Dopamine D1 receptor-mediated intracellular responses in the hypothalamus after co-administration of caffeine with MDMA

Abstract: Markers of dopamine D₁ receptor activation were determined to elucidate intracellular mechanisms associated with the combined effects of caffeine and 3,4 methylenedioxymethamphetamine (MDMA), reported previously to produce increased toxicity, when compared with either drug alone. Caffeine (10 mg/kg) and MDMA (15 mg/kg) were administered to male Sprague Dawley rats alone and in combination. One hour after drug administration, core body temperature and phosphorylation of the dopamine D₁‐related intracellular markers, cAMP response element binding protein (CREB), the dopamine and c‐AMP‐regulated phosphoprotein of 32 kDa (DARPP‐32) and expression of the immediate early gene and cellular activation marker c‐fos were determined in the hypothalamus. Co‐administration of caffeine with MDMA increased core body temperature when compared with MDMA or caffeine treatment alone. Pre‐treatment with the dopamine D₁ receptor antagonist SCH 23390 (1 mg/kg, i.p.), 30 min. prior to caffeine and MDMA administration, produced a hypothermic response to MDMA that was unaffected by caffeine. Co‐administration of caffeine with MDMA increased p‐CREB, p‐DARPP‐32 and c‐fos expression when compared with either treatment alone. Pre‐treatment with SCH‐23390 attenuated the changes in p‐CREB, p‐DARPP and c‐fos. The results show an enhanced intracellular response when caffeine is combined with MDMA but not with either agent alone suggestive of synergistic intracellular actions convergent on a dopamine D₁ receptor signalling pathway. A dopamine‐related synergy associated with the combined administration of caffeine and MDMA may have important use and safety implications for recreational drug users.     

Caffeine promotes hyperthermia and serotonergic loss following co-administration of the substituted amphetamines, MDMA ("Ecstasy") and MDA ("Love")

The present study determined the effect of caffeine co-administration on the core body temperature response and long-term serotonin (5-HT) loss induced by methylenedioxymethamphetamine (MDMA; "Ecstasy") and its metabolite methylenedioxyamphetamine (MDA; "Love") to rats. In group-housed animals, caffeine (10 mg/kg) enhanced the acute toxicity of MDMA (15 mg/kg) and MDA (7.5 mg/kg), resulting in an exaggerated hyperthermic response (+2 degrees C for 5 h following MDMA and +1.5 degrees C for 3 h following MDA) when compared to MDMA (+1 degree C for 3 h) and MDA (+1 degree C for 1 h) alone. Co-administration of caffeine with MDMA or MDA was also associated with increased lethality. To reduce the risk of lethality, doses of MDMA and MDA were reduced in further experiments and the animals were housed individually. To examine the effects of repeated administration, animals received MDMA (10 mg/kg) or MDA (5 mg/kg) with or without caffeine (10 mg/kg) twice daily for 4 consecutive days. MDMA and MDA alone induced hypothermia (fall of 1 to 2 degrees C) over the 4 treatment days. Co-administration of caffeine with MDMA or MDA resulted in hyperthermia (increase of up to 2.5 degrees C) following acute administration compared to animals treated with caffeine or MDMA/MDA alone. This hyperthermic response to caffeine and MDMA was not observed with repeated administration, unlike caffeine + MDA, where hyperthermia was obtained over the 4 day treatment period. In addition, 4 weeks after the last treatment, co-administration of caffeine with MDA (but not MDMA) induced a reduction in 5-HT and 5-hydroxyindole acetic acid (5-HIAA) concentrations in frontal cortex (to 61% and 58% of control, respectively), hippocampus (48% and 60%), striatum (79% and 64%) and amygdala (63% and 37%). However, when caffeine (10 mg/kg) and MDMA (2.5 mg/kg) were co-administered four times daily for 2 days to group-housed animals, both hyperthermia and hippocampal 5-HT loss were observed (reduced to 68% of control). Neither MDMA nor MDA alone induced a significant reduction in regional 5-HT or 5-HIAA concentrations following repeated administration. In conclusion, caffeine promotes the acute and long-term toxicity associated with MDMA and MDA. This is a serious drug interaction, which could have important acute and long-term health consequences for recreational drug users.     

