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Tolerance to repeated microinjection of morphine into the periaqueductal gray is associated with changes in the behavior of off- and on-cells in the rostral ventromedial medulla of rats.

Abstract：

:Although the administration of opioids is the most effective treatment for pain, their efficacy is limited by the development of tolerance. The midbrain periaqueductal gray matter (PAG) participates in opioid analgesia and tolerance. Microinjection of morphine into PAG produces antinociception, probably through neurons in the rostral ventromedial medulla (RVM), namely through the activation of off-cells, which inhibit nociception, and the inhibition of on-cells, which facilitate nociception. After its repeated microinjection into the PAG morphine loses effectiveness. The present study sought to determine whether tolerance to PAG morphine administration is associated with changes in the behavior of RVM neurons. Morphine (0.5 microg/0.4 microl) or saline (0.4 microl) was microinjected into the ventrolateral PAG twice daily. Initially morphine caused a latency increase in the hot plate test (antinociception) but this effect disappeared by day 3 (tolerance). On day 4, each rat was anesthetized with halothane and recordings were made from off- and on-cells in the RVM, i.e. from neurons that decrease or increase their firing, respectively, just before a heat-elicited tail flick. In contrast to saline-pretreated rats, PAG microinjection of morphine in tolerant animals did not change the baseline activity of off- or on-cells, did not prevent the off-cell pause or the on-cell activation upon tail heating, and did not lengthen the tail flick latency. However, microinjection of kainic acid into the PAG (1) caused off-cells to become continuously active and on-cells to become silent, and (2) prevented the tail flick, i.e. exactly what morphine did before tolerance developed. These results demonstrate a correspondence between neuronal and behavioral measures of tolerance to PAG opioid administration, and suggest that tolerance is mediated by a change in opioid-sensitive neurons within the PAG.

journal_name

Pain

journal_title

Pain

authors

Tortorici V,Morgan MM,Vanegas H

doi

10.1016/s0304-3959(00)00367-5

keywords:

subject

Has Abstract

pub_date

2001-01-01 00:00:00

pages

237-44

issue

2-3

eissn

0304-3959

issn

1872-6623

pii

S0304395900003675

journal_volume

pub_type

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Tolerance to repeated microinjection of morphine into the periaqueductal gray is associated with changes in the behavior of off- and on-cells in the rostral ventromedial medulla of rats.