The atypical antipsychotic,remoxipride, blocks phencyclidine-induced disruption of prepulse inhibition in the rat

The effect of various typical (haloperidol) and atypical (clozapine, raclopride, remoxipride) antipsychotics on phencyclidine (PCP)-induced disruption of sensorimotor gating was tested in rats using an acoustic startle paradigm. Clozapine (4–40 µmol/kg), haloperidol (1–5 µmol/kg) and raclopride (1–12 µmol/kg) failed to reverse PCP-induced disruption of prepulse inhibition (PPI) of the acoustic startle response. In contrast, remoxipride (12–60 µmol/kg) caused a dose-dependent block of this effect. PCP-induced disruption of PPI is a widely accepted animal model of a corresponding behavioural deficit observed in schizophrenia although little evidence has been presented that it is in fact sensitive to antipsychotic agents. The present results indicate that remoxipride behaves in a unique way in this model compared to clozapine, haloperidol and raclopride.     

The atypical antipsychotic,aripiprazole, blocks phencyclidine-induced disruption of prepulse inhibition in mice

Rationale The psychotomimetic drug, phencyclidine, induces schizophrenia-like behavioural changes in both humans and animals. Phencyclidine-induced disruption of sensory motor gating mechanisms, as assessed by prepulse inhibition of the acoustic startle, is widely used in research animals as a screening model for antipsychotic properties in general and may predict effects on negative and cognitive deficits in particular. Dopamine (DA) stabilizers comprise a new generation of antipsychotics characterized by a partial DA receptor agonist or antagonist action and have been suggested to have a more favourable clinical profile. Objective The aim of the present study was to investigate the ability of first, second and third generation antipsychotics to interfere with the disruptive effect of phencyclidine on prepulse inhibition in mice. Results Aripiprazole blocked the phencyclidine-induced disruption of prepulse inhibition. The atypical antipsychotic clozapine was less effective, whereas olanzapine, and the typical antipsychotic haloperidol, failed to alter the effects of phencyclidine on prepulse inhibition. Conclusions The somewhat superior efficacy of clozapine compared to haloperidol may be explained by its lower affinity and faster dissociation rate for DA D2 receptors possibly combined with an interaction with other receptor systems. Aripiprazole was found to be more effective than clozapine or olanzapine, which may be explained by a partial agonist activity of aripiprazole at DA D2 receptors. In conclusion, the present findings suggest that partial DA agonism leading to DA stabilizing properties may have favourable effects on sensorimotor gating and thus tentatively on cognitive dysfunctions in schizophrenia.     

Neonatal neurosteroid administration results in development-specific alterations in prepulse inhibition and locomotor activity

Rationale Early life exposure to stress and to GABA_A receptor modulators have well-defined and persistent behavioral effects. A single neonatal injection of the GABAergic neurosteroid allopregnanolone (3α-hydroxy,5α-pregnane-20-one, 10 mg/kg, i.p.) alters the localization of prefrontal cortex (PFC) interneurons in adulthood. Such displacement could result in disinhibited behavior associated with impaired development of the mesocortical dopamine system.Objectives To determine if there is a critical window in which allopregnanolone levels may impact the development and mature function of the mesocorticolimbic circuitry.Methods Behavioral measures, including prepulse inhibition (PPI) and total locomotor activity, after amphetamine exposure were assessed at postnatal day 20 (P20) (prepuberty), P40 (puberty), P60 (postpuberty), and P80 (adulthood) in animals previously exposed to allopregnanolone (10 mg/kg) on P2 and P5. PFC tyrosine hydroxylase immunoreactivity was stereologically measured.Results P2 administration of allopregnanolone resulted in an increased locomotor response to amphetamine (14, 28% on P20 and P80, respectively) and reduced PPI (28, 22% on P20 and P80, respectively) at P20 and P80, whereas allopregnanolone administration on P5 increased locomotor response to amphetamine (20%) and reduced PPI (37%) at P80. Clozapine (7.5 mg/kg) pretreatment reversed the PPI deficit in P2-exposed animals. The total length of tyrosine hydroxylase immunopositive fibers in PFC was not altered by neonatal neurosteroid exposure, but more fibers were located in layers V/VI vs I–III.Conclusions Altering neonatal allopregnanolone levels disrupts PFC-dependent behavior, indicating that allopregnanolone might be important for normal PFC circuitry development. The temporal exposure differences (P2 vs P5) and ontological-dependent effects (P20 and P80, but not P40 or P60) suggest critical windows of vulnerability to neurosteroid insult across development.     