The role of 5-HT in the impairment of thermoregulation observed in rats administered MDMA (‘ecstasy’) when housed at high ambient temperature

Rationale Administration to rats of a neurotoxic dose of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) produces an impairment in thermoregulation which is reflected in a prolonged hyperthermic response to a subsequent dose of MDMA given to rats housed at high ambient temperature.Objective We wished to examine whether the impaired thermoregulation was associated with decreased cerebral 5-HT content produced by the prior neurotoxic dose of MDMA.Methods Rats were injected with drugs decreasing 5-HT function [the tryptophan hydroxlase inhibitor p-chlorophenylalanine (PCPA), and 5-HT receptor antagonists] and rectal temperature was measured after administering MDMA to rats housed at 30°C.Results PCPA pretreatment decreased 5-HT and 5-HIAA concentrations in cortex, hippocampus and striatum by >80% and prolonged the hyperthermia induced in rats housed at 30°C by administering MDMA (5 mg/kg i.p.). A similar prolongation of the hyperthermic response to MDMA was seen when rats were pretreated with methysergide (10 mg/kg i.p.) or the 5-HT_1A antagonist WAY100635 (0.5 mg/kg s.c.).Conclusions Decreasing 5-HT function in diverse ways enhanced the hyperthermic response to MDMA given to rats housed at high ambient temperature. This suggests that loss of 5-HT acting on 5-HT_1A receptors leads to impaired thermoregulation in rats and suggests that the impairment seen in MDMA pretreated rats housed at high ambient temperature is due to a loss in 5-HT function. These data could have implications for recreational users of MDMA, who may have damaged serotoninergic neurons because of prior heavy or frequent use of the drug, when taking further doses of MDMA in hot environments such as dance clubs.     

The effect of caffeine on MDMA-induced hydroxyl radical production in the mouse striatum

BackgroundThe psychostimulant 3,4-methylenedioxymethamphetamine (MDMA) with a strong addictive potential is widely used as a recreational drug. Neurotoxicity of MDMA is related with the generation of highly reactive free radicals.MethodsMDMA was given in doses of 20 and 40 mg/kg ip alone or in combination with caffeine (CAF) 10 mg/kg ip. Extracellular concentration of hydroxyl radical was measured using microdialysis in freely moving mice and was assayed by HPLC with electrochemical detection.ResultsMDMA dose-dependently increased production of hydroxyl radical in the mouse striatum and its effect was reversed by caffeine.ConclusionsThe data show that caffeine may have neuroprotective properties as it decreased oxidative stress induced by MDMA.     

N-substituted piperazines abused by humans mimic the molecular mechanism of 3,4-methylenedioxymethamphetamine (MDMA, or 'Ecstasy').

3,4-Methylenedioxymethamphetamine (MDMA, or 'Ecstasy') is an illicit drug that stimulates the release of serotonin (5-HT) and dopamine (DA) from neurons. Recent evidence reveals that drug users are ingesting piperazine analogs, like 1-benzylpiperazine (BZP, or 'A2') and 1-(m-trifluoromethylphenyl)piperazine (TFMPP, or 'Molly'), to mimic psychoactive effects of MDMA. In the present study, we compared the neurochemistry of MDMA, BZP, and TFMPP in rats. The effects of MDMA, BZP, and TFMPP on transporter-mediated efflux of [3H]5-HT and [3H]MPP+ (DA transporter substrate) were determined in synaptosomes. The effects of drugs on extracellular levels of 5-HT and DA were examined using in vivo microdialysis in conscious rats. MDMA evoked transporter-mediated release of [3H]5-HT and [3H]MPP+. BZP released [3H]MPP+, whereas TFMPP was a selective releaser of [3H]5-HT. MDMA (1-3 mg/kg, i.v.) increased dialysate 5-HT and DA in a dose-related fashion, with actions on 5-HT being predominant. BZP (3-10 mg/kg, i.v.) elevated dialysate DA and 5-HT, while TFMPP (3-10 mg/kg, i.v.) elevated 5-HT. Administration of BZP plus TFMPP at a 1:1 ratio (BZP/TFMPP) produced parallel increases in dialysate 5-HT and DA; a 3 mg/kg dose of BZP/TFMPP mirrored the effects of MDMA. At a 10 mg/kg dose, BZP/TFMPP increased dialysate DA more than the summed effects of each drug alone, and some rats developed seizures. Our results show that BZP/TFMPP and MDMA share the ability to evoke monoamine release, but dangerous drug-drug synergism may occur when piperazines are coadministered at high doses.     

