Simulations to derive membrane resistivity in three phenotypes of guinea pig sympathetic postganglionic neuron

The Input-Output Relation of Primary Nociceptive Neurons is Determined by the Morphology of the Peripheral Nociceptive Terminals

The output from the peripheral terminals of primary nociceptive neurons, which detect and encode the information regarding noxious stimuli, is crucial in determining pain sensation. The nociceptive terminal endings are morphologically complex structures assembled from multiple branches of different geometry, which converge in a variety of forms to create the terminal tree. The output of a single terminal is defined by the properties of the transducer channels producing the generation potentials and voltage-gated channels, translating the generation potentials into action potential (AP) firing. However, in the majority of cases, noxious stimuli activate multiple terminals; thus, the output of the nociceptive neuron is defined by the integration and computation of the inputs of the individual terminals. Here, we used a computational model of nociceptive terminal tree to study how the architecture of the terminal tree affects the input-output relation of the primary nociceptive neurons. We show that the input-output properties of the nociceptive neurons depend on the length, the axial resistance (Ra), and location of individual terminals. Moreover, we show that activation of multiple terminals by a capsaicin-like current allows summation of the responses from individual terminals, thus leading to increased nociceptive output. Stimulation of the terminals in simulated models of inflammatory or neuropathic hyperexcitability led to a change in the temporal pattern of AP firing, emphasizing the role of temporal code in conveying key information about changes in nociceptive output in pathologic conditions, leading to pain hypersensitivity.SIGNIFICANCE STATEMENT Noxious stimuli are detected by terminal endings of primary nociceptive neurons, which are organized into morphologically complex terminal trees. The information from multiple terminals is integrated along the terminal tree, computing the neuronal output, which propagates toward the CNS, thus shaping the pain sensation. Here, we revealed that the structure of the nociceptive terminal tree determines the output of nociceptive neurons. We show that the integration of noxious information depends on the morphology of the terminal trees and how this integration and, consequently, the neuronal output change under pathologic conditions. Our findings help to predict how nociceptive neurons encode noxious stimuli and how this encoding changes in pathologic conditions, leading to pain.

Membrane properties and electrogenesis in the distal axons of small dorsal root ganglion neurons in vitro

Although it is generally thought that sensory transduction occurs at or close to peripheral nerve endings, with action potentials subsequently propagating along the axons of dorsal root ganglia (DRG) neurons toward the central nervous system, the small diameter of nociceptive axons and their endings have made it difficult to estimate their membrane properties and electrogenic characteristics. Even the resting potentials of nociceptive axons are unknown. In this study, we developed the capability to record directly with patch-clamp electrodes from the small-diameter distal axons of DRG neurons in vitro. We showed using current-clamp recordings that 1) these sensory axons have a resting potential of −60.2 ± 1 mV; 2) both tetrodotoxin (TTX)-sensitive (TTX-S) and TTX-resistant (TTX-R) Na+ channels are present and available for activation at resting potential, at densities that can support action potential electrogenesis in these axons; 3) TTX-sensitive channels contribute to the amplification of small depolarizations that are subthreshold with respect to the action potential in these axons; 4) TTX-R channels can support the production of action potentials in these axons; and 5) these TTX-R channels can produce repetitive firing, even at depolarized membrane potentials where TTX-S channels are inactivated. Finally, using voltage-clamp recordings with an action potential as the command, we confirmed the presence of both TTX-S and TTX-R channels, which are activated sequentially during action potential in these axons. These results provide direct evidence for the presence of TTX-S and TTX-R Na+ channels that are functionally available at resting potential and contribute to electrogenesis in small-diameter afferent axons.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.