NMDA-mediated modulation of dopamine release is modified in rat prefrontal cortex and nucleus accumbens after chronic nicotine treatment

Dopamine-Dependent Ketamine Modulation of Glutamatergic Synaptic Plasticity in the Prelimbic Cortex of Adult Rats Exposed to Acute Stress

Traumatic stress is the main environmental risk factor for the development of psychiatric disorders. We have previously shown that acute footshock (FS) stress in male rats induces rapid and long-lasting functional and structural changes in the prefrontal cortex (PFC), which are partly reversed by acute subanesthetic ketamine. Here, we asked if acute FS may also induce any changes in glutamatergic synaptic plasticity in the PFC 24 h after stress exposure and whether ketamine administration 6 h after stress may have any effect. We found that the induction of long-term potentiation (LTP) in PFC slices of both control and FS animals is dependent on dopamine and that dopamine-dependent LTP is reduced by ketamine. We also found selective changes in ionotropic glutamate receptor subunit expression, phosphorylation, and localization at synaptic membranes induced by both acute stress and ketamine. Although more studies are needed to understand the effects of acute stress and ketamine on PFC glutamatergic plasticity, this first report suggests a restoring effect of acute ketamine, supporting the potential benefit of ketamine in limiting the impact of acute traumatic stress.

Metamodulation of presynaptic NMDA receptors: New perspectives for pharmacological interventions

Metamodulation shifted the scenario of the central neuromodulation from a simplified unimodal model to a multimodal one. It involves different receptors/membrane proteins physically associated or merely colocalized that act in concert to control the neuronal functions influencing each other. Defects or maladaptation of metamodulation would subserve neuropsychiatric disorders or even synaptic adaptations relevant to drug dependence. Therefore, this "vulnerability" represents a main issue to be deeply analyzed to predict its aetiopathogenesis, but also to propose targeted pharmaceutical interventions. The review focusses on presynaptic release-regulating NMDA receptors and on some of the mechanisms of their metamodulation described in the literature. Attention is paid to the interactors, including both ionotropic and metabotropic receptors, transporters and intracellular proteins, which metamodulate their responsiveness in physiological conditions but also undergo adaptation that are relevant to neurological dysfunctions. All these structures are attracting more and more the interest as promising druggable targets for the treatment of NMDAR-related central diseases: these substances would not exert on-off control of the colocalized NMDA receptors (as usually observed with NMDAR full agonists/antagonists), but rather modulate their functions, with the promise of limiting side effects that would favor their translation from preclinic to clinic.

Synaptosomes and Metamodulation of Receptors

Synaptosomes are re-sealed pinched off nerve terminals that maintain all the main structural and functional features of the original structures and that are appropriate to study presynaptic events. Because of the discovery of new structural and molecular events that dictate the efficiency of transmitter release and of its receptor-mediated control in the central nervous system, the interest in this tissue preparation is continuously renewing. Most of these events have been already discussed in previous reviews, but few of them were not and deserve some comments since they could suggest new functional and possibly therapeutic considerations. Among them, the "metamodulation" of receptors represents an emerging aspect that dramatically increased the complexity of the presynaptic compartment, adding new insights to the role of presynaptic receptors as modulators of chemical synapses. Deciphering the mechanism of presynaptic metamodulation would permit indirect approaches to control the activity of presynaptic release-regulating receptors that are currently orphans of direct ligands/modulators, paving the road for the proposal of new therapeutic approaches for central neurological diseases.

Presynaptic release-regulating NMDA receptors in isolated nerve terminals: A narrative review

The existence of presynaptic, release‐regulating NMDA receptors in the CNS has been long matter of discussion. Most of the reviews dedicated to support this conclusion have preferentially focussed on the results from electrophysiological studies, paying little or no attention to the data obtained with purified synaptosomes, even though this experimental approach has been recognized as providing reliable information concerning the presence and the role of presynaptic release‐regulating receptors in the CNS. To fill the gap, this review is dedicated to summarising the results from studies with synaptosomes published during the last 40 years, which support the existence of auto and hetero NMDA receptors controlling the release of transmitters such as glutamate, GABA, dopamine, noradrenaline, 5‐HT, acetylcholine and peptides, in the CNS of mammals. The review also deals with the results from immunochemical studies in isolated nerve endings that confirm the functional observations.

