Presynaptic involvement of nitric oxide in dopamine D1/D5 receptor-induced sustained enhancement of synaptic currents mediated by ionotropic glutamate receptors in the rat hippocampus

TRPV1 channels in nitric oxide-mediated signalling: insight on excitatory transmission in rat CA1 pyramidal neurons

Nitric oxide (NO) is a fascinating signalling molecule implicated in a plethora of biological functions, especially at the synaptic level. Exploring neurotransmission in the hippocampus could be instrumental in the individuation of putative targets for nitric-oxide mediated neuromodulation, especially in terms of the potential repercussions on fundamental processes i.e. synaptic plasticity and excitability-related phenomena. Among these targets, endovanilloid signalling constitutes an object of study since Transient Receptors Vanilloid type 1 (TRPV1) channels possess a NO-sensitive gate modulating its activation. Also, NO has been referred to as a mediator for numerous endocannabinoid effects. Notwithstanding, the linkage between TRPV1 and NO systems in neuromodulation still remains elusive. To this end, we aim at investigating the involvement of TRPV1 in nitric oxide-mediated influence on hippocampal processes. Electrophysiological whole-cell recordings in CA1 pyramidal neurons were applied to evaluate excitatory neurotransmission in rat brain slices. Indeed, miniature excitatory postsynaptic currents (mEPSCs) were analysed upon pharmacological manipulation of TRPV1 and NO signalling pathways. In detail, only the administration of the specific TRPV1 exogenous agonist - capsaicin - reduced the frequency and amplitude of mEPSC similarly to the inhibitor of neuronal nitric oxide synthase (nNOS), 7-nitroindazole (7NI). In contrast, capsazepine, TRPV1 antagonist, does not influence excitatory transmission. The combined TRPV1 activation and nNOS blockade confirm the presence of a putative common mechanism. When we administered the endovanilloid-endocannabinoid ligand, i.e. anandamide, we unveiled a potentiation of neurotransmission that was selectively reverted by 7NI. Our data suggest that nitric oxide influences TRPV1 hippocampal signalling since these channels are not constitutively active, but can be "on-demand" activated to modulate excitation in CA1 pyramidal neurons, and that this effect is linked to nitric oxide production.

Pathologic role of nitrergic neurotransmission in mood disorders

Mood disorders are chronic, recurrent mental diseases that affect millions of individuals worldwide. Although over the past 40 years the biogenic amine models have provided meaningful links with the clinical phenomena of, and the pharmacological treatments currently employed in, mood disorders, there is still a need to examine the contribution of other systems to the neurobiology and treatment of mood disorders. This article reviews the current literature describing the potential role of nitric oxide (NO) signaling in the pathophysiology and thereby the treatment of mood disorders. The hypothesis has arisen from several observations including (i) altered NO levels in patients with mood disorders; (ii) antidepressant effects of NO signaling blockers in both clinical and pre-clinical studies; (iii) interaction between conventional antidepressants/mood stabilizers and NO signaling modulators in several biochemical and behavioral studies; (iv) biochemical and physiological evidence of interaction between monoaminergic (serotonin, noradrenaline, and dopamine) system and NO signaling; (v) interaction between neurotrophic factors and NO signaling in mood regulation and neuroprotection; and finally (vi) a crucial role for NO signaling in the inflammatory processes involved in pathophysiology of mood disorders. These accumulating lines of evidence have provided a new insight into novel approaches for the treatment of mood disorders.

Possible contributions of a novel form of synaptic plasticity in Aplysia to reward, memory, and their dysfunctions in mammalian brain

Recent studies in Aplysia have identified a new variation of synaptic plasticity in which modulatory transmitters enhance spontaneous release of glutamate, which then acts on postsynaptic receptors to recruit mechanisms of intermediate- and long-term plasticity. In this review I suggest the hypothesis that similar plasticity occurs in mammals, where it may contribute to reward, memory, and their dysfunctions in several psychiatric disorders. In Aplysia, spontaneous release is enhanced by activation of presynaptic serotonin receptors, but presynaptic D1 dopamine receptors or nicotinic acetylcholine receptors could play a similar role in mammals. Those receptors enhance spontaneous release of glutamate in hippocampus, entorhinal cortex, prefrontal cortex, ventral tegmental area, and nucleus accumbens. In all of those brain areas, glutamate can activate postsynaptic receptors to elevate Ca²⁺ and engage mechanisms of early-phase long-term potentiation (LTP), including AMPA receptor insertion, and of late-phase LTP, including protein synthesis and growth. Thus, presynaptic receptors and spontaneous release may contribute to postsynaptic mechanisms of plasticity in brain regions involved in reward and memory, and could play roles in disorders that affect plasticity in those regions, including addiction, Alzheimer’s disease, schizophrenia, and attention deficit hyperactivity disorder (ADHD).

