature. In this study we report that MDMA (7.5 mg/kg i.p.) given at normal ambient temperatures of 24 to 25 degrees C caused, in conscious freely moving rats, hypothermia (mean decrease from baseline of 1.1 +/- 0.06 degrees C at 40 min). Pretreating animals with a 0.5 mg/kg i.p. dose of the 5-hydroxytryptamine 1A (5-HT(1A)) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY 100635) not only prevented MDMA-induced hypothermia, but resulted in the development of hyperthermia (mean temperature increase from baseline of 0.74 +/- 0.2 degrees C at 120 min). After treatment with WAY 100635, MDMA also elicited an enhanced tachycardia (mean increases in heart rate from baseline of 110 +/- 16 beats/min at 90 min). To identify the location of 5-HT(1A) receptors responsible for hypothermia induced by MDMA, we first investigated the role of 5-HT(1A) receptors in the rostral raphe pallidus (rRP) in decreases in temperature evoked by the known 5-HT(1A) agonist 8-hydroxy-2-di-n-propylamino-tetralin (DPAT). Microinjections of 0.5 nmol of WAY 100635 into the rRP significantly attenuated DPAT (0.2 mg/kg i.p.)-elicited hypothermia. In parallel experiments, we found that microinjections of WAY 100635 into the rRP, while significantly augmenting MDMA-mediated tachycardia, did not alter body temperature. These results demonstrate that although hypothermia mediated by both MDMA and DPAT shares a common dependence on the activation of 5-HT(1A) receptors, the location of these receptors is different for each drug.     

Possible 5-hydroxytryptamine1 (5-HT1) receptor involvement in the stimulus properties of 1-(m-trifluoromethylphenyl)piperazine (TFMPP)

Male rats (N = 24) were trained to discriminate 1-(m-trifluoromethylphenyl)piperazine (TFMPP) (0.8 mg/kg) from saline in a two-lever, drug discrimination situation. 5-Hydroxytryptamine (5-HT) agonists such as fenfluramine (0.8-1.6 mg/kg), m-chlorophenylpiperazine (0.1-1.6 mg/kg) and RU 24969 (0.1-1.6 mg/kg) mimicked TFMPP; 8-hydroxy-2-(di-n-propylamino)tetralin (0.02-0.32 mg/kg) and quipazine (0.2-3.2 mg/kg) elicited saline lever responding; d-lysergic acid diethylamide (0.1-0.16 mg/kg) produced intermediate results. The 5-HT antagonists BC 105 (1.6-12.8 mg/kg), bromolysergic diethylamide (0.8-1.28 mg/kg), ketanserin (0.8-6.4 mg/kg), Ly 53857 (0.2-1.6 mg/kg) and pirenperone (0.08-0.64 mg/kg) failed to attenuate the TFMPP cue; metergoline (0.4-6.4 mg/kg) and spiperone (0.08-1.28 mg/kg) decreased drug lever responding by as much as 60%. These data suggest that 5-HT agonists are not identical and that drug discrimination procedures can differentiate among them. Given that there is strong evidence to support the existence of heterogeneous 5-HT receptors, the present results also suggest that TFMPP acts through mechanism(s) similar to those of the novel 5-HT1 agonists m-chlorophenylpiperazine and RU 24969; these actions can be differentiated from those underlying d-lysergic acid diethylamide, quipazine and 2,5-dimethoxy-4-methylamphetamine, which are attenuated by putative 5-HT2 antagonists. Thus, the authors propose a role for 5-HT1 receptors in mediating the stimulus effects of TFMPP, although further research is necessary to identify functional antagonists of such systems.     

Relationship between receptors mediating serotonin (5-HT) contractions in the canine basilar artery to 5-HT1, 5-HT2 and rat stomach fundus 5-HT receptors

The receptors responsible for contraction to serotonin (5-HT) in the canine basilar artery have not been definitively established to date. Several selective 5-HT2 receptor antagonists (spiperone, ketanserin and LY53857) did not inhibit markedly 5-HT-induced contractions in the canine basilar artery in doses higher than required for substantial inhibition of 5-HT2 receptor-mediated responses. These data suggest that the receptors mediating 5-HT-induced contractions in the basilar artery are not 5-HT2 receptors. Using a series of 5-HT antagonists with relatively high affinity at 5-HT1 sites, over a 1000-fold difference occurred in their ability to block 5-HT receptors in the canine basilar artery, in spite of the similar and high affinity of the antagonists at 5-HT1 binding sites. These data support the contention that 5-HT receptors in the canine basilar artery are not 5-HT1 receptors as defined by ligand binding studies in brain cortical membranes. Similarity of the contractile effects of 5-HT in the rat stomach fundus and in the basilar artery coupled to the previous observations that receptors mediating 5-HT-induced contractions in the fundus were not 5-HT1, 5-HT1A, 5-HT1B or 5-HT2 led us to consider the possibility that 5-HT receptors in the canine basilar artery may resemble those in the rat stomach fundus. The affinity of several 5-HT antagonists determined in the canine basilar artery correlated extremely well (correlation coefficient = 0.96) with the affinities obtained for the same antagonists in the rat stomach fundus.(ABSTRACT TRUNCATED AT 250 WORDS)     

A competitive interaction model predicts the effect of WAY-100,635 on the time course of R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin-induced hypothermia.

