5-HT2A/2C receptor blockade regulates progenitor cell proliferation in the adult rat hippocampus

Adult hippocampal neurogenesis is reported to be a target of antidepressants, drugs of abuse and animal models of depression, suggesting a role for this form of structural plasticity in psychopathology. Serotonergic neurotransmission, which is implicated in several psychiatric diseases, has been reported to regulate adult hippocampal neurogenesis. Amongst the serotonergic receptors, the serotonin2A/2C (5-HT2A/2C) receptors play an important role in the actions of antidepressants and the effects of hallucinogenic drugs of abuse. We have used the mitotic marker 5'-bromo-2-deoxyuridine to address the effects of the 5-HT2A/2C receptors on the proliferation of adult hippocampal progenitors following acute or chronic treatment with the hallucinogenic partial agonists, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI) and lysergic acid diethylamide (LSD) and the antagonist, Ketanserin. Acute, and chronic, DOI and LSD treatments induced a strong behavioral activation, but did not alter adult hippocampal progenitor proliferation. In striking contrast, Ketanserin treatment resulted in a biphasic regulation with a significant decline (22%) in progenitor proliferation following a single treatment, and a robust increase (46%) observed following chronic administration. These results indicate that hallucinogenic drugs that primarily target the 5-HT2A/2C receptors, in contrast to other drugs of abuse, may not alter adult hippocampal neurogenesis. In addition, our results that enhanced adult hippocampal progenitor proliferation results from a sustained blockade of the 5-HT2A/2C receptors suggest that the 5-HT2A/2C receptors may be an important target for the neurogenic effects of antidepressant treatment.     

Preliminary study on potential of the jPET-D4 human brain scanner for small animal imaging

Objective  One trend in positron emission tomography (PET) instrumentation over the last decade has been the development of scanners dedicated to small animals such as rats and mice. Thicker crystals, which are necessary to obtain higher sensitivity, result in degraded spatial resolution in the peripheral field-of-view (FOV) owing to the parallax error. On the other hand, we are developing the jPET-D4, which is a dedicated human brain PET scanner that has a capability for depth-of-interaction (DOI) measurement. Although its crystal width is about twice that of commercially available small animal PET scanners, we expect the jPET-D4 to have a potential for small animal imaging by making full use of the DOI information. In this article, we investigate the jPET-D4’s potential for small animal imaging by comparing it with the microPET Focus220, a state-of-the-art PET scanner dedicated to small animals. Methods  The jPET-D4 uses four-layered GSO crystals measuring 2.9 mm × 2.9 mm × 7.5 mm, whereas the microPET Focus220 uses a single layer of LSO crystals measuring 1.5 mm × 1.5 mm × 10.0 mm. First, the absolute sensitivity, counting rate performance and spatial resolution of both scanners were measured. Next a small hot-rod phantom was used to compare their imaging performance. Finally, a rat model with breast tumors was imaged using the jPET-D4. Results  Thanks to the thicker crystals and the longer axial FOV, the jPET-D4 had more than four times higher sensitivity than the microPET Focus220. The noise equivalent counting-rate performance of the jPETD4 reached 1,024 kcps for a rat-size phantom, whereas that of the microPET Focus220 reached only 165 kcps. At the center of the FOV, the resolution was 1.7 mm for the microPET Focus220, whereas it was 3.2 mm for the jPET-D4. On the other hand, the difference of resolution became smaller at the off-center position because the radial resolution degraded faster for the microPET Focus220. The results of phantom imaging showed that the jPET-D4 was comparable to the microPET Focus220 at the off-center position even as the microPET Focus220 outperformed the jPET-D4 except for the peripheral FOV. Conclusions  The jPET-D4 human brain PET scanner, which was designed to achieve not only high resolution but also high sensitivity by measuring DOI information, was proven to have a potential for small animal imaging.     

