Neuroscience. 2007 Jan 5;144(1):375-86. Epub 2006 Oct 19.

Sleep and GABA levels in the oral part of rat pontine reticular formation are decreased by local and systemic administration of morphine.

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Department of Anesthesiology, University of Michigan, 7433 Medical Sciences Building I, 1150 West Medical Center Drive, Ann Arbor, MI 48109-0615, USA.

Abstract

Morphine, a mu-opioid receptor agonist, is a commonly prescribed treatment for pain. Although highly efficacious, morphine has many unwanted side effects including disruption of sleep and obtundation of wakefulness. One mechanism by which morphine alters sleep and wakefulness may be by modulating GABAergic signaling in brain regions regulating arousal, including the pontine reticular nucleus, oral part (PnO). This study used in vivo microdialysis in unanesthetized Sprague-Dawley rat to test the hypothesis that mu-opioid receptors modulate PnO GABA levels. Validation of the high performance liquid chromatographic technique used to quantify GABA was obtained by dialyzing the PnO (n=4 rats) with the GABA reuptake inhibitor nipecotic acid (500 microM). Nipecotic acid caused a 185+/-20% increase in PnO GABA levels, confirming chromatographic detection of GABA and demonstrating the existence of functional GABA transporters in rat PnO. Morphine caused a concentration-dependent decrease in PnO GABA levels (n=25 rats). Coadministration of morphine (100 microM) with naloxone (1 microM), a mu-opioid receptor antagonist, blocked the morphine-induced decrease in PnO GABA levels (n=5 rats). These results show for the first time that mu-opioid receptors in rat PnO modulate GABA levels. A second group of rats (n=6) was used to test the hypothesis that systemically administered morphine also decreases PnO GABA levels. I.v. morphine caused a significant (P<0.05) decrease (19%) in PnO GABA levels relative to control i.v. infusions of saline. Finally, microinjections followed by 2 h recordings of electroencephalogram and electromyogram tested the hypothesis that PnO morphine administration disrupts sleep (n=8 rats). Morphine significantly (P<0.05) increased the percent of time spent in wakefulness (65%) and significantly (P<0.05) decreased the percent of rapid eye movement (REM) sleep (-53%) and non-REM sleep (-69%). The neurochemical and behavioral data suggest that morphine may disrupt sleep, at least in part, by decreasing GABAergic transmission in the PnO.

PMID:: 17055662; [PubMed - indexed for MEDLINE]
PMCID:: PMC2729685

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Figure 1

Dialysis administration of nipecotic acid to the pontine reticular nucleus, oral part (PnO) significantly increased GABA levels in the PnO. A. Representative chromatograms demonstrate excellent signal-to-noise ratios obtained during control dialysis (Ringer’s, gray line) and during dialysis administration of the GABA reuptake inhibitor nipecotic acid (NPA, black line). The increase in peak size during nipecotic acid administration confirms the presence of GABA. The inset shows a sagittal schematic modified from a rat brain atlas (Paxinos and Watson, 1998) and depicts placement of a microdialysis probe in the PnO. Arrows indicate dialysis administration of nipecotic acid via one port on the dialysis probe, and simultaneous collection of dialysis samples containing GABA via the second port on the dialysis probe. The unfilled area at the tip of the probe represents the 1 mm long dialysis membrane and is drawn to scale. B. Dialysis administration of nipecotic acid to the PnO of awake rat increased GABA levels (solid bar) to 185±20% of control (Pre-NPA Ringer’s). Control GABA levels (Pre-NPA Ringer’s) were normalized to 100% for each experiment. The number of samples analyzed for each dialysis condition was: pre-NPA Ringer’s=23, NPA=24, and post-NPA Ringer’s =22. C. A digitized image of one cresyl violet-stained coronal section illustrates a typical PnO microdialysis site located approximately 8.30 mm posterior to bregma. The arrow marks the most ventral part of the dialysis site. Note that this section was cut in a different plane from the atlas (Paxinos and Watson, 1998), and that the dorsal part of the section is more caudal than the ventral part. D. Coronal diagrams are modified from a rat brain atlas (Paxinos and Watson, 1998). Shaded cylinders represent the dialysis membranes, are drawn to scale, and show that the four dialysis sites used for the nipecotic acid study were localized to the PnO. Numbers at the top right of each brainstem schematic indicate stereotaxic coordinates in mm from bregma.

SLEEP AND GABA LEVELS IN THE ORAL PART OF RAT PONTINE RETICULAR FORMATION ARE DECREASED BY LOCAL AND SYSTEMIC ADMINISTRATION OF MORPHINE

Neuroscience. Neuroscience;144(1):375-386.