Behavioral evidence for mu-opioid and 5-HT2A receptor interactions

Electrophysiological studies have demonstrated a physiological interaction between 5-HT2A and mu-opioid receptors in the medial prefrontal cortex. Furthermore, behavioral studies have found that phenethylamine hallucinogens induce head shakes when directly administered into the medial prefrontal cortex. The receptor(s) by which morphine suppresses head shakes induced by serotonin agonists have not been characterized. We administered mu-opioid receptor agonists and antagonists to adult male Sprague-Dawley rats prior to treatment with the phenethylamine hallucinogen 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), which is known to induce head shakes via 5-HT2A receptors. The suppressant action of the moderately selective mu-opioid receptor agonist, buprenorphine (ID50 approximately 0.005 mg/kg, i.p.; a mu-opioid receptor partial agonist and kappa-opioid receptor antagonist) was blocked by naloxone and pretreatment with the irreversible mu-opioid receptor antagonist clocinnamox. Another mu-opioid receptor agonist fentanyl also suppressed DOI-induced head shakes. In contrast, a delta-opioid receptor agonist was without effect on DOI-induced head shakes. Thus, activation of mu-opioid receptors can suppress head shakes induced by hallucinogenic drugs.     

Role of endocannabinoid and glutamatergic systems in DOI-induced head-twitch response in mice

We previously reported that systemic administration of the endocannabinoid anandamide inhibited the head-twitches induced by the hallucinogenic drug 2,5-dimethoxy-4-iodoamphetamine (DOI) in mice, which is mediated via the activation of 5-HT_2A receptors. Endocannabinoid and glutamatergic systems have been suggested to modulate the function of 5-HT_2A receptors. In the present study, we further investigated the role of endocannabinoid and glutamatergic systems in DOI-induced head-twitch response in mice. An anandamide transport inhibitor AM404 (0.3-3 mg/kg, i.p.), a fatty acid amide hydrolase inhibitor URB597 (0.1-10 mg/kg, i.p.), a glutamate release inhibitor riluzole (0.3 and 1 mg/kg, i.p.), a natural glutamate analog l-glutamylethylamide (theanine, 1 and 3 mg/kg, p.o.) and an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist NBQX (0.01-0.3 mg/kg, i.p.) significantly inhibited DOI-induced head-twitch response. The AMPA receptor positive modulator aniracetam (30 or 100 mg/kg, p.o.) reversed inhibition of head-twitch response by NBQX and URB597. These findings indicated that endocannabinoid and glutamatergic systems participate in the mechanism of action of DOI to induce head-twitch response.     

Long-lasting effects of chronic stress on DOI-induced hyperthermia in male rats

Rationale Exposure to chronic stress can affect the serotoninergic (5-HT) system and behavioral measures associated with 5-HT. Repeated stress increases 5-HT receptor subtype 2 (5-HT₂) mediated behaviors in rodents, such as wet dog shakes and head twitch. Objectives The current study investigated whether exposure to chronic unpredictable stress would augment 5-HT_2A/C receptor-mediated hyperthermia. Furthermore, the persistence of these hyperthermic effects was investigated by testing rats up to 60 days after the stress procedure terminated. Methods For 2 or 10 days, rats were either not stressed (controls) or exposed to chronic unpredictable stress, i.e. two stressors per day of the following: cage rotation, cold exposure, swim, restraint, light cycle manipulations, single housing, and food and water deprivation. After the termination of stress (day 3 or 11), the 5-HT_2A/C receptor agonist DOI (1.5 mg/kg) or saline, was injected and the rectal temperature of the rats was monitored. In a separate experiment, the 5-HT₂ receptor antagonist, LY-53,587, was injected 30 min prior to the injection of DOI or saline. Finally, DOI was injected into rats 8, 30 or 60 days after the 10-day stress procedure ended. Results Rats exposed to 10 days, but not 2 days, of unpredictable stress exhibited higher rectal temperatures following DOI than non-stressed rats. The DOI-induced hyperthermia was attenuated by LY-53,587. The augmentation of DOI-induced hyperthermia in stressed rats persisted when examined 8, 30 and 60 days following the stress procedure. Conclusions The enhancement of 5-HT receptor function by chronic stress persists even after the environmental stressor is removed. This lasting increase in 5-HT receptor function may have implications for clinical disorders associated with stress, such as depression or post-traumatic stress disorder.     