Ethanol modulation of synaptic plasticity

Synaptic plasticity in the most general terms represents the flexibility of neurotransmission in response to neuronal activity. Synaptic plasticity is essential both for the moment-by-moment modulation of neural activity in response to dynamic environmental cues and for long-term learning and memory formation. These temporal characteristics are served by an array of pre- and post-synaptic mechanisms that are frequently modulated by ethanol exposure. This modulation likely makes significant contributions to both alcohol abuse and dependence. In this review, I discuss the modulation of both short-term and long-term synaptic plasticity in the context of specific ethanol-sensitive cellular substrates. A general discussion of the available preclinical, animal-model based neurophysiology literature provides a comparison between results from in vitro and in vivo studies. Finally, in the context of alcohol abuse and dependence, the review proposes potential behavioral contributions by ethanol modulation of plasticity.

Acute cocaine administration increases NO efflux in the rat prefrontal cortex via a neuronal NOS-dependent mechanism

An understanding of the neurochemical changes occurring following exposure to psychostimulants such as cocaine is critical for the development of novel pharmacotherapies aimed at disrupting the addictive cycle. It is well established that the acute effects of cocaine associated with drug-induced blockade of dopamine (DA) reuptake processes occur in reward-related areas of the brain including the medial prefrontal cortex (mPFC). Considerable evidence has accumulated indicating that the interaction between DA, glutamate, and nitric oxide (NO) is likely to play a critical role in the neuroplastic changes associated with psychostimulant exposure. However, the potential impact of cocaine on NO synthase (NOS) activity in the mPFC has not been examined. In this study, NO efflux was measured in the mPFC of anesthetized male rats using a NO-selective electrochemical microsensor. Acute systemic administration of cocaine significantly increased NO efflux in the mPFC in a time-dependent manner. Similar injections using vehicle did not affect NO efflux. The facilitatory effect of cocaine on NO efflux was transient and reproducible. The signal was derived from neuronal sources of NO, because it was attenuated by systemic administration of the neuronal NO synthase inhibitor 7-nitroindazole. These studies support a role for prefrontal cortical NO signaling in cocaine-induced changes in neurotransmission in reward-related circuits involved in addiction.

Dopamine selectively potentiates hippocampal mossy fiber to CA3 synaptic transmission

Dopamine has been implicated in various brain functions and the pathology of neurological diseases. In the hippocampus, dopamine has been shown to induce acute depression of synaptic transmission in the CA1 region, but it remains largely unknown how it works in the CA3 region. We here report that dopamine induces acute synaptic potentiation at the synapse formed by mossy fibers (MFs) on mouse hippocampal CA3 pyramidal cells, but not at converging associational/commissural synapses. Dopamine potentiated both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) components of MF synaptic responses similarly in respect of the magnitude and time course. The dopamine-induced potentiation was intact in the presence of picrotoxin, required activation of D(1)-like receptors and was apparently occluded by an activator of adenylate cyclase. The potentiation was accompanied by a decrease in magnitude of synaptic facilitation, suggesting the presynaptic site for the expression of the potentiation. The present study is the first demonstration of acute potentiation of hippocampal excitatory synaptic transmission by dopamine, which is most probably mediated by presynaptic D(1)-like receptor-cAMP cascades.

Dopamine has bidirectional effects on synaptic responses to cortical inputs in layer II of the lateral entorhinal cortex

Dopaminergic modulation of neuronal function has been extensively studied in the prefrontal cortex, but much less is known about its effects on glutamate-mediated synaptic transmission in the entorhinal cortex. The mesocortical dopamine system innervates the superficial layers of the lateral entorhinal cortex and may therefore modulate sensory inputs to this area. In awake rats, systemic administration of the dopamine reuptake inhibitor GBR12909 (10 mg/kg, ip) enhanced extracellular dopamine levels in the entorhinal cortex and significantly facilitated field excitatory postsynaptic potentials (fEPSPs) in layer II evoked by piriform cortex stimulation. An analysis of the receptor subtypes involved in the facilitation of evoked fEPSPs was conducted using horizontal slices of lateral entorhinal cortex in vitro. The effects of 15-min bath application of dopamine on synaptic responses were bidirectional and concentration dependent. Synaptic responses were enhanced by 10 microM dopamine and suppressed by concentrations of 50 and 100 microM. The D(1)-receptor antagonist SCH23390 (50 microM) blocked the significant facilitation of synaptic responses induced by 10 microM dopamine and the D(2)-receptor antagonist sulpiride (50 microM) prevented the suppression of fEPSPs observed with higher concentrations of dopamine. We propose here that dopamine release in the lateral entorhinal cortex, acting through D(1) receptors, can lead to an enhancement of the salience of sensory representations carried to this region from adjacent sensory cortices.