Use of an Animal Model to Evaluate Anxiolytic Effects of Dietary Supplementation with Tilia tomentosa Moench Bud Extracts

Anxiety disorders are common and complex psychiatric syndromes affecting a broad spectrum of patients. On top of that, we know that aging produces an increase in anxiety vulnerability and sedative consumption. Moreover, stress disorders frequently show a clear gender susceptibility. Currently, the approved pharmacological strategies have severe side effects such as hallucinations, addiction, suicide, insomnia, and loss of motor coordination. Dietary integration with supplements represents an intriguing strategy for improving the efficacy and the safety of synthetic anxiolytics. Accordingly, a recent article demonstrated that glyceric bud extracts from Tilia tomentosa Moench (TTBEs) exert effects that are consistent with anxiolytic activity. However, the effects of these compounds in vivo are unknown. To examine this question, we conducted behavioral analysis in mice. A total of 21 days of oral supplements (vehicle and TTBEs) were assessed by Light Dark and Hole Board tests in male and female mice (young, 3 months; old, 24 months). Interestingly, the principal component analysis revealed gender and age-specific behavioral modulations. Moreover, the diet integration with the botanicals did not modify the body weight gain and the daily intake of water. Our results support the use of TTBEs as dietary supplements for anxiolytic purposes and unveil age and gender-dependent responses.

Interactions of Glutamatergic Neurotransmission and Brain-Derived Neurotrophic Factor in the Regulation of Behaviors after Nicotine Administration

Nicotine causes tobacco dependence, which may result in fatal respiratory diseases. The striatum is a key structure of forebrain basal nuclei associated with nicotine dependence. In the striatum, glutamate release is increased when α7 nicotinic acetylcholine receptors expressed in the glutamatergic terminals are exposed to nicotine, and over-stimulates glutamate receptors in gamma amino-butyric acid (GABA)ergic neurons. These receptor over-stimulations in turn potentiate GABAergic outputs to forebrain basal nuclei and contribute to the increase in psychomotor behaviors associated with nicotine dependence. In parallel with glutamate increases, nicotine exposure elevates brain-derived neurotrophic factor (BDNF) release through anterograde and retrograde targeting of the synapses of glutamatergic terminals and GABAergic neurons. This article reviews nicotine-exposure induced elevations of glutamatergic neurotransmission, the bidirectional targeting of BDNF in the striatum, and the potential regulatory role played by BDNF in behavioral responses to nicotine exposure.

CXCR4 and NMDA Receptors Are Functionally Coupled in Rat Hippocampal Noradrenergic and Glutamatergic Nerve Endings

Previous studies had shown that the HIV-1 capsidic glycoprotein gp120 (strain IIIB) modulates presynaptic release-regulating NMDA receptors on noradrenergic and glutamatergic terminals. This study aims to assess whether the chemokine CXC4 receptors (CXCR4s) has a role in the gp120-mediated effects. The effect of CXCL12, the endogenous ligand at CXCR4, on the NMDA-mediated releasing activity was therefore investigated. Rat hippocampal synaptosomes were preloaded with [³H]noradrenaline ([³H]NA) or [³H]D-aspartate ([³H]D-Asp) and acutely exposed to CXCL12, to NMDA or to both agonists. CXCL12, inactive on its own, facilitated the NMDA-evoked tritium release. The NMDA antagonist MK-801 abolished the NMDA/CXCL12-evoked tritium release of both radiolabelled tracers, while the CXCR4 antagonist AMD 3100 halved it, suggesting that rat hippocampal nerve endings possess presynaptic release-regulating CXCR4 receptors colocalized with NMDA receptors. Accordingly, Western blot analysis confirmed the presence of CXCR4 proteins in synaptosomal plasmamembranes. In both synaptosomal preparations, CXCL12-induced facilitation of NMDA-mediated release was dependent upon PLC-mediated src-induced events leading to mobilization of Ca²⁺ from intraterminal IP₃-sensitive stores Finally, the gp120-induced facilitation of NMDA-mediated release of [³H]NA and [³H]D-Asp was prevented by AMD 3100. We propose that CXCR4s are functionally coupled to NMDA receptors in rat hippocampal noradrenergic and glutamatergic terminals and account for the gp120-induced modulation of the NMDA-mediated central effects. The NMDA/CXCR4 cross-talk could have a role in the neuropsychiatric symptoms often observed in HIV-1 positive patients.

