Oxytocin prevents neuronal network pain-related changes on spinal cord dorsal horn in vitro

Highlights

• Calcium spinal cord cells activities were scarce.

• Pre-treatment with NMDA enhances the spinal cord cells activities.

• Oxytocin did not interfere with the NMDA increased activity.

• Oxytocin blocks the NMDA produces coactivity.

Abstract

Recently, oxytocin (OT) has been studied as a potential modulator of endogenous analgesia by acting upon pain circuits at the spinal cord and supraspinal levels. Yet the detailed action mechanisms of OT are still undetermined. The present study aimed to evaluate the action of OT in the spinal cord dorsal horn network under nociceptive-like conditions induced by the activation of the N-methyl-d-aspartate (NMDA) receptor and formalin injection, using calcium imaging techniques. Results demonstrate that the spontaneous Ca²⁺-dependent activity of the dorsal horn cells was scarce, and the coactivity of cells was mainly absent. When NMDA was applied, high rates of activity and coactivity occurred in the dorsal horn cells; these rates of high activity mimicked the activity dynamics evoked by a neuropathic pain condition. In addition, although OT treatment increased activity rates, it was also capable of disrupting the conformation of coordinated activity previously consolidated by NMDA treatment, without showing any effect by itself. Altogether, our results suggest that OT globally prevents the formation of coordinated patterns previously generated by nociceptive-like conditions on dorsal horn cells by NMDA application, which supports previous evidence showing that OT represents a potential therapeutic alternative for the treatment of chronic neuropathic pain.

Introduction

Recent evidence has described oxytocin (OT) as an inhibitor of nociceptive activity in either the central nervous system [[1], [2], [3], [4], [5]] or peripheral nerve terminals [6]. OT has also been reported as a successful alternative for pain management in humans [7,8]. OT administration demonstrated a striking antinociceptive effect by reducing the pain-related signals and perception. Particularly, evidence shows that OT is capable of diminishing the pain-related electrical activity of spinal cord dorsal horn neurons and therefore inhibiting the transmission of antinociceptive information [1]. However, the intrinsic participation of OT and its effect on inhibiting the transmission of nociceptive information in the functional networks of sensory processing of the dorsal horn circuitry has remained elusive.

On the other hand, most works done on the spinal cord have pursued a better and more profound understanding of the histological aspects of this region. For several years’ electrophysiology has been the standard technique used to study the physiology of the neuronal components of the dorsal horn because of its excellent temporal resolution and sensitivity. Nevertheless, the functional networks of sensory processing were not completely accessible with the main techniques used in the last century. Calcium imaging techniques with epifluorescent or two-photon microscopy allow the observation and analysis of large areas with several neurons. Some studies done in intact [[9], [10], [11], [12], [13], [14], [15], [16]] or in vitro spinal cord tissue have been beneficial in understanding how cells function and respond to different stimuli [17,18].

To address the changes produced by OT in the neuronal network circuits of the dorsal horn, we used calcium imaging analysis on spinal cord slices to describe overall neuronal activity in spinal cord dorsal horn slices in vitro on a circuit that had not been altered by any stimulus. Afterward, we challenged the system with the application of N-methyl-d-aspartate (NMDA) to simulate the nociceptive-related activation of dorsal horn neurons [17,19]. Finally, we show that OT application inhibits the pain-related changes in neuronal circuit dynamics produced by NMDA.