Effects of d-amphetamine and DOI (2,5-dimethoxy-4-iodoamphetamine) on timing behavior: interaction between D1 and 5-HT2A receptors

RATIONALE: The dopamine-releasing agent d-amphetamine and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behavior. The selective D(1) dopamine receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), but not the D(2) dopamine receptor antagonist haloperidol, can antagonize the effect of d-amphetamine, and the selective 5-HT(2A) receptor antagonist (+/-)2,3-dimethoxyphenyl-1-(2-(4-piperidine)-methanol (MDL-100907) can antagonize the effect of DOI. However, it is not known whether the effect of d-amphetamine can be reversed by MDL-100907 and the effect of DOI by dopamine receptor antagonists. OBJECTIVE: The objective of this work is to examine the interactions of d-amphetamine and DOI with MDL-100907, SKF-83566, and haloperidol on timing performance. MATERIALS AND METHODS: Rats (n = 12-15 per experiment) were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices [T (50) (time corresponding to %B = 50); Weber fraction]. Rats were treated systemically with d-amphetamine or DOI, alone and in combination with haloperidol, SKF-83566, or MDL-100907. RESULTS: d-Amphetamine (0.4 mg kg(-1)) reduced T (50) compared to vehicle; this effect was antagonized by SKF-83566 (0.03 mg kg(-1)) and MDL-100907 (0.5 mg kg(-1)), but not by haloperidol (0.05, 0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced T (50); this effect was reversed by MDL-100907 (0.5 mg kg(-1)), but not by SKF-83566 (0.03 mg kg(-1)) or haloperidol (0.05 mg kg(-1)). CONCLUSIONS: The results suggest that both 5-HT(2A) and D(1) receptors, but not D(2) receptors, are involved in d-amphetamine's effect on timing behavior in the free-operant psychophysical procedure. DOI's effect on timing is mediated by 5-HT(2A) receptors, but neither D(1) nor D(2) receptors are involved in this effect.     

Tolerance to the effect of 2,5-dimethoxy-4-iodoamphetamine (DOI) on free-operant timing behaviour: interaction between behavioural and pharmacological mechanisms

RATIONALE: The psychostimulant d-amphetamine, the D(2/3) dopamine receptor agonist quinpirole and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behaviour. There is evidence that tolerance develops to the effects of psychostimulants on timing performance during chronic treatment; this tolerance is generally attributed to behavioural adaptation rather than to pharmacological desensitisation. There have been no previous investigations of tolerance to the effect of DOI on free-operant timing behaviour. OBJECTIVE: To demonstrate tolerance to DOI's effect on timing performance and to examine the nature of this tolerance. MATERIALS AND METHODS: Rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 80-s trials in which reinforcement was provided intermittently for responding on A in the first half and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 8-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices (T (50) [time corresponding to %B = 50]; Weber fraction). RESULTS: In experiment 1, DOI (0.25 mg kg(-1)) reduced T (50) compared to vehicle; tolerance to this effect was seen after repeated daily treatments with DOI if the rats were exposed to behavioural training during the period of treatment but not if the repeated treatments took place during a 'holiday' from behavioural training. In experiment 2, repeated treatment with DOI resulted in tolerance to the effect of DOI on T (50) and cross-tolerance to the effect of d-amphetamine (0.4 mg kg(-1)), but no cross-tolerance was seen to the effect of quinpirole (0.08 mg kg(-1)). CONCLUSIONS: The results indicate that behavioural adaptation is involved in the development of tolerance to DOI's effect on timing. The finding of cross-tolerance to d-amphetamine but not to quinpirole suggests that the reduction of T (50) in the free-operant psychophysical procedure may be brought about by two distinct pharmacological mechanisms, one activated by DOI and d-amphetamine, and the other by quinpirole.     

