Abdel-Mouttalib Ouagazzal

Effects of dopamine D1 and D2 receptor blockade on MK-801-induced hyperlocomotion in rats

Blocking glutamatergic transmission at the N-methyl-d-aspartate (NMDA) receptor complex with MK-801 (0.15–0.5 mg/kg, IP) was found to induce a robust, dose-dependent increase in locomotor activity. This behavioural activation was similar in intensity to that observed afterd-amphetamine (1 mg/kg, SC). The locomotor stimulation induced by MK-801 at 0.3 mg/kg was significantly inhibited by the D2 dopamine receptor antagonist raclopride (0.1–0.3 mg/kg, SC) and by the D1 receptor antagonist SCH 23390 (0.04 mg/kg, SC). The locomotor activity induced by a higher dose of MK-801 (0.5 mg/kg) was reduced by higher doses of raclopride or SCH 23390 administered alone (0.3 and 0.08 mg/kg, respectively), and was inhibited by simultaneous administration of ineffective doses. Raclopride significantly reducedd-amphetamine-induced locomotor activity at a dose (0.2 mg/kg) that also blocked the effects of a low dose of MK-801. In contrast, SCH 23390 blocked the effects ofd-amphetamine at a dose (i.e. 0.01 mg/kg) lower than that needed to block MK-801. These results suggest that the dopaminergic system may in part mediate the locomotor effects induced by the NMDA antagonist, MK-801, in rats. However, the locomotor activity induced by MK-801 appears to be less sensitive to dopaminergic receptor blockade than that induced byd-amphetamine, suggesting that the underlying mechanisms, although similar, are not identical.

Zinc alleviates pain through high-affinity binding to the NMDA receptor NR2A subunit

Zinc is abundant in the central nervous system and regulates pain, but the underlying mechanisms are unknown. In vitro studies have shown that extracellular zinc modulates a plethora of signaling membrane proteins, including NMDA receptors containing the NR2A subunit, which display exquisite zinc sensitivity. We created NR2A-H128S knock-in mice to investigate whether Zn2+–NR2A interaction influences pain control. In these mice, high-affinity (nanomolar) zinc inhibition of NMDA currents was lost in the hippocampus and spinal cord. Knock-in mice showed hypersensitivity to radiant heat and capsaicin, and developed enhanced allodynia in inflammatory and neuropathic pain models. Furthermore, zinc-induced analgesia was completely abolished under both acute and chronic pain conditions. Our data establish that zinc is an endogenous modulator of excitatory neurotransmission in vivo and identify a new mechanism in pain processing that relies on NR2A NMDA receptors. The study also potentially provides a molecular basis for the pain-relieving effects of dietary zinc supplementation.

Impaired Emotional-Like Behavior and Serotonergic Function During Protracted Abstinence from Chronic Morphine

BACKGROUND Opiate abuse is a chronic relapsing disorder, and maintaining prolonged abstinence remains a major challenge. Protracted abstinence is characterized by lowered mood, and clinical studies show elevated comorbidity between addiction and depressive disorders. At present, their relationship remains unclear and has been little studied in animal models. Here we investigated emotional alterations during protracted abstinence, in mice with a history of chronic morphine exposure. METHODS C57BL6J mice were exposed to a chronic intermittent escalating morphine regimen (20-100 mg/kg). Physical dependence (naloxone-precipitated withdrawal), despair-related behaviors (tail suspension test), and social behaviors were examined after 1 or 4 weeks of abstinence. Stress hormones and forebrain bioamine levels were analyzed at the end of morphine regimen and after 4 weeks of abstinence. Finally, we examined the effects of chronic fluoxetine during abstinence on morphine-induced behavioral deficits. RESULTS Acute naloxone-induced withdrawal was clearly measurable after 1 week, and became undetectable after 4 weeks. In contrast, social and despair-related behaviors were unchanged after 1 week, but low sociability and despair-like behavior became significant after 4 weeks. Chronic morphine regimen increased both corticosterone levels and forebrain serotonin turnover, but only serotonergic activity in the dorsal raphe remained impaired after 4 weeks. Remarkably, chronic fluoxetine prevented depressive-like behavioral deficits in 4-week abstinent mice. CONCLUSIONS During protracted abstinence, the immediate consequences of morphine exposure attenuate, whereas fluoxetine-sensitive emotional alterations strengthen with time. Our study establishes a direct link between morphine abstinence and depressive-like symptoms and strongly suggests that serotonin dysfunction represents a main mechanism contributing to mood disorders in opiate abstinence.

