As the major excitatory neurotransmitter in the mammalian central nervous system, glutamate plays a key role in many central pathologies, including gliomas, psychiatric, neurodevelopmental, and neurodegenerative disorders. Post-mortem and serological studies have implicated glutamatergic dysregulation in these pathologies, and pharmacological modulation of glutamate receptors and transporters has provided further validation for the involvement of glutamate. Furthermore, efforts from genetic, in vitro, and animal studies are actively elucidating the specific glutamatergic mechanisms that contribute to the aetiology of central pathologies. However, details regarding specific mechanisms remain sparse and progress in effectively modulating glutamate to alleviate symptoms or inhibit disease states has been relatively slow. In this report, we review what is currently known about glutamate signalling in central pathologies. We also discuss glutamate’s mediating role in comorbidities, specifically cancer-induced bone pain and depression.

Breast cancer cells release the signalling molecule glutamate via the system xC− antiporter, which is upregulated to exchange extracellular cystine for intracellular glutamate to protect against oxidative stress. Here, we demonstrate that this antiporter is functionally influenced by the actions of the neurotrophin nerve growth factor on its cognate receptor tyrosine kinase, TrkA, and that inhibiting this complex may reduce cancer-induced bone pain via its downstream actions on xCT, the functional subunit of system xC−. We have characterized the effects of the selective TrkA inhibitor AG879 on system xC− activity in murine 4T1 and human MDA-MB-231 mammary carcinoma cells, as well as its effects on nociception in our validated immunocompetent mouse model of cancer-induced bone pain, in which BALB/c mice are intrafemorally inoculated with 4T1 murine carcinoma cells. AG879 decreased functional system xC− activity, as measured by cystine uptake and glutamate release, and inhibited nociceptive and physiologically relevant responses in tumour-bearing animals. Cumulatively, these data suggest that the activation of TrkA by nerve growth factor may have functional implications on system xC−-mediated cancer pain. System xC−-mediated TrkA activation therefore presents a promising target for therapeutic intervention in cancer pain treatment.

We previously identified that several cancer cell lines known to induce nociception in mouse models release glutamate in vitro. Although the mechanisms of glutamatergic signalling have been characterized primarily in the central nervous system, its importance in the peripheral nervous system has been recognized in various pathologies, including cancer pain. We therefore investigated the effect of glutamate on intracellular electrophysiological characteristics of peripheral sensory neurons in an immunocompetent rat model of cancer-induced pain based on surgical implantation of mammary rat metastasis tumour-1 cells into the distal epiphysis of the right femur. Behavioural evidence of nociception was detected using von Frey tactile assessment. Activity of sensory neurons was measured by intracellular electrophysiological recordings in vivo. Glutamate receptor expression at the mRNA level in relevant dorsal root ganglia was determined by reverse transcription polymerase chain reaction using rat-specific primers. Nociceptive and non-nociceptive mechanoreceptor neurons exhibiting changes in neural firing patterns associated with increased nociception due to the presence of a bone tumour rapidly responded to sulphasalazine injection, an agent that pharmacologically blocks non-vesicular glutamate release by inhibiting the activity of the system x C À antiporter. In addition, both types of mechanoreceptor neurons demonstrated excitation in response to intramuscular glutamate injection near the femoral head, which corresponds to the location of cancer cell injection to induce the bone cancer-induced pain model. Therefore, glutamatergic signalling contributes to cancer pain and may be a factor in peripheral sensitization and induced tactile hypersensitivity associated with bone cancer-induced pain.

The aim of this work was to synthesize and evaluate [2 + 1] Tc(i) polypyridine complexes containing tetrazines, which along with the corresponding Re(i) complexes, represent a new class of isostructural nuclear and turn-on luminescent probes that can be derivatized and targeted using bioorthogonal chemistry. To this end, [2 + 1] complexes ofTc(i) of the type [Tc(CO)(N^N)(L)] (N^N = bathophenanthroline disulfonate (BPS) or 2,2'-bipyridine (bipy)), where the monodentate ligand (L) was a tetrazine linked to the metal through an imidazole derivative, were prepared. The desired products were obtained in nearly quantitative radiochemical yield by adding [Tc(CO)(N^N)(OH)] to the imidazole-tetrazine ligand and heating at 60 °C for 30 min. Measurement of the reaction kinetics between the tetrazine and (E)-cyclooct-4-enol revealed a second-order rate constant of 8.6 × 10 M s at 37 °C, which is suitable for in vivo applications that require rapid coupling. Stability studies showed that the metal complexes were resistant to ligand challenge and exhibited reasonable protein binding in vitro. Biodistribution studies of the more water-soluble BPS derivative in normal mice, one hour after administration of a bisphosphonate derivative of trans-cyclooctene (TCO-BP), revealed high activity concentrations in the knee (9.3 ± 0.3 %ID g) and shoulder (5.3 ± 0.7 %ID g). Using the same pretargeting approach, SPECT/CT imaging showed that the [2 + 1] tetrazine complex localized to implanted skeletal tumors. This is the first report of the preparation of Tc complexes of BPS and demonstration that their tetrazine derivatives can be used to prepare targeted imaging probes by employing bioorthogonal chemistry.

IntroductionPain is a common and debilitating comorbidity of metastatic breast cancer. The hippocampus has been implicated in nociceptive processing, particularly relating to the subjective aspect of pain. Here, a syngeneic mouse model was used to characterize the effects of peripheral tumors on hippocampal microglial activation in relation to cancer-induced pain (CIP).Materials and methodsMice were systemically treated with the colony-stimulating factor 1 receptor inhibitor Pexidartinib prior to intrafemoral (IF) or subcutaneous 4T1 carcinoma cell inoculation. Spontaneous and evoked nociceptive responses were quantitated throughout tumor development, and contralateral hippocampi were collected via endpoint microdissection for RNA analysis. Additionally, IF tumor-bearing animals were sacrificed on days 5, 10, 15, and 20 post 4T1 cell inoculation, and brain sections were immunofluorescently stained for Iba1, a marker of activated microglia.ResultsAblation of these neuroimmune cells with the CSF1R inhibitor Pexidartinib delayed the onset and severity of cancer-induced nociceptive behaviors in IF tumor-bearing animals, adding to the body of literature that demonstrates microglial contribution to the development and maintenance of CIP. Furthermore, in untreated IF tumor-bearing mice, nociceptive behaviors appeared to progress in parallel with microglial activation in hippocampal regions. Immunofluorescent Iba1+ microglia increased in the dentate gyrus and cornu ammonis 1 hippocampal regions in IF tumor-bearing animals over time, which was confirmed at the mRNA level using relevant microglial markers.ConclusionThis is the first experimental evidence to demonstrate the effects of peripheral tumor-induced nociception on hippocampal microglial activation. The increase in hippocampal microglia observed in the present study may reflect the emotional and cognitive deficits reported by patients with CIP.