Cholinergic modulation of dopamine pathways through nicotinic acetylcholine receptors

Nicotine addiction is highly prevalent in current society and is often comorbid with other diseases. In the central nervous system, nicotine acts as an agonist for nicotinic acetylcholine receptors (nAChRs) and its effects depend on location and receptor composition. Although nicotinic receptors are found in most brain regions, many studies on addiction have focused on the mesolimbic system and its reported behavioral correlates such as reward processing and reinforcement learning. Profound modulatory cholinergic input from the pedunculopontine and laterodorsal tegmentum to dopaminergic midbrain nuclei as well as local cholinergic interneuron projections to dopamine neuron axons in the striatum may play a major role in the effects of nicotine. Moreover, an indirect mesocorticolimbic feedback loop involving the medial prefrontal cortex may be involved in behavioral characteristics of nicotine addiction. Therefore, this review will highlight current understanding of the effects of nicotine on the function of mesolimbic and mesocortical dopamine projections in the mesocorticolimbic circuit.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Altered neurochemical levels in the rat brain following chronic nicotine treatment

Converging evidence shows that neurochemical systems are crucial mediators of nicotine dependence. Our present study evaluates the effect of 3-month chronic nicotine treatment on the levels of multiple quaternary ammonium compounds as well as glutamate and gamma aminobutyric acid in the rat prefrontal cortex, dorsal striatum and hypothalamus. We observed a marked decrease of acetylcholine levels in the dorsal striatum (22.88%, p<0.01), reflecting the impact of chronic nicotine in local interneuron circuits. We found decreases of carnitine in the dorsal striatum and prefrontal cortex (19.44%, p<0.01; 13.58%, p<0.01, respectively), but robust enhancements of carnitine in the hypothalamus (26.59%, p<0.01), which may reflect the alterations in food and water intake during chronic nicotine treatment. Finally, we identified an increase of prefrontal cortex glutamate levels (8.05%, p<0.05), supporting previous studies suggesting enhanced prefrontal activity during chronic drug use. Our study shows that quaternary ammonium compounds are regulated in a highly brain region specific manner during chronic nicotine treatment, and provides novel insights into neurochemical regulation during nicotine use.

Chronic administration of nicotine enhances NMDA-activated currents in the prefrontal cortex and core part of the nucleus accumbens of rats

Nicotine is an addictive substance of tobacco. It has been suggested that nicotine acts on glutamatergic (N-methyl-d-aspartate, NMDA) neurotransmission affecting dopamine release in the mesocorticolimbic system. This effect is reflected in neuroadaptative changes that can modulate neurotransmission in the prefrontal cortex (PFC) and nucleus accumbens (NAcc) core (cNAcc) and shell (sNAcc) regions. We evaluated the effect of chronic administration of nicotine (4.23 mg/kg/day for 14 days) on NMDA activated currents in dissociated neurons from the PFC, and NAcc (from core and shell regions). We assessed nicotine blood levels by mass spectrophotometry and we confirmed that nicotine increases locomotor activity. An electrophysiological study showed an increase in NMDA currents in neurons from the PFC and core part of the NAcc in animals treated with nicotine compared to those of control rats. No change was observed in neurons from the shell part of the NAcc. The enhanced glutamatergic activity observed in the neurons of rats with chronic administration of nicotine may explain the increased locomotive activity also observed in such rats. To assess one of the possible causes of increased NMDA currents, we used magnesium, to block NMDA receptor that contains the NR2B subunit. If there is a change in percent block of NMDA currents, it means that there is a possible change in expression of NMDA receptor subunits. Our results showed that there is no difference in the blocking effect of magnesium on the NMDA currents. The magnesium lacks of effect after nicotinic treatment suggests that there is no change in expression of NR2B subunit of NMDA receptors, then, the effect of nicotine treatment on amplitude of NMDA currents may be due to an increase in the quantity of receptors or to a change in the unitary conductance, rather than a change in the expression of the subunits that constitute it.