Dual synaptic sites of D1-dopaminergic regulation of ethanol sensitivity of NMDA receptors in nucleus accumbens

Regulation of NMDAreceptor-mediated synaptic transmission onto accumbal medium spiny neurons (MSN) may constitute an important site in drug reward and reinforcement in mesolimbic structures. Previously, we reported that D(1)-like dopamine receptors activate a postsynaptic cAMP/PKA/DARPP-32 signaling cascade culminating in phosphorylation of SER897-NR1 subunits and a reduction in the sensitivity to ethanol of NMDA receptor-mediated synaptic transmission. Here, we use a detailed electrophysiological analysis of D(1)-like receptor regulation of the ethanol sensitivity of accumbal NMDA receptors (NMDARs) through recordings of quantal Sr(2+)-supported NMDA miniature synaptic currents (mEPSCs) in reduced Mg(2+) (0.6 mM) and report dual presynaptic and postsynaptic components of D(1)-like regulation of ethanol sensitivity of NMDARs. Ethanol inhibited NMDA mEPSC amplitude and frequency in a dose-dependent manner (25-75 mM), indicating inhibitory effects on presynaptic and postsynaptic components NMDA receptor-mediated synaptic transmission. The presynaptic inhibitory effect was corroborated by analysing the ratio of paired-pulse facilitation (PPF) of Ca(2+)-supported NMDA EPSCs. Activation of D(1) receptors with the agonist, SKF 38393 (25 microM), reversed ethanol suppression of NMDA mEPSC frequency and amplitude. Furthermore, the Mg(2+)-dependent decay off-rate of NMDA mEPSCs was substantially reduced by ethanol in a manner strongly reversed by the D(1) agonist. D(1) receptor-mediated attenuation of both the presynaptic and postsynaptic actions of ethanol was completely blocked by a D(1) selective antagonist (SCH 23390). These data suggest that D(1)-like receptors modulate both the presynaptic and postsynaptic effects of ethanol on NMDA receptor-mediated synaptic transmission in nucleus accumbens (NAc) and that these interactions may contribute to ethanol-induced neuroadaptation of the reward pathway.

8,9-dihydroxy-1,2,3,11b-tetrahydrochromeno[4,3,2,-de]isoquinoline (dinoxyline), a high affinity and potent agonist at all dopamine receptor isoforms

The synthesis and preliminary pharmacological evaluation of 8,9-dihydroxy-1,2,3,11b-tetrahydrochromeno[4,3,2,-de]isoquinoline (5, now named dinoxyline) is described. This molecule was designed as a potential bioisostere that would conserve the essential elements of our beta-phenyldopamine D(1) pharmacophore (i.e., position and orientation of the nitrogen, hydroxyls, and phenyl rings). Previously, we have rigidified these elements using alkyl bridges, as exemplified in the dopamine D(1) full agonist molecules dihydrexidine (1) and dinapsoline (2). This approach has been modified and we now show that it is possible to tether these elements using an ether linkage. Preliminary pharmacology has revealed that 5 is a potent full D(1) agonist (K(0.5) <10 nM; EC(50)=30 nM), but also has high affinity for brain D(2)-like and cloned D(2) and D(3) receptors. Interestingly, whereas 1 and 2 and their analogues have only moderate affinity for the human D(4) receptor, 5 also has high affinity for this isoform. Moreover, although N-alkylation of 1 and 2 increases D(2) affinity, the N-allyl (15) and N-n-propyl (17) derivatives of 5 had decreased D(2) affinity. Therefore, 5 may be engaging different amino acid residues than do 1 and 2 when they bind to the D(2) receptor. This is the first example of a ligand with high affinity at all dopamine receptors, yet with functional characteristics similar to dopamine. These rigid ligands also will be useful tools to determine specific residues of the receptor transmembrane domains that are critical for agonist ligand selectivity for the D(4) receptor.