Pharmacodynamic interactions between MDMA and concomitants in MDMA tablets on extracellular dopamine and serotonin in the rat brain

3,4-methylenedioxymethamphetamine (MDMA) is a psychoactive stimulant abused by young people as the recreational drug ecstasy. Other compounds, either deliberately added or present as byproducts, are often found in MDMA tablets and can unexpectedly interact with each other. The aim of this study was to evaluate the pharmacodynamic effects of interactions caused by concomitants in MDMA tablets on extracellular dopamine and serotonin (5-HT) by microdialysis in the striatum of ethylcarbamate-anesthetized rats. Baseline levels of dopamine and 5-HT in the striatum were 16.5±7.7 and 3.5±1.7 nM (mean±standard deviation), respectively. After a single administration of MDMA (10 mg/kg, i.p.), a dramatic increase in extracellular dopamine (Cmax: 36.1-fold vs. baseline) and 5-HT levels (Cmax: 9.3-fold vs. baseline) was observed. When rats were co-administered with methamphetamine (1, 5 or 10 mg/kg) with MDMA, the dopamine levels induced by MDMA increased in a methamphetamine-dose-dependent manner (Cmax: 2.5-, 3.5-, and 3.8-fold vs. MDMA). A similar trend was observed in 5-HT levels (Cmax: 1.1-, 1.3-, and 1.8-fold vs. MDMA). In contrast, ketamine and caffeine showed synergistic effects on the monoamine levels induced by MDMA, whereas the individual administration of either of these compounds did not affect monoamine levels. Ketamine (1, 5 mg/kg) decreased the dopamine levels induced by MDMA (Cmax: 0.9- and 0.7-fold vs. MDMA) and increased the 5-HT levels induced by MDMA (Cmax: 1.4- and 1.6-fold vs. MDMA), and co-administration of caffeine (20 mg/kg) with MDMA increased dopamine levels (Cmax: 1.7-fold vs. MDMA). These results suggest that exposure to multiple drugs in addition to MDMA can have neurotoxic effects.     

3,4-Methylenedioxymethamphetamine and naloxone in rat rotational behaviour and open field

It has recently been shown that 3,4-Methylenedioxymethamphetamine (MDMA) has an anti-parkinsonian effect in rodent models of Parkinson's disease. The mechanism of this anti-parkinsonian action is unknown. Opioids have been suggested to play a role in MDMA-induced behaviour. We therefore investigated MDMA and naloxone in the rat rotational behavioural model. Male Sprague-Dawley rats were lesioned unilaterally with 6-hydroxydopamine at the medial forebrain bundle. Administration of R/S-MDMA (5 mg/kg, s.c.) produced ipsilateral rotations. Naloxone (2, 5, 10 mg/kg, s.c.) did not produce rotations on its own but reduced the number of MDMA-induced ipsilateral rotations. This effect was not dose-dependent. In contrast to reports on mice, in unlesioned animals, naloxone (10 mg/kg, s.c.) did not block MDMA (5 mg/kg, s.c.)-induced hyperactivity in an open field in our experiment. It is concluded that endogenous opioids play a role in MDMA's action in the rat rotational behavioural model.     