Sensitization to amphetamine,but not phencyclidine,disrupts prepulse inhibition and latent inhibition

Rationale Schizophrenia has been linked to dysregulation of dopamine and glutamate transmitter systems. Attempts to model aspects of schizophrenia in animals have made use of treatments that primarily affect dopaminergic (e.g., amphetamine, Amp) and glutamatergic (e.g., phencyclidine, PCP) function. In addition to exerting short-term acute effects, these agents also induce long-term effects, as seen, for example, in neurochemical and behavioural sensitization.Objectives The goal of this work was to compare Amp- and PCP-sensitized states on two measures of information processing that are impaired in schizophrenia, prepulse inhibition (PPI) of the acoustic startle reflex and latent inhibition (LI).Methods Rats received injections of Amp, PCP or saline 3 days per week for 3 weeks. The Amp dose increased from 1 to 3 mg/kg, at the rate of 1 mg/kg each week. The PCP dose was 3 mg/kg throughout. After various periods of withdrawal rats were tested for PPI and LI.Results Repeated intermittent treatment with Amp or PCP resulted in augmented locomotor responses to challenge with each drug, providing an operational index that sensitization had occurred. Rats sensitized to Amp showed disrupted PPI when tested drug free at 3, 21 and 60 days of withdrawal. Amp-sensitized rats also showed abolition of the LI effect. Rats sensitized to PCP did not show deficits in any of these behaviours when tested drug free.Conclusions Because disrupted PPI and LI have both been reported in schizophrenic patients, these results suggest that the Amp-sensitized state may represent a useful model for investigating the neural bases of information processing deficits in schizophrenia.     

mGluR5, but not mGluR1, antagonist modifies MK-801-induced locomotor activity and deficit of prepulse inhibition

Hypoglutamatergic theory of schizophrenia is substantiated by observation that high affinity uncompetitive antagonists of NMDA receptors such as PCP can induce psychotic symptoms in humans. Recently, metabotropic glutamate receptors of the mGluR5 type have also been discussed as possible players in this disease. However, less is known about the potential contribution of mGluR1 in schizophrenia. Therefore, the aim of the present study was to compare the effect of selective mGluR1 antagonist EMQMCM, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist (MTEP ([(2-methyl-1, 3-thiazol-4-yl) ethynyl] pyridine) either alone or in combination with (+)MK-801 in a prepulse inhibition (PPI) model and locomotor activity tests. Additionally, the effect of both mGluR1 and mGluR5 antagonists on (+)MK-801-evoked ataxia was tested. In contrast to (+)MK-801, which induced disruption of PPI, neither MTEP (1.25-5 mg/kg) nor EMQMCM (0.5-4 mg/kg) altered the PPI. However, MTEP, but not EMQMCM, enhanced disruption of PPI induced by (+)MK-801. Although neither mGluR1 nor mGluR5 antagonists given alone changed locomotor activity of rats, MTEP at 5 mg/kg potentiated the effect of (+)MK-801 while EMQMCM (up to 4 mg/kg) turned out to be ineffective. On the other hand, EMQMCM, but not MTEP, enhanced ataxia evoked by MK-801. The present results demonstrate that blockade of mGluR1 and mGluR5 evokes different effects on behavior induced by NMDA receptor antagonists.     

