The effects of the peptide-coupling agent, EEDQ, on 5-HT2A receptor binding and function in rat frontal cortex.

This ex vivo study in rat frontal cortex determined the influence of 5-HT receptor agonists and antagonists on EEDQ-induced depletion of 5-HT2A binding sites and reduction in their functional coupling to phospholipid hydrolysis. Twenty-four hours after EEDQ (6 mg/kg) administration a marked reduction (66%) of cortical 5-HT2A binding sites with no change in binding affinity was observed. The 5HT2A antagonists ritanserin (1 mg/kg), ketanserin (1 and 5 mg/kg), metergoline (3 mg/kg) or the 5HT2A agonist, DOI (3 and 10 mg/kg) also significantly reduced (by 15-44%) these binding sites 24 h after injection. Thirty minute pretreatment with ritanserin, ketanserin, metergoline or DOI (at the doses above) afforded 49-65% protection against the loss of 5-HT2A binding sites induced by EEDQ (6 mg/kg). DOI (10 mg/kg) pretreatment (-24 h) decreased by 26% the accumulation of [3H]inositol phosphates (IPs) evoked by 5-HT (100 microM), but did not affect that produced by DOI (100 microM). Ketanserin (5 mg/kg, -24 h) decreased 5-HT- and DOI-induced IP formation by 65% and 53%, respectively. The EEDQ (6 mg/kg, -24 h)-evoked reductions (-50%) of 5-HT- and DOI-induced IP formation were not altered by DOI (10 mg/kg) or ketanserin (5 mg/kg) given 30 min before EEDQ. G-protein-stimulated IP accumulation was unaffected by EEDQ (6 mg/kg). Overall, EEDQ reduces 5-HT2A binding sites and function in rat frontal cortex, whereas its effects on binding were attenuated by various 5-HT receptor antagonists and agonists, its effects on function was unaltered by these drugs.     

Cessation of chronic nicotine administration enhances wet-dog shake responses to 5-HT2 receptor stimulation in rats

RATIONALE: The involvement of central serotonergic systems has been hypothesized clinically to contribute to nicotine withdrawal symptoms. However, involvement of the serotonin2 (5-HT(2)) receptor system in nicotine withdrawal is not clear. OBJECTIVES: The changes in wet-dog shake responses induced by (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a selective 5-HT(2) receptor agonist, following nicotine cessation was investigated in rats. METHODS: DOI (1 mg/kg SC) was administered 24 h after the final treatment of saline or nicotine (0.5 mg/kg per day SC) for 7 or 21 days. RESULTS: Cessation of nicotine administration for 7 or 21 days increased DOI-induced wet-dog shake responses. A single administration of nicotine (0.5 mg/kg SC) had no effect on DOI-induced wet-dog shakes. The enhancement by the cessation of nicotine treatment for 7 days was abolished by coadministration of nicotine. Mecamylamine (3 mg/kg IP), a nicotinic receptor antagonist, precipitated DOI-induced wet-dog shake responses in rats chronically treated with nicotine but not with saline. CONCLUSIONS: These findings suggest that cessation of chronic nicotine produced increased sensitivity to 5-HT(2) receptor systems, and that the 5-HT(2) receptor systems may be involved in the nicotine withdrawal symptoms.     

Effect of nicotine cessation on the central serotonergic systems in mice: involvement of 5-HT(2) receptors