Pharmacological studies on ginger. V. Pharmacological comparison between (6)-shogaol and capsaicin

Pharmacological actions of (6)-shogaol and capsaicin were studied. Both (6)-shogaol (0.5 mg/kg, i.v.) and capsaicin (0.1 mg/kg, i.v.) caused a triad such as a rapid fall in blood pressure, bradycardia and aponea in rats. Both drugs-induced marked pressor responses in blood pressure, which occurred after the rapid fall, were markedly reduced by a spinal destruction. In pithed rats, both drugs-induced peripheral pressor responses were markedly reduced with the combined treatment of [D-Arg1, D-Pro2, D-Trp7,9, Leu11]-substance P (0.5 mg/kg, i.v.), phentolamine (10 mg/kg, i.v.) and the section of sciatic nerves. In isolated guinea-pig trachea, (6)-shogaol (100 microM) and capsaicin (10 microM) induced contractile responses which were slightly inhibited by substance P antagonist (10 microM), but exhibited also a tachyphylaxis. Furthermore, although (6)-shogaol (3.6 microM) showed positive inotropic and chronotropic actions on isolated atria in rats, this effect of (6)-shogaol disappeared by repeated injections or pretreatment (100 mg/kg, s.c.) of (6)-shogaol. These results suggest that (6)-shogaol and capsaicin have similar actions, and that both drugs may cause a peripheral action by releasing an unknown active substance from nerve ends.     

The effects of concurrent administration of +/-3,4-methylenedioxymethamphetamine and cocaine on conditioned place preference in the adult male rat

Conditioned place preference (CPP), a commonly used model for studying the role of contextual cues in drug reward and drug seeking, was employed to explore possible behavioral interactions between (+/-)3,4-methylenedioxymethamphetamine (MDMA; "ecstasy") and cocaine. On each of four occasions, adult male rats received one of three doses of MDMA (0 mg/kg, 5 mg/kg, 10 mg/kg; administered subcutaneously [s.c.]) combined with one of three doses of cocaine (0 mg/kg, 2.5 mg/kg, 5 mg/kg; administered intraperitoneally [i.p.]), and were then tested in a CPP paradigm. The results showed MDMA-induced CPP at a unit dose of 5 mg/kg, but at the 10 mg/kg dose there was a return to baseline (control) performance levels. For cocaine alone, CPP increased in a linear fashion as the drug dose was increased. Concurrent administration resulted in antagonism of each drug, but there was evidence that this pattern was reversible at higher doses of the respective drugs. These data are instructive insofar as they suggest that the behavioral and neurochemical effects of MDMA and cocaine presented in isolation are dramatically altered when the two drugs are presented in combination.     

Serotonergic-dopaminergic mediation of MDMA's discriminative stimulus effects in a three-choice discrimination

(+/-)3,4-Methylenedioxymethamphetamine (MDMA; "Ecstasy") is a common drug of abuse that is often described as both a psychostimulant and a hallucinogen. Two-choice drug discriminations (i.e. drug vs. nondrug) in nonhumans comparing the discriminative stimulus properties of MDMA to psychostimulants or hallucinogens have produced somewhat inconsistent findings. The relative contribution of serotonergic versus dopaminergic actions to MDMA's discriminative stimulus effects may depend on the training stimulus conditions employed. We have previously demonstrated that rats can learn to discriminate the effects of MDMA and D-amphetamine in a three-choice drug discrimination procedure, and that LSD produced nearly complete substitution for MDMA under these conditions, and fenfluramine fully substituted for MDMA. In the present study, 12 rats were trained to discriminate LSD (0.08 mg/kg) and MDMA (1.5 mg/kg) from saline in a three-choice drug discrimination procedure under a fixed-ratio (FR) 10 schedule of food reinforcement. D-Amphetamine produced only partial substitution for MDMA while fenfluramine produced complete stimulus generalization. Low doses of D-amphetamine and fenfluramine produced greater stimulus generalization when administered in combination than when given alone. The serontonin(2) antagonist MDL-100,907 only partially blocked the MDMA cue, but completely antagonized LSD discrimination. The dopamine antagonist haloperidol also failed to block MDMA discrimination. These results indicate that 5-HT release is a salient feature to MDMA's discriminative stimulus effects but that MDMA produces a compound discriminative stimulus.     

