Following on unilateral injection of 6 -hydroxydopamine(6 -OHDA ) into the substantia nigra, rats circle in the opposite direction when challenged with apomorphine. In this study we examined the effects of norepinephrine, prazosin, yohim bine to investigate the possible involvement of the brain noradrenergic system in the apomorphine-induced rotational behavior of the unila teral substantia nigra-lesioned rats with 6 -OHDA . The results are as follows : 1) Apom orphine induced time-dependant contralateral rotation in the unilateral 6 -OHDAlesioned rats. Maximum development of supersensitivity to apomorphine was observed 4-5 weeks after denervation. 2) Norepinephrine increased the rotatory effect of apomorphine. 3) Prazosin, an a 1-adrenoceptor antagonist decreased the rotatory effect of apomorphine, and the time-dependant rotatory effect of apomorphine returned to the control level after the removal of prazosin. 4) Y ohim bine, a presynaptic a2-adrenoceptor antagonist enhenced the rotatory effect of apomorphine and the time-dependant rotatory effect of apomorphine returned to the control level after the removal of yohimbine. From the above results, it is suggested that the rotatory effect of apomorphine is modulated by the brain noradrenergic system.
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