In the present study, the changes in head twitch responses induced by (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT(2) receptor agonist, following nicotine cessation or during nicotinic receptor antagonist-precipitated withdrawal were investigated in mice. DOI-induced head twitch responses did not change 1.5 h after the final nicotine (0.5 mg/kg per day, s.c. for 7 days) administration, but it increased 24 h later. The increase in head twitch responses induced by nicotine (0.05-1 mg/kg per day) was dose-dependent. The 5-HT turnover ratio (5-HIAA/5-HT) in mouse whole brain was significantly decreased 24 h after the final nicotine administration (0.5 mg/kg per day for 7 days). Mecamylamine, a non-competitive nicotinic receptor antagonist, and dihydro-beta-erythroidine, a competitive alpha(4)beta(2) nicotinic receptor antagonist, precipitated DOI-induced head twitch responses in mice repeatedly treated with nicotine but not with saline, indicating the involvement of alpha(4)beta(2) nicotinic receptors. The present findings suggest cessation of repeated nicotine administration results in increased sensitivity to 5-HT(2) receptor systems and decreased 5-HT turnover, and that these phenomena may be related to the manifestation of nicotine withdrawal symptoms.     

Effects of the 5-HT2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on plasma glucose and glucagon levels of rats

Effects of the 5-hydroxytryptamine (5-HT)2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on plasma glucagon levels were studied in rats. Systemic injection of DOI induces significant increases in plasma glucagon levels. Hyperglucagonemia induced by DOI was dose-dependently prevented by the 5-HT2A receptor antagonist ketanserin. Adrenodemedullation abolished hyperglucagonemia elicited by DOI. Previous report demonstrated that the peripheral 5-HT2A receptor agonist induces hyperglycemia in rats but does not increase plasma glucagon levels at doses inducing hyperglycemia. Therefore, our findings suggest that DOI-induced glucagon release was elicited by stimulation of the central 5-HT2A receptor, which in turn increasing adrenaline release.     

Temporal differential adaptation of head-twitch and ear-scratch responses following administration of challenge doses of DOI

Previously, we reported that administration of the 5-HT_2A/C receptor agonist, DOI [(±)-1-(2,5-Dimethoxy-4-iodophenyl)-2aminopropane], can simultaneously produce the head-twitch response (HTR) and the ear-scratch response (ESR) in mice. Our recent studies have indicated that the HTR is a 5-HT_2A receptor-mediated phenomenon, whereas the ESR is probably a 5-HT_2C receptor-mediated event. The HTR and ESR exhibit subsensitivity to a challenge dose of DOI (2.5 mg/kg) administered 24 h after its acute or termination of its chronic (2.5 mg/kg, once daily for 13 days) administration. When the dose interval for the challenge dose of DOI was increased to 48 h, both the acute- and chronically treated mice exhibited a simultaneous supersensitive HTR response and a subsensitive ESR effect. The purpose of the present study was to investigate the dose-response effects of lower challenge doses of DOI 48 h following their respective first injections as well as determining the effects of repeated DOI injections at 2-h intervals for 8 h. Thus, in the present study, initial administration of DOI produced a dose-and time-dependent increase in the mean frequencies of both HTR and ESR. Significant HTRs were observed after administration of the lowest tested dose of DOI (0.25 mg/kg), whereas a robust frequency of ESR was only evident at 1 mg/kg or greater doses of DOI. A 48-h challenge administration of lower doses of DOI (0.25 and 0.5 mg/kg) did not significantly affect their respective first injection HTR scores. However, larger challenge doses of DOI (1 and 2.5 mg/kg) produced supersensitivity in the mean HTR score (+46% and +40%, respectively, p < 0.05) and subsensitivity in the mean ESR frequency (−92% and −67%, respectively, p < 0.05) relative to their first injection control values. All administered doses of DOI (0.25, 0.5, or 1 mg/kg) eventually significantly reduced their first injection (control) HTR scores when injected repeatedly at 2-h intervals. Significant HTR reductions were attained quicker for the larger DOI doses. It appears that a mouse needs to receive either cumulatively or in a single injection about 1 mg/kg dose of DOI prior to exhibiting a significant reduction in the HTR score in response to further administration of DOI. As with their initial first injection ESR scores, repeated administration of lower doses of DOI (0.25 and 0.5 mg/kg) did not produce a significant effect. However, the 1 mg/kg challenge dose of DOI, 2 h following its first injection, nearly completely attenuated its first injection control ESR score. Further repeated injections of DOI at 2-h intervals did not cause additional alteration in the mean ESR score. The present results indicate that adaptation mechanisms for the DOI-induced HTR and ESR are different, and this difference is probably a reflection of the adaptation mechanisms of the 5-HT_2A- and 5-HT_2C-receptor function.     