Withdrawal from continuous amphetamine administration abolishes latent inhibition but leaves prepulse inhibition intact

Rationale Schizophrenia has been associated with dysregulation of dopamine (DA) transmission and impairment in a number of experimental tasks, including sensorimotor gating assessed using prepulse inhibition (PPI) and selective attention assessed using latent inhibition (LI). We have demonstrated in previous studies that after withdrawal from escalating (ESC) dosages of amphetamine (AMPH), animals exhibited disruption of LI but no alteration of PPI. Moreover, these animals always showed behavioural sensitization to an AMPH challenge. Objective In this study, we were interested in testing whether a different administration schedule would elicit disruption of both LI and PPI. Methods Animals were treated with continuous AMPH release (via osmotic mini-pumps at a dosage of 10 mg kg⁻¹ day⁻¹ for 7 days) and tested for their performance in L and PPI during withdrawal in a drug free state. Rats received AMPH treatment during the induction phase in their home cages or in the activity chambers. Following withdrawal, the expression of behavioural sensitization to an AMPH challenge was tested in both cases in the activity chambers. Results Animals pretreated with AMPH from both groups did not exhibit behavioural sensitization. Withdrawal from continuous administration induced LI attenuation with no effect on PPI. Conclusions These findings are similar to what was previously found with respect to an ESC AMPH regime. The only difference between the schedules was that the ESC AMPH schedule led to behavioural sensitization whereas the continuous AMPH did not. It is suggested that the expression of sensitization may not be a prerequisite for observed LI disruption.     

Lack of phencyclidine-induced effects in mice with reduced neuronal nitric oxide synthase

RATIONALE: Phencyclidine (PCP) is widely used as an animal model of schizophrenia, because in humans it can induce positive and negative symptoms associated with schizophrenia. PCP is an antagonist of N-methyl-D-aspartate receptors, which are associated with the nitric oxide (NO) system. OBJECTIVE AND METHODS: The primary objective was to determine whether neuronal NO synthase (nNOS) is involved in PCP-induced behaviours and neuronal activation, as measured by the expression of c-Fos. After characterizing a PCP mouse model (dose-response study, Experiment 1), we measured PCP-induced effects in mice treated with nNOS antisense oligodeoxynucleotides (AS-ODNs) (Experiment 2), and in nNOS knockout (nNOS-/-) mice (Experiment 3). RESULTS: PCP 5 mg/kg induced the maximum behavioural effects of all doses tested, consisting of hyperlocomotion, stereotyped turning behaviour, without the presence of ataxia. PCP also induced an increase in Fos-like immunoreactivity (Fos-LIR) in the frontal cortex, as well as in the midline limbic (thalamic and hypothalamic nuclei) areas. In the AS-ODN-treated mice, PCP induced less behaviour when compared to water-treated controls. In the nNOS-/- mice, PCP induced less behaviour and a decrease in Fos-LIR in the frontal cortex and midline limbic areas, when compared to wild-type littermate controls. CONCLUSIONS: Our findings suggest that the frontal cortex and midline thalamic brain regions are involved in PCP-induced effects in mice. Furthermore, we show that an intact nNOS system is necessary to obtain PCP-induced effects. This may implicate nNOS as a viable drug target in the treatment of schizophrenia.     

Effects of tryptophan deficiency on prepulse inhibition of the acoustic startle in rats