Reinstatement of MDMA (ecstasy) seeking by exposure to discrete drug-conditioned cues

The widely used recreational drug MDMA (ecstasy) supports self-administration in animals, but it is not known whether MDMA-associated cues are able to reinstate drug seeking in a relapse model of drug addiction. To assess this possibility, drug-na?ve rats were trained to press a lever for MDMA infusions (0.30 mg/kg/infusion, i.v.) paired with a compound cue (light and tone) in daily 2 h sessions. Responding was reinforced contingent on a modified fixed-ratio 5 schedule of reinforcement. Conditioned cue-induced reinstatement tests were conducted after lever pressing was extinguished in the absence of MDMA and the conditioned cues. Conditioned cues reinstated lever pressing after extinction, and the magnitude of reinstatement was positively correlated with the level of responding during MDMA self-administration. These results show for the first time that conditioned cues can trigger reinstatement of MDMA-seeking behavior in rats, and that individual differences in the pattern of MDMA self-administration can predict the magnitude of reinstatement responding.     

Effect of GBR 12909 and fluoxetine on the acute and long term changes induced by MDMA ('ecstasy') on the 5-HT and dopamine concentrations in mouse brain

We examined the long term effect of 3,4 methylenedioxymethamphetamine (MDMA, 10, 20 and 30 mg/kg, i.p.) on the cerebral 5-hydroxytryptamine (5-HT) and dopamine content in Swiss Webster mice. Three injections of MDMA (20 or 30 mg/kg, i.p.) given 3 h apart produced a marked depletion in the striatal content of dopamine and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) 7 days later. None of the doses administered altered the concentration of 5-HT or its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in several brain areas. Pre-treatment with the dopamine uptake inhibitor GBR 12909 (10 mg/kg, i.p.), 30 min before each of the three MDMA (30 mg/kg, i.p.) injections, completely prevented the long term loss in the striatal catechol concentrations. However, GBR 12909 (10 mg/kg, i.p.) not only failed to prevent the acute effects induced by MDMA (30 mg/kg x 3, i.p.) on dopamine metabolism 30 min later, but in fact potentiated them. The 5-HT uptake inhibitor, fluoxetine (10 mg/kg, i. p.) failed to prevent both the acute and long term dopaminergic deficits. MDMA (30 mg/kg x 3) altered the body temperature of the mice biphasically, producing a rapid hyperthermia followed by prolonged hypothermia. In contrast, MDMA (20 mg/kg x 3) produced an initial hypothermia followed by hyperthermia. The present experiments therefore appear to rule out any direct relationship between the neurotoxic effects of MDMA and its acute effects on body temperature in mice. Fluoxetine administered 30 min before each MDMA (30 mg/kg) injection prevented these temperature changes, while GBR 12909 was without effect. This suggests that the neuroprotective effect of GBR 12909 against MDMA-induced neurotoxicity is not directly related to its ability to inhibit the MDMA-induced acute effects on dopamine metabolism or alter the MDMA-induced temperature change. The data illustrate major differences in the neurotoxic profile of MDMA in mice and rats.     

Synergism between caffeine and dl-phenylpropanolamine on brown adipose tissue thermogenesis in the adult rat

Caffeine produces enhanced oxygen consumption, an effect that may reflect an action of caffeine on brown adipose thermogenesis. In Experiment 1, adult male rats were anesthetized with 1.2 g/kg urethane and treated (IP) with either 0.9% saline or 10, 20 or 40 mg/kg caffeine (n = 4 each group). Interscapular BAT (IBAT) and rectal temperatures were recorded every minute for 10 minutes prior to and 30 minutes following drug injection. Stable IBAT and rectal temperatures were observed prior to and after saline injection whereas rats treated with 20 and 40 mg/kg caffeine exhibited moderate increases in IBAT, but not rectal, temperature. In Experiment 2, adult male rats were treated with either 0.9% saline or 10 mg/kg caffeine, anesthetized with urethane (1.2 g/kg) and treated (30 minutes after pretreatment injections) with either 0.9% saline or 10 mg/kg dl-phenylpropanolamine (dl-PPA). A combination of caffeine and dl-PPA produced significantly greater BAT thermogenesis than just dl-PPA alone. The implications of these data for the inclusion of caffeine in over-the-counter diet-pills are discussed.     