Repeated administration of low doses of cocaine enhances the sensitivity of 5-HT2 receptor function.

The acute and chronic effects of cocaine were evaluated on the 5-hydroxytryptamine (5-HT)-receptor 5-HT2 mediated behavioral function, the head-twitch response (HTR), in mice. In a recent study, we reported that the (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI)-induced HTR was dose dependently reduced by cocaine via indirect stimulation of serotonergic 5-HT1A and adrenergic alpha 2 receptors. In the present investigation, the HTR was evoked by the nonselective 5-HT agonist 5-methoxy-N,N-dimethyltryptamine hydrogen oxolate (5-MeO-DMT). Cocaine by itself failed to produce HTR but dose dependently inhibited the 5-MeO-DMT-induced behavior. Cocaine's effects were not due to 5-HT3 antagonism since acute administration of the more potent 5-HT3 antagonist (ICS-205,930) failed to produce or modify the 5-MeO-DMT-induced behavior. During withdrawal from chronic cocaine treatment (5-20 mg/kg), 5-MeO-DMT-induced HTR was enhanced. Depending upon the cocaine dose used, the induced supersensitivity persisted up to 172 h following cessation of cocaine treatment. The mechanisms of cocaine-induced supersensitivity were further investigated using the more selective 5-HT2 agonist DOI. Withdrawal from a low-dose (0.03-1.25 mg/kg) chronic cocaine treatment caused the DOI-induced HTR to increase, whereas withdrawal from a 5- and 10-mg/kg cocaine regimen had no significant effect. The maximal effect persisted up to 36 h following termination of cocaine treatment. Relative to vehicle-exposed controls, withdrawal from cocaine treatment enhanced the inhibitory potency of the 5-HT1A agonist (+-)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT) on DOI-induced HTR.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of the 5-HT1C/5-5-HT2 receptor agonists DOI and alpha-methyl-5-HT on plasma glucose and insulin levels in the rat

Administration of the 5-HT1C/5-HT2 receptor agonist 1-(2,5-dimethoxy-4- iodophenyl)-2-aminopropane (DOI, 0.125-2.0 mg/kg i.v.) triggered dose-dependent increases in plasma glucose; plasma insulin levels remained unchanged. Pretreatment with the 5-HT1C/5-HT2 receptor antagonists LY 53857, ritanserin, or the mixed 5-HT2/alpha 1-adrenoceptor antagonist ketanserin either diminished or prevented the hyperglycemic effect of DOI (0.5 mg/kg). Administration of the mixed 5-HT1C receptor agonists/5-HT2 receptor antagonists 1-(3-chlorophenyl)-piperazine (mCPP) or 1-(3-trifluoromethyl)phenyl)piperazine level (TFMPP) did not affect plasma glucose levels. However, pretreatment with mCPP or TFMPP decreased DOI-induced hyperglycemia in a dose-dependent manner. The alpha 2-adrenoceptor antagonist idazoxan and the ganglionic blocker hexamethonium both decreased DOI-induced hyperglycemia, Whilst the alpha 1-adrenoceptor antagonist prazosin amplified the rise in plasma glucose elicited by DOI. The peripherally acting 5-HT1C/5-HT2 receptor agonist alpha-methyl-5-HT (0.5-1.0 mg/kg i.v.) triggered a rise in plasma glucose levels that was associated with an increase in plasma insulin levels. Pretreatment with LY 53857 diminished alpha-methyl-5-HT-induced hyperglycemia. These data indicate that 5-HT2 receptors, but not 5-HT1C receptors, and catecholaminergic systems, mediate DOI-induced hyperglycemia. Moreover, it is suggested that the inhibition of insulin release by DOI is centrally mediated, and that activation of peripheral 5-HT2 receptors may affect glycemia.     