Rationale Serotonin (5-HT) plays a key role in the pathophysiology of psychotic disorders, presumably through a modulation of dopamine (DA) transmission. Reduction of 5-HT signaling has been suggested to enhance dopaminergic responses in animal models of psychosis. An intriguing naturalistic strategy to reduce 5-HT brain content is afforded by the dietary restriction to its precursor, l-tryptophan (TRP). Objective We investigated the impact of a TRP-deficient diet in rats on the prepulse inhibition of the startle (PPI), a measure of sensorimotor gating which is typically impaired by psychotomimetic substances. Materials and methods After either short-term (6 h) or long-term (14 days) TRP deprivation, rats were tested for startle reflex and PPI. Moreover, we assessed the impact of both TRP deprivation regimens on PPI reduction induced by the psychotomimetic substance d-amphetamine (AMPH). Results Both TRP-deficient regimens failed to significantly affect PPI responses. However, chronic, but not short-term, TRP-deficient diet induced a significant sensitization to the effects of AMPH (1.25–2.5 mg/kg, subcutaneous). The enhanced predisposition to PPI disruption elicited by prolonged TRP deprivation was completely reversed 24 h after reinstatement of TRP in the diet, as well as pretreatment with antipsychotic drugs haloperidol (0.1 mg/kg, intraperitoneal) and clozapine (5 mg/kg, intraperitoneal), which exert their therapeutic action mostly through blockade of DA D₂ receptors. Conclusions The present results confirm and extend previous findings on the impact of serotonergic signaling in the modulation of DA transmission in schizophrenia and point to chronic TRP deprivation as a potential model of environmental manipulation that may produce a sensitization to psychotic-like symptoms induced by dopaminergic activation. Fabio Fadda and Roberto Stancampiano contributed equally to the study.     

Reversal of phencyclidine-induced prepulse inhibition deficits by clozapine in monkeys

Rationale Prepulse inhibition (PPI) of the acoustic startle reflex is a measure of sensorimotor gating, which occurs across species and is deficient in severe neuropsychiatric disorders such as schizophrenia. In monkeys, as in rodents, phencyclidine (PCP) induces schizophrenia-like deficits in PPI. In rodents, in general, typical antipsychotics (e.g. haloperidol) reverse PPI deficits induced by dopamine (DA) agonists (e.g. apomorphine), but not those induced by N-methyl-d-aspartate (NMDA) receptor antagonists [e.g. phencyclidine (PCP)], whereas atypical antipsychotics (e.g. clozapine) reverse PPI deficits induced by DA agonists and NMDA antagonists. However, some discrepancies exist with some compounds and strains of rodents.Objectives This study investigated whether a typical (haloperidol, 0.035 mg/kg) and an atypical (clozapine, 2.5 mg/kg) antipsychotic could be distinguished in their ability to reverse PCP-induced deficits in PPI in eight monkeys (Cebus apella).Methods First, haloperidol dose was determined by its ability to attenuate apomorphine-induced deficits in PPI. Then, haloperidol and clozapine were tested in eight monkeys with PCP-induced deficits of PPI. Experimental parameters were similar to standard human PPI procedures, with 115 dB white noise startle pulses, either alone or preceded by 120 ms with a prepulse 16 dB above the 70 dB background noise.Results Clozapine reversed PCP-induced PPI deficits. In contrast, haloperidol did not significantly attenuate PCP-induced PPI deficits even at doses that significantly attenuated apomorphine effects.Conclusions In this primate model, clozapine was distinguishable from haloperidol by its ability to attenuate PCP-induced deficits in PPI. The results provide further evidence that PPI in nonhuman primates may provide an important animal model for the development of novel anti-schizophrenia medications.     

Repeated administration of amphetamine induces sensitisation to its disruptive effect on prepulse inhibition in the rat

 Male Sprague-Dawley rats were repeatedly treated with amphetamine (AMP, 1 mg/kg, SC) at 3- day intervals for 15 days and tested for prepulse inhibition of acoustic startle after each treatment. This treatment regimen induced sensitisation in the animals as evidenced by a progressive increase in the disruptive effect of AMP on prepulse inhibition. Persistent changes in brain function was indicated, since an increase in disruptive effect was observed in sensitised animals also after a 22-day-long drug- and test-free period. The development of sensitisation was blocked by pretreatment with haloperidol (HPD, 0.1 mg/kg, SC), which suggests that sensitisation to the disruptive effect of AMP was dependent on dopamine (DA) D₂ receptor activation. Furthermore, the development of sensitisation was blocked by adrenalectomy, which suggests that sensitisation was dependent also on circulating adrenal hormones. Increased DA-ergic activity has been implicated in the pathophysiology of schizophrenia and AMP-induced sensitisation to the neuronal functions that modulate prepulse inhibition may be an experimental model to investigate this hypothesis. Received: 20 May 1997 / Final version: 14 August 1997     