Icilin-induced wet-dog shakes in rats are dependent on NMDA receptor activation and nitric oxide production

Icilin is a cold channel agonist that produces vigorous wet-dog shaking in rats. The shaking is accompanied by an increase in the level of extracellular glutamate in the brain. Hence, we hypothesized that icilin-induced wet-dog shakes are dependent on increased glutamatergic transmission and nitric oxide (NO) production. Rats injected with icilin (0.5, 1, 2.5, 5 mg/kg, i.p.) displayed a dose-related increase in wet-dog shakes. Pretreatment with LY 235959 (1, 2 mg/kg, i.p.), a NMDA receptor antagonist, or L-NAME (50 mg/kg, i.p.), a NO synthase (NOS) inhibitor, attenuated icilin-induced wet-dog shakes. The shaking was also reduced by intracerebroventricular L-NAME (1 mg/rat, i.c.v.) administration, indicating that the stimulant effect of icilin is dependent on central NO production. Pretreatment with 6,7-dinitroquinoxaline-2,3(1H,4H)-dione (DNQX) (10, 20 mg/kg, i.p.), an AMPA receptor antagonist, or ceftriaxone (200 mg/kg, i.p. for 5 days), a beta-lactam antibiotic and glutamate transporter subtype 1 (GLT-1) activator, did not alter the incidence of icilin-induced shaking. The present data reveal that icilin produces behavioral stimulation by a mechanism requiring NMDA receptor activation and nitric oxide production and suggest that glutamate and NO signaling play important roles in cold channel pharmacology.     

Role of TRPV1 and cannabinoid CB1 receptors in AM 404-evoked hypothermia in rats

AM 404 inhibits endocannabinoid uptake and enhances the cannabinoid CB(1)-mediated effects of endogenous cannabinoids. Accumulating evidence also suggests that AM 404 acts at sites other than the endocannabinoid system. One site is the transient receptor potential vanilloid 1 cation channel (TRPV1). A useful endpoint for discriminating between TRPV1- or CB(1)-mediated effects of AM 404 is hypothermia. This is because TRPV1 or CB(1) receptor activation produces a significant hypothermia in rats. The present study investigated the effects of AM 404 (1, 5, 10 and 20 mg/kg, i.p.) on body temperature in rats and the involvement of TRPV1 and CB(1) receptors in the effects of AM 404. Doses of 10 and 20 mg/kg of AM 404 produced significant hypothermia. Pre-treatment with capsazepine (30 mg/kg, i.p.) blocked the hypothermia caused by 10 and 20 mg/kg of AM 404. Pre-treatment with SB 366791 (2 mg/kg, i.p.), a new TRPV1 antagonist, also abolished the hypothermia evoked by AM 404 (20 mg/kg, i.p.). In contrast, pre-treatment with SR 141716A (Rimonabant), a CB(1) antagonist, or AA-5-HT, a fatty acid amide hydrolase (FAAH) blocker, did not affect AM 404-evoked hypothermia. The present data demonstrate that AM 404 evokes a significant hypothermia in rats that is dependent on TRPV1 receptor activation.     

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.     

The antidepressant effects of curcumin in the forced swimming test involve 5-HT1 and 5-HT2 receptors