Regulation of 5-HT2 receptors in rat cortex. Studies with a putative selective agonist and an antagonist

The phenylisopropylamine derivative 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI) has been suggested recently as a selective serotonin2 (5-HT2) receptor agonist. Because of the potential importance of such a tool for investigations of 5-HT2 receptor regulation, receptor binding studies were performed in rats after acute and chronic treatment with DOI, the selective 5-HT2 antagonist ketanserin, or vehicle. Single injections of 5 or 10 mg/kg DOI reduced the Bmax of cortical sites labeled with [3H]1-(2,5-dimethoxy-4-bromo-phenyl)-2-aminopropane and [3H]ketanserin (9-32 or 32-46%, respectively). Chronic daily treatment with DOI (3-9 mg/kg) further down-regulated 5-HT2 sites in cortex identified with either [3H]ketanserin (-60%) or with [3H]DOB (-75%), without altering Kd values or affecting 5-HT1 sites. In vitro addition to the [3H]ketanserin or [3H]DOB binding assay of 10 nM to 1 microM DOI resulted in competitive inhibition, suggesting that down-regulation found in vivo was not secondary to residual drug. Chronic treatment with ketanserin (10 mg/kg) also down-regulated both [3H]ketanserin (-38%) and [3H]DOB (-58%) sites in cortex without charges in 5-HT1 sites. In naive cortex, competition experiments revealed a Ki (nM) for ( +/- )-DOI of 1.7 +/- 0.02 at sites labeled by [3H]DOB, and a KH and KL of 4.8 +/- 1.5 and 53 +/- 2 nM at sites labeled by [3H]ketanserin. These data indicate that in chronic treatment, DOI, like ketanserin, is highly selective for 5-HT2 vs 5-HT1 sites at behaviorally useful doses. However, a representative putative 5-HT2 selective agonist and antagonist have similar effects on 5-HT2 receptors labeled by agonist or antagonist radioligands.     

Functional role of the endocannabinoid system and AMPA/kainate receptors in 5-HT2A receptor-mediated wet dog shakes

These experiments sought to determine the influence of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and the endocannabinoid system in the functional expression of the serotonin (5-HT) type 2A receptor-mediated wet dog shake response. Male Long-Evans rats were pretreated with either 1 mg/kg i.p. of the 5-HT(2A/2C) receptor antagonist ketanserin; 1, 10 or 30 mg/kg i.p. of the AMPA/kainate antagonist 6,7-dinitroquinnoxaline-2,3-dione (DNQX); 1, 5 or 10 mg/kg i.p. of the endocannabinoid uptake inhibitor AM404; or 1, 5 or 10 mg/kg i.p. of the cannabinoid CB(1) receptor antagonist AM 251 prior to injection of the 5-HT(2A/2C) receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI, 1 mg/kg i.p.). Results demonstrated that 10 mg/kg of AM404 significantly reduced the expression of DOI-induced wet dog shakes, but lower doses were ineffective. Administration of AM251 did not induce wet dog shakes behavior when administered alone, but significantly potentiated DOI-induced wet dog shaking behavior at a dose of 10 mg/kg. Pretreatment with DNQX significantly reduced the expression of DOI-induced wet dog shakes at all doses tested. These data suggest that AMPA/kainate receptors play a role in the mediation of 5-HT(2A) receptor activity, whereas the endocannabinoid system may act as a regulatory buffer system during periods of elevated activity, but not under basal conditions.     