Disparate effects of pramipexole on locomotor activity and sensorimotor gating in Sprague-Dawley rats

Prepulse inhibition (PPI) of acoustic startle and locomotor activity are both widely studied in the preclinical development of dopaminergic agents, including those acting at D3 dopamine receptors. In mice, the dopamine D3 receptor-preferential agonist pramipexole (PPX) alters locomotor activity in a biphasic manner at doses that have no effect on PPI. The present study examined the time-course of PPX effects on locomotion and PPI in rats. In adult male Sprague-Dawley rats, PPX (0, 0.1, 0.3, 1.0 mg/kg) was injected prior to measurement of locomotor activity for 90 min in photobeam chambers. Based on disparate early vs. late effects of PPX on locomotion, the effects of PPX (0 vs. 0.3 mg/kg) on PPI were tested 20 and 80 min after injection. All doses of PPX decreased locomotor activity for 30 min compared to vehicle, and the higher doses stimulated hyperlocomotion later in the session; the late hyperlocomotion, but not the early hypolocomotion, was blocked by the D2-selective antagonist, L741626 (1.0 mg/kg sc). In contrast to its locomotor effects, PPX caused a similar reduction in PPI at 20 and 80 min after administration. These findings suggest both a temporal and pharmacological dissociation between PPX effects on locomotor activity and PPI; these two behavioral measures contribute non-redundant information to the investigation of D3-related behavioral pharmacology.     

Activation of a nitric-oxide-sensitive cAMP pathway with phencyclidine: elevated hippocampal cAMP levels are temporally associated with deficits in prepulse inhibition

Rationale Schizophrenic patients show deficits in pre-attentive information processing as evidenced, for example, by disrupted prepulse inhibition, a measure of sensorimotor gating. A similar disruption can be observed in animals treated with the psychotomimetic agent, phencyclidine (PCP). However, the mechanism by which PCP alters brain function has not been fully elucidated. Recent studies have demonstrated that certain behavioural and neurochemical effects of PCP in rats and mice are blocked by nitric oxide (NO) synthase inhibition, suggesting an important role for NO in the effects of PCP.Objective The aim of the present study was to investigate the effects of PCP on cAMP production in the ventral hippocampus and the role of NO in these effects using in vivo microdialysis in rats. Furthermore, the effects of PCP on acoustic startle reactivity and prepulse inhibition of acoustic startle were compared with changes in cAMP levels in the ventral hippocampus.Results Significant increases in cAMP levels were observed in the ventral hippocampus following both local infusion (10^–4 mol/l and 10^–3 mol/l) and systemic administration (2 mg/kg) of PCP. The PCP-induced changes in prepulse inhibition and startle reactivity were associated in magnitude and duration with the increase in cAMP levels in the hippocampus. Furthermore, systemic administration of the NO synthase inhibitor, l-NAME (10 mg/kg), blocked both the changes in cAMP levels and the behavioural responses induced by PCP.Conclusions These findings indicate that the effects of PCP on prepulse inhibition and startle reactivity are associated with an increase in cAMP levels in the ventral hippocampus, and that this change in cAMP response may be linked to the production of NO.     

Interaction of glutathione depletion and psychotropic drug treatment in prepulse inhibition in rats and mice