Mu and Delta Opioid Receptors Oppositely Regulate Motor Impulsivity in the Signaled Nose Poke Task

Impulsivity is a primary feature of many psychiatric disorders, most notably attention deficit hyperactivity disorder and drug addiction. Impulsivity includes a number of processes such as the inability to delay gratification, the inability to withhold a motor response, or acting before all of the relevant information is available. These processes are mediated by neural systems that include dopamine, serotonin, norepinephrine, glutamate and cannabinoids. We examine, for the first time, the role of opioid systems in impulsivity by testing whether inactivation of the mu- (Oprm1) or delta- (Oprd1) opioid receptor gene alters motor impulsivity in mice. Wild-type and knockout mice were examined on either a pure C57BL6/J (BL6) or a hybrid 50% C57Bl/6J–50% 129Sv/pas (HYB) background. Mice were trained to respond for sucrose in a signaled nose poke task that provides independent measures of associative learning (responses to the reward-paired cue) and motor impulsivity (premature responses). Oprm1 knockout mice displayed a remarkable decrease in motor impulsivity. This was observed on the two genetic backgrounds and did not result from impaired associative learning, as responses to the cue signaling reward did not differ across genotypes. Furthermore, mutant mice were insensitive to the effects of ethanol, which increased disinhibition and decreased conditioned responding in wild-type mice. In sharp contrast, mice lacking the Oprd1 gene were more impulsive than controls. Again, mutant animals showed no deficit in associative learning. Ethanol completely disrupted performance in these animals. Together, our results suggest that mu-opioid receptors enhance, whereas delta-opioid receptors inhibit, motor impulsivity. This reveals an unanticipated contribution of endogenous opioid receptor activity to disinhibition. In a broader context, these data suggest that alterations in mu- or delta-opioid receptor function may contribute to impulse control disorders.

Locomotor activation induced by MK-801 in the rat : postsynaptic interactions with dopamine receptors in the ventral striatum

The effects of bilateral 6-hydroxydopamine-induced destruction of the dopamine nerve terminals in the ventral striatum (nucleus accumbens) or pharmacological blockade of dopamine receptors with haloperidol injected locally into this area were examined on the locomotor hyperactivity induced by systemic administration of the non-competitive NMDA receptor antagonist, MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5,10-imine hydrogen maleate salt). The locomotor stimulation induced by two doses of MK-801 (0.15 and 0.3 mg/kg, i.p.) was not attenuated by 6-hydroxydopamine bilateral lesions to the ventral striatum, either 7 or 14 days after the operation. The same lesion however reduced the locomotor activation induced by 0.5 mg/kg d-amphetamine 14 days after surgery. Bilateral intra-accumbens injection of haloperidol at a dose (2.5 micrograms/side) that blocked d-amphetamine-induced hypermotility did not reduce the locomotor response to 0.3 mg/kg MK-801, while 5 micrograms/side haloperidol decreased the MK-801-induced locomotor stimulation. These results suggest that the locomotor response to MK-801 is dependent on an interaction between dopaminergic and excitatory amino acid transmission occurring postsynaptically rather than presynaptically in the ventral striatum.

Nociceptin Receptor Impairs Recognition Memory via Interaction with NMDA Receptor-Dependent Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase Signaling in the Hippocampus

Strong evidence suggests a role for nociceptin/orphanin FQ (N/OFQ) neuropeptide and its receptor (NOP) in cognition. However, the signaling mechanisms underlying N/OFQ modulation of memory are less understood. Here, we show that intracerebroventricular or intrahippocampal infusions of N/OFQ impair long-term memory formation in the mouse object recognition task. The synthetic NOP receptor agonist, (1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5]decan-4-one (Ro64-6198), administered systemically, also produced amnesic effects that were blocked by coinfusion of the NOP receptor antagonist, [Nphe1,Arg14,Lys15]nociceptin-NH2 (UFP-101), into the dorsal hippocampus. In contrast, Ro64-6198 had no effect on short-term memory or recall performances. Immunoblotting analysis revealed a strong suppressive action of Ro64-6198 on learning-induced upregulation of hippocampal extracellular signal-regulated kinase (ERK) phosphorylation, which is crucial for long-term information storage. Accordingly, pharmacological inhibition of ERK activation after systemic injection of SL327 [α-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzene acetonitrile], a selective inhibitor of the upstream kinase MEK (mitogen-activated protein kinase kinase), abolished long-term recognition memory formation. The noncompetitive NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine maleate (MK-801), given systemically, also suppressed ERK activation and disrupted recognition memory. In contrast, no effect of MK-801 was observed on recall, as for Ro64-6198. When administered concurrently at subthreshold doses, Ro64-6198 and MK-801 synergistically suppressed hippocampal ERK activation and impaired long-term memory formation. Under resting conditions, neither Ro64-6198 nor MK-801 affected spontaneous ERK activity in the hippocampus at the amnesic doses whereas at higher doses, only MK-801 had a suppressive effect. We conclude that N/OFQ-NOP receptor system negatively regulates long-term recognition memory formation through hippocampal ERK signaling mechanisms. This modulation may in part take place by inhibiting glutamatergic function at the NMDA receptor.