Inhibition of DOI-induced wet dog shakes in the guinea-pig by 5-HT2 receptor antagonists

The preferential 5-HT( 2)/5-HT(1C) receptor agonist DOI (0.1-4 mg/kg s.c.) caused an increase in locomotor activity, grooming and 'wet-dog' shakes (WDS) in the adult guinea-pig. The DOI-induced WDS behaviour was potently inhibited by several antagonists that have high affinity for the 5-HT(2) binding site. The WDS response is likely to be centrally-mediated since the effects of peripherally administered DOI were poorly antagonized by the peripherally-acting 5-HT(2) receptor antagonist BW501C67. Although these studies do not exclude an effect of DOI at 5-HT(1C) receptors, the high potency of ketanserin and spiperone in attenuating the effects of DOI would suggest an effect at the 5-HT(2) receptor. The present data suggest that antagonism of the directly-acting agonist DOI may be useful for assessing the selectivity and duration of action of centrally-acting 5-HT(2) receptor antagonists in the guinea-pig.     

DOI-induced inhibition of copulatory behavior in male rats: reversal by 5-HT2 antagonists.

Relatively little is known regarding the role of 5-HT2 receptor activity in male rat sexual behavior. Previous work has yielded equivocal results, and both facilitation and inhibition of copulation have been reported to follow administration of selective 5-HT2 antagonists. In the present series of experiments, the ability of a variety of 5-HT2 antagonists to block inhibition induced by the 5-HT2/5-HT1C agonist DOI was examined. Systemic ritanserin, pirenperone and ketanserin all potently blocked DOI-induced (1.0 mg/kg SC) inhibition of mounts, intromissions and ejaculations. None of these drugs influenced the sexual behavior of the male rats when given alone in doses that effectively blocked DOI-induced inhibition. In addition, unlike ritanserin and ketanserin, pirenperone produced a biphasic effect on DOI-induced inhibition, exhibiting a diminished blockade at higher doses. This may be due to activity at receptors other than 5-HT2. Overall, the present data suggest that activity at 5-HT2 receptors mediates an inhibition of male rat sexual behavior.     

Facilitation of 8-OHDPAT-induced forepaw treading of rats by the 5-HT2 agonist DOI

The potency of the serotonin 1A (5-HT1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), to induce forepaw treading was increased 20-fold after co-treatment with the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). DOI induced head twitches which were inhibited by 8-OHDPAT. The putative 5-HT1B agonist, 1-(3-trifluoromethylphenyl)piperazine (TFMPP), had a weak effect on the responses to DOI or 8-OHDPAT. The forepaw treading induced by 8-OHDPAT plus DOI was inhibited by high doses of (-)-alprenolol, ketanserin or ritanserin, but was not influenced by the beta-adrenoceptor antagonist, ICI 118.551, or the 5-HT3 antagonist, ICS 205-930. A non-effective dose of (-)-alprenolol increased the inhibitory effect of ketanserin and ritanserin. These results indicate a complex and different interaction between 5-HT1A and 5-HT2 receptors in the expression of two behavioural responses mediated by 5-HT.     

The 5-HT(2) receptor activation enhances impulsive responding without increasing motor activity in rats

The effects of 5-HT(2) receptor ligands on the performance of rats were investigated using a 5-choice serial reaction time (5-CSRT) task. Systemic administration of DOI (0.03 to 0.3 mg/kg subcutaneously [SC]), a 5-HT(2) receptor agonist, did not impair choice accuracy of well-performing rats under either baseline conditions or more demanding conditions of the task, in which the stimulus duration or intensity were reduced or the intertrial interval (ITI) was decreased. DOI (0.1 mg/kg or 0.15 mg/kg) increased premature responding (the probability of intertrial interval hole pokes) in all testing conditions, except under conditions of a short ITI when the rats did not make any hole responses. Ketanserin (0.1 to 0.3 mg/kg SC), a 5-HT(2A) receptor antagonist, had no marked effect on performance. When combined with ketanserin (0.2 mg/kg SC), however, DOI (0.1 mg/kg) did not increase premature responding. The lowest doses of DOI (0.05 and 0.1 mg/kg) that increase premature responding had no effect on open-field performance. Further, the effects of systemically administered DOI were not reproduced by bilateral administration of DOI into the anterior cingulate cortex. These data indicate that excessive activation of 5-HT(2A/2C) receptors interferes with response control rather than visual attention. Furthermore, the DOI-induced enhancement of impulsive responses are not due to locomotor hyperactivity, and the anterior cingulate cortex is not the primary site of action for this enhancement of premature responding.     