Oxidative stress has been implicated in several psychiatric illnesses, including schizophrenia. Glutathione is the brain's primary antioxidant and decreased levels of brain glutathione are reported in schizophrenia. Prepulse inhibition (PPI) is a measure of sensory gating, and PPI is reduced in schizophrenia. This study aimed to investigate the effects of brain glutathione depletion on PPI regulation.Rats and mice were treated with the glutathione-depleting agent, 2-cyclohexene-1-one (CHX), and tested for baseline PPI and its disruption by treatment with amphetamine and MK-801.Treatment with CHX caused significant depletion of GSH in frontal cortex and striatum of rats and mice. Baseline PPI and startle were not altered. However, the disruption of PPI after treatment with amphetamine was absent in CHX-treated rats. In contrast, the effect of MK-801 was not altered by CHX-treatment, nor was there any effect of CHX treatment in mice.These data show an interaction of glutathione depletion with the effects of amphetamine treatment on PPI in rats. This effect could reflect loss of plasticity in PPI regulation caused by the additive effects of CHX-induced glutathione depletion and additional oxidative stress caused by amphetamine-induced dopamine release. The significance of these results for schizophrenia is discussed.     

Nitric oxide synthase inhibition blocks amphetamine-induced locomotor activity in mice.

Effects of N(G)-nitroarginine methyl ester (L-NAME), a nonspecific inhibitor of nitric oxide (NO) synthase, on amphetamine-induced locomotor activity were investigated in Swiss-Webster mice. Locomotor activity was measured for 30 min immediately following amphetamine (1, 2 and 4 mg/kg, i.p.) or saline treatments. L-NAME (15 and 30 mg/kg) and a combination of L-arginine (1000 mg/kg) and L-NAME (30 mg/kg) were injected 30 min before amphetamine (2 mg/kg) to other groups of the mice. L-Arginine was injected 30 min before L-NAME treatment when they were combined. L-NAME (30 mg/kg) and L-arginine (1000 mg/kg) were also tested for ability to depress or stimulate locomotor activity in the absence of amphetamine. Amphetamine caused a dose-dependent increase in locomotor activity of the mice. L-NAME blocked the amphetamine-induced locomotor stimulation dose dependently. L-Arginine pretreatment prevented the inhibitory effects of L-NAME on amphetamine-induced locomotor stimulation. L-NAME and L-arginine did not cause any significant change in locomotor activity in mice not treated with amphetamine. These results suggest that amphetamine-induced locomotor stimulation in mice is modulated by NO.     

Phencyclidine during pregnancy in the rat: effects on locomotor activity in the offspring

Either 5 or 10 mg/kg of phencyclidine hydrochloride (PCP) was administered by gastric intubation to gravid rats during the last two weeks of gestation. Intubation controls received the vehicle and all offspring were fostered to untreated controls at birth. PCP produced a decrement in maternal weight gain and a small but nonsignificant reduction in birth weight that was no longer evident at weaning. There were no maternal deaths nor were resorptions or stillbirths increased by PCP exposure. Offspring were tested for differences in locomotor activity from birth to weaning at 30 days of age and during adulthood. No behavioral differences were found among the preweanling or adult offspring. Results are compared with other prenatal studies of PCP toxicity and teratogenicity.     

Subchronic and chronic PCP treatment produces temporally distinct deficits in attentional set shifting and prepulse inhibition in rats

Rationale We have previously demonstrated that subchronic (five daily administrations of 2.6 mg/kg PCP) and chronic intermittent administration of 2.6 mg/kg PCP to rats produces hypofrontality and other neurochemical changes akin to schizophrenia pathology (Cochran et al., Neuropsychopharmacology, 28:265–275, 2003). Objectives We sought to determine whether behavioral alterations related to discrete aspects of schizophrenia are also induced by these PCP treatment regimes. Materials and methods Following administration of vehicle or PCP according to the protocols described above, rats were assessed for attentional set shifting ability, prepulse inhibition (PPI), or social interaction and the locomotor response to a challenge dose of amphetamine. Results Ability to shift attentional set was impaired 72 h after the last PCP administration following the subchronic and chronic intermittent treatment regimes. PPI was disrupted after each acute administration of PCP in animals under the subchronic treatment regime. However, PPI deficits were not sustained 72 h after the last of five daily administrations. In subchronic and chronic PCP treated animals, no change was found in social interaction behavior, and there was little change in baseline or amphetamine-stimulated locomotor activity, employed as an indicator of dopaminergic hyperfunction. Conclusions The temporally distinct behavioral effects of these PCP treatment regimes suggest that PPI deficits relate directly to acute NMDA receptor antagonism, whereas the more enduring set shifting deficits relate to the longer term consequences of NMDA receptor blockade. Therefore, these subchronic and chronic PCP treatment regimes produce hypofrontality (Cochran et al., Neuropsychopharmacology, 28:265–275, 2003) and associated prefrontal cortex-dependent deficits in behavioral flexibility which mirror core deficits in schizophrenia.     