Functional interactions between glutamate and dopamine in the rat striatum

The functional role of NMDA receptors in a spontaneous (locomotion) and a conditioned behaviour (reaction-time task) known to preferentially involve dopamine transmission in the ventral or the dorsal part of the striatum, respectively, was studied in the rat. The non-competitive NMDA receptor antagonist MK-801 systemically injected produced a dose-dependent increase in locomotor activity and impaired the performance of the animals trained to release a lever after a visual stimulus within a time limit by increasing the number of anticipatory errors (lever releases occurring before the stimulus onset). Similar behavioural changes were obtained after bilateral striatal microinjections of the competitive NMDA-antagonist APV into the ventral or dorsal striatum, respectively, suggesting that MK-801-induced behavioral effects after systemic injection might be mediated through a blockade of EAA transmission within the striatum. Dopamine injected in the same striatal locations induced effects similar to APV on locomotion and reaction-time performance, in agreement with the proposal for a functional antagonism between the glutamatergic and the dopaminergic transmission at striatal level. The conjoint administration of APV and dopamine directly into the striatum did not alter the behavioural effect induced by each compound injected alone showing that these effects are not additive. This latter observation actually suggests the occurrence of a functional interaction between the two neuronal systems probably acting on a common striatal target relaying dopaminergic and glutamatergic antagonistic influences on locomotion and conditioned motor behaviours.

Effects of age-related hearing loss on startle reflex and prepulse inhibition in mice on pure and mixed C57BL and 129 genetic background

The present study examined the developmental course of the age-related hearing loss and its consequences on the expression of acoustic startle reflex (ASR) and prepulse inhibition (PPI) generated by white-noise bursts in 129S2/SvPas (129) and C57BL/6J (C57) mouse strains and their F(1) hybrids. Auditory brainstem responses (ABR), ASR and PPI were assessed at various time points: 6, 28, 41 and 94 weeks. Both parental strains showed marked ABR threshold shifts with age, with C57 mice having the most pronounced deficits. By contrast, the hybrids displayed only minor hearing loss with age. The time courses of ASR and PPI varied considerably between the mouse strains. From 6 to 41 weeks of age, ASR and PPI elicited by weak stimuli (70-90dB) increased in C57 mice, whereas the startle responses to intense stimuli (95-120dB) declined progressively. In 129 and hybrid mice, PPI levels remained relatively stable during the first year, but a progressive increase of ASR was observed in the hybrids for intense stimuli (95-120dB). When animals reached 94 weeks of age, marked deterioration of ASR was observed in all strains, while deficits in PPI were only seen in 129 and C57 mice. These findings show that the time course and the severity of the hearing loss vary considerably between 129, C57 strains and their hybrids, thus suggesting a marked heterogeneity in the genetic mechanisms underlying deafness in mice. They also demonstrate that the age-related hearing loss may have complex consequences on auditory behavioral performances depending of the severity of the deficits, the genetic background as well as the stimuli parameters.

Endogenous nociceptin/orphanin-fq in the dorsal hippocampus facilitates despair-related behavior

Nociceptin/orphanin‐FQ (N/OFQ) peptide and its receptor (NOP: N/OFQ opioid peptide receptor) are highly expressed in the hippocampus, but their functional role remains poorly understood. We recently showed that hippocampal N/OFQ inhibits learning and memory abilities in mice. Here, we investigated whether the endogenous peptide also regulated emotional responses at the level of the hippocampus. Bilateral infusions of the selective NOP receptor antagonist, UFP‐101 (1–3 nmol/side), into the dorsal hippocampus produced antidepressant‐like effects in the mouse forced swim and tail suspension tests comparable with those obtained with the prototypical antidepressant, fluoxetine (10–30 mg/kg, intraperitoneal). In the light‐dark test, neither UFP‐101 (1–3 nmol/side) nor N/OFQ peptide (1–3 nmol/side) modified anxiety measures when injected at behaviorally active doses in the dorsal hippocampus. These findings show a clear dissociation in the involvement of hippocampal N/OFQ system in anxiety‐ and despair‐related behaviors. We conclude that the dorsal hippocampus is a brain region in which there is an important N/OFQ modulation of mnemonic processes and adaptive emotional responses associated to despair states.

Activation of nociceptin opioid peptide (NOP) receptor impairs contextual fear learning in mice through glutamatergic mechanisms.

The present study investigated whether the selective nociceptin opioid peptide (NOP) receptor agonist, Ro64-6198, impairs acquisition of fear conditioning through glutamatergic mechanisms. Systemic administration of Ro64-6198 (0.3 and 1mg/kg) or the non-competitive NMDA receptor antagonist, MK-801 (0.03 and 0.1mg/kg) prior to conditioning severely impaired contextual but not cued fear learning in C57BL/6N mice. When administered together at sub-effective doses, Ro64-6198 (0.5mg/kg) and MK-801 (0.05mg/kg), synergistically impaired contextual fear learning, but left cued fear learning intact. We next used the immediate shock deficit paradigm (ISD) to examine the effects of Ro64-6198 and MK-801 on contextual memory formation in the absence of the foot-shock. As expected, naive mice that were shocked briefly after being placed in the training chamber displayed no contextual fear conditioning. This learning deficit was elevated by prior exposure of mice to the training context. Furthermore, administration of Ro64-6198 and MK-801, either separately at amnesic doses (1mg/kg and 0.1mg/kg, respectively) or concomitantly at sub-effective doses (0.5mg/kg and 0.05mg/kg, respectively) significantly reduced the facilitating effects of context preexposure. These findings demonstrate the existence of functional antagonism between NOP and NMDA receptors that predominantly contributes to modulation of conditioned fear learning which involves spatial-processing demands.