Evidence that 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced hyperthermia in rats is mediated by stimulation of 5-HT2A receptors

The effects of various 5-HT receptor subtype-selective antagonists were studied on phenylisopropylamine hallucinogen1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-induced hyperthermia in Wistar rats, in an attempt to characterize the 5-HT receptor subtype mediating DOI-induced hyperthermia. Intraperitoneal administration of DOI to rats produced hyperthermia with a peak effect at 60 min. Pretreatment with propranolol (-adrenoceptor antagonist that also has binding affinity for 5-HT_1A, 5-HT_1B and 5-HT_2C sites), MDL-72222 or ondansetron (5-HT₃ antagonists) did not attenuate DOI-induced hyperthermia. In contrast, pretreatment with metergoline (5-HT₁/5-HT₂ antagonist), ketanserin, LY53857, mesulergine, mianserin and ritanserin (5-HT_2C/5-HT_2A antagonists), as well as spiperone (5-HT_1A/5-HT_2A/D₂ antagonist), significantly attenuated DOI-induced hyperthermia. Furthermore, daily administration of DOI (2.5 mg/kg per day) for 17 days did not produce either tolerance to its hyperthermic effect or modifym-CPP-induced hyperthermia in rats. These findings suggest that DOI-induced hyperthermia in rats is mediated by stimulation of 5-HT_2A receptors.     

Serotonin-glutamate interaction in rat cerebellum: involvement of 5-HT1 and 5-HT2 receptors

The effects of serotonin (5-HT) on the release of endogenous glutamate (GLU) in rat cerebellum were investigated in slices depolarized with 35 mM K+. The Ca2+-dependent release of GLU was potently inhibited by 5-HT in a concentration-dependent way. Release was also inhibited by the 5-HT1 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). The inhibition by 10 nM 5-HT was partly (35-40%) counteracted by the 5-HT2 receptor antagonist ketanserin but was fully blocked by the mixed 5-HT1/5-HT2 receptor antagonist methiothepin. The effect of 8-OH-DPAT was not affected by ketanserin but was totally antagonized by methiothepin, while the effect of DOI was entirely suppressed by ketanserin. Ketanserin or methiothepin themselves increased (by 23 and 55%, respectively, at 10 nM) the K+-evoked release of GLU. In conclusion the release of endogenous GLU in rat cerebellum can be inhibited by 5-HT through receptors of the 5-HT1 and 5-HT2 type. The enhancement of GLU release by ketanserin or methiothepin could suggest a tonic inhibition. The possible localization of the 5-HT receptors involved in the interaction with the GLU systems is discussed.     

Repeated cocaine modifies the neuroendocrine responses to the 5-HT1C/5-HT2 receptor agonist DOI.

Endocrine responses to the serotonin (5-HT) 5-HT1C/5-HT2 agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) were utilised to evaluate cocaine-induced alterations in postsynaptic 5-HT receptor function. Rats received cocaine HCl (0, 5 or 15 mg/kg i.p.) twice daily for 7 days. Effects of DOI (0, 0.5, 2 or 10 mg/kg i.p.) on plasma adrenocorticotropic hormone (ACTH), corticosterone, prolactin, oxytocin and renin concentrations were assessed 42 h after the final cocaine injection. DOI dose dependently increased the plasma concentrations of each hormone. Cocaine potentiated the DOI-induced elevations of plasma ACTH, corticosterone and prolactin concentrations. In contrast, the oxytocin response was reduced, and the renin response was unaltered by cocaine exposure. The data suggest that 5-HT2 receptor-mediated responses for ACTH, corticosterone and prolactin secretion become supersensitive following repeated cocaine. In contrast, the 5-HT2 receptor-mediated response for oxytocin secretion is subsensitive. The cocaine-induced changes in postsynaptic 5-HT receptor function are likely a consequence of deficits in the function of 5-HT nerve terminals, that we have documented previously.     

