Comparative Analysis of Dorsal Root, Nodose and Sympathetic Ganglia for the Development of New Analgesics

Abstract

Interoceptive and exteroceptive signals, and the corresponding coordinated control of internal organs and sensory functions, including pain, are received and orchestrated by multiple neurons within the peripheral, central and autonomic nervous systems. A central aim of the present report is to obtain a molecularly informed basis for analgesic drug development aimed at peripheral rather than central targets. We compare three key peripheral ganglia: nodose, sympathetic (superior cervical), and dorsal root ganglia in the rat, and focus on their molecular composition using next-gen RNA-Seq, as well as their neuroanatomy using immunocytochemistry and in situ hybridization. We obtained quantitative and anatomical assessments of transmitters, receptors, enzymes and signaling pathways mediating ganglion-specific functions. Distinct ganglionic patterns of expression were observed spanning ion channels, neurotransmitters, neuropeptides, G-protein coupled receptors (GPCRs), transporters, and biosynthetic enzymes. The relationship between ganglionic transcript levels and the corresponding protein was examined using immunohistochemistry for select, highly expressed, ganglion-specific genes. Transcriptomic analyses of spinal dorsal horn and intermediolateral cell column (IML), which form the termination of primary afferent neurons and the origin of preganglionic innervation to the SCG, respectively, disclosed pre- and post-ganglionic molecular-level circuits. These multimodal investigations provide insight into autonomic regulation, nodose transcripts related to pain and satiety, and DRG-spinal cord and IML-SCG communication. Multiple neurobiological and pharmacological contexts can be addressed, such as discriminating drug targets and predicting potential side effects, in analgesic drug development efforts directed at the peripheral nervous system.

Anatomical Analysis of Transient Potential Vanilloid Receptor 1 (Trpv1+) and Mu-Opioid Receptor (Oprm1+) Co-expression in Rat Dorsal Root Ganglion Neurons

“…Note the cells strongly expressing Trpv1 (arrowheads) or Trpa1 (arrows), as well as cells co-expressing both markers (asterisks). Cell counts by marker positivity were performed to evaluate the findings of our peripheral ganglionic comparison study between the nodose, DRG and superior cervical ganglia (Sapio et al, 2020b ) ( Figure 5C ). We found that the most prominent populations of cells were Trpv1 +/ Trpa1- ( Trpa1 -; 233 cells, 53.8%) followed closely by the faintly labeled Trpv1 +/ Trpa1 + (190 cells, 43.9%) population.…”

“…The largely differential expression is also consistent with previous studies showing partial depletion of Trpa1 with Trpv1 agonists (Pecze et al, 2009 ) or coexpression of these receptors. However, TRPA1 is apparently expressed in low expressing TRPV1 population, which could explain why these neurons are resistant to TRPV1 agonist actions and could also explain the failure to deplete TRPA1 -expressing neurons with TRPV1 agonists in some experiments (Isensee et al, 2014 ; Sapio et al, 2018 , 2020b ). The functional implication of co-expression of these receptors is being investigated.…”

“…Multiple windows on the sections of DRG were selected to capture representative fields for quantification. Sample sizes were estimated from previous experiments (Sapio et al, 2020b ).…”

Anatomical differences in nociceptor neurons sensitivity

“…They detect a broad range of thermal, mechanical, and chemical threats and respond by means of protective reflexes and by modulating the cells of the microenvironment. Nociceptor neurons can be differentiated based on their expression profiles (Prescott et al, 2020 ; Usoskin et al, 2015 ; Kupari et al, 2019 ; Kaelberer et al, 2020 ; Sapio et al, 2020 ), degree of myelination, type of cues to which they are sensitive (Crosson et al, 2019 ), the reflexes they initiate, the anatomical location of their soma, or the anatomical site they innervate (Mazzone & Undem, 2016 ). For instance, dorsal root ganglia (DRG) neurons are derived from the neural crest and innervate the skin, muscles, and joints, while the Jugular Nodose Complex (JNC) neurons originate from the neural crest (Jugular) and placode (Nodose) and innervate internal organs (Chavan et al, 2017 ; Baker & Schlosser, 2005 ).…”

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

“…Because the persistence of aberrant and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain (2,(6)(7)(8)(9)(10)(11)(12)(13)(14), the peripheral nervous system offers a range of actual and potential drug targets. It has been argued that targeting the peripheral nervous system with substances that do not readily cross the blood-brain barrier, may circumvent the doselimiting side effects seen with centrally acting agents (15). For example, adverse centrally-mediated effects of gabapentin include dizziness, somnolence, fatigue, ataxia, and nystagmus (16).…”