Curcuma longa is a main constituent of many traditional Chinese medicines, such as Xiaoyao-san, used to manage mental disorders effectively. Curcumin is a major active component of C. longa and its antidepressant-like effect has been previously demonstrated in the forced swimming test. The purpose of this study was to explore the possible contribution of serotonin (5-HT) receptors in the behavioral effects induced by curcumin in this animal model of depression. 5-HT was depleted by the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA, 100 mg/kg, i.p.) prior to the administration of curcumin, and the consequent results showed that PCPA blocked the anti-immobility effect of curcumin in forced swimming test, suggesting the involvement of the serotonergic system. Moreover, pre-treatment of pindolol (10 mg/kg, i.p., a beta-adrenoceptors blocker/5-HT(1A/1B) receptor antagonist), 4-(2'-methoxy-phenyl)-1-[2'-(n-2'-pyridinyl)-p-iodobenzamino-]ethyl-piperazine (p-MPPI, 1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist), or 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane, 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist) was found to prevent the effect of curcumin (10 mg/kg) in forced swimming test. On the other hand, a sub-effective dose of curcumin (2.5 mg/kg, p.o.) produced a synergistic effect when given jointly with (+)-8-hydroxy-2-(di-n-propylamino)tetralin, (8-OH-DPAT, 1 mg/kg, i.p., a 5-HT(1A) receptor agonist), anpirtoline (0.25 mg/kg, i.p., a 5-HT(1B) receptor agonist) or ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist with higher affinity to 5-HT(2A) receptor) or R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, 1 mg/kg, i.p., a 5-HT(2A) receptor agonist). Taken together, these results indicate that the antidepressant-like effect of curcumin in the forced swimming test is related to serotonergic system and may be mediated by, at least in part, an interaction with 5-HT(1A/1B) and 5-HT(2C) receptors.     

Delta(9)-tetrahydrocannabinol prolongs the immobility time in the mouse forced swim test: involvement of cannabinoid CB(1) receptor and serotonergic system

In the present study, we investigated the effect of Delta(9)-tetrahydrocannabinol (THC), the principal psychoactive component of marijuana, on immobility time during the forced swim test. THC (2 and 6 mg/kg, i.p.) significantly prolonged the immobility time. In addition, THC at the same doses did not significantly affect locomotor activity in the open-field test. The selective cannabinoid CB(1) receptor antagonist rimonabant (3 mg/kg, i.p.) significantly reduced the enhancement of immobility by THC (6 mg/kg). Similarly, the selective serotonin (5-HT) reuptake inhibitor (SSRI) citalopram (10 mg/kg, i.p.) and 5-HT(1A/7) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.3 mg/kg, i.p.) significantly reduced this THC-induced effect. Moreover, the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide dihydrochloride (WAY100635, 1 mg/kg, i.p.) and the postsynaptic 5-HT(1A) receptor antagonist MM-77 (0.1 mg/kg, i.p.) reversed this reduction effect of 8-OH-DPAT (0.3 mg/kg). In contrast, the selective 5-HT(7) receptor antagonist (R)-3-[2-[2-(4-methylpiperidin-1-yl)ethyl]pyrrolidine-1-sulfonyl]phenol hydrochloride (SB269970) had no effect on this reduction effect of 8-OH-DPAT. WAY100635 (1 mg/kg) also reversed the reduction effect of citalopram (10 mg/kg). These findings suggest that the 5-HT(1A) receptors are involved in THC-induced enhancement of immobility.     

Anxiolytic-like action of MTEP expressed in the conflict drinking Vogel test in rats is serotonin dependent

The purpose of the present study was to investigate whether the anxiolytic-like action of a selective and brain penetrable group I metabotropic glutamate (mGlu5) receptor antagonist 3-[(2-methyl-1,3-tiazol-4-yl)ethynyl]-pyridine (MTEP) is dependent upon the serotonergic system. Experiments were performed on male Wistar rats. The Vogel conflict drinking test was used to detect anxiolytic-like activity. MTEP administered intraperitoneally at doses of 1, 3 and 6 mg/kg induced anxiolytic-like effect. The potential anxiolytic effect of MTEP (1 mg/kg) was inhibited by a nonselective 5-HT receptor antagonist metergoline (2 mg/kg i.p.) and 5-HT2A/2C receptor antagonist ritanserin (0.5 mg/kg i.p.), but not by a 5-HT1A receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl)cyclohexane-carboxamide (WAY 100635) (0.1 mg/kg i.p). The anxiolytic effect of MTEP (6 mg/kg) was attenuated by ritanserin (1 mg/kg i.p.). Moreover, MTEP-induced a dose-dependent release of serotonin in the frontal cortex. The obtained results suggest that the potential anxiolytic effect of the mGlu5 receptor antagonist MTEP is due to the increased serotonin release with subsequent activation of 5-HT2A/2C receptors, most probably located postsynaptically, but not by the 5-HT1A receptors.     

