Gi protein functions in thalamic neurons to decrease orofacial nociceptive response

VGLUT2 Is a Determinant of Dopamine Neuron Resilience in a Rotenone Model of Dopamine Neurodegeneration

Parkinson's disease (PD) is characterized by progressive dopamine (DA) neuron loss in the SNc. In contrast, DA neurons in the VTA are relatively protected from neurodegeneration, but the underlying mechanisms for this resilience remain poorly understood. Recent work suggests that expression of the vesicular glutamate transporter 2 (VGLUT2) selectively impacts midbrain DA neuron vulnerability. We investigated whether altered DA neuron VGLUT2 expression determines neuronal resilience in rats exposed to rotenone, a mitochondrial complex I inhibitor and toxicant model of PD. We discovered that VTA/SNc DA neurons that expressed VGLUT2 are more resilient to rotenone-induced DA neurodegeneration. Surprisingly, the density of neurons with detectable VGLUT2 expression in the VTA and SNc increases in response to rotenone. Furthermore, dopaminergic terminals within the NAc, where the majority of VGLUT2-expressing DA neurons project, exhibit greater resilience compared with DA terminals in the caudate/putamen. More broadly, VGLUT2-expressing terminals are protected throughout the striatum from rotenone-induced degeneration. Together, our data demonstrate that a distinct subpopulation of VGLUT2-expressing DA neurons are relatively protected from rotenone neurotoxicity. Rotenone-induced upregulation of the glutamatergic machinery in VTA and SNc neurons and their projections may be part of a broader neuroprotective mechanism. These findings offer a putative new target for neuronal resilience that can be manipulated to prevent toxicant-induced DA neurodegeneration in PD.SIGNIFICANCE STATEMENT Environmental exposures to pesticides contribute significantly to pathologic processes that culminate in Parkinson's disease (PD). The pesticide rotenone has been used to generate a PD model that replicates key features of the illness, including dopamine neurodegeneration. To date, longstanding questions remain: are there dopamine neuron subpopulations resilient to rotenone; and if so, what are the molecular determinants of this resilience? Here we show that the subpopulation of midbrain dopaminergic neurons that express the vesicular glutamate transporter 2 (VGLUT2) are more resilient to rotenone-induced neurodegeneration. Rotenone also upregulates VGLUT2 more broadly in the midbrain, suggesting that VGLUT2 expression generally confers increased resilience to rotenone. VGLUT2 may therefore be a new target for boosting neuronal resilience to prevent toxicant-induced DA neurodegeneration in PD.

Neural Pathways of Craniofacial Muscle Pain: Implications for Novel Treatments

Craniofacial muscle pain is highly prevalent in temporomandibular disorders but is difficult to treat. Enhanced understanding of neurobiology unique to craniofacial muscle pain should lead to the development of novel mechanism-based treatments. Herein, we review recent studies to summarize neural pathways of craniofacial muscle pain. Nociceptive afferents in craniofacial muscles are predominantly peptidergic afferents enriched with TRPV1. Signals from peripheral glutamate receptors converge onto TRPV1, leading to mechanical hyperalgesia. Further studies are needed to clarify whether hyperalgesic priming in nonpeptidergic afferents or repeated acid injections also affect craniofacial muscle pain. Within trigeminal ganglia, afferents innervating craniofacial muscles interact with surrounding satellite glia, which enhances the sensitivity of the inflamed neurons as well as nearby uninjured afferents, resulting in hyperalgesia and ectopic pain originating from adjacent orofacial tissues. Craniofacial muscle afferents project to a wide area within the trigeminal nucleus complex, and central sensitization of medullary dorsal horn neurons is a critical factor in muscle hyperalgesia related to ectopic pain and emotional stress. Second-order neurons project rostrally to pathways associated with affective pain, such as parabrachial nucleus and medial thalamic nucleus, as well as sensory-discriminative pain, such as ventral posteromedial thalamic nuclei. Abnormal endogenous pain modulation can also contribute to chronic muscle pain. Descending serotonergic circuits from the rostral ventromedial medulla facilitate activation of second-order neurons in the trigeminal nucleus complex, which leads to the maintenance of mechanical hyperalgesia of inflamed masseter muscle. Patients with temporomandibular disorders exhibit altered brain networks in widespread cortical and subcortical regions. Recent development of methods for neural circuit manipulation allows silencing of specific hyperactive neural circuits. Chemogenetic silencing of TRPV1-expressing afferents or rostral ventromedial medulla neurons attenuates hyperalgesia during masseter inflammation. It is likely, therefore, that further delineation of neural circuits mediating craniofacial muscle hyperalgesia potentially enhances treatment of chronic muscle pain conditions.