Prolonged nicotine exposure down-regulates presynaptic NMDA receptors in dopaminergic terminals of the rat nucleus accumbens

The presynaptic control of dopamine release in the nucleus accumbens (NAc) by glutamate and acetylcholine has a profound impact on reward signaling. Here we provide immunocytochemical and neurochemical evidence supporting the co-localization and functional interaction between nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartic acid (NMDA) receptors in dopaminergic terminals of the NAc. Most NAc dopaminergic terminals possessed the nAChR α4 subunit and the pre-exposure of synaptosomes to nicotine (30 μM) or to the α4β2-containing nAChR agonist 5IA85380 (10 nM) selectively inhibited the NMDA (100 μM)-evoked, but not the 4-aminopyridine (10 μM)-evoked, [(3)H] dopamine outflow; this inhibition was blunted by mecamylamine (10 μM). Nicotine and 5IA85380 pretreatment also inhibited the NMDA (100 μM)-evoked increase of calcium levels in single nerve terminals, an effect prevented by dihydro-β-erythroidine (1 μM). This supports a functional interaction between α4β2-containing nAChR and NMDA receptors within the same terminal, as supported by the immunocytochemical co-localization of α4 and GluN1 subunits in individual NAc dopaminergic terminals. The NMDA-evoked [(3)H]dopamine outflow was blocked by MK801 (1 μM) and inhibited by the selective GluN2B-selective antagonists ifenprodil (1 μM) and RO 25-6981 (1 μM), but not by the GluN2A-preferring antagonists CPP-19755 (1 μM) and ZnCl2 (1 nM). Notably, nicotine pretreatment significantly decreased the density of biotin-tagged GluN2B proteins in NAc synaptosomes. These results show that nAChRs dynamically and negatively regulate NMDA receptors in NAc dopaminergic terminals through the internalization of GluN2B receptors.

Repeated treatment with nicotine induces phosphorylation of NMDA receptor NR2B subunit in the brain regions involved in behavioral sensitization

Protein phosphorylation is an important mechanism for the post-translational modulation of N-methyl-d-aspartate (NMDA) receptor functions. In the present study, we investigated the levels of NR2B phosphorylation at Tyr1472 and Ser1303 in the nucleus accumbens, striatum, frontal cortex, and hippocampus of rats that exhibit behavioral sensitization to nicotine. Repeated treatment of rats with nicotine (0.6mg/kg, s.c., for 7 days) produced locomotor sensitization accompanied by increased NR2B phosphorylation at Tyr1472 in the nucleus accumbens and striatum, brain regions involved in behavioral sensitization. In contrast, no changes in NR2B phosphorylation were observed after a single treatment with nicotine in these brain regions. In addition, no changes in NR2B phosphorylation at Ser1303 were observed after repeated treatment with nicotine in any examined brain regions. These results suggest that repeated treatment with nicotine induces NR2B phosphorylation at Tyr1472 in the nucleus accumbens and striatum, which might contribute to the development of synaptic and behavioral plasticity in response to nicotine.

Chronic nicotine exposure selectively activates a carrier-mediated release of endogenous glutamate and aspartate from rat hippocampal synaptosomes

The effect of chronic nicotine treatment on the release of endogenous glutamate (GLU), aspartate (ASP) and GABA evoked in vitro by KCl, 4-aminopyridine (4AP) and nicotinic agonists in synaptosomes of rat hippocampus was investigated. Rats were chronically administered with nicotine bitartrate or saline vehicle each for 14 days using osmotic mini-pumps. Hippocampal synaptosomes were stimulated with KCl, 4AP, nicotine or with choline (Ch) and 5-iodo-A-85380 dihydrochloride (5IA85380). The GLU and ASP overflow evoked by Ch, nicotine, KCl and 4AP were increased in treated animals while the nicotine-evoked GABA overflow was reduced and that evoked by Ch, KCl and 4AP was unaffected. The 5IA85380-evoked overflow of the three aminoacids (AAs) was always reduced. The increase of ASP and GLU overflow evoked by KCl, 4AP or Ch was blocked by dl-threo-β-benzyloxyaspartic acid (dl-TBOA), a carrier transporter inhibitor, and by inhibitors of the Na(+)/Ca(2+) exchangers 2-[[4-[(4-nitrophenyl)methoxy]phenyl]methyl]-4-thiazolidinecarboxylic acid ethyl ester (SN-6) and 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate (KB-R7943). In conclusion long-term nicotine treatment may selectively increase GLU and ASP overflow elicited by KCl, 4AP and Ch through the activation of a carrier-mediated release mechanism and completely abolished the stimulatory effects of α4β2 nAChRs which modulate the release of all the three AA.