Mechanism of tolerance development to 2,5-dimethoxy-4-iodoamphetamine in rats: down-regulation of the 5-HT2A, but not 5-HT2C, receptor

  Rationale: Defining the mechanism of tolerance development to hallucinogenic drugs will help to explain their mechanism of action. Objectives: The present study was conducted to determine first, if tolerance develops to the discriminative stimulus (DS) properties of the hallucinogen, 2,5 dimethoxy-4-iodo-amphetamine (DOI) and second, the mechanism mediating tolerance. Methods: Rats were trained to discriminate 0.75 mg/kg DOI from saline on a concurrent VI-30-min schedule of reinforcement with a 15-min time-out for incorrect responses. To evaluate tolerance development, rats were assigned to one of four groups and treated with either chronic saline or chronic DOI. Prior to chronic treatment, two groups were tested for choice behavior following vehicle administration while the remaining two groups were tested following the administration of 0.375 mg/kg DOI. One group from each pre-test condition was injected with either saline or DOI (1 mg/kg) for 8 days. Twenty-four hours after the last chronic injection the pre-test treatments were replicated. Using receptor autoradiography, the density of 5-HT_2A and 5-HT_2C receptors was measured in independent groups of rats that had received identical treatment conditions. Results: Animals receiving chronic DOI showed a 60% decrease in DOI lever responding (from 100% to 40%) when tested on 0.375 mg/kg DOI, while animals receiving chronic saline showed no change in percent choice (100%) on the DOI lever. Significant changes in binding were observed in 5-HT_2A receptors but not 5-HT_2C receptors. The results of tests with antagonists were consistent with the changes in binding. Conclusions: These results suggest that behavioral tolerance to DOI reflects neuroadaptive changes in 5-HT_2A receptors. Received: 17 July 1998 / Final version: 19 January 1999     

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.     

Characterization of DOI, a putative 5-HT2 receptor agonist in the rat

DOI (1-100 micrograms/kg i.v.) induced an increase in mean blood pressure in the anaesthetized rat. Similarly, in the pithed rat, DOI (1-100 micrograms/kg i.v.) induced a dose-dependent increase in mean blood pressure, as did 5-HT. However, in contrast to 5-HT, DOI did not change the heart rate in either intact or pithed rats. In the pithed rat, the dose-pressor response curves to both 5-HT and DOI were unaffected by MDL 72222 (5-HT3 receptor antagonist), spiroxatrine or (+/-)-pindolol (5-HT1A receptor antagonists), idazoxan (alpha 2-adrenoceptor blocking agent) and AR-C 239 (alpha 1-adrenoceptor blocking agent). Only the selective 5-HT2 receptor antagonist. LY 53857, significantly and dose dependently shifted to the right the dose-response curves to both 5-HT and DOI. These results indicated that DOI possesses 5-HT2 agonistic properties and that the pressor response induced by DOI in the pithed rat is mediated via 5-HT2 receptors.     

Investigation of the effects of IVth ventricular administration of the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), on autonomic outflow in the anaesthetized cat.

1. The effects of IVth ventricular injections of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on renal, splanchnic and cardiac sympathetic nerve activities, phrenic nerve activity, arterial blood pressure, heart rate, femoral arterial conductance, tracheal and intragastric pressures were investigated in alpha-chloralose anaesthetized, neuromuscular blocked and artificially ventilated cats. 2. Cumulative doses of DOI (80, 160 and 320 nmol kg-1) injected into the IVth ventricle caused an increase in mean arterial blood pressure, a fall in femoral arterial conductance, an increase in tracheal pressure and a decrease in the rate of phrenic nerve bursts but did not affect any of the other variables recorded. 3. Even after i.v. administration of the peripheral 5-HT2 antagonist BW501C67 (2 mg kg-1) following the highest dose of DOI there was still a significant pressor response, a fall in femoral arterial conductance and small increase in tracheal pressure. 4. In control experiments, intravenous infusion of noradrenaline to raise blood pressure to the levels obtained during the cumulative doses of DOI caused large falls in renal, splanchnic and cardiac nerve activities which were all significantly lower than those recorded during the cumulative doses of DOI. 5. The results of this study provide evidence for a brainstem site of action of DOI in producing hypertension and further support the hypothesis that central 5-HT2 receptors are involved in the control of skeletal muscle and skin vascular beds.     

Imaging brain phospholipase A2 activation in awake rats in response to the 5-HT2A/2C agonist (+/-)2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI).