Dissociable effects of 6-OHDA-induced lesions of neostriatum on anorexia,locomotor activity and stereotypy: The role of behavioural competition

The effect of 6-OHDA-induced lesions of neostriatum on locomotor activity, stereotypy and anorexia induced by amphetamine (0.5 mg/kg, 1.5 mg/kg and 5.0 mg/kg IP) was examined. Lesioned rats demonstrated attenuated stereotypy and anorexia but enhanced locomotor activity to amphetamine. Biochemical analysis of dopamine and noradrenaline in specific forebrain areas demonstrated significant dopamine depletion in neostriatum. Dopamine levels in mesolimbic, frontal cortex and hypothalamic areas, and noradrenaline in frontal cortex and hypothalamic areas, were not significantly reduced. The data were interpreted in terms of a response incompatibility hypothesis. It is proposed that stereotyped responses mediated by nigrostriatal dopamine neurones are incompatible with eating. In addition, it is suggested that a second form of competition, at the neuro-anatomical level, occurs between mesolimbic and nigrostriatal systems for motor output pathways and the ultimate expression of behaviour. The role of noradrenaline in amphetamine anorexia is also discussed.     

Comparison of the effects of nitric oxide synthase inhibition and guanylate cyclase inhibition on vascular contraction in vitro and in vivo in the rat

We have investigated the differences between the nitric oxide synthase inhibitor (NOSI), L-NMMA, and the guanylate cyclase inhibitors (GCI), methylene blue and LY 83583, in their abilities to increase vasoconstrictor responses in vitro and in vivo. In rat small mesenteric arterial rings, 1 h exposure to the NOSI, L-NMMA (100 μM), and the GCI, methylene blue (10 μM), alone or in combination with L-NMMA, caused a significant reduction in the maximum relaxation to ACh in mesenteric arteries pre-contracted with the thromboxane mimetic U46619 (10 μM). Hence, both NOSI and GCI inhibit endothelium-dependent relaxations to ACh in rat small mesenteric artery. However, 1 h exposure to L-NMMA and L-NNA (both 100 μM), but not methylene blue (10 μM), significantly increased the contractile response to U-46619 (10 μM) in rat small mesenteric artery. It was decided to investigate further this difference between NOSI and methylene blue. In rat small mesenteric arterial rings, L-NMMA (10 μM) and LY 83583 (1–10 μM) significantly increased the contractile response to KCl (40 mM) or to noradrenaline (10 μM), when administered during the contraction. However, methylene blue (1–10 μM) increased the contractile response to KCl but not noradrenaline. In rat aortic rings, L-NMMA (100 μM), methylene blue (1–10 μM) and LY 83583 (1–10 μM) significantly increased the contractile response to KCl (40 mM) or to noradrenaline (1 μM). In the pithed rat preparation, L-NMMA (40.3 μmol kg^–1, i.v.) significantly increased the pressor response both to bolus injection of noradrenaline (3.13 nmol kg^–1) and to spinal pressor nerve stimulation. However, methylene blue (3.13–15.6 μmol kg^–1) or LY 83583 (4.0–40.0 μmol kg^–1), failed to affect pressor responses to either NA or pressor nerve stimulation. Hence, there are differences between NOSI and GCI in their abilities to increase vasoconstrictor responses, especially when comparing responses in vitro and in vivo. This suggests that nitric oxide has actions in addition to activation of guanylate cyclase to modulate vasoconstrictor responses, presumably by membrane hyperpolarisation, and that this action may be more important in vivo. Received: 2 May 1997 / Accepted: 19 June 1997     

