Metabotropic Glutamate Receptors 7 within the Nucleus Accumbens are Involved in Relief Learning in Rats

Chronic unilateral inhibition of GABA synthesis in the amygdala increases specificity of conditioned fear in a discriminative fear conditioning paradigm in rats

Neural activity in the amygdala is critical for fear learning. In anxiety disorder patients, bilateral hyperactivity of the amygdala can be observed. This hyperactivation is often associated with the facilitation of fear learning and/or over-generalization of conditioned fear. In contrast, hypoactivity of the amygdala, e.g. by pharmacological interventions, attenuates or blocks fear learning. To date, little is known about how neural excitability of the amygdala affects specificity or generalization of fear. Therefore, the present study utilized chronic inhibition of GABA synthesis in the amygdala to increase excitability and investigated the effect on the specificity of fear learning. In rats, unilateral cannulas aiming at the amygdala were implanted. The cannulas were connected to subcutaneously implanted osmotic mini pumps that delivered either the GABA synthesis inhibitor L-allylglycine or its inactive enantiomer D-allylglycine. Following one week of chronic GABA synthesis manipulation, the rats were submitted to a discriminative fear conditioning protocol. In addition, anxiety-like behavior in the light-dark box was measured. Our data show that chronic unilateral L-AG infusions into the amygdala improve the specificity of learned fear, support safety learning, and reduce fear generalization and anxiety. This data demonstrates that moderately increased amygdala excitability can be beneficial for the specificity of fear learning and highlights the potential application for therapeutic interventions.

Recent Advances in the Modulation of Pain by the Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluR or mGlu) are G-protein coupled receptors activated by the binding of glutamate, the main classical neurotransmitter of the nervous system. Eight different mGluR subtypes (mGluR1-8) have been cloned and are classified in three groups based on their molecular, pharmacological and signaling properties. mGluRs mediate several physiological functions such as neuronal excitability and synaptic plasticity, but they have also been implicated in numerous pathological conditions including pain. The availability of new and more selective allosteric modulators together with the canonical orthosteric ligands and transgenic technologies has led to significant advances in our knowledge about the role of the specific mGluR subtypes in the pathophysiological mechanisms of various diseases. Although development of successful compounds acting on mGluRs for clinical use has been scarce, the subtype-specific-pharmacological manipulation might be a compelling approach for the treatment of several disorders in humans, including pain; this review aims to summarize and update on preclinical evidence for the roles of different mGluRs in the pain system and discusses knowledge gaps regarding mGluR-related sex differences and neuroimmune signaling in pain.

International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

The Modulation of Pain by Metabotropic Glutamate Receptors 7 and 8 in the Dorsal Striatum

The dorsal striatum, apart from controlling voluntary movement, displays a recently demonstrated pain inhibition. It is connected to the descending pain modulatory system and in particular to the rostral ventromedial medulla through the medullary dorsal reticular nucleus. Diseases of the basal ganglia, such as Parkinson's disease, in addition to being characterized by motor disorders, are associated with pain and hyperactivation of the excitatory transmission. A way to counteract glutamatergic hyperactivation is through the activation of group III metabotropic glutamate receptors (mGluRs), which are located on presynaptic terminals inhibiting neurotransmitter release. So far the mGluRs of group III have been the least investigated, owing to a lack of selective tools. More recently, selective ligands for each mGluR of group III, in particular positive and negative allosteric modulators, have been developed and the role of each subtype is starting to emerge. The neuroprotective potential of group III mGluRs in pathological conditions, such as those characterized by elevate glutamate, has been recently shown. In the dorsal striatum, mGluR7 and mGluR8 are located at glutamatergic corticostriatal terminals and their stimulation inhibits pain in pathological conditions such as neuropathic pain. The two receptors in the dorsal striatum have instead a different role in pain control in normal conditions. This review will discuss recent results focusing on the contribution of mGluR7 and mGluR8 in the dorsal striatal control of pain. The role of mGluR4, whose antiparkinsonian activity is widely reported, will also be addressed.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Role of the mesolimbic dopamine system in relief learning