Caffeine does not effectively ameliorate, but rather may worsen the ethanol intoxication when assessed by discrete avoidance in mice.

Ethanol disrupted the discrete lever-press and shuttle avoidances in mice at doses over 1.6 and 2.4 g/kg, p.o., respectively, eliciting a dose-dependent decrease in the % of avoidance with no significant change or slight increase in the response rate. Caffeine increased the response rate of both the avoidances at the doses of 1-30 mg/kg, p.o., but disrupted the avoidance at 100 mg/kg. Caffeine (10 mg/kg) reduced the decreased % of avoidance by ethanol (1.6 and 2.4 g/kg) with a significant increase in the response rate. In contrast, the % of avoidance was significantly lower after the combined administration of ethanol (3.2 g/kg) with caffeine than after ethanol (3.2 g/kg) alone. Unlike ethanol, diazepam (2 mg/kg, s.c.) and pentobarbital (10 mg/kg, s.c.) significantly decreased both the response rate and the % of avoidance. Caffeine (10 mg/kg) ameliorated the decreased response rate and the % of avoidance produced by diazepam and pentobarbital. The present results suggest that caffeine does not effectively ameliorate, but rather may worsen the ethanol intoxication.     

The serotonin uptake inhibitor citalopram reduces acute cardiovascular and vegetative effects of 3,4-methylenedioxymethamphetamine ('Ecstasy') in healthy volunteers

MDMA (3,4-methylenedioxymethamphetamine) or 'Ecstasy' is a widely used recreational drug that produces a state of heightened mood but also cardiovascular and vegetative side-effects. In animals, MDMA releases serotonin and, to a lesser extent, dopamine and norepinephrine. The release of serotonin can be blocked by serotonin uptake inhibitors such as citalopram. It is unknown to what extent this mechanism is also responsible for the physiological side-effects of MDMA seen in humans. We investigated the effect of citalopram pretreatment (40 mg i.v.) on vegetative and cardiovascular effects of MDMA (1.5 mg/kg p.o.) in a double-blind placebo-controlled study in 16 healthy volunteers. MDMA moderately increased blood pressure and heart rate, slightly elevated body temperature and produced a broad range of acute and short-term side-effects. Citalopram reduced all these MDMA-induced physiological changes except for body temperature. These findings suggest that physiological effects of MDMA in humans are partially due to an interaction of MDMA with the serotonin carrier and a subsequent release of serotonin.     

Nicotine antagonizes caffeine- but not pentylenetetrazole-induced anxiogenic effect in mice

Rationale Nicotine and caffeine are widely consumed licit psychoactive drugs worldwide. Epidemiological studies showed that they were generally used concurrently. Although some studies in experimental animals indicate clear pharmacological interactions between them, no studies have shown a specific interaction on anxiety responses. Objectives The present study investigates the effects of nicotine on anxiety induced by caffeine and another anxiogenic drug, pentylenetetrazole, in mice. The elevated plus-maze (EPM) test was used to evaluate the effects of drugs on anxiety. Methods Adult male Swiss Webster mice (25–32 g) were given nicotine (0.05–0.25 mg/kg s.c.) or saline 10 min before caffeine (70 mg/kg i.p.) or pentylenetetrazole (15 and 30 mg/kg i.p.) injections. After 15 min, mice were evaluated for their open- and closed-arm time and entries on the EPM for a 10-min session. Locomotor activity was recorded for individual groups by using the same treatment protocol with the EPM test. Results Nicotine (0.05–0.25 mg/kg) itself did not produce any significant effect in the EPM test, whereas caffeine (70 mg/kg) and pentylenetetrazole (30 mg/kg) produced an anxiogenic effect, apparent with decreases in open-arm time and entry. Nicotine (0.25 mg/kg) pretreatment blocked the caffeine- but not pentylenetetrazole-induced anxiety. Administration of each drug and their combinations did not produce any effect on locomotor activity. Conclusions Our results suggest that the antagonistic effect of nicotine on caffeine-induced anxiety is specific to caffeine, instead of a non-specific anxiolytic effect. Thus, it may extend the current findings on the interaction between nicotine and caffeine.     