Influence of the 5-HT(2C) receptor antagonist SB242,084 on behaviour produced by the 5-HT(2) agonist Ro60-0175 and the indirect 5-HT agonist dexfenfluramine

Ro60-0175 has been described as a selective agonist at the 5-HT(2C) receptor, yet it has only 10- fold higher affinity at the 5-HT(2C) compared to the 5-HT(2A) subtype, and equivalent affinity for the 5-HT(2B) receptor. The selective 5-HT(2C) receptor antagonist SB242,084 (0.5 mg kg(-1) i.p.), blocked the hypoactivity and penile grooming induced by Ro60-0175 (1 mg kg(-1) s.c.). The combination of SB242,084 (0.5 mg kg(-1) i.p.) and Ro60-0175 (3 - 10 mg kg(-1)) produced a completely different pattern of behaviours including wet-dog shakes, hyperactivity and back muscle contractions. These latter effects were blocked by the selective 5-HT(2A) receptor antagonist MDL100,907 (0.5 mg kg(-1) i.p.), but not the 5-HT(2B) receptor antagonist SB215,505 (3 mg kg(-1) p.o.). The indirect 5-HT releaser/reuptake inhibitor dexfenfluramine (1 - 10 mg kg(-1) i.p.) produced a mild increase in locomotor activity, penile grooming, and occasional back muscle contractions and wet-dog shakes. Pre-treatment with SB242,084 (0.5 mg kg(-1)), blocked the incidence of penile grooming, and markedly potentiated both the dexfenfluramine-induced hyperactivity, the incidence of back muscle contractions, and to a lesser extent wet-dog shakes. Some toxicity was also evident in animals treated with dexfenfluramine (10 mg kg(-1))/SB242,084 (0.5 mg kg(-1)), but not in any other treatment groups. The hyperactivity and toxicity produced by the dexfenfluramine (10 mg kg(-1))/SB242,084 (0.5 mg kg(-1)) combination was replicated in a further study, and hyperthermia was also recorded. Both hyperthermia and toxicity were blocked by MDL100,907 (0.5 mg kg(-1)) but not SB215,505 (3 mg kg(-1)). An attenuation of the hyperlocomotor response was also observed following MDL100,907. These findings suggest that 5-HT(2C) receptor activation can inhibit the expression of behaviours mediated through other 5-HT receptor subtypes.     

Evidence that central 5-HT2A and 5-HT2B/C receptors regulate 5-HT cell firing in the dorsal raphe nucleus of the anaesthetised rat

1. Systemic administration of phenethylamine-derived, 5-hydroxytryptamine(2) (5-HT(2)) receptor agonists inhibits the firing of midbrain 5-HT neurones, but the 5-HT receptors involved are poorly defined, and the contribution of peripheral mechanisms is uncertain. This study addresses these issues using extracellular recordings of 5-HT neurones in the dorsal raphe nucleus of anaesthetised rats. 2. The 5-HT(2) receptor agonists DOI ((+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride) and DOB ((+/-)-2,5-dimethoxy-4-bromoamphetamine hydrobromide), caused a dose-related (10-100 micro g kg(-1) i.v.) inhibition of 5-HT neuronal activity, with the highest dose reducing firing rates by >80%. 3. Pretreatment with the 5-HT(2) receptor antagonist ritanserin (1 mg kg(-1) i.v.) completely blocked the action of DOI. The 5-HT(2A) receptor antagonist MDL 100,907 (0.2 mg kg(-1) i.v.) blocked the action of both DOI and DOB. In comparison, the 5-HT(2B/C) receptor antagonist SB 206553 (0.5 mg kg(-1) i.v.) caused a small, but statistically significant, shift to the right in the dose response to DOI and DOB. 4. Pretreatment with the peripherally acting 5-HT(2) receptor antagonist BW 501C67 (0.1 mg kg(-1) i.v.) had no effect on the DOI-induced inhibition of 5-HT cell firing, but completely blocked the DOI-induced rise in mean arterial blood pressure. 5. These data indicate that the inhibition of 5-HT cell firing induced by systemic administration of DOI and DOB is mediated predominantly by the 5-HT(2A) receptor-subtype, but that 5-HT(2B/C) receptors also play a minor role. Moreover, central and not peripheral mechanisms are involved. Given evidence that 5-HT(2) receptors are not located on 5-HT neurones, postsynaptic 5-HT feedback mechanisms are implicated.     

