Hypophysectomy does not block sensitization to the dopamine agonist quinpirole or its modulation by the MAOI clorgyline
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Repeated administration of the dopamine agonist quinpirole induces behavioral sensitization in rats that is characterized by a four- to eight-fold increase in the amount of locomotion compared to an acute dose of quinpirole, in the absence of any increases in mouthing behavior. The monoamine oxidase (MAO) inhibitor, clorgyline, switches behavioral sensitization to quinpirole from that of locomotion to self-directed mouthing. The mechanism by which clorgyline produces this switch in behavioral sensitization is unknown, but is independent of the known effects of clorgyline, namely, inhibition of MAO, inhibition of striatal dopamine uptake, or stimulation of sigma and I(2) receptors. Because clorgyline also inhibits hypothalamo-pituitary-adrenal (HPA) axis function, and increased HPA activity facilitates the behavioral effects of psychostimulant drugs, the effects of clorgyline on quinpirole sensitization are possibly due to an inhibition of HPA function. Therefore, the present study examined whether HPA activity is required for sensitization to quinpirole, and whether clorgyline exerts its effects on quinpirole sensitization via inhibition of HPA function. Control and hypophysectomized rats were administered clorgyline (1 mg/kg, s.c.) or vehicle 90 min before each injection of quinpirole (0.5 mg/kg x 8, twice weekly) or saline. To assess the level of sensitization reached by control and hypophysectomized rats, test injections of quinpirole (0.0, 0.07, and 0.2 mg/kg) were administered. Chronic quinpirole administration produced equivalent levels of locomotor sensitization in control and hypophysectomized rats. Clorgyline was equally effective in blocking the development of locomotor sensitization in control and hypophysectomized rats, and in sensitizing self-directed mouthing. The present study suggests that (1). HPA function is not necessary for the development of quinpirole sensitization and, (2). clorgyline does not produce its effects on behavioral sensitization to quinpirole via an inhibition of HPA activity. Moreover, the observation that quinpirole sensitization develops normally in the absence of any pituitary endocrine function suggests that pituitary-gonadal and pituitary-thyroid axes activity are also not necessary for quinpirole sensitization to occur.
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