Incorporation coefficients k(*) of intravenously injected [(3)H]arachidonic acid from blood into brain reflect the release from phospholipids of arachidonic acid by receptor-initiated activation of phospholipase A(2) (PLA(2)). In unanesthetized adult rats, 2.5 mg/kg intraperitoneally (i.p.) (+/-)2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), which is a 5-HT(2A/2C) receptor agonist, has been reported to produce the behavioral changes of what is known as the 5-HT(2) syndrome, but only a few small regional decrements in brain glucose metabolism. In this study, 2.5 mg/kg i.p. DOI, when administered to unanesthetized rats, produced widespread and significant increases, of the order of 60%, in k(*) for arachidonate, particularly in neocortical brain regions reported to have high densities of 5-HT(2A) receptors. The increases could be entirely blocked by chronic pretreatment with mianserin, a 5-HT(2) receptor antagonist. The results suggest that the 5-HT(2) syndrome involves widespread brain activation of PLA(2) via 5-HT(2A) receptors, leading to the release of the second messenger, arachidonic acid. Chronic mianserin, a 5-HT(2) antagonist, prevents this activation.     

The 5-HT4 receptor agonist, tegaserod, is a potent 5-HT2B receptor antagonist in vitro and in vivo

1 Tegaserod (Zelnorm) is a potent 5-hydroxytryptamine4 (5-HT4) receptor agonist with clinical efficacy in disorders associated with reduced gastrointestinal motility and transit. The present study investigated the interaction of tegaserod with 5-HT2 receptors, and compared its potency in this respect to its 5-HT4 receptor agonist activity. 2 Tegaserod had significant binding affinity for human recombinant 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi=7.5, 8.4 and 7.0, respectively). The 5-HT2B receptor-binding affinity of tegaserod was identical to that at human recombinant 5-HT4(c) receptors (mean pKi=8.4) in human embryonic kidney-293 (HEK-293) cells stably transfected with the human 5-HT4(c) receptor. 3 Tegaserod (0.1-3 microm) inhibited 5-HT-mediated contraction of the rat isolated stomach fundus potently (pA2=8.3), consistent with 5-HT(2B) receptor antagonist activity. Tegaserod produced, with similar potency, an elevation of adenosine 3',5' cyclic monophosphate in HEK-293 cells stably transfected with the human 5-HT4(c) receptor (mean pEC50=8.6), as well as 5-HT4) receptor-mediated relaxation of the rat isolated oesophagus (mean pEC50=8.2) and contraction of the guinea-pig isolated colon (mean pEC50=8.3). 4 Following subcutaneous administration, tegaserod (0.3 or 1 mg kg(-1)) inhibited contractions of the stomach fundus in anaesthetized rats in response to intravenous dosing of alpha-methyl 5-HT (0.03 mg kg(-1)) and BW 723C86 (0.3 mg kg(-1)), selective 5-HT2B receptor agonists. At similar doses, tegaserod (1 and 3 mg kg(-1) subcutaneously) evoked a 5-HT4 receptor-mediated increase in colonic transit in conscious guinea-pigs. 5 The data from this study indicate that tegaserod antagonizes 5-HT2B receptors at concentrations similar to those that activate 5-HT4 receptors. It remains to be determined whether this 5-HT2B receptor antagonist activity of tegaserod contributes to its clinical profile.     

Renal vasodilatation induced by DOI, a 5-HT2 receptor agonist, in the canine kidney

An intrarenal infusion of (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a selective 5-HT2 agonist, at a rate of 5 micrograms/min in anesthetized dogs resulted in an increase in renal blood flow (RBF) without any transient decrease as usually observed during the infusion of 5-HT. During the infusion, urine flow (UF) and urinary sodium excretion rates (UNaV) increased along with RBF while the mean arterial pressure and glomerular filtration rate did not change. After pretreatment with ritanserin, a selective 5-HT2 antagonist, DOI failed to increase RBF, UF and UNa V. It is concluded that DOI produces renal vasodilatation mediated via 5-HT2 receptors and has a diuretic action.     

R-zacopride, a 5-HT3 antagonist/5-HT4 agonist, reduces sleep apneas in rats.