Hypothalamic paraventricular 5-hydroxytryptamine: receptor-specific inhibition of NPY-stimulated eating and energy metabolism

The feeding effects of 5-hydroxytryptamine (5-HT)(1) and 5-HT(2) receptor agonists injected into the hypothalamic paraventricular nucleus (PVN) immediately prior to PVN administration of neuropeptide Y (NPY) were examined. The impact of these same compounds on NPY-induced alterations in energy metabolism was also assessed in an attempt to characterize further the potential interactive relationship of PVN NPY and 5-HT on feeding and whole body calorimetry. Specifically, several experiments examined the effect of various 5-HT receptor agonists on NPY-stimulated eating and alterations in energy substrate utilization [respiratory quotient (RQ)]. This included the 5-HT(1A) receptor agonist 8-OH-DPAT, the 5-HT(1B/1A) agonist RU 24969, the 5-HT(1D) agonist L-694,247, the 5-HT(2A/2C) agonist DOI, the 5-HT(2B) agonist BW 723C86 and the 5-HT(2C) agonist mCPP. In feeding tests conducted at the onset of the dark cycle, drugs were administered 5 min prior to PVN injection of NPY and food intake was measured 2 h postinjection. The metabolic effects of NPY following a similar pretreatment were monitored using an open-circuit calorimeter measuring the volume of oxygen consumed (VO(2)), carbon dioxide produced (VCO(2)) and RQ (VCO(2)/VO(2)). PVN injection of NPY (100 pmol) potentiated feeding and evoked reliable increases in RQ. Only DOI (2.5--5 nmol) pretreatment antagonized NPY-induced eating and blocked the peptide's effect on energy substrate utilization. Direct PVN pretreatment with spiperone (SPRN), a 5-HT(2A) receptor antagonist, and ketanserin (KTSN), a 5-HT(2A/2C) antagonist, but not SDZ SER 082, a 5-HT(2B/2C) antagonist, or the 5-HT(2C) antagonist RS 102221, blocked the effect of DOI in both feeding and metabolic tests providing additional evidence that activation of PVN 5-HT(2A) receptors inhibits NPY's action on feeding and substrate utilization.     

Regulation of 5-HT(2A) receptor mRNA levels and binding sites in rat frontal cortex by the agonist DOI and the antagonist mianserin

In the present study we have characterized the time course of effect of administration of the serotonin(2) (5-HT(2)) receptor antagonist mianserin, or the 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), on 5-HT(2A) receptor binding sites and mRNA levels in rat frontal cortex. Radioligand binding and ribonuclease protection assays were performed with separate hemispheres of frontal cortex from each animal to examine concomitant changes in 5-HT(2A) receptor sites and mRNA levels. The decrease in cortical 5-HT(2A) receptor sites in response to chronic DOI administration was not accompanied by changes in 5-HT(2A) receptor mRNA. A single injection of DOI produced a transient decrease in 5-HT(2A) receptor mRNA levels detected 1 h post-injection. The density of 5-HT(2A) receptor sites, however, was not significantly reduced following a single injection of DOI. The down-regulation of cortical 5-HT(2A) receptor sites in response to a single injection of mianserin was accompanied by reductions in 5-HT(2A) receptor mRNA levels. Following 4 days of mianserin administration, however, we did not observe a change in 5-HT(2A) receptor mRNA levels, although 5-HT(2A) receptor density was decreased. Thus, changes in receptor mRNA may initially contribute to the down-regulation of 5-HT(2A) receptors in response to acute mianserin administration. Sustained changes in 5-HT(2A) receptor mRNA, however, appear not to be involved in maintaining the down-regulation of 5-HT(2A) receptor number with chronic mianserin administration. Mechanisms other than the regulation of receptor mRNA levels appear to underlie the down-regulation of 5-HT(2A) receptor sites in response to chronic administration of the agonist DOI.     

