Effects of morphine and morphine withdrawal on adrenergic neurons of the rat rostral ventrolateral medulla

Brain Research
S C BarabanP G Guyenet

Abstract

In urethane anesthetized rats, iontophoretic application of morphine or alpha-methylnoradrenaline (alpha-MNE) inhibited (80-100%) the discharges of all putative adrenergic (C1) cells of the rostral ventrolateral medulla (RVLM). The effect of morphine was blocked selectively by naloxone while that of alpha-MNE was blocked selectively by the alpha 2-adrenergic antagonist idazoxan. Putative C1 cells were inhibited (75-100%) by low i.v. doses of clonidine (10-15 micrograms/kg). Most cells (7/10) were also inhibited by morphine i.v. (81% at 7 mg/kg). Two cells were slightly excited at doses below 2 mg/kg and inhibited at higher doses. Three cells were excited only. All effects of morphine i.v. were reversed by naloxone (1 mg/kg, i.v.). Intravenous administration of naloxone to morphine-dependent rats increased significantly the firing rate of all putative C1 adrenergic cells (from 5.8 +/- 0.9 spikes/s to 12.3 +/- 1.5 spikes/s; n = 8). During withdrawal these cells could still be inhibited (80-100%) by i.v. injection of clonidine (15 micrograms/kg). C-Fos expression induced by naltrexone-precipitated withdrawal was examined in the brainstem of freely moving morphine-dependent rats pretreated with clonidine or saline before injection ...Continue Reading

Citations

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