Precipitation by yohimbine of the withdrawal syndromes of clonidine, guanfacine, and methyldopa in the spontaneously hypertensive rat

In conscious spontaneously hypertensive (SH) rats, continuously infused with clonidine (500 micrograms/kg/day s.c.) for 12 days, yohimbine (3 or 10 mg/kg i.p.) dose dependently precipitated an overshoot of heart rate and other withdrawal symptoms, such as blood pressure upswings, diarrhea, ptosis, body shivering, jumping, and wet-dog shakes. Naloxone (3 or 30 mg/kg i.p.) or prazosin (1 mg/kg i.p.) did not evoke withdrawal signs in clonidine-treated SH rats. In SH rats treated for 12 days with guanfacine (10 mg/kg/day s.c.), the administration of yohimbine elicited a withdrawal pattern similar in severity and appearance to that observed in clonidine-treated rats. In methyldopa-treated (200 mg/kg/day) SH rats, the yohimbine-precipitated withdrawal symptoms were much less pronounced than those observed in the clonidine- and guanfacine-treated animals, despite the equipotency of the infusions in reducing mean arterial pressure. These results indicate that the clonidine-withdrawal syndrome can be precipitated by acute alpha 2-adrenoceptor blockade. The previously observed minor severity of the guanfacine-discontinuation syndrome in the rat should be attributed to the longer half-life of this drug as compared to clonidine. On the other hand, alpha-methylnoradrenaline much less intensively triggers the mechanisms underlying withdrawal symptoms than clonidine or guanfacine.     

The effects of nicotine on locomotor activity in non-tolerant and tolerant rats.

1--Rats were tested for locomotor activity in photocell cages, for 80 min starting immediately after subcutaneous injection of (-)-nicotine bitartrate or 0.9% w/v NaCl solution (saline). In non-tolerant subjects, nicotine (0.1 to 0.4 mg/kg base) depressed activity and induced ataxia in the first 20 min, but increased activity later in the session; these actions were dose-dependent. 2--Tolerance was studied by comparing rats given nicotine (0.4 mg/kg s.c.) every day with control rats given saline instead. Each week, every subject was tested once with nicotine (0.4 mg/kg) and once with saline. With daily or even weekly injections of nicotine, the initial depressant action of the drug was replaced by a dose-dependent stimulant action which occurred throughout the session. In these tolerant animals, little ataxia was seen except when a larger dose of 0.8 mg/kg was given. Tolerance to the depressant action of nicotine persisted for at least 3 weeks. 3--In non-tolerant subjects, mecamylamine (0.5, 1.0 mg/kg s.c.) prevented the initial depressant action of nicotine (0.4 mg/kg). In tolerant rats, the locomotor stimulant action of nicotine (0.4 mg/kg) was prevented by mecamylamine (0.1, 0.32, 1.0 mg/kg s.c.) in a dose-related way; the quaternary ganglion blocker, hexamethonium (0.2, 1.0, 5.0 mg/kg s.c.) had little or no such effect. Neither mecamylamine nor hexamethonium altered activity when given alone. 4--It is suggested that a few treatments with nicotine can unmask a stimulant action of the drug, probably of central origin, which possibly reflects a stimulation of nicotine receptors.     

Effects of the co-administration of mirtazapine and paroxetine on serotonergic neurotransmission in the rat brain.