The relief from an aversive event is rewarding. Since organisms are able to learn which environmental cues can cease an aversive event, relief learning helps to better cope with future aversive events. Literature data suggest that relief learning is affected in various psychopathological conditions, such as anxiety disorders. Here, we investigated the role of the mesolimbic dopamine system in relief learning. Using a relief learning procedure in Sprague Dawley rats, we applied a combination of behavioral experiments with anatomical tracing, c-Fos immunohistochemistry, and local chemogenetic and pharmacological interventions to broadly characterize the role of the mesolimbic dopamine system. The present study shows that a specific part of the mesolimbic dopamine system, the projection from the posterior medial ventral tegmental area (pmVTA) to the nucleus accumbens shell (AcbSh), is activated by aversive electric stimuli. 6-OHDA lesions of the pmVTA blocked relief learning but fear learning and safety learning were not affected. Chemogenetic silencing of the pmVTA-AcbSh projection using the DREADD approach, as well as intra-AcbSh injections of the dopamine D2/3 receptor antagonist raclopride inhibited relief learning. Taken together, the present data demonstrate that the dopaminergic pmVTA-AcbSh projection is critical for relief learning but not for similar learning phenomena. This novel finding may have clinical implications since the processing of signals predicting relief and safety is often impaired in patients suffering from anxiety disorders. Furthermore, it may help to better understand psychological conditions like non-suicidal self-injury, which are associated with pain offset relief.

Metabotropic glutamate receptor subtype 7 in the dorsal striatum oppositely modulates pain in sham and neuropathic rats

The study investigated the role of the metabotropic glutamate receptor subtype 7 (mGluR7) in pain signalling in the dorsal striatum of sham and neuropathic rats. Supraspinal circuitries involved in the dorsal striatum control of pain were also explored. In the sham rats, microinjection of N,N'-bis(diphenylmethyl)-1,2-ethanediamine (AMN082), a selective mGluR7 positive allosteric modulator, into the dorsal striatum, facilitated pain, increased the activity of the ON cells and inhibited the activity of the OFF cells in the rostral ventromedial medulla, and decreased glutamate levels in the dorsal striatum. Conversely, AMN082 inhibited pain and the activity of the ON cells while increased the activity of the OFF cells in rats with spared nerve injury (SNI) of the sciatic nerve. AMN082 also decreased glutamate levels in the dorsal striatum of SNI rats. The effect of AMN082 on mechanical allodynia and glutamate release was blocked by 6-(2,4-dimethylphenyl)-2-ethyl-6,7-dihydro-4(5H)-benzoxazolone (ADX71743), a selective mGluR7 negative allosteric modulator. Moreover, in the sham rats, AMN082 increased the activity of total nociceptive convergent neurons in the dorsal reticular nucleus while in the SNI rats, such activity was decreased. The administration of lidocaine into the subthalamic nucleus abolished the effect of AMN082 on the total nociceptive convergent neurons in the sham rats but not in the SNI rats. Thus, the dual effect of mGluR7 in facilitating or inhibiting pain responses may be due to the recruitment of different pathways of the basal ganglia, the indirect or direct pathway, in physiological or pathological conditions, respectively.

The influence of AMN082, metabotropic glutamate receptor 7 (mGlu7) allosteric agonist on the acute and chronic antinociceptive effects of morphine in the tail-immersion test in mice: Comparison with mGlu5 and mGlu2/3 ligands