The objective of this investigation was to characterize quantitatively the pharmacodynamic interaction between N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide (WAY-100,635) and R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) in vivo. The 8-OH-DPAT-induced change in body temperature was used as a pharmacodynamic endpoint. Four groups of rats each received 1 mg/kg 8-OH-DPAT in 5 min during computer-controlled infusions of physiological saline or WAY-100,635, targeted at steady-state concentrations of 20, 85, and 170 ng/ml. Body temperature was monitored continuously with a telemetric system, and frequent blood samples were obtained to determine the pharmacokinetics of both drugs. Large differences in pharmacokinetics were observed between WAY-100,635 and R-8-OH-DPAT, reflected in values of the terminal elimination half-life of 33 and 143 min, respectively. Infusion of WAY-100,635 had no influence on the pharmacokinetics of R-8-OH-DPAT. With regard to the pharmacodynamics, clear antagonism of the R-8-OH-DPAT-induced hypothermia was observed. The complex pharmacological effect versus time profiles of R-8-OH-DPAT were analyzed on the basis of an indirect physiological response model with set point control coupled to a competitive interaction model for an agonist and antagonist acting at a common receptor. This model converged, yielding precise estimates of the pharmacodynamic parameters of both WAY-100,635 and R-8-OH-DPAT, which were independent of the infusion rate of WAY-100,635. The estimated in vivo binding constant of WAY-100,635 was 0.98 ng/ml (2.3 nM), which is very similar to the reported value from in vitro receptor binding assays. The findings of this investigation show that, in contrast to earlier reports in the literature, WAY 100,635 behaves as a pure competitive antagonist at the 5-hydroxytryptamine(1A) receptor in vivo.     

Reduction of 5-hydroxytryptamine (5-HT)(1A)-mediated temperature and neuroendocrine responses and 5-HT(1A) binding sites in 5-HT transporter knockout mice

The aim of the present study was to determine whether alterations in 5-hydroxytryptamine (5-HT)(1A) receptors would be found in knockout mice lacking the serotonin transporter (5-HTT). Hypothermic and neuroendocrine responses to the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) were used to examine the function of 5-HT(1A) receptors. Initial studies evaluated the dose-response and time course of 8-OH-DPAT-induced hypothermia and hormone secretion in normal CD-1 mice (the background strain of the 5-HTT knockout mice). 8-OH-DPAT dose-dependently produced hypothermic responses that peaked at 20 min postinjection. 8-OH-DPAT-induced hypothermia was blocked by the 5-HT(1A) antagonist WAY-100635. 8-OH-DPAT dose-dependently increased the concentrations of plasma oxytocin, corticotropin, and corticosterone. In the 5-HTT knockout (-/-) mice, the hypothermic response to 8-OH-DPAT (0.1 mg/kg s.c.) was completely abolished. Furthermore, 5-HTT-/- mice had significantly attenuated plasma oxytocin and corticosterone responses to 8-OH-DPAT. No significant changes in the hypothermic or hormonal responses to 8-OH-DPAT were observed in heterozygous (5-HTT+/-) mice. [(3)H]8-OH-DPAT- and [(125)I]MPPI [4-(2'-methoxyphenyl)-1-[2'-[N-(2"-pyridinyl)-iodobenzamido]ethyl] pip erazine]-binding sites in the hypothalamus and [(125)I]MPPI-binding sites in the dorsal raphe were significantly decreased in 5-HTT-/- mice. The results indicate that lack of the 5-HTT is associated with a functional desensitization of 5-HT(1A) receptor responses to 8-OH-DPAT, which may be a consequence, at least in part, of the decrease in density of 5-HT(1A) receptors in the hypothalamus and dorsal raphe of 5-HTT-/- mice.     

The time-course for up- and down-regulation of the cortical 5-hydroxytryptamine (5-HT)2A receptor density predicts 5-HT2A receptor-mediated behavior in the rabbit

5-Hydroxytryptamine (serotonin; 5-HT)2 receptor agonists such as (+/-)-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) injected systemically or directly into frontal cortex, elicit stereotyped head movements that are mediated by 5-HT2A receptors. Chronic administration of 5-HT2A receptor antagonists can produce either a down-regulation, e.g., d-2-bromolysergic acid diethylamide (BOL) or an up-regulation, e.g., alpha-phenyl-10(2-phenylethyl)-4-piperidinemethanol (MDL11,939) of cortical 5-HT2A receptors in the rabbit with no change in the density of the 5-HT2C receptor. We examined the degree to which the time course for the onset and offset of changes in cortical 5-HT2A receptor density was correlated with functional changes as measured by the magnitude of DOI elicited, 5-HT2A receptor-mediated head movements (head bobs). First, the magnitude of DOI-elicited head bobs was measured over 1 to 8 days after chronic BOL (5.8 micromol/kg), MDL11,939 (10 micromol/kg), or vehicle administration. Second, rabbits were injected with BOL, MDL11,939, or vehicle once daily for 8 days, and then, 1 to 8 days after the cessation of drug or vehicle, DOI-elicited head bobs were determined. Samples of frontal cortex were obtained for each animal immediately following behavioral testing, and 5-HT2A receptor density was measured using [3H]ketanserin. Thus, each animal provided a value for receptor density and number of head bobs, and these two measures showed a high degree of correlation between 0.94 for BOL and 0.95 for MDL11,939. This study establishes that the density of 5-HT2A receptors in cortex reflects their functional status.