Ryanodine receptors contribute to cGMP-induced late-phase LTP and CREB phosphorylation in the hippocampus

We previously found that the nitric oxide (NO)-cGMP-cGMP-dependent protein kinase (PKG) signaling pathway acts in parallel with the cAMP-cAMP-dependent protein kinase (PKA) pathway to produce protein and RNA synthesis-dependent late-phase long-term potentiation (L-LTP) and cAMP response element-binding protein (CREB) phosphorylation in the CA1 region of mouse hippocampus. We have now investigated the possible involvement of a downstream target of PKG, ryanodine receptors. L-LTP can be induced by either multiple-train tetanization, NO or 8-Br-cGMP paired with one-train tetanization, or the cAMP activator forskolin, and all three types of potentiation are accompanied by an increase in phospho-CREB immunofluorescence in the CA1 cell body area. Both the potentiation and the increase in phospho-CREB immunofluorescence induced by multiple-train tetanization or 8-Br-cGMP paired with one-train tetanization are reduced by prolonged perfusion with ryanodine, which blocks Ca(2+) release from ryanodine-sensitive Ca(2+) stores. By contrast, neither the potentiation nor the increase in immunofluorescence induced by forskolin are reduced by depletion of ryanodine and inositol-1,4,5-triphosphate (IP3)-sensitive Ca(2+) stores. These results suggest that NO, cGMP, and PKG cause release of Ca(2+) from ryanodine-sensitive stores, which in turn causes phosphorylation of CREB in parallel with PKA during the induction of L-LTP.

Nitric oxide as modulator of neuronal function

The gas NO is a messenger that modulates neuronal function. The use of NO donors and NO synthase inhibitors as pharmacological tools revealed that this free radical is probably implicated in the regulation of excitability and firing, in long-term potentiation and long-term depression, as well as in memory processes. Moreover, NO modulates neurotransmitter release. In vivo and in vitro studies have shown that, in all brain structures investigated, endogenous NO modulates the release of several neurotransmitters, such as acetylcholine, catecholamines, excitatory and inhibitory amino acids, serotonin, histamine, and adenosine. In most cases, enhanced NO level in the tissue increases the release of neurotransmitters, although decreasing effects have also been observed. Cyclic 3'-5' guanosine monophosphate and glutamate mediate the modulation of transmitter release by NO. Recent observations suggest that the release of some transmitters is dually influenced by NO. Thus, besides modulation by presynaptically located auto- and heteroreceptors, NO released from nitrergic neurons seems to play a universal role in modulating the release of transmitters in the brain.

Ceramide-induced sustained depression of synaptic currents mediated by ionotropic glutamate receptors in the hippocampus: an essential role of postsynaptic protein phosphatases

Ceramide, a sphingomyelin-derived second messenger, mediates cellular signals of cytokines such as tumor necrosis factor-alpha that are rapidly produced in the brain in response to vigorous neuronal activity and tissue injury. Using whole-cell patch-clamp recordings, the present study examined whether ceramide modulated excitatory postsynaptic currents mediated by ionotropic glutamate receptors in CA1 pyramidal neurons of rat hippocampal slices. Application of N-acetyl-D-sphingosine, a synthetic cell-permeable ceramide analog, promptly produced a slight increase of excitatory postsynaptic current amplitude lasting for about 3 min. However, this transient enhancement was followed by a profoundly delayed-onset, sustained depression of synaptic excitatory postsynaptic currents in a concentration-dependent fashion (1-30 microM). This ceramide-induced sustained depression was not associated with changes in paired-pulse facilitation, a phenomenon resulting from an alteration of presynaptic transmitter release. Dihydro-N-acetyl-D-erythro-sphingosine (10 microM), an inactive analog of N-acetyl-D-sphingosine, did not affect synaptic excitatory postsynaptic currents, indicating the specificity of N-acetyl-D-sphingosine's action. The induction of ceramide-induced sustained depression was primarily dependent on the activation of postsynaptic protein phosphatases, being considerably blocked by loading 30 nM okadaic acid (a potent inhibitor of protein phosphatases 1 and 2A) into neurons. In addition, following a stable establishment of ceramide-induced sustained depression, a protocol for inducing long-term depression caused no additional decreases in excitatory postsynaptic current amplitude, and vice versa. The study suggests that ceramide induces a long-term depressed modulation on synaptic transmission mediated by ionotropic glutamate receptors in the hippocampus, possibly through the activation of postsynaptic protein phosphatases 1 and 2A. In addition, ceramide-induced sustained depression seems to share some common mechanisms with long-term depression, such as the cascades of events resulting from the activation of protein phosphatases. Collectively, the long-term depressed modulation of ceramide on ionotropic glutamate receptor-mediated functions may be particularly important in various physiological and/or pathological conditions, in which the ceramide signaling pathway is activated in the mammalian brain.