Preclinical data indicated that the metabotropic glutamate receptors 5 (mGlu5) and glutamate receptors 2/3 (mGlu2/3) are involved in modulating morphine antinociception. However, little is known about the role of metabotropic glutamate receptors 7 (mGlu7) in this phenomenon. We compared the effects of AMN082 (0.1, 1 or 5mg/kg, ip), a selective mGlu7 allosteric agonist, LY354740 (0.1, 1 or 5mg/kg, ip), an mGlu2/3 agonist and MTEP (0.1, 1 or 5mg/kg, ip), a selective mGlu5 antagonist, on the acute antinociceptive effect of morphine (5mg/kg, sc) and also on the development and expression of tolerance to morphine analgesia in the tail-immersion test in mice. To determine the role of mGlu7 in morphine tolerance, and the association of the mGlu7 effect with the N-methyl-d-aspartate (NMDA) receptors regulation, we used MMPIP (10mg/kg, ip), a selective mGlu7 antagonist and MK-801, a NMDA antagonist. Herein, the acute administration of AMN082, MTEP or LY354740 alone failed to evoked antinociception, and did not affect morphine (5mg/kg, sc) antinociception. However, these ligands inhibited the development of morphine tolerance, and we indicated that MMPIP reversed the inhibitory effect of AMN082. When given together, the non-effective doses of AMN082 and MK-801 did not alter the tolerance to morphine. Thus, mGlu7, similarly to mGlu2/3 and mGlu5, are involved in the development of tolerance to the antinociceptive effects of morphine, but not in the acute morphine antinociception. Furthermore, while mGlu7 are engaged in the development of morphine tolerance, no interaction exists between mGlu7 and NMDA receptors in this phenomenon.

Requirement of NF-kappa B Activation in Different Mice Brain Areas during Long-Term Memory Consolidation in Two Contextual One-Trial Tasks with Opposing Valences

NF-kappa B is a transcription factor whose activation has been shown to be necessary for long-term memory consolidation in several species. NF-kappa B is activated and translocates to the nucleus of cells in a specific temporal window during consolidation. Our work focuses on a one trial learning tasks associated to the inhibitory avoidance (IA) setting. Mice were trained either receiving or not a footshock when entering a dark compartment (aversive vs. appetitive learning). Regardless of training condition (appetitive or aversive), latencies to step-through during testing were significantly different to those measured during training. Additionally, these testing latencies were also different from those of a control group that only received a shock unrelated to context. Moreover, nuclear NF-kappa B DNA-binding activity was augmented in the aversive and the appetitive tasks when compared with control and naïve animals. NF-kappa B inhibition by Sulfasalazine injected either in the Hippocampus, Amygdala or Nucleus accumbens immediately after training was able to impair retention in both training versions. Our results suggest that NF-kappa B is a critical molecular step, in different brain areas on memory consolidation. This was the case for both the IA task and also the modified version of the same task where the footshock was omitted during training. This work aims to further investigate how appetitive and aversive memories are consolidated.

Relief learning requires a coincident activation of dopamine D1 and NMDA receptors within the nucleus accumbens

Relief learning is the association of a stimulus with the offset of an aversive event. Later, the now conditioned relief stimulus induces appetitive-like behavioral changes. We previously demonstrated that the NMDA receptors within the nucleus accumbens (NAC) are involved in relief learning. The NAC is also important for reward learning and it has been shown that reward learning is mediated by an interaction of accumbal dopamine and NMDA glutamate receptors. Since conditioned relief has reward-like properties, we hypothesized that (a) acquisition of relief learning requires the activation of dopamine D1 receptors in the NAC, and (b) if D1 receptors are involved in this process as expected, a concurrent dopamine D1 and NMDA receptor activation may mediate this learning. The present study tested these hypotheses. Therefore, rats received intra-NAC injections of the dopamine D1 receptor antagonist SCH23390 and the NMDA antagonist AP5, either separately or together, at different time points of a relief conditioning procedure. First, we showed that SCH23390 dose-dependently blocked acquisition and the expression of conditioned relief. Next, we demonstrated that co-injections of SCH23390 and AP5 into the NAC, at doses that were ineffective when applied separately, blocked acquisition but not consolidation or expression of relief learning. Notably, neither of the injections affected the locomotor response of the animals to the aversive stimuli suggesting that their perception is not changed. This data indicates that a co-activation of dopamine D1 and NMDA receptors in the NAC is required for acquisition of relief learning.