Elsevier

Brain Research

Volume 366, Issues 1–2, 26 February 1986, Pages 203-210
Brain Research

Putative pain modulating neurons in the rostral ventral medulla: Reflex-related activity predicts effects of morphine

https://doi.org/10.1016/0006-8993(86)91296-5Get rights and content

Abstract

Three physiologically-defined classes of neurons are found in the rostral ventromedial medulla (RVM), a region which contributes to the antinociceptive action of opiates. The off-cell exhibits an abrupt pause just prior to the occurrence of the tail flick reflex (TF). The on-cell shows a burst of activity beginning just prior to the occurrence of the TF. Neutral cell firing does not change in relation to the TF. Systematic administration of morphine has been shown to produce a consistent increase in the activity of off-cells. In the present studies, the effects of systemically-administered morphine on spontaneous and TF-related activity of on-cells and neutral cells were examined in lightly-anesthetized rats. Measures of spontaneous activity were obtained before and after morphine (1.25–2.5 mg/kg, i.v.). On cell exhibited an irregular cyclic rate of spontaneous discharge similar to that previously reported for off-cells. In contrast, neutral cells had a nearly constant level of spontaneous activity. After administration of morphine, spontaneous activity ceased for 8 of 8 on-cells, and heat-related activity was eliminated. Administration of naloxone resulted in a return of the periodic firing pattern and the burst associated with the TF. Seven of 8 neutral cells showed no change in firing rate and one showed a decreased rate after morphine administration. These results show that the effect of systemic opiates on an RVM neuron can be predicted based on whether a cell increases or decreases its firing just prior to the occurrence of a nocifensive reflex. They add to the existing body of evidence implicating both on-and off-cells in modulation of nociceptive transmission and nocifensive reflexes at spinal levels and indicate that these neurons contribute to opiate-induced analgesia.

References (33)

Cited by (145)

  • 5.15 - The Brainstem and Nociceptive Modulation

    2020, The Senses: A Comprehensive Reference: Volume 1-7, Second Edition
  • Activity correlations between on-like and off-like cells of the rostral ventromedial medulla and simultaneously recorded wide-dynamic-range neurons of the spinal dorsal horn in rats

    2016, Brain Research
    Citation Excerpt :

    Fig. 5B shows the decrease in spike activity of the same neuron pair in 15s-bin histograms during 80 min after morphine microinjection into RVM. This decrease in off-like cell activity induced by morphine during deep barbiturate anesthesia is in contrast with the activation of off-cells when μ-opioid agonists are administered systemically or locally into the RVM to lightly anesthetized rats (Barbaro et al., 1986; Fields et al., 1983b; Heinricher et al., 1994). In spite of such unusual behavior, in the absence of morphine the neurons herein labelled as off-like cells showed a stable behavior and were reliably inhibited by mechanical stimulation of their receptive field (Fig. 6A,B).

View all citing articles on Scopus
View full text