Androgen influence on prefrontal dopamine systems in adult male rats: localization of cognate intracellular receptors in medial prefrontal projections to the ventral tegmental area and effects of gonadectomy and hormone replacement on glutamate-stimulated extracellular dopamine level

Although androgens are known to modulate dopamine (DA) systems and DA-dependent behaviors of the male prefrontal cortex (PFC), how this occurs remains unclear. Because relatively few ventral tegmental area (VTA) mesoprefrontal DA neurons contain intracellular androgen receptors (ARs), studies presented here combined retrograde tracing and immunolabeling for AR in male rats to determine whether projections afferent to the VTA might be more AR enriched. Results revealed PFC-to-VTA projections to be substantially AR enriched. Because these projections modulate VTA DA cell firing and PFC DA levels, influence over this pathway could be means whereby androgens modulate PFC DA. To assess the hormone sensitivity of glutamate stimulation of PFC DA tone, additional studies utilized microdialysis/reverse dialysis application of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate receptor subtype-selective antagonists which act locally within the PFC and tegmentally via inhibition or disinhibition of PFC-to-VTA afferents to modulate intracortical DA levels. Here, we compared the effects of these drug challenges in control, gonadectomized, and gonadectomized rats given testosterone or estradiol. This revealed complex effects of gonadectomy on antagonist-stimulated PFC DA levels that together with the anatomical data above suggest that androgen stimulation of PFC DA systems does engage glutamatergic circuitry and perhaps that of the AR-enriched glutamatergic projections from PFC-to-VTA specifically.

Pre-synaptic nicotinic receptors evoke endogenous glutamate and aspartate release from hippocampal synaptosomes by way of distinct coupling mechanisms

BACKGROUND AND PURPOSE

The present work aimed to investigate whether and through which mechanisms selective α7 and α4β2 nicotinic receptor (nAChR) agonists stimulate endogenous glutamate (GLU) and aspartate (ASP) release in rat hippocampus.

EXPERIMENTAL APPROACH

Rat hippocampal synaptosomes were purified on Percoll gradients and superfused in vitro to study endogenous GLU and ASP release. The synaptosomes were superfused with selective α7 and α4β2 nAChR agonists and antagonists. The excitatory amino acid (EAA) content of the samples of superfusate was determined by HPLC after pre-column derivatization and separation on a chromatographic column coupled with fluorimetric detection.

KEY RESULTS

Choline (Ch), a selective α7 receptor agonist, elicited a significant release of both GLU and ASP which was blocked by the α7 receptor antagonist methyllycaconitine (MLA), but was unaltered by the α4β2 receptor antagonist dihydro-β-erythroidine (DHβE). The stimulant effect of Ch was strongly reduced in a Ca(2+) -free medium, was not inhibited by Cd(2+) and tetrodotoxin (TTX), but was antagonized by dantrolene, xestospongin C and thapsigargin. 5-Iodo-A-85380 dihydrochloride (5IA85380), a selective α4β2 receptor agonist, elicited EAA release in a DHβE-sensitive, MLA-insensitive fashion. The 5IA85380-evoked release was dependent on extracellular Ca(2+) , blocked by Cd(2+) and TTX, but unaffected by dantrolene.

CONCLUSIONS AND IMPLICATIONS

Our study shows for the first time that rat hippocampal synaptosomes possess α7 and α4β2 nAChR subtypes, which can enhance the release of endogenous GLU and ASP via two distinct mechanisms of action. These results extend our knowledge of the nicotinic modulation of excitatory synaptic transmission in the hippocampus.