The effects of R-zacopride, a benzamide with potent 5-HT3 receptor antagonist and 5-HT4 receptor agonist properties, on spontaneous apneas were studied in 10 Sprague-Dawley rats by monitoring respiration and sleep for 6 h. R-zacopride (0.5, 1.0 and 10.0 mg/kg) suppressed spontaneous central apneas during non-rapid-eye-movement (NREM) sleep by 50% (P=.05 for 0.5 mg/kg, P=.02 for 1.0 mg/kg and P=.001 for 10.0 mg/kg dose vs. control), and during rapid-eye-movement (REM) sleep by 80% by all doses tested (P<.0007) for at least 2 h after intraperitoneal injection. We conclude that R-zacopride, over a 20-fold dose range, significantly reduces central apnea expression during NREM and REM sleep in the rat. The efficacy of this compound to suppress central apneas most probably arises from its antagonist actions at 5-HT3 receptors or from its mixed agonist/antagonist profile at 5-HT4/5-HT3 receptors.     

5-HT3 receptor antagonism by anpirtoline, a mixed 5-HT1 receptor agonist/5-HT3 receptor antagonist.

1. The aim of this study was to provide evidence that anpirtoline, which is an agonist at 5-HT1B and 5-HT1D receptors and also displays submicromolar affinity for 5-HT1A recognition sites, in addition, acts as an antagonist at 5-HT3 receptors. 2. In radioligand binding studies on rat brain cortical membranes, anpirtoline inhibited specific binding of [3H]-(S)-zacopride to 5-HT3 receptor recognition sites (pKi: 7.53). 3. In N1E-115 neuroblastoma cells in which [14C]-guanidinium was used as a tool to measure cation influx through the 5-HT3 receptor channel, the 5-HT-induced influx was concentration-dependently inhibited by anpirtoline. In this respect, anpirtoline mimicked other 5-HT3 receptor antagonists; the rank order of potency was ondansetron > anpirtoline > metoclopramide. 4. The concentration-response curve for 5-HT as a stimulator of [14C]-guanidinium influx was shifted to the right by anpirtoline (apparent pA2: 7.78). 5. In urethane-anaesthetized rats, anpirtoline inhibited (at lower potency than zacopride and tropisetron) the 5-HT- or phenylbiguanide-induced bradycardia (Bezold-Jarisch reflex), but did not induce this reflex by itself. 6. Intravenous infusion of cisplatin in the domestic pig caused a consistent emetic response which was antagonized by anpirtoline. 7. It is concluded that anpirtoline, which was previously characterized as a 5-HT1 receptor agonist also proved to be a 5-HT3 receptor antagonist in several experimental models and, hence, exhibits a unique pattern of properties at different 5-HT receptors.     

Mechanisms involved in the hyperglycemic effect of the 5-HT1C/5-HT2 receptor agonist, DOI.

Previous experiments have indicated that 5-HT2 receptors and catecholaminergic systems mediate the rise in plasma glucose levels elicited by acute administration of the 5-HT1c/5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). On this basis, we investigated the location of these serotonin receptors and the nature of this catecholaminergic involvement. Administration of DOI (0.4 mg/kg i.v.) to conscious rats (bearing jugular catheters) elicited a rapid rise in plasma glucose which was associated with a decreased insulin response to a glucose bolus (300 mg/kg i.v.). Pretreatment with the peripherally acting 5-HT1c/5-HT2 receptor antagonist, BW 501C67 (0.5 mg/kg i.v. 10 min beforehand) prevented the rise in plasma glucose triggered by the peripherally acting 5-HT1c/5-HT2 receptor agonist, alpha-methyl-5-HT (0.75 mg/kg i.v.), but amplified the rise elicited by DOI. Pretreatment with chlorisondamine (1 mg/kg i.v. 10 min beforehand) or adrenalectomy 20 h beforehand prevented the DOI-induced hyperglycemia. On the other hand, pretreatment with dexamethasone (0.35 mg/kg s.c. 2 h and 20 min beforehand) did not affect the DOI-induced hyperglycemia. It is concluded that the hyperglycemic effect of DOI administration is mediated by centrally located 5-HT2 receptors and, in turn, adrenal epinephrine release.     