Acute immobilization stress reduces (±DOI)-induced 5-HT2A receptor-mediated head shakes in rats

Acute immobilization stress induced by taping four limbs, applying tail pinch stress and electric foot shock stress immediately reduced the frequency of head shakes induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((±)DOI), a 5-HT_2A/C agonist in rats. Immobilization stress due to the use of cylinder restraint and forced swimming did not affect 5-HT_2A-mediated behavior. Acute immobilization stress did not affect [³H]ketanserin binding to the 5HT_2A receptor in the prefrontal cortex and hippocampus. Presynaptic serotonergic lesions with 5,7-dihydroxytryptamine (5,7-DHT) did not affect the reduction in 5-HT_2A-mediated behavior after acute immobilization stress. The decreases in head shake frequency after acute immobilization stress by taping were attenuated by pretreatment with diazepam (2.5 mg/kg IP): This attenuation was reversed by pretreatment with flumazenil (10 mg/kg IP). The reduction in (±)DOI- induced 5-HT_2A-mediated behavior caused by stress may be related to a change in agonist affinity to the receptor or changes in other neurotransmitter systems or the effect of PI turnover.     

5-HT<Subscript>2A/2C</Subscript> receptor signaling via phospholipase A<Subscript>2</Subscript> and arachidonic acid is attenuated in mice lacking the serotonin reuptake transporter

Subjects The serotonin reuptake transporter (SERT) helps to regulate brain serotonergic transmission and is the target of some antidepressants. To further understand SERT function, we measured a marker of regional brain phospholipase A₂ (PLA₂) activation in SERT knockout mice (SERT–/–) and their littermate controls (SERT+/+).Methods Following administration of 1.5 mg/kg s.c. (±)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), a 5-HT_2A/2C receptor agonist, to unanesthetized mice injected intravenously with radiolabeled arachidonic acid (AA), PLA₂ activation, represented as the regional incorporation coefficient k* of AA, was determined with quantitative autoradiography in each of 71 brain regions.Results In SERT+/+ mice, DOI significantly increased k* in 27 regions known to have 5-HT_2A/2C receptors, including the frontal, motor, somatosensory, pyriform and cingulate cortex, white matter, nucleus accumbens, caudate putamen, septum, CA1 of hippocampus, thalamus, and hypothalamus. In contrast, DOI did not increase k* significantly in any brain region of SERT–/– mice. Head twitches following DOI, which also were measured, were robust in SERT+/+ mice but were markedly attenuated in SERT–/– mice.Conclusions These results show that a lifelong elevation of the synaptic 5-HT concentration in SERT–/– mice leads to downregulation of 5-HT_2A/2C receptor-mediated PLA₂ signaling via AA and of head twitches, in response to DOI.     

Attenuation of nicotine’s discriminative stimulus effects in rats and its locomotor activity effects in mice by serotonergic 5-HT<Subscript>2A/2C</Subscript> receptor agonists

Rationale Reports have indicated that administration of nicotine inhibits, while withdrawal of chronically administered nicotine augments effects of serotonergic 5HT_2A/2C agonists.Objective It was our objective to determine whether 5HT_2A/2C agonists can modulate the discriminative stimulus effects of nicotine in rats or its locomotor activity effects in mice.Methods Adult male Sprague–Dawley rats were trained to discriminate 0.3 mg/kg nicotine base from saline in a two-lever, fixed-ratio (FR10), food-reinforced, operant-conditioning task during daily (Monday–Friday) 15-min experimental sessions. After characterizing a dose–response curve for nicotine, we tested the ability of the 5HT_2A/2C agonists (+/–)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCL (DOI; 0.18–1.0 mg/kg) and 1-(4-bromo-2, 5-dimethoxyphenyl)-2-aminopropane (DOB; 0.1–1.0 mg/kg), the 5HT_2C agonist 6-chloro-2-(1-piperazinyl)pyrazine hydrochloride (MK 212; 0.1 mg/kg–1.0 mg/kg), and the 5HT_1A agonist (±)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 0.01 mg/kg–1.0 mg/kg) to modulate nicotines discriminative stimulus effects. After finding that DOI was able to attenuate the percentage nicotine lever responding (%NLR), we tested for it to also reverse nicotines effects on locomotor activity in mice.Results The 5HT_2A/2C agonists—in particular DOI—dose dependently attenuated %NLR. The effects of DOI were reversed by the 5HT_2A/2C antagonist ketanserin. MK 212 and 8-OH-DPAT had irregular effects among rats and only reduced %NLR to below 50% levels at doses markedly suppressing responding. DOI also dose dependently blocked nicotines acute rate-lowering locomotor activity effects.Conclusions These results indicate that activation of serotonin 5HT_2A/2C receptors can blunt the discriminative stimulus and locomotor activity effects of nicotine and presents the possibility that activation of these receptors might also be able to attenuate other effects of nicotine.     