Behavioral evidence for interactions between a hallucinogenic drug and group II metabotropic glutamate receptors.

Recent electrophysiological studies in our laboratory have demonstrated a physiological interaction between 5-HT(2A) and metabotropic glutamate2/3 (mGlu2/3) receptors in the medial prefrontal cortex. Several behavioral studies have found that phenethylamine hallucinogens with partial agonist activity at 5-HT(2A) receptors induce head shakes when directly administered into the medial prefrontal cortex. The purpose of the present experiments was to examine whether an interaction occurs between mGlu2/3 and 5-HT(2A) receptors on a behavioral level using head shakes induced by phenethylamine hallucinogens as a model of 5-HT(2A) receptor activation. Administration of the mGlu2/3 agonist LY354740 (0.3-10 mg/kg, ip) suppressed head shakes induced by the phenethylamine hallucinogen 1-(2, 5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Conversely, administration of the mGlu2/3 antagonist LY341495 (1 mg/kg, ip) enhanced the frequency of DOI-induced head shakes. Taken together, these results raise the possibility that the psychomimetic properties of hallucinogenic drugs may be mediated in part, via increased glutamate release following activation of 5-HT(2A) receptors.     

Activation of 5-HT2 receptors induces glycogenolysis in the rat brain

The effect of 5-HT(2) receptor activation on brain glycogen and the extracellular concentration of glucose was investigated in the present study. An injection of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2 mg/kg, i.p.) or mescaline (10 mg/kg, i.p.) at an ambient temperature of 29 degrees C produced a 35-45% decrease in brain glycogen that persisted for at least 2 h. DOI also increased the extracellular concentration of glucose in the striatum by 60%. Maintenance of rats at 22 degrees C significantly attenuated DOI-induced glycogenolysis, as well as DOI-induced hyperthermia, and the increase in the extracellular concentration of glucose in the striatum. DOI-induced hyperthermia, glycogenolysis and increase in the extracellular concentration of glucose also were attenuated in rats treated with the 5-HT(2) receptor antagonist, 6-methyl-1-(methylethyl)-ergoline-8beta-carboxylic acid 2-hydroxy-1-methylpropyl ester maleate (LY-53,857) (3 mg/kg, ip). These results support the conclusion that 5-HT(2) receptor activation promotes glycogenolysis and that hyperthermia exerts a prominent role in this process.     

Effects of 5-HT3 receptor antagonists on behavioural measures of naloxone-precipitated opioid withdrawal

The effect of the selective 5-HT3 receptor antagonists, ondansetron and MDL 72,222, against various behaviours elicited by naloxone-precipitated morphine withdrawal were examined. Rats made dependent upon morphine by the subcutaneous implantation of a 75 mg pellet, when challenged with naloxone (0.5 mg/kg SC), 3 or 4 days later exhibited a wide range of behaviours including wet dog shakes, paw shakes, salivation and a marked weight loss. Pre-treatment with ondansetron (0.01-1 mg/kg SC) or MDL 72,222 (1-3 mg/kg SC) failed to affect the incidence of these responses except weight loss, which was attenuated by both treatments. At doses similar to and below those required to elicit the withdrawal syndrome, naloxone produced a single-trial place aversion in morphine dependent rats. The place aversion produced by naloxone (0.05 mg/kg SC) was antagonized by pre-treatment of ondansetron (0.1-1 mg/kg SC) and MDL 72,222 (1 mg/kg SC) prior to conditioning. Chlordiazepoxide (10 mg/kg IP) but not gepirone (3-10 mg/kg SC) was similarly effective. It is concluded that 5-HT3 antagonists may attenuate some but not all behavioural signs associated with morphine withdrawal. Reasons for this apparent selectivity are discussed.     

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.     