Effect of destruction of noradrenergic neurones with DSP4 on performance on a free-operant timing schedule

This experiment examined the effect of destroying central noradrenergic neurones, using the selective neurotoxin DSP4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine], on performance in a free-operant timing schedule. Rats received either systemic treatment with DSP4 or vehicle-alone injections. They were trained to press levers for a sucrose reinforcer. Training sessions consisted of 40, 50-s trials in which reinforcers were available on a variable-interval 25-s schedule; in the first 25 s of each trial, reinforcers were only available for responses on lever A, whereas in the last 25 s reinforcers were available only for responses on lever B. Data were collected from probe trials (four per session), in which no reinforcers were delivered, during the last ten of 60 training sessions. Both groups showed decreasing response rates on lever A, and increasing response rates on lever B, as a function of time from the onset of the trial. Quantitative indices of timing behaviour were derived from a two-parameter logistic function fitted to the relative response rates on lever B (response rate on lever B, expressed as a percentage of overall response rate); this function accounted for > 90% of the data variance in each group. The DSP4-treated group showed a significantly lower value of the indifference point (i.e. the time corresponding to 50% responding on lever B) than the control group. The slope of the function and the rate of switching between response alternatives did not differ significantly between the two groups. The concentrations of noradrenaline were markedly reduced in the neocortex and hippocampus of the DSP4-treated group, but the concentrations of dopamine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid were not significantly altered. It is suggested that results may be consistent with a role of the dorsal ascending noradrenergic pathway in behavioural “arousal”. Received: 23 April 1997/Final version: 13 October 1997     

Effects of repeated treatment with 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) on the autoregulatory control of dorsal raphe 5-HT neuronal firing and cortical 5-HT release.

These experiments were designed to examine the effects of repeated 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) treatment on the autoregulatory control of cortical 5-HT release and dorsal raphe nucleus (DRN) 5-HT neuronal cell firing. Repeated DOI treatment decreased the behavioural responsiveness (wet-dog shakes) of 5-HT2 receptors and attenuated the inhibitory effects of the 5-HT1A receptor agonist, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), on both cortical 5-HT release and DRN 5-HT neuronal firing. In contrast, the inhibitory effect of acute DOI on cortical 5-HT release and DRN 5-HT neuronal firing was unaffected by repeated DOI treatment. The results demonstrate that changes in the responsiveness of 5-HT2 receptor function may influence the responsiveness of presynaptic 5-HT1A receptors regulating 5-HT neuronal function. The results also provide further evidence that the inhibition of cortical 5-HT release and DRN 5-HT neuronal firing produced by DOI is not mediated by 5-HT2 receptor activation.     

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.     

Effects of clozapine and 2,5-dimethoxy-4-methylamphetamine [DOM] on 5-HT2A receptor expression in discrete brain areas

Activation of 5-HT2A receptors has been shown to be an essential component of the discriminative stimulus effects of indoleamine and phenethylamine hallucinogens. The objective of the present study was to determine the neuroanatomical location of the 5HT2A receptors which may be responsible for the stimulus effects of the phenethylamine hallucinogen [-]2,5-dimethoxy-4-methylamphetamine (DOM). It was hypothesized that brain areas containing altered 5-HT2A receptor expression in the context of a similar alteration in DOM-induced stimulus control might be important in mediating the stimulus effects of DOM. Fisher 344 rats were treated with either clozapine (25 mg/kg/day) or DOM (2 mg/kg/day) for 7 days, and the consequences of these drug treatment regimens on DOM-induced stimulus control and on 5-HT2A receptor expression in several brain areas were determined. Chronic administration of clozapine was associated with a wide-spread decrease in levels of 5-HT2A/2C receptors. Conversely, treatment with DOM had varied effects including a neuroanatomically selective decrease in 5-HT2A/2C receptor levels that was restricted to the olfactory nucleus. Both chronic treatment with DOM and clozapine decreased the stimulus effects of DOM. The present findings suggest a role for the olfactory nucleus in producing the stimulus effects of DOM.     

DOI, a 5-HT2 receptor agonist, induces renal vasodilation via nitric oxide in anesthetized dogs

