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Simultaneous changes in hypothalamic catecholamine levels and plasma corticosterone concentration in the rat after
M V Milanés1, M M Puig, M L Vargas
1Department of Physiology and Pharmacology, University School of Medicine, Muricia, Spain.
Abstract:
The effects of morphine on plasma corticosterone and hypothalamic noradrenaline (NA) and dopamine (DA) content were studied in naive and in morphine-tolerant rats. Acutely administered morphine (30 mg/kg i.p.) significantly increased the plasma levels of corticosterone and significantly reduced the hypothalamic NA and DA content. In chronically morphine-treated rats (subcutaneously implanted with pellets for 7 days), a challenge dose of morphine (30 mg/kg intraperitoneally (i.p.)) did not modify the plasma corticosterone levels and inhibited the morphine-induced decreases in hypothalamic NA and DA content. These results suggest that: (1) In naive rats, the morphine-induced activation of hypothalamus-pituitary-adrenocortical (HPA) axis is mediated by catecholaminergic neurons in the hypothalamus; (2) In tolerant rats morphine did not modify the plasma corticosterone concentrations, presumably by attenuating hypothalamic noradrenergic and dopaminergic activity. (3) Hypothalamic catecholamines have a role in regulating the HPA axis during morphine tolerance.