The selective AMPA receptor antagonist GYKI 53784 blocks action potential generation and excitotoxicity in the guinea pig cochlea

The role of AMPA receptors in cochlear synaptic transmission and excitotoxicity was investigated by comparing the actions of a selective AMPA antagonist GYKI 53784 (LY303070) with additional AMPA/kainate antagonists, GYKI 52466 and DNQX, and the NMDA antagonist, D-AP5, in several electrophysiological, neurotoxicological and histochemical tests. GYKI 53784 had the same potency as DNQX and was 10 times more potent than GYKI 52466 in reducing auditory nerve activity. The NMDA antagonist D-AP5 had no effect on auditory nerve activity. When single-fiber activity was blocked with GYKI 53784, the effects of AMPA or kainate were also antagonized. GYKI 53784 completely blocked excitotoxicity (i.e. destruction of the afferent nerve endings) induced by AMPA and kainate. The histochemical detection of Co(2+) uptake was used to study Ca(2+) influx within the primary auditory nerve cells. Application of AMPA induced no significant Co(2+) uptake into the cells, suggesting that these receptors normally have a very low permeability to Ca(2+). Application of kainate induced significant Co(2+) uptake that was blocked by the AMPA receptor antagonist GYKI 53784 suggesting that kainate stimulated Ca(2+) entry through AMPA receptor channels. Results suggest that AMPA-preferring receptors are functionally located at the sensory cell-afferent synapse whereas NMDA and kainate receptors are not.     

Effect of ACTH,adrenalectomy and the combination treatment on the density of 5-HT2 receptor binding sites in neocortex of rat forebrain and 5-HT2 receptor-mediated wet-dog shake behaviors

The effect of ACTH and/or adrenalectomy on serotonin (5-HT)₂ receptor binding sites was evaluated in the neocortex of rat forebrain. One day after the adrenalectomy or sham operation, ACTH (50 µg/day) was injected subcutaneously into adult male SD rats for 10 consecutive days. Saturation analysis showed that subchronic ACTH treatment significantly increased the B_max values for³H-ketanserin binding without any change in the K_d values. Moreover, this ACTH-induced increase in the B_max values was prevented by adrenalectomy. The concentrations of 5-HT and 5-hydroxyindole acetic acid (5-HIAA) measured by HPLC-ECD were not altered by these manipulations. Ten-day administration of corticosterone (20 and 50 mg/kg) also increased 5-HT₂ receptor density in the neocortex of rat forebrain. 5-HT₂ (and 5-HT_1C) receptor agonist, (±)DOI-induced wet-dog shakes in ACTH and/or adrenalectomy-treated rats were also examined. Ten-day administration of ACTH enhanced (±)DOI-induced wet-dog shakes and this increase was prevented by adrenalectomy. These results indicate that subchronic adrenocorticotropinadrenal axis activation of rats increases both the number of 5-HT₂ receptors in neocortex of forebrain and the wet-dog shake responses induced by (±)DOI.     

Effects of a 5-HT7 receptor antagonist DR4004 on the exploratory behavior in a novel environment and on brain monoamine dynamics in mice

The present study examined whether serotonin (5-hydroxytryptamine; 5-HT)7 receptors play a role in the modulation of emotionality in mice using the selective 5-HT7 receptor antagonist 2a-[4-(4-phenyl-1,2,3,6-tetrahydropyridyl)butyl]-2a,3,4,5-tetrahydrobenzo (c,d)indol-2-(1H)-one (DR4004). The emotionality of mice was evaluated in terms of exploratory activity in the hole-board test. The mice treated with DR4004 (2.5-10 mg/kg, i.p.) displayed a dose-dependent decrease in locomotor activity by moving less distance in the hole-board, and statistically significant decreases were observed at 5 and 10 mg/kg. On the other hand, DR4004 (10 mg/kg, i.p.) did not affect spontaneous motor activity. In a neurochemical study, decreases in amygdaloid dopamine and 5-HT turnover were observed in mice in which locomotor activity in the hole-board test was attenuated following the administration of DR4004 (10 mg/kg, i.p.). Also, a simple linear regression analysis revealed that locomotor activity on the hole-board was significantly correlated with dopamine and 5-HT turnover in amygdala. Furthermore, co-injection of the selective dopamine reuptake inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)piperazine (GBR12909; 1.25-5 mg/kg, i.p.) or the selective 5-HT reuptake inhibitor fluvoxamine (20 mg/kg, i.p.) significantly reversed the DR4004 (10 mg/kg, i.p.)-induced decrease in locomotor activity in the hole-board test. These findings constitute the behavioral evidence that 5-HT7 receptors may play a role in the modulation of emotionality. Furthermore, it is also suggested that amygdaloid dopamine and 5-HT neuronal systems may be involved in this modulation.     

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.