We have previously reported that (+/-)-1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 receptor agonist, induced renal vasodilation in anesthetized dogs. The present study was designed to investigate whether DOI-induced renal vasodilation might be mediated by increased nitric oxide (NO) release/production in renal tissue. The experiments were performed in anesthetized dogs. A 23-gauge needle was inserted into the left renal artery for infusion of drug solutions. Renal blood flow was measured with an electromagnetic flowmeter. The microdialysis probes were implanted into the renal cortex to collect the dialysate for measurement of guanosine 3',5'-cyclic monophosphate (cGMP) and nitrite/nitrate (NO2/NO3) concentration. Intrarenal infusion of DOI at a rate of 5 microg/kg/min resulted in a significant increase, by 30 +/- 4%, in renal blood flow, indicating renal vasodilation. The renal interstitial concentrations of NO2/NO3 and cGMP also increased by 70 +/- 6% and 60 +/- 6%, respectively. These changes induced by DOI were completely abolished by the intrarenal pretreatment with N(w)-nitro-L-arginine methyl ester (L-NAME, a NO synthase inhibitor, 100 microg/kg/min) or sarpogrelate (100 microg/kg/min, a highly selective 5-HT2 receptor antagonist). DOI infusion increased urine volume and urinary excretion of Na+, which were also blocked by L-NAME or sarpogrelate. These results suggest that DOI caused renal vasodilation due to increased NO release/production by stimulation of 5-HT2 receptors in the kidney. The natriuretic effect of DOI might also be related to increased intrarenal NO production.     

Evidence for involvement of 5-HT1C and 5-HT2 receptors in the food intake suppressant effects of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)

Administration of various doses of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) to rats produced dose-related decreases in 1-h food intake in the food-deprived paradigm. Pretreatment with spiperone (5-HT_1A/5-HT₂/D₂ antagonist), propranolol or CGP361A (-adrenoceptor antagonists that also have binding affinities for 5-HT_1A and 5-HT_1B sites) and MDL-72222 (5-HT₃ antagonist) did not attenuate DOI-induced suppression of food intake. In contrast, pretreatment with metergoline (5-HT₁/5-HT₂ antagonist) completely blocked whereas mesulergine, mianserin and ritanserin (5-HT_1C/5-HT₂ antagonists) partially blocked DOI's effect on food intake. On the other hand, pretreatment with MDL-72222 but not with m-chlorophenylpiperazine (m-CPP) significantly potentiated DOI-induced suppression of food intake. Furthermore, the food intake suppressant effects of various doses of DOI were found to be similar in the Fawn-Hooded (FH) rat strain as compared to the Wistar rat strain. These findings suggest that DOI-induced suppression of food intake is mediated by stimulation of both 5-HT_1C and 5-HT₂ receptors.     

The selective 5-HT2 receptor antagonist amperozide attenuates 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane-induced inhibition of male rat sexual behavior.

This study was aimed at exploring the role of 5-HT2/5-HT1C neurotransmission in male rat sexual behavior. The administration of the 5-HT2/5-HT1C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (1 mg/kg), suppressed sexual activity in most of the animals. The suppressive effect of DOI was antagonized by treatment with amperozide, a selective 5-HT2 receptor antagonist, in doses which did not by themselves affect sexual activity. In addition, several other serotonin antagonists were tested with varying affinity profiles for 5-HT2/5-HT1C receptors, including ketanserin, ritanserin, and mesulergine. All these compounds antagonized the suppressive action of DOI. In contrast, no antagonizing effect was obtained by treatment with (-)-alprenolol, a 5-HT1A antagonist. The present findings suggest that 5-HT2/5-HT1C receptors might be involved in the neural control of male rat sexual behavior, presumably by exerting an inhibitory influence on the behavior.     

5-HT1 and 5-HT2 binding characteristics of 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane analogues

1-(2,5-Dimethoxy-4-bromophenyl)-2-aminopropane (DOB; 1a) is a purported serotonin (5-HT) agonist that binds selectively to central 5-HT2 binding sites. Systematic removal of any or all of the aromatic substituents had relatively little effect on 5-HT1 binding but reduced 5-HT2 binding by approximately 2 or more orders of magnitude. Demethylation of the 2-methoxy group of 1a, or introduction of an N-n-propyl group, doubled 5-HT1-site affinity but decreased 5-HT2-site affinity by 3- and 30-fold, respectively. In tests of stimulus generalization, using rats trained to discriminate DOM from saline, the 2-demethyl and N-propyl derivatives were found to produce stimulus effects similar to those of DOB. In addition, the S-(+) isomer of the iodo analogue of 1a was found to possess one-third the affinity of its R-(-) enantiomer at 5-HT2 sites and also resulted in DOM-stimulus generalization. Of the DOB analogues examined, DOB (1a) possesses optimal selectivity for 5-HT2 binding.