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.     

Involvement of diazepam-insensitive benzodiazepine receptors in the suppression of DOI-induced head-twitch responses in diabetic mice

Rationale We previously reported that the head-twitch responses induced by the 5-HT₂ receptor agonist (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) (DOI-HTRs) were decreased in streptozotocin-induced diabetic mice.Objectives We examined the involvement of γ-aminobutyric acid (GABA)/benzodiazepine system on the suppression of DOI-HTRs in diabetic mice.Results The benzodiazepine receptor antagonist flumazenil (0.1–1 mg/kg, i.v.) dose-dependently and significantly increased DOI-HTRs in diabetic mice to the same levels as in nondiabetic mice. However, flumazenil (0.1–1 mg/kg, i.v.) did not affect DOI-HTRs in nondiabetic mice. The benzodiazepine receptor agonist diazepam (0.1–1 mg/kg, i.p.) had no effect on DOI-HTRs in either nondiabetic or diabetic mice. The GABA_A receptor antagonist bicuculline (0.1–1 mg/kg, i.p.) and the benzodiazepine receptor partial inverse agonist Ro 15-4513 (0.1–1 mg/kg, i.v.) dose-dependently and significantly suppressed DOI-HTRs in nondiabetic mice to the same levels as in diabetic mice. Ro 15-4513-induced reduction of DOI-HTRs in nondiabetic mice was completely antagonized by flumazenil (1 mg/kg, i.v.), but not diazepam (0.3 mg/kg, i.p.).Conclusions We suggest that the abnormal diazepam-insensitive benzodiazepine receptor function partly underlies the suppression of DOI-HTRs in diabetic mice.     

Early Adoption of a Multitarget Stool DNA Test for Colorectal Cancer Screening

Objectives

To characterize early adoption of a novel multitarget stool DNA (MT-sDNA) screening test for colorectal cancer (CRC) screening and to test the hypothesis that adoption differs by demographic characteristics and prior CRC screening behavior and proceeds predictably over time.

Hallucinogenic drug interactions at human brain 5-HT2 receptors: implications for treating LSD-induced hallucinogenesis

It has been shown that the hallucinogenic potencies of LSD, the phenylisopropylamines, such as DOB (4-bromo-2,5-dimethoxyphenylisopropylamine) and DOI (4-iodo-2,5-dimethoxyphenylisopropylamine), and the indoleaklylamines, such as DMT (dimethyltryptamine) and 5-OMe-DMT (5-methoxy-dimethyltryptamine), strongly correlate with their in vitro 5-HT₂ receptor binding affinities in rat cortical homogenates. In order to ascertain if this correlation applies to human 5-HT₂ receptors as well, we examined the affinities of 13 psychoactive compounds at ³H-ketanserin-labelled 5-HT₂ receptors in human cortical samples. Both radioligand binding and autoradiographical procedures were used. As in rat brain d-LSD was the most potent displacer of ³H-ketanserin specific binding with a K _i of 0.9 nM. The phenylisopropylamine DOI also displayed high affinity (K _i of 6 nM). Stereospecific interactions were found with DOB; (-_ DOB had a K _i of 17 nM while (+) DOB had a K _i of 55 nM. The behaviorally active compound DOM (4-methyl-2,5-phenylisopropylamine) had an affinity of 162 nM while its behaviorally less active congener iso-DOM had an affinity of 6299 nM. The indolealkylamines 5-OMe-DMT and DMT competed with moderate affinities (207 and 462 nM, respectively). In general, Hill coefficients were significantly less than unity which is consistent with an agonist interaction with 5-HT₂ receptors. MDMA, a substituted amphetamine analog was inactive with a K _i of greater than 10 M. A strong correlation was found for the hallucinogen affinities and human hallucinogenic potencies (r=0.97). Also, human and rat brain 5-HT₂ receptor affinities were strongly correlated (r=0.99). These results strongly support the hypothesis that the hallucinogenic effects of these drugs in humans are mediated in whole or in part via 5-HT₂ receptors. Furthermore, these studies imply that treatment with 5-HT₂ receptor antagonists may be effective in reversing the hallucinogenic effects caused by the ingestion, of LSD and LSD-like drugs.