Cannabinoids of diverse structure inhibit two DOI-induced 5-HT(2A) receptor-mediated behaviors in mice

We have recently shown that the selective cannabinoid CB(1) receptor antagonist SR 141716A produces robust frequencies of head-twitch response (HTR) and ear-scratch response (ESR) in drug-naive mice. Both behaviors were potently blocked by the selective 5-HT(2A/C) receptor antagonist SR 46349B. Selective 5-HT(2A/C) agonists such as DOI also produce these behaviors in mice. The purpose of the present study was to: (1) investigate whether Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and its analogs [Delta(8)-tetrahydrocannabinol (Delta(8)-THC), HU-210, CP 55,940, and WIN 55,212-2] can prevent the DOI-induced behaviors and (2) to see whether any correlation exists in the ID(50) potency order of these cannabinoids in inhibiting the DOI-induced HTR and ESR relative to their published ED(50) potency profiles in producing the tetrad of behaviors in mice. Thus, at 0 min, different groups of mice were injected intraperitoneally with either vehicle or varying doses of the following cannabinoids: Delta(9)-THC (0.25-20 mg/kg), Delta(8)-THC (2.5-20 mg/kg), HU-210 (0.02-0.5 mg/kg), CP 55,940 (0.004-0.5 mg/kg), and WIN 55,212-2 (0.5-10 mg/kg). Twenty minutes later, each mouse received an intraperitoneal injection of DOI (1 mg/kg) and the frequencies of DOI-induced behaviors (mean +/- S.E.M.) were recorded for the next 20 min. The tested cannabinoids reduced the frequencies of both DOI-induced HTR and ESR in a dose-dependent fashion. HU-210 was the most potent inhibitor of HTR, whereas CP 55,940 was most effective against ESR. The ID(50) potency order of cannabinoids in blocking the HTR is: HU-210 > CP 55,940 > WIN 55,212-2 > Delta(9)-THC > Delta(8)-THC, which is identical to their published order of potency in producing the tetrad of behaviors in mice. On the other hand, they had the following ID(50) potency order against the ESR: CP 55,940 > HU-210 > WIN 55,212-2 > Delta(9)-THC > Delta(8)-THC. The tested cannabinoids were 3-30 times more potent in preventing the ESR than the HTR. The data show that cannabinoids inhibit 5-HT(2A) receptor-mediated functions in a potent but differential manner.     

A novel behavioral model that discriminates between 5-HT2A and 5-HT2C receptor activation

2,5-Dimethoxy-4-iodoamphetamine (DOI), a serotonin (5-HT)2A/2C receptor agonist, elicits shaking behaviors in rodents, which have been reliably quantified as behavioral correlates of 5-HT2A receptor activation. Such studies are lacking in the rabbit. As part of our research examining the role of the 5-HT2 receptor in rabbits, we analyzed the behavioral effects of systemically administered DOI in rabbits. DOI (0.01-3 micromol/kg) or vehicle was injected, and two distinct behaviors, head bobs (vertical head movements) and body shakes (wet dog shakes), were counted for 90 min following the injection. DOI dose-dependently increased the number of head bobs and body shakes. The selective 5-HT2A receptor antagonist ketanserin (1-3 micromol/kg), 1 h before DOI (0.3 micromol/kg) challenge, significantly attenuated head bobs, but not body shakes. In contrast, the selective 5-HT2C receptor antagonists SDZ SER 082 (1-3 micromol/kg) and SB 206553 (1 micromol/kg) 30 min before challenge, significantly reduced body shakes but not head bobs produced by the same dose of DOI. This study establishes that, in rabbits, DOI mediates head bobs via 5-HT2A receptors and body shakes via 5-HT2C receptors. Thus, the rabbit provides a novel behavioral assay that discriminates between 5-HT2A and 5-HT2C receptor activation.