The alpha(2)-adrenoreceptor antagonist mirtazapine, which is also a 5-HT(2), 5-HT(3) and H(1) receptors antagonist and the selective serotonin (5-HT) reuptake inhibitor paroxetine are effective antidepressant drugs which enhance 5-HT neurotransmission via different mechanisms. The present studies were undertaken to determine whether the mirtazapine-paroxetine combination could induce an earlier and/or a greater effect on the 5-HT system than either drug alone. Using in vivo electrophysiological paradigms, the firing activity of dorsal raphe 5-HT neurons was decreased by 70% in rats treated with paroxetine (10 mg/kg/day, s.c.) for 2 days and was back to normal after 21 days. In contrast, a 2-day treatment with mirtazapine (5 mg/kg/day, s.c.) did not alter the firing of 5-HT neurons whereas it was increased by 60% after 21 days of treatment. A low dose of mirtazapine (5 mg/kg/day, s.c.x2 days) failed to offset the decremental effect of paroxetine on the 5-HT neuron firing activity, but a higher dose (10 mg/kg/day, s.c.x2 days) did attenuate the decremental effect of paroxetine. In the dorsal hippocampus, neither mirtazapine (5 mg/kg/day, s.c.) nor a paroxetine (10 mg/kg/day, s.c.) treatment altered the responsiveness of 5-HT(1A) receptors to microiontophoretically-applied 5-HT. Both in controls and in rats treated for 2 days with paroxetine alone, the administration of the 5-HT(1A) antagonist WAY 100635 (25-100 microg/kg, i.v.) did not change the firing activity of dorsal hippocampus CA(3) pyramidal neurons. However, WAY 100635 increased significantly the firing activity of these neurons in rats treated with mirtazapine alone but to a greater extent with both mirtazapine and paroxetine for 2 days. After 21 days of treatment, WAY 100635 increased to a greater degree the firing rate of CA(3) pyramidal neurons in rats which received the combination over rats given either drug alone. It is concluded that the mirtazapine-paroxetine combination shortened the delay in enhancing the tonic activation of postsynaptic 5-HT(1A) receptors and produced a greater activation of the postsynaptic 5-HT(1A) receptors than either drug given alone. The present results suggested that mirtazapine may have a faster onset of action than a SSRI, and that the co-administration of mirtazapine and paroxetine may accelerate the antidepressant response and as well as being more effective than either drug alone.     

Yohimbine induces sympathetically mediated renin release in the conscious rat

The preferential alpha 2-adrenergic antagonist yohimbine (4 mg/kg s.c.) caused a time-related increase in serum renin activity and heart rate in conscious Sprague-Dawley rats. Although mean arterial pressure was not decreased significantly over the 2-h period, heart rate was elevated significantly at 15 and 30 min post-injection. In contrast, serum renin activity remained elevated for up to 2 h with a 9-fold and 9.7-fold increase occurring at 30 and 60 min post-injection, respectively. Yohimbine (0.3, 1, 3 and 10 mg/kg s.c.) elicited a dose-related increase in serum renin activity and heart rate (30 min post-injection). The 1 mg/kg dose of yohimbine did not alter blood pressure whereas the 3 mg/kg dose caused a variable decrease in mean arterial pressure. The highest dose of yohimbine (10 mg/kg) significantly lowered blood pressure. The beta-adrenergic receptor antagonist propranolol (1.5 mg/kg s.c.), blocked the renin release and tachycardia caused by yohimbine (1 and 3 mg/kg s.c.), and the ganglionic blocking agent chlorisondamine partially inhibited the renin release elicited by 3 mg/kg (s.c.) of yohimbine. The prostaglandin synthetase inhibitors indomethacin (5 mg/kg s.c.) and meclofenamate (5 mg/kg s.c.) impaired the ability of yohimbine (3 mg/kg) to elevate SRA but did not alter the hemodynamic effects of yohimbine. Thus, the increase in renin release caused by yohimbine appears to be mediated by the sympathetic nervous system. Because the smaller doses of yohimbine increase renin release in the absence of a decrease in mean arterial pressure, it is unlikely that yohimbine stimulates renin release by baroreflex-mediated activation of the renal sympathetic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)