Interactions between ethanol and cocaine,amphetamine, or MDMA in the rat: thermoregulatory and locomotor effects

Rationale (±)-3,4-methylenedioxymethamphetamine (MDMA, ecstasy) is often taken recreationally with ethanol (EtOH). In rats, EtOH may potentiate MDMA-induced hyperactivity, but attenuate hyperthermia. Objective Experiment 1 compared the interactions between EtOH (1.5 g/kg) and MDMA (6.6 mg/kg) with EtOH + cocaine (COCA; 10 mg/kg) and EtOH + amphetamine (AMPH; 1 mg/kg) on locomotor activity and thermoregulation. Experiment 2 used a weaker dose of MDMA (3.3 mg/kg) and larger doses of COCA (20 mg/kg) and AMPH (2 mg/kg). Materials and methods Drug treatments were administered on four occasions (2, 5, and 2 days apart, respectively; experiment 1) or two (2 days apart; experiment 2). Results All psychostimulants increased activity, and EtOH markedly increased the effect of MDMA. AMPH alone-related hyperactivity showed modest sensitization across treatment days, while MDMA + EtOH activity showed marked sensitization. AMPH, COCA, and MDMA induced hyperthermia of comparable amplitude (+1 to +1.5°C). Co-treatment with EtOH and AMPH (1 mg/kg) or COCA (10 mg/kg) produced hypothermia greater than that produced by EtOH alone. Conversely, EtOH attenuated MDMA-related hyperthermia, an effect increasing across treatment days. These results demonstrate that the interaction between MDMA and EtOH may be different from the interaction between EtOH and AMPH or COCA. Conclusion Because of potential health-related consequences of such polydrug misuse, it is worth identifying the mechanisms underlying these interactions, especially between EtOH and MDMA. Given the different affinity profiles of the three drugs for serotonin, dopamine, and norepinephrine transporters, our results appear compatible with the possibility of an important role of serotonin in at least the EtOH-induced potentiation of MDMA-induced hyperlocomotion.     

Dopamine agonist effects on startle and sensorimotor gating in normal male subjects: time course studies

RATIONALE: Prepulse inhibition (PPI) of startle is a measure of sensorimotor gating that is deficient in schizophrenia and in rodents treated with dopamine (DA) agonists. Reduced PPI is reported in normal humans treated with direct or indirect DA agonists. To facilitate future studies, we assessed the time course of DA agonist effects on PPI in humans, for both direct (bromocriptine: 1.25, 2.5 mg; pergolide: 0.025, 0.1 mg) and indirect DA agonists (amphetamine: 20 mg; amantadine: 200 mg) ( n=6-10/dose). METHODS: Baseline (no drug) levels of acoustic and tactile startle, as well as uni- and cross-modal PPI, were assessed in 63 normal adult males. Seven to ten days later, subjects were tested in five sessions over 3.5 h after ingestion of placebo or active drug in a double-blind design. RESULTS: Expected drug effects were observed in both autonomic (for example, increased heart rate and blood pressure with amphetamine), somatic (for example, "queasiness" with direct DA agonists), and psychological measures (for example, "happiness", less "drowsiness" with amphetamine). Drugs increased (bromocriptine) or decreased (amantadine) startle magnitude, and caused either no change or modest, time-dependent effects on PPI. Amantadine increased PPI over the test session, a pattern not observed with other DA agonists or placebo. No consistent effects on PPI were observed with either bromocriptine, pergolide, or amphetamine. Drug effects on startle did not consistently correlate with self-assessment measures. CONCLUSIONS: Despite evidence of "bioactivity", under the specific experimental conditions of this study, neither direct nor indirect DA agonists had robust effects on startle or PPI. In some cases (for example, amantadine), a time course was identified that will facilitate future studies of DA agonist effects on PPI in humans.